Young ages of two diogenites and their genetic implications

Abstract

THE achondrite meteorites known as eucrites and diogenites are thought to be igneous rocks derived from the crust and mantle, respectively, of a common parent planet or asteroid^1,2. Several eucrites have been dated at 4.5–4.6 Gyr by the Rb–Sr method^4–6, and a Rb–Sr age of 4.45±0.18 Gyr (λ = 1.42x 10⁻¹¹yr⁻¹) has been reported3 for three diogenites. From these data, one cannot judge whether diogenites and eucrites are the same age. Here we present precise Rb–Sr data for two diogenites (Tatahouine and Johnstown) that yield an age of 4.394 ±0.011 Gyr—significantly younger than our age of 4.52 ±0.15 Gyr for the Juvinas eucrite. The low initial ⁸⁷Sr/⁸⁶Sr ratio of these diogenites, and their rare-earth element abundance patterns, are inconsistent with the interpretation that the young ages reflect secondary metamorphism. We suggest that our data raise doubts about a genetically close relationship between eucrites and these diogenites, and that there exist groups of diogenites that are genetically distinct from one another. Our results also suggest that igneous activity in the assumed parent body continued for about one hundred million years after its formation 4.52 Gyr ago. The parent body may thus have had a complicated evolutionary history.