Size distributions in two porous chondritic micrometeorites

Abstract

Quantitative size measurements of granular units [GUs], and nanometer-sized minerals in these units, in two porous chondritic micrometeorites are investigated for the information that they carry on the processes in the early solar system. The matrix of these micrometeorites consist of loosely packed, 0.1 μm-sized, GUs. These objects were a major component of the solar nebula dust that accreted into protoplanets. The matrix in micrometeorite W7010 *A2 has a fractal dimension with a small coefficient, D = 0.83, that supports efficient sticking of carbon-rich GUs during accretion. The fractal nature of the matrix provides a way to calculate the density using the aggregate size. The resulting very low density for porous chondritic micrometeorites is 0.08–0.14 g cm −3, which supports the view that they are the solid debris from unconsolidated solar system bodies (e.g., comet nuclei, or regolith deposits on carbon-rich asteroids). Chondritic GUs contain ultrafine olivines, pyroxenes, and sulfides, embedded in hydrocarbons and amorphous carbons. Nanocrystals in the micrometeorites W7010 *A2 and U2015 *B show log normal size distributions. The high incidence of disk-shaped grains, a changeover from disk-shaped to euhedral grains, the unevolved nature of the size distributions, and multiple populations for grains < 127 nm in size, are consistent with continuous post-accretion nucleation and growth in amorphous GUs, including coarsening via Ostwald ripening.