In situ AFM study on barite (0 0 1) surface dissolution in NaCl solutions at 30 °C

This paper reports in situ observations on barite (0 0 1) surface dissolution behavior in 0.1–0.001 M NaCl solutions at 30 °C using atomic force microscopy (AFM). The step retreating on barite (0 0 1) surfaces changed with increasing NaCl solution concentrations. In solutions with a higher NaCl concentration (⩾0.01 M), many steps showed curved or irregular fronts during the later experimental stage, while almost all steps in solutions with a lower NaCl concentration exhibited straight or angular fronts, even during the late stage. The splitting phenomenon of the initial 〈h k 0〉 one-layer steps (7.2 Å) into two half-layer steps (3.6 Å) occurred in all NaCl solutions, while that of the initial [0 1 0] one-layer steps observed only in the 0.1 M NaCl solution. The step retreat rates increased with an increasing NaCl solution concentration. We observed triangular etch pit and deep etch pit formation in all NaCl solutions, which tended to form late in solutions with lower NaCl concentrations. The deep etch pit morphology changed with increasing NaCl solution concentrations. A hexagonal form elongated in the [0 1 0] direction was bounded by the {1 0 0}, {3 1 0}, and (0 0 1) faces in a 0.001 M NaCl solution, and a rhombic form was bounded by the {5 1 0} and (0 0 1) faces in 0.01 M and 0.1 M NaCl solutions. An intermediate form was observed in a 0.005 M NaCl solution, which was defined by {1 0 0}, a curved face tangent to the [0 1 0] direction, {3 1 0}, and (0 0 1) faces: the intermediate form appeared between the hexagonal and rhombic forms in solutions with lower and higher NaCl concentrations, respectively. The triangular etch pit and deep etch pit growth rates also increased with the NaCl solution concentration. Combining the step and face retreat rates in NaCl solutions estimated in this AFM study as well as the data on the effect of water temperature on the retreat rates reported in our earlier study, we produced two new findings. One finding is that the retreat rates increase by approximately two-fold when the NaCl solution concentration increases by one order of magnitude, and the other finding is that the retreat rate increase due to a one order of magnitude increase in the NaCl concentration corresponds to an increase of approximately 8 °C in water temperature. This correlation may help to understand and evaluate increasing dissolution kinetics induced by the different mechanisms where barite dissolution is promoted by the catalytic effect of Na⁺ and Cl⁻ ions (through an increase in the NaCl solution concentration) or by an increase in the hydration of Ba²⁺ and SO₄²⁻ (through an increase in water temperature).     

Factors affecting the dissolution kinetics of volcanic ash soils: dependencies on pH,CO2, and oxalate

Laboratory experiments were conducted with volcanic ash soils from Mammoth Mountain, California to examine the dependence of soil dissolution rates on pH and CO₂ (in batch experiments) and on oxalate (in flow-through experiments). In all experiments, an initial period of rapid dissolution was observed followed by steady-state dissolution. A decrease in the specific surface area of the soil samples, ranging from 50% to 80%, was observed; this decrease occurred during the period of rapid, initial dissolution. Steady-state dissolution rates, normalized to specific surface areas determined at the conclusion of the batch experiments, ranged from 0.03 μmol Si m⁻² h⁻¹ at pH 2.78 in the batch experiments to 0.009 μmol Si m⁻² h⁻¹ at pH 4 in the flow-through experiments. Over the pH range of 2.78–4.0, the dissolution rates exhibited a fractional order dependence on pH of 0.47 for rates determined from H⁺ consumption data and 0.27 for rates determined from Si release data. Experiments at ambient and 1 atm CO₂ demonstrated that dissolution rates were independent of CO₂ within experimental error at both pH 2.78 and 4.0. Dissolution at pH 4.0 was enhanced by addition of 1 mM oxalate. These observations provide insight into how the rates of soil weathering may be changing in areas on the flanks of Mammoth Mountain where concentrations of soil CO₂ have been elevated over the last decade. This release of magmatic CO₂ has depressed the soil pH and killed all vegetation (thus possibly changing the organic acid composition). These indirect effects of CO₂ may be enhancing the weathering of these volcanic ash soils but a strong direct effect of CO₂ can be excluded.     

Dissolution study of tremolite and anthophyllite: pH effect on the reaction kinetics

The effect of pH on the kinetics of tremolite and anthophyllite dissolution was investigated at 25 °C in batch reactors over the pH range of 1–13.5, in inorganic buffered solutions. Dissolution rates were obtained based on the release of Si and Mg. Results obtained in this study show different behaviors for both minerals. For tremolite, dissolution rates show a noticeable dependence on pH between 1 and 8, decreasing as pH increases and reaching a minimum around neutral conditions. At basic pH this dependence becomes even stronger, but dissolution takes place together with collateral effects of saturation and carbonation. A preferential release of Ca and Mg is observed in acid media, lowering the Mg/Si ratio to the extent that Mg solubility decreases with pH. For anthophyllite, dissolution rates also show a strong dependence on pH, between 1 and 9.5. At the same pH, anthophyllite dissolves up to 8 times faster than tremolite. For pH > 9.5 this dependence is smooth, and it is probably associated with effects of saturation and carbonation. Dissolution is also non-stoichiometric with a faster release of Mg with respect to Si in acid media. SEM observations show differences in the breakage mechanism of the fibers. The anthophyllite particle breakage during dissolution consists of the splitting of bundle fibers parallel to the fiber longitudinal direction. However, for tremolite, other than fiber splitting, particles shorten induced by coalescence of etch pits developed perpendicular to c axe.     

