¹Chen, Ying Wang,²Shiyong Liao,²Le Zhang,¹Pengli He,³Lei Jin,¹Yuri Amelin,^1,2Yi-Gang Xu
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.70075]
¹State Key Laboratory of Deep Earth Processes and Resources, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, China
²Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing, China
³State Key Laboratory of Lunar and Planetary Sciences, Macau University of Science and Technology, Macau, China
Published by arrangement with John Wiley & Sons

Mesosiderites are widely believed to have originated from a metal-silicate mixing event triggered by planetesimal collisions in the early solar system. However, a key unresolved issue in this model is the physical state (liquid vs solid) of the metallic materials involved, which complicates our understanding of mesosiderite formation. Melt pockets and comb plessites in the Dong Ujimqin Qi mesosiderite provide critical insights into this issue. The melt pockets exhibit quenched textures of dendritic troilite-metal intergrowths, typically cooled at a rate of >9500°C s−1 above 950°C. In contrast, the Ni profile in kamacite, pentlandite, taenite, and cordierite inside melt pockets points to a subsequent burial-induced slow cooling process, which starts below 780°C with a maximal estimated rate of ~2°C Myr−1. The two-stage cooling pathway of melt pockets aligns well with thermal fingerprints expected from the catastrophic disruption and reassembly of the mesosiderite parent body. More importantly, the impact has led to shock deformation of metal nodules to varying degrees, as reflected by the extension of kamacite polygonization associated with melt pockets into some comb plessite domains. This provides vital evidence that the metal nodules remained partially solid during the mixing process. Accordingly, we propose a revised mesosiderite formation model that involves an impact mixing with a partially solidified metal planetesimal. The revised model better accounts for several issues regarding the formation of mesosiderites, such as three-orders-of-magnitude variations of bulk Ir concentrations, slow metallographic cooling rates, bimodal size distribution of the metallic nodule and matrix, and deficient olivine materials.

¹Ryota Fukai et al. (>10)
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.70077]
¹Japan Aerospace Exploration Agency, Sagamihara, Japan
Published by arrangement with John Wiley & Sons

Analyzing primitive extraterrestrial samples from asteroids is key to understanding the evolution of the early solar system. The OSIRIS-REx mission returned samples from the B-type asteroid Bennu, providing a valuable opportunity to compare them with the Ryugu samples collected by the Hayabusa2 mission. This study examines the representativeness of a fraction of the Bennu samples, which was allocated from NASA to JAXA, by nondestructive characterization of their physical and spectral properties without atmospheric exposure. The reflectance and observed spectral features in the visible-to-infrared range of the Bennu sample resemble those from the spectroscopic analysis of different fractions. Additionally, we found differences in the slope of the visible range and band-center of ~2.7 μm band between the samples and the asteroid surface, which could be explained by the degree of space weathering. A comparative analysis of the Bennu and Ryugu samples revealed spectral similarities, including absorption features indicative of Mg-rich phyllosilicates, organics, and carbonates, without any evidence of sampling bias or terrestrial alteration. This finding can be used as a benchmark for subsequent Ryugu–Bennu comparative studies.