Solubility of silica polymorphs in electrolyte solutions, I. Activity coefficient of aqueous silica from 25° to 250°C, Pitzer''s parameterisation

Within the framework of Pitzer's specific interaction model, interaction parameters for aqueous silica in concentrated electrolyte solutions have been derived from Marshall and co-authors amorphous silica solubility measurements. The values, at 25°C, of the Pitzer interaction parameter (λ_SiO2(aq)−i) determined in this study are the following: 0.092 (i = Na⁺), 0.032 (K⁺), 0.165 (Li⁺), 0.292 (Ca²⁺, Mg²⁺), −0.139 (SO₄²⁻), and −0.009 (NO₃⁻). A set of polynomial equations has been derived which can be used to calculate λ_SiO2(aq)−i for these ions at any temperature up to 250°C. A linear relationship between the aqueous silica-ion interaction parameters (λ_SiO2(aq)−i) and the surface electrostatic field (Z_i/r_e,i) of ions was obtained. This empirical equation can be used to estimate, in first approximation, λ_SiO2(aq)−i if no measurements are available. From this parameterisation, the calculated activity coefficient of aqueous silica is 2.52 at 25°C and 1.45 at 250°C in 5 m NaCl solution. At lower concentrations, e.g. 2 m NaCl, the activity coefficient of silica is 1.45 at 25°C and 1.2 at 250°C. Hence, in practice, it is necessary to take into account the activity coefficient of aqueous silica (λ_SiO2(aq)≠1) in hydrothermal solutions and basinal brines where the ionic strength exceeds 1. A comparison of measured [Marshall, W.L., Chen, C.-T.A., 1982. Amorphous silica solubilities, V. Prediction of solubility behaviour in aqueous mixed electrolyte solutions to 300°C. Geochim. Cosmochim. Acta 46, 289–291.] and computed amorphous silica solubility, using this parameterisation, shows a good agreement. Because the effect of individual ions on silicate and silica polymorph solubilities are additive, the present study has permitted to derive Pitzer interaction parameters that allow a precise computation of γ_SiO2(aq) in the Na---K---Ca---Mg---Cl---SO₄---HCO₃---SiO₂---H₂O system, over a large range of salt concentrations and up to temperatures of 250°C.     

The facies distribution of carbonate sediments on Porcupine bank, northeast Atlantic

Porcupine Bank (51–54°N, 12–15°W), approximately 10,000 km² in area and 150–500 m deep, is effectively removed from present-day terrigenous sedimentation by the broad Irish Shelf to the east, and deep water to the north, west and south. Water temperatures range from 10° to 14.5°C. Carbonate sediments presently accumulate on the bank, mixing with relict quartz sands and lithic pebbles, cobbles and boulders of glacial origin. Traction current structures are absent but biogenic hollows, pits, mounds and trails are prevalent. The lithofacies have a crudely concentric zonation with pelagic foraminiferal oozes in the deepest waters ( > 500 m), passing to foraminiferal (pelagic and benthic) glauconitic quartz sands over the bulk of the bank (500-200 m) where scattered patches of Lophelia pertusa coral branches are found, which in turn pass into a narrow zone associated with the lithic boulder field at the bank crest (< 200 m) which has coarse sands composed of quartz and skeletal fragments of benthic organisms-molluscs, echinoids, bryozoans and serpulids. This facies pattern is closely similar to that on Rockall Bank, 500 km to the north, and supports the construction of a depth-related facies model for modern open-sea temperate carbonate sediments: > 500 m, pelagic foraminiferans; 500-200 m, benthic foraminiferans plus Lophelia coral patches; 200-100 m, bivalve-echinoderm; 100-50 m, bryozoan-bivalve-gastropod-serpulid; <50 m, calcareous red algae-bivalve-gastropod-barnacle.     