Dissolution kinetics and biodurability of tremolite particles in mimicked lung fluids: Effect of citrate and oxalate

The effect of citrate and oxalate on tremolite dissolution rate was measured at 37 °C in non-stirred flow-through reactors, using modified Gamble’s solutions at pH 4 (macrophages), 7.4 (interstitial fluids) and 5.5 (intermediate check point) containing 0, 0.15, 1.5 and 15 mmol L⁻¹ of citrate or oxalate. The dissolution rates calculated from Si concentration in the output solutions without organic ligands depend on pH, decreasing when the pH increases from −13.00 (pH 4) to −13.35 (pH 7.4) mol g⁻¹ s⁻¹ and following a proton-promoted mechanism. The presence of both ligands enhances dissolution rates at every pH, increasing this effect when the ligand concentration increases. Citrate produces a stronger effect as a catalyst than oxalate, mainly at more acidic pHs and enhances dissolution rates until 20 times for solutions with 15 mmol L⁻¹ citrate. However, at pH 7.4 the effect is lighter and oxalate solutions (15 mmol L⁻¹) only enhances dissolution rates eight times respect to free organic ligand solutions. Dissolution is promoted by the attack to protons and organic ligands to the tremolite surface. Magnesium speciation in oxalate and citrate solutions shows that Mg citrate complexes are more effective than oxalate ones during the alteration of tremolite in magrophages, but this tendency is the opposite for interstitial fluids, being oxalate magnesium complexes stronger. The biodurability estimations show that the destruction of the fibers is faster in acidic conditions (macrophages) than in the neutral solutions (interstitial fluid). At pH 4, both ligands oxalate and citrate reduce the residence time of the fibers with respect to that calculated in absence of ligands. Nevertheless, at pH 7.4 the presence of ligands does not reduce significantly the lifetime of the fibers.     

Kinetics of chromate reduction by pyrite and biotite under acidic conditions

The removal of chromate from aqueous solutions, using finely ground pyrite and biotite, was investigated by batch experiments. The kinetics and mechanism of chromate reduction are discussed here. Chromate reduction by pyrite was about 100 times faster than that by biotite, and was also faster at pH 3 than 4. When pyrite was used, more than 90% of the initial chromate was reduced within 4 h at pH 4, and within 40 min. at pH 3. However, with biotite more than 400 h was required for the reduction of 90% of the initial chromate. The results indicate that the rate of chromate reduction was strongly depending on the amount and dissolution rate of the Fe(II) in the minerals. The reduction of chromate at pH 4 resulted in the precipitation of (Cr, Fe)(OH)_3(s), which is believed to have limited the concentrations of dissolved Cr(III) and Fe(III) to less than the expected values. When biotite was used, the amounts of decreased Fe(II) and reduced Cr(VI) showed no stoichiometric relationship, which implies that not only was there chromate reduction by Fe(II) ions in the acidic solution, but also heterogeneous reduction of Fe(III) ions by structural Fe(II) in biotite. However, the results from a series of the experiments using pyrite showed that the concentrations of the decreased Fe(II) and the reduced Cr(VI) were close to the stoichiometric ratio of 3:1. This was because the oxidation of pyrite rapidly created Fe(II) ions, even in oxygenated solutions, and the chromate reduction by the Fe(II) ions was significantly faster than the Fe(II) ion oxygenation. When compared with the experimental sets controlled at an initial pH of 3, the pH of the biotite batch, which was not controlled, increased to 3.4. Because of the increase in the pH, Cr(VI) was not completely removed, and 25% (1.2–1.3 mg/L Cr(VI)) of the initial concentration remained for up to 1000 h. The pH increase is, in most cases, caused by the hydrolysis of clay minerals. However, in the pyrite batches, there was no difference in the variations of the chromate reduction in relation to the pH control. There was also no difference in the capacity and rate of Cr(VI) reduction in 0.01 M NaCl or Na₂SO₄ solutions. In the 0.01 M NaH₂PO₄ solution pyrite experiment, the Cr(VI) was not completely removed, despite the maintenance of the pH at 3. The dominant Fe species was about 10 mg/L Fe(III) and few Fe(II) ions existed in solution. The Fe phosphate (Fe₃(PO₄)₂ or FePO₄) coatings on the surface of pyrite prevented access of O₂ or Cr(VI). Therefore, the surface coatings are likely to have caused the deterioration of the Cr(VI) reduction capacity in the NaH₂PO₄ solution.     

