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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 USEDAssumptions: 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