Cosmic-ray production rates for 3He, 21Ne, 38Ar, 83Kr, and 126Xe in chondrites based on 81Kr-Kr exposure ages

A noble gas study of the Kobe (CK4) chondrite by a stepwise heating method.

“…By using the estimated amounts of cosmogenic and radiogenic components released from 900 to 1600°C, we calculated cosmic-ray exposure ages (T 3 Eugster (1988) with the elemental composition of Kobe C (Hirota et al, 2000) or Kobe E (Oura et al, 2000). An averaged chemical composition for CK chondrites (Lodders and Fegley, 1998) was used where the elemental abundance for Kobe was not available.…”

“…However, they do not seem to be paired because the other noble gas signatures such as He concentration are different each other. The longer T 126 age (62.6 Ma) indicates that there would be some difficulty in obtaining appropriate production rate with a method described in Eugster (1988) because of heavy shielding with the low cosmogenic 22 Ne/ 21 Ne ratio of Kobe. On the other hand, the ages T 3 (14.0 Ma) and T 4 (1.2 Ga) are shorter than the other exposure ages and the T 40 age, respectively (T 3 /T 21 = 0.37, T 4 /T 40 = 0.41).…”

Noble gas and oxygen isotopic compositions of Towada H6-chondrite from Aomori Prefecture, Japan

“…The ( 22 Ne/ 21 Ne) c ratio of 1.177 for the Towada meteorite is higher than the average chondrite value of 1.11 (e.g., Eugster, 1988). This may imply a small preatmospheric size while the meteorite was in space, because contribution of secondary neutron-produced 21 Ne through 24 Mg(n,α) 21 Ne reaction becomes small in smaller object compared to that in ordinary size meteoroids (e.g., Masarik et al, 2001).…”

“…The sample of 0.0306 g used for noble gas measurements may not represent the bulk Towada meteorite, at least with respect to Ca and Fe which are the main target elements producing 38 Ar by cosmic-ray bombardment. Ca and Fe concentrations play a significant role in the calculation of T 38 ages using the equation of Eugster (1988). There is also the possibility that 38 Ar was lost during weathering as pointed out for E-chondrites by Okazaki et al (2000) and Patzer and Schultz (2001).…”

“…The production rates of 3 He, 21 Ne and 38 Ar, abbreviated as P 3 , P 21 and P 38 , were calculated using the formulae proposed by Eugster (1988), inserting 1.177 ( 22 Ne/ 21 Ne) c in the correction factor for shielding depth and the correction factors for target element composition of H-chondrites summarized in Eugster (1988). The calculated cosmic-ray exposure ages using 3 He (T 3 ), 21 Ne (T 21 ) and 38 Ar (T 38 ) are 7.68, 31.1 and 18.7 m.y., respectively.…”