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Alteration of mantle peridotite in the Samail ophiolite forms secondary minerals, mainly serpentine and Mg‐rich carbonates. Magnesium accounts for ∼25 – 30% of peridotite mass and its mobility can be used to trace this alteration. We report the first set of Mg isotope measurements from peridotites and their alteration products in Oman. Partially serpentinized peridotites have Mg isotope ratios that are indistinguishable from estimates for the average mantle and bulk silicate earth (δ²⁶Mg = −0.25 ± 0.04‰). However, more extensively altered peridotite samples show large shifts in Mg isotopic composition. The range ofδ²⁶Mg values for our suite of alteration products from the mantle section is ∼4.5‰ (from −3.39‰ to 1.19‰), or >60% of the total range of terrestrial variability inδ²⁶Mg values. Serpentine veins are typically enriched in²⁶Mg (maxδ²⁶Mg value = 0.96‰) whereas Mg‐carbonate veins are associated with low²⁶Mg/²⁴Mg ratios (magnesiteδ²⁶Mg = −3.3‰, dolomiteδ²⁶Mg = −1.91‰). Our preferred explanation for the range inδ²⁶Mg values involves coprecipitation of serpentine and carbonates at water‐to‐rock ratios >10³. The coincidence of alteration products characterized byδ²⁶Mg values that are both lower and higher than bulk silicate Earth and the finite¹⁴C ages of the carbonates suggest that both serpentinization and carbonation are ongoing in Oman. Rates of calcite precipitation in travertines inferred from Δ²⁶Mg_cal‐flsuggest that travertine formation in Oman sequesters a total of 10⁶–10⁷ kg CO₂/yr, consistent with previous estimates.

In hyperalkaline () fluids that have participated in low‐temperature (<150) serpentinization reactions, the dominant form of C is often methane (), but the origin of thisis uncertain. To assessorigin in serpentinite 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 (D and) relative abundances less than equilibrium, consistent with substantial microbialproduction. Furthermore, we observed an inverse relationship between dissolved inorganic C concentrations andacross fluids bearing microbiological evidence of methanogenic activity, suggesting that the apparent C isotope effect of microbial methanogenesis is modulated by C availability. An additional source ofis evidenced by the presence of‐bearing fluid inclusions in the Samail Ophiolite and our measurement of highvalues of ethane and propane, which are similar to those reported in studies of‐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 ofin the ophiolite may further enrichin¹³C. We conclude that substantial microbialis produced under varying degrees of C limitation and mixes with abioticreleased from fluid inclusions. This study lends insight into the functioning of microbial ecosystems supported by water/rock reactions.

The Oman Drilling Project “Multi‐Borehole Observatory” (MBO) samples an area of active weathering of tectonically exposed peridotite. This article 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 measurable¹⁴C. Ongoing hydration is accompanied by SiO₂addition. 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 + O₂combined with oxidation of ferrous iron in serpentine, brucite, and olivine. fO₂deep in Holes BA1A, BA1D, and BA4A is 3–4 log units above the H₂O‐H₂limit, controlled by equilibria involving serpentine and brucite. Variations in alteration are correlated with texture, with reduced, low SiO₂assemblages in mesh cores recording very low water/rock ratios, juxtaposed with adjacent veins recording much higher ratios. The proportion of reduced mesh cores versus oxidized veins increases with depth, and the difference in fO₂recorded in cores and veins decreases with depth.