Ch 6: Internal Constitution of the Earth

Ch 6: Internal Constitution of the Earth

Mantle composition

Geological background 88 elements found in the Earth's crust -- of these, only 8 make up 98%: oxygen, silicon, aluminum, iron, calcium, magnesium, potassium and sodium In the whole earth, only 4 elements dominate: iron, oxygen, silicon and magnesium These elements go up to make minerals. A mineral is a naturally occurring, inorganic solid with a characteristic chemical composition and a crystalline structure Even though there are more than 2500 minerals knows, only nine minerals make up most of the rocks of the Earth's crust -- these are the "rock-forming minerals"

The rock-forming minerals Minerals containing silicon and oxygen are called silicates. These make up more than 95% of the crust. The seven most abundant silicates in the crust are feldspar, quartz, pyroxene, amphibole, mica, clay minerals, and olivine. Olivine and pyroxene are the main constituents of the uppermost mantle. Most silicates are formed from SiO4 tetrahedra (a silicon atom surrounded by four oxygens) arranged in a variety of ways. Exceptions are quartz and feldspar which are socalled framework silicates. The silicate tetrahedron is exceptionally stable and allows close-packed structures to be formed.

The mantle: 0 670 km 2900 km olivine + pyroxene garnet + spinel silicate perovskite + magnesio-wustite liquid iron Depth, km 200 400 Orthopyroxene Clinopyroxene Majorite Garnet 1200 C Olivine 1400 C Wadsleyite 10 Pressure, GPa solid iron 600 Ringwoodite 1600-2000 C 20 crust upper mantle lower mantle outer core inner core Ca-P Al-P + St Perovskite Magnesiowüstite 0.0 0.2 0.4 0.6 0.8 1.0 Volume Fraction

Pyrolite composition:

DIRECT SAMPLES (Peridotite) Common mantle minerals: Olivine Orthoppyroxene Clinopyroxene Al-phase Spinel Garnet

High pressure experiments

Multi- anvil press: (i) (ii) (iii) Uniaxial Press Upper Guide Block Tungsten Carbide Anvil Guide Blocks Split-Sphere Anvils Anvils 6 4 Second-stage assembly Presure Medium 2000 Ton Jack 2 1 Pre-formed gasket Spacer Lower Guide Block 32 mm

Large volume multi anvil cells: 3 orders of magnitude higher than DACs! Large volume: House probes, synthesize larger specimens, some experiments require large V (e.g. ultrasonic interferometry) Hydrostatic Pressure: Closer, since squeezing from 8 directions, But, not easily used with gas pressure medium Pressures: Top of lower mantle at best with sintered diamonds and synchrotron radiation

Diamond anvil cells:

Diamond Anvil Cells: Why Diamonds? Can use: Steel, tungsten carbide, boron carbide, sapphire, cubic zirconia, sintered diamond, or single-crystal diamond Single crystal diamond: 1) Strongest material known 2) Transparent (IR, optical, UV, and X-ray) 3) Non-magnetic insulator: σ, χ

Creating Temperature: 3 ways: 1) External heating 2) Internal heating 3) IR Laser Heating

Measuring Material Parameters Measurement of Pressure Ruby Chips Fluorescence Method Freq. shift of ruby with increasing pressure Linear to 30 GPa Calibrated to 100 GPa by Raman Spec. Calibrated to >200 GPa by Gold Accurate to 15-20% at 200 GPa Diffuses with temperature (>700K) Ruby and Diamond Fluorescence overlap between 120-180 GPa KEY: Allows sampling at multiple points in pressure medium

Laser heating - use black body radiation T: temperature I: intensity λ: wavelength Cs: constants ε: emissivity Perfect black body: ε = 1 Grey body: ε < 1 ε is wavelength dependent But dependence not known for many materials! (known for Fe)

Measuring Material Parameters In-Situ X-Ray Diffraction Provides Crystal Structure, Density and melting points Synchrotron Radiation provides highly collimated x-ray source Braggs Law: 2q = angle of diffraction λ = 2dsin(q) d = spacing of crystal planes λ = wavelength of X-ray

Phase transformations in the upper mantle

Redrawn from Ita & Stixrude (1992) by Christiansen (1995)

Bina & Helffrich (1994)

Christiansen (1995)

Iron and the core

Low pressure phase diagram of iron:

Iron can exist in three crystal forms:

Melting Temperature: Pure Fe Boehler(1993) Yoo et al (1993) Nguyen & Holmes(2009) Brown & McQueen (1986) Belonoshko et al (2000 Alfe et al (2002) Alfe (2009) Sola & Alfe (2009) Anzellini et al (2013) CMB ICB (slide courtesy Chris Davies)

Structures of minerals

Spinel structure (ringwoodite)

Undistorted (cubic) perovskite structure