周怀阳
同济大学 海洋与地球科学学院, 上海 200092
Zhou Huaiyang
中图分类号: P756.5
文献标识码: A
文章编号: 1001-8166(2017)12-1245-08
收稿日期: 2017-10-25
修回日期: 2017-11-25
网络出版日期: 2017-12-20
版权声明: 2017 地球科学进展 编辑部
作者简介:
First author:Zhou Huaiyang(1961-),male,Changshu City, Jiangsu Province, Professor. Research areas include marine geology and geochemistry.E-mail:zhouhy@tongji.edu.cn
作者简介:周怀阳(1961-),男,江苏常熟人,教授,主要从事海洋地质和地球化学研究.E-mail:zhouhy@tongji.edu.cn
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摘要
一般认为,地球物理上的莫霍面是地幔和地壳的分界面。被海水覆盖的洋壳通常只有6~7 km厚,相对于较厚的陆壳来说,洋壳不仅年轻得多,也活跃得多。然而,洋壳的地质结构以及莫霍面的地质意义到底是什么,至今仍是固体地球科学领域最大的悬疑之一。现有的结果表明,依据分布在大陆边缘的蛇绿岩套确定的Penrose模型基本不适用于慢速和超慢速扩张洋中脊的实际洋壳结构,对快速扩张洋中脊的适用性也有一定的疑问。解答洋壳结构及其莫霍面地质意义最有效的手段仍是大洋钻探。20世纪60年代初,试图挺进莫霍面的“莫霍钻”计划揭开了地球科学最伟大的大洋钻探计划的序幕。新世纪以来,随着各种研讨和计划的切实开展,人类真正实现莫霍钻梦想的日子似乎正在来临。
关键词：
Abstract
Knowledge of ocean crust is one of bases to understand the deep and the surface of our planet. Since the definition of the Earth crust based on the geophysical discovery of Moho, marvelous efforts have been made to understand the geological significance of the Moho and the structure of the ocean crust. Up to date, it becomes clear that the Penrose model built up on ophiolite is unsuitable for the explanation of the ocean crust structure along slow and ultraslow spreading ridges, and probably also questionable for that of fast spreading ridges. The only effective way to solve the problem is to drill into the geophysical detected Moho and get samples. With the development of modern technology and more logic scientific strategy, that largely improved from the milestone Mohole projects carried out about half a century ago. The time to realize the Mohole dream seams coming.
Keywords：
地球是人类的家园,地球内部的资源是人类文明进步、社会可持续发展的主要依靠。探索地球内部的结构构造和物质组成、了解地球的演化历史是地球科学家的神圣职责和光荣使命。然而,在科学技术迅猛发展、人类触角已经伸到外太空、深入到宇宙内部的今天,人类对地球内部的了解却仍然停留在十分肤浅的状态。
目前我们有关地球内部是分层的知识主要来源于地球物理的探测。将地球划分为地壳和地幔的莫霍面就是地球浅表层最重要的一个地球物理界面。洋壳的莫霍面深度要比陆壳的薄很多,只有5~8 km。在地震波剖面上,可看到莫霍面所处深度有一个明显的亮的地震反射面。地震波纵波速度在穿过这个面的前后会发生跳跃性变化,从7.6 km/s跃变为8.1 km/s。
在打穿洋壳、获得洋壳和地幔的样品之前,人们试图通过研究分布在一些大陆边缘、陆地上的蛇绿岩套来为洋壳的地质结构和莫霍面的岩性地质模型提供参照物。根据单个或多个蛇绿岩套综合研究的结果[1],1972年确定的Penrose模型认为,从蛇绿岩套的顶部到底部,岩石类型依次从沉积岩、玄武岩、辉长岩递变为橄榄岩,其中,大多数蛇绿岩套的岩石都显示出明显的层状构造,这些岩性的密度、地震波速度及其层状的构造等特征与地球物理对地球内部的探测结果看起来似乎十分吻合(图1)。根据Penrose模型,莫霍面就是岩浆深成结晶的辉长岩与橄榄岩之间的界面。橄榄岩也因此可称为地幔岩。岩浆是从地幔岩中解压部分熔融产生的,很多橄榄岩也显示出亏损岩浆组分的状态,因此橄榄岩有时也被称为残余橄榄岩。Penrose模型也意味着洋壳内部的岩石主要为平行于莫霍面的层状构造。
人们一度以为,洋壳的地质结构应该像Penrose模型描述的那样有规律、那样简单。但是,实际情况是,Penrose模型建立40多年以来,几乎每一项有关洋壳调查的结果都能指出Penrose模型的不可比拟性,或者说,这个Penrose模型从来没有在真正的大洋洋壳上被验证过。
绵延全球海底7万多千米的洋中脊是大洋地壳生长的主要区域。地球物理探测也表明,离洋中脊轴部2 km范围内即可有莫霍反射面显示,而且其莫霍面深度与离洋中脊轴部更远处的莫霍面深度差不多,说明大部分洋壳的岩浆增生只发生在大洋中脊轴部相当狭窄的区域内。
在不同的区域,洋中脊有不同的扩张(生长)速率。根据扩张速率的大小,全球洋中脊也可分为快速扩张洋中脊、中速扩张洋中脊、慢速扩张洋中脊和超慢速扩张洋中脊。由于一些世界海洋科技强国离快速扩张洋中脊比较近,对快速扩张洋中脊开展的研究工作积累也比较多,我们至今有关洋壳的大部分认识都来源于对快速扩张洋中脊的调查研究。
目前人们一般认为,Penrose模型应该主要适用于快速扩张洋中脊形成的洋壳。快速扩张洋中脊的海底几乎完全由岩浆喷出海底形成的火山岩(玄武岩,图1中层2)覆盖。在个别“构造窗”(有构造切割或剥蚀的地方,如Hess Deep)和一些转换断层处,有时可见到一些岩墙(辉绿岩)、深成岩(辉长岩)和地幔橄榄岩,展现出快速扩张洋中脊处洋壳相对比较简单的层状结构。
在慢速与超慢速扩张洋中脊上,在Penrose模型中标为层2的火山岩呈明显不连续甚至呈点状或零星分布,在有的洋脊段上,大面积缺失火山岩,而且,被喻为下洋壳(层3)最基本岩性的辉长岩也很少出现[2]。所谓地幔橄榄岩的出露面积在一些洋脊段上可达到1/2~1/4[3]。结合对一些拆离断层和推覆杂岩体岩石组构的观察,目前一般认为这些缺乏层2的区域是由构造拉伸作用形成的,可能原有的层2是被拆离断层等构造作用移位或剥蚀掉了。然而,有的地方并不能确认有拆离断层的存在。根本的问题也是至今不能回答的问题是,在这些地区出露地幔橄榄岩的下面究竟是什么?
