Petrology of a portion of the Mare Fecunditatis regolith

Abstract

1300 microprobe analyses of glasses, pyroxenes, feldspars, oxides, olivines, troilite and metal in two 0.025 g samples of the Luna 16 return were made in order to characterize the Mare Fecunditatis regolith. Pyroxenes display a very wide compositional range, extreme fractionation, and metastable crystallization. Solid solution of Ti, Al, and Cr is appreciable and most pyroxenes plot along an Al:Ti line 2:1, similar to Apollo 11 clinopyroxenes. Orthopyroxenes are very rare. Zoning in plagioclase is varied but not extensive; compositions from An 75 to An 100 are dominant. The compositional distribution is indistinguishable from Apollo 12 and 11 low-K basaltic plagioclases. No potassic feldspars were found. Ilmenite is the dominant oxide phase, with minor ulvospinel, rare chrome spinel and spinel. The latter resemble Apollo 14 pink spinels. Olivines range from Fo 75 to Fo 11 but the majority range from Fo 60 to Fo 70 thus more iron-rich than olivines from other maria. On the basis of preferred compositions, a tentative classification of glasses has been made. Twenty-three percent of the glasses are Al-rich, Fe, Cr-poor, have Ca/Al ratios similar to many Apollo 14 glasses and are considered to have a non-mare origin. Their compositions are essentially the same as that of the high Al component at all Apollo landing sites. Glasses equivalent in composition to Fra Mauro basalts (KREEP) and to granite are extremely rare. The majority of glasses, mare-derived, are substantially higher in Fe, Ti, and Cr and lower in Ca and Al. They are divisible into a major group, Fecunditatis type A basaltic glasses, with less than 5% TiO 2, and a smaller group, Fecunditatis type B basaltic glasses, with more than 5% TiO 2. The type A glasses are richer in Al, and lower in Fe than glasses at the Apollo 11 or 12 sites. Type B glasses are similar to the high Fe basaltic glasses from the Apollo 11 regolith. If the type A glasses reflect the characteristic basalts at the Mare Fecunditatis site, then these are intermediate in major element chemistry between Apollo 11 and 12 basalts and the aluminous non-mare basaltic rocks.