Slab-derived melts interacting with peridotite: Toward the origin of diamond-forming melts

Abstract

It was recently shown that partial melting of carbonated metapelites, subducted to a depth of 200 km, yields the formation of two immiscible melts, CO2-bearing phonolitic and K-rich carbonate. These melts resemble silicic and low-Mg carbonatitic melt inclusions in diamonds from kimberlites and placers worldwide. Here we studied the interaction of these melts with natural garnet lherzolite at 6 GPa. We found that the CO2-bearing phonolite melt reacts with peridotite consuming olivine to produce orthopyroxene and garnet, while K2O and CO2 enter carbonate melt. The latter has Ca# 24–29 and appears in equilibrium with garnet lherzolite. The SiO2 content in the carbonate melt varies from 2 to 18 wt% as temperature increases from 1200 to 1500 °C. Our results imply that the slab-derived immiscible silicic and low-Mg carbonatitic melts react with peridotitic mantle producing the high-Mg carbonatitic melt, which makes up the majority of carbonatitic inclusions in diamonds. Thus, the melt entrapped by diamonds may decipher genetic signatures of different mantle lithologies: silicic and low-Mg carbonatitic inclusions correspond to eclogite or recycled pelite, while high-Mg carbonatitic inclusions correspond to peridotite.