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

We have performed a series of electron irradiation experiments with 300 keV electrons on thin (Mg1.8Fe0.2SiO4) olivine samples in situ in a transmissi on electron microscope. The irradiation-induced modifications occur in two distinct stages: firstly, stage 1 is the breakdown of olivine into small Mg O crystallites and amorphous SiO2-rich phase; secondly, stage 2 is the impo rtant O loss that leads to the reduction of SiO2. The phase separation requ ires an electron fluence of approximately 3 x 10(20) e(-)cm(-2). This bulk dissociation process does not imply elemental diffusion over large distance s. A moderate stoichiometric loss of MgO also occurs during this first stag e. In stage 2, bulk diffusion of chemical species is evidenced from the ins ide 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 exponentia l factor directly proportional to the electron fluence and inversely propor tional to the sample thickness.