Hydrothermal Alteration Zoning and Kinetic Process of Mineral-Water Interactions

This study reports the kinetic experimental results of albite in water and in KCI solution at 22 MPa in the temperature range of 25 to 400℃. 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℃ (close to the critical point 374℃). At temperatures from 25 to 300℃, the incongruent dissolution of albite is reflected by the fact that sodium and aluminum are easily dissolved into water; from 300 to 400℃ 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℃, 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℃, while ma     

Mineral Surface after Reaction with Aqueous Solution at High Temperatures and Pressures

This work presents new experimental results on surface chemistry of reacting minerals and interface kinetics between mineral and aqueous solutions. These experiments were carried out using a flow reactor (packed bed reactor) of an open system as well as a continuous stirred tank reactor, CSTR. The authors measured reaction rates of such minerals as zeolite, albite and carbonate (rhodochrosite, dolomite) in various solutions, and tested corresponding mineral surface by using SEM, XPS, SIMS, etc. This paper mainly presents the experimental results of zeolite dissolution in water and in low pH solutions at room temperature, and dolomite dissolution at elevated temperatures. The results show that the release rates of Si, Al and Na of zeolite are different in most cases. The incongruent dissolution of zeolite is related to surface chemical modifications. The Na, Al and Si release rates for dissolution of albite and zeolite in water and various solutions were measured as a function of temperature, flow veloci     

Dependence of albite dissolution kinetics on ph and time at 25°c and 70°c

The dissolution rate of albite has been measured as a function of pH and time at 25°C and 70°C in a single-pass flow-through leaching apparatus. Run times extended to 50 days in each experiment. Limited data were obtained at 25°C in the pH range 4–10. More extensive data were obtained at 70°C over the pH range 1.39–11.75.Dissolution rates were defined by release of Si, and in some cases also by Al and Na releases. Speciationsolubility calculations indicate the solutions were well undersaturated for all the likely possible secondary minerals. The fluid was maintained far from equilibrium with albite in all runs. Analysis of the data shows a general consistency with the transition state theory model of Helgesonet al. (1984).Feldspars leached at low and high pH at 70°C showed extensive development of prismatic etch pits demonstrating a surface reaction-controlled dissolution process.     

The effect of oxalate on the dissolution rates of oligoclase and tremolite

The effect of oxalate, a strong chelator for Al and other cations, on the dissolution rates of oligoclase feldspar and tremolite amphibole was investigated in a flow-through reactor at 22°C. Oxalate at concentrations of 0.5 and 1 mM has essentially no effect on the dissolution rate of tremolite, nor on the steady-state rate of release of Si from oligoclase. The fact that oxalate has no effect on dissolution rate suggests that detachment of Si rather than Al or Mg is the rate-limiting step. At pH 4 and 9, oxalate has no effect on the steady-state rate of release of Al, and dissolution is congruent. At pH 5 and 7, oligoclase dissolution is congruent in the presence of oxalate, but in the absence of oxalate Al is preferentially retained in the solid relative to Si.Large transient “spikes” of Al or Si are observed when oxalate is added to or removed from the system. The cause of the spikes is unknown; we suggest adsorption on feldspar surfaces away from sites of active dissolution as a possibility. Solutions in the reactors are undersaturated with respect to both gibbsite and kaolinite, so neither the spikes nor the incongruent dissolution can be explained by formation of a secondary precipitate.The rate of dissolution of tremolite is independent of pH over the pH range 2–5, and decreases at higher pH. The rate of dissolution of oligoclase in our experiments was independent of pH over the pH range 4–9. Since the dissolution rate of these minerals is independent of pH and organic ligand concentration, the effect of acid deposition from the atmosphere on the rate of supply of cations from weathering of granitic rocks should be minor.     

