PHYS3070 Physics of the Earth: from seismic structure to geodynamics ‘Geophysics … has the rigour of physics and the vigour of geology’ C. M. R. Fowler Goal: to connect the seismological investigation of the Earth’s internal structure (Hrvoje Tkalcic’s segment) with Paul Tregoning’s component concerning geodetic observations of surface deformations Ian.Jackson@anu.edu.au WebCT http://rses.anu.edu.au/people/jackson_i/PHYS3070/ Part I: Elasticity, equations-of-state & interpretation of seismological models Tensor stress & strain Constitutive law & elastic waves Elasticity & interatomic forces Geophysical thermodynamics Lattice vibrations & thermal energy Anharmonicity & thermal expansion Finite strain & cohesive energy @ high pressure Mie-Grüneisen equation-of-state & thermal pressure Anelasticity & seismic wave attenuation Interpretation of seismological models The End References Fowler, C.M.R. (2005) The Solid Earth, Cambridge University Press, 2nd edn. (Hancock: QC806.F625 2005 - 2 hr reserve) Lowrie, W. (1997), Fundamentals of Geophysics, Cambridge University Press (Hancock: QC806.L67 1997 - 2 hr reserve) Stacey, F.D. (1992) Physics of the Earth, 3rd Edn., Brookfield Press (ES: QC806.S65 1992) Poirier, J.P. (1991) Introduction to the Physics of the Earth's Interior, Cambridge University Press, (ES: QE509.P64 1991) Davies, G. F. (1999) Dynamic Earth, Cambridge University Press (ES & Hancock: QE509.4 .D38 1999) Jackson, I. (ed.) (1998) The Earth's Mantle: Composition, Structure and Evolution, Cambridge University Press. Turcotte, D.L. & Schubert, G. (1982) Geodynamics: Applications of continuum physics to geological problems. Wiley, 1982 (ES:QE501.T83 ) Gross Earth seismological models Fowler Fig. 8.3 Fowler Fig. 8.1 Inversion of traveltime versus angular distance & free-oscillation data for spherically averaged structure Seismological models for the transition zone of the Earth’s mantle Bulk sound speed, V! / km s-1 9.0 8.5 pyrolite model NJPB 8.0 7.5 7.0 TNA Nth Atl 6.5 6.0 200 SNA 300 400 500 600 700 800 900 Depth / km Seismological models for bulk sound speed Vφ = (KS/ρ)1/2 vs depth Jackson & Rigden, In The Earth’s Mantle, 1998 Lateral variations of seismic wave speeds in the upper mantle Surface-wave tomographic model of Fishwick et al. (2005) VS variations (%) at 200 km depth Recap on elasticity: tensor strain Displacement gradient tensor eij = ∂ ui/∂ xj (i, j = 1, 2, 3) ui = eijxj for homogeneous deformation Rigid-body rotation: e21 = -e12 = δθ ≠ 0 ∴define strain as symmetrical part of eij εij = (1/2)(∂ ui/∂ xj+∂ uj/ ∂ x i) Recap on elasticity: tensor stress σij is the component of force parallel to xi per unit area oriented normal to xj (exerted on the infinitesimal element by the surrounding medium; tension +ve) Rotational equilibrium requires σji = σij Translational equilibrium the wave equation ρ∂ 2ui/∂ t 2 = ∂ σij/∂ x j Elasticity: constitutive law & elastic waves Generalisation of Hooke’s law: σij = cijkl εkl For isotropic medium: σij = λδijεkk + 2µεij Kronecker ‘delta’ δij = 1 (for i = j), 0 (for i ≠ j) Einstein