Mineral-water Interactions Research Articles

Response to the Comment by J. Horita and R.N. Clayton on “The studies of oxygen isotope fractionation between calcium carbonates and water at low temperatures”

The apparent inconsistency in calcite–water fractionation does occur between the arithmetic combination of Zhou and Zheng [Zhou G.-T., and Zheng Y.-F. (2003) An experimental study of oxygen isotope fractionation between inorganically precipitated aragonite and water at low temperatures. Geochim. Cosmochim. Acta 67, 387–399] and the experimental determination of Zhou and Zheng [Zhou G.-T., and Zheng Y.-F. (2005) Effect of polymorphic transition on oxygen isotope fractionation between aragonite, calcite and water: a low-temperature experimental study. Am. Mineral 90, 1121–1130]. To resolve this issue is to acknowledge whether or not the isotope salt effect of dissolved minerals would occur on oxygen isotope exchange between water and the minerals of interest. The question is whether or not a term of mineral–water interaction should be taken into account when calculating mineral–water 10 3ln α factors by an arithmetic combination between theoretical 10 3ln β factors for mineral and water, respectively. The hydrothermal experiments of Hu and Clayton [Hu G.-X., and Clayton R.N. (2003) Oxygen isotope salt effects at high pressure and high temperature, and the calibration of oxygen isotope geothermometers. Geochim. Cosmochim. Acta 67, 3227–3246] demonstrate the absence of isotope salt effect on the oxygen isotope fractionation between calcite and water, and this abnormal behavior reasonably explains the so-called inconsistency in the calcite–water fractionations of Zhou and Zheng (2003, 2005). We argue that the mineral–water correction is still necessary for calculation of fractionations in mineral–water systems. New experimental data for oxygen isotope fractionations involving dolomite and cerussite are consistent with the calculations of Zheng [Zheng Y.-F. (1999a) Oxygen isotope fractionation in carbonate and sulfate minerals. Geochem. J. 33, 109–126], but also shed light on the assumptions used in modifying the increment method. We argue that the modified increment method has developed into a theoretical mean of predictive power for calculation of oxygen isotope fractionation factors for crystalline minerals of geochemical interest.

Rapid fluid-rock interaction in oilfield reservoirs

We report here on production data from a North Sea oilfield that demonstrates that significant mineral-fluid interactions took place in the reservoir on a time scale of months. A time series of produced water compositions from the BP-operated Miller Field, in the UK sector of the North Sea, was generated over a period of more than 7 yr when seawater was being injected for secondary recovery and pressure maintenance. Deviation of the produced water chemistry from linear mixing of seawater and formation water indicates that mineral-water interactions took place in the reservoir. Of particular significance is the evidence for rapid reaction of injected water with silicates. The safety case for CO 2 storage in such reservoirs is greatly facilitated if it can be shown to react with the host pore waters and rocks on a human time scale, and the results of this study indicate that this is indeed the case.

Hydrogeochemical properties of CO2-rich thermal–mineral waters in Kayseri (Central Anatolia), Turkey

The present study highlights the hydrogeological and hydrogeochemical characteristics of the CO2-rich thermal–mineral waters in Kayseri, Turkey. These waters of Dokuzpinar cold spring (DPS) (12–13°C), Yesilhisar mineral spring (YMS) (13–16°C), Acisu mineral spring (ACMS) (20–22.5°C), Tekgoz thermal spring (TGS) (40–41°C), and Bayramhaci thermal-mineral spring (BTMS) (45–46.5°C) have different physical and chemical compositions. The waters are located within the Erciyes basin in the Central Anatolian Crystalline complex consisting of three main rock units. Metamorphic/crystalline rocks occur as the basement, sedimentary rocks of Upper Cretaceous-Quaternary age form the cover, and volcanosedimentary rocks Miocene-Quaternary in age represent the extrusive products of magmatism acting in that period. All these units are covered unconformably by terrace and alluvial deposits, and travertine occurrences have variable permeability. Dokuzpinar cold spring, YMS and ACMS localized mainly along the faults within the region have higher Na+ and Cl− contents whereas TGS and BTMS have higher amounts of Ca2+ and HCO3−. The high concentrations of Ca2+ and HCO3− are mainly related to the high CO2 contents resulting from interactions with carbonate rocks. Whereas the high Na+ content is derived from the alkaline rocks, such as syenite, tuff and basalts, the Cl− is generally connected to the dissolution of the evaporitic sequences. These waters are of meteoric-type. BTMS deviates from meteoric water line. The content is related to the increases in the δ18O compositions due to mineral–water interaction (re-equilibrium) process. CO2-dominated YMS and ACMS with low temperatures have higher mineralizations. Yesilhisar mineral spring, ACMS, TGS and BTMS are oversaturated in terms of calcite, aragonite, dolomite, goethite and hematite, and undersaturated with respect to gypsum, halite and anhydrite. Yesilhisar mineral spring, ACMS and BTMS are also characterized by recent travertine precipitation. Dokuzpinar cold spring is undersaturated in terms of the above minerals. The higher ratios of Ca/Mg and Cl/HCO3, and lower ratios of SO4/Cl in BTMS than TGS suggest that TGS has shallow circulation compared to BTMS, and/or has much more heat-loss enroute the surface. The sequence of hydrogeochemical and isotopic compositions of the waters is in an order of DPS>YMS>ACMS>TGS>BTMS and this suggests a transition period from a shallow circulation to a deep circulation path.

