佟小雪
, 王长乐
Tong Xiaoxue, Wang Changle
中图分类号: P618.31
文献标识码: A
文章编号: 1001-8166(2018)02-0152-14
通讯作者:
收稿日期: 2017-10-12
修回日期: 2017-11-27
网络出版日期: 2018-02-20
版权声明: 2018 地球科学进展 编辑部
基金资助:
作者简介:
First author:Tong Xiaoxue(1993-), female, Tangshan City, Hebei Province, Ph.D student. Research areas include BIF iron ore deposit.E-mail:xiaoxuetong1993@mail.iggcas.ac.cn
作者简介:佟小雪(1993-),女,河北唐山人,博士研究生,主要从事BIF型矿床研究.E-mail: xiaoxuetong1993@mail.iggcas.ac.cn
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摘要
条带状铁建造 (BIF)原生矿物组成有助于约束其沉积相和沉积环境,当前主要认为三价铁氢氧化物或铁硅酸盐微粒 (主要成分为铁蛇纹石或黑硬绿泥石) 可能是BIF原生矿物的主要成分,在后期成岩或变质作用过程中转变为赤铁矿、磁铁矿、菱铁矿等矿物。根据BIF的矿物组合可将其沉积相划分为氧化物相、硅酸盐相和碳酸盐相。通过沉积地层学和地球化学等方法研究,以古元古代大氧化事件为标志将沉积相总结为“缺氧还原”和“分层海洋”2种相模式:大氧化事件前,古海洋整体处于缺氧还原环境,BIF沉积相从远岸到近岸呈赤铁矿相—磁铁矿相—碳酸盐相分布,如南非West Rand群BIF (2.96~2.78 Ga) 和Kuruman BIF (约2.46 Ga);大氧化事件期间及之后,古海洋上部氧化、下部还原,BIF沉积相与之前截然相反,从远岸到近岸呈碳酸盐相—磁铁矿相—赤铁矿相分布,如中国袁家村BIF (2.2~2.3 Ga) 和加拿大Sokoman铁建造 (约1.88 Ga)。总体看来,只有特定的沉积环境才能形成这种特殊的地质历史上不再重复出现的沉积建造,而原生矿物组成的甄别和推导、沉积相的形成机制、BIF沉淀条件的准确限定和微生物活动与BIF的关联等问题是推测古海洋环境的关键所在,也是目前亟待解决的问题。
关键词：
Abstract
The primary mineral compositions of BIF are regarded as ferric oxyhydroxide or iron silicate nanoparticles (mainly greenalite and stilpnomelane ) whichcan transform into minerals like hematite, magnetite and siderite. On the basis of predominant iron minerals, three distinctive sedimentary facies are recognized in BIF: oxide facies, silicate facies and carbonate facies. Marked by the Great Oxidation Event (GOE, 2.4~2.2 Ga), sedimentary facies can be divided into two models: “anoxic and reducing” model and “stratified ocean” model. The ancient ocean was anoxic and reducing before GOE, and under this circumstance, BIF was distributed from the distal to proximal zones transforming from hematite facies through magnetite facies to carbonate facies, such as West Rand Group BIF (2.96~2.78 Ga) and Kuruman BIF (~2.46 Ga) in south Africa. However, the ancient ocean was a stratified ocean during and after GOE, which means that shallow seawater was oxidizing while deeper seawater was reducing, leading to an opposite sedimentary facies distribution compared to the former one: BIF was distributed from the distal to proximal zones transforming from carbonate facies through magnetite facies to hematite facies, such as Yuanjiacun BIF in China (~2.3 Ga) and Sokoman iron formation in Canada (~1.88 Ga). Overall, BIF is an unrepeatable formation in geological history, which can only form in specific sedimentary environment. The key point to speculate the paleo-ocean environment, namely the problems to be solved at the moment, is to identify and derive the primary mineral compositions, to make sure the genetic mechanism of sedimentary facies especially silicate facies, to restrict the sedimentary conditions and to study microbial activities contacting with BIF.
Keywords：
条带状铁建造 (Banded Iron Formation, BIF) 主要分布于早前寒武纪 (3.2~1.9 Ga)的古老克拉通内,通常由富铁和富硅条带组成,且全铁的质量百分含量大于15%[1,2]。BIF是早期地球最具独特性且规模巨大的海洋化学沉积岩,故对其研究可用来揭示古海洋和大气的氧化还原状态和原始生命活动[3~8]。然而,BIF在沉积后普遍会经历成岩、变质及热液等作用改造[5],导致其矿物组成的连续转变、矿物颗粒粒径的变化和原始沉积结构的破坏,且现代海洋缺乏可与BIF类比的岩相建造,从而致使BIF原生矿物及演变机制、沉积相成因及过程等方面的研究存在极大争议,造成目前对古海洋环境存在不同认识[9~14]。因此,对BIF现存矿物成因的研究、原始矿物组成的推导和沉积相模式的建立是当前研究古海洋环境和生物活动的重点和难点。
为此,在系统总结近年来国内外BIF成因资料的基础上,本文对BIF中矿物特征和成因、沉积相及沉积模式等方面的研究进展进行了详细梳理和综合整理,指出了当前存在的问题和今后的研究方向,以期为早前寒武纪古环境的研究提供有用信息。
根据BIF的构造—沉积环境,Gross[15]将其划分为阿尔戈玛型 (Algoma型) 和苏必利尔湖型 (Superior型)。二者矿物组成有一定差异,Superior型BIF通常变质程度较低 (常为绿片岩相),主要由燧石 (石英)、磁铁矿、赤铁矿、菱铁矿、铁蛇纹石和黑硬绿泥石等矿物组成;而Algoma型BIF变质程度相对较高 (常为高绿片岩相—角闪岩相,部分可至麻粒岩相),因而其矿物组成除石英和磁铁矿之外,还含有少量阳起石和镁铁闪石等[7,16]。除以上2种主要形成于早前寒武纪 (3.2~1.9 Ga) 的BIF之外,还有新元古代 (0.8~0.6 Ga ) 的拉皮坦型 (Rapitan型) BIF,其成因与“雪球事件”密切相关,未经历作用或变质程度较低 (绿片岩相),典型矿床如巴西Urucum地区BIF、埃及Um Anab和 Wadi Karim地区BIF以及中国江西新余地区BIF等[17~22]。值得注意的是,该类型BIF条带不发育、与冰碛岩相关且主要矿物为赤铁矿和碧玉,明显区别于早前寒武纪的BIF,因此,本文着重论述典型Algoma型 和Superior型BIF矿物及沉积相的相关问题。
BIF现存矿物是成岩或变质作用过程中的产物[23~26],不同类型的沉积相与变质相直接控制BIF形成不同的矿物组合[27]。鉴于Superior型BIF变质程度普遍较低且分布范围广,因此目前关于矿物成因的研究主要集中于古元古代Superior型BIF,如西澳哈默斯利Dales Gorge BIF,南非德兰士瓦BIF和中国山西袁家村BIF等[12,13,28,29]。下面将对BIF主要组成矿物的成因进行阐述。
燧石在BIF中普遍存在,在较强变质的BIF中多以石英形式出现。石英和燧石常被认为是早期沉淀的硅质在成岩或变质过程中重结晶的产物[5]。由于显生宙之前海洋中普遍缺乏能消耗硅的生物,因而前寒武纪海水硅浓度异常高 (约2 mmol),大约为现代海洋 (约0.1 mmol) 的数十倍,接近无定形硅的饱和浓度[30,31]。因此,硅可因蒸发作用或温度变化导致过饱和而沉淀[32,33],或者与固相含铁矿物 (如三价铁氢氧化物) 同沉淀[34,35]。当前,可利用燧石或石英的原位硅和氧同位素特征 (δ30Si和δ18O) 来判别其成因,如Delvigne等[36] 发现南非2.95 Ga Pongola BIF单条带的硅同位素值 (δ30Si) 自下而上由-2.27‰升至-0.53‰,认为这种趋势由2个连续的同位素分馏过程造成:①在沉淀过程中,溶解的硅[Si(OH)4] 被三价铁氢氧化物吸附沉淀,此过程优先吸附28Si,造成古海水中δ30Si升高;②在早期成岩过程中,发生去吸附作用,此过程将所有的硅质都释放至孔隙水中,导致硅局部过饱和而形成硅质层,此过程优先沉积28Si。上述2个过程会造成硅质层的δ30Si为负值,残余的孔隙水会逐渐富30Si,致使随后再沉积的硅质层中δ30Si相对于下部硅质层较高。然而,Krapež等[23]和Pickard等[37]发现哈默斯利盆地的BIF中层状燧石中普遍存在侵蚀面,条带不连续或呈透镜体、结核状产出,且局部可见交代部分含铁矿物的现象,进而推测燧石可能形成于压实之前的早期成岩过程,是在水岩界面附近硅质交代早期沉积物的产物,而非直接沉淀成因。
赤铁矿在BIF中分布较为广泛,形态多样,关于其是否为原生成因是当前关注的热点。早期学者发现BIF中赤铁矿可呈微小球状体 (3~5 nm)[38,39],与现代红海沉积物中的赤铁矿形态相似[40],据此认为赤铁矿可能是原始沉积的三价铁氢氧化物在早期成岩过程中发生脱水的产物[5,41,42],且脱水过程应当发生于中性—弱碱性 (pH=7~8)、低碳的环境[43]。
近年来部分学者认为BIF中赤铁矿成因具有多样性[10,12,25]。如Rasmussen等[10]通过详细的岩相学观察,发现哈默斯利盆地Dales Gorge BIF中黑硬绿泥石和铁白云石等矿物微粒从边缘到中心被赤铁矿交代,说明赤铁矿应形成于其他矿物的氧化作用。Sun等[12]同样基于岩相学和矿物学研究发现,加拿大2.7 Ga Abitibi绿岩带Hunter Mine BIF和南非2.46 Ga Kuruman BIF中的赤铁矿可分为以下3类:①分布于富铁条带中,由纳米级赤铁矿微粒 (3~5 nm) 组成集合体 (约200 nm);②位于富铁和富硅条带过渡带内,以次微米级赤铁矿微粒分散于燧石基质中;③ 赋存于富硅条带中,呈针状、放射状和纤维状,并沿裂隙交代黑硬绿泥石或碳酸盐。结合矿物组构分析,进一步发现前2种形态的赤铁矿一般呈纳米—微米级粒度、非定向分布,且与典型赤铁矿的晶格间距一致,因而推测二者应为原生矿物,是由早期形成的三价铁氢氧化物脱水形成,而第三种为交代成因的次生赤铁矿[12]。
磁铁矿是BIF中最主要的含铁矿物,目前大部分学者认为其是三价铁氧化物或氢氧化物在成岩和变质过程中发生转变的产物[5,38,39,44,45]。该认识的主要岩相学证据是在BIF中可见磁铁矿沿赤铁矿边缘生长的现象[6,46]。关于磁铁矿具体的形成机制主要有以下4种认识:
(1) 形成于微生物异化还原反应过程
大量岩相学、同位素证据及实验模拟结果表明,磁铁矿为三价铁氢氧化物经微生物异化还原作用 (Dissimilatory Iron Reduction, DIR) 形成[39,44,47~49],其可能的反应式为:CH3COO-+24Fe(OH)3(水铁矿) + OH-→8Fe3O4(磁铁矿) + 2HC+37H2O[50]。主要证据有以下3点:①Johnson等[49]通过实验模拟发现,由DIR作用形成的磁铁矿的δ56Fe值与哈默斯利和德兰士瓦BIF中的δ56Fe值相似,均为负值,并认为在2.5 Ga左右磁铁矿主要是通过DIR作用形成;②Li等[44]通过对Dales Gorge BIF中磁铁矿的晶体化学分析显示,其晶格常数、Fe2+/Fe3+值与实验中DIR作用产生的磁铁矿极为相似;③Li等[39]通过实验发现微生物DIR作用可使磁铁矿增大将近1 μm。
(2) 形成于三价铁氢氧化物与海水中Fe2+反应过程
在温度低于200 ℃且贫硫、贫有机碳的条件下,三价铁氢氧化物与Fe2+反应也可形成磁铁矿[45,49],反应式为:2Fe(OH)3(水铁矿) + Fe2+ → Fe3O4(磁铁矿) + 2H2O + 2H+。部分BIF中磁铁矿铁同位素值可提供佐证,如加拿大1.88 Ga Sokoman BIF中磁铁矿的铁同位素值为-0.5‰~0.5‰,反映磁铁矿可能由Fe2+离子还原三价铁氢氧化物而形成[11]。
(3) 形成于早期菱铁矿的氧化过程
实验证明[51],菱铁矿在一定温压条件下(T=450 ℃,P=2 MPa)发生氧化形成磁铁矿,反应方程式如下:12FeCO3(菱铁矿) + 2H2O → 4Fe3O4(磁铁矿)+11CO2 + CH4。部分学者发现磁铁矿有沿着菱铁矿或铁白云石边缘生长的现象,为该反应的发生提供了证据[26]。
(4) 形成于赤铁矿和菱铁矿的变质反应过程
赤铁矿和菱铁矿在温度和压力较高的条件下发生反应可形成磁铁矿,方程式为:3FeCO3(菱铁矿) + Fe2O3(赤铁矿) → Fe3O4(磁铁矿) + CO2(T=480~650 ℃,P=5~12 MPa)[52]。由于磁铁矿广泛分布于不同变质级别的BIF中,且目前并未发现其同时交代赤铁矿和菱铁矿的现象,因此,该种途径可能仅为磁铁矿形成的次要机制,而成岩过程中三价铁氢氧化物与Fe2+或微生物反应可能为磁铁矿形成的主要方式。
Li等[14]通过实验模拟缺氧环境下三价铁氢氧化物与热液Fe2+反应,认为早太古代BIF中的磁铁矿是原生矿物,可由富Fe2+的热液与沉淀的三价铁氢氧化物在水体中反应直接形成。实验中可见一种处于亚稳态的中间产物——绿锈 (Green Rust,GR,一种含Fe2+,Fe3+和OH-的盐类矿物),该物质在大于50 ℃条件下会转变为磁铁矿。仔细来看,该实验结果仍存在3点争议:①反应的发生并非仅局限于水体中,也可能发生于沉积后早期成岩过程;②未考虑前寒武纪海洋高Si浓度的影响;③实验反应的温度较低,一般来讲,热液相比于太古代海水 (55~85 ℃)[53],温度明显偏高 (>250 ℃)[54]。
菱铁矿是较低变质级别BIF中常见的碳酸盐矿物,在后期成岩或变质作用过程中可转化为铁白云石和方解石等[5,55]。早期学者认为菱铁矿类似于海相碳酸盐岩,当水中存在溶解的二氧化碳时,可能与Fe2+结合形成菱铁矿[56~58]。
当前,大部分岩相学、地球化学和同位素证据说明菱铁矿是成岩作用的产物[46,59]。如菱铁矿沿磁铁矿边缘生长;菱铁矿中可见燧石和赤铁矿的包裹体[5,46];Fischer等[59]研究发现南非Kaapvaal克拉通的BIF中菱铁矿的碳同位素值 (-14‰~-3‰) 与其上覆碳酸盐矿物的碳同位素值 (0‰) 明显不同。随后,Heimann 等[55] 通过对比南非2.46 Ga Kuruman BIF中的菱铁矿及其下部Gamohaan建造中钙镁碳酸盐岩的铁、碳和氧同位素发现,菱铁矿的δ56Fe值多变,介于+1‰~-1‰;δ13C值为-2.6‰~-12‰,低于海相碳酸盐岩的值;而δ18O值偏高,可达+21‰。这些特征说明菱铁矿并非直接沉积成因,可能是由早期形成的三价铁氢氧化物在有机碳存在的情况下经微生物DIR作用而生成。
硅酸盐矿物在BIF中广泛分布,矿物种类主要取决于其经历的变质程度高低。低级变质条件下,多形成铁蛇纹石、黑硬绿泥石和铁滑石等矿物,在Superior型BIF中常见,如哈默斯利盆地Dales Gorge BIF[10] 和山西袁家村BIF[28];高级变质条件下,多形成黑云母、角闪石甚至辉石等矿物,常见其交代早期硅酸盐或碳酸盐矿物,多存在于Algoma型BIF中,如内蒙古固阳地区BIF以角闪石为主[60],而西非南部喀麦隆Bikoula BIF[61]和中国辽北清原地区小莱河BIF[62]则以辉石为主。
大部分学者认为硅酸盐应是在成岩或变质作用过程中形成的。近年来,部分学者认为铁硅酸盐微粒可能是原生矿物,如铁蛇纹石和黑硬绿泥石等[5,24,31,63,64]。如Rasmussen等[24]应用扫描电镜、透射电镜等方法研究西澳2.5 Ga Dales Gorge BIF发现,在燧石结核中存在大量黑硬绿泥石球形微粒,大小较为均一 (5~20 μm),边部被硅质环绕,推测黑硬绿泥石的形成应早于硅质胶结,可能为原生沉积矿物。随后Rasmussen等[63]对西澳哈默斯利盆地其他BIF、含铁石英岩和含铁泥岩等进行观察发现,在薄层状燧石条带中同样发现有大量分散分布的纳米级铁硅酸盐微粒 (主要成分为黑硬绿泥石和铁蛇纹石)。然而Haugaard等[65]对哈默斯利盆地Joffre BIF及其围岩进行研究发现,二者均有黑硬绿泥石颗粒和火山碎屑,因此认为黑硬绿泥石可能与火山灰物质的混染有关,而非原生沉积。Tosca等[64]通过实验模拟认为,铁蛇纹石可在碱性、富硅铁及缺氧环境下形成,该实验条件可与前寒武纪海洋环境类比,说明铁蛇纹石可能为原始成因。由于铁蛇纹石的形成往往会消耗一定硅质,可能造成古海水中硅质不饱和而缺乏二氧化硅的沉淀,因而在一些硅酸盐相BIF局部可见硅酸盐和磁铁矿条带而无石英条带的现象。
