Chondrules in the Parent Asteroids of Micrometeorites

Introduction: Two main types of unmelted mi-crometeorite are recognised: (1) fine-grained micro-meteorites (fgMMs), dominated by phyllosilicates, and (2) coarse-grained micrometeorites (cgMMs), dominated by anhydrous silicates and glass with magmatic textures [1]. Fine-grained MMs have close affinities to the matrices of CI, CM2 and CR2 chondrites, however, the other major components of these meteorites, chon-drules, are exceedingly rare amongst micrometeorites [2]. Kurat et al., [3] described one particle that, on the basis of shape, texture and minor element chemistry, is probably a chondrule fragment. Genge and Grady [1] have further proposed, on the basis of texture and mineralogy , that most cgMMs represent fragments of chondrules. The preservation of glass in the mesostases of most cgMMs, however, is not consistent with derivation from a parent asteroid that has experienced hy-drous alteration to the extent of the CM2 or CI parent bodies. We report the first discovery of a chondrule fragment from within a fgMM. Texture and mineralogy: Particle SP96-100-001 was collected from the South Pole water well [4] and is dominated by a core of porous fine-grained matrix. The matrix consists of micron-sized subhedral anhy-drous silicate grains embedded in a network-like structure of much-finer grained materials which probably represent the thermal decomposition products of phyllosilicates. Several large (<10 µm) anhedral oli-vine grains are also present in the matrix and appear to be clastic fragments. The unmelted core is surrounded by a <30 µm thick melted rim consisting of sub-micron fayalitic olivine grains in a glassy matrix and evidently formed during entry heating. Adjacent to the unmelted matrix is a large 50 µm diameter igneous object consisting of euhedral zoned forsterite phenocrysts within a glassy mesostasis. Some of the euhedral forsterites are truncated at the interface with the fine-grained matrix indicating that this is a fragment of a larger grain. Close to the external surface sub-micron fayalitic olivine phenocrysts occur in the mesostasis and are similar to those in the melted rim surrounding the fine-grained matrix. These suggest remobilisation of the mesostasis during entry heating and indicate that the igneous texture predates atmospheric entry. Implications: The texture and mineralogy of the igneous object found within particle SP96-100-001 is similar to that of Fe-poor cgMMs and of Type I chon-drules [5]. The occurrence of this object embedded in fine-grained matrix closely resembles that of chondrule fragments and thus implies that cgMMs are derived as