Hydrochemistry of Waters of Volcanic Rocks: The Case of The Volcanosedimentary Rocks of Thrace, Greece

This work is referred to the characterization of the environmental hydrochemistry in the broader Sapes area – Thrace region, on the basis of physico-chemical properties of surface and groundwaters occurring in the volcanosedimentary formations of this area, where gold mining activities are planned to operate. Volcanic rocks are considerably altered where they are in contact with hydrothermal solutions. Aquifers are formed within these formations. Surface and ground waters are strongly metalliferous and their hydrochemical facies present similar but complex water types. Certain characteristic chemical types are the following: Ca-Mg-HCO3-SO4, Ca-Mg-SO4-HCO3. Ca-SO4, Ca-Mg-SO4. Ca-Na-Cl-HCO3, Na-Cl. A small majority of the water samples present the following order of anion dominance HCO3− > SO42− > Cl−. Calcium is the dominant cation. Bicarbonates and sulfate ions are the dominant anions. The order of dominance for the heavy metals in surface and ground waters is as follows: Fe > Mn > Zn > Ni > Cu. The saturation index of waters regarding minerals is low. Computer simulation indicates that calcite and dolomite are common minerals in all water samples which are saturated in respect to quartz and argillaceous-siliceous minerals. The most pronounced property of waters is their acidic character. The high metal concentrations are related to water with low pH. Sulfide minerals control the low pH values of waters which is an important control factor for the evolution of the water chemical composition. The abundance of sulfates is attributed to the dissolution of the minerals pyrite (FeS2) and alunite (KAl3(SO4)2(OH)6. The water–mineral interactions are responsible for the chemical composition of waters. Water quality problems can be successfully handled by the use factor analysis. 17 chemical parameters can be substituted by five factors which successfully represent the hydrochemical processes as well as their geographic distribution. Volcanic rocks in the study area have the potential to produce acid drainage.

Crystal structure control of the dissolution of rare earth elements in water-mineral interactions

Leaching experiments with pyroxene and plagioclase minerals and whole-rock basalt and distilled water were performed to clarify rare earth element (REE) behavior during water-rock interactions. A system was developed to recover the leachate quickly under nearly neutral conditions (pH ∼6) with little re-precipitation or adsorption. The release ratios (amount of an element in the leachate from the minerals or rock relative to the amount in the original minerals or rock) of REE were comparable to those of Mg and Ca. Rare earth elements were easily leached. More heavy REE (HREE) than light REE (LREE) were leached from the plagioclase, and more LREE than HREE from the pyroxene. Very little europium was leached from the plagioclase. These release patterns show an inverse relationship with the elemental partition coefficient between the minerals and magma. The trends in the REE release ratios thus depend mostly on the compatibility of each element with the crystal structure of the mineral, rather than on chemical characteristics such as the solubility of the element itself. During the basalt-water interaction, a negative Eu anomaly in the leachate increased with time because Eu was initially leached from interstitial minor phases and later was dominantly from the plagioclase which is the major host mineral of Eu.