Influence of pH and temperature on the early stage of mica alteration

Mineral dissolution and precipitation reactions actively participate in controlling fluid chemistry during water–rock interaction. In this study, the changes in the biotite and muscovite basal surface nano-morphology were evaluated during interaction with fluids of different pH (pH = 1.1, 3.3 and 5.7) at different temperatures (T = 25°, 120°, and 200 °C). Results show that at the nanometre scale resolution of the atomic force microscope (AFM), dissolution generates etch pits with a stair-shaped pattern over the (0 0 1) surface. The flux of dissolved elements decreases when pH increases. However, at pH 5.7, a change was found in the flux after 42 h of reaction when abundant gibbsite and kaolinite coat the dissolving mineral surface. This phenomenon was widely observed at edges of the etch pits by AFM. It was also found that an increase in temperature produces an enhancement in the elemental flux in both micas. Dissolution regime changes after less than one day of interaction at high temperature because of abundant coating formation over the etch pits and edges. The results demonstrate the key role of nanometre size neogenic phases in the control of elemental flux from mica surfaces to solution. The formation of nanometre size coatings, blocking the sites active for dissolution, appears to control the alteration of phyllosilicates even at the early stage of the interaction.     

Mobilisation of Iron from rocks in a fractured aquifer: Lithological and geochemical controls

Iron mobilisation from aquifer rocks in an important fractured aquifer system in South Africa is resulting in clogging of boreholes by Fe oxide minerals. Leach experiments using natural waters were conducted to determine the effects of redox conditions, pH lithology and presence of organic acids on the rate and extent of Fe dissolution from aquifer rocks, with the aim of clarifying the association of Fe clogging with geological formations that show Fe staining on weathering. The results indicate that the greatest amount of Fe (>30 mmol/kg rock) is leached from arenaceous rocks with low total Fe contents (49.0–75.0 mmol/kg) under anoxic conditions. Rocks with the highest Fe contents (>800 mmol/kg) generated low concentrations of Fe (<10 mmol/kg) even under favourable conditions of 0 mg/L DO and pH 3. The extent of Fe dissolution from the rocks was found to be most strongly dependent on the redox conditions, and the form of Fe present in the rock, with ascorbate-extracted amorphous Fe being the most mobile. The rate of dissolution is affected by pH and the presence of natural organic acids in the leachate. However, the effect of organic acids was only noticeable on arenaceous rocks.     

Dissolution kinetics of soil clays in sulfuric acid solutions: Ionic strength and temperature effects

Significant amounts of sulfuric acid (H₂SO₄) rich saline water can be produced by the oxidation of sulfide minerals contained in inland acid sulfate soils (IASS). In the absence of carbonate minerals, the dissolution of phyllosilicate minerals is one of very few processes that can provide long-term acid neutralisation. It is therefore important to understand the acid dissolution behavior of naturally occurring clay minerals from IASS under saline–acidic solutions. The objective of this study was to investigate the dissolution of a natural clay-rich sample under saline–acidic conditions (pH 1–4; ionic strengths = 0.01 and 0.25 M; 25 °C) and over a range of temperatures (25–45 °C; pH 1 and pH 4). The clay-rich sample referred to as Bottle Bend clay (BB clay) used was from an IASS (Bottle Bend lagoon) in south-western New South Wales (Australia) and contained smectite (40%), illite (27%), kaolinite (26%) and quartz (6%). Acid dissolution of the BB clay was initially rapid, as indicated by the fast release of cations (Si, Al, K, Fe, Mg). Relatively higher Al (pH 4) and K (pH 2–4) release was obtained from BB clay dissolution in higher ionic strength solutions compared to the lower ionic strength solutions. The steady state dissolution rate (as determined from Si, Al and Fe release rates; R_Si, R_Al, R_Fe) increased with decreasing solution pH and increasing temperature. For example, the highest log R_Si value was obtained at pH 1 and 45 °C (−9.07 mol g⁻¹ s⁻¹), while the lowest log R_Si value was obtained at pH 4 and 25 °C (−11.20 mol g⁻¹ s⁻¹). A comparison of these results with pure mineral dissolution rates from the literature suggests that the BB clay dissolved at a much faster rate compared to the pure mineral samples. Apparent activation energies calculated for the clay sample varied over the range 76.6 kJ mol⁻¹ (pH 1) to 37.7 kJ mol⁻¹ (pH 4) which compare very well with the activation energy values for acidic dissolution of monomineralic samples e.g. montmorillonite from previous studies. The acid neutralisation capacity (ANC) of the clay sample was calculated from the release of all structural cations except Si (i.e. Al, Fe, K, Mg). According to these calculations an ANC of 1.11 kg H₂SO₄/tonne clay/day was provided by clay dissolution at pH 1 (I = 0.25 M, 25 °C) compared to an ANC of 0.21 kg H₂SO₄/tonne clay/day at pH 4 (I = 0.25 M, 25 °C). The highest ANC of 6.91 kg H₂SO₄/tonne clay/day was provided by clay dissolution at pH 1 and at 45 °C (I = 0.25 M), which is more than three times higher than the ANC provided under the similar solution conditions at 25 °C. In wetlands with little solid phase buffering available apart from clay minerals, it is imperative to consider the potential ANC provided by the dissolution of abundantly occurring phyllosilicate minerals in devising rehabilitation schemes.     

