Hello Matt,
the method described by Evans (2004) is much more simple. It does not use
the theorem of Duhem and therefore we don't need to express all phases in
equilibrium with the garnet to solve the system (and the effective
composition).
This is "simply" based on the Henry' law for the manganese and Rayleigh's
equation (first garnet mode given by thermocalc or other softwares) but I
don't know if someone already tried to apply this method in doing their
pseudosections (and P-T path)...
Best regards,
Jean-Luc
----- Original Message -----
From: "Matthew Kohn" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Monday, July 22, 2013 4:22 AM
Subject: Re: [geo-metamorphism] garnet fractionation
Hi folks,
This message reminds that next month is the 25th anniversary of Frank
Spear's paper on - you guessed it! - the effect of garnet fractional
crystallization on (reactive) bulk compositions and mineral assemblages.
This approach first fully quantified chemical impacts in a fundamentally
predictable fashion and became the cornerstone of all subsequent Gibbs
method - type models.
His paper also addresses the effects of diffusional relaxation of garnet
growth zoning as rocks heat.
Here's the reference: Spear, FS (1988) Metamorphic fractional
crystallization and internal metasomatism by diffusional homogenization of
zoned garnets. Contributions to Mineralogy and Petrology, v. 99, 507-517.
It's founded on Duhem's theorem, which constrains P-T-X-M in rocks, and also
published 25 years ago: Spear, FS (1988) The Gibbs method and Duhem's
theorem - the quantitative relationships among P, T, chemical potential,
phase composition and reaction progress in igneous and metamorphic systems.
Contributions to Mineralogy and Petrology, v. 99, 249-256.
Best,
Matt
On Jul 20, 2013, at 1:24 AM, Jean-Luc Régnier wrote:
> Dear All,
>
> I got the opportunity to do a small application (.exe only for windows
> unfortunately, but source code for Python27) calculating the effective
> bulk composition after garnet fractionation. The program is based on the
> work of Evans* (2004) with some modifications and might be useful for
> people building pseudosections/P-T path from bulk rock analyses and garnet
> analyses.
> I would be happy to discuss with people working on this subject if any ?
> Best regards,
> Jean-Luc
>
> * Evans, T. P. (2004). A method for calculating effective bulk composition
> modification due to crystal fractionation in garnet bearing schist:
> implications for isopleth thermobarometry. Journal of Metamorphic Geology
> 22, 547–557.
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Dr. Matthew J. Kohn, University Distinguished Professor
Department of Geosciences, Boise State University
1910 University Dr.; MS1535
Boise, ID 83725-1535
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http://earth.boisestate.edu/mattkohn/
phone: (208)-426-2757 fax: (208)-426-4061
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