Understanding the salinisation process for groundwater in an area of south-eastern Australia, using hydrochemical and isotopic evidence

Increases in groundwater salinity down gradient are a common occurrence in catchments throughout Australia, exacerbating the effects of dryland salinisation in discharge areas. Groundwater hydrochemical and isotopic data from the Willaura catchment in south-eastern Australia, which is affected by both primary (natural) and secondary (human-induced) salinity, allowed the factors that cause the down gradient trend in salinisation within this catchment to be determined. The overall salinity of the groundwater (0.8–70 mS/cm) is controlled by evapotranspiration, which concentrates cyclic salts in the unsaturated zone, greatly increasing the salinity of the soil–water to 3700–6400 mg/L Cl −. The progressive addition of this saline soil–water to fresher groundwaters recharged on the catchment margins causes a gradual increase in salinity down gradient. The groundwater stable isotope compositions show that most recharge occurs in winter and spring. Groundwater salinities increase around the saline lakes of the discharge zone, due to direct evaporation from shallow watertables and minor dissolution of halite (as indicated by Cl −/Br − ratios). Mineral–water interactions within the aquifers cause a slight overall reduction in salinity; conversion of kaolinite to smectite and illite, and cation exchange of Na + for Ca 2+ on smectites, cause Na +/Cl −, Mg 2+/Cl −, K +/Cl −, Si/Cl − and HCO 3 −/Cl − ratios to progressively decline and pH to gradually increase down gradient. Oxidation of organic matter causes a strong decrease in the redox potential of the groundwaters down gradient. The groundwater ages increase from modern (containing tritium) on the catchment margins to ∼8000 years (based on 14C dating) at the end of the ∼25 km flow path. The older groundwaters are isotopically heavier than the younger waters, probably due to recharge during a wetter climate in the early Holocene. The groundwater ages show clearly that the progressive addition of saline soil–water, causing groundwaters to become more saline down gradient, has occurred throughout the Holocene and may explain the occurrence of primary salinisation within many Australian catchments.

Experimental studies of REE fractionation during water–mineral interactions: REE release rates during apatite dissolution from pH 2.8 to 9.2

Rare earth elements (REE) release rates were measured during the open-system dissolution of natural apatite at 25 °C, 2.8 < pH < 9.2, and at ionic strengths ranging from 0.001 to 0.02 mol/kg. All inlet solutions were Ca, REE, and P free. In agreement with previous studies, apatite dissolution rates determined from outlet solution Ca concentrations decrease monotonically with increasing pH. Outlet solution molar REE / Ca ratios differ from that of the dissolving apatite during the experiments, this behaviour is interpreted to stem from dissolution or precipitation of minor quantities of rhabdophane (REE(PO 4)· nH 2O). This interpretation is supported by solute speciation calculations, which indicate that REE are retained in the solid phase when the outlet fluids are supersaturated with respect to Nd-rhabdophane ( K sp ≈ 10 − 24.5 ± 0.5 ), but preferentially released when the outlet fluids are undersaturated with respect to Nd-rhabdophane. The relative concentrations of the REE in the outlet solutions suggest that the secondary rhabdophane is slightly enriched in light REE (Ce to Eu). The results of this study suggest that rhabdophane dissolution and precipitation during apatite–fluid interaction plays a major role in controlling surface and ground water REE signatures.

The influence of water removal on the strength and toughness of cortical bone

Although the effects of dehydration on the mechanical behavior of cortical bone are known, the underlying mechanisms for such effects are not clear. We hypothesize that the interactions of water with the collagen and mineral phases each have a unique influence on mechanical behavior. To study this, strength, toughness, and stiffness were measured with three-point bend specimens made from the mid-diaphysis of human cadaveric femurs and divided into six test groups: control (hydrated), drying in a vacuum oven at room temperature (21 °C) for 30 min and at 21, 50, 70, or 110 °C for 4 h. The experimental data indicated that water loss significantly increased with each increase in drying condition. Bone strength increased with a 5% loss of water by weight, which was caused by drying at 21 °C for 4 h. With water loss exceeding 9%, caused by higher drying temperatures (⩾70 °C), strength actually decreased. Drying at 21 °C (irrespective of time in vacuum) significantly decreased bone toughness through a loss of plasticity. However, drying at 70 °C and above caused toughness to decrease through decreases in strength and fracture strain. Stiffness linearly increased with an increase in water loss. From an energy perspective, the water–mineral interaction is removed at higher temperatures than the water–collagen interaction. Therefore, we speculate that loss of water in the collagen phase decreases the toughness of bone, whereas loss of water associated with the mineral phase decreases both bone strength and toughness.

