Home – Home – Jornals – Russian Geology and Geophysics 2018 number Неопубликованное

Experiments were carried out with naturally-doped pelite containing 3 wt.% CaCO₃ in the pressure range of 3.0-7.8 GPa and temperatures of 750-1090 °C, so that the total carbonate content reached 7 wt.%, similar to that in the model global averaged subducted sediment (GLOSS). As a result of dehydration and decarbonation of the pelite under thermal conditions of subduction zones, residue is formed, consisting of garnet, clinopyroxene, phengite, coesite, ±Mg-Fe carbonate, and kyanite, along with accessory minerals such as rutile, monazite, and zircon. Additionally, a mobile phase is generated, which significantly evolves with increasing P-T parameters along the warm subduction geotherm. At 3.0 GPa and 750 °C, a granite-like melt (H₂O+CO₂ - 19 wt.%) enriched in SiO₂ and Al₂O₃ is formed, with a K₂O/Na₂O ratio of 1.2. At 5.5-7.8 GPa and 850-940 °C, a supercritical fluid-melt (H₂O+CO₂ around 40 wt.%) enriched in SiO₂ and K₂O, but poor in Al₂O₃, is produced, with a K₂O/Na₂O ratio reaching 9.5. The supercritical fluid-melt (SCFM) forming in the carbonate-bearing pelite can effectively transport LILE and LREE, including trace elements that are markers for both deluted aqueous fluids (Ba and U) and markers for granite-like melts (Sr, LREE, and Th). During the fractionation of trace elements, important roles are played by host minerals: phengite (LILE), monazite (LREE), and rutile (HFSE). An increase in carbonate concentration in the pelite leads to a slight decrease in the partition coefficients of the most incompatible elements due to the increase of the mobile phase fraction and concentration CO₂ within it. The SCFM obtained in equilibrium with rutile-bearing residue retains the characteristic negative Nb anomaly typical of marine sediments and is capable of transmitting it to arc magmas if involved in their generation.