Analcime equilibria

The stability fields of analcime and analcime+quartz have been investigated using conventional hydrothermal techniques, over the approximate range of conditions 160–600 °C and 500–5000 bars fluid pressure. The dehydration of analcime (Na₂Al₂Si_3·3O_11·6 · nH₂O) to albite, nepheline and H₂O occurs at temperatures of 492±5 °C at 500 bars, 538±5 °C at 1000 bars, 578±5 °C at 2000 bars and 598±5 °C at 3000 bars. In the presence of quartz, analcine dehydrates to highly disordered albite and H₂O at about 200 °C and 2000 bars, 196°±5 °C and 3000 bars, about 190 °C and 4000 bars, and 183±5 °C at 5000 bars P_fluid. The synthetic phase equilibria appear to be compatible with field observations that primary analcimes occur as phenocrysts or in groundmass in some volcanic and hypabyssal rocks and secondary analcimes in sedimentary, hydrothermally altered and low-grade metamorphic rocks.     

Enrichment of SiO2 in rhyolites by fractional crystallization: An experimental study of peraluminous granitic rocks from the St. Francois Mountains, Missouri, USA

Experiments were conducted to test the hypothesis that higher silica rhyolites of the St. Francois Mountains, Missouri, USA are products of fractional crystallization of lower silica granitic magmas. Experiments were carried out at pressures of 0.5 and 1.0 kb under water-saturated conditions, temperatures ranged from 800° to 925°C and the oxygen fugacity for all experiments were maintained at, or near, the nickel-nickel oxide (NNO) buffer level.

Results of experiments with Butler Hill granite indicate that fractionation of near liquidus silicate phases, orthopyroxene and plagioclase, causes an enrichment of SiO₂ in the residual melts. Mineralogical and chemical compositions of the experimental charges are similar to that of higher silica Grassy Mountain rhyolites of the St. Francois Mountains. Experiments also show that at pressures of 1.0 Kb or higher orthopyroxene reacts with the hydrous melt to produce biotite which is a common phase in the plutonic rocks of the St. Francois Mountains.     

Gas pressures and redox reactions in geothermal fluids in Iceland

The gas and redox chemistry of 100–300 °C geothermal fluids in Iceland has been studied as a function of fluid temperature and fluid composition. The partial pressures of CO₂ in dilute (mCl<500 ppm) and saline (mCl>500 ppm) geothermal fluids above 200 °C are controlled by the mineral buffer clinozoisite+prehnite+calcite+quartz. Two buffers are considered to control the H₂S and H₂ partial pressures above 200 °C depending on fluid salinity, epidote+prehnite+pyrite+pyrrhotite for dilute fluids and pyrite+prehnite+quartz+magnetite+anhydrite+clinozoisite+quartz for saline fluids. Below 200 °C, the partial pressures of CO₂, H₂S and H₂ also seem to be buffered but other minerals must be involved. Zeolites are expected to replace prehnite and epidote. Redox potential calculated on the assumption of equilibrium for the H⁺/H₂ redox couple decreases in dilute geothermal fluids with increasing temperature from about −0.5 V at 100 °C to −0.8 V at 300 °C, whereas saline geothermal fluids at 250 °C display a redox potential of about −0.45 V. A systematic discrepancy between redox couples of about 0.05–0.09 V is observed in the redox potential for the dilute geothermal fluids, whereas redox potentials agree within 0.02–0.04 V for saline geothermal waters. The discrepancies in the calculated redox potential for dilute geothermal fluids are thought to be due to a general lack of equilibrium between CH₄, CO₂ and H₂ and between H₂S, SO₄ and H₂. It is, accordingly, concluded that an overall equilibrium among redox species has not been reached for dilute geothermal fluids whereas it appears to be more closely approached for the saline geothermal fluids. The latter conclusion is based on limited database and should be treated with care. Since the various redox components are not in an overall equilibrium in geothermal fluids in Iceland these fluids cannot be characterised by a unique hydrogen fugacity, oxygen fugacity or redox potential at a given temperature and pressure.     

