First-order Earth structure: PREM

? ?

Equatorialsection:

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 vS-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 fromLLSVP-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 Degree 2

Spherical harmonics modelingPower spectra Cumulative power spectra

Slab m

odel

Tomog

r.

mod

elsTomographic 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 ( > 300-500 Ma)

Paleogeographic relocation → LIPs cluster near LLSVP-margins

- long-term stability- dense and hot

Large igneous provinces (LIPs) - age span: 16-297 Ma

- irregular distribution

SC

–1%

slow

+2.5% fast

–3%slow

AfricaPacific

Burke & Torsvik, 2004, EPSLTorsvik et al., 2006, GIJBurke et al. 2007, EPSLTorsvik et al. 2008, EPSL

SC

Plume generation along the margins of LLSVPs: evidence from relocated LIPs

Additional kimberlite and LIP dataTorsvik et al. (2010, Nature)

LLSVP-stability may exceed 540 Ma

horizontal flow (and PGZ)

S-wave modelNE part of Pacific LLSVPSamoa quakes, recorded in N-America

S-wave model

Double crossing of thepv-ppv-transition

Large lateral variation

Lay et al.(2006, Science)

Bin 1-3

Mantle flow model

Seismological image ofPlume Generation Zones (PGZ)

The Scd and Scd2 may be

ascribed to double-crossing ofthe post-perovskite boundary

- large thermal gradient in the D”- large dp/dT-slope of phase bound.

D”-discontinuitiesLay 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 Al2O3-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 creepin 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 relationsperidotite - basalt

K0 (GPa) Mg-pv 230-260 (perid. is stiffer ?) Ca-pv 236softest: ferroper. 158-152 (FeO-MgO)stiffest: silica 314-325 (stish. - PbO2)

K’: poorly constrained

Basalt: pv, Ca-pv, SiO2, Al-phase

high , possibly higher K0

Peridotite: pv, fp, Ca-pv

low , possibly lower K0

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 intersectsthe curved liquidus here,the magma ocean will strartcrystallizing in the middle

z

Scenario with two magma ocean

Labrosse et al (2007, Nature) Stixrude et al. (2009, Earth Planet. Sci. Lett.)

Core

Core

Core

Inner magma ocean: melt density > crystal density (pv, fp) (Fe/Mg)melt > (Fe/Mg)crystals

Stage 1

Stage 2

Stage 3

Fe-rich cumulatesstarting 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)

Lee et al. (2010, Nature)

Melt accumulation zone

Solidified, thermally equilibrated melt sink to the CMB

Unresolved issue: pseudo-invariant melt compositions at 20-30 GPa - liquidus phase variation can guide - systematic experimentation on a range of model compositions

increasing

MgO

bas.komatiite

possi

bly 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 %): CaCl2- to PbO2-structure

- p-T-condition of transition, including Clapeyron slope - compositional relations (silica-phases may contain up to 12% Al2O3)

Possible silica analogue compositions : TiO2, ZrO2, CaCl2 and PbO2 (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 (5500 - 9900 kg/m3) 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 convectiveand thermal regime of the core

Pv: HIGH entropyPost-pv: LOW entropy

pv-ppv transition has large, positive dp/dT-slope

Crystal structures

MgSiO3 (Murakami et al. 2004)

Analogue system:CaIrO3

Phase boundary:not well constrainedby DAC-experiments

DFT-model of pv-ppv in CaIrO3Stølen & Trønnes (2007, PEPI)

dp/dT = 19 MPa/K

K: negative (reaction: pv→ppv)

G: positive: negative)

Similar DFT-results for MgSiO3(e.g. Wookey et al. 2005, Nature)

vs2 = G/

v(bulk sound

speed)

vp2 = (KG)

Consistent with theanti-correlated vS and v

Experiments: Boffa-Ballaran et al. 2007, Am Min.

DFT: Stølen & Trønnes 2007, PEPICompressibility ofpv and ppv, CaIrO3

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 VSH) - 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 K0) 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 FeAlO3-component precludes this)

Could other phase transitions in basalt-rich material be responsible ? - Possible candidate: CaCl2- to PbO2-structure of SiO2

- 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

PbO2

CaCl2

CMB

core

Earth dynamics –a modified working hypothesis

Schematicequatorial section

??

The plume generation zone:density-driven separation of basalt and peridotite

Modified from Trønnes (2009/10, Mineral. Petrol.)

CaIrO3-based analogues (for MgSiO3)

- space group match: pv: Pbnm, ppv: Cmcm

- phase transition at 1-3 GPa and 1400-1600 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 MgSiO3-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 CaIrO3-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: Al2O3, FeAlO3, MgAlO2.5

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 vS and v