THERMAL PROCESSING OF SILICATE DUST IN THE SOLAR NEBULA: CLUES FROM PRIMITIVE CHONDRITE MATRICES

摘要

The most abundant matrix minerals in chondritic meteorites—hydrated phyllosilicates and ferrous olivine crystals—formed predominantly in asteroids during fluid-assisted metamorphism. We infer that they formed from minerals present in three less altered carbonaceous chondrites that have silicate matrices composed largely of micrometer- and nanometer-sized grains of crystalline forsterite Mg_2SiO_4 and enstatite MgSiO_3 and amorphous, ferromagnesian silicate. Compositional and structural features of enstatite and forsterite suggest that they formed as condensates that cooled below 1300 K at ～1000 K hr~(-1). Most amorphous silicates are likely to be solar nebula condensates also, as matrix, which is approximately solar in composition, is unlikely to be a mixture of genetically unrelated materials with different compositions. Since chondrules cooled at 10-1000 K hr~(-1) and matrix and chondrules are chemically complementary, most matrix silicates probably formed close to chondrules in transient heating events. Shock heating is favored, as nebular shocks capable of melting millimeter-sized aggregates vaporize dust. The crystalline and amorphous silicates in the primitive chondrite matrices share many characteristic features with silicates in chondritic interplanetary dust particles, suggesting that most of the crystalline silicates and possibly some amorphous silicates in the interplanetary dust particles are also nebular condensates. Except for small amounts of refractory oxides from the innermost region where refractory inclusions formed and presolar dust, most of the crystalline silicate dust that accreted into chondritic asteroids and long-period comets appears to have formed from shock heating at ～2-10 AU. Forsterite crystals around young stars may have a similar origin.