FURTHER EVIDENCE FOR A LOW U PB, SOURCE IN THE MOON - U-TH-PB, SM-ND,AND AR-AR ISOTOPIC SYSTEMATICS OF LUNAR METEORITE YAMATO-793169/

The coarse-grained lunar meteorites, Yamato-793169 and Asuka-881757, r epresent a new type of low-Ti mare basalt. This paper reports the resu lts of a U-Th-Pb, Sm-Nd, and Ar-Ar isotopic study of Yamato-793169 per formed as part of a consortium studies of lunar basaltic meteorites. T he isotopic study was carried out on a small sample (100 mg) so that o nly three density fractions could be separated. These fractions were l eached with dilute acid in order to eliminate terrestrial Pb contamina tion. However, the leaching procedure did not completely remove this c ontamination in some fractions and also apparently caused a fractionat ion of elements (U, Th, Pb) due to preferential leaching effects, prod ucing a secondary disturbance of the systematics. Furthermore, the Ar- Ar analyses indicate that the isotopic systematics in this meteorite m ight have been disturbed sometime later than 750 Ma. For these reasons , the ages obtained using different isotopic systems disagree with eac h other and a precise formation age could not be obtained for this met eorite. However, using what results there are, two reasonable interpre tations can be made: (1) the Sm-Nd system yielded an age of 3.4 Ga tha t could be interpreted as the formation age, assuming that this system is possibly the least disturbed during the metamorphic event(s) that this meteorite experienced at least once, and that all other isotopic systems and their corresponding ages were disturbed, and (2) the U-Pb system yielded a nearly concordant age of 3.8 Ga that could be interpr eted as the formation age and the Sm-Nd isotopic systematics were some how disturbed. We prefer the second interpretation for the reasons dis cussed below. On a concordia diagram, the CDT (Canon Diablo troilite)- corrected U-Pb isotopic data yield a discordia line similar to the lun ar catastrophic array, indicating that the source of the meteorite for med during early lunar differentiation (similar to 4.4 Ga) and that th e basalt was generated near 3.9 Ga. Total Ar-Ar age on plagioclase is 3.26 Ga, which seems to be too old if the formation age is 3.4 Ga, bec ause low temperature fractions lost large amounts of radiogenic Ar-40 during the late thermal event. If we assume a formation age of 3.8 Ga, the estimated source U-238/Pb-204 (mu) is 21.6 +/- 3.5 and epsilon(Nd ) is 3.9 +/- 0.3. These results indicate that the source of Yamato-793 169 is more depleted than Apollo 12 and 15 LT basalts, but less deplet ed than Asuka-881757. Therefore, Yamato-793169 may represent a new typ e of LT- or VLT-like mare basalt that is different from Asuka-881757. The wide variety of lead and neodymium isotopic characteristics among LT and VLT mare basalts indicate that the lunar mantle was very hetero geneous with respect to trace element abundances.