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At mid-ocean ridges (MORs), seawater carrying dissolved sulfate (SO4) infiltrates the oceanic crust. Hydrothermal fluid emissions from such systems have much lower δ34S and sulfur is mostly present as reduced sulfide, albeit in lower total sulfur concentrations than in seawater. This has been explained by anhydrite formation and sulfate reduction based on petrographic evidence and mass balance considerations. Here, we utilize the chemical and stable isotope (δ34S, δ18O) systematics in natural anhydrite and pyrite from various locations along the submarine and on-land section of the Mid-Atlantic ridge near Iceland to quantify the key variables that control anhydrite formation and sulfate recycling in the oceanic crust. Hydrothermal anhydrite exhibited δ34S values of +20.6 ± 1.0‰ and δ18O values between +2.4 to +25.3‰. Volcanogenic anhydrite in encrustations showed δ34S values of −1.7 to +21.4‰ and δ18O values between +1.4 and +38.0‰. Hydrothermal pyrite exhibited δ34S values ranging from +3.4 and +19.7‰. Comparison of the natural dataset with results from geochemical isotope modelling revealed that δ34S and δ18O values of anhydrite and pyrite were dependent on the isotope composition of the source fluid, extent of water–rock interaction, temperature, and redox conditions. Departures of δ34S and δ18O values in anhydrite from the source fluid were caused by progressive fluid-basalt interaction where lower δ34S and δ18O values reflected a change in sources of S and O from solely fluid to basaltic origin. The δ18O values of anhydrite were additionally affected by temperature. Quantitative formation of anhydrite mainly occurred at temperatures < 150 °C, whereas at elevated temperatures (>200 °C) reduction of seawater-sulfate to H2S and subsequent pyrite precipitation were found to limit anhydrite formation. Extending our calculations to the oceanic crust revealed that the majority of seawater-sulfate is sequestered into anhydrite (3–38 Tg S yr−1) in vicinity of MORs at < 200 °C at shallow depth (

Publisher's version, Supercritical fluids exist in the roots of many active high-temperature geothermal systems. Utilization of such supercritical resources may multiply energy production from geothermal systems; yet, their occurrence, formation mechanism, and chemical properties are poorly constrained. Flow-through experiments at 260°C and 400-420°C were performed to study the chemical and mineralogical changes associated with supercritical fluid formation near shallow magmatic intrusions by conductive heating and boiling of conventional subcritical geothermal fluids. Supercritical fluids formed by isobaric heating of liquid geothermal water had similar volatile element concentrations (B, C, and S) as the subcritical water. In contrast, mineral-forming element concentrations (Si, Na, K, Ca, Mg, and Cl) in the supercritical fluid were much lower. The results are consistent with the observed mineral deposition of quartz, aluminum silicates, and minor amount of salts during boiling. Similar concentration patterns have been predicted from geochemical modeling and were observed at Krafla, Iceland, for the IDDP-1 supercritical fluid discharge. The experimental results confirm previous findings that supercritical fluids may originate from conductive heating of subcritical geothermal reservoir fluids characterized by similar or lower elemental concentrations with minor input of volcanic gas., We would like to thank Ríkey Kjartansdóttir, Andri Ísak Thórhallsson, and Helgi Arnar Alfreðsson for their valuable help. This work was funded by the Swiss National Science Foundation (CRSII2_1418431/1, Sinergia COTHERM), Georg (11-04-003), and The Energy Research Fund of Landsvirkjun in 2018 and 2019.

Supercritical fluids with temperatures of ~400–500 °C have been reported from several active geothermal fields worldwide. Although the utilization of such fluids may multiply power production from new and already exploited geothermal systems, the fluid origin and chemical controls on their composition remain unclear. We performed flow-through high-temperature (400–420 °C) experiments at 34–69 bar to study the chemical and mineralogical changes associated with supercritical fluid formation upon boiling of subcritical geothermal fluids of varying chemical composition. Based on geochemical modeling and laboratory results, we propose that an important mechanism of supercritical fluid formation is conductive heating and boiling of subcritical geothermal groundwater by a magmatic intrusion. Such supercritical fluids will display low concentrations of mineral-forming elements (Si, Na, K, Ca, Mg, Al), with their concentrations being controlled by the solubility of salts, oxides, and aluminum silicates in high-temperature (>400 °C) and low-density (ρ

This thesis aims to provide a better understanding of the role of mountain building in the carbon cycle. The amount of CO2 released into the atmosphere due to metamorphic processes is largely unknown. To constrain the quantity of CO2 released, fluid-driven reactions in metamorphic rocks can be studied by tracking fluid-rock interactions along ancient fluid flow pathways. The thesis is divided into two parts: 1) modeling of fluid flow rates and durations within shear zones and fractures during greenschist- and blueschist-facies metamorphism and 2) the assessment of possible mechanisms of fluid infiltration into rocks during greenschist- to epidote-amphibolite-facies metamorphism and controlling chemical and mineralogical factors of reaction front propagation.On the island Syros, Greece, fluid-rock interaction was examined along a shear zone and within brittle fractures to calculate fluid flux rates, flow velocities and durations. Petrological, geochemical and thermodynamic evidence show that the flux of CO2-bearing fluids along the shear zone was 100-2000 times larger than the fluid flux in the surrounding rocks. The time-averaged fluid flow velocity and flow duration along brittle fractures was calculated by using a governing equation for one-dimensional transport (advection and diffusion) and field-based parameterization. This study shows that fluid flow along fractures on Syros was rapid and short lived.Mechanisms and controlling factors of fluid infiltration were studied in greenschist- to epidote-amphibolite-facies metabasalts in SW Scotland. Fluid infiltration into metabasaltic sills was unassisted by deformation and occurred along grain boundaries of hydrous minerals (e.g. amphibole) while other minerals (e.g. quartz) prevent fluid infiltration. Petrological, mineralogical and chemical studies of the sills show that the availability of reactant minerals and mechanical factors, e.g. volume change in epidote, are primary controls of reaction front propagation.

