Ion microprobe U-Th-Pb dating and REE analyses of phosphates in the nakhlites Lafayette and Yamato-000593/000749

Abstract

U, Th, and Pb isotopes and rare earth elements (REEs) in individual phosphate grains from martian meteorites Lafayette and Yamato-000593/000749 were measured using a sensitive highresolution ion microprobe (SHRIMP). Observed U-Pb data of 12 apatite grains from Yamato (Y-) 000593/000749 are well represented by linear regressions in both conventional 2D isochron plots and the 3D U-Pb plot (total Pb/U isochron), indicating that the formation age of this meteorite is 1.53 +/- 0.46 Ga (2-sigma). On the other hand, the data of nine apatite grains from Lafayette are well represented by planar regression rather than linear regression, indicating that its formation age is 1.15 +/- 0.34 Ga (2-sigma) and that a secondary alteration process slightly disturbed its U-Pb systematics as discussed in the literature regarding Nakhla. The observed REE abundance patterns of the apatites in Lafayette and Yamato-000749, normalized to CI chondrites, are characterized by a progressive depletion of heavy REEs (HREEs), a negative Eu anomaly, similarity to each other, and consistency with previously reported data for Nakhla. Considering the extensive data from other radiometric systems such as Sm- Nd, Rb-Sr, Ar-Ar, and trace elements, our results suggest that the parent magmas of the nakhlites, including the newly found Y-000593/000749, are similar and that their crystallization ages are ~1.3 Ga.