Evolution of a Cooling Planet Magma ocean Thick buoyant crust Melting at base Heat pipes Eclogite at base Delamination Plate instability *prior to all this is accretional zone refining & differentiation

The Earth started out HOT! ‘Standard Models’ of geochemistry invoke a volatile-rich lower mantle, with Helium & Water leaking into the Transition Region & Upper Mantle from below (Wasserburg, DePaolo, Allegre, O’Nions, Kellogg, Bercovici, Karato, Helffrich, Hart) The transition Zone may be a filter, but it filters downgoing material Volatiles were zone-refined up, and some came in as Late Veneer Deep mantle is the dense depleted residue

STANDARD MODEL Standard Assumptions: upper mantle is homogeneous, isothermal [‘the convecting mantle’] & subsolidus; anomalous magmatism requires hot deep thermal plumes from a deep Thermal Boundary Layer (TBL)

Basalt, eclogite, harzburgite & magmas are less dense than lower mantle; lower mantle is dense residue of differentiation UPPER MANTLE (basalt, peridotite, eclogite, kimberlite)

Rocks and minerals arranged by density: crust & upper mantle delaminates when crust > 50 km thick warmer than MORB

Part of accretional differentiation is irreversible The buoyant and volatile products of early differentiation are excluded upwards (radial zone refining) The dense residues (restites) get trapped at depth as pressure increases and coefficient of thermal expansion decreases Layers that differ enough in intrinsic density & viscosity cannot be mixed back

Fertile patches in upper mantle are subducted seamounts etc. & delaminated lower continetal crust=melting anomalies

The transition zone is a crust-slab-water filter but it filters from above, not below. Most recycled material bottoms out above 650-km depth

Density crossover PREM is Denser than pyrolite Ponding of eclogite

ECLOGITE CAN BE BROUGHT BACK UP BY A VARIETY OF MECHANISMS Buoyancy, melting, entrainment, displacement ___ These should NOT be called ‘plumes’, e.g.’splash plumes’!

There are many things in the mantle other than old slabs Delaminated lithosphere & crust Cumulates Trapped melts Young plate, subducted ridges… If these differ from ‘normal’ mantle by more than ~3% and are large (~10 km) they will settle to various depths The ‘convecting mantle’ is stratified and blobby Some of these can cause non-plume melting anomalies

DENSITY & SHEAR VELOCITY magma eclogite Density Vs STABLE STRATIFICATION density

Is there any evidence for a blobby laminated mantle? Plenty! reflections, conversions, scatterers, low- velocity zones… Mafic blobs at depths of neutral buoyancy or trapped at phase changes have a chance to warm up and can be the source of melting anomalies

Dueker

Phase changes are flat and stack-up. Chemical boundaries & blobs are variable depth. Phase changes V V V Chemical boundaries Chemical discontinuities & blobs

Low-velocity zone atop the 410-km seismic discontinuity in the northwestern United States Teh-Ru Alex Song, Don. V. Helmberger & Stephen P. Grand 400-km

MANTLE IS NOT SIMPLE

Lower mantle (LM) is denser than pyrolite; therefore eclogite can be trapped in TZ Lower mantle is chondritic minus {volatiles-crust-upper mantle}, e.g.SiO2-rich LM is (depleted, refractory, residual; formed during accretion) K.Lee et al. Perovskite is too dense Pyrolite & low-FeO is too light

Some eclogites equilibrate above 400-km depth

THE ALTERNATE TO A TURBULENT WELL- STIRRED MANTLE IS ONE OF NEUTRAL DENSITY

Mantle stratification irregular chemical discontinuities expected difficult to see in tomography can be seen in receiver functions

In a petrologically realistic planet the products of differentiation are not mixed back in; the mantle becomes stratified (pink and red are mafic rocks & melts)

Geochemical & geodynamic models are dominated by simplistic 1 & 2 layer models The idea of a homogeneous (‘the convecting’) mantle is based on low resolution techniques (global tomography, sampling at ridges, 2D Boussinesq convection simulations) Higher resolution (receiver functions, reflections, xenoliths, inclusions, seamounts) methods paint a different picture

NMORB,DMORB,EMORB,TMORB, OIB,AOB,DMM,EM,HIMU,DUPAL, LONU,PHEM,FOZO… Kimberlites, carbonatites, abyssal peridotites, continental mantle…are underappreciated sources of enrichment Eclogites come in many flavors and densities The mantle is not just 1 or 2 reservoirs or components

WHEN DID PLATE TECTONICS BEGIN? When did water get into the mantle?

Is Sea Ice Tectonics ‘Plate Tectonics’? Sea ice has ‘plates’, collisions (pressure ridges), break-ups (leads), rifts, sutures, rapid motions, shallow underthrusting when thin…but no subduction tectonics

Low seismic velocities can be partial melts, eclogite, CO2

Cold eclogite can be negatively buoyant but it can have low shear wave velocities & low melting point (Gpa)

Old oceanic plate is likely to sink deeper than subducted seamount chains & younger plates

Eclogite,arclogite,garnet pyroxenite(GtPx)…can be trapped

Slide 2

QUANTATIVE & STATISTICAL TOMOGRAPHIC INTERPRETATIONS DO NOT SUPPORT WHOLE MANTLE CONVECTION Decorrelation of past subduction reconstructions and tomography(Scrivner,Ray, Wen,Anderson,Becker,Boschi) Change in spatial patterns (Tanimoto) Change in spectral characteristics (Gu,Dziewonski) Flat slabs (Zhou,Fukao)

Sinking & rising blobs DYNAMIC ISOLATED SLUGGISH Tri-partite mantleDensity variability

The large “megaplumes” under s.Africa and Pacific are cold & dense!

Dense but low velocity Buoyant & high velocity

Dense Domes Not Megaplumes

The pyrolite model has problems; A transition zone that is slower than dry pyrolite & unacceptably low temperatures in deep mantle. A denser lower mantle where velocities increase with depth less fast than pyrolite would alleviate the problems. This would require (1) a change in transition zone composition (eclogite) (2) a gradual change in physical state of the lower mantle, e.g., a superadiabatic temperature gradient (3) more SiO 2,FeO than upper mantle (chondritic Mg/Si minus crust and upper mantle)

SUBDUCTION? WATER INTO MANTLE? ECLOGITE FORMATION? THIN OCEANIC CRUST? KIMBERLITES? DELAMINATION?

Complications in lower mantle Post-perovskite phases of pyroxenes Low-spin transitions Iron partitioning into isolated phases Pressure lowers expansivity & raises conductivity Radiative transfer Chemical layers and megablobs

Dry peridotite can only melt in shallow mantle

Asthenospheric return flow vectors, with entrained mafic blobs, explain ‘hotspot’ tracks and relative motions between ‘hotspots’

The fate of eclogite depends on composition.MORB is SiO2-rich and becomes stishovite-rich & dense MORB-eclogite at high pressure

NORMAL TEMPERATURE FLUCTUATIONS ARE ~25 %

THE END GAME OF PLATE TECTONICS

MANTLE IS A TOP-DOWN SYSTEM

Archean Catastrophe? Not if plates & volatiles rather than mantle viscosity are the control parameters

Bottom Lines Temperature is not the only or even the main parameter in controlling; Seismic velocity Melting Viscosity Density (geologists know this but seismologists, geochemists & geodynamicists do not!)