Die Hoch-Tiefquarz-Inversion als petrologisches Hilfsmittel

It was shown by Keith and Tuttle (1952) that the high-low inversion of quartz from different sources can occur at somewhat variable temperatures; the authors suggest the determination of the inversion temperature (= t _i) as a “finger print method” for the geologic conditions of formation of the mineral. The results of the d.t.a. investigations reported here on the inversion temperature of 145 quartz samples from 130 localities (72 minerals from igneous rocks, 31 which had been formed authigenically in sediments, 17 from metamorphic rocks, 25 detritical quartzes from sediments) have been gained by exactly standardized methods of preparation and investigation (precision of t _i-determination: ±0.1° C).

1. The mean of t _i of all investigated quartz samples from igneous rocks lies at 573.2° C, the mean t _i of all quartzes which have been formed authigenically in sediments at 563.6° C. The t _i of all investigated specimens ranges from 536.9° to 576.3° C.
2. There is a boundary between the t _i of igneous and authigenically formed quartzes: the t _i of all investigated quartzes that were formed authigenically is lower than 570.9° C, the t _i of all investigated quartzes from igneous rocks, however, lies above this temperature.
3. There has no dependence been observed between t _i and temperature of formation as described by Keith and Tuttle. There is, however, a destinct dependence between the lowering of t _i and the degree of substitution of Al³⁺ and alkali metals for Si⁴⁺. The amount of foreign ions present in the lattice of quartz depends much more strongly on the concentrations of these ions in the solutions resp. melts from which the quartz crystals have formed than on temperature.
4. Besides substitution physical properties of the quartz crystals influence the variation of t _i . In the case of authigenically formed quartzes t _i will be the lower the worse the degree of crystallization. The microcrystalline quartz which had been formed authigenically in siliceous nodules in slates shows the lowest t _i of all investigated specimens. The influence of lattice defects and the factors which are responsible for them is discussed especially with regard to the t _i of metamorphic quartzes.
5. Inclusions of other minerals, liquids or gasses have no influence on the t _i of quartz.
6. The quartzes from a metamorphic sequence (Beaume valley, France) ranging through all subfacies of the greenschist and the amphibolite facies show characteristic differences in their t _i which can be explained partly by an “effect of recrystallization”.
7. The efficiency of t _i -determinations as a petrologic tool is outlined.

     

Das Hoch-Tief-Umwandlungsverhalten mikrokristalliner Quarze

The optical and x-ray investigation of 51 microcrystalline quartz samples show that they are built up of fibrous chalcedony (agates) or finest crystalline, isometric grained quartz (flints, cherts, jasper or Chrysoprase). These samples were examined by standardized d.t.a. — methods in order to get their inversion characteristics.

1. A third of the samples conteins low-temperature cristobalite in addition to quartz (in two samples even as main component); the high-low inversion of this mineral lies between 80 and 245° C according to its defect character. In five samples of jasper low- tridymite is occurring as minor component.
2. In contrary to macrocrystalline quartz crystals microcrystalline quartzes generally show no sharp inversion point. The inversion takes place over an interval of nearly 50° C, to be seen in the d.t.a. curve as a broad, only slightly endothermic effect with hardly visible peak or several small minima. In the curves of 15 samples no inversion could be detected. This broad inversion peak is caused by different defect characters of the crystals or even of parts of the crystals which invert at different temperatures (Flörke, 1955).
3. There is no dependance of t _i on the size of crystallites at least for crystals with diameter greater than 0.05 μ. The temperature of the most prominent inversion minimum allows a division of the microcrystalline quartz crystals analogous the macrocrystalline classification into samples with t _i below and above 570° C. The lowest (518±2° C) and the highest t _i (578,7+0,3° C) were measured at different parts of the same sample of a Chrysoprase.
4. The intensity of the brown and red coloured parts of agate and jasper grows with increasing Fe₂O₃-content, but there is no connection between t _i and chemical composition.
5. As in macrocrystalline quartzes of caverns and veins there are also some t _i -differences between the marginal and the central parts of agates or jaspers. The explanation lies in the mechanism of formation: higher t _i in the margin (e.g. in one globular jasper) points to a formation by silification, whereas higher t _i in the center indicates a formation by the filling up of cavities and veins.
6. The variation in t _i of microcrystalline quartzes and the comparison of these t _i ? with those of associated low-temperature cristobalite shows a connection between temperatures of formation and inversion: samples with low t _i (i.e. with a great number of defects) should be formed at low temperatures, that is by sedimentary or diagenetic processes, and samples with t _i above 570° C (i.e. nearly without defects in the structure) should have crystallized out of hydrothermal solutions. The weak inversion peaks below 570° which in d.t.a. curves often appear besides a main peak at 570° C represent the small amount of diagenetically formed microcrystalline quartz with a large degree of defect character.

