Evolution Of The Iron-silicate Ratio In Ordinary Chondrites And Genetic Types Of Iron Meteorites

摘要

The problem concerning formation of chondrites is central in understanding planet evolution in the solar system. There are fundamentally different hypotheses explaining the formation of silicate chondrales and the matrix enriched in metallic iron composing chondrites. Most cosmochemists admitting the melt nature of chondrules suggest their formation either during the preaccretionary period or as a result of impact melting of matter on the surface of parental bodies. This breaks the genetic link between chondrules and the host matrix, whose formation is related to the condensation in the protoplanetary nebula. Chondrules and the chon-drite matrix are regarded as independent objects that formed and evolved under different conditions as a result of distinct mechanisms. The proximity of the equilibrium ratios between silicates in chondrules and the metallic phase in the matrix is due to superimposed metamorphism, which causes the formation of equilibrium chondrites of petrological types 4-7, which "experienced oxidation of metallic Fe during heating" [8]. However, oxidation Fe + 0.5O_2 + MgSiO_3 = MgFeSiO_4 occurs at decreasing temperature (being regressive), rather than increasing temperature, which explains the noted in [8] small increase in the FeO/(FeO + MgO) ratio in silicates and Ni content in the iron phase of chondrite with an increase in the number of the petrological type.