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Abiotic Methane Synthesis and Serpentinization in Olivine-Hosted Fluid Inclusions

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Abiotic Methane Synthesis and Serpentinization in Olivine-Hosted Fluid Inclusions Abiotic methane synthesis and serpentinization in olivine-hosted fluid inclusions Frieder Kleina,1, Niya G. Grozevab, and Jeffrey S. Seewalda aDepartment of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA 02543; and bMassachusetts Institute of Technology–Woods Hole Oceanographic Institution Joint Program in Oceanography, Cambridge, MA 02139 Edited by Kenneth A. Farley, California Institute of Technology, Pasadena, CA, and approved July 22, 2019 (received for review May 6, 2019) The conditions of methane (CH4) formation in olivine-hosted second- deep-sea hydrothermal fluids associated with ultramafic rocks may ary fluid inclusions and their prevalence in peridotite and gabbroic be leached from fluid inclusions (3, 6–8). Many important questions rocks from a wide range of geological settings were assessed using remain regarding fluid inclusion prevalence, formation, internal confocal Raman spectroscopy, optical and scanning electron micros- fluid–mineral interaction, and their contributions of CH4 to venting copy, electron microprobe analysis, and thermodynamic modeling. fluids and global reservoirs. Moreover, because fluid inclusions may Detailed examination of 160 samples from ultraslow- to fast-spreading form in olivine-rich rocks that interact with water on celestial bodies midocean ridges, subduction zones, and ophiolites revealed that hy- elsewhere in our solar system, their formation may have key impli- drogen (H2) and CH4 formation linked to serpentinization within cations for the maintenance of microbial life beyond Earth. olivine-hosted secondary fluid inclusions is a widespread process. Fluid Here we examined the chemical and mineralogical composi- inclusion contents are dominated by serpentine, brucite, and magne- tion of fluid inclusions in olivine-bearing gabbros and partially tite,aswellasCH4(g) and H2(g) in varying proportions, consistent with serpentinized peridotites from ultraslow-, slow-, and fast-spreading serpentinization under strongly reducing, closed-system conditions. midocean ridges, a backarc basin, subduction zone forearcs, and Thermodynamic constraints indicate that aqueous fluids entering the ophiolites (Fig. 1 and SI Appendix, Table S1). We assessed the upper mantle or lower oceanic crust are trapped in olivine as second- distribution and composition of secondary fluid inclusions in olivine ary fluid inclusions at temperatures higher than ∼400 °C. When tem- by means of confocal Raman spectroscopy, scanning electron mi- peratures decrease below ∼340 °C, serpentinization of olivine lining croscopy, transmitted and reflected light microscopy, and electron the walls of the fluid inclusions leads to a near-quantitative consump- microprobe analysis. Complementing these efforts, we used ther- EARTH, ATMOSPHERIC, modynamic reaction path models to assess the geochemical envi- AND PLANETARY SCIENCES tion of trapped liquid H2O. The generation of molecular H2 through precipitation of Fe(III)-rich daughter minerals results in conditions that ronments present within the inclusions during fluid entrapment, are conducive to the reduction of inorganic carbon and the formation serpentinization, and CH4 formation. of CH . Once formed, CH and H canbestoredovergeological 4 4(g) 2(g) Methane Abundance in Oceanic Peridotite and Gabbro timescales until extracted by dissolution or fracturing of the olivine = = host. Fluid inclusions represent a widespread and significant source Examination of gabbro (n 43) and peridotite (n 117) in thin of abiotic CH and H in submarine and subaerial vent systems on sections with relict olivine revealed the presence of fluid inclu- 4 2 sions in rocks from each of the field locations shown in Fig. 1. All Earth, and possibly elsewhere in the solar system. of the olivine-bearing gabbro samples and 77% of the peridotite samples contain fluid inclusions hosted in olivine. Image analyses abiotic methane | fluid inclusions | serpentinization | methane seeps | of some of the most inclusion-rich samples revealed more than carbon cycling 3 × 106 inclusions per cm3. Inclusions vary in size from <100 nm to ∼30 μm in diameter and are heterogeneously distributed on a he formation of molecular hydrogen (H2) and abiotic hy- millimeter to centimeter scale (Figs. 2 and 3 and SI Appendix, Tdrocarbons such as methane (CH4) has far-reaching impli- cations for our understanding of the deep Earth carbon cycle, as Significance well as the origin and maintenance of life on Earth and beyond. Elevated concentrations of H2 and CH4 are associated with the Our findings highlight the ubiquitous occurrence of methane hydrous alteration of olivine-rich (ultramafic) rocks