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.