Mineral Reaction Rates Research Articles

Water-Rock interaction in the Long Valley Caldera (USA)

Water-rock interactions within the main thermal aquifer in the Long Valley Caldera are evaluated using water chemistry data from a new suite of samples. The results reflect the impact of increased geothermal production and major CO2 loss, which appears to drive calcite precipitation in the aquifer. The study provides qualitative information on the rates of mineral reactions and the response times of chemical geothermometers to declining temperatures.

How predictable are dissolution rates of crystalline material?

The large discrepancy between field and laboratory measurements of mineral reaction rates is a long-standing problem in earth sciences, often attributed to factors extrinsic to the mineral itself. Nevertheless, differences in reaction rate are also observed within laboratory measurements, raising the possibility of intrinsic variations as well. Critical insight is available from analysis of the relationship between the reaction rate and its distribution over the mineral surface. This analysis recognizes the fundamental variance of the rate. The resulting anisotropic rate distributions are completely obscured by the common practice of surface area normalization. In a simple experiment using a single crystal and its polycrystalline counterpart, we demonstrate the sensitivity of dissolution rate to grain size, results that undermine the use of “classical” rate constants. Comparison of selected published crystal surface step retreat velocities (Jordan and Rammensee, 1998) as well as large single crystal dissolution data (Busenberg and Plummer, 1986) provide further evidence of this fundamental variability. Our key finding highlights the unsubstantiated use of a single-valued “mean” rate or rate constant as a function of environmental conditions. Reactivity predictions and long-term reservoir stability calculations based on laboratory measurements are thus not directly applicable to natural settings without a probabilistic approach. Such a probabilistic approach must incorporate both the variation of surface energy as a general range (intrinsic variation) as well as constraints to this variation owing to the heterogeneity of complex material (e.g., density of domain borders). We suggest the introduction of surface energy spectra (or the resulting rate spectra) containing information about the probability of existing rate ranges and the critical modes of surface energy.

Clay-Microbe Interactions and Implications for Environmental Mitigation

Clay minerals are ubiquitous in soils, sediments, and sedimentary rocks, and they play important roles in environmental processes. Microbes are also abundant in these geological media, and they interact with clays via a variety of mechanisms, such as reduction and oxidation of structural iron and mineral dissolution and precipitation through the production of siderophores and organic acids. These interactions greatly accelerate clay mineral reaction rates. While it is certain that microbes play important roles in clay mineral transformations, quantitative assessment of these roles is limited. This paper reviews some active areas of research on clay–microbe interactions and provides perspectives for future work.

Formation and stability of Pb-, Zn- & Cu-PO 4 phases at low temperatures: Implications for heavy metal fixation in polar environments

Low temperatures and frequent soil freeze–thaw in polar environments present challenges for the immobilisation of metals. To address these challenges we investigated the chemical forms of Pb, Zn and Cu in an Antarctic landfill, examined in vitro reaction kinetics of these metals and orthophosphate at 2 and 22 °C for up to 185 days, and subjected the products to freeze–thaw. Reaction products at both temperatures were similar, but the rate of production varied, with Cu-PO 4 phases forming faster, and the Zn- and Pb-PO 4 phases slower at 2 °C. All metal-orthophosphate phases produced were stable during a 2.5 h freeze–thaw cycle to −30 °C. Metal immobilisation using orthophosphate can be successful in polar regions, but treatments will need to consider differing mineral stabilities and reaction rates at low temperatures.

Reactive transport modeling for CO 2 geological sequestration

One way to reduce carbon dioxide (CO 2) releases to the atmosphere, is to capture it from power plants and then inject it into deep geological formations. Understanding water-gas-mineral reactions is the core of assessing the long term storage and risks in geological sequestration. Because of various limitations of laboratory and field tests, reactive transport modeling has been one important supplementary or independent tool to study the water–gas–mineral reactions at different components of geological sequestration. In this paper, we reviewed the applications of reactive transport modeling to three aspects of CO 2 geological sequestration: the behavior of CO 2 in storage formation, the storage security issues related to caprock integrity or wellbore cement degradation, and the change of the shallow groundwater in response to the potential leakage of CO 2. Key finding are summarized and further research needs are identified. The lack of the detailed mineralogical composition information at the storage formation, caprock and overlying aquifers is currently the first obstacle for quantitative prediction of geochemical evolution. The thermodynamic properties of relevant minerals under high temperature and pressure have to be refined. Reliable reaction rates of minerals dissolution/precipitation at field are still the bottle neck of the calculating the long term mineral trapping of CO 2. Issues related to relatively fast reactions such sorption and ion exchange need special attention when the impact of CO 2 leakage on shallow groundwater was evaluated. The role of organic compounds in water-gas-mineral reactions needs further study.

