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Agnieszka Płonka Leszek Czechowski

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)

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Agnieszka Płonka Leszek Czechowski

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  1. Interactions between mantle convection and dense material accumulation on the core-mantle boundaries in large terrestrial planets Agnieszka Płonka Leszek Czechowski

  2. PLAN • Characteristics of theEarth’score-mantleboundary (CMB) • Theprocess of densematerialaccumulation on theEarth’s CMB – causes and consequences • Numerical model used • Results and plans for future • Conclusions

  3. Core- mantleboundary 2900 km • Above: mantleconvection • Below: geodynamo • Plumeformation • Subductedslabsgraveyard • Phasetransitions • Problemswithdeterminingheatflow, viscosity and thermal conductivity • Thermal and chemical diversity • Understandingthislayer – understandingEarth? (heatflow controls major processes) Methodology: - seismology - numericalsimulations - high pressurematerialphysics

  4. Density and viscosity problem • Viscosity as a function of temperature and pressureisgiven by (H- pressure – dependantactivation energy): • Density and viscosity of the CMB maydifferup to severalorders of magnitude • Viscosityisstrongly • temperature – dependant and • CMB isthermallydiverse • Problemswithheatflow • estimation and choosinggood • numerical model From: Hirose, Lay, 2008

  5. Densematerialaccumulation (c-continents, BAM – BasalMelange) From: Czechowski, 1992

  6. Densematerialaccumulation (c-continents, BAM – BasalMelange) • Primeval? • Generatedin time? could be also a result of accumulation of materialfromsubductingslabs Ifprimeval: moreradioactiveelements and probablyenrichedin iron (seismicobservations!) From: Tackley, 2012

  7. Seismicsignature and possible chemical compound • Ultra – Low – VelocityZones (5- 10 % velocityloss) correlatedwithc-continents • Iron enrichment? • Plumesrisingfromtheiredges From: Tackley, 2012

  8. Our model (dimensionlessversion) • Diffusionequations: (gravitationindirection y, e – diffusioncoefficient, 0 <Za, b < 1– relativevalues of upper and lowerfractionrespectively , H - constant) Densitydistributionisapproximatedlineraly by: Where - mantledensity Equation for fractiondistribution:

  9. Equation for thermal conductionisgiven by: Function f describeshereradiogenicheatproductioninthemantle ( ) and boundaryfractions ( , ): We do not knowthevalue of . Streamfunctioniscalculated by: • denoteshere Rayleigh numberincase of internal heating, theotherparameters (characterizinggravitationaldifferentiation) aregiven by

  10. Initialcondition and parametersused Assumptions: whole-mantleconvection, no phasetransitions Time unit: d2/κ = 300 Gyr Velocity unit: κ/d = 0,3*10-12 m/s Viscosityisgiven by ParameterstakenfromTackley , 2012

  11. RESULT SCHEME: Streamfunction : 0.1 - 7*10-8 m/s Temperaturedistribution: 0,5 - 1800 K

  12. Results • Rayleigh numberis dominant overdensitygradients: Same density gradient (0,005), different Ra: Ra ~ 4*106 Ra ~ 105

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

  14. In case of low Rayleigh numberthereis no visibledifferencebetweendifferentratios of heatproduction: Ratio 0,5 Ratio 5

  15. Conclusions • CMB iscrucial and diverse • Rayleigh numberis dominant overdensitydifferences and heatsourcedistribution • Theheatproductioninbothfractionsdoes not make anyvisibledifferenceinthestreamfunction(in thecase of low Rayleigh number) PLANS • Repeatingsimulationswithhigher Rayleigh number • Usingmantlethatisalreadymixed by convection as initialcondition • We want to determinethe role of radioactive heating inc-continents

  16. Thankyou for attention Thankyou for attention

  17. Equation for fractiondistributionisgiven by: Where and We changetheunitsintodimensional by transformations: Where

  18. C-Cont DYNAMICS? B>1 stable 0,5<B<1 – mid-case B<0,5 – unstable B – chembuoyanc/therm a - initialdens. Z: Tackley, 2012, za LeBars &Davaille, 2004b

  19. Incorp. In plumes Stabledensity – 2 % contrast (but for different model?) Compositionaffectsplumeshape! Plumeslikesharpedges • Q – material • C – constant? (exp.) • Κ- therm, diffusivity • H – initialthickness • B – as before.

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