Dissolution mechanisms of silicate minerals yielded by intercomparison with glasses and radiation damage studies

The behaviour of two silicates (albite and olivine) toward aqueous dissolution has been investigated with emphasis to two aspects: (1) similarities and differences with silicate glasses; and (2) influence of ion bombardment which transforms the initially crystalline structure to a glass-like one. The occurrence on leached materials of hydrated layers enriched with metallic elements is checked by using high-energy ion beam analytical techniques.

It is shown that for both amorphous and crystalline silicates, dissolution involves competing processes responsible for either hydrated-layer generation (hydration by ion exchange and/or water permeation), or its destruction (hydrolysis of siloxane bonds and detachment of silica units). At moderate temperature ( 100°C), the latter processes dominate for crystalline silicates and the resulting dissolution is congruent. However, these silicates tend to behave like glasses upon ion bombardment above a critical dose. At higher temperature (> 200°C), hydration is markedly enhanced and thick altered layers are generated even on undamaged minerals such as albite and orthoclase, where alkali cations are easily exchangeable.     

Identifying when microbes biosilicify: The interconnected requirements of acidic pH, colloidal SiO2 and exposed microbial surface

This study demonstrates discernible biosilicification of natural microbial mats through batch laboratory experiments. Identification of the geochemical requirements for this process to occur includes thermodynamically favorable, but sluggish silica reaction kinetics associated with acidic conditions, and the necessity for colloidal silica rather than dissolved silicic acid species. This study provides the first results to bridge the apparent literature discrepancy between widespread, in-situ observations of microbial silicification, and the inability to demonstrate a detectable microbial impact in this process under well-constrained laboratory conditions. We compared the silica scavenging abilities of three natural microbial mats collected from Yellowstone National Park (YNP) hotsprings, relative to those of both abiotic particle (TiO₂) and solution controls at constant, near-saturated aqueous silica concentrations, while experimental pH and temperature conditions were varied, using both dissolved and colloidal SiO₂ forms. We specifically evaluated three microbial mats sampled from YNP sites all exhibiting saturation with respect to amorphous SiO₂, but possessing variable pH and temperature conditions that should reflect differential kinetics (and therefore biological opportunity) relative to silica polymerization: (1) most biologically favorable, acidic-mesophile (AM: pH 3, T = 35 °C); (2) biologically possible, but less opportune, alkaline, mesophile (ALK-M: pH 8, T = 35 °C) and (3) unlikely to be biologically favorable, alkaline-thermophile (ALK-T: pH 8, T = 80 °C). Comparison of field and laboratory results substantiates the requirements for thermodynamically favorable, but kinetically slower SiO₂ polymerization conditions. Results show that acidic moderate temperature conditions were required for an observable biosilicification impact. Moreover, they also identified for the first time, the necessity specifically for colloidal silica forms which are surface bound under acidic pH conditions, to distinguish discernible biosilicification compared to mineral particle controls. Results also highlight the important influence of mat surface characteristics in this process, specifically the extent of live, non-mineralized, exposed biological mat surface. Greater colloidal SiO₂ scavenging abilities are associated with non-mineralized microbial mat surfaces than with mineral particle surfaces or microbial mat surfaces encrusted with authigenic silica. These results are the first to demonstrate that biosilicification can be a microbially mediated, discernible geobiological process, shedding new light on the longstanding argument in the literature, and opening the door for more sensitive evaluation of this phenomenon in natural systems.     

P-T evolution of metasediments from the Eclogite Zone, south-central Tauern Window, Austria

Petrologic data on the paragenesis of (I) kyanite-zoisite marbles and (II) garnet-chloritoid quartz-mica schists are presented with the goal of providing constraints on the pressure-temperature evolution of the Eclogite Zone, Tauern Window, Austria. The peak metamorphic assemblages in the two rock types are: (I) kyanite + zoisite + dolomite + quartz; zoisite + muscovite + dolomite + calcite + quartz; and (II) garnet + chloritoid + kyanite + muscovite + quartz + epidote ± dolomite ± Zn-staurolite. The estimated peak metamorphic conditions are 19 ± 2 kbar, 590 ± 20°C.

