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Ignimbrites within calderas host intrusions with hazardous and/or economically significant hydrothermal systems. The Hvítserkur ignimbrite at Breiðuvík caldera, north-eastern Iceland, is intruded by basaltic dykes. Our data show that the ignimbrite immediately adjacent to the dyke is hard, dark-coloured, recrystallised quartz, plagioclase, and alkali feldspar with a low permeability and porosity and frequent macrofractures. At 1-2 m from the dyke, the ignimbrite is hard, dominantly glassy with pervasive perlitic microfractures, has high permeability, but low porosity and frequent macrofractures. A narrow zone of pervasive unlithified clay exists 2 m from the dyke. Beyond this, the ignimbrite is soft and zeolite-rich, has low permeability, high porosity and fewer macrofractures. The dyke intrusion promoted a narrow zone of welding, fracturing and perlitisation in the ignimbrite resulting in high permeability and focussed alteration. Our study shows how intrusions and their thermal aureoles create vertical pathways for, and horizontal barriers to, geothermal fluid flow.

The collapse of a volcanic flank can be destructive and deadly. Hydrothermal alteration is common to volcanoes worldwide and is thought to promote volcano instability by decreasing rock strength. However, some laboratory studies have shown that not all alteration reduces rock strength. Our new laboratory data for altered rhyodacites from Chaos Crags (Lassen volcanic center, California, USA) show that pore- and crack-filling mineral precipitation can reduce porosity and permeability and increase strength, Young's modulus, and cohesion. A significant reduction in permeability, by as much as four orders of magnitude, will inhibit fluid circulation and create zones of high pore fluid pressure. We explored the consequences of pore fluid pressurization on volcano stability using large-scale numerical modeling. Upscaled physical and mechanical properties for hydrothermally altered rocks were used as input parameters in our modeling. Results show that a high-pore-pressure zone within a volcano increases volcano deformation and that increasing the size of this zone increases the observed deformation. Hydrothermal alteration associated with mineral precipitation, and increases to rock strength, can therefore promote pore pressurization and volcano deformation, increasing the likelihood of volcano spreading, flank collapses, and phreatic/phreatomagmatic explosions. We conclude that porosity-decreasing alteration, explored here, and porosity-increasing alteration can both promote volcano instability and collapse, but by different mechanisms. Hydrothermal alteration should therefore be monitored at volcanoes worldwide and incorporated into hazard assessments.

The tensile strength of volcanic rock exerts control over several key volcanic processes, including fragmentation, magma chamber rupture, and dyke propagation. However, and despite this importance, values of tensile strength for volcanic rocks are relatively rare. It is also unclear how their tensile strength is modified by rock physical properties such as porosity, pore size, and pore shape and ongoing processes such as hydrothermal alteration. We present here the results of systematic laboratory and numerical experiments designed to better understand the influence of porosity, microstructural parameters (pore size, shape, and orientation), and hydrothermal alteration on the tensile strength of volcanic rocks. Our data show that tensile strength is reduced by up to an order of magnitude as porosity is increased from 0.01 to above 0.3, highlighting that porosity exerts a first-order control on the tensile strength of volcanic rocks. Our data also show that pore diameter, pore aspect ratio, and pore orientation can also influence tensile strength. Finally, our data show that hydrothermal alteration can decrease tensile strength if associated with mineral dissolution and weak secondary minerals, or increase tensile strength if associated with pore- and crack-filling mineral precipitation. We present a series of theoretical and semi-empirical constitutive models that can be used to estimate the tensile strength of volcanic rocks as a function of porosity or alteration intensity. To outline the implications of our data, we show how tensile strength estimations can influence predictions of magma overpressures and assessments of the volume and radius of a magma chamber, and we explore the influence of alteration using discrete element method modelling in which we model the amount and distribution of damage within variably-altered host-rock surrounding a pressurised dyke. It is our hope that the experiments, models, and understanding provided by our study prove useful for modellers that require the tensile strength of volcanic rocks for their models.

International audience; The tensile strength of volcanic rocks is an important parameter for understanding and modelling a wide range of volcanic processes, and in the development of strategies designed to optimise energy production in volcanic geothermal reservoirs. However, despite the near-ubiquity of hydrothermal alteration at volcanic and geothermal systems, values of tensile strength for hydrothermally altered volcanic rocks are sparse. Here, we present an experimental study in which we measured the tensile strength of variably altered volcanic rocks. The alteration of these rocks, quantified as the weight percentage of secondary (alteration) minerals, varied from 6 to 62.8 wt%. Our data show that tensile strength decreases as a function of porosity, in agreement with previous studies, and as a function of alteration. We fit existing theoretical constitutive models to our data so that tensile strength can be estimated for a given porosity, and we provide a transformation of these models such that they are a function of alteration. However, because porosity and alteration influence each other, it is challenging to untangle their individual contributions to the measured reduction in tensile strength. Our new data and previously published data suggest that porosity exerts a first-order role on the tensile strength of volcanic rocks. Based on our data and observations, we also suggest that (1) alteration likely decreases tensile strength if associated with mineral dissolution, weak secondary minerals (such as clays), and an increase in microstructural heterogeneity and (2) alteration likely increases tensile strength if associated with pore- and crack-filling mineral precipitation. Therefore, we conclude that both alteration intensity and alteration type likely influence tensile strength. To highlight the implications of our findings, we provide discrete element method modelling which shows that, following the pressurisation of a dyke, the damage within weak hydrothermally altered host-rock is greater and more widespread than for strong hydrothermally altered host-rock. Because the rocks in volcanic and geothermal settings are likely to be altered, our results suggest that future modelling should consider the tensile strength of hydrothermally altered volcanic rocks.

