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Under normal oxidizing conditions this will yield a

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Unformatted text preview: involves no oxidation of H2S, therefore, there is an excess of S in the water column and sediments. Monday, 13 August, 12 7CH2O+4SO4+Fe2+ → 2FeS2+7HCO3-+3H++2H2O From Leventhal (1995) after Raiswell and Berner (1985, 1986) Monday, 13 August, 12 From Goodfellow et al. (2003) Monday, 13 August, 12 6 HW HW . (<10m to sulphide) 5 HW . (<10m to sulphide) (Zr-enriched) S 4 FW 3 2 ents) m d Sedi xidize rn O ode .36 (M /C = 0 S 1 0 0 1 2 3 Corg Monday, 13 August, 12 4 5 Mn and Anoxia • • Mn has three valence states: Mn4+, Mn3+, and Mn2+. • Under anoxic conditions Mn4+ is reduced to Mn2+ and Mn3+ and MnO2 becomes soluble. With this anoxic waters are enriched in Mn with the anoxic sediments being depleted in Mn (Jacobs and Emerson, 1982; Jacobs et al. 1985, 1987; Calvert and Pedersen 1993, Quinby-Hunt and Wilde, 1994. • Thus, shales formed under anoxic conditions have low Mn contents (<0.1%). Monday, 13 August, 12 Under oxidizing ocean conditions Mn occurs in Mn4+ state and readily precipitates as MnO2 into oxidized sediments. Modified from: Goodfellow and Peter, 1999; Goodfellow et al., 2003) Monday, 13 August, 12 REE and Anoxia • Most REE behave similarly in ocean waters due to similar trivalent state. • Ce is different in that it occurs in both Ce3+ and Ce4+ states. • In oxidized seawater Ce occurs as Ce4+ and is preferentially associated with Mn-oxides and Mn-nodules (e.g., Elderfield et al., 1981). This leads to negative Ce anomalies in seawater and positive Ce-anomalies in Mnnodules (Elderfield et al., 1981; Elderfield and Greaves, 1982; Elderfield, 1988; Alibo and Nozaki, 1999; . • In contrast, in anoxic waters and sediments exhibit flat to positive Ce anomalies with Ce/Ce* ~1 [Ce/Ce* < 1 = negative anomaly, >1 = positive anomaly] Monday, 13 August, 12 1 Rock / Chondrite .1 Oxygenated Seawater .01 Anoxic Seawater Black Smoker Fluids White Smoker Fluids .001 .0001 .00001 Monday, 13 August, 12 La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu 1 Rock / Chondrite Rock / Chondrite .1 Oxygenated Seawater .01 1000 Anoxic Seawater Mn-Nodules Black Smoker Fluids White Smoker Fluids .001 .0001 100 .00001 Monday, 13 August, 12 La La Ce Ce Pr Pr Nd Nd Pm Sm Pm Sm Eu Eu Gd Gd Tb Tb Dy Dy Ho Ho Er Er Tm Tm Yb Yb Lu Lu From Goodfellow et al., 2003 Monday, 13 August, 12 Transition Metals • Most transition metals have multiple REDOX states and have variable behaviour depending on whether basins are oxic or anoxic. • Furthermore, many elements enriched due to adsorption onto organic matter. • Key elements enriched in anoxic sediments include: V, Ni, Zn, Mo, U, Re, Ag. • Ratios of these elements to Al2O3 of La (representing the detrital component in the sediment) can be particularly useful in identifying anoxic sedimentary rocks and anoxic basinal settings. Monday, 13 August, 12 Monday, 13 August, 12 Monday, 13 August, 12 Shale Geochemistry: Summary • • Shales are outstanding monitors of crustal evolution. Outstanding for outlining anoxic basins that typically host massive sulphide deposits. Can be recognized by: • • • • • • Monday, 13 August, 12 high S/C ratios (>>0.32) low MnO contents (<0.01) Flat Ce anomalies and Ce/Ce* ~1 Elevated V, Ni, U, Mo, etc. contents. High V/Al(La), Ni/Al(La). Low Th/U Summary • • • • Monday, 13 August, 12 Basic tenents of sediment geochemistry. Major elements - weathering, sedimentary maturity, CIW. Provenance of clastic sedimentary rocks. Geochemistry of shales....
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