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

Jörg Hermann*

*Corresponding author for this work

    Research output: Contribution to journalArticlepeer-review

    127 Citations (Scopus)

    Abstract

    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 ∼730 °C. 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 ∼43 kbar for the measured Si=3.60 of phengite in the natural white schists. 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 ∼750 °C 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.

    Original languageEnglish
    Pages (from-to)163-182
    Number of pages20
    JournalLithos
    Volume70
    Issue number3-4
    DOIs
    Publication statusPublished - Oct 2003

    Fingerprint

    Dive into the research topics of 'Experimental evidence for diamond-facies metamorphism in the Dora-Maira massif'. Together they form a unique fingerprint.

    Cite this