27Al, 29Si,and 23Na MAS NMR study of an Al,Si ordered alkali feldspar solid solution series

Solid-state ²⁷Al, ²⁹Si and ²³Na MAS NMR spectra have been obtained for an Al,Si ordered low albite to low microcline ion exchange series for which unit-cell parameters and ²⁹Si NMR data have previously been reported. ²⁷Al δ_i vary continuously with composition from 63.4 (±0.5) ppm for albite to 58.9 (±0.5) ppm for microcline, and parallel the ²⁹Si chemical shifts assigned to the T2m-site. The ²⁷Al and ²⁹Si chemical shifts for this series correlate well with composition-dependent lattice parameters, most notably cell volume and the angle [201]¹b. The linewidths of the ²⁹Si and ²⁷Al resonances indicate a significant amount of structural disorder in the intermediate compositions due to Na, K substitution. The 1 σ width of the distribution of average Si-O-T angles for each T-site is estimated to be about 1° for the Or33 sample. The average ²³Na δ_i varies monotonically from -8.5 (±1) ppm for albite to -24.3 (±1)ppm for Or83. Similarly, the average ²³Na nuclear quadrupole coupling constant decreases from 2.60 to 1.15 (±0.05) MHz and the asymmetry parameter of the electric field gradient increases from 0.25 to 0.6 with increasing K-content from albite to Or83. The observed variations in the quadrupole coupling parameters are consistent with simple electrostatic calculations. Higher resolution ²³Na spectra of the intermediate compositions obtained at 11.7 T indicate the presence of an inhomogeneous linebroadening which is related to the distribution of Na-environments. A model based on a random distribution of local compositions does not simulate the spectra, suggesting that the distribution of Na is skewed toward Na-rich clusters. Observation of the ²³Na NMR lineshape of Or49 after short periods of heat treatment indicate that ²³Na NMR is very sensitive to the changes in the Na, K distribution accompanying the early stages of exsolution. Reversible changes occur after heating at 530° C for 3 h, whereas heating at 600° C produces no changes, possibly bracketing the position of the coherent spinodal for Al, Si ordered alkali feldspars at this composition.     

Kinetics of feldspar and quartz dissolution at 70–80°C and near-neutral pH: effects of organic acids and NaCl

Effects of the organic acid (OA) anions, oxalate and citrate, on the solubility and dissolution kinetics of feldspars (labradorite, orthoclase, and albite) at 80°C and of quartz at 70°C were investigated at pH 6 in separate batch experiments and in media with different ionic strength (0.02–2.2 M NaCl). Although it has been shown that OAs can increase rates of feldspar dissolution, prior experiments have focused primarily on dilute, highly undersaturated and acidic conditions where feldspar dissolution kinetics are dominated by H⁺ adsorption and exchange reactions. Many natural waters, however, are only weakly acidic and have variable ionic strength and composition which would be expected to influence mineral surface properties and mechanisms of organic ligand-promoted reactions.Oxalate and citrate (2–20 mM) increased the rate of quartz dissolution by up to a factor of 2.5. Quartz solubility, however, was not increased appreciably by these OAs, suggesting that Si–OA complexation is not significant under these conditions. The lack of significant OA–SiO₂ interaction is important to understanding the effects of OAs on the release of both Si and Al from feldspars. In contrast to quartz, both the rates of dissolution and amounts of Si and Al released from the three feldspars studied increased regularly with increasing OA concentration. Feldspar dissolution was congruent at all but the lowest OA concentrations. Total dissolved Al concentrations increased by 1–2 orders of magnitude in the presence of oxalate and citrate, and reached values as high as 43 mg/l (1.6 mM). Si concentrations reached values up to 65 mg/l (2.3 mM) in feldspar–OA experiments. Precipitation of authigenic clays was observed only in experiments without or at very low concentrations of OAs. The high concentrations of dissolved Si attained during dissolution of feldspars in OA solutions, relative to Si concentrations in quartz–OA experiments, is attributed to concomitant release of Si driven by strong Al–OA interactions.Modeling of the dependence of feldspar dissolution rates on OA concentration in natural diagenetic environments is complicated by the competing effects of overall solution chemistry and ionic strength on the dissolution mechanism. Results of experiments using labradorite (An₇₀) indicate that in OA-free solutions, dissolution is progressively slower at increasing NaCl concentrations (up to 2.2 M), in agreement with prior experiments on the effects of alkali metals on feldspar dissolution. The combined effects of oxalate and NaCl on labradorite dissolution rates are such that the rate increase due to oxalate is suppressed by the addition of NaCl. Thus, feldspar dissolution kinetics should be most significantly affected by a given concentration of OAs in low ionic strength solutions.     

