Li-Sr-Nd isotope signatures of the plume and cratonic lithospheric mantle beneath the margin of the rifted Tanzanian craton (Labait)

Lithium concentrations and isotopic compositions of olivine and ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd of coexisting clinopyroxene from peridotite xenoliths from the Quaternary Labait volcano, Tanzania, document the influence of rift-related metasomatism on the ancient cratonic mantle. Olivines show negative correlations between Fo content and both δ⁷Li and Li concentrations. Olivines in iron-rich peridotites (Fo_85–87) have high Li concentrations (3.2–4.8 ppm) and heavy δ⁷Li (+5.2 to +6.6). In contrast, olivines in ancient, refractory peridotites have lower Li concentrations (∼2 ppm) and relatively light δ⁷Li (+2.6 to +3.5). This reflects mixing between ancient, refractory cratonic lithosphere and asthenosphere-derived rift magmas. A uniquely fertile, deformed, high-temperature garnet lherzolite, interpreted to be from the base of the lithosphere, has a ⁸⁷Sr/⁸⁶Sr of 0.7029 and ¹⁴³Nd/¹⁴⁴Nd of 0.51286, similar to HIMU oceanic basalts. It provides the best estimate of the Sr–Nd isotope composition of the upwelling mantle (i.e., plume, sensu lato) underlying this portion of the East African Rift, and is slightly less radiogenic compared to previous estimates of the plume that were based on rift basalts. Although elevated δ⁷Li are not exclusive to HIMU source regions, the data collectively indicate that the plume beneath Labait has HIMU characteristics in Sr, Nd and Li isotope composition. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

The effects of metamorphism on O and Fe isotope compositions in the Biwabik Iron Formation, northern Minnesota

The Biwabik Iron Formation of Minnesota (1.9 Ga) underwent contact metamorphism by intrusion of the Duluth Complex (1.1 Ga). Apparent quartz–magnetite oxygen isotope temperatures decrease from ∼700°C at the contact to ∼375°C at 2.6 km distance (normal to the contact in 3D). Metamorphic pigeonite at the contact, however, indicates that peak temperatures were greater than 825°C. The apparent O isotope temperatures, therefore, reflect cooling, and not peak metamorphic conditions. Magnetite was reset in δ¹⁸O as a function of grain size, indicating that isotopic exchange was controlled by diffusion of oxygen in magnetite for samples from above the grunerite isograd. Apparent quartz–magnetite O isotope temperatures are similar to calculated closure temperatures for oxygen diffusion in magnetite at a cooling rate of ∼5.6°C/kyr, which suggests that the Biwabik Iron Formation cooled from ∼825 to 400°C in ∼75 kyr at the contact with the Duluth Complex. Isotopic exchange during metamorphism also occurred for Fe, where magnetite–Fe silicate fractionations decrease with increasing metamorphic grade. Correlations between quartz–magnetite O isotope fractionations and magnetite–iron silicate Fe isotope fractionations suggest that both reflect cooling, where the closure temperature for Fe was higher than for O. The net effect of metamorphism on δ¹⁸O–δ⁵⁶Fe variations in magnetite is a strong increase in δ¹⁸O_Mt and a mild decrease in δ⁵⁶Fe with increasing metamorphic grade, relative to the isotopic compositions that are expected at the low temperatures of initial magnetite formation. If metamorphism of Iron Formations occurs in a closed system, bulk O and Fe isotope compositions may be preserved, although re-equilibration among the minerals may occur for both O and Fe isotopes. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Enhancing understanding of breakdown and collapse in the Yorkshire Dales using ground penetrating radar on cave sediments

Historical evidence shows block breakdown and collapse are actively occurring in large fault aligned caverns in the Yorkshire Dales karst. Deployment of ground penetrating radar at two such sites has provided detailed images of the sedimentary sequences below the present day cavern floor but no large blocks are imaged within the sediments. Solutional processes must be removing limestone from the sediment to allow continued cavern growth. Possible mechanisms to account for the lack of large blocks within the sediment fill are discussed.     

