The composition of melt inclusions in minerals at the garnet–spinel transition zone

High-pressure melts are preserved as inclusions in automorphous spinel crystals from reaction coronas surrounding pyrope garnets in lherzolites from Lashaine volcano, northern Tanzania. The inclusions are primary; i.e. they contain liquids that have formed in thermodynamic equilibrium with their host spinels and the trapping pressures of the melts correspond to those of the host spinel growths. Calibrations of the spinel–garnet phase equilibrium transition suggest that the spinel containing the melt inclusions formed at pressures around 2.5 GPa, and mass balance calculations indicate that the inclusions contain melts formed by low-degree of melting (F≈0.04) during garnet breakdown. Despite the possibility that full equilibration was not attained with all the mineral phases found in the nodules, the melt inclusions, after homogenization by high-T experiments, display compositions remarkably consistent with experimental melts obtained at pressures around 2.5 GPa on natural and model peridotite systems. They are characterized by lower SiO2 (47.5 wt% on average) and higher MgO and FeO contents, at a given Na2O+K2O content, than low-degree melt inclusions formed around 1.0 GPa and preserved in spinel-peridotite minerals. This provides direct evidence for the hypothesis that for low-degree melts, the decrease in SiO2 with pressure is superimposed on a decrease of the effect of alkali oxides on the silica activity coefficient, thereby generating melts with a composition more akin to erupted alkali basalt than at shallow depths.