INTERACTIONS BETWEEN MANTLE CONVECTION AND DENSE MATERIAL ACCUMULATION ON THE CORE-MANTLE BOUNDARIES IN LARGE TERRESTRIAL PLANETS

Agnieszka Płonka Leszek Czechowski

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

CORE- 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

DENSITY 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

DENSE MATERIAL ACCUMULATION (C-CONTINENTS, BAM – BASAL MELANGE)

From: Czechowski, 1992

DENSE MATERIAL ACCUMULATION (C-CONTINENTS, BAM – BASAL MELANGE) 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

SEISMIC 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

OUR MODEL (DIMENSIONLESS VERSION) Diffusion equations:

(gravitation in direction y, e – diffusion coefficient, 0 <Za, 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:

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

INITIAL CONDITION AND PARAMETERS USED

Assumptions: whole-mantle convection, no phase transitions

Time unit: d2/κ = 300 Gyr Velocity unit: κ/d = 0,3*10-12 m/s

Viscosity is given by

Parameters taken from Tackley , 2012

RESULT SCHEME:

Stream function : 0.1 - 7*10-8 m/s

Temperature distribution: 0,5 - 1800 K

RESULTS Rayleigh number is dominant over density gradients:

Same density gradient (0,005), different Ra:

Ra ~ 105

Ra ~ 4*106

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

In case of low Rayleigh number there is no visible difference between different ratios of heat production:

Ratio 0,5

Ratio 5

CONCLUSIONS

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

Thank you for attention

Thank you for attention

Equation for fraction distribution is given by:

Where and

We change the units into dimensional by transformations:

Where

C-CONT DYNAMICS?

Z: Tackley, 2012, za Le Bars &Davaille, 2004b

B>1 stable 0,5<B<1 – mid-caseB<0,5 – unstable

B – chem buoyanc/therm

a - initial dens.

INCORP. IN 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