探索原生矿物信息是BIF研究最基本的内容之一,这不仅有助于理解BIF的沉淀机制和条带的形成过程[24,33],而且可精细约束BIF各沉积相的内在联系[56]。当前,关于BIF原始沉积物的认识主要有以下4种观点:
(1)认为BIF早期最主要的原生矿物为三价铁的氢氧化物和硅质,可能有少量含铁碳酸盐或硅酸盐[6,26,31,38,66],这些物质经由后期的成岩和变质作用演变为现今可见的矿物组合 (图1)。BIF中常见赤铁矿微粒和燧石,可支持此观点[1,41,67,68]。赤铁矿一般粒径较细,小于0.5 μm,常孤立分布于硅质条带中,沿原始沉积层理展布[68],综合说明其应为原始沉积的三价铁的氢氧化物脱水的产物[41,42,46]。
(2)认为BIF原始沉积的产物主要由贫铝含水的铁硅酸盐和铁的氢氧化物组成,可与现代海底黑烟囱附近的富铁沉积物类比[23,69]。Krapež等[23]通过分析哈默斯利Brockman BIF现存矿物成分和形态结构等,认为原始矿物应由富铁蒙脱石、铁氢氧化物和菱铁矿等组成。富铁蒙脱石可在后期地质作用过程中转化为铁蛇纹石或黑硬绿泥石[70],硅质在早期成岩过程中对原始沉积物进行交代。
图1 华北克拉通古元古代袁家村BIF矿物共生次序图 (据参考文献[26]修改)
Fig.1 Schematic paragenetic sequence for the Yuanjiacun BIF in the North China Craton (modified after reference[26])
(3)与上述观点不同的是,强调铁硅酸盐颗粒为BIF最为主要的原生矿物。Rasmussen等[13,24,63] 采取光学显微镜、扫描电镜和透射电镜对西澳哈默斯利盆地和南非德兰士瓦盆地BIF展开详细的岩相学观察,结果显示BIF中均存在铁硅酸盐微粒,成分为黑硬绿泥石或铁蛇纹石。因这些铁硅酸盐矿物常见被石英包裹,推测其为原生沉积矿物,沉积成岩过程中被硅质胶结。并且,Rasmussen等[10,13,25]进一步的岩相学证据显示,赤铁矿可呈铁硅酸盐晶形假象 (图2),且在铁硅酸盐微粒的部分空洞中存在自形的赤铁矿晶体,因而推测赤铁矿可能为铁硅酸盐微粒后期发生氧化所致。Konhauser等[31]和Tosca等[64]通过对前寒武纪海洋环境的模拟实验发现,铁硅酸盐可在古海水中由二价铁离子和无定形硅直接反应生成。
(4)认为GR可能是BIF的原始沉积物[71],其分子式是[F
图2 反射光下赤铁矿从边缘向中心逐步交代黑硬绿泥石微粒[
(a)燧石 (ch) 条带中的黑硬绿泥石微粒 (stp);(b),(c)黑硬绿泥石微粒边缘被赤铁矿 (hem) 交代;(d),(e)赤铁矿包裹黑硬绿泥石核;(f)赤铁矿完全交代黑硬绿泥石
Fig.2 Reflected Light (RL) image showing that hematite replaces stilpnomelane microgranules from margin to cores and finally turns into solid hematite microgranules[
(a) Stilpnomelane microgranule (stp) in chert (ch). (b),(c) Stilpnomelane microgranules (stp) surrounded by thin, discontinuous rims of hematite (hem). (d),(e) Thick rims of hematite (hem) around stilpnomelane core. (f) Hematite microgranule
尽管BIF沉积后会经历一系列成岩和变质作用,但其稀土元素和铁同位素组成仍可用来指示古海洋的氧化还原条件[49,72]。
研究表明,形成于古元古代大氧化事件 (the Great Oxidation Event, GOE) 之前的大部分BIF缺乏Ce负异常,说明当时古海水整体处于缺氧状态,如西非中太古代的Bikoula BIF,中国鞍本地区BIF (约2.55 Ga) 等[61,73~77]。少量BIF显示出Ce负异常,如巴西米纳斯吉拉斯地区2.65 Ga的Itabira BIF[78]和中国五台地区2.54 Ga的王家庄BIF,说明局部水体为弱氧化[79]。
GOE期间及之后,全球大气氧含量急剧升高,该时段形成的BIF通常显示出正、负Ce异常,较大变化范围的Y/Ho值和LREE/HREE值,说明古海洋是上部氧化、下部还原的分层海洋,其稀土元素特征主要受控于水体中的铁锰氢氧化物载体的形成和溶解[73],如中国山西2.3 Ga的袁家村BIF[28],加拿大苏必利尔湖地区1.88 Ga 的Gunflint BIF和Biwabik BIF[73]。具体机制为,Mn2+在水体上部发生氧化,形成铁锰氢氧化物,同时优先吸附轻稀土 (LREE),Ho和Ce4+,促使表层水体具有负Ce异常、高Y/Ho值和低LREE/HREE值[54,80];当铁锰氢氧化物进入下部缺氧水体时,会发生还原性溶解并释放吸附元素,从而导致局部水体LREE,Ho和Ce4+浓度的显著升高,显示出正Ce异常、低Y/Ho值和高LREE/HREE值[81]。
但也有个别学者认为3.2 Ga之前就出现了海洋的氧化还原分层现象。如Huston等[4] 认为3.2 Ga之前,既存在层状硫酸盐矿床 (形成环境一般较为氧化且高硫),也存在阿尔戈玛型BIF (缺氧还原且低硫),定性推测3.2 Ga前的古海洋是分层的;Satkoski等[82]通过对南非3.2 Ga Manzimnyama BIF铁同位素研究发现,沉积于浅水环境下BIF的δ56Fe平均为0.18‰,接近热液的δ56Fe值 (0‰),说明近岸处水体氧化程度较高;而较深水环境下BIF的δ56Fe值相对较高,平均为0.55 ‰,说明远岸处水体氧化程度较低。
James[1]最早根据矿物组合将BIF沉积相划分为氧化物相、硅酸盐相、碳酸盐相和硫化物相。其中,氧化物相的主要含铁矿物为磁铁矿和赤铁矿,碳酸盐相的主要含铁矿物是菱铁矿和铁白云石,硅酸盐相的矿物组成较复杂,主要取决于沉积后的变质程度,主要为黑硬绿泥石、铁蛇纹石和角闪石等。硫化物相以出现黄铁矿为特征,由于其具有较高的硅质碎屑含量,所以常被描述为黄铁矿化的黑色页岩。虽然硫化物相与其他各相BIF联系紧密,但对于其中黄铁矿的沉积成因仍存在较大争议[83]。当前,大部分学者认为硫化物相,包括黄铁矿化页岩和含硫化物燧石等,不能当作BIF沉积相的一类[6,67,84]。因此,本文将BIF主要划分为以下3相,主要特征见表1。
表1 BIF沉积相主要特征 (据参考文献[1]修改)
Table 1 Principal characters of BIF sedimentary facies (modified after reference[1])
| 沉积相 | 碳酸盐相 | 硅酸盐相 | 氧化物相 | |
|---|---|---|---|---|
| 磁铁矿亚相 | 赤铁矿亚相 | |||
| 岩石学特征 | 灰色,风化后呈土黄色, 燧石和碳酸盐互层 | 浅绿—墨绿色,不同比例的 燧石、硅酸盐形成条带 | 深灰色,磁铁矿 和燧石互层 | 灰黑色或灰红色,赤铁矿与灰色燧石或浅红色的碧玉形成条带 |
| 形成位置 | 近岸处或海底喷口附 近,与碳质来源有关 | 一般为陆架中部,有机碳贫 乏环境 | 深水盆地环境或 陆架中部 | 近岸浅水处或深水有机碳贫乏环境 |
| 主要含铁 矿物 | 菱铁矿和铁白云石等 富铁碳酸盐 | 随变质程度的变化而不同,可能为铁蛇纹石、黑硬绿泥石、铁滑石、绿泥石、角闪石等 | 磁铁矿 | 赤铁矿 |
| 其他含铁 矿物 | 黄铁矿、磁铁矿、黑硬 绿泥石、铁滑石 | 含铁碳酸盐、磁铁矿 | 铁蛇纹石、黑硬绿泥石、铁滑石、含铁碳酸盐 | 磁铁矿 |
| 金属矿物含量范围 | 20%~35% | 20%~30% | 25%~35% | 30%~40% |
| 区别性特征 | 可见沉积缝合线构造 | 硅酸盐矿物种类可判定 变质程度 | 强磁性 | 常见鲕粒结构 |
| 典型矿床 | 南非Kuruman BIF,南 非West Rand 群的BIF | 加拿大Michipicoten BIF, 中国鞍本地区BIF | 西格陵兰Isua BIF,中国冀东司家营BIF等 | 西澳哈默斯利盆地BIF,中国袁家村BIF等 |
BIF的沉积作用与古海洋的氧化还原条件密切相关,总体看来,BIF的沉积相模式以古元古代大氧化事件为界 (2.4~2.2 Ga) 可分为“缺氧还原”和“分层海洋”环境2类。
5.2.1 “缺氧还原”环境下的沉积相模式
在GOE之前,古海洋整体处于缺氧还原状态,Fe2+氧化成Fe3+的机制主要为生物产氧氧化、微生物新陈代谢氧化、紫外线光氧化等,国内外学者应用不同的氧化机制建立沉积相模式,以南非Kuruman BIF(约2.46 Ga)和West Rand群BIF(2.96~2.78 Ga)为例,沉积相从远岸到近岸依次为赤铁矿—磁铁矿—碳酸盐相。
Klein等[56]基于南非2.46 Ga Kuruman BIF的岩相地层学和地球化学研究,认为BIF的沉积与海侵过程相关,大陆有机碳的输入控制了沉积相的展布。其中,在海侵序列中,海平面上升,导致热液中的Fe2+上升到透光区内,与生物光合作用产生的氧气反应形成铁氧化物或氢氧化物,最终沉积转变为赤铁矿相和磁铁矿相BIF;由于透光带位置较浅,致使少量碳质可到达深部大陆架,从而在近岸处可形成碳酸盐相BIF。在海退过程中,透光带位置较深,近岸处可沉积大量微生物灰岩,进而促使深水盆地的碳质输入增强,从而在远岸处形成炭质页岩。
Beukes等[67]基于最新地层关系和地球化学数据,对早期Kuruman BIF沉积模型进行重新修改 (图3)。相比于早期模型,主要有4点不同:①透光区无绝对数值限定;②浅部碳酸盐岩和深部BIF均存在Eu正异常,说明高温热液对整个盆地水体有强烈影响,深部和浅部水体在较大范围内发生充分混合;③浅水碳酸盐岩缺乏Ce负异常,说明古海洋表面氧逸度较低,推测Fe2+的氧化机制为微需氧的化能自养铁氧化细菌氧化,而非自由氧;④三价铁氢氧化物是唯一含铁原生沉积物,菱铁矿应为成岩作用过程中微生物异化还原作用所致。
南非中太古代(2.96~2.78 Ga) West Rand群BIF与Kuruman BIF具有相似的沉积相分布,Smith等[85]针对性地开展了岩相学、沉积学和地球化学特征研究,认为沉积相与热液柱位置、碳质供应和微生物作用等密切相关 (图4)。具体来看,海侵过程中,富Fe2+海底热液上升运移,在微需氧铁氧化细菌作用下氧化形成三价铁氢氧化物沉积[31,66]。这些细菌可存在于热液与周围海水的接触界面附近。在远岸处,热液由于浮力作用未与海底三价铁氢氧化物长时间接触,且有机碳输入较少,因此三价铁氢氧化物可直接脱水转变为赤铁矿;而在近岸处,有机碳含量逐渐升高,三价铁氢氧化物与之反应形成菱铁矿 (或有少量磁铁矿),并且,热液柱与沉积物的长期接触可造成Fe2+与三价铁氢氧化物发生非氧化还原反应形成磁铁矿[45]。
图3 Kuruman BIF的沉积模式 (据参考文献[67]修改)
Fig.3 Model for deposition of Kuruman banded iron formation (modified after reference[67])
5.2.2 “分层海洋”环境下的沉积模式
在GOE期间及之后,古海洋上部氧化下部还原,BIF沉积相从远岸到近岸主要为碳酸盐—磁铁矿—(硅酸盐)—赤铁矿相。下面以GOE期间2.3 Ga袁家村BIF和GOE之后1.88 Ga Sokoman铁建造为例简述相应的沉积相模式。
Wang等[28]对中国山西袁家村BIF进行了详细的岩相学和地球化学研究发现, BIF从远岸到近岸为碳酸盐—硅酸盐—磁铁矿—赤铁矿相分带 (图5)。赤铁矿相局部发育鲕粒和豆粒状赤铁矿,且具有Ce负异常、LREE/HREE值变化范围较大等特征,综合说明其是在相对氧化的高能浅水环境下沉积。其余相的BIF整体以条纹、条带状构造为主,无Ce异常或正Ce异常,可能形成于浪基面以下的稳定还原水体。其中,富碳酸盐相BIF具有较大的Eu正异常,说明其距热液喷口相对较近,可能位于盆地深部,且距离海岸线越远,碳酸盐矿物含量越高,说明沿着这个方向有机碳的输入逐渐增加[67,87],可能是海底热液喷口提供大量营养物(如V,Fe,Co和Zn),导致附近发育微生物而发生DIR作用,形成菱铁矿等矿物。而在远离热液喷口的地方,有机碳的输入速度急剧降低,从而致使三价铁氢氧化物聚集在这些地方,随后转变为赤铁矿或磁铁矿。
图4 West Rand群中与页岩相关铁建造的沉积模型[
反应(1)引自参考文献[45],反应(2)引自参考文献[66]
Fig.4 Simplified depositional model for the shale-associated iron formation of the West Rand Group[
Reaction (1) adapted from reference[45] and reaction (2) taken from reference[66]
图5 袁家村BIF沉积相的沉淀模型[
反应(2)和(3)分别源自参考文献[67]和[45];反应(4)据参考文献[55]修改;反应(6)来自参考文献[66]; 反应(7)来自参考文献[86]
Fig.5 Conceptual depositional model for the Paleoproterozoic Yuanjiacun BIF[
Reactions (2) and (3) are adapted from reference [67] and [45], respectively; Reaction (4) is modifed after reference[55]; Reaction (6) is from reference[66], and reaction (7) is adapted from reference[86]
加拿大1.88 Ga的Sokoman铁建造与中国袁家村BIF较为相似,Pufahl等[81]通过详细的岩相学研究发现,该建造从远岸到近岸依次为富磁铁矿—铁硅酸盐铁建造、富赤铁矿的粒状铁建造 (Granular Iron Formation, GIF)、含叠层石的燧石岩等 (图6)。富赤铁矿GIF发育内碎屑结构和板状交错层理,应是受波浪和潮汐再作用而形成,且存在明显的Ce负异常,说明其可能形成于富氧的潮间带或潮上带。富磁铁矿—硅酸盐铁建造呈条带状—条纹状构造,缺乏Ce负异常,局部发育正Ce异常和相对较高的LREE/HREE值,综合说明其应形成于浪基面下部的缺氧还原水体[11]。相比于袁家村BIF,区别主要为以下2点:一是此模式中碳酸盐相缺乏,可能是由于碳质供给不足;二是此模式中发育富赤铁矿GIF,且磁铁矿—硅酸盐相BIF的正Ce异常较强,说明此时存在明显的氧化还原分层。
综合上述,古海洋氧化还原状态、碳质来源、热液作用、微生物作用等是影响BIF沉积相分布的主要因素。GOE前后BIF的沉积模式截然不同,可能是由于古海洋氧化还原条件发生变化以及不同的有机碳来源,如在 Kuruman 盆地中,碳质来源于近岸处陆架上光合作用的生物,而袁家村盆地的碳质来源可能是海底热液喷口附近发育的微生物[22]。
图6 加拿大Sokoman铁建造沉积相图(据参考文献[81]修改)
SWB为风暴浪基面;FWB为好天气浪基面
Fig.6 Conceptual depositional model for Sokoman iron formation in Canada (modified after reference[81])
SWB: Storm Wave Base. FWB: Fair-weather Wave Base
总体而言,当前早前寒武纪BIF沉积环境的研究工作主要基于现存矿物成因、原生矿物组合及演化、沉积相模式的建立等方面,并取得了一系列研究进展,但仍存在一些有待解决的问题:
(1) BIF原生矿物的精细识别。原生矿物的形成条件是恢复古海洋环境的重要依据,但目前对三价铁氢氧化物、铁硅酸盐微粒等观点仍存在较大争议,可应用SEM与TEM结合单矿物微区原位技术[88]来综合分析矿物成分,或进行实验模拟来探索原生矿物信息。
(2) 硅酸盐相形成机制的详细分析。氧化物相和碳酸盐相主要与古海洋氧化还原状态和碳质来源相关,而硅酸盐相BIF的形成机制仍不清楚,是今后应该探讨的方向。
(3) 微生物活动与BIF沉积作用的关联。微生物在BIF中难寻踪迹,其种类、形态、营养物质来源、生存环境及对BIF沉积作用的影响等问题,是当前研究的热点和难点,可进行微生物实验,模拟铁沉淀过程,探索微生物活动对铁沉淀机制的影响。
(4) BIF沉淀条件的准确限定。古海洋的pH,Eh和水体深度等方面缺少综合、定量的研究。不同的氧化还原指标 (稀土或铁同位素等) 对氧的敏感度不同,反映出的氧化还原状态有所区别,因此,可使用多种指标综合分析古海洋的氧化—还原条件, 如应用稀土元素和U元素含量变化,Fe和U-Th-Pb同位素分析等方法。
致 谢:感谢辽宁省冶金地质勘查局地质勘查研究院文屹、姚良德、刘明军工程师和中国科学院地质与地球物理研究所张帮禄、柯强博士在文章撰写过程中的帮助,感谢审稿人对文章提出的宝贵修改意见及建议,在此表示衷心感谢!
The authors have declared that no competing interests exist.
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Sedimentary facies of iron-formation [J]. |
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Distribution of banded Iron-formation in space and time [J]. |
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Barite, BIFs and bugs: Evidence for the evolution of the Earth’s early hydrosphere [J].
