From EPMA analysis, I have done cation calculation and also have done end-member calculation for Garnet , Orthopyroxene, Clinopyroxene, Plagioclase . From this calculation how do I calculate Pressure and Temperature condition of basic granulite rock ?
Dear Mr. Patra,
Please re-think your question. How are you going to use a geothermometer for pressure calculations ?
Best regards
H.G.Dill
For temperature estimates, I would recommend looking into the garnet-pyroxene exchange thermometers or the two-pyroxene solvus thermometer. I know you said this is a basic granulite, but is there any quartz in your sample at all? Many of the net transfer equilibria (e.g. GASP) require quartz (and sometimes an Al2SiO5 polymorph) to be present.
Hello Biswa, if your intention is to calculate the PT condition of the grt-opx (cpx) -plg-qtz balance I recommend taking a look in this paper -
Pattison et al 2003 (https://doi.org/10.1093/petrology/44.5 .867).
One of the big problems of doing geothermobarometry in granulite facies is the post-peak diffusion effect that tends to be relatively high for Fe and Mg and as many thermometers and barometers calibrations uses those elements to perform the calculations, this ends up generating an underestimation of the P-T results.
To get around the problem of underestimating peak conditions due to the retrograde diffusion of Fe-Mg in the rock, you can use the approach of Pattinson et al, 2003; Chacko et al (1996) and Fitzsimons and Harley (1994).
In this method, the composition of garnet and orthopyroxene is adjusted in order to converge to single pressure-temperature conditions using different calibrations. The Pressure is calculated with Fe/Mg independent garnet-orthopyroxene-plagioclase-quartz (GAPES) barometer of Newton and Perkins (1989) for example. The Fe/Mg ratio of orthopyroxene and garnet are is gradually corrected until the barometer of Harley (1984b) reaches the initial pressure calculated with Newton and Perkins (1989) calibration. That adjusted composition is finally is calculated with Fe-Mg garnet-orthopyroxene thermometer of Harley (1984a). (All references in Pattinson's paper)
To discuss more about the topic I would need textural information about the relationship between grt-cpx-opx ... did the cpx grow after the grt breaking down? is there evidence of equilibria between all phases in thin section? I hope I helped,
regards Lucas