Li and B isotopic variations in an Allende CAI: Evidence for the in situ decay of short-lived Be-10 and for the possible presence of the short-lived nuclide Be-7 in the early solar system

摘要

The concentrations and isotopic compositions of lithium, beryllium, and boron, analyzed in situ by ion microprobe in 66 spots of a type B1 Ca-Al-rich inclusion (CAI 3529-41) from the Allende meteorite, are reported. Large variations are observed for both the Li and the B isotopic ratios with Li-7/Li-6 ranging from 9.2 +/- 0.22 to 12.22 +/- 0.43 (a approximate to 250 parts per thousand range in delta(7)Li values) and B-10/B-11 ranging from 0.2468 +/- 0.0057 to 0.4189 +/- 0.0493 (a 410 parts per thousand range in delta(11)B values). The verylow Li concentrations (< 1 ppb) observed in several anorthite and fassaite grains require that a correction for the contribution of spallogenic Li produced during irradiation of the Allende meteoroid by galactic cosmic rays (GCR) be made (after this correction Li-7/Li-6 ranges from 9.2 +/- 0.22 to 13.44 +/- 0.56, i.e., a approximate to 350 parts per thousand range in delta(7)Li values). In 3529-41, the B-10/B-11 ratios are positively correlated with Be-9/B-11 in a manner indicating the in situ decay of short-lived Be-10 (half-life = 1.5 Ma) with a Be-10/Be-9 ratio at the time of formation of the CAI of 8.8 +/- 0.6 x 10(-4), which is in agreement with previous findings [McKeegan, K.D., Chaussidon, M., Robert, F., 2000. Incorporation of short-lived Be-10 in a calcium-aluminum-rich inclusion from the Allende meteorite. Science 289, 1334-1337]. The present detailed investigation demonstrates that only minor perturbations of the Be-10-B-10 system are present in 3529-41, contrary to the Al-26/Mg-26 system for which numerous examples of isotopic redistribution following crystallization were observed [Podosek, F.A., Zinner, E.K., MacPherson, G.J., Lundberg, L.L., Brannon, J.C., Fahey, AJ., 1991. Correlated study of initial Sr-87/Sr-86 and Al-Mg systematics and petrologic properties in a suite of refractory inclusions from the Allende meteorite. Geochim. Cosmochim. Acta 55, 1083-1110]. Petrographically based criteria were developed to identify within the 66 analyzed spots in 3529-41, those where post-magmatic perturbation of the Li and Be distributions occurred. Li and Be concentrations measured in different analytical spots are compared with those predicted by using experimentally determined partition coefficients according to a model of closed-system crystallization of the CAI melt. These criteria show that 56% of the spots in melilite, 38% in anorthite, and 8% in fassaite suffered post-crystallization perturbations of Li and/or Be distributions. In the remaining spots, which do not show obvious indication of redistribution of Li or Be, the Li-7/Li-6 isotopic variations (corrected for GCR exposure) are positively correlated with Be-9/Li-6 suggesting the in situ decay of now-extinct Be-7. The derived isochron implies that at the time of its formation, the CAI melt had a Be-7/Be-9 ratio of 0.0061 +/- 0.0013 and a Li-7/Li-6 ratio of 11.49 +/- 0.13. In contrast, all the spots in 3529-41, which do show evidence for post-magmatic redistribution of Li and Be, have relatively constant Li-7/Li-6, averaging 11.72 +/- 0.56, which is consistent with mass balance calculations for Li isotopic homogenization in the CAI after the decay of Be-7.