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Spatial heterogeneity of S and C stores in an inland acid sulfate soil
Goldschmidt Conference Abstracts 2006
A683
The search for evidence of chemical interactions between the core and mantle
Spatial heterogeneity of S and C stores in an inland acid sulfate soil
R.J. WALKER1, T. IRELAND1, A.D. BRANDON2
L.J. WALLACE1,2, D.C. MCPHAIL1,2, S. WELCH1,2, D. KIRSTE1,2, S. BEAVIS2,3, S. LAMONTAGNE2,4, R.W. FITZPATRICK2,4
1
Department of Geology, University of Maryland, College Park, MD 20742, USA ([email protected]; tireland@geol. umd.edu) 2 NASA-JSC, 2101 NASA Road 1, Mail Code KR, Houston TX 77058, USA ([email protected]) Coupled 187Os-186Os systems and 182W have been proposed to be potentially useful in the search for evidence for chemical/isotopic interaction between the core and silicate Earth. The premise behind Os as a diagnostic tool is that the outer core has evolved to an isotopic composition that is distinct from anything produced in the mantle. Also, for the core to have evolved to a unique composition, the inner core must have formed sufficiently early and solid metal-liquid metal distribution coefficients must be sufficiently large. Whether or not these conditions are met by the core is debatable. Previous work has reported coupled enrichments consistent with predicted outer core contributions to some Hawaiian picritic lavas (Brandon et al., 1999). Although sources for similar coupled enrichments have been proposed for some mantle processes, such sources have not yet been confirmed by measurement. Higher precision measurements of Hawaiian picritic lavas reveal greater complexities than earlier studies reported. For example, the correlation between 187Os and 186Os is less well defined for Mauna Kea lavas than previously revealed, indicating mixing processes were more complicated than simple two component mixing between ambient upper mantle and core, or between ambient upper mantle and an enriched mantle component. Tungsten could provide independent or corroborating evidence for core-mantle interaction. Mass balance arguments suggest that the core should be depleted in 182W relative to the silicate Earth by about 2 parts per 10,000. One previous study reported no resolvable depletion of 182W in Hawaiian picrites that do show coupled 187Os-186Os enrichments (Schersten et al., 2004). Our measurements of W concentrations in Hawaiian picritic lavas, however, reveal generally high concentrations that are inconsistent with derivation from a W depleted mantle source. The excess W may reflect the presence of recycled W-rich crust in the mantle sources of at least some of the lavas. At present there are insufficient data to assess whether W concentration correlates with other indicators of crustal contamination or whether any Hawaiian picrites are sufficiently depleted in W so as to be useful in the search for a core component.
1
Department of Earth and Marine Sciences, The Australian National University, Australia ([email protected]) 2 Cooperative Research Centre for Landscape Environments and Mineral Exploration (CRC LEME), Australia 3 Centre for Resource and Environmental Studies, The Australian National University, Australia 4 CSIRO Land and Water, Adelaide, Australia Sulfate has been concentrated in the Loveday Basin of the lower Murray floodplains, Australia, by evaporation of saline waters. The presence of labile organic carbon and a permanent water cover have resulted in biologically mediated sulfate reduction and the formation of environmentally significant sulfide deposits (Lamontagne et al., 2004). Unlike coastal acid-sulfate-soils, the alkalinity produced by sulfate reduction has not been flushed from these sediments and the basin has sufficient carbonate neutralising capacity to buffer acidity produced from sulfide oxidation. Sulfur (gypsum, pyrite and monosulfides) and carbon (carbonate and organic carbon) are largely concentrated in the upper 40 cm of the basin. However, their distribution and form can vary greatly throughout these surface sediments due to the development of cracks and peds following several wetting and drying cycles. During dry periods, gypsum and carbonate salts are concentrated within oxidised crack sediments and covering peds as salt efflorescence. Conversely, pyrite framboids are concentrated below the oxidised surface of crack sediments and within the cores of peds. Despite the excess of stored alkalinity at the scale of the wetland, acidic micro-environments characterised by the presence of jarosite occur locally where pyrite and carbonate are separated. Upon rewetting, biological materials rapidly grow and accumulate in sediment cracks, turning the oxidised sediment surface back into a substrate for sulfate reduction. The dynamic cycling of sulfur and carbon in surface sediments of the Loveday Basin is controlled by physical, chemical and biological processes. Repeated wetting and drying of sediments has resulted in cyclic oxidation and reduction leading to the heterogeneity of the sulfur and carbon stores.
Reference References Brandon, A.D. et al., 1999. Earth Planet. Sci. Lett. 174, 25–42. Schersten, A. et al., 2004. Nature 427, 234–237.
