Jollands, Michael
Description
Knowledge of the rates and mechanisms of diffusion in olivine can
be used,
among other things, to elucidate timescales of magmatic
processes, determine closure
temperatures of radiogenic systems, quantify the robustness of
melt inclusions and aid
in development of point defect models. Firstly, however, the
diffusion process itself
must be well understood. This is only possible when diffusion
studies are closely
coupled with considerations of...[Show more] equilibrium thermodynamics.
In this thesis, the diffusion of Mg2+, Be2+, Sc3+, Zr4+, Hf4+,
Cr2+, Cr3+, Ti3+,
Ti4+ and H+ in olivine are presented at various temperatures
between 900-1600 °C,
and pressures from 1 bar to 2.5 GPa under controlled oxygen
fugacity, silica activity
and crystal orientation.
The interface concentration (at the crystal edge) of trace
element diffusants
should be at equilibrium with external conditions (such as silica
activity, oxygen
fugacity, temperature and pressure). This assumption must be
satisfied in order to
draw any conclusions regarding diffusion rates.
The silica activity of a system affects both the concentration
and diffusion rate
of trace elements in olivine, with all cations (except H+ and
Be2+) diffusing faster at
high silica activity than when silica activity is low. Oxygen
fugacity affects the
diffusion rate and interface concentrations of Ti and Cr –
these both (generally)
diffuse faster and at higher concentrations in more reducing
conditions, where Cr2+
and Ti3+ are more prevalent.
The concentration of trivalent cations in olivine can affect
their diffusion rate
– the rate of Sc3+ diffusion is positively correlated with its
interface concentration.
The ionic radius of cations that substitute onto the M sites in
olivine strongly
affect the diffusive anisotropy (the difference in diffusion
coefficients between the
fastest and slowest axes), relating to preferential ordering onto
the M1 sites, which are
aligned into closely spaced chains along one crystallographic
axis.
There is no apparent causal relationship between ionic charge and
diffusivity
of M-site cations in olivine. More important are atomic mass,
site preference and
ionic radii.
Taken together, the results from these studies show firstly that
diffusion in
olivine is more complex than previously thought, and secondly
that the effect of
thermodynamic variables must be considered when studying
diffusion.
Items in Open Research are protected by copyright, with all rights reserved, unless otherwise indicated.