Supplementary material from Chemical and isotopic analyses of hydrocarbon-bearing fluid inclusions in olivine-rich rocks.

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 δ¹³C_CH4 values of −16.7‰ to −4.4‰ and δ¹³C_C2H6 values of −20.3‰ to +0.7‰. Ultramafic rocks from Zambales exhibit δ¹³C_CH4 values of −12.4‰ to −2.8‰ and δ¹³C_C2H6 values of −1.2‰ to −0.9‰. 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 the theme issue ‘Serpentinite in the Earth system’.