Experimental determination of the solubility of the assemblage paragonite,albite, and quartz in supercritical H2O

The concentrations of Na, Al, and Si in an aqueous fluid in equilibrium with natural albite, paragonite, and quartz have been measured between 350°C and 500°C and 1 to 2.5 kbar. Si is the dominant solute in solution and is near values reported for quartz solubility in pure H₂O. At 1 kbar the concentrations of Na and Al remain fairly constant from 350°C to 425°C but then decrease at 450°C. At 2 kbar, Na increases slightly with increasing temperature while Al remains nearly constant. Concentrations of Si, Na, and Al all increase with increasing pressure at constant temperature.The molality of Al is close to that of Na and is nearly a log unit greater than calculated molalities assuming Al(OH)⁰₃ is the dominant Al species. This indicates a Na-Al complex is the dominant Al species in solution as shown by Anderson and Burnham (1983) at higher temperature and pressure. The complex can be written as NaAl(OH)⁰₄ ± nSiO₂ where n is the number of Si atoms in the complex. The value of n is not well constrained but appears to be less than or equal to 3.The results indicate Al can be readily transported in pure H₂O solutions at temperatures and pressures as low as 350°C and 1 kbar.     

An experimental study of dissolution kinetics of Calcite, Dolomite, Leucogranite and Gneiss in buffered solutions at temperature 25 and 5°C

Laboratory experiments were carried out continuously for 30–35 days at 25 and 5°C in three different buffer solutions of pH 4.0, 2.2 and 8.4 to calculate dissolution rates of two minerals, calcite (CC) and dolomite (DM) and two rocks, leucogranite (LG) and gneiss (GN) from the Himalayan range. Calculated rates in terms of release of targeted elements versus time (Ca for CC; Mg for DM; Si for LG and GN) demonstrate direct correlation with temperature. Dissolution rates are higher at 25°C compared to 5°C. CC and DM were experimented only at pH 8.4 and results show that both undergo congruent dissolution with CC dissolving ∼5 times faster than DM. Ca and Mg exhibit average apparent activation energies (E _a) of 13.98 and 9.98 kcal mol⁻¹ respectively at pH 8.4 which reflects greater sensitivity of CC dissolution than DM dissolution towards an increase in temperature. Scanning Electron Microscope attached with Energy Dispersive X-Ray Analyser (SEM-EDX) data indicates that dissolution is controlled primarily by surface-reaction processes, with dislocation sites contributing maximum to the dissolution. As compared to CC and DM dissolution, LG and GN undergo relatively slower incongruent dissolution with precipitation of some secondary minerals as revealed from X-ray diffractometer (XRD) results. Rates of dissolution of LG is maximum at pH 2.2, moderate at pH 8.4 and least at pH 4.0, whereas GN shows maximum dissolution at pH 2.2, moderate at pH 4.0 and least at pH 8.4. A comparison in dissolution behavior of LG and GN at experimental conditions reveals that increase in Si-release rate in the temperature range between 5 and 25°C is maximum at pH 8.4 (∼3.4–4.5 times), moderate at pH 4.0 (∼3–1.8 times) and least at pH 2.2 (∼1.0–1.5 times). Within the experimental temperature range, calculated values of E _a for Si release during LG and GN dissolution advocates positive correlation with pH. A substantial decrease in initial values of Brunauer–Emmett–Teller (BET) surface area of DM, LG, and GN has been encountered at the end of the experiment, except for CC for which an increase is observed. The study clearly demonstrates the dissolution behavior of pure minerals and rocks under controlled conditions. The dissolution rates assume enormous significance for the release of trace elements from rocks/minerals to the reacting water.     

