Boron isotopic composition in early solar system solids

Abundance and isotopic composition of boron (B) are determined in refractor y silicate phases from the primitive carbonaceous chondirite Efremovka usin g an item microprobe. These refractory phases represent some of the earlies t solids to condense from the solar nebula. All the analysed phases (anorth ite, 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 iso topic anomalies within individual meteoritic chondrules(1) vp hose formatio n in the nebula postdates refractory silicates. Although B isotopes could h ave been homogenized during the formation of refractory phases, plausibilit y considerations suggest that our data are consistent with a uniform distri bution of B isotopes of normal composition in the solar nebula.