Brine Rock Interactions Research Articles

CO2-brine-rock interactions as a result of long term experiment onto rock samples from Chabowo anticline, Poland

Presented work concern investigation of 20-months CO 2 -brine-rock interactions. Experiment were carried out at laboratory scale in designed for this purpose equipment. Research materials (reservoir and cap rocks) were taken form the Chabowo 3 well located within Chabowo Anticline. The experiment were carried out at room temperature (about 25oC) and pressure about 6 MPa. For samples before and after experiment a numerous investigation were carried out i.e. petrophysical (porosity, surface are, threshold diameter, average capillary, framework and bulk densities) and petrology-mineralogical characteristics. Also brine used for examination before and after experiment were analysed (chemical analysis). Petrophysical result have shown a significant decrease of porosity (form 17.0 % to 7.5%) and surface area ( form 0.395 m 2 /g to 0.196 m 2 /g) in case of sandstone. This changes are caused probably by a crystallization of halite in pore spaces, because mineralogical analysis (XRD) have shown presence of halite (3-5% vol. in the rock) after experiment. More detailed results were achieved for mineralogical analysis of thin sections. These results have shown in sandstone samples a small increase of porosity as small decrease of feldspar and cements (carbonate and matix) what indicate on dissolution of this phases. For claystone after experiment XRD clay fraction have shown a lack of clay minerals such as chlorite, montmorillonite, what also can be caused by interaction this rock between CO 2 and brine. Dissolution of some minerals is approved by chemical analysis of brine where amount of most tested ions increase after experiment for both sandstone and claystone.

Experimental Investigation of Brine-Rock Interactions for Groundwater Quality Predictions in Mine Areas

The study of brine-rock interactions was carried out with the purpose of determining parameters for geomigration models used to predict groundwater contamination in mine areas. Comprehensive tests of clay fraction mineralogical composition, microstructure, salinity, exchange capacity and exchanged cation contents of rocks, as well as updating of experimental laboratory techniques for brine–rock interactions in static and dynamic conditions were carried out. Data obtained from experimental laboratory tests permitted us to determine the specificities of cation exchange in the system brine-rock, to define the kinds of exchange “two-stage” isotherms, to estimate the cation exchange parameters, and to establish the possibility of brine chemical composition change when filtering through the rocks.

A mechanistic understanding of plagioclase dissolution based on Al occupancy and T–O bond length: from geologic carbon sequestration to ambient conditions

A quantitative description of how the bulk properties of aluminosilicates affect their dissolution kinetics is important in helping people understand the regulation of atmospheric CO2 concentration by silicate weathering and predict the fate and transport of geologically sequestered CO2 through brine-rock interactions. In this study, we employed a structure model based on the C1 space group to illustrate how differences in crystallographic properties of aluminosilicates, such as T-O (Tetrahedral site-Oxygen) bond length and Al/Si ordering, can result in quantifiable variations in mineral dissolution rates. The dissolution rates of plagioclases were measured under representative geologic carbon sequestration (GCS) conditions (90 °C, 100 atm of CO2, 1.0 M NaCl, and pH ∼ 3.1), and used to validate the model. We found that the logarithm of the characteristic time of the breakdown of Al-O-Si linkages in plagioclases follows a good linear relation with the mineral's aluminum content (nAl). The Si release rates of plagioclases can be calculated based on an assumption of dissolution congruency or on the regularity of Al/Si distribution in the constituent tetrahedra of the mineral. We further extended the application of our approach to scenarios where dissolution incongruency arises because of different linkage reactivities in the solid matrix, and compared the model predictions with published data. The application of our results enables a significant reduction of experimental work for determining the dissolution rates of structurally related aluminosilicates, given a reaction environment.

