M. Borcsik

Dissolution kinetics of strained calcite

Abstract Interaface-limited dissolution of minerals occurs non-uniformly with preferential attack at sites of excess surface energy such as dislocations, edges, point defects, microfractures, etc. Strained crystals are predicted to show higher dissolution rates due to the increased internal energy associated with dislocations and due to enhanced nucleation of dissolution pits at dislocation outcrops on the surface. Using calcite strained to different degrees, we have observed a measurable rate enhancement of two to three times relative to unstrained crystals at temperatures from 3 to 80°C. This rate enhancement is large compared to that predicted from the calculated increase in crystal activity due to strain energy, but small compared to the three orders of magnitude difference in dislocation densities for the crystals tested (106–109 cm−2). Measurements over a range of pH (4.5–8.3) and temperature (3–80°C) showed that the rate enhancement increased with increasing pH and decreasing temperature. Calculations based on the excess free energy of screw dislocations suggest that dissolution rate enhancement should become significant above a critical defect density of roughly 107 cm−2, in apparent agreement with our observations. Crystal dissolution comprises several parallel processes operating in parallel at active sites. The small relative enhancement of dissolution rate with defect density reflects the greater quantity of dissolved material delivered to solution from receding edges and ledges relative to material coming from point defects and dislocations. Our data, coupled with existing information on other minerals, suggest that generally applicable kinetic measurements can be made on low-strain, macroscopic mineral specimens. However, kinetic data on highly strained minerals should include measurement of defect density because of the rate vs. strain correlation. Selective dissolution can be expected to occur in naturally-deformed rocks, where heterogeneity in dislocation distribution could cause solution transfer and deformation.

Adsorption of Co and selected actinides by Mn and Fe oxides in soils and sediments

Abstract Iron and Mn oxides and associated radionuclides in soils and sediments from the radioactive waste burial grounds at Oak Ridge National Laboratory have been selectively extracted using wet chemical techniques. Product-moment-correlation analyses have demonstrated that 60 Co and various actinides, principally 244 Cm, 241 Am and 238 Pu are dominantly associated with Mn oxides. Correlation coefficients between these radionuclides and Fe oxides and organic C are generally very low. The important role of Mn oxides in radionuclide adsorption is attributed to their unique surface and colloidal properties. The data illustrate the importance of the Mn oxide component of soils and sediments in controlling transition metal and actinide solubility. These results suggest two major implications for the disposal of radioactive waste. First, in order to minimize future 60 Co and actinide mobilization from disposal sites, a chemical environment in which Mn oxides are least soluble should be maintained. Second, the liberal use of Mn oxides in waste management operations might improve long-term retention of these radionuclides. Deep-sea Mn modules, which may in the future be mined for their trace metal contents, could serve as a ready supply of Mn oxide for waste disposal applications.

Dating ultra-deep mine waters with noble gases and 36Cl, Witwatersrand Basin, South Africa

Abstract Concentrations and isotopic ratios of dissolved noble gases, 36Cl, δD and δ18O in water samples from the ultra-deep gold mines (0.718 to 3.3 km below the surface) in the Witwatersrand Basin, South Africa, were investigated to quantify the dynamics of these ultra deep crustal fluids. The mining activity has a significant impact on the concentrations of dissolved gases, as the associated pressure release causes the degassing of the fissure water. The observed under saturation of the atmospheric noble gases in the fissure water samples (70–98%, normalized to ASW at 20°C and 1013 mbar) is reproduced by a model that considers diffusive degassing and solubility equilibration with a gas phase at sampling temperature. Corrections for degassing result in 4He concentrations as high as 1.55 · 10−1cm3STP4He g−1, 40Ar/36Ar ranging between 806 and 10331, and 134Xe/132Xe and 136Xe/132Xe ratios above 0.46 and 0.44, respectively. Corrected 134(136)Xe/132Xe and 134(136)Xe/4He-ratios are consistent with their production ratios, whereas the nucleogenic 4He/40Ar, and 134(136)Xe/40Ar ratios generally indicate that these gases are produced in an environment with an average [U + Th]/K-content 2–3 times above that of crustal average. In two scenarios, one considering only accumulation of in situ produced noble gases, the other additionally crustal flux components, the model ages for 14 individual water samples range from 13 to 168 Ma and from 1 to 23 Ma, respectively. The low 36Cl-ratios of (4–37) · 10−15 and comparatively high 36Cl-concentrations of (8–350) · 10−15 atoms 36Cl l−1 reflect subsurface production in secular equilibrium indicating an age in excess of 1.5 Ma or 5 times the half-life of 36Cl. In combination, the results suggest residence times of the fluids in fissures in this region (up to 3.3 km depth) are of the order of 1–100 Ma. We cannot exclude the possibility of mixing and that small quantities of younger water have been mixed with the very old bulk.

