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

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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


Figure 14-9. (Winter) From Hart (1984) Nature, 309, 753-756.


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.


Trace Elements: REEs

Figure 14-2. (Winter) After Wilson (1989) Igneous Petrogenesis. Kluwer.


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


Pb Isotopes

Pb produced by radioactive decay of U & Th

238U 234U 206Pb

235U 207Pb

232Th 208Pb


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


Figure 14-8. After Wilson (1989) Igneous Petrogenesis. Kluwer. Data from Hamelin and Allègre (1985), Hart (1984), Vidal et al. (1984).


  • Can map the geographic distribution of the isotopic data

Figure 14-9. From Hart (1984) Nature, 309, 753-756.


  • 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


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).


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