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

      Terada, K.; Sano, Y. (The Meteoritical Society, 2004-01-01)
      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.
    • Ion microprobe U-Th-Pb dating of phosphates in martian meteorite ALH 84001

      Terada, K.; Monde, T.; Sano, Y. (The Meteoritical Society, 2003-01-01)
      Phosphates in martian meteorites are important carriers of trace elements, although, they are volumetrically minor minerals. PO4 also has potential as a biomarker for life on Mars. Here, we report measurements of the U-Th-Pb systematics of phosphates in the martian meteorite ALH 84001 using the Sensitive High Resolution Ion MicroProbe (SHRIMP) installed at Hiroshima University, Japan. Eleven analyses of whitlockites and 1 analysis of apatite resulted in a total Pb/U isochron age of 4018 +/- 81 Ma in the 238U/206Pb-207Pb/206Pb-204Pb/206Pb 3-D space, and a 232Th-208Pb age of 3971 +/- 860 Ma. These ages are consistent within a 95% confidence limit. This result is in agreement with the previously published Ar-Ar shock age of 4.0 +/- 0.1 Ga from maskelynite and other results of 3.8-4.3 Ga but are significantly different from the Sm-Nd age of 4.50 +/- 0.13 Ga based on the whole rock and pyroxene. Taking into account recent studies on textural and chemical evidence of phosphate, our result suggests that the shock metamorphic event defines the phosphate formation age of 4018 +/- 81 Ma, and that since then, ALH 84001 has not experienced a long duration thermal metamorphism, which would reset the U-Pb system in phosphates.