The relevance of vein texture in understanding the past hydraulic behaviour of a crystalline rock mass: reconstruction of the palaeohydrology of the Mecsekalja Zone,south Hungary

This study reconstructs the palaeohydrogeologic evolution of the shallow‐to‐moderate Mesozoic subsidence history for the Mecsekalja Zone (MZ), a narrow metamorphic belt in the eastern Mecsek Mountains, Hungary. Brittle deformation of the MZ produced a vein system with a cement history consisting of five sequential carbonate generations and one quartz phase. Vein textures suggest different fluid‐flow mechanisms for the parent fluids of subsequent cement generations. Combined microthermometric and stable‐isotope measurements permit reconstruction of the character of subsequent fluid generations with different flow types, as defined by vein textures, yielding new information regarding the hydraulic behaviour of a metamorphic crystalline complex. Textural observations and geochemical data suggest that fracture‐controlled flow pathways and externally derived fluids were typical of some flow events, while percolation through the rock matrix and the relationship to the Cretaceous volcanism and dyke emplacement were typical of others. The difference in the mode of calcite deposition from pervasive fluids (i.e. pervasive carbonatisation along grain boundaries versus deposition in antitaxial veins) between two calcite generations related to the volcanism inspired a stress‐dependent model of antitaxial vein growth. Textural and isotope variations in a vein generation produced by the same parent fluid indicate rock‐dependent hydraulic behaviour for different rock types, distinct action of the contemporaneous fracture systems and different extents of fluid–rock interaction. Cathodoluminescence microscopy and fluid‐inclusion microthermometry shed light on the possible role of hydraulic fracturing in the formation of massive calcite. The time of formation was estimated from the isotope composition of the oldest calcite generation and its presumptive relationship with the sedimentary sequences to the north, whereas microthermometry permitted conciliation of the reconstructed flow sequence with the Mesozoic subsidence history of the Mórágy Block (including the MZ).     

Apatite grain boundary morphology and its response to low-temperature fluid infiltration in crystalline basement

Apatite grain boundaries on fractured rock surfaces have been examined in an amphibolite facies regional metamorphic granite gneiss from the central Swiss Alps. The morphology of apatite has been characterized using a scanning electron microscope and matched to surface textures in adjoining silicates. Apatites show a wide variety of different surface features ranging from planar crystal faces, to small-scale ridges and dimples, to extensive irregular pitting. Many of these features form in response to the periodic infiltration of fluids along open grain boundaries during the cooling history of the gneiss. Apatite shows evidence of both dissolution and re-precipitation that is controlled by the nature of the grain boundary, the structure of the adjoining silicate phase and the alteration of the host rock. Fracturing occurs in a range of retrograde conditions and is common both within the apatite and along grain boundaries. This coupled to the evidence of fluid interaction with mineral surfaces suggests that extensive permeable networks may be typical of cooling crystalline basement rocks. Grain boundary textures have the potential to reveal a unique record of fluid infiltration in the crust that would be very difficult to decipher using traditional petrographic methods.     

Monitoring fluid chemistry in iron oxide–copper–gold‐related metasomatic processes,eastern Mt Isa Block,Australia

Most researchers in the Proterozoic eastern Mt Isa Block, NW Queensland, Australia, favour magmatic fluid and salt sources for sodic‐(calcic) alteration and iron oxide–copper–gold mineralization. Here we compare spatial, mineralogic and stable isotope data from regional alteration assemblages with magmatic and magmatic‐hydrothermal interface rocks in order to track chemical and isotopic variations in fluid composition away from inferred fluid sources. Tightly clustered δ¹⁸O values for magnetite, quartz, feldspar and actinolite for igneous‐hosted samples reflect high temperature equilibration in the magmatic‐hydrothermal environment. In contrast, these minerals record predominantly higher δ¹⁸O values in regional alteration and Cu–Au mineralization. This dichotomy reflects partial equilibration with isotopically heavier wallrocks and slightly lower temperatures. Increases in Si concentrations of metasomatic amphiboles relative to igneous amphiboles in part reflect cooling of metasomatic fluids away from igneous rocks. Variations in X_Mg for metasomatic amphiboles indicate local wallrock controls on amphibole chemistry, while variations in X_Cl/X_OH ratios for amphiboles (at constant X_Mg) indicate variable aH₂O/aHCl ratios for metasomatic fluids. Biotite geochemistry also reflects cooling and both increases and decreases in aH₂O/aHCl for fluids away from plutonic rocks. Decreased aH₂O/aHCl ratios for metasomatic fluids reflect in part scavenging of chlorine out of meta‐evaporite sequences, although this process requires already saline fluids. Local increases in aH₂O/aHCl ratios, as well as local decreases in δ¹⁸O values for some minerals (most notably haematite and epithermal‐textured quartz), may indicate ingress of low salinity, low δ¹⁸O fluids of possible meteoric origin late in the hydrothermal history of the region. Taken together, our observations are most consistent with predominantly magmatic sources for metasomatic fluids in the eastern Mt Isa Block, but record chemical and isotopic variations along fluid flow paths that may be important in explaining some of the diversity in alteration and mineralization styles in the district.     

