First-order Earth structure: PREM ? ? Equatorial section: Trønnes (2010) LLSVP: Large low shear-velocity province: thermochemical pile (?) - Material, origin, age Origin and composition of LLSVPs in the lowermost mantle Reidar Trønnes, Natural History Museum, Univ. of Oslo Dziewonski & Anderson (1981)

Harvard model, Masters and Laske, website Seismic tomography models Large v S -amplitudes at the top and bottom of the mantle

Two large anti-podal, slow provinces - LLSVP Africa – Pacific (near equator - 180º apart) S-wave models, lowermost mantle (D”-zone) The main, degree-2 velocity anomaly was recognized about 35 years ago ! e.g. Dziewonsky et al. (1977, JGR) Dziewonski & Anderson (1984, Am Sci) Dziewonski et al. (2010, EPSL)

Earth’s rotation axis related to mantle mass distribution and geoid Steinberger and Torsvik (2010, GGG) Calculated rotation axis from LLSVP-contributions, only Actual rotation axis Combined contributions: LLSVPs + shallow slab mass contributions

Comparison of seismic tomography (LLSVPs) and slab-sinking model at 2800 km depth Dziewonski et al. (2010, EPSL) Lithgow-Bertelloni & Richards (1998, Rev. Geophys.) Degree 2 Spherical harmonics modeling Power spectra Cumulative power spectra Slab model Tomogr. models Tomographic models Slab model Tentative conclusions 1. The observed degree-2 pattern is only partly reproduced by calculated slab-accumulation 2. The LM-structure may thus be old ( > Ma)

Paleogeographic relocation → LIPs cluster near LLSVP-margins - long-term stability - dense and hot Large igneous provinces (LIPs) - age span: Ma - irregular distribution SC –1% slow +2.5% fast –3% slow Africa Pacific Burke & Torsvik, 2004, EPSL Torsvik et al., 2006, GIJ Burke et al. 2007, EPSL Torsvik et al. 2008, EPSL SC Plume generation along the margins of LLSVPs: evidence from relocated LIPs Additional kimberlite and LIP data Torsvik et al. (2010, Nature) LLSVP-stability may exceed 540 Ma

 horizontal flow (and PGZ) S-wave model NE part of Pacific LLSVP Samoa quakes, recorded in N-America S-wave model Double crossing of the pv-ppv-transition Large lateral variation Lay et al. (2006, Science) Bin 1-3 Mantle flow model Seismological image of Plume Generation Zones (PGZ)

The Scd and Scd 2 may be ascribed to double-crossing of the post-perovskite boundary - large thermal gradient in the D” - large dp/dT-slope of phase bound. D”-discontinuities Lay and Helmberger (1983, GJRAS): S-wave triplication: S, Scd and ScS (in certain areas, at least)

pv-ppv transition - wide phase loop: pv and ppv coexist through the entire D"-zone and -the Al 2 O 3 -component stabilizes pv and widens the phase loop Catalli et al. (2009, Nature)

With this model: - D”-discontinuities: rheological changes (steadily increasing ppv-fraction with depth) - High T may facilitate diffusion creep below the lower discontinuity. - Additionally: the lower discontinuity could also be caused by back-reaction to pv. Possible rheological explanation for sharp D”-discontinuities (Amman et al. 2010) Strong alignment and dislocation creep in ppv at a ”critical” phase proportion (40-50% ?) But probably no ppv inside LLSVPs - hot and rich in basaltic material (?)

Garnero & McNamara (2008, Science) Locally steep thermochemical pile margins Requirements: - moderate density contrast (2-5 %) - pile material: higher bulk modulus than ambient mantle High thermal conductivity and low thermal expansivity in the lowermost mantle may help to stabilize the thermochemical piles

Possible LLSVP-material Basalt-rich - separated from subducted lithosphere - age: 3-0 Ga Perdotitic (or komatiitic) with elevated Fe/Mg-ratio - cumulates, deep-level partial melts - age: mainly Hadean

Mantle mineralogy Irifune & Tsuchiya (2007, Treatise on Geophys.) Shim et al. (2011, this meeting)

Density relations peridotite - basalt K 0 (GPa) Mg-pv (perid. is stiffer ?) Ca-pv 236 softest: ferroper (FeO-MgO) stiffest: silica (stish. -  PbO 2 ) K’: poorly constrained Basalt: pv, Ca-pv, SiO 2, Al-phase high , possibly higher K 0 Peridotite: pv, fp, Ca-pv low , possibly lower K 0 Irifune & Tsuchia, 2007, Treatise on Geophys. Density contrast: 1-3% - sufficient ?

