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

Abstract

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 high‐resolution 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 (2σ). 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 (2σ) 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.