Melting of Fe-bearing and Fe-free carbonated pelites under fluid-saturated conditions at 6 GPa

Abstract

Abstract Although the induced melting of pelitic sediments, i.e., the devolatilization of hydrous and carbonate minerals, has been widely studied at fluid-undersaturated conditions, the flush dissolution of carbonated pelite has not been fully understood. In addition, the role of iron in the melting of carbonated pelite has not received much attention. To address these issues, we conducted two sets of experiments for carbonated pelite with an iron-bearing (LH-gloss) and an iron-free (LHIF-gloss) starting bulk composition at 5.5 to 6 GPa, 800 to 1600 °C at fluid-saturated conditions. The phase assemblages for both experiments at 800 °C are composed of garnet + clinopyroxene + coesite + kyanite + phengite + aragonite + magnesite ± lawsonite ± rutile. Higher jadeite component and lower diopside–hedenbergite solid solution (Di–Hdss) in omphacitic clinopyroxene are observed in the LH-gloss experiments; also, garnet remains stable to higher temperatures (800–1400 °C) in the LH-gloss than in the LHIF-gloss (900–1200 °C). Carbonate- and phengite-out temperature boundaries are overlapping in the respective system, with the temperature boundary in the LH-gloss (800–900 °C) slightly lower than that in the LHIF-gloss experiments (900–1000 °C). The different stability fields of volatile-bearing minerals can be ascribed not only to variable bulk XH2O [molar ratio H2O/(H2O + CO2)], which in turn depends on bulk H2O, CO2 and K2O contents, but also to bulk FeO*(Total Fe as FeO) content. Both the characteristic “fish egg” texture and the strong increase in the amount of dissolved solids in the liquid phase over a narrow temperature interval at 6 GPa testify to the possible existence of supercritical fluid. The marked solvent power of supercritical fluid can explain the earlier disappearance of experimental products including phengite, aragonite and magnesite. For the produced liquid phase (supercritical fluid or melt), the K2O/Na2O weight ratio decreases, whereas that of the SiO2/CaO increases with increasing temperature, placing potassium-rich carbonatitic supercritical fluids in the low-temperature and sodium-rich (carbonated) silicate melts in the high-temperature sections of both systems. The produced ultrapotassic (supercritical) liquid, when liberated from the subducting slab, may evolve into a melt parental to carbonatites and possibly result in the formation of diamonds. While those (carbonated) silicate melts, especially the more oxidized and buoyant ones produced by the melting of ferrous iron-depleted carbonated pelite, are expected to intensely interact with the overlying peridotite during the upward migration, which could lead to the formation of the metasomatic garnet pyroxenite.