Vol 27, No 2 (2019)

ARTICLES
A technique for calculating the enthalpy of silicate melt of arbitrary composition
Bychkov D.A., Koptev-Dvornikov E.V.

Abstract

The Kirchhoff equation was used to develop a method for calculating the enthalpy of silicate melt of any arbitrarily chosen composition with regard for the latent heat of fusion. To do this, the following two problems were solved: (a) the coefficients of the heat capacity equation of a silicate melt were revised and (b) the partial molar enthalpies of formation of melt components from elements at standard state were determined. The uncertainty in calculating the heat capacity of silicate melt at 95% significance level is no greater than ± 0.7 J · mol–1 · K–1. A weak but statistically significant temperature dependence of the heat capacity was determined. The enthalpies of melts calculated using the method are comparable with the values obtained by other techniques. The calculation error at 95% significance level is ±2 relative %.

Петрология. 2019;27(2):123-137
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High pressure metamorphism in the peridotitic cumulate of the Marun-Keu complex, Polar Urals
Liu Y.Y., Perchuk A.L., Ariskin A.A.

Abstract

The Marun-Keu Complex of high-pressure rocks comprises granitoids, gneisses, schists, gabbroids and peridotites, which are unevenly and variably metamorphosed to the eclogite facies. A representative sample of garnet–amphibole lherzolite from the Mount Slyudyanaya area shows a cumulate texture and well preserved magmatic mafic minerals (olivine and pyroxenes) but practically no preserved plagioclase. The eclogite-facies metamorphism produced corona textures of newly formed minerals: amphibole, garnet, orthopyroxene and spinel. The metamorphic parameters of the garnet–amphibole lherzolite were estimated by geothermobarometry and by modeling phase equilibria at Р ~ 2.1 GPa and T ~ 640–740°C and are well consistent with our earlier estimate of the formation conditions of eclogites in the area. Computer simulation of the crystallization process of the gabbroic melt with the COMAGMAT program package, using literature data on the composition of the least altered plagioclase peridotites and gabbroids from the Marun-Keu Complex, shows that the mafic and ultramafic rocks are genetically interrelated: they crystallized in a single magmatic chamber. According to the modeling, the origin of the cumulate texture in the lherzolite was controlled by the peritectic reaction Ol + melt → Opx at a pressure of 0.7–0.8 GPa and a temperature of 1255–1268°C. Differences between thermodynamic parameters in the eclogites and garnet peridotites are discussed within the framework of a tectonic model for subduction and subsequent exhumation of the Baltica paleocontinent.

Петрология. 2019;27(2):138-160
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Non-subduction petrological mechanisms for the growth of the neoarcheam continental crust of the Kola–Norwegian terrane, Fennoscandian shield: geological and isotope-geochemical evidence
Vrevskii A.B.

Abstract

The paper reports new data on the composition and age of the Neoarchean calc-alkaline volcanic rocks of the Uraguba–Kolmozero–Voron’ya greenstone belt (UKV GB). Petrological-geochemical modeling indicates a polygenetic origin of primary melts of the basalt–andesite–dacite association and non-subduction geodynamic mechanisms for the crustal growth in the largest greenstone belt of the Kola–Norwegian Block of the Fennoscandian shield.

Петрология. 2019;27(2):161-186
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Djerfisherite in monticellite rocks of the Krestovskaya Intrusion (Polar Siberia)
Panina L.I., Isakova A.T.

