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


Can generate tholeiitic and alkaline basalts from a chemically uniform mantle
Can Generate tholeiitic and alkaline basalts from a mantlechemically 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
Trace Elements: REEs mantle

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


Isotope geochemistry
Isotope Geochemistry mantle

  • 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 Isotopes mantle

Pb produced by radioactive decay of U & Th

238U 234U 206Pb

235U 207Pb

232Th 208Pb


Pb is quite scarce in the mantle 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. mantleAfter Wilson (1989) Igneous Petrogenesis. Kluwer. Data from Hamelin and Allègre (1985), Hart (1984), Vidal et al. (1984).


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 too enriched for any known mantle process, and they correspond to

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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