Dissolution Of Plagioclase Research Articles

Hydro-Geochemistry of Deep Aquifers in Coal Mines in Northern Anhui Province, China: Implications for Water Rock Interaction and Water Source Discrimination

Thirty-eight groundwater samples from three deep seated aquifers (the Quaternary, the Coal bearing and the Taiyuan Fm aquifers) had been collected from three coal mines in northern Anhui Province, China for analyzing their major ions, and the statistical methods (correlation, cluster and discriminant analysis) had been processed for evaluating the source of ions and relationship between aquifers. The results suggest that: (1) the chemical signatures of the groundwater are mainly controlled by dissolution of plagioclase, K-feldspar and calcite; (2) the aquifers had been mixed with each other, especially the mixing between the Quaternary aquifer and the Taiyuan Fm aquifer. Discriminant analysis has produced two functions and a diagram, which can be used for water source discrimination.

Integrating topography, hydrology and rock structure in weathering rate models of spring watersheds

Weathering rate models designed for watersheds combine chemical data of discharging waters with morphologic and hydrologic parameters of the catchments. At the spring watershed scale, evaluation of morphologic parameters is subjective due to difficulties in conceiving the catchment geometry. Besides, when springs emerge from crystalline massifs, rock structure must be accounted in formulas describing the area of minerals exposed to the percolating fluids, for a realistic evaluation of the rates. These particular features are not included in the available approaches and for that reason a new model was developed, coined THROW model. This is a lumped approach that integrates (T)opography, (H)ydrology, (RO)ck structure and (W)eathering in a single algorithm. The study area comprises several stream watersheds and spring sites of the Vouga River basin (northern Portugal), shaped on granites. Firstly, the THROW model couples a terrain modeling analysis with hydrologic models based on discharge rates, to determine hydraulic conductivities (K), effective porosities (ne) and annual recharges (Vr) at the stream watershed scale. Subsequently, these parameters are used in a water balance model to estimate concomitant groundwater travel times (t). The mean K [(4.7 ± 3.2) × 10−7 m s−1] and ne [(2.0 ± 1.3) × 10−2] values are adopted as proxies for the spring watersheds and a firm regression equation is defined between time and stream watershed area (A). Secondly, two more runs of terrain modeling analysis are executed to extrapolate morphologic parameters for the spring watersheds. The first run hinges on scaling properties of the drainage networks, known as Horton laws, and is used to scale watershed areas across stream orders (i). The scaling function is described by another regression equation. The second run evaluates the order of a spring watershed, defined as equivalent order (ieq) and equated to the mean order of the surrounding stream watersheds. Having calculated the ieq, spring watershed areas and travel times were downscaled using the regression equations (A < 10 km2 and t = 1.4–2.8 year). Standing on the physical and hydrologic parameters of the spring watersheds, the THROW model finally calculates plagioclase weathering rates in the vicinity of the spring sites. The SiB model (Pacheco and Van der Weijden, 1996) was used before to estimate the contribution of plagioclase dissolution to the chemical composition of these springs (Van der Weijden and Pacheco, 2006). The chemical data were now coupled with K, ne and t in a rate equation to estimate chemical weathering rates of plagioclase in the basin. In the THROW model, the rate equation describes the exposed surface area as a function of fracture spacings, openings and porosities (Pacheco and Alencoao, 2006). The calculated rates (WPl = (2.5 ± 1.2) × 10−14 mol m−2 s−1) are consistent with previous reports and with results of experimental kinetic models. The SiB results predict formation of halloysite and gibbsite along the flow path, which were indeed close to equilibrium with the dissolved Al and Si activities. âo Develop a weathering model that incorporates rock structure in the rate equation. âo Conceive a weathering model especially designed for fracture artesian springs. âo Create a model that integrates topography, hydrology, rock structure and weathering

Impacts of glacial/interglacial cycles on continental rock weathering inferred using Sr/Ca and 87Sr/86Sr ratios in Michigan watersheds

