Lithium isotope compositions of U.S. coals and source rocks: Potential tracer of hydrocarbons
Lithium isotope compositions of U.S. coals and source rocks: Potential tracer of hydrocarbons
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DOI：
10.1016/j.chemgeo.2020.119694
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发表时间：
2020-09
期刊：
影响因子：
3.9
通讯作者：
Z. Teichert;M. Bose;L. Williams
中科院分区：
文献类型：
--
作者：
Z. Teichert;M. Bose;L. Williams
富含有机质岩石中的干酪根含有微量的锂（Li），而锂作为全球锂地球化学循环的贡献者一直被忽视。本研究考察了干酪根富集的各种煤采用二次离子质谱（西姆斯）技术，对22个煤样和4个烃源岩进行了Li同位素组成（δ 7 Li ‰）分析，并对影响煤样和烃源岩Li同位素组成的因素进行了探讨（I、II、III型），以确定干酪根原位的δ 7 Li，而不进行可能改变其原始同位素组成的相的化学分离。与大多数天然矿物和流体相比，所调查的煤的δ 7 Li值明显较轻（<0‰）。在未成熟煤中，油中镜质组反射率（VRo）≤ 0.5%，干酪根δ 7 Li值平均为-23.4 ± 1.1‰，且随热品位升高而变重（VRo ~1.3%）。δ 7 Li与VRo的线性相关关系表明，干酪根在热成熟过程中，δ 7 Li可能优先释放到孔隙流体中。值得注意的是，在成岩温度下形成的自生粘土将Li从孔隙流体中取代到硅酸盐层中，因此，粘土的Li同位素组成可以记录受有机Li源影响的流体同位素组成。下巴肯页岩的NanoSIMS同位素图和绿色河页岩的西姆斯测量显示与C主导区域相关的同位素轻Li，烃源岩δ 7 Li重、Si优势区。我们的结论是，干酪根是一个来源的同位素轻锂，有助于在热成熟和烃类生成过程中的流体。干酪根可能是孔隙流体中Li的重要贡献者，其相对于其他陆地沃茨和矿物的明显轻的Li同位素组成可以为全球地球化学循环提供有机输入的示踪剂。
Kerogen in organic-rich rocks contains trace amounts of lithium (Li) that has been overlooked as a contributor to the global Li geochemical cycle. This study examined a variety of coals where kerogen is concentrated (>50% organic carbon) and hydrocarbon source rocks of different ages, depositional environments and thermal maturity to determine their range of Li isotopic compositions (δ7Li‰) and factors that influence their compositions.Using Secondary Ion Mass Spectrometry (SIMS), we analyzed 22 coals and 4 hydrocarbon source rocks (Types I, II, III), to determine the δ7Li of kerogenin situ, without chemical isolation of phases that can alter their original isotopic compositions. The δ7Li values of the coals surveyed are distinctly isotopically light (<0‰) compared to most natural minerals and fluids. In immature coals, with a vitrinite reflectance in oil (VRo) of ≤0.5%, kerogen δ7Li values average –23.4 ± 1.1‰ and become heavier with increasing thermal grade to temperatures of gas generation (VRo ~1.3%). The linear correlation between δ7Li and VRo suggests that6Li may be preferentially released to pore fluid from kerogen during thermal maturation. Notably, authigenic clays forming at diagenetic temperatures substitute Li from pore fluids into silicate layers, therefore, the Li isotopic composition of the clays may record fluid isotopic compositions influenced by organic-Li sources.NanoSIMS isotopic maps of the Lower Bakken Shale, and SIMS measurements of the Green River Shale show isotopically light Li associated with C-dominated areas, and heavier δ7Li with Si-dominated areas of the hydrocarbon source rocks. We conclude that kerogen is a source of isotopically light Li that contributes to fluids during thermal maturation and hydrocarbon generation. Kerogen may be a significant contributor of Li to pore fluids and its distinctly light Li isotopic composition relative to other terrestrial waters and minerals could provide a tracer of organic inputs to the global geochemical cycle.