Reproduction of I-type cosmic spherules and characterization in an Fe-Ni-O system

The chemical composition and texture of cosmic spherules are influenced by atmospheric conditions and the characteristics of their parent interplanetary particles. The objective of this study was to reproduce I-type cosmic spherules, which consist mainly of Fe oxide and Fe-Ni metal, and compare their textural characteristics with those of natural I-type cosmic spherules. Thus, a series of rapid heating and quenching experiments were performed on free falling iron meteorite powders obtained from Canyon Diablo, in the United States. The experiments were conducted using a high-temperature furnace with controlled gas flow rates at oxygen fugacities of FMQ + 2.4, FMQ, and FMQ - 2.5 log unit. The resulting Fe-Ni metal and oxide phases showed the nonequilibrium state of the melted spherules formed during quenching. Two types of magnetite crystals in different orientations were found in iron oxide. As temperatures decreased, the molten metal was oxidized to form immiscible molten iron oxide that then covered the former. As the oxide melt increased at the expense of metal, magnetite began to crystallize from the iron oxide melt, as the liquidus phase, either on the surface or within the melt phase. The characteristics of the run products obtained under different oxygen fugacities were similar to those of natural I-type cosmic spherules, which have textures and compositions that may contain information regarding the oxygen content of the upper atmosphere. Our study suggests that CO2-bearing molecules in the atmosphere could form iron oxide with a texture similar to natural I-type cosmic spherules. During this process, rapid crystallization of magnetite plays an important role in texture formation in disequilibrium states.