Phase relations in carbonate component of carbonatized eclogite and peridotite along subduction and continental geotherms

Abstract

Phase relations in the carbonate constituent of mafic and ultramafic suites are decisive in the deep carbon cycle and partial melting in the Earth's mantle. Here we present an experimental study of phase relations in the “Na‑carbonatite” and “K‑carbonatite” systems, which represent the carbonate constituent of mantle eclogites and peridotites, respectively.The phase relations in the “Na‑carbonatite” system indicate that the carbonate constituent of the mafic suite consists of Na2Ca4(CO3)5 + dolomite + eitelite, Na2Mg(CO3)2, at shallow mantle pressures and changes to Na2Ca4(CO3)5 + magnesite at depths exceeding 170 km. At the same time, the “K‑carbonatite” system indicates that the carbonate constituent of the ultramafic suite consists of K2Mg(CO3)2 + dolomite + magnesite at shallow mantle pressures and changes to K2Mg(CO3)2 + magnesite + aragonite at depths exceeding 170–180 km.The “Na‑carbonatite” solidus is located near 850 °C / 3 GPa and 1100 °C / 6.5 GPa and has a Clapeyron slope of 14 MPa/°C. The “K‑carbonatite” solidus is situated near 825 °C / 3 GPa and 1000 °C / 6.5 GPa and has a Clapeyron slope of 20 MPa/°C. Both solidi are hotter than subduction geotherms. Consequently, under anhydrous conditions, the carbonate constituent of mafic and ultramafic suites can survive subduction into the deep mantle without melting. However, warming even to moderate temperatures corresponding to continental geotherms should be accompanied by partial melting yielding alkaline carbonatite melt.