Granulite facies metamorphism and metasomatism in the gabbro-anorthosites of the Kolvitsa massif,Kola Peninsula

The investigation of the Kolvitsa gabbro-anorthosite massif showed that its melanocratic layers conformable with metamorphic banding are mafic differentiates transformed into eclogite-like rocks during prograde metamorphism. During the peak and retrograde stage of the Svecofennian metamorphism in the White Sea region at t = 910–750°C and P = 14-7 kbar, the infiltration of Fe-, CO₂-, Si-, and Na-bearing fluids with X_{H2 O} < 0.4X_{H_2 O} < 0.4 resulted in metasomatic alterations of the melanocratic gabbro-anorthosite interlayers, dissolution of a number of elements, and their reprecipitation with the formation of cross-cutting zoned metasomatic veins with abundant magnetite and ilmenite. The high content of hematite in the ilmenite suggests that the veins were formed at an increase in oxygen fugacity from logf_O2 = - 14.5\log f_{O_2 } = - 14.5 to logf_O2 = - 11\log f_{O_2 } = - 11, which caused the Fe²⁺ → Fe³⁺ transition and iron precipitation. The increase in at the conditions corresponding to the metamorphic peak was probably related to the neutralization of solutions during their infiltration through the gabbro-anorthosites. The reprecipitation of components and the formation of cross-cutting veins occurred owing to interaction between the melanocratic layers in the gabbro-anorthosites and a fluid phase and, contrary to previous models, did not involve the fluid transport of components from the zones of charnockite formation and granitization located far away from the sites of reprecipitation. This is demonstrated by the similarity of mineral compositions and major component contents in the melanocratic gabbro-anorthosite layers and cross-cutting metasomatic veins and regular distribution of trace elements.     

Oxygen potential of diamond formation in the lower mantle

Thermodynamic calculations have shown that when a metallic phase arising due to ferroan ion disproportionation is contained in lower-mantle rocks, carbon occurs as iron carbide and the oxygen fugacity corresponds to the equilibrium of ferropericlase with Fe-Ni alloy. The typical values of oxygen fugacity in zones of diamond formation in the lower mantle lie between the iron-wüstite buffer and six logarithmic units above this level. The processes that proceed in the lower mantle give rise to variation of $f_{O_2 }$ within several orders of magnitude above the elevated $f_{O_2 }$ values, which are necessary for the formation of diamond, as compared with a common level typical of the lower mantle. The mechanisms responsible for redox differentiation in the lower mantle comprise the subduction of oxidized crustal material, mechanical separation of metallic phase and silicate-oxide mineral assemblage enriched in ferric ions, as well as transfer of fused silicate material presumably enriched in Fe³⁺ through the mantle.     

Garnet inclusions in diamond and the state of the upper mantle

Temperature and pressure estimates for Earth's upper mantle generally are based on indirect information derived from phase equilibria studies and the measurement of temperature and pressure dependent physical and chemical properties for relevant mantle materials. This paper describes an alternative approach, based on solid-inclusion piezothermometry, which utilizes the thermoelastic properties of direct mantle derived mineral samples. In particular, this study provides the theoretical development, based on the Murnaghan equation of state for solids, for a simple method of calculating isomeke lines for host and inclusion minerals of cubic symmetry which may be extrapolated accurately to upper mantle pressure and temperature conditions. The method is demonstrated for the particular case of garnet inclusions in diamond, for which adequate laboratory thermoelastic data are available. A specific application is made in the evaluation of the depth of formation of the D1 garnet-diamond inclusion system described by Harris et al. (1970). The pressure and temperature conditions of inclusion formation lie along the calculated isomeke line within the range constrained by recent graphite-diamond phase equilibria data. However, because the isomeke line for the garnet-diamond system and the graphite-diamond phase transition are very similar in slope, a further constraint is required. Assuming, therefore, that temperature in the upper mantle is bounded by the “Oceanic” and “Shield” geotherms of Clark and Ringwood (1964), the present results indicate that the D1 garnet-diamond system formed within the depth range 138 to 155 km (about 45 to 53 kbar pressure). This result, which relates to the genesis of kimberlite xenoliths, is generally consistent with the results of other studies which utilize phase equilibria data.     

