Dear Jean-Claude,
Just as a clarification: your mineral assemblage from the Himalayas
(orthoamphibole + kyanite + cordierite + biotite + quartz ± garnet ±
staurolite ± chlorite) would be described in FMASH rather than in KFMASH
as it contains only one potassic phase - right? Orthoamphibole is not
stable in the presence of additional muscovite or K-feldspar.
Using clinoamphibole (cummingtonite - grunerite) from the THERMOCALC
database instead of orthoamphibole gives a chlorite-absent invariant
point at 648°C, 8.2 kbar for your assemblage. Orthoamphibole should give
a similar result at somewhat lower temperatures.
Additionally, the KFMASH-invariant point including garnet + staurolite +
chlorite + chloritoid + biotite + muscovite + quartz also lies at
unreasonably low pressures, so the thermodynamic data / mixing model of
another phase, like staurolite, might be responsible to some degree for
low-pressure results.
Cheers,
Alexander
Dr. Alexander Proyer
Institut für Mineralogie und Petrologie
Universität Graz
Universitätsplatz 2
A-8010 Graz, AUSTRIA
Tel. (++43) 316-380-5522 Fax: (++43) 316-380-9865
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