Dave,
I agree with your comments about fO2, but why should Fe3+/Fe2+
remain unchanged during metamorphism? Both of them can surely
evolve depending on the evolution of the fluid (or a melt in the case
of granulites) that is usually present.
eric
On Nov 17, 2005, at 2:56 AM, Dave Kelsey wrote:
> Dear all,
>
> I have recently joined the Metamorphic mailing list, as well as the
> THERMOCALC mailing list. I am very familiar with the use of
> THERMOCALC and metamorphic processes in all metamorphic facies, but
> particularly the granulite facies. I have worked extensively with
> Roger Powell (and Tim Holland) and was involved in the generation
> of the a-x (mixing) model for sapphirine.
>
> I am interested in garnering the opinions of the group with regard
> to the consideration of ferric iron in metamorphism and phase
> equilibria modeling. There is a widespread notion that ferric iron
> is 'imposed' on a chemical system as a proxy for fugacity rather
> than the alternative that ferric iron is an inherent part (i.e. an
> integral component) of the chemical system. My personal belief
> (which of course is largely a function of the rp school of thought)
> is that ferric iron is an inherent part of a chemical system, and
> that a given chemical volume has a fixed amount of ferric iron
> within it regardless of the imposed P-T conditions. Interpreting
> (or using) a fixed fO2 as being the same as fixed Fe2O3 is not
> correct. Implicit in assuming a fixed fO2 is that the total Fe2O3
> (component) is variable and vice versa. In simpler terms, a fixed
> fO2 implies that somehow the chemical volume is being oxidised (or
> reduced) as it is being metamorphosed. It therefore follows that a
> variable fO2 is basically a proxy for describing the distribution
> of a FIXED amount of Fe2O3 amongst solid-solution minerals. That
> is, as pressure and temperature change, the composition of minerals
> change, and thus the oxidation state of individual minerals changes
> too!
>
> I put this discussion forward more as a note of caution for those
> studies that have considered, for example, KFMASH, with the
> oxidation state being superimposed on the system. This actually
> implies that at the first-order scale, changes in mineral
> assemblage and composition are driven by chemical means (e.g.
> chemical potential gradients, in this case being oxidation) and not
> changes in pressure and temperature. Thus, if changes in pressure
> and temperature are assumed to be the driving force for mineral
> reaction in a given sample, it is more appropriate to consider the
> system as containing Fe2O3, e.g. KFMASHO (one would usually include
> TiO2 as well)
>
> I would like finish by saying that I do not wish to exclude the
> fact that open system processes do occur (e.g. melt loss, fluid
> infiltration) that will lead to fixed activities (or fixed fugacities)
>
> cheers and happy pursuits!
> dave k
>
>
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