Modeling of zoning patterns in garnet: Thermodynamic and kinetic aspects

Results from the modeling of compositional zoning patterns in garnet porphyroblasts from the medium-grade metapelitic schist of northern Ladoga area are considered. The P-T pseudosections in the model KMnFMASH system were calculated for this purpose using THERMOCALC software (Powell et al., 1998). Particular emphasis is placed upon the effect of garnet growth kinetics on the model zoning profiles for Mn (Gulbin, 2013). They fit the observed profiles if intergranular diffusion-controlled growth is assumed for porphyroblasts. Additionally, a model of metamorphic fractional crystallization is used to characterize the oscillations in both the garnet core and rim. Starting from the assumption that a reservoir, where garnet grows, consists of chlorite, and that this mineral is intensely replaced with biotite and staurolite at the onset of crystallization, a partial release of Mn from the chlorite structure and the concentration of this component in intergranular space is inferred. In terms of the model under consideration, the coefficient of the Mn partition between garnet and reservoir temporarily increases at the early stage of garnet growth, giving rise to the enrichment of the intermediate zone of porphyroblasts in Mn. In addition to the modeling of garnet growth zoning, its subsequent diffusion modification is estimated on the basis of intracrystalline diffusion profile simulation. The reverse zoned, Mn-rich and Mg-poor garnet rims are related to retrograde growth of garnet at the late stage of porphyroblast formation. The data obtained are used to constrain metamorphic evolution and the P-T-t path of staurolite-bearing rocks in the northern domain of the studied area.     

Occurrence of Precambrian bauxites

Discovery of metamorphically altered diaspore bauxite containing 55.43 to 63.13 percent alumina in Patom Highlands of Siberia is believed to be an indication that Precambrian formations can be considered as a potential source of bauxite. — E. A. Alexandrov.     

Problem of lateritic bauxites of Siberia

On paleoclimatic-geological premises and by available indications in the field, prospecting for bauxites of the platformal type is recommended in certain Siberian districts in which conditions were favorable for bauxitization at the end of the Triassic, in the Barremian-Aptian, the Turonian, and the Eocene. -- V.P. Sokoloff.     

Thermodynamic and kinetic control on anaerobic oxidation of methane in marine sediments

The free energy yield of microbial respiration reactions in anaerobic marine sediments must be sufficient to be conserved as biologically usable energy in the form of ATP. Anaerobic oxidation of methane (AOM) coupled to sulfate reduction (SRR) has a very low standard free energy yield of ΔG^° = −33 kJ mol⁻¹, but the in situ energy yield strongly depends on the concentrations of substrates and products in the pore water of the sediment. In this work ΔG for the AOM-SRR process was calculated from the pore water concentrations of methane, sulfate, sulfide, and dissolved inorganic carbon (DIC) in sediment cores from different sites of the European continental margin in order to determine the influence of thermodynamic regulation on the activity and distribution of microorganisms mediating AOM-SRR. In the zone of methane and sulfate coexistence, the methane-sulfate transition zone (SMTZ), the energy yield was rarely less than −20 kJ mol⁻¹ and was mostly rather constant throughout this zone. The kinetic drive was highest at the lower part of the SMTZ, matching the occurrence of maximum AOM rates. The results show that the location of maximum AOM rates is determined by a combination of thermodynamic and kinetic drive, whereas the rate activity mainly depends on kinetic regulation.     

Chemical aspects of quick-clay formation

In connection with the systematical examination of the slope stability in the Go¨ta River Valley in Sweden it was impossible to get a satisfactory explanation of the cause of the high sensitivity of quick clays which could be used in the continued examination as a basis for geotechnical investigations.

It was concluded that the quick-clay problem must be regarded as a problem of colloid chemical nature. Many of the results obtained pointed out the fact that the surface properties of the grains will play an important role, and in this connection the problem with the possible existence of surface-active agents (dispersing agents) in natural quick clays has been studied.

