Fine-Grained Scoriaceous and Unmelted Micrometeorites: Sources and Relationships with Cosmic Spherules

185 fine-grained micrometeorites (FgMMs) from the Novaya Zemlya micrometeorite collection were studied to elucidate the sources of cosmic dust. The compositions of the micrometeorites were compared with those of carbonaceous chondrites and cosmic spherules. FgMMs were weakly or moderately heated when decelerated in the atmosphere, and some of them thereby retained their original textures (unmelted FgMMs), while others consist mostly of porous degassing products of phyllosilicates (scoriaceous FgMMs). According to the composition of their matrix, we divide the FgMMs into two groups: high- and low-Fe. The textures and compositions of the FgMMs are generally similar to those of the matrix of carbonaceous chondrites. The FgMM group with high-Fe matrix corresponds to the composition of CI, CM, CO, and CR chondrites, which may have been a source of these meteorites. Conversely, the low-Fe group of FgMMs differs from chondrites of these groups and is likely related to the carbonate-poor matrix of the Tagish Lake carbonaceous chondrite or is presented by a material that is still absent from meteorite collections. We have compared the compositions of the FgMMs with the compositions of various types of cosmic spherules (CSs), which were produced by melting of micrometeoroids (cosmic dust particles) at their deceleration in the atmosphere. Both original and published data on CS compositions are used. Porphyritic and barred textural varieties of CSs could have been produced from micrometeoroids similar in the compositions to the high-Fe FgMMs, whereas low-Fe micrometeoroids might have been precursors of cryptocrystalline and glass CSs.