EVOLUTION OF THE UPPER-MANTLE OF THE EARTHS MOON - NEODYMIUM AND STRONTIUM ISOTOPIC CONSTRAINTS FROM HIGH-TI MARE BASALTS

Isotopic studies of mare basalts have led workers to conclude that the ir sources are heterogeneous on both large and small scales. Furthermo re, these studies have led workers to postulate that depletion within the lunar mantle occurred early in its evolution and was a result of a ccumulation of mafic minerals from a LREE-enriched magma ocean. High-T i basalts from the Apollo 11 and 17 landing sites and ilmenite basalts from Apollo 12 are secondary evidence of this extreme, early depletio n event. KREEPy rocks are the complementary enriched component in the Moon. A total of fourteen high-Ti basalts have now been analyzed from the Apollo 11 landing site for neodymium and strontium isotopes. A Sm- Nd internal isochron on basalt 10058 yields an age of 3.70 +/- 0.06 Ga , similar to Ar-40/Ar-39 ages of other Group B1 basalts. A compilation of all previously determined ages on Apollo 11 high-Ti basalts indica tes four distinct phases of volcanism at 3.85 +/- 0.02 Ga (Group B2), 3.71 +/- 0.02 Ga (Group B3), 3.67 +/- 0.02 Ga (Group B1), and 3.59 +/- 0.04 Ga (Group A). Whole-rock Sm-Nd isotopic data for all Apollo 11 h igh-Ti basalts form a linear array, which yields the age of the Moon ( 4.55 +/- 0.30 Ga). A similar regression of all uncontaminated high-Ti basalts from the Moon (both Apollo 11 and Apollo 17) yields an age of 4.46 +/- 0.17 Ga. Both arrays are interpreted as average source ages o f the high-Ti basalts and are consistent with the formation of these s ources by precipitation of cumulates from a magma ocean early in the h istory of the Moon. These new strontium and neodymium isotopic data, c oupled with previously published data, are consistent with a two-compo nent model for the upper mantle of the Moon. These two components incl ude mafic accumulates precipitated from a magma ocean prior to 4.4 Ga and small amounts (<2%) of trapped, KREEPy, late-stage, magma ocean di fferentiates. The mafic accumulate evolves from 4.5 Ga, with Sm-147/Nd -144 = 0.318 and Rb-87/Sr-86 = 0.005 to extremely radiogenic neodymium isotopic ratios and very unradiogenic stronitium isotopic ratios. The KREEPy trapped liquid has a Sm-147/Nd-144 = 0.168 and Rb-87/Sr-86 = 0 .235 and thus, evolves toward very unradiogenic neodymium and radiogen ic strontium isotopic ratios. Because the KREEPy trapped liquid is enr iched in both rubidium and the REEs by over an order of magnitude comp ared to the mafic accumulate, trapping of even small proportions of th is liquid in the accumulate will control the radiogenic isotopic compo sition of the source. The apparent heterogeneity in the source regions of mare basalts could be caused by trapping of variable, yet small, p roportions of this LILE-enriched liquid in the cumulate pile.