Element Fractionation During Transformation of Carbonate-Bearing Pelite Under Subduction Zone P - T Conditions

Experiments are conducted on pelite doped with 3 wt.% CaCO3 over a pressure range of 3.0–7.8 GPa and temperatures of 750–1090 °C. The goal is that the total carbonate content in initial samples should reach 7 wt.%, consistent with that of the global subducting sediment model (GLOSS). Dehydration and decarbonation of the pelite under the thermal regimes of subduction zones produce a residue composed of garnet, clinopyroxene, phengite, coesite, ± Mg–Fe carbonate, and kyanite, along with accessory rutile, monazite, and zircon. In addition, a mobile phase forms that evolves significantly with increasing P-T conditions along the average subduction geotherm. At 3.0 GPa and 750 °C, a melt with a granite-like composition forms. This melt is enriched in SiO2 and Al2O3, has a K2O/Na2O ratio of 1.2, and contains up to 19 wt.% H2O + CO2. At 5.5–7.8 GPa and 850–940 °C, a supercritical fluid-melt forms (H2O + CO2 ≈ 40 wt.%) that is enriched in SiO2 and K2O, poor in Al2O3, and has a K2O/Na2O ratio reaching 9.5. The supercritical fluid-melt generated in carbonate-bearing pelite can efficiently transport large-ion lithophile elements (LILE) and light rare earth elements (LREE), including trace element markers of both dilute aqueous fluids (Ba and U) and granite-like melts (Sr, LREE, and Th). Host minerals play an important role in trace element fractionation: phengite (LILE), monazite (LREE), and rutile (high field strength elements, 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 a higher proportion of the mobile phase and an increased CO2 concentration within it. The supercritical fluid-melt equilibrated with rutile-bearing residue retains the negative Nb anomaly characteristic of marine sediments and can transfer this anomaly to arc magmas if it participates in their generation.