Electron-irradiation-induced phase transformation and fractional volatilization in (Mg, Fe)(2)SiO4 olivine thin films

摘要

We have performed a series of electron irradiation experiments with 300 keV electrons on thin (Mg1.8Fe0.2SiO4) olivine samples in situ in a transmission electron microscope. The irradiation-induced modifications occur in two distinct stages: firstly, stage 1 is the breakdown of olivine into small MgO crystallites and amorphous SiO2-rich phase; secondly, stage 2 is the important O loss that leads to the reduction of SiO2. The phase separation requires an electron fluence of approximately 3 x 10(20) e(-)cm(-2). This bulk dissociation process does not imply elemental diffusion over large distances. A moderate stoichiometric loss of MgO also occurs during this first stage. In stage 2, bulk diffusion of chemical species is evidenced from the inside to the outside of the irradiated area and atoms, mostly O, are desorbed from the surface, leading to a marked change in the composition. Rates of elemental loss follow first-order kinetic exponential laws, with exponential factor directly proportional to the electron fluence and inversely proportional to the sample thickness. [References: 20]