Proper accounting for anisotropy changes everything! The PREM Preamble! 11 4 DECEMBER 2009 VOL 326 SCIENCE plus ~10 other conflicting interpretations 7 color pictures, intended to help in visualizing tomographic models, can be misinterpreted. We have a responsibility, as scientists, to do more than present laterally truncated color-saturated 2D images. The first principle is that you must not fool yourself and you are the easiest person to fool. Richard P. FeynmanRichard P. Feynman Color graphics have become very sophisticated; there is no mistaking what authors want you to seeis seeing believing? Filtering, smoothing, interpolating, color pallet, orientation, vertical exaggeration, truncation, parameterization, reference model, contour cutoffs No power versus the Farallon slab (times 10) This picture caused mantle geochemists & modelers to drop layered models in favor of enforced whole mantle convection It is well established that oceanic plates sink into the lower mantle geochemists interpretation of tomographic images The evidence stagnant slabs volatiles SpainItaly Slip-free subadiabat Color pictures can lie but some are well constrained Slab ponding is the key! TRADE OFFS Isotropic 6- parameters, 1-anisotropic parameter inversion (VSV) 6-parameter anisotropic inversion (density, VSV,VSH, VPH, VPV, ETA) The same data, number of parameters & starting model are used in both inversions artefacts HAWAII MYTHS; anisotropy is more complicated than isotropy; too many parameters Hawaii Pacific This is what PREM would have looked like if anisotropy (& Vp) had not been taken into account properly, cf. Hawaii PREM (BLUE) Laske et al. Pseudo-anisotropy The most subtle bleeding artefact VSV ! RED PAUSE 23 parameters Cold warm TZ Through-going slabs Though-going hot conduits Stagnant slabs Ridge adiabat Midplate geotherm surface volatiles TEMPERATURE 200 The transition region is the key to a number of geophysical problems Francis Birch 1952 Whole-mantle & bottoms-up convection Layered convection 410 & 650 are correlated Internal boundaries allow for a fixed reference system Hints from fluid dynamics Whole mantle convection paradox Long-lived slabs in the transition region cool off the top of the lower mantle. Stagnant long-lived blobs in lower mantle influence upper mantle. Apparent continuity of tilted o S 2 feature does not imply whole-mantle convection High velocity ABA (ADAMS & BARBARAS ANCHOR) paradox Slabs at 650 km Heated from below cooled from above & internally heated CMB (constant T) 650 Lattice dynamics (T) (V,T) T CMB (t) (V,T) U(z,t), Th(z,t) T(x,t) Narrow buoyant jets Broad passive upwellings Constant T Materials displaced out of TZ become broad Ridge Feeding Upwellings (RFUs) 12 conflicting models for Hawaii in Nature & Science alone Why? mysteries remain. - Humphreys & Schmandt Physics Today, August 2011 paradigm paradox physics *Likewise for 3D effects, anisotropy, thermodynamics, quantitative seismology "When you really understand physics*, there are no paradoxes." S.Chandrasekhar Physics Today, March 2007 Mysteries & paradoxes Decreasing T with depth Subsolidus melting Subadiabaticity Volcanoes on Correlated 410 & 650 depths thick sides of transforms Deep structure of continents Blue Hawaii Fixed reference system Heatflow, He, Pb, Th Residual topography Seafloor flattening Slabs under hotspots High Vs gradients >12 conflicting models for Hawaii in Nature & Science alone mysteries remain. - Humphreys & Schmandt Physics Today, August 2011 paradigm paradox physics *3D effects, anisotropy, thermodynamics, quantitative seismology Paradoxes, & differences in mantle dynamic & geochemical models, can be traced to 1. 