The presence of relatively abundant bedded sulfate deposits before 3.2 Ga and after 1.8 Ga, the peak in iron formation abundance between 3.2 and 1.8 Ga, and the aqueous geochemistry of sulfur and iron together suggest that the redox state and the abundances of sulfur and iron in the hydrosphere varied widely during the Archean and Proterozoic. We propose a layered hydrosphere prior to 3.2 Ga in which sulfate produced by atmospheric photolytic reactions was enriched in an upper layer, whereas the underlying layer was reduced and sulfur-poor. Between 3.2 and 2.4 Ga, sulfate reduction removed sulfate from the upper layer, producing broadly uniform, reduced, sulfur-poor and iron-rich oceans. As a result of increasing atmospheric oxygenation around 2.4 Ga, the flux of sulfate into the hydrosphere by oxidative weathering was greatly enhanced, producing layered oceans, with sulfate-enriched, iron-poor surface waters and reduced, sulfur-poor and iron-rich bottom waters. The rate at which this process proceeded varied between basins depending on the size and local environment of the basin. By 1.8 Ga, the hydrosphere was relatively sulfate-rich and iron-poor throughout. Variations in sulfur and iron abundances suggest that the redox state of the oceans was buffered by iron before 2.4 Ga and by sulfur after 1.8 Ga.
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| [5] |
Some Precambrian Banded Iron-Formations (BIFs) from around the world: Their age, geologic setting, mineralogy, metamorphism, geochemistry, and origin [J]. |
| [6] |
Iron formation: The sedimentary product of a complex interplay among mantle, tectonic, oceanic and biospheric processes [J].
Iron formations are economically important sedimentary rocks that are most common in Precambrian sedimentary successions. Although many aspects of their origin remain unresolved, it is widely accepted that secular changes in the style of their deposition are linked to environmental and geochemical evolution of Earth. Two types of Precambrian iron formations have been recognized with respect to their depositional setting. Algoma-type iron formations are interlayered with or stratigraphically linked to submarine-emplaced volcanic rocks in greenstone belts and, in some cases, with volcanogenic massive sulfide (VMS) deposits. In contrast, larger Superior-type iron formations are developed in passive-margin sedimentary rock successions and generally lack direct relationships with volcanic rocks. The early distinction made between these two iron-formation types, although mimimized by later studies, remains a valid first approximation. Texturally, iron formations were also divided into two groups. Banded iron formation (BIF) is dominant in Archean to earliest Paleoproterozoic successions, whereas granular iron formation (GIF) is much more common in Paleoproterozoic successions. Secular changes in the style of iron-formation deposition, identified more than 20 years ago, have been linked to diverse environmental changes. Geochronologic studies emphasize the episodic nature of the deposition of giant iron formations, as they are coeval with, and genetically linked to, time periods when large igneous provinces (LIPs) were emplaced. Superior-type iron formation first appeared at ca. 2.6 Ga, when construction of large continents changed the heat flux at the core-mantle boundary. From ca. 2.6 to ca. 2.4 Ga, global mafic magmatism culminated in the deposition of giant Superior-type BIF in South Africa, Australia, Brazil, Russia, and Ukraine. The younger BIFs in this age range were deposited during the early stage of a shift from reducing to oxidizing conditions in the ocean-atmosphere system. Counterintuitively, enhanced magmatism at 2.50 to 2.45 Ga may have triggered atmospheric oxidation. After the rise of atmospheric oxygen during the GOE at ca. 2.4 Ga, GIF became abundant in the rock record, compared to the predominance of BIF prior to the Great Oxidation Event (GOE). Iron formations generally disappeared at ca. 1.85 Ga, reappearing at the end of the Neoproterozoic, again tied to periods of intense magmatic activity and also, in this case, to global glaciations, the so-called Snowball Earth events. By the Phanerozoic, marine iron deposition was restricted to local areas of closed to semiclosed basins, where volcanic and hydrothermal activity was extensive (e.g., back-arc basins), with ironstones additionally being linked to periods of intense magmatic activity and ocean anoxia.
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| [7] |
Research progress of Precambrian iron formations abroad and some problems deserving further discussion [J].国外前寒武纪铁建造的研究进展与有待深入探讨的问题 [J].
形成于早前寒武纪的铁建造,是一种富铁〔w(TFe)15%〕的硅质化学沉积岩,其主要矿物组成是铁氧化物(磁铁矿和赤铁矿)及石英。根据铁建造的岩相学特征,将其划分为条带状铁建造和粒状铁建造;根据铁建造的沉积环境,将条带状铁建造划分为与火山岩有关的Algoma型和与细碎屑-碳酸盐岩有关的Superior型2种类型。铁建造的出现,起始于38亿年前,主要集中于28~18亿年,在18亿年之后有一个连续的缺失,但在8亿年左右因雪球事件而重新少量出现。Algoma型铁建造主要发育于中-新太古代,而Superior型则集中出现于古元古代;前者多形成于前寒武纪克拉通化之前,与海相火山活动和陆壳巨量增生密切相关,而后者多形成于克拉通化之后,与稳定发育的克拉通盆地和大气氧含量增加有关。Algoma型铁矿具有单个矿体规模较小、品位较低和多层发育等特征,而Superior型铁矿则具有单个矿体规模较大、品位较高、层位稳定等特征。由于铁建造在地质历史上大规模发育且不重复出现,所以,开展铁建造的研究不仅具有经济价值,而且具有重要的科学意义。铁建造的研究趋势是,在世界范围内进一步深化地球早期构造(地幔柱与早期板块构造)演化、水圈及大气圈组成与演化、地球早期生物活动,以及铁建造成因和时空分布规律等方面的研究。
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| [8] |
Study of the Precambrian BIF-iron deposits in the North China craton: Progresses and questions [J].华北克拉通前寒武纪BIF铁矿研究:进展与问题 [J]. |
| [9] |
Petrology and geochemistry of the ~2.9 Ga Itilliarsuk banded iron formation and associated supracrustal rocks, West Greenland: Source characteristics and depositional environment [J].
Here we present new field, petrographic and geochemical data from the 652.902Ga Itilliarsuk banded iron formation (BIF) and associated lithologies within the Itilliarsuk supracrustal belt, south-eastern Nussuuaq, West Greenland. The supracrustals represent a volcanic–sedimentary sequence, which rests unconformably on a basement of tonalite–trondhjemite–granodiorite (TTG) lithologies. Felsic metagreywackes, meta-semipelites and thinly bedded ferruginous shales were identified intercalated with the Itilliarsuk BIF. Other associated rocks include metapelites, acidic metavolcanics and metagabbroic sills. The supracrustals have experienced amphibolite-facies metamorphism, which has resulted in complete resetting of the U–Pb system with an apparent age of 189502±024802Ma. This tectono-metamorphic event corresponds well with the Paleoproterozoic Rinkian orogeny known from this region. The Itilliarsuk-(oxide-facies) BIF has been divided into two segments on the basis of major and trace elements chemistry: a shaley-BIF with a strong clastic component and a more chemically pure BIF. The shaley-BIF contains high terrigenous influx as reflected by elevated Al 2 O 3 (up to 1202wt.%), TiO 2 , high field strength elements (HFSE) and transition metals. The chemically pure BIF is characterised by alternating high iron (656802wt.%) and high silica (656402wt.%) bands with low total rare earths and yttrium (REY), Al 2 O 3 , TiO 2 and HFSE contents, suggesting a low detrital component. The least altered bands of the BIF record diagnostic Archaean seawater features with Post-Archaean Average Shale (PAAS)-normalised positive La- and Eu-anomalies, enrichment in heavy rare earth elements (HREE) relative to light rare earth elements (LREE) [(Pr/Yb) PAAS 02>021] CHON ) metasediments with affinities to TTG-suites, primarily extrusives, whereas the meta-semipelites and metapelites contain a larger mafic contribution with higher content of Fe 2 O 3 , MgO, Cr, Ni and HREEs. This suggests that the BIF was deposited in a highly unstable basin, presumably in a palaeo-continental slope or outer shelf environment, with frequent fluctuations of epiclastic and volcanogenic sediments derived from adjacent bimodal sources. The T DM model ages and the use of Th–Sc–Zr and La–Th–Sc tectonic discrimination plots indicate that the metasediments were sourced from a juvenile ocean island arc setting.
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| [10] |
Replacement origin for hematite in 2.5 Ga banded iron formation: Evidence for post-depositional oxidation of iron-bearing minerals [J].
Abstract Banded iron formations (BIFs) are central to interpretations about the composition of the Precambrian ocean, atmosphere, and biosphere. Hematite is an important component of many BIFs, and its presence has been used as evidence for the former presence of hydrous ferric oxyhydroxides that formed from the oxidation of dissolved ferrous iron in seawater. However, textural evidence for the origin of hematite is equivocal. New petrographic results show that hematite in unmineralized BIF from the ca. 2.5 Ga Dales Gorge Member of the Brockman Iron Formation, Hamersley Group, Western Australia, including morphologies previously interpreted to represent ferric oxyhydroxide precipitates, formed via fluid-mediated replacement of iron-silicates and iron-carbonates along sedimentary layering. The lateral transition from stilpnomelane- and siderite-rich laminae to hematite-dominated laminae is interpreted to reflect rogressive stages of in situ alteration of reduced mineral assemblages by oxygen-bearing fluids rather than changes in the chemistry of the water column during deposition. Although morphologies previously ascribed to 鈥減rimary鈥 hematite are present, they are related to mineral replacement reactions, raising doubts about the petrographic criteria used to identify original hematite. Hematite replacement in unmineralized BIF postdated deposition and possibly metamorphism, and predated modern weathering. From a regional perspective, it appears to be a distal signature of the processes that were responsible for iron-ore mineralization, which involved the deep infiltration of oxygen-bearing meteoric fluids. The mineral replacement reactions recorded in the Dales Gorge Member are unlikely to be unique and probably occurred in BIFs elsewhere at some point in their history. The observation that at least some of the hematite in unmineralized BIF did not form directly from ferric oxyhydroxides implies that hematite is not a reliable proxy for the composition of the precursor sediment or the redox chemistry of the ocean. The oxidation of ferrous-rich phases after deposition suggests that the precursor sediments of BIF originally had a more reduced bulk composition. This raises the possibility that, in an ocean with negligible molecular oxygen and elevated Si and Fe, the growth of iron-rich clay minerals was favored over hematite.
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| [11] |
The role of sedimentology, oceanography and alteration on the δ56Fe value of the Sokoman Iron Formation, Labrador Trough, Canada [J].
The Fe isotopic composition of 31 whole rock (-0.46 81 δ 56 Fe 81 0.47‰) and 21 magnetite samples (-0.29 81 δ 56 Fe 81 0.22 ‰) from suboxic and anoxic lithofacies was controlled primarily by the physical oceanography of the paleoshelf. Despite low-grade metamorphism recorded by the δ 18 O values of paragenetically related quartz and magnetite, the Sokoman Formation preserves a robust primary Fe isotopic signal. Coastal upwelling is interpreted to have affected the isotopic equilibria between Fe 2+ aq and Fe-(oxyhydr)oxide in open marine versus coastal environments, which controlled the Fe isotopic composition of lithofacies. Unlike previous work that focuses on microbial and abiotic fractionation processes with little regard for paleoenvironment, our work demonstrates that depositional setting is paramount in governing the Fe isotopic composition of iron formations irrespective of what Fe-bearing minerals precipitated.
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| [12] |
Primary hematite in Neoarchean to Paleoproterozoic oceans [J].
Banded iron formations (BIFs) are iron- and silica-rich chemical sedimentary rocks formed throughout the Archean and Paleoproterozoic Eras. The presence of hematite (Fe2O3) and magnetite (Fe3O4) in BIFs has led to the widespread assumption that Fe(II) oxidation must have occurred in the ancient oceans via either a biological or chemical mechanism. However, it is unclear whether the ferric iron now present in BIF represents the original ferric oxyhydroxide [e.g., ferrihydrite, Fe(OH)3] precipitated in the water column, or if it is the result of later-stage circulation of oxidizing fluids through the sediment pile. In this study, we conducted high-resolution microscopic investigations on BIF from the 2728 Ma Abitibi greenstone belt located in the Superior Province of the Canadian Shield and the 2460 Ma Kuruman Iron Formation in South Africa to ascertain the timing and paragenesis of the hematite. Three types of hematite are identified by high-resolution electron microscopic characterization and selected area electron diffraction: (1) 3鈥5 nm ultrafine hematite particles in the iron oxide鈥搑ich bands (H1); (2) submicrometer subhedral to euhedral hematite crystals randomly distributed in the chert matrix of transitional zones between iron oxide鈥 and chert-rich bands (H2); and (3) needle-like, radial and fibrous hematite that replaced stilpnomelane or carbonates and is distributed along fractures or layer boundaries (H3). We interpret the first two types as primary minerals dehydrated from precursor ferric oxyhydroxides. H1 remains ultrafine in size, while H2 has undergone an Ostwald coarsening process facilitated by internal fluids produced during amorphous silica to quartz transformation. H3 is a later-stage mineral formed by external fluid-mediated replacement of iron silicates or carbonates. These results indicate that a significant fraction of the hematite in the BIF originated from ferric oxyhydroxide precursors. Importantly, this implies that photosynthetic Fe(II) oxidation, by either a direct or indirect biological mechanism, did exist in seawaters from which some BIF material was deposited.
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| [13] |
Dust to dust: Evidence for the formation of “primary” hematite dust in banded iron formations via oxidation of iron silicate nanoparticles [J].
Conventional models for the deposition of banded iron formations (BIFs) envisage the oxidation of upwelled ferrous iron and the precipitation of ferric oxide/hydroxide particles in surface waters that settled to form laterally extensive layers of iron-rich sediment. A fundamental tenet of this model is that fine-grained hematite (so-called dusty hematite) in least-altered BIFs represents the dehydration product of original oxide/hydroxide precipitates. However, this premise has never been proven. We have investigated the origin of the earliest-formed iron oxides in chert in well-preserved BIFs of the 2.63–2.45 billion-year-old Hamersley Group, Australia. We find that laminated chert in BIFs show progressive stages of in situ alteration from gray–green chert, containing iron-silicate nanoparticles, to red chert with abundant hematite dust. Analysis of textures by transmission electron microscopy of samples from the transition zone between gray–green and red chert reveals that dusty hematite formed after partial dissolution of iron-silicate nanoparticles by the precipitation of iron oxides in resulting cavities. These observations suggest that hematite dust is not a relict of an original seawater precipitate but the end-product of post-depositional oxidation. Our observations are consistent with paleomagnetic results from the Hamersley Group, which record two major phases of magnetic remanence carried by hematite that post-date deposition by more than 200 million years. Our results may provide an alternative explanation for the origin of jasper in BIFs deposited before the start of the Great Oxidation Event about 2.4 billion years ago. If correct, it follows that hematite dust is not a reliable proxy for paleoenvironmental conditions or biological processes in early Precambrian seawater. Furthermore, our results suggest that the primary iron precipitate in BIFs was iron-silicate mud that was silicified at or just below the sediment–water interface, a hypothesis that requires neither dissolved oxygen nor photosynthetic life, but was an inorganic, chemical process, reflecting anoxic oceans enriched in iron and silica.
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| [14] |
The formation of magnetite in the early Archean oceans [J].
Banded iron formations (BIFs) are iron- and silica-rich chemical sedimentary rocks that were deposited throughout much of the Precambrian. The biological oxidation of dissolved Fe(II) led to the precipitation of a ferric oxyhydroxide phase, such as ferrihydrite, in the marine photic zone. Upon burial, ferrihydrite was either transformed into hematite through dehydration or it was reduced to magnetite via biological or abiological Fe(III) reduction coupled to the oxidation of buried microbial biomass. However, it has always been intriguing as to why the oldest BIFs are characteristically magnetite-rich, while BIFs formed after the Neoarchean are dominated by hematite. Here, we propose that some magnetite in early Archean BIF could have precipitated directly from seawater through the reaction of settling ferrihydrite and hot, Fe(II)-rich hydrothermal fluids that existed in the deeper waters. We conducted experiments that showed the reaction of Fe(II) with biogenic ferric iron mats under strict anoxic conditions lead to the formation of a metastable green rust phase that within hours transformed into magnetite. Our model further posits that with the progressive cooling and oxidation of the Earth's oceans, the above reaction shuts off, and magnetite was subsequently restricted to reactions associated with diagenesis and metamorphism.
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| [15] |
A classification of iron formations based on depositional environments [J].
Two groups of ferruginous sediments are recognized: (1) chemically precipitated iron-formations composed mainly of thinly banded chert and iron minerals; (2) ironstones commonly consisting of oolitic chamosite-siderite-goethite beds with appreciable clay and detrital constituents. Both groups form under a wide range of depositional environments and have distinctive lithological and mineralogical facies. Two principal types of siliceous iron-formation are recognized, Lake Superior and Algoma, based on the characteristics of their depositional basins and the kinds of associated rock. The Lake Superior type was deposited with quartzite, dolomite and black shale in continental-shelf environments, and the Algoma type with volcanic and greywacke rock assemblages along volcanic arcs, rift zones and deep-seated fault and fracture systems. Factors pertinent to the classification of depositional environments for chemical precipitation of iron and silica in iron formations include neritic, continental-shelf and deep-ocean basin environments; proximity to volcanic centres, rift zones, fault systems; type of associated sedimentary and volcanic rock; mineralogy, sedimentary features and lithological facies of the iron formation.
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| [16] |
A compare geochemistry study for Algoma-and Superior-type banded iron formations [J].Algoma型和Superior型硅铁建造地球化学对比研究 [J].
前寒武纪条带状硅铁建造 (BIFs)是世界上最重要的铁矿资源类型和地球早期特有的化学沉积建造类型,广泛分布于太古代-古元古代(3.2～1.8Ga),记录了地球早期岩石 圈、水圈、大气圈和生物圈的状态及演化。前人根据BIFs的岩石组合和构造地质环境将其划分为Algoma型和Superior型。本文对比研究了 Algoma型和Superior型BIFs的硅、氧、铁和多硫同位素特征。不同时代和不同类型BIFs的硅氧同位素组成非常相似,强烈亏损 30Si,δ30SiNBS-28为较大的负值。二者的铁同位素和硫同位素非质量分馏效应明显不同。Algoma型BIF的Δ33S多为负值,而 Superior型BIF的Δ33S多为正值;Algoma型BIF富集重铁同位素,δ56FeIRMM-144多为高正值,而Superior型BIF 相对富集轻铁同位素,δ56FeIRMM-144多为负值或小正值。研究提出无论是Algoma型,还是Superior型BIFs都是由地球早期的海底 火山热液喷气作用形成的,二者属于同一成矿系统,相对而言,Algoma型BIF与火山活动关系更密切,距离同期火山活动中心更近,多形成于深水盆地,环 境更加还原。
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| [17] |
Sedimentology and geochemistry of the glaciogenic late Proterozoic Rapitan Iron-Formation in Canada [J].