构成下洋壳(层3)的辉长岩无疑是硅酸盐岩浆在洋壳内部比较缓慢地冷却结晶形成的。按照Penrose模型,作为深成岩的辉长岩不仅呈层状,而且厚度巨大,是洋壳的主体组成部分。对Oman蛇绿岩套的研究证实,可以出现宽20 km、高度达4 km的岩浆房。由此结晶的辉长岩体的成分变化比较小,其中有明显的陡倾叶理和定向的层状构造。
在1987年东太平洋快速扩张洋中脊第一个真正岩浆房探测结果公布之前,根据Penrose模型,人们相信洋中脊下面存在一个大的“洋葱头”岩浆房[4]。“大岩浆房”从其顶部向下冷却,岩浆逐渐结晶,晶体沉入岩浆房的底部,晶体从下往上逐渐堆积起来。一段时间内,该大“洋葱头”模型被认为可以比较好地吻合Penrose模型中巨厚层3(下洋壳)的形成。然而,对岩浆喷发十分旺盛的东太平洋快速扩张洋中脊的地球物理探测发现,实际上,轴部岩浆房很小。在东太平洋洋中脊9°~3°N和13°~21°S,有超过60%的洋中脊轴部区域下方探测到有岩浆房[5]。轴部岩浆房大致1 km宽、50 m厚,沿轴部能够连续分布几十千米。一次较大体积的岩浆火山喷发(如200×106 m3)[6]就能够将轴部岩浆房约4 km长的部分完全清空。这意味着,如果要保持岩浆房不变的话,需要有合适的机制不断地补充岩浆房中的岩浆。
经过近几十年对慢速和超慢速扩张洋中脊的研究,可以基本判定,Penrose模型不适合慢速扩张洋中脊的洋壳结构。慢速扩张洋中脊的慢速扩张及其宽阔而深凹的洋中脊轴部,一般主要归因于较少的岩浆供应和强烈的构造拉伸作用。在占全球大约一半长度的洋中脊上,基本上不存在广泛连续的洋壳层状结构构造。有的区域,只在几百米的范围内,洋壳结构在时间、空间上都极度不均匀(图2)。
在慢速扩张的大西洋洋脊和超慢速扩张的西南印度洋洋中脊的部分区段,其深部也可以出现相似于快速扩张洋中脊的岩浆房[8,9,10,11,12],说明至少一部分下洋壳可能是在浅层岩浆房内结晶形成的。同时,探测到的微震深至少8 km,表明同一洋中脊其他区域的岩石圈都是冷的[13]。也就是说,慢速扩张洋中脊的岩浆供应在时空上可能都是间歇性的。
至今被大家一般认可能够描述下洋壳的是“韭葱头”模型,小体积的硅酸盐熔体温度比较高,在地下结晶相对缓慢,在晶体结晶并堆积的同时,随地幔流作用和板块运动,产生“拖拉”移动,形成下洋壳。根据对岩浆结晶深度认识的分歧,有2种模型(图3)。一种被称为“冰川”模型。4 km左右厚的辉长质下洋壳就起源于很小很薄的“韭葱”状岩浆房,绝大多数结晶作用发生在浅侵位的岩浆房中,结晶作用产生的潜热贡献给上覆的热液循环系统,浅位形成的硅酸盐晶粥一边向深处沉降、一边随板块扩张向外运动。对西南印度洋洋中脊Atlantis Bank辉长岩体中锆石年龄的研究给这个模型提供了有力的支持[14]。辉长岩体中75%的锆石年龄,与周围火山岩磁条带的年龄相似,意味着辉长岩的结晶作用发生于浅部。另外有约25%的锆石比磁条带的年龄老0.6~2.5 Ma,表明这少部分岩石的结晶作用发生于深部,是后来的构造拉伸作用使之到达浅部并最终出露于海底。另一种被称为“席状”模型[15,16],即下洋壳是由不同深度上的小岩浆体结晶形成的,几乎所有的硅酸盐结晶作用都是原位发生的,热液活动在下洋壳广泛分布,吸收结晶潜热,并阻止形成较大的熔融区域。探测到的轴部岩浆房深度在时间和空间上的大范围变化,似乎给这个模型提供了有力的证据。在远离洋中脊段末端,洋脊轴部很小的空间范围内就能够发生岩浆房深度的变化,一些岩浆房深度的变化可能与洋中脊段末端岩浆供应的减少有关,但是一些小型洋中脊段末端的岩浆充足,而且对地幔层析成像中也没有发现洋中脊段边界的熔体供应减少。例如,东太平洋隆(East Pacific Rise,EPR)13°N,洋脊轴部岩浆房的深度变化范围为400 m~2 km,而海底表面的地形并未发生相应的改变。Mutter等[17]在东太平洋隆长15 km的区域下方发现轴部岩浆房变浅的深度大于150 m,这一区域近期应该曾发生过岩浆喷发。
图1 基于Oman蛇绿岩剖面岩石地层学的Penrose模型与地震数据比较
Fig.1 Penrose model of oceanic crust based on Oman ophiolite and its comparison to seismic data
图2 平行轴向横穿慢速扩张洋中脊的洋壳剖面简化图[
洋脊段末端之下的浅莫霍标注在图中,反映沿着大西洋洋中脊北部内高角之下的地震观察
Fig.2 Simplified, interpreted, axis-parallel section through slow-spread crust[
Note the shallower Moho beneath segment ends, reflecting seismic observations beneath inside high corners along the northern Mid-Atlantic Ridge
基于对慢速扩张洋中脊橄榄岩中辉长岩体的观察与热分析,Cannat[7,18]认为下洋壳是由一系列小的岩浆体侵位到不同深度处的橄榄岩中冷却结晶形成的。该“布丁”模型与上述“席状”模型基本类似,但差别是,下洋壳不仅由结晶的深成岩构成,还包括所谓的地幔成分。
图3 2种洋壳增生的端元模型
(a)“ 冰川”模型,塑性熔体是从洋壳轴部岩浆房下沉并扩张向外,形成下洋壳;(b)“席状”模型,在洋壳中有不同深度的多个熔融体,除了浅层岩浆房岩体之外,深部熔体通过水力破碎传输,结合基底岩床塑性流动;红色箭头代表热液活动范围与方向