Michigan soils have developed on thick glacial-drift deposits that include different proportions of ground granite and gneiss from the Canadian Shield region (with radiogenic Sr) and carbonate sedimentary rocks from within the Michigan Basin (with non-radiogenic Sr). This study of the Cheboygan, Huron and Kalamazoo watersheds shows how Sr-isotope and Sr/Ca ratios in soil waters, ground waters, and soils reflect relative weathering intensities of the dominant minerals in Michigan soils, including carbonates from the Michigan Basin, and amphibole, plagioclase and K-feldspar derived from the Canadian Shield.Soil water 87Sr/86Sr ratios evolve quickly to the carbonate weathering end-member (0.709–0.711) once a calcite and dolomite layer is reached at depth (~100–200cm) in the Huron and the Kalamazoo watersheds. Dissolution of plagioclase and amphibole controls shallow soil water 87Sr/86Sr ratios (0.711–0.713), with minor contributions from K-feldspar weathering. In contrast, soils in the previously studied Cheboygan watershed are completely depleted in carbonate minerals and contain little plagioclase and amphibole in the top 300cm of the profile. As a result, soil waters in this watershed are ionically dilute with high 87Sr/86Sr ratios (0.72 and 0.74), dominantly contributed by K-feldspar dissolution. Subsequent dissolution of plagioclase and amphibole at greater depths sharply increases soil water and ground water Mg2+, Ca2+, and Sr2+ concentrations, and lowers the Sr-isotopic ratios to ~0.709 for the Cheboygan watershed.Similarly, along hydrologic flow paths, soil water Sr/Ca ratios move from the silicate end-member (defined by amphibole and plagioclase) towards the carbonate end-member. The Sr-isotopic compositions and Sr/Ca ratios of soil waters thus reveal the types, directions and extent of chemical weathering processes in Michigan soils, augmenting information from previous soil water chemistry and soil mineralogy studies.This work also highlights the two-fold impacts of glacial/interglacial cycles on the riverine and oceanic Sr isotopes: due to the great extent of continental glaciation, Paleozoic carbonate minerals from the Michigan Basin were redistributed widely within the interior of the North American continent, leading to elevated Sr fluxes with lower Sr-isotopic ratios in natural waters after glacial retreat. The glacial ice also ground up the ancient Precambrian Canadian Shield, accelerating mineral weathering rates and releasing highly radiogenic Sr from K-feldspar.

Amphibole-bearing Multiphase Solid Inclusions in Olivine and Plagioclase from a Layered Gabbro: Origin of the Trapped Melts

This study presents new data regarding the petrographic and mineralogical characteristics of amphibole-bearing multiphase solid inclusions (MSI) enclosed in olivine and plagioclase from a well-characterized Japanese layered gabbro, the Murotomisaki Gabbroic Intrusion. Bulk compositions of the MSI were obtained using wavelength-dispersive (WDS) and energy-dispersive (EDS) techniques combined with modal analysis. Many MSI in olivines are spherical or polygonal in shape and are mainly composed of pargasitic amphibole, orthopyroxene, and mica (biotite and aspidolite). The MSI in olivine phenocrysts in the chilled margins of the intrusion are composed of plagioclase and hornblende rather than pargasite. Many of them are found to be enriched in olivine components in their bulk compositions, and therefore they cannot be regarded as simple trapped basaltic melts or their derivatives. The MSI in plagioclase are more irregular in shape and occur in the resorbed part of calcic plagioclase cores; they are mainly composed of pargasite and sodic plagioclase. Attempts were made to obtain the original composition of the MSI melts by applying a correction for Fe^Mg exchange reactions with the host olivine. It was found that after the correction, the MSI in the chilled margin were closest to the published initial basaltic melt composition, whereas the olivine-hosted MSI from the olivine gabbros were still fairly high in olivine components despite the correction. Such post-entrapment modification was probably minimal for the plagioclase-hosted MSI because plagioclase is essentially free of Mg and Fe, and therefore plagioclase-hosted MSI are believed to represent the compositions of the original trapped melts. These are characteristically enriched in plagioclase components. We believe that the anomalous compositions of the trapped melts (after the corrections) were generated by local dissolution of olivine and plagioclase caused by the introduction of water from lower horizons of the crystallization boundary layer in a magma chamber. The MSI are therefore considered to provide new evidence for the dissolution of silicates by hydrous fluxing in a lower crystallizing boundary layer of a magma reservoir during solidification of basaltic magma.

Dados de campo e petrográficos de granitoides dos tipos I e A de 1,90 a 1,88 Ga da região central do Cráton amazônico, NE do Estado do Amazonas, Brasil