Planned Experimental Studies into the Nucleation and Precipitation Kinetics of Sulfate-bearing Minerals under Conditions Relevant to CO2 Geological Storage

Carbon Capture and Storage (CCS) has been widely recognized as one of the main technologies to mitigate climate change. Deep-saline aquifers are among the preferred potential repositories for CO2 currently being emitted to the atmosphere from burning of fossil fuels. Large-scale injection of supercritical CO2 into subsurface reservoirs induces a complex interplay of multiphase flow, capillary trapping, dissolution and chemical reactions that may have significant impacts on both, short-term injection performance and long-term fate of CO2 storage. To ensure the viability of geological CO2 storage, a holistic understanding of reactions at supercritical CO2-water-rock interfaces and the environmental factors affecting these interactions is required.The major objectives of this contribution are to study the kinetics of CO2 brine-rock interactions and derive respective chemical reaction rate expressions under the typical conditions of CO2 storage sites. These rate laws may be used in modeling the storage sites and operating them in a way that will minimize scaling and thus potential reduction of injectivity.A new experimental system was set up at the Water-Rock interaction laboratory, Ben-Gurion University of the Negev that allows studying the interaction between CO2, brine, and minerals under CO2 supercritical conditions. All equipment is fully capable to deal with adjustable PTX conditions enabling continuous and computerized data acquisition of various variables including pressure, temperature, stirring speed, in-situ measurement of pH and redox potential. The experimental setup allows the measurement of dissolution and precipitation rates of diverse minerals under a wide range of environmental conditions as found in CO2 storage sites offering the possibility to collect liquid and solid samples for further chemical and morphological analyses.

An experimental study of CO2–brine–rock interaction at in situ pressure–temperature reservoir conditions

A detailed investigation of CO2–brine–rock interactions, through a core flooding laboratory experiment, was carried out under simulated reservoir conditions (100°C and 24MPa). Changes in the ionic chemistry of the outlet solution, combined with core scanning electron microscopy (SEM) and bulk-rock X-ray diffraction (XRD) analysis of the core pre- and post-experiment reveal new insights into CO2–brine–rock interactions. Minerals such as potassium (K) feldspar, albite, calcite, and ankerite are variably dissolved after the experiments. Calcite is the mineral most affected by dissolution, followed by ankerite, whereas dissolution of feldspar minerals is minimal. Small amounts of kaolinite and solid phases were generated as a result of the experiment. The solid phases are mainly comprised of C, O, Na, Cl, Al, and Si, and are presumed to be the transitional products in the formation of carbonate minerals. The very low fluid penetration rate in the experiment resulted in significantly reduced contact area between the acidic CO2 fluid and minerals in the cores, resulting in low rates of feldspar dissolution. Core permeability decreased substantially throughout the experiment, although core porosity remained unchanged. The permeability reduction is the result of precipitation of new mineral phases (e.g., kaolinite and solid phases), and potentially also the presence of clay particles released by the dissolution of carbonate cement, which have then been transported in the fluid flow path and accumulated at pore throats. The results provide new insights into CO2 trapping mechanisms in depleted oil and gas reservoirs, and into the potential formation damage that may result from massive injections of CO2 into reservoirs during enhanced oil recovery programs.

Multiphase flow behaviour during CO 2 geo-sequestration: Emphasis on the effect of cyclic CO 2–brine flooding

Geo-sequestration is considered to have a significant global potential for reducing the amount of anthropogenic greenhouse gas emissions, in particular carbon dioxide. In simple terms CO 2 geo-sequestration involves capturing the gas at its emitting source and then compressing and injecting it into a deep underground geological reservoir with a known seal where it will exist as a dense supercritical fluid. In this way the unwanted gas will be sequestered for perpetuity. It is known that during many long term CO 2 geo-sequestration processes three will be parts of the underground reservoir subjected to cyclic CO 2–brine flooding. The multiphase flow characteristics of the fluids–rock system can change after each flooding cycle or in other words each flooding stage would be different from the other from the multiphase flow behaviour point of view. This continual change is due to various factors including: the capillary hysteresis effect, chemical reactions between the in-site created solute and the host formation rock, the effect of the stress applied to the reservoir and any formation damage or enhancements caused by the CO 2 or alternating CO 2–brine injection process. To the authors' knowledge there has not been an extended experimental study conducted to date on the effects of such cyclic flooding pattern on the multiphase flow characteristics of the system for more than two flooding cycles. This paper presents in details the equipment, procedure, protocol and the results of an experimental work carried out to understand the change in supercritical CO 2–brine–rock interactions subjected to this cyclic flooding pattern under in-situ reservoir conditions of pressure and temperature, with emphasis on change in multiphase flow characteristics of the system. The experiments were run on a number of sandstone samples with different permeability and porosity values to allow an evaluation of the change in experimental results with change in the sample properties. With this knowledge, we can better understand the significance of change in fluid flow characteristics during cyclic flooding which occurs in many CO 2 geo-sequestration processes in one form or the other.

