| Solid-state diffusion controls a variety of dynamic processes within the Earth, e.g., the
rheological behavior of rocks as well as the element and isotopic exchange on different
spatial scales. Modeling these processes requires well-defined diffusion coefficients along
with appropriate physical models. In the last decade a significant increase in number and
improvement of quality of diffusion data for the major mantle minerals olivine,
clinopyroxene, and orthopyroxene have been achieved mainly due to improved experimental
and analytical techniques. A reliable application of these data to various conditions
within the Earth is ultimately linked to a basic and quantitative understanding of
the diffusion mechanisms and the parameters affecting the concentration of the
relevant point defects. These have to be identified by a combination of experimental
studies and point defect thermodynamic models. The availability of a large body of
systematic diffusion data makes olivine the mineral of choice to explore this avenue.
Diffusion coefficients for monovalent (Li, H), divalent (Fe, Mg, Mn, Ni, Co, Ca, Sr),
trivalent (REE, Cr) and tetravalent (Si, Hf) cations as well as O are known typically
as a function of temperature, and often as a function of other variables such as
pressure, oxygen fugacity or water fugacity. The large amount of experimental
data can be reproduced using a quantitative point defect model, which explicitly
considers the various minor and trace elements in olivine (Dohmen and Chakraborty,
2007). This method was further developed to consider also H related defects and this
provides now a parameterized equation to predict Fe-Mg diffusion (and potentially
also Ca, Mn, Ni, etc.) in olivine over the whole range of conditions in the Earth’s
upper mantle. The approach is perfectly general and can be extended to any other
mineral provided enough data are available. Much less is known about diffusion
mechanisms and point defects in other major mantle minerals like orthopyroxene or
clinopyroxene (diopside). There is also an increasing data set available for these
minerals, but the role of thermodynamic parameters other than temperature (like
pressure, oxygen fugacity and water fugacity) are almost unknown. Olivine will
serve here as a reference case to discuss and evaluate these effects. However, a
significant difference compared to olivine is the ability of the pyroxene structure to
accommodate higher concentrations of heterovalent ions (e.g., Fe3+, Al3+, Cr3+,
H+-¦). Thus, heterovalent cations may or may not increase the concentrations of the
relevant point defects e.g. vacancies on the metal sites of pyroxenes, depending on the
charge balancing mechanism. Despite the larger concentrations of impurities in
pyroxenes, the new data sets for Fe-Mg diffusion in clinopyroxene and orthopyroxene
(Chakraborty et al. 2008, ter Heege et al. 2006) show that diffusivities are significantly
slower. The effect of oxygen fugacity is also smaller than in olivine, suggesting, in
combination, that the concentration of vacancies on the metal site is lower. It could be
concluded that vacancy formation in pyroxenes is energetically much less favorable than
in olivine. Detailed experimental and also computational studies should help to
quantify these effects to provide a full description of diffusion in the earth’s upper
mantle.
ter Heege et al. (2006) EOS Trans. AGU 87(52) MR21A-0004. Dohmen and Chakraborty
(2007) Phys. Chem. Minerals, 34, 409. Chakraborty et al. (2008) EOS Trans. AGU 89(53)
MR21C-0004. |