REE STUDIES OF THREE LUNAR HIGHLAND METEORITES: CLUES TO THE ANCIENT LUNAR CRUST

Measurements of the incompatible rare earth elements (REE) in returned lunar samples have been an important key to our understanding of early lunar evolution, but such interpretations have been hampered in part by the limited area of the Moon sampled by the Apollo and Luna programs. Lunar meteorites, representing a random sampling of the lunar surface, provide important complementary data; however such samples are often strongly altered by the shock event that removed them from the lunar surface. We have measured REE and other trace element abundances in situ from three lunar highland meteorites (Dhofar 081, NWA 482, and DAG 400) including melt-produced clasts, dark surrounding matrix, shock veins, and pure anorthite crystals. Calculations of the melt in equilibrium with the anorthites indicate they crystallized from a magma with enriched (10 x CI) but unfractionated REE. This is different from the REE calculated for magmas in equilibrium with Apollo ferroan anorthosite samples, and different from the REE observed in the other components of these meteorites. No simple mixing model can relate either these anorthites or this magma to the REE of the other components of these meteorites. Furthermore, the REE apparently did not equilibrate among these phases during the shock melting event. Thus the shock event that destroyed the original rock must have quenched too rapidly to redistribute the major and trace elements. It should be possible therefore to determine the original mineral composition of the melt clasts and matrix from their bulk element abundances, and model their evolution based on their measured REE abundances. In this way we may have a clue to the earliest rock-forming events in the Moon.