summation convention εkk = ε11+ ε22+ ε33 = Δ (known as the dilatation) The wave equation ρ∂ 2ui/∂ t 2 = ∂ σij/∂ xj becomes ρ∂ 2ui/∂ t 2 = λδij∂ 2uk/∂ x j∂ x k + µ(∂ 2ui/∂ x j∂ x j + ∂ 2uj/∂ x j∂ x i) Substit’n of trial plane-wave solution ui = ui0f(n.r - vt) = ui0f(nmxm- vt) system of 3 eqns. linear in ui0 for given nm 3 eigenvalues given by cubic in ρv2 1 compressional mode ρvP2 = λ + 2µ = K +(4/3)G 2 orthogonal shear modes ρvS2 = µ = G ‘Geophysical’ thermodynamics: the internally consistent framework for understanding the Earth’s internal structure First law of thermodynamics (conservation of energy embodying the equivalence of heat and work and their relationship with internal energy E) dE = dQ − PdV; substitute for entropy S defined by dS = dQ/T, to get dE(S,V) = TdS − PdV Similar expressions for the other thermodynamic potentials: Enthalpy H = E + PV dH(S,P) = TdS + VdP Helmholz free energy F = -kT ln Z = E − TS dF(V,T) = − PdV − SdT Gibbs free energy G = H − TS = E + PV − TS dG(P,T) = VdP − SdT Z is the partition function of statistical mechanics References: Stacey, Appendix E; Poirier, Ch. 1 Useful thermodynamic identities Tools for functional analysis: For Z = Z(X,Y), dZ = (∂ Z/∂ X)Y dX + (∂ Z/∂ Y)X dY, ∂ 2Z/∂ X∂ Y = ∂ 2Z/∂ Y∂ X, & (∂ Z/ ∂ X)W = (∂ Z/∂ X)Y + (∂ Z/∂ Y)X (dY/dX)W (∂ Z/ ∂ Y)X = (∂ Z/ ∂ W )X / (∂ Y/ ∂ W )X Applications in deriving thermodynamic identities: E(S,V): (∂ E/∂ S)V = T (∂ E/∂ V)S = −P H(P,S): (∂ H/∂ S)P = T (∂ H/∂ P)S = V F(V,T): (∂ F/∂ V)T = −P (∂ F/∂ T)V = −S G(P,T): (∂ G/∂ P)T = V (∂ G/∂ T)P = − S (∂ T/∂ V)S = −(∂ P/∂ S)V (∂ T/∂ P)S = (∂ V/∂ S)P (∂ P/∂ T)V = (∂ S/∂ V)T (∂ V/∂ T)P = −(∂ S/∂ P)T Applications of thermodynamic identities Thermal pressure (∂ P/∂ T)V = (∂ S/∂ V)T = αKT (dPth)V = (γ/V) dEth (Grüneisen parameter: γ = αKTV/CV) Isobaric & isochoric derivatives (∂ Z[X,Y]/ ∂ X)W with X = T, Y = V & W = P: (∂ Z/ ∂ T )P = (∂ Z/∂ T)V + (∂ Z/∂ V)T (∂ V /∂ T )P = (∂ Z/∂ T)V − αKT (∂ Z/∂ P)T e.g., CP=(∂ Q/∂ T )P=T(∂ S/∂ T )P = T(∂ S/∂ T )V + VαT(∂ S/∂ V )T = CV[1 + αγT] Isothermal & adiabatic derivatives (∂ Z[X,Y]/ ∂ X)W with X = V, Y = T & W = S: (∂ Z/ ∂ V )S = (∂ Z/∂ V)T + (∂ Z/∂ T)P(∂ T /∂ V )S e.g., (∂ P/ ∂ V )S = (∂ P/∂ V)T + (∂ P/∂ T)V(∂ T /∂ V )S hence KS = KT[1 + α γT] Adiabatic temperature gradient (∂ T/∂ P)S = (∂ V/∂ S)P = (∂ V/∂ T)P/ (∂ S/∂ T)P = γT/KS (∂ lnT/∂ lnρ)S = − (∂ lnT/∂ lnV)S = γ Stacey Appendix E, Poirier Ch. 1 Elasticity & interatomic forces Turcotte & Schubert Figs.7.4 & 7.5 Volume per ion pair V = 2r3 Internal energy E = -C0/r +D0/rn = -C(V/V0)-1/3 + D (V/V0)-n/3 P = -dE/dV [strictly -(∂ F/∂ V )T with F = E - TS] Bulk modulus KT = -V(∂ P/∂ V )T Pressure derivative of KT’ = (∂ K T/∂ P)T ∴ K0V0 = (C/9)(n-1) ∝ d2E/dV2 ~ constant for isostructural compounds & K’0 = (n+7)/3 Interplanar forces & lattice vibrations Poirier, Fig. 3.1 Sequence of identical parallel planes of atoms interacting with neighbours: Fn = - K [un-un-1] + K [un+1-un] = M ∂ 2un/∂ t 2 Trial solution un = u0 sin(kxn - ωt) with xn= na Dispersion relation for lattice vibrations Poirier, Fig. 3.2 