     

Effects of chemical environment on the solubility and incorporation mechanism for hydrogen in olivine

To investigate the solubility and the sites of incorporation of hydrogen in olivine as a function of point defect concentration, two-stage high-temperature annealing experiments have been carried out. The first annealing stage (the dry preannealing stage) was conducted at a total pressure of 0.1 MPa, a temperature of 1300° C and various oxygen fugacities in the range 10^?11–10^?4 MPa for times > 12 h. In these heat treatments, the samples were buffered against either orthopyroxene or magnesiowustite, or they remained unbuffered. The second annealing stage (the hydrothermal annealing stage) was performed at 300 MPa and 900–1050 ° C under a hydrogen fugacity of ～ 158 MPa for 1–5 h. Infrared spectra from the annealed samples revealed two distinct groups of bands. Group I bands occurred at wavenumbers in the range 3450–3650 cm^?1, while Group II bands occurred in the range 3200–3450 cm^?1. The hydrogen solubility associated with Group I bands is proportional to f _{O 2} to the 1/6 power for samples preannealed in contact with orthopyroxene, to the 1/3 power for samples preannealed in contact with magnesiowustite, and to the 1/13 power for samples preannealed in the absence of a solid-state buffer. The hydrogen concentration for Group II bands varies with f _{o 2} to the 1/3 power for opxbuffered samples, to the 1/2 power for mw-buffered samples, and to the 1/3 power for unbuffered samples. The dependence of hydrogen solubility on oxygen fugacity and orthopyroxene activity suggests that hydrogen is incorporated into the olivine structure via association with point defects. The presence of two distinct groups of absorption bands indicates that hydrogen is associated with two distinct lattice defects. The following point defect model for the mechanism of incorporation of hydrogen in olivine is consistent with these results: Hydrogen ions responsible for the Group I bands are associated with doubly charged oxygen interstitials, while hydrogen ions responsible for the Group II bands are associated with singly charged oxygen interstitials. Furthermore, the infrared bands observed in naturally derived olivines are present in spectra from our hydrothermally annealed crystals. Thus, the mechanisms of incorporation of hydrogen in olivine under geological conditions are the same as those operative under laboratory conditions. The maximum solubility reached in these experiments was ～ 360H/10⁶Si, which corresponds to ～ 0.002 wt% of H₂O. This value is a lower bound for the solubility of hydrogen in olivine under upper mantle conditions.     

Juvenile CO2 in enderbites of Tromøy near Arendal, southern Norway: a fluid inclusion and stable isotope study

The enderbites from Tromøy in the central, granulite facies part of the Proterozoic Bamble sector of southern Norway contain dominantly CO₂ and N₂ fluid inclusions. CO₂ from fluid inclusions in quartz segregations in enderbites was extracted by mechanical (crushing) and thermal decrepitation and the δ¹³C measured. Measurement was also made on samples washed in 10% HCl, oxidized with CuO at high temperatures, and step-wise extracted with progressive heating. Results between the different techniques are systematic. The main results show δ¹³C of -4.5±1.5% for crushing and -7±2% for thermal decrepitation. δ¹³C is about constant for CO₂ extracted at different temperatures and points to a homogeneous isotopic composition. Due to the presence of carbonate particles and/or induced contaminations for the extraction by thermal decrepitation, the results for the crushing experiments are assumed the most reliable for fluid-inclusion CO₂. Very low values of δ¹³C have not been found in enderbite samples and δ¹³C combined with δ¹⁸O of the host quartzes (8-11%) indicates juvenile values. In addition, the fluid inclusions were examined by microthermometry and Raman analysis and host quartz by acoustic emission and cathodoluminescence. CO₂ fluid inclusions have varying densities with a frequency maximum of 0.92 g cm^-3 and generally do not concur with trapping densities at granulite conditions. Textures show that CO₂ must have been trapped in fluid inclusions in one early event, but transformed to different extents during late isothermal uplift without important fractionation of isotope compositions. The present data support a model of intrusion and crystallization of a CO₂-rich enderbitic magma at granuiite conditions.     