Atmospheric Mercury Accumulation Rates Between 5900 and 800 Calibrated Years BP in the High Arctic of Canada Recorded by Peat Hummocks

In this paper, we present the first comprehensive long-term record of preanthropogenic rates of atmospheric mercury accumulation in dated peat deposits for the High Arctic of Canada. Geochemical studies of two peat hummocks from Bathurst Island, Nunavut reveal substantial inputs from soil dust (titanium), marine aerosols (bromine), and mineral-water interactions (uranium). Mercury, however, was supplied to these peat mounds exclusively by atmospheric deposition. Mercury concentration measurements and age dating of the peat profiles indicate rather constant natural "background" mercury flux of ca. 1 microgram per square meter per year from 5900 to 800 calibrated years BP. These values are well within the range of the mercury fluxes reported from other Arctic locations, but also by peat cores from southern Canada that provide a record of atmospheric Hg accumulation extending back 8000 years. Thus, preanthropogenic Hg fluxes in the Arctic were not significantly different from atmospheric Hg fluxes in the temperate zone. In preindustrial times, therefore, the High Arctic was no more important as a sink for global atmospheric mercury than was the temperate zone.

Hydrothermal Alteration Zoning and Kinetic Process of Mineral‐Water Interactions

AbstractThis study reports the kinetic experimental results of albite in water and in KCl solution at 22 MPa in the temperature range of 25 to 400°C. Kinetic experiments have been carried out in an open flow‐through reaction system (packed bed reactor). Albite dissolution is always incongruent in water at most temperatures, but becomes congruent at 300°C (close to the critical point 374°C). At temperatures from 25 to 300°C, the incongruent dissolution of albite is reflected by the fact that sodium and aluminum are easily dissolved into water, from 300 to 400°C it is reflected by silicon being more easily dissolved in water than Al and Na. Maximum albite dissolution rates in the flow hydrothermal systems have been repeatedly observed at 300°C, independent of flow rates.The kinetic experiments of albite dissolution in a KCl aqueous solution (0.1 mol KCl) indicate that the dissolution rate of albite increases with increasing temperature. Maximum silicon release rates of albite have been observed at 400°C, while maximum aluminum release rates of albite at 374°C. The reaction rates of albite also depend on the potassium concentration in the aqueous solution.These results can be used to interpret the mechanism for forming hydrothermal alteration. The kinetic experiments of mineral‐aqueous solutions interactions in the hydrothermal system from 25 to 400°C and at 22 MPa indicate that the formation of the feldspar‐mica‐kaolinite zoning occurring in some ore deposits may depend not only on the mineral stability but also on the kinetics of feldspar hydration, which is affected by the water property variation when crossing the critical point.

Interpretation and prediction of triple-layer model capacitances and the structure of the oxide-electrolyte-water interface

The interpretation of mineral-water interactions in processes such as surface-charge development, adsorption of aqueous ions onto surfaces, and the kinetics of dissolution at the scale of the oxide-electrolyte-water interface has been greatly facilitated by models of the electric double layer. In models that explicitly account for adsorption of the electrolyte ions, a critical parameter is the integral electric capacitance that expresses the charge at the surface relative to the drop in electric potential at some distance away from the surface where the electrolyte ions adsorb. Despite the widespread application of such surface complexation models, much uncertainty surrounds the choice of values of the integral capacitance because it appears to depend on the specific oxide and type of electrolyte, yet it cannot be directly measured.In the present study, it is shown that triple-layer model capacitances (C1), obtained in a consistent manner from regression of surface-charge data referring to a wide range of ionic strengths, electrolyte types, and mineral surfaces, fall into two groups: on rutile, anatase, and magnetite, values of C1 increase with decreasing crystallographic radius of the electrolyte cation from Cs+ to Li+; on quartz, amorphous silica, goethite, hematite, and alumina, values of C1 increase with decreasing hydrated electrolyte cation radius from Li+ to Cs+. The triple-layer model capacitances on both groups of solids can be described by a model of the mineral-water interface with physically reasonable parameters consistent with X-ray standing-wave studies of the rutile-water interface (Fenter et al., 2000). The model specifies a layer of chemisorbed water molecules at the surface and a layer of adsorbed electrolyte cations. On rutile, anatase, and magnetite, the layer of chemisorbed water molecules is interpenetrated by the layer of electrolyte cations that adsorb close to the surface as dehydrated, inner-sphere complexes. On quartz, amorphous silica, goethite, hematite, and alumina, the layer of chemisorbed water molecules varies from 0.0 to as much as 6.0 Å and the electrolyte cations form hydrated, outer-sphere complexes. The model capacitances are consistent with interfacial dielectric constants ranging from 20 to 62. Triple-layer model capacitances can now be predicted for oxides in either alkali or alkaline earth electrolyte solutions that have not been studied experimentally. In addition, predictions can be made of the structure of the oxide-electrolyte-water interface for many oxides and electrolytes.