in many (CH4)-rich fluid inclusions in olivine-bearing rocks that, collec- natural environments, a process that entails a number of redox- tively, may constitute one of the largest reservoirs of abiotic dependent dissolution–precipitation reactions collectively known CH4 on Earth. Because serpentinization in olivine-hosted fluid as serpentinization. Large quantities of H2 are generated during inclusions takes place in isolation from the surrounding rock, aqueous oxidation of ferrous iron-bearing mineralsP which results hydrogen (H ) and CH can form in any rock type containing = + 2 4 in the reduction of dissolved inorganic carbon ( CO2 CO2(aq) olivine that hosts aqueous fluid inclusions, including those + − + 2- H2CO3 HCO3 CO3 ). Due to its important roles in a broad that do not undergo serpentinization on a macroscopic scale. array of biogeochemical processes, few aspects of deep-sea hy- Serpentinization and associated CH4 formation within olivine- drothermal vent systems and alkaline springs and gas seeps on hosted fluid inclusions has likely supplied microbial ecosystems land have attracted more attention than the origin of abiotic CH4 with abiotic CH throughout most of Earth’s history and may be – 4 (1 5). Field observations have revealed that the abundance of abi- asourceofH2 and CH4 on other planetary bodies in our solar otic CH4 in hydrothermal systems hostedinmaficrocks(basalt, system, even those where liquid water is no longer present. diabase, gabbro) is substantially lower than in hydrothermal systems hosted in ultramafic rocks (peridotite or peridotite plus gabbro), but Author contributions: F.K., N.G.G., and J.S.S. designed research, performed research, an- alyzed data, and wrote the paper. the pathways of abiotic CH4 synthesis have remained elusive. Re- cently, McDermott et al. (3) usedP carbon isotopic and mass balance The authors declare no conflict of interest. constraints to demonstrate that CO2 reduction by H2 does not This article is a PNAS Direct Submission. yield CH4 during convection of hydrothermal fluids at the Von Published under the PNAS license. 1 Damm hydrothermal field, suggesting that abiotic CH4 formation To whom correspondence may be addressed. Email: [email protected]. and convective seawater circulation are decoupled. This challenged This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. the paradigm of significant abiotic CH4 formation during active fluid 1073/pnas.1907871116/-/DCSupplemental. circulationandledtothesuggestionthatabioticCH4 observed in www.pnas.org/cgi/doi/10.1073/pnas.1907871116 PNAS Latest Articles | 1of7 Downloaded by guest on September 28, 2021 Figs. S1, S3, and S4). Most inclusions occur along planes, which Combining this value with CH4 stored in upper-mantle peridotite indicate a secondary origin via annealing of fluid-filled fractures suggests that the fluid-inclusion–hosted lithospheric CH4 reservoir in the olivine host (9, 10). With the exception of 3 sites, i.e., Hess created at slow- and ultraslow-spreading midocean ridges exceeds Deep, the Romanche Fracture Zone, and the Mid-Atlantic Ridge theamountofpreindustrialCH4 in the atmosphere (∼2 Pg) (15). Kane Fracture Zone Area (MARK) which do not contain de- This estimate does not include the potential contribution of tectable CH4 in olivine-hosted fluid inclusions despite being rich in CH4 hosted in fluid inclusions in the faster-spreading Pacific H2, olivine-hosted fluid inclusions from all other sites examined in lithosphere. Our analysis of 2 sites in the Pacific (Hess Deep this study contain CH4(g) or CH4(g) and H2(g) (Figs. 1 and 3 and SI and Cocos Plate) yield contrasting results, which precludes a Appendix, Figs. S1 and S2 and Table S1). The partial pressure of meaningful assessment at this point. However, the oceanic CH4(g) within olivine-hosted fluid inclusions determined using lithosphere in the Pacific contains additional, potentially mas- empirical calibrations of the Raman shifts of CH4(g) (11) (SI Ap- sive amounts of abiotic CH4 that remain to be quantified when pendix,TableS1) are 0.4 to 55 MPa, with an average of 11.5 MPa. more gabbro and peridotite samples from layered oceanic crust Using the ideal gas law and the assumption that fluid inclusions become available. are spherical, we calculated that an inclusion with a diameter of −5 −2 10 μm contains 8.4 × 10 to 1.2 × 10 nmol CH4(g). Serpentinization and CH4(g) Formation within Olivine-Hosted The CH4(g) content of individual fluid inclusions can be used Fluid Inclusions in conjunction with their abundance to estimate the CH4(g) Electron microscope and confocal Raman analyses indicate that content of olivine-rich rocks. A rock containing 105 inclusions per iron-bearing serpentine (chrysotile, lizardite, antigorite), iron-bearing 3 cm would contain 2.5 to 363 nmol CH4(g) per gram of olivine. For brucite,
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