Accurate Rates of the Complex Mechanisms for Growth and Dissolution of Minerals Using a Combination of Rare-Event Theories

Mineral growth and dissolution are often treated as occurring via a single reversible process that governs the rate of reaction. We show that multiple distinct intermediate states can occur during both growth and dissolution. Specifically, we used metadynamics, a method for efficiently exploring the free-energy landscape of a system, coupled to umbrella sampling and reactive flux calculations to examine the mechanism and rates of attachment and detachment of a barium ion onto a stepped barite (BaSO(4)) surface. The activation energies calculated for the rate-limiting reactions, which are different for attachment and detachment, precisely match those measured experimentally during both growth and dissolution. These results can potentially explain anomalous non-steady-state mineral reaction rates observed experimentally and will enable the design of more efficient growth inhibitors and facilitate an understanding of the effect of impurities.

A coupled electrochemical–geochemical model of corrosion for mild steel in high-pressure CO2–saline environments

Wellbore systems are designed to isolate fluids in the subsurface and are typically engineered for a 30–50 year service life. In the geologic sequestration of CO2, wellbores will have to perform for 100s of years. As a consequence, one of the key questions in the viability of sequestration is whether long-term wellbore integrity is feasible in the high-salinity, high-CO2 fluids likely to be present in CO2 storage reservoirs. Isolation in wellbores is usually accomplished by a combination of Portland cement and steel. In this study, we focus on predicting the corrosion rate of steel under typical CO2 sequestration conditions.We have developed a mechanistic model for predicting corrosion rates of mild steel used in most wellbore systems. The model includes an aqueous geochemistry and an electrochemistry module. The water chemistry module uses the Pitzer formulation for activity and Duan et al.’s (2006) model for CO2 solubility. The electrochemical module accounts for both mass transfer processes and electrochemical kinetics. The electrochemistry includes the primary oxidation reaction (the dissolution of iron) and the primary reduction reactions (the formation of H2 gas from carbonic acid, hydrogen ion, and/or water).At high CO2 pressures, the dominant corrosion reaction is Fe+2H2CO3=Fe2++2HCO3−+H2(g) and is driven by CO2 solubility rather than solution acidity. This result shows that typical buffering reactions between dissolved CO2 and minerals (e.g., carbonates) will not significantly reduce corrosion rates in contrast to many mineral reaction rates that are strongly dependent on pH. For similar reasons, high salinity solutions reduce corrosion rates significantly mainly due to the “salting-out” effect of reduced CO2 solubility. For example, an increase in salinity from 5 to 20wt% salt results in a 50% reduction in corrosion rate as confirmed in our experiments.Numerical simulations are tabulated that provide predicted rates of corrosion in wellbore environments over a wide range of temperature, partial pressure of CO2 and salt concentration.

Wellbore integrity at the Krechba Carbon Storage Site, In Salah, Algeria: 2. Reactive transport modeling of geochemical interactions near the Cement–Formation interface

Abstract A reactive transport modeling approach was used to assess key geochemical reactions between wellbore cement, formation mineralogy, and injected supercritical CO 2 at the Krechba natural gas field at In Salah, Algeria. Characterization of these reactions is important for understanding changes in porosity and permeability and the propagation or sealing of fractures in the formation or wellbore cement. Experiments involving the reaction of CO 2 with reservoir mineral assemblages and wellbore cement under in situ conditions, combined with geochemical modeling, were used to identify candidate reactive mineral phases and reaction rates specifically applicable to CO 2 injection at In Salah. These findings informed a reactive transport model which considered advective transport of CO 2 along the wellbore-formation interface and diffusive transport of CO 2 and brine constituents into juxtaposed wellbore cement. Model results indicate shallow carbonation of the cement along the interface, leading to changes in cement porosity. Diffusive transport of cations such as Ca, Fe, and Al between the cement and formation materials results in mineralogical changes in the formation material immediately adjacent to the cement, including localized dissolution of calcite and precipitation of siderite, magnesite, gibbsite, smectite, and amorphous silica. In contrast to the cement, the modeled porosity changes in the formation material appear to be minor. Taken together, these results suggest (1) significant retardation of the rate of advance of CO 2 along the interface, and (2) relatively minor impacts to permeability.

