Ion microprobe U‐Th‐Pb dating of phosphates in martian meteorite ALH 84001

Abstract

Ion microprobe U-Th-Pb dating of phosphates in martian meteorite ALH 84001 Pb 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.

High-spatial resolution U-Pb dating of phosphate minerals in Martian meteorite Allan Hills 84001

“…200 ppm (Sano et al, 1999), the concentrations of U in the observed merrillite grains are significantly lower and show smaller variations. The variations of U concentrations observed in individual grains are even smaller than those of grain-to-grain differences reported previously for apatite and merrillite in ALH 84001 (Terada et al, 2003). Although the variation is rather small, the observed distributions of U contents are not random but instead show some trend.…”

“…Diffusion parameters of U or Pb in merrillite are not well known. However, it might be reasonable to apply those of apatite in this case because previous data of U-Th-Pb analyses of apatite and merrillite plot on the same regression lines (Sano et al, 2000;Terada et al, 2003), which indicates diffusion behaviors of U-Th-Pb system are not significantly different between the two. Therefore, we apply apatite parameters to our results here.…”

“…We again calculated the 3D total U/Pb isochron age based on the results obtained in this work (Table S1), together with those reported by Terada et al (2003). Concordia-constrained linear regression using 35 data gives the isochron age of 4015 ± 60 Ma (95% confidence limit, MSWD = 2.2).…”

“…Dashed curves are error envelopes. Previous U-Pb study of phosphates (Terada et al, 2003) and the isotopic ratios of terrestrial common Pb (Stacey and Kramers, 1975) and initial Pb estimated for ALH 84001 (Bouvier et al, 2009) Fig. S3).…”

“…However, because the phosphates are volumetrically minor and because their grain sizes are limited, minimally or non-destructive analytical methods must be used for accurate dating, even though enriched shergottites contain phosphate mineral grains up to several hundred micrometers. Currently, in-situ U-ThPb dating by SHRIMP is a major analytical method for the radiogenic system in terrestrial (Sano et al, 1999(Sano et al, , 2006aLan et al, 2013) and extraterrestrial phosphates (Sano et al, 2000;Terada et al, 2003; tremely old brecciated Martian meteorites (NWA 7034/ 7475/7533) have been found recently (Agee et al, 2013;Humayun et al, 2013).…”