Fig.3 Tow types of oceanic crust accretion models (“Glacial”(a) and “sheet”(b) )
(a) A “gabbro-glacier” model, in which melt crystallizes in a small sill at the base of the sheeted dike complex from which cumulates subside down; (b) A “sheeted sill” model, in which the lower oceanic crust forms through the crystallization of multiple sills. Arrow in red represents hydrothermal fluid activities
岩浆房下方区域为剪切波和压缩波的低速区,一般认为这些区域为包含小于等于20%熔体的部分熔融区。假设低速区的底部是深成岩,可以从轴部岩浆房深度与洋壳厚度的深度比中得出上部玄武质洋壳与整体洋壳厚度的体积比。对快速扩张洋中脊6~7 km厚的洋壳来说,60%~85%的洋壳都是深成的。但是,用地球物理数据解释洋壳的地质结构,存在很大的不确定性。其中最大的不确定性是如何根据压缩波波速确定熔体的分布及其变化。例如,有少量熔体分布的高纵横比的岩盘,与大量熔体以球体形式单独分布导致的地震波速的降低是相同的[19]。这意味着随洋壳深度的加深,P波波速加快的原因既可能是熔体平均含量降低,也可能是熔体的分布发生了变化。基于Dunn等[20]的层析成像模型以及对海底依从性的研究结果,Crawford等[21]认为,波速随深度加大而加快,可能是深部熔体作为岩浆体独立分布而浅层熔体连续分布在间隙中造成的。但他们也强调,剪切波和压缩波波速的比值表明下洋壳的熔体也可能主要分布于高纵横比的裂隙中,而不是作为独立的岩床存在。所有这些不确定性妨碍了对低速带中熔体分布细节的确定。
同样,自从地球物理莫霍面发现以来,地球科学家一直没有停止过对其地质意义的探索。是什么样的物质组成和结构构造造成了地震波速在地球物理莫霍面处的跃增?接近莫霍面的地方究竟是辉长岩还是强烈蛇纹石化的橄榄岩?下洋壳可以是完全的深成结晶岩,而将大约10%逐渐蛇纹石化的地幔橄榄岩与深成岩加以混合也能够解释这些地球物理数据。对此,Hess[22,23]很早就提出过莫霍面是橄榄岩蛇纹石化前锋的设想。Muller等[24]根据Atlantis Bank精细的地球物理资料,又重提了这一观点。此外,相对于快速扩张洋中脊,慢速扩张洋中脊处的地球物理莫霍面界限并不清晰,这可能意味着辉长岩到橄榄岩的过渡更为平缓;随着慢速扩张洋中脊处洋壳年龄变老,其下洋壳的地震波波速有规律的加快,而快速扩张洋中脊不存在这一现象。
也许,并不存在全球一律的洋壳结构以及壳幔关系。在快速扩张洋中脊最近完成的IODP345航次的结果[25]表明,原始的快速扩张的下洋壳中出现了较多的斜方辉石,与洋中脊玄武岩形成的所谓标准模型有偏差。局部保存完好的橄榄石骨架结构也表明,至少部分下洋壳并不受制于固相线上下应力的影响。也就是说,矿物和结构构造观察表明,Oman蛇绿岩套可能也不是快速扩张洋壳的理想模型。洋壳增生的2种端元模型都需要进一步的检验。
洋壳的结构是由地球内部的动力学决定的,是地球内部的热能、物质组成、运动机制相互协调的产物,同时,洋壳的结构及其形成过程与地球表层的热液活动、生命活动也休戚与共(图4)。如果上述“席状”模型是可能的,地球物理莫霍面的成因是橄榄岩的蛇纹石化,或者实际情况远远超出人们基于已有知识的猜想,那么,人们有关地球深部生物圈、热液循环、碳循环及其他影响表生地球的生物地球化学过程的观念将随之发生根本性的改变,进而影响各国政府有关资源与环境政策的制订与执行。
人类期望打穿洋壳的梦想可以追溯到20世纪50年代末至60年代初。与美国针对太空的“星球大战”计划几乎同时起步,美国自然科学基金委批准资助“莫霍钻”计划,希望通过钻探的办法,获得洋壳以及莫霍面上的样品。1961年4月在墨西哥岸外瓜达卢佩岛附近水深3 600 m处,首次成功钻井,在170 m沉积层下取得了14 m长的玄武岩岩芯,迈出了向莫霍面进军的第一步。后来因预算太高、加上技术和管理的问题,1966年美国国会取消了该项计划。但是,这次钻探为后来在古海洋领域取得辉煌成就的大洋钻探开辟了道路。几十年来,人们从来没有放弃打穿地球莫霍面的梦想,历次大洋钻探的长远科学计划里,无不将“莫霍钻”作为目标之一。新世纪之后,除原有美国主导的“决心”号大洋钻探船之外,日本新建的“地球号”钻探船加入行列,并直接以打穿洋壳的“莫霍钻”为首要目标。
至今历次有关莫霍钻研讨[26,27]涉及的科学问题主要包括:①是什么物理特性造成了莫霍不连续面?这个界面的地质学本质是什么?②洋中脊处的洋壳,特别是下洋壳是怎样形成的?是哪些过程影响了洋壳形成之后的演化?这些岩浆、构造、热液、地球化学和化学过程的地球物理信号是什么?③我们能够推断出什么样的全球洋壳成分?洋壳与海洋和生物的相互作用有多大?这些相互作用对全球化学循环的影响?④生命极限是什么?控制生命极限的因素有哪些?在整个洋壳中,生物群落组分是如何随着深度和不断演化的物理和化学环境发生改变的?⑤最上部地幔的物理与化学性质是什么?它们与上覆的岩浆岩洋壳是如何联系的?
