Sr stable isotope composition of Earth, the Moon, Mars, Vesta and meteorites
Résumé
High-precision stable Sr isotopic variations (88Sr/86Sr) are reported in a variety of terrestrial samples, martian and lunar meteorites, HED, undifferentiated primitive meteorites, chondrules and refractory inclusions. Almost all the whole-rock samples are isotopically indistinguishable at a 50 parts per million (ppm) level. The exceptions are CV and CO chondrites which are isotopically light and for which we believe that their isotopic composition is controlled by the proportion of refractory material. Five separated chondrules and one refractory inclusion from Allende are isotopically light, with δ88/86Sr fractionations up to −1.73‰, whereas the matrix is enriched in the heavy isotopes (δ88/86Sr = + 0.66‰). The depletion in heavy isotopes observed in chondrules and refractory inclusions could be attributed to the condensation of a material already depleted in Sr, however, in that case more than 60% of the original material would be unaccounted. We propose instead that isotopic fractionation by electromagnetic sorting of ionized heavy Sr from neutral Sr in the early solar system for the origin of the fractionation observed in refractory inclusions and redistribution of Sr by aqueous alteration for the origin of the fractionation observed in chondrules and matrix. We conclude that CV and CO chondrites are not the primary building blocks for Earth and Mars. Research Highlights ► The Sr of most planetary materials is isotopically indistinguishable at a 50 ppm level. ► Exceptions are CV and CO chondrites which are isotopically light. ► Their isotopic composition is controlled by the proportion of refractory material. ► Chondrules and refractory inclusion from Allende are isotopically light. ► The Sr isotopic composition of CAIs can be explained by electromagnetic separation in the early solar system. ► The Sr isotopic composition of chondrules and matrix can be explained by aqueous alteration.