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Transcript
Eclogite Engine Don L. Anderson Caltech Can we bridge geophysics,geochemistry & geodynamics? Continents; the missing link The lower crust is transient It is recycled 6 times faster than upper crust Recent arc growth estimates are 5X previous estimates! Therefore, a huge previously unaccounted for flux THE UPPER MANTLE ECLOGITE CYCLE All the components of so-called hotspot or plume magmas originate in crust, lithosphere, cumulates or mantle wedge The Isotope Zoo EM1, EM2, HIMU, DUPAL, LONU, High 3He/4He, C-, FOZO, Os… Mantle is a Top-Down System QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. delamination Sources of dissipation The top boundary condition is not simple Many ways to get big chunks of mafic matter into upper mantle Subduction of seamount chains, aseismic ridges Delamination of island arcs Delamination of compressional mountain belts Estimating mantle sources & sinks, continental addition & loss, through time, depends critically on when deep subduction, got underway 1 Gyr of oceanic crust subduction=70 km of eclogite But lots of other things get put into the mantle OIB ~2 Km3/yr 20 km3/yr David Scholl Piles up @ 70 km/Gyr MASS BALANCE Recycling rate of oceanic crust (basaltic + gabbroic) ~20 km3/yr Midplate extrusive magmatism 1-2 km3/yr (4 to 8 current Hawaii’s; 10x more average Hawaii’s) (Factor of 10 to 20 mismatch for Hofmann & White recycling mechanism) Underside erosion of continents at marine margins 2.5 km3/yr Erosion plus delamination at continental colli sions 2-3 km3/yr Production rate of magmas in arcs 3-9 km3/yr [Scholl (2006) gives 4-5 km3/yr] Growth rate of arc garnet pyroxenite cumulates 1.5-6 km3/yr Island arcs and oceanic plateaus (and Hawaii’s) can also delaminate Midplate volcanisms & arcs can be upper mantle RIDGE-TRENCH ANNIHILATION Abandonment of young oceanic crust and mantle wedge BAB Future suture EM EM12 HIM U Trapped crust FOZ O HI 3HE/U LOMU Mantle wedge All of the isotope and trace element signatures of OIB are manufactured in the subduction factory, wedge & crust ROOT FORMATION Delamination cycle dense roots fall off warm up in ambient mantle rise possible mechanism for Atlantic & Indian ocean plateaus & DUPAL anomaly 1 DELAMINATION 2 SPREADING 3 heating UPWELLING 4 SPREADING 5 ridge mafic melt ERUPTABLE MELTS 50 km LMP ERUPTABLE 100 km MELTS 200 km BUOYANT MELT PONDED MELT LMP 300 km ultramafic melt jadeite eclogite mafic melt ultramafic melt UMR AVERAGE dunite sp.perid. Gt.Lhz. mafic melt ultramafic melt PYROLITE eclogite Hawaii Lherzolite densit SHEAR y(-0.68) VELOCITY (-0.40) 3.20 3.24 (-0.40) (-0.15) 3.29 3.30 3.35 (-0.18) (+0.00) 3.38 3.43 3.47 4.82 4.28 4.68 4.90 4.52 4.83 4.58 4.72 ECLOGITE AT DEPTH HAS LOW SHEAR VELOCITY Non-uniform heat leakage from the top & peeling off of crust Dual eruptions QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. unstable Qui ck Time™ and a TIF F ( Uncompres sed) dec ompres s or are needed to s ee this pic ture. The eclogite engine, as in any engine, involves changes in volume due to compression, heating and phase changes.One cannot consider T but not P. The cycle is cooling, phase change (eclogite), compression (sinking), heating, phase change (melting), decompression (rising), heat exchange… [The continent may move, rather than the blob.] Delaminated roots warm quickly • will start to melt before reaching same T as surrounding mantle • already in TBL, so starts off warm • when 30% melt, garnet mostly gone & will start to rise Peridotite melts Eclogite melts Dry peridotite can only melt in very shallow mantle; hence adiabatic ascent at ridges Eclogite can melt much deeper, and much more, even when colder; hence, “midplate magmatism” Delamination rate of arc cumulates 1.5-6 km3/yr is non-negligible. It is also likely that larger chunks are inv olved than would be the case with subduction of normal oceanic crust. They are also hotter. Thus, this material may be responsible for fertile melting anomalies, in addition to contributing trace element and isotopic signatures to their melts. To the arc lower crustal delamination rates must be added the colli sional mountain belt delamination. The breakup of Gondwana and the uncovering of the Indian and Atlantic oceans provides the best opportunity for seeing the re-emergence of these fertile blobs, after they have