Uranium diagenesis in sediments underlying bottom waters with high oxygen content

We measured U in sediments (both pore waters and solid phase) from three locations on the middle Atlantic Bight (MAB) from the eastern margin of the United States: a northern location on the continental shelf off Massachusetts (OC426, 75 m water depth), and two southern locations off North Carolina (EN433-1, 647 m water depth and EN433-2, 2648 m water depth). These sediments underlie high oxygen bottom waters (250-270 μM), but become reducing below the sediment-water interface due to the relatively high organic carbon oxidation rates in sediments (EN433-1: 212 μmol C/cm²/y; OC426: 120 ± 10 μmol C/cm²/y; EN433-2: 33 μmol C/cm²/y). Pore water oxygen goes to zero by 1.4-1.5 cm at EN433-1 and OC426 and slightly deeper oxygen penetration depths were measured at EN433-2 (∼4 cm).All of the pore water profiles show removal of U from pore waters. Calculated pore water fluxes are greatest at EN433-1 (0.66 ± 0.08 nmol/cm²/y) and less at EN433-2 and OC426 (0.24 ± 0.05 and 0.13 ± 0.05 nmol/cm²/y, respectively). Solid phase profiles show authigenic U enrichment in sediments from all three locations. The average authigenic U concentrations are greater at EN433-1 and OC426 (5.8 ± 0.7 nmol/g and 5.4 ± 0.2 nmol/g, respectively) relative to EN433-2 (4.1 ± 0.8 nmol/g). This progression is consistent with their relative ordering of ‘reduction intensity’, with greatest reducing conditions in sediments from EN433-1, less at OC426 and least at EN433-2. The authigenic U accumulation rate is largest at EN433-1 (0.47 ± 0.05 nmol/cm²/y), but the average among the three sites on the MAB is ∼0.2 nmol/cm²/y. Pore water profiles suggest diffusive fluxes across the sediment-water interface that are 1.4-1.7 times greater than authigenic accumulation rates at EN433-1 and EN433-2. These differences are consistent with oxidation and loss of U from the solid phase via irrigation and/or bioturbation, which may compromise the sequestration of U in continental margin sediments that underlie bottom waters with high oxygen concentrations.Previous literature compilations that include data exclusively from locations where [O₂]_bw < 150 μM suggest compelling correlations between authigenic U accumulation and organic carbon flux to sediments or organic carbon burial rate. Sediments that underlie waters with high [O₂]_bw have lower authigenic U accumulation rates than would be predicted from relationships developed from results that include locations where [O₂]_bw < 150 μM.     

Iron and manganese diagenesis in deep sea volcanogenic sediments and the origins of pore water colloids

Volcanogenic sediments are typically rich in Fe and Mn-bearing minerals that undergo substantial alteration during early marine diagenesis, however their impact on the global biogeochemical cycling of Fe and Mn has not been widely addressed. This study compares the near surface (0-20 cm below sea floor [cmbsf]) aqueous (<0.02 μm) and aqueous + colloidal here in after ‘dissolved’ (<0.2 μm) pore water Fe and Mn distributions, and ancillary O_2(aq), and solid-phase reactive Fe distributions, between two volcanogenic sediment settings: [1] a deep sea tephra-rich deposit neighbouring the volcanically active island of Montserrat and [2] mixed biosiliceous-volcanogenic sediments from abyssal depths near the volcanically inactive Crozet Islands archipelago. Shallow penetration of O_2(aq) into Montserrat sediments was observed (<1 cmbsf), and inferred to partially reflect oxidation of fine grained Fe(II) minerals, whereas penetration of O_2(aq) into abyssal Crozet sediments was >5 cmbsf and largely controlled by the oxidation of organic matter. Dissolved Fe and Mn distributions in Montserrat pore waters were lowest in the surface oxic-layer (0.3 μM Fe; 32 μM Mn), with maxima (20 μM Fe; 200 μM Mn) in the upper 1-15 cmbsf. Unlike Montserrat, Fe and Mn in Crozet pore waters were ubiquitously partitioned between 0.2 μm and 0.02 μm filtrations, indicating that the pore water distributions of Fe and Mn in the (traditionally termed) ‘dissolved’ size fraction are dominated by colloids, with respective mean abundances of 80% and 61%. Plausible mechanisms for the origin and composition of pore water colloids are discussed, and include prolonged exposure of Crozet surface sediments to early diagenesis compared to Montserrat, favouring nano-particulate goethite formation, and the elevated dissolved Si concentrations, which are shown to encourage fine-grained smectite formation. In addition, organic matter may stabilise authigenic Fe and Mn in the Crozet pore waters. We conclude that volcanogenic sediment diagenesis leads to a flux of colloidal material to the overlying bottom water, which may impact significantly on deep ocean biogeochemistry. Diffusive flux estimates from Montserrat suggest that diagenesis within tephra deposits of active island volcanism may also be an important source of dissolved Mn to the bottom waters, and therefore a source for the widespread hydrogenous MnO_x deposits found in the Caribbean region.     

