Origin of lunar ultramafic green glasses: Constraints from phase equilibrium studies

Phase equilibria experiments on an Apollo 14B green glass composition are u sed to develop a petrogenetic model fur the Apollo 14B green glasses. New c ompositional data for the Apollo 14B green glasses are presented, which rep lace the original data set of Delano (1986). Near-liquidus phase relations of the 14B green glass are determined from 1.3 to 2.7 GPa and 1380 to 1570 degreesC. The liquidus is saturated with olivine to 2.4 GPa and orthopyroxe ne above 2.4 GPa. A multiple saturation point with olivine and orthopyroxen e occurs at 1560 degreesC and 2.4 GPa. Crystallization of the ol + opx satu ration boundary involves 40 wt% ol and 60 wt% opx, and occurs over a 140 de greesC interval before cpx appears at a reaction boundary. Crystallization/ melting models are developed to reproduce the compositional variations disp layed by the Apollo 14B glasses using these phase equilibrium constraints. Fractional crystallization alone cannot account for the observed variation in the major element abundance of the Apollo 14B green glasses. Assimilatio n is also necessary, in particular, to fit variations in TiO2, FeO, Na2O an d Al2O3. The 14B glasses show evidence of simultaneous high-Ti assimilation and fractional crystallization of olivine, followed by simultaneous KREEP assimilation and olivine fractionation. The data requires the high-Ti assim ilant to be at a greater depth than the KREEP assimilant, but only constrai ns the total assimilation/fractional crystallization process to occur betwe en approximately 2.3 and 0.5 GPa. Assimilation of a high-Ti cumulate within this depth range is consistent with an overturn/hybrid mantle remelting mo del. Highlands crust assimilation is precluded for the Apollo 14B green gla sses. Copyright (C) 2000 Elsevier Science Ltd.