Rubidium-strontium studies on enstatite chondrites: Whole meteorite and mineral isochrons

Eight of the eleven known enstatite chondrite falls have been analyzed for K, Rb, and Sr concentrations and Sr isotopic composition. Several density fractions having a grain size of less than 37 microns have also been separated from one of them, Indarch, and Rb-Sr analyses made. The total meteorite samples show Sr87/Sr86 ratios ranging from 0.7200 to 1.001 and density fractions from 0.7600 to 1.015. All the total samples lie on a well-defined straight line on the Sr-Rb evolution diagram, which has a slope of 0.0652±0.0012 (2σ) and an intercept at 0.6993±0.0010 (2σ). Assuming that these objects are cogenetic, the straight line with the above slope gives an age of 4.54±0.13×109 years (λ = 1.39 × 10−11 year−1) and an initial Sr87Sr86 ratio of 0.6993±0.0015, where the errors are extreme limits. The mineral concentrates also lie on a well-defined internal isochron with a slope of 0.0654±0.0011 and an intercept of 0.7005±0.0017 (2σ). The corresponding age and initial Sr87Sr86 ratio are 4.56±0.15×109 years and 0.7005±0.0030, respectively. K/Rb ratios for the total samples vary from 250 to 900. The density fractions show a more uniform ratio of about 250. The mean Sr content of enstatite chondrites is about 30% less than that of the ordinary chondrites.