Geochemical investigation of two lunar mare meteorites: Yamato-793169: and Asuka-881757

We report new data for the bulk compositions of two meteorites derived from the mare lava plains of the Earth\u27s Moon : Yamato-793169 and Asuka-881757. These meteorites are remarkably similar to one another, and clearly of lunar origin, but not quite like any previously studied lunar rock. In the Ti-based classification system for mare basalts, Y-793169 and A-881757 are both low-Ti. However, viewed in relation to their extraordinarily low mg ratios, their Ti contents are also marginally consistent with kinship to previously-studied VLT (very-low-Ti) mare basalts. In another popular (albeit arbitrary) classification system, A-881757 is low-Al, and Y-793169 is marginally high-Al. Resemblances to VLT mare basalts are also evident from their unfractionated middle-REE/heavy-REE ratios and high Sc/Sm ratios. On a plot of Sc vs. Ti, Y-793169 and A-881757 plot near one another, and well apart from any Apollo/Luna mare basalt. They resemble low-Ti basalts in terms of moderate overall REE contents and V/Sm and Cr/Sm ratios, and they even resemble high-Ti basalts in having high Sc coupled with low V. The high Sc contents probably reflect partial melting of a Sc-rich source (or sources) in the lunar mantle. Neither meteorite shows the enrichments in K and Ge that remain peculiar to mare basalts from Apollo 14,apparently due to assimilation of the K-rich granitic and Ge-rich KREEPy materials that are uncommonly abundant in the Apollo 14 region. A set of remarkable coincidences between Y-793169 and A-881757 (from our data : their bizarrely high Sc despite low Ti, and extraordinarily low mg; other workers have demonstrated similar ages, ^U/^Pb, and times of blast-off from the Moon) suggests that these two meteorites might be paired, in the sense of having left the Moon in a single cratering-ejection event. The geochemical contrasts between Y-793169 and A-881757 and previously available mare basalts show that the full diversity of the lunar maria, and the factors that govern variability among mare basalts, are still poorly understood. The diversity and lack of systematics among lunar mare basalt compositions, reinforced by Y-793169 and A-881757,support the magma ocean cumulate model for genesis of the mare source regions. The relationship between geochemistry and age among mare basalts seems more complex that previously supposed, probably because the compositional stratification that develops in the pile of magma ocean cumulates is too disorderly to conform with any model for depth-time-temperature evolution of the lunar interior