Abstract The Rapitan iron-formation in the Northwest Territories and Yukon of Canada was formed between 755 and 730 Ma. The mineralogy and major element geochemistry is very simple and is distinctly different from that of most major banded iron-formation types of Archean and Early Proterozoic age. The rare earth element chemistry appears to be much less distinctly influenced by hydrothermal input into an ocean system than that of these earlier sequences. This suggests a hydrothermal input that was highly diluted by ocean waters at Rapitan time. The sedimentologic setting of the formation strongly suggests an origin as part of glaciomarine conditions. The iron-formation was deposited during a major transgressive event with a rapid rate of sea-level rise during an interglacial period. The occurrence, during Late Proterozoic time, of several, mineralogically and chemically very similar iron-formations worldwide that are in close association with major glaciogenic sequences, and paleomagnetic data for that same period which suggest widespread continental glaciers within a few degrees of the equator, lead to the concept of a "snowball-type Earth'; an earth that would have resembled a highly reflective "snowball' with floating pack ice over most of the ocean surface. -from Authors
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| [18] |
Geochemistry and mineralogy of neoproterozoic banded iron-formations and some selected, siliceous manganese formations from the Urucum district, Mato Crosso Do Sul, Brazil [J].
ABSTRACT This study characterizes the precursor mineralogy and geochemistry of the Neoproterozoic iron ore deposits as well as some associated Na-containing manganese assemblages of the Urucum district, Mato Grosso do Sul, Brazil. It is based on ten mineralogically well characterized samples (six of banded iron-formation (BIF), three of manganese formations, and a sample of a veinlet that crosscuts the manganese assemblage), which were carefully selected so as avoid as much as possible supergene enrichment and secondary alteration (weathering), both of which are pervasive in the Urucum district. The six BIF samples are representative of the extensive Urucum BIF sequence from which the rich iron ores were developed by supergene enrichment and are considered to be precursors to the iron ore. The manganese-rich samples are part of unusual siliceous manganese horizons that contain complex silicate assemblages with braunite, cryptomelane, some pyrolusite, and authigenic aegirine. The Urucum BIF sequence is distinctive because it consists almost entirely of hematite and chert (jasper) with almost all of the iron present as only Fe 2O 3. This is in sharp contrast to the iron chemistry of much older (Archean and Early Proterozoic) BIF, in which a very large proportion of the iron occurs as ferrous iron in magnetite, carbonates, and silicates. As such, the Urucum BIF are essentially identical to those of the Neoproterozoic Rapitan sequence (755-730 Ma) of the Yukon and Northwest Territories of Canada. The Urucum sequence contains abundant dropstones, whereas the Rapitan sequence is set among diamictites but also contains dropstones. The 未 13C values of carbonates at Urucum are low, ranging from -5.2 to -7.0 per mil, which reflects their deposition in a glaciomarine setting. The REE concentrations of the BIF, as well as three Mn formation samples, are very similar and are almost completely lacking positive Eu anomalies (relative to NASC). This is in sharp contrast to the pronounced positive Eu anomalies of Archean and Early Proterozoic iron-formations. The general trend of the REE profiles (in NASC plots), with some enrichment of the heavy REE, is qualitatively very similar to that of modern seawater. The source of the Fe, Mn, and Si is concluded to be from typical ocean water with some deep-sea hydrothermal component. The reappearance of the Neoproterozoic Urucum sequence with BIF and interlayered manganese formations, together with the Rapitan sequence of similar Neoproterozoic age, after an absence of such sedimentary sequences in the geologic record for about 1.1 billion years, is considered to reflect ocean stagnation (with anoxic conditions , which may have been caused by a near-global ice cover, referred to as "snowball Earth."
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| [19] |
Petrology and geochemistry of the Banded Iron Formation (BIF) of Wadi Karim and Um Anab, Eastern Desert, Egypt: Implications for the origin of Neoproterozoic BIF [J].
Banded iron formation (BIF) is exposed among the Precambrian rocks in the Wadi Karim and Um Anab areas in the Eastern Desert of Egypt. The BIF conformably alternates with Neoproterozoic arc metavolcanic rocks, which comprise metabasalts and mafic to intermediate metapyroclastic rocks. The BIF of Wadi Karim belongs to the oxide and mixed carbonate-oxide facies, while that of Um Anab belongs to the oxide facies only. Iron bands are generally composed of iron-rich mesobands rhythmically alternating with jasper, chert or carbonate mesobands as in the Wadi Karim area, or chert mesobands as in the Um Anab area. The BIF of Wadi Karim is composed essentially of magnetite, hematite and microcrystalline quartz, in addition to ankerite in the carbonate-bearing bands, while that of Um Anab is composed of magnetite and microcrystalline quartz. The positive correlation between Al 2O 3 and TiO 2 in the studied BIFs indicates that these chemical sediments incorporate minor detrital components. The limited areal extent of the studied BIF and its association with metavolcanic rocks are features of Algoma-type BIF of Archean greenstone belts. The spatial and temporal association between the BIF and the arc metavolcanic rocks indicates a genetic relationship between volcanic activity and BIF. The REY patterns of the studied BIFs suggest their precipitation after mixing of reduced bottom water carrying hydrothermal component with oxidized seawater in variable proportions. The absence of strong positive Eu anomaly, which characterizes BIF of Archean greenstone belts, in the BIF of the Eastern Desert may be attributed to contribution of Fe and Si through low-temperature hydrothermal solutions and to a high oxygen level of the atmosphere in the Neoproterozoic. It is proposed that the deposition occurred in an oceanic island arc setting, probably intra-arc or back-arc basins. In contrast with the majority of the Neoproterozoic iron formations (IFs), the studied BIFs are intimately associated with volcanic rocks and show no direct evidence of forming in response to glaciation. Consequently we attribute their formation to volcanic activity accompanying the break up of Rodinia rather than to snowball Earth condition. Thus, the Neoproterozoic IFs can be subdivided into Algoma and Rapitan-types. The Algoma-type, of volcanic association, is represented by African and Arabian Shield IFs, whereas the Rapitan-type, of glacial association, comprises deposits from all continents. The Wadi Karim and Um Anab BIFs constitute an additional evidence for the widespread return of IFs in the Cryogenian and Ediacaran after disappearance for 鈭1 Ga reflecting the recurrence of anoxic ferruginous conditions in the Neoproterozoic deep sea.
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| [20] |
Metallogeny of iron deposits and resource potential of major iron minerogenetic units in China [J].
中国铁矿成矿规律及重点矿集区资源潜力分析 [J].
铁矿是中国重要的大宗金属矿产资源,对其进行成矿规律总结和潜力分析具有重要的理论和实际意义。本文总结了中国铁矿资源的禀赋特征;将中国铁矿床分为沉积变质型、岩浆型、接触交代-热液型、火山岩型、沉积型和风化淋滤型6种成因类型和40个矿床式,并建立了鞍山式沉积变质型、大庙式岩浆型、蒙库式海相火山岩型、大西沟式沉积型铁矿床的成矿模式;划分了36个中国铁矿成矿区带,编制了成矿区带图,总结了不同类型铁矿和不同时代铁矿的空间分布规律;总结了矿床类型、矿床规模和矿石类型的时间分布规律;最后,探讨了7个重点成矿区带的资源潜力。
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| [21] |
Neoproterozoic iron formation: An evaluation of its temporal, environmental and tectonic significance [J].
Neoproterozoic iron formation (NIF) provides evidence for the widespread return of anoxic and ferruginous basins during a time period associated with major changes in climate, tectonics and biogeochemistry of the oceans. Here we summarize the stratigraphic context of Neoproterozoic iron formation and its geographic and temporal distribution. It is evident that most NIF is associated with the earlier Cryogenian (Sturtian) glacial epoch. Although it is possible that some NIF may be Ediacaran, there is no incontrovertible evidence to support this age assignment. The paleogeographic distribution of NIF is consistent with anoxic and ferruginous conditions occurring in basins within Rodinia or in rift-basins developed on its margins. Consequently NIF does not require whole ocean anoxia. Simple calculations using modern day iron fluxes suggest that only models that invoke hydrothermal and/or detrital sources of iron are capable of supplying sufficient iron to account for the mass of the larger NIF occurrences. This conclusion is reinforced by the available geochemical data that imply NIF record is a mixture of hydrothermal and detrital components. A common thread that appears to link most if not all NIF is an association with mafic volcanics.
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| [22] |
Formation Mechanism of Different Types of Banded Iron Formations of China: Constraints from Iron, Silicon, Oxygen and Sulfur Isotopes China[D]. 我国不同类型条带状破铁建造形成机制的铁硅氧硫同位素地球化学制约[D] . |
| [23] |
Hydrothermal and resedimented origins of the precursor sediments to banded iron formation: Sedimentological evidence from the early Palaeoproterozoic Brockman Supersequence of Western Australia [J].
ABSTRACT The Early Palaeoproterozoic Brockman Supersequence comprises banded iron formation (BIF), bedded chert, limestone, mudrock, sandstone, breccia, tuffaceous mudstone, ashfall tuff and, in sections not reported here, basalt and rhyolite. Density current rhythms are preserved in sandstones, mudrocks, tuffaceous mudstones and limestones. Relics of similar rhythms in BIF imply that its precursor sediments were also deposited by density currents. Hemipelagic deposits are siliciclastic or mixed siliciclastic鈥搗olcaniclastic mudstones. Bedded chert, chert nodules and the chert matrix of BIF preserve evidence for formation by diagenetic replacement. For bedded chert (and chert nodules), silica replacement occurred before compaction close to or at the sediment鈥搘ater interface, indicating that it is siliceous hardground. The chert matrix of BIF formed during compaction but before burial metamorphism. Original sediments were resedimented from two sources: (1) limestone, mudrock, sandstone, breccia and tuffaceous mudstone from a shelf; and (2) BIF from within the basin realm. Shelf sediments were resedimented to basin-floor fans during third-order lowstands. The precursor sediments to BIF are interpreted to have been granular hydrothermal muds, composed of iron-rich smectite and particles of iron oxyhydroxide and siderite that were deposited on the flanks of submarine volcanoes and resedimented by density currents. Resedimentation occurred by either bottom currents or gravity-driven turbidity currents, and the resulting sediment bodies may have been contourite drifts. The concept that BIF records high-frequency alternating precipitation from ambient sea water of iron minerals and silica is negated by this study. Instead, it is postulated that the precursor sediments to BIF originated in much the same way as modern Red Sea hydrothermal iron oxide deposits, implying that at least the particles of iron oxyhydroxide originated from the oxidation of vent fluids by sea water. Several orders of cyclicity in basin filling establish a relationship between rising to high sea levels, episodic sea-floor hydrothermal activity and BIF that is reminiscent of the link between eustacy and spreading-ridge pulses.
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| [24] |
Iron silicate microgranulesas precursor sediments to 2.5-billion-year-old banded iron formations [J].
Banded iron formations (BIFs) are chemical sedimentary rocks comprising alternating layers of iron-rich and silica-rich minerals that have been used to infer the composition of the early Precambrian ocean and ancient microbial processes. However, the identity of the original sediments and their formation is a contentious issue due to postdepositional overprinting and the absence of modern analogues. Petrographic examination of the ca. 2.5 Ga Dales Gorge Member of the Brockman Iron Formation (Hamersley Group), Western Australia, reveals the presence of abundant silt-sized microgranules composed of stilpnomelane. The microgranules are most common in the least-altered BIF where they define sedimentary laminations, implying a depositional origin. We suggest that the precursor mineral was an iron-rich silicate that formed either in the water column or on the seafloor. The microgranular texture may have developed due to clumping of amorphous mud, forming silt-sized floccules. The microgranules were resedimented by dilute density currents and deposited in lamina sets comprising a basal microgranular-rich lamina overlain by amorphous mud with dispersed microgranules. The lamina sets collectively define plane-lamination structure, probably of the lower flow regime. The microgranular textures are preserved only where early diagenetic silica prevented the compaction of lamina sets. Episodic resedimentation of iron silicates alternating with periods of nondeposition and seafloor silicification provides an explanation for some of the characteristic banding in BIF. We propose that for most of the early Precambrian, the persistence of ferruginous oceans with elevated silica concentrations favored the widespread growth of iron silicate minerals, which in environments starved of continental sediments formed extensive deposits of the precursor sediment to iron formation.
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| [25] |
Hematite replacement of iron-bearingprecursor sediments in the 3.46-b.y.-old Marble Bar Chert, Pilbara craton, Australia [J].
Abstract The history of atmospheric oxygen prior to the Great Oxidation Event (2.45-2.2 Ga) is not well understood. Hematite in the Marble Bar Chert from a NASA-funded drill hole (ABDP1) in the Pilbara craton, Australia, has been cited as evidence for an oxygenated ocean 3.46b.y.ago. However, isotopic data from the same drill hole have been used to argue for an anoxic ocean. It is generally agreed that the hematite is primary, representing either a direct hydrothermal precipitate or a dehydration product of iron oxyhydroxides that formed during anoxygenic photosynthesis. Here we present new petrographic evidence from the Marble Bar Chert (in drill hole ABDP1) that shows that hematite in jasper bands formed via mineral replacement reactions. The hematite mostly occurs as sub-micron-sized inclusions within chert (so-called "dusty" hematite) that are typically arranged into polygonal clusters surrounded by a rim of clear quartz, resembling shrinkage structures. The lateral transition from laminated chert enclosing minute inclusions of greenalite, siderite, and magnetite to chert dominated by dusty hematite provides evidence for in situ replacement of iron-bearing minerals. The presence of hematite-rich bands containing octahedral crystals with residual cores of magnetite indicates that some of the hematite was derived from the replacement of magnetite. This interpretation is supported by the widespread occurrence of magnetite in jasper displaying progressive stages of replacement, from unaltered octahedral inclusions in quartz to hematite pseudomorphs along quartz grain boundaries. The occurrence of dusty hematite in fractures, sedimentary laminae, and the outer margins of polygonal clusters containing greenalite is consistent with fluid-mediated oxidation of iron-rich precursor minerals. The presence of syn-sedimentary chert breccias comprising rotated fragments of laminated chert indicates that the precursor sediment was silicified shortly after deposition. The abundance of "dusty" greenalite inclusions, which are texturally the earliest components of the laminated chert, suggests that the precursor sediment contained an iron-rich clay mineral. Our results show that hematite has replaced ferrous-rich minerals after deposition and provide a mechanism to explain the origin of hematite in the Marble Bar Chert, which is consistent with the origin of hematite in adjacent basalts. A secondary origin for hematite invalidates arguments for an oxygen-bearing ocean similar to 3.46 b.y.ago and provides a viable explanation for the formation of Archean jasper bands. Our findings show that misinterpretations about the origin of hematite in early Precambrian cherts could lead to false conclusions about the chemistry of the ancient ocean and atmosphere.
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| [26] |
Analysis of sedimentary facies and depositional environment of the Yuanjiacun banded iron formation in the Lüliang area,Shanxi Province [J].山西吕梁袁家村条带状铁建造沉积相与沉积环境分析 [J].
山西吕梁作为华北克拉通上条带状铁建造(BIF)的重要产区之一,位于华北中央构造带中。袁家村BIF分布于吕梁岚县袁家村一带,极有可能是华北克拉通内最为典型的Superior型BIF。与华北克拉通其他大多数BIF相比,袁家村BIF具有明显的差异性,其中包括它的形成时代(2.3~2.1Ga)、铁建造类型和低级变质程度(低绿片岩相)等。因此,研究袁家村BIF具有特殊的研究意义,可为探讨大氧化事件之后古海洋氧化还原状态以及国内Superior型BIF的成因提供研究基础。袁家村BIF产于吕梁群袁家村组变沉积岩系的下部,前人根据上覆和下伏含火山岩地层的时代,推测袁家村组的形成时代为2.3~2.1Ga。BIF整体产状陡倾,沿北北东-北东东向呈L形带状分布。依据原生矿物的共生组合及产出特征,可将BIF沉积相划分为氧化物相(60%)、硅酸盐相(30%)和碳酸盐相(10%)。氧化物相是本区BIF最主要的沉积相,主要矿物为赤铁矿、磁铁矿和石英,从而可进一步划分为赤铁矿(24%)和磁铁矿(36%)亚相;硅酸盐相BIF以大量硅酸盐矿物出现为特征,散布于研究区,主要矿物组成除了石英和磁铁矿之外,还有铁黑硬绿泥石、绿泥石、铁滑石、镁铁闪石和阳起石等。在与碳酸盐相BIF构成过渡相的BIF中,还可发现大量的铁白云石。而碳酸盐相主要矿物为菱铁矿、铁白云石和石英等,主要发育于研究区的南部。依据含铁岩系构造格局特点复原获得了原始沉积相分布略图,沉积相主要呈南北向延展,自东向西显示出相变规律,西边为碳酸盐相,东边为氧化物相,其间是过渡的硅酸盐相。通过袁家村BIF的岩相学和含铁矿物化学成分的研究,可大致推测原始沉积的矿物组成为无定形硅胶、水铁矿、与铁蛇纹石和黑硬绿泥石组成类似的铁硅酸盐凝胶、富Al的粘土碎屑和含铁、镁、钙的碳酸盐软泥。这些沉积物在随后的成岩期和绿片岩相的区域变质作用下发生矿物之间的相互转变。BIF中主要含铁矿物的PO-P-Eh 2CO2和pH相关图解说明除了赤铁矿之外,其他矿物均是在较低氧逸度环境中形成的,且所有矿物共存的水体系为中性到弱碱性。袁家村BIF氧化物相中发育豆粒、内碎屑结构和板状交错层理等原始沉积构造,指示氧化相部分是在相对高能的浅水环境下沉积的。但BIF大部分应该形成于浪基面以下(200m)较为深水的环境中,沉淀可能同时发生于上部氧化和下部还原的水体之中,由于还原弱酸性的深部富铁海水在海侵的过程中上升到浅部相对氧化和弱碱性的浅水环境中,因为Eh、pH及氧逸度等物化条件的骤然变化,最终导致铁质的沉淀和沉积相自上而下的变化。
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| [27] |
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| [28] |
Depositional environment of the Paleoproterozoic Yuanjiacun Banded Iron Formation in Shanxi Province, China [J].
