Cosmogenic nuclides in the solar gas‐rich H3–6 chondrite breccia Frontier Mountain 90174

Abstract— We re‐evaluated the cosmic‐ray exposure history of the H3‐6 chondrite shower Frontier Mountain (FRO) 90174, which previously was reported to have a simple exposure history, an irradiation time of about 7 Ma, and a pre‐atmospheric radius of 80–100 cm (Welten et al. 2001). Here we measured the concentrations and isotopic compositions of He, Ne, and Ar in 8 aliquots of 6 additional fragments of this shower, and 10 Be and 26 Al in the stone fractions of seven fragments. The radionuclide concentrations in the stone fractions, combined with those in the metal fractions, confirm that all samples are fragments of the FRO 90174 shower. Four of the fragments contain solarwind‐implanted noble gases with a solar 20 Ne/ 22 Ne ratio of ˜12.0, indicating that FRO 90174 is a regolith breccia. The concentrations of solar gases and cosmogenic 21 Ne in the samples analyzed by us and by Welten et al. (2001) overlap with those of the FRO H‐chondrites from the 1984 season, suggesting that many of these samples are also part of the large FRO 90174 chondrite shower. The cosmogenic 21 Ne concentrations in FRO 90174 show no simple correlation with 10 Be and 26 Al activities. We found 21 Ne excesses between 0.3‐1.1 × 10 −8 cm 3 STP/g in 6 of the 17 samples. Since excess 21 Ne and trapped solar gases are not homogeneously distributed, i.e., we found in one fragment aliquots with and without excess 21 Ne and solar 20 Ne, we conclude that excess 21 Ne is due to GCR irradiation of the regolith before compaction of the FRO 90174 object. Therefore, the chondrite shower FRO 90174 did not simply experience an exposure history, but some material was already irradiated at the surface of an asteroid leading to excess 21 Ne. This excess 21 Ne is correlated to implanted solar gases, clearly indicating that both processes occurred on the regolith.