Experimental study on weathering of seafloor volcanic glass by bacteria (Pseudomonas fluorescens) – Implications for the contribution of bacteria to the water–rock reaction at the Mid-Oceanic Ridge setting

The biologically mediated weathering of the ocean crust has received increasing attention in recent decades, but the rates and the possible mechanism of elemental release during microbe–basalt interactions occurring below the seafloor have not been studied in detail. In this study, we established an experimental weathering study of seafloor natural basaltic glass comparing the effect of microbial activity (Pseudomonas fluorescens) in P-rich and P-poor media with parallel controls containing either nonviable cells or organic acid. The changes in the chemical parameters, including pH, bacterial densities, and ion concentrations (Ca, Mg, Si, Mn, Al, Fe, and P) in the solution, were examined during the different batch experiments. The results showed that the pH decreased from 7.0 to 3.5 and the bacterial density increased from 10⁵ to 10⁸ cells/ml during the first 120 h, and the cell numbers remained constant at 10⁸ cells/ml and the pH increased from 3.5 to 6 between 120 h and 864 h in the P-bearing reactors containing bacteria. In contrast, during all the experimental time, the pH remained close to neutral condition in the abiotic control systems and the dissolution rates increased markedly with a decrease in pH and became minimal at near-neutral pH in P-bearing reactors containing bacteria, where Ca, Si, and Mg release rates were 2- to 4-fold higher than those obtained in chemical systems and biotic P-limited systems. Furthermore, the surfaces of the natural volcanic glass from the biotic systems were colonized by bacteria. Simultaneously, the etch pits were observed by Scanning Electron Microscope, which further indicate that the bacteria may promote the mineral dissolution for energy gain. Some elements (e.g., Fe, Mn, and Al) releasing from natural volcanic glass are likely an important source of the elemental budget in the ocean, and thus the element release and its possible mechanism conducted in this experimental study have potential implications on the biogeochemical cycling process in the Mid-Oceanic Ridge setting.     

Phosphorus and iron cycling in deep saprolite,Luquillo Mountains,Puerto Rico

Rapid weathering and erosion rates in mountainous tropical watersheds lead to highly variable soil and saprolite thicknesses which in turn impact nutrient fluxes and biological populations. In the Luquillo Mountains of Puerto Rico, a 5-m thick saprolite contains high microorganism densities at the surface and at depth overlying bedrock. We test the hypotheses that the organisms at depth are limited by the availability of two nutrients, P and Fe. Many tropical soils are P-limited, rather than N-limited, and dissolution of apatite is the dominant source of P. We document patterns of apatite weathering and of bioavailable Fe derived from the weathering of primary minerals hornblende and biotite in cores augered to 7.5 m on a ridgetop as compared to spheroidally weathering bedrock sampled in a nearby roadcut.Iron isotopic compositions of 0.5 N HCl extracts of soil and saprolite range from about δ⁵⁶Fe = 0 to ? 0.1‰ throughout the saprolite except at the surface and at 5 m depth where δ⁵⁶Fe = ? 0.26 to ? 0.64‰. The enrichment of light isotopes in HCl-extractable Fe in the soil and at the saprolite–bedrock interface is consistent with active Fe cycling and consistent with the locations of high cell densities and Fe(II)-oxidizing bacteria, identified previously. To evaluate the potential P-limitation of Fe-cycling bacteria in the profile, solid-state concentrations of P were measured as a function of depth in the soil, saprolite, and weathering bedrock. Weathering apatite crystals were examined in thin sections and an apatite dissolution rate of 6.8 × 10^? 14 mol m^? 2 s^? 1 was calculated. While surface communities depend on recycled nutrients and atmospheric inputs, deep communities survive primarily on nutrients released by the weathering bedrock and thus are tightly coupled to processes related to saprolite formation including mineral weathering. While low available P may limit microbial activity within the middle saprolite, fluxes of P from apatite weathering should be sufficient to support robust growth of microorganisms in the deep saprolite.     

