地球科学进展, 2020, 35(3): 246-258 DOI: 10.11867/j.issn.1001-8166.2020.013

综述与评述

锂同位素地球化学在大陆地热体系研究中的应用

余小灿,, 刘成林, 王春连

中国地质科学院矿产资源研究所,自然资源部成矿作用与资源评价重点实验室,北京 100037

Application of Lithium Isotope Geochemistry to the Study of the Continental Geothermal System

Yu Xiaocan,, Liu Chenglin, Wang Chunlian

MLR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037, China

收稿日期: 2019-10-29   修回日期: 2020-01-17   网络出版日期: 2020-04-07

基金资助: 中央级公益性科研院所基本科研业务费项目“华南中新生代蒸发岩盆地卤水钾锂成矿机理”.  KY1603
国家重点基础研究发展计划项目“中国陆块海相成钾规律及预测研究”.  2011CB403007

Received: 2019-10-29   Revised: 2020-01-17   Online: 2020-04-07

作者简介 About authors

余小灿(1988-),男,湖北荆门人,博士后,主要从事地下卤水型钾锂矿床与地球化学研究.E-mail:xiaocany1988@163.com

YuXiaocan(1988-),male,JingmenCity,HubeiProvince,Postdoctoralfellow.Researchareasincludeundergroundbrine-typepotassiumandlithiumdepositsandgeochemistry.E-mail:xiaocany1988@163.com

摘要

盐湖富锂(Li)卤水是世界锂产品的主要原材料,而大陆高盐度地热流体常含有较高浓度的Li。大陆地热体系是地热形成机理研究的重点,由于岩石的复杂性较少受到关注,且该领域Li同位素应用研究尚未得到较为系统的认识。论述了近年来Li同位素地球化学在大陆地热研究中的最新应用和进展,提出了该领域研究存在的问题,并展望了未来的研究方法与方向。大陆地热流体研究应高度重视Li-B-Sr-U多同位素方法的应用,同时也应结合不同温度条件下的水岩反应实验研究。并且,未来大陆地热体系研究更应重视各类沉积物/岩石Li同位素组成及其时空分布特征、储层岩石矿物学以及水岩反应过程中次生矿物形成时的Li同位素行为研究,以期揭示地热体系中复杂的流体演化机制,为该体系内Li资源的勘查、开发和利用提供科学的参考。

关键词: 锂同位素 ; 分馏机制 ; 多同位素 ; 地热流体 ; 水岩反应

Abstract

The lithium-rich brine in salt lakes is the main raw material of the world’s lithium products, while the continental geothermal fluids with a high salinity often contain a high concentration of lithium. Continental geothermal system is the focus in the study of geothermal formation mechanism. However, less attention is paid to the system due to the complexity of lithology, and the application of lithium isotopes in this field has not been systematically recognized. The newest application and progress of lithium isotope geochemistry in continental geothermal research in recent years were discussed, the problems in this field were put forward, and future research methods and directions were expected. The study of continental geothermal fluids should attach great importance to the application of Li-B-Sr-U multi-isotopic method, and should also be combined with water-rock reaction experiments under different temperature conditions. Moreover, in the future, the research on continental geothermal system should pay more attention to the various sediment/rock lithium isotopic compositions and their spatio-temporal distribution characteristics in the regional or geothermal field’s scales, mineralogy of reservoir rock, and behaviors of lithium isotopes related to the formation of secondary minerals in the process of water-rock interaction, in order to reveal the complex process of fluid evolution in the geothermal system and provide scientific reference for the exploration, exploitation and utilization of lithium resources in the system.

Keywords: Li isotopes ; Fractionation mechanism ; Multiple isotopes ; Geothermal fluids ; Water-rock interaction

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本文引用格式

余小灿, 刘成林, 王春连. 锂同位素地球化学在大陆地热体系研究中的应用. 地球科学进展[J], 2020, 35(3): 246-258 DOI:10.11867/j.issn.1001-8166.2020.013

Yu Xiaocan, Liu Chenglin, Wang Chunlian. Application of Lithium Isotope Geochemistry to the Study of the Continental Geothermal System. Advances in Earth Science[J], 2020, 35(3): 246-258 DOI:10.11867/j.issn.1001-8166.2020.013