The SW Presidente Figueiredo district, which is located in the northeastern Amazonas State of the central Amazonian Craton, Brazil, consists of 1890 to 1898 Ma I-type granitoids (Terra Preta Granite, Água Branca Suite), A-type hornblende-bearing syenogranites (Canoas Syenogranite, Mapuera Suite), felsic to intermediate volcanic rocks (Iricoumé Group), and 1883 to 1889 Ma rapakivi granites (São Gabriel Granite, Mapuera Suite) and related rocks (quartz-gabbro-anorthosite and diorite), in addition to Castanhal quartz-monzonite, mylonites, and hornfels. The quartz-diorite facies of Terra Preta Granite were formed by mingling processes between a synplutonic quartz-gabbro dike and a hornblende granodiorite. Partially assimilated globules of Canoas hornblende-bearing syenogranite and their clear contacts with Terra Preta hornblende granodiorite suggest that Canoas Syenogranite is slightly younger than Terra Preta Granite. Canoas Syenogranite xenoliths inside São Gabriel Granite show that the granite is younger than the Canoas Syenogranite. New geologic and petrographic evidence improve the petrological understanding of these rocks and suggest that, in addition to fractional crystallization, assimilation and magma mingling played a role, at least at the local scale, in the evolution and compositional variation of the plutons. Such evidence is found in Terra Preta Granite mingled quartz-diorite, felsic material associated with the Canoas Syenogranite and in the intermediate microgranular enclaves, which exhibit primary biotite in hornblende-bearing rocks, plagioclase dissolution, corrosion of feldspars rims, alkali feldspar mantles, second apatite generation, and high xenocrystal contents in intermediate enclaves formed from the fragmentation of mafic intrusions. Petrographic analyses show that a deformational event recorded in the western part of the study area (with progressive deformation from E to W) is dated between the 1.90 Ga postcollisional magmatism and the intrusions of the São Gabriel Granite and related mafic/intermediate rocks (intra-plate). However, it is extremely necessary to obtain absolute ages for this metamorphic event.

Weathering of plagioclase across variable flow and solute transport regimes

The study area is situated in a fault zone with fractured granites and metasediments. In a conceptual model, infiltrating water first passes the bedrock cover of soil and saprolite and then partly enters the fractures. Weathering reactions of minerals occur in small pores and fissures in the bedrock cover zone to continue in the larger fractures. Pumping tests were carried out in a number of boreholes to measure the drawdown as a function of pumping time. From the results, values of transmissivity (T) could be derived. In combination with the storage coefficient (S) for similar fault zones, the hydraulic diffusivity (D = T/S) could be computed. Water samples, collected from the boreholes, represent fluid packets with a history of weathering reactions in the bedrock cover and in the larger fractures. The major element composition of these samples was used by means of the SiB mass balance algorithm (Pacheco and Van der Weijden, 1996) to calculate the moles L−1 of dissolved plagioclase (oligoclase with An ≈ 0.20) and the moles L−1 of secondary phases (gibbsite, halloysite, smectite) precipitated along the flow paths of the samples. These results were then used to calculate the net dissolved silica concentrations ([H4SiO40]) related to dissolution of plagioclase followed by precipitation of each of the secondary phases. An interpretation of a plot of each of these [H4SiO40] ’s versusD is that at D < 0.7 m2 s−1, dissolution of plagioclase is followed by precipitation of halloysite in the large fractures of the fault zone (open system), whereas at D ⩾ 0.7 m2 s−1 precipitation of both halloysite and smectite occurs in the rock matrix with small fissures and pores (semi-open system). Before being pumped, the percolating fluids travelled 0.01–13.7 years. During these periods, plagioclase weathered at rates (WPl) of 10−(12.9±1.1) moles m−2 s−1, which are approximately 2.2 orders of magnitude higher than solid-state weathering rates reported in various field studies. In this study, it is suggested that part of the apparent discrepancy between the results is due to changes in hydraulic diffusivity of the weathering environments occurring over the geologic times.

The effect of curvature on weathering rind formation: Evidence from Uranium-series isotopes in basaltic andesite weathering clasts in Guadeloupe

To quantify rates of rind formation on weathering clasts under tropical and humid climate and to determine factors that control weathering reactions, we analyzed Uranium series isotope compositions and trace element concentrations in a basaltic andesite weathering clast collected from Basse-Terre Island in Guadeloupe. U, Th, and Ti elemental profiles reveal that Th and Ti behave conservatively during rind formation, but that U is added from an external source to the rind. In the rind, weathering reactions include dissolution of primary minerals such as pyroxene, plagioclase, and glass matrix, as well as formation of Fe oxyhydroxides, gibbsite and minor kaolinite. Rare earth element (REE) profiles reveal a significant Eu negative anomaly formed during clast weathering, consistent with plagioclase dissolution. Significant porosity forms in the rind mostly due to plagioclase dissolution. The new porosity is inferred to allow influx of soil water carrying externally derived, dissolved U. Due to this influx, U precipitates along with newly formed clay minerals and oxyhydroxides in the rind. The conservative behavior of Th and the continuous addition of U into the rind adequately explain the observed systematic trends of (238U/232Th) and (230Th/232Th) activity ratios in the rind. Rind formation rates, determined from the measured U-series activity ratios with an open system U addition model, increase by a factor of ∼1.3 (0.18–0.24mm/kyr) from a low curvature to a high curvature section (0.018–0.12mm−1) of the core–rind boundary, revealing that curvature affects rates of rind formation as expected for diffusion-limited rind formation. U-series geochronometry thus provides the first direct evidence that the curvature of the interface controls the rate of regolith formation at the clast scale. The weathering rates determined at the clast scale can be reconciled with the weathering rates determined at the watershed or soil profile scale if surface roughness equals values of approximately 1300–2200.

