COMMENT ON "THE ROLE OF H3O+ IN THE CRYSTAL STRUCTURE OF ILLITE" BY F. NIETO, M. MELINI, AND I. ABAD

The arguments of Nieto et al. (2010) in favor of the incorporation of H3O+ rather than H2O in interlayer positions of illite are disputable. Stoichiometric arguments suggest that the excess water in the Silver Hill illite is in the form of H2O. Moreover, recent thermodynamic models assuming the incorporation of interlayer H2O in illite provide reasonable estimates of temperature and water content using the AEM/TEM analyses of Nieto et al. (2010)

Fluid flow and CO2–fluid–mineral interactions during CO2-storage in sedimentary basins

Modelling the progress of geochemical processes in CO2 storage sites is frustrated by uncertainties in the rates of CO2 flow and dissolution, and in the rates and controlling mechanisms of fluid–mineral reactions that stabilise the CO2 in geological reservoirs. Dissolution of CO2 must be controlled by the complexities of 2-phase flow of CO2 and formation brines and the smaller-scale heterogeneities in the permeability in the reservoirs which increase the fluid contact areas. The subsequent fluid mineral reactions may increase storage security by precipitating CO2 in carbonate minerals but the consequences of fluid–mineral reactions on caprock rocks or potential leakage pathways up fault zones are less certain as the CO2-charged brines may either corrode minerals or decrease permeabilities by precipitating carbonates. Observations from CO2-injection experiments and natural analogues provide important constraints on the rates of CO2 and brine flow and on the progress of CO2 dissolution and mineral–fluid reactions. In these experiments brines in contact with the propagating plume appear to rapidly saturate with CO2. Dissolution of the CO2 drives the dissolution of oxide and carbonate minerals, on times scales of days to weeks. These reactions buffer fluid pH and produce alkalinity such that carbonate dissolution moves to carbonate precipitation over time-scales of weeks to months. The dissolution of Fe-oxide grain coatings and the release of Fe to solution is important in stabilising insoluble Fe–Mg–Ca carbonate minerals but the rate limiting step for carbonate mineral precipitation is the transport of CO2-charged brines and silicate mineral dissolution rates. Observations from CO2-EOR experiments and natural analogues suggest that the silicate mineral dissolution reactions are initially fast in the low pH fluids surrounding the CO2 plume but that reaction progress over months to years drives minerals towards thermodynamic equilibrium and dissolution rates slow over 2–5 orders of magnitude as equilibrium is approached. The sluggish dissolution of silicate minerals is likely to preside over the long-term fate of the CO2 in geological reservoirs. Observations from injection experiments and natural analogues suggest that the potentially harmful trace elements mobilised by the drop in pH are immobilised as adsorbed and precipitated phases as fluid pH is buffered across mineral reaction fronts. There are very few observations of caprock exposed to CO2-rich brines. Preliminary examination of core recently recovered from scientific drilling of a natural CO2 accumulation in Utah suggests that the diffusion of CO2 into reservoir caprocks drives dissolution of Fe-oxides but subsequent precipitation of carbonate minerals likely retards the diffusion distance of the CO2. At this site thin siltstone layers are shown to be effective seals to the CO2-charged fluids, which has significant implications for the long term security of CO2 in geological reservoirs

Atomistic investigation of the pyrophyllitic substitution and implications on clay stability

Predicting phase relations and reactions involving phyllosilicates is of critical importance when modeling geological reservoirs and making thermobarometric estimates at temperatures below 350 {degrees}C. However, it is difficult to perform experimental studies of phase relations of phyllosilicates because of very slow reaction rates. Simple methods of estimations of thermodynamic properties of minerals such as oxide summation techniques are insufficient to constrain activity models for minerals. In this study, we use a recent atomistic technique that allows constraining activity models in minerals independently from kinetics. Among the various solid solutions occurring in phyllosilicates, the magnitude and the thermodynamic significance of the pyrophyllitic substitution has strong implications on the stability of clay minerals at surface conditions. We have applied a lattice energy estimation method combined with Monte Carlo simulations to estimate the energy of mixing along the muscovite-pyrophyllite solid solution at 25 {degrees}C and 1 bar. The results suggest a strongly positive and asymmetric Gibbs free energy of mixing, concordant at first order with previous thermodynamic models issued from phase relations and with field observations. The calculated solvus implies that pyrophyllite-rich phyllosilicates are unstable, unless another phenomenon such as hydration stabilizes them. Calculated structures at low muscovite contents present large variations of interlayer occupancies due to short- and long-range ordering. The observed ordering suggests that sub-layers in illite/smectite mixed layers minerals are not independent as the alternation of K-rich and K-depleted sublayers minimizes the Gibbs free energy of the mineral

COMMENT ON "THE ROLE OF H3O+ IN THE CRYSTAL STRUCTURE OF ILLITE" BY F. NIETO, M. MELINI, AND I. ABAD

The arguments of Nieto et al. (2010) in favor of the incorporation of H3O+ rather than H2O in interlayer positions of illite are disputable. Stoichiometric arguments suggest that the excess water in the Silver Hill illite is in the form of H2O. Moreover, recent thermodynamic models assuming the incorporation of interlayer H2O in illite provide reasonable estimates of temperature and water content using the AEM/TEM analyses of Nieto et al. (2010)