Micro-Fourier Transform Infrared (FT-IR) and δD value investigation of hydrothermal vein quartz: Interpretation of fluid inclusion δD values in hydrothermal systems

Some recent studies have suggested that the hydrogen isotopic composition (δD) of hydrothermal fluids, released in vacuo by thermal decrepitation of quartz, are not always accurately revealed. We report the results of a step-heating δD value study of vein quartz, hosted by Lower Palaeozoic rocks in SW England, which was analyzed by micro-FT-IR for hydrogen speciation, before and after fluid extraction at temperatures between 750 and 1500 °C. The δD values of individual aliquots of released water vary between −3‰ and −208‰, with the lowest values generally corresponding to the highest temperature fractions and samples of relatively low yield. The data show significant departures from geologically reasonable δD. Micro-FT-IR analyses show that a variety of OH species are present within the vein quartz, with significant intra and inter sample variation. Typically a broad absorption due to molecular water, in the region 3400 cm⁻¹ is observed, along with bands attributed to Li-OH and Al-OH. On heating, the broad absorption due to molecular water is reduced, accompanied by a measurable loss of Li-OH species. The latter becomes more pronounced in the higher temperature fractions (>750 °C). These data support earlier studies which indicated that contributions from the contrasting OH reservoirs in quartz can significantly influence the reported δD values. These new data also suggest that the incorporation of OH released from Li-OH sites in the quartz may be the most important factor in the generation of the anomalous values for these samples.     

Hydrogen speciation in synthetic quartz

The dominant hydrogen impurity in synthetic quartz is molecular H₂O. H-OH groups also occur, but there is no direct evidence for the hydrolysis of Si-O-Si bonds to yield Si-OH HO-Si groups. Molecular H₂O concentrations in the synthetic quartz crystals studied range from less than 10 to 3,300 ppm (H/Si), and decrease smoothly by up to an order of magnitude with distance away from the seed. OH^? concentrations range from 96 to 715 ppm, and rise smoothly with distance away from the seed by up to a factor of three. The observed OH^? is probably all associated with cationic impurities, as in natural quartz. Molecular H₂O is the dominant initial hydrogen impurity in weak quartz. The hydrolytic weakening of quartz may be caused by the transformation H₂O + Si-O-Si → 2SiOH, but this may be a transitory change with the SiOH groups recombining to form H₂O, and the average SiOH concentration remaining very low. Synthetic quartz is strengthened when the H₂O is accumulated into fluid inclusions and cannot react with the quartz framework.     

Determination of the first ionization constant of silicic acid from quartz solubility in borate buffer solutions to 350°C

The solubility of quartz has been determined in borax buffer solutions having total boron concentrations of 0.10, 0.20, 0.40 and 0.60 mol kg^?1 and over the temperature range 130–350°C at the saturated vapour pressure of the system. The first ionization constant of silicic acid was calculated from the solubility data and varied from ?logK₁ = 8.88 (± 0.15) at 130°C to ?logK₁ = 10.06 (± 0.20) at 350°C. The solubility of quartz in these solutions was due to the presence of the three species, H₄SiO₄, H₃SiO₄^? and NaH₃SiO₄°. The equilibrium constant for the reaction, Na⁺ + H₃SiO₄^? = NaH₃SiO₄° extended from log K_as = 1.18?1.40 (± 0.20) over the temperature interval 135–301°C. The formation of NaH₃SiO₄° ion pairs was concluded to contribute significantly to the solubility of quartz in alkaline hydrothermal solutions when pH > 8 and sodium concentration exceeds 0.10 mol kg^?1.     

