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

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)

-MORB + harzburgite -Pyrolite -MORB alone -Ancient Fe-enriched oceanic crust -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 Brandenburg and van Keken (2007) Labrosse et al. (2007) Tackley (2011) Scenarios of LLSVP origin

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

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

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

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

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)

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

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

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

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

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

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

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

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 semantics (LLSVP, BAR, MOP) Compositional Models Compute Physical Properties Compute Physical Properties Geodynamic & Seismic Test