Petrology最新文献

The possibility of changing the ratio of the concentrations of NaCl and CaCl₂ salts in fluid phases formed as a result of heterogenization of the H₂O–CO₂–NaCl–CaCl₂ fluid with a decrease in P-T parameters has been studied. A well-known experimental fact regarding the ternary systems H₂O–CO₂–NaCl and H₂O–CO₂–CaCl₂ is the greater tendency of the H₂O–CO₂–CaCl₂ system to separate into coexisting predominantly aqueous-salt and aqueous-carbon dioxide phases compared to the similar system H₂O–CO₂–NaCl. This experimental fact can be interpreted as a greater affinity of NaCl for CO₂ compared to CaCl₂. Using a recently developed numerical thermodynamic model of the H₂O–CO₂–NaCl–CaCl₂ quaternary fluid system, it was possible to identify geologically significant consequences of this difference in the interaction of NaCl and CaCl₂ with CO₂. Multistage heterogenization of the H₂O–CO₂–NaCl–CaCl₂ fluid with a significant decrease in P-T parameters ultimately leads to the formation of aqueous-carbon dioxide fluid phase f2, the salt component of which is significantly enriched in NaCl and depleted in CaCl₂ compared to the initial fluid. The fluid phase f1 formed at each stage of heterogenization has a predominantly water-salt composition with the ratio of the mole fractions of NaCl and CaCl₂ salts, differing little from that in the initial fluid. However, the total mole fraction of salt in the f1 phase, as a rule, significantly exceeds that in the original fluid. The density of phase f1 significantly exceeds the density of phase f2. During the process of multistage heterogenization of the fluid phase f1, there is no formation of a fluid with a significant enrichment of CaCl₂ compared to the initial ratio of the mole fractions of NaCl and CaCl₂. At the same time, successive multiple separation of the f2 phase leads to the enrichment of its salt component in NaCl. Under favorable conditions, this process can lead to the formation of a fluid with almost pure NaCl salt. Changes in the salt composition of the fluid H₂O–CO₂–NaCl–CaCl₂ are considered in application to the evolution of fluid composition along the regressive branch of the P-T trend of HP metamorphism and syngranulite metasomatism in the Lapland granulite belt.

Xenoliths in kimberlites are the most promising material for studying the composition and structure of the lower levels of the continental crust. This study is aimed at the estimation of P–T parameters and fluid regime of metamorphism for garnet–biotite–feldspar and orthopyroxene–garnet–biotite–feldspar rocks found as xenoliths in kimberlites of the Yubileynaya and Sytykanskaya pipes, Yakutian kimberlite province. The seven studied samples show inverse dependences of relative contents of garnet and orthopyroxene, orthopyroxene and biotite, garnet and plagioclase, plagioclase and potassium feldspar. This indicates a consistent series of transformations of the assemblage garnet + plagioclase + orthopyroxene ± quartz to the assemblage garnet + biotite + potassium feldspar. In this process, the replacement of plagioclase by potassium feldspar was the leading reaction. Evidence of this reaction is specific reaction textures in the rocks, negative correlations between the contents of the minerals, and petrochemical characteristics of the rocks. Modeling of the mineral assemblages of the xenoliths using the pseudosection approach (PERPLE_X) revealed two groups of rocks corresponding to different depth levels of the Siberian cratonic crust. For rocks without orthopyroxene or with this mineral as single relics, the pressure was estimated at 9.5–10 kbar, and it is 6–7 kbar for orthopyroxene-bearing samples. The xenolith rocks have close metamorphic peak temperatures of 750–800°C. They experienced 200–250°C cooling and 3–4 kbar decompression, regardless of the level of the crust at which they had initially occurred. This indicates that the metamorphic evolution of the rocks during their exhumation was probably associated with collisional processes during the amalgamation of individual terrains of the Siberian craton. Xenoliths enriched in K-feldspar might have been products of metamorphic reactions with participation of aqueous–(carbonic)–salt fluids, which were sourced from basaltic magmas in the lower crust. The most strongly metasomatized rocks were located closest to the place of accumulation of crystallizing magmas.