Analysis of the delta O-18 and delta C-13 values of carbonate rocks from Islay, Scotland reveals structural channelling of metamorphic fluids through the axial region of a major en echelon anticlinal fold system. Metamorphic fluid flow produced axial planar veins with higher vein density in the axial region of the fold. Fluid: rock ratios were more than 30: 1 within this axial region, at least four times greater than the regional mean ratio of 7.6 +/- 1.5:1 for carbonate rocks on Islay. This supports the interpretation that metamorphic fluids were channelled through the axial region of the Islay Anticline. Fluid: rock ratios were calculated using a model for coupled delta O-18 and delta C-13 exchange with a metamorphic fluid. The metamorphic fluid was calculated to have delta O-18 and delta C-13 values of 15.3 parts per thousand and -6.1 parts per thousand, respectively and X-CO2 of 0.2. This is in isotopic and chemical equilibrium with chlorite- and graphite-bearing metamudstones that are structurally below the folded metacarbonate rocks on Islay. Devolatilization of these metamudstones is therefore a likely source mechanism for this metamorphic fluid. Removal of the effects of metamorphic fluid flow on delta C-13 values recorded by metacarbonate rocks on Islay allows us to re-evaluate evidence used to reconstruct Neoproterozoic climate. This evidence includes a large negative delta C-13 excursion reported from the Lossit Limestone Formation. This unit underlies the Port Askaig Formation, which is dominated by diamictites that have been interpreted as glacial tillites. This 'Islay anomaly' has been correlated with other such anomalies worldwide and together with overlying tillites has been cited as evidence of major (worldwide) glaciation events. In this study, we show that the magnitude of this negative delta C-13 anomaly can partly be explained by exchange with metamorphic fluids. However, we also show that extremely negative delta C-13 values in the Bonahaven Dolomite Formation, which overlies the Port Askaig Formation and has been interpreted as a 'cap carbonate', cannot be attributed to metamorphic fluid flow.

In this study we investigate a group of metabasaltic sills from the SW Scottish Highlands metamorphosed at epidote-amphibolite facies conditions that provide useful insight into the mechanisms and characteristics of fluid infiltration during metamorphism. The sills are amphibole and garnet bearing and exhibit a strong foliation in the sill margins that developed pre- to syn-peak metamorphism. Fluid infiltration caused hydration and carbonation in the sills, expressed as (1) replacement of garnet and amphibole by chlorite and calcite and (2) replacement of amphibole and epidote to form chlorite and calcite. Using garnet-amphibole and garnet-chlorite geothermometers we show that these reactions occurred after peak metamorphism at T = 290 to 400 °C. Reaction textures show that the fluid infiltration into the sill that caused hydration and carbonation occurred in the absence of deformation. The fluid infiltration was mineralogically controlled with greater fluid access in areas of abundant fine-grained elongate minerals such as amphibole and chlorite. The replacement of garnet by chlorite most likely occurred by an interface-coupled dissolution-precipitation mechanism as evidenced by perfect pseudomorphic textures of garnet, porosity generation behind the reactive interface and fracturing ahead of this interface. Porosity generated in the product chlorite enhanced fluid access to the replacement front. The study shows that deformation was not required for extensive fluid infiltration and alteration during metamorphism. Fluid flow uses a pre-existing foliation to gain access to the rock, taking advantage of the anisotropic shape of the aligned minerals.

Two types of blue halo (types I and II) composed of blueschist-facies minerals are centered around a brittle, normal shear zone in greenschist-facies rocks on the island of Syros, Aegean Sea, Greece. The shear zone is steeply dipping and cuts a near-horizontal layer of greenschist-facies rocks (albite + epidote + actinolite + chlorite + quartz). Type I and II blue haloes are 0·3 m and c. 1 m wide respectively, and are seen on both sides of the shear zone. The inner type I haloes are composed of nearly pure glaucophane schist and were formed by metasomatic addition of Na2O and SiO2, and to a lesser extent of K2O and large ion lithophile elements (LILE), coupled with loss of CaO, Al2O3 and MnO. The outer type II haloes consist of a carbonated blueschist-facies assemblage (glaucophane + calcite + phengite + epidote + garnet + quartz). These experienced only slight metasomatic changes (i.e. addition of K2O and LILE), which cannot alone explain halo formation. We present petrological, geochemical and thermodynamic evidence that this assemblage was preserved at greenschist-facies conditions because XCO2 was elevated by flow of a CO2-bearing fluid along the shear zone, which was approximately contemporaneous with greenschist-facies hydration in the surrounding rocks. We further note that the flux of CO2-bearing fluid along the shear zone was rapid with respect to the fluid flux in the surrounding rocks. Mass-balance calculations reveal that the fluid flux within the shear zone was at least 100–2000 times greater than the fluid flux within the surrounding rocks. Mineral textures show greenschist-facies minerals replacing blueschist minerals in the type II haloes, supporting our interpretation that the blueschist-facies minerals were preserved during greenschist-facies retrogression. A simplified P–T vs XCO2 pseudosection confirms that preservation of carbonated blueschist can occur at greenschist-facies conditions in the presence of a CO2-bearing fluid.