筹备“莫霍钻”的研讨分2个方面:技术上主要探讨超深钻和高温,科学上主要选择钻探海区。 JAMSTEC在技术上已经对4 000 m水深条件下的“地球号”立管钻探进行了技术改进,对高温条件下的钻头进行研究,都为“莫霍钻”的实施筹备条件。至于选址,选在太平洋中脊侧翼,要求洋壳年龄已逾2 000万年,以便温度下降到250 ℃以下。
图4 洋中脊剖面的理想结构图解(未按比例)(据参考文献[
列举了可能影响贯穿过洋脊剖面热液循环的强度和类型因素,比如断层,海山, 基底地形和不透水沉积物以及可能的生命;箭头表示热(红色)和流体(蓝色)流动
Fig.4 Schematic architecture of a mid-ocean ridge flank (not to scale) (modified after reference[26])
Parameters illustrated that may influence the intensity and style of hydrothermal circulation through the ridge flanks, such as faults, seamounts, basement topography, and impermeable sediments, which isolate the crust from the oceans. Hypothetical change in microbial community structure with the depth limit of life increases with crustal age is also shown on the right. Arrows indicate heat (red) and fluid (blue) flow
希望实施“莫霍钻”的预选区能够满足以下所有科学要求:①洋壳快速扩张形成(半扩张速率大于40 mm/a)。②简单的地质构造:所选区域海底地势平坦,基底面平稳,远离断裂带、残余的相互有重叠的扩张盆地、海山和其他板块内后期火山作用形成的构造体。与主板块建设或破坏边界的连接将提供重要的科学信息。③相对于目前所认识的“正常”快速扩张太平洋层状洋壳,所选区域洋壳的地震速度结构不存在异常。④应用多通道地震(MCS)技术成像能够得出一个截然的、强烈的和单一反射的莫霍面。⑤在地震反射数据中能够得到较强的广角莫霍面反射(PmP),具有明显可识别的下莫霍面反射(Pn)。⑥较明显的上地幔地震各向异性。⑦洋壳形成的原始纬度大于±15°。
所选区域应具有相当高的上洋壳地震速度,较高的上洋壳地震速度表明大量火山岩的形成,能够保证深钻孔的起始定位。
满足①~⑤的要求是成功的关键。 ⑥~⑦的要求是非常可取的但不是必需的,具有一定的灵活性。几个技术约束限制了潜在站点的范围:
(1) 目前水深大于3 000 m的钻井泥浆重新循环技术(提升管或其他替代技术)还未被测试。先前的科学大洋钻井经验主要限于温度低于200 ℃。 高于约250 ℃的温度可能会限制钻头和测井工具的选择,而且可能会减少岩芯柱的获取,还可能增加钻孔失败的风险。因此,需要对钻井设备的坚固性进行重新设计。基于板块冷却模型,处于莫霍面深度的洋壳年龄大于15~20 Ma,这需要钻井设备满足这些要求。
(2) 为了达到钻入地幔橄榄岩的目的,位于莫霍面上部的洋壳厚度至少比钻井系统的最大穿透/测井/获取岩芯深度少几百米。
(3) 所选区域在一年中至少有8个月天气条件良好,即需要海洋平静和海底洋流较弱。
沉积物厚度应超过50 m,以便能够支持立管硬件和其他海底设施(重返式锥体/最上面的套管柱)。
(4) 为了后勤保证的现实性,目标区域应靠近(少于约1 000 km)重要港口设施。
基于上述的科学要求和技术限制,目前,选择了太平洋盆地的3个区域(图5)。
预选点一(A)处于科克斯板块(Site 1256),它的岩石圈年龄介于15~25 Ma。这个区域的西部是15 Ma的洋壳,该区域包括大洋钻探计划(ODP)的1256D钻孔和一个正在进行的综合性大洋钻探计划(IODP)的深钻站位。位于1256站位附近的15~17 Ma洋壳区域有多通道地震数据和广角海底地震仪(OBS)数据。这个区域位于超高速扩张洋壳(半扩张速率为110 mm/a),而且在一个过道中间包含了完整的一个板块生命周期。这样使其成为研究洋壳从扩张中心到俯冲整个演化过程的绝佳区域。这一地区洋壳结构与东太平洋隆起和中美洲俯冲过程直接相关。
图5 太平洋莫霍钻3个候选海区[
A:科克斯板块区;B:近南部和下加利福尼亚;C:夏威夷外区
Fig.5 Bathymetric map showing the three selected areas for large-scale MoHole site[
A:Cocos plate region; B: Off Southern/Baja California region; C:Off Hawaii region
预选点二(B)所在的近南部和下加利福尼亚区域是东太平洋板块的一部分,位于20°S~33°N和130°~118° W。洋壳的年龄为20~35 Ma。这个区域的地球物理信息极其缺少。目前研究最多的地区位于圣地亚哥最北端,“Deep Tow”站位位于32°25'N,125°45'W(31~32 Ma)。这个地方已有深层扫描和测深、3.5 kHz剖面仪、磁力和单通道地震等方法获得的历史数据。
预选点三靠近夏威夷,位于奥胡岛以北的弯弧上,水深为4 000~4 300 m。 洋壳年龄约为80 Ma,有适中的半扩张速率(35~40 mm/a)。 这个站点在莫霍面深度的温度估计只有100~150 ℃,但是洋壳结构可能受热点火山活动(板块下方或洋壳中侵入体)的影响,而且洋壳年龄较大,可能很难将洋壳深部的地球化学与现代海洋化学或海洋条件的变化进行关联研究。
借助于国家重点基础研究发展计划(973项目)研究和大洋协会对西南印度洋硫化物的勘探, 2015年5月13~16日在美国伍兹霍尔海洋研究所,举行了“印度洋壳幔科学钻探”的美—中联合IODP工作会[28]。可以说,我国在某种意义上已经介入慢速洋脊莫霍钻的讨论。
相比于几十万公里外月球上的样品,地球内部几公里深处的莫霍面距离我们很近,但似乎又遥不可及。半个多世纪以来,在向莫霍面挺进的道路上,在通过实践不断检验与修正已有模型或者提出新的模型过程中,我们收获了很多新的知识。科学家相信,向地球深处每多钻进一点,我们对地球结构及其历史的认识就会加深一点、更新一些。人类生活在地球上,但地球内部对人类来说,仍是一个未知的全新的秘界。
The authors have declared that no competing interests exist.
| [1] |
Penrose field conference on ophiolites [J]. |
| [2] |
An ultraslow-spreading class of ocean ridge [J].