Deep-level Hadean melting in hot plumes at >300 km depth, followed by downward or upward migration to 410 km depth, crystallization, cooling and sinking to CMB (possible plume initiation by density overturn of cumulate sequences) Compared to ambient” peridotite: - Similar mineralogy (in D” mainly pv/ppv and fp) - Higher density (possibly higher than basalt) - Higher bulk modulus → LLSVP-requirements may easily be fulfilled Peridotite (or komatiite) with elevated Fe/Mg-ratios Origins: Magma ocean cumulates from late-stage, residual melts - crystallization near CMB - crystallization in TZ or UM, followed by density-driven sinking to CMB

Depend on relative slopes of peridotite liquidus and melt isentropes Dense cumulates from crystallization of lower mantle magma ocean If melt adiabat intersects the curved liquidus here, the magma ocean will strart crystallizing in the middle z

Scenario with two magma ocean Labrosse et al (2007, Nature) Stixrude et al. (2009, Earth Planet. Sci. Lett.) Core Inner magma ocean: melt density > crystal density (pv, fp) (Fe/Mg) melt > (Fe/Mg) crystals Stage 1 Stage 2 Stage 3 Fe-rich cumulates starting point for thermochemical piles Cumulates with lower Fe/Mg Cumulates with higher Fe/Mg

Sinking of solidified melts from 410 km depth melts formed in hot plumes at 300–900 km → Intermediate age span: Hadean-Archean (between scenarios 1 and 2) Based on: Zhang & Herzberg (1994, JGR) Tønnes & Frost (2002, EPSL) Ito et al. (2004, PEPI) Suggested by: Lee et al. (2010, Nature)

Melt accumulation zone Solidified, thermally equilibrated melt sink to the CMB

Unresolved issue: pseudo-invariant melt compositions at GPa - liquidus phase variation can guide - systematic experimentation on a range of model compositions increasing MgO bas. komatiite possibly more basaltic-komatiitic Further experiments with D.J. Frost, BGI-Bayreuth

Geochemistry The relations of ULVZs and LLSVPs with possible long-lived, enriched (fertile) mantle reservoirs Better data on phase transitions and EoS in basaltic material Na-Al-phase (15-20 %): Ca-ferrite to Ca-titanite structure (??) Silica phase (10-15 %): CaCl 2 - to  PbO 2 -structure - p-T-condition of transition, including Clapeyron slope - compositional relations (silica-phases may contain up to 12% Al 2 O 3 ) Possible silica analogue compositions : TiO 2, ZrO 2, CaCl 2 and  PbO 2 (at var. T) Other important tasks For all minerals, better data on: - thermal conductivity, incl. radiative conductivity - Fe-spin transitions (in the minerals pv, ppv and fp) - thermal expansivity (and EoS in general) - mechanical propertis, diffusivity, deformation style (viscocity)

Large thermal boundary layer at CMB Mantle-core mixing is prevented by contrasts in density ( kg/m 3 ) and viscosity Large T-increase → viscosity decrease in the D” From: Steinberger and Calderwood (2006, GJI) CMB

The plume generation zone: density-driven separation basalt – peridotite Trønnes (2010, Mineral. Petrol.) Thermochemical piles (LLSVPs) 3 possible origins – 3 different age scenarios Mechanism 1: Segregation and accumulation of basaltic parts of subducted slabs → slow growth over most of Earth history Independant evidence for long-term stability: e.g. several studies by Torsvik et al., Dziewonsky et al. (2010, EPSL)

Terrestrial planets with liquid cores: ”Mantle is the master - core is the slave” (Dave Stevenson, Caltech) In spite of viscocity decrease in D”: The rheology of the mantle imposes the convective and thermal regime of the core

Pv: HIGH entropy Post-pv: LOW entropy pv-ppv transition has large, positive dp/dT-slope Crystal structures

MgSiO 3 (Murakami et al. 2004) Analogue system: CaIrO 3 Phase boundary: not well constrained by DAC-experiments

