Heat Capacity of High Pressure Minerals and Phase Equilibria of Cretan Blueschists.

[摘要] Low temperature heat capacity data are obtained for sphene (titanite) and TiO2-II.The standard entropies are 127.2 ± 0.2 and 48.8 ± 0.10 J/mol.K respectively, obtained by integrating the heat capacity over temperature from 0 to 298.15 K.The entropy of sphene supports an earlier determination by King et al. (1954), and refutes claim by studies of Xirouchakis et al. (1997) and Tangeman and Xirouchakis (2001) that the entropy required revision to a much lower value.When combined with experimental phase equilibria, the new data allows improved calculation of sphene-bearing thermobarometers.For TiO2-II, the newly determined entropy is very close to that of rutile.The heat capacity curves cross slightly below room temperature, and given the uncertainty in extrapolating heat capacity data, it is impossible to constrain the slope of the TiO2-II phase boundary.Conditions of metamorphism were determined in the high-pressure blueschist belt on Crete.Metabasaltic assemblages were studied, including albite+chlorite+epidote ±quartz±lawsonite±pumpellyite±glaucophane±omphacite.The temperatures and pressures obtained from phase equilibrium calculations for measured mineral compositions are 305 ± 30 ˚C at 7.5 ± 1.5 kbar and 280 ± 30 ˚C at 7.0 ± 1.5 kbar.This result is significantly lower than previously reported by Theye et al (1992), on carpholite and chloritoid–bearing equilibria.The thermodynamic data of carpholite and chloritoid used in the work of Theye et al. (1992) has to be revised to bring that study into accord with this determination.Fast exhumation on Crete was proposed by Thomson et al. (1996) when zircon fission tracks yielded similar ages to Ar/Ar on phengites.The temperatures obtained in this study suggest the phengite formed at 300˚C so the ages are expected to be nearly coincident, and the exhumation rate is not constrained.The presence of metamorphic aragonite described by Theye and Seidel (1993) was confirmed in this study using X-ray diffraction.The PT determined for Crete in this study is very close to the calcite/aragonite transition, and requires a counter-clockwise PT path to metastably preserve aragonite (Carlson and Rosenfeld et al. 1991).This path is similar to those obtained for the aragonite-bearing metagraywackes in the Franciscan.

[发布机构] University of Michigan

[效力级别] Blueschist

[关键词] Phase Equilibria;Blueschist;Metamorphic Petrology;Crete;Sphene;TiO2-II;Science;Geology