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 so-called framework silicates. The silicate tetrahedron is exceptionally stable and allows close-packed structures to be formed.

Pyrolite composition:

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

High pressure experiments

Multi-anvil press:

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: , 

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

Measurement of Pressure 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

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

In-Situ X-Ray Diffraction 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 d = spacing of crystal planes  = wavelength of X-ray

Phase transformations in the upper mantle

Structures of minerals

Spinel structure (ringwoodite)

Undistorted (cubic) perovskite structure

Fitting seismic models

Matching seismic observations with the pyrolite model:

Red=density Blue=Vc Green=Vs Xpv=0.65: “pyrolite”

Shock wave data and the core

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) Density jump is accounted when considering compositional convection Anzellini et al (2013) (slide courtesy Chris Davies) CMB ICB