46th Lunar and Planetary Science Conference (2015) 2750.pdf MULTIPLE GENERATIONS OF FRACTIONATED HIBONITE-RICH CAIS SAMPLED THE SOLAR NEBULA AT DIFFERENT DEGREES OF ISOTOPIC HETEROGENEITY L. Kööp1,2,4, A. M. Davis1,2,3,4, P. R. Heck2,4, N. T. Kita5, A. N. Krot6, P. Mane7, K. Nagashima6, D. Nakashima5,8, C. Park6,9, T. J. Tenner5, M. Wadhwa7, 1Dept. of Geophysical Sciences, 2Chicago Center for Cosmochemistry, 3Enrico Fermi Institute, Univ. of Chicago, Chicago, IL, 4Robert A. Pritzker Center for Meteoritics and Polar Studies, Field Museum of Natural History, Chicago, IL, 5WiscSIMS, Dept. of Geoscience, Univ. of Wisconsin, Madison, WI. 6HIGP/SOEST, Univ. of Hawai‘i at Mānoa, Honolulu, HI, 7School of Earth and Space Exploration, Arizona State Univ., Tempe, AZ, 8Tohoku Univ., Sendai, Japan, 9Korea Polar Research Institute, Incheon, Korea (E-mail: [email protected]) Introduction: Calcium-aluminum-rich inclusions (CAIs) are the oldest dated objects that formed in the solar system . A few of these show large massdependent enrichments in the heavy isotopes of some elements, attesting to a formation by melt distillation of low-temperature precursors, and are often classified as fractionated (F) CAIs ( and references therein). Some fractionated CAIs also have large mass-independent anomalies of nucleosynthetic origin and are classified as FUN (Fractionated and Unidentified Nuclear effects) CAIs. Other CAIs with large unidentified nuclear (UN) anomalies lack fractionation effects. While F and FUN CAIs are often not petrologically distinct from regular CAIs, UN effects are most commonly found in hiboniterich CAIs like platy hibonite crystals (PLACs) . We have studied 80 hibonite-rich CAIs separated from the Murchison meteorite for their UN characteristics [4, 5] and incorporation of 26Al at their time of formation. Here, we report the mass-dependent and massindependent isotope effects in O, Mg, Ca and Ti in 7 of these hibonite-rich CAIs that have F(UN) characteristics. We also infer their initial 26Al/27Al ratios [(26Al/27Al)0] from model and internal isochrons. Methods: Al-Mg and O-isotope analyses were performed with the WiscSIMS Cameca ims-1280 at UW. The results for O isotopes have been previously reported . For Mg isotopes, most hibonites were analyzed with a ~12 µm primary O– beam and mono-EM detection . Ca and Ti isotopes were analyzed with the Cameca ims1280 at UH ; part of the results have been previously reported . Intrinsic isotopic fractionation is expressed as FTi and FCa as defined in . The major element compositions were determined by EPMA (Cameca SX-50). Trace elements (REE, Mg, Ti) were measured with the Cameca ims-6f at ASU. NIST glasses were used to obtain relative sensitivity factors and oxide interferences on the heavy REE were corrected using the ion to oxide ratios published by . Results: Mineralogy and morphology: Grains 2-5-1, 2-6-6, and 2-8-7 are stubby crystals of hibonite (70 to 300 µm in size). All contain refractory metal nuggets (RMNs); those in 2-5-1 and 2-6-6 were large enough for analysis and are depleted in Mo and W. 2-2-1 and 2-8-3 are single crystals with platy morphologies. In contrast to other hibonites, their surfaces have abundant hexago- nal pits. 1-9-1 is a platy CAI fragment consisting of a spinel and a hibonite layer. 