Water in the structure of minerals from mantle peridotites as controlled bythermal and redox conditions in the upper mantle

Hydrous species and the amount of water (OH^? ions and crystal hydrate H₂O) in structures of nominally anhydrous rock-forming minerals (olivine, ortho- and clinopyroxenes) were studied with Fourier spectroscopy in peridotite nodules (19 samples) from Cenozoic alkali basalts of the Baikal-Mongolia region (Dariganga Plateau, Taryat Depression, and Vitim Plateau). Single-crystal samples oriented relative to the crystallographic axes of minerals were examined with an FTIR spectrometer equipped with an IR microscope at the points of platelets free from fluid inclusions. FTIR spectra were measured in regions of stretching vibrations of OH^? and H₂O (3800–3000 cm^?1) and deformation vibrations of H₂O (1850–1450 cm^?1). The water content in mineral structures was determined from integral intensities. To estimate the conditions of entrapment and loss of structural water in minerals, their chemical composition, including Fe²⁺ and Fe³⁺ contents, was determined with an electron microprobe analysis and Mössbauer spectroscopy. The bulk chemical composition of some nodules was determined with XRF and ICP MS. The total water content (OH^? + H₂O) varies from 150 to 1140 ppm in olivines, from 45 to 870 ppm in clinopyroxenes, and from 40 to 1100 ppm in orthopyroxenes. Both water species in the mineral structures are retained down to a depth of 150–160 km in wide temperature and pressure ranges (1100–1500 °C, 32–47 kbar) at the oxygen fugacity of ?1.4 to ?0.1 log units relative to that of the quartz-fayalite-magnetite buffer.     

Mineralogy of PGE in the mafic-ultramafic massifs of the Kola region

Summary During the last few years nearly 50 minerals and unnamed platinum-group element (PGE) phases have been identified by the authors in nickel-bearing mafic-ultramafic massifs of the Kola region. About 20 established PGE minerals have been found whilst more than 20 previously undescribed and unnamed PGE phases can be reported. In addition high concentrations of these elements in sulfarsenides of the cobaltitegersdorffite series have been established. The PGE phases form tiny (5–70 µm, seldom larger) mono- and polymineral inclusions in the main sulfides of the nickel-copper mineralization; at the contacts with silicates and within the silicates themselves. The details of the composition of platinum mineralization in various massifs, its formation under magmatic and hydrothermal conditions and wide isomorphism in certain groups of minerals have been established. Specific features of PGE mineralization of the region as a whole are: the predominance of bismuth tellurides and arsenides of palladium and platinum, a highly subordinated role of their sulphides and higher concentrations of several PGE in sulfarsenides.

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Platingruppen-Minerale in den mafischen-ultramafischen Intrusionen der Kola Region

Zusammenfassung In den vergangenen Jahren wurden etwa 50 z.T. noch unbekannte Minerale von Platingruppen-Elementen (PGE) in Ni-führenden mafisch-ultramafischen Gesteinen der Kola Region bestimmt (ca. 20 bereits bakannte und mehr also 20 bisher nicht beschriebene bzw. noch unbenannte PGE-Minerale). Außerdem wurden hohe Konzentrationen dieser Elemente in Sulfarseniden der Cobaltit-Gersdorffit-Reihe nachgewiesen.Die PGE-Minerale kommen als mono- und polymineralische Einschlüsse in den Sulfiden der Ni-Cu-Mineralisation, am Kontakt der Sulfide zu den Silikaten und in den Silikaten selbst vor. Details über die PGE-Mineralisation in verschiedenen Gebieten, ihre Bildung unter magmatischen und hydrothermalen Bedingungen und die Isomorphiebeziehungen innerhalb bestimmter Mineralgruppen werden dargestellt.Spezifische Eigenheiten der PGE-Mineralisation dieser Region sind: Das Vorherrschen von wismutvtelluriden, sowie von Pd- und Pt-Arseniden, die untergeordnete Rolle von Pd- und Pt-Sulfiden und erhöhte Gehalte bestimmter PGE in den Sulfarseniden.

