Melting of metasedimentary rocks at ultrahigh pressure-Insights from experiments and thermodynamic calculations

Diamondiferous quartzofeldspathic rocks from the Kokchetav Massif, Kazakhstan, and the Erzgebirge, central Europe, are rare witnesses of melting of sedimentary material at great depths. To better understand the melting process, two powdered samples from these localities were objects of experiments conducted in a piston-cylinder apparatus at pressures of 3-5 GPa. In addition, thermodynamic calculations in the system Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O using a haplogranitic melt model yielded isochemical phase equilibria diagrams (i.e., pseudosections) at ultrahigh pressure. The solidus for both rocks was located close to 1000 degrees C and 1100 degrees C at 3 GPa and 5 GPa, respectively, in the experiments. Initial potassic to ultrapotassic melts form by phengite breakdown. At similar to 200 degrees C above the solidus, clinopyroxene disappears from the restite assemblage, and coexisting melts are granitic. The restite consists of garnet + coesite (+/- kyanite) up to temperatures close to the liquidus, which occurs at a temperature similar to 350 degrees C above the solidus. The results of the thermodynamic calculations approximate the pressure-temperature conditions and phase relations around the solidus but increasingly deviate from the experimental results with rising temperature. According to the experiments, the melt that crystallized to diamondiferous saidenbachite from the Erzgebirge should have been as hot as 1400 degrees C, whereas the ultrahigh-pressure rocks from the Kokchetav Massif experienced temperatures of at least 1200 degrees C. These high melting temperatures are derived for rocks with no free water, which is the most likely scenario for continental crust taken to mantle depths where diamond forms.