Dear Colleagues,
A paper of interest to metamorphic petrologists has been published in
Geological Materials Research, see abstract below. Figure 5 in this paper
is a pseudosection that includes a "rollover" feature (best viewed with
Acrobat Reader 5.0) to emphasize the stability regions of particular
minerals. The full (pdf) version of the paper may be downloaded from:
http://gmr.minsocam.org/papers/v3/v3n1/v3n1abs.html
Best wishes,
John
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John B. Brady
Department of Geology, Smith College, Northampton, MA 01063
Voice: 413-586-9014 Fax: 413-585-3786
http://www.science.smith.edu/departments/geology
http://gmr.minsocam.org
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Metapelite phase equilibria modeling in MnNCKFMASH: The effect of variable
Al2O3 and MgO/(MgO+FeO) on mineral stability
Douglas K. Tinkham, Carlos A. Zuluaga, and Harold H. Stowell
Department of Geological Sciences, University of Alabama,
Tuscaloosa, AL 35487-0338, USA
<[log in to unmask]>
(Received July 26, 2001; Published December 17, 2001)
Abstract
A series of MnO-Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O (MnNCKFMASH) metapelite
pseudosections highlights the dependence of predicted mineral assemblages
on bulk rock Al2O3 and Mg# (MgO/(MgO+FeO)). T-XAl pseudosections portray
the dependence of staurolite, biotite, and aluminum silicate on Al2O3
content, allowing the distinction between high-Al and low-Al pelite, as
commonly portrayed with KFMASH modeling. The MnNCKFMASH system also shows
the effect of Al2O3 on plagioclase and zoisite stability, which cannot be
done in the KFMASH system. Comparison of MnNCKFMASH to KFMASH
pseudosections highlights the consequence of ignoring the important rock
constituents MnO, Na2O, and CaO when constructing pseudosections. KFMASH
cannot model important phases such as plagioclase and zoisite, and there
are significant differences in predicted garnet, biotite, and chloritoid
stability in the two different systems. In particular, KFMASH does not
model garnet stability appropriately at low pressures and temperatures
because it cannot account for the stabilizing effect of Mn. The comparisons
also show that the method of calculating a KFMASH bulk rock composition
equivalent to a real rock composition is problematic and has significant
implications for the predicted pseudosection assemblage stability.
Comparison of the MnNCKFMASH pseudosections to natural assemblages observed
in the Waterville Fm., Maine, indicates that the MnNCKFMASH system comes
very close to modeling naturally developed mineral assemblages
successfully. The only major discrepancy between predicted and observed
assemblages is the inability to predict the paragenesis staurolite +
andalusite using an average or natural Waterville Fm. composition.
Garnet thermobarometric results from the Waterville Fm. are in poor
agreement with pseudosection topology for an average Waterville Fm.
composition. This suggests that if quantitative P-T path information is to
be derived through a combination of pseudosections and thermobarometry,
samples will have to be investigated on an individual basis in more detail
than was done in this study.
Keywords: metapelite, phase equilibria, pseudosections, mineral assemblage,
thermodynamics, bulk composition, Waterville Fm., metamorphic petrology,
thermobarometry
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