Mechanisms and rates of plagioclase carbonation reactions

Plagioclase is one of the most abundant sources of calcium in the earth’s crust, and it may play an important role for CO2 storage. This study address’ the carbonation of anorthite-rich plagioclase (An67–An73) in a system with fluid transport, and under stagnant conditions.A combined approach of flow-through column and batch experiments has been used. Experimental conditions ranging from 100 to 250°C and 20 to 120bar and different preparations of the starting material were applied. The overall carbonation reaction consists of plagioclase dissolution coupled to a number of precipitation reactions. The flow-through column experiments at 250°C showed stoichiometric dissolution of the plagioclase. Al-hydroxide (“proto Al-hydroxide”) nucleated on the plagioclase as the first phase to precipitate. A secondary porosity developed between the shrinking plagioclase and the enclosing “proto Al-hydroxide”. Calcite, as the second phase to precipitate, filled the primary pore space. A reaction front was developed separating the zone at the inlet where all the plagioclase had dissolved and the less reacted outlet of the column. Redissolution of the calcite and formation of euhedral boehmite crystals occurred when a sufficient amount of plagioclase had dissolved. Clay minerals were not precipitated in the column experiments. Between 11% and 30% of the plagioclase was dissolved within 72–168h of reaction. A much higher extent of plagioclase dissolution was observed in the high pressure experiments compared to the low pressure. However, a smaller share of the released Ca was trapped as calcite in the high pressure experiments. Both observations are consistent with a more rapid progression of the dissolution front at high pressure.The batch experiments showed conversion of the plagioclase to a mixture of Al-hydroxide, possibly gibbsite, clays and calcite. A range in conversion from below the detection limit to 91% was observed within reaction periods of 24–72h. Crystallinity of the feldspar was the most important factor contributing to increased reaction rates. A general positive effect of increasing temperature on the conversion is observed for all materials, whereas pressure and the addition of CaCl2 did not have any effect.The carbonation of plagioclase at stagnant conditions is slow compared to olivine at temperatures around 200°C. However, industrial operations involving high fluid flows of CO2–water mixtures induce gradients in pH or solute concentrations, which may lead to increased reaction rates and changes in porosity/permeability.

Artificial chemical weathering of gabbro under earth surface conditions- - : The influence of acid rain on rocks and minerals (part 4)

In order to predict the effect of acid precipitation on building stone, artificial chemical weathering of polished plates of Muroto Gabbro from Kochi, Shikoku Island, Japan, was conducted. An improved Soxhlet extraction apparatus was used with distilled water and HNO 3 , H 2 SO 4 and HCl solutions at pH 4 at 55°C for different periods of time up to 780 days. The gabbro was composed mainly of olivine (Fo 66 Fa 34 ), plagioclase (Ab 33 An 66 Or 1 ), and clinopyroxene (En 47 Fs 13 Wo 40 ). Morphological and chemical changes of each mineral surface were studied by SEM, EPMA and optical microscopic techniques. Alteration products were collected from the surface of each mineral on the polished plates by hand picking and examined using XRD. The leached sample solutions were analyzed for major and minor elements using ICP-AES, ICP-MS and ion-chromatography. It is evident that olivine changed into smectite, chlorite and talc by exposure to the various acid solutions used in the experiment. Smectite appeared to be formed from plagioclase during the artificial chemical weathering. However, altered products from clinopyroxene were not detected. Molar ratios of each element in the leached solutions to those in the fresh gabbro varied depending on the experimental period. The ratios of elements such as Mg, Mn, Fe, Ni, Zn and Co, which reflect the dissolution of olivine, were relatively high. Similarly, the ratios of Ca, Na, Sr, Ce, Sm and La, which reflect the dissolution of plagioclase, were relatively high. It seems, therefore, that olivine and plagioclase are easily weathered. It was also estimated from the molar ratios that the reactivity order of the acid solutions for olivine and plagioclase is H 2 SO 4 , HNO 3 and HCl.