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Contribution to the Fifth International Platinum Symposium, Helsinki, August 1989.

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With 2 Figures     

Fundamentals of the mantle carbonatite concept of diamond genesis

In the mantle carbonatite concept of diamond genesis, the data of a physicochemical experiment and analytical mineralogy of inclusions in diamond conform well and solutions to the following genetic problems are generalized: (1) we substantiate that upper mantle diamond-forming melts have peridotite/eclogite–carbonatite–carbon compositions, melts of the transition zone have (wadsleyite ? ringwoodite)–majorite–stishovite–carbonatite–carbon compositions, and lower mantle melts have periclase/wüstite–bridgmanite–Ca-perovskite–stishovite–carbonatite–carbon compositions; (2) we plot generalized diagrams of diamondforming media illustrating the variable compositions of growth melts of diamonds and paragenetic phases, their genetic relationships with mantle matter, and classification relationships between primary inclusions; (3) we study experimentally equilibrium diagrams of syngenesis of diamonds and primary inclusions characterizing the diamond nucleation and growth conditions and capture of paragenetic and xenogenic minerals; (4) we determine the fractional phase diagrams of syngenesis of diamonds and inclusions illustrating regularities in the ultrabasic–basic evolution and paragenetic transitions in diamond-forming systems of the upper and lower mantle. We obtain evidence for physicochemically similar melt–solution ways of diamond genesis at mantle depths with different mineral compositions.     

基于岩石圈厚度和地幔横向黏度变化的地幔对流模型重估云南地区剪切波各向异性源的深度

在已有模型的基础上,考虑岩石圈厚度和软流层横向黏度的变化,本文建立了更接近地球实际情形的地幔对流模型,然后重新推测了导致云南地区剪切波各向异性的软流层源的深度.结果 表明:岩石圈厚度和软流层横向黏度变化对云南地区的软流层各向异性源的深度及软流层的变形程度和机制具有重要影响;软流层各向异性对云南西南部区域、东部区域北纬2...     

苏鲁大别及其周围地区深部P波速度结构特征的初步分析

本分析了苏鲁大别及其周围地区200km深的地震波速度结构。对上地幔50至200km不同深度的水平切面的速度结构特征进行了详细研究。结果表明,位于大别山地区下的西南侧存在着一个呈西北一东南延展的带状高速区;在苏鲁东海地区的下方,及东北方面也有高速度区域存在。这两个高速度区不相连。在大别山区东北端区与苏鲁超高压变质带的南端地区深部的两个高速区之间,即郯庐断裂的中部存在一个西北-东南延展的低速度区。大别山区下方深部呈现明显的速度梯度变化带。大别山区东端边界也是速度梯度变化带。郯庐断裂的北段,即与苏鲁超高压变质带为邻的部分下方,似乎是一个速度分界带或速度不连续带。郯庐断裂的北段下方的速度结构与其两侧地区的速度结构明显不同。该段的两侧速度结构常常存在着明显差异。     

Fluid regime and diamond formation in the reduced mantle: Experimental constraints

