Seminar Michael Walter

Abstract

A primary role of subducted lithospheric plates in diamond and inclusion formation, both oceanic crust and lithospheric slab mantle, is recorded by diamonds and their included minerals. Subducting lithosphere stagnates in the transition zone and shallow lower mantle, and fluids and melts released from slab lithologies upon heating may infiltrate and react with their surroundings,  precipitating diamonds and their inclusions. Phase  relations of carbonated oceanic crust suggest melting in the deep upper mantle and transition zone at depths like those derived from barometry of Ca-rich, Cr-poor ‘eclogitic’ majorite garnet and
Ti-rich Ca-perovskite. Similarly, subducted, depleted slab mantle will dehydrate at deep transition zone and shallow lower mantle depths. Reaction of hydrous melts with surrounding depleted lithosphere can plausibly account for the compositions of low-Al MgSiO3 and low-Ti CaSiO3 inclusions, and for the wide range in Mg# of  ferropericlase inclusions and the positive correlation between Mg# and Ni content. The narrative relating subducted oceanic plates stagnating in the transition zone and shallow lower mantle to the origin of sublithospheric diamonds recalls the megalith model of Ringwood. Many unanswered questions remain to further develop our understanding of these unique samples from the deep mantle, and what these samples reveal about deep mantle volatile element recycling processes.