1-10-3 is a hibonite aggregate with inclusions of corundum (<10 µm). Chemical and isotopic characteristics: The results are summarized in Table 1 and Figures 1 and 2. The main observations are as follows: (1) Based on the inferred (26Al/27Al)0, the hibonite-rich CAIs can be divided into three groups: 26Al-free, with subcanonical ratio, and with approximately canonical ratios (Table 1). (2) All except the canonical CAIs are depleted in Mg and Ti relative to PLAC and SHIB hibonites . (3) All CAIs show fractionation effects in at least one element, but never simultaneously in all four studied elements. (4) The most Mg- and Ti-rich hibonites (canonical CAIs) have the highest degrees of Ca fractionation, but lack corresponding effects in Ti, Mg and O (Fig. 2b). (5) Correlated fractionation effects in Ca and Ti are only observed in four CAIs (Fig. 2b), i.e., 1-10-3, two of the 26 Al-free CAIs (2-6-6, 2-5-1) and the subcanonical CAI 2-8-7. (6) Nucleosynthetic anomalies are only observed in 26Al-free CAIs and are more pronounced in 48Ca (δ48Ca of up to 43±5‰) than in 50Ti (Fig. 2a). (7) Most analyzed grains have Δ17O (=δ17O–0.52×δ18O) values between –23‰ and –25‰ (Fig. 1), like many FUN and normal CAIs from unmetamorphosed chondrites [e.g., 9, 10]. Only the CAI with the largest nucleosynthetic effects (26Al-free CAI 2-5-1) has a distinct Δ17O of –14±1‰. (8) Only CAI 2-6-6 shows significant frac- Table 1: Characteristics of the studied CAIs. Felement indicates degree of fractionation. Abbreviations: subc – subcanonical, aggr. – aggregate, cor – corundum, sp – spinel, n.a. – not analyzed, amu – atomic mass unit. Name Classification 26 Al/ 27Al (×10-5) Δ 17O (‰) FO (‰/amu) FCa (‰/amu) FTi (‰/amu) FMg (‰/amu) MgO (wt%) TiO2 (wt%) Morphology Additional phases REE 2-5-1 2-6-6 1-9-1 1-10-3 2-8-7 26 Al-free subc. ~0 ~0 ~0 n.a. ~ 0.3 –14 –23 –23 –24 –25 ~ 24 ~ 4 ~ 27 ~ 3 ~ 26 ~ 10 ~ 5 ~ 15 < 5 ~ 15 ~ 10 < 5 <5 <5 > 10 ~ 0 ~ 10 ~0 ~0 n.a. < 0.5 < 0.5 0.5 0.5 < 0.5 < 0.5 < 0.5 ~ 1.5 ~ 1 0.5 stubby platy aggr. stubby cor, RMN RMN sp RMN RMN Ce depletion n.a. n.a. 2-8-3 2-2-1 canonical ~5 ~5 –25 –24 ~7 ~5 > 15 > 15 ~0 ~0 ~0 ~0 ~1 ~1 ~2 ~2 platy perovskite low n.a. 46th Lunar and Planetary Science Conference (2015) 2750.pdf nounced in 1-9-1) and (2) the steep distribution of δ48Ca vs. δ50Ti compared to PLACs (Fig. 2a), which could Mg-depleted hibonites: indicate dilution of 50Ti anomalies in the Ti-poor frac0 0 Mg-depleted hibonites: 1-9-1 e ne tionated CAIs. Alternatively, isotopically normal Mg n lliin MM 1-10-32-6-6 A atiioon tionnat CCCA 2-5-1 1-9-1 rcatcio could have entered the hibonite lattice without Ti, possif C l triafra -20 2-6-6 2-5-1 reisal -20 Teesrtr bly facilitated by an overabundance of Ti4+ (and vacanr r 2-8-7 2-8-7 Te Corundum-hibonite cies) relative to Mg2+ in the hibonite lattice, as hinted at Others: -40 Canon. fract. CAIs by our SIMS analyses. Enhanced Ti may be a result of -40 Others: (2-2-1 & 2-8-3) crystallization in a melt with high Ti/Mg due to preferCanon. fract. CAIs -40 -20 18 0 20 PLACs  ential evaporation of the more volatile Mg. That the exδ O (‰) PLACs  -40 -20 18 0 20 cess Ti is present as Ti3+ is unlikely, given that Ce depleδ O (‰) tions in hibonite (Table 1) and W and Mo depletions in Figure 1. O isotope diagram, modified after . PLAC-like CAIs (Kööp et al.RMNs 2014 c) indicate formation under oxidizing conditions. a b Fit through PLAC data The mass-independent effects suggest that the sam40 15 ples record at least three different stages of solar nebular Mg-depleted hibonites: 20 history. The range of resolvable anomalies in 48Ca and 10 2-6-6 50 Ti indicates that the 26Al-free fractionated CAIs 1-9-1 0 2-5-1 formed early in a heterogeneous nebula, that was evolv5 2-8-7 -20 ing towards a uniform Δ17O of ~ –24‰ prior to the arri1-10-3 val of 26Al. The subcanonical CAI may have formed 0 -40 Others: during admixture of 26Al (although formation