been heated by ambient mantle. It has been proposed that the Indian ocean and the south Atlantic plateaus and island chains may be due to these mafic patches that were injected into the mantle from the base of the Gondwana crust (Anderson, 2005). LIPs are associated with continental breakup • reconstruction at ~ 30 Ma • dual volcanism – on breakup – ~ 30 Myr later • oceanic plateaus form ~ 1,000 km offshore • = rising of delaminated root? MANTLE DELAMINATION WEDGE COUNTERFLOW CHANNEL EROSION Slow upwelling Slow upwelling Broad upwellings NO PLUME NECESSARY Input into asthenosphere: delamination, seamount chains, broad upwelling, abandoned mantle wedges Fertile blob & asthenospheric counterflow model ^ LVZ 3 2 1 4 --------------------------------------------------------------------------- 1 delaminated crust, 2 wedge, 3 broad upwelling, 4 young oceanic crust Test of the model; lots of scattering in the upper mantle There is no mass balance or geochemical reason for anything to come from below 1000 km ECLOGITE ENGINE 0 Myr 40-80 Myr CRUST ^ >>>>>>>>> >>> <<< <<< 1 COLD DENSE BLOB 10 Myr 2(z) ^ 5 MAGMA-RICH ZONE RESIDUE ASTHENOSPHERE <1 Blobs deliver heat & magma to surface 50 Myr S L 4 PARTIALLY MOLTEN BLOB A B HOT ^ 13 Myr 2 HOT WARM BLOB PHASE CHANGE HOT COLD BLOB low U, Th 40 Myr 3 30 Myr high U, Th HOT OLD SLABS PHASE CHANGE HOT 100+ Myr NOTICE! Oceanic crust not involved LOWER MANTLE Blobs gain heat from mantle TRI-CYCLING THROUGH THE EARTH RIDGE-TRENCH ‘CYCLE’ (the escalator) SUPERCONTINENT CYCLE (the trolley) SLAB-PLUME ‘CYCLE’ (the tube) SUBTERRANEAN CYCLE (delamination, orogenic cycle; the elevator) Elevated water contents in Scale matters >10-km blobs; gravity takes over; heating is slow I ––––––10 cm or 10 km ? –– I piclogite QuickTime™ and a TIFF (U ncompres sed) decompressor are needed to see this picture. Garnet and clinopyroxene QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. Olivine and opx Eclogite melts in the mantle react with peridotite to form pyroxenite The olivine of mantle peridotite is consumed by reaction with melts derived from recycled crust & cumulates, to form a secondary pyroxenitic source ‘Hotspot’ basalts such as Hawaii form from olivine-free mantle QuickTim e™ and a TIFF (Uncom pres s ed) decom pres s or are needed to s ee this picture. QuickT ime™ and a TI FF (Uncompressed) decompressor are needed to see this picture. GALILEO THERMOMETER QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. Blobs sink to level of neutral buoyancy But they don’t stay there In mantle, they heat up & melt And bring mantle heat & magma to base of plate Not core heat Rates of generation of the continental crust The rates at which basaltic magma is added to the continental crust have recently been revised upwards range to 3.7 km3 yr-1 The “plume affinity” of basaltic rocks in juvenile crust has been used to support deep-seated disturbances in the Earth, as opposed to shallow-level processes. But this evidence could also be used in support of the delamination model. The average residence times of the lower crust may be at least five times less than the upper crust THE ECLOGITE CYCLE Mass balance seems o.k. Petrology & major elements seem o.k. Isotopes seem o.k. including stable isotopes Dynamics seems o.k. Thermodynamics seem fine Energetics seem o.k. What caused these? QuickTime™ and a TIFF (U ncompressed) decompressor are needed to see this picture. LIPs CFB SDR NAVP CRB OPB BABB OJP New parameters Scale (size) Homologous temperature Stress Buoyancy parameter Architecture Fertility Entrainability MASS BALANCE km3/yr TOTAL MIDPLATE MAGMA VOLUMES Growth rate of arc garnet pyroxenite cumulates Underside erosion & delamination of continents Production rate of magmas in arcs LOWER CRUSTAL LOSS = ‘HOTSPOT’ MAGMATISM CYCLE TIME 40-80 MYR 10-30 KM CHUNKS 3-6 1.5-6 4-6 4-5 KEY ELEMENTS Mantle is source of heat Thick basalt piles are the source of material True Top-Down system Mantle is heterogeneous Heterogenity washed out by Central Limit Theorem (ridges, tomography), not by mantle convection Mantle is gumbo, not fruit cake Unification of geodynamics & geochemistry of mantle Delamination of lower continental crust & subduction of seamount chains fertilize the mantle These become low meltng point blobs It is these, not plumes, that explain ‘anomalies’ such as midplate volcanism, swells and ‘hotspots’