Calculation of Peridotite Partial Melting from Thermodynamic Models of Minerals and Melts. II. Isobaric Variations in Melts near the Solidus and owing to Variable Source Composition

We examine several issues related to peridotite partial melting by composition (Na2O decreases, CaO/Al2O3 increases) are similar to the effects of increasing total extent of melting of a fixed source performing thermodynamic calculations using the MELTS algorithm. composition, and the two phenomena may therefore be difficult to MELTS calculations suggest that the high alkali content of neardiscriminate on the basis of these melt variables alone. The effects of solidus melts leads to their having high SiO2 contents at 1 GPa, but source variability on partial melting depend on whether compositionally that near-solidus enrichments in SiO2 become less pronounced with distinct domains experience the same or different temperature–pressure increasing pressure, such that 2% partial melts at 3 GPa have only paths. If they experience the same paths, as might be expected for ~1 wt % more SiO2 than higher melt fractions (5–15%) at that small-scale heterogeneities, then enhanced melting of the enriched sources pressure. Calculated near-solidus solid–liquid partitioning of TiO2 can yield partial melts with depleted characteristics such as low Na2O differs from that calculated and observed at high melt fraction, both and high CaO/Al2O3 relative to partial melts of depleted sources at at low and high pressure, reflecting variations in melt and mineral the same pressure and temperature. The differing compatibility of K compositions as melting proceeds. Specifically, near the solidus, high and Ti in spinel peridotite minerals causes the K2O/TiO2 ratio of liquid SiO2 and an abundant supply of charge coupling ions (Na, partial melts to vary inversely with the total extent of melting (and Al) in clinopyroxene (cpx) makes Ti more compatible in the solid. with FeO∗), so K2O/TiO2 may not be a reliable indicator of source We infer that similar effects are likely for other highly charged cations heterogeneity. Moreover, enriched high K2O/TiO2 sources probably such as the high field strength elements (HFSE), U, and Th. To produce melts richer in FeO than depleted low K2O/TiO2 sources at investigate the effects of heterogeneous source regions on major element the same conditions. Consequently, the inverse correlation between chemistry of basaltic partial melts, we perform melting calculations regionally averaged K2O/TiO2 and melt FeO∗ observed by Shen for a range of peridotite compositions. For partial melts in equilibrium & Forsyth (1995, Journal of Geophysical Research 100, with spinel lherzolite residues, CaO/Al2O3 and CaO content increase 2211–2237) is probably more strongly influenced by variations in with increasing temperature and decrease with increasing Na2O in the extent of melting than by source heterogeneity. melt and Al/(Cr + Al) in coexisting spinel. Thus, at any given melt fraction, the CaO/Al2O3 ratios and CaO contents of partial melts are inversely correlated with the fertility or enrichment of the source and only weakly dependent on the source CaO/Al2O3 or CaO