5:15 PM - 6:30 PM
[PPS12-P05] Be-B systematics of refractory inclusions in CO and CH chondrites
Keywords:early solar system, irradiation processes, Be-B isotopes
Previous studies have demonstrated that a short-lived radionuclide 10Be, which decays to 10B with a half-life of 1.4Myr , was present in the early solar system with initial 10Be/9Be ratios ranging from 10-4 to 10-2 [5-13]. However, most of the data come from refractory inclusions in CV3 chondrites. To further investigate the distribution of 10Be and irradiation conditions in the early solar system, we conducted Be-B isotopic measurements using a NanoSIMS 50 (at AORI, Univ. of Tokyo) on compact melilite-rich CAIs in primitive chondrites, Y81020 (C03.05) and SaU290 (CH3).
The melilite-rich CAI in Y81020 yields an isochron with the initial 10Be/9Be ratio comparable to those of CV CAIs within uncertainties. The results suggest that CO CAIs have also experienced irradiation processes similar to CV CAIs. In contrast, a melilite-rich CAI in SaU290 shows no resolvable excesses in 10B from the terrestrial value. Previous studies have demonstrated that hibonite-rich inclusions in CMs and FUN inclusions in CVs typically show lower 10Be/9Be ratios than those of most normal CAIs [7, 9, 12, 13]. In addition, these inclusions are known to have low 26Al abundances, which is interpreted as their formation prior to the injection of 26Al into the solar system [e.g., 14]. These observations may suggest that FUN-like inclusions record irradiation history in the protosolar molecular cloud [9, 15] and/or heterogeneous distribution of 10Be in the early solar system [12, 13]. A substantial fraction of CH CAIs also has little 26Al , suggesting possible relevance to FUN-like inclusions. The low 10Be/9Be ratio of the CH CAI observed in this study could, therefore, support the above hypothesis.
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