Some phase equilibrium systematics of lherzolite melting: I

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 % Na 2 O. Lherzolitic liquids with higher concentrations of Na 2 O have negative mass balance coefficients, regardless of Mg′, implying that there is a limit of ∼2 wt % Na 2 O in anhydrous partial melts of peridotites with ∼0.3 wt % bulk Na 2 O in the upper garnet‐lherzolite stability field. Examination of liquidus equilibria in the NCMAS system demonstrates that coupling of Na 2 O and SiO 2 concentrations in liquids saturated with lherzolite assemblages permits high‐Na 2 O, high‐SiO 2 melts at pressures ∼1.0 GPa, whereas only high‐Na 2 O, low‐SiO 2 melts are possible in the garnet‐lherzolite stability field. Because the bulk partition coefficient for Na 2 O increases with pressure, the concentration of Na 2 O 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 Na 2 O concentration decreases with increasing pressure more rapidly than in previous models. Thus, for example, 1% melting of lherzolite with Na 2 O bulk concentration typical of estimated terrestrial mantle (∼0.3 wt %), can produce a liquid with ∼6 wt % Na 2 O at 1.0 GPa but only ∼2% Na 2 O at 3.0 Gpa. In calculated melts of the DPUM and PUM compositions at 1.0 Gpa, the TiO 2 concentration decreases between 10 and 1% melting in response to an increase in D TiO2 cpx , consistent reported experimental observations. The increase in D TiO2 cpx appears to be a response to increasing alkalis in the melt. However, TiO 2 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 D TiO2 cpx 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.