Boron isotopic composition in early solar system solids

摘要

Abundance and isotopic composition of boron (B) are determined in refractory silicate phases from the primitive carbonaceous chondirite Efremovka using an item microprobe. These refractory phases represent some of the earliest solids to condense from the solar nebula. All the analysed phases (anorthite, melilite and fassaite) have very low B content ranging from 0.1 to 1.3 ppm; anorthite has relatively higher B content than melilite and fassaite. The measured values of the B-11/B-10 ratio in anorthite grains are similar and indistinguishable from the normal ratio of 4.04558 within experimental uncertainties. This is at variance with the reported large magnitude B isotopic anomalies within individual meteoritic chondrules(1) vp hose formation in the nebula postdates refractory silicates. Although B isotopes could have been homogenized during the formation of refractory phases, plausibility considerations suggest that our data are consistent with a uniform distribution of B isotopes of normal composition in the solar nebula. [References: 24]