Condition for solution: ω = 2(K/M)1/2 |sin(ka/2)| Phase speed for longitudinal vibrations c = fλ = ω/k In general, group velocity u = dω/dk ≠ c dispersion Limit as k 0, u = c = a(K/M)1/2 Lattice vibrations: acoustic & optic branches Redrawn afetr Kittel Poirier Fig. 3.4 Redrawn after Kittel p atoms per unit cell 3p modes of vibration Acoustic modes: un and un+1 in-phase as k 0 Optic modes: un and un+1 out-of-phase as k 0 Quantisation & lattice vibrational energy Periodic boundary conditions for crystal of length L = Na require k = m(2π/L) Poirier Fig. 3.4 with m = 0, ±1, ± 2,…., ± N/2) Density of (k,ω) states in reciprocal (k) space is g(k) = 1/dk = L/2π = Na/2π such that ∫ g (k)dk = N 1st approx’n to crystal lattice: collection of independent harmonic oscillators of frequency ν, energy quantum hν, & (equilibrium) phonon occupancy quantum number p(ν,T) = [exp(hν/kBT) - 1]-1 ~ kBT/ hν @ high T (Bose-Einstein statistics) Evib(T) = Σ(i=1,N) hνi p(νi,T) = ∫ p(ν,T) hν g(ν) dν The Debye model of lattice vibrations Key assumptions: all modes acoustic & non-dispersive (ω = vDk) & uniformly distributed within spherical BZ of radius kD to match actual BZ volume density of states g(ν) = 9mν2/νD3 Normalisation: (∫ ( 0,νD) g(ν)dν = 3m, m atoms per unit cell Poirier Fig. 3.9 Poirier, Fig. 3.5 ω∝k g(ω) ∝ ω2 g(ω) for MgO vs. Debye model Debye model: thermal energy & specific heat ED(T) = ∫ p (ν,T) hν gD(ν) dν = (9nRT/x3) ∫ (0,x) ξ3 dξ / [exp(ξ) - 1] (per mol) with n atoms per formula unit, x = θ/T & Debye temperature θ = hνD/kB Poirier, Fig. 3.6 CV(T) = (∂ Q/∂ T )V = (∂ E/∂ T )V ~ T3 as T 0, ~ 3R for T >> θD ~T3 θD 3R Anharmonicity & thermal expansion Harmonic Poirier Fig. 3.12 Anharmonic Poirier Fig. 3.13 Asymmetry of potential well results in time-averaged inter-atomic spacing greater than static equilibrium value, reduced inter-planar stiffness constants, K & reduced vibrational frequencies ν ~ (K/M)1/2 Quasi-harmonic approximation: νi = νi(V) with γi = -dlnνi/dlnV = −dlnθD/dlnV = γD Finite strain & cohesive energy @ high pressure KT = -V(∂ P/∂ V )T = K0, constant integrates to P = - K0 ln (V/V0) but incompressibility must increase with P: e. g., K’0 = (∂ K T/∂ P)T0 = (n+7)/3 for rocksalt lattice Eulerian finite strain εij = (1/2)(∂ui/∂Xj +∂uj/∂Xi) - (1/2)∑k (∂uk/∂Xi)(∂uk/∂Xj) (Poirier, p. 60) P isotropic compressional finite strain ε: V0/V = ρ/ρ0 = (1-2ε)3/2 Taylor series expansion of Helmholz free energy F = E - TS: F(V,T) = a0 + a1f + a2f2 + a3f3 + ….. with ai = ai(T) & f = -ε 3rd-order Eulerian finite strain isotherm P(V) Now P = − (∂ F/∂ V )T = − (∂ F/∂ f )T/ (∂ V /∂ f )T KT = − V(∂ P/∂ V )T = − V(∂ P/∂ f )T / (∂ V /∂ f )T KT’ = (∂ K T/∂ P)T = (∂ K T/∂ V )T / (∂ P/∂ V )T = (∂ K T/∂ f )T / [(∂ V /∂ f )T (∂ P/∂ V )T] = −(V/KT)(∂ K T/∂ f )T /(∂ V /∂ f )T etc. ∴ P = (1/3V0)(1 + 2f)5/2(2a2f + 3a3f2) (P = 0 for strain f = 0) KT = (1/9V0)(1 + 2f)5/2[ 2a2 + (14a2+6a3)f + 27a3f2) KT’ = (1/3)[24a2 + 6a3+ (98a2 + 96a3)f + 243a3f2)/ [2a2+ (14a2+6a3)f + 27a3f2] Initial conditions: KT = KT0, KT’ = KT’0 a2 = 9KT0V0/2, a3 = (9KT0V0/2)(KT’0 − 4) Hence 