Investigating dissolution of mechanically activated olivine for carbonation purposes

Mineral carbonation is one of several alternatives for CO2 sequestration and storage. The reaction rates of appropriate minerals with CO2, for instance olivine and serpentine with vast resources, are relatively slow in a CO2 sequestration context and the rates have to be increased to make mineral carbonation a good storage alternative. Increasing the dissolution rate of olivine has been the focus of this paper. Olivine was milled with very high energy intensity using a laboratory planetary mill to investigate the effect of mechanical activation on the Mg extraction potential of olivine in 0.01 M HCl solution at room temperature and pressure. Approximately 30–40% of each sample was dissolved and water samples were taken at the end of each experiment. The pH change was used to calculate time series of the Mg concentrations, which also were compared to the final Mg concentrations in the water samples. Percentage dissolved and the specific reaction rates were estimated from the Mg concentration time series. The measured particle size distributions could not explain the rate constants found, but the specific surface area gave a good trend versus dissolution for samples milled wet and the samples milled with a small addition of water. The samples milled dry had the lowest measured specific surface areas (<4 m2/g), but had the highest rate constants. The crystallinity calculated from X-ray diffractograms, was the material parameter with the best fit for the observed differences in the rate constants. Geochemical modelling of mechanically activated materials indicated that factors describing the changes in the material properties related to the activation must be included. The mechanically activated samples in general reacted faster than predicted by the theoretical models. Mechanical activation as a pre-treatment method was found to enhance the initial specific reaction rates by approximately three orders of magnitude for a sample milled dry for 60 min in a planetary mono mill compared to an unactivated sample. Wet milling in the planetary mill did not produce samples with the same maximum reaction rate as dry milling, but wet milling in general might be easier to implement into a wet carbonation process. Mechanical activation in a planetary mill is likely to consume too much energy for CO2 sequestration purposes, but the increase in obtained olivine rate constants illustrates a potential for using milling as a pre-treatment method.

Controls on rind thickness on basaltic andesite clasts weathering in Guadeloupe

A clast of low porosity basaltic andesite collected from the B horizon of a soil developed on a late Quaternary volcaniclastic debris flow in the Bras David watershed on Basse-Terre Island, Guadeloupe, exhibits weathering like that observed in many weathered clasts of similar composition in other tropical locations. Specifically, elemental profiles measured across the core–rind interface document that primary minerals and glass weather to Fe oxyhydroxides, gibbsite and minor kaolinite in the rind. The earliest reaction identified in the core is oxidation of Fe in pyroxene but the earliest reaction that creates significant porosity is plagioclase dissolution. Elemental loss varies in the order Ca ≈ Na > K ≈ Mg > Si > Al > Fe ≈ P ≫ Ti, consistent with the relative reactivity of phases in the clast from plagioclase ≈ pyroxene ≈ glass > apatite > ilmenite. The rind surrounds a core of unaltered material that is more spherical than the original clast. The distance from the core–rind boundary to a visually prominent rind layer, L, was measured as a proxy for the rind thickness at 36 locations on a slab cut vertically through the nominal center of the clast. This distance averaged 24.4 ± 3.1 mm. Maximum and minimum values for L, 35.8 and 20.6 mm, were observed where curvature of the core–rind boundary is greatest (0.12 mm − 1) and smallest (0.018 mm − 1) respectively. Extrapolating from other rinds in other locations, the rate of rind formation is estimated to vary by a factor of about 2 (from ∼ 4 to 7 × 10 − 14 m s − 1) from low to high curvature. The observation of a higher rate of rind formation for a higher curvature interface is consistent with a diffusion-limited model for weathering rind formation. The diffusion-limited model predicts that, like rind thickness, values of the thickness of the reaction front ( h) for a given reaction, defined as the zone over which a parent mineral such as plagioclase completely weathers to rind material, should also increase with curvature. Values of h were quantified as a function of interface curvature using bulk chemical analysis (500 < h < 2000 μm). Values of h were also quantified by measuring loss of matrix glass and increase in porosity as a function of curvature. In contrast to rind thickness, h shows no consistent increase with curvature. This contradiction is attributed to the mm-scale roughness of the interface which is related to phenocryst grain size. Therefore, the overall rind formation rate is strongly affected by curvature measured at the scale of the clast, while mineral reaction rates documented by reaction front thickness are strongly affected by curvature at the scale of phenocrysts. Similarly, the weathering advance rate (m s − 1) for the entire Bras David watershed can be extrapolated from the clast weathering rate if roughness at the watershed scale equals values of approximately 400–800.