Solubility of the buffer assemblage pyrite + pyrrhotite + magnetite in NaCl solutions from 200 to 350°C

Abstract In the design of hydrothermal solubility studies it is important that the system be completely defined chemically. If the solubilities of minerals containing m metallic elements are to be determined in hydrothermal NaCl solutions, the phase rule requires that a total of m + 6 independent intensive parameters be controlled or measured in order to determine completely the system. In this study the solubility of the univariant assemblage pyrite + pyrrhotite + magnetite has been determined in vapor saturated hydrothermal solutions from 200 to 350°C for NaCl concentrations ranging from 0.0 to 5.0 molal. At any temperature, oxygen and sulfur fugacities were buffered by the chosen assemblage. System pH was determined from excess CO 2 partial pressures and computed ionic equilibria. Equilibrium constants were calculated by regression analysis of solubility data. The results show that more than 10 ppm of each mineral can dissolve in typical hydrothermal solutions under geologically realistic conditions. Solubilities were best represented by the species Fe 2+ and FeCl + at 200 and 250°C; Fe 2+ , FeCl + and FeCl 2 0 at 300°C; and Fe 2+ and FeCl 2 0 at 350°C. Ore deposition would occur by lowering temperature, diluting chloride concentration, or by raising pH through wall rock alteration reactions.

The origin and age of biogeochemical trends in deep fracture water of the Witwatersrand Basin, South Africa

Water residing within crustal fractures encountered during mining at depths greater than 500 meters in the Witwatersrand basin of South Africa represents a mixture of paleo-meteoric water and 2.0–2.3 Ga hydrothermal fluid. The hydrothermal fluid is highly saline, contains abiogenic CH 4 and hydrocarbon, occasionally N 2 , originally formed at ∼ 250–300°C and during cooling isotopically exchanged O and H with minerals and accrued H 2 , 4 He and other radiogenic gases. The paleo-meteoric water ranges in age from ∼ 10 Ka to > 1.5 Ma, is of low salinity, falls along the global meteoric water line (GMWL) and is CO 2 and atmospheric noble gas-rich. The hydrothermal fluid, which should be completely sterile, has probably been mixing with paleo-meteoric water for at least the past ∼100 Myr, a process which inoculates previously sterile environments at depths > 2.0 to 2.5 km. Free energy flux calculations suggest that sulfate reduction is the dominant electron acceptor microbial process for the high salinity fracture water and that it is 10 7 times that normally required for cell maintenance in lab cultures. Flux calculations also indicate that the potential bioavailable chemical energy increases with salinity, but because the fluence of bioavailable C, N and P also increase with salinity, the environment remains energy-limited. The 4 He concentrations and theoretical calculations indicate that the H 2 that is sustaining the subsurface microbial communities (e.g. H 2 -utilizing SRB and methanogens) is produced by water radiolysis at a rate of ∼1 nM yr −1 . Microbial CH 4 mixes with abiogenic CH 4 to produce the observed isotopic signatures and indicates that the rate of methanogenesis diminishes with depth from ∼ 100 at < 1 kmbls, to < 0.01 nM yr −1 at > 3 kmbls. Microbial Fe(III) reduction is limited due to the elevated pH. The δ13C of dissolved inorganic carbon is consistent with heterotrophy rather than autotrophy dominating the deeper, more saline environments. One potential source of the organic carbon may be microfilms present on the mineral surfaces.

