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 CH<INF>4</INF>-rich secondary inclusions in olivine, with less abundant inclusions in plagioclase and clinopyroxene. Olivine-hosted secondary inclusions are chiefly composed of CH<INF>4</INF> and minor H<INF>2</INF>, in addition to secondary minerals including serpentine, brucite, magnetite and carbonates. Secondary inclusions in plagioclase are dominated by CH<INF>4</INF> with variable amounts of H<INF>2</INF> and H<INF>2</INF>O, while those in clinopyroxene contain only CH<INF>4</INF>. 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<INF>4</INF> and C<INF>2</INF>H<INF>6</INF> in samples from Cayman (4-313 nmol g-1 CH<INF>4</INF> and 0.02-0.99 nmol g-1 C<INF>2</INF>H<INF>6</INF>), with lesser amounts in samples from Zambales (2-37 nmol g-1 CH<INF>4</INF> and 0.004-0.082 nmol g-1 C<INF>2</INF>H<INF>6</INF>). Mafic and ultramafic rocks at Cayman exhibit delta13C<INF>CH<INF>4</INF></INF> values of -16.7 parts per thousand to -4.4 parts per thousand and delta13C<INF>C<INF>2</INF>H<INF>6</INF></INF> values of -20.3 parts per thousand to +0.7 parts per thousand. Ultramafic rocks from Zambales exhibit delta13C<INF>CH<INF>4</INF></INF> values of -12.4 parts per thousand to -2.8 parts per thousand and delta13C<INF>C<INF>2</INF>H<INF>6</INF></INF> values of -1.2 parts per thousand to -0.9 parts per thousand. Similarities in the carbon isotopic compositions of CH<INF>4</INF> and C<INF>2</INF>H<INF>6</INF> 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<INF>4</INF> and C<INF>2</INF>H<INF>6</INF> 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'.