Calculation of Peridotite Partial Melting from Thermodynamic Models of Minerals and Melts. III. Controls on Isobaric Melt Production and the Effect of Water on Melt Production

melting. This too can be demonstrated both by MELTS calculations We present a rigorous calculation of the isobaric entropy (S) change and by calculations in simple model systems. Productivities for of the melting reaction for peridotite (∂S/∂F ) P , where F is the systems enriched in incompatible components are systematically melt fraction. Calculations at 1 and 2 GPa for fertile and depleted lower than those depleted in such components, though the total melt peridotite show that (∂S/∂F ) P varies as a function of extent of produced at any given temperature will be greater for an enriched melting, temperature, and residual mineral assemblage. Changes in system. Exhaustion of clinopyroxene from peridotite residua decreases reaction stoichiometry cause discontinuous changes in (∂S/∂F ) P . calculated productivity by about a factor of four, and therefore Although calculated (∂S/∂F ) P varies by about a factor of two extensive partial melting of harzburgitic residues is inhibited. Cal(from ~0·25 to ~0·5 J/K per g), such variations have relatively culated isothermal addition of water to hot peridotite causes melting little effect on the formation of melt during adiabatic upwelling and to increase roughly linearly with the abundance of water added to a characteristic value suitable for peridotite partial melting at least up to 3 GPa is 0·3 J/K per g. Calculated variations in isobaric the system, in agreement with the trend recognized earlier for Mariana trough basalts. Melt production for calculated addition of melt productivity, (∂F/∂T )P, are large and have a significant effect on calculated adiabatic productivity, (∂F/∂P )S. For partial melting a subduction fluid (45 wt % H2O, 45% Na2O, 10% K2O) is only slightly greater than for pure water. If water addition to of fertile peridotite, MELTS calculations suggest that near-solidus productivities are greatly reduced relative to productivities at higher peridotite is not forced to be isothermal by an externally imposed heat sink or by buffering from low variance chemical reactions, then melt fraction, owing to the incompatible behavior of Na2O and the effect of this component on the liquidus temperature of partial melts. it will approach isenthalpic conditions, which will reduce melt production per increment of water added by about a factor of two. This behavior can also be demonstrated in simple model systems. Calculated near-solidus productivity for fractional or incremental For heating of peridotite containing minor amounts of H2O, calculations suggest that the extent of melting will remain small batch melting of peridotite is lower than for batch melting, but after a small amount of melting (~2%), productivity for the fractional (<5%) until the temperature is sufficient to generate significant melt for an equivalent dry peridotite. Small degrees of melting deep or incremental batch melting case is greater than that of batch