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Vol 32, No 1 (2024)
Articles
“В кильватере большого корабля”: плаванье продолжается!
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Modelling of Multicomponent Fluid Flow in Deforming and Reacting Porous Rock
Abstract
We propose a coupled hydro-chemo-mechanical model and its 1D numerical implementation. We demonstrate its application to model filtration of a multicomponent fluid in deforming and reacting host rocks, considering changes in the densities, phase proportions and chemical compositions of coexisting phases. We presented 1D numerical implementation on the example of soapstone formation from serpentinite during H₂O-СО2 fluid filtration with low concentration of СО2 coupled with viscous deformation of mineral matrix, considering MgO-SiO2-H₂O-СО2 system. The numerical results show porosity wave propagation by viscous (de)compaction mechanism accompanied with the formation of an elongated zone with higher filtration properties. After the formation of such a channel, the formation and propagation of reaction fronts occurs associated with the transformation of the mineral composition of the original rock. During H2O-CO2 fluid filtration, starting from 1 weight percent of dissolved CO2, carbonization of hydrated serpentinite starts, specifically antigorite transforms to magnesite and talc.
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Fault Tectonites of the Yenisei Shear Zone (Yenisei Ridge): Evidence and Thermomechanical Numerical Model of Generation of Tectonic Overpressure
Abstract
Based on the proposed numerical model of the stress-strain state of polymineral rocks, which describes the formation of blastomylonites in the Yenisei Regional Shear Zone (PRSZ) in the Yenisei Ridge, the possibility of local tectonic overpressure exceeding the lithostatic pressure in rocks subjected to shear deformations is shown. For tectonites of the southern (Angara-Kan block) and northern (Isakovka terrane and Garevka complex) segments of the PRSZ, estimates of the maximum overpressure were obtained from 2–3 to 4–5 kbar, which range from 25 to 50% of the lithostatic pressure. It is shown that excess pressures can be preserved in a local volume on a geological time scale sufficient for their fixation in metamorphic minerals. Model values of overlithostatic pressure in garnet-amphibole tectonites and geobarometric estimates of peak values during stress metamorphism allow us to offer new evidence of pressure inhomogeneity in natural mineral associations. Using the results of numerical modeling for the evolution of fault metabasite blastomylonites, it was established that the overpressure at the stage of syn-deformation metamorphism in the shear zone are possible at temperatures up to 600–650°C and not reaching 800°C; the presence of fluid or partial melt prevents the occurrence of overpressure. The amount of excess pressure due to shear stresses depends on the mineral composition and structure of the rock.
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Clorine Solubility in Silicate Melts: New Experiments and Thermodynamic Mixing Model
Abstract
We present new experimental data on Cl solubility in model basalt melts of eutectic compositions diopside (Di)–albite (Ab) and Di–anorthite ± quartz (Qtz). The starting glasses were equilibrated with aqueous fluid H₂O-NaCl-CaCl₂ at 4 kbar in the temperature range 900–1200°C. The experiments show that the Cl solubility decreases with increasing NaCl in the fluid. Ca-Na partitioning between melts and fluid is weekly temperature dependent and resembles that of the plagioclase-fluid system. The new experimental data, along with the previously published results on the model granite melting in the presence of (Na, K)Cl brines (Aranovich et al., 2013) are used to calibrate an empirical thermodynamic model for the salt species (NaCl, KCl, CaCl₂) in silicate melt. Calculations show that Cl solubility in the haplogranite melt decreases with increasing K/Na ratio in the fluid (and, correspondingly, melt). At high pressure (10 kbar) Cl solubility in the granite increases with increasing H₂O content. Calculated phase diagram for a simple pseudo-ternary system Ab–H₂O–NaCl demonstrates complex phase relations and, correspondingly, evolution of the H₂O and NaCl concentrations in the melt. Literature data on the variations of H₂O and NaCl in the melt and fluid inclusions in Qtz from the granite of Badzhal tin deposit is used to illustrate complex evolution of a fluid-magmatic system.
