The Oman Drilling Project established an “Active Alteration” multi-borehole observatory in dunite and harzburgite undergoing low-temperature serpentinization in the Samail ophiolite. The highly serpentinized rocks are in contact with strongly reducing fluids. Distinct hydrological regimes, governed by differences in rock porosity and fracture density, give rise to steep redox (Eh +200 to -750 mV) and pH (pH range 8.5 to 11.2) gradients within the 300 to 400 meter deep boreholes. The serpentinites and fluids host an active subsurface ecosystem. Microbial cell abundances vary at least 6 orders of magnitude, from ≤3.5*101 cells/g to 2.9*107 cells/gram. Low levels of biological sulfate reduction (2-1000 fmol/cm3/day) can be detected in rock cores, particularly in rocks in contact with reduced groundwaters with pH <10.5. Thermodesulfovibrio is the predominant sulfate reducer identified via metagenomic sequencing of adjacent groundwater communities. We infer that transport and reaction of microbially generated sulfide with the serpentine and brucite assemblages gives rise to optical darkening and sulfide overprinting, including the formation of tochilinite-vallerite group minerals, potentially serving as an indicator that this system is inhabited by microbial life. Olivine mesh-cores replaced with ferroan brucite and minor awaruite, abundant veins containing hydroandradite garnet and polyhedral serpentine, and late-stage carbonate veins are suggested as targets for future spatially-resolved life-detection investigations. The high-quality whole-round core samples that have been preserved can be further probed to define how life distributes itself and functions within a system where chemical disequilibria are sustained by low-temperature water/rock interaction, and how biosignatures of in-situ microbial activity are generated.

The potential for molecular hydrogen (H-OH- groundwaters bearing up to 4.05 μmol⋅L-1 H2 , 3.81 μmol⋅L-1 methane (CH4) and 946 μmol⋅L-1 sulfate (SO42-) revealed an ecosystem dominated by Bacteria affiliated with the class Thermodesulfovibrionia, a group of chemolithoheterotrophs supported by H2 oxidation coupled to SO42- reduction. In shallower, oxidized Mg2+-HCO3- groundwaters, aerobic and denitrifying heterotrophs were relatively more abundant. High δ13C and δD of CH4 (up to 23.9 ‰ VPDB and 45 ‰ VSMOW , respectively), indicated microbial CH4 oxidation, particularly in Ca2+-OH- waters with evidence of mixing with Mg2+-HCO3- waters. This study demonstrates the power of spatially resolving groundwaters to probe their distinct geochemical conditions and chemosynthetic communities. Such information will help improve predictions of where microbial activity in fractured rock ecosystems might occur, including beyond Earth.

The Oman Drilling Project “Multi-Borehole Observatory” (MBO) samples an area of active weathering of tectonically exposed peridotite. This paper reviews the geology of the MBO region, summarizes recent research, and provides new data constraining ongoing alteration. Host rocks are partially to completely serpentinized, residual mantle harzburgites and replacive. Dunites show evidence for “reactive fractionation”, in which cooling, crystallizing magmas reacted with older residues of melting. Harzburgites and dunites are 65-100% hydrated. Ferric to total iron ratios vary from 50 to 90%. In Hole BA1B, alteration extent decreases with depth. Gradients in water and core composition are correlated. Serpentine veins are intergrown with, and cut, carbonate veins with measurable 14C. Ongoing hydration is accompanied by SiO2 addition. Sulfur enrichment in Hole BA1B may result from oxidative leaching of sulfur from the upper 30 m, coupled with sulfate reduction and sulfide precipitation at 30-150 m. Oxygen fugacity deep in Holes BA3A, NSHQ14 and BA2A is fixed by the reaction 2H2O = 2H2 + O2 combined with oxidation of ferrous iron in serpentine, brucite and olivine. fO2 deep in Holes BA1A, BA1D and BA4A is 3-4 log units above the H2O-H2 limit, controlled by equilibria involving serpentine and brucite. Variations in alteration are correlated with texture, with reduced, low SiO2 assemblages in mesh cores recording very low water/rock ratios, juxtaposed with adjacent veins recording much higher ratios. The proportion of reduced mesh cores vs oxidized veins increases with depth, and the difference in fO2 recorded in cores and veins decreases with depth.

Serpentinized peridotite is actively reacting with groundwater in the subsurface in the Samail ophiolite in Oman. Although these rocks are partially to completely serpentinized, they are steeped in a groundwater aquifer containing hyperalkaline fluids rich in H production that can continue at low temperature, even after primary olivine and pyroxene are exhausted, thereby sustaining habitable conditions for microbial life.

In high-pH ($\text{pH}>10$) fluids that have participated in low-temperature ($<150\,^{\circ}\text{C}$) serpentinization, the dominant form of C is often methane (CH$_{4}$), but the origin of this CH$_{4}$ is uncertain. To assess CH$_{4}$ origin during low-temperature serpentinization, we pumped fluids from aquifers within the Samail Ophiolite, Oman. We determined fluid chemical compositions, analyzed taxonomic profiles of fluid-hosted microbial communities, and measured isotopic compositions of hydrocarbon gases. We found that 16S rRNA gene sequences affiliated with methanogens were widespread in the aquifer. We measured clumped isotopologue ($^{13}$CH$_{3}$D and $^{12}$CH$_{2}$D$_{2}$) relative abundances less than equilibrium, consistent with substantial microbial CH$_{4}$ production. Further, we observed an inverse relationship between dissolved inorganic C concentrations and $\delta^{13}\text{C}_{\text{CH}_{4}}$ across fluids bearing microbiological evidence of methanogenic activity, suggesting that the apparent C isotope effect of microbial methanogenesis is modulated by C availability. A second source of CH$_{4}$ is evidenced by the presence of CH$_{4}$-bearing fluid inclusions in the Samail Ophiolite and our measurement of high $\delta^{13}\text{C}$ values of ethane and propane, which are similar to those reported in studies of CH$_{4}$-rich inclusions in rocks from the oceanic lithosphere. In addition, we observed 16S rRNA gene sequences affiliated with aerobic methanotrophs and, in lower abundance, anaerobic methanotrophs, indicating that microbial consumption of CH$_{4}$ in the ophiolite may further enrich CH$_{4}$ in $^{13}$C. We conclude that substantial microbial CH$_{4}$ is produced under varying degrees of C limitation and mixes with abiotic CH$_{4}$ released from fluid inclusions. This study lends insight into the functioning of microbial ecosystems supported by water/rock reactions.