Dugald,                                         9/26/01
        Regarding:

"Although medium-grade Bt generally lacks concentric zoning, there is
independent
evidence that it fails to equilibrate in divariant KFMASH assemblages.
According to the phase diagrams, XFe in Bt ought to vary smoothly across
each prograde zone and inflect at each of the isograds, but in fact it
tends to be constant across each prograde zone and change abruptly at each
isograd (e.g. see Chinner 1965 Min Mag 34, 132-143; Mohr & Newton 1983 Am J
Sci 283, 97-134). This suggests that Bt tends to approach equilibrium
during the univariant KFMASH reaction at each isograd (by means of the
general catalytic dissolution and regeneration of matrix minerals that
facilitates the growth of porphyroblastic minerals), while also suggesting
that Bt becomes progressively more metastable in respect to XFe with
increasing distance upgrade from each major isograd (due to the
sluggishness of intracrystalline diffusion)."

        Re-read Guidotti (1974) BGSA (the paper that was of at least
some use to you for developing your bathograd concept); see also
Guidotti et al. (1988) AJS (on Bt composition as a function of
grade), Guidotti (1978) Amer. Mineral. (on Ms composition as a
function of grade). I think you'll see that at least in W. Maine
these solution phases did just as you'd expect according to phase
diagrams. Indeed, Bt (and the coexisting Chl, Guidotti et al. 1991,
Amer Mineral) becomes more Mg-rich until the isograd that brings in
Sil (essentially St + Chl = Sil + Bt). Then Bt switches and starts
becoming more Fe-rich. Coincidentally, over this same range of
grades, Ms becomes more Na-rich until the isograd, and then it
switches and becomes more K-rich.
        Maybe the W. Maine rocks are unusual in that they "seem" to
obey the predictions based on phase diagrams? Nonetheless, this
systematic behavior has been an important element in my arguing that
these rocks at least "approached" chemical equilibrium, even if they
didn't fully achieve it. At any rate, I hope readers won't infer that
your comments that the behavior of Bt is as troublesome as is Grt.
                                        Cheers,  Charlie
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>At 12:04 PM 25/09/2001 +0100, Dave Waters wrote:
>>... The assemblage Grt+Bt+Ms+Mt+Qtz will buffer fO2
>>effectively, i.e. retard changes and hold it in a narrow range of
>>values, because of the preponderant mass of condensed phases compared to
>>the amount of O2 (or H2) that can normally be added or removed in a
>>volatile phase. Except at extremely high fluid:rock ratio, only trivial
>>changes in the annite and almandine contents of biotite and garnet would
>>be needed to restore equilibrium, at an imperceptibly changed fO2.
>
>But those trivial changes would be difficult to achieve for kinetic
>reasons.  Equilibration that requires intracrystalline diffusion is
>kinetically more difficult than equilibration that requires only
>dissolution and growth of minerals through H2O-assisted grain-boundary
>diffusion. Medium-grade Grt commonly has concentric compositional zoning -
>proof of disequilibrium in respect to intracrystalline diffusion. Although
>medium-grade Bt generally lacks concentric zoning, there is independent
>evidence that it fails to equilibrate in divariant KFMASH assemblages.
>According to the phase diagrams, XFe in Bt ought to vary smoothly across
>each prograde zone and inflect at each of the isograds, but in fact it
>tends to be constant across each prograde zone and change abruptly at each
>isograd (eg see Chinner 1965 Min Mag 34, 132-143; Mohr & Newton 1983 Am J
>Sci 283, 97-134). This suggests that Bt tends to approach equilibrium
>during the univariant KFMASH reaction at each isograd (by means of the
>general catalytic dissolution and regeneration of matrix minerals that
>facilitates the growth of porphyroblastic minerals), while also suggesting
>that Bt becomes progressively more metastable in respect to XFe with
>increasing distance upgrade from each major isograd (due to the
>sluggishness of intracrystalline diffusion).
>
>    Accordingly, I am skeptical that the assemblage Grt+Bt+Mu+Mt+Qtz can
>equilibrate fast enough to slide-buffer fO2 in a transient fluid. The
>larger the number of additional minerals (Chl, St, Ky, Sil, Crd, Kfs,...)
>the better would be its chances of doing so. In my experience low-variance
>assemblages are not uncommon, and unfailingly they record maximal
>petrological information.
>
>Cheers, Dugald
>
>Dugald M Carmichael                    Phone/V-mail: 613-533-6182
>Dept of Geological Sciences and Geological Engineering
>Queen's University                              FAX: 613-533-6592
Kingston  ON  K7L3N6             E-mail: [log in to unmask]

--
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C.V. GUIDOTTI                                           Univ. Phone
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UNIVERSITY OF MAINE                                     Univ. Fax
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