evolution of a cooling planet
DESCRIPTION
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!. - PowerPoint PPT PresentationTRANSCRIPT
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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
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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
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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)
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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)
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Rocks and minerals arranged by density: crust & upper mantle
• delaminates when crust > 50 km thick
• warmer than MORB
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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
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Fertile patches in upper mantle are subducted seamounts etc. & delaminated lower continetal crust=melting anomalies
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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
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Densitycrossover
PREM isDenser than pyrolite
Ponding of eclogite
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ECLOGITE CAN BE BROUGHT BACK UP BY A VARIETY OF MECHANISMS
QuickTime™ and aTIFF (LZW) decompressor
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Buoyancy, melting, entrainment, displacement
- - - - - -
- - -
___
These should NOT be called ‘plumes’, e.g.’splash plumes’!
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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
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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
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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
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Dueker
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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
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Low-velocity zone atop the 410-kmseismic discontinuity in thenorthwestern United States
Teh-Ru Alex Song, Don. V. Helmberger & Stephen P. Grand
400-km
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MANTLE IS NOT SIMPLE
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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
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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
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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
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Mantle stratification
• irregular chemical discontinuities expected
• difficult to see in tomography
• can be seen in receiver functions
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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)
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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
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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
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WHEN DID PLATE TECTONICS BEGIN?
When did water get into the mantle?
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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
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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
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Low seismic velocities can be partial melts, eclogite, CO2
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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)
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Old oceanic plate is likely to sink deeper than subducted seamount chains & younger plates
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QuickTime™ and aTIFF (LZW) decompressor
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Eclogite,arclogite,garnet pyroxenite(GtPx)…can be trapped
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Slide 2
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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)
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Sinking & rising blobsDYNAMIC
DYNAMIC
ISOLATED
SLUGGISH
Tri-partite mantle Density variability
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The large “megaplumes” under s.Africa and Pacific are cold & dense!
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Dense but low velocity
Buoyant & high velocity
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Dense DomesNot Megaplumes
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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)
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SUBDUCTION?WATER INTO MANTLE?
ECLOGITE FORMATION?THIN OCEANIC CRUST?
KIMBERLITES?DELAMINATION?
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QuickTime™ and aTIFF (Uncompressed) decompressor
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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
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Dry peridotite can only melt in shallow mantle
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Asthenospheric return flow vectors, with entrained mafic blobs, explain ‘hotspot’ tracks and relative motions between ‘hotspots’
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The fate of eclogite depends on composition.MORB is SiO2-rich and becomes stishovite-rich & dense
MORB-eclogite at high pressure
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NORMAL TEMPERATURE FLUCTUATIONS ARE ~25 %
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THE END GAME OF PLATE TECTONICS
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MANTLE IS A TOP-DOWN SYSTEM
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Archean Catastrophe?
Not if plates & volatiles rather than mantle viscosity are the control parameters
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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!)