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The Story of Chemical Plumes and the Hawaii Mess

The Story of Chemical Plumes and the Hawaii Mess. Zoning of the plume. If it’s thermal zoning, will it be chemical too? Should be Inner - deep Outer- shallow If the plume is zoned, different isotopic components should be idetifiable. MORB melts. Trace Elements: REEs.

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The Story of Chemical Plumes and the Hawaii Mess

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  1. The Story of Chemical Plumes and the Hawaii Mess

  2. Zoning of the plume • If it’s thermal zoning, will it be chemical too? • Should be • Inner - deep • Outer- shallow • If the plume is zoned, different isotopic components should be idetifiable.

  3. MORB melts

  4. Trace Elements: REEs Figure 14-2. After Wilson (1989) Igneous Petrogenesis. Kluwer.

  5. Trace Elements: REEs La/Yb (REE slope) correlates with the degree of silica undersaturation in OIBs • Highly undersaturated magmas: La/Yb > 30 • OIA: closer to 12 • OIT: ~ 4 • (+) slopes  E-MORB and all OIBs  N-MORB (-) slope and appear to originate in the lower enriched mantle

  6. MORB-normalized Spider Diagrams Figure 14-3. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Data from Sun and McDonough (1989).

  7. 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

  8. Simple Mixing Models Ternary All analyses fall within triangle determined by three reservoirs Binary All analyses fall between two reservoirs as magmas mix Figure 14-5. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

  9. Sr - Nd Isotopes Figure 13-12. Data from Ito et al. (1987) Chemical Geology, 62, 157-176; and LeRoex et al. (1983) J. Petrol., 24, 267-318.

  10. m Figure 14-6. After Zindler and Hart (1986), Staudigel et al. (1984), Hamelin et al. (1986) and Wilson (1989).

  11. Mantle Reservoirs 1.DM (Depleted Mantle) = N-MORB source Figure 14-6. After Zindler and Hart (1986), Staudigel et al. (1984), Hamelin et al. (1986) and Wilson (1989).

  12. 2.BSE (Bulk Silicate Earth) or the Primary Uniform Reservoir Figure 14-6. After Zindler and Hart (1986), Staudigel et al. (1984), Hamelin et al. (1986) and Wilson (1989).

  13. 3.EMI = enriched mantle type I has lower 87Sr/86Sr (near primordial) 4.EMII = enriched mantle type II has higher 87Sr/86Sr (> 0.720, well above any reasonable mantle sources Figure 14-6. After Zindler and Hart (1986), Staudigel et al. (1984), Hamelin et al. (1986) and Wilson (1989).

  14. OAHU Pali

  15. Pali Significance of the Koolau component From Lassiter &Hauri, 1996

  16. Koolau component-recycled crust? Eiler et al., 1996 Lassiter and Hauri, 1998

  17. 2mm (a,b,c), 1mm (d) Plg-lherzolite-a,b,c, Sp lherzolite-d a b d c

  18. Sr-Nd isotopes @Pali, Salt Lake Crater & Koolau

  19. ???

  20. Assumptions: characteristic distances in PL: grain size~0.1 cm, average cpx-plg separation ~0.3 cm; ccpx/col~100, temperature ~10000C. Geologic scenario: depleted shallow mantle + enriched melt percolating through. Observations: O isotopes in high-T equilibrium, radiogenic isotopes not. Time-scales of events (t~x2/D): 1. Sr, Nd attain equilibrium with a melt in 103-104 yrs. “Metasomatic” melting event must be on the order of a few thousand years. 2. Without a melt phase, cpx-plag would equilibrate in these rocks in ~10-100 Ma. Can’t tell whether the exotic melt phase introduced during plume magmatism or late MORB. 3. Oxygen isotopes should equilibrate with melt in less than 5,000 yrs (olivine is slowest).

  21. Significance of “Pali plagioclase component”: • Definitely recycled; • Definitely related to Koolau surface lavas; • Definitely reacts pervasively with the Pacific MORB mantle at this location; • Could be old oceanic crust recycled within the plume or local contamination; • Mass balance works against local contamination if the Koolau volcano has the same isotopes throughout • Temperature of alteration must have been low - also a problem for local contamination; • If recycled and old, why not see the old Nd signatures? Why the ultradepleted nature of the REEs?

  22. The mess • We cannot really distinguish shallow now from shallow way back; • Chemical dynamics is not capable of telling deep plumes from shallow plumes; • The sizes of heterogeneities are also largely unconstrained; • The OIB isotopic experiment has failed. See you in Hawaii.

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