2 nd law violations (external sources of heat, material, information, e.g. Maxwell demons) 2. Applying wrong or incomplete physics (being in the wrong paradigm; this is more serious since residents of different paradigms & disciplines cannot communicate) internal constitution of the mantlethermodynamics, self-consistencyself-compression, self- organization fluid dynamic scaling relations, dimensional analysis, physics of high presssure physically realizable constitutive relations, thermal history, effects of internal heating, secular cooling & anisotropy PHYSICS OF THE MANTLE Harvard Harvard Harvard physics seismology models self- mean isolated, no external Maxwell demon (e.g. thermo-scoflaw) KELVIN AESOP ARCHIMEDES CLASSICS tectonics seismology Fluid dynamics complexity myths Just-so stories models Rheology time U,Th,K chaos entropy Allowing for BL anisotropy, cooling & self consistent thermodynamic properties & geotherms one derives a boundary layer model (Region B) that is essentially the complete opposite of presently popular models (which focus on Region D for intraplate volcanoes). EFFECT OF CLASSICAL PHYSICS ON THE GEOTHERMS & GEODYNAMICS Temperature dependence of thermal conductivity Hot concave-up conductive geotherms Radioactivity Subadiabaticity in the convecting mantle Secular cooling Thermal bump in upper mantle Thick boundary layer (BL) Temperature dependent viscosity Thick long-lived BL High potential temperatures (Tp) at base of BL Sub-solidus melting No horizontal isotherms Boundary Layers (BLs) & Mantle Geotherms Mantle temperature increases rapidly (non- linearly) with depth for ~200 km (in BLs) & decreases thereafter! Bullen, Birch, J.T.Wilson, Peltier, Jarvis, Jeanloz, Morris, Moore, Tackley, Sinha, Butler, Yuen, Hofmeister T depth 8 BOTTOM LINES- first order physics In a cooling radioactive isolated planet the geotherm is adiabatic almost nowhere! Condensed matter & complexity physics (self-compression & self-organization) rule out whole-mantle convection, bottoms-up & deep anchor/reservoir models for Hawaii, Iceland, Samoa, Yellowstone etc. Regions D & D are important parts of the mantle but have little to do with volcanoes & plate tectonics Evidence against whole-mantle convection 1.Dynamic topography; shape and amplitude 2.Correlated 410 & 650 depths 3.Polar ring of geoid lows 4.Convection simulations disagree with 1 st order seismic features (Schuberth et al.) 5.Short-lived geoid 6.Unphysical scaling relations 7.Birch (1952) 8.Violations of 2 nd Law (constant CMB temperature) SHALLOW Hawaii Reprinted: Canadian Journal of Earth Sciences Vol. 51, No. 3, (Special) 5 March 2014 PLATE Low-velocity zone Intra-plate magmas (e.g.Hawaiian tholeiites) are derived from the low-velocity zone (LVZ) part of the sheared surface boundary layer (LLAMA*). They are shear-driven not buoyancy-driven jets. The upper 220 km of the mantle (REGION B) is a thermal, shear & lithologic boundary layer & the source of midplate magmas. 200 km FOZO 1600C (OIB components) *LLAMA=laminated lithologies & magma accumulations LLAMA AFTER KAWAKATSU ANISOTROPY ADIABATICITY ANHARMONICITY ANELASTICITY 2 nd Law, & If you have tears, prepare to shed them nowYou all do know this mantle is not isotropic, adiabatic, linear, homogeneous, steady-state, heated from below, frequency independentideally elastic or fluid& this makes all the difference Classical physics . et tu, you must honor When Anisotropy is properly taken into account & when Artefacts/models/Assumptions/Approximations that violate the 2 nd Law are eliminated (injection, jets, constant CMB temperature, Maxwell demons) 2. ABSOLUTE WAVESPEED, 3. ARTEFACTS, 4. ASSUMPTIONS & APPROXIMATIONS whatever remains, however improbable, must be the truth. 