Chemical weathering of a marine terrace chronosequence, Santa Cruz, California. Part II: Solute profiles, gradients and the comparisons of contemporary and long-term weathering rates

The spatial and temporal changes in hydrology and pore water elemental and ⁸⁷Sr/⁸⁶Sr compositions are used to determine contemporary weathering rates in a 65- to 226-kyr-old soil chronosequence formed from granitic sediments deposited on marine terraces along coastal California. Soil moisture, tension and saturation exhibit large seasonal variations in shallow soils in response to a Mediterranean climate. These climate effects are dampened in underlying argillic horizons that progressively developed in older soils, and reached steady-state conditions in unsaturated horizons extending to depths in excess of 15 m. Hydraulic fluxes (q_h), based on Cl mass balances, vary from 0.06 to 0.22 m yr⁻¹, resulting in fluid residence times in the terraces of 10-24 yrs.As expected for a coastal environment, the order of cation abundances in soil pore waters is comparable to sea water, i.e., Na > Mg > Ca > K > Sr, while the anion sequence Cl > NO₃ > HCO₃ > SO₄ reflects modifying effects of nutrient cycling in the grassland vegetation. Net Cl-corrected solute Na, K and Si increase with depth, denoting inputs from feldspar weathering. Solute ⁸⁷Sr/⁸⁶Sr ratios exhibit progressive mixing of sea water-dominated precipitation with inputs from less radiogenic plagioclase. While net Sr and Ca concentrations are anomalously high in shallow soils due to biological cycling, they decline with depth to low and/or negative net concentrations. Ca/Mg, Sr/Mg and ⁸⁷Sr/⁸⁶Sr solute and exchange ratios are similar in all the terraces, denoting active exchange equilibration with selectivities close to unity for both detrital smectite and secondary kaolinite. Large differences in the magnitudes of the pore waters and exchange reservoirs result in short-term buffering of the solute Ca, Sr, and Mg. Such buffering over geologic time scales can not be sustained due to declining inputs from residual plagioclase and smectite, implying periodic resetting of the exchange reservoir such as by past vegetational changes and/or climate.Pore waters approach thermodynamic saturation with respect to albite at depth in the younger terraces, indicating that weathering rates ultimately become transport-limited and dependent on hydrologic flux. Contemporary rates R_solute are estimated from linear Na and Si pore weathering gradients b_solute such that

     