Shear strength characterization of municipal solid waste at the Suzhou landfill, China

The current practice of slope stability analysis for a municipal solid waste (MSW) landfill usually overlooks the dependence of waste properties on the fill age or embedment depth. Changes in shear strength of MSW as a function of fill age were investigated by performing field and laboratory studies on the Suzhou landfill in China. The field study included sampling from five boreholes advanced to the bottom of the landfill, cone penetration tests and monitoring of pore fluid pressures. Twenty-six borehole samples representative of different fill ages (0 to 13 years) were used to perform drained triaxial compression tests. The field and laboratory study showed that the waste body in the landfill can be sub-divided into several strata corresponding to different ranges of fill age. Each of the waste strata has individual composition and shear strength characteristics. The triaxial test results showed that the MSW samples exhibited a strain-hardening and contractive behavior. As the fill age of the waste increased from 1.7 years to 11 years, the cohesion mobilized at a strain level of 10% was found to decrease from 23.3 kPa to 0 kPa, and the mobilized friction angle at the same strain level increasing from 9.9° to 26°. For a confinement stress level greater than 50 kPa, the shear strength of the recently-placed MSW seemed to be lower than that of the older MSW. This behavior was consistent with the cone penetration test results. The field measurement of pore pressures revealed a perched leachate mound above an intermediate cover of soils and a substantial leachate mound near the bottom of the landfill. The measurements of shear strength properties and pore pressures were utilized to assess the slope stability of the Suzhou landfill.     

Iron isotopes constrain biologic and abiologic processes in banded iron formation genesis

The voluminous 2.5 Ga banded iron formations (BIFs) from the Hamersley Basin (Australia) and Transvaal Craton (South Africa) record an extensive period of Fe redox cycling. The major Fe-bearing minerals in the Hamersley-Transvaal BIFs, magnetite and siderite, did not form in Fe isotope equilibrium, but instead reflect distinct formation pathways. The near-zero average δ⁵⁶Fe values for magnetite record a strong inheritance from Fe³⁺ oxide/hydroxide precursors that formed in the upper water column through complete or near-complete oxidation. Transformation of the Fe³⁺ oxide/hydroxide precursors to magnetite occurred through several diagenetic processes that produced a range of δ⁵⁶Fe values: (1) addition of marine hydrothermal , (2) complete reduction by bacterial dissimilatory iron reduction (DIR), and (3) interaction with excess that had low δ⁵⁶Fe values and was produced by DIR. Most siderite has slightly negative δ⁵⁶Fe values of ∼ −0.5‰ that indicate equilibrium with Late Archean seawater, although some very negative δ⁵⁶Fe values may record DIR. Support for an important role of DIR in siderite formation in BIFs comes from previously published C isotope data on siderite, which may be explained as a mixture of C from bacterial and seawater sources.Several factors likely contributed to the important role that DIR played in BIF formation, including high rates of ferric oxide/hydroxide formation in the upper water column, delivery of organic carbon produced by photosynthesis, and low clastic input. We infer that DIR-driven Fe redox cycling was much more important at this time than in modern marine systems. The low pyrite contents of magnetite- and siderite-facies BIFs suggests that bacterial sulfate reduction was minor, at least in the environments of BIF formation, and the absence of sulfide was important in preserving magnetite and siderite in the BIFs, minerals that are poorly preserved in the modern marine record. The paucity of negative δ⁵⁶Fe values in older (Early Archean) and younger (Early Proterozoic) BIFs suggests that the extensive 2.5 Ga Hamersley-Transvaal BIFs may record a period of maximum expansion of DIR in Earth’s history.     

