Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.
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 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 Piles up @ 70 km/Gyr David Scholl
RIDGE-TRENCH ANNIHILATIONAbandonment of young oceanic crust and mantle wedge BAB Future suture EM2 LOMU Trapped crust FOZO EM1 Mantle wedge HI 3HE/U HIMU All of the isotope and trace element signatures of OIB are manufactured in the subduction factory, wedge & crust
Delamination cycle • dense roots • fall off • warm up in ambient mantle • rise • possible mechanism for Atlantic & Indian ocean plateaus & DUPAL anomaly
SHEAR VELOCITY density ECLOGITE AT DEPTH HAS LOW SHEAR VELOCITY
Non-uniform heat leakage from the top & peeling off of crust 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.] Dual eruptions unstable heating
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 Eclogitemelts 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”
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 WEDGE COUNTERFLOWCHANNEL EROSION DELAMINATION NO PLUME NECESSARY Slow upwelling Slow upwelling Broad upwellings Input into asthenosphere: delamination, seamount chains, broad upwelling, abandoned mantle wedges
Fertile blob & asthenospheric counterflow model ^ LVZ 2 4 3 1 --------------------------------------------------------------------------- 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
Blobs deliver heat & magma to surface Blobs gain heat from mantle NOTICE! Oceanic crust not involved at all !
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)
Scale matters >10-km blobs; gravity takes over; heating is slow I ––––––10 cm or 10 km ? –– I piclogite Garnet and clinopyroxene 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
GALILEO THERMOMETER And bring mantle heat & magma to base of plate Blobs sink to level of neutral buoyancy But they don’t stay there In mantle, they heat up & melt Not core heat
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 Rates of generation of the continental 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? 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 3-6 • Growth rate of arc garnet pyroxenite cumulates 1.5-6 • Underside erosion & delamination of continents 4-6 • Production rate of magmas in arcs 4-5 • LOWER CRUSTAL LOSS = ‘HOTSPOT’ MAGMATISM • CYCLE TIME 40-80 MYR • 10-30 KM CHUNKS
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’