OH defects in quartz in the system quartz–albite–water and granite–water between 5 and 25?kbar

The incorporation of OH defects in quartz from the systems quartz–water, quartz–albite–water and granite–water at pressures between 5 and 25?kbar and temperatures between 800 and 1,000?°C was investigated by IR spectroscopy. The two most important OH absorption features can be assigned to hydrogarnet defects (absorption band at 3,585?cm^?1) and coupled substitutions involving Al³⁺ (Al–H defects, absorption bands at 3,310, 3,378 and 3,430?cm^?1). Al incorporation in quartz is controlled by mineral/melt partitioning (D _Al ^Qz/Melt ?=?0.01) and exhibits a negative pressure dependence. This trend is not clearly reflected by the concentration of Al–H defects, which shows positive deviations from the theoretical 1:1 correlation of Al/H for some samples. In contrast to the Al–H defects, formation of hydrogarnet defects appears to be positively correlated to pressure and water activity, and may be used a petrological indicator. The overall water concentration exhibits only minor changes with pressure and temperature, but a clear correlation of water activity (controlled by various amounts of dissolved salts) and hydrogarnet substitution could be established.     

Incipient metamorphism and deformation in the Variscides of SW Dyfed, Wales: first steps towards isotopic equilibrium

Incipient metamorphism accompanying thrusting, folding and cleavage development has been investigated in a varied sequence of Palaeozoic sediments near the Variscan front in SW Dyfed, Wales. The aim was to evaluate a critical stage in the progression from heterogeneous sediment, whose detrital phases are neither in equilibrium with one another, nor with pore fluids, through indurated sedimentary rock to metamorphic rock comprising newly formed crystals that equilibrated with one another as they grew. Quartz veins are widely developed in the area, especially in the more psammitic lithologies, while finer grained rocks became cleaved during tectonic deformation. Mineralogical constraints and fluid inclusion measurements suggest maximum temperatures around 200-310d? C (slightly higher in the Marloes-Musselwick Thrust Sheet than in other parts of the structural succession) at depths of the order of 6-13 km. Quartz veins yield distinctly heavier oxygen isotopic compositions than detrital quartz grains in the adjacent wall rocks, although care must be taken in interpreting the data because slivers of detrital grains may become incorporated into veins, while matrix detrital grains may incorporate veinlets or rims of newly formed quartz. It is concluded that vein quartz grew in isotopic equilibrium with a fluid phase whose isotopic composition was primarily controlled by exchange with phyllosilicates, not detrital quartz grains. Vein and matrix quartzes from the Marloes-Musselwick Thrust Sheet are distinctly lighter (δ¹⁸O_veins=+14 to +18% and δ¹⁸O_matrix=+11 to +14%) than those from other thrust sheets (δ¹⁸O =+17 to +20% and +14 to +17%, respectively). We conclude that vein quartz and phyllosilicate grains in cleavage domains probably attained equilibrium with a locally buffered pore fluid at the peak of metamorphism, but many relict grains of different chemical and isotopic composition remained elsewhere in the rock. Local fluid migration along veins and through cleavage lamellae facilitated the attainment of equilibrium, but there is little evidence for large-scale infiltration of externally derived fluids. With further metamorphism the quartz in these rocks would attain an isotopic composition intermediate between that of the heavy vein material and light detritus which coexist here.     

The stability of annite+quartz: reversed experimental data for the reaction 2 annite+3 quartz=2 sanidine+3 fayalite +2 H2O

