Download The Mantle and Creation of the Oceanic Crust The Mantle

The Mantle and Creation of the
Oceanic Crust
EAS 302 Lecture 28
The Mantle
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The Earth’s mantle is 84% of the volume
of the Earth and 68% of its mass.
Convection in the mantle drives plate
tectonics.
Melting and degassing of the mantle have
created the crust and atmosphere.
Therefore, an understanding of the
Earth’s evolution and behavior requires
an understanding of the mantle.
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Ways of Studying the Mantle
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Seismic waves and other geophysical
approaches (gravity, electrical
conductivity).
Direct samples:
 “peridotite massifs”
 Oceanic peridotites
 Xenoliths (pieces of mantle carried to the surface
in volcanic eruptions)
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Indirect samples:
 Basalts and other mantle-derived magmas
Seismic Structure of the Mantle
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Upper mantle seismic
velocities consistent with
densities of ultramafic
rock: peridotite
Uppermost mantle:
 olivine, orthopyroxene,
and clinopyroxene +
plagioclase, spinel, or
garnet (depends on P)
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Transition Zone: 400-670
km depth: series of
phase changes
Lower mantle: Si in
octahedral coordination
in Mg-perovskite +
ferripericlase ((Mg, Fe)O)
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Mineralogy of the Upper Mantle
Silica Tetrahedron
Seismic Transition Zone
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At 300 km, pyroxenes dissolve in garnet,
forming “majorite” garnet
At 400 km, olivine undergoes a structural
change to “β” form.
At 500 km, olivine changes to “γ” or
“spinel” structure.
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Lower Mantle: Silicates Under High
Pressure
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Under extreme pressure,
Si becomes octahedrally
rather than tetrahedrally
coordinated (i.e,
surrounded by 6 oxygens
rather than 4).
This transition accounts
for jump in seismic
velocity at 660 km.
Fewer cations can be
accommodated in this
structure, so some oxides
phases form:
(Mg,Fe)2SiO4 →
Structure of Mg-perovskite
(Mg,Fe)SiO3+ (Mg,Fe)O
Convection and the Rayleigh Number
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Whether a fluid convects or not depends on
whether buoyancy forces exceed viscous forces.
The ratio of these forces is the Rayleigh Number:
Ra =
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αΔTgρd 3
Kη
Where α is the coefficient of thermal expansion,
∆T is the temperature change over the height of
the fluid, d is the height, g is the acceleration of
gravity, ρ is density, K is thermal conductivity and
η is viscosity.
Convection will occur if Ra exceeds 5000.
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Rayleigh Number and Convection in
the Mantle
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For the mantle, α = 3 x 10-5 K-1, ∆T = 3000
K, g = 10 m/s, ρ = 4660 kg/m3, K = 10-6
m2s-1, η = 1021 poise, and d = 2.9 x 106 m.
Therefore, Ra is ~ 108, so convection
must occur.
Details of the convection pattern still
unclear
Sources of heat
 Radioactive decay (U, Th, K)
 Accretional heat
 Latent heat of crystallization of the inner core
 Mantle is heated both from within and below.
Picturing the Mantle with Seismic
Waves
Red = slow seismic
velocity and high
temperature
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How tomography works
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Tomography begins with
measuring seismic
velocities based on travel
times of earthquake
waves.
 From this, identify ray
paths with anomalously
slow or fast average
velocities.
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When ray paths intersect,
it is possible to deduce
where in the path the
velocities are anomalous.
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Geodynamics: Modeling Mantle
Convection
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Problem of modeling
is similar to
modeling
atmospheric
circulation - dense
(cold) areas sink and
light (hot) areas rise.
What do the models tell us
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Whole mantle is probably involved in
convection
The phase change at 660 km slows, but
does not stop convecting material
Fully 3 dimensional, spherical model of
mantle convection including plate
tectonics is still not achievable with
present computers.
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Boundary Layers and Plate Tectonics
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Asthenosphere is the convecting part of the
mantle.
Lithosphere is a thermal and mechanical
boundary layer. It is a thermal boundary layer in
that heat is conducted, not convected, through
it; it is a mechanical boundary layer in that it
responds to stress by brittle fracture rather than
plastic flow.
Oceanic lithosphere is created as
asthenosphere cools to be point that it becomes
rigid. Oceanic lithosphere progressively
thickens away from mid-ocean ridges. This
lithosphere also contacts as it cools.
Plate Tectonics and Convection
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Plate Tectonics is the convection-driven motion
of the boundary layer.
Plates are part of the convection system.
 Volcanism at mid-ocean ridges releases heat to the surface.
 Subduction of cool lithosphere cools the Earth’s interior.
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Forces acting on plates
 “Ridge push” (active upwelling pushing plates)
 “Drag”: asthenosphere drags lithosphere above it.
 “Slab pull”: Subducting lithosphere pulls plates away from
ridges.
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“Slab pull” is the most important force acting on
plates.
