Melting relations of model lherzolite in the system CaO‐MgO‐Al2O3‐SiO2 at 2.4–3.4 GPa and the generation of komatiites

Abstract

Isobarically invariant phase relations in the CaO‐MgO‐Al2O3‐SiO2 system (CMAS) involving the lherzolite phase assemblage in equilibrium with liquid have been determined at 2.4–3.4 GPa. These phase relations form the solidus of model lherzolite in the CMAS system. Our data, which include determinations of all phase compositions, are in excellent agreement with the 3.0 and 4.0 GPa points of Milholland and Presnall [1991] and Davis and Schairer [1965], respectively. The invariant transition on the P‐T solidus curve from spinel‐ to garnet‐lherzolite at 3.0 GPa, 1575°C [Milholland and Presnall, 1991], is confirmed, but we observe that the theoretically required temperature depression on the solidus curve at this point is not experimentally detectable. Composition trends along the solidus take a sharp turn at the transition. In the spinel‐lherzolite stability field, melt compositions become increasingly Fo‐normative and less En‐normative with increasing pressure, but become less Fo‐normative and more pyroxenitic as pressure increases in the garnet‐lherzolite stability field. Calculated melting reactions indicate that forsterite is in reaction relationship with the melt up to 3.0 GPa. Orthopyroxene is also in reaction relationship at pressures higher than just over 2.8 GPa and is the only phase in reaction relationship with the melt in the garnet‐lherzolite stability field. Comparison of the normative compositions and the CaO/Al2O3 values of the komatiites of Gorgona Island and of the Reliance Formation in Zimbabwe with the compositions of liquids along the solidus of model lherzolite in the CMAS system indicates that the former komatiites were generated at pressures close to 3.7 GPa and the latter at close to 4.5 GPa, assuming that the melt generation occurred in the presence of the complete garnet‐lherzolite assemblage.