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
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
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
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
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
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)
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
Eclogite,arclogite,garnet pyroxenite(GtPx)…can be trapped
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)
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)
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
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
Archean Catastrophe?
Not if plates & volatiles rather than mantle viscosity are the control parameters