Alexander N. Krot, Changkun Park and Kazuhide Nagashima

Hawai‘i Institute for Geophysics and Planetology, School of Ocean, Earth Science and Technology, University of Hawai‘i at Mānoa, Honolulu, HI, 96822, USA

Amoeboid olivine aggregates (AOAs) in CH carbonaceous chondrites are texturally and mineralogically similar to those in other carbonaceous chondrite groups. They show no evidence for alteration and thermal metamorphism in an asteroidal setting and consist of nearly pure forsterite (Fa_❤; in wt%, CaO = 0.10.8, Cr₂O₃ = 0.040.48; MnO <0.5), anorthite, Al-diopside (in wt%, Al₂O₃ = 0.78.1; TiO₂ <1), Fe,Ni-metal, spinel, and, occasionally, low-Ca pyroxene (Fs₁Wo₂₃), and calcium-aluminum-rich inclusions (CAIs). The CAIs inside AOAs are composed of hibonite, grossite, melilite (Åk₁₃₄₄), spinel, perovskite, Al,Ti-diopside (in wt%, Al₂O₃ up to 19.6; TiO₂ up to 13.9), and anorthite. The CH AOAs, including CAIs within AOAs, have isotopically uniform ¹⁶O-rich compositions (average Δ¹⁷O = 23.4±2.3‰, 2SD) and on a three-isotope oxygen diagram plot along ∼slope-1 line. The only exception is a low-Ca pyroxene-bearing AOA 1-103 that shows a range of Δ¹⁷O values, from 24‰ to 13‰. Melilite, grossite, and hibonite in four CAIs within AOAs show no evidence for radiogenic ²⁶Mg excess (δ²⁶Mg). In contrast, anorthite in five out of six AOAs measured has δ²⁶Mg corresponding to the inferred initial ²⁶Al/²⁷Al ratio of (4.3±0.7)10⁵, (4.2±0.6)10⁵, (4.0±0.3)10⁵, (1.7±0.2)10⁵, and (3.0±2.6)10⁶. Anorthite in another AOA shows no resolvable δ²⁶Mg excess; an upper limit on the initial ²⁶Al/²⁷Al ratio is 510⁶. We infer that CH AOAs formed by gas-solid condensation and aggregation of the solar nebula condensates (forsterite and Fe,Ni-metal) mixed with the previously formed CAIs. Subsequently they experienced thermal annealing and possibly melting to a small degree in a ¹⁶O-rich gaseous reservoir during a brief epoch of CAI formation. The low-Ca pyroxene-bearing AOA 1-103 may have experienced incomplete melting and isotope exchange in an ¹⁶O-poor gaseous reservoir. The lack of resolvable δ²⁶Mg excess in melilite, grossite, and hibonite in CAIs within AOAs reflects heterogeneous distribution of ²⁶Al in the solar nebula during this epoch. The observed variations of the inferred initial ²⁶Al/²⁷Al ratios in anorthite of the mineralogically pristine and uniformly ¹⁶O-rich CH AOAs could have recorded (i) admixing of ²⁶Al in the protoplanetary disk during the earliest stages of its evolution and/or (ii) closed-system Mg-isotope exchange between anorthite and Mg-rich minerals (spinel, forsterite, and Al-diopside) during subsequent prolonged (days-to-weeks) thermal annealing at high temperature (∼1100°C) and slow cooling rates (~0.01 K hr¹) that has not affected their O-isotope systematics. The proposed thermal annealing may have occurred in an impact-generated plume invoked for the origin of non-porphyritic magnesian chondrules and Fe,Ni-metal grains in CH and CB carbonaceous chondrites about 5 Myr after formation of CV CAIs.

Reference
Krot AN, Park C and Nagashima K (in press) Amoeboid olivine aggregates from CH carbonaceous chondrites. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.04.050]
Copyright Elsevier

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F. Zambon^a et al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website.

^aINAF-IAPS Istituto di Astrofisica e Planetologia Spaziali, Via del Fosso del Cavaliere, 100, 00133 Rome, Italy

Vesta spectra have prominent near-infrared absorption bands characteristic of pyroxenes, indicating a direct link to the howardite, eucrite and diogenite meteorites. Many localized dark and bright materials are present on Vesta’s surface. Here we focus on the bright material (BM) units to determine their spectral properties, their origin, the presence of mineralogical phases different from pyroxenes, and whether different bright units share a common lithology. VIR, the Visible and Infrared spectrometer onboard Dawn, allows us to first do a detailed analysis of the spectral properties of a large number of bright material units on Vesta including examples of the different morphological classes. The spectral parameters used are band centers, band depths, and Band Area Ratio (BAR) for the pyroxene bands at ∼0.9 and ∼1.9 μm. The mineralogies of most bright regions are consistent with those of the howardite, eucrite and diogenite meteorites typical of Vesta’s surface. We find that bright material units exhibit the full range of HED pyroxene composition, from eucrites to diogenites. Large part of the bright materials are eucrite-rich, according with the Vesta’s mineralogy. In most cases, the bright materials have the same mineralogy of the surrounding terrain, but have larger band depth values. The band depths can be related to the abundance of the absorbing minerals, the abundance of Fe²⁺, grain size, and/or to the abundance of opaque materials. We found a positive correlation between albedo and band depth, which suggests that the grain size is not the main factor responsible for the higher albedo. The analysis of the band parameters indicates that most of the bright materials, excluding the few olivine-rich units, represent fresh uncontaminated Vestan pyroxenes from a variety of lithologies exposed from beneath the surface by impacts.

Reference
Zambon et al. (in press) Spectral Analysis of the Bright Materials on the Asteroid Vesta. Icarus
[doi:10.1016/j.icarus.2014.04.037]
Copyright Elsevier

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De¹ et al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website.

¹School of Earth and Space Exploration, Arizona State University, Tempe, AZ, USA

There is evidence that the peak brightness of a Type Ia supernova is affected by the electron fraction Y_e at the time of the explosion. The electron fraction is set by the aboriginal composition of the white dwarf and the reactions that occur during the pre-explosive convective burning. To date, determining the makeup of the white dwarf progenitor has relied on indirect proxies, such as the average metallicity of the host stellar population. In this paper, we present analytical calculations supporting the idea that the electron fraction of the progenitor systematically influences the nucleosynthesis of silicon group ejecta in Type Ia supernovae. In particular, we suggest the abundances generated in quasi-nuclear statistical equilibrium are preserved during the subsequent freeze-out. This allows potential recovery of Y_e at explosion from the abundances recovered from an observed spectra. We show that measurement of ²⁸Si, ³²S, ⁴⁰Ca, and ⁵⁴Fe abundances can be used to construct Y_e in the silicon-rich regions of the supernovae. If these four abundances are determined exactly, they are sufficient to recover Y_e to 6%. This is because these isotopes dominate the composition of silicon-rich material and iron-rich material in quasi-nuclear statistical equilibrium. Analytical analysis shows the ²⁸Si abundance is insensitive to Y_e, the ³²S abundance has a nearly linear trend with Y_e, and the ⁴⁰Ca abundance has a nearly quadratic trend with Y_e. We verify these trends with post-processing of one-dimensional models and show that these trends are reflected in the model’s synthetic spectra.

Reference
De et al. (2014) On Silicon Group Elements Ejected by Supernovae Type Ia. The Astrophysical Journal 787:149.
[doi:10.1088/0004-637X/787/2/149]

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