Biotite–Brine Interactions under Acidic Hydrothermal Conditions: Fibrous Illite, Goethite, and Kaolinite Formation and Biotite Surface Cracking

To ensure safe and efficient geologic CO(2) sequestration (GCS), it is crucial to have a better understanding of CO(2)-brine-rock interactions under GCS conditions. In this work, using biotite (K(Mg,Fe)(3)AlSi(3)O(10)(OH,F)(2)) as a model clay mineral, brine-biotite interactions were studied under conditions relevant to GCS sites (95 °C, 102 atm CO(2), and 1 M NaCl solution). After reaction for 3-17 h, fast growth of fibrous illite on flat basal planes of biotite was observed. After 22-70 h reaction, the biotite basal surface cracked, resulting in illite detaching from the surface. Later on (96-120 h), the cracked surface layer was released into solution, thus the inner layer was exposed as a renewed flat basal surface. The cracking and detachment of the biotite surface layer increased the surface area in contact with solution and accelerated biotite dissolution. On biotite edge surfaces, Al-substituted goethite and kaolinite precipitated. In control experiments with water under the same temperature and pressure, neither macroscopic fibrous illite nor cracks were observed. This work provides unique information on biotite-brine interaction under acidic hydrothermal conditions.

CO 2–brine–rock interaction — First results of long-term exposure experiments at in situ P–T conditions of the Ketzin CO 2 reservoir

Sandstone samples of the Stuttgart Formation at Ketzin have been experimentally treated with CO 2 and synthetic reservoir brine in high-quality steel autoclaves at simulated in situ P–T conditions (5.5 MPa, 40 °C). In order to observe mineralogical changes induced by CO 2, untreated samples are compared to CO 2-treated ones. Most samples show an analogous petrography of mainly quartz and plagioclase. Heterogeneities are related to minor mineral phases, such as K-feldspar, hematite, muscovite, biotite, illite, chlorite and opaque phase(s). These are attributed to the variability of the fluvial reservoir sandstone. The samples are weakly consolidated. Analcime, dolomite and anhydrite are only found as cement phases. During the experiments dissolution of anhydrite and plagioclase is observed. SEM micrographs of CO 2-treated samples show corrosion textures on mineral surfaces of intermediate plagioclase, as well as precipitation of euhedral albite crystals. Overall, the data indicate preferred dissolution of calcium-rich plagioclase, K-feldspar and anhydrite and stabilization or precipitation of albite.

Brine-rock interaction in the Athabasca basement (McArthur River U deposit, Canada): consequences for fluid chemistry and uranium uptake

Terra Nova, 22, 303–308, 2010 Abstract The nature of uranium source rocks, transport conditions and deposition processes are still highly controversial for world-class unconformity-related U deposits. This article presents the first detailed chemistry of brines associated with the giant McArthur River U deposit, Canada. LA-ICP-MS analysis of individual fluid inclusions suggests mixing between a Na–Ca–Mg–K–Sr–Ba brine and a Ca–Mg–Na–K–Sr–Ba brine. The brines share a common origin (evaporated seawater) and show evidence for contrasting interaction with basement rocks. The Na-rich brine lost Mg and K in alteration haloes around U ores, while the Ca-rich brine results from Na–Ca exchange and Sr–Ba gain. U concentrations (0.3–530 μg g−1) are anomalously high compared with usual basinal fluids, this indicating that U uptake occurred within basement rocks. The two brine end-members have mixed within the main U deposit area, which could be one of the major driving forces for U deposition.

