Fayalite-sekaninaite paralava from the Ravat coal fire (central Tajikistan)

We studied ferrous paralava, a high-temperature rock, produced by complete fusion of the sedimentary protolith in the Ravat natural coal fire which has been on for over two thousand years. The paralava was sampled from the Fan-Yagnob coal deposit at the Kukhi-Malik site in the vicinity of former Ravat Village in central Tajikistan. This rock contains fayalite, sekaninaite, hercynite, Ti-magnetite, tridymite, and siliceous glass. Low-Ca pyroxene (clinoferrosilite), globules of sulfides (mainly pyrrhotite) and Fe-Ti oxides, secondary greenalite (after fayalite) and hematite are minor. Paralava includes xenoliths of partially molten clinkers (up to 20 vol.%) composed of mullite, cordierite, tridymite, and relict detrital quartz. We found relatively high Fe contents (100?Fe/(Fe+Mg) > 60) in mafic minerals, high K₂O enrichment (up to 1.4 wt.%) in sekaninaite, and an unusually low CaO content (0.5 wt.%) in the rock. The Ravat paralava appears to be derived from a mixture of pelitic rocks (50–70%) and iron-rich rocks (30–50%), but without participation of calcareous material, which explains the low CaO and the absence of plagioclase and Ca-bearing pyroxene. The primary melt was as hot as >1210 °C, and the coal-fired fayalite-sekaninaite paralava crystallized at 1200–1100 °C, at a relatively low oxygen fugacity (near the QFM buffer), outside the zone of active aeration. Large-scale crystallization of ferrospinels and fayalite led to increasing Al₂O₃ and SiO₂ in the melt whence sekaninaite and tridymite crystallized as later phases. The residual melt progressively acquired a more silicic-aluminous composition, rich in K₂O, CaO, and P₂O₅, and became quenched to glass at >1080–1090 °C, when temperature dropped abruptly, possibly, by roof collapse or opening of large cracks, as it usually happens in underground coal fires.     

The redox state of the continental lithospheric mantle of the Baikal-Mongolia region

The thermal and redox state of the upper mantle beneath the Baikal-Mongolia region was estimated on the basis of the investigation of the chemical composition (including iron oxidation state) of major minerals (olivine, orthopyroxene, clinopyroxene, and spinel) in spinel and garnet-spinel peridotite xenoliths from the Cenozoic alkali basalts of the volcanic fields of the Dariganga Plateau, Tariat Depression, and Vitim Plateau. At temperatures of 1030–1500°C and pressures of 29–47 kbar, the Δlog$ f_{O_2 } $ f_{O_2 } values relative to the FMQ buffer (calculated using the olivine-spinel oxygen barometer) range from −0.9 to −1.7 for the xenoliths of the Dariganga Plateau, from −0.9 to −1.8 for the Tariat Depression, and from −0.8 to −0.1 for the Vitim Plateau. The oxygen fugacity of peridotites from all of the areas is, in general, lower than that of the WM buffer. Oxygen fugacity is usually below the CCO and EMOD/G buffers in the peridotites of the Dariganga Plateau and the Tariat Depression and higher than these buffers in the peridotites of the Vitim Plateau. The T-PΔlog$ f_{O_2 } $ f_{O_2 } relationships in the xenoliths suggest the existence of spatial heterogeneity in the thermal and redox state of the upper mantle of the Baikal-Mongolia region. This heterogeneity is probably related to the influence of the plume that was responsible for the Late Mesozoic-Cenozoic intraplate magmatism of this region and reflects the different distance of the respective mantle domains from the plume head. The C-O-H fluids in equilibrium with the upper mantle peridotites are composed mainly of water and carbon dioxide. The mantle of the Dariganga Plateau and the Tariat Depression (Δlog$ f_{O_2 } $ f_{O_2 } < −0.9) is characterized by the dominance of H₂O, whereas CO₂-rich fluids are characteristic of the more oxidized mantle of the Vitim Plateau (Δlog$ f_{O_2 } $ f_{O_2 } is mostly higher than −0.8).     

