Cosmic‐ray exposure history of two Frontier Mountain H‐chondrite showers from spallation and neutron‐capture products

• Published in: Meteoritics & Planetary Science
• Main Authors: WELTEN, K. C., NISHIIZUMI, K., MASARIK, J., CAFFEE, M. W., JULL, A. J. T., KLANDRUD, S. E., WIELER, R.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2001
• Subjects: Antarctic; The Antarctic; Frontier Mountain; Antarc*
• Online Access: http://dx.doi.org/10.1111/j.1945-5100.2001.tb01872.x; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1111%2Fj.1945-5100.2001.tb01872.x; https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1945-5100.2001.tb01872.x

Abstract— We measured the concentrations of 10 Be, 26 Al, 36 Cl, 41 Ca and 14 C 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 36 Cl (half‐life = 3.01 times 10 5 years) and 41 Ca (1.04 times 10 5 years) in the stone fraction. The measured activities of neutron‐capture 36 Cl and 41 Ca, as well as spallation produced 10 Be and 26 Al, were compared with Monte Carlo‐based model calculations. The largest shower, FRO 90174, includes eight fragments with an average terrestrial age of (100 ± 30) × 10 3 years; the neutron‐capture saturation activities extend to 27 dpm/kg stone for 36 Cl and 19 dpm/kg stone for 41 Ca. The concentrations of spallation produced 10 Be, 26 Al and 36 Cl constrain the radius ( R ) to 80–100 cm, while the neutron‐capture 41 Ca activities indicate that the samples originated from the outer 25 cm. With a pre‐atmospheric radius of 80–100 cm, FRO 90174 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) × 10 3 years; they contain small contributions of neutron‐capture 36 Cl, but no excess of 41 Ca. 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 10 Be/ 21 Ne and 26 Al/ 21 Ne ratios, the cosmic‐ray exposure ages of the two showers are 7.2 ± 0.5 Ma for FRO 90174 and 8 ± 1 Ma for FRO 90001. These ages coincide with the ...