Development of unusual rock weathering features in the Cordillera Blanca,Peru

Mylonite textures in granodiorite boulders are responsible for higher rates of surface denudation of host rocks and the progressive development of unusual rock weathering features, termed weathering posts. These textures are characterized by smaller grain sizes, higher biotite content, and a higher biotite axial ratio in host rocks relative to weathering posts. Elemental concentrations do not show a significant difference between weathering posts and the host rocks in which they are found, and this reflects the absence of a weathering residue on the rock surfaces. Chemical weathering loosens the bonds between mineral grains through the expansion of biotite, and the loosened grains fall off or are blown off the boulder surface and continue their chemical alteration in the surrounding soil. The height of weathering posts on late Quaternary moraines increases at a linear rate of ~ 1.45 ± 0.45 cm (1000 yr)^? 1 until post heights reach the diameter of host rocks. Such a rate of boulder denudation, if unrecognized, would generate significant errors (> 20%) in cosmogenic exposure ages for Pleistocene moraines. Given the paucity of boulders with diameters that significantly exceed 1.5 m, the maximum age of utility of weathering posts as a numeric age indicator is ~ 100 ka.     

In situ atomic force microscopy measurements of biotite basal plane reactivity in the presence of oxalic acid

We have used a direct imaging technique, in situ atomic force microscopy (AFM), to observe the dissolution of the basal biotite surface by oxalic acid over a range of temperatures close to ambient conditions, using a specially designed AFM liquid cell and non-invasive intermittent contact mode of operation. From the 3-dimensional nanometre-resolution data sets, we observe a process characterised by the slow formation of shallow etch pits in the (0 0 1) surface and fast growth of etch pits from the resulting steps, which represent proxies for the {h k 0} surface. Measurements of dissolution rates as a function of temperature allow a determination of an apparent activation energy (E_a,app) for the process, via mass-loss calculations from image analysis. We obtain a value of E_a,app = 49 ± 2 kJ mol⁻¹, which is consistent with separate calculations based on planar area etch pit growth, and measurements of etch pit perimeters, indicating that this value of E_a,app is representative of {h k 0} surface dissolution. The measurement of etch pit perimeters also enables an estimation of apparent activation energy as a function of step density indicating substantially higher apparent activation energy, up to E_a,app = 140 kJ mol⁻¹, on extrapolation towards a pristine surface with no defects. We suggest that this higher value of E_a,app represents the slow formation of etch pits into the (0 0 1) surface.     

Zircon geochronology of the Koraput alkaline complex: Insights from combined geochemical and U–Pb–Hf isotope analyses,and implications for the timing of alkaline magmatism in the Eastern Ghats Belt,India

Zircon formation and modification during magmatic crystallization and high-grade metamorphism are explored using TIMS and LA-ICP-MS U–Pb geochronology, Lu–Hf isotope chemistry, trace element analysis and textural clues on zircons from the Koraput alkaline intrusion, Eastern Ghats Belt (EGB), India. The zircon host-rock is a granulite-facies nepheline syenite gneiss with an exceptionally low Zr concentration, prohibiting early magmatic Zr saturation. With zircon formation occurring at a late stage of advanced magmatic cooling, significant amounts of Zr were incorporated into biotite, nearly the only other Zr-bearing phase in the nepheline syenite gneisses. Investigated zircons experienced a multi-stage history of magmatic and metamorphic zircon growth with repeated solid-state recrystallization and partial dissolution–precipitation. These processes are recorded by complex patterns of internal zircon structures and a wide range of apparently concordant U–Pb ages between 869 ± 7 Ma and 690 ± 1 Ma. The oldest ages are interpreted to represent the timing of the emplacement of the Koraput alkaline complex, which significantly postdates the intrusion ages of most of the alkaline intrusion in the western EGB. However, Hf model ages of TDM = 1.5 to 1.0 Ga suggest an earlier separation of the nepheline syenite magma from its depleted mantle source, overlapping with the widespread Mesoproterozoic, rift-related alkaline magmatism in the EGB. Zircons yielding ages younger than 860 Ma have most probably experienced partial resetting of their U–Pb ages during repeated and variable recrystallization events. Consistent youngest LA-ICP-MS and CA-TIMS U–Pb ages of 700–690 Ma reflect a final pulse of high-grade metamorphism in the Koraput area and underline the recurrence of considerable orogenic activity in the western EGB during the Neoproterozoic. Within the nepheline syenite gneisses this final high-grade metamorphic event caused biotite breakdown, releasing sufficient Zr for local saturation and new subsolidus zircon growth along the biotite grain boundaries.     

Mechanisms controlling acid neutralization and metal mobility within a Ni-rich tailings impoundment