The temperature and compositional parameters of the parental magma of ore-bearing apophysis DV10 of the Yoko-Dovyren massif are estimated by the method of geochemical thermometry based on results of thermodynamic modeling of the equilibrium crystallization of the melts of 24 rocks. The thermometric calculations were carried out using the COMAGMAT-5.3 program with increments of 0.5 mol % to a maximum degree of crystallization 75–85%, under oxygen fugacity controlled by the QFM buffer. The model crystallization sequence of minerals was as follows: olivine (Ol) + Cr-Al spinel (Spl) → plagioclase (Pl) → high-Ca pyroxene (Cpx) → orthopyroxene (Opx). Silicate−sulfide immiscibility was calculated to occur mostly before the onset of plagioclase crystallization, which is consistent with initial sulfide saturation of the parental magma. The calculation results demonstrate the convergence and intersection of the model liquid lines of descent at temperatures of about 1185^oC. When applied to the average composition of apophysis DV10, this temperature indicates the existence of suspension of the original crystals, including 52.1 wt % cumulus olivine (Fo_83.6), 2.3 wt % plagioclase (An_79.7), 0.24 wt % clinopyroxene (Mg# 88.8), 1 wt % aluminochromite (Cr# 0.62), and about 0.2% sulfide liquid in a moderately magnesian melt (53.6 wt % SiO₂, 7.4 wt % MgO). Therewith the sulfur concentration at sulfide saturation (SCSS) was estimated at 0.083 wt %. This heterogeneous system had a viscosity of 4.71 log units (Pa s) and integral density of 2929 kg/m³. Such rheological properties do not contradict the possibility of the migration and emplacement of the protocumulus mush from the main Dovyren chamber. However, a more probable scenario is the localized accumulation of olivine in the trough-shaped part of the DV10 subchamber, which preceded or occurred in parallel to the accumulation of segregated sulfides.

We propose a coupled hydro-mechanical-chemical model and its 1D numerical implementation. We demonstrate its application to the model filtration of a multicomponent fluid in deforming and reacting host rocks, considering changes in the densities, phase proportions, and the chemical compositions of the coexisting phases. The presented 1D numerical implementation is illustrated by the example of soapstone formation from serpentinite during the filtration of Н₂О−CО₂ fluid with a low CО₂ concentration coupled with the viscous deformation of the mineral matrix, considering the MgO−SiO₂−Н₂О−CО₂ system. The numerical results show the propagation of a porosity wave by means of a viscous (de)compaction mechanism accompanied by the formation of an elongated zone with higher filtration properties. After the formation of such a channel, the formation and propagation of the reaction fronts occur and are associated with the transformation of the mineral composition of the original rock. During H₂O−CO₂ fluid filtration, starting with 1 wt % dissolved CO₂, carbonization of hydrated serpentinite starts, specifically antigorite transforms to magnesite and talc.

Long-term strength of the lithosphere is often assumed to be equivalent to its average deviatoric stress level. However, this definition is only correct for a homogeneous visco-elastic material, in which no localized (in space and/or time) weakening and deformation processes occur. Here, I instead propose to define the large-scale-long-term strength of the lithosphere as the measure of its mechanical resistance to irreversible deformation, which corresponds to the amount of mechanical energy irreversibly spent (i.e., dissipated) for producing unit irreversible (i.e., inelastic, visco-plastic) deformation. According to this new definition, strength is the ratio of the integrated (through given lithospheric volume and time) mechanical energy dissipation to the integrated irreversible visco-plastic strain. With this new definition, the large-scale-long-term strength of the lithosphere stands as a strain-averaged rather than a volume-time-averaged quantity. As the result, an interesting behavior can occur when, due to localization of irreversible deformation along volumetrically minor weak structures, strength of the lithosphere can be significantly lower than its average long-term deviatoric stress level characteristic for volumetrically dominant strong elastic regions. This definition is applicable for both homogeneous and heterogeneous (i.e., localized in space and/or time) lithospheric deformation and provides a useful framework for analyzing various geodynamic settings on regional and global scale. In particular, I show some implications of this new lithospheric strength theory for better understanding of (i) intense melt-induced weakening of the lithosphere by magmatic processes, (ii) very low strength of plate interface in subduction zones and (iii) low brittle/plastic strength of tectonic plates predicted by global mantle convection models with plate tectonics. Although this work focuses on evaluating the long-term-large-scale brittle/plastic strength and deformation parameters, the proposed approach can also be applied for quantifying the effective ductile (i.e., viscous) strength and respective long-term-large-scale rheological properties.