International audience; The heat flux of an active volcano provides crucial information on volcanic unrest. The hydrothermal activity often responsible for volcanic unrest can be accompanied by an increase in the extent and intensity of hydrothermal alteration, which could influence the thermal properties of the volcanic edifice. Therefore, an understanding of the influence of alteration on the thermal properties of rocks is required to better interpret volcano heat flux data. We provide laboratory measurements of thermal conductivity, thermal diffusivity, and specific heat capacity for variably altered (intermediate to advanced argillic alteration) andesites from La Soufrière de Guadeloupe (Eastern Caribbean). We complement these data with previously published data for altered basaltic-andesites from Merapi (Indonesia) and new data for altered rhyodacites from Chaos Crags (USA). Our data show that thermal conductivity and thermal diffusivity decrease as a function of increasing porosity, whereas the specific heat capacity does not change systematically. Thermal conductivity decreases as a function of alteration (the percentage of secondary minerals) for the rocks from La Soufrière and Merapi (from ~1.6 to ~0.6 W·m−1·K−1 as alteration increases from ~1.5 to >75 wt%), but increases for the rocks from Chaos Crags (from ~1.1 to ~1.5 W·m−1·K−1 as alteration increases from ~6 to ~15 wt%). Although the thermal diffusivity of the rocks from Chaos Crags increases from ~0.65 to ~0.75–0.95 mm2·s−1 as alteration increases from ~6 to ~15 wt%, the thermal diffusivity of the rocks from La Soufrière and Merapi does not appear to be greatly influenced by alteration. The specific heat capacity is not significantly affected by alteration, although there is a slight trend of increasing specific heat capacity with alteration for the rocks from La Soufrière. We conclude that the decrease in thermal conductivity as a function of alteration in the rocks from La Soufrière and Merapi is the result of the low conductivity of the secondary mineral assemblage, and that a combination of the high thermal conductivity of cristobalite and the reduction in porosity as a result of the void-filling mineral precipitation can explain the increase in thermal conductivity in the rocks from Chaos Crags. Calculations show that an increase in alteration of a dome or edifice can result in decreases and increases in heat flow density, depending on the type of alteration. Therefore, alteration-induced changes in the thermal properties of dome or edifice rocks should be considered when interpreting volcano heat flux data. We conclude that it is important not only to monitor the extent and evolution of alteration at active volcanoes, but also the spatial distribution of alteration type.

The recently discovered giant Huoshaoyun zinc-lead deposit in the Karakorum Range, northwestern Tibet, China, consists of a major zinc-lead carbonate and a minor lead-dominant sulfide mineralization with metal reserves of zinc and lead of over 16 million tonnes. The zinc-lead carbonate mineralization is composed of smithsonite and cerussite, with laminated, massive, veined and botryoidal textures, showing sedimentary and metasomatic structures. The lead-dominant sulfide mineralization is composed mainly of galena and minor sphalerite and pyrite, with laminated, brecciated, and veined textures. The laminated sulfide mineralized zones occur at the top of the deposit, whereas the zinc-lead carbonate mineralized bodies are located below the sulfide zones. Lead-dominant sulfide veins cut the zinc-lead carbonate ores and the host limestone. The sequence of formation of the lead-zinc ores with various mineral colors in thin sections is as follows: (1) light-colored to yellowish, fine- to coarse- and subhedral to euhedral smithsonite; (2) yellowish to reddish, fine- to coarse- and subhedral to euhedral smithsonite; (3) light-colored veined smithsonite; (4) white coarse-grained, xenomorphic cerussite; (5) colorless euhedral to sub-angular quartz crystals; (6) euhedral to anhedral lead-zinc sulfides and gypsum. The mineralization events of the Huoshaoyun deposit are: (1) early hydrothermal sedimentary smithsonite mineralization; (2) hydrothermal replacive smithsonite mineralization; (3) vein-type smithsonite mineralization; (4) open space filling and replacive cerussite mineralization; (5) lead-zinc sulfide mineralization. The fluid inclusions of cerussite indicate that the hydrothermal fluids of the cerussite ore-forming stage are characterized by a temperature range of at least 186–206 °C and low salinities (0.7–1.2 wt% NaCl eq.). The source of oxygen and carbon for the zinc-lead carbonate mineralization is possibly consistent with the involvement of marine water and a magmatic fluid, and is probably dominated by the magmatic fluid. The source of sulfur for the lead-dominant sulfide mineralization is possibly related to a magma reservoir. The zinc-lead-carbonate portion of the deposit represents a primary hypogene non-sulfide mineralization.

