1. I NTERACTIONS BETWEEN MANTLE CONVECTION AND DENSE MATERIAL ACCUMULATION ON THE CORE - MANTLE BOUNDARIES IN LARGE TERRESTRIAL PLANETS Agnieszka Płonka Leszek Czechowski

2. PLAN Characteristics of the Earth’s core-mantle boundary (CMB) The process of dense material accumulation on the Earth’s CMB – causes and consequences Numerical model used Results and plans for future Conclusions

3. C ORE - MANTLE BOUNDARY Above: mantle convection Below: geodynamo Plume formation Subducted slabs graveyard Phase transitions Problems with determining heat flow, viscosity and thermal conductivity Thermal and chemical diversity Understanding this layer – understanding Earth? (heat flow controls major processes) Methodology: - seismology - numerical simulations - high pressure material physics 2900 km

4. D ENSITY AND VISCOSITY PROBLEM Viscosity as a function of temperature and pressure is given by (H- pressure – dependant activation energy): Density and viscosity of the CMB may differ up to several orders of magnitude Viscosity is strongly temperature – dependant and CMB is thermally diverse Problems with heat flow estimation and choosing good numerical model From: Hirose, Lay, 2008

5. D ENSE MATERIAL ACCUMULATION ( C - CONTINENTS, BAM – B ASAL M ELANGE ) From: Czechowski, 1992

6. D ENSE MATERIAL ACCUMULATION ( C - CONTINENTS, BAM – B ASAL M ELANGE ) Primeval? Generated in time? could be also a result of accumulation of material from subducting slabs If primeval: more radioactive elements and probably enriched in iron (seismic observations!) From: Tackley, 2012

7. S EISMIC SIGNATURE AND POSSIBLE CHEMICAL COMPOUND Ultra – Low – Velocity Zones (5- 10 % velocity loss) correlated with c- continents Iron enrichment? Plumes rising from their edges From: Tackley, 2012

8. O UR MODEL ( DIMENSIONLESS VERSION ) Diffusion equations: (gravitation in direction y, e – diffusion coefficient, 0 <Z a, b < 1– relative values of upper and lower fraction respectively, H - constant) Density distribution is approximated lineraly by: Where - mantle density Equation for fraction distribution:

9. Equation for thermal conduction is given by: Function f describes here radiogenic heat production in the mantle ( ) and boundary fractions (, ): We do not know the value of. Stream function is calculated by: denotes here Rayleigh number in case of internal heating, the other parameters (characterizing gravitational differentiation) are given by

10. I NITIAL CONDITION AND PARAMETERS USED Assumptions: whole-mantle convection, no phase transitions Time unit: d 2 /κ = 300 Gyr Velocity unit: κ/d = 0,3*10 -12 m/s Viscosity is given by Parameters taken from Tackley, 2012

11. RESULT SCHEME: Stream function : 0.1 - 7*10 -8 m/s Temperature distribution: 0,5 - 1800 K

12. R ESULTS Rayleigh number is dominant over density gradients: Same density gradient (0,005), different Ra: Ra ~ 10 5 Ra ~ 4*10 6

13. Same Ra, different density gradient (0,005 and 0,02):

14. In case of low Rayleigh number there is no visible difference between different ratios of heat production: Ratio 0,5 Ratio 5

15. C ONCLUSIONS CMB is crucial and diverse Rayleigh number is dominant over density differences and heat source distribution The heat production in both fractions does not make any visible difference in the stream function (in the case of low Rayleigh number) PLANS - Repeating simulations with higher Rayleigh number - Using mantle that is already mixed by convection as initial condition - We want to determine the role of radioactive heating in c-continents

16. Thank you for attention

17. Equation for fraction distribution is given by: Where and We change the units into dimensional by transformations: Where

18. C-C ONT DYNAMICS? Z: Tackley, 2012, za Le Bars &Davaille, 2004b B>1 stable 0,5<B<1 – mid-case B<0,5 – unstable B – chem buoyanc/therm a - initial dens.

19. I NCORP. I N PLUMES Q – material C – constant? (exp.) Κ- therm, diffusivity H – initial thickness B – as before. Stable density – 2 % contrast (but for different model?) Composition affects plume shape! Plumes like sharp edges