High temperature strontium stable isotope behaviour in the early solar system and planetary bodies

Abstract

This study presents comprehensive strontium stable isotope (88Sr/86Sr) data, measured by multiple-collector inductively-coupled plasma mass spectrometry (MC–ICP-MS), for a suite of carbonaceous chondrites, differentiated meteorites, lunar, martian and terrestrial samples.Carbonaceous chondrites comprise a mixture of refractory inclusions and chondrules that have light δ88Sr values between −0.35 to +0.05‰ and matrix material that possesses a heavy δ88Sr composition of 0.65‰, confirming the results of earlier studies. Consequently, bulk carbonaceous chondrites are relatively heterogeneous in composition ranging from +0.12 to +0.35‰, most likely reflecting clast–matrix variability. The light δ88Sr compositions of the refractory inclusions are consistent with mass dependent fractionation of other refractory elements (Ca and Eu) and are most likely produced by non-equilibrium fractionation (undercooling in the nebula gas) during condensation of hibonite from the solar nebula (Simon and DePaolo, 2010. Stable calcium isotopic composition of meteorites and rocky planets. Earth Planet. Sci. Lett. 289, 457–466).Carbonaceous chondrites, angrites and martian meteorites have indistinguishable compositions at the level of analytical uncertainty of this study. However, statistical analysis indicates that melts derived from the Earth's mantle have heavier δ88Sr values than bulk carbonaceous chondrites and martian meteorites, but compositions indistinguishable from eucrites (δ88Sr=+0.26±0.12‰). Moreover, terrestrial basalts and andesites have restricted δ88Sr values (+0.30±0.07‰), suggesting that mantle melting delivers rather homogenous melts to the Earth's surface with respect to δ88Sr.In contrast, glasses from evolved terrestrial rocks and lunar basalts extend to very light δ88Sr values ~−0.20‰. The Sr stable isotope composition covaries with europium anomaly (Eu/Eu*), as an index of plagioclase fractionation, and δ88Sr can be successfully modelled by the heavy isotopes of Sr being preferentially partitioned into plagioclase with a fractionation factor of ~1.0007 for 88Sr/86Sr. Our results demonstrate that Sr stable isotopes may be significantly fractionated at high temperatures and their measurement can provide insights into planetary evolution and magmatic processes.