Crystal Growth Mechanisms of Quartz Overgrowths in a Cambrian Quartz Arenite

ABSTRACT Mechanisms of crystal growth during quartz cementation can be investigated by determining the relationship between growth increments and crystal radii. Crystal growth is theoretically described as (1) dr/dt = k for polynuclear growth and spiral growth, where dr/dt is the rate, r is the radius of the crystal, and k is some constant, (2) dr/dt = kr for mononuclear growth, and (3) dr/dt = k/r for diffusion-limited growth. Empirical results in this study, where r = detrital grain radius and dr/dt = quartz overgrowth thickness, define another crystal growth mechanism, size-dependent growth described by the rate equation dr/dt = kr. Grain radii and quartz overgrowth thicknesses were measured in 16 thin sections of quartz arenite from the Galesville Sandstone (Cambrian, Wisconsin, U.S.A.) and plotted to determine correlation coefficients between radius and rate. Correlation coefficients ranged from 0.145 to 0.855. Ten of 16 samples show statistically significant correlations between detrital grain radii and overgrowth thicknesses representing dr/dt = kr. Standard deviations (sorting) of detrital grain size correlate with the correlation coefficient between radius and rate. Samples indicating no correlation may in fact grow by dr/dt = kr, but for smaller grain size ranges the overgrowth thicknesses cannot be measured precisely enough to detect small variations. Size-dependent growth (dr/dt = kr) is a new type of mineral-water interaction that has also been observed in ancient dolomites as well as metamorphic garnets and other various minerals grown in the laboratory and now in quartz-cemented sandstone. This indicates that crystal growth theory may not include some significant growth mechanisms.

Geochemistry of the peat bog at Etang de la Gruère, Jura Mountains, Switzerland, and its record of atmospheric Pb and lithogenic trace metals (Sc, Ti, Y, Zr, and REE) since 12,370 14C yr BP

A 650 cm core from a Swiss bog represents 12,370 14C years of peat accumulation and provides the first complete record of atmospheric Pb deposition for the entire Holocene. Excess, non-atmospheric Sr in the peat was calculated by normalizing Sr/Sc to crustal abundance; this was used to differentiate between the ombrogenic section of the bog in which inorganic solids are supplied exclusively by atmospheric deposition and the minerogenic zone where mineral-water interactions contribute metals to the peat. While sediment dissolution contributes significantly to the Sr inventory of the minerogenic section of the peat profile, there is no measurable effect of this process on the Pb burden. Isotopic analyses (204Pb, 206Pb, 207Pb, 208Pb) show that effectively all of the Pb in the peat profile was supplied exclusively from the atmosphere.To separate natural and anthropogenic Pb, Sc was selected over Ti, Y, Zr, Hf and REE as a conservative, reference element which is supplied by soil dust aerosols derived from rock weathering. Enrichment factors (EF) were calculated using the Pb/Sc ratio in the peat samples, normalized to the “natural, background” Pb/Sc which is found in peats dating from 8030 to 5320 14C yr BP. The results show that anthropogenic source have dominated the supply of atmospheric Pb to the peat core continuously since 3000 14C yr BP. The aerosols supplied to the bog can be divided into 3 classes: a) Pre-Anthropogenic (older than 3000 14C yr BP with Pb EF < 2 and 206Pb/207Pb > 1.194); b) Pre-Industrial (dating from 3000–240 14C yr BP, with Pb EF ≥ 2 but <20 and 206Pb/207Pb 1.193–1.179); c) Industrial (more recent than 240 14C yr BP, with Pb EF > 20 and 206Pb/207Pb < 1.179).Elevated soil dust fluxes are observed at 5320, 8230 and 10,590 14C yr BP; the latter corresponds to the Younger Dryas. Aluminum, Zr, Hf, and REE/Sc ratios also are elevated at the same depths, suggesting differences in particle size, wind strength, or source regions. Pre-Anthropogenic aerosols deposited since 8230 14C yr BP reveal Pb/Sc which is significantly higher, and 206Pb/207Pb which is less radiogenic, than during the early Holocene. While the trend toward increasing Pb/Sc could be due to chemical weathering and soil development, this could not explain the shift in Pb isotopic composition. The changes which took place at 8230 14C BP, therefore, may be related to a large scale climatic reorganization which, at present, is poorly understood.

