¹Kevin Righter,²Alexander P. Holmwood
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13465]
¹NASA Johnson Space Center, 2101 NASA Parkway, Houston, Texas, 77058 USA
²Department of Geosciences, Hamilton College, Clinton, New York, 13323 USA
Published by arrangement with John Wiley & Sons

The Allan Hills 76005 polymict eucrite pairing group consists of 15 paired masses recovered during six different field seasons in the Transantarctic Mountains. Although this group has been well studied in general, most of the meteorites contain a significant portion of dark clasts that have not been well characterized. The Dawn mission to Vesta discovered dark materials that provide insight into its evolution. The ALH dark clasts are thus of great interest to understanding the evolution of Vesta. Here, 45 different dark clasts from 15 different thin sections from the pairing group are characterized in detail to better understand their nature and origin. Five different textural types of dark clasts are recognized among this group—skeletal, vitrophyric, pilotaxitic, fan spherulitic, and troilite‐silica‐plagioclase‐rich clasts with aphyric or blobby textures. Mineralogy of the clasts is dominated by plagioclase and pyroxene, with minor troilite, silica, ilmenite, chromite, and rare Fe‐Ni metal. All of the textures can be produced by rapid cooling rates on the order of 60–2500°C h⁻¹. Bulk compositions of the clasts are demonstrably eucritic, and not chondritic, howarditic, or diogenitic. The combination of mineralogy, composition, and textures strongly suggests that the dark clasts are eucritic impact melts. Several craters on Vesta have associated orange deposits that have been proposed as impact melt breccias. The ALH pairing group may thus represent material that originated near Oppia or Octavia craters.

¹Noriyuki Kawasaki,¹Sohei Wadaa,²Changkun Park,³Naoya Sakamoto,^1,3,4Hisayoshi Yurimoto
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.03.045]
¹Department of Natural History Sciences, Hokkaido University, Sapporo 060-0810, Japan
²Division of Earth-System Sciences, Korea Polar Research Institute, Incheon 21990, Republic of Korea
³Isotope Imaging Laboratory, Creative Research Institution, Hokkaido University, Sapporo 001-0021, Japan
⁴Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara 252-5210, Japan
Copyright: Elsevier

Fine-grained Ca-Al-rich inclusions (FGIs) in CV chondrites are suggested to be condensates formed directly from the solar nebular gas. Al−Mg mineral isochrons of seven FGIs from reduced CV chondrites Efremovka, Vigarano, Thiel Mountains 07007, and Northwest Africa 8613 were obtained via in situ Al–Mg isotope measurements using secondary ion mass spectrometry. The slopes of the mineral isochrons for seven FGIs exhibit statistically significant variations in initial ²⁶Al/²⁷Al ratios, (²⁶Al/²⁷Al)₀, ranging from (5.19 ± 0.17) to (3.35 ± 0.21) × 10⁻⁵, which correspond to a relative age spread of 0.44 ± 0.07 Myr. Inferred upper limit of (²⁶Al/²⁷Al)₀ for the FGIs is identical to the Solar System (²⁶Al/²⁷Al)₀ of ∼5.2 × 10⁻⁵as determined by whole-rock Al–Mg isochron studies for CAIs in CV chondrites. The intercepts of the mineral isochrons, the initial ²⁶Mg/²⁴Mg ratios the FGIs formed with, are consistent with Mg-isotope evolution path of a solar-composition nebular gas. The observed variations in (²⁶Al/²⁷Al)₀ for FGIs are essentially similar to those (∼5.2 to ∼4.2 × 10⁻⁵) for coarse-grained, igneous CAIs of CV chondrites that are formed by melting and solidification. If ²⁶Al was distributed homogeneously in the forming region, then our data indicate that thermal processes of condensation and melting for CAI formation occurred contemporaneously and continued for at least ∼0.4 Myr at the very beginning of the Solar System. Alternatively, the observed variations in (²⁶Al/²⁷Al)₀ also indicate the possibility of heterogeneous distributions of ²⁶Al in the forming region, corresponding to a range of over at least 3.4 × 10^–5 < (²⁶Al/²⁷Al)₀ < 5.2 × 10^–5.