Petrology最新文献

The Kem-Ludy Islands is an understudied fragment of the Early Precambrian Belomorian eclogite province. This study presents detailed petrological analyses on four mafic rocks in a sheared gneiss matrix: eclogite, quartz–clinopyroxene–plagioclase–amphibole–garnet granofels, garnet amphibolite, and quartz-bearing gabbronorite. These rocks exhibit varying degrees of metamorphic transformations, mineral assemblages, and mineral chemistry. Epidote inclusions corresponding to an early metamorphic episode were identified in the garnet cores from the eclogite, granofels, and garnet amphibolite. Peak eclogite facies conditions reconstructed using mineral geothermobarometry yielded T ~ 670°C and P_min ~ 1.3 GPa. The granofels retains indirect evidence of the eclogite stage, while the garnet amphibolite and quartz-bearing gabbronorite completely lack such signatures. Amphibolite-facies metamorphism significantly varies among the rock, primarily due to differential fluid influence. Reconstruction of P−T conditions for the amphibolite stage, done for each rock using mineral geothermobarometry, phase equilibrium modeling (PerpleX), and multiequilibrium thermobarometry (TWQ), yields values of T = 610–730°C and P = 0.4–0.8 GPa. For the eclogite, a P–T path of near-isothermal (~670°C) decompression from ~1.3 to 0.6 GPa was established from two metamorphic stages. This P−T path is similar to those previously determined for some eclogites and metaultramafic rocks from the tectonic mélange in the Gridino area differs from the evolution for eclogites from the Salma and Kuru-Vaara localities.

In addition to CO₂ coming from external mantle sources, CO₂ generated by the transformation of carbonaceous material of the protolith (internal sources) is actively involved in the processes of high-grade metamorphism in the crust. The paper presents results of experiments at 500 MPa and 900°C on the partial melting of plagioclase-free garnet–two mica (+ quartz, apatite, ilmenite) schist containing 0, 4.2, 10.1, 14.6, and 18.6 wt % graphite. The melting of graphite-free rock results in peraluminous melts corresponding to alkali-calcic ultrapotassic granites. As the graphite content is increased, the A/CNK and A/NK indices decrease, the MALI index of the melts increases, and their compositions shift toward alkalic granites. The estimated content of H₂O + CO₂ in the melts decreases with an increase in the graphite content in the starting system. The peritectic phases are hercynite–magnetite spinel, orthoamphibole (gedrite), sillimanite, and potassium feldspar. The decrease in the Fe³⁺/ΣFe ratio in Fe–Mg minerals with an increase in the graphite content in the starting mixtures suggests more reducing conditions. This conclusion is confirmed by the logfO₂ values calculated from the equilibrium of spinel, sillimanite, and quartz in the experimental products, which range from ~NNO + 0.5 for experiments in the absence of graphite to lower than ~NNO − 1.5 for experiments in the presence of more than 14 wt % graphite. The interaction of Fe₂O₃ and, possibly, H₂O, released as a result of peritectic melting reactions of the initial schist minerals (primarily micas) with graphite facilitates the formation of CO₂. Modeling of phase relations showed that, along with oxygen fugacity, water activity could be an additional factor influencing phase compositions in the presence of graphite. The Raman spectroscopy of the quenched melts and bubbles in them demonstrates that CO₂ is not only the predominant component of the free fluid phase accompanying the melts but is also partially dissolved in the melt as molecular CO₂ and ({text{CO}}_{3}^{{2 - }}) complexes with alkali and alkali-earth cations. Experiments demonstrate that under conditions of high-grade metamorphism, graphite-bearing metapelites can serve as an effective internal source for CO₂ accompanying granite melts during anatexis.

This paper presents data on the distribution of phosphorus and other elements (Ti, Al, Cr, Ni, Ca, Fe, Mg) in olivine crystals in meso- and adcumulates from the Yoko-Dovyren and Monchegorsk layered intrusions, including dunites, troctolites, anorthosites, and olivine-bearing chromitites. Olivine was found to contain skeletal and oscillatory cores, which could be formed (1) in the upper boundary layer of the magma chamber, (2) due to local mixing of magmas with different temperatures during magma emplacement process, and (3) at the boundary between the cumulate pile and the main magma volume of the chamber. Specific boundaries between olivine grains cutting the compositional zoning of one of the contacting crystals were observed. They were interpreted as indicators of pressure solution, which is one of mechanisms of compaction of original cumulate pile. Olivine grains from dunites of the Yoko-Dovyren massif and Mt. Travyanaya of the Monchegorsk massif, which demonstrate a lognormal crystal size distribution (CSD), often display signatures of resorption and subsequent overgrowth of small olivine crystals. The examination of phosphorus zoning of olivine confirmed an experimentally based conclusion that the lognormal CSD could be produced due to more extensive dissolution of smallest grains. In the case of the Yoko-Dovyren massif, this dissolution was attributed to the reaction of nonequilibrium intercumulus melt with the original cumulate crystals, whereas the overgrowth of the olivine grains occurred during further cooling. In the case of Mt. Travyanaya (Monchepluton), the dissolution was probably related to the peritectic reaction of olivine with melt producing orthopyroxene, and the additional growth of the olivine rim was caused by the inverse reaction caused by the diminishing the pyroxene stability field during magma ascent and decompression. In all types of the examined cumulates, except for chromitites, widespread relicts of intercumulus pores filled with phosphorus-rich olivine were discovered. These pores were suggested to form near the lower solidification front, when the compaction of the evolving cumulate pile was stopped. The enrichment of the pore olivine in phosphorus may indicate an increase in the degree of supersaturation/supercooling of the porous melt with respect to olivine accompanied by an increase in the olivine growth rate at the late stages of solidification.

Experimental data on the solubility of niobium and tantalum oxides and oxyfluorides in fluoride solutions were reviewed. Experimental solubility data for pyrochlore (CaNa)Nb₂O₆F and microlite (CaNa)Ta₂O₆F were used to calculate their thermodynamic properties at 300–800°C. The thermodynamic properties of albite, andalusite, muscovite, paragonite, and pyrophyllite were refined in the temperature range 300–550°C. The influence of solution composition and aluminosilicate mineral association on the solubility of pyrochlore and microlite in the supercritical region of physicochemical parameters was modeled by thermodynamic calculations. The calculations showed that the solubility of the minerals is very low, and niobium and tantalum cannot be removed by metamorphosed solutions. Mechanisms were proposed for HF accumulation, which could play an important role in niobium and tantalum dissolution, as well as recrystallization and replacement of ore mineral phases over limited distances.

Feldspars of the KGaSi₃O₈–RbGaSi₃O₈ series were synthesized at 550°C and 1.5 kbar, and their exchange reactions with 1 M KCl + 1 M RbCl solution were investigated. Using the experimental data, Margules parameters were calculated for the description of the excess mixing energies of the (K,Rb)GaSi₃O₈ solid solution: W^G1 = W^G2 = 4.46 ± 0.11 kJ/mol. We refined the unit-cell parameters of the solid solutions; the excess mixing volume is described by the Margules model: W^V1 = 2.647 ± 0.05 and W^V2 = –0.883 ± 0.04 cm³/mol. The results were compared with feldspars studied previously. Empirical relations were proposed for the calculation of the energy Margules model and unit-cell volumes for various minerals with the feldspar structure.