Rubidium-Strontium studies on enstatite chondrites: whole meteorite and mineral isochrons

Abstract

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 (2s) 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 (2s). The corresponding age and initial Sr⁸⁷Sr⁸⁶ ratio are 4.56 ± 0.15 x 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.