Closed-system Conditions Research Articles

Stable-isotope compositions of lower Cambrian PbZnBa deposits and their host carbonates, Southwestern Sardinia, Italy

The major PbZnBa deposits exploited in Italy are in the lower Cambrian carbonates of southwestern Sardinia where the strata-bound ores occur as lenses in the Dolomia Rigata Member and in the cement or in the matrix of breccias in the C lcare Ceroide of the Gonnesa Formation. This study presents new isotope data both on the ore minerals (galena, sphalerite, pyrite, barite) and on the carbonate host rocks which outcrop in the Iglesiente district. The isotopic compositions of barite and sulfide minerals are compatible with the origin of sulfur components from Cambrian seawater sulfate. Bacterial sulfate reduction is interpreted to have been the mechanism responsible for sulfide formation and would have occurred mostly during earliest diagenesis in closed-system conditions. The isotopic compositions of the Dolomia Rigata are characteristic of early-diagenetic intertidal dolomites, partly affected by later recrystallization. The isotopic compositions of the Calcare Ceroide indicate that two major diagenetic events have affected the limestones. Early diagenesis was induced by massive influence of fresh waters in the eastern areas of the platform. Late diagenesis, on the contrary, produced by the progressive burial of the series, was active on the whole platform.

Isotopic fractionation of Kr and Xe implanted in solids at very low energies

We report results on the implantation of Kr and Xe in W under closed system conditions at very low energies (50–500 eV). Investigation of the fraction of gas trapped as a function of time reveals the existence of competing trapping and release mechanisms and analysis of recovered trapped gas and residual gas phases shows that both elemental and isotopic fractionation result from these mechanisms. We determined the mass dependence for the overall implantation process to be at or near m 1, with heavier isotopes enriched in the implanted gas. This mass dependence is inferred to result from implantation and a combination of diffusive and gas sputtering release mechanisms. Our results reaffirm the conclusion of Bernatowicz and Fahey (1986) that previously observed isotopic fractionation of Kr and Xe in carbonaceous material synthesized in electrical discharges owes its origin to low energy ion implantation and also suggest that this process may be relevant to incorporation of noble gases in early solar system materials. We also discuss the implication of our results for noble gas mass spectrometry.

Hydrochemistry, origin and evolution of sedimentary subsurface fluids: III. Early diagenetic mineralization reactions in high sedimentation—rate basins

The early diagenetic evolution of pore-water chemistry is closely linked to mineralization reactions which consume significant portions of the metabolites released by bacterial organic matter decomposition. These reactions are most intense in high-sedimentation rate basins and include the precipitation of iron-sulfides and various carbonates leading to concretion growth. Early diagenetic pyrite is typically framboidal attesting to its recrystallization from precursor mackinawite, greigite or amorphous FeS which are the favored phases at high supersaturation levels during the initial sulfate reduction stages. The sulfur isotopic composition of early diagnetic pyrite can be used to differentiate diffusion-controlled, open-system conditions with isotopically light sulfide (δ 34S = − 35 to − 20‰) from closed system conditions, under which Raleigh distillation produces increasingly heaver sulfide (δ 34 S = − 35 to + 18‰). Alabandite (Mn-sulfide) is a rare authigenic sulfide in Mn-rich environments such as certain restricted, semi-stagnant basins (Baltic Sea). pH-buffering by hydrogen sulfide and hydrogen ion uptake by the reduction of manganese and iron oxides and hydroxides in the nitrate and sulfate reduction zones raise the pH sufficiently to cause supersaturation of the porewaters with respect to Ca-, Mg-, Fe- and Mn-carbonates and complex solid solutions of them. Fe-carbonates cannot form in the sulfate reduction zone in the presence of dissolved sulfide which competes for the dissolved iron. Likewise, dolomite formation appears to be inhibited or slowed down in the presence of substantial dissolved sulfate. The appearance of siderite and ankerite therefore signals carbonate precipitation below the sulfate reduction zone. Supporting evidence for the early diagenetic origin of many carbonate concertions is provided by their high carbonate contents (70 to 90% reflecting the porosity existing at the time of precipitation, called “minus-cement porosity”), isotopic composition, clay fabrics, and preservation of original bedding features including the shapes of fossils and fecal pellets. In these environments increasing carbon isotope ratios (δ 13 C = − 20 to + 15‰) indicate concretion growth below the sulfate reduction zone, i.e., in the methane generation zones. Continuing concretion growth at greater burial depth explains pore water profiles with constantly low Ca and downward decreasing Mg concentrations. Dissolved ammonia and phosphate profiles reguire adsorption and ion-exchange reactions as additional removal machanisms (besides apatite precipitation) in order to explain their downward decrease after they have reached maximum concentrations below the alkalinity maximum. Classification of early diagnetic environments into oxic and anoxic and further subdivision of the latter into sulfidic and non-sulfidic (with suboxic or post-oxic and methanic as further subcategories of the non-sulfidic environment) according to Berner (1981) is preferred over the previous classification in terms of pH/Eh fields. The temperature range of the early diagenetic stage extends from O to about 75°C, at which temperature thermocatalytic organic matter decomposition replaces the earlier bacterially mediated reactions and causes a whole set of new diagenetic reactions (such as feldspar dissolution, smectite to illite transformation) to start.

