Mesosiderite formation via accreting a partially solidified metal planetesimal: Evidence from Dong Ujimqin Qi

Abstract 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.