Abstract New investigations of the Southwest Indian and Arctic ridges reveal an ultraslow-spreading class of ocean ridge that is characterized by intermittent volcanism and a lack of transform faults. We find that the mantle beneath such ridges is emplaced continuously to the seafloor over large regions. The differences between ultraslow- and slow-spreading ridges are as great as those between slow- and fast-spreading ridges. The ultraslow-spreading ridges usually form at full spreading rates less than about 12 mm yr(-1), though their characteristics are commonly found at rates up to approximately 20 mm yr(-1). The ultraslow-spreading ridges consist of linked magmatic and amagmatic accretionary ridge segments. The amagmatic segments are a previously unrecognized class of accretionary plate boundary structure and can assume any orientation, with angles relative to the spreading direction ranging from orthogonal to acute. These amagmatic segments sometimes coexist with magmatic ridge segments for millions of years to form stable plate boundaries, or may displace or be displaced by transforms and magmatic ridge segments as spreading rate, mantle thermal structure and ridge geometry change.
|
| [3] |
Thin crust as evidence for depleted mantle supporting the Marion Rise [J].
Abstract The global ridge system is dominated by oceanic rises reflecting large variations in axial depth associated with mantle hotspots. The little-studied Marion Rise is as large as the Icelandic Rise, considering both length and depth, but has an axial rift (rather than a high) nearly its entire length. Uniquely along the Southwest Indian Ridge systematic sampling allows direct examination of crustal architecture over its full length. Here we show that, unlike the Icelandic Rise, peridotites are extensively exposed high on the rise, revealing that the crust is generally thin, and often missing, over a rifted rise. Therefore the Marion Rise must be largely an isostatic response to ancient melting events that created low-density depleted mantle beneath the Southwest Indian Ridge rather than thickened crust or a large thermal anomaly. The origin of this depleted mantle is probably the mantle emplaced into the African asthenosphere during the Karoo and Madagascar flood basalt events.
|
| [4] |
A model for oceanic crystal structure developed [J].
The plastic upward flow of asthenosphere at mid-ocean ridge crests implies a linear basaltic magma chamber at the base of the ocean crust at ridge crests. This in turn implies a mature oceanic crust composed of a unit of lavas and dykes, overlying a unit of isotropic gabbro, formed by upward cooling of the magma chamber, overlying a unit of cumulate gabbros and ultramafics, overlying residual mantle from which the basaltic magma has been removed. Analysis of this structure shows that it is asymmetric, with lavas and cumulates dipping towards the spreading centre, and dykes predominantly chilled away from the spreading centre. Increase in spreading rate leads to an increase in lava dips, to a separation of lavas and dykes into distinct units, to a thinning of the dyke unit, and to a narrowing of individual dykes. Metamorphic facies and seismic layering may, but do not necessarily, correspond with this lithological layering. The degree of rifting of oceanic crust and the depth of the median valley are related to an interplay of spreading rate and viscosity of the rising asthenospheric material. Measurement of such parameters as direction and extent of one-way chilling of dykes, dyke thickness, lava dip, and degree of separation of lavas and dykes, in sequence-type ophiolite complexes should allow the determination of direction and rate of spreading at the mid-ocean ridges at which they were formed. The model implies cogenetic liquid-solid relationships between ocean-floor rocks, and suggests changes in the shape of the liquid-solid phase diagram over the pressure range 0.5芒聙聯2 kb, as well as extensive development of ocean-floor metamorphic rocks.
|
| [5] |
Multi-channel seismic imaging of a crustal magma chamber along the East Pacific Rise [J].
A reflection observed on multi-channel seismic profiles along and across the East Pacific Rise between 8°50′ N and 13°30′ N is interpreted to arise from the top of a crustal magma chamber located 1.2–2.4 km below the sea floor. The magma chamber is quite narrow (<4 – 6 km wide), but can be traced as a nearly continuous feature for tens of kilometres along the rise axis.
|
| [6] |
Volcanic eruptions on mid-ocean ridges: New evidence from the superfast spreading East Pacific Rise. Volcanic eruptions on mid-ocean ridges: New evidence from the superfast spreading East Pacific Rise, 17°-19°S [J]. |
| [7] |
How thick is the magmatic crust at slow spreading oceanic ridges? [J].
Thermal models based on the geometry of axial segmentation, and mechanical models of axial valley formation, suggest that the axial lithosphere of slow spreading ridges can be thicker than the crust modeled from gravity data, or measured from seismic data. This is particularly the case in thin crust ridge regions that are near axial discontinuities. The temperature field modeled in these thin crust ridge regions is such that some of the melts extracted from the asthenosphere should crystallize in the mantle, before they reach the crust. This prediction is consistent with observations made on gabbroic and ultramafic samples collected in thick lithosphere/thin crust regions of the Mid-Atlantic Ridge, and along the ultra-slow Southwest Indian Ridge. A geological model is proposed for the lithosphere created in these thick lithosphere/thin crust ridge regions. This model suggests that crustal thicknesses measured in seismic surveys of these regions do not directly reflect the melt production in the asthenosphere beneath the ridge: some of the melt gets trapped in the mantle, and part of the crust is made of variably fractured and serpentinized residual ultramafics. Potential consequences on the way we interpret along-axis crustal thickness variations at slow spreading ridges are discussed and evaluated.
|
| [8] |
Evidence for accumulated melt beneath the slow-spreading Mid-Atlantic Ridge [J].
The analysis of data from a multi-component geophysical experiment conducted on a segment of the slow-spreading (20 mm yr) Mid-Atlantic Ridge shows compelling evidence for a significant crustal magma body beneath the ridge axis. The role played by a crustal magma chamber beneath the axis in determining both the chemical and physical architecture of the newly formed crust is fundamental to our understanding of the accretion of oceanic lithosphere at spreading ridges, and over the last decade subsurface geophysical techniques have successfully imaged such magma chambers beneath a number of intermediate and fast spreading (60-140 mm yrfull rate) ridges. However, many similar geophysical studies of slow-spreading ridges have, to date, found little or no evidence for such a magma chamber beneath them. The experiment described here was carefully targeted on a magmatically active, axial volcanic ridge (AVR) segment of the Reykjanes Ridge, centred on 57 degrees 43 minutes North. It consisted of four major components: wide-angle seismic profiles using ocean bottom seismometers; seismic reflection profiles; controlled source electromagnetic sounding; and magneto-telluric sounding. Interpretation and modelling of the first three of these datasets shows that an anomalous body lies at a depth of between 2 and 3 km below the seafloor beneath the axis of the AVR. This body is characterized by anomalously low seismic P-wave velocity and electrical resistivity, and is associated with a seismic reflector. The geometry and extent of this melt body shows a number of similarities with the axial magma chambers observed beneath ridges spreading at much higher spreading rates. Magneto-telluric soundings confirm the existence of very low electrical resistivities in the crust beneath the AVR and also indicate a deeper zone of low resistivity within the upper mantle beneath the ridge.
|
| [9] |
Discovery of axial magma chamber reflections beneath the Lucky Strike Hydrothermal vents and volcano and its relationship with median Valley Faults [ |
| [10] |
Three-dimensional seismic structure of the Mid-Atlantic Ridge (35°N): Evidence for focused melt supply and lower crustal dike injection [J].