The precambrian continental crust: Its structure, composition and evolution as revealed by deep drilling in the U.S.S.R.

Abstract Petrologic-geochemical studies of the Kola superdeep and other deep borehole sections drilled in the Precambrian basement of the East-European Platform are discussed. Proterozoic metasediments and metavolcanic rocks that constitute the Pechenga palaeorift and the underlying Archaean granite—gneiss basement were detected by the Kola borehole in the 0–6842 m and 6842–12064 m intervals, respectively. Zones of density decrease by hydrofracture, resulting from progressive metamorphism under closed system conditions, were identified in the upper crust at a 4500–9000 m depth. Sharp textural variations of rocks, an increase in porosity and the presence of mineralizing fluids are specific features of these zones which may have contributed to tectonic deformation and metamorphically produced ore formation. Deep boreholes drilled into the more eroded Archaean basement of the Tatar Dome provided information that was used for constructing a petrologic-geochemical model of the lower crust to a depth of 25–30 km. Adacitic—trondhjemitic composition for the primeval continental crust was calculated by extrapolation of geochemical trends established for the lower crust. We propose that the lower crust formed as a result of an initial melting of the Earth's chondrite shell and only existed in pre-Archaean times. We also present data on temperatures and distribution of radiogenic elements in the Kola and some other borehole sections. A heat-production model is proposed for the Archean crust. According to the model, the crustal contribution to the heat flow is 25 mW · m −2 , or 52% of the total heat flow, and that of the mantle is 22–24 mW · m −2 , or 48% of the total flow.

Isotope geochemistry of frost-blister ice, North Fork Pass, Yukon, Canada

Oxygen-18 and deuterium abundances vary with depth in ice from a frost blister at a spring site near North Fork Pass, Yukon. They indicate that the ice was formed by continuous downward freezing under nearly ideal, closed-system conditions in a water-filled cavity. Weighted mean values of δ18O and δ2H for the ice samples indicate that the frost–blister ice was formed from groundwater similar to that of nearby springs. The slope of the regression line (5.1) for the 18O and 2H data of the frost–blister ice suggests that nonequilibrium fractionation conditions existed during freezing. Tritium concentrations in the ice show irregular variations with depth; the weighted mean 3H concentration for the ice is, however, similar to that of the spring water.