Reversals for the reaction 2 annite+3 quartz=2 sanidine+3 fayalite+2 H₂O have been experimentally determined in cold-seal pressure vessels at pressures of 2, 3, 4 and 5?kbar, limiting annite +quartz stability towards higher temperatures. The equilibrium passes through the temperature intervals 500–540°?C (2?kbar), 550–570°?C (3?kbar), 570–590°?C (4?kbar) and 590–610°?C (5?kbar). Starting materials for most experiments were mixtures of synthetic annite +fayalite+sanidine+quartz and in some runs annite+quartz alone. Microprobe analyses of the reacted mixtures showed that the annites deviate slightly from their ideal Si/Al ratio (Si per formula unit ranges between 2.85 and 2.92, Al^VI between 0.06 and 0.15). As determined by Mössbauer spectroscopy, the Fe³⁺ content of annite in the assemblage annite+fayalite +sanidine+quartz is around 5–7%. The experimental data were used to extract the thermodynamic standard state enthalpy and entropy of annite as follows: H ⁰ _f,?Ann =?5125.896±8.319 [kJ/mol] and S ⁰ _Ann=432.62±8.89 [J/mol/K] (consistent with the Holland and Powell 1990 data set), and H ⁰ _f,Ann =?5130.971±7.939 [kJ/mol] and S ⁰ _Ann=424.02±8.39 [J/mol/K] (consistent with the TWEEQ data base, Berman 1991). The preceeding values are close to the standard state properties derived from hydrogen sensor data of the redox reaction annite=sanidine+magnetite+H ₂ (Dachs 1994). The experimental half-reversal of Eugster and Wones (1962) on the annite +quartz breakdown reaction could not be reproduced experimentally (formation of annite from sanidine+fayalite+quartz at 540°?C/1.035?kbar/magnetite-iron buffer) and probable reasons for this discrepancy remain unclear. The extracted thermodynamic standard state properties of annite were used to calculate annite and annite+quartz stabilities for pressures between 2 and 5?kbar.     

Transformation of curite into metaautunite paragenesis and electrokinetic properties

Genesis of metaautinute [Ca(UO₂/PO₄)₂ · 7H₂O] starting from curite hints at the existence of an intermediate hydrogen autunite stage [HUO₂PO₄ · 4H₂O]. The substitution of protons in hydrogen autunite by Ca²⁺ ions is proved by electrokinetic measurements. As a consequence of the similarity between X-ray powder patterns of hydrogen autunite and meta-autunite a glycolation method has been applied in order to distinguish the two species. The cell dimensions have been determined from Guinier X-ray diffraction patterns. Both minerals are tetragonal with a=6.981±0.005 Å and c=8.448±0.005 Å for metaautunite and a=7.084±0.005 Å and c=8.777±0.005 Å for hydrogen autunite. For both minerals, the zeta-potential is mostly negative and is strongly influenced by temperature, pH and concentration of cations in the suspension. The surface conductivity has been calculated from the value of the zetapotential. The electrokinetic properties of metaautunite are very similar to those of metatorbernite.     

The mechanisms of the formation and growth of water bubbles and associated dislocation loops in synthetic quartz

The development of water bubbles in synthetic quartz has been monitored by measurements of (i) the intensity of the light scattered and (ii) the increase in volume of the crystal, both as a function of temperature and time. These macroscopic measurements have been complemented by observations of the resulting microstructures, using transmission electron microscopy (TEM). A mechanism is proposed on the assumption that hydrogen is incorporated in the quartz structure by means of (4 H)_Si defects. On heating, these defects diffuse and clusters develop. A cluster of n(4 H)_Si produces a water bubble of (n?1)H₂O, without any change of volume of the crystal. At any temperature T there is a critical bubble diameter above which the “steam” pressure P exceeds the pressure p for a spherical bubble in mechanical equilibrium. If P becomes greater than p, then the bubble increases in volume until P=p, the increase in volume being achieved by the pipe diffusion of Si and O away from the bubble site into a linked edge dislocation loop. This process produces the observed increase in volume of the crystal. The two diffusion processes take place virtually simultaneously and continue until all the (4 H)_Si defects have been trapped in the bubbles. Values of the diffusion constant and the activation energy for the diffusion of the (4 H)_Si defects are deduced. The relevance of these observations to the hydrolytic weakening of quartz is briefly discussed.     

Raman spectroscopy of natural silica in Chicxulub impactite,MexicoSpectroscopie Raman de silices naturelles dans l'impactite de Chicxulub,Mexique