1 引 言

锂(Li)元素位于元素周期表第Ⅰ主族,是最轻的金属元素,具有高热容量、高电离能,且在碱金属元素中,其具有高熔点和沸点。Li元素被广泛应用于陶瓷、电子与电气、冶金、医疗和光学等行业。近20年来,Li成为一种战略性新兴矿产资源,是制造新能源汽车电池的重要原料[1]。早期,Li资源主要来自岩浆成因的伟晶岩矿床,少量来自蒸发岩[2]。目前,卤水型锂矿已成为Li资源的主要来源[2,3,4,5,6,7,8]。卤水型锂矿可进一步分为盐湖卤水型和深层地下卤水型锂矿,世界上主要的卤水型锂矿包括美国的西尔斯湖、索尔顿海湖盐湖卤水和克莱顿峡谷地下卤水、玻利维亚的乌尤尼盐湖、智利的阿塔卡玛盐湖、阿根廷的翁布雷穆埃尔托盐湖、澳大利亚盐湖以及中国西藏的扎布耶和当雄错盐湖、青海的察尔汗和台吉乃尔盐湖[2,9,10,11,12,13,14,15,16]。基于南美和北美西部高原地区地下富Li卤水Li同位素研究[17,18,19],表明地下富Li卤水中Li的富集与高温水岩反应具有密切的关系。

Li有2个稳定同位素,分别为7Li(92.4%)和6Li(7.6%),它们之间的相对质量差高达约17%。由于7Li和6Li扩散的差异性,不同环境条件下,Li同位素值呈现不同的变化范围。在低温条件下,Li同位素值组成变化跨度可超过50‰;高温条件下,Li同位素值的组成变化也是非常大的[20]。过去Li同位素值的表示方法用δ6Li表示,目前国际上已使用δ7Li来表示:

δL 7i= 7Li/ 6Lisample- 7Li/ 6Listandard 7Li/ 6Listandard×103

国际上一直使用L-SVEC(碳酸锂)作为Li同位素标准参考物质[21],但这种标样不再生产,因此,IRMM-016(碳酸锂)作为一个理想的标准物质替代标样L-SVEC[22]

Li同位素的分馏不仅发生在地表或近地表[23],而且在海水与玄武岩之间[24],甚至在更深的俯冲带均可发生[25,26,27,28]。随着分析测试技术的快速发展,Li同位素地球化学在宇宙行星[29,30,31,32]、大陆风化[33,34,35,36]、洋壳蚀变与海底热液[24,37,38]、板块俯冲与壳幔演化[25,39,40]、流体的来源与演化[41,42,43,44,45,46]以及生物与人类行为[47,48,49]等领域取得了显著成果。地热是一种洁净且可再生的天然资源,在世界上已被广泛地用于发电、取暖和温室养殖等方面[50]。按地质构造成因特征,可分为沉积盆地型和隆起山地型地热资源[51]。地热体系是相对独立的热能生成、运移、聚集和保持的自然系统,是一个相对独立的单元,是地热形成机理研究的重点[52]。水化学和传统稳定同位素已在地热水形成机理研究中广泛应用[53],相对而言,在中国非传统Li同位素在地热水成因研究中较为薄弱。此外,大陆地热体系由于岩石的复杂性而较少受到关注,且大陆地热活动中的Li同位素行为尚未得到较好的认识。为更好地呈现国际上对大陆地热体系Li同位素地球化学研究成果,本文归纳和总结了大陆地热体系中Li同位素组成及其应用前景,并对该研究领域的未来提出展望和建议,以期为未来大陆地热体系的研究以及资源的开发、利用提供科学的参考。

2 Li同位素分馏机制

2.1 平衡分馏

平衡热力学和相关实验研究表明Li同位素分馏有温度依赖性。随着温度的降低,两相间的分馏系数会渐渐发生变化[54]。这种性质被解释为键能控制的7Li和6Li在两相间的配分,而配位数的高低决定了键能的高低,配位数越低,键能越高,7Li优先进入高键能的配位点[55]。具体可分为高温和低温条件下的同位素分馏机制。