The Paleoproterozoic(~2.38鈥2.21 Ga) Yuanjiacun banded iron formation(BIF),located in Shanxi Province,is a Superior-type BIF in the North China craton.This BIF is within a metasedimentary rock succession of the Yuanjiacun Formation,in the lower L眉liang Group,which has undergone lower greenschist-facies metamorphism.Iron oxide(magnetite and hematite),carbonate,and silicate facies are all present within the iron-rich layers.The eastward transition from carbonate-into oxide-facies iron formations is accompanied by a change in mineralogical composition from siderite in the west through magnetite-ankerite and magnetite-stilpnomelane assemblages in the transition zone to magnetite and then hematite in the east.These distinct lateral facies are also observed vertically within the BIF,i.e.,the iron mineral assemblage changes upsection from siderite through magnetite into hematite-rich iron formation.The oxide-facies BIF formed near shore,whereas carbonate(siderite)-and silicate-facies assemblages formed in deeper waters.Based on detailed analyses of these variations on a basinal scale,the BIF precipitated during a transgressive event within an environment that ranged from deep waters below storm wave base to relatively shallow waters.The BIF samples display distinctively seawater-like REEs + Y profiles that are characterized by positive La and Y anomalies and HREEs enrichment relative to LREEs in Post-Archean Australian shale-normalized diagrams.Consistently positive Eu anomalies are also observed,which are typical of reduced,high-temperature hydrothermal fluids.In addition,slightly negative to positive Ce anomalies,and a large range in ratios of light to heavy REEs,are present in the oxide-facies BIF.These characteristics,in combination with consistently positive 未~(56)Fe values,suggest that deposition of the BIF took place along the chemocline where upwelling of deep,anoxic,iron-and silicarich hydrothermal fluids mixed with shallower and slightly oxygenated seawater.The ankerite displays highly depleted 未~(13)C values and the carbonate-rich BIF has a high content of organic carbon,suggesting dissimilatory Fe(III) reduction of a ferric oxyhydroxide precursor during burial of biomass deposited from the water column;that same biomass was likely tied to the original oxidation of dissolved Fe(II).The fact that the more ferric BIF facies formed in shallower waters suggests that river-sourced nutrients would have been minimal,thus limiting primary productivity in the shallow waters and minimizing the organic carbon source necessary for reducing the hematite via dissimilatory Fe(III) reduction.By contrast,in deeper waters more proximal to the hydrothermal vents,nutrients were abundant,and high biomass productivity was coupled to increased carbon burial,leading to the deposition of iron-rich carbonates.The deposition of the Yuanjiacun BIF during the onset of the Great Oxidation Event(GOE;ca.2.4鈥2.2 Ga) confirms that deep marine waters during this time period were still episodically ferruginous,but that shallow waters were sufficiently oxygenated that Fe(II) oxidation no longer needed to be tied directly to proximal cyanobacterial activity.
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| [29] |
Geneses and evolutions of iron-bearing minerals in banded iron formations of >3760 to ca. 2200 million-year-old: Constraints from electron microscopic, X-ray diffraction and Mössbauer spectroscopic investigations [J]. |
| [30] |
The silica cycle in the Precambrian [J].
The silica cycle in the Precambrian is reconstructed mainly from inorganic reactions, with due consideration for interactions with the biosphere insofar as they result in silica-organic matter reaction. No evidence is found for deposition of a layered amorphous silica, but abundant evidence exists for diagenetic silicification in the Neoproterozoic, the time period under analysis. The evidence of Neoproterozoic rocks favors tectonic and weathering regimes similar to those of the early Phanerozoic and hydrothermal inputs significantly altered at certain periods. One flux would have been very different: the present diffusional influx from interstitial waters into the oceans would have been altered to an efflux from the oceans to interstitial waters. Reactions of dissolved silica with inorgnic phases would have controlled silica concentrations at a level of about 60 pm. It is argued that the major removal of silica from the Neoproterozoic ocean took place by diagenetic reactions.
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| [31] |
Decoupling photochemical Fe(II) oxidation from shallow-water BIF deposition [J].
Oxidized Fe minerals in Archean–Paleoproterozoic banded iron formations (BIFs) are commonly taken to indicate the presence of biogenic O 2 or photosynthetic Fe(II)-oxidizing bacteria in the oceans' photic zone. However, at least one viable abiogenic oxidation mechanism has been proposed. Prior to the rise of atmospheric oxygen and the development of a protective ozone layer, the Earth's surface was subjected to high levels of ultraviolet radiation. Bulk ocean waters that were anoxic at this time could have supported high concentrations of dissolved Fe(II). Under such conditions, dissolved ferrous iron species, such as Fe 2+ and Fe(OH) + , would have absorbed radiation in the 200–400nm range, leading to the formation of dissolved ferric iron [Fe(III)], which in turn, would have hydrolyzed to form ferric hydroxide [Fe(OH) 3 ] at circumneutral pH [Cairns-Smith, A.G., 1978, Precambrian solution photochemistry, inverse segregation, and banded iron formations. Nature 76, 807–808; Braterman, P.S., Cairns-Smith, A.G., and Sloper, R.W., 1983, Photo-oxidation of hydrated Fe 2 -Significance for banded iron formations. Nature 303, 163–164]. This process has been invoked to account for BIF deposition without need for biology [Fran04ois, L.M., 1986, Extensive deposition of banded iron formations was possible without photosynthesis. Nature 320, 352–354]. Here, we evaluate the potential importance of photochemical oxidation using a combination of experiments and thermodynamic models. The experiments simulate the chemistry of ambient Precambrian seawater mixing with Fe(II)-rich hydrothermal fluids with, and without, UV irradiation. We find that if Fe(II) was effused from relatively shallow seamount-type vent systems directly into an anoxic photic zone, the photochemical contribution to solid-phase precipitation would have been negligible. Instead, most of the Fe(II) would have precipitated rapidly as an amorphous precursor phase to the ferrous silicate mineral greenalite ((Fe) 3 Si 2 O 5 (OH) 4 ), and/or the ferrous carbonate, siderite (FeCO 3 ), depending on different simulated atmospheric pCO 2 levels. Conversely, in experiments where Fe(II) was exposed either to phototrophic Fe(II)-oxidizing bacteria or to O 2 , ferric hydroxide formed rapidly, and the precipitation of ferrous iron phases was not observed. If, as suggested on mass balance grounds, BIF deposition requires that Fe be sourced from shallow seamount-type systems, then we are driven to conclude that oxide-facies BIF are the product of a rapid, non-photochemical oxidative process, the most likely candidates being direct or indirect biological oxidation, and that a significant fraction of BIF could have initially been deposited as ferrous minerals.
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| [32] |
A model for the deposition of the microbanded Precambrian iron formations [J]. |
| [33] |
Alternating Si and Fe deposition caused by temperature fluctuations in Precambrian oceans [J].
Precambrian banded iron formations provide an extensive archive of pivotal environmental changes and the evolution of biological processes on early Earth. The formations are characterized by bands ranging from micrometre- to metre-scale layers of alternating iron- and silica-rich minerals. However, the nature of the mechanisms of layer formation is unknown. To properly evaluate this archive, the physical, chemical and/or biological triggers for the deposition of both the iron- and silica-rich layers, and crucially their alternate banding, must be identified. Here we use laboratory experiments and geochemical modelling to study the potential for a microbial mechanism in the formation of alternating iron-silica bands. We find that the rate of biogenic iron(III) mineral formation by iron-oxidizing microbes reaches a maximum between 20 and 25C. Decreasing or increasing water temperatures slow microbial iron mineral formation while promoting abiotic silica precipitation. We suggest that natural fluctuations in the temperature of the ocean photic zone during the period when banded iron formations were deposited could have led to the primary layering observed in these formations by successive cycles of microbially catalysed iron(III) mineral deposition and abiotic silica precipitation.
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| [34] |
Chemical factors in the deposition and diagenesis of banded iron-formation [ |
| [35] |
An iron shuttle for deep-water silica in Late Archean and early Paleoproterozoic iron formation [J].
Iron formations are typically thinly bedded or laminated sedimentary rocks containing 15% or more of iron and a large proportion of silica (commonly > 40%). In the ca. 2590-2460 Ma Campbellrand-Kuruman Complex, Transvaal Supergroup, South Africa, iron formation occurs as a sediment-starved deepwater facies distal to carbonates and shales. Iron minerals, primarily siderite, define the lamination. The silica primarily occurs as thin beds and nodules of diagenetic chert (now microcrystalline quartz), filling pore space and replacing iron formation minerals and co-occurring deepwater lithologies. Mechanisms proposed to explain precipitation of the iron component of iron formation include photosynthetic oxygen production, anoxygenic photosynthesis, abiotic photochemistry, and chemoautotrophy using Fe(II) as an electron donor. The origin and mechanism of silica precipitation in these deposits have received less attention. Here we present a conceptual model of iron formation that offers insight into the deposition of silica. The model hinges on the proclivity of dissolved silica to adsorb onto the hydrous surfaces of ferric oxides. Soluble ferrous iron is oxidized in the surface ocean to form ferric hydroxides, which precipitate. Fe(OH)3 binds silica and sinks from the surface ocean along with organic matter, shuttling silica to basinal waters and sediments. Fe(III) respiration in the sediments returns the majority of iron to the water column but also generates considerable alkalinity in pore waters, driving precipitation of siderite from Fe2+ and respiration-influenced CO2. Silica liberated during iron reduction becomes concentrated in pore fluids and is ultimately precipitated as diagenetic mineral phases. This model explains many of the mineralogical, textural, and environmental features of Late Archean and earliest Paleo-proterozoic iron formation.
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| [36] |
In order to determine the origin of silicon (Si) in banded iron formation (BIF), we have undertaken a multi-tracer study combining REE+Y data, Ge/Si ratios and Si isotopes (δ 30 Si) on stratigraphically resolved layers from a 652.9502Ga BIF from the Pongola Supergroup, South Africa. Si in both Si-rich and Fe-rich layers has a common origin, represented by a seawater reservoir strongly influenced by continent-derived freshwaters (6510%) and very limited (<0.1%) high-T hydrothermal fluids as indicated by Eu anomalies and Y/Ho ratios. The coevolution of δ 30 Si signatures of Si- and Fe-rich layers of the BIF coupled with similar Eu and Y anomalies in both types of layers is in accordance with a common silica precipitation promoted by Si adsorption onto Fe-oxyhydroxides from Archaean seawater. An increase in δ 30 Si values from 612.27‰ to 610.53‰ stratigraphically upwards in the BIF is inferred to be the result of two successive isotopic fractionation processes during (1) silicon adsorption onto the Fe-oxyhydroxide precursor and (2) silica precipitation at the sediment–water interface from pore fluid triggered by the local silica saturation consecutive to an early diagenetic Si desorption from the precursor Fe-oxyhydroxide. The first fractionation process depleted the parental water in 28 Si while the second released 30 Si back into the parental water, resulting in an increase of the δ 30 Si value of the parental water reservoir over time.
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| [37] |
Deep-marine depositional setting of banded iron formation: Sedimentological evidence from interbedded clastic sedimentary rocks in the early Palaeoproterozoic Dales Gorge Member of Western Australia [J]. |
| [38] |
Genesis of iron-bearing minerals in banded iron formation mesobands in the Dales Gorge Member, Hamersley Group, Western Australia [J].
Abstract The Hamersley Iron Ore Province of Western Australia is recognized as containing one of the major Precambrian iron formations in the world--the Brockman Iron Formation. The excellent exposure in drill core of the Dales Gorge Member of this formation has provided a unique
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| [39] |
Experimental low-grade alteration of biogenic magnetite indicates microbial involvement in generation of banded iron formations [J].
During the deposition of banded iron formation (BIF), the downward flux of ferric oxyhydroxides and phytoplankton biomass should have facilitated Fe(III) reduction during burial, with the end product being ferrous iron-containing minerals including magnetite. Although earlier studies have attempted to quantify the significance of this pathway based on models of the ancient Fe cycle, the only direct evidences of a biological role in magnetite formation in BIF are their iron isotope compositions and unique crystallography which are reminiscent of biologically-generated magnetite. However, the biogenesis hypothesis lacks an explanation as to why modern biogenic magnetite crystals are generally a few hundred nm or smaller in size, yet the magnetite crystals in BIF are mostly tens of micrometers or larger in size. In this study, we demonstrate that biogenic magnetite crystals can grow in size upon reaction between oxyhydroxide and microbial biomass after compression and heating to 1 kbar and 150 degrees C, respectively. The magnetite crystals previously produced by Thermoanaerobacter spp. TOR39 reach sizes in excess of 700 nm after the P-T experiments, while new magnetite grains >400 nm formed from the superparamagnetic magnetite-dominated end product of Shewanella sp. culture. This study indicates that the large magnetite crystals observed in BIF can be derived through a three-stage sequence, beginning with dissimilatory iron reduction of an initial ferric iron-rich sediment coupled to the oxidation of dead phytoplankton biomass, followed by magnetite crystal aging, and ultimately pressure-temperature induced abiotic alteration of the biogenic magnetite during metamorphism. (C) 2012 Elsevier B.V. All rights reserved.
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| [40] |
Synthesis of clay-sized iron oxides under marine hydrothermal conditions [J].
Goethite, lepidocrocite, magnetite and akaganeite were synthesized in 0.8 屑, 2 屑 and 5 屑 NaCl solutions at various temperatures (25, 40, 60潞C) under slightly acidic to slightly alkaline pH with or without Si additions. Elevated temperatures prevent complete oxidation of initial Fe2+solutions and magnetite and siderite precipitate, accompanied by goethite and lepidocrocite. At higher salinity, O2solubility is reduced and its distribution is limited, leading to coprecipitation of lepidocrocite, akaganeite and goethite. Lepidocrocite morphology changes from plates at pH 5.5 through rods at pH 7 to multi-domainic crystals at pH 8.2, due to enhanced crystal growth along the c axis. Salinity and temperature have opposite effects on lepidocrocite crystallinity. Goethite crystals are multi-domainic and twinning appears only at elevated temperatures. Increases in temperature and salinity improve goethite crystallinity as observed by IR spectra. Addition of Si up to Si/Fe = 0.1 retards crystal growth and Si-OH-stretching bands appear. At Si/Fe = 1 most of the precipitate is short range ordered. Platy and rod-shaped lepidocrocite from the Thetis and Atlantis II Deeps, were probably formed under the slightly acidic conditions of the hydrothermal brines. The Si concentration was greater in Atlantis II Deep than in Thetis Deep, leading to larger lepidocrocite and goethite crystals in the latter. Multi-domainic goethite could have precipitated throughout. Pure phase goethite might have precipitated in the less concentrated brine, whereas mixtures of goethite and lepidocrocite might have precipitated in the more concentrated brine, depending mainly on oxidation rate and oxygen mobility within the brine.
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| [41] |
Hematite nanospheres of possible colloidal origin from a Precambrian banded iron formation [J].
Abstract Exceptionally small spheres (nanospheres) of hematite (diameters between 120 and 200 nanometers) occur in the Marra Mamba Iron Formation of the Hamersley Basin, Australia. The nanospheres are clustered into small aggregates and may have formed by structural ordering and dehydration of colloidal iron hydroxide particles. Individual spheres consist of numerous thin, curved hematite platelets surrounding a central void that is approximately half the diamter of the sphere; this texture suggests that they formed by a volume reduction of the original colloidal particles by approximately 12.5%. The occurrence of hematite nanospheres supports the hypothesis that some ofthe iron was deposited colloidally during the development ofbanded iron formations, approximately 2.5 billion years ago.
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| [42] |
Genetic modelling for banded iron-formation of the Hamersley Group, Pilbara Craton, Western Australia [J].
Abstract The banded iron-formation (BIF) of the Hamersley Group, Pilbara Craton, Western Australia, particularly from the well studied Dales Gorge Member, is unique in its lateral stratigraphic and petrological continuity throughout an area exceeding 60,000 km2, enabling reasonable estimates for the annual input of components to the depository. In the model of this paper, varying supply of materials for the medley of mesoband types, particularly of iron and silica in the oxide BIF, can be accommodated by the interaction of two major oceanic supply systems: (1) surface currents and (2) convective upwelling from mid-oceanic ridge (MOR) or hot-spot activity, both modified by varied input of pyrochastic material. (1) The surface currents were saturated in silica and carried minimal iron due to photic precipitation, but were periodically recharged by storm mixing. Precipitation from them gave rise to the banded chert-rich horizons, including the varves, whose regular and finely laminated iron/silica distribution resulted from seasonal meteorological influences. (2) Precipitation from convection driven upwelling of high iron solution from MOR or hot-spot activity periodically overwhelmed the delicate seasonal patterns of (1) to produce the iron-dominated mesobands. A wide range of intermediate mesoband types resulted where the deep water supply was modified by varied MOR activity, or by partial blocking of upwelling waters by surface currents (such as by the present El Ni帽o). During these periods of oxide-dominated BIF, silica was deposited from saturated solution mainly by evaporative concentration, and iron by oxidation due to photolysis and photosynthetically produced oxygen.
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| [43] |
Effect of pH on the formation of goethite and hematite from ferrihydrite [J].
ABSTRACT Storage of ferrihydrite in aqueous suspensions at 24oC and pHs between 2.5 and 12 for as long as three years resulted in the formation of goethite and hematite. The proportions and crystallinity of these products varied widely with the pH. Maximum hematite was formed between pH 7 and 8, and maximum goethite at pH 4 and at pH 12. We relate the proportions of goethite and hematite to the activity of the Fe(III) ion species in solution; conditions favorable for the formation of goethite are unfavorable for that of hematite and vice versa. -from Authors
|
| [44] |
Mineral ecophysiological data provide growing evidence for microbial activity in banded iron formations [J]. |
| [45] |
Nonredox transformations of magnetite-hematite in hydrothermal systems [J].
Abstract The transformation of magnetite to hematite, or hematite to magnetite, in nature has generally been considered a redox reaction and linked to a specific redox state of fluid; however, a nonredox reaction, Fe2O3(hm) + Fe2+ H2O = Fe3O4(mt) + 2H(+), may have been the principal mechanism for the transformations of iron oxides in nature, especially in hydrothermal environments. For example, the transformation of goethite and/or hematite (primary precipitates) to magnetite in banded iron-formations (BIFs) probably occurred through nonredox reactions with Fe2+-bearing hydrothermal fluids during the accumulation of a BIF sequence, rather than through redox reactions involving organic matter during and/or after the BIF deposition. The proposed mechanisms for the transformation of magnetite to hematite provides new exploration strategies for hematite-rich secondary ores, extending the target for orebodies to much deeper zones below the paleosurface. Another important implication of the proposed mechanism is that the presence or absence of magnetite and/or hematite in geologic formations may or may not provide meaningful information on the redox state of fluid.