Systematic isolations of dispersing agents have been made by different methods, and the subsequent extracts have been analysed chromatographically. It is found to be easy to isolate dispersing substances by most of the methods tested, but it is very difficult to find any substances with a known composition. Instead, this part of the examination will approach the humic problem. In one case it has been possible to break up the complexes into individual components of known composition, i.e., in the case of fatty acids and soaps. Quick clay contains homologuous series of normal saturated fatty acids and for this reason soaps seem to be components which contribute to the quick-clay formation.     

Investigation of bauxites with the help of electron-probe

Summary 25 representative karstic and lateritic bauxite samples from Europe, Asia and Africa were studied by a JEOL-JXA-5 electron-probe X-ray microanalyser. The main directions of the investigation were: determination of the microtextures and the distribution of the main elements in bauxite (Al, Fe, Ti, Si). Conclusions on the genesis of the textural constitution and on bauxitization are given. Further possibilities of using the electron-probe in solving different problems connected with bauxites are enumerated.

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Bauxit-Untersuchungen mit der Elektronstrahl-Mikrosonde

Zusammenfassung Es wurden 25 typische Karst- und Lateritbauxite aus Europa, Asien und Afrika mit Hilfe einer JEOL-JXA-5 Elektronstrahl-Mikrosonde untersucht. Die Hauptrichtungen der Untersuchung waren die Bestimmung der Mikrostruktur und die Verteilung der Hauptelemente in den Bauxiten (Al, Fe, Ti, Si). Es wurden mehrere Schlüsse bezüglich der Entwicklung der Strukturelemente und der Bauxitbildung gezogen. Weitere Anwendungs-möglichkeiten zur Lösung verschiedener Bauxit-Probleme werden angegeben.

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With 30 Figures     

Beneficiation of ferruginous bauxites by high-gradient magnetic separation

The possibility of increasing the concentration of gibbsite in ferruginous bauxite by means of high-gradient magnetic separation (HGMS) has been studied. The effects of particle size, type of matrix, magnetic field and flow rate were tested. Enrichments of up to 13% were obtained under the experimental conditions used. The importance of selection of optimum operating conditions is evidenced by the results and empirical equations for data correlation are suggested.     

试论遵义高铁铝土矿与低铁铝土矿的分带性及形成机制

遵义铝土矿带早石炭世沉积型铝土矿及其含铝岩系,皆沉积-堆积在紫云运动(晚泥盆世至早石炭世)时期形成的略微向南倾斜的夷平面上,其中的高铁铝土矿与低铁铝土矿具有明显分带性.铝土矿及其含矿岩系的原始物质,主要是由下奥陶统湄潭组伊利石页岩形成的含三水铝石的红土风化壳.铝土矿的分带性与红土风化壳物质搬运的距离有关,近处、高处为高铁铝土矿,远处、低处为低铁铝土矿.总体来看,搬运距离较短,属近源搬运、沉积-堆积而成的.     

矿物之间的元素和同位素平衡:地质测温和等时线定年的热力学和动力学控制

在特定的地质事件过程中,矿物等时线放射体系是否达到并且保持了平衡是变质岩Sm-Nd和Rb-Sr同位素年代学中的一个重要问题。在这个问题上矿物对O同位素测温与矿物等时线定年相似,因此两者之间可以相互制约。在岩浆岩和变质岩中,矿物中Sm-Nd、Sr和O之间的扩散速率在无水的条件下一般具有可比性,因此矿物之间O同位素的平衡状态可以用来对Sm-Nd和Rb-Sr定年的有效性进行检验。对大别-苏鲁造山带超高压变质岩的Sm-Nd和Rb-Sr等时线矿物进行O同位素测温,得到Sm-Nd等时线有时给出三叠纪年龄,有时给出非三叠纪年龄;对应的矿物O同位素分馏分别处于平衡和不平衡状态。对于引起非三叠纪等时线年龄的原因,一方面可以是由于榴辉岩相变质过程中同位素体系没有达到平衡,另一方面则可能角闪岩相退变质作用打破了平衡。等时线矿物中初始同位素比值的均一化速率主要受慢扩散矿物的影响,而矿物等时线时钟的启动主要受高母/子比值矿物控制。因此在变质作用过程中,只有当高母/子比值矿物同时具有快的放射成因同位素扩散速率,才可能得到有效的矿物等时线来用于变质年龄的测定。根据不同矿物中不同元素在扩散速率上的差异,能够定量估计大陆碰撞过程中榴辉岩相变质的持续时间。应用增量方法和离子孔隙度经验模型,不仅分别能够从理论上准确计算所有固体矿物的氧同位素分馏系数和获得不同矿物中元素的扩散参数,而且分别能够定量预测热力学平衡条件下共生矿物之间的¹⁸O富集顺序和相同条件下矿物中元素扩散速率的相对快慢。     

