Experimentally derived high-pressure cumulates from hydrous arc magmas and consequences for the seismic velocity structure of lower arc crust

This contribution explores the consequences of experimentally derived ultramafic plutonic rocks for estimates of continental crust composition. The formation of high-pressure cumulates from primitive to evolved hydrous arc magmas was investigated in the range 0.8 - 1.5 GPa and 1350 - 800 degrees C, to provide constraints on solid assemblages that form the roots of island arc crust. At pressures exceeding 0.8 GPa ( 25km), 40 - 60% of ultramafic cumulates are produced to obtain andesite to dacite compositions that are typical for evolved island- arc volcanic and plutonic rocks. Calculated cumulate densities are comparable or higher than upper mantle values, while seismic velocities ( V-p) vary continuously between 7.3 and 8 km/ s, resulting in a broad crust- mantle transition. The Moho represents a plagioclase saturation boundary. Our results are consistent with seismic imaging from continental margins and island arcs, revealing that beneath an active volcanic front a sharp geophysical Moho signature is sometimes absent.