Chemical evolution of metamorphic fluids in the Central Alps,Switzerland: insight from LA‐ICPMS analysis of fluid inclusions

The chemical evolution of fluids in Alpine fissure veins (open cavities with large free‐standing crystals) has been studied by combination of fluid inclusion petrography, microthermometry, LA‐ICPMS microanalysis, and thermodynamic modeling. The quartz vein systems cover a metamorphic cross section through the Central Alps (Switzerland), ranging from subgreenschist‐ to amphibolite‐facies conditions. Fluid compositions change from aqueous inclusions in subgreenschist‐ and greenschist‐facies rocks to aqueous–carbonic inclusions in amphibolite‐facies rocks. The fluid composition is constant for each vein, across several fluid inclusion generations that record the growth history of the quartz crystals. Chemical solute geothermometry, fluid inclusion isochores, and constraints from fluid–mineral equilibria modeling were used to reconstruct the pressure–temperature conditions of the Alpine fissure veins and to compare them with the metamorphic path of their host rocks. The data demonstrate that fluids in the Aar massif were trapped close to the metamorphic peak whereas the fluids in the Penninic nappes record early cooling, consistent with retrograde alteration. The good agreement between the fluid–mineral equilibria modeling and observed fluid compositions and host‐rock mineralogy suggests that the fluid inclusions were entrapped under rock‐buffered conditions. The molar Cl/Br ratios of the fluid inclusions are below the seawater value and would require unrealistically high degrees of evaporation and subsequent dilution if they were derived from seawater. The halogen data may thus be better explained by interaction between metamorphic fluids and organic matter or graphite in metasedimentary rocks. The volatile content (CO₂, sulfur) in the fluid inclusions increases systematically as function of the metamorphic grade, suggesting that the fluids have been produced by prograde devolatilization reactions. Only the fluids in the highest grade rocks were partly modified by retrograde fluid–rock interactions, and all major element compositions reflect equilibration with the local host rocks during the earliest stages of postmetamorphic uplift.     

Modeling thermal convection in supradetachment basins: example from western Norway

The juxtaposition of fault‐bounded sedimentary basins, above crustal‐scale detachments, with warmer exhumed footwalls can lead to thermal convection of the fluids in the sediments. The Devonian basins of western Norway are examples of supradetachment basins that formed in the hanging wall of the Nordfjord‐Sogn Detachment Zone. In the central part of the Hornelen and Kvamshesten basins, the basin‐fill is chiefly represented by fluvial sandstones and minor lacustrine siltstones, whereas the fault margins are dominated by fanglomerates along the detachment contact. Prominent alteration and low‐greenschist facies metamorphic conditions are associated with the peak temperature estimates of the sediments close to the detachment shear zone. Fluid circulation may have been active during the burial of the sediments, and we quantify the potential role played by thermal convection in redistributing heat within the basins. Different models are tested with homogeneous and layered basin‐fill and with material transport properties corresponding to sandstones and siltstones. We found that thermally driven fluid flow is expected in supradetachment basins as a transient process during the exhumation of warmer footwalls. We demonstrate that the fluid flow may have significantly affected the temperature distribution in the upper five kilometers of the Devonian basins of western Norway. The temperature anomaly induced by the flow may locally reach about 80°C. The sedimentary layering formed by sand‐ and siltstones strata does not inhibit fluid circulation at the scale of the basin. The presence of fluid pathways along the detachment has an important impact on the flow and allows an efficient drainage of the basin by channelizing fluids upward along the detachment.     

