Listvenite Formation During Mass Transfer into the Leading Edge of the Mantle Wedge: Initial Results from Oman Drilling Project Hole BT1B

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 degrees 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 degrees-500 degrees. 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.