2.1.1 高温分馏

Wunder等[56]分析了在2.0 GPa压力条件下,500~900 ℃温度变化范围内,锂辉石与碱性、酸性流体间的Li同位素分馏过程。实验表明,7Li优先进入流体相,且在一定压力条件下,随着温度升高,辉石与流体间的分馏系数渐渐变小。Wunder等[57]调查了蛇纹石和流体间的同位素分馏(200~500 ℃),研究发现,含Li的蛇纹石(叶蛇纹石和利蛇纹石)与流体间的Li同位素分馏显示了与锂云母(300~500 ℃)[58]相似的分馏机制,进一步证实了7Li优先进入流体相,相对于固相,流体呈现了高的Li同位素值。然而,在十字石、纤维蛇纹石与流体间的同位素分馏实验中[57,58,59],发现截然相反的Li同位素分馏,即矿物有高于流体的δ7Li值。这种同位素分馏行为被解释为不同矿物中Li的配位数不同所造成的,Li在十字石和纤维蛇纹石中的配位数小于流体的,键能高,而7Li优先进入高键能的低配位点。同时,Wunder等[60]发现尽管Li在锂辉石和锂云母中均为六面体配位,但它们与流体间显示了不同的同位素分馏行为。显然,这不能用配位数差异来解释,而是由于键价的不同而产生的。在相同配位数的条件下,平均Li-O键较短的矿物具有较大的Li总键价,而7Li优先进入高Li总键价的矿物中。因此,由于锂云母总键价高于锂辉石,所以锂云母与流体间的同位素分馏小于锂辉石与流体间的同位素分馏。

除温度外,压力对矿物与流体间的Li同位素分馏的影响也是非常重要的。Wunder等[60]测量了在500~625 ℃温度范围与1、4和8 GPa压力条件下锂辉石与流体间的分馏值(Δ7Lisolid-fluid)。结果表明,在1和4 GPa压力条件下,Δ7Lisolid-fluid值约为-3.5‰,压力并未对其分馏产生影响,而在8 GPa压力条件下,Δ7Lisolid-fluid值约为-1.9‰。

2.1.2 低温分馏

前人对低温条件下的Li同位分馏机制进行了大量研究,其分馏机制主要归因于次生矿物形成过程中的吸附或并入作用等[24,37,61,62,63,64,65,66]。三水铝矿、蒙脱石和氢氧化铁胶体与流体间因吸附作用而发生的Li同位素分馏被实验所证实[63]。研究发现,在22 ℃温度下,三水铝矿的吸附作用可使流体的δ7Li值高出初始流体8‰~12‰,分馏系数α三水铝矿—流体为0.986。其次,氢氧化铁胶体吸附作用产生了一定的同位素分馏,而蒙脱石吸附作用几乎不发生同位素分馏。Wimpenny等[66]认为三水铝矿与流体间的Li同位素分馏可以分为2个阶段:首先是三水铝矿结构的扩张造成Li扩散进入层间和充填八面体空位,导致流体与三水铝矿的同位素分馏值可达16‰(α三水铝矿—流体约为0.984);其次,当八面体空位被充填完毕后,流体与三水铝矿间的同位素分馏微弱。在低温条件下(25~90 ℃),蒙脱石的分馏系数变化很小,从90 ℃时的0.990降低到25 ℃的0.983[65]。这种现象被解释为低温条件下黏土矿物贫穷结晶的结果,因为这些贫穷结晶的黏土晶体有大比例的共棱八面体,会造成不同程度的Li同位素分馏。同时,在成岩作用阶段,蒙脱石向伊利石转化时也会发生Li同位素分馏。在蒙脱石向伊利石转化的实验中,新生成的不同粒级的黏土矿物有不同的Li同位素组成,且粗粒级(>2 μm)的黏土与流体间的Li同位素分馏值可达11‰[67]

2.2 动力分馏

动力分馏是指在不完整或单向的过程中(例如蒸发或沿一个化学势梯度扩散)一个同位素被运移的速度高于另一个,由于两个同位素扩散系数的差异从而导致同位素的分馏。在矿物或岩石中因扩散性差异所产生的动力分馏被前人研究所证实[68,69,70]。辉石矿物内诱导的扩散实验[71]表明,在某种条件下,Li呈现了典型平滑变化的扩散曲线,归因于Li占据了快速扩散的间隙位置;在其他条件下,Li含量呈现了阶梯状变化的曲线,归因于在更加有限的扩散性下Li发生分馏进入金属活性位。同时,橄榄岩中的Li同位素分馏实验数据[72,73]表明,本质上较慢的Li扩散机制起主导地位,然而在橄榄石中Li的平均扩散速率比Fe和Mg的扩散速率高出一个数量级。

其次,由于温度梯度所发生的元素再分配也会引起同位素分馏[74,75,76]。安山岩活塞式液压缸部分熔融实验[75]表明,在冷端(411 ℃)和热端(952 ℃)之间存在Li同位素分馏,分馏值达18.5‰。同样地,Richter等[76]完成了玄武质熔体的高温高压实验,发现冷端(411 ℃)和热端(952 ℃)之间的Li同位素分馏值约为8‰。

