Strontium-Rubidium Age of an Iron Meteorite

The isotopic compositions and concentrations of rubidium and strontium were determined in silicate nodules contained in Weekeroo Station meteorite, a brecciated coarse octahedrite. The strontium had a Sr(87):Sr(86) range from 0.729 to 0.768, showing considerable enrichment in Sr(87) in coinparison with achondrites. Data for six samples of nodules lie on a straight line on the Sr-Rb evolution diagram, with an initial Sr(87):Sr(86) ratio of 0.696 to 0.702; the slope is 0.0674, corresponding to an age of 4.7 x 10(9) years for lambda = 1.39 x 10(-11) year(-1). These data agree with the previously assigned ages for the formation of stony meteorites and the earth; they support the conclusion that the major period of chemical and physical differentiation in the solar system occurred in a narrow interval at about this time. This result disagrees with the Ar(40)-K(40) ages of 5 to 13 x 10(9) years determined from other iron meteorites. A wide variety of isotopic-age investigations now seem experimentally feasible on iron meteorites that contain silicates.