Kurt Bucher

Water-Rock Interaction

Preface. Groundwater -- field and experimental studies. Groundwater Evolution in an Arid Coastal Region of the Sultanate of Oman based on Geochemical and Isotopic Tracers C.E. Weyhenmeyer. A combined isotopic tool box for the investigation of water-rock interaction: An overview of Sr, B, O, H isotopes and U-series in deep groundwaters from the Vienne granitoid (France) P. Negrel, et al. Water-rock reaction experiments with Black Forest gneiss and granite K. Bucher, I. Stober. The distribution of rare earth elements and yttrium in water-rock interactions: field observations and experiments P. Moller. Hydrothermal and volcanic settings. A case study of gas-water-rock interaction in a volcanic aquifer: the south-western flank of Mt. Etna (Sicily) A. Aiuppa, et al. Modelling chemical brine-rock interaction in geothermal reservoirs M. Kuhn, et al. Mineral deposits. Water-rock reactions in a barite-fluorite underground mine, Black Forest (Germany) I. Stober, et al. Extensional Veins and Pb-Zn Mineralisation in basement rocks: The role of penetration of formation brines S. Gleeson, B. Yardley. Experimental studies. Interaction of polysilicic and monosilicic acid with mineral surfaces M. Dietzel.

Composition of fluids in the lower crust inferred from metamorphic salt in lower crustal rocks

Knowledge of the rheological properties of the lower crust and the metamorphic processes that operate there is important for our understanding of orogenic processes and granite genesis. The rheological properties critically depend on whether fluids are present in the lower crust, and, if present, on their composition. Fluid-inclusion and phase-equilibria, studies of lower crustal granulites have shown that fluids with low water activities (due to the presence of dissolved components such as CH4, N2, CO, CO2 and chlorides) are present at least episodically in the lower crust. Here we report the occurrence of a solid salt solution (NaCl–KCl) found together with chlorine-rich amphibole and biotite in lower crustal granulites. A desiccation mechanism explains how salt and chlorine-rich minerals formed from an originally water-rich fluid through a short-lived series of hydration reactions in the granulites, during which chlorine was progressively enriched in the fluid. Consequently, it would appear that fluid was present in the lower crust in only small amounts and was not stable over geologically long periods of time, leading to the conclusion that the lower crust is devoid of a free fluid phase during most of its history.

Blueschists, eclogites, and decompression assemblages of the Zermatt-Saas ophiolite: High-pressure metamorphism of subducted Tethys lithosphere

Abstract The Zermatt-Saas ophiolite of the Swiss Alps represents a complete sequence of Mesozoic Tethys oceanic lithosphere. The ophiolite was subducted during early phases of the Alpine orogeny and the mafic rocks were transformed to eclogites and blueschists. Metabasalts locally preserve pillow structures in which glaucophanite forms rims on eclogitic pillow cores. Omphacite-garnet-glaucophane-epidote-ferroan dolomite-Mg-chloritoid-talc-paragonite-chlorite. rutile form characteristic coeval blueschist- and eclogite-facies assemblages. Omphacite + garnet + glaucophane + epidote + rutile represents an equilibrium assemblage that formed during deformation and in the period when the rocks reached the greatest depth of subduction. In rocks containing this assemblage, an additional significant mineral pair is Mg-chloritoid + talc. Coarse chloritoid (XMg ~ 0.45) and talc formed in dispersed clusters after the last penetrative deformation. The assemblage may require > 2.7 GPa pressure to form. It developed at maximum pressure conditions corresponding to the return-point of the ophiolite in the subduction zone. Coarse paragonite and chlorite replaced parts of the earlier formed assemblages and removed free H2O from the rocks. Exhumation of the HP to UHP ophiolite rocks was accompanied by development of symplectite rims and other replacement products along grain boundaries of the eclogite minerals by decompression reactions in a fluid-deficient regime. Particularly noteworthy is the formation of margarite, paragonite, chlorite, albite, barroisite, and preiswerkite. The latter mineral, a very rare Na-biotite, formed as a result of the decomposition of chloritoid + paragonite and is associated with magnetite and hercynite. Omphacite breakdown produced diopside-albite-barroisite symplectites. Calculated equilibrium assemblage phase diagrams for metabasite compositions indicate P-T conditions of ~2.5.3.0 GPa and ~550.600 °C. The conditions of the subduction-related metamorphism denote P and T at the return-point, which coincide with the upper P-T limit of antigorite. Antigoriteserpentinites constitute the largest volume of rocks within the ophiolite. We suggest that the P-T conditions recorded by the exhumed mafic rocks are coupled to those of antigorite breakdown in the serpentinite that released large amounts of dehydration water in the subducted serpentinite slab facilitating exhumation of the Zermatt-Saas eclogites and blueschists.