Secondary alteration of the kyanite-zoisite marbles was accomplished in two stages. The early stage resulted in the production of margarite, paragonite, secondary muscovite and chlorite and the later stage resulted in the formation of sudoite (a di/trioctahedral Mg---Al layer silicate) and kaolinite. The early alteration is bracketed at conditions between 3 and 10 kbar, 450–550°C and the later alteration between 200 and 350°C, P 3 kbar.

The P-T path is characterized by maximum burial to approximately 19 kbar (60–70 km) (at≈590°C), followed by nearly isothermal decompression to approximately 10 kbar (30 km), and then more gradual decompression with cooling to approximately 3 kbar (10 km). Alteration was apparently accomplished by the influx of H₂O-rich fluids, with the composition of the fluid locally buffered by the mineral assemblage.     

An electron microscopic study of anorthoclase spherulites

Spherulites consisting of fibrous alkali feldspar and silica minerals are produced by devitrification of rhyolite glass under hydrothermal conditions. The alkali feldspars (Ab_72.5Or_23.0An_4.5, Ab_81.7Or_14.0An_4.3) in spherulites from two localities in Japan consist of triclinic anorthoclase showing fine cross-hatched twinning and monoclinic sanidine showing fine cross-hatching not attributable to twinning. The cross-hatching, which corresponds to albite and pericline twinning, is produced in the process of transition from a monoclinic to a triclinic phase. The spherulite may develop at a temperature lower than about 200°C because the co-existing silica mineral is not quartz, but metastable tridymite. According to the phase diagram of the alkali feldspars by MacKenzie (1952), the alkali feldspars should have been triclinic during growth. However, the textures show that the alkali feldspar grew as a disordered monoclinic phase. Because of the high growth rate, the Al/Si disordered structure was produced during growth and afterwards transformed into a triclinic structure with cross-hatched twinning.     

Constraining the potential temperature of the Archaean mantle: A review of the evidence from komatiites

The maximum potential temperature of the Archaean mantle is poorly known, and is best constrained by the MgO contents of komatiitic liquids, which are directly related to eruptive temperatures. However, most Archaean komatiites are significantly altered and it is difficult to assess the composition of the erupted liquid. Relatively fresh lavas from the SASKMAR suite, Belingwe Greenstone Belt, Zimbabwe (2.7 Ga) include chills of 25.6 wt.% MgO, and olivines ranging to Fo_93.6, implying eruption at around 1520°C. A chill sample from Alexo Township, Ontario (also 2.7 Ga) is 28 wt.% MgO, and associated olivines range to Fo_94.1, implying eruption at 1560°C. However, inferences of erupted liquids containing 32–33 wt.% MgO, from lavas in the Barberton Greenstone Belt, South Africa (3.45 Ga) and from the Perseverance Complex, Western Australia (2.7 Ga) may be challenged on the grounds that they contain excess (cumulate) olivine, or were enriched in Mg during alteration or metamorphism. Re-interpretation of olivine compositions from these rocks shows that they most likely contained a maximum of 29 wt.% MgO corresponding to an eruption temperature of 1580°C. This composition is the highest liquid MgO content of an erupted lava that can be supported with any confidence. The hottest modern magma, on Gorgona Island (0.155 Ga) contained 18–20% MgO and erupted at circa 1400°C.

If 1580°C is taken as the temperature of the most magnesian known eruption, then the source mantle from which the liquids rose would have been at up to 2200°C at pressures of 18 GPa corresponding to a mantle potential temperature of 1900°C. These temperatures are in excess of the mantle temperatures predicted by secular cooling models, and thus komatiites can only be formed in hot rising convective jets in the mantle. This result requires that Archaean mantle jets may have been 300°C hotter than the Archaean ambient mantle temperature. This temperature difference is similar to the 200–300°C temperature difference between present day jets and ambient mantle temperatures. An important subsidiary result of this study is the confirmation that spinifex rocks may be cumulates and do not necessarily represent liquid compositions.     

