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

Carbonate droplets containing mafic silicate glass + or - CO (sub 2) occur within mineral inclusions and late microveins in spinel-bearing xenoliths (dunite, dunite-wehrlite and pyroxenite) from La Gomera. Primary carbonates are Mg-calcite (X (sub Ca) 0.89-0.93), dolomite (X (sub Ca) 0.46-0.54) with low Na (sub 2) O (< or = 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 (sub 2) (40-55 wt%) with low Al (sub 2) O (sub 3) , TiO (sub 2) , CaO and alkalis and a high content (> 10 wt.%) of volatiles (i.e. H (sub 2) O). The composite carbonate droplets represent a quenched liquid, resulting from unmixing within the inclusions of a carbonate-rich melt into separate carbonate- and silicate-rich phases. If not protected within inclusions, these melts are ephemeral, unstable in the P-T field of spinel peridotites (10-18 kbar, 900-1000 degrees C). Mafic glass remnants in microveins represent a residual degassed hydrous mafic silicate fraction after decarbonation.