Paragenesis of unusual Fe-cordierite (sekaninaite)-bearing paralava and clinker from the Kuznetsk coal basin,Siberia, Russia

Sekaninaite (X_Fe > 0.5)-bearing paralava and clinker are the products of ancient combustion metamorphism in the western part of the Kuznetsk coal basin, Siberia. The combustion metamorphic rocks typically occur as clinker beds and breccias consisting of vitrified sandstone–siltstone clinker fragments cemented by paralava, resulting from hanging-wall collapse above burning coal seams and quenching. Sekaninaite–Fe-cordierite (X_Fe = 95–45) is associated with tridymite, fayalite, magnetite, ± clinoferrosilite and ±mullite in paralava and with tridymite and mullite in clinker. Unmelted grains of detrital quartz occur in both rocks (<3 vol% in paralavas and up to 30 vol% in some clinkers). Compositionally variable siliceous, K-rich peraluminous glass is <30% in paralavas and up to 85% in clinkers. The paralavas resulted from extensive fusion of sandstone–siltstone (clinker), and sideritic/Fe-hydroxide material contained within them, with the proportion of clastic sediments ≫ ferruginous component. Calculated dry liquidus temperatures of the paralavas are 1,120–1,050°C and 920–1,050°C for clinkers, with calculated viscosities at liquidus temperatures of 10^1.6–7.0 and 10^7.0–9.8 Pa s, respectively. Dry liquidus temperatures of glass compositions range between 920 and 1,120°C (paralava) and 920–960°C (clinker), and viscosities at these temperatures are 10^9.7–5.5 and 10^8.8–9.7 Pa s, respectively. Compared with worldwide occurrences of cordierite–sekaninaite in pyrometamorphic rocks, sekaninaite occurs in rocks with X_Fe (mol% FeO/(FeO + MgO)) > 0.8; sekaninaite and Fe-cordierite occur in rocks with X_Fe 0.6–0.8, and cordierite (X_Fe < 0.5) is restricted to rocks with X_Fe < 0.6. The crystal-chemical formula of an anhydrous sekaninaite based on the refined structure is | \textK_0.02 |(\textFe_1.54^{2 +} \textMg_0.40 \textMn_0.06 )_{\Upsigma 2.00}^M [(\textAl_1.98 \textFe_0.02^{2 +} \textSi_1.00 )_{\Upsigma 3.00}^T1 (\textSi_3.94 \textAl_2.04 \textFe_0.02^{2 +} )_{\Upsigma 6.00}^T2 \textO₁₈ ]. \left| {{\text{K}}_{0.02} } \right|({\text{Fe}}_{1.54}^{2 + } {\text{Mg}}_{0.40} {\text{Mn}}_{0.06} )_{\Upsigma 2.00}^{M} [({\text{Al}}_{1.98} {\text{Fe}}_{0.02}^{2 + } {\text{Si}}_{1.00} )_{\Upsigma 3.00}^{T1} ({\text{Si}}_{3.94} {\text{Al}}_{2.04} {\text{Fe}}_{0.02}^{2 + } )_{\Upsigma 6.00}^{T2} {\text{O}}_{18} ].     

Fluorine mineralisation from burning coal spoil-heaps in the Russian Urals

Summary Anhydrous iron, aluminum and fluorine-rich paralavas were found in the burned spoil-heaps of the Chelyabinsk coal basin, Russia. The rocks contain tridymite, anorthite, ferroan fluorine-bearing cordierite, fluorine-bearing mullite, periclase, fluorapatite, micas of the F-biotite–F-phlogopite series, fluortopaz, fluorite, and sellaite. The fluorine-rich minerals formed as a result of local thermal reactions of sedimentary carbonates and silicates with gaseous fluorine. During coal combustion fluorine concentrates in the annealed ankeritic marls where the increase of F is hundreds of times over its concentration in the initial sedimentary rocks. The formation of MgF₂ and CaF₂ promotes local melting at relatively low temperatures (T < 1000 °C) with the residuum consisting of two immiscible liquids. One crystallises as the fluorides, the other as fluorine-substituted analogues of the hydrosilicates, which under the extremely dry conditions, produce minerals containing extremely high F-contents. Received April 10, 2000; revised version accepted February 24, 2001     

Carbonate-silicate equilibria in the high-magnesia ultrapotassic volcanics of the Toro-Ankole Province (Eastern African rift zone)