Paleomagnetism of Khatyrka and Maynitsky superterranes, Koryak highlands, far eastern Russia

We have completed a paleomagnetic reconnaissance study of sedimentary and volcanic extrusive rocks collected from two major tectonic zones in northeastern Russia. Paleomagnetic sites were sampled within the fault-bounded structural units of the Khatyrka and Maynitsky superterranes and an overlap sequence of the Khatyrka superterrane. These sampling localities were chosen to allow both within-site and between-site fold tests. Stepwise thermal demagnetization within the temperature range 200–640°C showed a characteristic linear demagnetization path between thermal demagnetization steps of 400°C and 530°C. For thermal steps above 550°C, the magnetic intensity of many samples began to increase rapidly with magnetic directions, which were random between heating steps, suggesting the formation of new magnetic phases in these samples. Paleomagnetic samples collected from basalts and sediments of the Khatyrka superterrane and basalts and gabbros of the Maynitsky superterrane pass fold tests and show significant poleward motion of these superterranes since the formation of their rocks. The observed paleomagnetic paleolatitudes between 24°N or S and 32°N or S can be compared with expected paleolatitudes of 57°N to 79°N. Paleomagnetic results from sites collected from overlapping Senonian rocks pass a fold test at the 99% confidence level and give a pole position not significantly different from that expected from the apparent polar wander path for the Eurasia or North America plates, suggesting that these sedimentary units overlapping the Khatyrka superterrane were deposited along the ancient northeast margin of the Eurasian plate. The declination, in stratigraphie coordinates, shows a maximum clockwise rotation of about 20° when compared with the Eurasian APWP.     

Experimental abiotic synthesis of methanol in seafloor hydrothermal systems during diking events

The abiotic synthesis of organic compounds in seafloor hydrothermal systems is one mechanism through which the subsurface environment could be supplied with reduced carbon. A flow-through, fixed-bed laboratory reactor vessel, the Catalytic Reactor Vessel (CRV) system, has been developed to investigate mineral–surface promoted organic synthesis at temperatures up to 400°C and pressures up to 30 MPa, conditions relevant to seafloor hydrothermal systems. Here we present evidence that metastable methanol can be directly synthesized from a gas-rich CO₂–H₂–H₂O mixture in the presence of a mineral substrate. Experiments have been performed without a substrate, with quartz, and with a mixture of quartz and magnetite. Temperatures and pressures in the experiments ranged from 200°C to 350°C and from 15 to 18 MPa, respectively. Maximum conversion of 5.8×10⁻⁴% CO₂ to CH₃OH per hour was measured using a mixture of magnetite and quartz in the reactor. After passivation of the stainless steel reactor vessel, experiments demonstrate that methanol is formed at temperatures up to 350°C in the presence of magnetite, and that the formation rate decreases over time. The experiments also show a loss of surface reactivity at 310°C and a regeneration of surface reactivity with increased temperature up to 350°C. Concentrations of CO₂ and H₂ used in the experiments simulate periodic, localized and dynamic conditions occurring within the seafloor during and immediately following magmatic diking events. High concentrations of CO₂ and H₂ may be generated by dike injection accompanied by exsolution of CO₂ and reaction of dissolved H₂O with FeO in the magma to form H₂. The experiments described here examine how the ephemeral formation of an H₂–CO₂-rich vapor phase within seafloor hydrothermal systems may supply reactants for abiotic organic synthesis reactions. These experiments show that the presence of specific minerals can promote the abiotic synthesis of simple organic molecules from common inorganic reactants such H₂O, CO₂ and H₂ under geologically realistic conditions.     

