eclogite engine

Eclogite Engine

Eclogite Engine

Don L. AndersonCaltech

Don L. AndersonCaltech

Can we bridge geophysics,geochemistry &

geodynamics?

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

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

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Sources of dissipation The top boundary condition is not simple

delamination

Many ways to get big chunks of mafic matter into upper mantle

Subduction of seamount chains, aseismic ridgesDelamination of island

arcsDelamination of

compressional mountain belts

Estimating mantle sources & sinks, continental addition & loss, through time, depends critically on when deep

subduction, got underway

David Scholl

1 Gyr of oceanic crust subduction=70 km of eclogiteBut lots of other things get put into the mantle

OIB

~2 Km3/yr

20 km3/yrPiles 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 collisions 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 recycling

RIDGE-TRENCH ANNIHILATIONAbandonment of young oceanic crust and mantle wedge

BAB

Mantle wedge

Trapped crust

Future suture

EM1EM2

HIMU

FOZO

LOMU

HI 3HE/U

All of the isotope and trace element signatures of OIB are manufactured in the subduction factory, wedge & crust

dense roots fall off warm up in ambient mantle

rise possible mechanism for Atlantic & Indian ocean plateaus & DUPAL anomaly

ROOT FORMATION

1

DELAMINATION ridge

2

SPREADING

3heating

UPWELLING

4

SPREADING

5

Delamination cycle

ERUPTABLE mafic melt (-0.68)

MELTS ultramafic melt (-0.40)50 km jadeite 3.20 4.82

LMP eclogite 3.24 4.28ERUPTABLE 100 km mafic melt (-0.40)MELTS ultramafic melt (-0.15)

UMR AVERAGE 3.29 4.68dunite 3.30 4.90

sp.perid. 3.35 4.52200 km Gt.Lhz. 4.83

BUOYANT MELT mafic melt (-0.18)PONDED MELT ultramafic melt (+0.00)

PYROLITE 3.38LMP eclogite 3.43 4.58

300 km Hawaii Lherzolite 3.47 4.72

SHEARVELOCITY

ECLOGITE AT DEPTH HAS LOW SHEAR VELOCITY

density

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.]

heating

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Dual eruptions

unstable

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

Dry peridotite can only melt in very shallow mantle; hence adiabatic ascent at ridgesEclogite can melt much deeper, and much more, even when colder; hence, “midplate magmatism”

Peridotite melts

Eclogitemelts

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 collisional 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?

NO PLUME

NECESSARY

COUNTERFLOWCHANNEL

Slow upwellingSlow upwelling

DELAMINATION

MANTLE

WEDGE

EROSION

Input into asthenosphere: delamination, seamount chains,

broad upwelling, abandoned mantle wedges

Broad upwellings

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€

⇒

12

34

1 delaminated crust, 2 wedge, 3 broad upwelling,

4 young oceanic crust

Fertile blob & asthenospheric counterflow model

LVZ

^

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 5 COLD MAGMA-RICH

10 Myr DENSE BLOB ZONE

ρ2 ( )z ρ1

RESIDUE

ρ< ρ1ASTHENOSPHERE 50 Myr

S

L 4 PARTIALLY

A MOLTEN BLOB 40 Myr

B HOTρ<ρ3 ^

13 Myr 2 HOT WARM BLOB HOT

PHASE CHANGE COLD BLOB

, low U Th 3 30 Myrρ3

ρ4 , high U Th

ρ>ρ3

HOT HOT

ρ4<ρ<ρ5 OLD SLABS 100+ Myr

PHASE CHANGE ρ5

LOWER MANTLE

Blobs gain heat from mantle

Blobs deliver heat & magma to surface

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)

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Elevated water contents in

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Garnet and clinopyroxene

Olivine and opx

I ––––––10 cm or 10 km ? –– I

Scale matters>10-km blobs; gravity takes over; heating is slow

piclogite

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

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GALILEO THERMOMETER

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Blobs sink to level of neutral buoyancyBut 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?

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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 ELEMENTSMantle is source of heatThick basalt piles are the source of materialTrue Top-Down system

Mantle is heterogeneousHeterogenity washed out by Central Limit Theorem (ridges, tomography), not

by mantle convectionMantle 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’