Variations in the carbon isotope composition and production yield of various pyrolysis products under open and closed system conditions

Production rates and carbon isotopic compositions of various pyrolysis products were investigated for three sediments from the Williston Basin under open and closed pyrolysis conditions in the temperature range of 300–600°C. Both parameters do not show any significant differences for kerogens and carbon dioxides with the analytical procedure. Contrary to open system pyrolysis, however, decreasing yields of pyrolysates and higher amounts of gaseous hydrocarbons (C 2–4 compounds) at temperatures of 500 and 600°C, point to their decomposition to give ultimately methane. Moreover, these pyrolysis products display distinct carbon isotopic variations under open and closed pyrolysis conditions. They are due to a kinetic isotope effect, i.e. the preferential cleavage of 12C- 12C over 13C- 12C bonds, but the extent of the shift in isotopic composition seems to depend primarily on the reservoir size and the type of source material.

Interstitial cristobalite-type compounds (Na 2O) ≤0.33Na[AlSiO 4

The series of interstitial compounds (Na 2O) ≤0.33Na[AlSiO 4] exhibits the cubic symmetry of the cristobalite type Na[AlSiO 4]-carnegieite host lattice as concentrations of 0.01–0.33 Na 2O are intercalated at elevated temperatures under closed system conditions (950–1100 K). The tetrahedral framework structure of the host lattice expands correspondingly from a = 7.3001(5) to 7.3029(1)Å. The structure model of the end member, (Na 2O) 0.33Na[AlSiO 4), proposed from structure factor calculations based on X-ray powder diffraction data, shows unique (Na,O)(Na,O) distances in the high-temperature form of 3.17 Å. A total of 6 2 3 sodium atoms and 1 1 3 oxygens statistically occupying the cage like 12-fold oxygen-coordinated position of space group F 43m form a diamond-type sublattice in the high-cristobalite-type “host lattice” [AlSiO 4] 1− 3 d∝ . Superstructure characteristics on X-ray Guinier powder diffraction data indicate complex ordering mechanisms following long-term annealing (200 hr, 370–970 K). At temperatures &2;1350 K in an open system, intercalated Na 2O is released with the subsequent collapse of the Na[AlSiO 4] host lattice from cubic to triclinic, the regular symmetry of Na[AlSiO 4]-carnegieite at temperatures <960 K.

The kinetics of calcite dissolution and precipitation in geologically relevant situations of karst areas: 2. Closed system

A general theory of dissolution and precipitation rates of calcite in the pure CaCO 3CO 2H 2O system under closed-system conditions is presented. The three rate-limiting processes, surface-controlled dissolution or precipitation at the CaCO 3 surface, diffusion of the molecular and ionic species, and slow conversion of CO 2 into HCO − 3 are treated simultaneously. Dissolution and precipitation rates are calculated as a function of the Ca 2+ concentration in the solution at various temperatures T and partial pressures of CO 2, P CO 2, in the solution. The rates can in all cases be approximated by a linear dependence R = α([Ca 2+] eq − [Ca 2+]), where α = α( T, P CO 2, δ) and 2δ is the thickness of the water film enclosed by two calcite surfaces. Values of α are summarized in two tables which provide the geologist with data from which dissolution rates of pure limestone can be derived easily. The calculated rates for turbulent flow are one order of magnitude higher than those for laminar flow. This provides the first theoretical explanation of the so-called hydraulic jump. We have carried out dissolution experiments to determine the time dependence of the Ca 2+ concentration in stagnant and turbulently stirred H 2O films at given P CO 2, δ and T. From the time constants of the exponential behaviour of [Ca 2+]( t) the values of α have been determined. They are in good agreement with the theory.

