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

Outer Core Inner Core F-layer PREM AK135 PREM2

 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

Case 1 Case 3 Chen et al GRL T increase near ICB (Greff-Lefftz and Legros, 1999 Science) Case 2

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

Snow crystallizes as T drops below liquidus Fe snow settles through liquid Light elements released percolate upwards Chemical stratification accumulates with time

 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

 Compare PREM with F layer model  Updated model improves eastern residuals

 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

 Can we evolve and sustain the F layer as snow accumulates the core?  Is this model consistent with seismological observations?  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, X L, X S,

 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?

 Previous studies find a larger PKIKP-PKiKP difference 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 Inner core boundary F layer – 150km above ICB

 Use PKIKP and PKiKP to look at inner core boundary region  Compare observed PKIKP-PKiKP travel time differences with those in PREM  Differences indicate a deviation in the velocity structure from PREM

PKP Cdiff – PKP DF differential travel time (Zou et al., 2008)

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

 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)

Snow crystallizes as T drops below liquidus Fe snow settles through liquid Light elements released percolate upwards Chemical stratification accumulates with time

 Compare PREM with F layer model  Updated model improves eastern residuals