Dear Richard,
There are several issues that should be considered (Juergen has raised
some of them), and there are several levels at which one can attack the
problem.
Among the issues, it is necessary to ensure that one is not seeing
artefacts from sectioning effects (sudden changes in slopes of
profiles), inclusions (affecting concentrations of individual elements)
etc. In general, it is best to consider several profiles and several
grains before inferring anything about timescales. There is, of course,
the primary concern of whether one is looking at a single stage
overgrowth at all. I am assuming that you have sorted these aspects out
through your studies of the thin section(s).
Different levels of attacking the problem include full multicomponent
simulations, with or without variable temperature, pressure etc., using
different kinds of initial and boundary conditions, and so on and so
forth. Theria-G can be adopted to do some of these, the literature that
John has pointed to are some simpler possibilities. At the very simplest
level one can take effective binary diffusion coefficients at a
characteristic temperature to get an *estimate* of timescales. This has
been done, for example, in:
Ganguly, J., Chakraborty, S., Sharp, T. and Rumble, D. III (1996)
Constraint on the time scale of biotite grade metamorphism during
Acadian orogeny from a natural garnet-garnet diffusion couple. American
Mineralogist, 81, 1208-1216.
For this, you do not need a program - you can do it using Excel and
simple error functions. If you need help, please write me off line and I
can help you set up a spreadsheet. I hope this helps,
Best wishes,
Sumit
On 5/21/2011 2:07 PM, Richard Palin wrote:
> Dear group,
>
> I have an aluminosilicate garnet from a metapelitic schist which has
> experienced two metamorphic episodes, the first being MP-HT and the
> latest being MT-HP (kyanite-grade, ~680C and 11kbar). Subsequently,
> the garnet profile currently shows a core-rim chemical relationship
> with a transition region between; the shape of this profile strongly
> suggests that this transition is the result of intracrystalline
> diffusion between these two initially separate chemical regions (see
> attached zoning profiles).
>
> Given that conditions of peak metamorphism are well constrained, I was
> wondering if anyone knew of a program which could simply model the
> diffusion ("relaxtion") of divalent cations along this line. The main
> aim is to extract an order-of-magnitude estimation of the time spent
> at peak T; i.e. with an initial step-function, I want to see how long
> the garnet would need to stay at 680C to match what is now seen.
>
> A literature search brought up Theria_G (Gaidies et al. 2008) and,
> although it comes close, I couldn't see how it could answer this
> particular question. I should not that this is in no way an in-depth
> study of diffusion in garnet; I am assuming that variables which are
> normally considered (such as pressure, temperature and radius) are
> fixed. The objective is to extract an estimate of time from this
> profile, and any suggestions would be greatly appreciated.
>
> Thanks,
>
> Richard Palin
>
> D.Phil Research Student,
> Himalayan Studies Group
>
> University of Oxford,
> Department of Earth Sciences,
> South Parks Road,
> Oxford,
> OX1 3AN,
> United Kingdom
>
> http://www.earth.ox.ac.uk/~richardp/
>
> ----------------------------------------------------------------
> This message was sent using IMP, the Internet Messaging Program.
>
--
***************** Sumit Chakraborty ****************************************
http://www.gmg.ruhr-uni-bochum.de/petrologie
Institut fuer Geologie, Mineralogie und Geophysik;
Ruhr-Universitaet Bochum;
D-44780 Bochum; Germany
Email: [log in to unmask]
Tel: +49-(0)234-322 -4395 / -8521 / -8155
Fax: +49-(0)234-321 4433
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