Petrology, mineralogy, and genesis of lunar crystalline igneous rocks

Abstract

Information on the mineralogy and petrology of the Apollo 11 crystalline basaltic rocks, obtained by about 35 groups of investigators, is summarized and used as a basis for speculation. The textural assemblage indicates near-surface, rapid crystallization from low-viscosity basaltic magmas under low oxygen pressures. The basalts are subsilicic and subalkaline, but only locally titaniferous. Terrestrial alkali basalts could, through alkali depletion, give similar assemblages under reducing conditions, but probably no primary lunar basalt has yet been sampled. Crystal accumulation, shown by Apollo 12 specimens, would be the main control of local variations in basalt composition. Apollo 11 specimens are more residual in composition but may in part be from remelted ilmenitic cumulates. Strong fractionation within rocks leads to a residuum with both granitic and syenitic affinities, with iron enrichment and titanium depletion as a predominant fractionation trend. The mineral phases are spectacular in character and diversity, and at least 30 phases have been recognized. Crystal-liquid fractionation gives early phases rich in Mg, Ca, Cr, and Ti, and late phases rich in Fe, Mn, Na, K, P, Si, and Zr. The Fe-Ni-S immiscible liquids show sulfur loss and nickel gain in the Apollo 12 suite. Rapid-quench, metastable crystallization was soon followed by equilibrium fractionation and then by subsolidus equilibration between phases. Basaltic fragments may be derived from subhighlands layered plutons, mare lava flows, and small crystallized pools of impact-generated melts. Primary basalt magmas probably originated from partial melting of a pyroxenite mantle. Early, large-scale melting of the outer pyroxenite shell could have produced a primitive basalt crust that differentiated to give a highlands and subhighlands crust. Some surface basalts may be derived from the latter, rather than from later partial melting of the mantle.