Volcanoes are inherently unstable structures that spread and frequently experience mass wasting events (such as slope failure, rockfalls, and debris flows). Hydrothermal alteration, common to many volcanoes, is often invoked as a mechanism that contributes significantly to volcano instability. We present here a study that combines laboratory deformation experiments, geophysical data, and large-scale numerical modelling to better understand the influence of hydrothermal alteration on volcano stability. La Soufrière de Guadeloupe (France) is a hazardous andesitic volcano that hosts a large hydrothermal system and therefore represents an ideal natural laboratory for our study. Uniaxial and triaxial deformation experiments were performed on samples prepared from 17 variably-altered (alteration minerals include quartz, cristobalite, tridymite, hematite, pyrite, alunite, natro-alunite, gypsum, kaolinite, and talc) blocks collected from La Soufrière de Guadeloupe. Our uniaxial compressive strength experiments show that strength and Young’s modulus decrease as a function of increasing porosity and increasing alteration. Triaxial deformation experiments show that cohesion decreases as a function of increasing alteration, but that the angle of internal friction does not change systematically. We first combined recent muon tomography data with our laboratory data to create a 3D strength map of La Soufrière de Guadeloupe. The low-strength zone beneath the southern flank of the volcano exposed by our 3D strength map is coincident with the hydrothermal system. We then assigned laboratory-scale and upscaled mechanical properties (e.g., Young’s modulus, cohesion, and angle of internal friction) to zones identified by a recent electrical survey of the dome of La Soufrière de Guadeloupe. Numerical modelling (using the software LaMEM) was then performed on a cross-section of the volcano informed by the recent electrical data, and on a cross-section in which we artificially increased the size of the hydrothermally altered zone. Our modelling shows (1) the importance of using upscaled values in large-scale models and (2) that hydrothermal alteration significantly increases the surface velocity and strain rate of the volcanic slope. We therefore conclude, using models informed by experimental data, that hydrothermal alteration decreases volcano stability and thus expedites volcano spreading and increases the likelihood of mass wasting events and associated volcanic hazards. Hydrothermal alteration, and its evolution, should therefore be monitored at active volcanoes worldwide.

The nature of sub-volcanic alteration is usually only observable after erosion and exhumation at old inactive volcanoes, via geochemical changes in hydrothermal fluids sampled at the surface, via relatively low-resolution geophysical methods or can be inferred from erupted products. These methods are spatially or temporally removed from the real subsurface and thus provide only indirect information. In contrast, the ICDP deep drilling of the Mt Unzen volcano subsurface affords a snapshot into the in situ interaction between the dacitic dykes that fed dome-forming eruptions and the sub-volcanic hydrothermal system, where the most recent lava dome eruption occurred between 1990 and 1995. Here, we analyse drill core samples from hole USDP-4, constraining their degree and type of alteration. We identify and characterize two clay alteration stages: (1) an unusual argillic alteration infill of fractured or partially dissolved plagioclase and hornblende phenocryst domains with kaolinite and Reichweite 1 illite (70)-smectite and (2) propylitic alteration of amphibole and biotite phenocrysts with the fracture-hosted precipitation of chlorite, sulfide and carbonate minerals. These observations imply that the early clay-forming fluid was acidic and probably had a magmatic component, which is indicated for the fluids related to the second chlorite-carbonate stage by our stable carbon and oxygen isotope data. The porosity in the dyke samples is dominantly fracture-hosted, and fracture-filling mineralization is common, suggesting that the dykes were fractured during magma transport, emplacement and cooling, and that subsequent permeable circulation of hydrothermal fluids led to pore clogging and potential partial sealing of the pore network on a timescale of ~ 9 years from cessation of the last eruption. These observations, in concert with evidence that intermediate, crystal-bearing magmas are susceptible to fracturing during ascent and emplacement, lead us to suggest that arc volcanoes enclosed in highly fractured country rock are susceptible to rapid hydrothermal circulation and alteration, with implications for the development of fluid flow, mineralization, stress regime and volcanic edifice structural stability. We explore these possibilities in the context of alteration at other similar volcanoes., info:eu-repo/semantics/published

International audience; Volcanoes are unstable structures that deform laterally and frequently experience mass wasting events. Hydrothermal alteration is often invoked as a mechanism that contributes significantly to volcano instability. We present a study that combines laboratory experiments, geophysical data, and large-scale numerical modeling to better understand the influence of alteration on volcano stability, using La Soufrière de Guadeloupe (Eastern Caribbean) as a case study. Laboratory experiments on variably altered (advanced argillic alteration) blocks show that uniaxial compressive strength, Young's modulus, and cohesion decrease as a function of increasing alteration, but that the internal friction angle does not change systematically. Simplified volcano cross sections were prepared (a homogenous volcano, a volcano containing the alteration zone identified by a recent electrical survey, and a volcano with an artificially enlarged area of alteration) and mechanical properties were assigned to zones corresponding to unaltered and altered rock. Numerical modeling performed on these cross sections, using a hydro-thermo-mechanical modeling code, show (a) the importance of using upscaled values in large-scale models and (b) that alteration significantly increases volcano deformation and collapse volume. Finally, we combined published muon tomography data with our laboratory data to create a 3D strength map, exposing a low-strength zone beneath the southern flank of the volcano coincident with the hydrothermal system. We conclude that hydrothermal alteration decreases volcano stability and thus expedites volcano spreading and increases the likelihood of mass wasting events and associated volcanic hazards. Hydrothermal alteration, and its evolution, should therefore be monitored at active volcanoes worldwide.