1. AGE of Earth (Kelvins secular cooling effect); modulated & slowed by internal heating ( Lord Rutherford, d.1937). average convecting mantle geotherm is subadiabatic. Plate tectonics places cold slabs in the interior, resulting in cooling from within, reenforcing subadiabaticity. The core & the core-mantle boundary (CMB) are cooling (thermodynamics & geodynamo). THE SURFACE BOUNDARY LAYER IS 200 K HOTTER & THE BASE OF THE MANTLE IS 400 K COLDER THAN ASSUMED IN CANONICAL MODELS 1. MANTLE IS A TOP-DOWN SYSTEM, HEAT IS REMOVED FROM THE TOP; EARTH IS COOLING (Lord Kelvin, d.1907) DEEP UPWELLINGS ARE COLD! CMB Upwellings (volcanoes) can be induced from the top & by squeezing Passive upwellings 2. Ancient conventional wisdom The mantle is a thermodynamic engine Engines, in contrast to models, must obey thermodynamics Standard approximations in geodynamics do not work for physically realizable thermodynamic systems When analyzed canonical models are perpetually motion machines eclogite harzburgite cold The plate tectonic top-down bi-cycle Eclogite is intrinsically dense & stays at the base of the TZ. Harzburgite (lithosphere) is intrinsically light & returns to surface. They both displace older material (MORB source) up & cool off top of lower mantle colder warm Eureka! cooling ^ Passive upwellings are broad & sluggish, to compensate for narrow fast downwellings Ridge crests occur above ~2000 km broad 3D passive upwellingshotspots are satellite shear-driven magma segregations km Near-ridge hotspots sample deep & are coolish compared to midplate volcanoes Similar features occur intraplate French et al. SCIENCE 2013 Off-ridge red/yellow regions are also >1000 km broad, therefore PASSIVE UCB thanks, Barbara Similar features occur under & near ridges 20 broad (~1000-km) passive upwellings rising at 1-2 cm/yr balance the global downward flux (slabs, delamination) These are NOT plumes (buoyancy- driven); they are background Important physics; 1.Maxs & mins in the geotherm 2.Decreasing temperatures with depth, time & subadiabaticity 4.Layered self-organized convection 5.Full anisotropy 6.Maxwell, Gruneisen, Debye & fluid dynamic scaling relations 7.Melts in the shallow mantle & interaction with seismic waves 8.Non buoyant upwellings & Aesop, Archimedes, Boltzmann, Born, Brillouin, Chaos These models are impossible (for an isolated planet) from classical physics (Debye, Gruneisen) & modern complexity theory (self-organization); they are riddled with numerous The isolation & 2 nd law constraints-b oundaries held at constant T by external sources-& thermodynamic consistency, are the most common violations of physics. Maxwell demon Current conventional wisdom Externally heated No pressure effects Thin boundary layer No temperature effects Free slip No internal heating No secular cooling Conduit to CMB Constant T CMB PHYSICS-FREE MODEL Physics Today 2011 jet Saturated images showing broad slab-like features Modern mantle geochemistry & geodynamics are based on these images & cartoons Geochemists & geodynamicists* are particularly impressed by color pictures provided by seismologists Whole-mantle convection, hole-in-the- floor, water filter, mantle jets *usual disclaimer about audience Sediments volatiles Intraplate volcanoes Midocean ridges Residual slab components LOWER MANTLE *Essentially the classical model of Birch, Tatsumoto, Wilson RIDGETRENCH OIB BAB Slabs displace ancient depleted materials out of TZ that become broad Ridge Feeding & Midplate Upwellings (RFUs) Contaminated MORB OIB MORB OIB Slab fluids 31 DD Classical (e.g. Birchean) views of the mantle are based on self- compression, thermodynamic self-consistency, thermal history Alumina, lime & alkalies Garnets & jadeite radioactivity ISOLATED TZ 3.5 sec S-time variation due to angle of incidence alone ~1.3 sec between S & SKS Pseudo- Anisotropy (3 moduli) VpVSH & VSV No angular dependence (unrealizable; bad physics) Laminations plus xl orientation Holtzman & Kendall Physically realizable patterns Polar plots Unaccounted for anisotropy in Region B gives artefacts (steaking, bleeding) in teleseismic travel-time tomography (TTTT) Constant conductivity No radioactive heating No thermal overshoot Horizontal isotherm subsolidus physics LVZ= No melt in ambient mantle isothermal adiabatic 10 violations of Heating from below No secular cooling TBL = The canonical 1988 ambient mantle geotherm Jet T max assigned AmbientFrom CMB A B B C C D D Crust LID Lower Mantle Tp BL LVZ GLGL Region B Moho-220 km Region D Subadiabatic