Although I am coeval with Chuck Guidotti and no less prone to decrepitude
:-(, I am not yet willing to defer to Dave Pattison's opinions. :-)
At 01:56 PM 5/21/99 -0600, Dave wrote:
>... Currently
>existing thermodynamic data sets do a moderate to poor job of handling
>natural low pressure mineral assemblage sequences. Rectification of this
>deficiency is a worthwhile goal, because variations in mineral assemblages
>are far more useful in providing information on subtle P-T variations in
>low-P (And-Sil type) settings than geothermobarometry - see my chapter on
>metapelites in the Am Min 'Contact metamorphism' volume (v. 26, 1991) to
>see why.
I have reread this chapter without seeing why. The accuracy of both the
grid and the thermobarometers depends on the same thing - accurate
determination of the thermodynamic properties of minerals and their
inferred-to-have-been-coexisting fluid and/or liquid. The so-called
"constraints of Schreinemakers' analysis" (op cit p.109) are nothing more
than the constraints of stable equilibrium. It is not possible to "rectify"
the grid without simultaneously making it possible to rectify the
thermobarometers. But even if/when you youngsters have perfectly accurate
thermobarometers and perfectly accurate grids for the system TiKNFMASH at
specified mol fractions of Mn in Grt, Ca in Plg, Zn in St, Fe3+ in Ilm
(etc.), and even if/when you can make perfectly accurate mineral analyses,
there will still be no certain way to prove that "geothermobarometric
results... which are inconsistent with the observed phase relations" (op cit
p.109) are due to metastable mineral compositions rather than unstable phase
relations.
Dugald M. Carmichael Phone/V-mail: 613-533-6182
Geological Sciences, Queen's University FAX: 613-533-6592
Kingston ON K7L3N6 E-mail: [log in to unmask]
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