The formation of Mg,Fe-silicates by reactions between amorphous magnesiosilica smoke particles and metallic iron nanograins with implications for comet silicate origins

摘要

This thermal annealing experiment at 1000?K for up to 167?h used a physical mixture of vapor phase-condensed magnesiosilica grains and metallic iron nanograins to test the hypothesis that a mixture of magnesiosilica grains and an Fe-source would lead to the formation of ferromagnesiosilica grains. This exploratory study found that coagulation and thermal annealing of amorphous magnesiosilica and metallic grains yielded ferromagnesiosilica grains with the Fe/(Fe?+?Mg) ratios in interplanetary dust particles. Furthermore, decomposition of brucite present in the condensed magnesiosilica grains was the source for water and the cause of different iron oxidation states, and the formation of amorphous Fe~(3+)-ferrosilica, amorphous Fe~(3+)-Mg, Fe-silicates, and magnesioferrite during thermal annealing. Fayalite and ferrosilite that formed from silica/FeO melts reacted with forsterite and enstatite to form Mg, Fe-silicates. The presence of iron in different oxidation states in extraterrestrial materials almost certainly requires active asteroid-like parent bodies. If so, the possible presence of trivalent Fe compounds in comet P/Halley suggests that Halley-type comets are a mixture of preserved presolar and processed solar nebula dust. The results from this thermal annealing experiment further suggest that the Fe-silicates detected in the impact-induced ejecta from comet 9P/Temple 1 might be of secondary origin and related to the impact experiment or to processing in a regolith.