东乌旗中铁陨石硅酸盐地球化学特征及对成因的启示

GEOCHEMISTRY OF SILICATES IN THE DONG UJIMQIN QI MESOSIDERITE AND THE IMPLICATIONS FOR ITS ORIGIN

东乌旗中铁陨石是1995年在内蒙发现的、为数不多的降落型中铁陨石,由金属-硫化物和硅酸盐相紧密混合而成。硅酸盐相由角砾部分(主要为角砾状橄榄石和辉长岩岩屑)和非角砾部分组成。岩相学特征、主量元素成分及REE地球化学特征表明角砾状橄榄石、非角砾橄榄石和辉长质硅酸盐(辉长岩岩屑和除橄榄石外的非角砾部分)三者可能分别结晶自不同的岩浆源区。进一步的分析表明Mg含量非常高的角砾状橄榄石可能来自一个分异小行星的橄榄岩质地幔,非角砾橄榄石和辉长质硅酸盐可能和HED陨石具有相同的母体Vesta小行星。冲击碰撞导致两个小行星的物质发生了混合。冲击作用产生的高温(约1 200—1 450℃)使得被撞击小行星表壳玄武质和辉长质岩石发生混合重熔,熔体结晶后形成中铁陨石的辉长质硅酸盐。该温度下非角砾橄榄石和角砾状橄榄石均未发生重熔。该模型较好地解释了角砾状橄榄石、非角砾橄榄石和辉长质硅酸盐三者之间的不平衡现象。

The Dong Ujimqin Qi meteorite is a rare mesosiderite that fell 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 and gabbro ） with a gabbroic matrix .Petrographic and REE characteristics and major element compositions show that the olivine clasts , matrix olivine and gabbroic rocks （ including gabbro clasts and gabbroic matrix ） may have crystal-lized from different magmas .Olivine clasts, with high Mg contents, may be derived from the dunite mantle of a dif-ferentiated asteroid , whereas matrix olivine and gabbroic rocks may be from the same parental asteroid , e.g, the Vesta asteroid .An impact event may have led to the mixture of materials from these two asteroids .During the bom-bardment , temperature rose to 1 200-1 450℃and the 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 its melting point .This model explains the disequilibrium between olivine clasts , matrix olivine and gabbroic rocks .