AN EXTREMELY LOW U PB SOURCE IN THE MOON - U-TH-PB, SM-ND, RB-SR, ANDAR-40 AR-39 ISOTOPIC SYSTEMATICS AND AGE OF LUNAR METEORITE ASUKA 881757/

We have undertaken U-Th-Pb, Sm-Nd, Rb-Sr, and Ar-40/Ar-39 isotopic stu dies on Asuka 881757, a coarse-grained basaltic lunar meteorite whose chemical composition is close to low-Ti and very low-Ti (VLT) mare bas alts. The Pb-Pb internal isochron obtained for acid leached residues o f separated mineral fractions yields an age of 3940 +/- 28 Ma, which i s similar to the U-Pb (3850 +/- 150 Ma) and Th-Pb (3820 +/- 290 Ma) in ternal isochron ages. The Sm-Nd data for the mineral separates yield a n internal isochron age of 3871 +/- 57 Ma and an initial Nd-143/Nd-144 value of 0.50797 +/- 10. The Rb-Sr data yield an internal isochron ag e of 3840 +/- 32 Ma (lambda(Rb-87) = 1.42 x 10(-11) yr-1) and a low in itial Sr-87/Sr-86 ratio of 0.69910 +/- 2. The Ar-40/Ar-39 age spectra for a glass fragment and a maskelynitized plagioclase are relatively f lat and give a weighted mean plateau age of 3798 +/- 12 Ma. We interpr et these ages to indicate that the basalt crystallized from a melt 3.8 7 Ga ago (the Sm-Nd age) and an impact event disturbed the Rb-Sr syste m and completely reset the K-Ar system at 3.80 Ga. The slightly higher Pb-Pb age compared to the Sm-Nd age could be due to the secondary Pb (from terrestrial and/or lunar surface Pb contamination) that remained in the residues after acid leaching. Alternatively, the following int erpretation is also possible; the meteorite crystallized at 3.94 Ga (t he Pb-Pb age) and the Sm-Nd, Rb-Sr, and K-Ar systems were disturbed by an impact event at 3.80 Ga. The crystallization age obtained here is older than those reported for low-Ti basalts (3.2-3.5 Ga) and for VLT basalts (3.4 Ga), but similar to ages of some mare basalts, indicating that the basalt may have formed from a magma related to a basin-formi ng event (Imbrium?). The age span for VLT basalts from different sampl ing sites suggest that they were erupted over a wide area during an in terval of at least approximately 500 million years. The impact event t hat thermally reset the K-Ar system of Asuka 881757 must have been pos t-Imbrium (perhaps Orientale) in age. The lead isotopic composition of Asuka 8817 5 7 is nonradiogenic compared with typical Apollo mare bas alts and the estimated U-238/Pb-204 (mu) value for the basalt source i s 10 +/- 3. This source-mu value is the lowest so far measured for lun ar rocks. A large positive epsilon(Nd) value (7.4 +/- 0.5) and the tim e averaged Sm-147/Nd-144 ratio for the basalt source are similar to th ose for some Apollo 12, 15, and 17 basalts, suggesting a LREE-depleted mantle, which is consistent with the global magma ocean hypothesis. T he U-Th-Pb, Sm-Nd, and Rb-Sr data on Asuka 881757 suggest that the bas alt was derived from a low U/Pb, low Rb/Sr, and high Sm/Nd source regi on, mainly composed of olivine and orthopyroxene with minor amounts of plagioclase (or clinopyroxene) and with sulfides enriched in volatile chalcophile elements. The basalt source may be deep in origin and dif ferent in chemistry from those previously estimated from studies of Ap ollo and Luna mare basalts, indicating heterogeneous sources for mare