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

Abstract

We examine several issues related to peridotite partial melting by performing thermodynamic calculations using the MELTS algorithm. MELTS calculations suggest that the high alkali content of near-solidus melts leads to their having high SiO_2 contents at 1 GPa, but that near-solidus enrichments in SiO_2 become less pronounced with increasing pressure, such that 2% partial melts at 3 GPa have only ∼1 wt % more SiO_2 than higher melt fractions (5–15%) at that pressure. Calculated near-solidus solid–liquid partitioning of TiO_2 differs from that calculated and observed at high melt fraction, both at low and high pressure, reflecting variations in melt and mineral compositions as melting proceeds. Specifically, near the solidus, high liquid SiO_2 and an abundant supply of charge coupling ions (Na^+, Al^(3+)) in clinopyroxene (cpx) makes Ti more compatible in the solid. We infer that similar effects are likely for other highly charged cations such as the high field strength elements (HFSE), U, and Th. To investigate the effects of heterogeneous source regions on major element chemistry of basaltic partial melts, we perform melting calculations for a range of peridotite compositions. For partial melts in equilibrium with spinel lherzolite residues, CaO/Al_(2)O_3 and CaO content increase with increasing temperature and decrease with increasing Na_(2)O in the melt and Al/(Cr + Al) in coexisting spinel. Thus, at any given melt fraction, the CaO/Al_(2)O_3 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/Al_(2)O_3 or CaO content. At fixed melt fraction, these effects of source depletion on melt composition (Na_(2)O decreases, cao/Al_(2)O_3 increases) are similar to the effects of increasing total extent of melting of a fixed source composition, and the two phenomena may therefore be difficult to discriminate on the basis of these melt variables alone. The effects of source variability on partial melting depend on whether compositionally distinct domains experience the same or different temperature–pressure paths. If they experience the same paths, as might be expected for small-scale heterogeneities, then enhanced melting of the enriched sources can yield partial melts with depleted characteristics such as low Na_(2)O and high cao/Al_(2)O_3 relative to partial melts of depleted sources at the same pressure and temperature. The differing compatibility of K and Ti in spinel peridotite minerals causes the K_(2)O/TiO_2 ratio of partial melts to vary inversely with the total extent of melting (and with FeO*), so K_(2)O/TiO_2 may not be a reliable indicator of source heterogeneity. Moreover, enriched high K_(2)O/TiO_2 sources probably produce melts richer in FeO than depleted low K_(2)O/TiO_2 sources at the same conditions. Consequently, the inverse correlation between regionally averaged K_(2)O/TiO_2 and melt FeO* observed by Shen & Forsyth (1995, Journal of Geophysical Research 100, 2211–2237) is probably more strongly influenced by variations in extent of melting than by source heterogeneity.

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