Low-quality pore waters containing high concentrations of dissolved H⁺, SO₄, and metals have been generated in the East Tailings Management Area at Lynn Lake, Manitoba, as a result of sulfide-mineral oxidation. To assess the abundance, distribution, and solid-phase associations of S, Fe, and trace metals, the tailings pore water was analyzed, and investigations of the geochemical and mineralogical characteristics of the tailings solids were completed. The results were used to delineate the mechanisms that control acid neutralization, metal release, and metal attenuation. Migration of the low-pH conditions through the vadose zone is limited by acid-neutralization reactions, resulting in the development of distinct pore-water pH zones at depth; the neutralization reactions involve carbonate (pH ⩾ 5.7), Al-hydroxide (pH ⩾ 4.0), and aluminosilicate solids. As the zone of low-pH pore water expands, the pH will then be primarily controlled by less soluble solids, such as Fe(III) oxyhydroxides (pH < 3.5) and the relatively more recalcitrant aluminosilicates (pH ⩾ 1.3). Precipitation/dissolution reactions involving secondary Fe(III) oxyhydroxides and hydroxysulfates control the concentrations of dissolved Fe(III). Concentrations of dissolved SO₄ are principally controlled by the formation of gypsum and jarosite. Geochemical extractions indicate that the solid-phase concentrations of Ni, Co, and Zn are associated predominantly with reducible and acid-soluble fractions. The concentrations of dissolved trace metals are therefore primarily controlled by adsorption/complexation and (or) co-precipitation/dissolution reactions involving secondary Fe(III) oxyhydroxide and hydroxysulfate minerals. Concentrations of dissolved metals with relatively low mobility, such as Cu, are also controlled by the precipitation of discrete minerals. Because the major proportion of metals is sequestered through adsorption and (or) co-precipitation, the metals are susceptible to remobilization if low-pH or reducing conditions develop within the tailings.     

Paleo- to Eoarchean crustal evolution in eastern Hebei,North China Craton: New evidence from SHRIMP U–Pb dating and in-situ Hf isotopic study of detrital zircons from paragneisses

In the Caozhuang complex in eastern Hebei, North China Craton, the Paleo- to Eoarchean crustal evolution was earlier revealed by the preservation of detrital zircon grains older than (or as old as) 3.8 Ga in fuchsite-quartzite. In order to test if the Eoarchean antiquity is also preserved in rocks other than the fuchsite quartzite, we collected two paragneisses, a hornblende gneiss and a garnet–biotite gneiss, from Huangbaiyu village and dated their detrital zircon grains. The zircon dating of the hornblende gneiss yielded concordant ²⁰⁷Pb/²⁰⁶Pb ages ranging from 3684 to 3354 Ma. However, an older date of 3782 Ma with 18% discordancy was also obtained. Detrital zircon grains from the garnet–biotite gneiss gave a similar ²⁰⁷Pb/²⁰⁶Pb age range, from 3838 to 3342 Ma. The metamorphic domains of the zircon grains from both samples, including the strongly recrystallized cores and rims, recorded an overprinting metamorphism at ca. 2.5 Ga, which correlates with the most widespread tectono-thermal event in the North China Craton. In situ zircon Hf-isotope analyses on the dated zircon grains yielded a wide range of model ages (T_DM1) from 4.0 to 3.3 Ga with corresponding ε_Hf(T) from −36.0 to +4.8. This suggests that the evolution of the crustal segment in this area has involved multiple phases of juvenile crustal addition as well as recycling of older crustal rocks. The new geochronological results imply the presence of a significant amount of Eoarchean crustal fragments in the eastern Hebei area. The sedimentary protoliths of the paragneisses and other high-grade metamorphic rocks in the Caozhuang complex were probably deposited between 3.4 and 2.5 Ga.     

Surface potential and dixanthogen stability on chalcopyrite surface

Contact angle measurements, ultraviolet (UV) spectroscopy and voltammetry were used in this investigation to study the effect of various externally applied potentials on the stability of dixanthogen at synthetic and natural chalcopyrite surfaces.The results of contact angle measurements indicate that synthetic and natural chalcopyrites have similar wettabilities. Their hydrophobic potential range in a 7 × 10^− 4 M PAX solution was + 0.04 V to + 0.44 V vs. NHE at pH 10 and was − 0.01 V to + 0.54 V at pH 7.UV spectroscopic results show that dixanthogen began to form on the chalcopyrite surface at a potential of + 0.04 V vs. NHE at pH 10 and − 0.01 V vs. NHE at pH 7. The amount of dixanthogen formed on the chalcopyrite surface initially increased with the increase of the applied potential and then decreased with the increasing potential. The optimum potential range for dixanthogen formation on the chalcopyrite surface was from + 0.14 V to + 0.34 V vs. NHE at pH 10 and from + 0.14 V to + 0.44 V vs. NHE at pH 7. It was difficult to completely remove the dixanthogen film on the chalcopyrite surface.     

Hydrogeochemistry in the Vouga River basin (central Portugal): Pollution and chemical weathering

