Geodetic Estimate of Water in the Wharton Basin Upper Mantle

Abstract:

The formation of oceans at the Earth’s surface and hence the origin of life can be directly linked to the fate of water after planetary formation, but how much water is now confined in the upper mantle remains elusive. Current estimates of water in olivine from geochemistry are between 200 and 3600 H/10⁶ Si. Here, we exploit the water-sensitive rheology of olivine to estimate the water content in the Wharton Basin asthenosphere in the Indian Ocean. We explore the role of water stratification in the upper mantle and the transient behavior of olivine flow in the context of postseismic deformation following the 2012 Mw 8.6 Wharton Basin earthquake using geodetic data from the Sumatra GPS network. We build a model that incorporates afterslip in the brittle upper mantle and viscous flow in the asthenosphere. We introduce a formulation of the transient rheology of olivine in the form of a flow law coupled to a state evolution that characterizes the internal stress of the mineral before steady-state. We find that the Wharton Basin asthenosphere contains about 1000 H/10⁶ Si, representing 10% of water saturation of olivine at 100 km depth. If these results can be extrapolated to other depths, this indicates an equivalent of about 0.7 ocean mass is now present in the Earth’s upper mantle.