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

Composition, mean pressure, mean melt fraction, and crustal thickness of mo del mid-ocean ridge basalts (MORBs) are calculated using MELTS. Polybaric, isentropic batch and fractional melts from ranges in source composition, po tential temperature, and final melting pressure are integrated to represent idealized passive and active flow regimes. These MELTS-derived polybaric m odels are compared with other parameterizations;the results differ both in melt compositions, notably at small melt fractions, and in the solidus curv e and melt productivity a sa result of the self-consistent energy balance i n MELTS. MELTS predicts a maximum mean melt fraction (similar to0.08) and a limit to crustal thickness (less than or equal to 15 km) for passive flow. For melting to the base of the crust, MELTS requires an similar to 200 deg reesC global potential temperature range to explain the range of oceanic cr ustal thickness; conversely, a global range of 60 degreesC implies conducti ve cooling to similar to 50 km. Low near-solidus productivity means that an y given crustal thickness requires higher initial pressure in MELTS than in other models. MELTS cannot at present be used to model details of MORB che mistry because of errors in the calibration particularly Na partitioning. S ource heterogeneity cannot explain either global or local Na-Fe systematics or the FeO-K2O/TiO2 correlation but can confound any extent of melting sig nal in CaO/Al2O3.