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Archaean geodynamics
Jeroen van HunenDurham University, UK
Thanks to: Jon DavidsonTaras GeryaArie van den Berg
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In this talk
Cooling history of the EarthArchaean tectonic stylesArchaean mantle dynamicsPossible links with surface processes
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Today’s dynamics of the solid Earth
Plate tectonics dominate dynamicsMantle plumes origin at CMBWhole-mantle convectionCore dynamics magnetic fieldDespite radiogenic heating the Earth cools down
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What was different in the past?
2)
was 100-300
K hotter
Archaean mantle: 1)
Produced 3 x as
much radiogenic heat
today
early Earth?
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Secular changes in mantle temperature
Significantly hotter mantle
Archean
mantle was 100-300 K hotter
Significantly hotter Archeanmantle
(Nisbet
et al., 1993,
Abbott et al., 1994)
Wet, slightly hotter Archean
mantle
(Grove and Parman, 2004)
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today
early Earth?
Consequences of more radiogenic heatToday’s surface heat flux of 80 mW/m2:
50% = from radiogenic heat production50% = Earth cooling
To have Earth coolingin Archaean, we needa cooling mechanismmore efficient than plate tectonics (PT)
(Sleep, 2000; Turcotte
and Schubert, 2002)
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today
early
Earth?
Consequences of a hotter mantle
1.
Weaker plate and mantle material:η = exp (T)~1 order of magnitude for every 100 K
2.
More melting at mid-ocean ridges thicker oceanic crust:
(van Thienen
& al., 2004)
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Consequences of more meltingmore meltingthicker crust + harzburgitelower average density ρless slab pullno subduction? (e.g. today Ontong Java)NO PLATE TECTONICS?
very low ρ
low ρlow ρ
very low ρ
normal ρnormal ρ
lithosphere
today Archean
crustharzburgite
peridotite
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Effect of basalt-eclogite transition
Thick basaltic crust cannot subductBasalt transforms
to eclogite
below40 km depthThis does help
subduction somewhat, but enough?
(Cloos, 1993)
No subduction
Subduction
density basaltic crust
density eclogitic crust
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Computer model simulations
ΔTmantle
= 0oC 100oC 200oC 300oC
colors =
viscosity
black =
basalt
white =
eclogite
viscosity
time
(van Hunen & van den Berg, 2008)
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Computer model simulations
colors =
viscosity
black =
basalt
white =
eclogite
time
(van Hunen & van den Berg, 2008)
viscosity
For low Tmantle subduction looks like today’s
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Computer model simulations
colors =
viscosity
black =
basalt
white =
eclogite
time
(van Hunen & van den Berg, 2008)
viscosity
For higher Tmantle frequent slab break-off occurs …
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Computer model simulations
colors =
viscosity
black =
basalt
white =
eclogite
time
(van Hunen & van den Berg, 2008)
viscosity
… or subduction completely stops.
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Are these subduction velocities enough to cool early Earth?
Summary of many model calculations
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1
32
Are these subduction velocities enough to cool early Earth?
Possible parameterizations of vsubd
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Model 1: subduction for all TmFlat vsubd rate:
cooling since early ArcheanCooling curve similar to Korenaga,’06
and Labrosse & Jaupart,’07.
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Increasing vsubd
with Tpot
:‘Thermal catastrophe’
Model 2: Rapid plate tectonics efficient cooling thermal catastrophe
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Peak in vsubd
: Recent rapid cooling since Proterozoic
Model 3: Inefficient subduction hotter Archaean mantle
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Observational evidence for Archaean PT
Seismic reflectors
Ophiolites
(?)
(Calvert et al., 1995; Furnes et al., 2007)
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Paleo-latitudes of old continents varied over timeOnly during supercontinent (formation/breakup)Episodic plate
tectonics?Data sparse!
(O’Neill et al., 2007; Silver and Behn, 2008)
Observational evidence for Archaean PTPaleo-magnetism
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(Modern) PT absent in Archaean?
(Stern, 2008)
Missing key characteristics of PT
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Alternative tectonics: models and requirements
Possible models:Magma oceanStagnant lid convection
Any model should be dynamically feasible:Based on graviational instabilityForces / stresses should be sufficient to drive the type of tectonics
.. and should provide a cooling mechanism for the Earth:Surface heat flow at least that of PT?