geotherm Deep Tp is colder than B slabs TZ OIB & Back-arc magmas MORB No infinite energy source; no 2 nd Law violations Decaying T boundary condition Anderson, J. Petr Upper Mantle km Tp BL A sheared melange Region C Slab graveyard Engines involve self-consistent changes in volume due to compression, heating & changes of state. The eclogite cycle is cooling, phase change (eclogite), compression (sinking), heating, phase change (melting), decompression (rising), shearing, squeezing, heat exchange This top-down physics differs from thermal convection but it removes heat from a planet. Aesop, Archimedes, Advection; the Eureka solution to mantle dynamics 200 Myr of oceanic crust accumulation TRANSITION ZONE (TZ) REGION B Super- adiabatic boundary layer Thermal max 600 km 300 km Tp decreases with depth 600 km Physics-Based Archimedian Paradigm* (RIP) (* Birch, Tatsumoto, J. Tuzo Wilson) Shear strain fixed Hawaii source MORB source Shear-driven magma segregation Sources deeper than ~ km are effectively fixed (e.g.J.T.Wilson) OIB (T), (V,T), T CMB (t), (V,T), U(z,t), Th(z,t) Pebbles Old Greeks Slabs The Eureka Solution Archimedes & Birch squeezin g self-consistent thermodynamic & transport properties, self-compression (Bridgman, Birch, Maxwell, Kelvin, Grneisen, Debye, Born), physical scaling relations, physically realizable constitutive relations, effects of internal heating & secular cooling QUANTITATIVE (robust) SEISMOLOGY surface waves, normal modes, waveforms & theory (in addition to body waves & relative wavespeed tomography) are essential (the ~20 contradictory models for Hawaii are based on subsets of data & limiting assumptions). THE PHYSICS OF GEOPHYSICS; A HARVARD LEGACY THANK YOU Boundary layer motions Sub-boundary layer Ridges are LVAs Ridges are HVAs W-ward drift Boundary layer motions Sub-boundary layer Ridges are LVAs Ridges are HVAs W-ward drift Ridges are HVAs (cold upwellings) Vs cold hot Mantle under large long- lived plates is hot Sources deeper than 150 km are relatively fixed J.Tuzo Wilson (1963) Tatsumoto (1976) RIDGEisland FERTILE RIDGE SOURCE Canonical geochemical model (1973-present) Deep jets/condui ts * *plus U & Th Lower mantle (Region D) starts ** ** eclogite (e.g. fertile) By physics we mean Archimedes, Rayleigh, Debye, Born, lattice vibrations, entropy Questions for students Why do tornados & dust devils only occur when the ground is heating up, not when equally hot ground is cooling? Explain relevance to a cooling CMB & unstable lower BLs (e.g. RT, plume)? Why are all BLs in Earth ~200-km thick? LIL Sheared mlange ridge UPPER MANTLE Tp km Ancient eclogite cumulates Modern slab fragments LVZ TZ LIL Athermal Anisotropy (NOT just SV or SH data, or isotropic perturbations) Advection-Shearing (driven from the top) Artefacts (streaking, smearing, bleeding, neglect of Vp in shear wave data) Absolute wavespeeds Boundary Layers Bullen Parameter (AdiabaticityNOT) Conservation Laws (2 nd Law violations, consistency, constant T boundaries (CMB, thin plate, whole mantle convection)) B REGIONS B & C C geotherm Some impossible assumptions & approximations in Standard & Thin Plate Models; Constant properties ( ) Adiabaticity (below a thin plate) Homogeneity (vigorous convection) Unphysical scaling relations ( Vp, Vs, T) Unrealizable forms of anisotropy Maxwell demons & violations of the 2 nd Law (definition of impossible in physics) Horizontal isotherms; 1D & 2D approximations Fluid injection simulations Forced whole mantle convection Non cooling interfaces (LAB, CMB) External sources(matter, heat, information) ISOTHERMAL ADIABATIC ISOTHERM 100 km HOMOGENEOUS jet STANDARD CONCEPTUAL MODEL (1988) No physics, no seismology (used as reference model; Herzberg, Asimow, Humphreys, Schmandt, Victor Campe.g) No U, Th, K No secular cooling Ambient T constrained (100 km) boundary layer geotherm; >200 o C hotter (conductivity decreases with temperature) Potential temperatures in D are much lower; (radioactive heating & global secular cooling yield subadiabats) Excess temperatures do not require jets or conduits to the CMB or local hot spots Broad (>300-km) sub-BL tomographic features are passive Now, phase in quantitative seismology *A.M. Dziewonski, D.L. Anderson, Preliminary reference Earth model, Phys. Earth Planet. Inter. 