I am currently studying unusual high-Mg metapelites from the High
Himalayan Crystalline Sequence containing the assemblage orthoamphibole
+ kyanite + cordierite + biotite + quartz ± garnet ± staurolite ±
chlorite. The orthoamphibole is a gedrite, with a composition (Na0.5)
(Mg3.1 Fe2.4) Al1.3 [Si6 Al2 O22] (OH)2.
In order to ensure a maximum consistency in the analysis of the P-T
stability of such assemblages compared to other more "classical"
Barrovian assemblages found in the same sequence, I'm trying to include
the orthoamphibole in a KFMASH grid I calculated with THERMOCALC,
according to the latest dataset of Holland & Powell (1998). Using a
simple ideal activity model for the anthophyllite - Fe-anthophyllite -
gedrite solution, two FMASH invariants involving orthoamphibole +
staurolite + cordierite + garnet + kyanite and orthoamphibole +
staurolite + cordierite + chlorite + kyanite appear to be unstable, as
they occur at low pressures (around 3 kbar), in the stability fields of
andalusite and sillimanite. In contrast, these two invariants are
stable in the KFMASH grid of Xu et al. (1994), and they plot close to
the ky = sill equilibria around 620 °C / 6 kbar. This discrepancy is
most likely the consequence of a significant non-ideality in the
orthoamphibole. Yet, I can't find informations about orthoamphibole
non-ideality in the THERMOCALC documentation or in related papers (e.g.
Will & Powell, 1992; Xu et al., 1994; Holland & Powell 1998).
I would consequently greatly appreciate if someone could tell me where
to find such informations (activity coefficients, DQF parameters)
allowing to calculate a non-ideal activity model for the anthophyllite
- Fe-anthophyllite - gedrite solution. Better still, if someone has an
already coded version of such an activity model for THERMOCALC, I would
be extremely grateful if I could get a copy.
Bonjour chez vous !
Holland, T. J. B. & Powell, R. 1998. An internally consistent
thermodynamic data set for phases of petrological interest. Journal of
Metamorphic Geology 16, 309-343.
Xu G., Will T.M. & Powell R. 1994. A calculated petrogenetic grid for
the system K2O-FeO-MgO-Al2O3-SiO2-H2O, with particular reference to
contact-metamorphosed pelites. Journal of Metamorphic Geology 12,
Will T.M. & Powell R. 1992. Activity-composition relationships in
multicomponent amphiboles: An application of Darken's quadratic
formalism. American Mineralogist 77, 954-966.