Calcite Dissolution in Deionized Water from 50°C to 250°C at 10 MPa: Rate Equation and Reaction Order

Carbonate minerals and water （or geofluids） reactions are important for modeling of geochemical processes and have received considerable attention over the past decades. The calcite dissolution rates from 50℃ to 250℃ at 10 MPa in deionized water with a flow rate varying from 0.2 to 5 mL/min were experimentally measured in a continuous flow column pressure vessel reactor. The dissolution began near the equilibrium with c/ceq 〉 0.3 and finally reached the equilibrium at 100℃-250℃, so the corresponding solubility was also determined as 1.87, 2.02, 2.02 and 1.88×10^-4.mol/L at 100℃, 150℃, 200℃ and 250℃ respectively, which was first increasing and then switching to decreasing with temperature and the maximum value might occur between 150℃ and 200℃. The experimental dissolution rate not only increased with temperature, but also had a rapid increase between 150℃ and 200℃ at a constant flow rate of 4 mL/min. The measured dissolution rates can be described using rate equations of R = k（1-c/ceq）n or R = kc-n. In these equations the reaction order n changed with temperature, which indicates that n was a variable rather than a constant, and the activation energy was 13.4 kJ/mol calculated with R = k（1-c/ceq）n or 18.0 kJ/mol with R = kc^-n, which is a little lower than the surface controlled values. The varied reaction order and lower activation energy indicates that calcite dissolution in this study is a complex interplay of diffusion controlled and surface controlled processes.     

Petrology, geochronology, and tectonics of shear zones in the Zermatt–Saas and Combin zones of the Western Alps

Metre to tens‐of‐metre wide, steeply dipping, greenschist facies shear zones that cut blueschists and eclogites of the Combin and Zermatt–Saas Zones at Täschalp and in adjacent areas of the western Alps were sites of extensive recrystallization driven by fluid flow and deformation. Rb–Sr data imply that these shear zones formed at 42–37 Ma with a systematic younging of structures northward toward, and into, the hangingwall of the Mischabel Structure. Shearing commenced at 400–475 °C and 400–500 MPa and continued as pressures and temperatures fell to 300–350 °C and 300–350 MPa. Individual shear zones were active for 2–3 Myr with later lower grade stages of shearing concentrated into narrow zones. Fluids that infiltrated the shear zones were water rich (X_H2O > 0.9). Alteration zones around albite veins and at the margins of serpentinite bodies are penecontemporaneous with these shear zones and formed at approximately the same conditions. The eclogites were exhumed from c. 64 km at 44 Ma to 14–16 km at 42–41 Ma implying exhumation rates of 2–5 cm yr^?1. Rapid exhumation was probably achieved by extension aided by buoyancy, following subduction of continental crust, and rapid erosion. The shear zones form part of a regional‐scale extensional system responsible for a significant portion of the exhumation of the subducted oceanic crust.     

Experimental Study on Water‐rock Reactions with CO2 Fluid in a Deep Sandstone Formation under High Temperature and Pressure

Qiongdongnan Basin has a tectonic geological background of high temperature and high pressure in a deep reservoir setting,with mantle-derived CO2.A water-rock reaction device was used under high temperature and high pressure conditions,in conjunction with scanning electron microscope(SEM)observations,to carry out an experimental study of the diagenetic reaction between sandstone at depth and CO2-rich fluid,which is of great significance for revealing the dissolution of deep clastic rock reservoirs and the developmental mechanism of secondary pores,promoting deep oil and gas exploration.In this study,the experimental scheme of the water-rock reaction system was designed according to the parameters of the diagenetic background of the deep sandstone reservoir in the Qiongdongnan Basin.Three groups of single mineral samples were prepared in this experiment,including K-feldspar samples,albite samples and calcite samples.Using CO2 as a reaction solution,a series of diagenetic reaction simulation experiments were carried out in a semi-closed high temperature and high pressure simulation system.A field emission scanning electron microscope(SEM)was used to observe the microscopic appearance of the mineral samples after the water-rock reaction,the characteristics of dissolution under high temperature and high pressure,as well as the development of secondary pores.The experimental results showed that the CO2-rich fluid has an obvious dissolution effect on K-feldspar,albite and calcite under high temperature and high pressure.For the three minerals,the main temperature and pressure window for dissolution ranged from 150℃to 300℃and 45 MPa to 60 MPa.Scanning electron microscope observations revealed that the dissolution effect of K-feldspar is most obvious under conditions of 150℃and 45 MPa,in contrast to conditions of200℃and 50 MPa for albite and calcite.Through the comparative analysis of experimental conditions and procedures,a coupling effect occurred between the temperature and pressure change and the dissolution strength and calcite.Under high temperature and high pressure,pressure changed the solubility of CO2,furthermore,the dissolution effect and strength of the sandstone components were also affected.The experiment revealed that high temperature and high pressure conditions with CO2-rich fluid has a significant dissolution effect on aluminosilicate minerals and is conducive to the formation of secondary pores and effective reservoirs.Going forward with the above understanding has important implications for the promotion of deep oil and gas exploration.     