Calcium isotope constraints on the uptake and sources of Ca 2+ in a base-poor forest: A new concept of combining stable (δ 44/42Ca) and radiogenic ( εCa) signals

Plant-available reserves of major base cations, Ca 2+ and Mg 2+, decreased markedly in soils over the past century, thus posing a potential threat to forest ecosystem health. Trees are thought to obtain dissolved Ca 2+ ions mainly from an easily accessible soil–water reservoir also termed the ‘exchangeable cation pool’. The status of Ca reserves in this soil pool is sensitive to anthropogenic perturbations such as soil acidification induced by acid rain and/or excessive timber harvesting. Here we show that in a base-poor forest of the northeastern USA (i.e. Wachusett Mountain, Massachusetts) the ‘exchangeable Ca pool’ of deeper mineral soils has a unique isotope signature that is significantly enriched in the radiogenic 40Ca, due to the dissolution of K-rich silicate minerals such as biotite. Using a simple isotope mass balance, and assuming that the input of Ca from biotite has a ε Ca signature of ∼16, the results of our calculation indicate that the weathering of biotite may supply a sizeable fraction, up to 25%, of Ca 2+ ions into the ‘exchangeable cation pool’ of deeper mineral soils. Importantly, samples of local vegetation (i.e. woody tissues of red oak) show no detectable excess of the radiogenic 40Ca, and based on our model the upper limit of a possible biotite-derived Ca contribution in vegetation is estimated at ∼5%. We also found no evidence of the radiogenic 40Ca signal in the samples of forest floor and the uppermost organic-rich soils (0–15 cm depth), which in turn suggest that over the long-term development of the forest and its organic matter accumulation, the vegetation growth must have also relied primarily on the non-radiogenic Ca sources. Based on our experimental data, such sources may include (i) wet atmospheric deposition, (ii) the organically-complexed Ca in topsoil horizons, and (iii) chemical weathering and/or fungal-mediated dissolution of apatite and Ca-rich plagioclase. Hence, our stable and radiogenic Ca isotope data indicate that the studied base-poor forest is able to bypass the ‘exchangeable cation pool’ of deeper (i.e. below 15 cm) mineral soils, and still manages to meet its nutritional requirements with respect to Ca. Another important implication of this study is that the organically-complexed Ca in the topsoil horizon (0–15 cm depth) has to be tightly bound to the ion exchange sites, otherwise the large radiogenic 40Ca signatures present in the ‘exchangeable cation pool’ of deep mineral soils would be swamped by the downward gravitational flux of non-radiogenic Ca from the decaying organic matter and litterfall. Hence, the limited mobility of the organically-complexed Ca in soils and its tight biological cycling could explain the lack of a significant impact of vegetation on the Ca isotope systematics observed in large rivers.

The major ion, δ 44/40Ca, δ 44/42Ca, and δ 26/24Mg geochemistry of granite weathering at pH = 1 and T = 25 °C: power-law processes and the relative reactivity of minerals

We dissolved Boulder Creek Granodiorite in a plug flow reactor for 5794 h at pH = 1 and T = 25 °C. The primary purpose of the experiment was to identify controls on dissolved δ 44/40Ca, δ 44/42Ca, and δ 26/24Mg values during granite weathering. Herein, we also examine the origin of Ca and Mg isotopic variability among minerals composing the Boulder Creek Granodiorite, and we constrain fundamental characteristics of granite weathering important for quantifying the elemental and isotopic geochemistry of the reactor output. Nine Ca-bearing minerals display an 8.80‰ range of δ 44/40Ca values and a 0.51‰ range of δ 44/42Ca values. Three Mg-bearing minerals display a 1.53‰ range of δ 26/24Mg values. These ranges expressed at the mineralogical scale are higher than the ranges thus far reported for bulk igneous rocks. Most of the δ 44/40Ca variability reflects 40Ca enrichment in K-feldspar, and to a lesser extent, biotite, due to the radioactive decay of 40K over the 1.7 Ga age of the rock, whereas the entire range of δ 44/42Ca values reflects mass-dependent isotope fractionation during igneous differentiation and crystallization. The range of δ 26/24Mg values may represent either fractionation during the chloritization of biotite or interaction of the Boulder Creek Granodiorite with Mg-rich metamorphic fluids having low δ 26/24Mg values. The elemental and isotopic composition of the reactor output varied substantially during the experiment. We synthesize the mineralogical and fluid data using coupled mass-conservation equations solved at non-steady-state. Model calculations reveal an intricate balance between increasing specific surface area and decreasing mineral concentrations. While surface area normalized dissolution rate constants were time-invariant, specific surface area increased as a power-law function of time through positive feedbacks between mechanical disaggregation, chemical dissolution, and mineral depletion. Variations in dissolved δ 44/40Ca, δ 44/42Ca, and δ 26/24Mg values reflect conservative mixing rather than fractionation. Apatite and calcite initially control δ 44/40Ca and δ 44/42Ca values, followed by biotite, titanite, epidote, hornblende, and plagioclase. The release of radiogenic 40Ca clearly defines the period where biotite dissolution dominates. The brucite layer of chlorite initially controls δ 26/24Mg values, followed by biotite, the TOT layer of chlorite, and hornblende. Through direct isotopic tracking, these results demonstrate that trace minerals, such as apatite and calcite in the case of Ca and brucite in the case of Mg, dominate elemental release during the incipient stages of granite weathering. The results further show that biotite dissolution dominates the middle stages of granite weathering and that plagioclase dissolution only becomes important during relatively late stages. The Ca and Mg isotope variations associated with these stages are distinct and potentially resolvable in soil mineral weathering studies.