The thermal reaction of Venezuelan lateritic bauxites with glycerol

Goethite contained in lateritic bauxites or in artificial mineral admixtures is completely transformed into peculiar Fe-alkoxide compounds by reaction with glycerol at 245°C. Magnetic spinel oxides result from hydrolysis by boiling water of alkoxides obtained from the artificial admixtures. On the other hand, the production of magnetic spinel materials is not observed in alkoxides derived from natural lateritic samples. This may be caused by the high amount of Al substituting for Fe in these Venezuelan lateritic goethites.     

Some aspects of ore formation in ultramafics

The relationship between petrogenesis and ore formation during the development of ultrabasics is discussed. It is emphasized that it is different for deep-seated pyrope peridotite and shallower spinel-enstatite peridotite facies. The influence of depth (pressure) on the composition of fluids which accompany ultrabasic magmatism is considered. It is inferred that the pressure is responsible for H₂O and CO₂ prevailing in fluid inclusions in diamonds. Regular compositional correlations are established for chrome-spinellids (chromites) and rock-forming chrome-spinellids, as well as general correlations between the compositions of chromite and sulphide mineralization and surrounding ultrabasics. These correlations indicate that liquid immiscibility was responsible for the separation of ore-forming magmas from ultramafic silicate melts of various compositions. The general specificity of orecontent in ultramafic complexes is explained by means of the hypothesis on basic-ultrabasic immiscibility in magmas under the influence of fluid components, with hydrogen and hydrogen sulphide playing an important role. Rhythmic layering in massifs is explained by repeated separation of the developed magmatic layers as well as by differentiation crystallization proceeding under the effect of temperature and magmatic composition gradients. Rhythmic layering reflects the departure of the magmatic systems from equilibrium on fast cooling, much in the same way as zoning in crystals.     

Thermodynamic characterization of boltwoodite and uranophane: Enthalpy of formation and aqueous solubility

Boltwoodite and uranophane are uranyl silicates common in oxidized zones of uranium ore deposits. An understanding of processes that impact uranium transport in the environment, especially pertaining to the distribution of uranium between solid phases and aqueous solutions, ultimately requires determination of thermodynamic parameters for such crystalline materials. We measured formation enthalpies of synthetic boltwoodites, K(UO₂)(HSiO₄)·H₂O and Na(UO₂)(HSiO₄)·H₂O, and uranophane, Ca(UO₂)₂(HSiO₄)₂·5H₂O, by high temperature oxide melt solution calorimetry. We also studied the aqueous solubility of these phases from both saturated and undersaturated conditions at a variety of pH. The combined data permit the determination of standard enthalpies, entropies and Gibbs free energies of formation for each phase and analysis of its potential geological impact from a thermodynamic point of view.     

