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Deciphering metamorphic evolution of rocks by means of Hf isotopes
A596
Goldschmidt Conference Abstracts 2006
Deciphering metamorphic evolution of rocks by means of Hf isotopes
Transformations of mercury, iron, and sulfur during the reductive dissolution of iron oxyhydroxide by sulfide
J. SLAMA1, J. KOSLER2, R.B. PEDERSEN2
A.J. SLOWEY1, G.E. BROWN JR.1,2
1
Department of Geochemistry, Charles University in Prague, Czech Republic and Institute of Geology, Academy of Sciences of the Czech Republic, Czech Rebulic (slama@ natur.cuni.cz) 2 Department of Earth Science, University of Bergen, Norway ([email protected]) Hafnium isotopes in minerals can be used as a tracer of mineral reactions in metamorphic rocks. If the reaction products behave as a closed system with respect to Lu and Hf, their present-day Hf isotopic composition (176Hf/177Hf) only depends on the Hf isotopic composition of the reacting phases and the partitioning of Lu and Hf during the reaction. The calculated Hf ‘‘reaction model ages’’ between individual metamorphic minerals can then be used not only to test the participation of minerals in the reactions, but also to constrain the timing of reactions. In addition, Hf isotopes can also be used to trace the involvement of Hf-bearing accessory phases, such as zircon or ilmenite, in metamorphic reactions. We have studied Hf isotopic mass balance in a pyroxene granulite from southern part of the Bohemian Massif in the Czech Republic. The rock has a simple mineralogy with several mafic Hf-bearing mineral phases that were involved in a decompression reaction Grt + Qtz = Opx + Plg. The P–T history of the granulite and the metamorphic evolution in the region are well constrained from numerous previous studies. The peak of granulite metamorphism has been dated at 343 ± 2 Ma (U–Pb zircon). The subsequent isothermal decompression and near isobaric cooling is recorded in a corona-forming reaction around garnet. The Hf isotopes in major minerals acting in the decompression reaction were used to model their Hf isotopic evolution and to determine that the garnet decompression reaction took place approximately 10 Ma after the metamorphic peak (see Figure). The results of this study point to a rapid decompression of the lower crust in the southern Bohemian Massif during the Variscan orogeny.
1
Department of Geological & Environmental Sciences, Stanford University, Stanford, CA 94305-2115, USA 2 Stanford Synchrotron Radiation Laboratory, SLAC, MS 69, Menlo Park, CA 94025, USA Methylmercury can accumulate in fish to concentrations unhealthy for humans and other predatory mammals. Most sources of mercury (Hg) emit inorganic species to the environment. Therefore, ecological harm occurs when inorganic Hg is converted to methylmercury. Sulfate and iron-reducing bacteria (SRB and FeRB) methylate Hg, but the effects of processes involving oxidized and reduced forms of sulfur and iron on the potential bioavailability of inorganic Hg are poorly understood. Under abiotic conditions, using a laboratory flow reactor, , bisulfide was added at 40 to 250 lM h 1 to 5 g L 1 goethite (a-FeOOH) suspensions to which Hg(II) was adsorbed (30 to 100 nmol m 2) at pH 7.5. Dissolved Hg initially decreases from 103 or 104 (depending on initial conditions) to 10 1 nM, during which the concentration of Hg(II) adsorbed to goethite decreased by 80% and metacinnabar (b-HgS) formed, according to Hg LIIIedge extended X-ray absorption fine structure (EXAFS) spectroscopy. The apparent coordination of oxygens surrounding Hg, measured with EXAFS, increased during this period, suggesting desorption of monodentate, inner sphere (IS) bound Hg while bidentate, IS Hg persisted on the goethite surface. Further sulfidation increased dissolved Hg concentrations by one to two orders of magnitude (0.5 to 10 or 30 nM), suggesting that ferrous iron formed by sulfide-promoted dissolution of goethite either desorbed Hg or dissolved b-HgS. Rapid accumulation of Fe(II) in the solid phase (up to 40 lmol g 1) coincided with faster elevation of dissolved Hg concentrations. A secondary Fe-bearing phase resembling poorly formed green rust was observed in sulfidized solids with scanning electron microscopy. These results suggest that surface-bound or mineralized Fe(II) strongly affects Hg speciation and therefore may affect Hg bioavailability under conditions where Fe(III) is reduced. Examination of interrelated processes involving sulfide and Fe(III) revealed new modes of Hg solubilization previously not considered in Hg methylation models. doi:10.1016/j.gca.2006.06.1107