Re-assessing the surface cycling of molybdenum and rhenium

We re-evaluate the cycling of molybdenum (Mo) and rhenium (Re) in the near-surface environment. World river average Mo and Re concentrations, initially based on a handful of rivers, are calculated using 38 rivers representing five continents, and 11 of 19 large-scale drainage regions. Our new river concentration estimates are 8.0 nmol kg⁻¹ (Mo), and 16.5 pmol kg⁻¹ (Re, natural + anthropogenic). The linear relationship of dissolved Re and in global rivers (R² = 0.76) indicates labile continental Re is predominantly hosted within sulfide minerals and reduced sediments; it also provides a means of correcting for the anthropogenic contribution of Re to world rivers using independent estimates of anthropogenic sulfate. Approximately 30% of Re in global rivers is anthropogenic, yielding a pre-anthropogenic world river average of 11.2 pmol Re kg⁻¹. The potential for anthropogenic contribution is also seen in the non-negligible Re concentrations in precipitation (0.03-5.9 pmol kg⁻¹), and the nmol kg⁻¹ level Re concentrations of mine waters. The linear Mo- relationship (R² = 0.69) indicates that the predominant source of Mo to rivers is the weathering of pyrite. An anthropogenic Mo correction was not done as anthropogenically-influenced samples do not display the unambiguous metal enrichment observed for Re. Metal concentrations in high temperature hydrothermal fluids from the Manus Basin indicate that calculated end-member fluids (i.e. Mg-free) yield negative Mo and Re concentrations, showing that Mo and Re can be removed more quickly than Mg during recharge. High temperature hydrothermal fluids are unimportant sinks relative to their river sources 0.4% (Mo), and 0.1% (pre-anthropogenic Re). We calculate new seawater response times of 4.4 × 10⁵ yr (τ_Mo) and 1.3 × 10⁵ yr (τ_Re, pre-anthropogenic).     

The behavior of molybdenum and its isotopes across the chemocline and in the sediments of sulfidic Lake Cadagno, Switzerland

Molybdenum (Mo) isotope studies in black shales can provide information about the redox evolution of the Earth’s oceans, provided the isotopic consequences of Mo burial into its major sinks are well understood. Previous applications of the Mo isotope paleo-ocean redox proxy assumed quantitative scavenging of Mo when buried into sulfidic sediments. This paper contains the first complete suite of Mo isotope fractionation observations in a sulfidic water column and sediment system, the meromictic Lake Cadagno, Switzerland, a small alpine lake with a pronounced oxygen-sulfide transition reaching up to H₂S ∼ 200 μM in the bottom waters (or about 300 μM total sulfide: ΣS²⁻ = H₂S + HS⁻ + S²⁻). We find that Mo behaves conservatively in the oxic zone and non-conservatively in the sulfidic zone, where dissolved Mo concentrations decrease from 14 nM to 2-8 nM across this transition. Dissolved Mo in the upper oxic waters has a δ⁹⁸Mo_oxic = 0.9 ± 0.1‰, which matches that of the riverine input, δ⁹⁸Mo_river = 0.9 ± 0.1‰. In the deeper sulfidic waters, a subaquatic source delivers Mo at 1.55 ± 0.1‰, but the dissolved Mo is even heavier at δ⁹⁸Mo_sulfidic = 1.8‰. Sediment traps in the sulfidic zone of the lake collect particles increasingly enriched in Mo with depth, with δ⁹⁸Mo values significantly fractionated at −0.8‰ to −1.2‰ both near the chemocline and in the deepest trap. Suspended particulates in the sulfidic waters carry lighter Mo than the ambient dissolved Mo pool by ∼0.3-1.5‰. Sedimentary Mo concentrations correlate with total organic carbon and yield Mo levels which are two orders of magnitude higher than typical crustal values found in rocks from the catchment area. Solid-phase Mo in the sediment shows a slightly positive δ⁹⁸Mo trend with depth, from δ⁹⁸Mo = 1.2‰ to 1.4‰ while the pore waters show dramatic enrichments of Mo (>2000 nM) with a relatively light isotope signature of δ⁹⁸Mo = 0.9-1.0‰.These data are explained if Mo is converted to particle-reactive oxythiomolybdates in the sulfidic waters and is fractionated during removal from solution onto particles. Isotope fractionation is expressed in the water column, despite the high sulfide concentrations, because the rate of Mo removal is fast compared to the slow reaction kinetics of thiomolybdate formation. However, elemental and isotopic mass balances show that Mo is indeed quantitatively removed to the lake sediments and thus the isotopic composition of the sediments reflects sources to the sulfidic water. This efficient Mo drawdown is expected to occur in settings where H₂S is very much in excess over Mo or in a restricted setting where the water renewal rate is slow compared to the Mo burial rate. We present a model for the Mo isotope fractionation in sulfidic systems associated with the slow reaction kinetics and conclude that quantitative removal will occur in highly sulfidic and restricted marine systems.     

