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
I don’t intend to dwell on the obvious:-
1. Ocean Island Basalt (OIB) mantle is less depleted
and more diverse than Mid Ocean Ridge Basalt
(MORB) mantle.
2. This diversity is widely attributed to subducted
crustal components.
3. There has to be a mechanism to return these
subducted components to shallow depths of
melting (~ 130 – 90 km in the case of Hawaii).
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.
Volcanic Growth Stages, (Stearns, 1946)
The
submarine
stage
Shield
building pre-shield
stage reflects
(Loihi)
is characterized
by low
Post-shield
stage issupply.
characterized
increased
magma
melting
and
magma
supply.
by
a return
low magma
supply,
Eruption
ofto
tholeiites
and picrites
Eruption
of alkalic
alkalic
basalts
eruption
of
lavas
as the
as the volcano
traverses
the
axial
followed
byplume
tholeiites
reflecting
volcano
nears
the margins
of the
zone of the
initiation of volcanism at the
plume.
margins of the plume.
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 300400 ka and onset of postshield 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!
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
Loihi
The distance from Loihi to Hualalai (94 km)
provides a constraint on its dimensions.
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.
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.
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.
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.
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.
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 (>14.5) 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.
Sr – Pb
Isotopes
•
•
•
There is a progressive increase in a Kea (or Loihi?) component
in Mauna Loa lavas as they age (0 to > 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.
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 (> 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 <100 ka Mauna Loa lavas.
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.
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).