Fluid-mineral reactions and trace metal mobilisation in an exhumed natural CO2 reservoir, Green River, Utah

Red sandstones near Green River, Utah (United States), have been bleached by diagenetic fluids. Field relationships, modeling, fluid inclusion and isotopic data suggest that the causal fluid was a CO2-charged brine, distinguishing this site from hydrocarbon-related bleaching elsewhere on the Colorado Plateau. Mineralogical and chemical profiles from unbleached to bleached sandstone show that bleaching is related to hematite dissolution and precipitation of a 1–2 cm band of secondary oxide and carbonate at the reaction front. Trace metals are mobilized by the fluid and concentrated near the reaction front. High-flux fluid pathways are more heavily altered with large-scale secondary calcite and iron oxide precipitation. Changes may be modeled by a reaction with stoichiometry 20Fe2O3 + 5CH4 + 64CO2 + 19H2O + 11H+ = 30Fe2+ + 10FeHCO3+ + 59HCO3–. The Fe-rich, reduced fluid precipitates iron-oxides and carbonate at the reaction front between bleached and unbleached sandstone. These findings make the site an analogue for processes occurring over long time scales in geological carbon storage projects. Trace metals moblized by CO2-charged brines are likely to be rapidly re-precipitated at reaction fronts

Controls of Sluggish, CO2-promoted, Hematite and K-feldspar dissolution kinetics in sandstones

CO2 sequestration is regarded as an important strategy for reducing anthropogenic CO2 emissions. Both the nature and rate of fluid–mineral reactions in CO2–water–rock systems are crucial, yet poorly constrained, parameters in understanding the fate of CO2 injected in geological formations. This study models reactions and reaction rates in an exhumed CO2-charged aquifer where CO2-rich brines have bleached red sandstones by dissolution of hematite grain coatings. We show that the vertical movement of the reaction front is dominated by diffusion and this allows calculation of reaction rates for the dissolution of hematite grain coatings and K-feldspar. Using mineral surface areas calculated from BET measurements, we estimate K-feldspar dissolution rates of View the MathML source; and hematite reaction rates of View the MathML source. The rates for K-feldspar are lower than previous, experimentally derived, estimates of K-feldspar dissolution rates by 1–2 orders of magnitude, likely explained by the proximity of the natural system to equilibrium. The inferred hematite reaction rates are 5–6 orders of magnitude slower than laboratory experiments and appear to be controlled by the chemical gradients imposed by the more sluggish K-feldspar dissolution. As the majority of potentially mobile trace metals are hosted in iron-oxide grain coatings, we argue that the rate of contaminant mobilization by CO2-charged brines will be lower than suggested by laboratory experiments

Chloritization of granites in shear zones: an open window on fluid pathways, equilibrium length-scales and porosity formation down to nanoscale

International audienceStrain localisation in the upper crust is strongly influenced by the presence of phyllosilicates (e.g. white mica, biotite, chlorite), systematically observed in shear zones in granites. Identifying reactions involving phyllosilicates at low-grade metamorphic conditions is crucial to understand crust mechanics and fluid-granite interactions during deformation. In the 305 Ma old basement of the Bielsa massif (Axial Zone, Pyrenees), extensive pre-orogenic (i.e. pre-Alpine) alteration related to feldspar sericitization and chloritization of biotite and amphibole occurred at temperatures of 270-350°C at 230-300 Ma. This event was followed by mylonitization and fracturing at 40-70 Ma, and fluid-rock interaction at 200-280°C marked by replacement and new crystallization of chlorite and white mica. In undeformed parts of the granite, compositional maps reveal in situ reaction, high local heterogeneities and low element mobility (migration over few µm) for most elements. Transmission electron microscopy (TEM) shows disconnected reaction-induced nanoporosity in chloritized amphiboles and ripplocations in chloritized biotite. Chloritization reaction varies over tens of nanometres, indicating high variability of element availability. Equilibrium is reached locally due to isolation of fluid in pockets. In samples with fractures, both elemental maps and TEM images show two chlorite groups: alpine chlorites in fractures have homogeneous composition while pre-alpine chlorites in the matrix show patchy compositions. Channelization of fluids in fractures and sealing by chlorite prevented replacement of the matrix chlorite. High element mobility was therefore limited to fractures. In mylonites, compositional maps show secondary chlorites up to 1 mm around cracks and only partial replacement of chlorite within the matrix. This suggests fluids could percolate from cracks to the matrix along chlorite grain boundaries. TEM images show nanocracks at the boundary of chlorite crystallites where replacement is localised. Crystallites were individually replaced by dissolution-reprecipitation reactions and not by intra-crystallite mineral replacement, explaining the patchy compositional variations. While fracturing did not allow chlorite sheets to be progressively re-oriented, a continuous, brittle-ductile deformation in mylonites did, making preferential fluid pathways progressively change. Despite high strain, chlorite replacement was not complete even in mylonites. Replacement appears to be controlled by matrix-fracture porosity contrasts and the location and connection of nanoporosity between crystallites, criteria that may be only transiently met in space during deformation. These mechanisms need to be taken into account when attempting to reconstruct the metamorphic history of shear zones as well as the evolution of their mechanical behaviour since they affect the scale of the thermodynamic equilibrium and the preservation of hydrothermal metamorphism in granites