25 (1981) J.-P. Montagner, T. Tanimoto, Global upper mantle tomography of seismic velocities and anisotropies, J. Geophys.Res. 96 (B12) (1991) 20337^ Anderson, D. L., Elastic wave propagation in layered anisotropic media, J. geophys. Res., 66, Anderson, D. L., Recent evidence concerning the structure and composition of the Earth's mantle, Phys. Chem. Earth, 6, Anderson, D. L. & Dziewonski, A. M., Upper mantle anisotropy: evidence from free oscillations, Ceophys. J. R. astr. Regan, J. & Anderson, D. L., Anisotropic models of the upper mantle, Phys. Earth Planet. Int., 35, Nataf, H. C. et al Measurements of mantle wave velocities and inversion for lateral heterogeneities and anisotropy, J. geophys. Res., 91, M. Panning and B. Romanowicz. Geophys. J. Int. (2006) 167, 361379 J.-P. Montagner /Earth and Planetary Science Letters 202 (2002) C. Beghein, J. Trampert /Earth and Planetary Science Letters 217 (2003) 151^162 Few seismologists [with the notable exception of present company*] have taken anisotropy into account in a fully self-consistent (Woodhousean) way. Common errors; assume Vp is unimportant in Rayleigh wave inversion, use SH data alone to derive SH models, invert for 1 parameter, invert P-SV & SH data independently, use isotropic perturbation & scaling relations is this a big deal? Anisotropy & recognition that Tp at ~200 km depth (bump) is higher than in D are the most far-reaching developments in mantle petrology & geochemistry since Birch & Bullen* established non-adiabaticity of the mantle [superadiabatic thermal gradient above 200 km, subadiabatic gradient below]. T depth High Tp in ambient shallow mantle is consistent with petrology (Hirschmann, Presnall) [the BL is mainly buoyant refractory harzburgite, not fertile pyrolite] *Raymond Jeanloz of UCB reopened this long-dormant issue but it is still widely ignored bump Low viscosity High attenuation Directional scattering Low velocity High SH Low SV Low viscosity High permeability SKS splitting ATTENUATING ANISOTROPIC ASTHENOSPHERE These are usually attributed to microscopic effects; oriented crystals, dislocations, diffusion 2% LVZ OIB components Non-MORB components are in the sheared boundary layer, not in D Magma is squeezed out of the mantle x LVZ 3.5 sec S-time variation due to angle of incidence alone ~1.3 sec between S & SKS Pseudo- Anisotropic (3 moduli) VpVSH & VSV No angular dependence (unrealizable) Laminations plus xl orientation Holtzman & Kendall Physically realizable patterns Polar plots x *Laminated Lithologies & Aligned Melt Accumulations Seismology of LLAMA* teleseismic rays Large lateral variations in relative delay times due to plate & LVZ structure, & subplate anisotropy bleed into deep mantle SKS very lateS early S late large relative delay times in BL = comparable to crustal delays underplate Hawaii 3. ABSOLUTE WAVESPEEDS ; Hawaii is blue!...in all well constrained inversions with anisotropy Models with lots of data (Katzman, Jordan, Maggi) Lots of rays fast HAWAII For example WHY? Bad (impossible) physics Externally heated No pressure effects Thin boundary layer No temperature effects Free slip No internal heating No secular cooling Conduit to CMB Constant T CMB An isolated cooling planet with internal heating is characterized by broad upwellings*. An accreting proto- planet & fluids heated from below are characterized by narrow heat pipes aka conduits, jets etc. Physics plus seismology plus plate tectonics * Mimics a cooling planet; broad passive upwellings COLD WARM REGION B TZ Ridges & near- ridge hotspots COOL SLABS DISPLACE ANCIENT DEPLETED MATTER OUT OF TZ Reheated harzburgite (lithosphere) also rises MORB Broad depleted