Thermodynamic constraints on the formation conditions of winonaites and silicate-bearing IAB irons

Silicate inclusions in IAB irons and related winonaite meteorites have textures, mineralogies and mineral chemistries that indicate a complex formation history of heating, followed by brecciation and metamorphism. Using olivine-orthopyroxene-chromite assemblages in five IAB iron silicate inclusions (Caddo County, Campo del Cielo, Copiapo, Lueders, and Udei Station) and one winonaite (Winona), we calculated closure temperatures and oxygen fugacities for these meteorites. Calculated olivine-chromite Fe-Mg exchange temperatures are compared to two-pyroxene temperatures. Olivine-chromite closure temperatures range from ∼590°C to ∼700°C, while two-pyroxene temperatures range from ∼900°C to ∼1200°C. Oxygen fugacities of these meteorites, determined for the first time in this study, range from 2.3 to 3.2 log units below the Fe-FeO buffer and define a line between the Fe-FeO and Cr-Cr₂O₃ buffers. Highly variable temperatures were experienced by these rocks on the hand sample, and sometimes even the thin section, scale consistent with the idea that the winonaite-IAB iron parent body experienced collisional fragmentation and reassembly after peak temperatures were reached. Although modest reduction likely occurred during cooling, the oxygen fugacities and mineral compositions recorded at peak metamorphic temperatures suggest that the chondritic precursor for this parent body was initially more reduced than ordinary chondrites.     

Chemical equilibrium and kinetic constraints for chondrule and CAI formation conditions

While many uncertainties remain, a kinetic evaporation-condensation model is used to show that type A chondrules, and compact Type A and B calcium-aluminum-rich inclusions (CAIs) could have formed from CI-like precursors under conditions that are consistent with predictions for 2-3 AU in a canonical solar nebula. Type B and Al-rich chondrules, and Type C CAIs, on the other hand, may have formed from fractionated precursors. Based primarily on chondrule and CAI isotopic compositions, previous studies have reached different conclusions because they did not take into account the effects of gas-melt exchange.Assuming CI-like precursor compositions, equilibrium silicate melts with elemental compositions like those of type A chondrules could have formed over a wide range of conditions (T, P_tot, solid/gas/solar). Metal is not predicted to be stable when T ≥ 1600°C. When T < 1600°C, the abundances and compositions of metal in chondrules appear to be less successfully reproduced than the silicates, e.g., at a given temperature more metal is predicted in type II chondrules than is generally observed, and under some conditions type IIs are predicted to be more metal-rich than type Is. These differences could be overcome if type Is formed from precursors that were more reduced than CI, and if type IIs formed after significant metal-silicate fractionation.The formation conditions of molten CAIs are much more restricted than for chondrules, perhaps in part explaining their lower abundances. The Mg, Si and O isotopic mass fractionations in non-FUN CAIs can be reproduced if they formed between ∼1400 to 1500°C in regions where CAI-like equilibrium melts were stable, but they did not quite reach equilibrium with the gas. CAI formation times at P_tot = 10⁻⁴-10⁻³ bars are consistent with estimates of Type B CAI cooling times, but pressures much below this require formation times that are too long. The isotopic mass fractionations in FUN CAIs can be explained if they formed at or below the ranges of solid/gas/solar ratios where CAI-like equilibrium compositions are stable. Under these conditions, FUN inclusions undergo less gas-melt exchange than non-FUN CAIs. The FUN CAI formation temperatures are consistent with formation at 1400 to 1500°C, but may have been higher.Two general explanations for the distribution of O mass independent fractionations (MIF) in chondrules/CAIs have been explored: creation of the MIF before chondrule/CAI formation, and creation of the MIF during chondrule/CAI formation. If the MIF was established before chondrule/CAI formation, the most promising explanation is that H₂O (presumably as ice) and silicate dust with MIFs of opposite sign are fractionated together from the remaining gas. On heating, the H₂O now in the gas exchanges with the melt.If the MIF was generated during chondrule/CAI formation, it must be generated in the H₂O, because it exchanges most rapidly with the melt, and mass balance requires creation of MIF of opposite sign in CO. Self-shielding from UV radiation is one possibility, but the effect may be quenched at high temperatures. Non-RRKM intramolecular kinetic isotope effects are another possibility, but a continuous source of radiation may be needed to prevent gas phase reactions from approaching equilibrium.     

