Significance of a primitive lunar basaltic composition present in Apollo 15 soils and breccias

Abstract

Distinctive spherules and fragments of ‘Green Glass’ previously described from Apollo 15 soils, have olivine-rich (30%) magnesian (100MgMg+Fe= 61) picritic compositions. ‘Green Glass’ has been interpreted as a representative composition of a significant lunar rock unit, the actual glass being produced by melting of this unit. Experimental studies of high pressure melting relations in a compositionally similar Apollo 12 basalt (12040) lead to prediction of the nature of liquidus phases of Apollo 15 Green Glass at various pressures. It is argued that the Apollo 15 Green Glass unit was a product of very high degrees of partial melting (30–60%) of pyroxenite (olivine-poor, dominated by subcalcic clinopyroxene) source rock with magma segregation (from residual olivine and orthopyroxene only) occurring at 15 kb, T = 1450°C. The source rock is similar to that predicted for Apollo 11, 12 and 15 mare basalts but the degree of melting is much greater than that postulated for even the more olivine rich mare basalts (e.g. 12009 - 10% melting, magma segregation 10 – 12 kb).