^1,2,3Levke Kööp, ^4,5Daisuke Nakashima, ^1,2,3Philipp R. Heck, ⁴Noriko T. Kita, ^4,6Travis J. Tenner, ⁷Alexander N. Krot, ⁷Kazuhide Nagashima, ^7,8Changkun Park, ^1,2,3,9Andrew M. Davis
Geochimica et Cosmochimica Acta (in Press) Link to Article [http://doi.org/10.1016/j.gca.2017.04.029]
¹Department of the Geophysical Sciences, The University of Chicago, Chicago, IL 60637, USA
²Chicago Center for Cosmochemistry, The University of Chicago, Chicago, IL 60637, USA
³Robert A. Pritzker Center for Meteoritics and Polar Studies, Field Museum of Natural History, Chicago, IL, USA
⁴Department of Geoscience, University of Wisconsin, Madison, WI 53706, USA
⁵Division of Earth and Planetary Material Sciences, Faculty of Science, Tohoku University, Aoba, Sendai, Miyagi 980-8578, Japan
⁶Chemistry Division, Nuclear and Radiochemistry, Los Alamos National Laboratory, MSJ514, Los Alamos, NM 87545, USA
⁷Hawai‘i Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawai‘i at Mānoa, Honolulu, HI
⁸Korea Polar Research Institute, Incheon 21990, Korea
⁹Enrico Fermi Institute, The University of Chicago, Chicago, IL 60637, USA.
Copyright Elsevier

Calcium-aluminum-rich inclusions (CAIs) are the oldest dated objects that formed inside the Solar System. Among these are rare, enigmatic objects with large mass-dependent fractionation effects (F CAIs), which sometimes also have large nucleosynthetic anomalies and a low initial abundance of the short-lived radionuclide 26Al (FUN CAIs). We have studied seven refractory hibonite-rich CAIs and one grossite-rich CAI from the Murchison (CM2) meteorite for their oxygen, calcium, and titanium isotopic compositions. The 26Al-26Mg system was also studied in seven of these CAIs. We found mass-dependent heavy isotope enrichment in all measured elements, but never simultaneously in the same CAI. The data are hard to reconcile with a single-stage melt evaporation origin and may require isotopic reintroduction or reequilibration for magnesium, oxygen and titanium after evaporation for some of the studied CAIs.

The initial 26Al/27Al ratios inferred from model isochrons span a range from <1×10–6 to canonical (∼5×10–5). The CAIs show a mutual exclusivity relationship between inferred incorporation of live 26Al and the presence of resolvable anomalies in 48Ca and 50Ti. Furthermore, a relationship exists between 26Al incorporation and Δ17O in the hibonite-rich CAIs (i.e., 26Al-free CAIs have resolved variations in Δ17O, while CAIs with resolved 26Mg excesses have Δ17O values close to –23‰). Only the grossite-rich CAI has a relatively enhanced Δ17O value (∼–17‰) in spite of a near-canonical 26Al/27Al. We interpret these data as indicating that fractionated hibonite-rich CAIs formed over an extended time period and sampled multiple stages in the isotopic evolution of the solar nebula, including: (1) an 26Al-poor nebula with large positive and negative anomalies in 48Ca and 50Ti and variable Δ17O; (2) a stage of 26Al-admixture, during which anomalies in 48Ca and 50Ti had been largely diluted and a Δ17O value of ∼ –23‰ had been achieved in the CAI formation region; and (3) a nebula with an approximately canonical level of 26Al and a Δ17O value of ∼ –23‰ in the CAI formation region.

¹Shun Ichimura, ¹Yusuke Seto, ¹Kazushige Tomeoka
Geochimica et Cosmochimica Acta (in Press) Link to Article [http://doi.org/10.1016/j.gca.2017.04.025]
¹Department of Planetology, Graduate School of Science, Kobe University, Nada, Kobe 657-8501, Japan
Copyright Elsevier

Nepheline is present as fine grains mainly in refractory inclusions and chondrules in CV and CO carbonaceous chondrites. The nepheline has been formed primarily by replacement of melilite and plagioclase in refractory inclusions and plagioclase and glass in chondrules. The nepheline formation is thought to have occurred during aqueous alteration and thermal metamorphism in the meteorite parent bodies. To verify this hypothesis, we performed the following experiments.

Hydrothermal experiments of gehlenite (Al-rich melilite) and plagioclase (An48) were carried out at 200 °C and ∼15 bar for 168 h using solutions of pH 0, 7, 13, and 14 with a uniform Na concentration. In the gehlenite experiments, various amounts of SiO2 were added. The results revealed that a Na zeolite, analcime, was produced from 10/3 and 10/6 mixtures of gehlenite/SiO2 at pH 7, 13, and14, and from a 10/10 mixture of gehlenite/SiO2 and plagioclase at pH 13 and 14. In particular, at pH 14, in addition to analcime, significant amounts of two other zeolites, fabriesite and hydroxycancrinite, were produced from the 10/6 mixture of gehlenite/SiO2, and fabriesite from plagioclase.

Isothermal heating experiments for 24 h showed that fabriesite, hydroxycancrinite, and analcime transform to nepheline at 600–650, 550–600, and 750–800 °C, respectively. Differential thermal analysis of these zeolites revealed that fabriesite and hydroxycancrinite exhibit exothermic peaks, which correspond to transformation to nepheline, and that the temperatures of those peaks decrease steadily with decreasing heating rate. Kinetic analysis using these data revealed that fabriesite and hydroxycancrinite transform to nepheline at temperatures more than 50–100 degrees lower than determined by the isothermal experiments if heated for durations > 102 and ∼ 1 yr, respectively. Analcime heated non-isothermally at a rate of 1 °C/min transformed to nepheline at temperature higher than that determined by the isothermal experiments, suggesting that its transformation temperature also decreases if it is heated for a much longer duration. From these experiments and analyses, we conclude that fabriesite, hydroxycancrinite, and possibly analcime are capable of transforming to nepheline by heating in meteorite parent bodies.

From our results, we propose that the nepheline in refractory inclusions and chondrules in meteorites formed by a two-stage alteration process: (1) formation of the Na zeolites from melilite, plagioclase, and glass by hydrothermal alteration at low temperature (probably