The distribution of nitrate N/N in marine sediments and the impact of benthic nitrogen loss on the isotopic composition of oceanic nitrate

We report ¹⁵N/¹⁴N ratios of porewater nitrate in sediments from the Bering Sea basin, where microbial nitrate reduction has been identified as a significant sink for fixed nitrogen (N). Strong ¹⁵N enrichment in porewater nitrate is observed as one goes deeper in the sediments and nitrate concentration decreases (δ¹⁵N generally reaches 25-35‰). Analysis of profiles with a one-dimensional diffusion-reaction model yields organism-scale isotope effects for dissimilatory nitrate reduction (ε_cell) of 11‰ to 30‰, in the same range as measured in previous studies of cultures and the marine and lacustrine water column. Estimates of ε_cell, while uncertain, show a negative correlation with bottom water [O₂]; we propose that this relates to the at the depth of denitrification. The N isotope effect at the scale of nitrate sediment-water exchange (ε_app) is ∼0‰ in two unreactive deep sites and is typically <3‰ at more reactive sites at various depths. ε_app is much lower than ε_cell because nitrate consumption is nearly complete at the sediment depth of denitrification, minimizing the escape of ¹⁵N-enriched nitrate from the sediments. In reactive sediments, this is due to rapid denitrification, while in less reactive sediments, it is due to greater diffusive distances for nitrate to the depth of denitrification. The data suggest that low bottom water [O₂] tends to yield more complete expression of ε_cell at the sediment-water scale, due to higher at the depth of denitrification. While porewater ammonium-N isotopes were not measured, our porewater model suggests that, in sediments with high organic matter supply and/or low-[O₂] bottom waters, the efflux and subsequent oxidation of ammonium enriched in ¹⁵N by incomplete nitrification can significantly enhance the total net isotope effect of sedimentary N loss (ε_sed, equivalent to ε_app but including ammonium fluxes). Model analysis of representative sedimentary environments suggests a global mean ε_sed of ∼4‰ (∼2‰ if restricted to seafloor below 1 km depth).     

Thermodynamic properties of chlorite CCa-2. Heat capacities, heat contents and entropies

The heat capacities of the international reference clay mineral chlorite CCa-2 from Flagstaff Hill, California, were measured by low temperature adiabatic calorimetry and differential scanning calorimetry, from 5 to 520 K (at 1 bar). The studied chlorite is a Fe-bearing trioctahedral chlorite with an intermediary composition between ideal clinochlore (Si₃Al)(Mg₅Al)O₁₀(OH)₈ and chamosite (Si₃Al)(Fe₅Al)O₁₀(OH)₈. Only few TiO₂ impurities were detected in the natural chlorite sample CCa-2. Its structural formula, obtained after subtracting the remaining TiO₂ impurities, is (Si_2.633Al_1.367)(Al_1.116Mg_2.952Mn_0.012Ca_0.011)O₁₀(OH)₈. From the heat capacity results, the entropy, standard entropy of formation and heat content of the chlorite were deduced. At 298.15 K, the heat capacity of the chlorite is 547.02 (±0.27) J mol⁻¹ K⁻¹ and the molar entropy is 469.4 (±2.9) J mol⁻¹ K⁻¹. The standard molar entropy of formation of the clay mineral from the elements is −2169.4 (±4.0) J mol⁻¹ K⁻¹.     