Coesite inclusions in garnet from eclogitic rocks in western Tianshan, northwest China: Convincing proof of UHP metamorphism

Abstract Coesite inclusions in garnet have been recognized in eclogitic rocks from western Tianshan, northwest China. The coesite grains exhibit distinct radial cracks in host porphyroblastic garnet; some coesite relics are well preserved, whereas others are partially replaced by quartz. Coesite has been identified optically and then confirmed by in situ Raman spectroscopy, showing the characteristic band at 522 cm-1 and subsidiary bands at 428, 326, 271, 178, 151, and 121 cm-1. The eclogitic rocks contain garnet, omphacite, and Na-Ca-amphibole, and they are rich in white mica (>30%) and graphite. Peak conditions of 570-630 °C and 2.7-3.3 GPa are constrained by garnet-clinopyroxene geothermometry and the occurrence of coesite. The presence of coesite and widespread quartz inclusions in garnet with radial cracks indicative of former coesite in these unique graphitic rocks confirms the previous suggestion of the UHP terrane for the western Tianshan, China.

Deep groundwater in the crystalline basement of the Black Forest region

Two major types of groundwater can be readily distinguished in the Variscian crystalline basement of the Black Forest in S–W Germany. Saline thermal water utilized in spas has its origin in 3–4 km deep reservoirs and developed its composition by 3 component mixing of surface freshwater, saltwater (of ultimately marine origin) and a water–rock reaction component. In contrast to the thermal water, CO2-rich mineral water, tapped and bottled from many wells in the Black Forest, has low salinities but a TDS distribution similar to that of thermal water. It developed its chemical composition entirely by reaction of CO2-rich water with the gneissic or granitic aquifer rock matrix. Particularly important is the contribution of various plagioclase dissolution and weathering reactions that may, at some locations, involve precipitation and dissolution of secondary calcite. Sodium/Ca ratios of water and of rock forming plagioclase in the basement rocks suggests that plagioclase weathering is strongly incongruent. Calcium is released to the water, whereas Na remains fixed to the albite feldspar component. The major element composition of 192 water samples used in this study also indicates a clear vertical stratification of the type of water chemistry; Ca–HCO3 near the surface, Na–Ca–HCO3–SO4 at intermediate depth and Na–Ca–Cl at great depth. The mean permeability of Black Forest granite is about K=10−6 m/s; it is significantly lower in gneisses (gneiss: mean K=5×10−8 m/s) leading to focused flow through granite. Highly permeable fracture and fault zones, particularly in granite, are utilized by high-TDS saline deep groundwater as ascent channels and flow paths. Although spatially closely associated, the topography driven upwelling system of saline deep water and the near-surface flow system of CO2-rich mineral waters are hydraulically and chemically unconnected.

Is water responsible for geophysical anomalies in the deep continental crust? A petrological perspective

Abstract It is common to ascribe conductivity and seismic anomalies in the lower crust to the presence of fluids. We note that fluids cannot be stored long in the lower crust for both mechanical reasons and because at temperatures above ca. 250°C reaction rates are so fast that fluids must be rapidly consumed by hydration reactions. To maintain a “wet” lower crust, therefore, fluid must be continually supplied. There are five sources for such fluids. Gravity-fed meteoric water may percolate many kilometers into the crust, but it cannot move to regions that are hotter than lowermost greenschist conditions because at these temperatures reactions consume water faster that water can be transported. Passive mantle degassing can only take place in areas of active magmatism, because in localities where the lower crust and upper mantle are cold, fluids are incompatible with known mantle mineralogy. Metamorphic devolatilization, evolution of igneous fluids, and tectonic introduction of fluids into the crust are all restricted to areas of active tectonism and are likely to proceed episodically. The fluid flow in the lower crust will vary according to the tectonic environment. In stable cratons fluids would be gravitationally driven but would be able to gain access only to the upper 10 km or so of the crust. Below this the crust would be “dry”. In extensional regimes the fluids would be thermally driven. Carbonic fluids of mantle origin may be present in the lower crust near underplated mantle melts, whereas fluids of meteoric or igneous origin will occur at shallow depths. In compressional regimes fluids may be driven by tectonic or thermal processes and may be of mantle, metamorphic, igneous, or meteoric origin. When considering the causes for lower crustal geophysical anomalies, therefore, one must consider the tectonic regime. Only in areas of active metamorphism or magmatism are fluids likely to play an important role. In cratonal regions the lower crust is “dry”. In such regions enhanced conductivity is likely to be caused by mineral films (graphite, magnetite, or sulfides) and reflectivity by lithologic variations due to either mylonitization or magmatic underplating.