3 Li同位素分析测试方法

3.1 Li的分离与纯化

由于地下水样品中其他元素(如Na、Mg、Ca等)含量是Li元素含量的数百甚至数千倍,所以Li元素的分离与纯化是一项极具挑战性的工作。目前,比较成熟的方法是阳离子交换色谱法,利用Li特效树脂(AG50W-X8或AG50W-X12)分离洗脱。常用的淋洗液有HCl、HNO3、HCl+乙醇以及HNO3+甲醇,在Li被洗出的过程中,会发生Li同位素的分馏[77],需保证百分之百的Li回收率。并且,不同的样品在Li的分离纯化过程中其析出峰值位置是不同的[27,39],因此不同的样品需要对离子交换柱进行矫正。相对于主量元素K、Ca和Mg来说,由于Na与Li具有相似的化学性质,它们之间的分离是最困难的,如果分离不当就会出现Na的大量拖尾,导致分离纯化的失败。

3.2 Li同位素分析测试方法

目前,能对Li同位素进行精确测量的主要仪器有四级杆电感耦合等离子质谱(Quadrupole Inductively Coupled Plasma Mass Spectrometry,Q-ICPMS)、扇形磁场电感耦合等离子质谱(Sector Field-Inductively Coupled Plasma Mass Spectrometry,SF-ICPMS)、热电离质谱(Thermal Ionization Mass Spectrometry,TIMS)、多接收电感耦合等离子质谱(Multiple-Collector Inductively Coupled Plasma Mass Spectrometry,MC-ICPMS)和二次离子质谱(Secondary Ion Mass Spectrometry,SIMS)。在ICPMS和SIMS多元化的发展之前,TIMS是高精度Li同位素测量的主要选择。由于TIMS测样耗时长且不易监控仪器质量偏差的变化等[78,79],近年来,MC-ICPMS受到越来越多学者的青睐,该方法所需样品量少、测样时间短且能避免测试过程中基质效应等因素产生的干扰[80,81,82,83],已成为高精度Li同位素测量的首选方法。SIMS是对样品进行原位Li同位素分析的方法,不需要对样品进行化学分离纯化,但其分析精度较差,且有许多问题要解决[84,85],例如合适标样的选择等,需进一步发展优化。其次,原子吸收光谱(Atomic Absorption Spectrometry, AAS)[86]、中子活化分析(Neutron Activation Analysis, NAA)[87]和共振电离质谱(Resonance Ionization Mass Spectrometry, RIMS)[88]也可进行Li同位素测试分析,但其应用面较窄。

4 大陆地热流体中Li同位素特征

4.1 地热流体中Li元素含量

Li是极易溶的碱金属元素,自然界中不同流体含有不同的Li元素含量(表1)。雨水和河水中Li含量较低,前者常小于3 μg/L[92],后者变化较大(0.2~20.0 μg/L)[91]。相对而言,海水具有相对稳定的Li含量(约185 μg/L)。地下水中有较高的Li含量,但其变化范围较大。与海底热液Li含量(0.1~10.0 mg/L)[23,37,61,106,107]相比,大陆地热水具有更高的Li含量(0.2~150.0 mg/L)[43,44,101,102,103]。大陆地热水中较高的Li浓度与高温条件下水岩反应有关[18,43,108]

表1   天然水体中Li元素含量及其同位素组成

Table 1  Concentration of lithium and its isotopic composition in natural waters

水体类型来源Li/(μg/L)δ7Li/‰参考文献
海水太平洋175+31.8±1.9[80]
海水标样BCR-403185+31.0±0.1[83]
西印度马尾藻海180+32.4±1.0[89]
日本海+29.6±0.2[90]
河水长江0.02~0.657.6~28.1[33]
亚马孙河0.6722.1[91]
密西西比河2.616.7[91]
雨水法国0.0006~0.04203.2~95.6[92]
青海大柴达木129.4[93]
美国夏威夷0.07514.3[63]
雪水青海大柴达木813.3[93]
阿尔卑斯山0.005~0.007[94]
南极≤0.05[95]
盐湖卤水死海13 60034.4[23]
青海大柴达木湖117 000~227 00022.2±1.5[93]
青海柴达木盆地720~408 8309.21~40.66[96]
加拿大地盾40~4 35033.2~41.8[41]
南美80 000~7 000 0009.0~13.3[19,97]
大陆地热水青海大柴达木湖热泉2 340~3 2502.29~4.33[93,96]
云南—西藏地热带4 240~25 000[97]
湖北江陵凹陷48 000~65 000[98,99]
美国Mono盆地热泉292~1 4903.0~17.1[100]
法国中央高原52~153 000-0.1~10.9[43,101]
法属群岛0.5~10 100.03.8~7.8[38]
新西兰陶波火山带200~32 500-3~2[102,103]
美国黄石公园270~6 5001.0~6.5[104]
日本御岳火山区0.57~2 370.00-5.17~12.60[105]
南美安第斯山脉7 000~147 4001.4~18.4[18]
海底热液Lau扩张中心和Juan de Fuca海岭100~10 0005~11[106,107]