Plates also influence motion of mantle at depth.
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East Pacific Rise (EPR)
Melting and Volcanism
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In most places, melting occurs as a
consequence of decompression of rising
mantle.
In this way, volcanoes are much like
clouds.
 Warm air rises because of its lower density.
 As it rises, it decompresses and cools.
 Clouds form when air rises far enough that a
phase boundary is crossed and water vapor
condenses.
 In a similar way, warm mantle rises because it is
less dense. Eventually, it may cross a phase
boundary, and partial melting occurs.
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Decompression Melting
Mid-Ocean Ridge Basalt (MORB)
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Compositionally uniform tholeiitic basalts
(tholeiites have more SiO2 than alkali basalt).
Larger extent of melting (~10%) and shallower
melting than alkali basalt
Compositional variation can be related to axial
depth and proximity to “hot spots”
Poor, i.e., depleted, in the incompatible elements
(e.g., light rare earth depleted)
Isotopic compositions indicate long standing (~
Ga) depletion of the mantle source.
Mid-ocean ridges thus appear to sample that
part of the mantle that is complimentary to the
continental crust. This is presumably the upper
mantle and is termed the “depleted mantle”.
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Formation of the Oceanic Crust
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Melts of the mantle beneath mid-ocean ridges rise
because they are less dense than the solid mantle.
These melts form the oceanic crust.
S-wave image of the East Pacific Rise
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Magma Chambers of the EPR
Oceanic Crust
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Three main layers:
 Layer 2A Lava flows
(magma erupted on the
surface)
 Layer 2B Sheeted Dike
Complex
 (magma crystallized on
way to surface)
 Layer 3 Gabbro
(magma that has
crystallized in magma
chamber).
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Total thickness:
~6km.
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Ridges and Rises
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Mid-Atlantic Ridge
(MAR) has rift valley,
EPR does not.
MAR has steep flanks,
EPR does not.
EPR has permanent
(“steady state”) magma
chamber, MAR does
not.
Why the difference?
Why are mid-ocean ridges ridges?
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Ridges stand above the surrounding seafloor by ~ 2 km.
The are not elevated because of a build-up of lava flows.
The oceanic crust is typically 6 km thick everywhere (if
anything, crust is thinner right at the axis).
Ridges and rises are elevated is because they are hot and
thermally expanded.
A thought experiment:
 Coefficient of thermal expansion, α, is only 10-5.
 If the outer 100 km (lithospheric thickness) is 200°C hotter, then:
 200˚C × 10-5 × 100 km = 2 km
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After formation, the lithosphere slowly cools and
thermally contracts. Consequently, the seafloor gets
progressively deeper.
The cooling deepens only on time (decreases with the
square root of age), not on spreading rate.
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Ridges and Rises: the difference is
spreading rate
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Graben forms on MAR
because of tension
 On EPR, volcanism is to
frequent for a graben to
develop
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Difference in flank
steepness is due to
difference in spreading
rate.
On the MAR, magma flux
(and therefore heat flux)
is not high enough to
keep the magma
chamber from freezing.
Spreading Rates and Sealevel Change
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Because faster spreading ridges remain higher
further from the ridge axis, they occupy volume
that would otherwise be occupied by seawater.
If global average seafloor spreading rates
increase, sealevel will rise.
Seawater then spills over onto the continental
margins.
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Hydrothermal Processes
 Basalt
fractures
as it cools,
allowing water to
penetrate the
young oceanic
crust.
 Water is heated
and reacts with
the oceanic
crust.
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Principle Hydrothermal Reactions
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Precipitation of
Anhydrite (CaSO4).
Removal of Mg from
seawater, acidification:
 Mg2+ + Mg2Si2O6 + 3H2O
→ Mg3Si2O5(OH)4 + 2H+
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Reduction of sulfate:
 SO42- + 8FeO → S2– +
4Fe2O3
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Dissolution of Fe, Mn, Zn,
Cu, etc.
 Fe(solid) + 3H+ →
Fe(diss) + 3H+(solid)
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Precipitation of sulfides
and hydroxides
 Cu2+ + S2– → CuS
 Mn + 2OH- → Mn(OH)2
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Implications of Ridge Crest
Hydrothermal Activity
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“Buffers” composition
of seawater (e.g.,
important ‘sink’ for
Mg)
Responsible for many
“base metal” (e.g., Cu,
Zn, Pb) ores
Metamorphoses and
“hydrates” oceanic
crust
Sustains unique
chemosynthetic
communities.
Did life originate at hydrothermal
vents?
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Energy source
 Chemosynthesis is simpler
than photosynthesis
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Chemical raw materials
 Variety of chemical raw
materials
 Also, variety of mineral
surfaces to catalyze reactions.
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Insulation from the hostile
surface environment
 Protection from UV radiation
 Some protection meteorite,
asteroid bombardment
 Highly variable climate
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Vent bacteria (Archaea) there
are among the simplest,
most primitive organisms
known.
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