David R. Janecky

Carbon dioxide reaction processes in a model brine aquifer at 200 °C and 200 bars: implications for geologic sequestration of carbon

The reactive behavior of supercritical CO2 under conditions relevant to geologic storage and sequestration of C is largely unknown. Experiments were conducted in a flexible cell hydrothermal apparatus to determine the extent of fluid–rock reactions, in addition to carbonate mineral precipitation, that may occur in a brine aquifer–aquitard system that simulates a saline aquifer storage scenario. The system was held at 200 C and 200 bars for 59 days (1413 h) to approach steady state, then injected with CO2 and allowed to react for another 80 days (1924 h). In addition to magnesite precipitation, silicate minerals (quartz, plagioclase, microcline and biotite) in the aquifer and the aquitard display textures (etch pits, mineralization) indicating significant reaction. Changes in elemental abundances in the brine following addition of CO2 include pH decrease and enrichment in Cl, partly due to supercritical CO2 desiccation of the brine. Geologic sequestration systems have potential for geochemical reactions that extend beyond simple aqueous dissolution of CO2 and precipitation of carbonate. These reactions may produce geochemical and geotechnical consequences for sequestration and provide important characteristics for monitoring and evaluation of stored CO2 .A n understanding of multi-phase equilibrium relationships between supercritical CO2 and aquifer–brine systems also raises new questions for a variety of geologic systems. Multi-phase fluid equilibria may, for example, account for the large amounts and heterogeneous distributions of calcite cement in a wide variety of geologic systems, particularly in sedimentary basin sandstones. Published by Elsevier Science Ltd.

Th and U isotopic systematics of basalts from the Juan de Fuca and Gorda Ridges by mass spectrometry

We have developed a mass spectrometric technique for the measurement of 230Th/232Th ratios in young volcanic rocks. We show that we can measure 230Th/232Th ratios on MORB samples of ∼- 1 gram with an accuracy and reproducibility of 0.5–1.0%. This represents an improvement of at least a factor of 5–10 in sample size and precision over conventional alpha spectrometry methods. Using this technique, we have measured distinct excesses of230Th activity relative to238U activity for axial samples from the Juan de Fuca (JDF) and Gorda Ridges. These enrichments are 13–15% but range up to 40% in one sample. Low boron concentrations and 234U/238U ratios corresponding to secular equilibrium verify the absence of detectable seawater contamination. From this we infer that primary magmatic processes are the source of the measured230Th activity excesses, and that Th is indeed more incompatible than U during partial melting of MORB sources. 230Th/232Th activity ratios for the axial samples from JDF appear to be relatively high for uncontaminated MORB, ranging from 1.35 to 1.41. From this we infer that the JDF basalts formed from a uniform source highly depleted in Th relative to U (Th/Uwt= 2.15–2.25). One Gorda axial sample has a somewhat lower 230Th/232Th activity ratio of 1.31, suggesting formation from a source only slightly less depleted (Th/U= 2.32 ± 0.02). Based on the small range in 30Th/232Th ratios for axial basalts from JDF, dating off-axis JDF basalts may be feasible.

Geochronology and petrogenesis of MORB from the Juan de Fuca and Gorda ridges by 238U– 230Th disequilibrium

A highly precise mass spectrometric method of analysis was used to determine238U—234U—230Th—232Th in axial and off-axis basalt glasses from Juan de Fuca (JDF) and Gorda ridges. Initial230Th activity excesses in the axial samples range from 3 to 38%, but generally lie within a narrow range of 12 to 15%. Secondary alteration effects were evaluated usingδ234U and appear to be negligible; hence the230Th excesses are magmatic in origin. Direct dating of MORB was accomplished by measuring the decrease in excess230Th in off-axis samples.238U—230Th ages progressively increase with distance from axis. Uncertainties in age range from 10 to 25 ka for U—Th ages of 50 to 200 ka. The full spreading rate based on U—Th ages for Endeavour segment of JDF is 5.9 ± 1/2 cm/yr, with asymmetry in spreading between the Pacific (4.0 ± 0.6 cm/yr) and JDF (1.9 ± 0.6 cm/yr) plates. For northern Gorda ridge, the half spreading rate for the JDF plate is found to be 3.0 ± 0.4 cm/yr. These rates are in agreement with paleomagnetic spreading rates and topographic constraints. This suggests that assumptions used to determine ages, including constancy of initial 230Th/232Th ratio over time, are generally valid for the areas studied. Samples located near the axis of spreading are typically younger than predicted by these spreading rates, which most likely reflects recent volcanism within a 1–3 km wide zone of crustal accretion. Initial230Th/232Th ratios and230Th activity were also used to examine the recent Th/U evolution and extent of melting of mantle sources beneath these ridges. A negative anomaly in 230Th/232Th for Axial seamount lavas provides the first geochemical evidence of a mantle plume source for Axial seamount and the Cobb-Eickelberg seamount chain and indicates recent depletion of other JDF segment sources. Large230Th activity excesses for lavas from northern Gorda ridge and Endeavour segment indicate formation from a lower degree of partial melting than other segments. An inverse correlation between230Th excess and 230Th/232Th for each ridge indicates that these lower degree melts formed from slightly less depleted sources than higher degree melts. Uniformity in230Th excess for other segments suggests similarity in processes of melt formation and mixing beneath most of the JDF-Gorda ridge area. The average initial230Th/232Th activity ratio of 1.31 for the JDF-Gorda ridge area is in agreement with the predicted value of 1.32 from the Th—Sr isotope mantle array.