A series of natural silica impactite samples from Chicxulub (Mexico) was investigated by Raman microprobe (RMP) analysis. The data yield evidence for high-pressure shock metamorphism in the rock. The impactite contains three polymorphs of silica: the original α-quartz, and two high-pressure varieties – coesite and disordered quartz representing various degrees of crystallinity. We found systematic changes in frequencies and half-widths of the Raman bands, caused by increasing irregularities of bond-lengths and bond-angles and a general breaking-up of the structure as a result of impact events. Therefore, RMP is an adequate tool for measuring the crystallinity of disordered quartz. The half-width Γ and the frequency ω of the symmetric SiOSi stretching vibrational band (A₁ mode) of the SiO₄ tetrahedra are the most amenable parameters for estimating the degree of crystallinity. In well-crystallized quartz, Γ=5 cm^?1 and ω=464 cm^?1, while in highly disordered quartz this line shifts up to ω=455 cm^?1 and broadens up to Γ=30 cm^?1. The Raman lineshapes appear to depend strongly on the degree of lattice disorder subsequent to impact events. To cite this article: M. Ostroumov et al., C. R. Geoscience 334 (2002) 21–26     

Quartz from Allchar as monitor for cosmogenic 26Al: Geochemical and petrogenetic constraints

Summary Results of a multidisciplinary study on quartz concentrates (mineralogically separated) and etched concentrates (stoichiometric quartz) from three locations at Allchar (Macedonia) are presented. The investigation of quality and composition of these quartz samples is of great interest because the same material has been previously used as monitor for ²⁶Al Acceleration Mass-Spectrometry (AMS) erosion rate estimates. Two genetically different types of quartz are distinguished in the studied samples which petrologically can be described as hydrothermally altered dacites or quartz latites; i.e. volcanic (Q_V) and hydrothermal (Q_H) quartz with relative proportions of Q_H:Q_V around 3:2. Q_H is genetically related to the Allchar Sb–As–Tl–S mineralization having very high Sb (85–785 ppm), As (7.6–78 ppm) and (Tl 3.3–4.0 ppm) contents. This type of quartz is also characterized by very high Li (129–138 ppm), Al (2424–2520 ppm) and Ti (153–219 ppm) concentrations. Q_V appears to be much less enriched in trace elements having Al and K contents ranging from 0 to 280 ppm and from 50 to 85 ppm, respectively. ²⁶Al AMS measurements were done on the samples containing two genetically different types of quartz but this had no effects on the interpretation and erosion rate determinations. However, the extremely high Al concentrations in the analyzed quartz have generally negative effects, mainly by decreasing ²⁶Al/²⁷Al ratios and thus causing an increase of the detection limit. The disagreement between the results of ²⁶Al AMS analyses and quantitative geomorphologic data for one location is probably caused by different geographical position with respect to the direction of cosmic rays.     

黑龙江平顶山金矿石英天然热发光特征

对平顶山金矿中52件石英样品的天然热发光曲线特征研究表明,含金或富含金石英的热发光曲线以双峰或多峰为其主要特征,不含金的石英以单峰为其特征,石英热发光曲线特征可作为指示金矿化的一个重要标志.     

Microscale oxygen isotope variations in 1.9 Ga Gunflint cherts: Assessments of diagenesis effects and implications for oceanic paleotemperature reconstructions

Variations in the oxygen isotope composition (δ¹⁸O) of five cherts from the 1.9 Ga Gunflint iron formation (Canada) were studied at the micrometer scale by ion microprobe to try to better understand the processes that control δ¹⁸O values in cherts and to improve seawater paleotemperature reconstructions. Gunflint cherts show clearly different δ¹⁸O values for different types of silica with for instance a difference of ≈15‰ between detrital quartz and microquartz. Microquartz in the five samples is characterized by large intra sample variations in δ¹⁸O values, (δ¹⁸O of quartz varies from 4.6‰ to 6.6‰ at the 20 μm scale and from ≈12‰ to 14‰ at 2 μm scale). Isotopic profiles in microquartz adjacent to hydrothermal quartz veins demonstrate that microquartz more than ≈200 μm away from the veins has preserved its original δ¹⁸O value.At the micrometer spatial resolution of the ion probe, data reveal that microquartz has preserved a considerable δ¹⁸O heterogeneity that must be regarded as a signature inherited from its diagenetic history. Modelling of the δ¹⁸O variations produced during the diagenetic transformation of sedimentary amorphous silica precursors into microquartz allows us to calculate seawater temperature (T_sw at which the amorphous silica precipitated) and diagenesis temperature (T_diagenesis at which microquartz formed) that reproduce the δ¹⁸O distributions (mean, range and shape) measured at micrometer scale in microquartz. The two critical parameters in this modelling are the δ¹⁸O value and the mass fraction of the diagenetic fluid. Under these assumptions, the most likely ranges for T_sw and T_diagenesis are from 37 to 52 °C and from 130 to 170 °C, respectively.     