The physical properties and mechanical behaviour of impactites are an important parameter in fluid flow models and slope stability and landscape evolution assessments for heavily impacted planetary bodies. We first present porosity, permeability, Young's modulus, and uniaxial compressive strength measurements for three suevites from the Ries impact crater (Germany). Porosity ranges from 0.18 to 0.43, permeability from 5.8 × 10−16 to 5.1 × 10−14 m2, Young's modulus from 1.4 to 8.1 GPa, and uniaxial compressive strength from 7.3 to 48.6 MPa. To explore their mechanical behaviour, we performed triaxial deformation experiments on these samples at a range of confining pressures. The brittle–ductile transition for the lowest (0.25) and highest (0.38) porosity suevite samples was at a confining pressure of ~30 and ~10 MPa, respectively (corresponding to, for example, depths of ~1 and ~4 km on Mars, respectively). Microstructural observations show that the dominant deformation micromechanism during brittle deformation is microcracking, and during ductile deformation is distributed cataclastic pore collapse. We show that a theoretically grounded permeability model for welded granular media accurately captures the permeability of the studied suevites, and we use micromechanical models to glean insight as to their mechanical behaviour. Finally, we upscale our laboratory measurements to provide physical property values for length scales more relevant for large-scale models, and we compare these data with those for basalt (a lithology representative of the surface of the inner Solar System bodies). These analyses show how macroscopic fractures serve to increase the permeability and decrease the strength and Young's modulus of suevite and basalt. We also find, for example, that basalt can be a factor of 2–5 stronger than suevite in the shallow crust. Our study suggests, therefore, that the rock masses comprising older, bombarded crusts are substantially weaker and more porous and permeable than the younger plains material on these bodies. These findings should be considered in large-scale fluid flow modelling and when providing crustal strength estimates or slope stability assessments for planetary bodies on which protracted records of impact bombardment are preserved.

An asteroid impact in southwest Germany ca. 15 Ma left a > 20 km-diameter blanket of poorly sorted clastic rock, called suevite. In the 15th century, the suevite was used to construct a Gothic church (St. George's church, Nordlingen). Visual inspection of the church highlights its strong weathering susceptibility. We investigate whether the suevite used to construct the church and the rocks used in its restoration (suevite and sandstone) are weaker when saturated with water, important for conservation strategies. We additionally assess, by comparing their physical properties, the suitability of the replacement stones. We show that the strength of the suevite used to build the church is reduced by half when water-saturated, considered here a consequence of its high smectite content. This helps explain the prevalent weathering of the stones forming the outer walls of the church. The replacement suevite has a higher wet/dry strength ratio than the original suevite and is therefore more resistant to weathering. Not only does the replacement sandstone have a low ratio of wet/dry strength, but also its physical properties are different from the original suevite, which has almost identical physical properties to those of the replacement suevite. Based on these results, and the appearance of the stones, we recommend that, where possible, natural suevite stones be used to replace the weathered blocks of suevite at St. George's church and at buildings built using suevite within the region.

Dome-forming volcanoes are among the most hazardous volcanoes on Earth. Magmatic outgassing can be hindered if the permeability of a lava dome is reduced, promoting pore pressure augmentation and explosive behaviour. Laboratory data show that acid-sulphate alteration, common to volcanoes worldwide, can reduce the permeability on the sample lengthscale by up to four orders of magnitude and is the result of pore- and microfracture-filling mineral precipitation. Calculations using these data demonstrate that intense alteration can reduce the equivalent permeability of a dome by two orders of magnitude, which we show using numerical modelling to be sufficient to increase pore pressure. The fragmentation criterion shows that the predicted pore pressure increase is capable of fragmenting the majority of dome-forming materials, thus promoting explosive volcanism. It is crucial that hydrothermal alteration, which develops over months to years, is monitored at dome-forming volcanoes and is incorporated into real-time hazard assessments., The permeability of a dome exerts a control on the outgassing efficiency of the underlying magma. The authors investigate the role of hydrothermal alteration on this process in the laboratory and use these data to model whether the overpressures generated are capable of promoting explosive behaviour.

International audience; Hydrothermal fluids can alter the chemical and physical properties of the materials through which they pass and can therefore modify the efficiency of fluid circulation. The role of hydrothermal alteration in the development of geothermal and epithermal mineral resources, systems that require the efficient hydrothermal circulation provided by fracture networks, is investigated here from a petrophysical standpoint using samples collected from a well exposed and variably altered palaeo-hydrothermal system hosted in the Ohakuri ignimbrite deposit in the Taupō Volcanic Zone (New Zealand). Our new laboratory data show that, although quartz and adularia precipitation reduces matrix porosity and permeability, it increases the uniaxial compressive strength, Young’s modulus, and propensity for brittle behaviour. The fractures formed in highly altered rocks containing quartz and adularia are also more planar than those formed in their less altered counterparts. All of these factors combine to enhance the likelihood that a silicified rock-mass will host permeability-enhancing fractures. Indeed, the highly altered silicified rocks of the Ohakuri ignimbrite deposit are much more fractured than less altered outcrops. By contrast, smectite alteration at the margins of the hydrothermal system does not significantly increase strength or Young’s modulus, or significantly decrease permeability, and creates a relatively unfractured rock-mass. Using our new laboratory data, we provide permeability modelling that shows that the equivalent permeability of a silicified rock-mass will be higher than that of a less altered rock-mass or a rock-mass characterised by smectite alteration, the latter of which provides a low-permeability cap required for an economically viable hydrothermal resource. Our new data show, using a petrophysical approach, how hydrothermal alteration can produce rock-masses that are both suitable for geothermal energy exploitation (high-permeability reservoir and low-permeability cap) and more likely to host high-grade epithermal mineral veins, such as gold and silver (localised fluid flow).

