Significance of the Seismic Anomaly on the Core-Mantle Boundary

1. Significance of the Seismic Anomaly on the Core-Mantle Boundary By Shawn Dubois

2. Contents • Detection • Characteristics • Nature of the seismic anomaly • What is the D’’ layer? • Significance

3. Detection • Measuring traveling times of seismic waves • Waves originate from earthquakes • The travel times can be used to calculate the seismic velocity of underground layers in the Earth

4. Detection • Snell’s Law: Sin(i1)/V1=Sin(i2)/2

5. Characteristics of the Anomaly • 200 to 300 km thick layer with a large seismic velocity drop • 5-40 km thick ULVZ (ultra low velocity zone) • Irregular surface • Anomaly referred to as D’’ layer

6. D’’ cross-section • D’’ layer with rough surface on the CMB • Peaks in D’’ surface thought to be linked to convection cells

7. Nature of seismic anomaly • Temperature dependent (increase of 500 K in a range of ~150 km) • Layer viscosity reduced by 4 orders of magnitude

8. What is the Nature of the D’’ Layer • Still open to debate • Subducted crustal material lying on the CMB • Perovskite ((Fe,Mg)SiO3)  Post-Perovskite phase change • Partial melting of the mantle • Partial crystallization of the core (increased amount of lighter material in the outer core due to crystallization of Fe onto the inner core) • Combination of some/all?

9. Perovskite Phase Change • At the conditions found just above the CMB (in the D’’ layer), perovskite can change from its Pv phase to a pPv phase. • Changes the anisotropy of the system

10. Material Transfer from the D’’ Layer to the Outer Core • Infiltration of iron along grain boundaries • Chemical reactions between liquid iron and solid oxides and silicates at high pressures • Extraction of FeO and Si from the lower mantle into the core, and returning of Fe3+ into the mantle

11. Reactions • 2(Mg,Fe)O [Mw] FeO [liq alloy] + MgO [Mw] • 2(Mg,Fe)SiO3  MgSiO3 + SiO2 + FeO [liq alloy] • (Fe1-x,Mgx)SiO3 + 3(1-x)Fe [liqalloy] xMgSiO3 + (1-x)FeO [liq alloy] + (1-x)Fe3Si [liq alloy] • 3Fe2+O [Mw or Pv] Feo [liqalloy] + Fe3+2O3 [perovskite] • These reactions might be causing the oxydation of the mantle over time.

12. Influences of Fluid Inflow • Fluid infiltration and reactions are relatively rapid (10^3 to 10^6 years) • Dispersal of the reaction products is longer • Thus, material accumulates as a result of the reaction. • Increases iron content in the mantle, thus increasing its conductivity. • Causes influences on the planetary magnetic field • Possible heterogenies in the D’’ layer may cause screening of the magnetic field

13. Possible Source of Mantle Plumes • The D’’ layer may be the source of mantle plumes. • Thickest beneath zones of upwelling in the mantle and thinnest under downwelling. • Heterogenies swept upwards from warmer areas, causing material to rise to the surface and cause a hot spot.

14. Problems • Difficulty to reproduce exact conditions in laboratories • Impossibility to sample in situ • Information limited greatly to seismic data • Any upwelled samples would have passed through phase boundaries and probably have been altered

15. Conclusion • Presence of a drastic decrease in seismic velocities along the CMB • Due mainly to large change in temperature and possibly to phase changes • Location of material exchange between the mantle and the core • Possible source of mantle plumes • Due to lack of information, it is impossible to develop accurate models