Autor: Welten, K. C.; Nishiizumi, K.; Masarik, J.; Caffee, M. W.; Jull, A. J. T.; Klandrud, S. E.; Wieler, R.
Názov: Cosmic-ray exposure history of two Frontier Mountain H-chondrite showers from spallation and neutron-capture products
Typ: publication
Dátum: 2000
Jazyk: en
Poznámka:
Abstrakt:
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 = 3.01×10⁵ years) and ⁴¹Ca (1.04×10⁵ 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
90174, includes eight fragments with an average terrestrial age of (100±30)×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 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)×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/²¹Ne and ²⁶Al/²¹Ne 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 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.
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