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!• ‘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

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)

HOT EARLY EARTH COLDER EARTH

BASALTBASALT, ECLOGITE

MELT

PERIDOTITE ECLOGITE

Basalt, eclogite, harzburgite & magmas are less dense than lower mantle; lower mantle is dense residue of differentiation

UPPER MANTLE (basalt, peridotite, eclogite, kimberlite)

Rocks and minerals arranged by density: crust & upper mantle

• delaminates when crust > 50 km thick

• warmer than MORB

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

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

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

Densitycrossover

PREM isDenser than pyrolite

Ponding of eclogite

ECLOGITE CAN BE BROUGHT BACK UP BY A VARIETY OF MECHANISMS

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

Buoyancy, melting, entrainment, displacement

- - - - - -

- - -

___

These should NOT be called ‘plumes’, e.g.’splash plumes’!

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

3 4 5 6pyroxenite 3.23eclogite 3.24UMR AVERAGE 3.29

LID bronzite 3.29dunite 3.30PHN1569 3.31sp.perid. 3.35

Gt.Lhz. 3.35 UPPER melt 3.30

opx 3.37 MANTLEPHN1611 3.42 _________36%ol,17%gt 3.43

LMP eclogite 3.43LMP eclogite 3.46

melt 3.20gt.perid. 3.35 _______Hawaii Lhz. 3.47magma(16 Gpa) 3.50 MAGMAmajorite (mj) 3.52mj 3.53DRY MORB MAGMA (1600 C)DRY KOMATIITE MELT (1600 C)garnet 3.57 _____

TZ beta(.1FeO) 3.59

400 km mj 3.61gr garnet 3.60

LMP eclogite 3.60LMP eclogite 3.61 LVZ

melt 3.40

500 km gamma(.1FeO) 3.68py gt 3.71komatiite(18 Gpa) 3.80 KOMATIITE ___

LO-T il(.1FeO) 3.92jd-mj 4.00

magma

eclogite

Density VsSTABLE STRATIFICATION

3.2

3.3

3.4

3.5

3.6

3.7

density

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

Dueker

Phase changes are flat and stack-up. Chemical boundaries & blobs are variable depth.

Phase changes V V V

Chemical boundaries Chemical discontinuities &

blobs

410 520 650

Low-velocity zone atop the 410-kmseismic discontinuity in thenorthwestern United States

Teh-Ru Alex Song, Don. V. Helmberger & Stephen P. Grand

400-km

MANTLE IS NOT SIMPLE

Lower mantle (LM) is denser than pyrolite; therefore eclogite can be trapped in TZ

Lower mantle is chondritic minus {volatiles-crust-upper mantle}, e.g.SiO2-rich

LM is (depleted, refractory, residual; formed during accretion)

K.Lee et al.

Perovskite is too dense

Pyrolite & low-FeO is too light

Figure 5-2: Rocks and minerals arranged by density

Rock type SHEAR VELOCITY (P=0) STP Vs (km/s)

density 3 4 5 6 km/s

(g/cc)granite 2.62

A' gabbro 2.87CRUST dolerite 2.93 usual max. crustal thickness

gneiss 2.98 50 km

A" eclogites & 3.45 unstablearc eclogites 3.46 root eclogite(arclogites,arcl) 3.48 " 3.62 Vp= 8.1 km/s

UPPER harzburgite 3.30MANTLE dunite 3.31 Vp= 8.4 km/s UPPER

pyrolite 3.38 Vp= 8.3 Km/s MANTLEperidotite 3.42

B arcl(highMgO) 3.45 stableeclogite 3.46 Vp=8.1 km/s eclogiteHawaii Lhz. 3.47arcl(highMgO) 3.48 8.1 km/s

3 4 5 6 km/s

β- (.1 )spinel FeO 3.59 X 410 kmTZ (.12 )FeO 3.60 9.3 /km s

(410 )pyrolite km "majorite "

