The Story of Chemical Plumes and the Hawaii Mess

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The Story of Chemical Plumes and the Hawaii Mess
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Zoning of the plume

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

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

• La/Yb (REE slope) correlates with the degree of
  silica undersaturation in OIBs
• Highly undersaturated magmas La/Yb gt 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

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MORB-normalized Spider Diagrams
Figure 14-3. Winter (2001) An Introduction to
Igneous and Metamorphic Petrology. Prentice Hall.
Data from Sun and McDonough (1989).
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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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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.
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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.
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Figure 14-6. After Zindler and Hart (1986),
Staudigel et al. (1984), Hamelin et al. (1986)
and Wilson (1989).
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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).
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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).
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• 3. EMI enriched mantle type I has lower
  87Sr/86Sr (near primordial)
• 4. EMII enriched mantle type II has higher
  87Sr/86Sr (gt 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).

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OAHU
Pali
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Pali
Significance of the Koolau component
From Lassiter Hauri, 1996
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Koolau component-recycled crust?
Eiler et al., 1996
Lassiter and Hauri, 1998
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2mm (a,b,c), 1mm (d)
Sp lherzolite-d
Plg-lherzolite-a,b,c,
a
b
d
c
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Sr-Nd isotopes _at_Pali, Salt Lake Crater Koolau
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???
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Assumptions characteristic distances in PL
grain size0.1 cm, average cpx-plg separation
0.3 cm ccpx/col100, 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 (tx2/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. Cant 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).
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• 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?

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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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