Experimental investigation of CO 2–brine–rock interactions at elevated temperature and pressure: Implications for CO 2 sequestration in deep-saline aquifers

Deep-saline aquifers are potential repositories for excess CO 2, currently being emitted to the atmosphere from anthropogenic activities, but the reactivity of supercritical CO 2 with host aquifer fluids and formation minerals needs to be understood. Experiments reacting supercritical CO 2 with natural and synthetic brines in the presence and absence of limestone and plagioclase-rich arkosic sandstone showed that the reaction of CO 2-saturated brine with limestone results in compositional, mineralogical, and porosity changes in the aquifer fluid and rock that are dependent on initial brine composition, especially dissolved calcium and sulfate. Experiments reacting CO 2-saturated, low-sulfate brine with limestone dissolved 10% of the original calcite and increased rock porosity by 2.6%. Experiments reacting high-sulfate brine with limestone, both in the presence and absence of supercritical CO 2, were characterized by the precipitation of anhydrite, dolomitization of the limestone, and a final decrease in porosity of 4.5%. However, based on favorable initial porosity changes of about 15% due to the dissolution of calcite, the combination of CO 2 co-injection with other mitigation strategies might help alleviate some of the well-bore scale and formation-plugging problems near the injection zone of a brine disposal well in Paradox Valley, Colorado, as well as provide a repository for CO 2. Experiments showed that the solubility of CO 2 is enhanced in brine in the presence of limestone by 9% at 25 °C and 6% at 120 °C and 200 bar relative to the brine itself. The solubility of CO 2 is enhanced also in brine in the presence of arkosic sandstone by 5% at 120 °C and 300 bar. The storage of CO 2 in limestone aquifers is limited to only ionic and hydraulic trapping. However, brine reacted with supercritical CO 2 and arkose yielded fixation and sequestration of CO 2 in carbonate mineral phases. Brine desiccation was observed in all experiments containing a discrete CO 2 phase, promoting porosity-reducing precipitation reactions in aquifers near saturation with mineral phases.

The formation of natural cryogenic brines

The source of salts in the Ca-chloridic, hypersaline brines (up to 190 g Cl L −1) occurring in crystalline basement rocks in the Canadian, Fennoscandian and Bohemian Shields and their evolution have been investigated and reported. The Cl-Br-Na relationship indicates that these waters have been concentrated from seawater, by freezing during glacial times. The Na/Cl ratio (0.25 to 0.35) in the more saline fluids is compatible with cooling down to −30°C, where the most saline waters have been concentrated by a factor of 25 to 30 relative to the parent seawater. The brines formed from seawater within cryogenic troughs, along the subarctic continental margins, around ice sheets. The depressions within which the brines formed are the cryogenic analogues of the classic, evaporitic lagoon. One million years suffice to saturate with brine a 2000km-radius by 1km-depth rock volume at an H 2O removal rate of only 2.8 mm/yr. Density-induced brine migration on a continental scale takes place via fissures below the ice. Our calculations, that were performed on a hypothetical ice sheet with dimensions compatible with the Laurentide ice sheet, demonstrate that during 1m.y., a 60m thick cryogenic sediment section could have formed. However, the precipitated minerals (mirabilite and hydrohalite) are repeatedly dispersed by the advance and retreat of the ice sheet, dissolved by melt water-seawater mixtures, and eroded during postglacial uplift, leaving almost no trace in the geological record. The cryogenic brines formed intermittently during and between glacial periods. The repeating advance and retreat of the ice sheets exerted a major control on the direction and intensity of brine flow. The cryogenic concentration of seawater and the migration of brine towards the center of the glaciostatic depression occurred mainly during the build up of the ice sheet, while reversal of the water flow from the center of the cryogenic basin outwards happened upon deglaciation. The flow of the waters in the subsurface was, inevitably, accompanied by significant dilution with melt water from the ice sheets. Using a “granitic” U concentration of 4 ppm and a (Ca-Mg mass balance based) rock/water ratio anywhere between 3.4 and 6.8 kg L −1, a few hundred thousand years of brine-rock interaction are sufficient for the growth of 129I in the most saline Canadian Shield brine to its present concentration (3.4×10 8 atoms 129I L −1). Hence, both the formation of the saline fluids and their emplacement in their present sites occurred most likely within the Pleistocene. The young age calculated for cryogenic brines in crystalline shields and the dynamic water flow therein should raise concern about the planning and construction of high-grade nuclear waste repositories in such rocks, which are already under way.