[1] We gathered seismic refraction and wide-angle reflection data from several active source experiments that occurred along the Mid-Atlantic Ridge near 3500°N and constructed three-dimensional anisotropic tomographic images of the crust and upper mantle velocity structure and crustal thickness. The tomographic images reveal anomalously thick crust (80900099 km) and a low-velocity 090008bull's-eye090009, from 4 to 10 km depth, beneath the center of the ridge segment. The velocity anomaly is indicative of high temperatures and a small amount of melt (up to 5%) and likely represents the current magma plumbing system for melts ascending from the mantle. In addition, at the segment center, seismic anisotropy in the lower crust indicates that the crust is composed of partially molten dikes that are surrounded by regions of hot rock with little or no melt fraction. Our results indicate that mantle melts are focused at mantle depths to the segment center and that melt is delivered to the crust via dikes in the lower crust. Our results also indicate that the segment ends are colder, receive a reduced magma supply, and undergo significantly greater tectonic stretching than the segment center.
|
| [11] |
Seismic observation of an extremely magmatic accretion at the ultraslow spreading Southwest Indian Ridge [J].
Abstract The oceanic crust is formed by a combination of magmatic and tectonic processes at mid-ocean spreading centers. Under ultraslow spreading environment, however, observations of thin crust and mantle-derived peridotites on the seafloor suggest that a large portion of crust is formed mainly by tectonic processes, with little or absence of magmatism. Using three-dimensional seismic tomography at an ultraslow spreading Southwest Indian Ridge segment containing a central volcano at 50°28′E, here we report the presence of an extremely magmatic accretion of the oceanic crust. Our results reveal a low-velocity anomaly (610.6 km/s) in the lower crust beneath the central volcano, suggesting the presence of partial melt, which is accompanied by an unusually thick crust (~9.565km). We also observe a strong along-axis variation in crustal thickness from 9.5 to 465km within 30–5065km distance, requiring a highly focused melt delivery from the mantle. We conclude that the extremely magmatic accretion is due to localized melt flow toward the central volcano, which was enhanced by the significant along-axis variation in lithosphere thickness at the ultraslow spreading Southwest Indian Ridge.
|
| [12] |
Evidence of an axial magma chamber beneath the ultraslow-spreading Southwest Indian Ridge [J].
Ultraslow-spreading ridges are a novel class of spreading centers symbolized by amagmatic crustal accretion, exposing vast amounts of mantle-derived peridotites on the seafloor. However, distinct magmatic centers with high topographies and thick crusts are also observed within the deep axial valleys. This suggests that despite the low overall melt supply, the magmatic process interacting with the tectonic process should play an important role in crustal accretion; however, this has been obscured due to the lack of seismic images of magma chambers. Using a combination of seismic tomography and full waveform inversion of ocean bottom seismometer data from the Southwest Indian Ridge at 50°28′E, we report the presence of a large low-velocity anomaly (LVA) 654–9 km below the seafloor, representing an axial magma chamber (AMC) in the lower crust. This suggests that the 9.5-km-thick crust here is mainly formed by a magmatic process. The LVA is overlain by a high-velocity layer, possibly forming the roof of the AMC and defining the base of hydrothermal circulation. The steep velocity gradient just below the high-velocity layer is explained by the ponding of magma at the top of the AMC; this could provide the overpressure for lateral dike propagation along the ridge axis, leading to a complex interaction between magma emplacement, tectonic, and hydrothermal processes, and creating a diversity of seafloor morphology and extremely heterogeneous crust.
|
| [13] |
Microearthquakes beneath Median Valley of Mid-Atlantic Ridge near 23°N: Tomography and tectonics [J].
Data from a microearthquake experiment in the median valley of the Mid-Atlantic Ridge near 2300°N in 1982 are used to measure earthquake source parameters, to determine the laterally heterogeneous seismic velocity structure across the inner floor, and to develop a kinematic tectonic model for this portion of the median valley. Fifty-three microearthquakes occurred over a 10-day period beneath the median valley inner floor and eastern rift mountains. Twenty of 23 well-located inner floor epicenters define a line of activity, about 17 km long, having a strike of N2500°E and located near an along-axis depression some 300090009400 m deeper than surrounding regions. Earthquakes with well-resolved hypocenters generally have focal depths of 40900098 km beneath the seafloor of both the inner floor and the rift mountains; the hypocentral locations are robust with respect to plausible lateral variations in seismic velocity structure. Composite fault plane solutions for inner floor events indicate normal faulting on planes dipping at angles near 4500°. Normal faulting mechanisms, although poorly constrained, are also indicated for the rift mountain microearthquakes. Seismic moments, approximate fault dimensions, and average stress drops for the largest events recorded are 10190900091020 dyn cm, 200090009400 m, and 109000970 bars, respectively. A twodimensional tomographic inversion of P wave travel time residuals from microearthquakes and local shots indicates a well-resolved lateral heterogeneity in crustal velocity structure across the median valley inner floor. P wave velocities at 10900095 km depth within a zone less than 10 km wide beneath the central inner floor are lower by several percent than in surrounding regions. The most likely explanation for the low velocities is that the region is the site of the most recent local magmatic injection and remains pervasively fractured as a result of rapid hydrothermal quenching of the newly emplaced crustal column. By this view, the seismic velocity structure at the ridge axis evolves, probably by the sealing of cracks and pores, within the first few hundred thousand years of crustal accretion. Consideration of the detailed Sea Beam bathymetry in this region of the inner floor, the characteristics of large earthquakes that the region has experienced during the past 25 years, and the results of the microearthquake and tomography analysis suggests that this section of the median valley has been undergoing continued horizontal extension and block faulting without significant crustal injection of magma for at least the past 104 years.
|
| [14] |
et al. Dating the growth of oceanic crust at a slow-spreading ridge [J].
