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Structure and dynamics of earth’s lower mantle

Structure and dynamics of earth’s lower mantle. Edward J. Garnero and Allen K. McNamara. Presented by : David de Vlieg Folkert van Straaten. R esearch on lower most mantle :. This part of the mantle has influence on the convection and chemistry of the entire mantle

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Structure and dynamics of earth’s lower mantle

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  1. Structureanddynamics of earth’slowermantle Edward J. Garnero and Allen K. McNamara Presentedby: David de Vlieg Folkert van Straaten

  2. Research on lower most mantle: • This part of the mantle has influence on the convectionandchemistry of the entiremantle • It playsan important role in the heat release of the core • It has influence on thermalstructureandevolution of the earth

  3. Keyscientificareastostudy the lowermantle • Seismology • Mineralphysics • Geodynamics • Geochemistry • to get a betterinsightintothe lowermantle, it is important to combine these areas

  4. Different theoriestoexplain the lowermantleanomalies • Anomalies are causedby a • Temperature effect • Chemical effect • It is verydifficulttodeterminehow important each effect is andhowtheyinfluenceeachother • During the remainder of the presentation we focus on the different theoriesexplaining the properties of the lowermantle

  5. Historicalperspectivelowermost mantle research • Discovery of a reducedseismicvelocitygradient as function of depth • This was interpreted as a lower most mantlethermalboundarylayerabove a hot core • 1980’s:seismologistsalsoobserved a first order discontinousincrease in velocitybetween 250 km and 350 km above the core-mantleboundary (CMB) • This was named the D” discontinuity

  6. Anomalies in shearvelocity Lowershearvelocity Highershearvelocity

  7. The D” discontinuity • D’’discontinuity does not have a specificstructuralcharacteristic, but is more a generaldepth shell of a few hundred kilometers • It shows a connectionwithsubductionand Hot spot regionsaboveit • Thiscanbeused as an argument fortotalmantleconvection • Convergent plate boundaries overlie D″ regions with higher than average velocities • hot-spot volcanoes overlie D″ regions with lower than average velocities. • combinedwithevidencefor high P- and S-wave velocities mimicking subduction slab shapes

  8. The LLSVP’s (large low-shear-velocity province’s) • Below Africaand the Pacific regionstwobroadregions of lowershearvelocityandhigherthanaveragedensityare observed • Africanregion is ca. 15000 km acrossand 1000 km high • Pacific region is ca. 15000 km acrossand 500 km high • Both show sharpedgeswithnormalmantle

  9. What are these LLSVP’s? • No agreement • Geodynamical view: Higherdensitymaterialwill go toupwellingregionsbyconvection • LLSVP’s have stabledensities • Too low densitywillcausebuoyancy • Too high densitywill flat out or even let the structuresdisappear

  10. Other way to look at LLSVP’s • Thermochemical view: LLSVP’s are in essence superplumes in different stadia, anddueto a thermochemicalbalanceverystable • thermochemicalsuperplumesmay heat up and rise because of excessthermal buoyancy • then cool and sink due to decreasedthermalbuoyancy • Smaller plumeswith the densermaterialcan form at the top of these structures

  11. MantlePiles • Mantlepiles are pileswithspecificchemicalproperties • They are accumulated in the Pacific andAfricanregion, which are dominant upwelling centers • Piles are passivelysweptandshapedbymantleconvection • Plumesmaybeoriginatefrompile tops, in particular at peaksandridges

  12. Causes of thislower-mantlechemicalheterogeneity • Lowermantleheterogeneitycouldbeexplainedby: • remnants of primordialmaterial • the result of chemicalreactionproductsfrom the CMB • remnants of subductedoceanicmaterial

  13. A way torecognise the chemicalproperties of a pile • Pilescomposedof a long-lived primordial layer will likely have sharp contacts at their top surface • Piles composed of accumulated subductedmaterial may have a rough or diffusive top

  14. Chemistry of llsvp’s • Volcanic hot spots tend to overlie LLSVP edges rather than their interiors • consistent withedgesandridges of thermochemical piles forming in regions of return flow and initiating plumes • This is stillcontroversial • Becausenumericalmodels of mantle convection show that plume morphologies are often more complicated than simple verticallycontinuouswhole-mantleconduits • Further geochemical research on ocean island basalts (OIB’s) is necessary

  15. Cause of D” discontinuity • Lateral variations in deep-mantletemperatureare expected but should be smooth • hence they do not explain a step velocity increase • D″ has interpreted as chemical dregs from subduction, • as a region of chemical reaction between the core and mantle, • Today most preferred: as a boundary between isotropic and anisotropic fabrics, or as a solid-state phase change

  16. D” discontinuityandchemicalproperties of LLSVP”s (1) • D’’-discountinuitycouldbe the result of the transitionfromperovskiteinto post-perovskite • Thistransitions has a positiveClapeyron curve • Sowhentemperatureincreases the pressureneededfor the transition must behigher

  17. Double crossing Perovskite, Post-Perovskite From: Ferroir

  18. D” discontinuityandchemicalproperties of LLSVP”s(2) • Due to this positive Clapeyron relation the discontinuity should deepen or even vanish in hot area’s • Near the core double crossing • This is not the case: Clear evidence is present for an S-wave discontinuity within the Pacific LLSVP • Prooffor a different chemicalcomposition! (maybehigheriron content)

  19. D” discontinuityandchemicalproperties of LLSVP”s(3) • Perovskiteto Post perovskite: exothermicreaction • Resulting in Plume formation • Higherconvection leads tolowertemperatures • Lowertemperaturesreaction

  20. D” discontinuityandchemicalproperties of LLSVP”s(4) • Todeterminewhich of the possibilities is the most probableyouneedtomeasure the discontinuitiesperfectly • Measuringanisotropyusinghorizontalandverticalcomponents of shear waves is a way to do this

  21. Anisotropyandmeasuring the D’’ discontinuity (1) • If the D’’ anisotropy is the result of the change fromperovskiteinto post perovskitean offset of depthbetween the onset of the anomalyand the discontinuity is expected • This is because the preferredlatticeorientation is onlyvisibleafter a sufficientamount of deformation

  22. Anisotropyandmeasuring the D’’ discontinuity(2) • mayexplainseismic observations under the central Atlantic which thought to be away from current downwellings • which there is evidence for a D″ discontinuity • but a weak seismic anisotropy

  23. Ultra-low velocity zones (1) • Directly above the CMB • 5 to 40 km thickthin patches in which P- and S-wave velocities are reduced by up to 10% and 30%, respectively • Partialmeltand a densityincrease up to 10%

  24. Ultra-low velocity zones (2) These ULVZ’scanbeusedto say somethingaboutLLSVP’s: • If the most lowermantle has anisochemicalcompositionULVZ’sshouldbe the thickest in the middle of a LLSVP (hottestregion) • If a LLSVP has a thermochemicalstructure the hottestregionsshouldbe at theiredgesandULVZ’sshouldbe the thickesthere

  25. Ultra-low velocity zones (3) • Most proofthatllsvp’s have a thermochemicalstructureinstead of a isochemicalstructure

  26. Thankyouforlistening • Are therestillquestions?

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