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 thickINTRODUCTION ness of model mid-ocean ridge basalts (MORBs) are calculated Comparison of observed basalt compositions with the using MELTS. Polybaric, isentropic batch and fractional melts predictions of polybaric mantle melting models places from ranges in source composition, potential temperature, and final important restrictions on melting processes in the mantle melting pressure are integrated to represent idealized passive and beneath mid-ocean ridges (Klein & Langmuir, 1987; active flow regimes. These MELTS-derived polybaric models are McKenzie & Bickle, 1988; Niu & Batiza, 1991, 1993; compared with other parameterizations; the results differ both in Kinzler & Grove, 1992b; Langmuir et al., 1992; Iwamori melt compositions, notably at small melt fractions, and in the et al., 1995; Kinzler, 1997). It is now well accepted that solidus curve and melt productivity, as a result of the self-consistent mid-ocean ridge basalts (MORBs) represent mixtures of energy balance in MELTS. MELTS predicts a maximum mean melts produced over a range of depths and that these melt fraction (>0·08) and a limit to crustal thickness (Ζ15 km) melts separate from their sources at low melt fraction for passive flow. For melting to the base of the crust, MELTS (McKenzie, 1984; von Bargen & Waff, 1986; Johnson et requires an >200°C global potential temperature range to explain al., 1990; Langmuir et al., 1992). However, significant the range of oceanic crustal thickness; conversely, a global range of uncertainties remain regarding the depth of initial melting 60°C implies conductive cooling to >50 km. Low near-solidus (related both to the range of mantle potential temperature productivity means that any given crustal thickness requires higher and to the influence of minor incompatible components initial pressure in MELTS than in other models. MELTS cannot on the solidus), the depth of final melting (and hence the at present be used to model details of MORB chemistry because of errors in the calibration, particularly Na partitioning. Source importance of spreading rate), the style of melt transport heterogeneity cannot explain either global or local Na–Fe systematics (e.g. the relative importance of fractional fusion vs equior the FeO–K2O/TiO2 correlation but can confound any extent of librium porous flow), and the effects of chemical heteromelting signal in CaO/Al2O3. geneities in mantle sources. These uncertainties remain for a number of reasons (among them disagreements over the selection of data, the appropriate scale for