Thermodynamic properties of anhydrous smectite MX-80, illite IMt-2 and mixed-layer illite-smectite ISCz-1 as determined by calorimetric methods. Part I: Heat capacities, heat contents and entropies

The heat capacities of the anhydrous international reference clay minerals, smectite MX-80, illite IMt-2 and mixed-layer illite-smectite ISCz-1, were measured by low temperature adiabatic calorimetry and differential scanning calorimetry, from 6 to 520 K (at 1 bar). The samples were chemically purified and Na-saturated. Dehydrated clay fractions <2 μm were studied. The structural formulae of the corresponding clay minerals, obtained after subtracting the remaining impurities, are K_0.026Na_0.435Ca_0.010(Si_3.612Al_0.388) (Al_1.593Mg_0.228Ti_0.011)O₁₀(OH)₂ for smectite MX-80, K_0.762Na_0.044(Si_3.387Al_0.613) (Al_1.427Mg_0.241O₁₀(OH)₂ for illite IMt-2 and K_0.530Na_0.135(Si_3.565Al_0.435)(Al_1.709Mg_0.218Ti_0.005)O₁₀(OH)₂for mixed-layer ISCz-1. From the heat capacity values, we determined the molar entropies, standard entropies of formation and heat contents of these minerals. The following values were obtained at 298.15 K and 1 bar:

	(J mol−1 K−1)	S0 (J mol−1 K−1)
Smectite MX-80	326.13 ± 0.10	280.56 ± 0.16
Illite IMt-2	328.21 ± 0.10	295.05 ± 0.17
Mixed-layer ISCz-1	320.79 ± 0.10	281.62 ± 0.15

Full-size table

     

The partitioning of molybdenum(VI) between aqueous liquid and vapour at temperatures up to 370 °C

We have conducted experiments to evaluate the vapour-liquid fractionation of Mo(VI) in the system MoO₃-NH₃-H₂O at 300-370 °C and saturated vapour pressure, using a two-chamber autoclave that allows separate trapping of the vapour and liquid. The measured total Mo concentrations in each phase were used to calculate a distribution coefficient, , which increases as the density of the vapour approaches that of the liquid, and is greater than one for pH ? 4. Molybdenum speciation in the vapour is described by a single complex, MoO₃H₂O. By contrast, thermodynamic modeling of the distribution of Mo species in the liquid indicates that bimolybdate (HMoO₄⁻) is the dominant aqueous species at the conditions of our experiments, and that molybdate (MoO₄²⁻) and molybdic acid (H₂MoO₄⁰) are present in smaller quantities. As vapour-liquid fractionation occurs between neutral species, it is governed by the reaction H₂MoO₄⁰(aq) = MoO₃ · H₂O(g). Fractionation is therefore controlled by the concentration of H₂MoO₄⁰ in the liquid, which increases with increasing temperature and decreasing pH. Owing to the pH dependence of , it cannot be used to describe Mo fractionation in aqueous vapour-liquid systems with compositions different than those of this study. We have therefore calculated a composition-independent (Henry’s Law) constant, , for each experimental point, using the measured total Mo concentration in the vapour and the modeled concentration of H₂MoO₄⁰ in the liquid. This constant may be applied to aqueous vapour-liquid systems of known liquid composition to estimate the concentration of Mo in a vapour for which little chemical information is available, and thereby supplement the available fractionation data for natural porphyry-forming systems. The results of this study demonstrate that at conditions typical of natural porphyry ore-forming systems, a significant amount of molybdenum fractionates into the vapour over the liquid, and the vapour may transport quantities of Mo in excess of that in the liquid at pH conditions below those of the muscovite-microcline reaction boundary.     

