Noble Gases In Grant And Carbo And The Influence Of S- And P-Rich Mineral Inclusions On The (41)K-(40)K Dating System

Cosmogenic He, Ne, and Ar were measured in the iron meteorites Grant (I I I A B)and Carbo (IID) to re-determine their preatmospheric geometries and exposure histories. We also investigated the influence of sulphur- and/or phosphorus-rich inclusions oil 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 (similar to 10 cm) from earlier results (similar to 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 oil literature (36)Cl 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 oil the production rates of cosmogenic Ne. In our samples, containing oil average 0.5% S and/or P, about 20% of (21)Ne was produced from these minor elements. Using measured (21)Ne concentrations and endmember (22)Ne/(21)Ne ratios for Fe + Ni and S + P, respectively, we show that it is possible to correct for (21)Ne produced from S and/or P. The thus corrected data are then used to calculate new (41)K-(40)K 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 (21)Ne concentrations and consequently decreases the (41)K-(40)K exposure ages. The discrepancies between (36)Cl-(36)Ar and (41)K-(40)K ages are accordingly reduced. The existence of a significant long-term variation of the GCR, which is based oil a former 30-50% difference between (41)K-(40)K and (36)Cl-(36)Ar ages, may warrant re-investigation.