Ridge-feeding upwellings Fractionation & contamination km 650 km Surface BL sources for intra-plate (non-MORB) geochemistry EM (DM) 13 Passive upwellings are broad & balance subduction flux. They are captured by spreading ridges, some are midplate active passive Upwellings are associated with plate tectonics even if there is no heating from below Top-Down Plate-Slab Driven Upwellings Ridges capture upwellings (Gabriele Marquart 2001) Broad passive upwellings Some candidates for broad passive off-ridge upwellings RITSEMA Midplate upwellings are fertile & interact with the surface boundary layer 300 km depth MID-ATLANTIC RIDGE (MAR) Ritsema & Allen Tp decreases with depth cool hot Depth (km) Ritsema & Allen Silveira & Stutzmann mid-Atlantic ridge Atlantic Deep ridge-feeding upwellings Off-ridge passive upwellings Too broad to be active or buoyancy-driven Ved Lekic et al. Just below the BL Passive fertile upwellings can melt at depths >300 km & become LVAs not = LVAs Hotspots Regions of extension, divergence, BL shear Passive upwellings are induced by Delamination Spreading Slabs (transition zone) Boundary layer shear (SDU) Secular cooling They are not driven by intrinsic buoyancy (in contrast to Rayleigh-Taylor instabilities ); orogenic & intraplate volcanoes operate in a higher temperature & shallower environment than midocean ridges & near-ridge hotspots. They are distinct from, but complementary to, small-scale convection, edge effects, propagating cracks etc. Ridge & intraplate volcanoes are not Rayleigh-Taylor instabilities; different physics entirely Hotspots are preferentially located, to a high degree of statistical significance, above regions of positive divergence of horizontal shear tractions beneath the lithosphere caused by downwellings (subduction) in the upper mantle & above low wavespeed regions associated with spreading. Upwellings & red regions noted in tomography are too broad* to be active, e.g. they are not buoyancy driven. Hotspots correlate with locations of slab-driven shallow divergence *Elinor Styles, Goes, van Keken, Ritsema, Hannah Smith LVZ MID-PLATE BOUNDARY LAYER VOLCANOES Leahy et al. Kawakatsu et al hotspot & back-arc magmas are extracted from the thermal bump region of the surface boundary layer Common Components (FOZO) 1600 C SOURCE Intraplate volcanoes inboard of Japan Trench (JT) Yellowstone, Snake River Plains, Karoo, Deccan occur in similar terranes (essentially, Back-Arc Basins). Slab graveyard fixed cold The correlated 410/650 paradox UPPER MANTLE & LOWER MANTLE ARE COOLED BY LONG-LIVED FLAT (STAGNANT) SLABS Cold slab European, African, Asian (Changbai), Yellowstone & most continental intra-plate volcanoes (hotspots) are underlain by slabs Cooled mantle 49 Center of LLAMA Scale change Region B Region C Degree 2 pattern in TZ is due to past subduction Degree km the key to a number of geophysical problems Francis Birch 1952 The Degree 2 Paradox Upper & lower mantle regions cooled by subduction 600-km 125-km 350-km Long-lived slabs in the transition region cool off the top of the lower mantle. Stagnant long-lived blobs in lower mantle influence upper mantle. Apparent continuity of tilted o S 2 feature does not imply whole-mantle convection High velocity ABA (ADAMS & BARBARAS ANCHOR) paradox Ditto for radioactive planets & those with self-compression pressure broadening Mesosphere (TZ) LID LVZLLAMA Intraplate (delamination, CRB, Deccan, Karoo, Siberia) magmas are shear-driven from the 200 km thick shear BL (LLAMA) ridge km Cold slabs SUMMARY OIB Ridges are fed by broad 3D upwellings plus lateral flow along & toward ridges subadiabatic Intraplate magmas are delivered to the Earths surface not by active buoyancy- driven upwellings but by shear-induced magma segregation (Conrad, Kohlstedt, Holtzman), magmafracture & passive upwellings. Active upwellings (aka jets, pipes, conduits) play little or no role in an isolated planet with no external sources of energy or material. This is a simple