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

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.