Equilibrium and kinetic aspects of soddyite dissolution and secondary phase precipitation in aqueous suspension

The dissolution and transformation of soddyite ([UO₂]₂SiO₄ · 2H₂O) have been examined in aqueous suspension at pH 6 and 0.01 M NaNO₃. Soddyite is an important component of the paragenetic sequence of secondary minerals that arises from the weathering of uraninite ore deposits and corrosion of spent nuclear fuel. A soddyite of high purity and crystallinity was synthesized in the laboratory for use in dissolution experiments. In batch experiments, rapid dissolution occurred over an initial period of several hours followed by continuing steady-state dissolution for up to 700 h. Up to 200 h, U and Si were released into solution at their stoichiometric 2:1 ratio in soddyite. A decrease in the dissolved U concentration was observed at longer times, indicating the precipitation of a new phase. Even after precipitation of the secondary phase, the continuing dissolution of soddyite could be inferred from increasing dissolved Si concentrations. Through the use of X-ray diffraction, Raman spectroscopy, and scanning electron microscopy, the precipitated phase was identified as a clarkeite-like sodium uranyl oxide hydrate. The sodium uranyl oxide hydrate was ultimately the solubility-controlling solid, despite being only a minor component. Soddyite dissolution rates were quantified in flow-through experiments, in which reaction products were flushed from the reactors, thereby avoiding reprecipitation of U. The measured dissolution rate at pH 6 was 0.71 μmol U m⁻² h⁻¹. A slower dissolution rate of 0.44 μmol U m⁻² h⁻¹ was observed when 100 μM dissolved Si was added to the reactor influent.     

Vertical zoning of oil and gas formation: historico-genetic aspects

Considerable variations in depth zoning of dispersed organic matter (DOM) catagenesis are caused by various physical and geological factors. The evolution of a sedimentary basin (SB) implies successive changes in organization levels of this system. In the process of evolution the system structure is determined by the interaction of its subsystems. Any parameter of an SB (physical properties of rocks, degree of OM catagenesis, temperature, formation pressure, phase ratio of hydrocarbons) is governed by the processes running in the system. Variations of these parameters in space and time characterize the structure of the changing system. The intensity of lithification of terrigenous rocks, OM catagenesis, and HC generation in time is approximated by a curvilinear relation, which becomes asympthotic at a particular stage. In other words, these processes drastically decay 150 ± 50 Myr after the main sedimentation had completed. For an SB system with a natural set of main subsystems (mineral, water, organic, hydrocarbon), the age is less important (at least throughout the Phanerozoic) than the duration of the process. Analysis is given to the formation of vertical HC zoning, which includes all the processes observable within an SB. The relationship of events and qualitative temporal and spatial changes during these processes is considered.     

贵州北部铝土矿地球化学特征及沉积环境分析

贵州北部产于二叠系中统梁山组底部含矿岩系中的铝土矿,厚度稳定、变化较小,矿石质量较好。近几年相继找到的大中型矿床接踵而至,是渝南—黔北铝土矿集区的重要组成部分。通过道真新民、岩坪、桃园和务川瓦厂坪等矿床中铝土矿(岩)微量元素、稀土元素地球化学特征及沉积环境地球化学标志的研究,论证了贵州北部铝土矿床具有相同的物源和陆相淡水沉积环境。     

Analysis of natural and beneficiated ferruginous bauxites by both X-ray diffraction and X-ray fluorescence

The quantitative determination of the main compounds Al (OH)₃ (gibbsite) as Al₂O₃ and hydrated Fe-oxides as Fe₂O₃ in natural and beneficiated ferruginous bauxites (low-grade bauxites) by X-ray diffraction (XRD) is presented employing the external standard method. The calibration parameters for the analysis by XRD were ascertained by using ten synthetic mixtures of the pure oxides Al₂O₃ and Fe₂O₃ which were calcined at 1200°C. The concentration ranges for Fe₂O₃ were 10–70 wt.% and those for alumina 30–90 wt.%. The standard deviation for the calibrations of the two oxides was 3.33 wt.%. The results obtained by XRD for both oxides were compared with those attained by X-ray fluorescence. The precision of the determinations by XRD was ascertained by analysing a natural sample consecutively 5 times. The accuracy was verified by the analysis of two international standard reference materials with low silica contents.     