after sig26 -40 -20 0 20 40 -5 0Al PLACs 5 10 15 50 δ Ti (‰) FTi (‰/amu) nificant 26Al decay cannot be excluded), in a nebula average uncertainty Figure 2. Ca- and Ti-isotope data for 3σfractionated characterized by a Δ17O of ~ –24‰, in which nucleosynhibonite-rich CAIs, with PLAC data from  for com- thetic anomalies had been homogenized to a level of < parison. a) Nucleosynthetic anomalies. b) Fractionation ~5‰. Finally, the canonical hibonites formed around the effects. Symbols as in Fig. 1; gray line in Fig. 2a is a fit same time as normal CAIs. Whether or not the subcathrough PLAC data. nonical level indicates early or late formation, the apDiscussion: The fractionated CAIs 2-2-1 and 2-8-3 proximately constant Δ17O value indicates that the oxyhave particularly enigmatic isotopic compositions (high gen isotopic evolution of the CAI-forming region apfractionation in Ca, yet not in O, Mg and Ti, ~canonical pears to have stagnated for a significant amount of time. Conclusions: The lack of Mg and Ti isotopic frac(26Al/27Al)0). Their identical morphological, chemical and isotopic characteristics suggest that they may have tionation in some hibonites with high Ca fractionation been liberated from the same CAI, possibly during acid suggests that late-stage reintroduction of Mg and Ti may have diluted isotopic effects in these elements. As Ca treatment. A general feature of the studied CAIs is that frac- abundance is fixed by mineral chemistry, this element tionation effects can be observed in all analyzed ele- likely provides a more robust record of the original ments, yet never simultaneously in the same CAI. This mass-dependent and -independent isotope effects in could be a result of complicated condensation and evap- hibonite. The variable nucleosynthetic anomalies and oration histories or of reintroduction of isotopically levels of 26Al incorporation suggest that these CAIs represent at least three different isotopic populations and normal Mg and/or Ti after CAI crystallization. Most fractionated hibonites studied here lack frac- sample different evolutionary stages of the nebula. This tionation in Mg. This has been observed in previous further implies that conditions favorable for melt distillastudies and attributed to quantitative Mg evaporation, tion existed over a significant period of nebula history. References:  Amelin Y. et al. (2002) Science, followed by Mg surface contamination . However, as Mg and Ti contents are generally coupled in the 297, 1678–1683.  MacPherson G. J. (2014) Treatise nd hibonites studied here, most prominently in the Mg- and on Geochemistry, 2 Ed.  Ireland T. (1990) GCA, 54, Ti-rich canonical CAIs, we argue against contamination, 3219-3237.  Kööp L. et al. (2014) LPS, 45, 2508.  but favor late-stage reintroduction of normal Mg (and Kööp L. et al. (2014) MAPS, 49, #5384.  Ushikubo T. possibly also Ti) to explain the lack of Mg fractionation et al. (2013) GCA, 109, 280–295.  Park C. et al. effects. Indicators that reintroduction of Mg also resulted (2014) LPS, 45, #2656.  Fahey A. J. et al. (1987) in addition of isotopically normal Ti include (1) the lack GCA, 51, 329–350.  Krot A. N. et al. (2010) ApJ, 713, of fractionation effects in Ti in the CAIs that show high 1159–1166.  Makide K. et al. (2009) GCA, 73, 5018-5050.  Ireland T. et al. (1992) GCA, 56, 2503– degrees of Ca fractionation (2-8-3, 2-2-1 and less pro2520. FCa (‰/amu) 48 δ Ca (‰) 17 17 δ O (‰) δ O (‰) tionation in Mg isotopes. All others are indistinguishable from or slightly lighter than measured standard values.
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