Insight Into Geological Evolution of the Mesosiderite Parent Body From Olivine Alteration and Merrillite Pb‐Pb Age in Dong Ujimqin Qi

Abstract

Mesosiderites are thought to be created by a catastrophic impact that mixes the silicate crust with the metallic core of a differentiated asteroid(s). The metal‐silicate mixing event greatly affects the subsequent geological evolution of the mesosiderite parent body. To gain a better understanding of this mixing event, we carried out studies on olivine alteration and merrillite Pb‐Pb thermochronology in the Dong Ujimqin Qi mesosiderite. The primary olivine in this meteorite has been altered through sulfidation reactions, leading to the formation of troilite‐orthopyroxene intergrowths. This alteration likely took place during metal‐silicate mixing, as the mixing environment can provide favorable chemical and thermal conditions for this reaction. Phosphate‐chromite veins crosscutting the troilite‐orthopyroxene intergrowths indicate a secondary alteration process likely induced by subsequent impacts. Additionally, the metal‐silicate mixing event likely contributed to the occurrence of abundant merrillites at the boundary between silicates and Fe‐Ni metals, as supported by the distinctly depleted incompatible elements resulting from the redox reaction between metals and adjacent silicates. The ion microprobe analyses for these merrillites yielded a Pb‐Pb age of 4,064 ± 120 Ma, which is interpreted as the record of the Pb isotopic closure of merrillite during prolonged cooling associated with the deep burial. Our two‐stage cooling model suggests that the mesosiderite parent body's burial potentially started around 4.52 Ga, which is consistent with the Sm‐Nd, Ar‐Ar, and Pb‐Pb thermochronological records in mesosiderites.