Seawater nutrient and carbonate ion concentrations recorded as P/Ca, Ba/Ca, and U/Ca in the deep-sea coral Desmophyllum dianthus

As paleoceanographic archives, deep sea coral skeletons offer the potential for high temporal resolution and precise absolute dating, but have not been fully investigated for geochemical reconstructions of past ocean conditions. Here we assess the utility of skeletal P/Ca, Ba/Ca and U/Ca in the deep sea coral D. dianthus as proxies of dissolved phosphate (remineralized at shallow depths), dissolved barium (trace element with silicate-type distribution) and carbonate ion concentrations, respectively. Measurements of these proxies in globally distributed D. dianthus specimens show clear dependence on corresponding seawater properties. Linear regression fits of mean coral Element/Ca ratios against seawater properties yield the equations: P/Ca_coral (μmol/mol) = (0.6 ± 0.1) P/Ca_sw(μmol/mol) - (23 ± 18), R² = 0.6, n = 16 and Ba/Ca_coral(μmol/mol) = (1.4 ± 0.3) Ba/Ca_sw(μmol/mol) + (0 ± 2), R² = 0.6, n = 17; no significant relationship is observed between the residuals of each regression and seawater temperature, salinity, pressure, pH or carbonate ion concentrations, suggesting that these variables were not significant secondary dependencies of these proxies. Four D. dianthus specimens growing at locations with Ω_arag ? 0.6 displayed markedly depleted P/Ca compared to the regression based on the remaining samples, a behavior attributed to an undersaturation effect. These corals were excluded from the calibration. Coral U/Ca correlates with seawater carbonate ion: U/Ca_coral(μmol/mol) = (−0.016 ± 0.003) (μmol/kg) + (3.2 ± 0.3), R² = 0.6, n = 17. The residuals of the U/Ca calibration are not significantly related to temperature, salinity, or pressure. Scatter about the linear calibration lines is attributed to imperfect spatial-temporal matches between the selected globally distributed specimens and available water column chemical data, and potentially to unresolved additional effects. The uncertainties of these initial proxy calibration regressions predict that dissolved phosphate could be reconstructed to ±0.4 μmol/kg (for 1.3-1.9 μmol/kg phosphate), and dissolved Ba to ±19 nmol/kg (for 41-82 nmol/kg Ba_sw). Carbonate ion concentration derived from U/Ca has an uncertainty of ±31μmol/kg (for ). The effect of microskeletal variability on P/Ca, Ba/Ca, and U/Ca was also assessed, with emphasis on centers of calcification, Fe-Mn phases, and external contaminants. Overall, the results show strong potential for reconstructing aspects of water mass mixing and biogeochemical processes in intermediate and deep waters using fossil deep-sea corals.     

Solvation processes in steam: Ab initio calculations of ion-solvent structures and clustering equilibria

Reports of the high ion content of steam and low-density supercritical fluids date back to the work of Carlon [Carlon H. R. (1980) Ion content of air humidified by boiling water.J. Appl.Phys.51, 171-173], who invoked ion and neutral-water clustering as mechanism to explain why ions partition into the low-density aqueous phase. Mass spectrometric, vibrational spectroscopic measurements and quantum chemical calculations have refined this concept by proposing strongly bound ion-solvent aggregates and water clusters such as Eigen- and Zundel-type proton clusters H₃O⁺·(H₂O)_m and the more weakly bound water oligomers (H₂O)_m. The extent to which these clusters affect fluid chemistry is determined by their abundance, however, little is known regarding the stability of such moieties in natural low-density high-temperature fluids. Here we report results from quantum chemical calculations using chemical-accuracy multi-level G3 (Curtiss-Pople) and CBS-Q theory (Peterson) to address this question. In particular, we have investigated the cluster structures and clustering equilibria for the ions and H₃S⁺·(H₂O)_m(H₂S)_n, where m ? 6 and n ? 4, at 300-1000 K and 1 bar as well as under vapor-liquid equilibrium conditions between 300 and 646 K. We find that incremental hydration enthalpies and entropies derived from van’t Hoff analyses for the attachment of H₂O and H₂S onto H₃O⁺, and H₃S⁺ are in excellent agreement with experimental values and that the addition of water to all three ions is energetically more favorable than solvation by H₂S. As clusters grow in size, the energetic trends of cluster hydration begin to reflect those for bulk H₂O liquids, i.e. calculated hydration enthalpies and entropies approach values characteristic of the condensation of bulk water (ΔH^o = −44.0 kJ mol⁻¹, ΔS^o = −118.8 J K mol⁻¹). Water and hydrogen sulfide cluster calculations at higher temperatures indicate that a significant fraction of H₃O⁺, and H₃S⁺ ions exists as solvated moieties.     