This work reports new data on phenocryst composition and carbonate-silicate equilibria in the volcanic rocks of kamafugitic affinity in the Toro-Ankole Province (East African rift zone). Inclusions of primary carbonates (calcite and dolomite) were found in olivines from ugandite and mafurite of the Bunyaruguru volcanic field. The initial compositions of melt inclusions in olivine from the ugandite were calculated from microprobe analyses and correspond to carbonatites. The find of barite and dolomite-barite inclusions in the olivine from the mafurite indicates that the melt contained sulfate sulfur, which is typical of oxidizing conditions. The calculation of the olivine-spinel equilibrium (T-$ f_{O_2 } $ f_{O_2 } ) showed that crystallization of phenocrysts in mafurite occurred at oxygen fugacities above the NNO buffer (ΔQFM ∼ 2.5) in a wide temperature range (1230–750°C).     

Mineralogy and formation conditions of ores in the Bereznyakovskoe ore field,the Southern Urals,Russia

The Bereznyakovskoe ore field is situated in the Birgil’da-Tomino ore district of the East Ural volcanic zone. The ore field comprises several centers of hydrothermal mineralization, including the Central Bereznyakovskoe and Southeastern Bereznyakovskoe deposits, which are characterized in this paper. The disseminated and stringer-disseminated orebodies at these deposits are hosted in Upper Devonian-Lower Carboniferous dacitic-andesitic tuff and are accompanied by quartz-sericite hydrothermal alteration. Three ore stages are recognized: early ore (pyrite); main ore (telluride-base-metal, with enargite, fahlore-telluride, and gold telluride substages); and late ore (galena-sphalerite). The early and the main ore stages covered temperature intervals of 320–380 to 180°C and 280–300 to 170°C, respectively; the ore precipitated from fluids with a predominance of NaCl. The mineral zoning of the ore field is expressed in the following change of prevalent mineral assemblages from the Central Bereznyakovskoe deposit toward the Southeastern Bereznyakovskoe deposit: enargite, tennantite, native tellurium, tellurides, and selenides → tennantite-tetrahedrite, tellurides, and sulfoselenides (galenoclausthalite) → tetrahedrite, tellurides, native gold, galena, and sphalerite. The established trend of mineral assemblages was controlled by a decrease in $ f_{S_2 } $ f_{S_2 } , $ f_{Te_2 } $ f_{Te_2 } and $ f_{O_2 } $ f_{O_2 } and an increase in pH of mineral-forming fluids from early to late assemblages and from the Central Bereznyakovskoe deposit toward the Southeastern Bereznyakovskoe deposit. Thus, the Central Bereznyakovskoe deposit was located in the center of an epithermal high-sulfidation ore-forming system. As follows from widespread enargite and digenite, a high Au/Ag ratio, and Au-Cu specialization of this deposit, it is rather deeply eroded. The ore mineralization at the Southeastern Bereznyakovskoe deposit fits the intermediate- or low-sulfidation type and is distinguished by development of tennantite, a low Au/Ag ratio, and enrichment in base metals against a lowered copper content. In general, the Bereznyakovskoe ore field is a hydrothermal system with a wide spectrum of epithermal mineralization styles.     

Unique Clinkers and Paralavas from a New Nyalga Combustion Metamorphic Complex in Central Mongolia: Mineralogy,Geochemistry, and Genesis