The mechanism of fluid infiltration in peridotites at Almklovdalen, western Norway

A major Alpine‐type peridotite located at Almklovdalen in the Western Gneiss Region of Norway was infiltrated by aqueous fluids at several stages during late Caledonian uplift and retrogressive metamorphism. Following peak metamorphic conditions in the garnet–peridotite stability field, the peridotite experienced pervasive fluid infiltration and retrogression in the chlorite–peridotite stability field. Subsequently, the peridotite was infiltrated locally by nonreactive fluids along fracture networks forming pipe‐like structures, typically on the order of 10 m wide. Fluid migration away from the fractures into the initially impermeable peridotite matrix was facilitated by pervasive dilation of grain boundaries and the formation of intragranular hydrofractures. Microstructural observations of serpentine occupying the originally fluid‐filled inclusion space indicate that the pervasively infiltrating fluid was characterized by a high dihedral angle (θ > 60°) and ‘curled up’ into discontinuous channels and fluid inclusion arrays following the infiltration event. Re‐equilibration of the fluid phase topology took place by growth and dissolution processes driven by the excess surface energy represented by the ‘forcefully’ introduced external fluid. Pervasive fluid introduction into the peridotite reduced local effective stresses, increased the effective grain boundary diffusion rates and caused extensive recrystallization and some grain coarsening of the infiltrated volumes. Grain boundary migration associated with this recrystallization swept off abundant intragranular fluid inclusions in the original chlorite peridotite, leading to a significant colour change of the rock. This colour change defines a relatively sharp front typically located 1–20 cm away from the fractures where the nonreactive fluids originally entered the peridotite. Our observations demonstrate how crustal rocks may be pervasively infiltrated by fluids with high dihedral angles (θ > 60°) and emphasize the coupling between hydrofracturing and textural equilibration of the grain boundary networks and the fluid phase topology.     

Experimental study of aqueous fluid infiltration into quartzite: implications for the kinetics of fluid redistribution and grain growth driven by interfacial energy reduction

To investigate the kinetics of interfacial energy‐driven fluid infiltration, experiments were carried out in a quartzite–water system at 621–925°C and 0.8 GPa. Infiltration couples were made by juxtaposing presynthesized dry quartzite cylinders and fluid reservoirs. The infiltration process was confirmed by the presence of pores at the quartzite grain edges. As predicted from theoretical considerations and previous experiments, wetting fluids such as pure water and NaCl aqueous solution infiltrated into quartzite, whereas nonwetting CO₂‐rich fluids did not. Newly precipitated quartz layers at the surfaces of the infiltrated sample proved that infiltration took place by a dissolution–precipitation mechanism. The enhancement of grain growth by fluid infiltration was observed over the entire range of experimental temperatures. The fluid fraction, gauged by the porosity of the run products, increases at the infiltration front and then decreases towards the fluid reservoir to form a high‐porosity zone with a maximum porosity of 2.3–2.9%. As infiltration proceeds, the high‐porosity zone advances like a travelling wave. This porosity wave is probably caused by a grain curvature gradient resulting from preferential grain growth in the infiltrated part of the quartzite, perhaps combined with other factors. The infiltration kinetics were modelled with a steady‐state diffusion model over the high‐porosity zone. The solubility difference between dissolving and precipitating grains was deduced to be 2 × 10^?2?3 × 10^?1 wt %. The experimentally obtained infiltration rate of aqueous fluid in the steady‐state diffusion regime (2 ± 0.5 × 10^?8 m sec^?1 at 823°C) is much faster than the estimated metamorphic fluid flux rates, so that interfacial energy‐driven fluid redistribution in quartz‐rich layers could significantly contribute to the fluid flux in high‐grade metamorphism, at least over a short distance. Cathodoluminescence observations of the run products revealed that the grain growth of quartzite in the presence of fluid proceeds extensively, which would promote the chemical equilibration between fluid and rock more effectively than would volume diffusion in quartz crystals.     