Contribution of long-term hydrothermal experiments for understanding the smectite-to-chlorite conversion in geological environments

The smectite-to-chlorite conversion is investigated through long-duration experiments (up to 9 years) conducted at 300 °C. The starting products were the Wyoming bentonite MX80 (79 % smectite), metallic iron and magnetite in contact with a Na–Ca chloride solution. The predominant minerals in the run products were an iron-rich chlorite (chamosite like) and interstratified clays interpreted to be chlorite/smectite and/or corrensite/smectite, accompanied by euhedral crystals of quartz, albite and zeolite. The formation of pure corrensite was not observed in the long-duration experiments. The conversion of smectite into chlorite over time appears to take place in several steps and through several successive mechanisms: a solid-state transformation, significant dissolution of the smectite and direct precipitation from the solution, which is over-saturated with respect to chlorite, allowing the formation of a chamosite-like mineral. The reaction mechanisms are confirmed by X-ray patterns and data obtained on the experimental solutions (pH, contents of Si, Mg, Na and Ca). Because of the availability of some nutrients in the solution, total dissolution of the starting smectite does not lead to 100 % crystallization of chlorite but to a mixture of two dominant clays: chamosite and interstratified chlorite/smectite and/or corrensite/smectite poor in smectite. The role of Fe/(Fe + Mg) in the experimental medium is highlighted by chemical data obtained on newly formed clay particles alongside previously published data. The newly formed iron-rich chlorite has the same composition as that predicted by the geothermometer for diagenetic to low-grade metamorphic conditions, and the quartz + Fe-chlorite + albite experimental assemblage in the 9-year experiment is close to that fixed by water–rock equilibrium.     

Theoretical approach to evaluating plagioclase dissolution mechanisms

The more rapid dissolution of Ca-rich feldspars relative to Na, K-rich feldspars has been attributed to the preferential leaching of Al deep within the feldspar structure. Evidence from surface microanalysis (e.g., Hellmann et al., 2003), however, shows that preferential dissolution of Al is confined to the top layers of the feldspar lattice and that the amorphous surface layer most likely results from precipitation versus dissolution. It is thus critical to examine the extent of preferential Al removal. Here we present a theoretical study of plagioclase dissolution behavior using parameterized Monte Carlo simulations. Two different dissolution mechanisms, a mechanism involving preferential leaching of Al and an interfacial dissolution-reprecipitation mechanism, are tested using compositions representing the entire plagioclase solid solution series. Our modeling results indicate that under the control of the preferential Al leaching mechanism, the influence of (Al, Si) disorder on the dissolution rate is significant. At a fixed composition, an increase in the degree of (Al, Si) disorder yields an increased dissolution rate, with an 8-fold increase in dissolution rate observed for highly disordered albite (An₀) compared to low albite. Increasing anorthite content tends to decrease the variation in the dissolution rate due to disorder. The difference in the dissolution rate of 293 tested oligoclase configurations with a composition of An₂₀ is 3-fold, and the difference is reduced to 2-fold among 107 andesine configurations of An₃₀. Furthermore, feldspar configurations with completely disordered (Al, Si) distributions yield a consistent log-linear dependence of dissolution rate on the anorthite content (An), while other feldspar configurations with modest degrees of (Al, Si) disorder exhibit rates less than this trend. In contrast, when Al removal is confined to the top surface layers, a variety of feldspar configurations with different (Al, Si) disorder but a single fixed composition have similar dissolution rates; and the dissolution rate of Ca-rich feldspars departs positively from its log-linear relationship with anorthite content. This departure occurs around An₈₀ and is in good agreement with previous experimental studies. Subsequent modeling results of aluminum inhibition, ΔG dependence, and formation of altered surface layers in the framework of the interfacial dissolution-reprecipitation mechanism are all comparable with experimental investigations, and these results suggest that an interfacial dissolution-reprecipitation mechanism governs the dissolution of plagioclase feldspars.     

