Some phase equilibrium systematics of lherzolite melting: I
Authors: Longhi John 
Published: October 2002

New piston‐cylinder experiments constrain the compositions of a series of synthetic picritic liquids that are in equilibrium with forsteritic olivine, orthopyroxene, clinopyroxene, and garnet or spinel from 2.4 to 3.4 Gpa. Mass balance calculations show that two of the liquid + crystal assemblages are consistent with those expected by 4.4 and 1.6 wt % anhydrous partial melting of a peridotite generally similar in composition to estimates of depleted upper mantle (DPUM). The liquids in these runs contain ≤2.0 wt % Na2O. Lherzolitic liquids with higher concentrations of Na2O have negative mass balance coefficients, regardless of Mg′, implying that there is a limit of ∼2 wt % Na2O in anhydrous partial melts of peridotites with ∼0.3 wt % bulk Na2O in the upper garnet‐lherzolite stability field. Examination of liquidus equilibria in the NCMAS system demonstrates that coupling of Na2O and SiO2 concentrations in liquids saturated with lherzolite assemblages permits high‐Na2O, high‐SiO2 melts at pressures ∼1.0 GPa, whereas only high‐Na2O, low‐SiO2 melts are possible in the garnet‐lherzolite stability field. Because the bulk partition coefficient for Na2O increases with pressure, the concentration of Na2O in batch melts of the same percent will necessarily decrease with pressure. Calculations of low‐degree anhydrous melting of DPUM with a revised melting model, BATCH, indicate that the Na2O concentration decreases with increasing pressure more rapidly than in previous models. Thus, for example, 1% melting of lherzolite with Na2O bulk concentration typical of estimated terrestrial mantle (∼0.3 wt %), can produce a liquid with ∼6 wt % Na2O at 1.0 GPa but only ∼2% Na2O at 3.0 Gpa. In calculated melts of the DPUM and PUM compositions at 1.0 Gpa, the TiO2 concentration decreases between 10 and 1% melting in response to an increase inDTiO2cpx, consistent reported experimental observations. The increase in DTiO2cpx appears to be a response to increasing alkalis in the melt. However, TiO2 concentration does not decrease with lower degrees of melting of DPUM and PUM at 3.0 GPa in part because the increase in alkali concentration with decreasing melting percentage is smaller and also because the effect of increasing alkalis in the liquid superposes on lower values of DTiO2cpx at higher pressures. These lower values are the result of a decrease in the wollastonite component in clinopyroxene coexisting with orthopyroxene with increasing pressure. Calculations of lherzolite melting also yield coefficients for the solidus reactions that are generally consistent with earlier studies: olivine is in reaction with the melt in the lower pressure portion of the spinel field (i.e., it crystallizes during melting) but changes sign at pressures approaching the spinel to garnet transition and remains a cotectic phase in the garnet field, whereas orthopyroxene becomes a reaction phase at pressures approaching the spinel to garnet transition and remains a reaction phase well into the garnet stability field. The orthopyroxene reaction relation causes orthopyroxene to be present in the melting interval but absent at the solidus of some lherzolites in the garnet stability field.