Geochemical Arguments Favoring an Hawaiian Plume

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Geochemical Arguments Favoring an Hawaiian
Plume J. Michael Rhodes University of
Massachusetts Dominique Weis University of
British Columbia Michael O. Garcia University of
Hawaii Marc Norman Australian National University
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• I dont intend to dwell on the obvious-
• Ocean Island Basalt (OIB) mantle is less depleted
  and more diverse than Mid Ocean Ridge Basalt
  (MORB) mantle.
• This diversity is widely attributed to subducted
  crustal components.
• There has to be a mechanism to return these
  subducted components to shallow depths of melting
  ( 130 90 km in the case of Hawaii).

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These cartoons illustrate the point that the
Hawaiian plume is thought to be concentrically
zoned in both temperature and composition. If
so, in the last 300 - 500 ka Mauna Loa and Mauna
Kea will have traversed about 30 - 50 km of the
plume. Over this time period we might expect to
see changes in magma composition reflecting
changes in melting, melt supply and changes in
source components.
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Volcanic Growth Stages, (Stearns, 1946)
The submarine pre-shield stage (Loihi) is
characterized by low melting and magma supply.
Eruption of alkalic basalts followed by
tholeiites reflecting initiation of volcanism at
the margins of the plume.
Shield building stage reflects increased magma
supply. Eruption of tholeiites and picrites as
the volcano traverses the axial zone of the plume
Post-shield stage is characterized by a return to
low magma supply, eruption of alkalic lavas as
the volcano nears the margins of the plume.
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Results from the Hawaii Scientific Drilling
Project confirm high magma supply rates, eruption
of tholeiites and picrites, between 600 and 400
ka. Followed by decline in eruption rates
between 300- 400 ka and onset of post-shield
volcanism and eruption of alkalic basalts.
Note. Model growth curve of DePaolo Stolper
(1996) was based on a simple geometric model of a
thermally zoned plume, prior to dating!
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Evolution of Hawaiian volcanoes from an alkalic
pre-shield stage, through a tholeiitic shield
stage, to an alkalic post shield stage is
consistent with movement of the Pacific plate
over a thermally zoned melting anomaly.
Hualalai
94 km
Loihi
The distance from Loihi to Hualalai (94 km)
provides a constraint on its dimensions.
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SiO2 TiO2 and CaO differ at a given value of MgO
between Hawaiian volcanoes. This is presumably a
consequence of differences in melting and melt
segregation processes in different parts of the
plume. Given thermal gradients we might expect
to see changes in these values as a volcano
transits the Hawaiian plume.
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This is the case for Mauna Kea
SiO2 in basalts (normalized to 17 MgO) is
dependent on depth of melt segregation and on the
extent of melting. Marked decrease in SiO2(17)
after 320 ka reflects a decline in melting and
melt production as the volcano enters the
post-shield stage. Increase in incompatible trace
data (e.g. Nb/Y) supports the interpretation.
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Not so for Mauna Loa!
In contrast Mauna Loa shows no obvious change in
SiO2(17) or Nb/Y in about 400 ka. This implies
that melting conditions have remained relatively
uniform as Mauna Loa transits about 30 to 40 km
of the Hawaiian plume.
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Magma production and evolution of Hawaiian
volcanoes is frequently presented like this
But perhaps the Mauna Loa data is telling us it
should really be like this with a wide, hot,
central core.
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How hot is the plume relative to ambient mantle?

• Compare Mauna Loa with MORB
• Maximum Fo in olivine in both magmas is close to
  Fo91.3
• TMauna Loa 1547 oC
• TMORB 1401 oC
• Difference 146 oC

You can play around with olivine compositions and
KD, but the results are the same a hotter Mauna
Loa magma relative to MORB, implying significant
differences in mantle potential temperatures.
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He Isotopes

• Both volcanoes exhibit a decline in 3He/ 4He with
  decreasing age
• Interpreted as a decline in an undegassed
  (primitive?) mantle component as the volcanoes
  approaches the plume margin.
• High (gt14.5) 3He/ 4He at Mauna Kea are spikes of
  Loihi-like lavas inter-layered with normal
  lavas. Kurtz et al. (2004) interpret this as
  evidence for an asymmetric plume.

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Sr Pb Isotopes

• There is a progressive increase in a Kea (or
  Loihi?) component in Mauna Loa lavas as they age
  (0 to gt 400 ka).
• Mauna Kea and Kilauea lavas are similar (0 to
  600 ka).
• Consistent with a zoned plume in which Mauna Loa
  is closer to the axis and Mauna Kea and Kilauea
  are closer to the margins.

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Pb Pb Isotopes
High-precision Pb data from Abouchami et al.
(2000, 2005) and unpublished data of Weis.

• Distinct bilateral asymmetry in the Pb data
  between Loa and Kea trends.
• Older Mauna Kea (gt 320 ka) overlaps with Kilauea
  long-lived ( 400 ka) heterogeneities sampled
  by the two volcanoes.
• Mauna Loa lavas become progressively more like
  Loihi (not Kea!) lavas with increasing age ( 100
  to 400 ka).
• Hualalai submarine tholeiites overlap with lt100
  ka Mauna Loa lavas.

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Implications for a Zoned Plume?

• Distinct bilateral asymmetry in the plume (not
  concentric).
• Mauna Kea would have been close to where Kilauea
  is today 500 600 ka ago. Implies long-lived,
  vertically stretched source components.
• Mauna Loa was closer to Loihi at 400 ka,
  consistent with greater proportion of Loihi
  components in Mauna Loa lavas at that time.

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Summary

• Evolution of Hawaiian volcanoes is consistent
  with a thermally zoned plume. This in itself
  requires that the mantle source is hotter than
  the surrounding mantle.
• Temperatures of Hawaiian primary magmas are
  hotter than MORB primary magmas.
• He isotopes are consistent with an undegassed
  (primitive?) mantle component in the plume
  center. Distribution, however, is asymmetric.
• Most isotopic data (Sr, Nd, Hf) can be reconciled
  with a concentrically zoned plume resulting from
  entrainment.
• Pb isotopic data require bilateral asymmetry in
  the plume with long-lived vertical
  heterogeneities.
• These inferences are consistent with (but derived
  independently!) recent plume models (Farnetani
  and Samuel, 2004).