Experimental evidence for diamond-facies metamorphism in the Dora-Maira massif

The peak metamorphic conditions of subducted continental crust in the Dora-Maira massif (Western Alps) have been revised by combining experimental results in the KCMASH system with petrologic information from whiteschists. Textural observations in whiteschists suggest that the peak metamorphic assemblage garnet+phengite+kyanite+coesite+/-talc originates from the reaction kyanite+talc<---->garnet+coesite+liquid. In the experimentally determined petrogenetic grid, this reaction occurs above 45 kbar at similar to730 degreesC. At lower pressures, talc reacts either to orthopyroxene and coesite or, together with phengite, to biotite, coesite and kyanite. The liberated liquid contains probably similar amounts of H2O and dissolved granitic components. The composition of the liquid in the whiteschists at peak metamorphic conditions, a major unknown in earlier studies, was probably very similar to the liquid composition produced in the experiments. Therefore, the experimentally determined petrogenetic grid represents a good model for the estimation of the peak metamorphic conditions in whiteschists. Experimentally determined Si-isopleths for phengite further constrain peak pressures to similar to43 kbar for the measured Si = 3.60 of phengite in the natural whiteschists. All these data provide evidence that the whiteschists reached diamond-facies conditions. The fluid-absent equilibrium 4 kyanite + 3 celadonite = 4 coesite + 3 muscovite + pyrope has been calibrated on the basis of garnet and phengite compositions in the experiments and serves as a geothermobarometer for ultra-high-pressure (UHP) metapelites. For graphite-bearing metapelites and kyanite-phengite eclogites, forming the country rocks of the whiteschists, peak metamorphic pressures of about 44 3 kbar were calculated from this barometer for temperatures of similar to750 degreesC estimated from garnet-phengite thermometry. Therefore, the whole ultra-high-pressure unit of the Dora-Maira massif most likely experienced peak metamorphic conditions in the diamond stability field. While graphite is common in the metapelites, diamond has not been found so far. The absence of metamorphic microdiamonds might be explained by the low temperature of metamorphism, the absence of a free fluid phase in the metapelites and a short residence time in diamond-facies conditions resulting in kinetic problems in the conversion of graphite to diamond. (C) 2003 Elsevier B.V. All rights reserved.