Dissolution of Columbia River Basalt under mildly acidic conditions as a function of temperature: Experimental results relevant to the geological sequestration of carbon dioxide

Abstract Increasing attention is being focused on the rapid rise of CO2 levels in the atmosphere, which many believe to be the major contributing factor to global climate change. Sequestering CO2 in deep geological formations has been proposed as a long-term solution to help stabilize CO2 levels. However, before such technology can be developed and implemented, a basic understanding of H2O–CO2 systems and the chemical interactions of these fluids with the host formation must be obtained. Important issues concerning mineral stability, reaction rates, and carbonate formation are all controlled or at least significantly impacted by the kinetics of rock–water reactions in mildly acidic, CO2-saturated solutions. Basalt has recently been identified as a potentially important host formation for geological sequestration. Dissolution kinetics of the Columbia River Basalt (CRB) were measured for a range of temperatures (25–90 °C) under mildly acidic to neutral pH conditions using the single-pass flow-through test method. Under anaerobic conditions, the normalized dissolution rates for CRB decrease with increasing pH (3 ⩽ pH ⩽ 7) with a slope, η, of −0.15 ± 0.01. Activation energy, Ea, has been estimated at 32.0 ± 2.4 kJ mol−1. Dissolution kinetics measurements like these are essential for modeling the rate at which CO2-saturated fluids react with basalt and ultimately drive conversion rates to carbonate minerals in situ.

A basin scale perspective on CO2 storage—short and long term risks

Abstract The CO 2 retention capacity of sediments is a very important control on the long term CO 2 content of the atmosphere. When CO 2 is stored at depths exceeding c. 800–1000 m, several important overburden sinks prevent CO 2 from returning to the atmosphere. In addition to seal capacity, CO 2 is immobilized by dissolution in overburden formation water, by sorption onto mineral surfaces and organic matter, by anionization during carbonate dissolution, and by mineralization. The purpose of this presentation is to show that the combined capacity of sinks in overlying sediments is most likely much in excess of the need for permanent storage, and that mineral reaction rates are fast enough to benefit from the sinks. There is however a need to improve the data background which is supporting this tentative conclusion. Basin scale natural CO 2 processes and the baselines associated with them, have so far been only weakly constrained. In particular better quantitative constraints are needed on the magnitude of natural CO 2 sources, reservoirs and sinks, before realistic retention capacities of overburden sediments may be estimated.

Acid Mine Drainage at the Abandoned Kettara Mine (Morocco): 2. Mine Waste Geochemical Behavior

