Elephant Moraine 87521: The first lunar meteorite composed of predominantly mare material

Abstract

The trace-element chemistry and detailed petrography of brecciated Antarctic meteorite EET87521 reveal that it is not, as originally classified, a eucrite. Its Fe/Mn ratio and bulk Co content are far higher than expected for a eucrite. An affiliation with the SNC meteorites appears almost equally improbable on the basis of far lower Ga Al and Na Ca ratios in EET87521. Only one known type of extraterrestrial material resembles EET87521 in all important respects for which constraints exist: very-low-Ti (VLT) lunar mare basalts. Even compared to VLT basalts, EET87521 is enriched in REE. However, other varieties of high-alumina, low-Ti mare basalt are known that contain REE at even higher concentrations than EET87521. Several clasts in EET87521 preserve clear vestiges of coarse-grained igneous, possibly orthocumulate, textures. Mineralogically, these coarse-grained clasts are diverse; e.g., olivine ranges from Fo 15 in one to Fo 67in another. One clast with an anomalously fine-grained texture is anor-thositic and contains exceptionally Mg-rich pyroxene and Na-poor plagioclase, along with the only FeNi-metal in the thin section. This clast is probably an impact melt breccia from the lunar highlands. Its FeNi-metals have compositions typical of metals incorporated into lunar soils and polymict breccias as debris from metal-rich meteorites. However, the low Ni and Ir contents of our bulk-rock analysis imply that the proportion of impact-projectile matter in our “chip” sample is probably small. The moderate degree of lithologic diversity among the lithic clasts and the bulk composition in general indicate that EET87521 is dominated by a single rock type: VLT mare basalt. The fundamental distinction between mare and nonmare lunar materials has traditionally been based primarily on Ti content, and secondarily on such characteristics as Ca Al ratio and REE pattern. However, EET87521, recent studies of Apollo 12 and Apollo 14 rocks, and the many apparent VLT basalts reported earlier as clasts in lunar-highlands meteorites, all tend to indicate that the dichotomy of lunar magmatism into distinct nonmare and mare styles may have been less abrupt than commonly envisaged.