Dehydrogenation as the mechanism of formation of the oriented spinel-pyroxene symplectites and magnetite-hematite inclusions in terrestrial and extraterrestrial olivines

Abstract

A new approach is developed to explain the nature of oriented pyroxene-spinel symplectites and magnetite-hematite inclusions in olivines. Symplectites are considered the products of dehydrogenation and solid-state substitution of OH-bearing olivine-like precursor phases occured as lamellar inclusions in olivine. According to the dehydrogenation model, the precursor of the oriented symplectic pyroxene-spinel intergrowths in olivine hydrous olivine [3(Mg,Fe)2SiO4] · (MgH2SiO4), while the appearance of hematite-magnetite intergrowths in olivine is related to the dehydrogenation and dehydration of humite-like interlayers [n(Mg,Fe)2SiO4] · [Mg(OH2)]. The dehydrogenation reactions are accompanied by the transformation of divalent transition metals into a trivalent state and interdiffusion of Fe, Cr, Ca, Mg, and Al between precursor phase and host olivine under the long-term decompression at temperatures >800°C. The transformation of a pre-existing phase into symplectite was driven through the topotaxic reaction, fulfilling the following conditions: inheritance of crystallographic orientation of precursor phase by symplectite; preservation of volume during reaction, i.e. tendency to zero volume effect of the reaction1; the mass conservation, which is consistent with a solid state reaction in a closed system without external input of chemical elements in olivine.