Ephemeral carbonate melts in the upper mantle: carbonate-silicate immiscibility in microveins and inclusions within spinel peridotite xenoliths, La Gomera, Canary Islands

摘要

Carbonate droplets containing mafic silicate glass ±CO_2 occur within mineral inclusions and late microveins in spinelbearing ultramafic xenoliths (dunite, dunite-werhlite and pyroxenite) from La Gomera, Canary Islands. Primary carbonates are Mg-calcite (X_(Ca) 0.89-0.93) and dolomite (X_(Ca) 0.46-0.54), with low Na_2O(≤0.1 wt.%) and variable MnO (0.2-8 wt.%). The mafic glass has high MgO (24-35 wt.%), FeO (1-18 wt.%) and SiO_2 (40-55 wt.%), with low Al_2O_3, TiO_2 CaO and alkalis. Mafic glass contains high amounts (> 10 wt%) of volatiles (i.e., H_2O). the composite carbonate droplets represent a quenched liquid, resulting from unmixing within the inclusions of a carbonaterich melt into separate carbonate- and silicate-rich phases. Modelling of initial bulk compositions suggests dolomitic melts, with high silica (≈10 wt.%) and H_2O, but low alkali contents. If not protected within inclusions, these melts are ephemeral, unstable in the P-T field of spinel peridotites (10-18 kbar; 900-1000 ℃). Mafic glass remnants in microveins represent a sresidual degassed hydrous mafic silicate fraction after decarbonation.