Dimitri Vlassopoulos

Magma mixing at Nisyros volcano, as inferred from incompatible trace-element systematics

Abstract Rocks of the calc-alkaline volcanic complex of Nisyros, at the eastern end of the Quaternary South Aegean volcanic arc, range in composition from basaltic andesite to rhyodacite. Major- and compatible trace-element systematics are dominated by crystal fractionation, involving olivine, clinopyroxene, plagioclase, and magnetite in the mafic to intermediate compositional range and plagioclase, clinopyroxene, hypersthene, Fe-Ti oxides, and apatite in the intermediate to silicic range, although there is a marked paucity of lavas with 60–66 wt.% SiO 2 . The occurrence of andesitic inclusions in pre- and post-caldera dacites as well as reversely zoned and/or partly resorbed phenocrysts, exotic mineralogy (e.g., forsteritic olivine), compositionally bimodal plagioclase phenocryst populations, and the observation of two distinct glass phases in melt inclusions and pumices all indicate that magma mixing processes have played a role in the petrogenesis of Nisyros. Incompatible trace-element systematics are consistent with mixing between mafic and silicic end members. Major-element binary mixing calculations between end members successfully reproduce the dacite composition. The eruptive sequence at Nisyros suggests the existence of a compositionally zoned magma chamber, in which mixing between underlying more mafic magma and overlying rhyodacitic melt result in the formation of dacitic magmas. Inherent mineral assemblages in basaltic andesites indicate that mixing between mafic liquids and between mafic and more silicic liquids played also a role in their petrogenesis.

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.

Immobilization of Hg(II) by Coprecipitation in Sulfate-Cement Systems

Uptake and molecular speciation of dissolved Hg during formation of Al- or Fe-ettringite-type and high-pH phases were investigated in coprecipitation and sorption experiments of sulfate-cement treatments used for soil and sediment remediation. Ettringite and minor gypsum were identified by XRD as primary phases in Al systems, whereas gypsum and ferrihydrite were the main products in Hg–Fe precipitates. Characterization of Hg–Al solids by bulk Hg EXAFS, electron microprobe, and microfocused-XRF mapping indicated coordination of Hg by Cl ligands, multiple Hg and Cl backscattering atoms, and concentration of Hg as small particles. Thermodynamic predictions agreed with experimental observations for bulk phases, but Hg speciation indicated lack of equilibration with the final solution. Results suggest physical encapsulation of Hg as a polynuclear chloromercury(II) salt in ettringite as the primary immobilization mechanism. In Hg–Fe solids, structural characterization indicated Hg coordination by O atoms only and Fe backscattering atoms that is consistent with inner-sphere complexation of Hg(OH)20 coprecipitated with ferrihydrite. Precipitation of ferrihydrite removed Hg from solution, but the resulting solid was sufficiently hydrated to allow equilibration of sorbed Hg species with the aqueous solution. Electron microprobe XRF characterization of sorption samples with low Hg concentration reacted with cement and FeSO4 amendment indicated correlation of Hg and Fe, supporting the interpretation of Hg removal by precipitation of an Fe(III) oxide phase.

Manganese(IV) Oxide Amendments Reduce Methylmercury Concentrations in Sediment Porewater

Manganese(iv) oxide (pyrolusite, birnessite) mineral amendments can reduce dissolved MeHg concentrations in sediment theoretically by inhibiting microbial sulfate reduction, which is a major methylation pathway in sediments. Anaerobic sediment slurry microcosms in which Hg methylation was stimulated by addition of labile organic carbon (acetate) and HgCl2 showed that manganese(iv) oxide reduced the percent MeHg in slurry porewater (filtered), by 1-2 orders of magnitude relative to controls. Sediment-water mesocosms with pyrolusite or birnessite either directly mixed into the top 5 cm or applied in a thin (5 cm) sand layer over sediment showed reductions in percent MeHg in porewater of 66-69% for pyrolusite and 81-89% for birnessite amendment. A thin sand layer alone resulted in 65% reduction. CO2 respirometry experiments showed that the amendments stimulated microbial activity. Microbial community census by PCR and DNA sequencing indicated that the addition of Mn(iv) oxides did not significantly alter the indigenous sediment microbial community structure, although a small increase in abundance of iron and manganese reducers was observed after a 2 week incubation period. The mechanism of decreasing MeHg relative to Hg concentrations in porewater likely involved an increase in the importance of Mn(iv) reduction (relative to sulfate reduction) in heterotrophic microbial metabolism in the sediments amended with Mn(iv) oxides. Manganese reduction was confirmed as the predominant biogeochemical redox process by microelectrode voltammetry profiling of the sediment microcosms, although adsorption to Mn oxide surfaces, enhanced MeHg demethylation, and abiotic reduction of Mn(iv) also may have been involved in reducing percent MeHg and suppressing net MeHg production. These results represent a novel approach for mitigating MeHg impacts from sediments with potential applicability to a range of aquatic settings including intertidal zones, tidal marshes, seasonal wetlands, reservoirs, and lakes.