Different microstructures in low grade shear zone formed at comparable temperatures: effect of pre and syn-kinematic fluid-rock interactions

International audienceDifferent microstructures and quartz recrystallization mechanisms can be observed in shear zones of granites that formed in similar greenschist-facies conditions. It is generally assumed that temperature plays a major role on quartz rheology and recrystallization. However, at low-grade conditions, fluid percolation also controls strain accommodation, by favouring the growth of weak phases as phyllosilicates. The relative importance of temperature over fluid-induced softening reactions on microstructures remains however poorly constrained mainly because comparative studies among low-grade shear zones are lacking.The present work focusses on two granitic massifs of the central Pyrenees deformed at greenschist-facies conditions but showing different structural styles. While in Bielsa granitoid, shear zones are spaced of ~ 100-200 m, in Maladeta strain is localised in shear zones spaced of ~ 1.5 km. The Bielsa granitoid, is pervasively altered at late-Variscan time, as suggested by petrography and trace elements variations uncorrelated to strain gradients, and then at Alpine time. Alpine mylonites are made of white mica at ~ 50 % vol. Quartz poorly recrystallizes by bulging. Geochemical whole-rock analyses show systematic variations of alkali, fluid and volume with increasing strain. These results point to a pervasive fluid-rock interaction before and during deformation in Bielsa. In contrast, in high strain rocks of Maladeta, the magmatic mineral assemblage is largely preserved. Quartz pervasively recrystallizes by sub-grain rotation and white mica is less abundant (20% vol). Consistently, geochemical whole-rock analyses show no or little major element transfer across Maladeta shear zone at constant volume. This point to a lower pre and syn-kinematic fluid-rock interaction in Maladeta than in Bielsa. Thermometry on metamorphic chlorite show similar temperature ranges for deformation in the two massifs (280-350°C).Variations of pre-kinematic hydrothermal alteration therefore strongly affect quartz recrystallisation mechanisms and microstructures, by controlling the abundance of weak phases as white mica. This process is observed despite very similar temperature ranges. Such variations may also explain the difference of structural style in the two massifs (distributed vs localised deformation) up to an outcrop scale

Deformation mechanisms in mafic amphibolites and granulites: record from the Semail metamorphic sole during subduction infancy

International audienceThis study sheds light on the deformation mecha- nisms of subducted mafic rocks metamorphosed at amphi- bolite and granulite facies conditions and on their impor- tance for strain accommodation and localization at the top of the slab during subduction infancy. These rocks, namely metamorphic soles, are oceanic slivers stripped from the downgoing slab and accreted below the upper plate man- tle wedge during the first million years of intraoceanic sub- duction, when the subduction interface is still warm. Their formation and intense deformation (i.e., shear strain ≥ 5) attest to a systematic and transient coupling between the plates over a restricted time span of ∼ 1 Myr and specific rheological conditions. Combining microstructural analyses with mineral chemistry constrains grain-scale deformation mechanisms and the rheology of amphibole and amphibolites along the plate interface during early subduction dynamics, as well as the interplay between brittle and ductile deforma- tion, water activity, mineral change, grain size reduction and phase mixing.Results indicate that increasing pressure and temperature conditions and slab dehydration (from amphibolite to gran- ulite facies) lead to the nucleation of mechanically strong phases (garnet, clinopyroxene and amphibole) and rock hard- ening. Peak conditions (850 ◦C and 1 GPa) coincide with a pervasive stage of brittle deformation which enables strain localization in the top of the mafic slab, and therefore pos- sibly the unit detachment from the slab. In contrast, dur- ing early exhumation and cooling (from ∼ 850 down to ∼ 700 ◦C and 0.7 GPa), the garnet–clinopyroxene-bearing am- phibolite experiences extensive retrogression (and fluid in- gression) and significant strain weakening essentially accommodated in the dissolution–precipitation creep regime including heterogeneous nucleation of fine-grained materi- als and the activation of grain boundary sliding processes. This deformation mechanism is closely assisted with con- tinuous fluid-driven fracturing throughout the exhumed am- phibolite, which contributes to fluid channelization within the amphibolites. These mechanical transitions, coeval with detachment and early exhumation of the high-temperature (HT) metamorphic soles, therefore controlled the viscosity contrast and mechanical coupling across the plate interface during subduction infancy, between the top of the slab and the overlying peridotites. Our findings may thus apply to other geodynamic environments where similar temperatures, lithologies, fluid circulation and mechanical coupling be- tween mafic rocks and peridotites prevail, such as in mature warm subduction zones (e.g., Nankai, Cascadia), in lower continental crust shear zones and oceanic detachments