Crustal delaminationDiapir tectonicsPlume tectonics
…
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Alternative tectonics: diapir/delamination tectonics
(Zegers
and van Keken, 2001; van Thienen
et al., 2004, 2005)
Mechanism:Crust built by eruptionsDeepest crust transforms to dense eclogites: delaminatesDownwellings melting TTG formation
Abundant melting releases latentheat
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Alternative tectonics: diapir/delamination tectonics
(Zegers
and van Keken, 2001; van Thienen
et al., 2004, 2005)
Why this model?Explains ovoid extrusions (e.g. Pilbara)No need for PT before late Archaean or ProterozoicEfficient cooling mechanism
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(van Hunen and van den Berg, 2008)
frequent slab break-off
modern subduction style
Absence of UHPM by slab break-off?
Did style of PT change over time?
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Today Archean
subduction ofcontinental crustgives UHPM
no subductionof continental crust: absenceof UHPM
UHPM
UHPM
Absence of UHPM by slab break-off?
(USGS website; Wortel
and Spakman, 2000; van Hunen and van den Berg, 2008)
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Did style of PT change over time?
(e.g. Abbott et al., 1994; van Hunen et al., 2004)
Evolution from flat to steep subduction?No, because:
1.
If too buoyant, slabs won’t subduct
at all2.
A hot, weak mantle is unable to support flat subduction
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Did style of PT change over time?Bulk continental crust:
Today: andesitesFormed in subduction zoneMantle wedge hydration and -melting
Archaean: tonalite-trondhjemite-granodiorite (TTGs)(slab?) melting of mafic crust (Similarities with adakites?)Interaction with a mantle wedge?
Suggested formation scenarios:
(e.g. Foley et al., 2002, 2003; Bédard, 2006)
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Archaean mantle dynamicsEpisodic continental crust formation
(McCulloch and Bennett, 1994)
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Archaean mantle dynamicsRole of the mantle in tectonics style
Geochemical ↔ geodynamical viewpoint:Seismic tomography shows whole-mantle convectionOIB/MORB basalt chemistry requires district, separate reservoirs
(van der
Hilst, 2007)
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Archaean mantle dynamics
Mantle spinel perovskite phase transition endothermic: hampers vertical flowTransition from layered to whole mantle convection?Major avalanches during transition period?
(Tackley, 1996; Davies, website, ANU)
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Melting by episodic mantle avalanches?Sudden warming of upper mantle may give:
Wide-spread (re-)melting new continental crustUnfavourable PT conditions: intermittent PT?
(Davies, 1995)
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Alternative models for episodic mantle behaviour and crust formation
‘Stick-slip’ plate tectonics
Mush ocean – plate tectonics alternation
(O’Neill et al., 2007; Sleep, 2000, 2006)
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Role of mantle plumes
Archaean plumes (much) hotterPlumes (probably) form at the core-mantle boundary Plume size ~ ΔTWithout D’’-layer plumes larger?Plumes cool Earth’s core drive core dynamics
magnetic field shielding life on Earth
T
z
CORE
MANTLED’’
ΔT
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Mantle degassing pulses
Crust formation pulses
Mantle degassing pulses
Influence on ocean/atmosphere composition and climate?
(data from Condie, 1998; Farquhar et al., 2000; Parman, 2007; Grocke
et al., in prep.)
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Links with the hydrosphere: water budgetToday: regassing > degassingEarly Earth:
Subduction rates? More volcanism more degassingHotter mantle faster slab dehydration less regassing?
(Wallmann, 2001; Rüpke
et al., 2004)
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Archaean sea level and emerged continentsConstant continental freeboard (±200 m)Very early ocean presentContinental growth model (?)
ΔT < 110-210 K unless orogenieswere weaker2-3% late
Archaean continent emergence
‘Archaean water world’
(Harrison et al., 2005; Flament
et al., 2008)
1300 1350 1400 1450 1500T(oC)
0
25
42
% c
ontin
enta
l are
a
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Archaean plate tectonics seems viable, but probably looked different:
No Archaean UHPM/blueschistsWeaker plates, more break-offDifferent continental crust Less regassing
However, …No widespread shallow flat subductionPlate speed higher/lower?
Alternative or additional tectonics possible / required: diapir/plume/delamination dynamics
Conclusions
Archaean Present
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ConclusionsEpisodic mantle dynamics / plate tectonics (?) / crust formation:
As evidenced by crustal record, mantle geochemistry
Expected from dynamic modelling
Influence on:De/regassing rates?
Ocean size / sea level?
Ocean / atmosphere
composition?
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Thank you.