The composition and potential diamond productivity of C–O–H fluids that could exist in the reduced regions of the Earth’s upper mantle and in the mantles of Uranus and Neptune were studied in experiments at 6.3 GPa and 1400–1600 °C and durations of 15–48 h. Hydrogen fugacity in the fluid phase was controlled by the Mo–MoO₂ or Fe–FeO buffers, using a specially modified double-capsule method. The oxygen fugacity in the samples was controlled by adding different amounts of water, stearic acid, anthracene, and docosane to a graphite charge. At high P–T conditions, the degree of decomposition of the heavy hydrocarbons added to the charge was 99.9%. The composition of the fluids coexisting with graphite/diamond in the buffered experiments varied from H₂O  H₂ > CH₄ (at fO₂ somewhat lower than the “water maximum”) to H₂ > CH₄ > (C₂H₄ + C₂H₆)>C₃H₈ (in C–H system). In the C–H system the maximum concentrations of major species in the synthesized fluid were: H₂ = 79 mol.% and CH₄ = 21 mol.%. The composition of the H₂-rich fluids, which were synthesized at 6.3 GPa and 1400–1600 °C for the first time, differs considerably from that of the ultra-reduced CH₄-rich fluids stable at 2.0–3.5 GPa and 1000–1300 °C. Thermodynamic calculations of the reduced C–O–H fluids at the P–T conditions of the experiments revealed CH₄-rich compositions (CH₄  H₂ > (C₂H₄ + C₂H₆)>C₃H₈), which however drastically differed from the synthesized compositions. The rates of diamond nucleation and growth in the experiments depended on the fluid composition. Diamond crystallization had a maximum intensity in the pure aqueous fluids, while in the H₂-rich fluids no diamond formation was observed. Only metastable graphite precipitated from the ultra-reduced fluids. The type of the initial hydrocarbon used for the fluid generation did not affect this process.     

On the composition and origin depth of basaltic magma in the upper mantle

The depths of mantle melting zones can be constrained by forward (in terms of physicochemical thermodynamics) or inverse (in terms of equilibrium thermodynamics) modeling. However, there is discrepancy in this respect between fluid-dynamic models of decompression melting in convecting upper mantle and thermodynamic models of basaltic magma sources beneath mid-ocean ridges. We investigate the causes of the mismatch in melting depth predictions with reference to the magmatic systems of the Basin and Range Province in the western margin of North America. The inverse solutions turn out to represent melts from different substrates (depth facies) in the lithospheric mantle, while modeling decompression melting in convecting fertile upper mantle refers to the depths the faults in spreading zones never reach. The discrepancy between forward and inverse solutions may be due to the fact that the respective depth estimates correspond to different levels of the same mantle–crust magmatic systems.     

苏浙皖地区茅口阶和上二叠统沉积相

苏浙皖地区茅口阶和上二叠统各组普遍穿时。进入茅口期,一改栖霞期广袤平坦的碳酸盐台地面貌,三角洲、浅海和碳酸盐台地分割清楚,来自华夏古陆和鲁东古陆的碎屑和清水的底栖生物成为塑造全区古地形的主要物质。冷坞海绵圆丘礁微相展布显示区域古地理格局,区内孤峰组生物—岩相变化揭示茅口期早时古地理差异。上二叠统陆屑明显减少,全区呈现碳酸盐台地和陆棚两个相带,其切变部位可能是江南断裂确切位置。     

Spatiotemporal relationships of dike magmatism in the Kola region, the Fennoscandian Shield

A brief geological and petrographic characterization of the Early Precambrian dike complexes of the Kola region is given along with data on new estimates of dike age and analysis of their distribution over the entire Fennoscandian Shield. The emplacement of dikes in the Archean core of the shield continued after consolidation of the sialic crust 2.74?C1.76 Ga ago. After the Svecofennian Orogeny, dikes continued to form in the west in the area of newly formed crust, while the amagmatic period began in the Archean domain. The intense formation of dikes in the Svecofennian domain lasted approximately for 1 Ga (1.8?C0.84 Ga). The younger igneous rocks in the crustal domains of different age are less abundant and localized at their margins. A similar distribution of dikes is characteristic of other shields in different continents. This implies that the formation of the sialic crust in the shields is not completed by its consolidation and formation of the craton. For 1 Ga after completion of this process, the crust is underplated by mantle-derived magmas. This process is reflected at the Earth??s surface in the development of mantle-derived mafic and anorogenic granitoid magmatism. The process of crust formation is ended as the subcratonic lithosphere cools and the amagmatic period of the craton history is started. Beginning from this moment, the manifestations of cratonic magmatism were related either to the superposed tectonomagmatic reactivation of the cold craton under the effect of crust formation in the adjacent mobile belts or to the ascent of mantle plumes.     