Mineral-water interaction and hydrogeochemistry in the Samkwang mine area, Korea.

The abandoned Samkwang mine area was chosen as the research site for radioactive waste disposal in Korea. The study site chiefly consists of Pre-cambrian granitic gneiss, which is considered as potential host rock of a radioactive waste repository. In this study, the chemical composition and residence time of groundwater collected at different depths and locations in the mine adits and boreholes nearby the mine were investigated. The factor analysis of hydrochemical data was applied to illustrate mineral-water reaction processes controlling the chemical composition of groundwater. Three chemical types of groundwater were recognized: Ca-HCO3 type of shallow groundwater and Ca(Mg)-SO4(HCO3) type of shallow and deep groundwater in the mine, and Na-HCO3 type of borehole groundwater. Tritium level indicates that the deep and borehole groundwater is older water recharged before 1950's, and that shallow groundwater is younger water recharged after 1950's. Thermodynamic equilibrium states between major minerals and water samples were calculated in the term of the activities of chemical constituents. The results of this study show that groundwater chemistry in the study area was evolved through the dissolution of calcite, chlorite, albite and sulfides, and the precipitation of clay and oxide minerals. The results of this study would be useful for the assessment of hydrochemical and mineralogical effects on the fate of radionuclides along groundwater pathways in the abandoned mine as a potential radioactive waste repository.

Carbonate-mineral/water interactions in sulfide-rich mine tailings

The chemical composition and mineralogy of coatings on carbonate minerals from mine tailings have been studied using aqueous geochemical methods, Time-of-Flight Laser-Ionization Mass Spectrometry (TOF-LIMS) and Transmission Electron Microscopy (TEM). The goal is to study major and trace element partitioning between the aqueous and solid phase, and to infer mechanisms that control the concentrations of elements in the pore water of sulfide-rich mine tailings. Pore-water samples and carbonate-mineral grains were collected from four geochemically distinct zones within the tailings. Oxidation of sulfide minerals near the surface results in a large range in pore-water pH (3.85 to 6.98) and aqueous concentrations of metals and sulfate. With increasing depth in the tailings, mineral-water interactions lead to increasing pH, and decreasing concentrations of metals and sulfate. Calculated mineral saturation indices, trends in the abundance of Ca, Fe, Mg and Mn in TOF-LIMS profiles through the secondary coatings, and electron diffraction patterns obtained from the coatings, suggest that precipitation/dissolution of jarosite-group minerals, gypsum, goethite, akaganéite, amorphous Fe oxyhydroxides and siderite control the aqueous Ca, Fe, Na, K and SO 4 concentrations. The occurrence of secondary coatings on primary minerals is widespread, and reactions with the secondary minerals, rather than the primary mineral substrate, probably represent the principal controls on trace-element distributions in the pore water. The data indicate that adsorption, surface-complexation and co-precipitation reactions are important controls on the concentrations of trace elements in the pore water. The occurrence of siderite coatings on the surface of ankerite grains suggests that Fe-bearing dolomite-structure carbonate minerals dissolve incongruently. This corroborates inferences made by previous workers that solubility differences between calcite and siderite lead to calcite dissolution and siderite precipitation during acid neutralization. The accumulation of secondary coatings will diminish the rate of mineral-dissolution reactions involving the primary minerals. Consequently, the use of kinetic rate constants obtained from the literature for simulating the dissolution of primary mineral phases such as the carbonates and alumino-silicates will likely overestimate the actual rates.

