Intra-crystalline thermometry and cooling rate of two-pyroxene granulites:valuation from Mossbauer and X-ray single crystal cation partitioning of Ca-poor and Ca-rich pyroxenes

Fe-Mg ordering and site distribution of Fe2+ and Fe3+ in coexisting orthopy roxene and clinopyroxene from the basic granulite 6a from the Sittampundi c omplex (Tamil Nadu, southern India), were determined by Mossbauer spectrosc opy (MS) and X-ray diffraction (XRD). Results were used to evaluate the the rmal history of the granulite host rock. The composition of both orthopyrox ene and clinopyroxene are consistent with the granulite facies metamorphism . The Mossbauer spectra and X-ray diffraction data show that Fe2+ of orthop yroxene is essentially located in M2 site. Moreover, the Mossbauer spectra for orthopyroxene revealed that Fe2+ is present in two different M2 sites. In the coexisting clinopyroxene, Fe2+ occurs in both M1 and M2 sites. The r atio Fe3+/Sigma Fe obtained from MS and XRD results, respectively, is of 0. 037 and 0.048 for orthopyroxene, and 0.211 and 0.258 for clinopyroxene. The intra-crystalline temperature of the orthopyroxene and clinopyroxene, yiel ded a temperature ranging from 235 to 273 degrees C for the former, and 408 degrees C for the latter. The corresponding estimated cooling rates, below 6 degrees C/Ma, appear to be very slow.