Oxygen and hydrogen isotopic studies of ore deposition and metamorphism at the Raul mine,Peru

The volcano-sedimentary sequence at the Raul mine, central Peru, consists of andesitic volcanics, graywackes, and siltstones, and has been metamorphosed to the upper greenschist-lower amphibolite facies at temperatures of 400–500°C. Isotopic data (O and H) have been collected from: (a) quartz and magnetite from stratiform ores, (b) amphiboles from amphibolite units that host stratiform ores, (c) calcite from late veins, (d) detrital quartz from graywackes, and (e) whole rocks.Interunit differences in quartz and magnetite δ¹⁸O values suggest that these minerals have resisted isotopic exchange during metamorphism, and that quartz-magnetite isotopic temperatures (380–414°C) represent primary formational temperatures. Calculated δ¹⁸O values of water in equilibrium with quartz and magnetite range from 9.1 to 12.6%..Amphibole δ¹⁸O and δD values show no interunit differences and suggest that the amphiboles have exchanged isotopes with a large metamorphic fluid reservoir. Calculated δ¹⁸O_H2O and δD_H2O values range from 8 to 12%. and ?3 to +42%., respectively.δ¹⁸O_H2O values calculated from δ¹⁸O calcite and fluid inclusion filling temperatures range from 7.5 to 10%.. Water extracted from fluid inclusions in calcite has a δD value of ?20%..δ¹⁸O values of metamorphosed graywackes and volcanic sediments are not atypical, but andesitic lavas are ¹⁸O-rich (8–10%.) compared to normal andesites.Waters involved in ore deposition, metamorphism, and late vein formation at Raul are all thought to have a common source, principally seawater. The δ¹⁸O_H2O and δD_H2O values could be produced by evaporation of seawater, shale ultrafiltration, and isotopic exchange with host rocks during deep circulation through the volcano-sedimentary pile.A model is proposed whereby coastal ocean water is restricted from the open sea by volcanic island arcs, and subsequently undergoes evaporation. Circulation of this water is initiated by heat associated with seafloor volcanism. ¹⁸O-enrichment in andesites may be produced by isotopic exchange with high ¹⁸O waters at elevated temperatures and sufficiently high water/rock ratios.     

Argillization processes at the El Berrocal analogue granitic system (Spain): mineralogy,isotopic study and implications for the performance assessment of radwaste geological disposal

The El Berrocal granite/U-bearing quartz vein (UQV) system has been studied as a natural analogue of a high-level radioactive waste repository. The main objective was to understand the geochemical behaviour of natural nuclides under different physicochemical conditions. Within this framework, the argillization processes related to fracturing and formation of the uranium–quartz vein were studied from a mineralogical and isotopic standpoint in order to establish their temperatures of formation and thus complete the geothermal history of the system. For this purpose, δ¹⁸O values were determined for pure mineral from the unaltered granite and quartz from the uranium–quartz vein, as well as for mixture samples from the hydrothermally altered granite (sericitised granite) and clayey samples from fracture fillings, including the clayey walls of the uranium–quartz vein. The isotopic signature of quartz from the uranium–quartz vein and the monophasic nature of its fluid inclusions led us to conclude that the isotopic signature of water in equilibrium with quartz was approximately in the range from −8.3‰ to −5.7‰ V-SMOV, its temperature of formation being around 85–120 °C. The δ¹⁸O values of pure sericite from the hydrothermally altered granite, calculated by means of the oxygen fraction molar method, indicate that its temperature of formation, in equilibrium with the aforementioned waters, is also in the range from 70 °C to approximately 120 °C. Clays from fracture fillings and clayey walls of the uranium–quartz vein are usually mixtures, in different proportions, of illite, approximately formed between 70 and 125 °C; two generations of kaolinite formed at approximately 90–130 °C and at around 25 °C, respectively; smectite, formed at ≤25 °C; and occasionally palygorskite, formed either between 30 and 45 °C or 19 and 32 °C, depending on the fractionation equation used. These data suggest that sericite from the hydrothermally altered granite, quartz from the uranium–quartz vein, illite and the first generation of kaolinite from the fracture fillings resulted from the same hydrothermal process affecting the El Berrocal granite in relation to fracturing. Under certain physicochemical conditions (T≈100 °C, pH≈8 and log [H₄SiO₄] between −4 and −3), illite and kaolinite can be paragenetic. As a result of weathering processes, smectite was formed from hydrothermal illite and inherited albite under alkaline weathering, while the second generation of kaolinite was formed from smectite, under acid conditions and close to the sulphide-rich uranium–quartz vein. Palygorskite is an occasional mineral formed probably either during the thermal tail of the above-described hydrothermal process or during weathering processes. In both cases, palygorskite must have formed from alkaline Si–Mg-rich solutions. Finally, these data and processes are discussed in terms of natural analogue processes, drawing some implications for the performance assessment of a deep geological radwaste repository (DGRR).     