Surface complexation of U(VI) on goethite (α-FeOOH)

Sorption of U(VI) to goethite is a fundamental control on the mobility of uranium in soil and groundwater. Here, we investigated the sorption of U on goethite using EXAFS spectroscopy, batch sorption experiments and DFT calculations of the energetics and structures of possible surface complexes. Based on EXAFS spectra, it has previously been proposed that U(VI), as the uranyl cation , sorbs to Fe oxide hydroxide phases by forming a bidentate edge-sharing (E2) surface complex, >Fe(OH)₂UO₂(H₂O)_n. Here, we argue that this complex alone cannot account for the sorption capacity of goethite (α-FeOOH). Moreover, we show that all of the EXAFS signal attributed to the E2 complex can be accounted for by multiple scattering. We propose that the dominant surface complex in CO₂-free systems is a bidentate corner-sharing (C2) complex, (>FeOH)₂UO₂(H₂O)₃ which can form on the dominant {101} surface. However, in the presence of CO₂, we find an enhancement of UO₂ sorption at low pH and attribute this to a (>FeO)CO₂UO₂ ternary complex. With increasing pH, U(VI) desorbs by the formation of aqueous carbonate and hydroxyl complexes. However, this desorption is preceded by the formation of a second ternary surface complex (>FeOH)₂UO₂CO₃. The three proposed surface complexes, (>FeOH)₂UO₂(H₂O)₃, >FeOCO₂UO₂, and (>FeOH)₂UO₂CO₃ are consistent with EXAFS spectra. Using these complexes, we developed a surface complexation model for U on goethite with a 1-pK model for surface protonation, an extended Stern model for surface electrostatics and inclusion of all known UO₂-OH-CO₃ aqueous complexes in the current thermodynamic database. The model gives an excellent fit to our sorption experiments done in both ambient and reduced CO₂ environments at surface loadings of 0.02-2.0 wt% U.     

Spectroscopic investigation of uranyl(VI) and citrate coadsorption to Al2O3

ATR-FTIR spectroscopy is used to understand the adsorption of uranyl-citrate complexes to Al₂O₃. Spectral data indicate that uranyl-citrate complexes partially dissociate upon adsorption, allowing full or partial hydrolysis of the uranyl ion. K_ads values determined for free citrate adsorption are similar to those for citrate in uranyl-citrate complexes, indicating that the complexation of uranyl by citrate does not significantly affect the ability of citrate to bond with the surface. The isotherm data also indicate enhanced citrate adsorption to Al₂O₃ in the presence of uranyl, suggesting that uranyl may be the central link between two citrate ligands, and that uranyl is associated with the surface through a bridging citrate ligand. Finally, uranyl-citrate complexes interact with citrate adsorbed to Al₂O₃ through outer sphere interactions.     

An absolute paleotemperature record from 10 to 6 Ka inferred from fluid inclusion D/H ratios of a stalagmite from Vancouver Island, British Columbia, Canada

By using continuous helium flow during the crushing of calcite speleothem samples, we are able to recover liberated inclusion waters without isotopic fractionation. A paleotemperature record for the Jacklah Jill Cave locality, Vancouver Island, BC, was obtained from a 30-cm tall stalagmite that grew 10.3-6.3 Ka ago, using δ¹⁸O values of the crushed calcite and of the inclusion water as inferred from its δD. It is found that the locality experienced mean annual temperature variations up to 11 °C over a 4-Ka period in the early Holocene. At the beginning of the period, local temperature quickly increased from a minimum of ∼1 °C to around 10 °C, but this early climate optimum, about 3 °C warmer than today, only lasted for ∼1200 years. About 8.6 Ka ago, temperature had declined to ∼7 °C, approximately the same as the modern cave temperature. Since then, the study area has experienced only minor temperature fluctuations, but there was a brief fall to ∼4 °C at around 7 Ka ago, which might be caused by a short lived expansion of local alpine glaciers. The long-term T-dependence of δD was 1.47‰/°C, identical to the value in modern precipitation.     