From deep to shallow fluid reservoirs: evolution of fluid sources during exhumation of the Sierra Almagrera,Betic Cordillera,Spain

The combination of structural, geochemical and palaeotopographic data proves to be an efficient tool to understand fluid transfers in the crust. This study discriminates shallow and deep fluid reservoirs on both sides of the brittle–ductile transition under an extensional regime and points out the role of major transcurrent fault activity in this palaeohydrogeological setting. Palaeofluids trapped in quartz and siderite–barite veins record the transfer of fluids and metal solute species during the Neogene exhumation of the Sierra Almagrera metamorphic belt. Ductile then brittle–ductile extensional quartz veins formed from a deep fluid reservoir, trapping metamorphic secondary brines containing low‐density volatile phases derived from the dissolution of Triassic evaporites. During exhumation, low‐salinity fluids percolated within the brittle domain, as shown by transgranular fluid inclusion planes affecting previous veins. These observations indicate the opening of the system during Serravalian to early Tortonian times and provide evidence for the penetration of surficial fluids of meteoric or basinal origin into the upper part of the brittle–ductile transition. During exhumation, synsedimentary transcurrent tectonic processes occurred from late Tortonian times onwards, while marine conditions prevailed at the Earth's surface. At depth in the brittle domain, quartz veins associated with haematite record a return to high‐salinity fluid circulation suggesting an upward transfer fed from a lower reservoir. During the Messinian, ongoing activity of the trans‐Alboran tectono‐volcanic trend led to the formation of ore deposits. Reducing fluids caused the formation of siderite and pyrite ores. The subsequent formation of galena and barite may be related to an increase of temperature. The high salinity and Cl/Br ratio of the fluids suggest another source of secondary brine derived from dissolved Messinian evaporites, as corroborated by the δ³⁴S signature of barite. These evaporites preceded the main sea‐level drop related to the peak of the salinity crisis (5.60–5.46 Ma).     

Devolatilization‐induced pressure build‐up: Implications for reaction front movement and breccia pipe formation

Generation of fluids during metamorphism can significantly influence the fluid overpressure, and thus the fluid flow in metamorphic terrains. There is currently a large focus on developing numerical reactive transport models, and with it follows the need for analytical solutions to ensure correct numerical implementation. In this study, we derive both analytical and numerical solutions to reaction‐induced fluid overpressure, coupled to temperature and fluid flow out of the reacting front. All equations are derived from basic principles of conservation of mass, energy and momentum. We focus on contact metamorphism, where devolatilization reactions are particularly important owing to high thermal fluxes allowing large volumes of fluids to be rapidly generated. The analytical solutions reveal three key factors involved in the pressure build‐up: (i) The efficiency of the devolatilizing reaction front (pressure build‐up) relative to fluid flow (pressure relaxation), (ii) the reaction temperature relative to the available heat in the system and (iii) the feedback of overpressure on the reaction temperature as a function of the Clapeyron slope. Finally, we apply the model to two geological case scenarios. In the first case, we investigate the influence of fluid overpressure on the movement of the reaction front and show that it can slow down significantly and may even be terminated owing to increased effective reaction temperature. In the second case, the model is applied to constrain the conditions for fracturing and inferred breccia pipe formation in organic‐rich shales owing to methane generation in the contact aureole.     

Salt tectonics and shallow subseafloor fluid convection: models of coupled fluid‐heat‐salt transport

Thermohaline convection associated with salt domes has the potential to drive significant fluid flow and mass and heat transport in continental margins, but previous studies of fluid flow associated with salt structures have focused on continental settings or deep flow systems of importance to petroleum exploration. Motivated by recent geophysical and geochemical observations that suggest a convective pattern to near‐seafloor pore fluid flow in the northern Gulf of Mexico (GoMex), we devise numerical models that fully couple thermal and chemical processes to quantify the effects of salt geometry and seafloor relief on fluid flow beneath the seafloor. Steady‐state models that ignore halite dissolution demonstrate that seafloor relief plays an important role in the evolution of shallow geothermal convection cells and that salt at depth can contribute a thermal component to this convection. The inclusion of faults causes significant, but highly localized, increases in flow rates at seafloor discharge zones. Transient models that include halite dissolution show the evolution of flow during brine formation from early salt‐driven convection to later geothermal convection, characteristics of which are controlled by the interplay of seafloor relief and salt geometry. Predicted flow rates are on the order of a few millimeters per year or less for homogeneous sediments with a permeability of 10^?15 m², comparable to compaction‐driven flow rates. Sediment permeabilities likely fall below 10^?15 m² at depth in the GoMex basin, but such thermohaline convection can drive pervasive mass transport across the seafloor, affecting sediment diagenesis in shallow sediments. In more permeable settings, such flow could affect methane hydrate stability, seafloor chemosynthetic communities, and the longevity of fluid seeps.     