Secular variation in seawater chemistry and the origin of calcium chloride basinal brines

CaCl 2 basinal brines, which are present in most Phanerozoic sedimentary basins, inherited their chemistries and salinities from evaporated paleoseawaters when the world oceans were Ca rich and SO 4 poor (CaCl 2 seas). CaCl 2 seas coincided with periods of rapid seafloor spreading, high influxes of mid-ocean-ridge brines rich in CaCl 2 , and elevated sea levels, conditions that favored accumulation of marine CaCl 2 brines in marginal and interior continental basins. Typical basinal brines in Silurian-Devonian formations of the interior Illinois basin, United States, show the same compositional trends as those of progressively evaporated CaCl 2 -rich Silurian seawater. Chemical deviations can be accounted for quantitatively by brine-rock reactions during burial (dolomitization, dolomite and K-feldspar cement). This explanation for the origin of CaCl 2 basinal brines contrasts with others that assume constancy of seawater chemistry and involve more complex brine-rock interactions.

The impact of brine-rock interaction during marine evaporite formation on the isotopic Sr record in the oceans: evidence from Mt. Sedom, Israel

The effect of brine-rock interaction on the composition of strontium in evaporitic basins and its impact on the 87Sr/ 86Sr ratios in contemporaneous seawater are examined for the Sedom (Dead Sea Rift Valley, or DSR), the Messinian (Mediterranean) and the Louann (Gulf of Mexico) evaporites. For that purpose, mineralogical, chemical and isotopic (Sr, S) analyses were performed on the Sedom Fm. evaporites (halite, anhydrite and dolomite). 87Sr/ 86Sr ratios are distinctively lower in the Sedom evaporites (dolomites: 0.7082–0.7083; halites: 0.7083–0.7087) than in the contemporaneous late Pliocene seawater (≈0.709). At the same time the sulfur isotope ratios (δ 34S ≈ 20‰) are consistent with deposition from late Cenozoic seawater. This duality, together with the variation of strontium isotopes between the dolomites and halites can be explained by modification of the 87Sr/ 86Sr ratio in the lagoon water by influx of Ca-Chloride brines. The brines were formed by dolomitization of marine carbonates of the DSR Cretaceous wall rocks (where 87Sr/ 86Sr ∼ 0.7077). Brine-rock interaction can similarly explain the anomalous 87Sr/ 86Sr ratios in the Messinian and Louann evaporites. It is concluded that this process causes significant changes in the 87Sr/ 86Sr ratios of evaporitic lagoons. A water and strontium mass balance of the Sedom data is used to show the impact on the strontium oceanic budget. Extrapolation to larger evaporitic basins indicates that the combined global riverine and hydrothermal influx of strontium can be matched by halite or gypsum precipitating lagoon of 2–3.5 × 10 5 km 2. Examples for such evaporitic sites include the Messinian, Louann and Zechstein basins.

High bromine contents and low [formula omitted] ratios in hydrothermally altered Archaean komatiitic rocks, West Greenland

In Archaean supracrustal rocks in West Greenland komatiitic and tholeiitic rocks have undergone alteration by circulating hydrothermal solutions, whereby the rocks were partly transformed into calc-silicate assemblages. The alteration, which also formed extensive stratabound scheelite mineralizations, took place partly at the sea floor and partly below the sea floor prior to metamorphism and deformation of the volcano-sedimentary pile. The altered komatiitic and tholeiitic rocks contain up to 418 ppm bromine and Cl Br ratios down to 1.6 compared with 60–1300 ppb bromine and Cl Br ratios of 430 in mid-ocean ridge basalts. Cl and Br was not detected by electron microprobe in any of the minerals found. It is suggested that the volcano-sedimentary pile was altered by hot brines which had attained high halogen contents by reaction with mafic and ultramafic rocks. During brine-rock interaction the Cl Br ratio of the brines decreased and the metal contents increased. The metalliferous bromine-enriched brines ascended towards the sea floor. The bromine and some of the metals precipitated in the upper part of the volcano-sedimentary pile and in the volcanic rocks at the sea floor. High bromine contents and low Cl Br ratios may indicate presence of major orebodies.

