Noble gas composition, cosmic-ray exposure age, 39Ar-40Ar, and I-Xe analyses of 
ungrouped achondrite NWA 7325

Hopp, Jens; Schroeter, Natalie; Pravdivtseva, Olga; Meyer, Hans-Peter; Trieloff, Mario; Ott, Ulrich

doi:10.1111/maps.13062

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Noble gas composition, cosmic-ray exposure age, ³⁹Ar-⁴⁰Ar, and I-Xe analyses of ungrouped achondrite NWA 7325

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Ott, Ulrich
Biogeochemistry, Max Planck Institute for Chemistry, Max Planck Society;

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Citation

Hopp, J., Schroeter, N., Pravdivtseva, O., Meyer, H.-P., Trieloff, M., & Ott, U. (2018). Noble gas composition, cosmic-ray exposure age, ³⁹Ar-⁴⁰Ar, and I-Xe analyses of ungrouped achondrite NWA 7325. Meteoritics and Planetary Science, 53(6), 1150-1163. doi:10.1111/maps.13062.

Cite as: https://hdl.handle.net/21.11116/0000-0001-A9EB-8

Abstract

Northwest Africa (NWA) 7325 is an anomalous achondrite that experienced episodes of large‐degree melt extraction and interaction with melt under reducing conditions. Its composition led to speculations about a Mercurian origin and provoked a series of studies of this meteorite. We present the noble gas composition, and results of 40Ar/39Ar and 129I‐129Xe studies of whole rock splits of NWA 7325. The light noble gases are dominated by cosmogenic isotopes. 21Ne and 38Ar cosmic‐ray exposure ages are 25.6 and 18.9 Ma, respectively, when calculated with a nominal whole rock composition. This 38Ar age is in reasonable agreement with a cosmic‐ray exposure age of 17.5 Ma derived in our 40Ar/39Ar dating study. Due to the low K‐content of 19 ± 1 ppm and high Ca‐content of approximately 12.40 ± 0.15 wt%, no reliable 40Ar/39Ar age could be determined. The integrated age strongly depends on the choice of an initial 40Ar/36Ar ratio. An air‐like component is dominant in lower temperature extractions and assuming air 40Ar/36Ar for the trapped component results in a calculated integrated age of 3200 ± 260 (1σ) Ma. This may represent the upper age limit for a major reheating event affecting the K‐Ar system. Results of 129I‐129Xe dating give no useful chronological information, i.e., no isochron is observed. Considering the highest 129Xe*/128XeI ratio as equivalent to a lower age limit, we calculate an I‐Xe age of about 4536 Ma. In addition, elevated 129Xe/132Xe ratios of up to 1.65 ± 0.18 in higher temperature extractions indicate an early formation of NWA 7325, with subsequent disturbance of the I‐Xe system.