Phase relations and adiabats in boiling seafloor geothermal systems

Observations of large salinity variations and vent temperatures in the range of 380–400°C suggest that boiling or two-phase separation may be occurring in some seafloor geothermal systems. Consideration of flow rates and the relatively small differences in density between vapors and liquids at the supercritical pressures at depth in these systems suggests that boiling is occurring under closed-system conditions. Salinity and temperature of boiling vents can be used to estimate the pressure-temperature point in the subsurface at which liquid seawater first reached the two-phase boundary. Data are reviewed to construct phase diagrams of coexisting brines and vapors in the two-phase region at pressures corresponding to those of the seafloor geothermal systems. A method is developed for calculating the enthalpy and entropy of the coexisting mixtures, and results are used to construct adiabats from the seafloor to the P-T two-phase boundary. Results for seafloor vents discharging at 2300 m below sea level indicate that a 385°C vent is composed of a brine (7% NaCl equivalent) in equilibrium with a vapor (0.1% NaCl). Brine constitutes 45% by weight of the mixture, and the fluid first boiled at approximately 1 km below the seafloor at 415°C, 330 bar. A 400°C vent is primarily vapor (88 wt.%, 0.044% NaCl) with a small amount of brine (26% NaCl) and first boiled at 2.9 km below the seafloor at 500°C, 520 bar. These results show that adiabatic decompression in the two-phase region results in dramatic cooling of the fluid mixture when there is a large fraction of vapor.

Surface oxidation-reduction kinetics associated with experimental basalt-water reaction at 25°C

Distributions of Fe(II) and Fe(III) during basalt-water interaction at 25°C were experimentally investigated under open- and closed-system conditions relative to O 2 and CO 2. After reaction, X-ray photoelectron spectroscopy analyses detected oxidized Fe on the surface of basalt, the concentration of which decreased as a function of reaction pH. Concurrent increases in Fe(II) and decreases in Fe(III) in solutions at pH's < 5 indicated continued surface oxidation by the reaction: (Fe(II) 1 z M z) basalt + Fe(III) solution→Fe(III) basalt + Fe(II) solution + 1 z M z solution where the electrical charge in solution is balanced by dissolution of a cation of charge z from the basalt. At neutral to basic pH, Fe(II) is oxidized to Fe(III) and precipitated as ferric oxyhydroxide in the presence of O 2. Fe(II) is also strongly sorbed on the basalt surface, resulting in low aqueous concentrations even under anoxic conditions. The rate of O 2 uptake increased with decreasing pH. Diffusion coefficients of the order 10 −14 cm 2 s −1, calculated using a one-dimensional diffusion model, suggest grain boundary diffusion in Fe-oxides.

Degradation studies of some polyesters and polycarbonates—2. Polylactide: Degradation under isothermal conditions, thermal degradation mechanism and photolysis of the polymer

The thermal degradation of polylactide has been studied at several temperatures in the range 230–440°C and the variation of product distribution with temperature has been examined. From experiments at 240–270°C, an energy of activation of 28·5 kcal mol −1 (119 kJ mol −1) has been calculated. Mass spectra have been obtained for polylactide and for the cold ring fraction of degradation products in TV A experiments. Both lactide and polylactide have also been heated under closed system conditions and the products have been identified. A mechanism is presented for the thermal degradation, based upon a hydroxyl end-initiated ester interchange process giving cyclic oligomers, lactide, acetaldehyde and carbon monoxide, together with a series of reactions at somewhat higher temperatures dependent upon chain homolysis, giving the same products and also carbon dioxide and methylketene. The photolysis of polylactide at 30°C, using the medium pressure mercury lamp, has been briefly examined.

Model considerations for the evolution of carbonate groundwaters in the Schwäbische Alb (Federal Republic of Germany)

The evolution of carbonate groundwaters may occur in a so-called open or closed system or under conditions between the extremes. In order to distinguish between the cases the chemistry of the water and the carbon isotope composition of the dissolved carbonate, the carbonate rock and the soil CO 2 of the recharge area must be known. Model calculations based on measurements of the above parameters show that ground-waters in the “Schwäbische Alb”, Federal Republic of Germany, are evolved essentially under closed-system conditions. The average measured carbon isotope composition of the water is δ 13C = −13.7% 0 vs. calculated values of δ 13C = −13.5% 0 and δ 13C = −18.5% 0 assuming a closed and an open system, respectively. In model calculations for closed-system conditions the origin of sulfate ions has to be considered.

