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

Liu, Ming-Chang ; McKeegan, Kevin D. ; Goswami, Jitendra N. ; Marhas, Kuljeet K. ; Sahijpal, Sandeep ; Ireland, Trevor R. ; Davis, Andrew M. (2009) Isotopic records in CM hibonites: implications for timescales of mixing of isotope reservoirs in the solar nebula Geochimica et Cosmochimica Acta, 73 (17). pp. 5051-5079. ISSN 0016-7037

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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 26Al decay, whereas PLAty-Crystals (PLACs) and Blue AGgregates (BAGs) either lack resolvable 26Mg-excesses or exhibit 26Mg deficits by up to ~4‰. High precision, multiple collector SIMS analyses show that 6 of 7 SHIBs investigated fall on a single correlation line implying 26Al/27Al = (4.5 ± 0.2) × 10-5 at the time of isotopic closure, consistent with the "canonical" 26Al abundance characteristic of internal isochrons in many calcium-aluminum-rich inclusions (CAIs). One SHIB sample exhibits Δ26Mg consistent with a "supracanonical" 26Al/27Al ratio of (6.4 ± 0.5) × 10-5. The PLAC hibonites contain highly anomalous titanium isotopic compositions, with δ50Ti values ranging from -80‰ to almost +200‰, whereas SHIBs generally lack large Ti isotopic anomalies. Eight out of 11 26Al-free PLAC hibonite grains record 10B/11B excesses that correlate with Be/B; the inferred initial 10Be/9Be ratio of (5.1 ± 1.4) × 10-4 is lower than the best-constrained 10Be/9Be of (8.8 ± 0.6) × 10-4 in a CV CAI. The data demonstrate that 10Be cannot be used as a relative chronometer for these objects and that most of the 10Be observed in CAIs must be produced by irradiation of precursor solids in the early solar system. The lack of 26Al in PLAC hibonites indicates that significant amounts of 26Al were not formed in the same spallogenic processes that made 10Be in PLAC precursors. This is most easily understood as indicating very early formation of the PLAC hibonites, prior to the incorporation and mixing of 26Al 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 16O-enriched, but are not derived from a homogeneous reservoir: Δ17O values span a range of ~-28‰ to -15‰. The ranges of 16O-enrichment in SHIBs and PLACs overlap and are less "anomalous" than the most 16O-enriched compositions found in meteorites [Kobayashi S., Imai H. and Yurimoto H. (2003) New extreme 16O-rich chondrule in the early solar system. Geochem. J. 37, 663-669]. Both PLACs and SHIBs formed in 16O-enriched reservoirs characterized by small-scale heterogeneities in the gas phase. If such heterogeneities were generated by an admixture of relatively 16O-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 × 105 years based on 26Al 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. Geophys. 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 26Mg-excess. Earth Planet. Sci. Lett. 254, 115-126]. The suite of mass-fractionated hibonites exhibit a range of isotopic properties, including 26Al/27Al 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.

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Deposited On:13 Nov 2010 13:20
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