Hydrogeology of crystalline rocks

Preface. 1. Water Conducting Features in Crystalline Rocks. Geological and hydraulic properties of water-conducting features in crystalline rocks M. Mazurek. Feldspars as microtextural markers of fluid flow I. Parsons, M.R. Lee. 2. Hydraulic Properties of Crystalline Rocks. Hydraulic properties of the Upper Continental Crust: data from the Urach 3 geothermal well I. Stober, K. Bucher. In-situ petrohydraulic parameters from tidal and barometric analysis of fluid level variations in deep wells: Some results from KTB K. Schulze, et al. The role of water-conducting features in the Swiss concept for the disposal of high-level radioactive waste M. Mazurek, et al. The scaling of hydraulic properties in granitic rocks D. Schulze-Makuch, P. Malik. 3. Hydrochemical Properties of Water in Crystalline Rocks. The composition of groundwater in the Continental crystalline crust K. Bucher, I. Stober. Evolution of fluid circulation in the Rhine graben: Constraints from the chemistry of present fluids L. Aquilina, et al. Occurrence and origin of Cl-rich amphibole and biotite in the Earths crust - implications for fluid composition and evolution K. Kullerud. Rare earth elements and yttrium as geochemical indicators of the source of mineral and thermal waters P. Moller. 4. Microbial Processes in Crystalline Rocks. The hydrogen driven intra-terrestrial biosphere and its influence on the hydrochemical conditions in crystalline bedrock aquifers K. Pedersen. Ancient microbial activity in crystalline bedrock - results from stable isotope analyses E.-L. Tullborg.

Metamorphic Processes in Rodingites of the Zermatt-Saas Ophiolites

Three types of rodingites can be distinguished in the serpentinite complex of the Zermatt-Saas ophiolites based on mineral assemblages, texture, and chemical characteristics of bulk-rock and mineral compositions. These three types of rodingites show a genetic relationship on the basis of their mineral evolution. The mineral assemblage Czo-Hgrs-Chl-Di ± Uvr (rodingite I) was developed during ocean-floor metamorphism and represents the earliest rodingitic rock. A second phase of oceanic alteration is characterized by the formation of andradite-rich hydrogarnet. Subsequent progressive metamorphism resulted in formation of vesuvianite and continued formation of hydroandradite, producing the assemblage Hgrs-Vsv-Hadr-Chl-Di (rodingite II). The low-variance mineral assemblage Hadr-Vsv-Chl (rodingite III) represents equilibrium conditions, and the most completely rodingitized rock. Complex variation in chemical composition and mineral assemblages (i.e., distribution of rodingite) is the result of variation in protoliths and degrees of rodingitization.

Proterozoic eclogites from the Lofoten islands, northern Norway

Abstract Massif lenses and bands of variably retrogressed eclogites occur within the granulite facies Proterozoic basement of the Lofoten islands in northern Norway. Eclogites are often related to shear and mylonite zones suggesting that deformation and fluid access was important for their formation. The eclogitic rocks are restricted to gabbroic or troctolitic rock compositions and show, in the least retrogressed samples, the typical assemblage garnet + omphacite + rutile. The widespread occurrence of eclogite shows that the whole group of the Lofoten islands was affected by high pressure metamorphism. Eclogites were found on three of the five major islands so far (Flakstadoy, Vestvagoy, Austvagoy). Thermobarometry reveals minimum conditions during eclogite formation of 680 °C/15 kbar on Flakstadoy and 540 °C/14 kbar on Austvagoy. Geological, petrological and geochemical data suggest the following succession of events: (a) Intrusion of mafic to mangeritic magmas (at about 1.8 Ga; Griffin et al., 1978). (b) Incomplete recrystallization under high-pressure granulite facies conditions. Associated with granulitization was the formation of Cl-rich hydrosilicates. Granulite formation was incomplete except for zones of intensive deformation. (c) Eclogites formed in response to a significant pressure increase in the same high strain zones. However, scattered eclogite lenses and boudins also occur in gneisses. (d) Post-eclogite retrogression occurred under middle to high pressure amphibolite facies conditions (probably the Leknes event at 1.1 Ga of Griffin et al., 1978). Eclogite formation was related to crustal thickening during the final stages of early Proterozoic magmatism and granulite facies metamorphism.

Chlorine stable isotope composition of granulites from Lofoten, Norway: Implications for the Cl isotopic composition and for the source of Cl enrichment in the lower crust

Abstract The Cl isotopic composition of lower crustal rocks, as revealed from granulites from the Lofoten Islands, Northern Norway, centres around the value of Standard Mean Ocean Chloride (SMOC). The investigated rocks and mineral separates of Cl-amphibole and Cl-biotite have δ 37 Cl values ( 37 Cl/ 35 Cl normalized to SMOC) of −1.12‰ to +0.79‰ with most values concentrated in the range of −0.3‰ to +0.11‰ and an average of −0.15‰ Samples from gabbroic and from dry granitic rocks (magmatic mangerites and charnockites) with highly Cl-enriched hydrosilicates and from a later amphibolite with low-Cl amphiboles show no distinct differences in their δ 37 Cl values. The results indicate that there was only minor involvement of mantle-derived Cl in fluid-involving processes in the middle and lower crust. Assuming a δ 37 Cl of 4.7‰ for the undegassed mantle [1], Cl isotopic values of the granulites do not support the influx of significant amounts of mantle derived Cl. The maximum possible mantle contribution to the sample with the highest δ 37 Cl is about 33%. Thus the common occurrence of Cl-enriched minerals in granulite facies terrains is regarded as a product of remobilized crustal Cl, either during magmatism or during later fluid activity. The results support the hypothesis that the crust can be regarded as a reservoir with Cl isotopic values very similar to SMOC.