Chemical Models of Solute Acquisition in Glacial Melt Waters

AbstractChemical models for the evolution of bulk melt waters can be constructed by assuming that rock mineral weathering and solute acquisition are controlled by carbonate equilibria. Different open- and closed- system models are defined by variations in the rates of weathering relative to the rate of hydrogen-ion supply by the dissolution and dissociation of CO2, and can be recognized by characteristic pH and ∑+relationships. The observed and inferred compositions of englacial and subglacial melt suggest that mixing is unlikely to be conservative and that closed-system conditions result where post-mixing weathering reactions occur. The final composition of bulk melt waters is determined by the mixing ratio between englacial and subglacial melt, the extent of post-mixing evolution, and opportunity for mixing with ground waters or re-equilibration with the atmosphere.

Chemical Models of Solute Acquisition in Glacial Melt Waters

AbstractChemical models for the evolution of bulk melt waters can be constructed by assuming that rock mineral weathering and solute acquisition are controlled by carbonate equilibria. Different open- and closed- system models are defined by variations in the rates of weathering relative to the rate of hydrogen-ion supply by the dissolution and dissociation of CO2, and can be recognized by characteristic pH and ∑+ relationships. The observed and inferred compositions of englacial and subglacial melt suggest that mixing is unlikely to be conservative and that closed-system conditions result where post-mixing weathering reactions occur. The final composition of bulk melt waters is determined by the mixing ratio between englacial and subglacial melt, the extent of post-mixing evolution, and opportunity for mixing with ground waters or re-equilibration with the atmosphere.

Dissolution of Technetium From Nuclear Waste Forms

ABSTRACTThe immobilization of 99Tc in both ceramic and borosilicate glass waste forms and the mechanism of dissolution of technetium from these waste forms are complicated by the range of technetium valence states. Metallic Tc,ionic Tc4+ oxides, and volatile molecular species containing Tc7+ all occur within the stability field of water. Tc4+ will substitute readily for Ti4+ in titanatebased ceramics in such phases as perovskite (CaTiO3) and spinel (Mg2TiO4). Tc4+ can be homogeneously distributed in borosilicate glass by re-melting the glass under closed system conditions to prevent the re-oxidation of technetium. The dissolution of technetium from both glass and ceramic waste forms follow similar kinetic behavior although the dissolution rate from the titanate is two orders of magnitude lower. Dissolution under highly reducing conditions is too slow to measure on the time scale of the experiments.

Geochemical Studies of Columbia River Basalts

ABSTRACTWe report our findings for two studies of Columbia River basalts that are relevant to United States radwaste disposal programs. A rhyodacite intrudes the Wanapum basalt and is exposed in a quarry along the Hood River, Oregon. Since the rhyodacite is different geochemically from the basalt, and was intruded at a high temperature, then this site can be used to evaluate the effects of the intrusion on the basalt. In such a study we assume the intrusive rock is a high temperature heat engine, and that cooling may be by convection, conduction, or both. Contact effects of the intrusion are not pronounced in the basalt, and are restricted to minor recrystallization within one meter of the contact, and to possibly some minor fracture filling. Traverses were made from the rhyodacite into the basalt, and samples at closely spaced intervals analyzed for K, U, Th by field and laboratory gamma spectrometry, and by neutron activation analysis for some 35 elements. Separate aliquots were analyzed by isotope dilution to inspect Sr isotopic variations. The chemical studies do not indicate migration of any elements from the rhyodacite into the basalt, nor for any in-basalt or in-rhyodacite migration. Despite a pronounced chemical gradient between the rocks, each has remained a closed, isochemical system. The Sr isotopic studies indicate that the rhyodacite possesses Sr (87/86) = 0.70407 ± 0.00008 while the intruded basalts possess Sr (87/86) = 0.7052 ± 0.0002. Even in samples of the contact, with intermingled rhyodacite and basalt, each rock preserves its own characteristic Sr (87/86) signature, thus again attesting to the closed system conditions. We conclude for this natural experiment that the Wanapum basalt is not readily penetrated by elements from a high temperature, molten source.

