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Evolution of a Cooling Planet

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!.

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Evolution of a Cooling Planet

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  1. 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

  2. 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

  3. 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)

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

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

  6. 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

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

  8. 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

  9. Ponding of eclogite PREM is Denser than pyrolite Density crossover

  10. 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’!

  11. 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

  12. DENSITY & SHEAR VELOCITY density STABLE STRATIFICATION Density Vs 3.2 3.3 3.4 3.5 3.6 3.7 eclogite magma

  13. 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

  14. Dueker

  15. Phase changesV V V 410 520 650 Chemical boundaries Chemical discontinuities & blobs Phase changes are flat and stack-up. Chemical boundaries & blobs are variable depth.

  16. Low-velocity zone atop the 410-kmseismic discontinuity in thenorthwestern United StatesTeh-Ru Alex Song, Don. V. Helmberger & Stephen P. Grand 400-km

  17. MANTLE IS NOT SIMPLE

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

  19. Some eclogites equilibrate above 400-km depth

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

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

  22. 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)

  23. 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

  24. NMORB,DMORB,EMORB,TMORB,OIB,AOB,DMM,EM,HIMU,DUPAL,LONU,PHEM,FOZO…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

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

  26. 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

  27. Low seismic velocities can be partial melts, eclogite, CO2

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

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

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

  31. Slide 2

  32. 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)

  33. Tri-partite mantle Density variability Sinking & rising blobs DYNAMIC ISOLATED SLUGGISH DYNAMIC

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

  35. Buoyant & high velocity Dense but low velocity

  36. Dense Domes Not Megaplumes

  37. 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 SiO2,FeO than upper mantle (chondritic Mg/Si minus crust and upper mantle)

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

  39. 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

  40. Dry peridotite can only melt in shallow mantle

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

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

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