consequence of the 2 nd Law of thermodynamics (Lord Kelvin) secular cooling implies subadiabaticity in an isolated cooling planet. Maxwell demon RIDGE Darwin R. Villagmez1(, Douglas R. Toomey1*, Dennis J. Geist2, Emilie E. E. Hooft & Sean C. Solomon NATURE Geoscience 2014 high 3 He/ 4 He High & variable 3 He/ 4 He is picked up in BL No He anomaly Mid-plate upwellings interact with the surface boundary layer INTRA-PLATE 3 He Hawaii, Galapagos etc. are broad passive upwellings that come up mid-plate Helium paradoxes In whole mantle convection simulations, both the surface & the core-mantle boundary move rapidly. Neither provides a stable reference system FREE-SLIP BOUNDARY HEATED FROM BELOW x Fixed hotspot paradox COLD WARM REGION B TZ Ridges & hotspots COOL No hotspots LVA STAGNANT SLABSA FIXED REFERENCE FRAME SLIP-FREE BOUNDARY 50 There may be a concentration of CaO, Al 2 O 3, KU, Thin the upper mantleBirch Fixed hotspot paradox Schuberth et al. (2009) geotherms for enforced whole mantle convection with hot core & constant CMB temperature but otherwise thermodynamically consistent CMB superadiabatic subadiabatic adiabat A subadiabatic mantle is stable and will not convect unless sinking material displaces it. Free-slip boundary SUBADIABATICITY HOT COLD Subadiabatic mantle Bullen, Birch, Jeanloz Why? The silo effect Petrology Geochemistry Tomography Seismology Geodynamics Tectonophysics Geology Physics Adiabaticity Isotropy Whole mantle convection Isothermal boundaries are assumptions, not approximations. Dropping them removes a host of paradoxes. Layered convection mimics whole mantle convection at long wavelengths Visual interpretations of color tomograms led some to infer whole mantle convection (e.g. Grand, van der Hilst, van DeCar, Solomon, Montelli). Later models (e.g. Ritsema) cannot be so interpreted. Boundary layer Midplate Ridge adiabat (passive) LLAMA(shea ring) Plate (conducting) Depth ToCToC T Depth B DD TZ CMB Geotherms based on 1. physics, 2. fluid dynamics, 3. petrology & broad-band seismology imply subadiabaticity UPPER MANTLE LOWER MANTLE The highest potential temperature (Tp) in the mantle is near 200 km. Tectonic processes (shear, delamination) access this. ridge midplate bump Tackley & others Presnall & Anderson O O Fluid dynamics (T) (V,T) T CMB (t) (V,T) U(z,t), Th(z,t) TpTp Both the top and lower boundary layers are rapidly moving in any whole mantle convection scheme However, slabs at 650 lie on a viscid boundary and provide a fixed reference frame Free-slip boundary Mantle reference frame Body wave only inversion Rabbit ear artifact West et al. Rabbit ears artifacts Wolfe et al Yellowstone Hawaii Rabbit ear artifacts? Cold ridge source hot Science 24 April 2009: Vol no. 5926, pp DOI: /science Prev | Table of Contents | Next REPORTS Seismic Evidence for Sharp Lithosphere- Asthenosphere Boundaries of Oceanic Plates Hitoshi Kawakatsu, Prakash Kumar, Yasuko Takei,Masanao Shinohara,Toshihiko Kanazawa, Eiichiro Araki, Kiyoshi Suyehiro The key papers Melt-rich layers Free-slip Slip-free 60 Myr later Non-fixed non- vertical upwellings No physics! Saturated images showing slab-like features Modern mantle geochemistry & geodynamics are based on these images & cartoons Geochemists at Lyon, Paris, Yale, Berkeley, Mainzare particularly impressed by these cartoons Whole-mantle convection, hole- in-the-floor, water filter 1988 TBL K(T) sub Lithosphere lid LVZ adiabat The 1988 Cambridge geotherm ignores K(T) & radioactivity Put physics & seismology into the geotherm and does not satisfy surface wave dispersion data When internal heating is taken into account the potential temperatures near the base of the mantle can be >300 C lower than at 200 km depth Schuberth et al. J.Tuzo Wilson (1963) Tatsumoto (1976) RIDGEisland FERTILE RIDGE SOURCE Canonical geochemical model (1973-present) Deep jets/condui ts The evidence for relatively cold mantle under and near ridges is 1. the depressed residual bathymetry & low subsidence rates, 2. the higher than predicted seismic wavespeeds and the lateral decrease of wavespeed away from ridges below 170 km depth. The evidence for deep adiabatic passive upwelling is the VSV>VSH anisotropy. The combination of high wavespeed, and possibly dense mantle below 170 km and ridge suction suggests that upwelling under ridges is passive and adiabatic. The ridge geotherm approximates a ~1300 C adiabat. 