Surface chemistry and dissolution kinetics of glassy rocks at 25°C

The weathering rates and mechanisms of three types of glassy rocks were investigated experimentally at 25 °C, pH 1.0 to 6.2, and reaction times as much as to 3 months. Changes in major element chemistry were monitored concurrently as a function of time in the aqueous solution and within the near surface region of the glass. Leach profiles, obtained by a HF leaching technique, displayed near-surface zones depleted in major cations. These zones increased in depth with increasing time and decreasing pH of reactions. Release rates into the aqueous solution were parabolic for Na and K and linear for Si and Al. A coupled weathering model, involving surface dissolution with concurrent diffusion of Na, K, and Al, produced a mass balance between the aqueous and glass phases. Steady state conditions are reached at pH 1.0 after approximately 3 weeks of reaction. Steady-state is not reached even after 3 months at pH 6.2.An interdiffusion model describes observed changes in Na diffusion profiles for perlite at pH 1.0. The calculated Na self-diffusion coefficient of 5 × 10^?19 cm²·s^?1 at 25°C approximates coefficients extrapolated from previously reported high temperature data for obsidian. The self-diffusion coefficient for H₃O⁺, 1.2 × 10^?20 cm²·s^?1, is similar to measured rates of water diffusion during hydration of obsidian to form perlite.     

Oscillatory zoning in eclogitic garnet and amphibole,Northern Serpentinite Melange,Cuba: a record of tectonic instability during subduction?

Exotic blocks of eclogite from distant localities along the Northern Serpentinite Melange of Cuba have comparable P–T histories that include high‐pressure prograde sections (450–600 °C, >15 kbar) associated with subduction of oceanic lithosphere, and retrograde sections within the albite–epidote amphibolite facies (<500 °C, <10 kbar) related to melange uplift. ⁴⁰Ar/³⁹Ar and Rb/Sr cooling ages (118–103 Ma) of one of the blocks indicate pre‐Aptian subduction and Aptian–Albian uplift. Detailed X‐ray imaging and profiling further reveals that minerals in these eclogite blocks (notably garnet and amphibole) display subtle but well defined oscillatory zoning that developed along the prograde trajectory of the rocks, previous to attainment of peak eclogitic conditions. The chemistry (e.g. coupled changes of Mg# and Mn in garnet, and of Si, Ti, Al and Na in amphibole) and geometry (euhedral to anhedral shapes) of the oscillations can be interpreted in terms of subtle fluctuations in P–T during the general prograde subduction‐related metamorphic path. A (near‐) equilibrium model is presented for the formation of oscillations at near peak conditions by means of recurrent dissolution‐growth reaction processes. This model for near‐peak conditions, and the chemical signatures of earlier oscillations (notably in amphibole), suggest that episodes of retrogression (upward movement?) affected parts of the subducting slab. It is proposed that these retrograde episodes record the tectonic rupture of the subducting slab and, probably, of the upper plate mantle, either due to the intrinsic dynamic behaviour of subduction systems or to the effects of the plate‐tectonic rearrangement of the Caribbean region during the Early Cretaceous.     

