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GEOL 4700 ‘Field’ Mineralogy

Earth’s Interior

http://ruby.colorado.edu/~smyth/G4700.html

GEOL 4700 ‘Field’ Mineralogy

Earth’s Interior

http://ruby.colorado.edu/~smyth/G4700.html

Earth’sInterior

Multi-anvil Press

5000 ton

(Bayerisches Geoinstitut)

Earth’sInterior

The Core is MetalThe Mantle is oxygen minerals

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There could be significant hydration in the Transition Zone

How do we know what is inside the Earth?

• Seismology: Earthquake waves

• Cosmochemistry: Meteorites

• Experiments: Laboratory synthesis

• Xenoliths

Review Mantle MineralogyMantle Mineralogy: Modal Fraction

Water Storage Capacity in Ocean Units

• Nominally Anhydrous Minerals (NAMs) can incorporate up to 10 times the total Ocean mass if saturated.

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)

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)

– Peridotite (Olivine + pyroxene)– Eclogite (Pyroxene + Garnet)

• Transition Zone (410 - 670 km)– Spinels and Spinelloids

• Lower Mantle (670 - 2900 km)– MgSiO3 - Perovskite – MgO - Periclase

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

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

Seismology• S-wave shadow

– No S-waves opposite-side earthquakes

– Core (outer) must be molten• P and S reflections

– Discontinuities at 410 and 670 km– Phase Changes

• P and S travel time anomalies– Tomography– Hot and cold regions

Ringwoodite

• 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

• 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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SeismologyP-waves S-waves

Velocity Structure and Earthquakes at N. Tonga(VanderHilst, Nature (1995)Red = slow = hot green = fast = cold

Xenoliths• 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

Xenoliths

Xenoliths

Xenolith-Peridotite

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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)

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Multi-anvil Press Sample Assembly

Multi-anvil Press Sample Assembly

Carbide cubes retain a lot of strain

Experiments• Multi-Anvil Press

– Synthesis experiments to 1000 km depth– Samples 1-5 mm3

• Diamond Anvil Cell– Spectroscopy– Ultrasound– X-ray diffraction

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 Octahedral Assembly:

After and Before

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Multi-Anvil Press Octahedron

Diamond-anvil Cell

Interior Terms

Interior Terms

Interior Terms

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Interior Terms

Interior Terms

Interior Terms

DAC on Diffractometer

Mica Crystal with Quartz @ 5GPa

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410km:Olivine Wadsleyite

Density 3.2 3.5 g/cm3

410km:Olivine Wadsleyite

Wadsleyite

Olivine

525 kmWadsleyite Ringwoodite

Ringwoodite Crystal

Ringwoodite Crystal with Quartz

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660 kmRingwoodite Perovskite + MgO

Density 3.6 4.2 g/cm3

660 kmRingwoodite Perovskite + MgOTetrahedral Si Octahedral Si

Mantle Density vs Depth

3000

3200

3400

3600

3800

4000

4200

4400

4600

4800

0 200 400 600 800 1000 1200

Lower Mantle

Transition Zone

Upper Mantle

Convection in solid mantle

• Driven by U, Th, K decay (internal engine)

• Moves about 2 cm/yr• Causes earthquakes• Moves plates• Splits and rejoins continents• One Cell or two?

Convection in solid mantle

Magnetic Field• Due to electrical currents from

Earth’s dynamo.• Convection in liquid metal outer core.• Convection is coupled to Earth’s

rotation.• Field is subject to sporadic reversals.

– Last reversal was 30,000 years ago.• Field holds ozone layer (UV shield) in

place.

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Magnetic Field

Reversals