Experimental Study of the Stability of a Dolomite + Coesite Assemblage in Contact With Peridotite: Implications for Sediment-Mantle Interaction and Diamond Formation During Subduction

Abstract

Carbonates carried by oceanic platescin the form of sediment or alteration products of basaltscare introduced into the mantle by subduction.The high-pressure^high-temperature stability of carbonates in a closed system has been constrained by a number of studies, but the effects of the interactions between subducted carbonates and the surrounding mantle on carbonate stability are poorly known. These interactions may, however, influence the stability depth of the subducted material, the composition of the interaction zone, and the rate of carbon transfer from the slab to the mantle. To determine the exchange mechanisms at the interface between a subducted dolomiteþ coesite assemblage and the overlying mantle, we performed experiments at 6 GPa and 9008C and 11008C in a multi-anvil apparatus. In a first series of experiments, we studied the reaction and migration processes operating along the interface between a cylinder of dolomiteþ coesite and a cylinder of garnet peridotite. In a second series of experiments, homogeneous mixtures of dolomiteþ coesite and garnet peridotite were equilibrated at high pressure and high temperature to characterize the phase relations as a function of the (dolomiteþ coesite)/peridotite ratio.We show that the destabilization temperature of a subducted dolomiteþ coesite assemblage is less than 9008C at 6 GPa when in contact with a garnet lherzolite or a harzburgite. A reaction zone composed mainly of clinopyroxene þmagnesite is produced at the interface between dolomiteþ coesite and peridotite. Carboncin the form of a fluid or in a carbonatitic meltcalso infiltrates the garnet peridotite to form magnesite and clinopyroxene. Moreover, graphite was observed in two experiments. We believe that it was produced by a redox reaction at the interface between dolomiteþ coesite and iron-bearing silicates in the peridotite. As our experimental conditions are in the stability field of diamond, this suggests a potential mechanism for diamond crystallization from subducted dolomiteþ coesite in contact with the mantle wedge.