Chemical and isotopic analyses of hydrocarbon-bearing fluid inclusions in olivine-rich rocks
Chemical and isotopic analyses of hydrocarbon-bearing fluid inclusions in olivine-rich rocks
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DOI：
10.1098/rsta.2018.0431
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发表时间：
2020-02-21
期刊：
影响因子：
5
通讯作者：
Sylva, Sean P.
中科院分区：
文献类型：
--
作者：
Grozeva, Niya G.;Klein, Frieder;Sylva, Sean P.
我们研究了两个活跃蛇纹石化系统中富含橄榄石的岩石中次生流体包裹体的矿物学、化学和同位素组成：冯达姆热液场（开曼中隆隆）和三描礼士蛇绿岩（菲律宾）。这些系统中的橄榄岩、方沸石和辉长岩含有丰富的橄榄石中富含 CH4 的次生包裹体，斜长石和单斜辉石中的包裹体含量较少。橄榄石中的次生包裹体主要由 CH4 和少量 H2 组成，此外还有蛇纹石、水镁石、磁铁矿和碳酸盐等次生矿物。斜长石中的次生包裹体主要是 CH4，并含有不同数量的 H2 和 H2O，而单斜辉石中的次生包裹体仅含有 CH4。我们通过破碎整个岩石并使用同位素比监测-气相色谱质谱法分析释放的挥发物来确定碳氢化合物丰度和稳定碳同位素组成。大块岩石气体分析显示，开曼群岛样品中含有大量 CH4 和 C2H6（4-313 nmol g-1 CH4 和 0.02-0.99 nmol g-1 C2H6），三描礼士省样品中含量较少（2-37 nmol g-1 CH4 和 0.004-0.082 nmol g-1 C2H6）。开曼的镁铁质和超镁铁质岩石的 delta13CCH4 值为 -16.7 ppm 至 -4.4 ppm，delta13CC2H6 值为 -20.3 ppm 至 +0.7 ppm。来自三描礼士省的超镁铁质岩石的 delta13CCH4 值为 -12.4 ppm 至 -2.8 ppm，delta13CC2H6 值为 -1.2 ppm 至 -0.9 ppm。深成岩、冯达姆热液和三描礼士气体渗漏中 CH4 和 C2H6 碳同位素组成的相似性表明，流体包裹体的浸出可能为深海喷口流体和蛇绿岩气体渗漏提供了非生物碳氢化合物的重要贡献。来自富含橄榄石岩石的各种热液场的 CH4 和 C2H6 同位素组成与冯达姆喷口流体中的相似，进一步支持了这样的观点，即受超镁铁质影响的喷口流体中的非生物碳氢化合物的很大一部分源自流体包裹体。本文是“地球系统中的蛇纹岩”讨论会议问题的一部分。
We examined the mineralogical, chemical and isotopic compositions of secondary fluid inclusions in olivine-rich rocks from two active serpentinization systems: the Von Damm hydrothermal field (Mid-Cayman Rise) and the Zambales ophiolite (Philippines). Peridotite, troctolite and gabbroic rocks in these systems contain abundant CH4-rich secondary inclusions in olivine, with less abundant inclusions in plagioclase and clinopyroxene. Olivine-hosted secondary inclusions are chiefly composed of CH4 and minor H2, in addition to secondary minerals including serpentine, brucite, magnetite and carbonates. Secondary inclusions in plagioclase are dominated by CH4 with variable amounts of H2 and H2O, while those in clinopyroxene contain only CH4. We determined hydrocarbon abundances and stable carbon isotope compositions by crushing whole rocks and analysing the released volatiles using isotope ratio monitoring-gas chromatography mass spectrometry. Bulk rock gas analyses yielded appreciable quantities of CH4 and C2H6 in samples from Cayman (4-313 nmol g-1 CH4 and 0.02-0.99 nmol g-1 C2H6), with lesser amounts in samples from Zambales (2-37 nmol g-1 CH4 and 0.004-0.082 nmol g-1 C2H6). Mafic and ultramafic rocks at Cayman exhibit delta13CCH4 values of -16.7 parts per thousand to -4.4 parts per thousand and delta13CC2H6 values of -20.3 parts per thousand to +0.7 parts per thousand. Ultramafic rocks from Zambales exhibit delta13CCH4 values of -12.4 parts per thousand to -2.8 parts per thousand and delta13CC2H6 values of -1.2 parts per thousand to -0.9 parts per thousand. Similarities in the carbon isotopic compositions of CH4 and C2H6 in plutonic rocks, Von Damm hydrothermal fluids, and Zambales gas seeps suggest that leaching of fluid inclusions may provide a significant contribution of abiotic hydrocarbons to deep-sea vent fluids and ophiolite-hosted gas seeps. Isotopic compositions of CH4 and C2H6 from a variety of hydrothermal fields hosted in olivine-rich rocks that are similar to those in Von Damm vent fluids further support the idea that a significant portion of abiotic hydrocarbons in ultramafic-influenced vent fluids is derived from fluid inclusions. This article is part of a discussion meeting issue 'Serpentinite in the Earth system'.