Sublithospheric diamond constraints on the state of deeply subducted slabs.

Abstract

Deep slab subduction is vital to Earth's geochemical cycles, but relevant geodynamic models lack natural mineralogical constraints. We report lower mantle inclusion assemblages in diamond carrying the geochemical signature of subducted slab mantle. Retrogressed stishovite implies its entrapment at temperatures at least 500°C cooler than ambient lower mantle. Three coexisting tuite inclusions with radiogenic strontium isotopic compositions and enriched trace element patterns suggest phosphorus-rich, slab crust-derived carbonatitic melts metasomatized the mantle of a buckled and overturned slab. Survival of carbonatitic crustal sources to lower mantle depths supports the cold slab temperatures implied by the stishovite. Cold slab temperatures produce a bimineralic stishovite-ferropericlase slab, a wholly new mantle lithology with elevated density and seismic velocity that is distinctly higher than slabs dominated by olivine high-pressure polymorphs. This new deep slab mineralogy likely controls the penetration of cold slabs into the lower mantle.