11/12/12 410-km McKenzie & Bickle (1988) argue that the mantle below 100-km depth is homogeneous, isothermal & adiabatic; this implies a cold subsolidus boundary layer & hot jets Anderson, Don L., Tanimoto, T., and Zhang,Y.-S., 1992, Plate tectonics and hotspots: The third dimension, Science, v. 256, p S Slide 2 Flat slabs at 650 km cool off the mantle & thicken the TZ Blue is thick, cold Flat slabs are the rule Slabs & possible locations of material displaced upwards by slabs X Heated from below Cooled from above Secular cooling CMB (constant T) 650 Lattice dynamics (T) (V,T) T CMB (t) (V,T) U(z,t), Th(z,t) T(x,t) Narrow buoyant jets Broad passive upwellings Constant T These & Maxwells relations completely change results of model calculations Heated from within SHALLOW Hawaii J.Tuzo Wilson (1963) Tatsumoto (1976) RIDGEisland FERTILE RIDGE SOURCE Canonical geochemical model (1973-present) Ridge Feeding Upwelling Broad passive upwelling Ekstrom, Nettles, Dziewonski What is unique about the mantle around Hawaii & hotspots, in general? Anisotropy, not temperature RFU Ekstrom, Nettles, Dziewonski MORB 1500 moles/a OIB 29 moles/a R/Ra>15 3 moles/a Subduction 300 moles/a pd from Parman 3 He flux The number of 3He atoms from high R/Ra hotspots is trivial! even if the mantle has a uniform & low amount of 3 He High 3He/4He will be not be evident if one samples near patches of high [3He] random samples will give some high 3 He/ 4 He ratios Samples near here will be overwhelmed by MORB High [ 3 He]; R~8 Ra Patches of high & low [U, Th, He] High (and low) R/Ra samples are found midplate, midcontinent, new rifts, new spreadinge.g. away from mature spreading centers rather than at plumes mol/a High 3 He/ 4 He 0.15 mol/a [ 3 He] cc/g [ 3 He]~ to cc/g 3 He fluxes OIB MORB PERISPHERE lets look [only] atfeatures where a plumemakesgeological sense and investigate those. Hofmann & Hart, Science 2007 Mantle is unsampled at most midplate sites; ambient mantle there is assumed by geochemists to be MORB-like Anomalous (non-MORB) samples are found in volcanoes and along rifts because those are the only places where one can sample the mantle Non-MORB chemistry is not unambiguous evidence for plumes. Cigar butts are only found under street lights! ex cathedra Let us shed some light Along-ridge profile Ridge-normal profile ridge R i d g e Ridge adiabat T RIDGE FEEDERS True intra-plate hotspots do not need deep feeders for their T & isotopes, but they may be near passive upwellings INVERTED GEOTHERMS BOUNDARY LAYERS TURNING HORIZONTAL, INSULATION (by large long-lived plates) HEATING WHILE RISING (Internal heating of passive upwellings) SUBDUCTION & SECULAR COOLING (cooling from below) Subadiabaticity explains high gradients * of seismic velocity below ~200-km depth 27 *The very large shear wave velocity gradient in seismological models below the LVZ is difficult to explain. STIXRUDE & LITHGOW-BERTELLONI paradox Mantle Shear-Wave Velocity Structure Beneath the Hawaiian Hot Spot Cecily J. Wolfe, Sean C. Solomon & The PLUME Team Low velocities continue downward to the mantle transition zone and continue to a depth of 1500 kilometersthe Hawaiian hot spot is the result of an upwelling high-temperature plume from the lower mantle. REPORTS Isotropic inversion of strongly damped and smoothed relative vertical travel time data; no absolute timing, no surface waves. The results show well known artifacts such as streaking, bleeding, & rabbit-earing. relative low-velocity features are attributed to high absolute temperature NATURE 1972 THANK YOU