87Rb87Sr isochron and 40K40Ar ages of the norton county achondrite

Abstract

Analysis of seven different portions of Norton County yielded Rb/Sr ratios ranging from 0.15 to 2.3 and permitted the determination of an isochron with high precision. The Rb and Sr concentrations were found to be as low as ∼0.16 ppm. All of the data lie to within their experimental errors on a line in the Sr evolution diagram of slope 0.0654 ± 0.0014, and intercept 0.700 ± 0.002. This determines an age of 4.7 ± 0.1 × 109 yr for λ = 1.39 × 10−11 yr−1.40K40Ar ages on samples containing 42, 53 and 107 ppm K gave ages of 4.2 – 4.5 × 109 yr which are compatible with the RbSr age. The K appears to be concentrated in alkali feldspar grains of about 5–50 microns.The 87Rb87Sr age and primary 87Sr/86Sr value is of sufficient precision to permit the age resolution of meteoritic objects which were formed 0.3 × 109 yr apart. No evidence of element redistribution was found for Norton Country as indicated by the consistency of our RbSr and KAr results.