Reconstruction of melt compositions recorded by orogenic peridotite dervied from convecting part of the mantle wedge

Abstract

Mantle wedge peridotite at South Bohemia of the Bohemian massif (Czech Republic) experienced transformations from earlier garnet peridotite to chlorite peridotite in the early exhumation, followed by subducted to high-pressure garnet peridotite condition at ca. 3 GPa and 850-1050ºC. We report on hydrous minerals and multiphase solid inclusions (MSI) in earlier stage UHP chromite to unravel the nature of melt in the mantle wedge. The UHP chromite includes MSI consisting of 40 % phlogopite, 47 % carbonates, and 3 % apatite. The reconstructed composition is ultrapotassic dolomitic carbonatite. The trace element pattern for the dolomitic carbonatite is enriched in Ba, Sr, LREE, and depleted in U, Th, Pb, HFSE and HREE, and it is quite similar to the carbonatite melt in equilibrium with pyrolite at the asthenosphere depth. Such alkali-rich dolomitic carbonatite in the mantle wedge was sourced either from deeply subducted oceanic crust or asthenospheric mantle.