Transcript Slide 1
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 234U 206Pb 235U 207Pb 232Th 208Pb 238U 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).