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| [46] |
Petrography and geochemistry of the Dales Gorge banded iron formation: Paragenetic sequence, source and implications for palaeo-ocean chemistry [J].
Banded iron formations (BIFs) have long been considered marine chemical precipitates or, as more recently proposed, the result of episodic density flows. In this study, we examined the mineralogy, petrography and chemistry of the Dales Gorge BIF to evaluate the validity of these models. Microbands reflect a compositionally variable primary precipitation/sedimentation pattern with diagenetic modifications. “Iron-rich” bands are characterized by massive anhedral aggregates and xenomorphic hematite, commonly showing overgrowths of subhedral magnetite with minor apatite and late diagenetic ankerite-Fe dolomite. “Iron-poor” bands consist of fine-grained quartz, siderite and Fe-talc in variable amounts. Amorphous silica, ferrihydrite (precursor of hematite), greenalite and possibly some siderite constitute the primary precipitates, while ankerite-Fe dolomite, Fe-talc and magnetite and the bulk of siderite are secondary mineral phases. Ankerite and Fe-dolomite most likely represent by-products of siderite dissolution and the subsequent reaction of dissolved bicarbonate with Ca, Mg and Fe. Ferroan-talc is thought to be formed from the following reactions: (i) greenalite + chert, and (ii) siderite + chert. Magnetite formed from the conversion of hematite, likely through bacterial Fe(III) reduction. Geochemical mineral analyses show that all the phases have very low concentrations of trace elements with the exception of Ba, As, Cr, Zn and Sr. This partitioning was presumably controlled by both sorptive reactions occurring during primary precipitation in the water column and secondary remobilization during diagenesis. Whole-rock analyses indicate two decoupled sources for BIF and S macrobands. While BIF macrobands have a major hydrothermal influence, data from the S macrobands supports a dominantly mafic provenance. Nonetheless, when all the lithologies (i.e., source rocks, S and BIF macrobands) are evaluated together, continuous geochemical trends can be observed. This suggests that at least part of the precursor material of BIF macrobands was sourced from the same material that gave origin to the S macrobands. Similar relationships are seen in other BIF successions, where the evolution from an ultamafic- to a mafic-dominated upper continental crust is distinctly reflected in BIF compositions through time. Interpretation of this data implies that any model developed to explain BIF deposition must consider: (i) processes involving low-temperature weathering of the continental and ocean basement rocks, mainly (ultra)mafic lithologies; and (ii) high temperature water–rock reactions associated with hydrothermal activity at spreading ridge centers or seamounts. In either case, the influence of the compositional change of the upper continental crust played a major role in the chemical compositions of BIFs through time.
|
| [47] |
Dissimilatory metal reduction [J]. |
| [48] |
Preservation of Fe isotope heterogeneities during diagenesis and metamorphism of banded iron formation [J].
We present the iron isotope composition of primary, diagenetic and metamorphic minerals in five samples from the contact metamorphosed Biwabik Iron Formation. These samples attained peak metamorphic temperatures of <200, <340, 65 500, <550, and <740°C respectively. δ56Fe of bulk layers ranges from –0.8 to +0.8‰; in some samples the layers may differ by >1‰ on the millimeter scale. Minerals in the lowest grade samples consistently show a sequence in which δ56Fe of magnetite > silicate ≥ carbonate. The intermineral Fe isotope differences vary in a fashion that cannot be reconciled with theoretical temperaturedependent fractionation factors. Textural evidence reveals that most, if not all, magnetite in the Biwabik Formation is diagenetic, not primary, and that there was tremendous element mobility during diagenesis. The short duration of contact metamorphism allowed diagenetic magnetite compositions to be preserved throughout prograde metamorphism until at least the appearance of olivine. Magnetite compositions therefore act as an isotope record of the environment in which these sediments formed. Larger-scale fluid flow and longer timescales may allow equilibration of Fe isotopes in regionally metamorphosed rocks to lower temperatures than in contact metamorphic environments, but weakly regionally metamorphosed rocks may preserve small-scale Fe isotopic heterogeneities like those observed in the Biwabik Iron Formation. Importantly, Fe isotope compositions that are characteristic of chemical sedimentation or hydrothermal processes are preserved at low grade in the form of large inter-mineral variations, and at high grade in the form of unique bulk rock compositions. This observation confirms earlier work that has suggested that Fe isotopes can be used to identify sedimentary processes in the Precambrian rock record.
|
| [49] |
Iron isotopes constrain biologic and abiologic processes in banded iron formation genesis [J].
Several factors likely contributed to the important role that DIR played in BIF formation, including high rates of ferric oxide/hydroxide formation in the upper water column, delivery of organic carbon produced by photosynthesis, and low clastic input. We infer that DIR-driven Fe redox cycling was much more important at this time than in modern marine systems. The low pyrite contents of magnetite- and siderite-facies BIFs suggests that bacterial sulfate reduction was minor, at least in the environments of BIF formation, and the absence of sulfide was important in preserving magnetite and siderite in the BIFs, minerals that are poorly preserved in the modern marine record. The paucity of negative δ 56 Fe values in older (Early Archean) and younger (Early Proterozoic) BIFs suggests that the extensive 2.502Ga Hamersley–Transvaal BIFs may record a period of maximum expansion of DIR in Earth’s history.
|
| [50] |
Iron reduction by bacteria: A potential role in the genesis of banded iron formations [J].
Abstract The recent discovery of bacteria that can grow anaerobically by coupling carbon oxidation to the dissimilatory reduction of manganese or iron oxides may have some relevance to our understanding of BIF formation. Such organisms provide a mechanism by which post-depositional reductive processes could lead to alternating iron-rich iron-poor layers, simultaneously releasing isotopically light carbon (from organic carbon oxidation) for incorporation into carbonate rocks. In this paper we discuss the apparently widespread distribution of such metal reducing microbes and deal in some detail with the metal reducing and carbon oxidizing abilities of one such bacterium, Shewanella putrefaciens strain MR-1, presenting a model of layer formation based on these abilities. -from Authors
|
| [51] |
Stability relations of siderite (FeCO3) in the system Fe-C-O [J].
ABSTRACT. Stability relations of siderite (FeCO.,) in the system Fe—C—O were determined between 500 and 2000 bars in a CO2+ CO atmosphere as a function of T, PF (I PC02+ P90), and fog, using solid-phase oxygen buffers. Siderite was synthesized for the experiments
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| [52] |
Experimental determination of siderite stability and application to Martian Meteorite ALH84001 [J].
The pressure-temperature equilibrium curve of the reaction siderite + hematite = magnetite + CO2 was determined in the range 5-12 kbar and 480-650 掳C by piston-cylinder experiments, with NaCl as a pressure medium. Silver oxalate was used as a CO2 source and samples were buffered at hematitemagnetite oxygen fugacity. Reaction progress was monitored by extent of CO2 gas loss and by X-ray diffraction (XRD) analysis. The data define a univariant curve, which is described by P = -14.599 + 0.025T + 0.000027 T2 with P in kbar and T in 掳C. Calculations based on these data give 螖H0f (298K) siderite = -760.6 卤 0.9 kJ (kilojoules) from the oxides. The formation of siderite requires a specific range of ambient oxygen and carbon dioxide fugacities, dependent upon temperature and pressure. The stable assemblage of siderite and magnetite, at a given temperature and pressure, implies more restrictive ranges of oxygen and carbon dioxide fugacities, defined by reactions among siderite, magnetite, graphite, and hematite. Experimental and thermodynamic investigation of the Fe-C-O system indicates that the formation of magnetite along with Ca-Fe-Mg carbonate globules by inorganic processes is possible and may be relevant to Martian meteorite ALH84001. Decarbonation of the siderite component of the carbonate, either by a transient heating event or by a change in oxygen fugacity of a coexisting fluid, may have formed the observed grains, although this study does not address the size or morphology of magnetite grains formed by this mechanism.
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| [53] |
Temperature and salinity history of the Precambrian ocean: Implications for the course of microbial evolution [J].
The temperature and salinity histories of the oceans are major environmental variables relevant to the course of microbial evolution in the Precambrian, the "age of microbes". Oxygen isotope data for early diagenetic cherts indicate surface temperatures on the order of 55-85 degrees C throughout the Archean, so-early thermophilic microbes (as deduced from the rRNA tree) could have been global and not just huddled around hydrothermal vents as often assumed. Initial salinity of the oceans was 1.5-2 x the modem value and remained high throughout the Archean in the absence of long-lived continental cratons required to sequester giant halite beds and brine derived from evaporating seawater. Marine life was limited to microbes (including cyanobacteria) that could tolerate the hot, saline early ocean. Because O-2 solubility decreases strongly with increasing temperature and salinity, the Archean ocean was anoxic and dominated by anaerobic microbes even if atmospheric O-2 were somehow as high as 70% of the modem level.Temperatures declined dramatically in the Paleoproterozoic as long-lived continental cratons developed. Values similar to those of the Phanerozoic were reached by 1.2 Ga. The first great lowering of oceanic salinity probably occurred in latest Precambrian when enormous amounts of salt and brine were sequestered in giant Neoproterozoic evaporite basins. The lowering of salinity at this time, together with major cooling associated with the Neoproterozoic glaciations, allowed dissolved O-2 in the ocean for the first time. This terminated a vast habitat for anaerobes and produced threshold levels of O-2 required for metazoan respiration. Non-marine environments could have been oxygenated earlier, so the possibility arises that metazoans developed in such environments and moved into a calcite and silica saturated sea to produce the Cambrian explosion of shelled organisms that ended exclusive microbial occupation of the ocean.Inasmuch as chlorine is a common element
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| [54] |
Comparing yttrium and rare earths in hydrothermal fluids from the Mid-Atlantic Ridge: Implications for Y and REE behaviour during near-vent mixing and for the Y/Ho ratio of Proterozoic seawater [J].
Mg-poor hydrothermal fluids from the high-temperature discrete flow at the Broken Spur site at the Mid-Atlantic Ridge show high Y concentrations between 1880 and 2639 pmol/kg, and almost chondritic Y/Ho molar ratios between 52 and 55. A sample contaminated with ambient seawater is lower in Y (661 pmol/kg), and yields an elevated Y/Ho ratio of 84. The diffuse flow at the TAG hydrothermal mound shows between 628 and 1785 pmol/kg of Y, and Y/Ho molar ratios between 57 and 65. Similar Y/Ho ratios in black-smoker fluids and MORBasalts argue against an important role of Rare Earths and Yttrium (REY) fluoride complexes in the solutions, but are compatible with a REY speciation dominated by chloride complexes and `free' REY 3+ ions. Close to the vent orifice, Y behaves conservatively during mixing of high-temperature hydrothermal fluid with entrained seawater. This is in marked contrast to the behaviour of the rare earth elements (REE) which are partly scavenged by Fe oxyhydroxides within less than 1 m distance from the vent orifice, resulting in a strong increase of the Y/Ho ratio. Non-conservative mixing behaviour of the REE may result in underestimation of REE concentrations in the hydrothermal end-member when calculations are based on conservative elements, such as Mg. An approach combining Mg concentration and Y/Ho ratio may reduce this problem. Despite the lower particle-reactivity of Y compared to the REE, there is no hydrothermal Y input into present-day oxic seawater, and marine hydrothermal vent sites are sinks for dissolved Y rather than sources. However, REE elemental and Nd isotopic systematics of Precambrian banded iron-formations reveal the existence of a high-temperature hydrothermal REY flux into Early Precambrian an- or suboxic seawater. The results of our study indicate a chondritic Y/Ho ratio of this black-smoker-type hydrothermal REY input, and suggest that Paleoproterozoic surface seawater showed super-chondritic Y/Ho ratios similar to those of present-day seawater.
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| [55] |
Fe, C, and O isotope compositions of banded iron formation carbonates demonstrate a major role for dissimilatory iron reduction in ~2.5 Ga marine environments [J].
Combined Fe, C, and O isotope measurements of ~022.502Ga banded iron formation (BIF) carbonates from the Kuruman Iron Formation and underlying BIF and platform Ca–Mg carbonates of the Gamohaan Formation, South Africa, constrain the biologic and abiologic formation pathways in these extensive BIF deposits. Vertical intervals of up to 10002m were sampled in three cores that cover a lateral extent of ~0225002km. BIF Fe carbonates have significant Fe isotope variability ( δ 56 Fe02=02+021 to 61021‰) and relatively low δ 13 C (down to 610212‰) and δ 18 O values ( δ 18 O02~02+0221‰). In contrast, Gamohaan and stratigraphically-equivalent Campbellrand Ca–Mg carbonates have near-zero δ 13 C values and higher δ 18 O values. These findings argue against siderite precipitation from seawater as the origin of BIF Fe-rich carbonates. Instead, the C, O, and Fe isotope compositions of BIF Fe carbonates reflect authigenic pathways of formation in the sedimentary pile prior to lithification, where microbial dissimilatory iron reduction (DIR) was the major process that controlled the C, O, and Fe isotope compositions of siderite. Isotope mass-balance reactions indicate that the low- δ 13 C and low- δ 18 O values of BIF siderite, relative to those expected for precipitation from seawater, reflect inheritance of C and O isotope compositions of precursor organic carbon and ferric hydroxide that were generated in the photic zone and deposited on the seafloor. Carbon–Fe isotope relations suggest that BIF Fe carbonates formed through two end-member pathways: low- δ 13 C, low- δ 56 Fe Fe carbonates formed from remobilized, low- δ 56 Fe aqueous Fe 2+ produced by partial DIR of iron oxide, whereas low- δ 13 C, high- δ 56 Fe Fe carbonates formed by near-complete DIR of high- δ 56 Fe iron oxides that were residual from prior partial DIR. An important observation is the common occurrence of iron oxide inclusions in the high- δ 56 Fe siderite, supporting a model where such compositions reflect DIR “in place” in the soft sediment. In contrast, the isotopic composition of low-Fe carbonates in limestone/dolomite may constitute a record of seawater environments, although our petrographic studies indicate that the presence of pyrite in most low-Fe carbonates may influence the Fe isotope compositions. The combined Fe, C, and O isotope data from Kuruman BIF carbonates indicate that BIF siderites that have negative, near-zero, or positive δ 56 Fe values may all record biological Fe cycling, where the range in δ 56 Fe values records differential Fe mobilization via DIR in the sediment prior to lithification. Our results demonstrate that the inventory of low- δ 56 Fe marine sedimentary rocks of Neoarchean to Paleoproterozoic age, although impressive in volume, may represent only a minimum of the total inventory of Fe that was cycled by bacteria.
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| [56] |
Geochemistry and sedimentology of a facies transition from limestone to iron-formation deposition in the Early Proterozoic Transvaal Supergroup, South Africa [J].
Abstract This study deals with the geochemistry and sedimentology of a facies transition from interbedded carbonate-shale to banded iron-formation in the Campbellrand carbonate sequence to the overlying Kuruman Iron Formation of the Transvaal Supergroup in South Africa which is approximately 2.3 Ga old. Four major lithologies are: 1) limestone and dolomite, 2) shale and interbedded shale carbonate, 3) siderite-rich banded iron-formation, and 4) iron oxide-rich banded iron-formation. On the basis of the geochemical data and a reconstruction of the depositional basin for the carbonate-shale to iron-formation transition, we conclude that the limestone-dolomite-shale lithologies originated in a water column quite distinct from that in which the iron-formations were precipitated. We propose a model with a stratified water column in which the surface waters were the site of much organic carbon productivity and the locus of cryptalgal limestones and intraclastic limestone deposition; with at somewhat greater depth (below the chemocline) deposition of pyritic carbonaceous shale. Our model depicts the deeper waters as the site for iron-formation deposition. -from Authors
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| [57] |
Primary and diagenetic controls of isotopic compositions of iron-formation carbonates [J].
Oxygen-isotopic abundances of microbanded carbonates are similar to those of under- and overlying massive marine carbonates, ranging from 17.6 to 21.0‰ vs. SMOW (619.6 to 61 12.9‰ vs. PDB). Millimeterscale variations in abundances of 13 C and 18 O are associated with diagenetic replacement of primary siderite by secondary ankerite and/or magnetite. It is shown that these isotopic variations cannot result from mineral-dependent fractionations, metamorphism, or the influence of large volumes of water in an open system.
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| [58] |
Trace element study of siderite-jasper banded iron formation in the 3.45 Ga Warrawoona Group, Pilbara Craton—Formation from hydrothermal fluids and shallow seawater [J].
Shale-normalised rare earth element and yttrium (REE + Y) patterns for siderite鈥搄asper couples in a banded iron formation of the 3.45 Ga Panorama Formation, Warrawoona Group, eastern Pilbara Craton, display distinct positive Y and Eu anomalies and weak positive La and Gd anomalies, combined with depleted light REE relative to middle and heavy REE. Ambient seawater and hydrothermal fluids are identified as major sources of REE + Y for the BIF. In the case of siderites, strong correlations between incompatible trace elements and trace element ratios diagnostic of seawater indicate variable input from a terrigenous source (e.g. volcanic ash). We propose a volcanic caldera setting as a likely depositional environment where jasper and siderite precipitated as alternating bands in response to episodic changes in ambient water chemistry. The episodicity was either driven by fluctuations in the intensity of hydrothermal activity or changes in magma chamber activity, which in turn controlled relative sea level. In this context, precipitation of jasper probably reflects background conditions during which seawater was saturated in silica due to evaporative conditions, while siderites were deposited most likely during intermittent periods of enhanced volcanic activity when seawater was more acidic due to the release of exhalative phases (e.g. CO 2).
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| [59] |
Isotopic constraints on the Late Archean carbon cycle from the Transvaal Supergroup along the western margin of the Kaapvaal craton, South Africa [J].