The Middle Miocene Upper Freshwater Molasse sediments represent the last cycle of clastic sedimentation during the evolution of the North Alpine Foreland Basin. They are characterized by small-scale lateral and temporal facies changes that make intra-basin stratigraphic correlations at regional scale difficult. This study provides new U–Pb zircon ages as well as revised 40Ar/ 39Ar data of volcanic ash horizons in the Upper Freshwater Molasse sediments from southern Germany and Switzerland. In a first and preliminary attempt, we propose their possible correlation to other European tephra deposits. The U–Pb zircon data of one Swiss (Bischofszell) and seven southern German (Zahling, Hachelstuhl, Laimering, Unterneul, Krumbad, Ponholz) tuff horizons indicate eruption ages between roughly 13.0 and 15.5 Ma. The stratigraphic position of the Unterneul and Laimering tuffs, bracketing the ejecta of the Ries impact (Brockhorizon), suggests that the Ries impact occurred between 14.93 and 15.00 Ma, thus assigning the event to the reversed chron C5Bn1r (15.032–14.870 Ma) which is in accordance with paleomagnetic evidence. We combine our data with published ages of tuff horizons from Italy, Switzerland, Bavaria, Styria, Hungary, and Romania to derive a preliminary tephrochronological scheme for the Middle Miocene in Central Europe in the age window from 13.2 to 15.5 Ma. The scheme is based on the current state of knowledge that the Carpathian–Pannonian volcanic field was the only area in the region producing explosive calc-alkaline felsic volcanism. This preliminary scheme will require verification by more high-quality ages complemented by isotopic, geochemical and paleomagnetic data.

The Sierra Mojada District comprises multiple types of near-surface mineral concentrations ranging from polymetallic sulfide zones, “nonsulfide Zn” (NSZ) deposits, and a silver-rich Pb carbonate deposit hosted by lower Cretaceous carbonate strata. Hypogene concentrations of Fe-Zn-Pb-Cu-Ag sulfides and sulfosalts are locally preserved and are associated with hydrothermal dolomite and silica. Alteration mineralogy and sulfur isotope data suggest primary Zn-Pb-Ag mineralization from circa 200 °C hydrothermal fluids. The NSZ deposits dominantly consist of smithsonite and hemimorphite associated with local Mn-Fe oxides. The Red Zinc Zone consists of strata-bound zones dominantly of hemimorphite that fills pores in residual and resedimented Fe oxides. The White Zinc Zone shows local dissolution features, including internal sediments interbanded with and cemented by smithsonite. Similar Pb isotopic compositions of smithsonite, hemimorphite, and cerussite to Sierra Mojada galena document that the NSZ deposits originated from polymetallic carbonate-replacement sulfide deposits, with flow of metal-bearing groundwater being controlled by local topography and structural features in this extensional terrane. Oxygen isotope values for Sierra Mojada smithsonite are relatively constant (δ18OVSMOW = 20.9 to 23.3‰) but are unusually low compared to other supergene smithsonites. Using δ18OVSMOW (− 8‰) of modern groundwater at nearby Cuatrocienegas, smithsonite formational temperatures are calculated to have been between 26 to 35 °C. Smithsonite precipitation was favored by near-neutral conditions typical of carbonate terranes, whereas hemimorphite precipitated by reaction with wallrock silica and locally, or episodically, more acidic conditions resulting from sulfide oxidation. Transition to, and stabilization of, the modern desert climate over the past 9000 years from the Late Pleistocene wetter, cooler climate of northern Mexico resulted in episodic drawdown of the water table and termination of local supergene metal mobilization.

International audience; Geothermal projects in the Upper Rhine Graben aim to harness thermal anomalies that have arisen due to hydrothermal circulation within the granitic basement and the overlying Permo-Triassic sedimentary units. We present here a systematic microstructural, mineralogical, and petrophysical characterisation of the lowermost unit of this Permo-Triassic sedimentary succession—the Buntsandstein—sampled from exploration well EPS-1 at the Soultz-sous-Forêts geothermal site (France). Twelve depths were sampled (from 1008 to 1414 m) and cylindrical cores were prepared perpendicular and parallel to bedding. These cores were described in terms of their microstructure, grain size and shape, specific surface area, pore size and pore throat size distribution, mineral content, porosity, P-wave velocity, and permeability. The Buntsandstein sandstones are predominantly feldspathic sandstones, often characterised by pores filled or partially filled (with clays (R3 illite–smectite), dolomite, siderite, and barite) as a consequence of diagenesis, tectonics, and the circulation of hydrothermal fluids. The porosity, dry P-wave velocity, and permeability of these sandstones vary from ~ 0.03 to 0.2, ~ 2.5 to 4.5 km s−1, and ~ 10−18 to 10−13 m2, respectively. Our data show that P-wave velocity decreases and permeability increases as porosity increases. P-wave velocities are significantly higher when measured parallel to bedding (by about 10 to 25%), and that saturation with water increases P-wave velocity (by about 5 to 50%, depending on sample orientation). The pervasive pore-filling precipitation has significantly reduced the permeabilities of the Buntsandstein sandstones, which are orders of magnitude less permeable than similarly porous unaltered sandstone. We also find that their permeability can be up to an order of magnitude more permeable when measured parallel to bedding than perpendicular to bedding. Although Buntsandstein units with low matrix permeabilities (as low as ~ 10−18 m2) require macroscopic fractures to attain the high permeability required to sustain regional hydrothermal circulation, matrix permeability is important for units with low fracture densities and high matrix permeabilities. We anticipate that these data will aid future fluid flow modelling and seismic investigations at geothermal sites within the Upper Rhine Graben.

