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200 Myr of oceanic crust accumulation

Shear-driven magma segregation. Super-adiabatic boundary layer. REGION B. Hawaii source. Thermal max. 300 km. Tp decreases with depth. Narrow downwellings. Broad passive upwellings. MORB source. TRANSITION ZONE (TZ). 600 km. 600 km. (RIP). 200 Myr of oceanic crust accumulation.

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200 Myr of oceanic crust accumulation

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  1. Shear-driven magma segregation Super-adiabatic boundary layer REGION B Hawaii source Thermal max 300 km Tp decreases with depth Narrow downwellings Broad passive upwellings MORB source TRANSITION ZONE (TZ) 600 km 600 km (RIP) 200 Myr of oceanic crust accumulation

  2. ridge hotspots Tp LIL Sheared mélange 200 400 km LIL LVZ UPPER MANTLE Ancient eclogite cumulates TZ Modern slab fragments ‘cold’ THE “NEW” PARADIGM “the canonical box”

  3. Density wavespeed Harzburgite (Hz) [with 1-2% melt] Vs piclogite ‘red’ but not hot Accumulated oceanic crust This is the stratigraphy for a density stratified mantle

  4. Ocean Island LITHOSPHERE LID MORB LVZ 220 km MORB

  5. island arc basalts backarc basin basalts underplate Ridge suction Sheared boundary layer LLAMA Low wavespeeds TZ Secondary downwellings Passive upwellings (not self-driven) Athermal explanation of tomographic results

  6. RIDGE The plate tectonic cycle Harzburgite (Hz) stays in or is returned to the shallow boundary layer (re-used ‘lithosphere”) MORB source is displaced & entrained up (passive upwelling) Dense cold eclogite stays at bottom of TZ Hz VERY COLD LLAMA Harzburgite Hz TZ piclogite Hz Au Revoirsevoir Oceanic crust accumulates at base (au revoir) Harzburgite rises out as it heats

  7. eclogite harzburgite 410 cold 650 cold

  8. RIDGE Shear wavespeed Temperature OIB 1 1600 C adiabat BL VSH>VSV Observed Seismic profile High-T conduction geotherm 2 5 220 km ~1600 C 6 VSV>VSH 3 Vs for self-compressed solid along adiabat Subadiabatic geotherm 4 7 Tp=~1300 C 650 km disconnect A mantle circulation model based on anisotropy, anharmonicity, absolute wavespeeds & gradients, allows for, and predicts, non adiabaticity

  9. European, African, Asian (Changbai), Yellowstone & most continental “hotspots” are underlain by slabs Cold slab Cooled mantle CAN BOTH UPPER MANTLE & LOWER MANTLE BE COOLED BY LONG-LIVED FLAT (STAGNANT) SLABS?

  10. Central Pacific Ritzwoller The laminated upper mantle Vs (T) T G1 Vs(T,f) f SH G2 Boundary layer SV VSH>VSV Vs(T) L Not pyrolite ~1600oC* f=V2/V1< 2% VSV=VSH (slow) Decrease of Vs with depth due to high conduction thermal gradient and the variation of melt-rich layer thicknesses and number (VSH)2~G1 , (VSV)2~G2/f *Note: contrary to some petrologists, there is nothing wrong with Tp=1600 C at 200 km if the boundary layer is harzburgite with ~2% melt rather than pyrolite.

  11. ridge Ridge-normal profile OIB B TZ MORB source Birch’s Transitional Layer Density barrier D’ D”

  12. The idea that ridges may be sourced deeper than OIB based on fixity (Wilson) and geochemistry (Tatsumoto) is more than 30 years old. Tatsumoto Model (1978) LLAMA Model ridge OIB source FERTILE DEPLETED MORB SOURCE BARREN LOWER MANTLE

  13. Along-ridge profile R I d g e ridge Ridge-normal profile

  14. SUMMARY Ridges are fed by broad 3D upwellings plus lateral flow along & toward ridges ridge OIB LID LVZ LLAMA 200 400 subadiabatic Mesosphere (TZ) km Cold slabs Intraplate (delamination, CRB, Deccan, Karoo, Siberia) magmas are shear-driven from the 200 km thick shear BL (LLAMA)

  15. TATSUMOTO

  16. large relative delay times in BL =comparable to crustal delays Seismology of LLAMA* S late *Laminated Lithologies & Aligned Melt Accumulations SKS very late S early underplate teleseismic rays Large lateral variations in relative delay times due to plate & LVZ structure, & subplate anisotropy …bleed into deep mantle

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