Petrology of basaltic and monomineralic soil fragments from the Sea of Fertility

Abstract

Basaltic and monomineralic fragments from the 150–425 μm size fractions of the Luna 16 core obtained from the Sea of Fertility were studied by optical petrographic, electron microprobe, and single-crystal X-ray diffraction techniques. Three textural varieties were identified in the basalts (intersertal, subophitic, and recrystallized). These textures, the assemblages, and individual mineral compositions (especially pyroxenes) indicate that these basalts crystallized under conditions very similar to those from Apollo 11. The pyroxenes have slightly lower Ti/Al (atomic) ratios than the Apollo 11 pyroxenes indicating that a small amount of R 2+AlSiAlO 6 is present. We interpret the presence of octahedral Al as being due to the low TiO 2/Al 2O 3 (weight %) ratio in the bulk rock. All observations made on the fine-grained basalt fragments are consistent with a rapid, near-surface, one-stage crystallization history. The assemblage metallic iron + fayalite + silica in some basaltic fragments suggest that they crystallized when oxygen fugacities were of the order of 10 −13 atm at 1200°C. A small contribution from the lunar highlands may be indicated by high calcium plagioclases (An 95–98) and by anorthite-orthopyroxene fragments. Several of the monomineralic pyroxene fragments may be derivatives of more deep-seated basalts.