Download Mantle Convection and Structure

Earth’s Mantle: A View
Through Volcanism’s
Window
William M. White
Dept. of Earth & Atmospheric Sciences
Cornell University
Ithaca NY USA
Terrestrial Volcanism
Subduction Zones
Mid-ocean ridges
Oceanic Islands/Hot spots
Terrestrial Volcanism
Mid-ocean ridges
 Decompression melting as
mantle upwells beneath
diverging lithospheric plates.
Subduction Zones
 Flux melting due to release of
water from sinking
lithosphere.
Hot Spots
 Decompression Melting of
convection plumes upwelling
from deep mantle.
Subduction Zones
Mid-ocean ridges
Oceanic Islands/Hot spots
Mid-Ocean Ridges
 MORB
 Basalt of uniform
composition created by
large (10%) extent of
melting at shallow depth.
 Depleted in incompatible
elements (including K, U,
Th) - probably as a result
of earlier melting
episodes.
 Isotope ratios indicate
ancient depletion.
Subduction
Zones
 Magmas of variable
composition, but with a
number of consistent
features.
 Melts of mantle
“contaminated” by fluids
released from the
subducting plate.
 Provide useful indicators of
what is being subducted.
Hot Spots
 Persistent, stationary
volcanism over 107 to 108
years.
 Basalts of variable
composition, generally
smaller degree, deeper
melts than MORB.
 Variable trace element and
isotopic composition,
generally less “depleted”,
with some hot spots derived
from an incompatible
element enriched source.
The Long Lens of Isotope
Ratios
 Chemical compositions provide an instantaneous view
of the mantle, but radiogenic isotopes provide a view
of the ancient mantle.
 Radiogenic isotope ratios, such as 87Sr/86Sr,
143Nd/144Nd, 206Pb/204Pb provide a time-integrated
measure of the corresponding parent/daughter ratios –
87Rb/86Sr, 147Sm/144Nd, 238U/204Pb.
 Isotope ratios tell us that the mantle has evolved
chemically since the Earth formed - that the hot spot
and mid-ocean ridge volcanoes tap chemical
reservoirs that have remained isolated for long
periods.
The Mantle Array
The “Mantle Zoo”
Age of Heterogeneity
Pb isotope systematics require mantle heterogeneity
be much younger than the Earth
Mantle Processes:
Depletion
Extraction of partial melts to form
continental and oceanic crust depleted
the mantle in incompatible elements
(such as K, U, Th).
Resulting depleted mantle is thought to
dominate the upper mantle* and be the
source of MORB
Acronym: DUM
*the thermodynamics of melting is such that melting, and melt extraction, is probably
only possible in the uppermost mantle. Furthermore, the melt-depleted residue has
lower density that virgin mantle.
Mantle Processes:
Enrichment
It is easy to imagine a geologically
reasonable depletion process, enrichment
has posed more of a dilemma. What
geologic process could enrich or re-enrich the
mantle in the incompatible elements removed
by melting?
The answer is ultimately obvious - you have
to introduce melts into the mantle.
But how?
A Plate Tectonic Solution
Plate tectonics continually produces melts at mid-ocean
ridges and returns them to the mantle at subduction
Hofmann & White
Hypothesis
From Hofmann & White ‘82
A Smoking Gun in Samoa ?
From Jackson et al. 2007
Oxygen Isotopes
Isotope ratios can also change due to
chemical processes.
For most elements, this effect is so small it is
almost immeasurable.
It can be large, however, for light elements
that form a variety of chemical bonds.
O is the best example (also, conveniently, is 50%
of most rocks).
Isotope fractionation is inversely proportional
to the square of temperature, so large
variations in stable isotope ratios can be
produced only at or near the surface of the
Earth.
Oxygen Isotope Variability
in OIB
From Eiler et al. ‘97
O, Sr, and Pb isotope
covariation in Samoan lavas
QuickTime™ and a
decompressor
are needed to see this picture.
From Workman et al. 2008
Mass Balance Considerations
Balancing continental
crust and depleted
mantle indicates about
1/4 to 1/2 the mantle is
“depleted”.
This assumes there are
no other significant
“enriched” or “depleted”
reservoirs in the Earth - a
questionable assumption.
The K-Ar Mass Balance
 Essentially all the Ar in the
atmosphere is 40Ar, and
essentially all of this was
produced by decay of K. It has
escaped from the Earth’s
interior.
 Ar is a gaseous element and will
escape to the atmosphere when
the mantle melts. Residual
mantle should have little Ar.
 Only about 1/2 the mantle need
be melted and degassed to
account for all the Ar in the
atmosphere.
Possible Mantle Structures
142Nd:
Do we really understand
mantle evolution?
 146Sm decays to 142Nd with a
half-life of 103 Ma - would have
completely decayed early in
Earth’s history.
 If the Earth has a chondritic
Sm/Nd ratio, the 142Nd/144Nd of
the Earth should be chondritic,
i.e., e142Nd = 0.
 Is there a missing ‘early
enriched’ reservoir in the deep
mantle?
 Do we not understand the early
solar system?
From Boyet & Carlson ‘05
Summary
 The mantle is chemically heterogeneous, particularly with
respect to K, U and Th.
 Variations in the concentrations of these elements could exceed
an order of magnitude.
 This heterogeneity results from removal of partial melts
and reintroduction of melts through subduction of crust
into other parts of the mantle.
 Significant uncertainties remain in both the mass fraction
of depleted and enriched reservoirs and in their physical
location in the mantle.
However, the most probable configuration would
include a large reservoir in the upper mantle that is
depleted in radioactive elements, with various enriched
reservoirs in the lower mantle.