LITHIUM-BERYLLIUM-BORON ISOTOPIC COMPOSITIONS IN METEORITIC HIBONITE: IMPLICATIONS FOR ORIGIN OF 10Be AND EARLY SOLAR SYSTEM IRRADIATION

NanoSIMS isotopic measurements of Li, Be, and B in individual hibonite grains extracted from the Murchison meteorite revealed that 10B excesses correlate with the Be-9/B-11 ratios in 26Al-free PLAty hibonite Crystals. From these data, an initial Be-10/Be-9 = (5.5 +/- 1.6) x 10(-4) (2s) and B-10/B-11 = 0.2508 +/- 0.0015 can be inferred. On the other hand, chondritic boron isotopic compositions were found in 26Al-bearing Spinel-HIBonite spherules, most likely due to contamination with normal boron. No 7Li excesses due to 7Be decay were observed. When combined with previously reported data, the newdata yield the best defined Be-10/Be-9=(5.3 +/- 1.0) x10-4 (2s) and B-10/B-11= 0.2513 +/- 0.0012 for PLACs. A comparison of this value and the best constrained Be-10/Be-9 = (8.8 +/- 0.6) x 10(-4) in CV Ca-Al-rich inclusions supports a heterogeneous distribution of Be-10 and its protosolar irradiation origin. We consider two possible irradiation scenarios that could potentially lead to the observedLi-Be-B isotopic compositions in PLACs. Although in situ irradiation of solids with hibonite chemistry seems to provide the simplest explanation, more high quality data will be needed for quantitatively constraining the irradiation history.