Experimental studies of mineralogical assemblages of metasedimentary rocks at Earth’s mantle transition zone conditions

Metasedimentary rocks, a major component of the continental crust, are abundant within ultra-high pressure (UHP) metamorphic terranes related to continental collisions. The presence of diamond, coesite, and relics of decompressed minerals in these rocks suggests that they were subducted to a depth of more than 150–250 km. Reconnaissance experiments at 9–12 GPa and 1000–1300 C on compositions corresponding to felsic rocks from diamond-bearing UHP terranes of Germany and Kazakhstan show that at higher pressures they consist of majoritic garnet, Al-Na-rich clinopyroxene, stishovite, solid solution of KAlSi3O8-NaAlSi3O8 hollandite, topaz-OH, and TiO2 with a-PbO2 structure. Comparison of our data with experiments conducted by others at similar P–T conditions shows differences, which are due to variations in bulk chemistry and the type of starting material (gel, oxides, minerals). These differences may affect correct establishment of the point of no return of subducted continental lithologies. This paper discusses the implication of the experimental data with regard to naturally existing UHP metamorphic rocks and their significance for our understanding of the deep subduction of continental material.