Cosmic-ray exposure history of two frontier mountain h-chondrite showers from spallation and neutron-capture products

K. C. Welten, K. Nishiizumi, J. Masarik, M. W. Caffee, A. J.T. Jull, S. E. Klandrud, R. Wieler

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We measured the concentrations of 10Be, 26Al, 36Cl, 41Ca and 14C in the metal and/or stone fractions of 27 Antarctic chondrites from Frontier Mountain (FRO), including two large H-chondrite showers. To estimate the pre-atmospheric size of the two showers, we determined the contribution of neutron-capture produced 36Cl (half-life = 3.01 x 105 years) and 41Ca (1.04 x 105 years) in the stone fraction. The measured activities of neutron-capture 36Cl and 41Ca, as well as spallation produced 10Be and 26Al, were compared with Monte Carlo-based model calculations. The largest shower, FRO 90 1 74, includes eight fragments with an average terrestrial age of (100 ± 30) x 103 years; the neutron-capture saturation activities extend to 27 dpm/kg stone for 36Cl and 19 dpm/kg stone for 41Ca. The concentrations of spallation produced 10Be, 26Al and 36Cl constrain the radius (R) to 80-100 cm, while the neutron-capture 41Ca activities indicate that the samples originated from the outer 25 cm. With a pre-atmospheric radius of 80-100 cm, FRO 90 1 74 is among the largest of the Antarctic stony meteorites. The large pre-atmospheric size supports our hypothesis that at least 50 of the ∼ 150 classified H5/H6-chondrites from the Frontier Mountain stranding area belong to this single fall; this hypothesis does not entirely account for the high H/L ratio at Frontier Mountain. The smaller shower, FRO 90001, includes four fragments with an average terrestrial age of (40 ± 10) x 103 years; they contain small contributions of neutron-capture 36Cl, but no excess of 41Ca. FRO 90001 experienced a complex exposure history with high shielding conditions in the first stage (150 < R < 300 cm) and much lower shielding in the second stage (R < 30 cm), the latter starting ∼ 1.0 million years (Ma) ago. Based on the measured 10Be/21 Ne and 26Al/21Ne ratios, the cosmic-ray exposure ages of the two showers are 7.2 ± 0.5 Ma for FRO 90 1 74 and 8 ± 1 Ma for FRO 90001. These ages coincide with the well-established H-chondrite peak and corroborate the observation that the exposure age distribution of FRO H-chondrites is similar to that of non-Antarctic falls. In addition, we found that corrections for neutron-capture 36Ar (from decay of 36Cl) result in concordant 21Ne and 38Ar exposure ages.

Original languageEnglish (US)
Pages (from-to)301-317
Number of pages17
JournalMeteoritics and Planetary Science
Issue number2
StatePublished - Jan 1 2001

ASJC Scopus subject areas

  • Geophysics
  • Space and Planetary Science


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