Error Propagation in Geochemical Calculations of Groundwater‐Mineral Interactions: A Case Study from the Pingdingshan Coalfield, Henan

AbstractThe WATEQ4F‐based “multiple step method”, or the method of running WATEQ4F repeatedly, was used to evaluate the error propagation in computations of water‐mineral interactions under the circumstances of constant and varied temperatures due to the uncertainties of input variables. The results show the following: the errors of water chemistry analysis can strongly affect the modelling results of water‐mineral reactions; different input variables (errors) have different effects on the saturation indices (S.I.) of different minerals; in many cases, the S.I. errors of minerals change with temperatures.

WEATHERING, BIOMASS PRODUCTION AND GROUNDWATER CHEMISTRY IN AN AREA OF DOMINANT ANTHROPOGENIC INFLUENCE, THE CHAVES-VILA POUCA DE AGUIAR REGION, NORTH OF PORTUGAL

Using a combination of a grouping algorithm and a geochemical mole balance algorithm, we could identify and quantify the contributions made by water-rock and water-biomass interactions to the composition of shallow groundwaters in northern Portugal, in an area with a sizable anthropogenic input (the Chaves-Vila Pouca de Aguiar region). The first algorithm, based on the mathematical concept of equivalent relations, allows for the grouping of water samples with similar chemistries. The second algorithm, which uses the stoichiometries and mole/charge balances in weathering reactions and biological processes, provides the possibility to relate the average chemistry of each identified group to water-mineral and water-biomass interactions. This algorithm hinges on ratios of dissolved silica and bicarbonate, constituents considered to be derived only from chemical weathering and biomass production. Background information on the petrology, mineralogy and chemistry of bedrocks and soils, on climatic differences, on the history of deforestation and reforestation of the study area, and on the application and composition of fertilizers and manure, was used to identify the most realistic weathering reactions and biological processes which contribute to the water chemistry. The concentrations of chloride, sulphate and nitrate were considered to represent sources other than chemical weathering or biomass changes, like cyclic salts, fertilizers and manure. Despite the high background concentrations derived from such sources, we were able to quantify the contributions of the identified weathering reactions and botanical uptake to the groundwater composition. The spatial distributions of the various groundwater groups shows a good correlation with the geology, land use and climatic differences in the area.

Mineral-Water Interactions in Acid Mine Drainage Environments: A TEM, AEM, and EFTEM Study

R. J. Harrison A. Putnis C. Putnis Background. Mineral-water interfacial interactions associated with working and abandoned sulphidebearing mines and mining wastes are among the most complex, dynamic, and environmentally important of all near-surface rock-water systems (e.g. Jambor and Blowes, 1994). These mining sites, numbering approximately 200,000 in the United States alone, typically release large amounts of heavy metals into the environment ranging up to hundreds of kilometers down hydrologic gradients in relatively short times. Contamination occurs when high solute concentrations of iron form during the weathering of pyrite and associated metal sulphides. Acidic effluent flowing from mine adits and waste piles mix with air and oxygenated surface water and precipitate iron oxides, oxyhydroxides, and/or hydroxysulphates. These phases form mineral/rock coatings and ironrich sediments that may contain very high concentrations of associated metals (up to percent levels of, for example, Cu, Zn, and Pb). Field and laboratory studies suggest that both the growth/dissolution of discrete metal-bearing precipitates and adsorption/desorption reactions are the most important processes in the attenuation/release of toxic metals. However, despite thermodynamic predictions, as well as mineralogical (e.g. XRD) and geochemical (e.g. extraction) techniques, many of the phases and interfaces which contain metals other than iron have generally not been identified with reliable certainty (e.g. Jambor, 1994). An understanding of the minerals and mineral coatings present, as well as their composition, degree of crystallinity, and microfabric are all critical in understanding toxic metal partitioning (including sorption/desorption reactions) under different Eh/pH environments in these complex natural systems. Further, modelling the formation of AMD environments, and the transport of toxic metals through and away from these sites, would be greatly aided by a Department of Geological Sciences, Virginia Polytechnic Institute, Blacksburg, VA 24061, USA

U-Pb systematics of monazite and xenotime: case studies from the Paleoproterozoic of the Grand Canyon, Arizona