The paper presents mineralogical and geochemical data on clinkers and paralavas and on conditions under which they were formed at the Nyalga combustion metamorphic complex, which was recently discovered in Central Mongolia. Mineral and phase assemblages of the CM rocks do not have analogues in the world. The clinkers contain pyrogenically modified mudstone relics, acid silicate glass, partly molten quartz and feldspar grains, and newly formed indialite microlites (phenocrysts) with a ferroindialite marginal zone. In the paralava melts, spinel microlites with broadly varying Fe concentrations and anorthite–bytownite were the first to crystallize, and were followed by phenocrysts of Al-clinopyroxene ± melilite and Mg–Fe olivine. The next minerals to crystallize were Ca-fayalite, kirschsteinite, pyrrhotite, minerals of the rhönite–kuratite series, K–Ba feldspars (celsian, hyalophane, and Ba-orthoclase, Fe³⁺-hercynite ± (native iron, wüstite, Al-magnetite, and fresnoite), nepheline ± (kalsilite), and later calcite, siderite, barite, celestine, and gypsum. The paralavas contain rare minerals of the rhönite–kuratite series, a new end-member of the rhönite subgroup Ca₄Fe ₈ ²⁺ Fe ₄ ³⁺ O₄ [Si₈Al₄O₃₆], a tobermorite-like mineral Ca₅Si₅(Al,Fe)(OH)O₁₆ · 5H₂O, and high- Ba F-rich mica (K,Ba)(Mg,Fe)₃(Al,Si)₄O₁₀F₂. The paralavas host quenched relics of microemulsions of immiscible residual silicate melts with broadly varying Si, Al, Fe, Ca, K, Ba, and Sr concentrations, sulfide and calcitic melts, and water-rich silicate–iron ± (Mn) fluid media. The clinkers were formed less than 2 Ma ago in various parts of the Choir–Nyalga basin by melting Early Cretaceous mudstones with bulk composition varies from dacitic to andesitic. The pyrogenic transformations of the mudstones were nearly isochemical, except only for volatile components. The CM melt rocks of basaltic andesitic composition were formed via melting carbonate–silicate sediments at temperatures above 1450°C. The Ca- and Fe-enriched and silicaundersaturated paralavas crystallized near the surface at temperatures higher than 900–1100°C and oxygen fugacity \(f_{O_2 }\) between the IW and QFM buffers. In local melting domains of the carbonate–silicate sedimentary rocks and in isolations of the residual melts among the paralava matrix the fluid pressure was higher than the atmospheric one. The bulk composition, mineral and phase assemblages of CM rocks of the Nyalga complex are very diverse (dacitic, andesitic, basaltic andesitic, basaltic, and silica-undersaturated mafic) because the melts crystallized under unequilibrated conditions and were derived by the complete or partial melting of clayey and carbonate–silicate sediments during natural coal fires.     

Oxygen, silicon, and Mn-Cr isotopes of fayalite in the Kaba oxidized CV3 chondrite: Constraints for its formation history

The iron-rich olivine end-member, fayalite, occurs in the matrix, chondrules, Ca-Al-rich inclusions (CAIs), silicate aggregates, and dark inclusions in the Kaba and Mokoia oxidized CV3 chondrites. In most occurrences, fayalite is associated with magnetite and troilite. To help constrain the origin of the fayalite (Fa_98-100), we measured oxygen and silicon isotopic compositions and Mn-Cr systematics in fayalite from two petrographic settings of the Kaba meteorite. One setting consists of big fayalite laths embedded in the matrix and radiating from a core of fine-grained magnetite and sulfide, while the other setting consists of small fayalite-magnetite-sulfide assemblages within or at the surface of Type I barred or porphyritic olivine chondrules. Oxygen in the big fayalite laths and small chondrule fayalites falls on the terrestrial fractionation line, and is distinct from that in chondrule forsterites, which are enriched in ¹⁶O (Δ¹⁷O = ∼−4‰). Oxygen in the big fayalite laths may be isotopically heavier than that in chondrule fayalites. Silicon isotopes suggest that forsterite is ∼1‰/amu heavier than adjacent fayalite within Kaba chondrules. However, we were unable to confirm large silicon isotopic differences among fayalites reported previously. The Mn-Cr data for big Kaba fayalites give an initial ⁵³Mn/⁵⁵Mn ratio of (2.07 ± 0.17) × 10⁻⁶, consistent with literature results on Mokoia chondrule fayalites. The combined data suggest that fayalites in both petrographic settings formed at about the same time, ∼9.7 Ma after the formation of CAIs. Our data indicate that those fayalite-magnetite-troilite assemblages replacing metal inside and around chondrules formed by aqueous alteration on the meteorite parent body. The formation site and mechanism for the big fayalite laths is less clear, but the petrographic setting indicates that they did not form in situ. None of the models that have been suggested for formation of these fayalites is entirely satisfactory.     