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Meta-Ultramafic Rocks of the Maksyutov Complex, Southern Urals: High-Pressure Si-Al Metasomatism and Carbonatization at the Crust-Mantle Interface in the Subduction Zone
Abstract
The eclogite-blueschists Maksyutov Complex is characterized by a complex fold-and-thrust structure that was developed during the Late Devonian collision between Baltica (East European Plate) and the Magnitogorsk Arc, that was formed during the Early Devonian intraoceanic subduction. Eclogites are the most studied rocks of the complex; their formation and exhumation are usually associated with the collisional stage of orogen development. At the same time, the origin of meta-ultramafic rocks, which together with eclogites form sheeted and lenses within metasedimentary rocks (shales and quartzites), still remains unknown. This paper presents the results of the first detailed petrological study of meta-ultramafic rocks, represented by antigorite-chlorite and magnesite-antigorite metaharzburgites, chlorite-antigorite metaorthopyroxenite. Mineral compositions and textural relationships between minerals in metaharzburgites indicate at least two stages of rock transformations. Minerals of the early mineral paragenesis (first stage) – olivine, accessory chromite and low-fluorine Ti-clinohumite – have a metamorphic genesis; ultrahigh-pressure (UHP) conditions of their formation are discussed. At the second stage, there was a partial replacement of olivine by orthopyroxene-bearing parageneses with Cr-Al antigorite and/or high-chromium chlorite. Based on the phase equilibria modeling using the Perple_X software package, it was found that the formation of antigorite-orthopyroxene paragenesis was associated with Si-Al metasomatism at: T ~ 630°С, P ~ 2 GPa, logaSiO₂ ~ –0.6, logaAl₂O₃ ~ – 2.5. It is important to note that the mineral paragenesis are highly sensitive to aSiO₂: a slight decrease in lgaSiO2 relative to the above value would lead to the growth of olivine with antigorite, and an increase would lead to the growth of orthopyroxene. The latter may explain the formation of meta-orthopyroxenites, which are widely distributed among the meta-ultramafic rocks of the Maksyutov Complex. Similar calculations performed for the range of XCO₂ = 0.01–0.05 in H₂O-CO₂ fluid showed replacing silicate minerals by magnesite under the established thermodynamic conditions. Carbonation and Si-Al metasomatism are specific features of high-pressure transformations of meta-ultramafic rocks, which have not been established in the associated eclogites, quartzites, and shales. Such selectivity of fluid influence on different rock types is interpreted as a result their different tectono-metamorphic evolution: meta-ultramafic rocks are fragments of the suprasubduction mantle, which were tectonically combined with the rocks of the subducting plate (eclogites and metasedimentary rocks).
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Metasomatism in the Precambrian Crust of the Siberian Craton: Results of a Study of Garnet(±orthopyroxene)-biotite-feldspar Xenolith Rocks From Yubileinaya and Sytykanskaya Kimberlite Pipes, Yakutia
Abstract
Xenoliths in kimberlites are the most perspective objects for studying the composition and structure of the lower levels of the continental crust. Present work is aimed at estimation of P-T fluid conditions of metamorphism for garnet-biotite-feldspar and orthopyroxene-garnet-biotite-feldspar rocks represented as xenoliths in kimberlites of the Yubileynaya and Sytykanskaya pipes, Yakutian kimberlite province. 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. Now it is represented by specific reaction textures in the rocks, negative correlations of the mineral contents, as well as in petrochemical characteristics of the rocks. Modeling of xenolith mineral assemblages using the pseudosection approach (PERPLE_X) revealed two groups of rocks corresponding to different depth levels of the Siberian cratonic crust. For rocks where orthopyroxene is absent or is present as single relics, pressure estimates are 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 were initially located. This points to the metamorphic evolution of the rocks during their exhumation, 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 metasomatized rocks were located closest to the place of accumulation of crystallizing magmas.
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Geochemical Thermometry of Ore-Bearing Gabbronorites from the Apophisis of the Yoko-Dovyren Massif: Composition, Amount of Olivine, and Conditions of Sulphide Saturation in the Parental Magma
Abstract
The temperature and compositional parameters of the parental magma of the ore-bearing apophysis DV10 from the Yoko-Dovyren massif are estimated based on the results of thermodynamic modeling the equilibrium crystallization of melts of 24 samples, following the method of geochemical thermometry. Thermometric calculations were carried out using the COMAGMAT-5.3 program with a step of 0.5 mol. % to a maximum degree of crystallization 75–85% under oxygen fugacity controlled by the QFM buffer. The model order of mineral crystallization corresponds to the sequence: olivine (Ol) + + Cr-Al spinel (Spl) → plagioclase (Pl) → high-Ca pyroxene (Cpx) → orthopyroxene (Opx). Silicate-sulfide immiscibility was modeled to occur mostlybefore the onset of plagioclase crystallization, being consistent with sulfide saturation of the parental magma. The results of calculations demonstrate the convergence and intersection of the model liquid lines of descent at temperatures of about 1185°C. When applied to the average composition of the DV10 apophysis, this temperature indicates the existence of a suspension of the original crystals (52.1 wt. % cumulus olivine (Fo83.6), 2.3 wt. % plagioclase (An79.7), 0.24% 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). At that the solubility of sulfide sulfur (SCSS) was estimated to be 0.083 wt. %. This heterogeneous system had a viscosity of 4.71 log. units (Pa · s) and an 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 includes a localized accumulation of olivine in the trough-like part of the DV10 subchamber, which preceded or occurred in parallel to the accumulation of segregated sulfides.
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