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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/VSdue to the occurrence of cracks. However, the anomalously low VP/VSobserved at the layer 2/3 transition cannot be explained by the crack model, because the effective medium theory predicts an increase in VP/VSdue to crack development. In this study, we present the first evidence that shows the low VP/VSin 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. The low velocity and high VP/VSstructure in the upper oceanic crust (“seismic layer 2”) is generally associated with the development of fluid‐filled thin cracks. However, the anomalously low VP/VSratios 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/VSratios 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/VSratios 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. Epidosites are characterized by low VP/VSdue to the presence of spheroidal poresLow VP/VSanomaly 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 Epidosites are characterized by low VP/VSdue to the presence of spheroidal pores Low VP/VSanomaly in the layer 2/3 transition can be explained by alterations and pore development during epidotization The unique characteristics of epidosites may be useful to detect the fluid upwelling zones in the oceanic crust

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 CO2to 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. 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 CO2from subducting sediments into the leading edge of the mantle wedge, and surprisingly low temperature ductile deformation at less than 200°C. Recycling of CO2via 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. The area at and around OmanDP Hole BT1B preserves nearly flat‐lying tectonic “stratigraphy” of the basal thrust of the Samail ophioliteRecycling of CO2from 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 The area at and around OmanDP Hole BT1B preserves nearly flat‐lying tectonic “stratigraphy” of the basal thrust of the Samail ophiolite Recycling of CO2from subducting sediment into the mantle wedge produced listvenites (carbonated peridotite) at <200°C Subduction beneath the ophiolite was accommodated, in part, by ductile deformation of serpentinite and listvenite at <200°C