¹K. Righter et al. (>10)
Meteoritics & Planetary Science (in Press) Open Access Link to Articlel [https://doi.org/10.1111/maps.70182]
¹Department of Earth and Environmental Sciences, University of Rochester, Rochester, New York, USA
Published by arrangement with John Wiley & Sons

Examination of 10 Bennu aggregate particles has revealed the presence of many phases which taken together can provide constraints on the oxygen fugacity (fO2) of Bennu samples. Phyllosilicates (saponite and serpentine), carbonates, oxides (magnetite, chromite), sulfides (pyrrhotite, pentlandite), phosphate (hydroxyapatite, Na-Mg-phosphate), and phosphides (schreibersite, andreyivanovite) all occur in most Bennu particles. The Bennu samples have experienced a high degree of aqueous alteration leaving only <1% of the original mineralogy unaltered. However, both the precursor anhydrous and alteration phases can present different constraints on fO2. Precursor phases include olivine, pyroxene, spinel, hibonite, chromite, phosphide, very rare Fe-Ni metal, apatite, and possibly MgS and MnS, all of which are typically <25 μm in size. Alteration phases include phyllosilicates, carbonates, magnetite, sulfides, sulfates, phosphates, chlorides, and fluorides. Detailed calculations of fO2 rely on having quantitative electron microprobe analyses of the phases involved in equilibria amongst both the precursor and alteration phases. In general, the absence of Fe-Ni metal, coupled with the stability of the Fe3O4 component in chromite, places a lower limit on the fO2. Concomitantly, the absence of Fe sulfates places an upper limit on fO2. Altogether, the textures, mineral compositions, and calculations suggest that some components in the Bennu samples (chondrules, inclusions) may have originally equilibrated at fO2 well below the iron-wüstite buffer but then experienced higher fO2 near or higher than the fayalite-magnetite-quartz (FMQ) buffer during aqueous alteration that produced coarser grained oxidized assemblages.

¹A. C. Singleton,¹G. R. Osinski
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.70184]
¹Department of Earth Sciences, University of Western Ontario, London, Ontario, Canada
Published by arrangement with John Wiley & Sons

The ~28 km Kamestastin (Mistastin) Lake impact structure is a relatively well-preserved and well-exposed complex impact structure. The central uplift of this structure is accessible as two islands in the middle of Kamestastin Lake. We present an updated, detailed geological map and description of Horseshoe and Bullseye islands that provides increased accuracy and detail of the target rock outcrop and contact locations. In addition, we document six occurrences of impact melt-poor breccia dikes and one occurrence of impact melt rock on Horseshoe Island for the first time. The impact melt rock outcrop is proposed to be a remnant of a veneer of impact melt on the original central peak, and the impact melt-bearing breccia dikes to have had a dynamic emplacement mechanism. We also carried out the first detailed, systematic shock study of the central uplift. Planar deformation features in quartz and diaplectic feldspar glass suggest local peak shock pressure of up to 45 GPa. These shock pressures are higher than the peak pressures recorded in the central uplifts of similarly sized impact structures. We suggest that this difference is due to the minimal erosion of the central uplift at the Kamestastin Lake impact structure.

¹Glenn J. MacPherson, ²Kazuhide Nagashima, ²Alexander N. Krot, ³Noriko T. Kita, ^3,4Takayuki Ushikubo, ²Elena Dobrică
Geochimica et Cosmochimica Acta (in Press) Link to Article [10.1016/j.gca.2026.06.023]
¹Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Washington DC 20560, the United States of America
²University of Hawai‘i at Mānoa, Honolulu, HI 96822, the United States of America
³WiscSIMS, Department of Geoscience, University of Wisconsin-Madison, Madison, WI 53706, the United States of America
⁴Kochi Institute for Core Sample Research, JAMSTEC, Nankoku, Kochi 783-8502 Japan
Copyright Elsevier

Using secondary ion mass-spectrometry, we analyzed the Al/Mg isotopic compositions of cleanly resolved Wark-Lovering (WL) rim layers on six calcium-aluminum-rich inclusions (CAIs) of different petrologic types from the Vigarano CV3 chondrite. In five cases, the inferred initial ²⁶Al/²⁷Al ratios [(²⁶Al/²⁷Al)₀] are within analytical error of the ratios of the host inclusions and of each other, and in the sixth case the value for the rim sequence is suspect and not deemed reliable. The results for the five reliable CAIs mean that the maximum age difference between the rim sequences and their host inclusions is at most 10⁵ years and possibly less. Petrologic observations show that the effects of the rim-forming process were not limited to the rims themselves but extended well into the inclusion interiors. The fact that the (²⁶Al/²⁷Al)₀ of WL rims are within analytical error of each other raises the possibility that all WL rim sequences formed during a single nebula-wide event, but the fact that the inclusions are diverse petrologically makes this unlikely. We propose instead that the diverse rims formed during multiple nebular reheating events after the initial formation of the inclusions, leading to surface melting, volatilization, and recondensation. Our isotopic data for some rim phases, especially forsterite and diopside, tend to be isotopically lighter (lower δ²⁵Mg) than their host inclusion interiors, suggesting that they formed predominantly by recondensation.