Carbonate droplets containing mafic silicate glass ± CO2 occur within mineral inclusions and late microveins in spinel-bearing ultramafic xenoliths (dunite, dunite-werhlite and pyroxenite) from La Gomera, Canary Islands. Primary carbonates are Mg-calcite (XCa 0.89–0.93) and dolomite (XCa 0.46–0.54), with low Na2O (≤ 0.1 wt.%) and variable MnO (0.2–8 wt.%). The mafic glass has high MgO (24–35 wt.%), FeO (1–18 wt.%) and SiO2 (40–55 wt.%), with low Al2O3, TiO2, CaO and alkalis. Mafic glass contains high amounts (> 10 wt %) of volatiles (i.e., H2O).
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. Modelling of initial bulk compositions suggests dolomitic melts, with high silica (≈ 10 wt.%) and H2O, 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°C). Mafic glass remnants in microveins represent a residual degassed hydrous mafic silicate fraction after decarbonation.