Noble gases in Grant and Carbo and the influence of S- and P-rich mineral inclusions on the 41K-40K dating system

Abstract

Cosmogenic He, Ne, and Ar were measured in the iron meteorites Grant (IIIAB) and Carbo (IID) to re-determine their preatmospheric geometries and exposure histories. We also investigated the influence of sulphur- and/or phosphorus-rich inclusions on the production rates of cosmogenic Ne. Depth profiles measured in Grant indicate a preatmospheric center location 117 mm left from the reference line and 9 mm below bar B, which is clearly different (~10 cm) from earlier results (~165 mm left from the reference line on bar F). For Carbo the preatmospheric center location was found to be 120 mm right of the reference line and 15 mm above bar J, which is in agreement with literature data. The new measurements indicate a spherical preatmospheric shape for both meteorites and, based on literature 36Cl data, the radii were estimated to be about 32 cm and 70 cm for Grant and Carbo, respectively. We demonstrate that minor elements like S and P have a significant influence on the production rates of cosmogenic Ne. In our samples, containing on average 0.5% S and/or P, about 20% of 21Ne was produced from these minor elements. Using measured 21Ne concentrations and endmember 22Ne/21Ne ratios for Fe + Ni and S + P, respectively, we show that it is possible to correct for 21Ne produced from S and/or P. The thus corrected data are then used to calculate new 41K-40K exposure ages--using published K data--which results in 564 +/- 78 Ma for Grant and 725 +/- 100 Ma for Carbo. The correction always lowers the 21Ne concentrations and consequently decreases the 41K-40K exposure ages. The discrepancies between 36Cl-36Ar and 41K-40K ages are accordingly reduced. The existence of a significant long-term variation of the GCR, which is based on a former 30-50% difference between 41K-40K and 36Cl-36Ar ages, may warrant re-investigation.