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4.2 地热流体中Li同位素组成

图1中可以看出,大陆地热流体的δ7Li值变化较大。例如,法国中央高原地热水的δ7Li值为-0.1‰~10.9‰[43,101];法属马提尼克岛和瓜德罗普岛的地热水的δ7Li值为3.8‰~7.8‰[38];新西兰陶波火山带的地热水δ7Li值为-3‰~2‰[102,103]、冰岛米瓦登湖受高温(200~300 ℃)地热水补给的河水δ7Li值为17.2‰~20.8‰[109];美国Mono盆地热泉水δ7Li值为3.0‰~17.1‰[100];美国黄石公园地热泉水δ7Li值为1.0‰~6.5‰[104];日本中部地区御岳火山附近地热水的δ7Li值为-5.17‰~12.60‰[105];中国的高温地热带主要位于藏南、川西以及滇西地区[97],其中地热水的δ7Li值未见报道,仅见青海大柴达木湖热泉的δ7Li值为2.29‰~4.33‰[93,96]。统计这些Li同位素数据(图2),可以发现大陆地热流体的δ7Li值主要集中于-1‰~11‰,而海底热液呈现了更加窄的变化范围(3‰~9‰)。

图1

图1   大陆地热体系Li同位素组成特征

数据来源:海底热液来自参考文献[38,106,107];法国地热水来自参考文献[101];新西兰地热水来自文献[102,103];法属群岛地热水来自参考文献[38];美国Mono盆地地热水来自参考文献[100];南美安第斯山脉地热水来自参考文献[18];日本御岳火山地区地热水来自参考文献[105]

Fig. 1   Characteristics of lithium isotopic compositions in continental geothermal system

Data source: Submarine hydrothermal fluids from references [38,106,107]; Geothermal waters in France, New Zealand, French Islands, America, South America and Japan from references [101], [102,103], [38], [100], [18] and [105] respectively


图2

图2   大陆地热流体与海底热液Li同位素值统计特征

数据来源:海底热液来自参考文献[38,106,107];大陆地热水来自参考文献[18,38,100~103,105]

Fig. 2   Statistical characteristics of lithium isotopic compositions in continental geothermal and submarine hydrothermal fluids

Data source: Hydrothermal fluids from references [38,106,107]; Continental geothermal waters from references [18,38,101~103,105]


此外,根据地热系统中热能的来源,可简单分为岩浆热源型和非岩浆热源型水热系统[110]。中国典型的岩浆热源型系统所排泄的地热水来自藏南和滇西地热带,储层温度大于150 ℃,其Li含量为4.24~25.00 mg/L[97]。国外较典型的火山地热带为新西兰陶波火山区,储层最高温度可达330 ℃,其Li含量为0.2~32.5 mg/L,δ7Li值为-3‰~2‰[102,103]。典型的非岩浆热源型系统所排泄的地热水为美国Mono盆地热泉水,储层温度小于100 ℃,其Li含量为0.292~1.490 mg/L,δ7Li值为3.0‰~17.1‰[100]。对比上述两种不同类型热源型地热系统的Li含量和δ7Li值可以发现,来自岩浆热源型水热系统的地热水含有较高浓度的Li,且具有较低并均一的δ7Li值,而非岩浆热源型系统所排泄的地热水具有相对较低的Li浓度和不均一的δ7Li值。这归因于岩浆热源型地热系统中,岩性较均一,在高温条件下岩石与流体间的Li同位素分馏值小,Li从具有相似δ7Li值的源岩中被淋滤出来,从而具有高浓度的Li和均一的δ7Li值[103];反之,非岩浆热源型系统地热水Li含量及其同位素呈现了相反的特征。

5 大陆地热流体中Li同位素地球化学应用

Li同位素地球化学已经得到了广泛地运用,取得了显著的成果。在大陆地热体系中,其主要用于储层温度的估算、Li的物质来源研究、示踪热流体与围岩间的水岩反应过程、揭示热流体的运移和循环机制。