DFT-model of pv-ppv in CaIrO 3 Stølen & Trønnes (2007, PEPI) dp/dT = 19 MPa/K  K: negative (reaction: pv→ppv)  G: positive  : negative) Similar DFT-results for MgSiO 3 (e.g. Wookey et al. 2005, Nature) v s 2 = G/  v    (bulk sound speed) v p 2 = (K   G)  Consistent with the anti-correlated v S and v 

Experiments: Boffa-Ballaran et al. 2007, Am Min. DFT: Stølen & Trønnes 2007, PEPI Compressibility of pv and ppv, CaIrO 3

DFT-computation of diffusion rates: pv, fp and ppv But then: Amman et al. (2010, Nature) Step 1: Testing of agreement between existing experimental data and computations for pv and fp. Result: good agreement So the D"-discontinuities disappear !?

Step 2: Computation of diffusion rates for ppv Result: strongly anisotropic diffusion in ppv with fast diffusion along a-axis low diffusion creep viscosity along a-axis

D”: - cold areas: strong seismic anisotropy (high V SH ) - low viscosity extensive deformation and LPO is likely deformation-related dislocation creep is likely Most of the lower mantle - no seismic anisotropy, small grain size and low stress (Solomatov et al. 2002) - high viscosity diffusion creep is likely Ammann et al. (2010, Nature), Hunt et al. (2009, Nature Geoscience): For dislocation creep: ppv may be 4 orders of magnitude weaker than pv Rheology changes dramatically at critical phase fraction of 30-50% ppv New model for D"-discontinuities Steinberger and Calderwood (2006, GJI) CMB

With this model: - D”-discontinuities: rheological changes (steadily increasing ppv-fraction with depth) - High T may facilitate diffusion creep below the lower discontinuity. - Additionally: the lower discontinuity could also be caused by back-reaction to pv.

Structure and dynamics of D” - Basalt is denser and stiffer (higher K 0 ) than peridotite (consistent with LLSVPs and PGZs) - Deformation / LPO of ppv at critical phase fraction may eplain the seismic D"-disc. in low-T areas Important unresolved issue: Seismic observation of discontinuities inside the hot LLSVPs (thermo-chemical piles, basaltic?) cannot bed due to the pv-ppv-transition (relative stabilization of pv by the FeAlO 3 -component precludes this) Could other phase transitions in basalt-rich material be responsible ? - Possible candidate: CaCl 2 - to  PbO 2 -structure of SiO 2 - Do we know the Clapeyron slope of this transition ? - Ohta et al. (2008, EPSL) indicate positive dp/dT, but this is not well established - Positive slope could explain a double-crossing scenario T p  PbO 2 CaCl 2 CMB core

Earth dynamics – a modified working hypothesis Schematic equatorial section ? ? The plume generation zone: density-driven separation of basalt and peridotite Modified from Trønnes (2009/10, Mineral. Petrol.)

CaIrO 3 -based analogues (for MgSiO 3 ) - space group match: pv: Pbnm, ppv: Cmcm - phase transition at 1-3 GPa and ºC - both phases are quenchable to ambient conditions, enabling single-crystal XRD: single-crystal structure refinement, DAC-compressibility, thermal expansivity - bulk and shear moduli changes for the pv-ppv-transition correspond to MgSiO 3 -based comp. - deformation mechanisms and slip systems may be similar to D” and MgSiO3-based comp. E.g. Walte et al. (2009), Hunt et al. (2009), Amman et al. (2010) But also contradictory indications for slip mechanisms, e.g. Miyagi et al. (2010) Substitutions in CaIrO 3 -based systems (possible studies of phase relations, mineral physics and deformation) Divalent A-site subsitutions for Ca: Sr, Ba - corresponding to Mg-Fe-substitutions Trivalent A- and B-site substitutions for Ca and Ir: In, Sc, Y - corresponding to end members: Al 2 O 3, FeAlO 3, MgAlO 2.5

Large T-gradients in D” large positive dp/dT-slope of pv to ppv transition  re-stabilization of pv near CMB the "double-crossing" scenario of Hernlund et al (2005, Nature) Thermal gradients S-wave speed Another characteristic feature: anti-correlated v S and v 