DEEP GLOBAL CYCLING OF CARBON CONSTRAINED BY THE SOLIDUS OF ANHYDROUS, CARBONATED ECLOGITE UNDER UPPER MANTLE CONDITIONS

Abstract

We present partial melting experiments that constrain the near solidus phase relations of carbonated eclogite from 2 to 8.5 GPa. The starting material was prepared by adding 5 wt.% CO2 in the form of a mixture of Fe–Mg–Ca–Na–K carbonates to an eclogite from Salt Lake crater, Oahu, Hawaii and is a reasonable approximation of carbonated oceanic crust from which siliceous hydrous fluids have been extracted during subduction. Melt-present versus melt-absent conditions are distinguished based on textural criteria. Garnet and clinopyroxene appear in all the experiments. Between 2 and 3 GPa, the subsolidus assemblage also includes ilmenite±calcio-dolomitess±CO2, whereas above the solidus (1050–1075 °C at 3 GPa) calcio-dolomitic liquid appears. From 3 to 4.5 GPa, dolomitess is stable at the solidus and the near-solidus melt becomes increasingly dolomitic. The appearance of dolomite above 3 GPa is accompanied by a negative Clapeyron slope of the solidus, with a minimum located between 995 and 1025 °C at ca. 4 GPa. Above 4 GPa, the solidus rises with increasing pressure to 1245±35 °C at 8.5 GPa and magnesite becomes the subsolidus carbonate. Dolomitic melt coexists with magnesite+garnet+cpx+rutile along the solidus from 5 to 8.5 GPa.