^1,2,3Zhan Zhou,¹Jiawei Zhao,^1,4Long Xiao,^1,5Jiahuai Sun
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2024.116205]
¹State Key Laboratory of Geological Processes and Mineral Resources, Planetary Science Institute, School of Earth Sciences, China University of Geosciences, Wuhan 430074, China
²Key Laboratory of Earth and Planetary Physics, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
³University of Chinese Academy of Sciences, Beijing 100049, China
⁴State Key Laboratory of Lunar and Planetary Sciences, Space Science Institute, Macau University of Science and Technology, Macau, China
⁵CAS Key Laboratory of Crust–Mantle Materials and Environments, School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China
Copyright Elsevier

High-pressure minerals formed during asteroid impact events are critical for unraveling the details of impact processes. Reidite, a high-pressure polymorph of zircon (ZiSiO₄), forms at ~20–50 GPa in shock-recovery experiments. However, high-contents of reidite in natural zircon (30–100%), which indicate a exceeding 40 GPa formation pressure, are rare in terrestrial and extraterrestrial materials. It is potentially associated with the extreme formation conditions, limiting the potential to use a shock thermobarometer in zircon. Here we report one outcrop of typical microstructures (reidite, granular zircon, and zirconia) in shocked zircon extracted from the outer suevite at the Ries impact crater, Germany. We describe a variety of complex habits of reidite with different proportions (0 − ~90%) of shocked zircon. As supported by previous shock-recovery experiments, these habits of reidite indicate a formation pressure of ~20–50 GPa, further constraining the application range of shock thermobarometer in natural zircon. The presence of diverse ZrSiO₄ phases at the centimeter or micrometer scale, as well as the co-occurrence of reidite, granular zircon, and zirconia at the grain scale reveal highly heterogeneous P-T conditions in outer suevite. We suggest that these thoroughly mixed materials have two types of origins: (1) The excavation flow (or cross-flow) fields mix materials with different shock levels from various positions within the crater. (2) The heterogeneous heating of impact melt result in the diversification of high-temperature phases in zircon. Furthermore, the extensive preservation of shock features of zircon such as reidite reveals that the outer suevite experienced rapid cooling during emplacement and was not exposed to a long-term overheated environment. This supports the radial flow hypothesis of emplacement rather than the FCI (fuel-coolant interaction) model. In general, this study indicates that zircon is a robust shock thermobarometer (0 − ~50 GPa) to help in understanding the formation history of parent rocks and unraveling the P-T conditions of the impact events.

^1,2Mutsumi Komatsu et al. (>10)
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.14234]
¹Division of Liberal Arts and Sciences, Saitama Prefectural University, Koshigaya, Saitama, Japan
²Department of Earth Sciences, Waseda University, Shinjuku, Tokyo, Japan
Published by arrangement with John Wiley & Sons

We present here an investigation of Ryugu particles recovered by the Hayabusa2 space mission and their extracted carbonaceous acid residues using Raman spectroscopy. Raman parameters of Ryugu intact grains and their acid residues are characterized by broad D (defect induced) and G (graphite) band widths, indicating the presence of polyaromatic carbonaceous matter with low thermal maturity. Raman spectra of Ryugu particles and CI (type 1) chondrites exhibit stronger laser-induced fluorescence backgrounds compared to Type 2 and Type 3 carbonaceous chondrites. The high fluorescence signatures and wide bandwidths of the D and G bands of Ryugu intact grains are similar to the Raman spectra observed in CI chondrites, reflecting the low structural order of their aromatic carbonaceous matter, and strengthening the link between Ryugu particles and CI chondrites. The high fluorescence background intensity of the Ryugu particles is due to multiple causes, but it is likely that the relative abundance of geometry-bearing macromolecular organic matter in total organic carbon contents makes a large contribution to the fluorescence intensities. Locally observed high fluorescence in the acid-extracted residues of Ryugu is due to nitrogen-bearing outlier phase. The high fluorescence signature is one consequence of the low degree of thermal maturity of the organic matter and supports evidence that the Ryugu particles have escaped significant parent body thermal metamorphism.