The solution behavior of H2O in peralkaline aluminosilicate melts at high pressure with implications for properties of hydrous melts

Solubility and solution mechanisms of H₂O in depolymerized melts in the system Na₂O-Al₂O₃-SiO₂ were deduced from spectroscopic data of glasses quenched from melts at 1100 °C at 0.8-2.0 GPa. Data were obtained along a join with fixed nominal NBO/T = 0.5 of the anhydrous materials [Na₂Si₄O₉-Na₂(NaAl)₄O₉] with Al/(Al+Si) = 0.00-0.25. The H₂O solubility was fitted to the expression, X_H2O=0.20+0.0020f_H2O-0.7X_Al+0.9(X_Al)², where X_H2O is the mole fraction of H₂O (calculated with O = 1), f_H2O the fugacity of H₂O, and X_Al = Al/(Al+Si). Partial molar volume of H₂O in the melts, , calculated from the H₂O-solulbility data assuming ideal mixing of melt-H₂O solutions, is 12.5 cm³/mol for Al-free melts and decreases linearly to 8.9 cm³/mol for melts with Al/(Al+Si) ∼ 0.25. However, if recent suggestion that is composition-independent is applied to constrain activity-composition relations of the hydrous melts, the activity coefficient of H₂O, , increases with Al/(Al+Si).Solution mechanisms of H₂O were obtained by combining Raman and ²⁹Si NMR spectroscopic data. Degree of melt depolymerization, NBO/T, increases with H₂O content. The rate of NBO/T-change with H₂O is negatively correlated with H₂O and positively correlated with Al/(Al+Si). The main depolymerization reaction involves breakage of oxygen bridges in Q⁴-species to form Q² species. Steric hindrance appears to restrict bonding of H⁺ with nonbridging oxygen in Q³ species. The presence of Al³⁺ does not affect the water solution mechanisms significantly.     

Experimental constraints on the mobility of Rhenium in silicate liquids

The volatization of Rhenium (Re) from melts of natural basalt, dacite and a synthetic composition in the CaO-MgO-Al₂O₃-SiO₂ system has been investigated at 0.1 MPa and 1250-1350 °C over a range of fO₂ conditions from log fO₂ = −10 to −0.68. Experiments were conducted using open top Pt crucibles doped with Re and Yb. Analysis of quenched glasses by laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS) normal to the melt/gas interface showed concentration profiles for Re, to which a semi-infinite one-dimensional diffusion model could be applied to extract diffusion coefficients (D). The results show Re diffusivity in basalt at 1300 °C in air is log D_Re = −7.2 ± 0.3 cm²/s and increases to log D_Re = −6.6 ± 0.3 cm²/s when trace amounts of Cl were added to the starting material. At fO₂ conditions below the nickel-nickel oxide (NNO) buffer Re diffusivity decreases to and to in dacitic melt. In the CMAS composition, . The diffusivity of Re is comparable to Ar and CO₂ in basalt at 500 MPa favoring its release as a volatile. Our results support the contention that subaerial degassing is the cause of lower Re concentrations in arc-type and ocean island basalts compared to mid-ocean ridge basalts.     

