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The paper summarizes authors' earlier and newly obtained experimental data on the formation of phlogopite and chromium-bearing potassium titanates of the crichtonite, magnetoplumbite, and hollandite groups, which are indicator minerals that characterize various stages of modal metasomatism in the upper mantle. Phlogopite-forming reactions were studied in the garnet–orthopyroxene system in the presence of H2O–KCl fluid at 3 and 5 GPa and 900–1000°C as simulating phlogopite formation in garnet peridotites and pyroxenites. Experiments have demonstrated regular variations in Ca and Cr contents in the garnet and Al in the pyroxenes, as well as the composition of the newly formed phlogopite, depending on the concentration of the potassium component (KCl or K2CO3) of the fluid. Experiments on the formation of potassium titanates (yimengite, mathiasite, and priderite) in the chromite–rutile/ilmenite–K2CO3–H2O–CO2 system at 3.5 and 5 GPa and 1200°C have proved that these minerals can be formed when chromite reacts with potassic aqueous–carbonic fluid, but these reactions require additional sources of titanium. These sources may be rutile and/or ilmenite, which are usually also produced by modal metasomatism of peridotites. The experiments thus confirm that the formation of titanates marks the most advanced stages of metasomatism in mantle peridotites. The experiments have shown that the formation of potassium titanates follows the formation of phlogopite, whereas assemblages of the titanates with phlogopite are produced at higher concentrations of the potassium component of the fluid than phlogopite alone. The relationships between the titanates are also a function of the activity of the potassium component in the fluid and, probably, pressure. The relationships and trends reproduced in the experiments clearly illustrate features of mineral assemblages and variations in compositions of minerals from metasomatized peridotites in the lithospheric mantle. [ABSTRACT FROM AUTHOR]

The results of experimental studies are presented for reactions in the orthopyroxene–garnet–phlogopite system in the presence of H2O–KCl fluid at 3–5 GPa and 900–1000°C, which model the processes of phlogopite formation in garnet peridotites and pyroxenites during alkaline metasomatism of the upper mantle. The experiments demonstrated regular variations in the composition of garnet, pyroxenes, and phlogopite depending on the KCl content of the fluid. With increasing KCl content of the fluid, enstatite and garnet become unstable, the Al2O3 content of enstatite decreases, and the amount of grossular and knorringite components in garnet are maximum at a KCl content of ~10 mol %. Our results illustrate well the regular variations in the compositions of the coexisting minerals and their zoning in phlogopite-bearing peridotites of the lithospheric mantle. [ABSTRACT FROM AUTHOR]

The results of experiments on the synthesis of exotic titanates (priderite and yimengite) simulating metasomatic conditions of alteration of the mantle minerals (chromite and ilmenite) are reported. Ba-free Cr-bearing priderite was synthesized for the first time. Experiments showed the possibility of crystallization of this mineral as a product of the reaction of high-Cr spinel and rutile with hydrous–carbonate fluid (melt) under the conditions of the upper mantle. In particular, the experimental data obtained provide an interpretation of the relationships between K‒Cr priderite and carbonate–silicate inclusions in chromites from garnet peridotite of the Bohemian massif. Experimental study of the reaction of chromite and ilmenite with potassic hydrous–carbonate fluid (melt) shows the presence of both titanate phases (priderite and yimengite), the mineral indicators of mantle metasomatism. This provides direct evidence for the formation of yimengite and K‒Cr priderite, as well as other titanates, due to mantle metasomatism of the upper mantle peridotite under the conditions of the highest activities of potassium. [ABSTRACT FROM AUTHOR]

The pseudo-binary system CaMgSi2O6-KAlSi2O6, modeling the potassium-bearing clinopyroxene (KCpx) solid solution, has been studied at 7 GPa and 1,100–1,650 °C. The KCpx is a liquidus phase of the system up to 60 mol% of KAlSi2O6. At higher content of KAlSi2O6 in the system, grossular-rich garnet becomes a liquidus phase. Above 75 mol% of KAlSi2O6 in the system, KCpx is unstable at the solidus as well, and garnet coexists with kalsilite, Si-wadeite and kyanite. No coexistence of KCpx with kyanite was observed. Above the solidus, KAlSi2O6 content of the KCpx coexisting with melt increases with decreasing temperature. Near the solidus of the system (about 1,250 °C) KCpx contains up to 5.6 wt% of K2O, i.e. about 22–26 mol% of KAlSi2O6. Such high concentration of potassium in KCpx is presumably the maximal content of KAlSi2O6 in the Fe-free clinopyroxene at 7 GPa. In addition to the major substitution MgM1CаМ2⇔AlМ1KМ2, the KCpx solid solution contains Ca-Eskola and only minor Ca-Tschermack components. Our experimental results indicate that the natural assemblage KCpx+grossular-rich garnet might be a product of crystallization of the ultra-potassic SiO2-rich alumino-silicate mantle melts (>200 km). [ABSTRACT FROM AUTHOR]