The origins of salinity in metamorphic fluids

Evaluation of data on formation waters and metamorphic fluids sampled by drilling or preserved in fluid inclusions reveals little correlation between fluid salinity and metamorphic grade, but a strong link to original sedimentary setting. Sediments and metasediments deposited originally in shallow marine environments can contain fluids with a very wide range of salinities, but they are commonly near twice seawater salinity or higher. With increasing metamorphic grade, a very wide range of salinities may develop, with the highest levels tracking halite saturation. Oceanic and accretionary prism sequences yield low‐salinity fluids, close to seawater values, almost irrespective of metamorphic grade until extreme conditions are reached where removal of water may increase fluid salinity. The salinities of metamorphic fluids exert a fundamental control on both fluid phase equilibria and metal‐transporting capability, and appear, to a large degree, to reflect the original presence or absence of highly saline formation waters and/or evaporites in the initial sedimentary sequence.     

Morphological evolution of three‐dimensional chemical dissolution front in fluid‐saturated porous media: a numerical simulation approach

This paper is concerned with the morphological evolution of three‐dimensional chemical dissolution fronts that occur in fluid‐saturated porous media. A fully coupled system between porosity, pore‐fluid flow and reactive chemical species transport is considered to describe this phenomenon. Using the newly presented concept of the generalized dimensionless pore fluid pressure‐gradient, which can be used to represent the interaction between solute advection, solute diffusion, chemical kinetics and the shape factor of the soluble mineral, a theoretical criterion has been established to assess the likelihood of instability at a chemical dissolution front in the reactive transport system. To simulate the chemical dissolution front evolution in a three‐dimensional fluid‐saturated porous medium, a numerical procedure combining both the finite difference method and the finite element method has been proposed. As the problem belongs to a complex system science problem, a small randomly generated perturbation of porosity is added to the initial porosity of a three‐dimensional homogeneous domain to trigger instability of a planar chemical dissolution front during its propagation within the fluid‐saturated porous medium. To test the correctness and accuracy of the proposed numerical procedure, a three‐dimensional benchmark problem has been constructed and the related analytical solution has been derived. This enables using the proposed numerical procedure for simulating the morphological evolution of a three‐dimensional chemical dissolution front from a stable, planar state into an unstable, fingering state. The related numerical results demonstrate that the proposed numerical procedure is useful for, and capable of, simulating the morphological instability of a three‐dimensional chemical dissolution front within a fluid‐saturated porous medium.     

Spatial coupling between spilitization and carbonation of basaltic sills in SW Scottish Highlands: evidence of a mineralogical control of metamorphic fluid flow

In a geochemical and petrological analysis of overprinting episodes of fluid–rock interaction in a well‐studied metabasaltic sill in the SW Scottish Highlands, we show that syn‐deformational access of metamorphic fluids and consequent fluid–rock interaction is at least in part controlled by preexisting mineralogical variations. Lithological and structural channelling of metamorphic fluids along the axis of the Ardrishaig Anticline, SW Scottish Highlands, caused carbonation of metabasaltic sills hosted by metasedimentary rocks of the Argyll Group in the Dalradian Supergroup. Analysis of chemical and mineralogical variability across a metabasaltic sill at Port Cill Maluaig shows that carbonation at greenschist to epidote–amphibolites facies conditions caused by infiltration of H₂O‐CO₂ fluids was controlled by mineralogical variations, which were present before carbonation occurred. This variability probably reflects chemical and mineralogical changes imparted on the sill during premetamorphic spilitization. Calculation of precarbonation mineral modes reveals heterogeneous spatial distributions of epidote, amphibole, chlorite and epidote. This reflects both premetamorphic spilitization and prograde greenschist facies metamorphism prior to fluid flow. Spilitization caused albitization of primary plagioclase and spatially heterogeneous growth of epidote ± calcic amphibole ± chlorite ± quartz ± calcite. Greenschist facies metamorphism caused breakdown of primary pyroxene and continued, but spatially more homogeneous, growth of amphibole + chlorite ± quartz. These processes formed diffuse epidote‐rich patches or semi‐continuous layers. These might represent precursors of epidote segregations, which are better developed elsewhere in the SW Scottish Highlands. Chemical and field analyses of epidote reveal the evidence of local volume fluctuations associated with these concentrations of epidote. Transient permeability enhancement associated with these changes may have permitted higher fluid fluxes and therefore more extensive carbonation. This deflected metamorphic fluid such that its flow direction became more layer parallel, limiting propagation of the reaction front into the sill interior.     

