4 LUNAR MARE METEORITES - CRYSTALLIZATION TRENDS OF PYROXENES AND SPINELS

We studied crystallization trends of pyroxene and spinel in four Antar ctic meteorites known to be derived from mare regions of the Moon: Y-7 93169 and A-881757 (YA meteorites) are unbrecciated igneous basalts, E ET 87521 is a fragmental breccia, and Y-793274 is a regolith breccia. All have relatively low bulk-rock TiO2 content, and the YA meteorites are uncommonly ancient. Our electron probe microanalysis (EPMA) data i ndicate that the YA meteorites and the dominant mare components of Y-7 93274 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 typical ly 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 configur ations of such Fe# vs. Ti# trends may reflect the bulk TiO2 contents o f the parent magmas (basalts). As a more systematic approach to this p roblem, we plot bulk-rock TiO2 as a function of the Fe# = 0.50 interce pt of each rock's pyroxene Fe# vs. Ti# trend. We call this intercept t he Fe#-normalized Ti#. Based on our data for EET 87521, the YA meteori tes, and Apollo 12 basalts 12031 and 12064, plus literature data for s everal other Ti-poor mare basalts, we find a strong correlation betwee n Fe#-normalized Ti# and the bulk TiO2 content of the parent basalt. T his correlation confirms that fragmental breccia EET 87521 is nearly p ure very low-Ti (VLT) basalt and that the YA meteorites, for which bul k-rock TiO2 results scatter due to unusually coarse grain size (A-8817 57) or scarcity of available sample (Y-793169), are pieces of an uncom monly Ti-poor, but not quite VLT, variety of low-Ti mare basalt. Extra polating from this correlation, the dominant mare component of regolit h breccia Y-793274 is probably of VLT affinity. Besides the normal mar e pyroxene trend of strong correlation between Fe# and Ti#, Y-793274 i ncludes two additional pyroxene compositional trends, both showing a w ide range of Ti# despite relatively constant (and low, by mare standar ds) Fe#. The most magnesian of these trends consists of a single clast with a mode of orthopyroxene + MgO-rich ilmenite. These two trends ar e of uncertain origin. Possibly one or both represents the highland co mponent of this regolith breccia, although, unlike most highland pyrox enes, these appear relatively unaltered by impact brecciation and meta morphism. Compositions of spinels in the coarse-grained A-881757 show an extraordinary distribution: chromite and ulvospinel components vary among grains but are nearly constant within grains. Despite its old a ge and unusually coarse grain sizes, mineralogical evidence (ie., hete rogeneity 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 slight ly more slowly than typical mare basalts and may have formed near the center of an uncommonly thick lava flow. Both of the VLT basaltic luna r meteorite breccias, EET 87521 and Y-793274, are composed dominantly of pyroxenes with exsolution coarser than normal for mare basalts. Pos sibly VLT basalt flows tend to be systematically thicker, and thus mor e slowly cooled, than more Ti-rich flows.