Four lunar mare meteorites: Crystallization trends of pyroxenes and spinels

Abstract

Abstract— We studied crystallization trends of pyroxene and spinel in four Antarctic meteorites known to be derived from mare regions of the Moon: Y‐793169 and A‐881757 (YA meteorites) are unbrecciated igneous basalts, EET 87521 is a fragmental breccia, and Y‐793274 is a regolith breccia. All have relatively low bulkrock TiO2 content, and the YA meteorites are uncommonly ancient. Our electron probe microanalysis (EPMA) data indicate that the YA meteorites and the dominant mare components of Y‐793274 and EET 87521 conform to a general trend for Ti‐poor (low‐Ti and very low‐Ti) mare basalts. Their pyroxenes show a strong correlation between Fe/(Fe + Mg) (Fe#) and Ti/(Ti + Cr) (Ti#), both ratios typically increasing from core to rim. These trends presumably reflect local crystallization differentiation of interstitial melt. Previous studies (M. J. Drake and coworkers) have suggested that the detailed configurations of such Fe# vs. Ti# trends may reflect the bulk TiO2 contents of the parent magmas (basalts). As a more systematic approach to this problem, we plot bulk‐rock TiO2 as a function of the Fe# = 0.50 intercept of each rock's pyroxene Fe# vs. Ti# trend. We call this intercept the Fe#‐normalized Ti#. Based on our data for EET 87521, the YA meteorites, and Apollo 12 basalts 12031 and 12064, plus literature data for several other Ti‐poor mare basalts, we find a strong correlation between Fe#‐normalized Ti# and the bulk TiO2 content of the parent basalt. This correlation confirms that fragmental breccia EET 87521 is nearly pure very low‐Ti (VLT) basalt and that the YA meteorites, for which bulk‐rock TiO2 results scatter due to unusually coarse grain size (A‐881757) or scarcity of available sample (Y‐793169), are pieces of an uncommonly Ti‐poor, but not quite VLT, variety of low‐Ti mare basalt. Extrapolating from this correlation, the dominant mare component of regolith breccia Y‐793274 is probably of VLT affinity. Besides the normal mare pyroxene trend of strong correlation between Fe# and Ti#, Y‐793274 includes two additional pyroxene compositional trends, both showing a wide range of Ti# despite relatively constant (and low, by mare standards) Fe#. The most magnesian of these trends consists of a single clast with a mode of orthopyroxene + MgO‐rich ilmenite. These two trends are of uncertain origin. Possibly one or both represents the highland component of this regolith breccia, although, unlike most highland pyroxenes, these appear relatively unaltered by impact brecciation and metamorphism. Compositions of spinels in the coarse‐grained A‐881757 show an extraordinary distribution: chromite and ulvöspinel components vary among grains but are nearly constant within grains. Despite its old age and unusually coarse grain sizes, mineralogical evidence (i.e., heterogeneity within both pyroxene and spinel; typical pyroxene exsolution scale very coarse by mare standards but exceeded by the pyroxenes of EET 87521 and Y‐793274) indicates that A‐881757 was cooled only slightly more slowly than typical mare basalts and may have formed near the center of an uncommonly thick lava flow. Both of the VLT basaltic lunar meteorite breccias, EET 87521 and Y‐793274, are composed dominantly of pyroxenes with exsolution coarser than normal for mare basalts. Possibly VLT basalt flows tend to be systematically thicker, and thus more slowly cooled, than more Ti‐rich flows.