Rapid generation of reaction permeability in the roots of black smoker systems, Troodos ophiolite, Cyprus

The deep levels of former black smoker hydrothermal systems are widespread in the Troodos ophiolite in Cyprus. They are marked by zones of hydrothermal reaction in the sheeted dyke unit close to the underlying gabbros. These zones are characterized by the presence of epidosite (epidote-quartz rock). In the reaction zones, the dykes are altered to a range of greenschist facies mineral assemblages, from a low degree of alteration with a five to seven phase metabasaltic assemblage to a high degree of alteration with a two to three phase epidosite assemblage. Individual dykes may contain the full range, with the epidosites forming yellow-green stripes within a darker background, often extending for more than several metres, parallel to the dyke margins. Field relations show that the alteration took place on a dyke-by-dyke basis and was not a regional process. SEM petrography reveals that the epidosites contain millimetre scale pores. The minerals surrounding the pores show euhedral overgrowths into the free pore space, indicating a former transient porosity of up to 20%. We conclude that the epidosites formed by reaction between newly intruded basaltic dykes and actively circulating black smoker fluid leading to extensive dissolution of primary dyke minerals. This reaction generated the porosity in the stripes and transiently led to a much increased permeability, allowing the rapid penetration of the black smoker fluid into the dykes and flow along them in fingers. As the system evolved, the same flow regime allowed mineral precipitation and partial infilling of the porosity. This mechanism allows rapid recrystallization of the rock with release of metals and other components into the fluid. This explains the depletion of these components in epidosites and their enrichment in black smoker vent fluids and the relatively constant composition of vent fluids as fresh rock is continually mined

Mid-Crustal Focused Fluid Movement: Thermal Consequences and Silica Transport

Numerical models have been constructed to assess the thermal consequences and silica transport that would result if water released by regional metamorphic dehydration or cooling plutons were focused into large-scale (10 km) fracture zones. Two fracture zone model geometries have been considered, in one the fracture zone is planar, and in the other the fracture zone is radially symmetric. In both models dispersion and collection of fluids is simulated. The model results indicate that for planar or radially symmetric fracture zones, hydrothermal flow rates must approach 0.1 g/s (per m crack length) or 1 kg/s, respectively, to produce significant thermal effects. Given that regional metamorphic fluid fluxes are probably < 10−9 kg/m2−s, generation of a thermal ano-maly by fluids released during metamorphic dehydration into a planar fracture zone requires an unrealistic degree of lateral flow (>50 km). The collection area required to produce a detectable heating effect about a radially symmetric fracture zone is smaller (a radius of ∼ ∼ 15 km), but also implausibly large. These scales suggest tha

Earthquake nucleation in the lower crust by local stress amplification

Deep intracontinental earthquakes are poorly understood, despite their potential to cause significant destruction. Although lower crustal strength is currently a topic of debate, dry lower continental crust may be strong under high-grade conditions. Such strength could enable earthquake slip at high differential stress within a predominantly viscous regime, but requires further documentation in nature. Here, we analyse geological observations of seismic structures in exhumed lower crustal rocks. A granulite facies shear zone network dissects an anorthosite intrusion in Lofoten, northern Norway, and separates relatively undeformed, microcracked blocks of anorthosite. In these blocks, pristine pseudotachylytes decorate fault sets that link adjacent or intersecting shear zones. These fossil seismogenic faults are rarely >15 m in length, yet record single-event displacements of tens of centimetres, a slip/length ratio that implies >1 GPa stress drops. These pseudotachylytes represent direct identification of earthquake nucleation as a transient consequence of ongoing, localised aseismic creep

Rates of retrograde metamorphism and their implications for the rheology of the crust: an experimental study

We have carried out experimental studies of the rate at which water is consumed by hydration reactions under mid-crustal conditions. Both pelitic and mafic assemblages are susceptible to extensive hydration in the laboratory on a time scale of weeks to months. Quantitative hydration rate determinations were made using enstatite–oligoclase ± diopside powder mixtures and a natural hypersthene hornfels. Under all conditions, the main hydration product was saponite clay with variable amounts of talc according to the initial proportion of enstatite to plagioclase. The experiments yield consistent rates for water consumption of around 10−8 g H2O per m2 of mineral surface per second at 400°C and 300 MPa (3 kbar). Additional experiments were run at 300°C and 500°C and at lower pressures (40 MPa), as well as with NaCl; rates appear to be faster at higher temperatures and in the presence of salt, but slower at low pressure. Comparison of powder and core experiments on the natural hornfels indicates that it is primarily the outer surface of the rock core that is available for hydration, with only minor infiltration along grain boundaries. The hydration rates reported here appear to be typical for the types of lithology that demonstrate moderate to high degrees of retrogression along joints and deformation zones in crystalline rocks of the upper crust. Assuming that the surface roughness and damage effects in a natural fault zone are comparable with those of the materials used here in the experiments, the measured hydration rates imply that a natural fracture in crystalline rocks of the middle crust that becomes filled with a water film 0·2 mm in thickness will dry out through incorporation of the water into hydrous phases on a time scale of the order of 10–100 years. This clearly implies that free water has only a short residence time in crystalline rocks of the middle crust or deeper, provided they have cooled below their original temperature of formation and therefore have the potential to undergo retrograde hydration. We infer that the strength of retrograde shear zones in the middle to lower crust will fluctuate through time, with episodes of water infiltration resulting in short periods of water weakening before the water is fully consumed and the rocks become stronger once more

