Browsing by Subject "XANES"

Now showing 1 - 3 of 3
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    Biogeochemical Interactions and Cycling of Sulfur, Iron, and Carbon in Sulfate-Impacted Riparian Wetlands and Wild Rice Waters
    (2018-08) Torgeson, Joshua
    Sulfide accumulation in the porewater of freshwater aquatic systems has been shown to inhibit the growth of many aquatic macrophytes, including wild rice. While interactions between sulfur (S), carbon (C), and iron (Fe) cycles are recognized, secondary “cryptic” S cycles are much less understood; these cycles favor reduction of sulfate over Fe, contrary to traditional thermodynamic expectations; these “cryptic” cycles have been suggested to occur at Second Creek through models by Ng et al. (2017). Using field observations, hydrologic monitoring, and geochemical analyses, we found that changes in hyporheic flux result in changes in porewater SO42- concentrations. Additionally, we have found that intermediate valence S species may act as primary sinks for excess dissolved sulfide. Our comparison study between a SO42--impacted stream and a less-impacted river demonstrates that the accumulation of porewater sulfide may be suppressed through limited TOC, excess sediment Fe, or through generation of S-intermediates.
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    Ferric Iron Partitioning between Pyroxene and Melt: Experiments, Microbeam analysis, and Consequences for Mantle Redox
    (2021-12) Rudra, Avishek
    Pyroxene is the chief reservoir of Fe3+ in upper mantle peridotite, but experiments exploring pyroxene/melt Fe3+ partitioning have been restricted to 100 kPa and pyroxene with low alumina. Here we present Fe3+ partitioning experiments between clinopyroxenes (cpx) and mafic melt at elevated pressures (1–2.5 GPa). Experiments were conducted with fO2 buffered and modulated by Ru+RuO2 and Fe-Pt alloy capsules, respectively, between ∆QFM -2.68 and +5.13. Fe3+/FeT of both cpx and melt were determined by Fe K-edge X-ray absorption near edge structure spectroscopy. The experimentally synthesized cpx compositions (Al2O3 = 2.36–6.01 wt.%, CaO = 19.33–22.21 wt.%) approximate those expected in basalt source regions. We find that Fe3+ is moderately incompatible in cpx and correlates with cpx Al2O3 content, increasing from 0.05±0.09 to 0.81±0.04. Comparison between experimentally synthesized cpx with those from natural peridotites indicates influences of both temperature and composition on Fe3+/FeT for cpx in spinel and garnet peridotites. The combined effects of decreased pyroxene Al2O3 concentration and pyroxene mode with progressive partial melting of peridotite diminishes the bulk partition coefficients of Fe3+, leading to greater Fe2O3 contents in high degree partial melts, and this accounts for an inverse relationship between Na2O and Fe2O3 observed in mid-ocean ridge basalts (MORB). Comparison to numerical experiments with pMELTS and the model of Jennings and Holland (2015) show that these models overpredict for partial melting of the mantle, and so they do not accurately determine the relationship between the fO2 and Fe2O3 of peridotite in basalt source regions. To estimate the Fe3+/FeT ratio of the mantle source of MORB, we modeled liquid Fe2O3 during isentropic batch melting of peridotite at three potential temperatures (1320 °C, 1400 °C, and 1440 °C) for peridotitic sources with Fe3+/FeT ratios between 0.02–0.06. A source with an Fe3+/FeT ratio of 0.038±0.007 matches most of the span of natural MORB. This ratio is similar to that typical of continental lithospheric mantle sampled by xenoliths, but lower than that surmised by several recent experimental and thermodynamic studies. Considering this source Fe3+/FeT but extending the partial melting calculations to higher pressures (2.5–4 GPa) reveals that bulk significantly decreases for garnet peridotite relative to spinel peridotite because the cpx become significantly less aluminous with increasing pressure. This results in high pressure partial melts with greater liquid Fe3+/FeT ratios. Therefore, elevated Fe3+/FeT ratios observed from some oceanic island basalts (OIB), such as those from Hawaii and Iceland, reflect in part the differences in conditions of melting and may not require mantle source regions more oxidized than those that produce MORB.
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    Solid-phase arsenic speciation in glacial aquifer sediments of west-central Minnesota, USA: a micro-X-ray absorption spectroscopy approach for quantifying trace-level speciation
    (2017-06) Nicholas, Sarah
    Abstract Arsenic (As) is a geogenic contaminant affecting groundwater in geologically diverse systems. The footprint of the Des Moines Lobe glacial advance in west-central Minnesota, is a regional nexus of drinking-water wells that exceed the US EPA maximum contaminant level for arsenic (As>10µgL-1). Arsenic release from aquifer sediments to groundwater is favored when biogeochemical conditions in aquifers fluctuate. The specific objective of this research was to identify the solid-phase sources and geochemical mechanisms of release of As in aquifers of the Des Moines Lobe glacial advance. The overarching hypothesis is that gradients in hydrologic conductivity and redox conditions found at aquifer-aquitard interfaces promote a suite of geochemical reactions leading to mineral alteration and release of As to groundwater. A microprobe X-ray absorption spectroscopy (µXAS) approach was developed and applied to rotosonic drill core samples to identify the solid-phase speciation of As in aquifer, aquitard, and aquifer-aquitard interface sediments. This approach addressed the low solid-phase As concentrations, as well as the fine-scale physical and chemical heterogeneity of the sediments. The solid-phase Fe and As speciation was interpreted using sediment and well-water chemical data to propose solid-phase As reservoirs and release mechanisms. The results are consistent with three different As release mechanisms: (1) desorption from Fe oxyhydroxides, (2) reductive dissolution of Fe oxyhydroxides, and (3) oxidative dissolution of Fe sulfides. The findings confirm that glacial sediments at the interface between aquifer and aquitard are geochemically active zones for As. The diversity of As release mechanisms is consistent with the geographic heterogeneity observed in the distribution of elevated-As wells. Supplementary file “Nicholas dissertation supplementary files 1 to 5.xlsx” was submitted to the UMN digital conservancy with this thesis. It is an excel workbook with five tables. Tables 1 and 2 are the complete provenance and citation information for all As and Fe reference spectra used. Tables 3, 4, and 5 are the reference spectra fits, fractions, and scores for the sampled spectra from cores OTT3, TG3, and UMRB2.