5.1 地热储层的温度计算

在地热体系中,地热水的溶质含量(K、Na、Ca、Mg、Li等)和氧同位素(δ18OH2O-SO4)可用于储层温度的估算[111,112,113,114,115,116,117],这是水化学在地热储库勘探中的重要应用之一。这些储层温度估算公式是基于经验、半经验或矿物与水的平衡反应实验所获得。

近年来,随着Li同位素地球化学在地热系统中的应用研究,发现储层温度控制着地热水的Li同位素组成,即它们之间存在较好的相关性[38,44,101]。对法国中央高原地区地热储库的Li-B-Sr多同位素体系研究[101]发现,相对B和Sr同位素变化范围,Li同位素值显示了更窄的波动,并且很少依赖于储层岩性特征。进一步利用地热储层温度估算公式计算出的温度与地热流体的δ7Li值投图,建立了储层温度和δ7Li值之间的相关性方程,即δ7Li=(-0.043±0.003)T+(11.9±0.5)。这些结果表明地热水的Li同位素组成可以作为地质温度计。Millot等[38]完成了海水与玄武岩在25~250 ℃范围的水岩反应实验,建立了Li同位素分馏值与温度之间的经验关系式,即Δsolution-solid=7 847/T-8.093。并用这个经验关系式计算了马提尼克岛和瓜德罗普岛火山岛弧区域地热体系的储层温度,这些温度结果与地热水溶质温度估算公式计算的一致,表明了Li同位素体系是一个用于地热储层温度计算的有利工具。Millot等[44]利用这个经验公式(Δsolution-solid=7 847/T-8.093)计算了法国巴黎盆地三叠系砂岩地热储层的温度,与化学温度估算公式计算的结果一致。以上研究说明无论是在火成岩地热体系还是沉积岩地热体系,温度控制着地热流体的Li同位素组成,δ7Li值可以用作地热储层温度的计算。

5.2 地热体系中Li的物质来源

由于地热流体温度或盐度等性质的差异,其Li元素的含量是不相同的,通常高温高盐度的地热流体含有较高浓度的Li。随着测试分析技术的不断进步和发展,Li同位素已成为地热流体中Li物质来源研究的有效手段和方法。

地热流体Li元素浓度和δ7Li值的较大波动表明其物质来源的复杂性。地热流体δ7Li值呈现了不同流体的混合,例如海水[44]、大气降水[44]以及深部流体[104,105]。然而,不同流体的混合并不能解释地热流体所有δ7Li值的波动,例如法国巴黎盆地[44]、新西兰陶波火山带[102,103]和冰岛米瓦登湖[109]等地热体系中的δ7Li值特征。地热流体δ7Li值与温度和水岩反应密切相关。

法国巴黎盆地三叠系砂岩储层地热水的δ7Li值为-0.1‰~10.9‰[4],远低于海水的δ7Li值(31.5‰),接近于当地的大气降水(9.2‰~22.9‰)[92]。可见,海水和雨水的混合来源并不能解释地热水的Li同位素特征,地热水与碎屑岩间的水岩反应改变了δ7Li值,有来自碎屑储层岩石释放Li的贡献[44]。Millot等[38]调查了马提尼克岛和瓜德罗普岛地区地热水的Li同位素组成,其δ7Li值为4‰~26‰,其中大陆地热水和泉水的δ7Li值为3.8‰~7.8‰。该地热系统δ7Li值较大的波动并不仅仅由于海水的加入所致,不同温度下水岩反应过程中储层岩石释放了Li,同时改变了δ7Li值。日本中部火山地区地震震中地热水的Li同位素组成为-5.17‰~1.55‰,显示下地壳的流体信息,表明有深部流体的补给[105]。新西兰陶波火山带的地热水δ7Li值(-3‰~2‰)显示了相对均一的变化范围[102,103]。如此相对均一且低的δ7Li值归因于Li从具有相同δ7Li值的岩石中淋滤而来。冰岛米瓦登湖高温(200~300 ℃)地热泉水显示了相对均一且较高的δ7Li值变化范围(17.2‰~20.8‰)[109],这与地热储层不同岩性岩石与地热水所发生的水岩反应过程有关。Négrel等[118]发现了法国南部始新世低温(20~50 ℃)砂岩储库地热水不均一的δ7Li值(6.5‰~28.6‰),其Li来源被解释为多源的,不仅有大气降水来源,还有各种基岩水岩反应过程中释放了Li。