3rd-order Eulerian (Birch-Murnaghan) isotherm P = 3KT0 (1 + 2f)5/2 [f + (3/2)(KT’0 - 4) f2] Mie-Grüneisen-Debye equation-of-state Finite-strain P(V) principal isotherm + Debye model for E(T,V) with θ(V) Construct F(V,T) = FBM(V,0) + FD(V,T) FD= ED− TSD with S = −(∂ F/∂ T )V FD− T (∂ F D/∂ T )V = ED ∴(∂[FD/T]/∂ T )V = −ED/T2 & FD = −T ∫(0,T) (ED/T2) dT = −T ∫(0,T) (ED/T2) dT ∫ by parts FD = 9nRT(θ/T)-3 ∫ (0,θ/T) ξ2 ln [1- exp(-ξ)] dξ P(V,T) = −(∂ F/∂ V )T = −(∂ FBM/∂ V )T − (∂ FD/∂ V )T ∴ P(V,T) = P(V,0) + (γD/V) ED(V,T) i.e., thermal pressure: δPTH(V,T) = (γD/V) δED(V,T) c.f. δPTH = (γ/V) δETH from (∂ P/∂ T )V = αKT = (γ/V)CV γ = γD The thermal pressure P(V,T) = P(V,0) + PTH(V,T) with PTH(V,T) = (γD/V) ED(V,T) Mie-Grüneisen-Debye EoS: completeness From F(V,T) = F(V,T0) + FD(V,T) = a2f2 + a3f3 + 9nRT[θ(f)]/T)-3 ∫ (0,θ(f)/T) ξ2 ln [1- exp(-ξ)] dξ [with θ(f) specified by γ0 = -(dlnθ/dlnV)0 & q0 = (dlnγ/dlnV)0] we have it all: P(V,T) = −(∂F/∂V)T, S(V,T) = −(∂F/∂T)V Hence E = F + TS, H = E + PV, G = F + PV CV = T(∂ S/∂ T )V KT(V,T) = −V(∂P/∂V)T , (∂P/∂T)V = αKT(V,T) = (γ/V)CV α, γ, KS = KT(1 + αγT), (∂ T /∂ P)S = γT/KS etc. Extension to shear strain (Stixrude & Lithgow-Bertelloni, Geophys. J. Int., 2005) Modelling the seismic properties of the Earth’s interior P(V,T), KT(V,T) & hence KS(V,T), G(V,T) from internally consistent finite-strain expansions of both static and thermal parts of the Helmholz free energy F ρ(z), VP(z), VS(z) For each mineral need F0, V0, KT0, KT’0, G0, G’0, θ0, γ0, q0, ηS0 constrained by experimental data and/or ab initio quantum-mechanical calculations Optimal finite-strain model constrained by diverse experimental data for MgO Kennett & Jackson, in prep. Elastic behaviour: essential characteristics Hookean elasticity: (i) Linearity: stress σ ∝ strain ε (ii) Instantaneity: strain appears(disappears) instantanously when stress is applied (removed) (iii) Recoverability: strain is fully recovered when stress is removed No dissipation: time-varying stress and strain in phase No dispersion: wave speeds are frequency independent Beyond elasticity: anelastic behaviour Relax requirement of instantaneity anelasticity Stress-induced diffusion of defects or redistribution of fluid occurs with characteristic timescale τ, typically thermally activated and contributes well-defined, delayed component of strain More strain for the same stress lower (relaxed) modulus = stress/strain Strain energy dissipation Delayed anelastic strain phase lag between stress σ(t) = σ0 sinωt & resulting strain ε(t) = ε0 sin(ωt − δ) Energy dissipated per cycle ΔE = ∫ (0,2π) σ dε = ωσ0ε0 ∫ (0,2π) sinωt cos(ωt − δ) dt Using cos(ωt − δ) = cosωt cosδ + sinωt sinδ & sin2ωt = 2sinωt cosωt and cos2ωt = 1 − 2sin2ωt obtain ΔE = (σ0ε0/2) ∫ (0,2π) [sin2ωt cosδ + (1 − cos2ωt)sinδ] d(ωt) = πσ0ε0sinδ Maximum energy stored Emax = ∫ (0,π/2) σ dεin phase = σ0ε0cosδ ∫ (0,π/2) sinωt d(sinωt) = (σ0ε0cosδ)/2 Quality factor Q Q = 2πEmax/ΔE = 1/tanδ or Q-1 = tanδ Seismic properties: laboratory methods Complementary experimental techniques probe a wide range of