^1,2,3Yingnan Zhang,^1,2,3Liping Qin
Earth and Planetary Science Letters 641, 118807 Link to Article [https://doi.org/10.1016/j.epsl.2024.118807]
¹CAS Key Laboratory of Crust-Mantle Materials and Environments, University of Science and Technology of China, Hefei 230026, China
²Deep Space Exploration Laboratory, Hefei 230088, China
³CAS Center for Excellence in Comparative Planetology, Hefei 230026, China
Copyright Elsevier

Thermal metamorphism for asteroids influences their structures and chemical compositions. As the only CCs with the petrologic type of 3-6, their metamorphis history and heat sources are unclear. The isotopic composition of molybdenum (Mo) was used as an indicator of oxidation state to investigate the oxidation and thermal metamorphic history of CK chondrites. CK chondrites are characterized by positively fractionated Mo isotopic composition relative to other chondritic groups, and the degree of enrichment in heavy Mo isotopes in CKs generally decreases with Mo content. Combined with numerical simulations for hexavalent Mo evaporation at elevated temperatures and thermodynamic calculations of the valence transformation of Mo, the Mo isotopic characteristic of CKs is proven in concordance with oxidative Mo lost during thermal metamorphism. This evaporation loss highlights the importance of the parent body process on the MVE depletion. Our results also require oxidation of CKs and subsequent thermal metamorphism to have occurred after the disintegration of the CK parent body. A viable heat source in this scenario could be solar radiation, and the oxidization may perform by the water accretion in orbits.

¹Birgit Schulz,¹Christian Vollmer,^2,3Jan Leitner,⁴Lindsay P. Keller,^5,6Quentin M. Ramasse
Geochimica et Cosmochimica Acta (in Press) Open Access Link to Article [https://doi.org/10.1016/j.gca.2024.06.013]
¹Institut für Mineralogie, Universität Münster, Corrensstr. 24, 48149 Münster, Germany
²Institut für Geowissenschaften, Ruprecht-Karls-Universität Heidelberg, Im Neuenheimer Feld 234-236, 69120 Heidelberg, Germany
³Max-Planck-Institut für Chemie, Hahn-Meitner-Weg 1, 55128 Mainz, Germany
⁴XI3, Astromaterials Research and Exploration Science (ARES) Division, NASA Johnson Space Center, Houston, TX 77058, United States
⁵SuperSTEM Laboratory, SciTech Daresbury Campus, Daresbury WA4 4AD, UK
⁶School of Chemical and Process Engineering, School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, UK
Copyright Elsevier

GEMS (glass with embedded metal and sulfides) are the dominant carrier of amorphous silicates in anhydrous interplanetary dust particles (IDPs) and one of the most suitable materials to study early solar system processes. Amorphous silicates in 105 GEMS from eight IDPs were analyzed regarding texture and chemical composition to reassess GEMS formation theories and genetic relationships to amorphous silicate material in meteorites. Petrography of bulk IDPs was investigated to understand GEMS’ relationships to other IDP components. Furthermore, carbon and nitrogen isotopic compositions were measured. Nearly all GEMS are aggregates of several subgrains with variable amount of nanophase inclusions and different Mg- and Si-contents, while single GEMS are rare. The subgrains within aggregates are typically surrounded by one or more carbon rims with high density. The chemical compositions of GEMS amorphous silicates are subsolar for all major element/Si ratios but exhibit wide heterogeneity. This is not influenced by silicon oil from the capturing process of IDPs as assumed before, as a penetration of the silicon oil is excluded by high resolution EELS (electron energy loss spectroscopy) areal density maps of silicon. Furthermore, low Fe-content in GEMS amorphous silicates shows that these are not altered by aqueous activity on the parent body as it is the case for amorphous silicate material in primitive meteorites. The subsolar element/Si ratios and the wide chemical heterogeneity point to a non-equilibrium fractional condensation origin either in the early solar nebula or in a circumstellar environment and are not in agreement with homogenization via sputtering in the ISM. The close association with carbon around GEMS subgrains and as double-rims around GEMS aggregates argue for a multi-step aggregation after formation of the smallest GEMS subgrains in the ISM or the early solar nebula. Carbon acting as matrix material connecting GEMS and other IDP components has lower areal density as seen from carbon EELS areal density maps and isotopic anomalies varying at the nanometer scale, pointing to different origins and processing of materials to varying extent or at changing temperatures.

To balance GEMS’ subsolar element/Si ratios, a supersolar component in IDPs was assumed to account for the overall chondritic composition of bulk IDPs. Nevertheless, our bulk IDP analyses revealed subsolar, but variable, element/Si ratios for complete particles as well, depending on type and amount of mineral phases in each particle. Pyroxenes in the investigated particles can occur as elongated euhedral crystals, but are overall rare. The dominant crystalline fraction in the investigated IDP samples are equilibrated aggregates (EAs) that show the same chemical compositions as GEMS, indicating that the EAs are recrystallized GEMS grains and formed after GEMS formation as secondary phases.