40Ar/39Ar age and thermal history of the Kirin chondrite

Abstract

The Kirin meteorite, a large (2800kg) H5 chondrite, fell in Kirin Province, China in 1976. A sample from each of the two largest fragments (K-1, K-2) yield40Ar/39Ar total fusion ages of 3.63 ± 0.02b.y. and 2.78 ± 0.02b.y. respectively.40Ar/39Ar age spectra show typical diffusional argon loss profiles. Maximum apparent ages of 4.36 b.y. (K-1) and ∼4.0 b.y. (K-2) are interpreted as possible minimum estimates for the age of crystallization of the parent body. The40Ar/39Ar ages found for gas released at low temperature are about 2.2 b.y. for K-1 and about 0.5 b.y. for K-2, suggesting that this meteorite may have suffered two discrete collisional events that caused degassing of radiogenic argon. Modelling of possible thermal events in the parent body indicates that samples K-1 and K-2 were at a depth of less than 3 m from the base of an impact melt of a thickness less than 7 m and separated by no more than ∼2 m from one another at the time of the heating event about 0.5 b.y. ago. Further, the duration of heating was probably less than a few years. Calculations from38Ar data yield exposure ages for samples K-1 and K-2 of about 5 m.y., similar to that found for many other H chondrites. Show more