Plagioclase and epidote buffering of cation ratios in mid-ocean ridge hydrothermal fluids: Experimental results in and near the supercritical region

Abstract Experiments have been performed with Na-Ca-K-Cl fluids of seawater chlorinity and diabase, basalt, and plagioclase bearing mineral mixtures at 350–425 °C and 250–400 bars to help constrain hydrothermal alteration processes at mid-ocean ridges. Dissolved Ca, Na, and pH for all experiments responded systematically to differences in dissolved SiO2 concentrations and the compositions of plagioclase reactants. Diabase alteration at low fluid/rock mass ratios (0.5 to 1) produces fluids undersaturated with respect to quartz during hydration of primary olivine and orthopyroxene, whereas basalt alteration under similar conditions yields fluids slightly supersaturated with respect to quartz during breakdown of glass to smectite and amphibole. Fluid chemistry in all experiments appears to approach a partial equilibrium state with the albite and anorthite components in plagioclase and approaches a pH consistent with plagioclase alteration to epidote. Trace element data from vent fluids, specifically B and Sr, together with major element chemistry (Ca, Na, SiO2, pH), provides evidence that the reaction zone for “black-smoker” fluids at mid-ocean ridges is composed of only slightly altered diabase and is characterized by small amounts of epidote, nearly fresh plagioclase and clinopyroxene, and partially to completely hydrated olivine and orthopyroxene. Fluids reacting with this rock may be undersaturated with respect to quartz so pressure estimates based on the quartz geobarometer should be regarded as minimums. Using equilibrium between plagioclase, the dominant reactant, and epidote, the dominant reaction product in experiments, we estimate that temperatures in reaction zones are in excess of 375°C for most vent systems. These temperatures are higher than measured vent temperatures, suggesting that hotspring fluids commonly loose heat during ascent to the seafloor.

Physical characteristics of the Endeavour Ridge hydrothermal plume during July 1988

Abstract We conducted CTD-transmissometer tows from 8 to 26 July, 1988 within 15 km of the central hydrothermal vent site ( ≈ 47°57′N, 129°06′W) on the Endeavour segment of Juan de Fuca Ridge. Anomalies of temperature, salinity and light attenuation reveal possible new vent sites 4 and 8 km northeast and 6 km south of the central vent site. As a result of widespread plume dispersion, background values of potential temperature, salinity and light attenuation below the 1900 m depth exceeded those for “pristine” ambient waters by 0.05°C, 0.05 psu and 0.03 m −1 , respectively. Maximum plume anomalies relative to the background waters were of the order of 0.10°C, 0.010 psu and 0.10 m −1 at core depths of 2000–2050 m. Heat and salt anomalies were detectable more than 5 km from the central vent site whereas light attenuation (particle) anomalies were confined to within 2.5 km of the vent site. Based on the background water property anomalies and moored current meter records, the mean (time-averaged) heat fluxes for the survey region were+2.3(±1.5) × 10 8 W in the along-ridge direction (20°T) and−7.7(±4.7) × 10 8 W in the cross-ridge direction (110°T). Mean along- and cross-ridge salt fluxes were+7(±5)and−25(±15)kg s −1 ; mean particle fluxes were+0.09(±0.06)and−0.29(±0.18)kg s −1 . Estimates of the instantaneous fluxes derived from coincident current and plume measurements indicate that heat fluxes from the central vent field may have been as high as1.2(±0.6)×10 10 W and corresponding particulate fluxes as high as6(±3)kg s −1 .

Neptunium and Plutonium Solubilities in a Yucca Mountain Groundwater

Solubilities of neptunium and plutonium were studied in J-13 groundwater (ionic strength of about 3.7 mmol; total dissolved carbonate of 2.8 mmol) from the proposed Yucca Mountain Nuclear Waste Repository site, Nevada, at three different temperatures (25, 60, and 90 °C) and pH values (6.0, 7.0, and 8.5). Experiments were performed from both over- and undersaturation at defined CO2 partial pressures. The solubility of 237Np from oversaturation ranged from a high of (9.40 ± 1.22) × 10-4 M at pH 6.0 and 60 °C to a low of (5.50 ± 1.97) × 10-6 M at pH 8.5 and 90 °C. The analytical results of solubility experiments from undersaturation (temperatures of 25 and 90 °C and pH values 6, 7, and 8.5) converged on these values. The 239/240Pu solubilities ranged from (4.70 ± 1.13) × 10-8 M at pH 6.0 and 25 °C to (3.62 ± 1.14) × 10-9 M at pH 8.5 and 90 °C. In general, both neptunium and plutonium solubilities decreased with increasing pH and temperature. Greenish-brown crystalline Np2O5·xH2O was identified as the solubil...