Monazite is accepted widely as an important U-Pb geochronometer in metamorphic terranes because it potentially preserves prograde crystallization ages. However, recent studies have shown that the U-Pb isotopic system in monazite can be influenced by a variety of processes that partially obscure the early growth history. In this paper, we attempt to interpret complex monazite and xenotime U-Pb data from three Paleoproterozoic granite dikes exposed in the Grand Canyon. Single-crystal monazite analyses from an unfoliated granite dike spread out along concordia from the crystallization age of the dike (defined by U-Pb zircon data to be 1685 ± 1 Ma) to 1659 ± 2 Ma, a span of 26 million years. Back-scattered electron (BSE) imaging reveals that magmatic domains within most crystals from this sample are truncated by secondary domains associated with prominent embayments at the grain margin. Fragments of a single crystal yield contrasting, concordant dates and fragments from the edges and tips of crystals yield the youngest dates. Based on these observations we suggest that the secondary domains formed at least 26 million years after the crystal formed. Monazite and xenotime dates from the second sample, a sheared dike that cross-cuts the previous dike, spread out along concordia over 16 million years and range up to 2.4% normally discordant. Again, BSE imaging reveals secondary domains that truncate both magmatic zoning and xenocrystic cores. Fragments sliced from specific domains of a previously imaged monazite crystal demonstrate that the secondary domain is 13 million years younger than the core domain. Textures revealed in BSE images suggest that the secondary domains formed by fluid-mineral interaction. Normal discordance appears to result from both radiation damage accumulated at temperatures below 300 °C and water-mineral interaction. Monazite data from the third sample exhibit dispersion in both the 207Pb/206Pb dates (1677–1690 Ma) and discordance (+ 1.6% to − 3.1%). Reverse discordance in these monazites cannot be explained by incomplete dissolution or excess (thorogenic) 206Pb. Sliced fragments from several crystals reveal dramatic intragrain U-Pb disequilibrium that does not correlate with either Th or U concentration or position within the crystal. We suggest that reverse discordance resulted from mechanisms that involve exchange or fractionation of elemental U or elemental Pb, and that neither the U-Pb dates nor the 207Pb/206Pb dates are reliable indicators of the rock's crystallization age. Given the large number of processes proposed in the recent literature to explain monazite U-Pb systematics from rocks of all ages, our results can be viewed as another cautionary note for single-crystal and multi-crystal monazite geochronometry. However, we suggest that because individual crystals can preserve a temporal record of primary and secondary monazite growth, micro-sampling of individual monazite crystals may provide precise absolute ages on a variety of processes that operate during the prograde, peak and/or retrograde history of metamorphic terranes.

Hydrochemical Evolution of Sodium-Sulfate and Sodium-Chloride Groundwater Beneath the Northern Chihuahuan Desert, Trans-Pecos, Texas, USA

Groundwater beneath the northern Chihuahuan Desert, Trans-Pecos, Texas, USA, occurs in both carbonate and siliciclastic aquifers beneath a thick unsaturated zone in shallow Rio Grande alluvium. Groundwater hydrochemical evolution was investigated by analyzing soils, soils leachates, bolson-fill sediments, water from the unsaturated zone, and groundwater from three major aquifers. Ionic relations, mineral saturation states, and geochemical modeling show that groundwater compositions are controlled by reactions in the unsaturated zone, mineralogy of unsaturated sediments and aquifers, position in the groundwater flow system, and extensive irrigation. Recharge to aquifers unaffected by irrigation is initially a Ca-HCO3 type as a result of dissolving carbonate surficial salts. With continued flow and mineral-water interaction, saturation with calcite and dolomite is maintained, gypsum is dissolved, and aqueous Ca and Mg are exchanged for adsorbed Na to produce a Na-SO4 water. Groundwater in Rio Grande alluvium is a Na-Cl type, reflecting river-water composition and the effects of irrigation, evapotranspiration, and surficial salt recycling. These results document two hydrochemical evolution paths for groundwater in arid lands. If recharge is dilute precipitation, significant changes in water chemistry can occur in unsaturated media, ion exchange can be as important as dissolution-precipitation reactions in determining groundwater composition, and mineral-water reactions ultimately control groundwater composition. If recharge is return flow of irrigation water that already contains appreciable solutes, mineral-water reactions are less important than irrigation-water composition in determining groundwater chemistry.