The chemistry and origin of zincian spinel associated with the Aggeneys Cu-Pb-Zn-Ag deposits,Namaqualand, South Africa

Zincian spinel or gahnite [(Zn,Fe,Mg)Al₂O₄] occurs in metamorphosed sulphide-rich rocks, garnet quartzites, quartz-magnetite rocks, aluminous metasediments, barite-magnetite rocks, quartz veins, and pegmatites associated with the Aggeneys base metal deposits, Namaqualand, South Africa. Zincian spinel in, sulphide-bearing rocks, is considered to have formed predominantly by desulphurization reactions involving a member of the system Fe-S-O and sphalerite with sillimanite or garnet. Gahnite in sulphide-free garnet quartzites, quartz-magnetite rocks and barite-magnetite rocks probably formed from Zn and Al that were hydrothermally derived whereas gahnite in aluminous metasediments was derived from the metamorphism of metalliferous shales, in which Zn may originally have been linked to organic material. Gahnite is Zn-rich in sulphide-bearing rock, but is Fe-rich in sulphide-free garnet quartzites and quartz-magnetite rocks. Although Zn-rich spinels represent guides to ore in the Aggeneys area and elsewhere in the Namaqualand Metamorphic Complex, Fe-rich spinels should not be discounted because Zn-rich and Fe-rich spinels occur within metres of sulphides at Aggeneys.     

Associations of Mn-bearing minerals as indicators of oxygen fugacity during the metamorphism of metalliferous deposits

The paper summarizes experimental and calculation data on the effect of oxygen fugacity on the origin of mineral assemblages in Mn-bearing rocks and demonstrates the possibility of application of these data to the reconstruction of conditions under which metalliferous deposits were metamorphosed. A new variant of the T-log\(f_{O_2 } \) diagram is proposed for the Mn-Si-O system, which differs from previous ones by the location of the lines for the formation (decomposition) of braunite and tephroite. These two minerals are the most universal indicators of oxygen fugacity during the metamorphism of Mn-bearing deposits, because these minerals are widespread in nature and can be formed in diverse environments: braunite at high \(f_{O_2 } \) values in the pore solution, and tephroite at low \(f_{O_2 } \) values. The occurrence of Mn oxides and rhodonite (pyroxmangite) in a rock makes it possible to constrain the oxygen fugacity range. An original T-log\(f_{O_2 } \) diagram is constructed for the Ca-Mn-Si-O system. As follows from this diagram, a Ca admixture expands the stability field of rhodonite toward higher oxygen fugacity values. Johannsenite can be formed in these rocks at even higher \(f_{O_2 } \). The stability of both minerals is constrained in the region of low \(f_{CO_2 } \). The paper reports data on the Fe-Si-O and Mn-Fe-Si-O systems and discusses the possibility of applying the results of experiments in the Mn-Al-Si-O system to the estimation of conditions under which andalusite, spessartine, and galaxite can be formed in Mn-bearing rocks. Data on the mineralogy of numerous Mn deposits metamorphosed under various PTX parameters indicate that the origin of Mn-bearing mineral assemblages depends not so much on the temperature and pressure as on the oxygen fugacity, which is, in turn, controlled primarily by the composition of the pristine sediments (the presence or absence of organic matter in them) and host rocks and depends on the permeability of the rocks to oxygen, the P-T conditions, and the duration of the metamorphic processes.     

Petrogenesis of Na-rich paralava formed by methane flares associated with mud volcanism, Altyn-Emel National Park, Kazakhstan

High-Na slag-like rocks (paralava) with 4.5–11 % Na₂O from the Altyn-Emel mud volcanic field, Kazakhstan, are the products of melting of sediment + salt mixtures by methane flares associated with mud extrusion. The main minerals of the paralavas are diopside and wollastonite which have quench morphologies. Other high-temperature phases (crystallizing from melt and vapour phase) are tridymite, cristobalite, chlorapatite, alkali feldspar, pyrrhotite, native iron and silicon, iron phosphides, titanite, rutile, and carbon. The paralavas lack the Na–Ca silicates devitrite and combeite, but have high-Na and Na–K glasses that have not been homogenized despite low viscosities of <10^?3.5 Pa s. The large number of ignition foci in the Altyn-Emel mud volcano field indicates gas venting from small, shallow reservoirs. The methane flares are inferred to have been small and the fire events short-lived. Fires were extinguished once overpressure released during eruption, methane venting stopped and melted rocks rapidly quenched. The periodicity of eruptions and methane flaring most likely depends on the recurrence of earthquakes (M < 5) which are frequent in this tectonically active area.     