Paleotemperatures from fluid inclusions in halite: method verification and a 100,000 year paleotemperature record, Death Valley, CA

Maximum homogenization temperatures of fluid inclusions (Th_max) in halite (laboratory-grown crystals and modern samples, Death Valley, CA) match maximum brine temperatures during halite precipitation. Maximum brine temperatures during halite precipitation in Death Valley, late April, 1993 (34.4°C) agree with Th_max (34°C) and correlate well with average maximum air temperatures in April (31.3°C) and May (37.6°C). Th_max may be used for paleoclimate interpretations based on the close relationship between saline lake temperatures and average air temperatures from modern settings. Lower homogenization temperatures, demonstrably below the temperatures at which halite grew, are interpreted to reflect collapse of some fluid inclusion walls due to the pressure difference between the inside and outside of inclusions. By only using Th_max, the problems of anomalously low homogenization temperatures due to possible collapse of fluid inclusions are avoided. Halite samples from 30 stratigraphic intervals, 90 to 0 m (100 to 0 ka), Core DV93-1, Death Valley, CA, were used to measure homogenization temperatures of fluid inclusions. Virtually all homogenization temperatures from Core DV93-1 are below the modern Th_max of 34°C (halite precipitation late April, 1993). Lacustrine halites, deposited in a perennial saline lake 35 to 10 ka, have Th_max between 19°C and 30°C, which suggests brine temperatures approximately 4°C to 15°C below modern late April values. Ephemeral saline lake halites precipitated 60 to 35 ka have Th_max between 23°C and 28°C, 6 to 11°C below modern values. The highest Th_max value in the 100 ka record (up to 35°C) is from a halite sample formed approximately 100 ka in a climate regime somewhat colder than the modern.     

Thermodynamic and kinetic aspects of formation of bauxites

Experiments in which cleavage nepheline samples were reacted with aqueous solutions at fixed pH's and temperature were carried out in the laboratory. The chemistry of the solution as a function of time was monitored, as well as the chemistry of the nepheline surfaces.

At 25°C, Al derived from the nepheline stays in solution due to slow precipitation kinetics of Al(OH)₃. At 60° and 80°C, precipitation of Al(OH)₃ is so rapid that Al concentration in solution is below 0.05 ppm. This indicates that precipitation kinetics favour the formation of bauxite deposits in tropical regions (i.e. T25°C), but not in temperate regions.

Precipitation products on the surface of the nepheline fragments at 60° and 80°C depend on the pH. At pH 3.0, an amorphous aluminium silicate (proto-kaolinite?) is formed. At pH>7.0, the precipitated phase contains, in addition to Al and Si, high amounts of Na and K (proto-muscovite?). The optimum pH for the formation of bauxite is in the range 5–7. These results are in agreement with thermodynamic calculations.     

Experimental constraints on pre-eruption conditions of pantelleritic magmas: Evidence from the Eburru complex, Kenya Rift

The phase relationships and compositions of a pantellerite from the Eburru complex in the Kenya Rift Valley have been determined at 150 MPa and under reducing conditions, 2 log units below the Ni–NiO solid buffer. The effects of temperature and melt water content on phase relationships have been explored. Alkali feldspar and quartz crystallise alone at temperatures above 700 °C, irrespective of melt water content. Below 700 °C, sodic amphibole and clinopyroxene also crystallise; the amphibole being the liquidus phase under water-rich conditions. The coexistence of amphibole phenocrysts with alkali feldspar and quartz in a crystal-poor pantellerite implies temperatures below 700 °C and melt water contents higher than 4 wt.%, possibly up to 5–6 wt.%. Pantellerites have lower liquidus temperatures than associated comendites, which supports a parent–daughter relationship between the two magma types. The melts produced in the experiments extend the compositional trend displayed by the natural rock series, and reproduce some extreme compositions occasionally observed in alkaline volcanic series, with FeO contents above 12 wt.% and Na₂O contents approaching 10 wt.%. Pantellerites are therefore the true near-minimum melt compositions of alkaline oversaturated magma series.     

