Vol 26, No 6 (2018)

This paper reports the results of high-pressure experimental modeling of interaction between glaucophane schist and harzburgite or websterite for the evaluation of the influence of mantle material on the input–output of components and character of metasomatic transformations at the crust–mantle boundary in the subduction zone. In all experiments, glaucophane schist (proxy for oceanic crust) containing volatile components (H₂O and CO₂) incorporated in hydrous minerals (amphiboles, phengite, and epidote) and calcite was loaded into the bottom of each capsule and overlain by mantle material. During the experiments at a temperature of 800°C and a pressure of 2.9 GPa, which correspond to the conditions of a hot subduction zone, the schist underwent partial (up to 10%) eclogitization with the formation of the anhydrous assemblage omphacite + garnet + quartz ± magnesite ± potassic phase. Carbonate and a potassic phase were formed only in the experiments with websterite in the upper layer. A reaction zone was formed at the base of the websterite layer, where newly formed omphacite, quartz, and orthopyroxene replaced in part initial pyroxenes. Orthopyroxene and phlogopite (or an unidentified potassic phase) were formed in the reaction zone at the base of the harzburgite layer; among the initial minerals, only orthopyroxene relicts were preserved. Above the reaction zones produced by diffusion metasomatism, new phases developed locally, mainly at grain boundaries: newly formed orthopyroxene and magnesite were observed in harzburgite, and omphacite and quartz, in websterite. Alterations along grain boundaries extended much further than the reaction zones, which indicates that fluid infiltration dominated over diffusion in the experiments. The experiments demonstrated that the H₂O–CO₂ fluid with dissolved major components released from the glaucophane schist can produce mineral assemblages of different chemical compositions in mantle materials: Na-bearing in websterite and K-bearing in harzburgite. The complementary components, K₂O and CO₂ for the websterite layer and Na₂O for the harzburgite layer, are fixed in the initial glaucophane schist layer. The distinguished separation of alkalis and CO₂ at the crust–mantle boundary can affect the character of metasomatism in the mantle wedge, primary magma compositions, and the chemical evolution of the rocks of the subducting slab.

This paper presents the results of a combined study of the composition and physical properties of fluid, melt and opaque mineral inclusions in olivine phenocrysts and glass from the magnesian basalt of Disko I., West Greenland. The rock is dominated by glass, which contains numerous fluid bubbles and large (up to 250 μm) opaque globules. The thermometric investigation of the melt inclusions showed that the temperature of olivine crystallization was no higher than 1220°C at a pressure of 0.15–0.20 GPa. The opaque globules in the groundmass glass have a eutectoid structure and are composed of troilite, Fe–Ni metal alloy, and sparse grains of wüstite and cohenite (Fe₃C). Cryometric measurements and Raman spectroscopy indicated a complex fluid composition and presence of reduced and oxidized gases: CH₄, N₂, H₂, CO₂, and H₂O, as well as highly ordered graphite. The nonequilibrium association of compounds in the fluid is related to the rapid cooling (quenching) of crystallizing magma preventing the equilibration of the gas system. At an estimated logfO₂ value of –13.95, methane is the only hydrocarbon phase that can exist at magmatic temperatures. The formation of organic substances detected in some gas bubbles in the groundmass glasses of the rock occurred at postmagmatic stages after a significant decrease in temperature.