Silicon isotope fractionation in bamboo and its significance to the biogeochemical cycle of silicon

A systematic investigation on silica contents and silicon isotope compositions of bamboos was undertaken. Seven bamboo plants and related soils were collected from seven locations in China. The roots, stem, branch and leaves for each plant were sampled and their silica contents and silicon isotope compositions were determined. The silica contents and silicon isotope compositions of bulk and water-soluble fraction of soils were also measured. The silica contents of studied bamboo organs vary from 0.30% to 9.95%. Within bamboo plant the silica contents show an increasing trend from stem, through branch, to leaves. In bamboo roots the silica is exclusively in the endodermis cells, but in stem, branch and leaves, the silica is accumulated mainly in epidermal cells. The silicon isotope compositions of bamboos exhibit significant variation, from −2.3‰ to 1.8‰, and large and systematic silicon isotope fractionation was observed within each bamboo. The δ³⁰Si values decrease from roots to stem, but then increase from stem, through branch, to leaves. The ranges of δ³⁰Si values within each bamboo vary from 1.0‰ to 3.3‰. Considering the total range of silicon isotope composition in terrestrial samples is only 7‰, the observed silicon isotope variation in single bamboo is significant and remarkable. This kind of silicon isotope variation might be caused by isotope fractionation in a Rayleigh process when SiO₂ precipitated in stem, branches and leaves gradually from plant fluid. In this process the Si isotope fractionation factor between dissolved Si and precipitated Si in bamboo (α_pre-sol) is estimated to be 0.9981. However, other factors should be considered to explain the decrease of δ³⁰Si value from roots to stem, including larger ratio of dissolved H₄SiO₄ to precipitated SiO₂ in roots than in stem. There is a positive correlation between the δ³⁰Si values of water-soluble fractions in soils and those of bulk bamboos, indicating that the dissolved silicon in pore water and phytoliths in soil is the direct sources of silicon taken up by bamboo roots. A biochemical silicon isotope fractionation exists in process of silicon uptake by bamboo roots. Its silicon isotope fractionation factor (α_bam-wa) is estimated to be 0.9988. Considering the distribution patterns of SiO₂ contents and δ³⁰Si values among different bamboo organs, evapotranspiration may be the driving force for an upward flow of a silicon-bearing fluid and silica precipitation. Passive silicon uptake and transportation may be important for bamboo, although the role of active uptake of silicic acid by roots may not be neglected. The samples with relatively high δ³⁰Si values all grew in soils showing high content of organic materials. In contrast, the samples with relatively low δ³⁰Si values all grew in soil showing low content of organic materials. The silicon isotope composition of bamboo may reflect the local soil type and growth conditions. Our study suggests that bamboos may play an important role in global silicon cycle.     

Atomistic simulation of the mechanisms of noble gas incorporation in minerals

Atomistic simulations have been carried out to investigate the mechanisms of noble gas incorporation in minerals using both the traditional two-region approach and the “supercell” method. The traditional two-region approach has been used to calculate defect energies for Ne, Ar, Kr and Xe incorporation in MgO, CaO, diopside and forsterite in the static limit and at one atmosphere pressure. The possibilities of noble gas incorporation via both substitution and interstitial mechanisms are studied. The favored mechanism varies from mineral to mineral and from noble gas to noble gas. In all minerals studied, the variation of the solution energies of noble gas substitution with atomic radius appears approximately parabolic, analogous to those for 1⁺, 2⁺, 3⁺ and 4⁺ trace element incorporation on crystal lattice sites. Noble gas solution energies thus also fall on a curve, similar to those previously observed for cations with different charges, but with much lower curvature.The “supercell” method has been used to investigate the pressure dependence of noble gas incorporation in the same systems. Results indicate a large variation of the solubility of the larger noble gases, Kr and Xe with pressure. In addition, explicit simulation of incorporation at the (0 0 1) surface of MgO shows that the solubility of the heavier noble gases may be considerably enhanced by the presence of interfaces.