Archaean geodynamics

Jeroen van HunenDurham University, UK

Thanks to: Jon DavidsonTaras GeryaArie van den Berg

In this talk

Cooling history of the EarthArchaean tectonic stylesArchaean mantle dynamicsPossible links with surface processes

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

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?

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)

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)

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)

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

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

Computer model simulations

ΔTmantle

= 0oC 100oC 200oC 300oC

colors =

viscosity

black =

basalt

white =

eclogite

viscosity

time

(van Hunen & van den Berg, 2008)

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

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 …

Computer model simulations

colors =

viscosity

black =

basalt

white =

eclogite

time

(van Hunen & van den Berg, 2008)

viscosity

… or subduction completely stops.

Are these subduction velocities enough to cool early Earth?

Summary of many model calculations

1

32

Are these subduction velocities enough to cool early Earth?

Possible parameterizations of vsubd

Model 1: subduction for all TmFlat vsubd rate:

cooling since early ArcheanCooling curve similar to Korenaga,’06

and Labrosse & Jaupart,’07.

Increasing vsubd

with Tpot

:‘Thermal catastrophe’

Model 2: Rapid plate tectonics efficient cooling thermal catastrophe

Peak in vsubd

: Recent rapid cooling since Proterozoic

Model 3: Inefficient subduction hotter Archaean mantle

Observational evidence for Archaean PT

Seismic reflectors

Ophiolites

(?)

(Calvert et al., 1995; Furnes et al., 2007)

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

(Modern) PT absent in Archaean?

(Stern, 2008)

Missing key characteristics of PT

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

…

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

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

(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?

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)

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

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)

Archaean mantle dynamicsEpisodic continental crust formation

(McCulloch and Bennett, 1994)

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)

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)

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)

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)

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

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.)

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)

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

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

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?

Thank you.