Calculation of Peridotite Partial Melting from Thermodynamic Models of Minerals and Melts, IV. Adiabatic Decompression and the Composition and Mean Properties of Mid-ocean Ridge Basalts

Abstract

Composition, mean pressure, mean melt fraction, and crustal thickness of model mid-ocean ridge basalts (MORBs) are calculated using MELTS. Polybaric, isentropic batch and fractional melts from ranges in source composition, potential temperature, and final melting pressure are integrated to represent idealized passive and active flow regimes. These MELTS-derived polybaric models are compared with other parameterizations; the results differ both in melt compositions, notably at small melt fractions, and in the solidus curve and melt productivity, as a result of the self-consistent energy balance in MELTS. MELTS predicts a maximum mean melt fraction (~0·08) and a limit to crustal thickness (<=15 km) for passive flow. For melting to the base of the crust, MELTS requires an ~200°C global potential temperature range to explain the range of oceanic crustal thickness; conversely, a global range of 60°C implies conductive cooling to ~50 km. Low near-solidus productivity means that any given crustal thickness requires higher initial pressure in MELTS than in other models. MELTS cannot at present be used to model details of MORB chemistry because of errors in the calibration, particularly Na partitioning. Source heterogeneity cannot explain either global or local Na–Fe systematics or the FeO–K2O/TiO2 correlation but can confound any extent of melting signal in CaO/Al2O3.