Nineteen uranium-lead zircon ages of lower crustal gabbros from Atlantis Bank, Southwest Indian Ridge, constrain the growth and construction of oceanic crust at this slow-spreading midocean ridge. Approximately 75% of the gabbros accreted within error of the predicted seafloor magnetic age, whereas approximately 25% are significantly older. These anomalously old samples suggest either spatially varying stochastic intrusion at the ridge axis or, more likely, crystallization of older gabbros at depths of approximately 5 to 18 kilometers below the base of crust in the cold, axial lithosphere, which were uplifted and intruded by shallow-level magmas during the creation of Atlantis Bank.
|
| [15] |
Geochemistry of gabbro sills in the crust-mantle transition zone of the Oman ophiolite: Implications for the origin of the oceanic lower crust [J]. |
| [16] |
Periodic formation of magma fractures and generation of layered gabbros in the lower crust beneath oceanic spreading ridges [M] |
| [17] |
Seismic images of active magma systems beneath the east pacific rise between 17°05' and 17°35'S [J]. |
| [18] |
Emplacement of mantle rocks in the seafloor at mid-ocean ridges [J].
This paper discusses the geological and geophysical data available on mid-ocean ridges with outcrops of serpentinized mantle peridotites, with the objective of better constraining the modes of emplacement of these rocks in the seafloor. Ridges with serpentinized peridotites outcrops are in most cases characterized by slow-spreading rates, and in every case by deep axial valleys. Such deep axial valleys are thought, based on geophysical constraints and on mechanical modelling results, to characterize ridges with a thick axial lithosphere. A predictable effect of a thick axial lithosphere is that it should prevent magmas from pooling at crustal depths in a long-lasting magma chamber: gabbro脙炉c magmas should instead form shortlived dike or sill-like intrusions. Samples from axial outcrops of serpentinized peridotites are often cut by dikelets of evolved gabbros which are interpreted as apophyses of such dike and sill-like intrusions. This observation leads to a discontinuous magmatic crust model, in which mantle-derived peridotites form screens for numerous gabbro脙炉c intrusions. This discontinuous magmatic crust is expected to form in magma-poor ridge regions, where there is not enough magma to produce a 4-to 7-km-thick magmatic crust, and where the uppermost kilometers of oceanic lithosphere therefore have to be at least partially made of tectonically uplifted mantle material. Because the dimensions of individual mantle-derived ultramafic screens may be smaller than seismic experiments detection limits, the discontinuous magmatic crust model discussed in this paper may produce a layer 3-type seismic signature, even without extensive serpentinization of its ultramafic component. It therefore provides an alternative to Hess's [1962] serpentinite layer 3 model, for the geological interpretation of seismic data from oceanic areas with frequent outcrops of deep crustal and mantle-derived rocks.
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| [19] |
Modelling the anisotropic seismic properties of partially molten rocks found at mid-ocean ridges [J].
The problem of modelling the seismic properties of mid-ocean ridge rocks of the axial magma chamber (AMC) and the low-velocity triangle (LVT) immediately below it has been addressed using two samples from Oman ophiolite as examples. The specimens are a layered gabbro from the lower oceanic crustal sequence and a harzburgite from the upper most mantle section at the palaeo ridge axis. The seismic properties have been simulated at a temperature of 1200掳C and pressure of 200 MPa so that the basalt melt is above its solidus. Various effective medium methods are discussed in the perspective of modelling rocks that have a strong background elastic anisotropy due to crystal preferred orientation (CPO) and an introduced anisotropy due to oriented melt filled inclusions. Calculations using various methods show a wide range of predicted melt fractions for a given seismic velocity. The self-consistent scheme (SCS) and differential effective medium (DEM) are compared in some detail. A tensorial model was developed using a poro-elastic method of Gassman at low seismic frequency and a standard DEM with isolated basalt inclusions at high frequency. At low frequency the basalt pore fluid is considered to be everywhere connected and the pressure uniform, whereas at high frequency basalt inclusions are isolated. Assuming the medium to be a standard linear solid the low and high frequency velocities were used to calculate the anisotropy of attenuation. Calculations with spherical basalt inclusions show that the seismic velocities decrease and attenuation increase with increasing melt fraction. The symmetry of the background anisotropy due to CPO is preserved, but gradually reduced with increasing melt fraction. For the seismic velocities observed for the axial magma chamber (AMC) with Vof 3.5 to 4.7 km/s the model predicts 50-70% melt. For the low-velocity triangle (LVT) below the AMC with Vof 4.8-5.6 km/s the model predicts 35-50% melt. Predictions from the observed attenuations ( Q) at the AMC of 0.05-0.02 and in the LVT of 0.02-0.01 are around 60% and 45% melt respectively. Seismic anisotropy has been modelled with ellipsoidal basalt 'pancake' shaped inclusions with their circular sections in the ( XY) foliation plane. Only a 2-3% of ellipsoidal basalt inclusions are required to over print the anisotropy of the background medium. There is a rapid decrease in Vand increase of Qfor propagation normal to the foliation ( Z) with increasing axial ratio of the inclusions. With increasing axial ratio the model predicts decreasing amounts of melt for given velocity Vin the Z direction. For the LVT only 15-25% melt is predicted for a 10:1 inclusion instead of the 35-50% of the spherical inclusion.
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| [20] |
Three-dimensional seismic structure and physical properties of the crust and shallow mantle beneath the East Pacific Rise at 9°30'N [J].
