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Geophysical surveys of the oceanic crust indicate that hydrothermal circulation universally occurs in seismic layer 2, which results in a low seismic velocity and high VP/VS due to the occurrence of cracks. However, the anomalously low VP/VS observed at the layer 2/3 transition cannot be explained by the crack model, because the effective medium theory predicts an increase in VP/VS due to crack development. In this study, we present the first evidence that shows the low VP/VS in the oceanic crust is caused by epidotization due to upward fluid flow in hydrothermal systems. Simultaneous measurements of elastic wave velocity and porosity of epidosites collected by the Oman Drilling Project show that quartz precipitation and spheroidal pores results in low VP/VS, in contrast to diabases that contain thin cracks. The presence of spheroidal pores in epidosites is supported by CT imaging, and is consistent with predictions from the effective medium model. Plain Language Summary: The low velocity and high VP/VS structure in the upper oceanic crust ("seismic layer 2") is generally associated with the development of fluid‐filled thin cracks. However, the anomalously low VP/VS ratios that have been observed in the layer 2/3 transition cannot be explained by the classical thin crack model. We provide the first direct evidence of low VP/VS ratios in epidotized rocks that are associated with the occurrence of spheroidal pores and quartz precipitation. Our theoretical calculations show that both the crack geometry and alteration minerals are required to explain the anomalously low VP/VS ratios in the transition zone. Knowledge of these unique physical properties of epidotized rocks will facilitate the detection of fluid upwelling zones in the oceanic crust, which are possibly linked to the hydrothermal ore resources. Key Points: Epidosites are characterized by low VP/VS due to the presence of spheroidal poresLow VP/VS anomaly in the layer 2/3 transition can be explained by alterations and pore development during epidotizationThe unique characteristics of epidosites may be useful to detect the fluid upwelling zones in the oceanic crust [ABSTRACT FROM AUTHOR]

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]

We quantified mineral proportions in listvenite (completely carbonated peridotite) in Hole BT1B drilled across the basal thrust of the Samail ophiolite by the International Continental Scientific Drilling Project Oman Drilling Project using 3D X‐ray Computed Tomography (XCT). We analyzed >250,000 XCT images from a continuous ∼200 m listvenite core. Histograms of the intensity of X‐ray attenuation of each XCT core‐slice image were fitted assuming that the listvenites are composed of magnesite, quartz, and dolomite. The XCT mineral peaks were confirmed by comparison with chemical mapping data obtained using an X‐ray fluorescence (XRF) core scanner. Listvenite matrix is composed almost entirely of magnesite and quartz, consistent with discrete XRD and XRF data. Veins are composed mostly of dolomite. The mean abundance of dolomite in listvenite from BT1B is 11 vol.%, whereas that in core sections within 15 m of the basal thrust is >50 vol.%, suggesting the basal thrust acted as a source of Ca‐ and CO2‐rich fluids. The SiO2:MgO:CaO molar ratio in the entire core from BT1B is 42:52:6, similar to that of onboard XRF data for discrete samples (41:54:5), whereas average Oman peridotites have ratios of (39:60:1), indicating Ca addition perhaps during carbonation. P‐ and S‐wave velocities and density of listvenite are close to those of peridotite and are higher than those of serpentinites. These results suggest that limited material transfer during carbonation and hydration of the Samail ophiolite, except for Ca, CO2, and H2O. Listvenites formed in the mantle wedge above subduction zones may be an overlooked reservoir for carbon in the Earth's interior. Plain Language Summary: To understand planetary‐scale fluid circulation from the surface to deep regions of Earth, it is important to constrain how fluids are captured/released by hydration/dehydration and carbonation/decarbonation of minerals and the associated volumes of these fluids. Listvenite, completely carbonated mantle material, occurs in the Samail ophiolite. Continuous ∼200 m drill core samples of listvenite overlying the basal thrust of the Samail ophiolite were obtained by the International Continental Scientific Drilling Project Oman Drilling Project. We evaluated the mineral fractions of listvenite from 3D X‐ray Computed Tomography (XCT) images of these core samples with confirmation using chemical mapping data from an X‐ray fluorescence core scanner. More than 125,000 XCT images were analyzed using this method. We found that dolomite (CaMg(CO3)2) abundance is highest within 15 m of the base of listvenite layer, which is the paleo‐plate boundary. These results suggest that the mantle carbonation involved CO2‐, H2O‐, and Ca‐rich fluids and that the paleo‐plate boundary acted as a pathway for these fluids. Because listvenite contains 36 mol% CO2 and has seismic properties different from serpentinites and similar to volatile‐free peridotites, listvenite in the mantle wedge overlying subduction zone may be an overlooked reservoir for carbon in the Earth's interior. Key Points: A method was developed to determine the downhole mineral fractions of drill cores from continuous X‐ray Computed Tomography (XCT) imagesQuartz‐magnesite‐dolomite fractions were estimated from XCT images of listvenite cores from International Continental Scientific Drilling Project Oman Drilling Project Hole BT1BResults indicate limited material transfer during carbonation and hydration of the Samail ophiolite, except for Ca, CO2, and H2O [ABSTRACT FROM AUTHOR]

Here we present some preliminary results constraining peridotite alteration in cores from several Oman Drilling Project (OmanDP) boreholes, from the Samail ophiolite in the Sultanate of Oman. Hole BT1B in Wadi Mansah east of Fanjah was drilled through 200 m of fully carbonated and/or serpentinized peridotites of the mantle wedge, through the basal thrust of the ophiolite, into the metamorphic sole. Holes BA1B, BA3A and BA4A were drilled into extensively serpentinized peridotite of the eastern Wadi Tayin massif. Fully carbonated peridotites (listvenites) and serpentinites of Hole BT1B reflect carbonation and hydration by fluids rising from the Cretaceous subduction zone at the base of the ophiolite, at about 100°C (Falk & Kelemen GCA 2015). Though there is extensive, 0.01-10 m scale chemical variation, the average, volatile-free composition of the listvenites is almost identical to that of partially serpentinized harzburgites throughout the ophiolite mantle section. This observation and thermodynamic modeling indicate that, during addition of CO2, H2O, Ca and K from subduction zone fluids, Mg/Fe/Si ratios were unchanged, suggesting very minor export of dissolved, major elements. In turn, this requires a large increase in solid volume. One possibility is that major volume expansion occurred at a serpentinization front, perhaps advancing via reaction-driven cracking, followed by smaller amounts of expansion at the carbonation front. Observed mineral parageneses and thermodynamic modeling require relatively CO2-rich aqueous fluids, and perhaps an exsolved, CO2-dominated fluid. CO2 contents were higher than for fluids in equilibrium with underlying metasediments at any likely depth, and thus suggest that fluids derived from higher temperature and pressure devolatilization migrated up the subduction zone to this site.Highly serpentinized peridotites from Holes BA1B, BA3A and BA4A record steep chemical gradients, with sharply decreasing f(O2) and f(CO2) downhole, beginning at the present-day erosional surface. Moreover, inferred conditions based on mineral parageneses are consistent with borehole water compositions. Oxygen fugacity reaches the limit for H2 production from H2O at < 100 meters below the surface. These borehole observations are consistent with previous observations (14C in carbonates, reduced sulfides, oxides and metals, highly alkaline and reduced fluids, H2 gas) and thermodynamic modeling indicating that carbonation and serpentinization-related redox reactions are ongoing. Given experimental data suggesting that serpentinization of olivine is slow, evidence for low temperature "serpentinization" raises interesting questions about natural rates, and the nature of reactants and products, in the present-day weathering environment. [ABSTRACT FROM AUTHOR]