Solar and galactic cosmic-ray records of the Fermo (H) chondrite regolith breccia

Bonino, G. ; Bhandari, N. ; Murty, S. V. S. ; Mahajan, R. R. ; Suthar, K. M. ; Shukla, A. D. ; Shukla, P. N. ; Cini Castagnoli1, G. ; Taricco, C. (2001) Solar and galactic cosmic-ray records of the Fermo (H) chondrite regolith breccia Meteoritics & Planetary Science, 36 (6). pp. 831-839. ISSN 1086-9379

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We demonstrate the presence of solar flare as well as neutron capture effects in the isotopic composition of rare gases in the Fermo regolith breccia acquired on its parent body based on the measurements of tracks, rare gases and radionuclides. The track density along a 3.2 cm long core decreases by a factor of about 6 and by more than a factor of 13 within the meteorite, indicating small (2-9 cm) and asymmetrical ablation. Rare gases show a large trapped component; the isotopic ratios, particularly 20Ne/22Ne ≅ 11 and 20Ne/36Ar = 10 are indicative of a solar component. The galactic cosmic-ray exposure age is determined to be 8.8 Ma. Activities of a dozen radionuclides ranging in half-life from 16 day 48V to 0.73 Ma 26Al are consistent with their expected production rates. Track, rare gas and radionuclide data show that the meteoroid was a small body (≤ 120 kg) and had a simple, one-stage exposure history to cosmic rays in the interplanetary space. However, 82Kr and 128Xe show an excess due to neutron irradiation on the parent body of the meteorite. The presence of solar gases and the neutron capture effects indicate several stages of irradiation on the parent asteroid. The chemical composition of Fermo confirms that it belongs to the H group of ordinary chondrites with lithic clasts having varying compositions. δ 15N is found to be 8.3 ± 1.2%0, close to the typical values observed in H chondrites.

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