Equilibriums between Cu,Fe, and Zn sulfides and oxides in chloride solution: A thermodynamic study

The results of thermodynamic modeling of equilibriums between Cu, Fe, and Zn sulfides and oxides pertaining to the Cu-Fe-Zn-S-O₂ system in water and aqueous chloride solution are presented. The system comprises solid phases of constant composition: pyrite, pyrrhotite, hematite, magnetite, wüstite, γ-iron, chalcocite, covellite, cuprite, native copper, chalcopyrite, and bornite, as well as more than 100 ions, complexes, and molecules in an aqueous solution. The GIBBS program with the UNITHERM thermodynamic dataset used in calculations allows numerical analysis of phase assemblages in a dry system and in equilibrium with an aqueous solution. How the temperature, pressure, and the composition of the solution in the system opened for oxygen and sulfur affects the composition of phase assemblages was considered in temperature and pressure ranges of 50–350 C and 100–1000 bar, respectively. Decrease in temperature leads to a shift in stability fields of the studied phases toward the region of elevated oxygen and sulfur partial pressures. Variation of temperature is an important factor affecting precipitation of ore minerals, primarily, Cu- and Zn-bearing. The calculation results are presented in tables and diagrams. Each point in the $ (\log m_{S_{tot} } - \log f_{O_2 } ) $ (\log m_{S_{tot} } - \log f_{O_2 } ) diagram is characterized by a single possible assemblage of phases equilibrated with a solution of the given composition within the considered temperature and pressure range. Since the composition of the mineral assemblage is controlled by physicochemical conditions at the moment of mineral formation, comparison of the calculation results with mineral assemblages at ore deposits makes it possible to estimate the parameters of ore deposition at the early stage of investigation, including oxygen and sulfur activity and, occasionally, the composition and salinity of the solution. These parameters control the formation of such assemblages.     

Kinetics of diffusion-controlled growth of fayalite

The rate of growth of fayalite (Fe₂SiO₄) has been measured at one atmosphere total pressure, temperatures from 1000° to 1120° C, and oxygen fugacities controlled by CO/CO₂ gas-mixing from 10^-9.9 to 10^-13.0atm, chosen to span the fayalite stability field. The fine-grained polycrystalline fayalite layer was formed by reacting the oxides FeO or Fe₃O₄ with a thin slice of single-crystal quartz. The rate of growth of the fayalite increases with increasing temperature and decreasing oxygen fugacity, and is consistent with a parabolic rate law, indicating that the growth rate is controlled by diffusion through the fayalite. Microstructural observations and platinum marker experiments suggest that the reaction phase is formed at the quartz-fayalite interface, and is therefore controlled by the diffusion of iron and oxygen. The parabolic rate constant was analyzed in terms of the oxide activity gradient to yield mean chemical diffusivities for the rate-limiting ionic species, assuming bulk transport through the fayalite layer. Given that iron diffusion in olivine polycrystals occurs either by lattice diffusion, which shows a positive dependence on oxygen activity, or by grain boundary diffusion, which would result in growth rates significantly faster than we observe, we conclude that the diffusivities derived in this study represent oxygen diffusion. However, since oxygen lattice diffusion in fayalite has been established to be much slower than our measurements, it is likely that the transport path for oxygen is along the grain boundaries. Thus, the mean grain boundary diffusivity of oxygen in fayalite $\bar D$ _O ^gb (m² s^-1), using the measured grain size of 0.25 μm, is then given by $$\bar D_O^{gb} {\mathbf{ }}\delta = 1.28 \times 10^{ - 3} f_{O_2 }^{ - 0.17} {\mathbf{ }}e^{ - 540/RT} $$ , where δ is the grain boundary width (in m), and the activation energy is in kJ/mol. Assuming δ=10^-9 m (Ricoult and Kohlstedt 1983), the oxygen grain boundary diffusivities are about a factor of 30 × slower than those reported by Watson (1986) for Fo₉₀ olivine.     

