Download 660 km

GEOL 4700
‘Field’ Mineralogy
Earth’s Interior
http://ruby.colorado.edu
/~smyth/G4700.html
GEOL 4700
‘Field’ Mineralogy
Earth’s Interior
Multi-anvil
Press
5000 ton
(Bayerisches
Geoinstitut)
Earth’s
Interior
http://ruby.colorado.edu
/~smyth/G4700.html
Earth’s
Interior
The Core is Metal
The Mantle is oxygen minerals
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There could be significant
hydration in the Transition Zone
Water Storage Capacity in
Ocean Units
• Nominally
Anhydrous
Minerals (NAMs)
can incorporate up
to 10 times the
total Ocean mass
if saturated.
How do we know what
is inside the Earth?
• Seismology: Earthquake waves
• Cosmochemistry: Meteorites
• Experiments: Laboratory synthesis
• Xenoliths
Major Parts
• Ocean water 0.025% of mass
• Crust (Above the Moho) 0.5% of mass
– Oceanic (7 to 10 km of basalt & gabbro)
– Continental (30 to 60 km of granite)
• Mantle - Moho to 2900 km - Solid Rock
– 65% of total mass
– Upper (Moho to 410km) (Olivine + pyroxene)
– Transition Zone (410 -670 km) (Silicate Spinels)
– Lower 670 to 2900 km (Perovskite + periclase)
• Core (2900 to 6367km) 35% of mass
– Outer Core (Liquid Metal)
– Inner Core (Solid Metal)
Mantle Mineralogy:
Modal Fraction
Review
Mantle
Mineralogy
Crust
• Oceanic Crust (Basalt and Gabbro)
– Thin (7 - 10 km)
– Dense (2.9 - 3.1 g/cm3)
– Young (< 250 my)
• Continental Crust (Granite + Diorite +
Seds)
– Thick (30 - 60 km)
– Light (2.7 - 2.8 g/cm3)
– Old (250 - 4000 my)
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Mantle
• Upper Mantle (Moho to 410 km)
Seismology
• S-wave shadow
– No S-waves opposite-side
– Peridotite (Olivine + pyroxene)
– Eclogite (Pyroxene + Garnet)
• Transition Zone (410 - 670 km)
– Spinels and Spinelloids
earthquakes
– Core (outer) must be molten
• P and S reflections
– Discontinuities at 410 and 670 km
– Phase Changes
• Lower Mantle (670 - 2900 km)
– MgSiO3 - Perovskite
– MgO - Periclase
• P and S travel time anomalies
– Tomography
– Hot and cold regions
Ringwoodite
Core
• Outer Core (2900 to 5150km)
– Molten iron metal (+ ~10% lighter
element)
– No S-wave transmission
• Inner Core (5150 - 6378 km)
– Solid Iron Metal
– May slowly rotate relative to
mantle
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Formula (Mg,Fe)2SiO4
Up to 2.4 %H2O by weight
Stable at depths of 525 to 660 km.
Third most abundant mineral in Earth
Wadsleyite
How do we know what’s there?
• Seismology
– S-wave shadow
– P and S reflections and conversions
• Meteorites:
– Earth formed from primitive meteorites
• Xenoliths from kimberlites & basalts
• Experiments
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•
•
•
Formula (Mg,Fe)2SiO4
Up to 3.2 %H2O by weight
Stable at depths of 410 to 525 km.
Fourth most abundant mineral in Earth
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Seismology
P-waves
S-waves
Xenoliths
Velocity Structure and Earthquakes at N.
Tonga
Xenoliths
(VanderHilst, Nature (1995)
Red = slow = hot
green = fast = cold
Xenoliths
Xenolith-Peridotite
• Xenoliths are ‘strange rocks’
• Xenoliths are inclusions in
kimberlites and basalts
• Xenoliths provide natural high
pressure rocks.
– Kimberlites bring diamonds and other
rock samples from as deep as 670 km.
– Basalts are derived from the lithospheric
mantle
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Breaking Xenoliths at a Diamond
Mine in South Africa
Meteorites
• Meteorites tell us of Earth’s bulk
composition
– Chondrites are primitive meteorites
– Earth is “chondritic” in composition.
• Composition is similar to sun minus
H, He and other volatiles.
Meteorites
Meteorites
Experiments
• Multi-Anvil Press
– Synthesis experiments to 1000 km depth
– Samples 1-5 mm3
• Diamond Anvil Cell
– Spectroscopy
– Ultrasound
– X-ray diffraction
Multi-anvil
Press
5000 ton
(Bayerisches
Geoinstitut)
5
Multi-anvil Press Sample
Assembly
Experiments
• Multi-Anvil Press
– Synthesis experiments to 1000 km depth
– Samples 1-5 mm3
• Diamond Anvil Cell
– Spectroscopy
– Ultrasound
– X-ray diffraction
Multi-anvil Press Sample
Assembly
Experiments
• Multi-Anvil Press
– Synthesis experiments to 1000 km depth
– Samples 1-5 mm3
• Diamond Anvil Cell
– Spectroscopy
– Ultrasound
– X-ray diffraction
Carbide cubes retain a lot of
strain
Multi-anvil Press
Octahedral Assembly:
After and Before
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Multi-Anvil Press Octahedron
Interior Terms
Diamond-anvil Cell
Interior Terms
Interior Terms
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Interior Terms
Interior Terms
DAC on Diffractometer
Mica Crystal with Quartz @ 5GPa
Interior Terms
8
525 km
Wadsleyite
410km:
Olivine
Density 3.2
Olivine
Ringwoodite
Ringwoodite Crystal
Wadsleyite
3.5 g/cm3
410km:Olivine
Ringwoodite Crystal with Quartz
Wadsleyite
Wadsleyite
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660 km
Ringwoodite
Density 3.6
Perovskite + MgO
4.2 g/cm3
Convection in solid mantle
• Driven by U, Th, K decay
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•
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660 km
Ringwoodite
Tetrahedral Si
Perovskite + MgO
Octahedral Si
(internal engine)
Moves about 2 cm/yr
Causes earthquakes
Moves plates
Splits and rejoins continents
One Cell or two?
Convection in solid mantle
Mantle Density vs Depth
4800
Magnetic Field
• Due to electrical currents from
4600
Earth’s dynamo.
• Convection in liquid metal outer core.
• Convection is coupled to Earth’s
rotation.
• Field is subject to sporadic reversals.
4400
4200
Lower Mantle
4000
3800
Transition Zone
3600
– Last reversal was 30,000 years ago.
3400
• Field holds ozone layer (UV shield) in
Upper Mantle
3200
place.
3000
0
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400
600
800
1000
1200
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Magnetic
Field
Reversals
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