Few existing studies illuminate the operation of the carbon cycle before the rise of atmospheric oxygen circa 2400 million years ago. Stable carbon isotopic measurements of shallow stromatolitic carbonates (650‰ VPDB) and basinal carbonate minerals (616‰) in iron formation have been used to infer a strong isotopic depth gradient in Archean ocean basins. From new diamond drill cores obtained by the Agouron Drilling Project from the Griqualand West structural basin in the Northern Cape Province, South Africa, we present δ 13 C data from carbonates and organic matter that offer fresh insights into the Late Archean carbon cycle. Three drill cores cover the development, progradation, and ultimate demise (by drowning) of the Campbellrand carbonate platform (ca. 2590–250002Ma); one captures the platform top shallow marine and intertidal paleoenvironments, the other two run through slope and basinal sections deposited adjacent to the platform margin, increasing in water depth (likely to >102km). Both shallow and deep-water carbonates precipitated on the seafloor consistently show δ 13 C values around 610.5‰, incompatible with a strong Late Archean isotopic depth gradient. A mathematical model suggests that these isotopic data are consistent with a reduced biological pump, increased dissolved inorganic carbon in seawater due to higher atmospheric P CO2 , or both. Certain horizons do show distinct isotopic variability. Such areas are commonly shaly, and they tend to be organic and/or iron rich. Strong C-isotopic variations occur on a cm scale and most likely stem from diagenetic remineralization of organic matter. In sediment-starved areas where iron formation developed, siderite tends to be 13 C-depleted, sometimes by as much as 6114‰. These observations suggest a carbon cycle in which iron respiration played a conspicuous role. Carbon isotope ratios from organic matter in shales are commonly >1‰ lighter than stratigraphically contiguous carbonates, but there is no clear water depth trend in the organic carbon isotopic data. Taken as a whole, the δ 13 C of organic matter can be explained by several non-unique sets of processes, including different autotrophic mechanisms of carbon fixation, heterotrophic recycling (including fermentation and methanotrophy), and post-depositional diagenesis. The most striking feature is the occurrence of organic δ 13 C values <6140‰, a feature that appears to be commonplace in Late Archean successions. Framed in the context of carbon cycle isotopic mass balance, both organic and carbonate carbon isotopic data suggest that the proportion of carbon buried as organic matter was not radically different before the appearance of free environmental oxygen.
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| [60] |
Formation age and genesis of the banded iron formations from the Guyang Greenstone Belt, Western North China Craton [J].
61The formation age of BIFs from the Guyang Greenstone Belt is 2.56-2.57Ga.61Geodynamic setting of Guyang GB is mantle plume erupting into the subduction zone.61Fe and Si were derived from the high-T hydrothermal fluids leaching oceanic crust.61Low-grade BIF was upgraded via the interaction with hydrothermal fluids.
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| [61] |
Petrography and geochemistry of the Mesoarchean Bikoula banded iron formation in the Ntem complex (Congo craton), Southern Cameroon: Implications for its origin [J].
Precambrian banded iron formations (BIFs) represent an important source of mineable iron, as well as an archive recording secular changes in the chemistry of the Earth’s early oceans. Here we report petrographic and geochemical characteristics of unweathered drill core samples from the Bikoula BIF, a virtually uncharacterized oxide facies iron formation, hosted in the Mesoarchean Ntem complex, southern Cameroon. The BIF is cross-cut with syenitic veins. The entire succession is highly deformed and metamorphosed under granulite facies conditions. The BIF is characterized by alternating micro-bands of magnetite, quartz and pyroxene. Sulfides (pyrite, pyrrhotite, and chalcopyrite), oligoclase, ferro-pargasite, biotite and ilmenite occur as minor phases. The presence of pyroxene, ferro-pargasite and oligoclase, relatively high contents of major elements such as Al 2 O 3 (0.76–7.5202wt.%), CaO (1.95–4.9002wt.%), MgO (3.78–5.5902wt.%), as well as positive correlations among Al 2 O 3 , TiO 2 , HFSEs, LILEs and transition metals (V, Cr, Ni, Cu and Zn), suggest that the BIF protolith included a significant amount of clastic material. Several samples have preserved seawater-like PAAS-normalized REE-Y patterns, including LREE depletion, and positive La and Y anomalies. Positive Eu anomalies observed in some of the analyzed samples indicate influx of hydrothermal fluids (possibly including Fe and Si) within the basin where the BIF precipitated. However, few samples show unusual negative Eu anomalies that likely result from a large proportion of clastic contamination. The lack of Ce anomalies suggests that the Bikoula BIF was deposited in a basin that was (at least partly) anoxic or suboxic, where it was possible to transport and concentrate dissolved Fe 2+ .
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| [62] |
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| [63] |
Precipitation of iron silicate nanoparticles in early Precambrian oceans marks Earth’s frst iron age [J].
Not Available
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| [64] |
An authigenic origin for Precambrian greenalite: Implications for iron formation and the chemistry of ancient seawater [J].
Persistent anoxia and the lack of a skeletal silica sink through the Precambrian would have promoted a variety of reactions between iron and dissolved silica through much of Earth鈥檚 early history. However, although both iron and silica have each left clear fingerprints in the Precambrian record, evidence for their interaction, and the attendant biogeochemical consequences, is cryptic. Here, experimental evidence is presented showing that Fe2+ and SiO2(aq) in anoxic seawater鈥揹erived solutions promote rapid nucleation of a hydrous Fe(II)-silicate gel at 25 掳C. By merging experimental data with crystallographic constraints, we observe that structural rearrangement and dehydration produce Fe-rich serpentine nanoparticles within the gel, which eventally aggregate to form the mineral greenalite. This nonclassical crystal growth pathway is consistent with the crystal structure of greenalite and with its syndepositional origin in iron formation. A mechanistic underpinning for greenalite precipitation also permits new constraints on the chemistry of ferruginous Precambrian waters. For example, greenalite may have nucleated from waters with a pH as high as 7.7鈥8.3, implicating alkalinity as a key trigger in coupling and decoupling Fe and Si during the anoxic deposition of several late Archean and Paleoproterozoic iron formations. The common, though not exclusive, association of greenalite with deeper-water iron formation facies (i.e., below the fair-weather wave base) suggests that the upwelling of silica-rich alkaline water masses played an important role in driving precipitation. More broadly, our results prompt a reconsideration of the inorganic reactions that determine the upper limits on water-column Fe2+ concentrations in nonsulfidic seawater. The primary precipitation of greenalite and/or siderite would set a ceiling for dissolved Fe2+ that is sensitive to pH, and higher than previously estimated. These results indicate that a better understanding of greenalite distributions in chemical and siliciclastic sediments will help to disentangle the coevolution of redox and acid-base chemistries through the Precambrian.
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| [65] |
The Joffre banded iron formation, Hamersley Group, Western Australia: Assessing the palaeoenvironment through detailed petrology and chemostratigraphy [J].
The Joffre Member of the Brockman Iron Formation is by volume the largest single known banded iron formation (BIF) in the world. Here we present detailed petrology and chemostratigraphy through the entire 35502m core section of this 652.4502billion year old unit. Oxide BIF and silicate–carbonate–oxide BIF dominate the lithology, with minor amounts of interbedded stilpnomelane mudrock, stilpnomelane-rich tuffaceous mudrock and calcareous mudrock. Besides chert and magnetite, the prominent mineralogy is riebeckite, ankerite, hematite, stilpnomelane and crocidolite. The BIF is characterized by an average of 5002wt.% SiO 2 and 44.502wt.% Fe 2 O 3 and an overall low abundance of Al 2 O 3 (<102wt.%), TiO 2 (<0.0402wt.%), and trace metals such as Cr (<1002ppm), Ni (<502ppm) and Mo (<0.502ppm). It has a high ∑REE (rare earth element) content (up to 4102ppm) and a fractionated shale-normalized (SN) seawater REY (rare earth element02+02yttrium) pattern having an enrichment of HREE (heavy rare earth elements) relative to LREE (light rare earth elements) with an average (Pr/Yb) SN of 0.24. The REY patterns also show a positive La SN anomaly, no Ce SN anomaly and a weakly developed positive Y SN anomaly. Iron isotopes (δ 56 Fe) with positive δ 56 Fe values of +0.04‰ to +1.21‰ suggest that a large part of the hydrothermal iron was partly oxidized in the upper water column and subsequently precipitated as ferric oxyhydroxides. No epiclastic grains have been found; rather submarine hydrothermal fluids and fine-grained volcanogenic detritus controlled BIF chemistry. The former source is reflected through a constant positive Eu SN anomaly throughout the core (average Eu SN anomaly of 1.6 with a peak of 2.1 between 100 and 15502m depth), while the latter source is best reflected through the stilpnomelane-rich tuffaceous mudrock consisting of volcanic ash-fall tuff with relict shards set in a stilpnomelane matrix. The mudrock is overlain by well-preserved wavy laminae and laminae sets of stilpnomelane microgranules that likely originated from re-worked volcanic ash formed either on the seafloor or in the water column prior to deposition. An enriched HREE-to-LREE pattern, a high iron content (653002wt.%), and a δ 56 Fe value of +0.59‰ collectively imply that the mudrock facies interacted with the Fe-rich seawater prior to deposition. The TiO 2 –Zr ratio of the BIF and the associated mudrocks suggest a felsic-only-source related to the same style of volcanics as the slightly younger Woongarra rhyolites. Given the observation that the dominant control on the seawater chemistry was associated with felsic volcanics, we speculate that the fine-grained pelagic ash particles may have sourced bio-available nutrients to the surface water. This would have facilitated enhanced biological productivity, including bacterial Fe(II)-oxidation which is now recorded as the positively fractionated 56 Fe iron oxide minerals in the Joffre BIF. Alongside submarine hydrothermal input to the basin, the dominant control on the ocean chemistry seems to have been through volcanic and pyroclastic pathways, thereby making the Joffre BIF poorly suited as a chemical proxy for the study of atmospheric oxygen and its weathering impact on local landmasses.
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| [66] |
Could bacteria have formed the Precambrian banded iron formations? [J].
Banded iron formations (BIFs) are prominent sedimentary deposits of the Precambrian, but despite a century of endeavor, the mechanisms of their deposition are still unresolved. Interactions between microorganisms and dissolved ferrous iron in the ancient oceans offer one plausible means of mineral precipitation, in which bacteria directly generate ferric iron either by chemolithoautotrophic iron oxidation or by photoferrotrophy. On the basis of chemical analyses from BIF units of the 2.5 Ga Hamersley Group, Western Australia, we show here that even during periods of maximum iron precipitation, most, if not all, of the iron in BIFs could be precipitated by iron-oxidizing bacteria in cell densities considerably less than those found in modern Fe-rich aqueous environments. Those ancient microorganisms would also have been easily supported by the concentrations of nutrients (P) and trace metals (V, Mn, Co, Zn, and Mo) found within the same iron-rich bands. These calculations highlight the potential importance of early microbial activity on ancient metal cycling.
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| [67] |
Origin and paleoenvironmental signifcance of major iron formations at the Archean-Paleoproterozoic boundary: Reviews [J].
Abstract This paper provides a critical review of advances made in understanding of sedimentary environments, geochemical processes, and biological systems that contributed to the deposition and diagenetic evolution of the exceptionally well-preserved and large
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| [68] |
Micro-and nanobands in Late Archean and Palaeoproterozoic banded-iron formations as possible mineral records of annual and diurnal depositions [J].
61I investigated the banding structures in early Precambrian banded-iron formations.61Mineralogical microbands of chert/jasper, magnetite and dolomite are observed.61The hematite microbands are made of spindle-like grains.61Chert/jasper microbands represent primary annual depositions from the ocean.61Nanobands in hematite are records of diurnal activities of photosynthesis.
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| [69] |
Black smokers and density currents: A uniformitarian model for the genesis of banded iron-formations [J].
The iron-rich hydrothermal fluids triggered the precipitation of dissolved ferrous iron accumulated by anoxic weathering to produce the huge deposits of BIF in the Archean and Paleoproterozoic until the rise in atmospheric oxygen stopped the accumulation of ferrous iron in the oceans leaving only the hydrothermal source for later deposits.
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| [70] |
McCulloch M T. Rare earth element and Nd isotopic compositions of the banded iron-formations and associated shales from Hamersley, Western Australia [J].
Major-element, trace-element, and Nd isotopic compositions were determined for the early Proterozoic banded iron formations (IFs) from the Hamersley Basin (Australia). The data place some constraints on the composition of the early Proterozoic oceans. It was found that the MgO contents measured in the IFs and those of modern metalliferous sediments are very similar. This implies either approximately constant amounts of seafloor hydrothermal alteration relative to ocean volumes or a higher continental Mg flux into the oceans during the Archean.
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| [71] |
A key role for green rust in the Precambrian oceans and the genesis of iron formations [J].
Iron formations deposited in marine settings during the Precambrian represent large sinks of iron and silica, and have been used to reconstruct environmental conditions at the time of their formation. However, the observed mineralogy in iron formations, which consists of iron oxides, silicates, carbonates and sulfides, is generally thought to have arisen from diagenesis of one or more mineral precursors. Ferric iron hydroxides and ferrous carbonates and silicates have been identified as prime candidates. Here we investigate the potential role of green rust, a ferrous-ferric hydroxy salt, in the genesis of iron formations. Our laboratory experiments show that green rust readily forms in early seawater-analogue solutions, as predicted by thermodynamic calculations, and that it ages into minerals observed in iron formations. Dynamic models of the iron cycle further indicate that green rust would have precipitated near the iron redoxcline, and it is expected that when the green rust sank it transformed into stable phases within the water column and sediments. We suggest, therefore, that the precipitation and transformation of green rust was a key process in the iron cycle, and that the interaction of green rust with various elements should be included in any consideration of Precambrian biogeochemical cycles.
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| [72] |
Metal stable isotopes in paleoceanography [J].
Considered esoteric only a few years ago, research into the stable isotope geochemistry of transition metals is moving into the geoscience mainstream. Although initial attention focused on the potential use of some of these nontraditional isotope systems as biosignatures, they are now emerging as powerful paleoceanographic proxies. In particular, the Fe and Mo isotope systems are providing information about changes in oxygenation and metal cycling in ancient oceans. Zn, Cu, Tl, and a number of other metals and metalloids also show promise. Here we review the basis of stable isotope fractionation as it applies to these elements, analytical considerations, and the current status and future prospects of this rapidly developing research area.
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| [73] |
Rare earth element and yttrium compositions of Archean and Paleoproterozoic Fe formationsrevisited: New perspectives on the significance and mechanisms of deposition [J].
Similar to modern redox-stratified basins, the REE02+02Y patterns in late Paleoproterozoic Fe formations record evidence of a shuttle of metal and Ce oxides across the redoxcline from oxic shallow seawater to deeper anoxic waters. Oxide dissolution—mainly of Mn oxides—in an anoxic water column lowers the dissolved Y/Ho ratio, raises the light to heavy REE ratio, and increases the concentration of Ce relative to the neighboring REE (La and Pr). Fe oxides precipitating at or near the chemocline will capture these REE anomalies and thus evidence for this oxide shuttle. In contrast, Archean Fe formations do not display REE02+02Y patterns indicative of an oxide shuttle, which implies an absence of a distinct Mn redoxcline prior to the rise of atmospheric oxygen in the early Paleoproterozoic. As further evidence for reducing conditions in shallow-water environments of the Archean ocean, REE data for carbonates deposited on shallow-water Archean carbonate platforms that stratigraphically underlie Fe formations also lack negative Ce anomalies. These results question classical models for deposition of Archean Fe formations that invoke oxidation by free oxygen at or above a redoxcline. In contrast, we add to growing evidence that metabolic Fe oxidation is a more likely oxidative mechanism for these Fe formations, implying that the Fe distribution in Archean oceans could have been controlled by microbial Fe uptake rather than the oxidative potential of shallow-marine environments.
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| [74] |
Characteristics of rare earth elements and geological significations of BIFs from Jidong, Wutai and Lüliang area [J].冀东、五台和吕梁地区条带状铁矿的稀土元素特征及其地质意义 [J].
详细报道了冀东、五台和吕梁地区条带状铁矿全岩样品的稀土元素分析结果。结果表明，研究区BIF具有非常相似的特征：稀土总量均较低；经页岩标准化的稀土元素配分模式均呈现轻稀土亏损、重稀土富集的特征；Y/Ho比值较高；具有明显的Eu、Y、La的正异常，且这些特征表明研究区BIF的稀土元素来源于火山热液和海水的混合溶液。虽然BIF均显示Eu正异常，但不同类型、不同沉积年龄BIF的铕异常程度不同：与吕梁地区Superior型铁矿相比，冀东和五台地区的Algoma型铁矿显示了更大的Eu正异常；并且自中太古代—新太古代—古元古代，BIF的铕正异常逐渐减小，这可能反映了随着BIF沉积年龄的减小，进入到该地区海水中的高温热液流体逐渐减少；同时，研究区BIF缺乏明显的Ce负异常，可能暗示在BIF沉积时海水的氧化还原状态为缺氧环境。
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| [75] |
Petrochemical characteristics and geological significations of banded iron formations in the Wutai Mountain of Shanxi and Qian’an of eastern Hebei [J].
山西五台山和冀东迁安地区条带状铁矿的岩石化学特征及其地质意义 [J].
详细报道了五台山地区白峪里、柏枝岩和峨口(又名山羊坪铁矿)3 个新太古代条带状铁矿床和冀东迁安地区条带状铁矿样品的岩石学和岩石化学特征,并与辽宁鞍山和山东韩旺以及国外同类矿床进行了对比.五台山地区和冀东迁安 地区条带状铁矿的微量元素和稀土元素的含量和配分特征与国内外同类矿床十分一致:4个地区条带状铁矿样品均富集Th、U、La、Ce、P、Sm等元素,亏 损K、Nb、Sr、Hf、Er、Ti等元素;稀土元素总量均较低,是太古宙海洋沉积特征之一,轻稀土元素轻微亏损,重稀土元素稍富集,具有明显的Eu的正 异常,部分具有Y正异常.Y的异常通常代表了海水的特征,Eu的正异常指示了高温海底热液的特征,由此可判断铁硅质建造形成于热海水环境.五台山地区与条 带状铁矿伴生的黄铁矿的δ34S值在零附近,表明其来源于地帽.由此可知所研究条带状铁矿床是慢源的火山喷发或火山喷气带来的硅铁质溶于海水后在特定条件 下经化学沉积而成.
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| [76] |
Geochemical characteristics of the Huoqiu BIF ore deposit in Anhui Province and their metallogenic significance: Taking the Bantaizi and Zhouyoufang deposits as examples [J].