The Landshut bentonites that formed from Ca- and Mg-poor rhyolitic tuffs in a fluviatile-lacustrine depositional environment of the Miocene Upper Freshwater Molasse, southern Germany, contain abundant palustrine, pedogenic and groundwater carbonates. Geochemical analyses of dolomites, calcites and smectites from bentonites of various environments by X-ray diffraction, thermal ionization mass spectrometry, inductively coupled plasma-mass spectrometry, and handheld X-ray fluorescence yield new insights into the compositions of fluids and sources of imported components involved in carbonate formation and bentonitization, as well as the timing of bentonite formation. Evaporated, Sr-rich brackish surface water with a molar Mg/Ca ratio of 2–5, derived mostly from the weathering of detrital carbonates, was involved in dolomite and bentonite formation in palustrine and some pedogenic environments. However, Sr-poor groundwater with a molar Mg/Ca ratio of ∼ 1 and a stronger silicate weathering component caused bentonite and calcite formation in strictly pedogenic and groundwater settings. The 87Sr/86Sr and molar Mg/Ca in the smectite interlayers indicate later cation exchange with water having more radiogenic Sr sources and smaller molar Mg/Ca ratios. The Rb-Sr data indicate the common presence of detrital illitic phases in the

The Ediacaran continental flood basalts and associated tuffs were studied to identify and quantify alteration processes by means of XRD and chemistry, supplemented by Mossbauer and FTIR spectroscopies, petrography, oxygen and iron isotopes, K-Ar dating, and organic geochemistry. Two superimposed alteration processes were identified: the Ediacaran hydrothermal alteration, induced by meteoric waters, heated and put in motion by the cooling basalt, and the Caledonian and/or Variscan potassic alteration. The degree of basalt alteration was quantified using as an index the sum of primary minerals in the bulk rock. The sequence of minerals dissolved and crystallized during the hydrothermal alteration was established. The alteration resulted in the loss of Ca (dissolution of plagioclases), compensated by the gain of water and Mg (crystallization of clays), and proceeded from the edges of the basalt flows in an oxidizing environment, evidenced by the increasing amount of hematite and Fe3+/Fe2+ ratio of the bulk rock. Cyanobacteria were active in the hydrothermal system, most probably responsible for the measured negative δ56Fe values and more reducing conditions at the stage of intense alteration. Chlorophaeite (palagonite), following quartz as the earliest petrographically identifiable basalt alteration product was found to vary systematically from fully isotropic to birefringent. The chlorophaeite was identified as a mixture of Fe-montmorillonite and Fe-saponite, identical with griffithite and oxysmectites, probably preceded by a finer-grained ferrosaponite at the isotropic stage. REE content of chlorophaeite indicates basaltic volcanic glass (sideromelane) as the major source of material. REE in clays are contained mostly in the dioctahedral smectite, while in the bulk rock mostly in phosphates. The smectite characteristics and Mg enrichment are indicative of the hydrothermal basalt alteration process, which perhaps was dominant also on Mars. Both mineral and chemical composition of tuffs vary continuously from basaltic to felsic, the latter close to the measured rhyodacite composition, dominated by quartz and feldspars. The basaltic tuffs resemble the most altered basalts but contain also abundant albite and chlorite, indicative of higher alteration temperatures, up to 220 °C. Tuff composition indicates stronger component of felsic volcanism in the trap formation than evidenced by the preserved bodies of effusive rocks.

Stable isotope (C–O) analysis was applied to smithsonites and cerussites from Zn(Pb) nonsulphide ores from Britain and Ireland, to determine the nature of the fluids responsible for the precipitation of secondary carbonates, and any relationship with palaeoclimatic conditions. The carbon isotope compositions of the Irish smithsonites show a major contribution of organic carbon whereas those of both Zn- and Pb-carbonates in the UK deposits indicate contribution of organic carbon and host-rock carbonates. δ18OVSMOW values for cerussite from both areas are about 13‰ lower than those of smithsonites, UK smithsonite δ18OVSMOW values are more variable than those for cerussites and the Irish smithsonites have unusually high values. Isoscape maps indicate that δ18O values for both carbonates are higher in the west and decrease toward the east. This trend correlates with a general decrease in the oxygen isotope compositions of rainwater moving eastward, and suggests that oxidation of sulphides occurred at temperatures of about 15–20°C, under climatic conditions warmer than today. Supplementary material: A comprehensive overview of the investigated sites, detailed data for the samples and SEM images are available at https://doi.org/10.6084/m9.figshare.c.4579535