Non-transparent uppermost mantle in the island-arc region of Japan

In Japan, the crust and uppermost mantle seismic character is yet unimaged although many refraction surveys have been recorded. The longest seismic profiles are analyzed. A remarkable feature, a long-duration coda wave after the PmP wave (reflected wave at the Moho boundary), is observed on the record sections. Several possible models are considered to explain the long-duration coda wave. The model with many scatterers located in the uppermost mantle explains the observed data well while the undulating Moho and continuous layering models do not account for some aspects of the observed data. The scatterer distributed uppermost mantle is not consistent with that of continental region which is often characterized as transparent. We estimate the scattering coefficient of the uppermost mantle and crust using simulations. The scattering coefficients obtained for upper crust, lower crust, and uppermost mantle are 0.01, 0.02, and 0.025, respectively. The scattering coefficient of the uppermost mantle is slightly larger than that of lower crust, which is characterized as being reflective. The many scatterers in the uppermost mantle might be related to magmatism in Japan. This will be one of the important observations for understanding formation processes of the Moho boundary and uppermost mantle in the island-arc environment.     

Fault rocks as indicators of progressive shear deformation in the Guingamp region, Brittany

The North Armorican Shear Zone is a major structural feature running from the island of Moiene in the west to Moncontour in the east of the Armorican Massif. In the region of Guingamp it cuts through a Precambrian migmatite complex and granitoid rocks of both Precambrian and Hercynian age. A variety of fault rocks are present in this part of the shear zone, and are thought to represent a time sequence in which deep level, ductile deformation gave way to higher level brittle displacements. Mylonite and cataclasite series rocks, and pseudotachylites are described and their conditions of formation considered. The Hercynian Quintin granite post-dates the main movement of the shear zone but is itself dextrally displaced during the late stages of shear movement.     

Zeolite minerals in the Jurassic dolerites of Tasmania: Their use as possible indicators of burial depth

Secondary zeolites in the Jurassic dolerites of Tasmania are re‐interpreted as burial assemblages reflecting stages of unloading of the Jurassic land surface.

Prehnite and laumontite occurrences suggest former burial depths exceeding 1600 to 2200 m. Scolecite and chabazite, described in detail, suggest burial depths between 800 to 1600 m about the end of Cretaceous time. These depths give an average erosional lowering of about 10–15 m/m.y. for the Tasmanian land surface over the last 165 m.y.     

The physicochemical conditions of diamond formation in the mantle matter: experimental studies

Experimental studies of diamond formation in the alkaline silicate-carbon system Na₂O–K₂O–MgO–CaO–Al₂O₃–SiO₂–C were carried out at 8.5 GPa. In accordance with the diamond nucleation criterion, a high diamond generation efficiency (spontaneous mass diamond crystallization) has been confirmed for the melts of the system Na₂SiO₃–carbon and has been first established for the melts of the systems CaSiO₃–carbon and (NaAlSi₃O₈)₈₀(Na₂SiO₃)₂₀–carbon. It is shown that in completely miscible carbonate-silicate melts oversaturated with dissolved diamond-related carbon, a concentration barrier of diamond nucleation (CBDN) arises at a particular ratio of carbonate and silicate components. Study of different systems (eclogite–K-Na-Mg-Ca-Fe-carbonatite–carbon, albite–K₂CO₃–carbon, etc.) has revealed a dependence of the barrier position on the chemical composition of the system and the inhibiting effect of silicate components on the nucleation density and rate of diamond crystal growth. In multicomponent eclogite-carbonatite solvent, the CBDN is within the range of carbonatite compositions (<50 wt.% silicates). Based on the experimental criterion for the syngenesis of diamond and growth inclusions in them, we studied the syngenesis diagram for the system melanocratic carbonatite–diamond and determined a set of the composition fields and physical parameters of the system that are responsible for the cogeneration of diamond and various mineral and melt parageneses. The experimental results were applied to substantiate a new physicochemical concept of carbonate-silicate (carbonatite) growth media for most of natural diamonds and to elaborate a genetic classification of growth mineral, melt, and fluid inclusions in natural diamonds of mantle genesis.