ABSTRACT The seismic structure of the crust and shallow mantle beneath the East Pacific Rise near 9掳30'N is imaged by inverting P wave travel time data. Our tomographic results constrain for the first time the three-dimensional structure of the lower crust in this region and allow us to compare it to shallow crustal and mantle structure. The seismic structure is characterized by a low-velocity volume (LVV) that extends from 1.2 km depth below the seafloor into the mantle. The cross-axis width of the LVV is narrow in the crust (5-7 km) and broad in the mantle (~18 km). Although the width of the top of the LVV is similar to previous estimates, its narrow shape at lower crustal depths and its significant widening in the mantle are previously unknown features of the rise velocity structure. In the rise-parallel direction the LVV varies in magnitude such that the lowest velocities are located between two minor rise axis discontinuities near 9掳28'N and 9掳35'N. From the seismic results we estimate the thermal structure and melt distribution beneath the rise. The thermal structure suggests that heat removal is relatively efficient throughout the crust yet inefficient at Moho and mantle depths. Estimates of the melt distribution indicate that magma accumulates at two levels in the magmatic system. One is at the top of the magmatic system and is capped by the shallow melt lens detected by seismic reflection surveys; the other is within the Moho transition zone and topmost portion of the mantle. The highest melt fractions occur within the upper reservoir, whereas the lower reservoir contains a lower melt fraction distributed over a broader area. By volume, however, there may be up to 40% more melt in the lower reservoir than in the upper reservoir. Along-axis variations in crustal melt content are similar to those in the mantle, supporting the hypothesis that the mantle, midway between the 9掳28'N and 9掳35'N devals, is presently delivering greater amounts of melt to the lower crust than to regions immediately to the north or south. We see no evidence (from seismic anisotropy) for diapiric mantle flow, suggesting that solid-state flow and melt migration are decoupled in the shallow mantle. Our results are not compatible with models that require a large, segment-scale redistribution of melt within the crust. Instead, our results imply that crustal magma chambers are replenished at closely spaced intervals along the rise.
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| [21] |
Variations in the distribution of magma in the lower crust and at the Moho beneath the East Pacific Rise at 9°~10°N [J].
Measurements of the seafloor deformation under ocean waves (compliance) reveal an asymmetric lower crustal partial melt zone (shear velocity less than 1.8 km/s) beneath the East Pacific Rise axis between 9° and 10°N. At 9°48′N, the zone is less than 8 km wide and is centered beneath the rise axis. The zone shifts west of the rise axis as the rise approaches the westward-stepping 9°N overlapping spreading center discontinuity and is anomalously wide at the northern tip of the discontinuity. The ratio of the compliance determined shear velocity to the compressional velocities (estimated by seismic tomography) suggests that the melt is well-connected in high-aspect ratio cracks rather than in isolated sills. The shear and compressional velocities indicate less than 18% melt in the lower crust on average. The compliance measurements also reveal a separate lower crustal partial melt zone 10 km east of the rise axis at 9°48′N and isolated melt bodies near the Moho beneath four of the 39 measurement sites (three on-axis and one off-axis). The offset of the central melt zone from the rise axis correlates strongly with the offset of the overlying axial melt lens and the inferred center of mantle melting, but its shape appears to be controlled by crustal processes.
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| [22] |
The evolution of ocean basins [M] |
| [23] |
History of ocean basins [M] |
| [24] |
Crustal structure of the Southwest Indian Ridge at the Atlantis II Fracture Zone [J].
The Southwest Indian Ridge is a slow spreading end-member of the mid-ocean ridge system. The deepest borehole penetrating the lower oceanic crust, Ocean Drilling Program hole 735B, lies on the eastern transverse ridge of the Atlantis II Fracture Zone at 5700°E. A wide-angle seismic survey in the vicinity of the borehole reveals a crustal structure that is highly heterogeneous. To the east of Atlantis Bank, on which hole 735B is located, the crust consists of a 20900092.5 km thick high-velocity-gradient oceanic layer 2 and a 10900092 km thick low-velocity-gradient layer 3. The transform valley has a 2.50900093 km thick crust with anomalously low velocities interpreted to consist largely of highly serpentinized mantle rocks. The seismically defined crust is thickest beneath the borehole, where layer 2 is thinner and the lower crust is inferred to contain 20900093 km of partially serpentinized mantle. The seismic velocity models are consistent with gravity data which show weak residual mantle Bouguer anomalies because the regions of thinner crust have lower crustal densities. Stress variations deduced from mass balances between the transform valley floor and the adjacent transverse ridges are much larger than the likely threshold for lithospheric failure and therefore indicate that the relief is supported dynamically. The variation of crustal thickness with spreading rate defined by data from the Southwest Indian Ridge and elsewhere is consistent with models of melt generation in which the upwelling mantle is cooled by conductive heat loss at very slow spreading rates, resulting in reduced melt generation under the spreading axis. Large segment-scale variations in crustal thickness suggest subcrustal along-axis migration of melt toward segment centers.
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| [25] |
Integrated Ocean Drilling Program Expedition 345 Preliminary Report, Exploring the Plutonic Crust at A Fast-Spreading Ridge: New Drilling at Hess Deep[R]. Washington DC:Integrated Ocean Drilling Program Management International , |
| [26] |
The MoHole: A crustal journey and mantle quest, workshop in Kanazawa, Japan, 3-5 June 2010 [J].
Drilling an ultra-deep hole in an intact portion of oceanic lithosphere, through the crust to the Mohorovi i discontinuity (the ‘Moho’), and into the uppermost mantle is a long-standing ambition of scientific ocean drilling (Bascom, 1961; Shor, 1985; Ildefonse et al., 2007). It remains essential to answer fundamental questions about the dynamics of the Earth and global elemental cycles. The global system of mid-ocean ridges and the new oceanic lithosphere formed at these spreading centers are the principal pathways for energy and mass exchange between the Earth’s interior, hydrosphere, and biosphere. Bio-geochemical reactions between the oceans and oceanic crust continue from ridge to subduction zone, and the physical and chemical changes to the ocean lithosphere provide inventories of these thermal, chemical, and biological exchanges.The 2010 MoHole workshop in Kanazawa, Japan followed from several recent scientific planning meetings on ocean lithosphere drilling, in particular the Mission Moho Workshop in 2006 (Christie et al., 2006; Ildefonse et al., 2007) and the “Melting, Magma, Fluids and Life” meeting in 2009 (Teagle et al., 2009). Those previous meetings reacheda consensus that a deep hole through a complete section of fast-spread ocean crust is a renewed priority for the ocean lithosphere community. The scientific rationale for drilling a MoHole in fast-spread crust was developed in the workshop reports (available online) and most thoroughly articulated in the 2007 IODP Mission Moho drilling proposal (IODP Prop 719MP; www.missionmoho.org).
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| [27] |
Mission Moho workshop: Drilling through the oceanic crust to the mantle [J].
No abstract available.brbrdoi:a href=http://dx.doi.org/10.2204/iodp.sd.4.02.2007 target=_blank10.2204/iodp.sd.4.02.2007/a
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| [28] |
Workshop Report for Scientific Drilling in the Indian Ocean Crust and Mantle[R].US- |
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