International audience; Geothermal energy exploitation in the Upper Rhine Graben currently targets high-temperature anomalies in the crystalline Paleozoic basement at depths up to 5 km. However, at certain locations (e.g. Rittershoffen, France), geothermal installations are actively targeting resources at shallower depths where the Paleozoic granite transitions into the overlying Permo-Triassic sandstones. We here investigate the variation in physical properties—including porosity, P-wave velocity, permeability, uniaxial compressive strength, and thermal properties—in rocks that locally extend across the Paleozoic–Permo-Triassic boundary in four locations to the west of the Upper Rhine Graben. The lithologies investigated include sandstones, breccia, granites, metagranites, dolomite, and altered and unaltered volcanic rocks and represent the variety of lithologies at this transition. We note that while the porosity, permeability, thermal conductivity, and P-wave velocity of the Permo-Triassic sedimentary cover and Paleozoic crystalline basement rocks are consistent with values determined for rocks from exploratory and production boreholes at Soultz-sous-Forêts (France), the other lithologies (belonging to neither the sedimentary nor basement sequences) are conspicuously lower in porosity and permeability. Further, the attendant strength of these other lithologies could make them relatively unamenable to fracturing, reducing the possibility of fracture-controlled permeability in these units. Indeed, we conclude that in areas where these low-permeability and high-strength rocks act to cap the crystalline basement, hydrothermal convection may be curtailed and geothermal exploitation may be rendered untenable.

The hydrogen and oxygen isotope compositions of bentonites and smectite-rich tuffs from the North Alpine Foreland Basin were measured for samples dated between 9 and 21 Ma. Assuming clay mineral formation shortly after the deposition of the volcanic ash layers and at surface temperatures estimated from paleoclimatic studies, the H and O isotope composition of water in equilibrium with the clay minerals can be calculated. A comparison with values derived from other proxies of paleoprecipitation in the Molasse Basin (such as pedogenic carbonates and mammal remains) and δ 18 O values of carbonate in the bentonite beds, suggests that the δ 18 O values of water estimated by this method are typical for foreland precipitation at this time and for this region. Therefore, the δ 18 O value of smectite in bentonites and altered tuff layers can be a useful tool for studies of paleoclimate and -topography. There are, however, indications that the clay minerals did not retain their initial hydrogen isotopic composition, but that a postformational exchange of H isotopes has occurred.

The titanium content of pyrope garnet can be quantified using the intensity of a Raman band at about 830 cm−1 that is normalized to the 363 cm−1 band using a spectrometer-specific calibration using 10 to 15 chromian pyropes from Bohemia, Czech Republic. An accuracy of 0.025 wt% could be achieved for TiO2 contents between 0.17 and 0.67 wt% TiO2 with a Raman spectrometer with a spectral resolution of better than 3.9 cm−1. The technique can be used in petrological and gemmological studies. Copyright © 2015 John Wiley & Sons, Ltd.

Neapolitan Yellow Tuff (NYT) has been used in construction in Naples (Italy) since the Greeks founded the city—then called Neapolis—in the sixth century BCE. We investigate here whether this popular building stone is weaker when saturated with water, an issue important for assessments of weathering damage and monument preservation. To this end, we performed 28 uniaxial compressive strength measurements on dry and water-saturated samples cored from a block of the lithified Upper Member of the NYT. Our experiments show that the strength of the zeolite-rich NYT is systematically reduced when saturated with water (the ratio of wet to dry strength is 0.63). Complementary experiments show that two other common Neapolitan building stones—Piperno Tuff and the grey Campanian Ignimbrite (both facies of the Campanian Ignimbrite deposit devoid of zeolites)—do not weaken when wet. From these data, and previously published data for tuffs around the globe, we conclude that the water-weakening in NYT is a consequence of the presence of abundant zeolites (the block tested herein contains 46 wt.% of zeolites). These data may help explain weathering damage in NYT building stones (due to rainfall, rising damp, and proximity to the sea or water table) and the observed link between rainfall and landslides, rock falls, and sinkhole formation in Naples, and the weathering of other buildings built from zeolite-rich tuffs worldwide.

The use of Mt. Epomeo Green Tuff (MEGT) as a building stone is widespread on Ischia Island (Italy). We assess here the fire resistance of MEGT by thermally stressing samples to temperatures up to 1000 °C. Porosity and uniaxial compressive strength increase and decrease from 44% and 4.5 MPa at ambient temperature to 48% and 1.5 MPa following exposure to 1000 °C, respectively. Complementary thermogravimetric and X-ray powder diffraction analyses, experiments that monitor acoustic emissions during heating/cooling, and microstructural observations highlight that these changes are the result of thermal microcracks, formed due to the breakdown of zeolites and clays (MEGT contains 35 wt.% analcime, 15 wt.% smectite, and 3 wt.% illite) at high temperature. Although the stability of structures built from MEGT will be jeopardised at high temperature, a very low thermal diffusivity requires that fires must burn for many hours to compromise the strength of a typical dimension stone: tuffs are tough in the event of fire.