Mineral reactions were studied in metamorphic rocks from the Meyeri tectonic zone, and the P–T path of the development of this structure was calculated. According to the P–T path, the Proterozoic granulite complex of the Svecofennian Belt was thrust onto low-temperature rocks of the Archean Karelian Craton. Relict staurolite and other minerals preserved as inclusions in the garnet porphyroblasts made it possible to identify the pre-peak stage of metamorphism with P–T parameters no higher than the low-temperature amphibolite facies of moderate and low pressure. The peak metamorphic conditions of the tectonic zone are estimated at T > 700°C and P ~ 7 kbar using the composition of relict minerals, while the temperature on the prograde trend of metamorphism was 500–600°C at a pressure of about 5 kbar. The post-peak stage began with a distinct decompressional P–T path at the aforementioned high temperatures, with a change from granulite hypersthene-containing assemblages to lower-temperature hydrous ones. The subsequent metamorphic retrogression was characterized by the development of numerous hydrous minerals as a result of the activation of fluids in the shear zone. The P–T path of the tectonic zone is clockwise and reflects the exhumation of the Svecofennian granulite complex during the orogenic events.

Metamorphosed dacitic porphyry dikes were first found in the western part of the Vorontsovka terrane, which is located in the Paleoproterozoic Volga–Don orogen at the margin of Archean Sarmatia and Volga–Ural cratons. The magmatic protolith age for the metadacites is ca. 2.07 Ga. These are ferrous, metaluminous calc-alkali I-type granitoids. The sodium specialization of the rocks and their low concentrations of Mg, Cr, Ni, and incompatible elements, with significant REE fractionation, the absence of Eu* anomalies, high Sr/Y ratio, remarkably high (Gd/Yb)_n values (>10), and the radiogenic Nd isotopic composition indicate that the dacitic melts were derived from a juvenile mafic source. According to petrogenetic estimations, such conditions could be caused by the partial melting of depleted N-MORB basites in equilibrium with an eclogitic residue. The dacitic magmas were likely generated by the partial melting of mafic rocks at lower levels of the significantly thickened crust (>60 km) in relation to collision processes.

Syn-emplacement serpentine is one of the most abundant late minerals in kimberlites; its multiple generations can be distinguished by various textural positions and parageneses. Composition of the primary kimberlite melt cannot be accurately determined if we do not recognize distinct origins for several textural varieties of serpentine. This study aims to find compositional indicators of the serpentine origin by characterizing millimetre-sized serpentine domains in hypabyssal kimberlites. Serpentine forms as segregations in the groundmass or when serpentine replaces olivine or metasomatized silicate xenoliths. The latter textural variety of serpentine has not been recognized previously; it develops in Si-rich basement xenoliths ranging from basalt to granite. This serpentine is associated with abundant diopside, pectolite, phlogopite and chlorite and less prominent amphibole, hydrogarnet, wollastonite, xonotlite and other rare Ca hydrosilicates. We report petrography and textures of reacted silicate xenoliths in Renard 65, Orapa AK15, BK1, Gahcho Kué 5034 and Jericho kimberlites and provide a global summary of the phase compositions in the xenoliths. This study discovered that NiO content < 0.05 wt %, Al₂O₃ content > 1.3 wt % and MnO > 0.3 wt % in serpentine are clear signs of formation after felsic xenoliths. Serpentine/chlorite replacing olivine always have 1.5–4 wt % more FeO than serpentine after silicate xenoliths. The compositional contrast results from the immobile behaviour of conserved Al, Ni and Mn. The proposed criteria were tested on a pyroclastic kimberlite with an enigmatic origin of round serpentinized clasts overgrown by fibrous clinopyroxene and identified the precursor of these clasts as felsic. We also determined mineralogical characteristics of serpentine parageneses that can be used for reconstruction of the initial xenolith lithology. Serpentine coexists with the more abundant calcic hydrosilicates (hydrogarnet, xonotlite, amphiboles) in reacted mafic xenoliths. There, serpentine and chlorite crystal structures show less ideal stoichiometry indicative of a higher volume of nanometre-scale interstratification with smectites. Serpentine-rich assemblages in reacted xenoliths formed metasomatically at T < 600°C due to skarn-like mass transfer with the host kimberlite that controlled the gain of Ca and Mg and desilication. These metasomatic assemblages are remarkably identical to rodingites. Serpentine production appeared to be limited by the availability of Si in and around silicate xenoliths, but by the H₂O availability in pseudomorphed olivine/monticellite.