To quantify and explain the contributions by pollution and chemical weathering to their composition, we studied the chemistries of springs and surface waters in the mountainous part of the Vouga River basin. Water samples were collected during a number of consecutive summer campaigns. Recharge rates were derived from monitored discharge rates within the basin. Very large contributions by meteoric, agricultural and domestic sources to the water chemistries were found, identified by the chloride, sulfate and nitrate concentrations: on average only 1/4 to 1/3 of the solutes could be attributed to chemical weathering. Two petrologic units characterize the river basin: granites and metasediments. The waters collected within metasediment units are distinct from those in granite terrain by a higher magnesium concentration. On that basis, it could be estimated that the Rio Vouga, when leaving the mountainous part of the basin, has for some 2/5 a signature determined by chemical weathering in the metasediments. The dominant primary minerals subject to chemical weathering are plagioclase (Pl) and biotite (in granite) or Pl and chlorite (in metasediment). Kaolinite, gibbsite and vermiculite are the major weathering products where annual precipitation (P) > 1000 mm y⁻¹, and kaolinite, vermiculite and smectite where P was lower. Using an algorithm based on the ratio of dissolved silica to bicarbonate, the contributions of chemical weathering of primary minerals could be unraveled. The results show that in granite the export rate (as mol ha⁻¹ y⁻¹ wt%mineral⁻¹) of oligoclase (Pl with An_10–30) was 5.0 ± 2.6 and of biotite 3.2 ± 2.6, while in metasediment these rates for albite (Pl with An_0–10) are 16.5 ± 8.9 and for chlorite are 0.5 ± 0.5. The observed decrease of dissolved silica in surface waters relative to springs was ascribed to (summer) uptake by aquatic biota.     

Sorption and precipitation of Co(II) in Hanford sediments and alkaline aluminate solutions

Sorption and precipitation of Co(II) in simplified model systems related to the Hanford site high-level nuclear waste tank leakage were investigated through solution studies, geochemical modeling, and X-ray absorption fine structure (XAFS) spectroscopy. Studies of Co(II) sorption to pristine Hanford sediments (ERDF and Sub), which consist predominantly of quartz, plagioclase, and alkali feldspar, show an adsorption edge centered at pH ≈ 8.0 for both sediments studied, with sorption >99% above pH ≈ 9.0. Aqueous SiO₂ resulting from dissolution of the sediments increased in concentration with increasing pH, though the systems remained undersaturated with respect to quartz. XAFS studies of Co(II) sorption to both sediment samples reveal the oxidation of Co(II) to Co(III), likely by dissolved O₂, although this oxidation was incomplete in the Sub sediment samples. The authors propose that Fe(II) species, either in aqueous solution or at mineral surfaces, partially inhibited Co(II) oxidation in the Sub sediment samples, as these sediments contain significantly higher quantities of Fe(II)-bearing minerals which likely partially dissolved under the high-pH solution conditions. In alkaline solutions, Al precipitated as bayerite, gibbsite, or a mixture of the two at pH > 7; an amorphous gel formed at pH values less than 7. Aqueous Co concentrations were well below the solubility of known Co-bearing phases at low pH, suggesting that Co was removed from solution through an adsorption mechanism. At higher pH values, Co concentrations closely matched the solubility of a Co-bearing hydrotalcite-like solid. XAFS spectra of Co(II) sorbed to Al-hydroxide precipitates are similar to previously reported spectra for such hydrotalcite-like phases. The precipitation processes observed in this study can significantly reduce the environmental hazard posed by ⁶⁰Co in the environment.     

Optical and thermoluminescence dating of Middle Stone Age and Kintampo bearing sediments at Birimi,a multi-component archaeological site in Ghana

We report the first luminescence ages for the archeological and geological sediments forming the substrate of the Birimi archaeological site in the Northern Region of Ghana. The site's significance rests on the fact that it contains a rich collection of artifact assemblages representative of three distinct cultures, and that, on the basis of artifact typology, the earliest assemblage is diagnostic of the Middle Stone Age (MSA). In situ occurrences of MSA artifacts are found at over 1 m below today's surface. They are overlain by a ceramic-rich complex of a sedentary or semi-sendentary Later Stone Age culture known as the Kintampo. The western half of the site is dominated by the industrial remains of Iron Age smelting activity.Elemental, mineralogical, and sedimentological analysis of the cultural and sub-cultural sedimentary horizons at the site revealed at least three distinct lithostratigraphic units. The quartz sediments are derived from the sandstone of the Gambaga escarpment, mass wasted and accreted fluvially at a rate of 3.2 cm/ka, forming a wide terrace at Birimi. Silts and finer fractions derive from windblown dust, likely from White Volta River and granitic sources to the north. Soil forming processes and wide fluctuations in moisture have progressively reduced the sediments at depth to the resistant quartz and kaolinite, with rich iron oxide coatings, and created two ironstone horizons composed of goethite-cemented quartz nodules.Multiple aliquot green-light stimulated optical ages for 125–150 μm quartz grains yielded ages of 23.6±2.9 and 40.8±11.8 ka for the MSA-bearing sediments, and 58.4±15.3 ka for the base of the terrace. Radiocarbon ages on charcoal from Kintampo-bearing units are 3.36–3.83 ka cal BP, and are supported by thermoluminescence (TL) ages on pottery sherds and burnt house daub fragments of this cultural complex. A 0.4 ka age on sediment from the site's surface confirms that the quartz zeroes well when exposed to natural light. Sediments bearing the Kintampo artifacts, however, yielded ages of 7.8–16.9 ka. These ages were obtained on sediments from large pits, some over 50 cm deep, and they deviate only slightly from the ages expected for naturally aggraded sediments at these depths. We conclude, therefore, that extensive digging of pits by the Kintampo dwellers was followed by rapid refilling, and that the bulk mobilization of the matrix did not permit the sedimentary quartz grains to experience any appreciable zeroing at that time.     