frequencies (c.f. mHz - Hz of teleseismic waves) Elastic wavespeeds: ultrasonic methods Mode-specific piezoelectric or ferroelectric transducers Generate & detect elastic waves Simple pulse transmission (time-of-flight) & interferometric methods Ultrasonic wave-propagation methods: representative results MgSiO3 perovskite Li & Zhang PEPI (2005) Silicate perovskite analogue ScAlO3 Data fitted to F(f,T) model V0,K0,K’0,G0,G’0,θ0,γ0,q0,ηS0 Jackson & Kung, PEPI, 2008 Opto-acoustic methods: Brillouin scattering Vi = Δω λ /2sin(θ/2) (i = P, S) from Doppler shift Δω Application in diamond-anvil apparatus G(P) to 100 GPa for MgSiO3 perovskite improved constraints on G' Murakami et al., EPSL (2007) 0 Pressure, GPa 100 Forced-oscillation method for laboratory study of anelasticity @ seismic frequencies Implementation within internally heated gas apparatus (Jackson & Paterson, PAGEOPH, 1993): P = 200 MPa, T to 1300°C oscillation periods 1-1000 s shear strains < 10-5 Underlying principle Specimen & reference assemblies & T profile Specimen encapsulation Seismic-frequency forced-oscillation data for dry melt-free polycrystalline olivine Jackson, Fitz Gerald, Faul & Tan, JGR, 2002 Modelling elasticity with interatomic potentials Putnis (1992) K (quartz) = 39.7 GPa c.f. 39.3 GPa (measured) Newton et al., Phys. Chem. Minerals (1980) Quantum chemistry: H atom to crystals Schrödinger equation: (-h2/2m)∇2ψ + Vψ = Eψ Hydrogen atom Analytical solution: s, p, d, … orbitals Energy levels consistent with observed line spectrum Multi-electron atoms Electron-electron interaction: no analytic solutions Aufbau and Pauli exclusion principles: self-consistent field atomic orbitals (a.o.) Small molecules Molecular orbitals as linear combinations of a.o. - coefficients chosen to minimise total energy; high e- density between atoms = chemical bonding Crystalline solids Zero K Density functional theory: ground-state energy a unique function of the spatial distribution of electron density; High-T Quasi-harmonic approach – lattice vibrational frequencies ν(V); anharmonic ab initio molecular dynamics. 1998 Nobel prize in chemistry to Kohn & Pople Seismic properties: ab initio constraints C11 C22 C33 C12 C13 C23 C44 C55 C66 Theory 493 546 470 142 146 160 212 186 149 Exp 482 537 485 144 147 146 204 186 147 MgSiO3 perovskite: singlecrystal elastic constants (GPa) ΔE = (1/2)Cε2 Oganov et al. (2001) Single-crystal elastic anisotropy: olivine triangular clusters of edge-sharing MO6 octahedra capped by SiO4 tetrahedra form stiff columns high VP along [100] Webb & Jackson, Am Mineral. (1990) Polyhedral structure of olivine (Putnis, 1992) Mainprice (2007) Treatise on Geophysics Bulk Earth composition & pressureinduced phase transformations Phase proportions, % 50 0 100 200 . px olivine SIO4 tetrahedron Depth, km 400 Pyrolite model upper-mantle composition = magma (basalt) +residue (harzburgite) Green & Ringwood (1960’s) wadsleyite garnet 600 ringwoodite -------------SIO6 octahedron Mg-perovskite 800 After Irifune, Nature (1994) mw Ca-pv Crystal structures of high-pressure