Speciation of aqueous palladium(II) chloride solutions using optical spectroscopies

Abstract Spectroscopic measurements of palladium(II) chloride solutions have been performed under ambient to elevated temperature conditions with systematic changes in pH and [Cl−1]. Spectral signatures from electronic absorption spectra (Uv/Vis) and Raman vibrational spectra were determined, and these were subsequently used to systematically map out species along several paths of the predominance diagram. The species PdCl x (H 2 O) 4−x 2−x (x = 2, 3, or 4) and Pd(OH)2 were observed, along with a precipitation product formed from > 10 μM [Pd2+]solutions under near-neutral conditions. The elemental composition of the precipitation product was examined with a scanning electron micrograph (SEM) and was found to contain chloride as well as palladium. While sub-stoichiometric amounts of available OH− ligand produced UV/ Vis solution spectra likely to be from mixed Pd-Cl-OH species, the pH field of these species would be vanishingly small at low, geologically relevant palladium ion concentrations where [OH−]is no longer the limiting reagent in the transformation. In mildly acidic media, where chloropalladium(II) species predominate, elevated temperatures (up to 90°C) cause lower charged palladium-chloride species to be favored, consistent with the lowered dielectric constant of water at higher temperatures.

A Raman spectroscopic study of arsenite and thioarsenite species in aqueous solution at 25 °C

The Raman spectra of thioarsenite and arsenite species in aqueous solution were obtained at room temperature. Solutions at constant ΣAs + ΣS of 0.1 and 0.5 mol kg-1 were prepared with various ΣS/ΣAs ratios (0.1–9.0) and pH values (~7–13.2). Our data suggest that the speciation of As under the conditions investigated is more complicated than previously thought. The Raman measurements offer evidence for at least six separate S-bearing As species whose principal bands are centered near 365, 385, 390, 400, 415 and 420 cm-1. The data suggest that at least two different species may give rise to bands at 385 cm-1, bringing the probable minimum number of species to seven. Several additional species are possible but could not be resolved definitively. In general, the relative proportions of these species are dependent on total As concentration, ΣS/ΣAs ratio and pH. At very low ΣS/ΣAs ratios we also observe Raman bands attributable to the dissociation products of H3AsO3(aq). Although we were unable to assign precise stoichiometries for the various thioarsenite species, we were able to map out general pH and ΣS/ΣAs conditions under which the various thioarsenite and arsenite species are predominant. This study provides a basis for more detailed Raman spectroscopic and other types of investigations of the nature of thioarsenite species.

Science-based cleanup of Rocky Flats

The chemical and physical interactions of radioactive compounds are key to understanding how they can contaminate the environment and, more importantly, how best to remove them.

Solubility and spectroscopic studies of the interaction of palladium with simple carboxylic acids and fulvic acid at low temperature

Abstract The interaction of Pd with some O-donor organic acid anions has been investigated using solubility measurements and a variety of spectroscopic techniques (UV-visible, Raman, FTIR, 13C NMR). Some of the ligands investigated (acetate, oxalate and fulvic acid) occur naturally in relatively high concentrations, whereas others (phthalate and salicylate) serve as models of potential binding sites on humic and fulvic acids. Solubility measurements show that the presence of acetate, phthalate, salicylate and fulvic acid (oxalate was not studied via solubility methods) can increase the mobility of Pd over various pH ranges, depending on the organic ligand. In the case of acetate, UV-visible and Raman spectroscopy (13C NMR results were inconclusive) provide strong evidence for the formation of electrostatically bound, possibly outer-sphere palladium acetate complexes. Oxalate was confirmed by UV-visible and FTIR spectroscopy to compete favorably with chloride (0.56 M NaCl) for Pd even at oxalate concentrations as low as 1 mM at pH = 6–7. Available data from the literature suggest that oxalate may have an influence on Pd mobility at free oxalate concentrations as low as 10−8–10−9 M. UV-visible spectroscopy provides evidence of an initially rapid, followed by a slower, reaction between PdCl42− and o-phthalate ion. Our findings lend support to the idea that similar binding sites on fulvic acid may be capable of complexing and solubilizing Pd in the natural environment. Although thermodynamic data are required to fully quantify the extent, it is concluded that simple carboxylic acid anions and/or fulvic and humic acids should be capable of significantly enhancing Pd transport in the surficial environment by forming truly dissolved complexes. On the other hand, flocculation of fulvic/humic acids, owing to changing ionic strengths or pH, or adsorption of these acids onto mineral surfaces, may also provide effective means of immobilizing Pd. These results have applications in exploration geochemistry and disposal of radioactive waste containing 107Pd.