Topography‐driven flow versus buoyancy‐driven flow in the U.S. midcontinent: implications for the residence time of brines

Topography‐driven flow is normally considered to be the dominant groundwater flow system in uplifted sedimentary basins. In the U.S. midcontinent region east of the Rocky Mountains, the presence of brines derived from dissolution of halite suggests that significant topography‐driven flushing has occurred to remove older brines that presumably formed concurrently with Permian evaporites in the basin. However, the presence of evaporites and brines in the modern basin suggests that buoyancy‐driven flow could limit topography‐driven flushing significantly. Here we used numerical models of variable‐density fluid flow, halite dissolution, solute transport, and heat transport to quantify flow patterns and brine migration. Results indicate the coexistence of large‐scale topography‐ and buoyancy‐driven flow. Buoyancy‐driven flow and low permeability evaporites act to isolate brines, and the residence time of the brines was found to be quite long, at least 50 Myr. The modern distribution of salinity appears to reflect near‐steady‐state conditions. Results suggest that flushing of original evaporatively‐concentrated brines occurred tens of millions of years ago, possibly concurrent with maximum uplift ca. 60 Ma. Simulations also suggest that buoyancy‐driven convection could drive chemical exchange with crystalline basement rocks, which could supply significant Ca²⁺, Sr²⁺, and metals to brines.     

Diverse origins and timing of formation of basinal brines in the Gulf of Mexico sedimentary basin

A review of five different field areas in the Gulf of Mexico sedimentary basin (GOM) illustrates some of the potentially diverse chemical and physical processes which have produced basinal brines. The elevated salinities of most of the formation waters in the GOM are ultimately related to the presence of the Middle Jurassic Louann Salt. Some of these brines likely inherited their salinity from evaporated Mesozoic seawater, while other saline fluids have been produced by subsequent dissolution of salt, some of which is occurring today. The timing of the generation of brines has thus not been restricted to the Middle Jurassic. The mechanisms of solute transport that have introduced brines throughout much of the sedimentary section of the GOM are not entirely understood. Free convection driven by spatial variations in formation water temperature and salinity is undoubtedly occurring around some salt structures. However, the driving mechanisms for the broad, diffusive upward solute transport in the northern Gulf rim of Arkansas and northern Louisiana are not known. In the Lower Cretaceous of Texas, fluid flow was much more highly focused, and perhaps episodic. It is clear that many areas of the Gulf basin are hydrologically connected and that large‐scale fluid flow, solute transport, and dispersion have occurred. The Na‐Mg‐Ca‐Cl compositions of brines in the areas of the Gulf Coast sedimentary basin reviewed in this article are products of diagenesis and do not reflect the composition of the evaporated marine waters present at the time of sediment deposition. Large differences in Na, Ca, and Mg trends for waters hosted by Mesozoic versus Cenozoic sediments may reflect differences in: (i) the sources of salinity (evaporated seawater for some of the Mesozoic sediments, dissolution of salt for some of the Cenozoic sediments); (ii) sediment lithology (dominantly carbonates for much of the Mesozoic sediments, and dominantly siliciclastics for the Cenozoic sediments); or (iii) residence times of brines associated with these sediments (tens of millions of years versus perhaps days).     

