In principle, the phengite barometer is buffered in your rocks,
but the experiments are unreversed. Two-feldspar would apply in
principle, but the system is not well calibrated at T < 600°C.
Without obtaining Fe3+/Fe2+ on both the muscovite and biotite, any
application of thermobarometry involving those phases must be
considered suspect. Be sure to check for Ba in the micas and
Kfeldspar. I would recommend finding some other assemblages!
Green and Usdansky (1986) published a correction to their
equation. However, we still cannot reproduce their T results with
their equations. My impression is that their system gives
temperatures that are too low.
>I have been studying gneisses from Erzgebirge, which have mineral
>assemblage quartz-Kfs-Pl-Ms-Bt. To constrain P-T conditions of their
>metamorphism, I can use phengite barometry - however, determination
>of temperatures seems to be difficult. At least muscovite is outside
>the compositional range defined by Hoisch (1989) biotite-muscovite
>thermometer, being too Mg-rich.
>I will be grateful to any advice or comment from those who have
>experience with this and other thermobarometers (e.g. Green and
>Usdansky 1986 based on Na-K exchange reaction between Pl and Ms,
>and more recent ones), which could be used for these rocks.
>Thank you in advance!
>Dr. Jana Kotkova
>Czech Geological Survey
>118 21 Praha 1