Practical Aspects of Mineral Thermobarometry

Garnet activity coding

FMAS

The simple coding for FMAS garnet is found in the example file thdkfm.txt, supplied with the thermodynamic data set in the file TC31win.zip.

g 2 
x(g) 0.85

p(py) 1 1 1 1 -1 x
p(alm) 1 1 0 1 1 x

sf 
w(g) 2.5 0 0

2
x(Mg) 1 1 1 1 -1 x
x(Fe) 1 1 0 1 1 x

py 1 1 x(Mg) 3
alm 1 1 x(Fe) 3

CFMAS

Extending this to three-component Fe-Mg-Ca garnet is simple, although there is a choice of interaction parameters (fixed or T-dependent) for the Mg-Ca interaction. This formulation maintains the convention of an x(Fe) = Fe/(Fe+Mg) parameter for each Fe-Mg phase.

W(gr,py) = 33 kJ mol^-1 is the value given in NCFMASH example data files distributed with THERMOCALC 3.1 (June 2001, in TC3win.zip). The T-dependent formulation of W(gr,py) = 41.4 - 0.0188T kJ mol^-1 is derived by recasting the Newton and Haselton (1981, Adv. Phys. Geochem. 1, 131-147) value of 3300 - 1.5T cal mol^-1 (cation unit) to kJ mol^-1 (macroscopic unit).

g 3
x(g) 0.95 % Fe/(Fe+Mg) 
z(g) 0.4  % Ca/(Fe+Mg+Ca) 

p(gr)  1 1   0 1  1 z
p(alm) 1 2   1 1 -1 z   0 1  1 x
p(py)  1 2   1 1 -1 z   1 1 -1 x

sf 
W(gr,alm)  0   0 0
W(gr,py)  33   0 0     % 41.4 -0.0188 0
W(alm,py)  2.5 0 0

3 
x(Fe,M1) 1 2   1 1 -1 z   0 1  1 x 
x(Mg,M1) 1 2   1 1 -1 z   1 1 -1 x
x(Ca,M1) 1 1   0 1  1 z

gr   1 1 x(Ca,M1) 3 
alm  1 1 x(Fe,M1) 3
py   1 1 x(Mg,M1) 3

MnCFMAS

This formulation of a four-component Fe-Mg-Ca-Mn garnet departs from the schemes used above. For P-T modelling in the full pelite system MnKNCFMASH it can be advantageous to recast the variables so that almandine is the dependent quantity, leaving py, gr and sps as potential monitors of evolving P-T trajectories. A rather simple set of interaction parameters is given here.

% Fe-Mg-Ca-Mn garnet recoded 27 Aug 03
g 4
prp(g) 0.06 % Mg/(Mg+Fe+Ca+Mn)
grs(g) 0.25 % Ca/(Mg+Fe+Ca+Mn)
sps(g) 0.10 % Mn/(Mg+Fe+Ca+Mn)

p(alm)  1 1   1 3 -1 prp -1 grs -1 sps
p(py)   1 1   0 1  1 prp
p(gr)   1 1   0 1  1 grs
p(spss) 1 1   0 1  1 sps

sf
W(alm,py)   2.5 0 0
W(alm,gr)     0 0 0
W(alm,spss)   0 0 0
W(py,gr)     33 0 0     % 41.4 -0.0188 0
W(py,spss)    0 0 0
W(gr,spss)    0 0 0

4 
x(Fe,M1)  1 1   1 3 -1 prp -1 grs -1 sps
x(Mg,M1)  1 1   0 1  1 prp
x(Ca,M1)  1 1   0 1  1 grs
x(Mn,M1)  1 1   0 1  1 sps

alm  1 1 x(Fe,M1) 3
py   1 1 x(Mg,M1) 3
gr   1 1 x(Ca,M1) 3
spss 1 1 x(Mn,M1) 3

^ Top

This page last modified 1 November 2004

Main Sections

THERMOCALC Stuff

Practical Aspects of Mineral Thermobarometry

Garnet activity coding

FMAS

CFMAS

MnCFMAS