安徽霍邱BIF铁矿地球化学特征及其成矿意义:以班台子和周油坊矿床为例 [J].URL 摘要
安徽霍邱铁矿田位于华北克拉通南缘,是一个大型BIF铁矿田.本文对霍邱矿田班台子矿区和周油坊矿区的铁矿石及其赋存的岩石共28件样品进行了详细的主微量元素地球化学分析.分析结果表明,班台子矿区的片麻岩和角闪岩的原岩属于一套亚碱性系列的岩石,具有大离子亲石元素(LILE)富集,高场强元素(HFSE)明显亏损的火山弧岩石的特征.班台子角闪岩具有低的K2O含量和Ti/V值,Ti/V=22.7 ～ 25.9,平均24.5,与岛弧拉斑玄武岩一致.弧后盆地玄武岩化学组成具有类似岛孤拉斑玄武岩的特征.BIFs的形成往往需要构造稳定的半深水-深水盆地,孤后盆地能够为BIFs韵律条带的产生提供稳定的沉积环境,因此霍邱BIFs铁矿的大量出现说明班台子矿区角闪岩形成于弧后盆地,代表了霍邱铁矿形成的构造环境.班台子矿区铁矿石的(Eu/Eu*)SN=1.57 ～1.82,与Superior型(简称S型)BIFs特征一致；而周油坊矿区假象镜铁矿的(Eu/Eu*)SN=1.93 ～3.41,与Algoma型(简称A型)BIFs特征比较吻合.正Eu异常的强弱反应了成矿位置距离海底火山热液喷气口的远近.因此,我们推断霍邱地区BIFs型铁矿形成位置与海底火山热液喷气口的距离比较特别,处于A型向S型过渡的位置.角闪岩和片麻岩及其赋存的铁矿石的Al2O3和TiO2良好的线性相关性说明铁矿石铁质部分来源于侵蚀的弧后盆地玄武岩.Y/Ho比值=31.05 ～56.67,平均为46.65,说明霍邱铁矿继承了海水与热液的混合特征,其中,海水的贡献更大一些.周油坊矿区的大理岩主要化学组成CaO为28.49％ ～29.10％,MgO为20.25％ ～ 21.22％以及少量的SiO2(2.45％～6.10％).与平均显生宙石灰岩相比,周油坊大理岩亏损LILE和HFSE；与后太古代平均澳大利亚页岩(PAAS)相比,周油坊假象镜铁矿稀土元素总量低,明显正Eu异常,Ce无明显异常,Y/Ho比值介于35.00～56.67,平均48.81.这些特征显示大理岩及其赋存的假象镜铁矿形成于缺氧的海洋环境,海水中的氧能使亚铁离子氧化成三价铁离子沉淀出Fe(OH)3,但不足以使Ce3+氧化成Ce4+.
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| [77] |
Geochemical characteristics of Banded Iron Formations in Liaoning-eastern Hebei area:Ⅱ. Characteristics of rare earth elements [J].辽冀地区条带状铁建造地球化学特征:Ⅱ.稀土元素特征 [J].
辽冀地区(鞍山-本溪地区和冀东地区)位于华北克拉通北东部,是我国早前寒武纪条带状铁建造(BIFs)最重要的分布区,主要为新太古代Algoma型。本文系统对比了辽冀地区28个铁矿床200件铁矿石样品的稀土元素特征。结果表明:(1)所有样品的稀土元素特征比较相似:稀土元素总量较低,Y/Ho比值较高;经太古宙后平均澳大利亚页岩(PAAS)标准化后呈现重稀土相对富集、轻稀土相对亏损的配分模式,La异常不明显,强烈的Eu正异常和明显的Y正异常,暗示研究区铁矿石成矿物质主要来源于海底高温热液和海水的混合溶液;与冀东地区BIFs相比,鞍本地区Eu异常更为明显,说明鞍本地区BIFs显示更多的热液特征;(2)铁矿石的Ce/Ce*变化范围为0.77~1.09,缺乏明显的Ce负异常,说明其沉积于还原的海水环境;(3)辽冀地区BIFs的稀土元素总量、Eu异常、Y异常和Y/Ho比值变化范围均比较大,可能与BIFs沉积过程中碎屑物质的加入有关;与鞍本地区相比,冀东地区BIFs的Eu正异常、Y正异常程度均小于鞍本地区,热液和海水特征均不明显,Y/Ho比值更接近球粒陨石(26~28),可能暗示冀东地区有更多的碎屑物质的加入。
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| [78] |
Episodic negative anomalies of cerium at the depositional onset of the 2.65 Ga Itabira iron formation, Quadriltero Ferrífero of Minas Gerais, Brazil [J].
Magnetite bands in sericitic phyllite of the Batatal Formation mark the transition to the overlying 2.65-Ga Itabira iron formation, or Cauê Itabirite, in the Quadrilátero Ferrífero of Minas Gerais, Brazil. The magnetite bands have low (non-crustal) Th/U ratios, suprachondritic Y/Ho ratios and sporadic anomalies of Ce that are characterised as truly negative. The data point to mild oxygenation of sea water at the depositional onset of the Itabira iron formation some 200Ma before the Great Oxidation Event.
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| [79] |
Source characteristics of the 2.5 Ga Wangjiazhuang banded iron formation from the Wutai greenstone belt in the North China Craton: Evidence from neodymium isotopes [J].
Here we first present samarium (Sm)–neodymium (Nd) isotopic data for the 652.502Ga Wangjiazhuang BIF and associated lithologies from the Wutai greenstone belt (WGB) in the North China Craton. Previous geochemical data of the BIF indicate that there are three decoupled end members controlling REE compositions: high-T hydrothermal fluids, ambient seawater and terrigenous contaminants. Clastic meta-sediment samples were collected for major and trace elements studies in an attempt to well constrain the nature of detrital components of the BIF. Fractionated light rare earth elements patterns and mild negative Eu anomalies in the majority of these meta-sedimentary samples point toward felsic source rocks. Moreover, the relatively low Th/Sc ratios and positive ε Nd ( t ) values are similar to those of the 652.502Ga granitoids, TTG gneisses and felsic volcanics in the WGB, further indicating that they are derived from less differentiated terranes. Low Chemical Index of Weathering (CIW) values and features in the A-CN-K diagrams for these meta-sediments imply a low degree of source weathering. Sm–Nd isotopes of the chemically pure BIF samples are characterized by negative ε Nd ( t ) values, whereas Al-rich BIF samples possess consistently positive ε Nd ( t ) features. Significantly, the associated supracrustal rocks in the study area have positive ε Nd ( t ) values. Taken together, these isotopic data also point to three REE sources controlling the back-arc basin depositional environment of the BIF, the first being seafloor-vented hydrothermal fluids ( ε Nd ( t )02020), the third being syndepositional detritus that received their features by weathering of a nearby depleted source (likely the arc) ( ε Nd ( t )02>020).
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| [80] |
Rare earth elements in the water column of Lake Vanda, McMurdo Dry Valleys, Antarctica [J].
Water collected under trace-element clean conditions was analyzed for its dissolved and total rare earth element (REE) concentrations by inductively coupled plasma mass spectrometry. Depth profiles are characterized by low dissolved REE concentrations (La, Ce, <15 pM) in surface waters that increase slightly (La, 70 pM; Ce, 20 pM) with increasing depth to 鈭55 m, the limit of the fresh oxic waters. Below this depth, a sharp increase in the concentrations of strictly trivalent REE (e.g., La, 5 nM) is observed, and a submaximum in redox sensitive Ce (2.6 nM) is found at 60- to 62-m depth. At a slightly deeper depth, a sharper Ce maximum is observed with concentrations exceeding 11 nM at a 67-m depth, immediately above the anoxic zone. The aquatic concentrations of REE reported here are 鈭50-fold higher than previously reported for marine oxic/anoxic boundaries and are, to our knowledge, the highest ever observed at natural oxic/anoxic interfaces. REE maxima occur within stable and warm saline waters. All REE concentrations decrease sharply in the sulfidic bottom waters. The redox-cline in Lake Vanda is dominated by diffusional processes and vertical transport of dissolved species driven by concentration gradients. Furthermore, because the ultraoligotrophic nature of the lake limits the potential for organic phases to act as metal carriers, metal oxide coatings and sulfide phases appear to largely govern the distribution of trace elements. We discuss REE cycling in relation to the roles of redox reactions and competitive scavenging onto Mn- and Fe-oxides coatings on clay sized particles in the upper oxic water column and their release by reductive dissolution near the anoxic/oxic interface.
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| [81] |
Oxygenation of the Earth’s atmosphere-ocean system: A review of physical and chemical sedimentologic responses [J].
The Great Oxidation Event (GOE) is one of the most significant changes in seawater and atmospheric chemistry in Earth history. This rise in oxygen occurred between ca. 2.4 and 2.302Ga and set the stage for oxidative chemical weathering, wholesale changes in ocean chemistry, and the evolution of multicelluar life. Most of what is known about this important event and the subsequent oxygenation history of the Precambrian Earth is based on either geochemistry or “data mining” published literature to understand the temporal abundance of bioelemental sediments. Bioelemental sediments include iron formation, chert, and phosphorite, which are precipitates of the nutrient elements Fe, Si, and P, respectively. Because biological processes leading to their accumulation often produce organic-rich sediment, black shale can also be included in the bioelemental spectrum. Thus, chemistry of bioelemental sediments potentially holds clues to the oxygenation of the Earth because they are not simply recorders of geologic processes, but intimately involved in Earth system evolution. Chemical proxies such as redox-sensitive trace elements (Cu, Cr, V, Cd, Mo, U, Y, Zn, and REE's) and the ratio of stable isotopes (δ56Fe, δ53Cr, δ97/95Mo, δ98/95Mo, δ34S, Δ33S) in bioelemental sediments are now routinely used to infer the oxygenation history of paleo-seawater. The most robust of these is the mass-independent fractionation of sulfur isotopes (MIF), which is thought to have persisted under essentially anoxic conditions until the onset of the GOE at ca. 2.402Ga. Since most of these proxies are derived from authigenic minerals reflecting pore water composition, extrapolating the chemistry of seawater from synsedimentary precipitates must be done cautiously. Paleoenvironmental context is critical to understanding whether geochemical trends during Earth's oxygenation represent truly global, or merely local environmental conditions. To make this determination it is important to appreciate chemical data are primarily from authigenic minerals that are diagenetically altered and often metamorphosed. Because relatively few studies consider alteration in detail, our ability to measure geochemical anomalies through the GOE now surpasses our capacity to adequately understand them. In this review we highlight the need for careful consideration of the role sedimentology, stratigraphy, alteration, and basin geology play in controlling the geochemistry of bioelemental sediments. Such an approach will fine-tune what is known about the GOE because it permits the systematic evaluation of basin type and oceanography on geochemistry. This technique also provides information on how basin hydrology and post-depositional fluid movement alters bioelemental sediments. Thus, a primary aim of any investigation focused on prominent intervals of Earth history should be the integration of geochemistry with sedimentology and basin evolution to provide a more robust explanation of geochemical proxies and ocean-atmosphere evolution.
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| [82] |
A redox-stratified ocean 3.2 billion years ago [J].
61We propose a discrete redoxcline existed between deep and shallow seawater at 3.2 Ga.61Oxygen in the photic zone was produced by cyanobacteria.61Cyanobacteria evolved before 3.2 Ga.
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| [83] |
Craton-scale distribution of Archean greenstone gold deposits: Predictive capacity of the metamorphic model [J].
Abstract In previous papers, the authors have outlined a metamorphic-replacement model for the genesis of gold deposits in Western Australia greenstone belts and have concentrated on developing genetic models specifically for the larger economic deposits. Here, they develop this theme further by examining the capacity of the metamorphic model to explain the observed regional distribution of Archean gold deposits and to define more prospective areas for future exploration. Emphasis is placed on mineralization within the Western Australian Shield, but other granitoid-greenstone terranes in Canada and southern Africa are also considered.
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| [84] |
Iron formation: The sedimentary product of a complex interplay among mantle, tectonic, oceanic, and biospheric processes—A reply [J]. |
| [85] |
The composition and depositional environments of Mesoarchean iron formations of the West Rand Group of the Witwatersrand Supergroup, South Africa [J].
ABSTRACT This paper documents the sedimentological setting, mineralogy, and geochemistry of several iron formation units interbedded with siliciclastic strata of the Mesoarchean Witwatersrand Supergroup, well known for its world-class conglomerate-hosted Au-U deposits. Four major iron formation beds, with associated magnetic mudstones, are present in two distinctly different lithostratigraphic associations, namely shale- and diamictite-associated iron formation. The shale association is represented by the Water Tower and Contorted Bed iron formations in the Parktown Formation of the Hospital Hill Subgroup in the lower part of the succession and the diamictite association by the Promise and Silverfield iron formations in the overlying Government Subgroup. The iron formation units have been subjected to lower greenschist facies metamorphism. Oxide (magnetite and limited hematite), carbonate, and silicate facies iron formations are recognized. The iron formations typically overlie major transgressive flooding surfaces in the succession and, in turn, form the base of progradational coarsening-upward increments of sedimentation comprising magnetic mudstone, nonmagnetic shale, and interbedded siltstone-quartzite. The upward transition from iron formation into magnetic mudstone is accompanied by a change in mineralogical composition from hematite-magnetite iron formation at the base in the most distal setting through magnetite-siderite- and siderite-facies iron formation in the transition zone to magnetic mudstone. The siderite with associated ankerite displays highly depleted 未13C values, suggesting crystallization via iron respiration in presence of organic carbon. The iron formations display positive post-Archean Australian shale-normalized Eu and Y anomalies with depletion in light rare-earth elements relative to heavy rare-earth elements, indicating precipitation from marine water with a high-temperature hydrothermal component. Integration of sedimentological, petrographic, and geochemical results indicates that the shale-associated iron formation was deposited during the peak of transgression, when reduced iron-rich hydrothermal waters entered the Witwatersrand Basin over a limited vertical extent due to neutral buoyancy, with the top of the plume occurring below the photic zone. It is suggested that chemolithoautotrophic iron-oxidizing bacteria, which would have been able to exploit the difference in chemistry between the iron-enriched plume water and ambient ocean water to fuel metabolic activity in the presence of limited free molecular oxygen, were responsible for precipitation of initial ferric iron oxyhydroxides. The vertical facies associations in the iron formations most likely developed in response to the limited vertical extent of the hydrothermal plume, with (from distal to proximal) hematite preserved where the base of the plume was not in contact with the basin floor, magnetite where the plume water was in contact with bottom sediment, iron-rich carbonates where organic carbon input was high, iron-rich alumosilicates where siliciclastic input became significant in more proximal settings, and iron-poor sediment above the top of the plume. Diamictite-associated iron formations in the Witwatersrand are inferred to have been deposited in a fashion similar to the shale-associated iron formations, with the exception that major transgressions and hydrothermal plume invasion were preceded by glacial ice cover. The climate warming and increased volcanic activity required could have been related to increased tectonic activity inferred for the Witwatersrand Supergroup during deposition of the glacially associated iron formations.
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| [86] |
Deposition of banded iron formations by anoxygenic phototrophic Fe (II)-oxidizing bacteria [J].
The mechanism of banded iron formation (BIF) deposition is controversial, but classically has been interpreted to reflect ferrous iron [Fe(II)] oxidation by molecular oxygen after cyanobacteria evolved on Earth. Anoxygenic photoautotrophic bacteria can also catalyze Fe(II) oxidation under anoxic conditions. Calculations based on experimentally determined Fe(II) oxidation rates by these organisms under light regimes representative of ocean water at depths of a few hundred meters suggest that, even in the presence of cyanobacteria, anoxygenic phototrophs living beneath a wind-mixed surface layer provide the most likely explanation for BIF deposition in a stratified ancient ocean and the absence of Fe in Precambrian surface waters.
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| [87] |
Biological carbon precursor to diagenetic siderite with spherical structures in iron formations [J].
During deposition of Precambrian iron formation, the combined sedimentation of ferrihydrite and phytoplankton biomass should have facilitated Fe(III) reduction during diagenesis. However, the only evidence for this reaction in iron formations is the iron and carbon isotope values preserved in the authigenic ferrous iron-containing minerals. Here we show experimentally that spheroidal siderite, which is preserved in many iron formation and could have been precursor to rhombohedral or massive siderite, forms by reacting ferrihydrite with glucose (a proxy for microbial biomass) at pressure and temperature conditions typical of diagenesis (17065°C and 1.265kbar). Depending on the abundance of siderite, we found that it is also possible to draw conclusions about the Fe(III):C ratio of the initial ferrihydrite-biomass sediment. Our results suggest that spherical to rhombohedral siderite structures in deep-water, Fe-oxide iron formation can be used as a biosignature for photoferrotrophy, whereas massive siderite reflects high cyanobacterial biomass loading in highly productive shallow-waters.
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| [88] |
LA-ICP-MS analysis of magnetite and application in genesis of mineral deposit [J].磁铁矿LA-ICP-MS分析在矿床成因研究中的应用 [J].
激光剥蚀电感耦合等离子体质谱(LA-ICP-MS),由于其原位、实时、低检测限、高空间分辨率等优点,在矿物原位微量元素分析方面具有独特的优势。磁铁矿作为多种矿床和岩石中的常见矿物,其化学组成一直是国内外学者关注的焦点。而大量的研究表明,在磁铁矿LA-ICP-MS分析过程中,基体效应不明显,一般采用富铁硅酸盐玻璃作为标样,就能够取得较为准确的结果。因此近年来磁铁矿原位微量元素研究进展迅速,并在反演成岩成矿条件、辅助判别矿床类型和间接指导找矿勘探等方面显示出广泛的应用前景。通过总结25个不同类型岩浆和热液矿床中磁铁矿微量元素数据,与前人在矿床类型判别上的研究进行了一定的对比,发现常用的磁铁矿判别图解可以用来区分多种不同类型的矿床,但是已经划分出的分类边界可能需要进一步细化和严格验证,并且事先仔细的岩相学观察是数据解释的重要基础。另外,通过磁铁矿微量元素分配对岩浆和热液过程一系列复杂物理化学条件(熔/流体成分、温度、冷却速率、压力、氧逸度、硫逸度和二氧化硅活度等)的响应进行了一定探讨。在岩浆阶段,磁铁矿成分与熔体组成及分异演化密切相关;而热液阶段,流体性质的变化也会显著改变磁铁矿的化学成分。并且后期流体的改造或者磁铁矿的亚固相再平衡作用会对磁铁矿的成因鉴别产生严重干扰。综述了近年来LA-ICP-MS在磁铁矿微量元素分析方面的发展以及在矿床学领域的重要应用,以期对成矿作用和成矿过程研究提供新的思路和方向。
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