New aspects on the formation of hydrocarbon source rocks

Episodic volcanism favors, as shown by examples from Europe, the USA, and Brazil, the formation of organic-rich sediments (hydrocarbon source rocks). The examples examined from Europe comprise the Lower Carboniferous Oil-Shale Group, Scotland; Upper Carboniferous Coal Measures, western Germany; Upper Jurassic Kimmeridge Shale, North Sea area; Lower Cretaceous Blättertonstein, northwest Germany; and Eocene Messel Oil Shale, Rheingraben. Examples from the USA are Devonian/Mississippian Gas Shales, Permian Phosphoria Shale, Middle Cretaceous Mowry Shale, Eocene Green River Shale, and Miocene Monterey Shale. Examples from Brazil are Permian Iratí Oil Shale and Oligocene Paraiba-Valley Oil Shale. Volcanism is reflected during sedimentation (1) by exhalation of SO₂ and/or CO₂, (2) volcanic ashfalls, (3) by generation of anoxia and preservation of organic matter, and (4) by halmyrolytic alteration of volcanic ashes into highly surface-active clay minerals (montmorillonite).

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Zusammenfassung Episodisch auftretender Vulkanismus begünstigt, wie an Beispielen aus Europa, aus den USA und aus Brasilien gezeigt wird, die Bildung von Sedimenten, die reich an organischer Substanz sind (Erdöl- und Erdgasmuttergesteine). Beispiele aus Europa sind: unterkarbonische Oil-Shale Group, Schottland; oberkarbonische Steinkohle, W-Deutschland; oberjurassischer Kimmeridge Shale, Nordsee-Raum; unterkretazischer Blättertonstein, NW-Deutschland; und eozäner Messel-Ölschiefer, Rheingraben. Beispiele aus den USA sind oberdevonische/unterkarbonische Gas-Shales, östliche USA; permische Phosphoria-Formation, mittlere Rocky Mountains; mittelkretazischer Mowry-Shale, nördliche Rocky Mountains; eozäner Green River Shale, südliche Rocky Mountains; und miozäner Monterey Shale, Kalifornien. Aus Brasilien sind permischer Irati Oil Shale und oligozäner Paraiba-Valley Oil Shale beschrieben. Der Vulkanismus äußert sich während der Sedimentation vor allen durch: (1) Exhalation von SO₂ und/oder CO₂, (2) Einwehung vulkanischer Asche, (3) Schaffung anoxischer Verhältnisse und Konservierung der organischen Substanz und (4) halmyrolytische Umwandlung der vulkanischen Aschen in oberflächenaktive Tone (Montmorillonit).

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Résumé Divers exemples pris en Europe, aux U.S.A. et au Brésil, montrent que des manisfestations volcaniques épisodiques favorisent la formation de sédiments riches en matières organiques (roches-mères de pétrole ou de gaz naturel). Les exemples d'Europe sont: l'Oil-Shale Group carbonifère inférieur en Ecosse, les formations houillères du Carbonifère supérieur en Allemagne, les shales kimmeridgiens en Mer du Nord, le Blättertonstein crétacé inférieur du NW de l'Allemagne, les schistes bitumineux éocènes de Messel dans le graben du Rhin. Les exemples aux U.S.A. sont: les »Gas Shales« dévoniens sup./carbonifères inf. de l'Est des U.S.A., la formation permienne Phosphoria dans les Rocheuses moyennes, le Mowry Shale méso-crétacé dans les Rocheuses septentrionales, le Green River Shale eocene au S des Rocheuses, le Monterey Shale miocène en Californie.Les exemples au Brésil sont: l'Irati Oil shale permien et le Paraiba-Valley Oil shale oligocène. Le volcanisme se manifeste au cours de la sédimentation par: (1) des exhalaisons de SO₂ et/ou de CO₂; (2) des chutes de cendres; (3) la production d'un milieu anaérobie avec conservation des matières organiques; (4) la transformation halmyrolitique des cendres en produits argileux gonflants (montmorillonite).

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