Chemical and isotopic analyses of hydrocarbon-bearing fluid inclusions in olivine-rich rocks

• Published in: Philosophical transactions of the Royal Society of London. Series A: Mathematical, physical, and engineering sciences physical, and engineering sciences, 2020-02, Vol.378 (2165), p.20180431-20180431
• Main Authors: Grozeva, Niya G, Klein, Frieder, Seewald, Jeffrey S, Sylva, Sean P
• Format: Article
• Language: English
• ISSN: 1364-503X; 1471-2962
• DOI: 10.1098/rsta.2018.0431

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 CH -rich secondary inclusions in olivine, with less abundant inclusions in plagioclase and clinopyroxene. Olivine-hosted secondary inclusions are chiefly composed of CH and minor H , in addition to secondary minerals including serpentine, brucite, magnetite and carbonates. Secondary inclusions in plagioclase are dominated by CH with variable amounts of H and H O, while those in clinopyroxene contain only CH . 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 CH and C H in samples from Cayman (4-313 nmol g CH and 0.02-0.99 nmol g C H ), with lesser amounts in samples from Zambales (2-37 nmol g CH and 0.004-0.082 nmol g C H ). Mafic and ultramafic rocks at Cayman exhibit δ C values of -16.7‰ to -4.4‰ and δ C values of -20.3‰ to +0.7‰. Ultramafic rocks from Zambales exhibit δ C values of -12.4‰ to -2.8‰ and δ C values of -1.2‰ to -0.9‰. Similarities in the carbon isotopic compositions of CH and C H 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 CH and C H 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'.