Caprock interaction with CO2: A laboratory study of reactivity of shale with supercritical CO2 and brine

Abstract Crushed rock from two caprock samples, a carbonate-rich shale and a clay-rich shale, were reacted with a mixture of brine and supercritical CO 2 (CO 2 –brine) in a laboratory batch reactor, at different temperature and pressure conditions. The samples were cored from a proposed underground CO 2 storage site near the town of Longyearbyen in Svalbard. The reacting fluid was a mixture of 1 M NaCl solution and CO 2 (110 bar) and the water/rock ratio was 20:1. Carbon dioxide was injected into the reactors after the solution had been bubbled with N 2 , in order to mimic O 2 -depleted natural storage conditions. A control reaction was also run on the clay-rich shale sample, where the crushed rock was reacted with brine (CO 2 -free brine) at the same experimental conditions. A total of 8 batch reaction experiments were run at temperatures ranging from 80 to 250 °C and total pressures of 110 bar (∼40 bar for the control experiment). The experiments lasted 1–5 weeks. Fluid analysis showed that the aqueous concentration of major elements (i.e. Ca, Mg, Fe, K, Al) and SiO 2 increased in all experiments. Release rates of Fe and SiO 2 were more pronounced in solutions reacted with CO 2 –brine as compared to those reacted with CO 2 -free brine. For samples reacted with the CO 2 –brine, lower temperature reactions (80 °C) released much more Fe and SiO 2 than higher temperature reactions (150–250 °C). Analysis by SEM and XRD of reacted solids also revealed changes in mineralogical compositions. The carbonate-rich shale was more reactive at 250 °C, as revealed by the dissolution of plagioclase and clay minerals (illite and chlorite), dissolution and re-precipitation of carbonates, and the formation of smectite. Carbon dioxide was also permanently sequestered as calcite in the same sample. The clay-rich shale reacted with CO 2 –brine did not show major mineralogical alteration. However, a significant amount of analcime was formed in the clay-rich shale reacted with CO 2 -free brine; while no trace of analcime was observed in either of the samples reacted with CO 2 –brine.

Soil chemistry and shale weathering on a hillslope influenced by convergent hydrologic flow regime at the Susquehanna/Shale Hills Critical Zone Observatory

Soil chemistry data (major and REEs) are presented from a swale transect for comparison to similar measurements on a planar transect published previously for the Susquehanna/Shale Hills Critical Zone Observatory. Similar reaction fronts are observed: plagioclase dissolution is indicated by Na and Ca depletion and a negative Eu anomaly; clay dissolution followed by particle loss is accompanied by depletion of Mg, K, Fe, Al and Si. However, in contrast to the planar transect, soils along the swale transect, especially in the topographically depressed site, do not show smooth elemental profiles. This documents both residuum soils and accumulation of colluvium sediments. The soils in the swale transect are thicker and on average wetter than those along the planar transect; however, the Ce anomaly observed in the swale soils is consistent with a generally oxic environment. Thus, preferential flowpaths are an important mechanism for water transport, preventing swale soils from water saturation.

Petrophysical and petrochemical effects of long-term CO2 -exposure experiments on brine-saturated reservoir sandstone

Sandstone samples of the Stuttgart Formation at Ketzin have been experimentally treated with CO2 and synthetic reservoir brine in high-quality steel autoclaves at simulated in situ P-T conditions of 5.5 MPa and 40 °C. In order to observe mineralogical changes induced by CO2, untreated samples are compared to CO2-treated ones. Samples show a comparable petrography of mainly quartz and plagioclase. Heterogeneities are related to minor mineral phases, such as K-feldspar, hematite, muscovite, biotite, illite, chlorite, and opaque phase(s). This variability of the Stuttgart Formation is primarily attributed to the sandstone sedimentation in a meandering, fluvial environment. The samples are weakly consolidated. Analcime, anhydrite and dolomite are only found as cement phases. SEM micrographs of CO2-treated samples show corrosion textures on mineral surfaces of anorthitic plagioclase and K-feldspar, as well as euhedral albite crystals. Overall, the data indicate preferred dissolution of calcium-rich plagioclase, K-feldspar and anhydrite, and stabilization or precipitation of albite. The petrophysical properties of the sandstone samples also show slight changes. Nuclear magnetic resonance (NMR) and mercury injection measurements indicate a slightly increased porosity and a shift to larger pore sizes; both suggesting dissolution processes.