Origin and chemical evolution of formation waters from Silurian-Devonian strata in the Illinois basin, USA

A suite of formation-water samples from Silurian-Devonian reservoirs in the Illinois basin has been analyzed for major, minor and trace element concentrations and for H, O and Sr isotopic compositions in order to interpret origin of salinity and geochemical evolution of brine compositions in this evaporite- and shale-poor cratonic basin. Although chloride concentrations range from 2000 to 137,000 mg/L, Cl Br ratios (291 ± 18) are consistent with those of seawater or seawater evaporated short of halite saturation ( Cl/ Br = 292). Thus, during Silurian-Devonian time, subaerially evaporated, penesaline brine entered the subsurface where it was chemically modified through brine-rock interactions. Cation Br ratios and mineralogy of associated strata indicate that Na and K were depleted through interaction with clay minerals, Ca was enriched and Mg depleted by dolomitization and Sr was enriched as a result of CaCO 3 recrystallization and dolomitization. Although significant dilution of the modified brine with meteoric water is supported by δD- δ 18 O covariance, original marine waters have not been completely expelled from Silurian-Devonian strata. Hydrogen and oxygen isotopes exhibit covariant relations with cation and anion concentrations, implying that isotopic exchange between H 2O and minerals has not greatly influenced the δD − δ 18 O trend. Brine 87Sr 86Sr ratios range from 0.7092 to 0.7108; when these ratios are plotted versus 1/Sr, a two-component mixing trend is suggested, although Sr concentrations have experienced local diagenetic modification. A 87Sr-enriched fluid may have accompanied petroleum migration from New Albany shales into adjacent Silurian-Devonian carbonates where it mixed with remnant evaporated seawater. This event probably preceded the influx of meteoric water, as /gdD and δ 18 O are not correlated with Sr isotopic compositions of formation waters.

Isotopic constraints on the origin of the Atlantis II, Suakin and Valdivia brines, Red Sea

The origin of three Red Sea submarine brine pools was investigated by analysis of the S and O isotope ratios of dissolved sulfate and Sr isotope ratios of dissolved Sr in the brines. Sulfur and O isotope ratios of sulfate and Sr isotope ratios of evaporitic source rocks for the brines were measured for comparison. The S, O and Sr isotope ratios of evaporites recovered from DSDP site 227 are consistent with an upper Miocene evaporites age. The Valdivia Deep brine formed by karstic dissolution of Miocene evaporites by overlying seawater and shows no signs of hydrothermal input. The Suakin Deep brines are derived from, or have isotopically exchanged with Miocene or older evaporites. There has been only minor dilution of the brine by overlying seawater. Strontium isotope ratios of Suakin brine may indicate addition of a minor (15%) amount of volcanic Sr to the brine, but there is no evidence of high temperature brine-rock interaction. The sulfate in the Atlantis II brine was apparently derived from seawater. The O isotope ratio of sulfate in the present Atlantis II brine could reflect isotopic exchange between seawater sulfate and the brine at approximately 255°C. Approximately 30% of the Sr in the Atlantis II brine is derived from the underlying basalt, probably by hydrothermal leaching. Atlantis II brine is the only known example from the Red Sea which has a significant high-temperature hydrothermal history.

Carbonate Sedimentation And Diagenesis Associated With Mg-Ca-Chloride Brines: The Permian San Andres Formation in the Texas Panhandle