Weathering and humidity cell tests were used to predict the potential for acid mine drainage (AMD) and to estimate the mineral reaction rates and depletion of fine and coarse tailings from the abandoned Kettara mine, Morocco. The geochemistry of the fine and coarse mine wastes was similar and, as expected by static tests, the wastes produced significant amounts of AMD. The sulfate production rate of both fine and coarse tailings was very high (2,000–8,000 and 2,400–560 mg SO4/kg/week, respectively) during the first weeks of kinetics tests. After 9 weeks, sulfate release became low, ranging between 600 and 78 mg SO4/kg/week for fine tailings and 500–120 mg SO4/kg/week for coarse tailings. Effluent water samples had low pH (2.9–4.2) and elevated concentrations of acidity, sulfate, iron, copper, and zinc. Most or all of the dissolved K, Na, Al, Mg, and Si in the AMD result from the acidic dissolution of silicates (chlorite, talc, muscovite, and albite). Fine tailings produce much higher concentrations of acidity and sulfate than coarse tailings. However, due to greater transport of oxygen and water within the coarse waste, coarse tailings could be of greater environmental significance than fine tailings. The coarse waste continued to release acid after 378 days of leaching, whereas the fine tailings naturally passivates. These laboratory results agree with field observations; the upper profile of the coarse waste rock dam is highly oxidized (75 cm) whereas oxidation in the fine tailings does not extend more than 5–15 cm beneath the surface. A comparison between weathering and humidity cell tests indicated that the general trend of dissolution of metals was essentially similar for both methods. However, sulfate depletion rates were higher for the weathering cell tests. These tests indicate that the Kettara tailings piles and dam will continue to release acid for a long time unless remedial action is taken.

Applicability of averaged concentrations in determining geochemical reaction rates in heterogeneous porous media

This work examines the applicability of averaged concentrations, a mathematical analog of field-measured solute concentrations averaged over a large number of pores, in determining mineral reaction rates in heterogeneous porous media. Pore-scale network models were used to represent sandstones with anorthite and kaolinite as reactive minerals that are heterogeneously distributed in space. Reaction rates calculated from averaged concentrations were compared to true reaction rates that take into account variabilities in individual pore properties. Simulations were run under the highly acidic conditions relevant to geological CO2 sequestration in deep brine formations under various mineralogical and flow conditions. Results show that, under conditions where incomplete mixing arises, the averaged concentrations and analogously the field-measured concentrations, do not accurately reflect reaction progress. Over the length scale of several millimeters, the anorthite dissolution rates can be overestimated by a factor of three. For kaolinite, due to its highly nonlinear reaction rate law, even the reaction direction may be incorrectly determined, with precipitation predicted as dissolution. The extent of errors introduced depends on the extent of incomplete mixing. Conditions that homogenize the concentration fields, such as small reactive mineral clusters, abundant reactive minerals, and very fast or very slow flow rates, minimize errors introduced from averaging. These results indicate that the averaging scheme may partly contribute to the often-cited laboratory-field rate discrepancy and have important implications for the interpretation of concentration data obtained from field investigation.

Redox evolution and mass transfer during serpentinization: An experimental and theoretical study at 200 °C, 500 bar with implications for ultramafic-hosted hydrothermal systems at Mid-Ocean Ridges

Highly reducing and high-pH vent fluids characterize moderately low temperature ultramafic-hosted hydrothermal systems, such as the recently discovered Lost City hydrothermal field at 30°N Mid-Atlantic Ridge Ridge (MAR). To better understand the role of mineral reaction rates on changes in fluid chemistry and mineralization processes in these and similar systems, we conducted an experimental study involving seawater and peridotite at 200 °C, 500 bar. Time series changes in fluid chemistry were monitored and compared with analogous data predicted using experimental and theoretical data for mineral dissolution rates. Although there was qualitative agreement between predicted and measured changes in the chemical evolution of the fluid for some species, the rate and magnitude of increase in pH, dissolved chloride and H 2 did not agree well with predictions based on theoretical modeling results. Experimental data indicate that dissolved H 2 abruptly and intermittently increased, reaching a value only approximately 20% of that predicted assuming magnetite as the primary Fe-bearing alteration phase. The distribution and valence of Fe in primary and secondary minerals reveal that the most abundant secondary mineral, serpentine, contained significant amounts of both ferric and ferrous Fe, with the less abundant brucite, also being Fe-rich ( X Fe = 0.3). Surprisingly, magnetite was present in only trace amounts, indicating that H 2 generation was largely accommodated by the formation of Fe–chrysotile. Accordingly, the diversity of Fe-bearing secondary minerals together with rates of serpentinization less than theoretically predicted, account best for the relatively low dissolved H 2 concentrations produced. Thus, the experimental data can be used to obtain provisional estimates of thermodynamic data for Fe-bearing minerals, enhancing the application of reaction path models depicting mass transfer processes during serpentinization at mid-ocean ridges. Similarly, the observed differences between theoretically predicted and experimentally measured pH values result from constraints imposed by complex patterns of mass transfer inherent to the experimental system. In particular, the experimental observation of a late stage increase in Na/Cl ratio likely results from the dissolution of a Na 2O component of clinopyroxene, which causes pH to increase sufficiently to induce precipitation of a Ca-bearing phase, perhaps portlandite. As with the redox variability observed during the experiment, this event could not be predicted, underscoring the need to use caution when modeling alteration processes in the chemically complex ultramafic-hosted hydrothermal systems at elevated temperatures and pressures.

