The conditions of methane (CH4) formation in olivine-hosted secondary fluid inclusions and their prevalence in peridotite and gabbroic rocks from a wide range of geological settings were assessed using confocal Raman spectroscopy, optical and scanning electron microscopy, electron microprobe analysis, and thermodynamic modeling. Detailed examination of 160 samples from ultraslow- to fast-spreading midocean ridges, subduction zones, and ophiolites revealed that hydrogen (H2) and CH4 formation linked to serpentinization within olivine-hosted secondary fluid inclusions is a widespread process. Fluid inclusion contents are dominated by serpentine, brucite, and magnetite, as well as CH4(g) and H2(g) in varying proportions, consistent with serpentinization under strongly reducing, closed-system conditions. Thermodynamic constraints indicate that aqueous fluids entering the upper mantle or lower oceanic crust are trapped in olivine as secondary fluid inclusions at temperatures higher than ∼400 °C. When temperatures decrease below ∼340 °C, serpentinization of olivine lining the walls of the fluid inclusions leads to a near-quantitative consumption of trapped liquid H2O. The generation of molecular H2 through precipitation of Fe(III)-rich daughter minerals results in conditions that are conducive to the reduction of inorganic carbon and the formation of CH4. Once formed, CH4(g) and H2(g) can be stored over geological timescales until extracted by dissolution or fracturing of the olivine host. Fluid inclusions represent a widespread and significant source of abiotic CH4 and H2 in submarine and subaerial vent systems on Earth, and possibly elsewhere in the solar system.

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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 4 -rich secondary inclusions in olivine, with less abundant inclusions in plagioclase and clinopyroxene. Olivine-hosted secondary inclusions are chiefly composed of CH 4 and minor H 2 , in addition to secondary minerals including serpentine, brucite, magnetite and carbonates. Secondary inclusions in plagioclase are dominated by CH 4 with variable amounts of H 2 and H 2 O, while those in clinopyroxene contain only CH 4 . 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 4 and C 2 H 6 in samples from Cayman (4–313 nmol g −1 CH 4 and 0.02–0.99 nmol g −1 C 2 H 6 ), with lesser amounts in samples from Zambales (2–37 nmol g −1 CH 4 and 0.004–0.082 nmol g −1 C 2 H 6 ). Mafic and ultramafic rocks at Cayman exhibit δ 13 C CH 4 values of −16.7‰ to −4.4‰ and δ 13 C C 2 H 6 values of −20.3‰ to +0.7‰. Ultramafic rocks from Zambales exhibit δ 13 C CH 4 values of −12.4‰ to −2.8‰ and δ 13 C C 2 H 6 values of −1.2‰ to −0.9‰. Similarities in the carbon isotopic compositions of CH 4 and C 2 H 6 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 4 and C 2 H 6 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’.

24The exposure of mantle peridotite to water at crustal levels leads to a 25 cascade of interconnected dissolution-precipitation and reduction-oxidation 26 reactions -a process referred to as serpentinization. These reactions have major 27 implications for microbial life through the provision of hydrogen (H2). To simulate 28 incipient serpentinization under well-constrained conditions, we reacted cm-sized 29 pieces of uncrushed harzburgite with chemically modified seawater at 300°C and 35 30 MPa for ca. 1.5 years (13441 hours), monitored changes in fluid chemistry over 31 time, and examined the secondary mineralogy at the termination of the experiment. 32Approximately 4 mol % of the protolith underwent alteration forming serpentine, 33 accessory magnetite, chlorite, and traces of calcite and heazlewoodite. Alteration 34 textures bear remarkable similarities to those found in partially serpentinized 35 abyssal peridotites. Neither brucite nor talc precipitated during the experiment. 36Given that the starting material contained ~4 times more olivine than 37 orthopyroxene on a molar basis, mass balance requires that dissolution of 38 orthopyroxene was significantly faster than dissolution of olivine. Coupled mass 39 transfer of dissolved Si, Mg, and H + between olivine and orthopyroxene reaction 40 fronts was driven by steep activity gradients and facilitated the precipitation of 41 serpentine. Hydrogen was released in significant amounts throughout the entire 42 experiment; however, the H2 release rate decreased with time. Serpentinization 43 consumed water but did not release significant amounts of dissolved species (other 44 than H2) suggesting that incipient hydration reactions involved a volume increase of 45 ~40%. The reduced access of water to fresh olivine surfaces due to filling of 46 This is a preprint, the final version is subject to change, of the American Mineralogist (MSA) Cite as Authors (Year) Title. American Mineralogist, in press. (DOI will not work until issue is live.) DOI: http://dx.doi. org/10.2138/am-2015-5112 10/20Always consult and cite the final, published document.

This thesis examines abiotic processes controlling the transformation and distribution of carbon compounds in seafloor hydrothermal systems hosted in ultramafic rock. These processes have a direct impact on carbon budgets in the oceanic lithosphere and on the sustenance of microorganisms inhabiting hydrothermal vent ecosystems. Where mantle peridotite interacts with carbon-bearing aqueous fluids in the subseafloor, dissolved inorganic carbon can precipitate as carbonate minerals or undergo reduction by H 2(aq) to form reduced carbon species. In Chapters 2 and 3, I conduct laboratory experiments to assess the relative extents of carbonate formation and CO 2 reduction during alteration of peridotite by CO 2(aq)-rich fluids. Results from these experiments reveal that formation of carbonate minerals is favorable on laboratory timescales, even at high H 2(aq) concentrations generated by serpentinization reactions. Although CO 2(aq) attains rapid metastable equilibrium with formate, formation of thermodynamically stable CH 4(aq) is kinetically limited on timescales relevant for active fluid circulation in the subseafloor. It has been proposed that CH 4 and potentially longer-chain hydrocarbons may be sourced, instead, from fluid inclusions hosted in plutonic and mantle rocks. Chapter 4 analyzes CH 4-rich fluid inclusions in olivine-rich basement rocks from the Von Damm hydrothermal field and the Zambales ophiolite to better understand the origin of abiotic hydrocarbons in ultramaficinfluenced hydrothermal systems. Comparisons of hydrocarbon abundances and stable isotopic compositions in fluid inclusions and associated vent fluids suggest that fluid inclusions may provide a significant contribution of abiotic hydrocarbons to both submarine and continental serpentinization systems.

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