A Large-scale isotope anomaly in the Southern Hemisphere mantle

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A Large-scale isotope anomaly in the Southern
Hemisphere mantle

• Stanley R. Hart

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Figure 14-9. (Winter) From Hart (1984) Nature,
309, 753-756.
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Can Generate tholeiitic and alkaline basalts from
a chemically uniform mantle

• Variables
• Temperature
• Pressure

Figure 10-2 (Winter) Phase diagram of aluminous
lherzolite with melting interval (gray),
sub-solidus reactions, and geothermal gradient.
After Wyllie, P. J. (1981). Geol. Rundsch. 70,
128-153.
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Trace Elements REEs
Figure 14-2. (Winter) After Wilson (1989) Igneous
Petrogenesis. Kluwer.
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Isotope Geochemistry

• Isotopes do not fractionate during partial
  melting of fractional melting processes, so will
  reflect the characteristics of the source
• OIBs, which sample a great expanse of oceanic
  mantle in places where crustal contamination is
  minimal, provide incomparable evidence as to the
  nature of the mantle

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Pb Isotopes

• Pb produced by radioactive decay of U Th
• 238U ? 234U ? 206Pb
• 235U ? 207Pb
• 232Th ? 208Pb

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• Pb is quite scarce in the mantle
• Mantle-derived melts susceptible to contamination
• U, Pb, and Th are concentrated in continental
  crust (high radiogenic daughter Pb isotopes)
• 204Pb is non-radiogenic, so 208Pb/204Pb,
  207Pb/204Pb, and 206Pb/204Pb increase as U and Th
  decay
• Oceanic crust has elevated U and Th content
  (compared to the mantle) as will sediments
  derived from oceanic and continental crust
• Pb is a sensitive measure of crustal (including
  sediment) components in mantle isotopic systems

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Figure 14-8. After Wilson (1989) Igneous
Petrogenesis. Kluwer. Data from Hamelin and
Allègre (1985), Hart (1984), Vidal et al. (1984).
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• Can map the geographic distribution of the
  isotopic data

Figure 14-9. From Hart (1984) Nature, 309,
753-756.
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• Isotopically enriched reservoirs (EMI, EMII, and
  HIMU) are too enriched for any known mantle
  process, and they correspond to crustal rocks
  and/or sediments
• EMI (slightly enriched) correlates with lower
  continental crust or oceanic crust
• EMII is more enriched, especially in radiogenic
  Sr (indicating the Rb parent) and Pb (U/Th
  parents) correlates with the upper continental
  crust or ocean-island crust

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A Model for Oceanic Magmatism
Continental Reservoirs
DM
OIB
EM and HIMU from crustal sources (subducted OC
CC seds)
Figure 14-10. Nomenclature from Zindler and Hart
(1986). After Wilson (1989) and Rollinson (1993).