The effect of methanogen growth on mineral substrates: will Ni markers of methanogen-based communities be detectable in the rock record?

Methanogens, thought to be present on early Earth, have a high requirement for Ni, suggesting that Ni utilization could be a potential biosignature for methanogens if enhanced Ni extraction from surrounding minerals accompanies methanogenic growth. To test the potential for such Ni extraction from minerals by methanogens, Ni release from Ni-containing silicate glass was measured in Ni-free growth medium in the presence of the methanogen Methanothermobacter thermoautotrophicus (average pH ∼7.0) and observed to be higher than an abiotic control (average pH ∼6.8). However, batch dissolution experiments and a siderophore assay indicate that cell exudates such as siderophores, low molecular weight organic acids, or lysates accompanying cell death are not responsible for the observed increase in Ni release rate. In addition, scanning electron microscopy (SEM) shows little to no evidence of direct microbe-mineral interactions such as biofilms or pitting. Instead, comparison with abiotic experiments suggests that changes in pH due to CO2 uptake may be responsible for enhanced dissolution in the presence of metabolizing cells. These results document that methanogens may not preferentially extract Ni from surrounding minerals although they may indirectly affect mineral reaction rates that are pH sensitive. Thus identifiable Ni biosignatures may not exist in the rock record to document the presence of methanogens on early Earth or Mars.

The effect of time on the weathering of silicate minerals: why do weathering rates differ in the laboratory and field?

The correlation between decreasing reaction rates of silicate minerals and increasing duration of chemical weathering was investigated for both experimental and field conditions. Column studies, using freshly prepared Panola Granite, produced ambient plagioclase weathering rates that decreased parabolically over 6 years to a final rate of 7.0×10 −14 mol m −2 s −1. In contrast, the corresponding plagioclase reaction rate for partially kaolinized Panola Granite, after reaching steady-state weathering after 2 months of reactions, was significantly less (2.1×10 −15 mol m −2 s −1). Both rates were normalized to plagioclase content and BET surface area. Extrapolation of decreasing rates for the fresh plagioclase with time indicated that several thousand years of reaction would be required to replicate the rate of the naturally weathered plagioclase under identical experimental conditions. Both rates would remain orders of magnitude faster than field weathering rates previously measured for a weathering profile in the Panola Granite. Additional trends in weathering rates with time were established from a tabulation of previously reported experimental and field rates for plagioclase, K-feldspar, hornblende and biotite. Discrepancies in the literature, produced by normalization of weathering rates with respect to surface areas measured by gas absorption (BET) and geometric methods, were overcome by developing a time-dependent roughness factor. Regression curves through the corrected rates produced strong correlations with time that were similar for the four silicate minerals. The average silicate weathering rate R (mol m −2 s −1) was described by the power function R=3.1×10 −13t −0.61 which was similar to the relationship describing the decrease in the fresh Panola plagioclase with time and suggesting control by transport-limited reaction. The time dependency of silicate weathering is discussed in terms of processes intrinsic to the silicate mineral and those extrinsic to the weathering environment. Intrinsic surface area, which increases with the duration of weathering, was shown to account for a third of the exponential decrease in the weathering rate shown by the above equation. Other factors, including progressive depletion of energetically reactive surfaces and accumulation of leached layers and secondary precipitates, must explain differences for fresh and weathered plagioclase reacting under identical experimental conditions. Extrinsic controls, including low permeability, high mineral/fluid ratios and increased solute concentrations, produce weathering reactions close to thermodynamic equilibrium under field conditions compared to highly unsaturated conditions during experimental reaction of fresh and weathered plagioclase. These differences explain the time-dependent difference in field and lab rates.