Fabric analysis of quartzites with negative magnetic susceptibility – Does AMS provide information of SPO or CPO of quartz?

We ask the question whether petrofabric data from anisotropy of magnetic susceptibility (AMS) analysis of deformed quartzites gives information about shape preferred orientation (SPO) or crystallographic preferred orientation (CPO) of quartz. Since quartz is diamagnetic and has a negative magnetic susceptibility, 11 samples of nearly pure quartzites with a negative magnetic susceptibility were chosen for this study. After performing AMS analysis, electron backscatter diffraction (EBSD) analysis was done in thin sections prepared parallel to the K₁K₃ plane of the AMS ellipsoid. Results show that in all the samples quartz SPO is sub-parallel to the orientation of the magnetic foliation. However, in most samples no clear correspondance is observed between quartz CPO and K₁ (magnetic lineation) direction. This is contrary to the parallelism observed between K₁ direction and orientation of quartz c-axis in the case of undeformed single quartz crystal. Pole figures of quartz indicate that quartz c-axis tends to be parallel to K₁ direction only in the case where intracrystalline deformation of quartz is accommodated by prism <c> slip. It is therefore established that AMS investigation of quartz from deformed rocks gives information of SPO. Thus, it is concluded that petrofabric information of quartzite obtained from AMS is a manifestation of its shape anisotropy and not crystallographic preferred orientation.     

Mössbauer and infrared spectroscopic studies of Belgian chloritoids

Three chloritoid samples from the Stavelot massif (Belgium) and one sample from the Serpont massif (Belgium) have been characterized by chemical analyses and differential X-ray diffraction. A classification of chloritoid is proposed. Mössbauer spectra at temperatures between 78 and 360 K and in external magnetic fields were obtained for a triclinic and for a monoclinic specimen. The spectra show a superposition of a weak Fe³⁺ doublet (less than 10%) and an intense Fe²⁺ doublet. It is found that a decomposition of the ferrous absorption into two distinct quadrupole doublets leads to smaller deviations between experimental and calculated line shapes, especially at low temperatures. This suggests that Fe²⁺ is present in both cis and trans O₂(OH)₄ octahedral positions in the trioctahedral layer. However, the structural data derived from the temperature dependence of isomer shifts and quadrupole splittings, are found to be inconsistent with known crystallographic data. It is therefore concluded that Fe²⁺ is present in only one type of lattice site and that the numerically imposed decomposition into two ferrous doublets is merely an artifact due to thickness saturation effects and to the distributive character of the hyperfine parameters. The negative sign of the electric field gradient further confirms the assignment of the Fe²⁺ doublet to a cis octahedral configuration. Finally, only minor differences between the Mössbauer results for triclinic and monoclinic chloritoid are observed. The infrared absorption spectra of the four samples are almost identical except in the region around 600 cm^?1 at which the monoclinic phase exhibits two absorption bands instead of one band for the triclinic samples. All absorption bands can be well assigned to the different vibrations. Inter-layer hydrogen bonding is evidenced by the occurrence of two v _O-H absorption bands. Furthermore, the specific nature of the infrared spectra enables a fast identification of chloritoid samples.