Carbonate melts in the hydrous upper mantle

Abstract

Carbonatite compositions resulting from melting of magnesian calcite + olivine + clinopyroxene were experimentally determined in the system CaO–MgO–SiO₂–CO₂–H₂O as a function of temperature and bulk H₂O contents at 1.0 and 1.5 GPa. The melting reaction and melt compositions were found to be highly sensitive to H-loss or -gain during experiments. We hence designed a new hydrogen-trap technique, which provided sufficient control to obtain consistent results. The nominally dry solidus temperatures at 1.0 and 1.5 GPa are 1225–1250 °C and 1275–1300 °C, respectively. At 1.0 GPa, the solidus temperature decreases with H₂O increasing to 3.5 wt% (1025–1050 °C), then remains approximately constant at higher H₂O concentrations. Our nominally dry solidus temperatures are up to 140 °C higher than in previous studies that did not take measures to limit hydrogen infiltration and hence suffered from H₂O formation in the capsule. The near-solidus anhydrous melts have 7–8 wt% SiO₂ and molar Ca/(Ca + Mg) of 0.78–0.82 (X_Ca). Melting temperatures decrease by as much as 200 °C with increasing ${\text{X}}_{{{\text{H}}_{{2}} {\text{O}}}}$ in the coexisting COH-fluid. Concomitantly, near-solidus melt compositions change with increasing bulk H₂O from siliceous Ca-rich carbonate melts to Mg-rich silico-carbonatites with up to 27.8 wt% SiO₂ and 0.55 X_Ca. The continuous compositional array of Ca–Mg–Si carbonatites demonstrates the efficient suppression of liquid immiscibility in the alkali-free system. Diopside crystallization was found to be sensitive to temperature and bulk water contents, limiting metasomatic transformation of carbonated upper mantle to wehrlite at 1.0–1.5 GPa to < 1175 °C and < 7 wt% bulk H₂O.