Dug and others
In ref to the Pattison & Tracy Am Min RiM Contact Metamorphism (v 26)
chapter, the pertinent pages discussing the grid vs thermobarometry are
179-180.
In ref to Dug's points:
1. Yes, if the equilibrium model holds, the grid and thermobarometers
should give the same result using the same data set. The fact that one gets
the scatter common in thermobarometry results from individual or closely
spaced samples, or thermobarometry results inconsistent with phase
equilibrium constraints (a few low-P examples are listed in the pages
above; there are many others from other P-T regimes), suggests that the
equilibrium model does not hold. Obvious potential reasons for this:
(1) non-simultaneous closure of all elements in all phases assumed to be in
equilibrium, such as due to selective resetting or alteration or zoning (a
'rock process' problem);
(2) incorrect selection of parts of minerals that may have been in
equilibrium (an 'analysis' problem);
(3) imprecise analyses (another 'analysis' problem)
(4) incorrect accounting for minor elements, Fe2/Fe3 etc. (another
'analysis' problem, although this must also be done to any grid to be
consistent).
2. Yes, if the ultimate thermo data set and perfect analyses are achieved,
and all the corrections Dug notes are taken care of, we are still left with
the first two problems. If the thermobarometry results are still
inconsistent with the phase equilibria, one has to answer why. I agree that
there is no certain way to distinguish metastable mineral compositions
(thermobarometry) from unstable phase relations (grid) for any individual
sample. But, I think that the context provided by consideration of a sample
with others from the same setting eg prograde sequence, and analysis of
mineral textures (admittedly an imperfect blend of art and science, but one
that cannot be avoided in analysing metamorphic rocks), provide means to
discriminate. A simple minded (but real) example is that a
thermobarometry-based 4.5 kbar estimate for an aureole that develops
metapelitic And+Kfs assemblages as part of a well defined low pressure
prograde sequence (Dug's bathozone 1, ie less than 2.2-3 kbar from phase
equilibrium analysis) is most likely wrong.
>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. :-)
>
>>... 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]
>
>
David R.M. Pattison, Department of Geology and Geophysics,
University of Calgary, Calgary, Alberta, T2N 1N4, Canada.
Phone: 403-220-3263 (my office); 403-220-5841 (dept. office)
Fax: 403-284-0074 email: [log in to unmask]
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