Monitoring petrophysical, mineralogical, geochemical and microbiological effects of CO2 exposure — Results of long-term experiments under in situ conditions

Abstract To study the long term effects of geochemically and microbially induced mineral dissolution and precipitation during CO 2 storage and their impact on storage reliability and efficiency, laboratory experiments under in situ P-T conditions were performed. Long term CO 2 exposure experiments were initiated in September of 2007 using fresh core material from the reservoir horizon at the CO2SINK project in Ketzin, Germany. Formation water-filled sandstone samples, obtained from a depth of 627 to 633 m, have been incubated individually in high pressure vessels together with sterile synthetic formation brine under simulated reservoir P-T conditions of 5.5 MPa and 40 °C. After 15, 21, and 24 months of CO 2 exposure, rock and fluid samples were analyzed with respect to geochemistry, microbial community, as well as mineralogical and petrophysical characteristics. The analysis of fluid samples revealed the dissolution of mineral phases during CO 2 exposure, indicated by increasing concentrations of Ca 2+ , Mg 2+ , K + and SO 4 2− . First mineralogical results are in consistence with the fluid chemical data and showed dissolution of anorthitic plagioclase, K-feldspar and anhydrite. However, the mineral dissolution can only partly be attributed to the influence of CO 2 . A proportion is due to the equilibration between rock and synthetic brine. The dissolution of minerals led to increasing porosities, as observed with NMR measurements and mercury injections. Decreasing porosities after 24 months of incubation together with decreasing inorganic fluid component concentrations, and precipitation of albite suggest secondary mineral precipitation. A 16S rDNA based fingerprinting method was used to identify the dominant species in DNA extracts of pristine sandstone samples. So far sequences belonging to facultative anaerobic, chemoheterotrophic bacteria were identified. During CO 2 exposure minor changes in the microbial community composition were observed. The majority of microbes were able to adapt to the changes in environmental conditions.

Reservoir characterization of a CO 2 storage aquifer: The Upper Triassic Stuttgart Formation in the Northeast German Basin

Ketzin, in the Northeast German Basin (NEGB), is the site for pilot injection of CO 2 (CO 2SINK project) into a saline aquifer (the Upper Triassic Stuttgart Formation) situated at a depth of about 630–700 m. This paper reports the baseline characterization of the reservoir formation based on new core material and well-logs obtained from one injection well and two observations wells, drilled at a distance from 50 m to 100 m from each other. The reservoir is lithologically heterogeneous and made up by fluvial sandstones and siltstones interbedded with mudstones showing remarkable differences in porosity. The thickest sandstone units are associated with channel sandstone, whose thickness varies over short lateral distances. In-depth petrographic, mineralogical, mineral-chemical, and whole-rock geochemical analysis were performed focusing on the sandstone intervals, which display the best reservoir properties for CO 2 injection. The dominantly fine-grained and well to moderately-well sorted, immature sandstones classify as feldspathic litharenites and lithic arkoses. Quartz (22–43 wt.%), plagioclase (19–32 wt.%), and K-feldspar (5–13 wt.%) predominate mineralogically. Muscovite plus illite and mixed-layer minerals are omnipresent (4–13 wt.%). Quartz, feldspar, as well as meta-sedimentary and volcanic rock fragments comprise the most abundant detrital components, which often are rimmed by thin, early diagenetic coatings of ferric oxides, and locally of clay minerals. Feldspar grains may be unaltered and optically clear, partially to completely dissolved, partially altered to sheet silicates (mainly illite), or albitized. Analcime and anhydrite constitute the most widespread, often spatially associated pore-filling cement minerals. Authigenic dolomite, barite, and coelestine is minor. The percentage of cements ranges in total from about 5 vol.% to 32 vol.%. Except of samples intensely cemented by anhydrite and analcime, total porosities of the sandstones range from 13% to 26%. The fraction of intergranular porosity varies between 12% and 21%. About 1–5% porosity has been generated by dissolution of detrital plagioclase, K-feldspar, and volcanic rock fragments. The comparatively large modal abundance of feldspars, micas, chlorite, clay minerals, Fe–Ti-oxides, and analcime account for the richness in Ti, Al, Fe, Mg, Na, and K, and the paucity in Si, of the Stuttgart sandstones relative to mature sandstones. Altogether, these sandstones are comparatively rich in minerals that may potentially react with the injected CO 2.

