Download Mantle Plumes and Intraplate Volcanism Volcanism on the Earth

Mantle Plumes and Intraplate
Volcanism
Origin of Oceanic
Island Volcanoes
EAS 302 Lecture 20
Volcanism on the Earth
• Mid-ocean ridges
(>90% of the volcanism)
–
“constructive” plate
margins
• Subduction-related
(much of the rest)
–
“destructive plate”
margins
• Volcanism in plate
interiors (usually)
–
, e.g., Yellowstone, Hawaii
not explained by the
plate tectonic paradigm.
Characteristics of Intra-plate
Volcanoes
• Not restricted to plate
margins.
• Occur at locations
that are stationary
relative to plate
motions, “hot
spots”(pointed out by
J. T. Wilson, 1963).
• Distinctive isotopic
and trace element
composition.
Hot Spot Traces on the Pacific
Ocean Floor
The Mantle Plume Model
• “ Hot spot” volcanoes are manifestations of
mantle plumes: columns of hot rock rising
buoyantly from the deep mantle
– This idea proposed by W. J. Morgan in 1971.
• Evidence
–
–
–
Maintain (almost) fixed positions relative to each
other; i.e., they are not affected by plate motions
A number of “hot spots” are associated with
“swells”, indicative of hot mantle below
Their magmas are compositionally distinct from
mid-ocean ridge basalts and therefore must be
derived from a different part of the mantle
Current Mantle Plumes
The Hawaiian Mantle Plume
Age of Hawaiian Volcanism
The Hawaiian “Swell”
Plumes at the surface
• In the last 100-200
km, the plume
begins to melt.
• Once it reaches the
base of the
lithosphere, it can
no longer rise and
spreads out.
Isotopic Compositions of
Oceanic Island Basalts
• Nd and Sr isotope ratios
12
12
DMM
distinct from MORB:
10
10
derived from separate
MORB
88
reservoir which is less
66
Society
depleted (and
ε Nd
44
Nd
sometimes enriched) in
HIMU
22
incompatible elements.
00
St. Helena
• Isotopic compositions
EM II
-2
-2
fall into just a few
Kerguelen
-4
-4
groups: there are only a
EM I
-6
-6
few “flavors” of plumes.
.702
.702 .703
.703 .704
.704 .705
.705 .706
.706 .707
.707 .708
.708
87
87Sr/
86Sr
Sr/86
Sr
Chemical Histories of
Mantle Plumes
• Apparently, the material in mantle plumes has
evolved through just a few pathways. What are
these?
No one is certain. Some ideas:
– 1. Mixing between primitive and depleted mantle
– 2. Recycling of oceanic crust and sediment
– 3. “Delamination” of the mantle lithosphere beneath the
continents, which could be enriched in incompatible
elements by melts or fluids migrating into it from below
– 4. Component of core material in some plumes?
In ideas 2 & 3 above, the material, being dense, would
sink to the core-mantle boundary where it would be slowly
reheated until it is buoyant enough to rise.
“Primitive” plume source
• Mantle Plumes
come from a
deep “primitive”
(i.e., chemically
unprocessed
since the Earth
formed)
reservoir.
• But their isotopic
and trace
element
compositions
are not
“primitive”.
•
Recycling of Oceanic Crust
and Sediment
Oceanic crust and
sediment subducts
and sinks to the deep
mantle.
– This is because it is
cold, and rich in iron.
• This material is heated
by the core, and
eventually becomes
hotter than overlying
mantle.
• When hot enough, it
rises buoyantly as
plumes.
Recycling of Subcontinental
Lithosphere
• Lithosphere
beneath
continents
enriched in
incompatible
elements by
melts from below.
• Collisions
“delaminate” this
lithosphere,
which sinks to
deep mantle.
Where do mantle plumes
come from?
• must be
generated at
thermal
boundary layer:
two possibilities
–
–
660 km
discontinuity
the coremantle
boundary
D’’ as the source of plumes?
• Hot spots
seem to be
located over
regions of
high seismic
wave velocity
gradient in
the lower
mantle.
Seismic Confirmation of Deep
Mantle Plumes
Montelli et al., Science 303: 338, 2004
Mantle Plumes, Large
Igneous Provinces, and
Climate
• Theory says that new
plumes need large heads to
initiate buoyant rise.
• When these “heads” reach
the surface, they produce
large pulses of volcanism,
know as “flood basalts”,
“plateau basalts”, “oceanic
plateaus” or “large igneous
provinces”.
• CO2 released by these
events may change climate.
Flood Basalts and LIPS
(Large Igneous Provinces)
Cretaceous Flood Basalts
• Numerous new plumes surfaced in the Cretaceous,
creating “oceanic plateaus” and flood basalts.
• Were these plumes responsible for the warm
Cretaceous climate?