Lamontagne, S., Hicks, W.S., Fitzpatrick, R.W., Rogers, S., 2004. CRC LEME open file report, 167. doi:10.1016/j.gca.2006.06.1488
doi:10.1016/j.gca.2006.06.1487
A683
The search for evidence of chemical interactions between the core and mantle
Spatial heterogeneity of S and C stores in an inland acid sulfate soil
R.J. WALKER1, T. IRELAND1, A.D. BRANDON2
L.J. WALLACE1,2, D.C. MCPHAIL1,2, S. WELCH1,2, D. KIRSTE1,2, S. BEAVIS2,3, S. LAMONTAGNE2,4, R.W. FITZPATRICK2,4
1
Department of Geology, University of Maryland, College Park, MD 20742, USA ([email protected]; tireland@geol. umd.edu) 2 NASA-JSC, 2101 NASA Road 1, Mail Code KR, Houston TX 77058, USA ([email protected]) Coupled 187Os-186Os systems and 182W have been proposed to be potentially useful in the search for evidence for chemical/isotopic interaction between the core and silicate Earth. The premise behind Os as a diagnostic tool is that the outer core has evolved to an isotopic composition that is distinct from anything produced in the mantle. Also, for the core to have evolved to a unique composition, the inner core must have formed sufficiently early and solid metal-liquid metal distribution coefficients must be sufficiently large. Whether or not these conditions are met by the core is debatable. Previous work has reported coupled enrichments consistent with predicted outer core contributions to some Hawaiian picritic lavas (Brandon et al., 1999). Although sources for similar coupled enrichments have been proposed for some mantle processes, such sources have not yet been confirmed by measurement. Higher precision measurements of Hawaiian picritic lavas reveal greater complexities than earlier studies reported. For example, the correlation between 187Os and 186Os is less well defined for Mauna Kea lavas than previously revealed, indicating mixing processes were more complicated than simple two component mixing between ambient upper mantle and core, or between ambient upper mantle and an enriched mantle component. Tungsten could provide independent or corroborating evidence for core-mantle interaction. Mass balance arguments suggest that the core should be depleted in 182W relative to the silicate Earth by about 2 parts per 10,000. One previous study reported no resolvable depletion of 182W in Hawaiian picrites that do show coupled 187Os-186Os enrichments (Schersten et al., 2004). Our measurements of W concentrations in Hawaiian picritic lavas, however, reveal generally high concentrations that are inconsistent with derivation from a W depleted mantle source. The excess W may reflect the presence of recycled W-rich crust in the mantle sources of at least some of the lavas. At present there are insufficient data to assess whether W concentration correlates with other indicators of crustal contamination or whether any Hawaiian picrites are sufficiently depleted in W so as to be useful in the search for a core component.
1
Department of Earth and Marine Sciences, The Australian National University, Australia ([email protected]) 2 Cooperative Research Centre for Landscape Environments and Mineral Exploration (CRC LEME), Australia 3 Centre for Resource and Environmental Studies, The Australian National University, Australia 4 CSIRO Land and Water, Adelaide, Australia Sulfate has been concentrated in the Loveday Basin of the lower Murray floodplains, Australia, by evaporation of saline waters. The presence of labile organic carbon and a permanent water cover have resulted in biologically mediated sulfate reduction and the formation of environmentally significant sulfide deposits (Lamontagne et al., 2004). Unlike coastal acid-sulfate-soils, the alkalinity produced by sulfate reduction has not been flushed from these sediments and the basin has sufficient carbonate neutralising capacity to buffer acidity produced from sulfide oxidation. Sulfur (gypsum, pyrite and monosulfides) and carbon (carbonate and organic carbon) are largely concentrated in the upper 40 cm of the basin. However, their distribution and form can vary greatly throughout these surface sediments due to the development of cracks and peds following several wetting and drying cycles. During dry periods, gypsum and carbonate salts are concentrated within oxidised crack sediments and covering peds as salt efflorescence. Conversely, pyrite framboids are concentrated below the oxidised surface of crack sediments and within the cores of peds. Despite the excess of stored alkalinity at the scale of the wetland, acidic micro-environments characterised by the presence of jarosite occur locally where pyrite and carbonate are separated. Upon rewetting, biological materials rapidly grow and accumulate in sediment cracks, turning the oxidised sediment surface back into a substrate for sulfate reduction. The dynamic cycling of sulfur and carbon in surface sediments of the Loveday Basin is controlled by physical, chemical and biological processes. Repeated wetting and drying of sediments has resulted in cyclic oxidation and reduction leading to the heterogeneity of the sulfur and carbon stores.
Reference References Brandon, A.D. et al., 1999. Earth Planet. Sci. Lett. 174, 25–42. Schersten, A. et al., 2004. Nature 427, 234–237.
Lamontagne, S., Hicks, W.S., Fitzpatrick, R.W., Rogers, S., 2004. CRC LEME open file report, 167. doi:10.1016/j.gca.2006.06.1488
doi:10.1016/j.gca.2006.06.1487