Apparent pyrrhotite-chalcopyrite solid solutions in charnockites: the Shevaroy Hills Massif, Tamil Nadu, S India and the Bamble Sector, SE Norway

Examples of apparent exsolution lamellae and lenticular blebs of chalcopyri te in pyrrhotite are described in orthopyroxene-bearing granulite facies ro cks from two, oxidized (log(10)/f(O2) = -14 to -11), widely separated, well characterized high grade terranes: the Bamble Sector, SE Norway (795 degre es C, 7.5 kbar) and the Shevaroy Hills Massif, Tamil Nadu, S India (750 deg rees C, 7.5 kbar). These exsolution features only occur in isolated pyrrhot ite grains and not in integral pyrrohotite-pyrite-chalcopyrite-magnetite gr ain clusters which essentially represent an oxidation equilibrium. Reintegr ation of these chalcopyrite exsolution features back into the pyrrhotite ho st indicate Cu contents ranging from 1 to 5 wt.% in good agreement with exp erimental observations which indicate that pyrrhotite can take up to 7 wt.% Cu at temperatures above 800 degrees C at pressures of similar to 1 bar. T his suggests that under high grade conditions these chalcopyrite exsolution features were in solid solution with pyrrhotite. Whether Cu stabilizes pyr rhotite at higher oxygen fugacities or these chalcopyrite-pyrrhotite grains represent a metastable phase is uncertain. One possibility is that the iso lated pyrrhotite grains with chalcopyrite lamellae could represent grains t hat were preferentially not exposed to infiltrating fluids, which oxidized the pyrrhotites in other areas of the sample. A second possibility is that either these grains had enough Gu to stabilize them during pervasive infilt ration of oxidizing fluids or that they represent a metastable phase with r espect to the overall oxygen fugacity of the sample. The two conclusions th at can be drawn from these observations are, firstly, that it is possible f or pyrrhotite and chalcopyrite to form a limited solid solution at granulit e facies temperatures and pressures under relatively high oxidizing conditi ons, i.e. 1.5 log units above fayalite-magnetite-quartz, at 800 degrees C a nd 8 kbar. Secondly, this limited solid solution should have some bearing o n the stability of pyrrhotite with respect to co-existing magnetite and pyr ite as a function of the oxidation state of the rock, be it inherited or fl uid induced.