Solubility of corundum + kyanite in H2O at 700°C and 10 kbar: evidence for Al‐Si complexing at high pressure and temperature

The solubility of the assemblage corundum + kyanite in H₂O was determined at 700°C and 10 kbar, using a piston‐cylinder apparatus and rapid‐quench/fluid‐extraction techniques. Weighted mean concentrations of total Al and Si were 5.80 ± 0.03 mmol kg^?1 H₂O and 0.308 ± 0.003 mol kg^?1 H₂O, respectively (1σ errors). The Al concentration is nearly five times higher than that of corundum solubility in pure H₂O. This difference is interpreted to arise from complexing between Si and Al to form HAlSiO_4,aq species. Charged or more polymerized species are also possible, but their abundance cannot be constrained based on these experiments. Assumption of a single aqueous aluminosilicate complex permits calculation of the thermodynamic consequences of Al‐Si interaction in high‐pressure fluids, as well as phase diagrams for the system Al‐Si‐O‐H. Formation of Al‐Si complexes leads to a large increase in dissolved Al with increasing Si in solution, such that Al concentration in equilibrium with kyanite + quartz is predicted to be 7.1 mmolal, higher than with kyanite + corundum. The elevated concentration of Si in deep‐crustal and mantle aqueous fluids suggests that Al must readily be dissolved and transported by Al‐Si complexing in high‐pressure metamorphic and metasomatic environments. The results provide a simple explanation for the common observation of kyanite + quartz segregations in eclogites and Barrovian metamorphic rocks.     

Pattern formation during healing of fluid-filled cracks: an analog experiment

The formation and subsequent healing of cracks and crack networks may control such diverse phenomena as the strengthening of fault zones between earthquakes, fluid migrations in the Earth's crust, or the transport of radioactive materials in nuclear waste disposal. An intriguing pattern-forming process can develop during healing of fluid-filled cracks, where pockets of fluid remain permanently trapped in the solid as the crack tip is displaced driven by surface energy. Here, we present the results of analog experiments in which a liquid was injected into a colloidal inorganic gel to obtain penny-shaped cracks that were subsequently allowed to close and heal under the driving effect of interfacial tension. Depending on the properties of the gel and the injected liquid, two modes of healing were obtained. In the first mode, the crack healed completely through a continuous process. The second mode of healing was discontinuous and was characterized by a 'zipper - like' closure of a front that moved along the crack perimeter, trapping fluid that may eventually form inclusions trapped in the solid. This instability occurred only when the velocity of the crack tip decreased to zero. Our experiments provide a cheap and simple analog to reveal how aligned arrays of fluid inclusions may be captured along preexisting fracture planes and how small amounts of fluids can be permanently trapped in solids, modifying irreversibly their material properties.     

Quartz recrystallization and fluid flow during contact metamorphism: a cathodoluminescence study

Cathodoluminescence (CL) images of quartz grains in the Appin Quartzite from the aureole of the Ballachulish Igneous Complex (Scotland) reveal a textural complexity that we interpret in the light of published models of the evolution of the contact aureole. Five distinct generations of quartz can be discriminated in CL. The oldest of these is a dark luminescing mottled quartz (Type 1 quartz) that occurs in the centres of pre‐existing grains, in samples collected from 210 m to 0.1 m from the contact. Dark mottled quartz is interpreted to be unrecrystallized material and has a regional metamorphic CL spectral signature. The onset of contact metamorphism resulted in grain growth visible in CL as a series of fine‐scale alternating bands of bright and dark luminescing material (Type 2 quartz), which we attribute to infiltration of repeated pulses of small amounts of H₂O along grain boundaries. Close to the intrusion, a subgrain‐scale network of intragranular, bright luminescing features could have resulted from either intragranular microcrack‐controlled infiltration of H₂O at high temperatures or intergranular cracking followed by grain growth (Type 3 quartz). Broad bands of bright material on grain boundaries in samples that are inferred to have undergone partial melting are interpreted as quartz crystallized from the melt phase (Type 4 quartz). The final stage in the textural development is marked by a series of aligned fractures, detected in CL by nonluminescing material (Type 5 quartz) and corresponding closely with trails of fluid inclusions. These fractures are interpreted as the pathways for late‐stage, low‐temperature, retrogressive fluids.     