Abstract

Djerfisherite in the monticellite rocks of the Krestovskaya Intrusion is found in primary melt inclusions, mono- and polysulfide globules, and in the djerfisherite–hydrogarnet segregations. Melt inclusions are represented by three types. Type I is observed in the cores of perovskite phenocrysts and monticellite grains and corresponds to one of the early crystallization stages of the parental larnite-normative alkali ultrabasic magma enriched in water and other volatiles. Daughter phases of the inclusions are clinopyroxene, serpentine, phlogopite, apatite, nepheline, hydrogarnet, magnetite, djerfisherite, pectolite, and calcite. In some type I inclusions, melt at 1230–1250°C was immiscibly split into two fractions: alkali silicate fraction and highly fluidized water-bearing low-silica fraction enriched in alkali, sulfur, and CO2. The types II and III inclusions in perovskite, monticellite, Ti-garnet, and melilite were formed through the spatial separation of immiscible phases. This follows from the similarity of the modal composition of types II and III melt inclusions to the normative composition of immiscible fractions of type I inclusions. Type II inclusions contain mainly water-bearing silicate daughter phases (hydrogarnet, serpentine, phlogopite, and pectolite), as well as djerfisherite, calcite, and magnetite, Type III inclusions contain clinopyroxene, nepheline, apatite, magnetite, djerfisherite, calcite, and pectolite. The djerfisherite–hydrogarnet segregations are confined to the Ti-magnetite and perovskite phenocrysts and fractures radiating from them in monticellite. The mineral composition of the djerfisherite–hydrogarnet segregations together with their surrounding is similar to the composition of type II inclusions containing similar water-bearing silicates, djerfisherite, calcite, and magnetite.

Such similarity gives grounds to relate the formation of the djerfisherite–hydrogarnet segregations, as type II inclusions, with the spatial separation and crystallization of highly fluidized low-silica melt enriched in water, alkalis, sulfur, and CO2. According to the homogenization experiment, the crystallization of highly fluidized melt at 990–1090°C was accompanied by silicate–sulfide immiscibility and the formation of globular, emulsion-like, and myrmekite structures in the djerfisherite–hydrogarnet segregations, as well as mono- and polysulfide globules with djerfisherite in the hydrogarnet–calcite–serpentine substrate. The formation of ferrobrucite–carbonate–hydrogarnet globules in the djerfisherite–hydrogarnet segregations was also related to melt liquation, which again confirms the magmatic origin of the latter. Sometimes, djerfisherite in the djerfisherite–hydrogarnet segregations becomes coarser and forms rims, bands, and veinlets, which is likely explained by the high mobility and low viscosity of sulfide melt. Scarce grains of heazlewoodite, godlevskite, and pentlandite hosted in the djerfisherite–hydrogarnet segregations frequently have the same shape as djerfisherite, which indirectly suggests their simultaneous crystallization from the same melt. The chemical composition of the djerfisherite from mono- and polysulfide globules, djerfisherite–hydrogarnet segregations, and type I inclusions, as most Yakutian kimberlites, is characterized by the high (12.1–16.7 wt %) Ni and low (0.1–0.9 wt %) Cu contents. The composition of the djerfisherite from types II and III inclusions differs in the lowered (3.3–1.6 wt %) Ni and elevated (40.9–53.2 wt %) Fe contents; type III inclusions have high Cu content: from 7.6 to 10.6 wt %.

Петрология. 2019;27(2):187-205
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Liquid immiscibility in fluid–magmatic systems: an experimental study
Kotelnikov A.R., Suk N.I., Kotelnikova Z.A., Yanev Y., Encheva S., Ananiev V.V.

Abstract

Liquid immiscibility was studied in melting experiments in the system trachyrhyolite–water + salt (NaF, Na2CO3) conducted at parameters imitating those of the volcanic process. The indicator components were La, Nb, Sr, W, Mo, Cr, Fe, Rb, and Cs. The experimental runs were carried out in a high gas pressure apparatus. At 1200oC and 5 kbar, melting, homogenization, and melt saturation with fluid components occurred. A decrease in the P-T parameters to 1000°C and 1 kbar led to the onset of liquid immiscibility in the form of droplets in the melt matrix. The composition of the droplets is similar to that of the melt matrix and differs from the latter only in concentrations of water, indicator components, and proportions of alkaline and alkali-earth elements. In the presence of salt (NaF), the melt exsolved into immiscible silicate and salt melts. No liquid immiscibility was detected when Na carbonate was added to the melt, but its agpaitic coefficient increased.

Петрология. 2019;27(2):206-224
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