Hydrogeochemistry of the New Jersey Coastal Plain: 2. Transport and deposition of iron, aluminum, dissolved organic matter, and selected trace elements in stream, ground- and estuary water☆

The geochemical controls on the transport and deposition of iron, organic carbon, and certain trace elements have been examined in bogs, streams, groundwater and estuaries of the New Jersey Coastal Plain. Surface waters are unusually dilute (total dissolved solids ∼ 25 ppm) and acidic (pH 4–5) and contain relatively high concentrations of organic C and Fe. Liquid chromatographic analyses show that from 10 to 70% of total “dissolved” Fe ( 0.2 μm) Fe-oxyhydroxide or -oxyhydroxides admixed with insoluble organic material. The proportion of organically-complexed Fe present varies seasonally with the DOC content of the water. These stream waters are near saturation with respect to amorphous Fe(OH)3; groundwaters are uniformly undersaturated with respect to amorphous Fe(OH3). Both stream and groundwaters are near saturation with respect to halloysite, slightly supersaturated with respect to gibbsite, and highly supersaturated with respect to kaolinite. Bog iron deposits are common in streams and swamps where Fe precipitation is catalyzed by iron-oxidizing bacteria. Fe and Al precipitate rapidly within both estuaries, with ionic strength rather than increased pH exerting the prime control on estuarine deposition. Estuarine floccules contain very little or no organic matter, suggesting that inorganic Fe- and Al-oxyhydroxides are the primary precipitates. The high contents of “dissolved” (< 0.2 μm) Fe and Al suggest that both are either organically complexed or in the form of small (< 0.2 μm) oxyhydroxide colloids in the estuaries.

Hydrogeochemistry of the New Jersey Coastal Plain: 1. Major-element cycles in precipitation and river water

Abstract The New Jersey Coastal Plain (Pine Barrens) is an extensive region where inorganic interactions between the soils and shallow groundwater are at a minimum. This study compares the geochemistry of precipitation and surface water to analyze transport mechanisms of major elements and to evaluate hydrogeochemical balances in this relatively unusual system. Precipitation composition is controlled by proximity to the ocean and seasonal variability of storm movements. Total dissolved solids in precipitation are high (11.5 ppm) but decrease inland; Na + and Cl − are more abundant in winter precipitation whereas K + , SO 2− 4 , Ca 2+ , and NO 3 − exhibit greater concentrations during the summer months. Na + and Cl − are derived from marine aerosols; SO 2− 4 is a product of SO 2 pollution in the atmosphere; other ions have significant contributions from continental aerosols. Dissolved solids in rivers are low (20 ppm) compared with average river water and pH is acidic (4.5). Both reflect paucity of mineral-water reactions in the uppermost aquifer (Cohansey Formation — quartz sand and gravel). However, regional variations in river-water geochemistry suggest cross-aquifer migration of deep groundwaters into the Mullica River basin. Mass balances confirm that river-water composition is controlled principally by precipitation. However, Ca 2+ , PO 3− 4 , and SO 2− 4 are depleted in river waters, probably as a result of removal by biota or cedar swamp processes. Mg 2+ shows an excess in the rivers, suggesting a contribution from deep groundwater. The poor chemical buffering in the waters of this region render them highly susceptible to contamination.

Solution-mineral equilibria in the system MgO-SiO2-H2O-MgCl2 at 500°C and 1 kbar

Abstract The quench pH of MgCl 2 solutions equilibrated at 500°C and 1 kbar pressure with talc + quartz is between 0.9 and 1.6, that of solutions equilibrated with forsterite + talc 4.2 to 7.7, and that of solutions equilibrated with brucite + forsterite between 6 and 9. The addition of NaCl and CaCl 2 does not affect the quench pH measurably. The low quench pH of solutions equilibrated with talc + quartz is due to the ionization of HCl O during the quench. The quench pH of solutions equilibrated with forsterite + talc is apparently controlled by the reaction of Mg-OH complexes with HCl O during cooling, that of brucite + forsterite solutions by the precipitation of magnesium hydroxychlorides and/or brucite during cooling. At 500°C and 1 kbar the solutions are neutral to slightly acid. The low quench pH of solutions equilibrated with talc + quartz shows that these can give rise to extensive proton metasomatism with wall rocks during cooling. This is not the case for solutions equilibrated at 500°C and 1 kbar with forsterite + talc or brucite + forsterite, and explains the lack of proton metasomatism around ultrabasic bodies. The SiO 2 concentration in solutions equilibrated with forsterite + talc is sufficient to lead to the precipitation of quartz on cooling. This may explain the occurrence of quartz with some ultramafic bodies. Mg 2 (OH) 3 Cl appears to be a stable phase in equilibrium with brucite + forsterite in runs containing more than 0.1 mole MgCl 2 /kg of solution. Much of the chlorine in altered ultramafic bodies may be present as a component of this and/or other magnesium hydroxychlorides.