doi:10.1016/j.gca.2006.06.1106
Goldschmidt Conference Abstracts 2006
Deciphering metamorphic evolution of rocks by means of Hf isotopes
Transformations of mercury, iron, and sulfur during the reductive dissolution of iron oxyhydroxide by sulfide
J. SLAMA1, J. KOSLER2, R.B. PEDERSEN2
A.J. SLOWEY1, G.E. BROWN JR.1,2
1
Department of Geochemistry, Charles University in Prague, Czech Republic and Institute of Geology, Academy of Sciences of the Czech Republic, Czech Rebulic (slama@ natur.cuni.cz) 2 Department of Earth Science, University of Bergen, Norway ([email protected]) Hafnium isotopes in minerals can be used as a tracer of mineral reactions in metamorphic rocks. If the reaction products behave as a closed system with respect to Lu and Hf, their present-day Hf isotopic composition (176Hf/177Hf) only depends on the Hf isotopic composition of the reacting phases and the partitioning of Lu and Hf during the reaction. The calculated Hf ‘‘reaction model ages’’ between individual metamorphic minerals can then be used not only to test the participation of minerals in the reactions, but also to constrain the timing of reactions. In addition, Hf isotopes can also be used to trace the involvement of Hf-bearing accessory phases, such as zircon or ilmenite, in metamorphic reactions. We have studied Hf isotopic mass balance in a pyroxene granulite from southern part of the Bohemian Massif in the Czech Republic. The rock has a simple mineralogy with several mafic Hf-bearing mineral phases that were involved in a decompression reaction Grt + Qtz = Opx + Plg. The P–T history of the granulite and the metamorphic evolution in the region are well constrained from numerous previous studies. The peak of granulite metamorphism has been dated at 343 ± 2 Ma (U–Pb zircon). The subsequent isothermal decompression and near isobaric cooling is recorded in a corona-forming reaction around garnet. The Hf isotopes in major minerals acting in the decompression reaction were used to model their Hf isotopic evolution and to determine that the garnet decompression reaction took place approximately 10 Ma after the metamorphic peak (see Figure). The results of this study point to a rapid decompression of the lower crust in the southern Bohemian Massif during the Variscan orogeny.
1
Department of Geological & Environmental Sciences, Stanford University, Stanford, CA 94305-2115, USA 2 Stanford Synchrotron Radiation Laboratory, SLAC, MS 69, Menlo Park, CA 94025, USA Methylmercury can accumulate in fish to concentrations unhealthy for humans and other predatory mammals. Most sources of mercury (Hg) emit inorganic species to the environment. Therefore, ecological harm occurs when inorganic Hg is converted to methylmercury. Sulfate and iron-reducing bacteria (SRB and FeRB) methylate Hg, but the effects of processes involving oxidized and reduced forms of sulfur and iron on the potential bioavailability of inorganic Hg are poorly understood. Under abiotic conditions, using a laboratory flow reactor, , bisulfide was added at 40 to 250 lM h 1 to 5 g L 1 goethite (a-FeOOH) suspensions to which Hg(II) was adsorbed (30 to 100 nmol m 2) at pH 7.5. Dissolved Hg initially decreases from 103 or 104 (depending on initial conditions) to 10 1 nM, during which the concentration of Hg(II) adsorbed to goethite decreased by 80% and metacinnabar (b-HgS) formed, according to Hg LIIIedge extended X-ray absorption fine structure (EXAFS) spectroscopy. The apparent coordination of oxygens surrounding Hg, measured with EXAFS, increased during this period, suggesting desorption of monodentate, inner sphere (IS) bound Hg while bidentate, IS Hg persisted on the goethite surface. Further sulfidation increased dissolved Hg concentrations by one to two orders of magnitude (0.5 to 10 or 30 nM), suggesting that ferrous iron formed by sulfide-promoted dissolution of goethite either desorbed Hg or dissolved b-HgS. Rapid accumulation of Fe(II) in the solid phase (up to 40 lmol g 1) coincided with faster elevation of dissolved Hg concentrations. A secondary Fe-bearing phase resembling poorly formed green rust was observed in sulfidized solids with scanning electron microscopy. These results suggest that surface-bound or mineralized Fe(II) strongly affects Hg speciation and therefore may affect Hg bioavailability under conditions where Fe(III) is reduced. Examination of interrelated processes involving sulfide and Fe(III) revealed new modes of Hg solubilization previously not considered in Hg methylation models. doi:10.1016/j.gca.2006.06.1107
doi:10.1016/j.gca.2006.06.1106