University:
Widener University
Course:
CHEM-255 | Organic Chemistry I
Academic year:

2023
Views:

112

Pages:

13
Author:

bobcat18l34

Earth’s Core-Mantle Boundary: Results of Experiments at High Pressures and Temperatures • Observations The Core-Mantle boundary and the lowermost mantle exhibit radial and lateral heterogeneities in seismic velocities, velocity gradients, discontinuities and scattering. • Previous interpretation The CMB is a thermal boundary layer which support a thermal gradient of over 1000K over ~200 Km. Suggested model • D’’ is also a chemical boundary layer between the silicate mantle and the iron core • • May support the 1000K gradient, by allowing density variations to counteract the thermal buoyancy forces Can also explain lateral heterogeneities Lab Experiments • Objective: Recreate the CMB in miniature, within a 200µm sample in a laser heated diamond-cell • Tools: – Iron foils embedded in a silicate matrix (x4) – Laser heating melted the iron (not the silicates) – Pressure resemble CMB’s ( ≥ 70 GPa) – Two comparison samples: one not heated and under pressure ; one heated and under low pressure. Experiment Results • All 4 samples had reaction zones between iron and silicates Experiment Results – Contd. • Resolution of microprobe readings ≤ 2µm (determined using the pressure-only sample). • Observed composition gradients: – Diffusion of O, Mg, Si : 4-7µm – Diffusion of Fe: at least 12 µm • Migration of elements much stronger under higher pressures Chemical gradient across the interface Compositional profiles (Variations from the unreacted sample) • Note – penetration of Fe into unmelted silicate matrix: – migration along grain boundaries? – Soret diffusion of heaviest component? Thermodynamics Proposed balanced equation: MgxFe1-xSiO3 + 3[(1-x)-s]Fe = xMgSiO3 + sSiO2 + [3(1-x)-2s]FeO + [(1-x)-s]FeSi By assuming amount of FeSi equal to SiO2 Mg0.9Fe0.1SiO3 + 0.15Fe = 0.9MgSiO3 + 0.2SiO2 + 0.05FeO + 0.05FeSi Sign of thermodynamic driving force: –By calculating the volume change –By experiments with higher Fe content Result: the reaction is favored under high pressure and higher Fe content (See Fig 8-9) Implications on the CMB • The proposed reaction inevitably takes place in D’’ • Can cause strong heterogeneity in density, elastic properties and electrical conductivity • Implications on the magnetic field –due to lateral variations in electric conductivity (Secular variations?) See Buffet, 1996; Busse&Witch, 1992 • Implications on mantle dynamics due to variations in thermal conductivity (Manga&Jeanloz, 1996) Length and time scales • Short time/length scale: – Mantle rocks in direct contact with core iron react rapidly. – The reaction zone will penetrate upwards on order of 101-102 meters, based on capillary rise along grain boundaries • Long time/length scales: – The reaction zone will be swept upwards by slow mantle convection. Will expose fresh rock to reaction. – Reacted material is denser – cannot rise very high Æ will form the D’’ layer Implications on outer core • Reaction products, mainly Oxygen, swept into the core by the rapid outer convection. • May explain why the density is lower by 10% of pure liquid iron Conclusions • Preferred model – – Vigorous chemical reaction between liquid iron of the outer core and the crystalline silicates of the lower mantle. – Dissolving of mantle O, Mg and Si into the core – Dissolving of core Fe into the mantle.