Albite vein formation during exhumation of high-pressure terranes: a case study from alpine Corsica

The formation of late‐stage veins can yield valuable information about the movement and composition of fluids during uplift and exhumation of high‐pressure terranes. Albite veins are especially suited to this purpose because they are ubiquitously associated with the greenschist facies overprint in high‐pressure rocks. Albite veins in retrogressed metabasic rocks from high‐pressure ophiolitic units of Alpine Corsica (France) are nearly monomineralic, and have distinct alteration haloes composed of actinolite + epidote + chlorite + albite. Estimated P–T conditions of albite vein formation are 478 ± 31 °C and 0.37 ± 0.14 GPa. The P–T estimates and petrographic constraints indicate that the albite veins formed after the regional greenschist facies retrogression, in response to continued decompression and exhumation of the terrane. Stable isotope geochemistry of the albite veins, their associated alteration haloes and unaltered hostrocks indicates that the vein‐forming fluid was derived from the ophiolite units and probably from the metabasalts within each ophiolite slice. That the vein‐forming fluid was locally derived means that a viable source of fluid to form the veins was retained in the rocks during high‐pressure metamorphism, indicating that the rocks did not completely dehydrate. This conclusion is supported by the observation of abundant lawsonite at the highest metamorphic grades. Fluids were liberated during retrogression via decompression dehydration reactions such as those that break down hydrous high‐pressure minerals like lawsonite. Albite precipitation into veins is sensitive to the solubility and speciation of Al, which is more pressure sensitive than other factors which might influence albite vein formation such as silica saturation or Na:K fluid ratios. Hydraulic fracturing in response to fluid generation during decompression was probably the main mechanism of vein formation. The associated pressure decrease with fracturing and fluid decompression may also have been sufficient to change the solubility of Al and drive albite precipitation in fracture systems.     

Hydrothermal alteration of hemi-pelagic sediments: experimental evaluation of geochemical processes in shallow subduction zones

Laboratory hydrothermal experiments provide unique information regarding the fate of volatile and/or incompatibles (e.g. B, Li, and As) during oceanic crust subduction. Examination of chemical redistribution between the subducted slab, mantle wedge, arc volcanics and overlying ocean water during subduction is critical to gain further insight into arc volcanism and chemical oceanic budgets. For instance, efficient mobilization of B at shallow depths may be a key aspect of its oceanic budget, and can help to explain the systematics of B-δ¹¹B and B-¹⁰Be in arc lavas. Fluid–rock interactions at elevated temperatures and pressures in accretionary prisms were studied using a rocking autoclave hydrothermal apparatus to monitor sediment–porewater interaction over the range of 25 to 350°C, at 800 bars. Clay-rich hemi-pelagic sediments from the décollement zone of Ocean Drilling Program Site 808, Nankai Trough, were reacted with NaCl–CaCl₂ solutions at water/rock ratios of ∼3.5 to 1.5 (w/w) to mimic alteration processes in the shallow subduction zone. Fluids were extracted at 25–50°C intervals and were analyzed for major and trace chemical constituents. The fluid chemistry changed significantly during the course of these experiments, but there was generally only minor modification of the solid phase; only B, Li, As, Br, and Pb are significantly depleted. During the heating cycle, dissolved Na, Mg and SO₄ decreased sharply and NH₄, SiO₂, K, B, and Li increased at T⩾300°C. Calcium drops gradually at low temperatures, but concentrations rise sharply at T⩾300°C. Decomposition of organic matter, SO₄²⁻ depletion, and Mg-fixation dominate at low temperatures; however, albitization of calcic plagioclase leads to marked Na depletions and Ca enrichments at T⩾300°C. Dissolved SiO₂ remained below saturation with respect to quartz and amorphous silica throughout the entire experiment. B and Li mobilization with large isotopic fractionations occurred at low temperature. Exchangeable B (δ¹¹B=∼15‰) is completely leached before reaching 150°C. Substantial O₂ exchange between fluids and the solid phase occurred at T⩾200°C in the spiked experiment, where δ¹⁸O varies more than 100‰ in the fluids. During retrograde cooling, dissolved Mg, SO₄, Ca, Si, K and Sr are released as a result of carbonate or anhydrite dissolution, and marked B re-adsorption occurred at temperatures below 60°C.     

