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

Abstract

81Kr-Kr exposure ages and the isotopic abundances of He, Ne, Ar, Kr, and Xe were measured for thirteen ordinary chondrites. Together with the 81Kr-Kr ages for four chondrites obtained earlier in our laboratory and two chondrites reported by other authors, this data base is used to derive cosmic-ray production rates for 3He, 21Ne, 38Ar, 83Kr, and 126Xe. Production rates are obtained as a function of the shielding dependent cosmogenic ratio ( 22Ne 21Ne ) c and of the chemical composition of CI, CM, CO, CV, H, L, LL, EH, and EL chondrites. The following production rate equations are proposed (P 3),P 21, and P 38in units of 10 −8 cm 3 STP/g per Ma, P 83and P 126 in 10 −12 cm 3 STP/g per Ma): P 3 = F[2.09 − 0.43( 22Ne 21Ne ) c], F CI = 1.01, F CM = 1.00, F CO = 0.99, F CV = 0.99, F H = 0.98 , F L=1.00, F LL=1.00, F eh = 0.97, and F EL = 1.00 . P 21 = 1.61F[21.77( 22Ne 21Ne ) c− 19.32] −1, F CI = 0.67, F CM = 0.79, F CO = 0.96, F CV = 0.96, F H = 0.93 , F L=1.00, F LL=1.00, F EH = 0.78, and F EL = 0.96 . P 38 = F[0.125 − 0.071( 22Ne 21Ne ] c, F CI, = 0.75, F CM = 0.88, F CO = 1.03, F CV = 1.10, F H = 1.08 , F L=1.00, F LL=1.00, F EH = 0.98, and F EL = 0.89 . P 83 = 0.0196F[0.62( 22Ne 21Ne ) c − 0.53] −1, F CI = 0.71, F CM = 0.94,if CO = 1.02, F CV = 1.13, F H = 1.00 , F L=1.00, F LL=1.00, F EH = 0.75, and F EL = 0.80 . P 126 = F[0.0174 − 0.0094( 22Ne 21Ne ) c], F CI = 0.66, F CM = 0.93, F CO = 1.18, F CV = 1.40, F H = 1.00 , F L = 1.00, F LL = 1.00, F EH = 0.72, and F EL = 0.72 . Exposure ages were calculated from cosmogenic 3He, 21Ne, and 38Ar concentrations in eleven chondrites which lack Kr data. The good agreement of the ages derived from different noble gases demonstrates the consistency of the adopted production rates. Finally, noble gas data from mineral separates and from stepped temperature fractions show that Kr and Xe extracted between 700 and 1200°C from a feldspar-rich separate have large excesses of the cosmogenic component. This should result in more accurate 81Kr-Kr and 126Xe ages than bulk chondrite analyses.