The graphite-COH fluid equilibrium in P,T, f_{O_2 } space

The oxygen fugacity ( ) of a C-O-H fluid in equilibrium with graphite has been determined in the range 10–30 kbar by equilibrating solid -buffer assemblages in graphite capsules containing C-O-H fluid. By using different buffers (Fe_xO-Fe₃O₄, Ni-NiO, Co-CoO, Mo-MoO₂), the of the graphite-saturated fluid is bracketed within a narrow range. This technique produces a calibration for the imposed on a sample contained within a graphite capsule. To achieve a thermodynamically-invariant system at fixed P and T, the was imposed on the system with an external buffer and the double-capsule technique. The experiments were performed in solid-media, high pressure apparatus with 19 mm tale-pyrex assemblies. A series of experiments at 10, 15, 20, 25, and 30 kbar, 800–1600° C, with imposed by the Fe₂O₃-Fe₃O₄-H₂O equilibrium were conducted. The experimental results have been fitted to the following equation:

Petrology of the early Proterozoic platinum-bearing massif of Fedorov Tundra,Kola Peninsula

The massif of Fedorov Tundra was formed as part of the Paleoproterozoic (2.5 Ga) Fedorov-Pana platinum-bearing layered complex as a result of consecutive emplacement of two intrusive phases. The emplacement of the first phase resulted in the formation of a large layered intrusive body composed of amphibole gabbro, gabbronorite, norite, pyroxenite, olivine pyroxenite, and harzburgite. The second phase gave birth to a gabbronorite intrusion smaller in volume and enriched in sulfides and PGM. Magmatic breccia has been observed in the contact zone between two phases. The rocks of the massif are referred to the series of normal alkalinity and to the quartz- and olivine-normative groups differing in saturation with silica. Using isoplethic and isobaric joins of the q-fo-fa-di-hd-ab-an-aq phase diagram, the stages of rock formation are considered. The thermodynamic conditions of rock crystallization were determined as T = 1000−800°C and $ P_{H_2 O} $ P_{H_2 O} = 1000−2500 bar for the first intrusive phase and T = 1000–900°C and $ P_{H_2 O} $ P_{H_2 O} = 800−1000 bar for the second intrusive phase.     

Neotectonics of the Deryugin Basin area (Sea of Okhotsk)

The data of the bottom “summit” surface were used for compiling the schematic structural-neotectonic map and map of the main neotectonic structural elements. Their comparison with the schematic paleogeographic maps of the lithophysical complexes for four periods (K₂-$ _{1 - 2} $ _{1 - 2} , $ \rlap{--} P_3 $ \rlap{--} P_3 -N₁¹, N₁^1–2, and N₁³-N₂) reveals that the largest part of the considered area was characterized by either a continental or relatively shallow-sea environment, except for the western areas occupied at that time by the relatively deep trough with its axis located substantially westward of the neotectonic Deryugin Basin and the Staritskii Trough. In the Late Pliocene, the deep paleotrough ($ \rlap{--} P_3 $ \rlap{--} P_3 -N₂²) and Deryugin Basin were likely occupied by shelf settings with continuing sedimentation. The paleogeographic environments of the area for the period from the terminal Pliocene to the late Riss (Taz) Glaciation (Q₂⁶; MIS6) are unknown so far. The most complete Quaternary section recovered by Core LV 28-34-2 consists of six units; the odd (1, 3, and 5) and even (2, 4, and 6) among them correspond to the warm and cold marine isotopic stages, respectively. Judging from the benthic foraminiferal assemblages, the water depths during cold periods were shallower as compared with the warm stages, which is explained by the respective ascending and descending bottom movements and, partially, by the eustatic sea level fluctuations. In the Late Pleistocene-Holocene (∼17 ka), the bottom of the Deryugin Basin and the summit part of the Institut Okeanologii Rise subsided with average rates of 8 and 3 cm/year, respectively.     

中国北方烧变岩的分布、特征及研究意义*

中国北方烧变岩分布广泛,见于以昆仑山—秦岭—大别山为界的中国北方地区多个沉积盆地边缘,但地质学家对其研究较为薄弱。野外调查显示中国北方烧变岩主要以4种方式分布: (1)大面积连片分布,(2)沿山脉走向的线状分布,(3)沿河流下切河谷分布,(4)为第四系黄土覆盖。烧变岩主要为侏罗系、石炭—二叠系煤层自燃所致。各类型的烧变岩在中国烧变岩区不同程度发育,具瓷化结构、白化结构、烧熔结构、残余构造、气孔构造、微柱状节理、角砾状构造、垮塌构造、裂缝充填等结构和构造。其颜色以砖红色、赭红色、棕色、钢灰色最为典型,岩石中发育鳞石英、方石英、堇青石、铁堇青石、莫来石等典型高温矿物。中新世以来,中国西部、北部地区构造活动强烈,干旱气候加剧,烧变岩也主要形成于这一时期,但煤层自燃与构造运动、环境变化、古野火事件的耦合关系还鲜有提及,其所蕴含的各种地质信息值得在今后的研究中予以关注。     