Stoichiometry of smectite dissolution reaction

The dissolution stoichiometry of smectite-rich bentonites SAz-1, STx-1 and SWy-1 was studied at 50°C and pH 2 and 3 using flow-through reactors. In addition to smectite, these samples contain considerable amounts of silica phases (quartz, cristobalite and/or amorphous silica). As a result, the molar Al/Si ratios of the bulk samples are significantly lower than those of the pure smectite.Smectite dissolution was highly incongruent during the first few hundred to few thousand hours of the experiments. Release rates of Si, Mg, Ca and Na underwent a distinct transition from an initial period of rapid release to significantly lower release rate at steady state. A reversed trend was observed for release of Al, which gradually increased from very low starting release rate to higher release rate at steady state. At steady state the ratio of released Al to released Si was found to be constant and independent of the experimental conditions. We suggest that this ratio represents the Al/Si ratio of the smectite itself, and it is not influenced by the presence of accessory phases in the sample.The rapid release of calcium, sodium and magnesium from the interlayer sites is explained by ion-exchange reactions, whereas the fast release of silicon is explained by dissolution of amorphous silica. We interpret the initial slow release of Al as the result of inhibition of smectite dissolution due to coating or cementation of the smectite aggregates by amorphous silica. As the silica is dissolved, the aggregates fall apart and more smectite surfaces are exposed, resulting in an increase in the smectite dissolution rate. Thereafter, the system approaches steady state, in which the major tetrahedral and octahedral cations of smectite are released congruently.     

Coupled alkali-feldspar dissolution and secondary mineral precipitation in batch systems: 1. New experiments at 200 °C and 300 bars

Batch reactor experiments were conducted to assess perthitic alkali-feldspar dissolution and secondary mineral formation in an initially acidic fluid (pH = 3.1) at 200 °C and 300 bars. Temporal evolution of fluid chemistry was monitored by major element analysis of in situ fluid samples. Solid reaction products were retrieved from two identical experiments terminated after 5 and 78 days. Scanning electron microscopy revealed dissolution features and significant secondary mineral coverage on feldspar surfaces. Boehmite and kaolinite were identified as secondary minerals by X-ray diffraction and transmission electron microscopy. X-ray photoelectron spectroscopy analysis of alkali-feldspar surfaces before and after reaction showed a trend of increasing Al/Si ratios and decreasing K/Al ratios with reaction progress, consistent with the formation of boehmite and kaolinite.Saturation indices of feldspars and secondary minerals suggest that albite dissolution occurred throughout the experiments, while K-feldspar exceeded saturation after 216 h of reaction. Reactions proceeded slowly and full equilibrium was not achieved, the relatively high temperature of the experiments notwithstanding. Thus, time series observations indicate continuous supersaturation with respect to boehmite and kaolinite, although the extent of this decreased with reaction progress as the driving force for albite dissolution decreased. The first experimental evidence of metastable co-existence of boehmite, kaolinite and alkali feldspar in the feldspar hydrolysis system is consistent with theoretical models of mineral dissolution/precipitation kinetics where the ratio of the secondary mineral precipitation rate constant to the rate constant of feldspar dissolution is well below unity. This has important implications for modeling the time-dependent evolution of feldspar dissolution and secondary mineral formation in natural systems.     

Temporal variations of aqueous constituents in a water-basalt-supercalcine system: Implications for the experimental assessment of nuclear waste forms

A Dickson-type rocking autoclave has been used for studying the interaction of a synthetic groundwater solution with the non-radioactive simulated nuclear-waste form supercalcine (SPC-4), and with a mixture of supercalcine plus basalt, at 300° and 200°C at 300 bars. Over the course of a month, the concentrations of the aqueous species of Mo, Na, Si, K, Rb, Al, Cr, and H⁺ displayed rapid change initially followed by a gradual approach to constant or nearly constant values, whereas the solids displayed very little alteration. These observations suggest that dissolution/precipitation reactions are quickly established even though the solids are very sluggish in attaining a completely altered or equilibrium state. The effects of various waste-form crystallinities and material preparations on the solution chemistry are quite different, but reproducible, and are discussed in terms of assessing potential waste forms.