Phreatic eruptions are possibly the most dramatic surface expressions of hydrothermal activity, and they remain poorly understood. The near absence of precursory signals makes phreatic eruptions unpredictable with respect to both time and magnitude. The Valley of Desolation (VoD), Dominica, located close to the Boiling Lake, the second largest high-temperature volcanic crater lake in the world, hosts vigorous hydrothermal activity with hot springs, mud pools, fumaroles, and steaming ground. A phreatic or phreatomagmatic eruption from this site is considered to be the most likely scenario for future volcanic activity on Dominica. Yet there is little information regarding the trigger mechanisms and eruption processes of explosive events at this active hydrothermal center, and only a very small number of studies have investigated hydrothermal activity in the VoD. We therefore conducted two field campaigns in the VoD to map hydrothermal activity and its surficial phenomena. We also investigated alteration processes and their effects on degassing and phreatic eruption processes. We collected in situ petrophysical properties of clay-rich unconsolidated samples, and together with consolidated rock samples, we investigated the range of supergene and hydrothermal alteration in the laboratory. In addition, we performed rapid decompression experiments on unconsolidated soil samples. Our results show that alteration leads to an increasing abundance of clay minerals and a decrease in both strength and permeability of the rocks. In the immediate vicinity of degassing acid-sulfate fluids, advanced argillic alteration yields a mineral zoning which is influenced by meteoric water. The water-saturated basal zone is dominated by kaolinite run 0whereas alunite formation is favored at and above the groundwater table where atmospheric oxidation of H2S to H2SO4 occurs (e.g., steam-heated alteration). Alteration effects may in turn inhibit degassing at the surface, increasing the potential for pressurization in the subsurface and thus lead to phreatic eruptions. Rapid decompression experiments, together with ballistic trajectory calculations, constrain estimates of the conditions prior to the 1997 small-scale phreatic event in the VoD. The results presented here may serve as a contribution to the understanding of the hazard potential of ongoing hydrothermal activity within the VoD. On a broader perspective, our results will help evaluate hydrothermal activity in similar areas worldwide which might also have the potential for phreatic eruptions, for instance Poas (Costa Rica) or Tongariro and Waimangu (New Zealand).

The sub-volcanic basement at Mt Etna (Italy) comprises thick sedimentary sequences. An understanding of the physical, mechanical, and microstructural properties of these sequences, and an appreciation of their variability, is important for an accurate assessment of the structural stability of Mt Etna. Here, we present a combined field and laboratory study in which we explore the extent of variability of the materials comprising the sedimentary basement of Mt Etna. To this end, we sampled twelve different lithological units that span the sediments of the Apenninic-Maghrebian Chain (from both the Sicilide and Ionides sequences) and the Hyblean Plateau. X-ray diffraction analysis of the blocks collected show that calcite and quartz are the predominant mineral phases. Textural analysis highlights the wide variability in rock microstructures, with features such as the presence/absence of fractures or veins, pore size and shape, and grain size and shape varying tremendously between the samples. One consequence of this microstructural, textural, and mineralogical variability is that the rock units are characterised by very different values of porosity, P-wave velocity, uniaxial compressive strength, and static Young’s modulus. For example, strength and Young’s modulus vary by a factor of twenty and an order of magnitude, respectively. Our study affirms the vast heterogeneity of the sub-volcanic sedimentary basement of Mt Etna and, on this basis, we urge caution when selecting potentially oversimplified input parameters for models of flank stability.

Stable isotope ratios can provide important evidence for estimating groundwater temperatures during the formation of clay minerals in response to chemical weathering of rocks at the landsurface. In this paper, we investigate weathering kaolins found in Buchan, NE Scotland. Stable oxygen and hydrogen isotopes for kaolins from weathered clasts in post-Cretaceous fluvial gravels and from clay-rich saprolites developed in Dalradian metamorphic and Caledonian igneous rocks indicate weathering under warm groundwater temperatures of 23 ± 5 °C. Comparisons with Cenozoic palaeotemperatures derived from North Sea sediments indicate that weathering took place under humid tropical climates of the Palaeocene–Eocene. The > 25 m deep weathering profiles in the Buchan Gravels indicate that these coarse, quartzite- and flint-bearing gravels are older than previously thought and were likely deposited by rivers which also fed thick Palaeocene–Early Eocene deep-water sand fans in the western North Sea basin. A later phase of kaolin weathering at lower temperatures of 15 ± 5 °C took place in the Middle Miocene. Kaolinitic weathering remnants are largely confined to elevations of > 100 m in eastern Buchan and define a well-preserved, deeply weathered Cenozoic landscape in north-east Scotland.

Field and microstructural data of the Pfahl shear zone in north-eastern Bavaria (Germany) reveal the intimate spatial-temporal connection between fragmentation, fluid influx and quartz crystallization. These processes and their interaction led to complex-structured quartz units: (i) a dense network of early quartz veins, (ii) two domains of fine-grained reddish to grayish quartz masses, (iii) an extended central zone of massive white quartz, and (iv) late cross-cutting closely spaced parallel fractures and partly open quartz veins. The fine-grained quartz domains result from repeated and coeval cataclasis, fluidization and quartz precipitation. Material transport in these domains is at least partly governed by the flow of mobile fluid-quartz-particle suspensions. The complex internal meso-to microstructures of the massive white quartz are generated by repeated processes of fragmentation and grain growth. In general, the brittle part of the Pfahl shear zone represents a key example of cyclic dissolution/precipitation and fragmentation on large scale.