40Ar/39Ar and U–Pb dating of the Fish Canyon magmatic system,San Juan Volcanic field,Colorado: Evidence for an extended crystallization history

The ∼ 5000 km³ Fish Canyon Tuff (FCT) is an important unit for the geochronological community because its sanidine, zircon and apatite are widely used as standards for the ⁴⁰Ar/³⁹Ar and fission track dating techniques. The recognition, more than 10 years ago [Oberli, F., Fischer, H. and Meier, M., 1990. High-resolution ²³⁸U–²⁰⁶Pb zircon dating of Tertiary bentonites and Fish Canyon Tuff; a test for age “concordance” by single-crystal analysis. Seventh International Conference on Geochronology, Cosmochronology and Isotope Geology. Geological Society of Australia Special Publication Canberra, 27:74], of a ≥ 0.4 Ma age difference between the U–Pb zircon ages and ⁴⁰Ar/³⁹Ar sanidine ages has, therefore, motivated efforts to resolve the origin of this discrepancy. To address this controversial issue, we initially performed 37 U–Pb analyses on mainly air-abraded zircons at ETH Zurich and nearly 200 ⁴⁰Ar/³⁹Ar measurements on hornblende, biotite, plagioclase and sanidine obtained at the University of Geneva, using samples keyed to a refined eruptive stratigraphy of the FCT magmatic system.Disequilibrium-corrected ²⁰⁶Pb/²³⁸U ages obtained for 29 single-crystal and three multi-grain analyses span an interval of ∼ 28.67–28.03 Ma and yield a weighted mean age of 28.37 ± 0.05 Ma (95% confidence level), with MSWD = 8.4. The individual dates resolve a range of ages in excess of analytical precision, covering ∼ 600 ka. In order to independently confirm the observed spread in zircon ages, 12 additional analyses were carried out at the Berkeley Geochronology Center (BGC) on individual zircons from a single lithological unit, part of them pre-treated by the “chemical abrasion” (CA) technique [Mattinson, J.M., 2005. Zircon U–Pb chemical abrasion (“CA-TIMS”) method: Combined annealing and multi-step partial dissolution analysis for improved precision and accuracy of zircon ages. Chemical Geology, 220(1–2): 47–66]. Whereas the bulk of the BGC results displays a spread overlapping that obtained at ETH, the group of CA treated zircons yield a considerably narrower range with a mean age of 28.61 ± 0.08 Ma (MSWD = 1.0). Both mean zircon ages determined at ETH and BGC are older than the ∼ 28.0 Ma ⁴⁰Ar/³⁹Ar eruption age of FCT – even when considering the possibility that the latter may be low by as much as ∼ 1% due to a miscalibration of the ⁴⁰K decay constants – and is thus indicative of a substantial time gap between magma crystallization and extrusion. The CA technique further reveals that younger FCT zircon ages are likely to be associated with chemically unstable U-enriched domains, which may be linked to crystallization during extended magma residence or may have been affected by pre-eruptive and/or post-eruptive secondary loss of radiogenic lead. Due to their complex crystallization history and/or age bias due to Pb loss, the FCT zircon ages are deemed unsuitable for an accurate age calibration of FCT sandine as a fluence monitor for the ⁴⁰Ar/³⁹Ar method.Even though data statistics preclude unambiguous conclusions, ⁴⁰Ar/³⁹Ar dating of sanidine, plagioclase, biotite, and hornblende from the same sample of vitrophyric Fish Canyon Tuff supports the idea of a protracted crystallization history. Sanidine, thought to be the mineral with the lowest closure temperature, yielded the youngest age (28.04 ± 0.18 Ma at 95% c.l., using Taylor Creek Rhyolite [Renne, P.R. et al., 1998. Intercalibration of standards, absolute ages and uncertainties in ⁴⁰Ar/³⁹Ar dating. Chemical Geology, 145: 117–152.] as the fluence monitor), whereas more retentive biotite, hornblende and plagioclase gave slightly older nominal ages (by 0.2–0.3 Ma). In addition, a laser step-heating experiment on a 2-cm diameter feldspar megacryst produced a “staircase” argon release spectrum (older ages at higher laser power), suggestive of traces of inherited argon in the system. Thermal and water budgets for the Fish Canyon magma indicate that the body remained above its solidus (∼ 700 °C) for an extended period of time (> 10⁵ years). At these temperatures, argon volume diffusion is thought to be fast enough to prevent accumulation of radiogenic Ar. If this statement were true, an existing isotopic record should have been completely reset within a few hundred years, regardless of the phase and initial age of the phenocryst. As these minerals are unlikely to be xenocrysts that were incorporated within such a short time span prior to eruption, we suggest that a fraction of radiogenic Ar can be retained > 10⁵ years, even at T ∼ 700 °C.