Isotopic records in CM hibonites: Implications for timescales of mixing of isotope reservoirs in the solar nebula

The magnesium isotopic compositions of 26 hibonite-bearing inclusions from the CM chondrite Murchison, as well as isotopic measurements on a subset of these samples for oxygen, titanium, and lithium-beryllium-boron are reported along with oxygen isotopic data for an additional 13 hibonites that were previously investigated for other isotope systems (magnesium, potassium, calcium, and titanium) and rare earth element concentrations. Magnesium isotopic compositions divide CM hibonites into two distinct populations which correlate perfectly with their mineralogy and morphology, as previously discovered by Ireland [Ireland T. R. (1988) Correlated morphological, chemical, and isotopic characteristics of hibonites from the Murchison carbonaceous chondrite. Geochim. Cosmochim. Acta 52, 2827-2839]: Spinel-HIBonite spherules (SHIBs) bear evidence of in situ Al-26 decay, whereas PLAty-Crystals (PLACs) and Blue AGgregates (BAGs) either lack resolvable Mg-26-excesses or exhibit Mg-26 deficits by up to similar to 47 parts per thousand. High precision, multiple collector SIMS analyses show that 6 of 7 SHIBs investigated fall on a single correlation line implying Al-26/Al-27 = (4.5 +/- 0.2) x 10(-5) at the time of isotopic closure, consistent with the canonical Al-26 abundance characteristic of internal isochrons in many calcium-aluminum-rich inclusions (CAIs). One SHIB sample exhibits Delta Mg-26* consistent with a supracanonical Al-26/Al-27 ratio of (6.4 +/- 0.5) x 10(-5). The PLAC hibonites contain highly anomalous titanium isotopic compositions, with delta Ti-50 values ranging from -80 parts per thousand to almost +200 parts per thousand, whereas SHIBs generally lack large Ti isotopic anomalies. Eight out of 11 Al-26-free PLAC hibonite grains record B-10/B-11 excesses that correlate with Be/B; the inferred initial Be-10/Be-9 ratio of (5.1 +/- 1.4) x 10(-4) is lower than the best-con strained Be-10/Be-9 of (8.8 +/- 0.6) x 10(-4) in a CV CAI. The data demonstrate that Be-10 cannot be used as a relative chronometer for these objects and that most of the Be-10 observed in CAIs must be produced by irradiation of precursor solids in the early solar system. The lack of Al-26 in PLAC hibonites indicates that significant amounts of Al-26 were not formed in the same spallogenic processes that made Be-10 in PLAC precursors. This is most easily understood as indicating very early formation of the PLAC hibonites, prior to the incorporation and mixing of Al-26 into the solar nebula, although an alternative scenario, which invokes irradiation under different solar flare conditions, cannot be ruled out. Lithium isotopes are normal within uncertainties, probably reflecting contamination and/or postcrystallization exchange. The oxygen isotopic compositions of SHIBs and PLACs are all highly O-16-enriched, but are not derived from a homogeneous reservoir: Delta O-17 values span a range of similar to-28 parts per thousand to -15 parts per thousand. The ranges of O-16-enrichment in SHIBs and PLACs overlap and are less anomalous than the most O-16-enriched compositions found in meteorites [Kobayashi S., Imai H. and Yurimoto H. (2003) New extreme O-16-rich chondrule in the early solar system. Geochem. J. 37, 663-669]. Both PLACs and SHIBs formed in O-16-enriched reservoirs characterized by small-scale heterogeneities in the gas phase. If such heterogeneities were generated by an admixture of relatively O-16-poor gas created by self-shielding during CO photolysis and transported to the hot inner regions of the accretion disk, then this process must have been initiated very early on, prior to the arrival of fresh radioactivity into the inner solar system. Oxygen isotope heterogeneities persisted throughout the formation interval of PLACs, CAI precursors, and SHIBs which could be as long as 3 x 10(5) years based on Al-26 records. One SHIB and one BAG exhibit mass fractionated oxygen isotopic compositions similar to those seen in FUN inclusions and in several platy hibonite crystals [Lee T., Mayeda T. K. and Clayton R. N. (1980) Oxygen isotopic anomalies in Allende inclusion HAL. Geopkys. Res. Lett. 7, 493-496; Ireland T. R., Zinner E. K., Fahey A. J. and Esat T. M. (1992) Evidence for distillation in the formation of HAL and related hibonite inclusions. Geochim. Cosmochim. Acta 56, 2503-2520; Ushikubo T., Hiyagon H. and Sugiura N. (2007) A FUN-like hibonite inclusion with a large Mg-26-excess. Earth Planet. Sci. Lett. 254, 115-126]. The suite of mass-fractionated hibonites exhibit a range of isotopic properties, including Al-26/Al-27 ratios from below detection to a canonical level and oxygen and titanium anomalies that are not exceptional by PLAC standards. This suggests that F (fractionation) processes-evaporation under (oxidizing) conditions-are not necessarily associated with sampling a special isotopic reservoir. (C) 2009 Elsevier Ltd. All rights reserved.