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The paper presents data on the formation of K–Na richterite in the enstatite + diopside association with K2CO3–Na2CO3–CO2–H2O fluid at 3 GPa and 1000°C as a model for the formation of this mineral in peridotites of the upper mantle. Richterite formation depends on the (H2O + CO2)/(K2CO3 + Na2CO3) and K2CO3/Na2CO3 ratios in the starting material. A high concentration of alkaline components in the fluid leads to the decomposition of clinopyroxene, the formation of olivine, and a change in the component composition of the pyroxene and amphibole. Fluids with a high potassium concentration are favorable for the formation of K-richterite similar in composition to that formed in metasomatized peridotites of the upper mantle. In some cases, such a fluid leads to the decomposition of amphibole and stabilization of alkaline melt. An increase in the activity of the sodium component results in richterite similar in composition to richterite from lamproites. The clarified relations can be used to assess the activities of fluid components and conditions for the formation of K-richterite. To update the data bank of the Raman spectra of minerals, the largest and most homogeneous amphibole crystals of different compositions were studied. [ABSTRACT FROM AUTHOR]

The paper presents experimental data on the interaction of amphibole with NaCl–H2O and (K, Na)Cl–H2O solutions at varying salt content. When interacting with H2O–NaCl fluid, amphibole remains the predominant mineral in all experiments, and the newly formed minerals are Na-phlogopite, plagioclase, and nepheline/sodalite. At > 0.6, the amphibole melts. When amphibole interacts with H2O–NaCl–KCl fluid at < 0.40 and ХKCl/(ХKCl + ХNaCl) in the fluid, defined as ХNaCl = 0.506 – 0.84ХKCl, the amphibole is replaced by the association of nepheline with sodic plagioclase, sodalite, and biotite. At ХKCl/(ХKCl + ХNaCl) > 0.3, nepheline, sodalite, and plagioclase become unstable, K-feldspar is formed, and biotite, clinopyroxene, and amphibole remain stable. At ХKCl/(ХKCl + ХNaCl) > 0.5, the association Cpx + Bt + Kfs + Grt (grossular–andradite) is stable. Thus, grossular–andradite garnet is an indicator of a high potassium activity in the fluid, whereas nepheline testifies that the sodium activity was high. Na → K exchange is typical of the amphibole and biotite, and Ca → Na exchange occurs in the clinopyroxene, and all of these minerals (but neither nepheline nor garnet) remain generally stable within a wide range of the K/Na ratio in the fluid. Clinopyroxene in the experiments spans Ca–Fe–Mg compositions with a varying, sometimes high, Al content, and the amphiboles belong to the pargasite–hastingsite series. With an increase in ( > 0.57), i.e., a decrease in the gross salinity of the fluid, melts are generated, and their composition varies from trachyte to phonolite. An increase in the ХKCl/(ХKCl + ХNaCl) ratio in the fluids leads to a decrease in alumina content of the melts. An increase in the total salinity of the fluid leads to an increase in the content of potassium in the melt and a decrease in the content of chlorine in it. The experiments have shown that interaction between amphibole and fluids containing high NaCl and KCl concentrations results in mineral associations typically produced by alkaline metasomatism of amphibole-bearing rocks and concomitant HCl enrichment in the fluid phase. The substitution of highly saline fluids for highly acidic ones leads to the leaching of Ca, Mg, Fe from the metamorphic rocks, and the transport and redeposition of these components. It follows that significant removal of FeO, MgO, CaO from rocks is sometimes a consequence of the interaction of the host rocks with saline aqueous solutions. [ABSTRACT FROM AUTHOR]

A tonalitic vein from a representative traverse section from the Paleoarchean Sand River biotite-amphibole gneiss consists of two domains - orthopyroxene-bearing and orthopyroxene-absent domains - which grade from center to the margin, along its length. The orthopyroxene-bearing domain is heavily altered with respect to the surrounding orthopyroxene-absent domain, which preserves a prominent magmatic texture typical of crystallized melt. Contrasting textures were observed in terms of monazite occurrence in fluorapatite in the two domains. While monazite occurs as inclusions in fluorapatite in the orthopyroxene-bearing domain, it prominently occurs as irregular rims along fluorapatite margins in the orthopyroxene-absent domain. The textural relation of monazite, occurring as both inclusions and irregular rims in the same fluorapatite grains, and different mineral chemical characteristics of both types of monazite, supports the operation of melting accompanying fluid-induced dehydration in the Sand River orthogneiss. T-aH2O pseudosection modeling reproduced the mineral assemblages of the domain representing melt petition formed during dehydration. The Paleoproterozoic timing of the events is characterized by a 40Ar/39Ar amphibole age of 2037 ± 10 Ma from the dehydrated gneiss. [ABSTRACT FROM AUTHOR]

The join CaMgSi2O6-KAlSi3O8 has been studied at 6 GPa (890-1,500°C) and 3.5 GPa (1,000-1,100°C). K-rich melts in the join produce assemblages Cpx + Grt, Cpx + Opx, Cpx + San, and Cpx + Grt + San at 1,100-1,300°C. AtN<~70 mol%, sanidine is unstable on the solidus and appears at the liquidus, ifN>90 mol%. This explains a scarcity of San in mantle Cpx-rich assemblages and its association with high-K aluminosilicate melt inclusions in diamonds. In absence of San, KCpx is the only host for potassium. The K-jadeite content in KCpx systematically increases with decreasing temperature and reaches 10-12 mol% near the solidus. However, KCpx coexists with San atN>70 mol% and<1,300°C, being formed via reaction San + L=KCpx. The KJd content in KCpx is controlled by the equilibrium San=KJd + SiOthat displaces to the right with increasing pressure and decreasing both the temperature andThis equilibrium is considered to be responsible for the formation of San lamellae in natural UHP Cpx. In our experiments at 3.5 GPa, garnet is absent whereas the KJd and Ca-Eskola contents in Cpx are low, and the join CaMgSi2O6-KAlSi3O8 is close to binary (with the eutectic Cpx + San + L). Different topologies of the join at 6 and 3.5 GPa define a sequence of mineral crystallization from K-rich aluminosilicate melts during cooling and decompression: from KCpx + Grt without San atP>4 GPa to Cpx + San atP<4 GPa. Similar sequence of assemblages is observed in some eclogitic xenoliths from kimberlites and Grt-Cpx rocks of the Kokchetav Complex (Northern Kazakhstan). [ABSTRACT FROM AUTHOR]