Abstract: The Dong Ujimqin Qi meteorite is a rare mesosiderite falling in Inner Mongolia, China in 1995. It consists of intimately mixed metal–sulfide and silicates. Silicates in the mesosiderite are composed of clasts (mainly olivine clasts and gabbro clasts) and gabbroic matrix. Petrographic and REE characteristics and major element compositions show that olivine clasts, matrix olivine and gabbroic rocks (including gabbro clasts and gabbroic matrix) may have crystallized from different magmas. Olivine clasts, with high contents of Mg, may be derived from the dunite mantle of a differentiated asteroid, whereas matrix olivine and gabbroic rocks may be from the same parental asteroid, i.e. the Vesta asteroid. An impact event led to mixture of materials from these two asteroids. During the bombardment, temperature rose to 1200—1450℃. Target basaltic and gabbroic rocks from the Vesta melted at the high temperature. Gabbroic rocks in the Dong Ujimqin Qi mesosiderite crystallized from the mixed melt. Olivine (both olivine clasts and matrix olivine), however, did not melt because the temperature was below their melting points. This model explains the disequilibrium between olivine clasts, matrix olivine and gabbroic rocks.

Key words: Dong Ujimqin Qi mesosiderite, olivine, gabbroic rocks, disequilibrium, origin