Geothermometry and mineralogy of two-pyroxene granulites: Evaluation from Mossbauer and X-ray single crystal cation partitioning of Ca-poor and Ca-rich pyroxenes

Fe-Mg ordering as well as Fe2+ and Fe3+ distribution at different sites in coexisting orthopyroxene and clinopyroxene determined by Mossbauer spectros copy and X-ray diffraction (sample 6a), have been used in deciphering the t hermal history of two-pyroxene granulites of Sittampundi Complex (India). O rthopyroxenes and clinopyroxenes are compositionally in the granulite field of metamorphism. For orthopyroxenes, the Mossbauer spectra show that Fe2is present at M1 and two different M2 sites, while Fe3+ is localised at M1 site alone. For the coexisting clinopyroxenes, Fe2+ occurs at both M1 and M 2. Fe3+ is present only at M1 site, but in Ca-rictt clinopyroxenes, under h igh fO(2) conditions, it is seen to replace silicon in tetrahedral site. From the distribution of Fe2+- Mg, using Mossbauer data, the blocking tempe rature of intracrystalline ordering in orthopyroxene has been calculated to range between 427 and 699 degrees C in agreement with the estimated temper ature obtained by means of X-ray data of about 500 degrees C; in clinopyrox ene it was 680 degrees C as obtained from X-ray data. The recorded inter-cr ystalline ordering temperature was determined as between 661 and 785 degree s C. The lowering of blocking temperature recorded by intracrystalline Fe2 Mg exchange suggests that the cation ordering in the M1 and M2 sites conti nued in the cooling path of the granulites below the temperature indicated by inter-crystalline distribution, and that the cooling (history) suffered by the host rocks was slow.