Chemical features of sediment-derived supercritical fluid-melt at subduction zone conditions

Abstract

Phase relations and partitioning of major and some trace elements are studied in samples of metasediment (natural schist) poor in H₂O (2.2 wt%) and CO₂ (0.9 wt%) in diamond trap experiments at pressures of 3.0 GPa, 5.5, and 7.8 GPa and temperatures from 750 to 1090 °C. At the applied P-T parameters simulating the conditions of warm and hot subduction, the run products comprise a mobile phase and a residue composed of garnet, phengite, clinopyroxene, coesite, kyanite, as well as accessory monazite, rutile, and zircon. The mobile phase is a hydrous melt of a granitic composition till 3.0 GPa and 750–900 °C but transforms into a supercritical fluid-melt (SCFM) with SiO₂ > Al₂O₃ ≈ K₂O > Na₂O ≈ CaO ≈ MgO ≈ FeO at 5.5 GPa and 850–1030 °C. The transformation is accompanied by increase in the contents of H₂O + CO₂ from 17–24 wt% to >40 wt%, at the weight ratio H₂O/(H₂O + CO₂) ∼0.75. SCFM remains almost invariable in its fraction and amounts of H₂O + CO₂ and solutes within the 5.5–7.8 GPa and 850–1090 °C intervals. At higher pressures and temperatures, the coefficients of Al and Na partitioning between the mobile phase and the residue (D = C_SCFM/C_Residue) become lower and that of K increases, whereby the K₂O/Na₂O ratio in SCFM becomes an order of magnitude greater than in hydrous melt. The revealed trend can be used as a criterion for discriminating between hydrous melt and SCFM proceeding from the bulk composition of multiphase inclusions in mantle minerals. The K₂O/H₂O ratio in metasediment-derived SCFM increases with pressure but does not vary much with temperature. This provides experimental evidence for the potash-depth relationship inferred earlier for primitive arc lavas. The partition coefficients of Sr, K, Rb and P become progressively higher with pressure and temperature to make these elements least compatible at 7.8 GPa and 940–1090 °C in the presence of SCFM. The partition coefficient of Li is >1 at moderate temperatures only. Thus, SCFM can originate in pre-devolatilized metasediment poor in H₂O and CO₂ subducting to depths of 150 to 240 km. The low contents of volatiles in the metasediment ensure stability of the residue monazite and phengite which are main carriers of LREE and LILE recycled by slabs into the mantle wedge. K-rich silicic SCFM penetrating into the mantle can be an important agent in the generation of arc and back-arc magmas, as well as in metasomatism of lithospheric mantle.