其次,南美和北美西部高原地区地下富Li卤水或热泉的Li同位素值(1.4‰~18.4‰)也落在了大陆地热体系Li同位素值(-1‰~17‰)[119]范围内,显示了一定温度范围内水岩反应所导致的Li同位素值特征,表明了地下富Li卤水中Li的富集与高温水岩反应具有密切的关系。

因此,地热流体中Li可以来源于海水、雨水或深部流体等,且Li的来源更受控于温度控制下的不同程度的水岩反应。在高温条件下,次成矿物的形成受到抑制,地热流体与岩石间的Li同位素分馏值小,地热水可以从围岩中林滤出大量的Li而使流体富集Li元素,特别是高盐度的地热流体。

5.3 热液流体运移与地震活动

地热水的运移需要一定动能,而地下热能的变化可以驱动地热流体的流动与运移,特别是火山区以及汇聚大陆边缘地热水的运移更加频繁。与传统同位素相比,Li同位素可以作为有效的工具预测深部地热流体流动与地震活动的关系。Nishio等[105]调查了日本中部火山地区地热水的Li同位素,发现来自地震震中的地热水样品以低的δ7Li值(-5.17‰~1.55‰)为特征,显示了下地壳深部流体的Li同位素组成性质,表明了Li来自下地壳,而其他地震不活跃地区的地热水样品呈现了更高的δ7Li值,显示了地热水与上地壳岩石水岩反应所呈现的Li同位素组成,代表了上地壳Li的信息。因此,这些以低δ7Li值为特征的地热流体来自下地壳深部地震的活动。当然,地热水低的δ7Li值也可以解释为一定温度下流体与岩石间Li同位素扩散交换的结果[102,103,120]。然而,Nishio等[46]利用日本Nankai增生楔泥火山中流体的Li同位素特征成功证明了深部流体流动和地震活动间的联系。

6 问题与展望

国际上,Li同位素已在地热领域中得到较好的应用,尤其是海底热液方面。由于大陆地热储库岩石的复杂性,为大陆地热体系Li同位素行为的揭示带来了较大的困难,对其的研究也相对薄弱。目前,国内也未见Li同位素在大陆地热体系应用的报道,在研究大陆地热流体中Li的成因时应高度重视一些方法的应用,同时,该领域存在的一些前沿科学问题也值得我们思考与探索。

6.1 相关方法的应用

6.1.1 多同位素(Li-B-Sr-U)方法的示踪

地热水中大多数元素的浓度受储层温度和岩石风化后的矿物组合控制,同时由于大陆地热储库岩性的复杂性,仅仅一个同位素的示踪往往会导致流体成因不完全的解释。因此,多同位素(Li-B-Sr-U)的示踪有重要的意义,不仅可以提供更加全面的地热水物源信息,而且还可以用于评价储层温度和岩性对地热水中这些同位素行为的控制作用[43,44,101,102,108]。在大陆地热体系中,多同位素应用最广泛的地区当属法国中央高原[43,101]和巴黎盆地[44]、欧洲西部的莱茵河地堑上部地区[108]以及新西兰陶波火山带[102,103]。Sr同位素信息可以反映沉积岩、变质岩或火成岩的贡献,B同位素的调查可以进一步证明涉及这些岩石的水岩反应过程和揭示水体的来源[43,44,101,102,118]。相对于Sr和B同位素而言,温度对地热流体Li同位素组成起到重要的控制作用,可以示踪不同温度下的水岩反应过程,特别是高温条件[38,101]