minerals (Mg,Fe)SiO3 perovskite (Mg,Fe)O magnesiowüstite CaSiO3 perovskite Gross Earth seismological models Fowler Fig. 8.3 Fowler Fig. 8.1 Inversion of traveltime versus angular distance & free-oscillation data for spherically averaged structure Lateral variations of seismic wave speeds in the Australasian upper mantle Surface-wave tomographic model of Fishwick et al., EPSL, 2005 VS variations (%) at 200 km depth Optimal geotherms and VS(z) profiles for contrasting tectonic provinces thermal boundary layer of varying depth ------------------------------------- common mantle adiabat Lab-based model inclusive of anelastic relaxation (Faul & Jackson, EPSL, 2005) Seismological models for the transition zone of the Earth’s mantle Vφ = (Ks/ρ)1/2 = [VP2-(4/3)VS2]1/2 Bulk sound speed, V! / km s-1 9.0 8.5 pyrolite model NJPB 8.0 TNA 7.5 7.0 Nth Atl 6.5 SNA 6.0 200 300 400 500 600 700 800 Depth / km Jackson & Rigden, Ringwood volume, CUP,1998 900 Composition, elasticity & temperature of the lower mantle Lower-mantle mineralogy for pyrolite composition G’ & dG/dT from ultrasonics: pyrolite & 1600 K adiabat OK (Li & Zhang, PEPI, 2005) Lower G’(Brillouin scattering): [SiO2] > pyrolite Murakami et al., EPSL (2007) Lower mantle: new developments Fe2+: 3s23p63d64s0 d-orbital degeneracy removed by the octahedral crystal field (Brown et al., Chemistry,1991) Pressure-induced spin-pairing in Fe Minerals Badro et al. , Science, 2003 & 2004 δρ/ρ ~1% ‘Post-perovskite’ CaIrO3 phase of MgSiO3 @ P > 120 GPa, T ~ 2500 K Murakami et al., Science, 2004 Oganov & Glass, J. Chem. Phys., 2006 etc. Earth’s core: composition & temperature Preferred hexagonal close-packed structure for pure Fe under innercore conditions Inner core conditions & properties Fowler Fig. 8.12b Core is significantly less dense & somewhat more compressible than pure Fe Constraints on core composition & temperature from ab initio calculations Mol fraction in liquid Fe Ab initio calculation of equations-of-state V(P,T) & Gibb’s free energies G(P,T) for both solid and liquid phases melting temperature Tm and element partitioning Si S O Can match densities of inner and outer core with thermodynamic equilibrium at inner-outer core boundary: P = 330 GPa,Tm = 5600 K Inner core: 8 mol% S/Si & 0.3% O Outer core: 10 mol% S/Si & 8% O Alfè et al., EPSL, 2002 Stable crystal structure for the inner core? ? or hexagonal close-packed symmetric 12-coordination body-centred cubic split 8-6 coordination with larger interstices Thermal/compositional stabilisation of the bcc phase? (Vocadlo et al., Nature, 2003) Part I: Elasticity, equations-of-state & interpretation of seismological models Tensor stress & strain Constitutive law & elastic waves Elasticity & interatomic forces Geophysical thermodynamics Lattice vibrations & thermal energy Anharmonicity & thermal expansion Finite strain & cohesive energy @ high pressure Mie-Grüneisen equation-of-state & thermal pressure Anelasticity & seismic wave attenuation Interpretation of seismological models The End Next: Part II Heat transport & geodynamics