Interpreting fracture development from diagenetic mineralogy and thermoelastic contraction modeling

Four sets of thin-section scale, Mode I (open mode), cemented microfractures are present in sandstone from the Eocene Misoa Formation, Maracaibo basin, Venezuela. The first set of microfractures is intragranular (F1), formed early during compaction and are filled with quartz cement precipitated at temperatures equal to or higher than 100 °C. The second set of microfractures (F2) is cemented by bituminite–pyrite, formed at temperatures between 60 and 100 °C, and are associated with kerogen maturation and hydrocarbon migration from underlying overpressured source rocks. The third set of microfractures (F3) is fully cemented by either quartz cement or calcite cement. The former has fluid inclusion homogenization temperatures between 149 and 175 °C. These temperatures are mostly higher than maximum burial temperatures (160 °C), suggesting that upward flow, caused by a pressure gradient, transported silica vertically which crystallized into the fractures. Upward decompression may have also caused a P_CO2 drop, which, at constant temperature, allowed simultaneous carbonate precipitation into the third microfracture set. The fourth set of thin-section scale microfractures (F4) is open or partially cemented by siderite–hematite and other iron oxides. The presence of hematite and iron oxides in microfractures is evidence for oxidizing conditions that may be associated with the uplift of the Misoa formation. In order to time and place constraints on the depth of formation of the fourth set of microfractures, we have coupled published quartz cementation kinetic algorithms with uniaxial strain equations and determined if, in fact, they could be associated with the uplift of the formation. Our results suggest that thermoelastic contraction, caused by the formation's uplift, erosion, and consequent cooling is a feasible mechanism for the origin of the last fracture set. Hence, we infer that meteoric water invasion into the fractures, at the end of the uplift, cause the precipitation of oxides and the transformation of siderite to hematite.     

Chrysophyte resting stages: a tool for reconstructing winter/spring climate from Alpine lake sediments

Chrysophyte algae produce siliceous resting stages (stomatocysts) that are indicators of past environmental conditions. The objective of this study was to assess their strength for climate reconstructions. Stomatocysts were collected using sediment traps exposed in 45 mountain lakes (1502-2309 m a.s.l., Austrian Alps). Bi-hourly water-temperature measurements were used to determine dates of freezing and break-up, spring and autumn mixing. Canonical correspondence analyses revealed that the stomatocyst assemblages were related to the dates of ice break-up and spring mixing. The two dates are controlled by winter/spring air temperature. We developed a weighted averaging-partial least squares (WA-PLS) stomatocyst/date-of-spring-mixing regression and calibration model (R² _boot=0.85), and reconstructed 'dates of spring mixing' for Jezero v Ledvici (1824 m a.s.l., Slovenian Alps) from AD 1842 to 1996. Sample-specific standard errors of prediction corresponded to 0.6°C - 1.0°C. Despite dating uncertainties and poor fits of fossil assemblages with the training set, reconstructed 'dates of spring mixing' were significantly correlated with the mean March-April air temperature, which is known to drive break-up dates. Furthermore, the record was in agreement with glacier advances during the Little Ice Age.     

Carbonatation processes at the El Berrocal natural analogue granitic system (Spain): inferences from mineralogical and stable isotope studies

The El Berrocal granite/U-bearing quartz vein system has been studied as a natural analogue of a high-level radioactive waste repository. The main objective is to understand the geochemical behaviour of natural radionuclides occurring under natural conditions. In this framework, the carbonatation processes have been studied from a mineralogical and isotopic ( and ) point of view, since carbonate anions are powerful complexing agents for U(VI) under both low-temperature hydrothermal and environmental conditions. The carbonatation processes in the system are identified by the presence of secondary ankerite, with minor calcite, scattered in the hydrothermally altered granite, and Mn calcite in fracture filling materials. The isotopic signatures of these carbonates lead us to conclude that ankerite and calcite from the former were formed at the end of the same hydrothermal process that altered the granite, at a temperature range of between 72° and 61°C for ankerite, and between 52° and 35°C for calcite. The effect of edaphic CO₂ on both carbonates, greater on calcite than on ankerite, is demonstrated. Calcites from fracture fillings are, at least, binary mixtures, in different proportions, of hydrothermal calcite, formed between 25° and <100°C, and supergenic calcite, formed at ≤25°C. According to their signatures, the effect of edaphic CO₂ in both calcites is also evident. It is assumed that: (i) hydrothermal calcite from fracture fillings and ankerite from the hydrothermally altered granite are the result of the same hydrothermal process, their chemical differences being due to the intensity of the water/rock interaction which was stronger in the altered granite than in the fractures; and (ii) all of these carbonatation processes are responsible for ancient and recent migration/retention of uranium observed in the hydrothermally altered granite and fracture fillings.     

