Partially molten asthenosphere beneath Anatolia and in high temperature regions globally

Abstract:

Beneath Anatolia, Sp receiver functions show clear positive velocity gradient phases at 100-150 km depth, as well as a negative velocity gradient consistent with a lithosphere-asthenosphere boundary (LAB) at depths around 70 km. The depth range of the positive velocity gradient phases agrees with the depth where the geotherm estimated from shear-velocity structure exceeds the solidus temperature, indicating the phase is consistent with the onset of hydrous melting. To better constrain mantle potential temperature (Tp) and mantle water content, the Sp-defined melting onset depths were jointly interpreted with primary magma equilibration P-T conditions from 116 volcanic rock samples in the Karacadag volcanic field. Mantle adiabats and solidii for different Tp and water content conditions were tested, and the combination that satisfies the incipient melting depth and agrees best with the petrologically defined P-T conditions was selected. With the preferred mantle conditions, the distribution of equilibration depths for primary magmas follows the modelled degree of melting, validating the assumption that melt equilibration P-T conditions directly represent the geotherm. Tp values across the region were calculated from the Sp-defined melting depth at a given location, assuming the preferred water content (110 wt ppm) from the joint modelling with Karacadag samples. Resulting Tp values lie between 1350-1450°C, reaching temperatures higher than ambient upper mantle. To further analyze the prevalence of hydrous melting in the upper mantle, Sp receiver functions from 430 globally distributed stations were analyzed. The resulting station stacks cluster into four groups, including one with strong positive velocity gradients at 100-150 km depths that are consistent with the onset of upper mantle hydrous melting discovered in Anatolia, and two typical of cratons. Stations with hydrous melting phases are typically correlated with shear velocities of less than 4.35 km/s at 150 km depth. More than 30% of the earth has comparable low velocities, suggesting widespread partially molten upper mantle.