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Origin of the F-layer by “snowfall” in the core

Origin of the F-layer by “snowfall” in the core. Matt Armentrout , Antonio Buono , Bin Chen, Stephanie Durand, Jodi Gaeman , John Hernlund , Jackie Li, Jeffrey Pigott , Jeroen Tromp, Claire Waller, Lauren Waszek , Allen Zhao Zheng. PREM. AK135. PREM2.

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Origin of the F-layer by “snowfall” in the core

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  1. Origin of the F-layer by “snowfall” in the core Matt Armentrout, Antonio Buono, Bin Chen, Stephanie Durand, Jodi Gaeman, John Hernlund, Jackie Li, Jeffrey Pigott, Jeroen Tromp,Claire Waller, Lauren Waszek, Allen Zhao Zheng

  2. PREM AK135 PREM2 Seismic Observations:Flattened Vp Gradient in the F-layer (~200 km) Outer Core F-layer Inner Core

  3. Previously proposed mechanism • A thermochemical F-layer (Gubbins et al. 2008, GJI) • Decreasing light element concentration with depth • No mechanism to create or sustain such gradient was proposed

  4. Proposed “snowing” at F-layer Case 1 Case 2 Case 3 T increase near ICB (Greff-Lefftz and Legros, 1999 Science) Chen et al. 2008 GRL

  5. Methods • Mineral physics • EOS • Melting relations • Seismology • Body wave travel times • Attenuation • Normal Mode Eigenfrequencies • Geodynamics • Core crystallization and evolution models • Snow settling dynamics

  6. F-Layer Dynamics • Snow crystallizes as T drops below liquidus • Fe snow settles through liquid • Light elements released percolate upwards • Chemical stratification accumulates with time

  7. Mineral Physics Model • 3rd order Birch-Murnaghan and Mie-Gruneisen-Debye EoS • pure Fe and FeS liquid endmembers • Large extrapolations but still fits well • Examine which parameters are insensitive 7.2 wt% sulfur

  8. Compare PREM with F layer model Updated model improves eastern residuals Comparison with observations

  9. Conclusions • Iron precipitation and light element depletion in the F-layer is a potential mechanism to explain seismic features • P-wave velocities calculated using mineral physics data are in good agreement with seismic models • Normal modes are not sensitive to the proposed velocity structure of the F-layer • Due to relatively low viscosities and convective vigor in the outer core, a stratified F-layer above the ICB is likely to be gravitationally stable

  10. Thermal Evolution of Core Derived from conservation equations (Mass, Momentum, Energy) Evolving IC, F, OC thicknesses and compositional gradients Input: Mineral physics data Output: Profiles for V,T, Φ, XL, XS, Future Direction:Core Crystallization • Can we evolve and sustain the F layer as snow accumulates the core? • Is this model consistent with seismological observations?

  11. Open Questions • What are melting relations in the Fe-L systems at core conditions? • How sensitive are the material properties of Fe-alloys to light element concentration (e.g. multi-component systems)? • What is the viscosity of core fluids? • What is the temperature and heat flux at the ICB and CMB? • How is the F-layer coupled to outer core convection, inner core growth, or the dynamo? • What are the effects of scattering and attenuation in the F-layer? • What is the relationship between snowfall and inner core boundary topography? • What is the light element composition of the core?

  12. Previous studies find a larger PKIKP-PKiKPdifference than in PREM in the eastern hemisphere, and a smaller difference in the west This is attributed to a faster velocity structure at the top of the inner core in the east Could also explain the difference with a slow velocity F layer in the lower outer core Hemispherical observations F layer – 150km above ICB Inner core boundary

  13. Use PKIKP and PKiKP to look at inner core boundary region Compare observed PKIKP-PKiKP travel time differences with those in PREM Differences indicate adeviation in the velocitystructure from PREM Seismology tests: body waves

  14. PKPCdiff – PKPDF differential travel time (Zou et al., 2008)

  15. CMB ICB Previously proposed mechanisms Slurry zone A slurry F-layer Inner core freezing must occur above the solid boundary (Loper and Roberts, 1981 PEPI) A thermochemical F-layer Gubbins et al. 2008, GJI

  16. Though the F layer model fits the data better than PREM, the PKIKP-PKiKP method is not ideal Cannot use PKIKP as a reference phase due to the hemispherical inner core structure, detected in PKIKP-PKPbc phases and normal modes (e.g. Deuss et al, 2010) Better method – use waveform modelling of PKiKP to search for scatter within F layer, and examine for precursors reflected from top of F layer (e.g. Poupinet & Kennett 2004) PKIKP-PKiKP problems

  17. F-Layer Dynamics • Snow crystallizes as T drops below liquidus • Fe snow settles through liquid • Light elements released percolate upwards • Chemical stratification accumulates with time

  18. Compare PREM with F layer model Updated model improves eastern residuals Comparison with observations

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