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-A l2O3-SiO2-Na2O over a pressure range of 7-35 kbar. These data permit t he melting behavior of a wide variety of model lherzolite compositions to be determined quantitatively by algebraic methods. Two P-T univari ant melting reactions, corresponding to plagioclase to spinel therzoli te 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 l ow-temperature regions and isobarically invariant melting intervals al ong 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 produ cing liquids that are quartz tholeiites at < 8 kbar and olivine tholei ites at > 8 kbar. Calculated equilibrium constants for plagioclase-liq uid equilibria show both temperature and pressure dependence. Plagiocl ase with anorthite content (AN) > 90 mol%, as observed in some oceanic basalts, can crystallize from liquids with < 1% Na2O. Melting of spin el lherzolite is not pseudo-invariant but occurs over large temperatur e 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 lherzo lite field, melts from garnet lherzolite are more silica rich for a gi ven degree of melting than melts from spinel lherzolite, and liquid co mpositions trend toward enstatite with increase in pressure. Source fe rtility (especially Na2O content) has a strong control on the temperat ure of melting and liquid composition. Less fertile sources produce sm aller amounts of liquids richer in normative silica. For certain bulk compositions (high SiO2 and low Al2O3), spinel is not a stable phase a long the lherzolite solidus.