Geochemistry of the Dakota Formation of Northwestern New Mexico: Relevance to Radioactive Waste Studies

The Dakota Formation of the San Juan Basin in northwestern New Mexico consists predominantly of well-cemented sandstones and arenaceous mudstones. Clay mineral-rich rocks, derived from volcanic ash, are mapped as bentonites. The likely physical conditions during burial were temperatures between 35 to 60/sup 0/C and a pressure of about 0.5 kbar. X-ray studies reveal a mixture of montmorillonite, kaolinite, illite, and mixed layer clay minerals. The typical cation exchange capacities range from 20 to 40 meq/100 g for most samples. Radiometric age determinations of clay minerals by the K-Ar method yield 90 to 94 millions of years before present (MYBP) and Rb-Sr ages yield 93 + or - 8 MYBP. These dates agree with paleontological ages and indicate closed-system conditions for potassium, argon, rubidium, and strontium in these rocks. Closed-system conditions for cesium are inferred based on its greater retentivity than rubidium and potassium in clay-rich rocks. Neutron activation analysis (NAA) of the Dakota samples indicates normal lanthanide abundances and distribution in the bentonitic rocks; local lanthanide enrichment is noted where local uranium accumulations are noted. The uranium has been derived from several sources and fixed in the Dakota Formation at various times from roughly 60 to near 0.25 MYBP.more » The chalcophile elements copper, antimony, and lead are often fixed with uranium in organic-rich rocks and apparently have not migrated since fixation, even under oxidizing conditions. No mobilization for the lanthanides and barium is noted either. Collectively, the radiometric ages and NAA data indicate the bentonite and bentonite-sand mix to be suitable for overpack in radioactive waste repositories.« less

Toxic Effects From Phenolic Foam At Various Temperatures

A rising temperature program appears to be a rea sonably precise means of integrating the effects of successive fixed temperatures on animal survival times upon exposure to off-gases from phenolic foam under closed-system conditions.

Integration of Toxic Effects With Foam Insulation

A rising temperature program appears to be a rea sonably precise means of integrating the effects of successive fixed temperatures on animal survival times upon exposure to off-gases from foam insulation under closed-system conditions.

Hydrothermal interactions of basalts with Cs and Sr of spent fuel elements: Implications to basalt as a nuclear waste repository

The immobilization reactions of Cs, Sr, U and Mo from spent fuel elements (SFE) in the presence of basalts and basalt phases were investigated under hydrothermal, closed-system conditions. The exact Cs and Sr phases in SFE are not known with certainty, but Cs 2MoO 4, CsI, Cs 2O, β-Cs 2U 2O 7 and SrZrO 3 are some of the predicted possibilities. These phases were mixed with basalts and basalt phases along with water, sealed in gold capsules and reacted at 100, 200 and 300°C under a confining pressure of 300 bars. The Cs and Sr concentrations of the product solutions were measured to determine the partitioning of these elements between the liquid and solid phases. Results varied substantially, depending on the particular basalt or basalt phase or Cs phase, the temperature and the duration of run. For example, a Deep Drill Hole (DDH-3) basalt-Cs 2MoO 4 reactions resulted in 15, 78 and 99% fixation of added Cs at 100, 200 and 300°C respectively. Fixation of up to 99.7 and 99.8% of the added Cs and Sr respectively were observed in some cases. X-Ray diffraction (XRD) analysis of the solid products revealed that Cs was immobilized by the formation of pollucite, (Cs, Na)AlSi 2O 6 in most cases and the formation of CsAlSiO 4 in some cases. The interactions of basalts and basalt phases with Cs 2MoO 4 also resulted in the immobilization of Mo by the formation of powellite, CaMoO 4 because Ca is available in the reaction mixtures. The U 6+ in β-Cs 2U 2O 7 was reduced to form uraninite, UO 2 by the divalent iron component of basalts. The exact nature of Sr reaction with basalts is not clear because no new Sr phases could be detected by XRD. The reaction products, pollucite, CsAlSiO 4, powellite and uraninite from SFE-basalt interactions serve to immobilize Cs, Mo and U because Cs in pollucite and CsAlSiO 4 is difficult to exchange and Mo and U in powellite and uraninite are quite insoluble.