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
(<10m to sulphide) 5 HW .
(<10m to sulphide)
(Zr-enriched) S 4 FW 3 2 ents) m
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:
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
Nd Pm Sm
Pm Sm Eu
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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