Can metamorphic reactions proceed faster than bulk strain?

Available constraints on metamorphic reaction rates derived from the study of natural systems are similar to, or slightly lower than, the bulk strain rates measured in the same rocks. Here, we explore whether this apparent relationship is merely coincidence or due to a more fundamental mechanistic link between reaction and strain. Grain boundary migration accommodated dislocation creep (GBMDC) or grain boundary diffusion creep (GBDC) (i.e. pressure solution), both of which involve dissolution-precipitation as we define it, will occur simultaneously with mineral reactions involving dissolution-precipitation in the presence of a non-zero deviatoric stress. The exact relationships between reaction and strain are different depending on whether GBMDC or GBDC is controlling strain, but the mechanistic link exists in both cases. We present theoretical arguments which show that bulk strain by GBMDC or GBDC, which may additionally be accommodated by processes not involving dissolution-precipitation, such as dislocation glide and climb or grain boundary sliding, should in most cases be somewhat faster than the bulk reaction rates as observed. With few exceptions, for natural metamorphic systems undergoing plastic deformation, strain rates provide an upper limit for bulk reaction rates occurring simultaneously in the same rocks. The data suggest that mineral reaction rates may typically be within one order of magnitude of the strain rate.

Climatic Variation and Solute Concentration and Flux in Meltwaters Draining from an Alpine Glacier

Measurements of electrical conductivity and discharge ofmeltwaters in the Gornera, which drains from the 83%glacierised basin containing Gornergletscher, PennineAlps, Switzerland, were undertaken between May andSeptember in both 1979 and 1998. Discharge in theGornera was 43% higher in 1998, average air temperatureduring the ablation season being 2.1 °C warmer andpreceding winter precipitation 28% lower than in 1979. Mean electrical conductivity of meltwater in 1998 wasreduced by 40%. In the same 60 day period in 1998,however, solute flux was augmented by only 2% bycomparison with 1979. Year-to-year climatic variations,reflected in discharge variability, strongly affectsolute concentration in glacial meltwaters, but havelimited impact on solute flux. Climatic conditionstranslate into meltwater quality through inter-relationships between mineral reaction rates, subglacialresidence time in contact with sediment, and discharge. Annual variability in solute flux depends on the extentto which volume of flow can offset decline in soluteconcentration brought about by reducing residence time.

Calculation of fluid fluxes in Earth’s crust

The movement of fluids in the crust and upper mantle not only lead to important mineral reactions but also play an essential role in the geochemical cycling of elements and in controlling global change. Numerous papers have focused on calculation of fluid fluxes driving metamorphic reactions in the earth’s crust. The extent of reaction in nature has been “inverted” to predict the total amount of fluid that was required to drive that much reaction. These models, although based on thermodynamic equilibrium, have extended the earlier concept of water-rock ratio. Any quantitative treatment of the fluid fluxes and the relationship between these fluxes and other variables such as temperature and mineral abundances requires a kinetic model. A simple model is presented that incorporates the essential dynamics of metamorphic processes including both heat flow by conduction and convection as well as fluid flow in and out of a representative volume. Overall mineral reactions can then take place within this rock volume in response to internal and external factors. The production and subsequent expulsion of excess fluids (H 2O and CO 2) as a result of these reactions leads to increased fluid fluxes, which the model can also handle. Using this kinetic model, the assumption of thermodynamic equilibrium can be tested and forward calculations can compare the numbers “inverted” for total integrated fluid fluxes based on equilibrium with the “actual” integrated fluid fluxes. Other effects such as changes in the temperature field or the presence of dispersion/diffusion can also be readily quantified with this kinetic model. The nontrivial consequences of heterogeneity in natural systems make the kinetic approach much more essential but at the same time much more “invertible” than earlier approaches. Ultimately, the effects of the rates of overall mineral reactions and their interplay with the other kinetic processes taking place in these open systems have to be evaluated to guide us in developing much more powerful and correct ways of extracting fluid velocities from petrologic field data.