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Determining the nature of the LLSVP

Determining the nature of the LLSVP. Post-AGU CIDER Workshop 2012 Maxim Ballmer, Jamie Barron,  Rohan Kundargi , Curtis Williams,  Rick Carlson, Jasper Konter, Jackie Li, Sujoy Mukhopadhyay. Motivation. Why LLSVPs?.

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Determining the nature of the LLSVP

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  1. Determining the nature of the LLSVP Post-AGU CIDER Workshop 2012 Maxim Ballmer, Jamie Barron, RohanKundargi, Curtis Williams, Rick Carlson, Jasper Konter, Jackie Li, SujoyMukhopadhyay

  2. Motivation Why LLSVPs? (1) “hidden” geochemical reservoir (2) secular evolution of the Earth Continental Crust Upper Mantle / Lower Mantle Early Enriched Reservoir = ??? = LLSVP ? Lee et al. (2010) Labrosse et al. (2007) Bulk Silicate Earth = ???

  3. What are the LLSVPs? Seismic tomography shows two large low shear velocity regions in the lowermost mantle: a degree 2 pattern centered on the Pacific and Africa Garnero et al. (2007)

  4. Scenarios of LLSVP origin Brandenburg and van Keken (2007) Labrosse et al. (2007) Tackley (2011) • Magma Ocean Product <= team BMO • - Perovskite/Magnesiowüstite • crystallization • Magma Ocean Cumulate • KREEP-like • Fe-rich differentiate • Early Enriched Reservoir • calculated from volumes of • LLSVPs and BSE-estimates • MORB + harzburgite • Pyrolite • MORB alone • Ancient Fe-enriched • oceanic crust

  5. Plan of Attack (1) Primitive Reservoir (2) Slab Graveyard (3) Hybrid Scenario Team BMO Mg/Si-group Calculate physical properties (Jackie Li’s code and/or BurnMan) Density Vp Vs Geodynamical test Seismological test

  6. Plan of Attack (1) Primitive Reservoir (2) Slab Graveyard (3) Hybrid Scenario Team BMO Mg/Si-group Calculate physical properties (Jackie Li’s code and/or BurnMan) Density Vp Vs Geodynamical test Seismological test

  7. Plan of Attack (1) Primitive Reservoir (2) Slab Graveyard (3) Hybrid Scenario Team BMO Mg/Si-group Calculate physical properties (Jackie Li’s code and/or BurnMan) Density Vp Vs Geodynamical test Seismological test calculate volumes of LLSVPs from seismic images

  8. LLSVP volume - choosing contours saw24b16 0.6% contour S362ANI 0.6% contour S40RTS 0.4% contour Choose contour so it goes through region of steep gradient at edge of LLSVP (compositional change should be associated with sharp gradient)

  9. LLSVP volume – choosing height From clustering analysis – all models show change in gradient for slow cluster at approximately the same depth, around 2100km. Lekic et al (2012)

  10. LLSVP volume – prel. results Calculate the volume within isosurface chosen for model, from CMB to 2100km depth next step: consider structure that is continuous from CMB only

  11. geodynamic test 2D models with a resolution of 96x192 elements Height above CMB Gyrs after solidification of basal reservoir dense, radio-active material 760 temperature [°C] 3800 Key ingredients: -basal layer with intrinsic negative buoyancy - and with excess radiogenic heat production that decays over model time

  12. geodynamic test 2D models with a resolution of 96x192 elements Height above CMB Gyrs after solidification of basal reservoir dense, radio-active material 760 temperature [°C] 3800 Key ingredients: -basal layer with intrinsic negative buoyancy - and with excess radiogenic heat production that decays over model time - As soon as basal-layer material enter the upper mantle, it looses its exceptional properties (density, )

  13. numerical parameter study Early Enriched Reservoir makes up 11% of the initial mantle Model time: 4 Gyrs Model time: 4 Gyrs 10 10 10 Initial heat production In basal layer [W/kg] 10 10 10 0.8 1.0 1.2 1.4 0.8 1.0 1.2 1.4 Δρ(basal layer) [g/cm3] Δρ(basal layer) [g/cm3]

  14. numerical parameter study Early Enriched Reservoir makes up 11% of the initial mantle Model time: 4 Gyrs Model time: 4 Gyrs 10 10 10 Initial heat production In basal layer [W/kg] 10 10 10 0.8 1.0 1.2 1.4 0.8 1.0 1.2 1.4 Δρ(basal layer) [g/cm3] Δρ(basal layer) [g/cm3]

  15. trace-element concentrations and isotopic ratios in Early Enriched Reservoir …

  16. numerical parameter study Early Enriched Reservoir makes up 11% of the initial mantle Model time: 4 Gyrs Model time: 4 Gyrs 10 10 10 Initial heat production In basal layer [W/kg] 10 10 10 0.8 1.0 1.2 1.4 0.8 1.0 1.2 1.4 Δρ(basal layer) [g/cm3] Δρ(basal layer) [g/cm3]

  17. outlook • good plan, but quite a lot of work remains to be done • rule out a couple specific hypotheses for the composition of the LLSVPs • perhaps rule out a family of hypotheses for their origin (e.g. the slab graveyard hypothesis) • present results at EGU 2013 • semantics (LLSVP, BAR, MOP) CompositionalModels Compute Physical Properties Geodynamic & Seismic Test

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