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This paper provides an overview of research on core from Oman Drilling Project Hole BT1B and the surrounding area, plus new data and calculations, constraining processes in the Tethyan subduction zone beneath the Samail ophiolite. The area is underlain by gently dipping, broadly folded layers of allochthonous Hawasina pelagic sediments, the metamorphic sole of the Samail ophiolite, and Banded Unit peridotites at the base of the Samail mantle section. Despite reactivation of some faults during uplift of the Jebel Akdar and Saih Hatat domes, the area preserves the tectonic "stratigraphy" of the Cretaceous subduction zone. Gently dipping listvenite bands, parallel to peridotite banding and to contacts between the peridotite and the metamorphic sole, replace peridotite at and near the basal thrust. Listvenites formed at less than 200°C and (poorly constrained) depths of 25–40 km by reaction with CO2‐rich, aqueous fluids migrating from greater depths, derived from devolatilization of subducting sediments analogous to clastic sediments in the Hawasina Formation, at 400°–500°. Such processes could form important reservoirs for subducted CO2. Listvenite formation was accompanied by ductile deformation of serpentinites and listvenites—perhaps facilitated by fluid‐rock reaction—in a process that could lead to aseismic subduction in some regions. Addition of H2O and CO2 to the mantle wedge, forming serpentinites and listvenites, caused large increases in the solid mass and volume of the rocks. This may have been accommodated by fractures formed as a result of volume changes, mainly at a serpentinization front. Plain Language Summary: This paper reports initial results from study of core from Oman Drilling Project Hole BT1B and the surrounding area. It provides insights into subduction zone processes, including large fluxes of recycled CO2 from subducting sediments into the leading edge of the mantle wedge, and surprisingly low temperature ductile deformation at less than 200°C. Recycling of CO2 via carbon mineralization in the hanging wall of subduction zones may produce an important, lithospheric reservoir in the global carbon cycle. Ductile deformation of serpentinite, and during or after transformation of peridotite to listvenites (mixtures of carbonates and opal or quartz) could explain aseismic subduction atop some subduction zones. Key Points: The area at and around OmanDP Hole BT1B preserves nearly flat‐lying tectonic "stratigraphy" of the basal thrust of the Samail ophioliteRecycling of CO2 from subducting sediment into the mantle wedge produced listvenites (carbonated peridotite) at <200°CSubduction beneath the ophiolite was accommodated, in part, by ductile deformation of serpentinite and listvenite at <200°C [ABSTRACT FROM AUTHOR]

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 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 versus oxidized veins increases with depth, and the difference in fO2 recorded in cores and veins decreases with depth. Plain Language Summary: The Oman Drilling Project developed a "Multi‐Borehole Observatory" (MBO) in an area of active weathering of tectonically exposed peridotite, to study the geochemistry, mechanics, and hydrology of peridotite alteration, which modifies the mineralogy, composition, density, and rheology of mantle lithologies, creates and sustains plate boundaries, and forms dramatic redox gradients. In turn, these redox conditions support a unique subsurface microbial ecosystem, form free H2 gas, facilitate methane generation, and potentially play a role in the origin of life on this and other planets. This paper provides an overview of the geology of the area within and surrounding the MBO, a summary of recent research on core and fluids from three new cored boreholes and four new rotary boreholes in the MBO, together with older Omani water monitoring well NSHQ14, and new data and calculations constraining ongoing peridotite alteration in this area. We constrain the igneous history of the mantle rocks, the extent to which they have been hydrated, carbonated, and oxidized. Highlights include new hypotheses on mechanisms of shallow sulfur depletion and deeper enrichment, and new insights into the interaction of water and minerals that controls the partial pressures of oxygen and hydrogen in this important geochemical environment. Key Points: Ongoing alteration decreases in intensity with increasing depth below the present erosional surfaceSulfur is accumulating in a supergene enrichment zone from 30 to 150 m below the present erosional surfaceDeep oxygen fugacity is fixed by H2 gas saturation = plus ongoing oxidation of Fe2+ in three holes and serpentine‐brucite equilibria in four holes [ABSTRACT FROM AUTHOR]

Mantle peridotite in Wadi Fins in eastern Oman exhibits three concentric alteration zones with oxide and sulfide mineralogy recording gradients in fO2 and fS2 (fugacity) of more than 20 orders of magnitude over 15-20 cm. The black cores of samples (approx. 5 cm in diameter) exhibit incomplete, nearly isochemical serpentinization, with relict primary mantle minerals (olivine, orthopyroxene and clinopyroxene) along with sulfide assemblages (pentlandite/heazlewoodite/bornite) recording low fO2 and moderate fS2. In addition to the black cores, two alteration zones are evident from their colouration in outcrop and hand samples: green and red. These zones exhibit non-isochemical alteration characterized by intergrowths of stevensite/lizardite. All three reaction zones are cut by calcite±serpentine veins, which are most abundant in the outer, red zones, sometimes are flanked by narrow red and/or green zones where they cut the black zones, and thus may be approximately coeval with all three alteration zones. The alteration zones record progressively higher fO2 recorded by Ni-rich sulfides and iron oxides/hydroxides. These alteration zones lost 20- 30% of their initial magnesium content, together with mobilization of iron over short distances from inner green zones into outer red zones, where iron is reprecipitated in goethite intermixed with silicates due to higher fO2. Thermodynamic modelling at 60°C and 50MPa (estimated alteration conditions) reproduces sulfide assemblages, fO2 changes and Mg and Fe mobility. [ABSTRACT FROM AUTHOR]