An integrated sulfur isotope model for Namibian shelf sediments

In this study the sulfur cycle in the organic-rich mud belt underlying the highly productive upwelling waters of the Namibian shelf is quantified using a 1D reaction-transport model. The model calculates vertical concentration and reaction rate profiles in the top 500 cm of sediment which are compared to a comprehensive dataset which includes carbon, sulfur, nitrogen and iron compounds as well as sulfate reduction (SR) rates and stable sulfur isotopes (³²S, ³⁴S). The sulfur dynamics in the well-mixed surface sediments are strongly influenced by the activity of the large sulfur bacteria Thiomargaritanamibiensis which oxidize sulfide (H₂S) to sulfate () using sea water nitrate () as the terminal electron acceptor. Microbial sulfide oxidation (SOx) is highly efficient, and the model predicts intense cycling between and H₂S driven by coupled SR and SOx at rates exceeding 6.0 mol S m⁻² y⁻¹. More than 96% of the SR is supported by SOx, and only 2-3% of the pool diffuses directly into the sediment from the sea water. A fraction of the produced by Thiomargarita is drawn down deeper into the sediment where it is used to oxidize methane anaerobically, thus preventing high methane concentrations close to the sediment surface. Only a small fraction of total H₂S production is trapped as sedimentary sulfide, mainly pyrite (FeS₂) and organic sulfur (S_org) (∼0.3 wt.%), with a sulfur burial efficiency which is amongst the lowest values reported for marine sediments (<1%). Yet, despite intense SR, FeS₂ and S_org show an isotope composition of ∼5 ‰ at 500 cm depth. These heavy values were simulated by assuming that a fraction of the solid phase sulfur exchanges isotopes with the dissolved sulfide pool. An enrichment in H₂S of ³⁴S towards the sediment-water interface suggests that Thiomargarita preferentially remove H₂³²S from the pore water. A fractionation of 20-30‰ was estimated for SOx (ε_SOx) with the model, along with a maximum fractionation for SR (ε_SR-max) of 100‰. These values are far higher than previous laboratory-based estimates for these processes. Mass balance calculations indicate negligible disproportionation of autochthonous elemental sulfur; an explanation routinely cited in the literature to account for the large fractionations in SR. Instead, the model indicates that repeated multi-stepped sulfide oxidation and intracellular disproportionation by Thiomargarita could, in principle, allow the measured isotope data to be simulated using much lower fractionations for ε_SOx (5‰) and ε_SR (78‰).     

Water speciation and diffusion in haploandesitic melts at 743-873 K and 100 MPa

Water is an important volatile component in andesitic eruptions and deep-seated andesitic magma chambers. We report an investigation of H₂O speciation and diffusion by dehydrating haploandesitic melts containing ?2.5 wt.% water at 743-873 K and 100 MPa in cold-seal pressure vessels. FTIR microspectroscopy was utilized to measure species [molecular H₂O (H₂O_m) and hydroxyl group (OH)] and total H₂O (H₂O_t) concentration profiles on the quenched glasses from the dehydration experiments. The equilibrium constant of the H₂O speciation reaction H₂O_m+O?2OH, K = (X_OH)²/(X_H2OmX_O) where X means mole fraction on a single oxygen basis, in this Fe-free andesite varies with temperature as ln K = 1.547-2453/T where T is in K. Comparison with previous speciation data on rhyolitic and dacitic melts indicates that, for a given water concentration, Fe-free andesitic melt contains more hydroxyl groups. Water diffusivity at the experimental conditions increases rapidly with H₂O concentration, contrary to previous H₂O diffusion data in an andesitic melt at 1608-1848 K. The diffusion profiles are consistent with the model that molecular H₂O is the diffusion species. Based on the above speciation model, H₂O_m and H₂O_t diffusivity (in m²/s) in haploandesite at 743-873 K, 100 MPa, and H₂O_t ? 2.5 wt.% can be formulated as