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

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.