陕北烧变岩水-岩作用的劣化特性

鄂尔多斯盆地东北缘陕晋蒙交界地区存在大量煤火高温烘烤形成的烧变岩。与原岩相比,烧变后的岩体结构破碎孔隙发育,在长期水–岩作用下,形成了大量危岩体。以陕北神木大柳塔地区的烧变岩为对象,开展了50次干湿循环试验,测试不同循环次数后烧变岩物理性质与力学性质及其变化,探讨不同烧变程度烧变岩劣化特性。结果表明：烧变岩质量、硬度、力学性质下降,表面亮度降低,粗糙度与红黄色度增强;基于低场核磁共振技术(NMR)测试,烧变岩孔隙体积比例曲线右移,最大孔径扩张;微孔向大孔转化,大孔含量与总孔隙率升高;烧变作用使岩体孔隙发育,且提高了烧变岩的抗侵蚀能力,降低了色度、质量、粗糙度、硬度、力学性质的变化程度。长期水–岩作用可对烧变岩的物理力学性质与孔隙结构造成明显改变,烧变程度与初始孔隙结构能够影响劣化效果。根据研究不同烧变岩的劣化过程,得出加固低烧变程度岩体表面、加固沉积层理与原有裂隙、堵塞高烧变程度岩体孔隙,可降低水-岩作用的侵蚀破坏强度。     

A New Approach to 40Ar/39Ar Dating of Combustion Events: A Case Study from the Late Pleistocene Coal Fires in Goose Lake Depression (Transbaikalia)

Doklady Earth Sciences - Based on the study results of combustion metamorphic (CM) rocks from the young (0.01–0.06 kyr) CM complex at the Goose Lake brown coal deposit, a new approach to...     

Modal mineralogy of CV3 chondrites by X-ray diffraction (PSD-XRD)

Using position sensitive detector X-ray diffraction (PSD-XRD) we determine a complete modal mineralogy for all phases present in abundances greater than 1 wt% in Vigarano, Efremovka, Mokoia, Grosnaja, Kaba and Allende. Reduced CV3 samples are comprised of (vol%): olivine (83-85%); enstatite (6.5-8.1%); anorthite (1.1-1.2%); magnetite (1.4-1.8%); sulphide (2.4-5.1%); Fe, Ni metal (2-2.2%). The oxidized samples are comprised of: olivine (76.3-83.9%); enstatite (4.8-7.8%); anorthite (1.1-1.7%); magnetite (0.3-6.1%); sulphide (2.9-8.1%); Fe, Ni metal (0.2-1.1%); Fe-oxide (0-2.7%) and phyllosilicate (1.9-4.2%). When our modal data is used to calculate a bulk chemistry that is compared to literature data a near 1:1 correlation is observed. PSD-XRD data indicates that olivine compositions may span almost the entire Fe-Mg solid solution series in all CV samples and that these contain a component (4-13%) of fine-grained olivine that is more Fe-rich (>Fa₆₀) than is typically reported. Modal mineralogy shows that there are mineralogic differences between CV3 samples classified as oxidized and reduced but that these sub-classes are most clearly distinguished by the relative abundance of metal and Ni content of sulphide, rather than abundance of magnetite. The most significant difference in modal mineralogy observed is the relative absence of phyllosilicate in reduced CV that essentially escaped aqueous alteration.Fayalite, ferrous olivine and magnetite are typically considered secondary alteration products. The abundances of these minerals overlap in oxidized and reduced samples and correlate positively supporting common conditions of formation in a relatively oxidizing environment. The abundances of fayalite, ferrous olivine and magnetite show no relationship to petrographic type and if these abundances were used as a proxy for alteration, Allende would be the least altered CV - contrary to all previous data. The implication is that thermal metamorphism on the parent body was de-coupled from formation of Fe-rich secondary minerals. Low temperature fluid-assisted metamorphism can also not easily explain the origin of fayalite, ferrous olivine and magnetite, since the reduced CVs appear to be largely unaffected by this process. Parent body models require an anhydrous low-temperature mechanism of secondary alteration. The alternative is that these phases formed prior to accretion of the final CV parent body.