The pressure path of solid inclusions in minerals: the retention of coesite inclusions during uplift

An inclusion model allows calculation of the stresses in and around minerals included in other minerals: during changes in pressure and temperature. The equations are applied to illustrate cooling and uplift histories of quarzo-feldspathic rocks from 500°C and various pressures to ambient conditions. Even in the absence of pore-fluid pressure, microfractures may open at external pressures of 200–400 MPa and temperatures of 200–400°C due to differential volume changes of the constituent minerals. Coesite included in garnet cannot have formed during progressive metamorphism from quartz at lithostatic pressures below the coesite stability field because of differential volume changes. Coesite inclusions are captured by their host minerals at ultra-high pressures and they persist to lower pressures because of the large volume increase occurring at the coesite to -quartz transition. The Pⁱ-T path followed by the SiO₂ inclusion traces the boundary between the stability fields of coesite and -quartz until radial fractures develop in the host at low external pressure.     

Cooling history of lunar Mg-suite gabbronorite 76255, troctolite 76535 and Stillwater pyroxenite SC-936: The record in exsolution and ordering in pyroxenes

We have determined cooling rates of orthopyroxene crystals from two Mg-suite lunar samples (gabbronorite 76255 and troctolite 76535) and one terrestrial sample (orthopyroxenite SC-936 from the Stillwater Complex), on the basis of their Fe–Mg ordering states. In addition, a cooling rate of 76255 was determined by modeling the formation of exsolution lamellae in pyroxenes. The M1–M2 site occupancies of the orthopyroxene crystals were determined by single crystal X-ray diffraction and the rate constant for the ordering reaction was used along with calibrations of the equilibrium intracrystalline fractionation of Fe and Mg as a function of temperature to calculate cooling rates. The closure temperatures (T_C) of cation ordering are 525 °C for 76255, 500 °C for 76535 and 350 °C for SC-936 corresponding to cooling rates of 4 × 10⁻² °C/year at the closure temperature for the lunar samples and 10⁻⁶ °C/year for the Stillwater sample. A cooling rate for 76255, determined by simulating the exsolution process, is 1.7 × 10⁻² °C/year at a closure temperature for exsolution of 700 °C. The Fe–Mg ordering cooling rate determined for 76535 reflects a complex thermal history superimposed on the initial plutonic provenance established for this sample [McCallum, I.S., Schwartz, J.M., 2001. Lunar Mg suite: thermobarometry and petrogenesis of parental magmas. J. Geophys. Res. 106, 27969–27983]. The preservation of a crystallization age of 4.51 Ga and a metamorphic age of 4.25 Ga for 76535 is consistent with a model in which excavation of this sample from the lower lunar crust took place while the sample was at a temperature above the closure temperatures for the Sm–Nd, U–Pb and Ar–Ar isotopic systems. Temperatures in excess of the isotopic closure temperatures (i.e., >600 °C) in the lower lunar crust were maintained by heat diffusing from concentrations of U- and Th-rich KREEP material at the base of the crust. On the other hand, 76255 formed at a much shallower depth in the lunar crust (2 km) and was well below its isotopic closure temperatures at the time of excavation, most likely during the Serenitatis basin-forming impact event. Both lunar samples were reheated during transport to the surface and deposition in hot ejecta blankets. The reheating was short lived but apparently sufficient to redistribute Fe and Mg in M sites in orthopyroxenes. For the lunar samples, the cooling rates based on Fe–Mg ordering represent final stage cooling within an ejecta blanket.