ABSTRACT The San Andres evaporitic sequence in the Palo Duro Basin includes several thick carbonate units in its lower part and many thin units in its upper part. Evaporites are concentrated in the northern part of the Northern Shelf of the Midland Basin, and carbonates predominate to the south. Within the Palo Duro Basin the carbonates in the lower part of the sequence differ from those in the upper part in containing skeletal lithofacies and in having lower Mn, Fe, and terrestrial organic content. The Br content in halite in the lower part of the sequence is consistently high, whereas halite in the upper part is commonly Br depleted. The Sr content in dolomite, calcite, and anhydrite and 18O values are about the same in the en ire San Andres sequence in the Palo Duro Basin. Depleted 13C values in dolomite and low pristane/ phytane ratios occur toward the south. Na/Cl and K/Cl ratios attributed to liquid inclusions in almost all carbonates are characteristic of marine brines evaporated beyond halite saturation. Na in the dolomite lattice is generally low and increases from north to south at the same stratigraphic levels. The lower part of the formation was deposited in a broad shelf basin or lagoon sufficiently deep to maintain long periods of steady-state circulation, during which little halite dissolution and no potash-magnesia mineral precipitation occurred. The upper part of the formation was deposited in smaller water bodies sensitive to fluctuations in inflow of marine water as well as meteoric water. Diagenesis of the carbonates occurred in contact with saline Mg-Ca-chloride brines evolved through anhydrite and halite precipitation. Skeletal mold formation, anhydrite cementation, dolomitization, and high Sr calcite cementation associated with celestite are all cogenetic products of such a brine-rock interaction. 18O of dolomite, calcite, and chert indicates apparent equilibrium relationships with the same solution. Temperatures have never exceeded 40 to 45° C, and imply the 18O of such a solution to be about 3. The somewhat depleted 18 UP>O composition of the proposed halite-saturated brine resulted from the reversal in the dependence of 18O on increased evaporation in highly saline brines. The proposed diagenetic model implies that, following early diagenetic processes, the rock sequence remained as a closed, but compacting, system unchanged by later fluid introduction. Low Sr values in dolomite are indicative of recrystallization and not fresh-water diagenesis.

Mediterranean Ridge: a young submerged chain associated with the Hellenic arc

The source of salts in the Ca-chloridic, hypersaline brines (up to 190 g Cl L−1) occurring in crystalline basement rocks in the Canadian, Fennoscandian and Bohemian Shields and their evolution have been investigated and reported. The Cl-Br-Na relationship indicates that these waters have been concentrated from seawater, by freezing during glacial times. The Na/Cl ratio (0.25 to 0.35) in the more saline fluids is compatible with cooling down to −30°C, where the most saline waters have been concentrated by a factor of 25 to 30 relative to the parent seawater.The brines formed from seawater within cryogenic troughs, along the subarctic continental margins, around ice sheets. The depressions within which the brines formed are the cryogenic analogues of the classic, evaporitic lagoon. One million years suffice to saturate with brine a 2000km-radius by 1km-depth rock volume at an H2O removal rate of only 2.8 mm/yr. Density-induced brine migration on a continental scale takes place via fissures below the ice.Our calculations, that were performed on a hypothetical ice sheet with dimensions compatible with the Laurentide ice sheet, demonstrate that during 1m.y., a 60m thick cryogenic sediment section could have formed. However, the precipitated minerals (mirabilite and hydrohalite) are repeatedly dispersed by the advance and retreat of the ice sheet, dissolved by melt water-seawater mixtures, and eroded during postglacial uplift, leaving almost no trace in the geological record.The cryogenic brines formed intermittently during and between glacial periods. The repeating advance and retreat of the ice sheets exerted a major control on the direction and intensity of brine flow. The cryogenic concentration of seawater and the migration of brine towards the center of the glaciostatic depression occurred mainly during the build up of the ice sheet, while reversal of the water flow from the center of the cryogenic basin outwards happened upon deglaciation. The flow of the waters in the subsurface was, inevitably, accompanied by significant dilution with melt water from the ice sheets.Using a “granitic” U concentration of 4 ppm and a (Ca-Mg mass balance based) rock/water ratio anywhere between 3.4 and 6.8 kg L−1, a few hundred thousand years of brine-rock interaction are sufficient for the growth of 129I in the most saline Canadian Shield brine to its present concentration (3.4×108 atoms 129I L−1). Hence, both the formation of the saline fluids and their emplacement in their present sites occurred most likely within the Pleistocene.The young age calculated for cryogenic brines in crystalline shields and the dynamic water flow therein should raise concern about the planning and construction of high-grade nuclear waste repositories in such rocks, which are already under way.