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

Abstract

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.