MELTING BEHAVIOR OF SIMPLIFIED LHERZOLITE IN THE SYSTEM CAO-MGO-AL2O3-SIO2-NA2O FROM 7 TO 35 KBAR

Phase equilibrium data have been collected for isobarically univariant melting of simplified lherzolite compositions in the system CaO-MgO-Al2O3-SiO2-Na2O over a pressure range of 7-35 kbar. These data permit the melting behavior of a wide variety of model lherzolite compositions to be determined quantitatively by algebraic methods. Two P-T univariant melting reactions, corresponding to plagioclase to spinel therzolite and spinel to garnet lherzolite, are identified as peritectic-type transitions and have positive Clapeyron slopes. The univariant curves move to higher pressures and temperatures with increasing Na2O in the liquid. The effect of the univariant curves on melting is to produce low-temperature regions and isobarically invariant melting intervals along lherzolite solidi. In the plagioclase lherzolite stability field, melting of four-phase model lherzolite is pseudo-invariant, occurring over small temperature intervals (approximately 5-degrees-C) and producing liquids that are quartz tholeiites at < 8 kbar and olivine tholeiites at > 8 kbar. Calculated equilibrium constants for plagioclase-liquid equilibria show both temperature and pressure dependence. Plagioclase with anorthite content (AN) > 90 mol%, as observed in some oceanic basalts, can crystallize from liquids with < 1% Na2O. Melting of spinel lherzolite is not pseudo-invariant but occurs over large temperature intervals (15-60-C), producing a wide range in liquid compositions, from alkali basalts and alkali picrites at low to moderate degrees of melting (< 1-10%) to olivine tholeiites and picrites at higher degrees of melting (> 10%). On the basis of limited data in the garnet lherzolite field, melts from garnet lherzolite are more silica rich for a given degree of melting than melts from spinel lherzolite, and liquid compositions trend toward enstatite with increase in pressure. Source fertility (especially Na2O content) has a strong control on the temperature of melting and liquid composition. Less fertile sources produce smaller amounts of liquids richer in normative silica. For certain bulk compositions (high SiO2 and low Al2O3), spinel is not a stable phase along the lherzolite solidus.