Element partitioning between silicate minerals and coexisting melts at pressures of 1–27 GPa, and implications for mantle evolution

Abstract

Major element partitioning data relevant to melt generation and separation at various depths in the mantle have been derived from the compositions of coexisting silicate minerals and quenched melts in our melting experiments on komatiites and C1 chondrite at 4–11 GPa and 20–27 GPa respectively, and from published experimental studies covering the 1–26 GPa range. In these experiments there is a general positive correlation between pressure, temperature and the normative olivine content of the melts. The K D Si/Mg (mineral/melt) for olivine/β-spinel, pyroxenes, and garnet/majorite increases markedly with increasing depth, especially in the 30–180 km range, and exceeds unity for majorite and perovskite. The K D Fe/Mg remains well below unity throughout the entire depth (pressure and temperature) range for all of the minerals (0.3–0.5 for β-spinel, majorite and perovskite). The K D Ca/Al for garnet/majorite remains low (0.1–0.3), decreases with increasing depth for olivine/β-spinel (⩽ 1 at pressures exceeding 10 GPa), and remains near unity for the pyroxenes. The Nernst mineral/melt distribution coefficient for Cr ( D Cr) decreases with increasing depth for all minerals. Whereas garnet/majorite D Cr remains above 1, the values for the pyroxenes and olivine/β-spinel are nearly constant at 0.3–0.5 in the 10–20 GPa range. All the distribution coefficients derived for perovskite overlap those of majorite. These partitioning relationships indicate that the ratio Mg/(Mg + Fe) of upper mantle peridotites is a largely depth-independent measure of the degree of melt depletion. Because the bulk K D Ca/Al for assemblages containing garnet/majorite is below unity at depths exceeding 150 km, melt separation will lead to a decreased Ca/Al ratio in residues and correspondingly Al-depleted magmas. Al-undepleted komatiites must form by melt separation from harzburgitic or dunitic residues following extensive melting. The higher than chondritic Ca/Al ratio of sampled upper mantle peridotites may result from minor fractionation of majorite and perovskite in combination with the addition of some Al-depleted komatiitic magma to the uppermost mantle. The bulk residue D Cr decreases with increasing depth of melting, and garnet/majorite-bearing residues will have D Cr near unity at depths exceeding 150 km. Extensive melting and melt extraction from garnet/majorite-free residues at such depths will lead to a bulk D Cr of 0.3–0.5. The chemistry of variably depleted mantle peridotites points to a bulk D Cr of about 1 during the melt extraction, indicating that a major part of the time-integrated melt extraction occurred in the deeper parts of the upper mantle. Much of the melt depletion occurred in the form of early komatiitic magmatism but high-MgO primary magma extraction followed by shallow-level fractionation may have also been important in recent geological time.