地热体系中Li同位素行为往往被解释为一定温度下流体与岩石所发生的水岩反应,而与水岩反应条件有关的流体停留时间等,尚未得到较好的约束。在评价地下水停留时间时,氢(3H)、碳(14C)和氦(4He)同位素常常被用到[121,122,123,124,125]。相比而言,由于水体与沉积物之间234U和238U的放射性不平衡[126],即铀(U)系不平衡原理,使得U同位素成为评价地下水混合、循环时间和流速的有效工具之一[44,108,127,128]。基于地下水的U同位素特征,Ivanovich等[127]建立了2种估算水体流速的模式:一种是在U含量稳定条件下,根据234U过剩衰变方程得出地下水的年龄,从而计算出水体的流速;另一种是在水体运输方程的基础上,考虑了放射性衰变和流动过程中发生的吸附作用2种因素的影响,从而得出水体的流速。其中,第二种模式计算出的流速与水力学模型计算出的结果一致。根据距离补给区不同位置地热水的234U/238U活度比,Millot等[44]得出了法国巴黎盆地三叠系地层流体循环的平均表观速度0.2 m/a,进而算出水体的停留时间约1.25 Ma,并与原油中稀有气体同位素估算的水体停留时间[129]一致。基于地下热液流体的U含量和活度比,Innocent等[128]进一步研究法国巴黎盆地东北部地下热水的流速,仅约为0.05 m/a。Sanjuan等[108]报道了莱茵河地堑上部地区地热水的U含量和活度比以及钍(Th)同位素特征,估算出深部地热卤水的最小运输时间约为1 000年。这些均说明了U同位素在估算地下流体流速和停留时间方面已经得到很好的应用,而热液体系中流体停留时间的确定对流体与岩石间水岩反应过程的理解具有重要的意义。

每个同位素都从不同方面揭示了水体来源或水岩反应过程,同时Li、B、Sr和U同位素的应用也突出了地热体系研究的复杂性,需要综合所有同位素的信息,从而更加全面地理解大陆地热体系。

6.1.2 不同温度下沉积物/岩石水岩反应实验研究

富Li卤水形成于干旱气候条件下的封闭盐湖盆地,同时在盆地内或周缘伴有火山或地热活动[18,97,130]。水体中Li同位素值(δ7Li)已成为富Li卤水成因研究的有效工具。通过Li同位素行为的研究,表明地热流体中Li的富集与高温条件下的水岩反应有关[43,44,101,102,103,108]。然而,温度对Li同位素的控制以及水岩反应过程中矿物相的变化更需要室内模拟实验的研究。Araoka等[17]对美国内华达州干盐湖中各种湖相沉积物开展了常温的淋滤实验研究,结果表明沉积物淋滤液的δ7Li值比河水和地下水对应的值低得多,接近火山岩的δ7Li值,并位于大陆热液流体的δ7Li值范围内,证明了Li主要通过热液活动中发生的水岩反应提供。但是这次淋滤实验并未能揭示温度以及主要矿物相溶解和次生矿物形成对Li同位素行为的控制。因此,不同温度下(25~300 ℃)水岩反应实验,例如水(盐水)与湖相沉积物、水(盐水)与玄武岩、水(盐水)与花岗岩,并结合矿物学的研究,更易于揭示和理解大陆地热体系中的Li同位素行为。

6.2 科学问题与展望

在以往地下水Li成因研究中,学者更关注的是水体本身的Li同位素组成,而忽视了区域或地热田尺度的各类沉积物/岩石Li同位素组成,这对地热体系中Li同位素行为的约束是不完整的。当然,大陆地热储库岩石的复杂性也为Li同位素行为解释带来了一定的困难。因此,针对单一岩性地热储库的Li同位素行为研究显得格外重要。

不同的地质构造背景可形成不同类型的地热系统,首先,各类型地热体系Li同位素组成背景及其时空分布特征是怎样的尚不清楚,其次,这些体系内地热流体循环运移过程中Li同位素行为也是不清楚的,制约着Li同位素在大陆地热体系中的应用研究。大陆地热储库岩石主要为碎屑岩、碳酸盐岩和火成岩等[53,119,131],由于储库岩石的复杂性,其主要问题是地热流体与岩石间水岩反应过程中主要矿物相溶解和次生矿物形成机制是不清楚的,且该过程中Li同位素分馏机制还无法得到清楚的解释。这就需要我们注重不同温度下的水岩反应实验研究,定量化不同温度下次生矿物形成或Li被吸附于矿物表面所导致的Li同位素分馏机制。因此,将来在该方面的工作研究对地热体系中Li 的成因与演化的解释具有重要的意义。同时,我们也应该关注储层岩石的矿物学研究,地球化学与矿物学的双重验证才能更好地揭示地质作用过程。

Li同位素在大陆地热体系中的应用研究,国外已取得较成熟的认识,同时也显示了Li同位素应用的无限潜力。然而,国内相关成果依然比较薄弱和匮乏。近年来,国内多家实验室已相继建立了高精度的Li同位素分析方法,可以开展不同地质样品的Li同位素测试工作。在此基础上,可以建立大陆地热体系Li同位素数据库,深入研究该体系内的Li同位素分馏机制,对今后大陆地热体系中Li同位素行为研究应用于地下卤水型锂矿的勘查和地热资源的开发和利用具有重要的理论和实际意义。

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