CO 2–brine–rock interaction — First results of long-term exposure experiments at in situ P–T conditions of the Ketzin CO 2 reservoir

Sandstone samples of the Stuttgart Formation at Ketzin have been experimentally treated with CO 2 and synthetic reservoir brine in high-quality steel autoclaves at simulated in situ P–T conditions (5.5 MPa, 40 °C). In order to observe mineralogical changes induced by CO 2, untreated samples are compared to CO 2-treated ones. Most samples show an analogous petrography of mainly quartz and plagioclase. Heterogeneities are related to minor mineral phases, such as K-feldspar, hematite, muscovite, biotite, illite, chlorite and opaque phase(s). These are attributed to the variability of the fluvial reservoir sandstone. The samples are weakly consolidated. Analcime, dolomite and anhydrite are only found as cement phases. During the experiments dissolution of anhydrite and plagioclase is observed. SEM micrographs of CO 2-treated samples show corrosion textures on mineral surfaces of intermediate plagioclase, as well as precipitation of euhedral albite crystals. Overall, the data indicate preferred dissolution of calcium-rich plagioclase, K-feldspar and anhydrite and stabilization or precipitation of albite.

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.

10.1007/s11495-008-1005-1

New data on the composition of minerals in corona textures around olivine and crystal-fluid inclusions in olivine from anorthosites of the Korosten’ pluton (sampled in the Golovino quarry), Ukrainian Shield were obtained using electron and ion microprobe analyses, Raman spectroscopy, scanning electron microscopy, and cryo- and thermometry. The corona textures developed around olivine grains in contact with plagioclase and consist of inner orthopyroxene rims around olivine and outer rims of orthopyroxene-clinopyroxene-orthoclase-plagioclase symplectites. The symplectites and orthopyroxene rims most probably developed nearly simultaneously and grew in the opposite directions from the original contact of the magmatic olivine and plagioclase and replaced both olivine and plagioclase. The Al2O3 and CaO concentrations in the symplectitic orthopyroxene increase toward the contact with magmatic plagioclase, whereas the Al2O3 and CaO concentrations in the symplectitic plagioclase simultaneously decrease and its Na2O and K2O increase. Optically discernible crystalline and fluid phases of crystal-fluid inclusions in olivine were identified as pyroxenes (orthopyroxene and clinopyroxene), actinolite, Ca-and Fe, Mg-carbonates, and magnetite, along with practically pure highdensity CO2. The mineral assemblages of corona texture in the Korsten’ anorthosites were produced by autometasomatic processes at a high CO2 activity, and the local variations in the chemistry of corona minerals were likely controlled by the content and chemistry of the interstitial fluid and primary minerals. The coronas developed under subsolidus conditions, via the reaction interaction of olivine and plagioclase under the effect of an integranular fluid, with the dissolution of olivine and plagioclase at T = 980–860°C and P > 5 kbar. Inasmuch as corona textures do not occur ubiquitously in the rocks, the origin of the former was most probably controlled by the amount of the intergranular fluid.

Hydrothermal alteration of deep fractured granite: Effects of dissolution and precipitation

This paper investigates the mineralogical effects of hydrothermal alteration at depth in fractures in granite. A fracture accompanied by an alteration halo and filled with clay was found at a depth of 200 m in a drill core through Toki granite, Gifu, central Japan. Microscopic observation, XRD, XRF, EPMA and SXAM investigations revealed that the microcrystalline clays consist of illite, quartz and pyrite and that the halo round the fracture can be subdivided into a phyllic zone adjacent to the fracture, surrounded by a propylitic zone in which Fe-phyllosilicates are present, and a distinctive outer alteration front characterized by plagioclase breakdown. The processes that result in these changes took place in three successive stages: 1) partial dissolution of plagioclase with partial chloritization of biotite; 2) biotite dissolution and precipitation of Fe-phyllosilicate in the dissolution pores; 3) dissolution of K-feldspar and Fe-phyllosilicate, and illite precipitation associated with development of microcracks. These hydrothermal alterations of the granite proceed mainly by a dissolution–precipitation process resulting from the infiltration of hydrothermal fluid along microcracks. Such infiltration causes locally high mobility of Al and increases the ratio of fluid to rock in the alteration halo. These results contribute to an understanding of how granitic rock becomes altered in orogenic fields such as the Japanese island arc.