Fluids in the continental crust

Fluids play a critical role in the geochemical and geodynamical evolution of the crust, and fluid flow is the dominant process associated with mass and energy transport in the crust. In this Perspectives, we summarise the occurrence, properties and role that fluids play in crustal processes, as well as how geoscientists’ understanding of these various aspects of fluids have evolved during the past century and how this evolution in thinking has influenced our own research careers. Despite the wide range of possible fluid sources in the crust, fluids in sedimentary, magmatic and metamorphic environments are all approximated by the system H2O – “gas” – “salt” and normally reflect equilibrium with rocks and melts at the relevant PT conditions. The “gas” component in many environments is dominated by CO2, but CH4, as well as various sulphur and nitrogen-rich gases, may also be important. The major “salt” components are usually NaCl and/or CaCl2, but salts of K, Mg and Fe can be major components in specific circumstances. While the activities of many fluid components can often be calculated assuming equilibrium with coexisting minerals, salinity is normally unbuffered and must be determined independently from observations of fluid inclusions. Solubilities of “gas” and “salt” in H2O generally rise with increasing temperature and/or pressure, but in many environments compositions are such that phase separation (immiscibility or boiling) leads to the development of salt-rich aqueous fluids coexisting with a volatile-rich phase. Chloride content, buffering assemblages, temperature and, to a lesser extent pressure, all play a role in determining the dissolved load of crustal fluids. In addition to equilibrium considerations, kinetic factors can play an important role in relatively shallow, low temperature environments. The most important distinction between relatively shallow basinal or geothermal fluids and deeper metamorphic or magmatic ones is the physical behaviour of the fluid(s). In regions where fluid pressure corresponds to hydrostatic pressure, extensive circulation of fluid is possible, driven by thermal or compositional gradients or gravity. In contrast, at greater depths where fluids are overpressured and may approach lithostatic pressure, fluid can only escape irreversibly and so fluxes are generally much more limited. Much of our understanding of crustal fluids has come from studies of ore-forming systems that are present in different crustal environments. Thus, studies of Mississippi Valley-Type deposits that form in sedimentary basins have shown that the fluids are dominantly high salinity (Na,Ca) brines that have significant metal-carrying capacity. Studies of active continental geothermal systems and their fossil equivalents, the epithermal precious metal deposits, document the importance of boiling or immiscibility as a depositional mechanism in this environment. Ore-forming fluids associated with orogenic gold deposits show many similarities to low salinity metamorphic fluids, consistent with their formation during metamorphism, but similar fluids are also found in some magmatic pegmatites, demonstrating the difficulty in distinguishing characteristics derived from the fluid source from those that simply reflect phase relationships in the H2O – “gas” – “salt” system. Magmatic fluids associated with silicic epizonal plutons are consistent with experimental and theoretical studies related to volatile solubilities in magmas, as well as the partitioning of volatiles and metals between the melt and exsolving magmatic fluid. Ore fluids are generally representative of crustal fluids in comparable settings, rather than unusual, metal-rich solutions. During progressive burial and heating of sediments and metamorphic rocks, there is continuous fluid release and loss and the rocks remain wet and weak. Fluid composition evolves continuously as a result of changing conditions. Once rocks begin to cool, fluid is consumed by retrograde reactions and in much of the crust the rocks are effectively dry with a notional water fugacity buffered by the coexisting high-T and retrograde phases. In this case rocks are strong and unreactive. Our understanding of crustal fluids has advanced by leaps and bounds during the past few decades, and we expect new and exciting results to continue to emerge as new analytical methods are developed that allow us to analyse smaller fluid inclusions in particular, and as theoretical models and experiments advance our understanding of how fluids interact with rocks and minerals in the crust, changing both chemical and physical characteristics

Geological disposal of nuclear waste: A primer

The back-end of the nuclear fuel cycle has become the Achilles Heel of nuclear power. After more than 50 years of effort, there are, at present, no operating nuclear waste repositories for the spent nuclear fuel from commercial nuclear power plants or for the high-level waste from the reprocessing of spent fuel. The articles in this issue of Elements describe the status of geological disposal in salt, crystalline rock, clay, and tuff, as presently developed in five countries

Tectonic settings of regional metamorphism. A discussion.

The proceedings of a discussion meeting held in London in 1986, in association with IGCP Project 235 on 'Metamorphism and Geodynamics'. The 13 papers discuss theory and observation of orogenic processes, and range from the regional distribution of metamorphic rocks (Canada, New Zealand, Europe, Taiwan, Tibet), through the derivation of palaeotemperature and palaeopressure information from such rocks, to observational and theoretical studies of regions that are undergoing deformation and metamorphism today. Papers are abstracted separately with the original journal citation. -after Publishe