Some eclogites equilibrate above 400-km depth

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

DENSITY GRAVITATIONAL STRATIFICATION OF THE MANTLE

BUOYANT CRUSTDENSE LOWER CRUST

HARZBURGITE

BASALT UNDERPLATE

PERISPHERE

PICLOGITE

PYROLITE

GARNETITE

PEROVSKITITE

DENSE DREGS

Mantle stratification

• irregular chemical discontinuities expected

• difficult to see in tomography

• can be seen in receiver functions

CHEMICAL STRATIFICATION OF CRUST AND MANTLE

MAGMA 1600 C

*δ ρ with respect to PREM

DENSITY ( =0)SHEAR VELOCITY P=LMP ( ρ )

low deficit VS

melting (δ ρ)*point KGVs3 4 5 6

basalt 2.59graniteplagioclase 2.64quartzgranodiorite 2.68 _______________

UPPER anorthositeCRUST gneiss 2.79

dioriteanorthosite 2.80 CRUSTALserpentinite &MINERALSgabbro 2.86 ROCKSmetabasaltdolerite 2.93gabbro _______________

LOWER gneiss restite 2.98CRUST amphibolite

-granulite mafic 3.10amphibole

^ mafic melt (-0.68) buoyant ^ ultramafic melt (-0.40) magma

50 km jadeite 3.20pyroxenite 3.23 pyroxenites

LMP eclogite 3.24100 km mafic melt (-0.40) buoyant ^

ultramafic melt (-0.15) magma UMR AVERAGE 3.29

bronzitedunite 3.30

1569PHN. .sp perid 3.35

200 km . .Gt Lhz peridotites^ mafic melt (-0.18) rises

ultramafic melt(+0.00) stableopx 3.37

1611PHNPYROLITE 3.3836% ,17%ol gt 3.43

3 4 5 6LMP eclogite 3.43 ,eclogitesLMP eclogite 3.46 ,garnetites

300 km Hawaii Lherzolite 3.47 ultramaficLMP eclogite 3.48 &melts^ mafic melt (-0.1) rises - iron richv "ultramafic (+0.03) sinks lherzolites

( )majorite mj 3.52mjgarnet 3.57

400 km (.1 )beta FeO gr garnet 3.60

pyrolite 3.60 low MgOLMP eclogite 3.60 &eclogitesLMP eclogite 3.61 mafic magmas

mafic melt (+0.00) stablepyrolite 3.67

500 km (.1 )gamma FeO 3.68( + )eclogite mj coe MORB

py garnet 3.71 eclogites( + )eclogite mj st 3.75

- (.1 )Mg ilmenite FeO 3.92mj 4.00

650 km ( .8)mw Mg-Mg perovskite 4.10

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)

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

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

WHEN DID PLATE TECTONICS BEGIN?

When did water get into the mantle?

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

THERMAL HISTORY CONSIDERATIONS Observed heat loss from Earth is actually 33 TW not 44 TW Low concentration of heat-producing elements in MORB source (which is much smaller than “the convecting mantle” or “upper mantle”) SCLM & perisphere have high radioactivities Kimberlite has very high U, Th & K & cannot be ignored U-contents in MORB vary by more than an order of magnitude There is no need for a hidden heat reservoir There are numerous minor sources of energy (tidal, differentiation, shrinking) There are large (25 %) temporal variations in heat flow Hot mantle does not imply high heat flow, smaller plates, faster plates, or thinner lithosphere

Low seismic velocities can be partial melts, eclogite, CO2

Eclogite 70% molten beforeperidotite starts to melt

• eclogite 70%molten atperidotite solidus

• eclogite sinkerswarmed byconduction

• rise before T hasrisen to that ofambient mantle

Cold eclogite can be negatively buoyant but it can have low shear wave velocities & low melting point

(Gpa)

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

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

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

Slide 2

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)

Sinking & rising blobsDYNAMIC

DYNAMIC

ISOLATED

SLUGGISH

Tri-partite mantle Density variability

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

Dense but low velocity

Buoyant & high velocity

Dense DomesNot Megaplumes

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)

SUBDUCTION?WATER INTO MANTLE?

ECLOGITE FORMATION?THIN OCEANIC CRUST?

KIMBERLITES?DELAMINATION?

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

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

Dry peridotite can only melt in shallow mantle

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

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

MORB-eclogite at high pressure

NORMAL TEMPERATURE FLUCTUATIONS ARE ~25 %

THE END GAME OF PLATE TECTONICS

MANTLE IS A TOP-DOWN SYSTEM

Archean Catastrophe?

Not if plates & volatiles rather than mantle viscosity are the control parameters

Bottom Lines

Temperature is not the only or even the main parameter in controlling;

Seismic velocity

Melting

Viscosity

Density

(geologists know this but seismologists, geochemists &

geodynamicists do not!)