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 spectrpscopy and X-ray diffraction (sample 6a), have been used in deciphering the thermal history of two-pyroxene granulites of Sittampundi Complex (India). Orthopyroxenes and clinopyroxenes are compositionally in the granulite field of metamorphism. For orthopyroxenes, the Mossbauer spectra show that Fe²⁺ is 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 M2. Fe³⁺ is present only at M1 site, but in Ca-rich clinopyroxenes, under high fO₂, conditions, it is seen to replace silicon in tetrahedral site.

From the distribution of Fe²⁺ - Mg, using Mossbauer data, the blocking temperature of intracrystalline ordering in orthopyroxene has been calculated to range between 427 and 699°C, in agreement with the estimated temperature obtained by means of X-ray data of about 500°C; in clinopyroxene it was 680°C as obtained from X-ray data. The recorded inter-crystalline ordering temperature was determined as between 661 and 785°C. The lowering of blocking temperature recorded by intracrystalline Fe²⁺ - Mg exchange suggests that the cation ordering in the M1 and M2 sites continued 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.