Non-Chondritic Stable Strontium (δ88Sr vs. δ84Sr) in the Earth, the Moon and Some Differentiated Asteroids

Abstract:

The determination of isotopic anomalies in early solar system materials provides important constraints on the nucleosynthetic origin and isotopic fractionation of elements during nebular condensation and subsequent processing in the protoplanetary disk. Several recent studies involving high-precision Sr isotope measurements have pointed towards the possibility of small excesses in the minor p-process isotope ⁸⁴Sr in chondritic meteorites. However, these data remain equivocal because of the reliance on internal normalization to a fixed value of 0.1194 for the ⁸⁶Sr/⁸⁸Sr in order to correct for instrumental mass fractionation. On the basis of such data alone, it is not possible to determine with certainty which isotopes have anomalous abundances. Here, we present new high-precision ⁸⁴Sr-⁸⁷Sr double spike data that return the true isotopic composition for all the Sr isotope ratios, free from any assumptions about normalization values. Our new data demonstrate that the Earth, the Moon, eucrites and diogenites plus several angrites lie on a single mass-dependent fractionation trend in three isotope space (δ⁸⁸Sr vs δ⁸⁴Sr). In contrast, bulk CI, CM and CV3 carbonaceous chondrites have highly variable δ⁸⁸Sr but lie on a separate mass-dependent fractionation line of identical slope which is offset towards excess ⁸⁴Sr by ca. 100 ppm. Several aliquots of the angrite Sahara 99555 yield high δ⁸⁴Sr values, but with ‘normal’ δ⁸⁸Sr similar to other angrites. Stable Sr variations in chondritic meteorites most probably reflect primary Sr isotope nucleosynthetic heterogeneity in the early solar system, whilst those for Sahara 99555 indicate later addition of distinct chondritic material. Our new Sr double-spike data clearly demonstrate fundamental differences between materials forming the asteroids and terrestrial planets versus chondritic materials (in particular CI meteorites) thought to be the closest representative of solar photosphere composition.