MICROSTRUCTURE OF EXPERIMENTALLY OXIDIZED OLIVINE FROM A MANTLE NODULE: II. PHASE COMPOSITION OF CRYSTALLINE MICROINCLUSIONS CONTAINING FE3+ AND OH-

Abstract

An olivine grain (Fa 8.2%) from a mantle nodule in kimberlite (Udachnaya pipe, Yakutia) was annealed in air for 9 h at 700 degreesC and studied by EMPA, FTIR, UV spectrometry, and TEM using HRTEM, SAED, CBED, AEM, and EELS. The FTIR and UV spectroscopic methods revealed the presence of OH- and Fe3+. Transmission electron microscopy (TEM) investigations provided no evidence for the presence of laihunite or Fe oxides (magnetite or hematite) as oxidation products in the grain. Electron microscope examinations revealed accumulations of hexagon-like inclusions, several thousand angstroms in size, which were confined to dislocation loops. The data obtained at a resolution of 4 nm allowed us to establish that the core of these inclusions was composed of alternating thin layers of feroxyhyte (FeOOH), bernalite, Fe(OH)(3), and beta -cristobalite SiO2 and that the envelope consisted of secondary olivine, whose structure was deformed (compressed) along the c direction of the unit cell. Strict regular orientation and crystallographic relationships were observed between the coexisting phases in the inclusion and the olivine matrix. Such a complex microstructure of inclusions is interpreted as a result of the solid-phase transformation of a precursor phase, which built up the inclusion and was represented by a hydrous magnesium silicate (HMS). The formation of the inclusions is considered as an exsolution of HMS from the olivine matrix containing OH-bearing point defects, which accumulated on dislocations during crystal growth. The subsequent HMS transformation with separation into the core and envelope phases can be made to occur in the course of laboratory annealing. A model is proposed for such transformations including the processes of oxidation and partial dehydration of HMS accompanied by (a) diffusion of Mg and Fe between olivine and Fe-free inclusions and (b) phase and chemical separation within the inclusions.