Fluid inclusion and stable isotope constraints on the genesis of the Jian copper deposit,Sanandaj–Sirjan metamorphic zone,Iran

The Jian copper deposit, located on the eastern edge of the Sanandaj–Sirjan metamorphic zone, southwest of Iran, is contained within the Surian Permo‐Triassic volcano‐sedimentary complex. Retrograde metamorphism resulted in three stages of mineralization (quartz ± sulfide veins) during exhumation of the Surian metamorphic complex (Middle Jurassic time; 159–167 Ma), and after the peak of the metamorphism (Middle to Late Triassic time; approximately 187 Ma). The early stage of mineralization (stage 1) is related to a homogeneous H₂O–CO₂ (XCO₂ > 0.1) fluid characterized by moderate salinity (<10 wt.% NaCl equivalent) at high temperature and pressure (>370°C, >3 kbar). Early quartz was followed by small amounts of disseminated fine‐grained pyrite and chalcopyrite. Most of the main‐ore‐stage (stage 2) minerals, including chalcopyrite, pyrite and minor sphalerite, pyrrhotite, and galena, precipitated from an aqueous‐carbonic fluid (8–18 wt.% NaCl equivalent) at temperatures ranging between 241 and 388°C during fluid unmixing process (CO₂ effervescence). Fluid unmixing in the primary carbonaceous fluid at pressures of 1.5–3 kbar produced a high XCO₂ (>0.05) and a low XCO₂ (<0.01) aqueous fluid in ore‐bearing quartz veins. Oxygen and hydrogen isotope compositions suggest mineralization by fluids derived from metamorphic dehydration (δ¹⁸O_fluid = +7.6 to +10.7‰ and δD = ?33.1 to ?38.5‰) during stage 2. The late stage (stage 3) is related to a distinct low salinity (1.5–8 wt.% NaCl equivalent) and temperatures of (120–230°C) aqueous fluid at pressures below 1.5 kbar and the deposition of post‐ore barren quartz veins. These fluids probably derived from meteoric waters, which circulated through the metamorphic pile at sufficiently high temperatures and acquire the characteristics of metamorphic fluids (δ¹⁸O_fluid = +4.7 to +5.1‰ and δD = ?52.3 to ?53.9‰) during waning stages of the postearly Cimmerian orogeny in Surian complex. The sulfide‐bearing quartz veins are interpreted as a small‐scale example of redistribution of mineral deposits by metamorphic fluids. This study suggests that mineralization at the Jian deposit is metamorphogenic in style, probably related to a deep‐seated mesothermal system.     

Granite‐related overpressure and volatile release in the mid crust: fluidized breccias from the Cloncurry District,Australia

The source and transport regions of fluidized (transported) breccias outcrop in the Cloncurry Fe‐oxide–Cu–Au district. Discordant dykes and pipes with rounded clasts of metasedimentary calc–silicate rocks and minor felsic and mafic intrusions extend several kilometres upwards and outwards from the contact aureole of the 1530 Ma Williams Batholith into overlying schists and amphibolites. We used analytical equations for particle transport to estimate clast velocities (≥20 m sec^?1), approaching volcanic ejecta rates. An abrupt release of overpressured magmatic‐hydrothermal fluid is suggested by the localization of the base of the breccias in intensely veined contact aureoles (at around 10 km, constrained by mineral equilibria), incorporation of juvenile magmatic clasts, the scale and discordancy of the bodies, and the wide range of pressure variation (up to 150 MPa) inferred from CO₂ fluid inclusion densities and related decrepitation textures. The abundance of clasts derived from depth, rather than from the adjacent wallrocks, suggests that the pressure in the pipes was sufficient to restrict the inwards spalling of fragments from breccia walls; that is, the breccias were explosive rather than implosive, and some may have vented to the surface. At these depths, such extreme behaviour may have been achieved by release of dissolved fluids from crystallizing magma, in combination with a strongly fractured and fluid‐laden carapace, sitting under a strong, low permeability barrier. The relationship of these breccias to the Ernest Henry iron‐oxide–Cu–Au deposit suggests they may have been sources of fluids or mechanical energy for ore genesis, or alternately provided permeable pathways for later ore fluids.