Archaean geodynamics. Jeroen van Hunen Durham University, UK. Thanks to: Jon Davidson Taras Gerya Arie van den Berg

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1 Archaean geodynamics Jeroen van Hunen Durham University, UK Thanks to: Jon Davidson Taras Gerya Arie van den Berg

2 In this talk Cooling history of the Earth Archaean tectonic styles Archaean mantle dynamics Possible links with surface processes

3 Today s dynamics of the solid Earth Plate tectonics dominate dynamics Mantle plumes origin at CMB Whole-mantle convection Core dynamics magnetic field Despite radiogenic heating the Earth cools down

4 What was different in the past? early Earth? Archaean mantle: 1) Produced 3 x as much radiogenic heat today 2) was K hotter

5 Secular changes in mantle temperature Significantly hotter mantle Significantly hotter Archean mantle (Nisbet et al., 1993, Abbott et al., 1994) Wet, slightly hotter Archean mantle (Grove and Parman, 2004) Archean mantle was K hotter

6 Consequences of more radiogenic heat Today s surface heat flux of 80 mw/m 2 : 50% = from radiogenic heat production 50% = Earth cooling To have Earth cooling in Archaean, we need a cooling mechanism more efficient than plate tectonics (PT) early Earth? today (Sleep, 2000; Turcotte and Schubert, 2002)

7 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: early Earth? today (van Thienen & al., 2004)

8 Consequences of more melting more melting thicker crust + harzburgite lower average density ρ less slab pull no subduction? (e.g. today Ontong Java) NO PLATE TECTONICS? very low ρ today Archean lithosphere low ρ normal ρ crust harzburgite peridotite very low ρ low ρ normal ρ

9 Effect of basalt-eclogite transition Thick basaltic crust cannot subduct Basalt transforms to eclogite below 40 km depth This does help subduction somewhat, but enough? Subduction density eclogitic crust density basaltic crust No subduction (Cloos, 1993)

10 Computer model simulations colors = viscosity time black = basalt white = eclogite viscosity ΔT mantle = 0 o C 100 o C 200 o C 300 o C (van Hunen & van den Berg, 2008)

11 Computer model simulations colors = viscosity time black = basalt white = eclogite viscosity For low T mantle subduction looks like today s (van Hunen & van den Berg, 2008)

12 Computer model simulations colors = viscosity time black = basalt white = eclogite viscosity For higher T mantle frequent slab break-off occurs (van Hunen & van den Berg, 2008)

13 Computer model simulations colors = viscosity time black = basalt white = eclogite viscosity or subduction completely stops. (van Hunen & van den Berg, 2008)

14 Summary of many model calculations Are these subduction velocities enough to cool early Earth?

15 Possible parameterizations of v subd Are these subduction velocities enough to cool early Earth?

16 Model 1: subduction for all T m Flat vsubd rate: cooling since early Archean Cooling curve similar to Korenaga, 06 and Labrosse & Jaupart, 07.

17 Model 2: Rapid plate tectonics efficient cooling thermal catastrophe Increasing v subd with T pot : Thermal catastrophe

18 Model 3: Inefficient subduction hotter Archaean mantle Peak in v subd : Recent rapid cooling since Proterozoic

19 Observational evidence for Archaean PT Seismic reflectors Ophiolites (?) (Calvert et al., 1995; Furnes et al., 2007)

20 Observational evidence for Archaean PT Paleo-magnetism Paleo-latitudes of old continents varied over time Only during supercontinent (formation/breakup) Episodic plate tectonics? Data sparse! (O Neill et al., 2007; Silver and Behn, 2008)

21 (Modern) PT absent in Archaean? Missing key characteristics of PT (Stern, 2008)

22 Alternative tectonics: models and requirements Possible models: Magma ocean Stagnant lid convection Any model should be dynamically feasible: Based on graviational instability Forces / 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 delamination Diapir tectonics Plume tectonics

23 Alternative tectonics: diapir/delamination tectonics Mechanism: Crust built by eruptions Deepest crust transforms to dense eclogites: delaminates Downwellings melting TTG formation Abundant melting releases latent heat (Zegers and van Keken, 2001; van Thienen et al., 2004, 2005)

24 Alternative tectonics: diapir/delamination tectonics Why this model? Explains ovoid extrusions (e.g. Pilbara) No need for PT before late Archaean or Proterozoic Efficient cooling mechanism (Zegers and van Keken, 2001; van Thienen et al., 2004, 2005)

25 Did style of PT change over time? Absence of UHPM by slab break-off? frequent slab break-off modern subduction style (van Hunen and van den Berg, 2008)

26 Absence of UHPM by slab break-off? Today Archean UHPM UHPM subduction of continental crust gives UHPM no subduction of continental crust: absence of UHPM (USGS website; Wortel and Spakman, 2000; van Hunen and van den Berg, 2008)

27 Did style of PT change over time? Evolution from flat to steep subduction? No, because: 1. If too buoyant, slabs won t subduct at all 2. A hot, weak mantle is unable to support flat subduction (e.g. Abbott et al., 1994; van Hunen et al., 2004)

28 Did style of PT change over time? Bulk continental crust: Today: andesites Formed in subduction zone Mantle 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)

29 Archaean mantle dynamics Episodic continental crust formation (McCulloch and Bennett, 1994)

30 Archaean mantle dynamics Role of the mantle in tectonics style Geochemical geodynamical viewpoint: Seismic tomography shows whole-mantle convection OIB/MORB basalt chemistry requires district, separate reservoirs (van der Hilst, 2007)

31 Archaean mantle dynamics Mantle spinel perovskite phase transition endothermic: hampers vertical flow Transition from layered to whole mantle convection? Major avalanches during transition period? (Tackley, 1996; Davies, website, ANU)

32 Melting by episodic mantle avalanches? Sudden warming of upper mantle may give: Wide-spread (re-)melting new continental crust Unfavourable PT conditions: intermittent PT? (Davies, 1995)

33 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)

34 Role of mantle plumes T z Archaean plumes (much) hotter Plumes (probably) form at the core-mantle boundary Plume size ~ ΔT Without D -layer plumes larger? Plumes cool Earth s core drive core dynamics magnetic field shielding life on Earth D ΔT MANTLE CORE

35 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.)

36 Links with the hydrosphere: water budget Today: regassing > degassing Early Earth: Subduction rates? More volcanism more degassing Hotter mantle faster slab dehydration less regassing? (Wallmann, 2001; Rüpke et al., 2004)

37 Archaean sea level and emerged continents Constant continental freeboard (±200 m) Very early ocean present Continental growth model (?) ΔT < K unless orogenies were weaker 2-3% late Archaean continent emergence % continental area Archaean water world T( o C) (Harrison et al., 2005; Flament et al., 2008)

38 Conclusions Archaean plate tectonics seems viable, but probably looked different: No Archaean UHPM/blueschists Weaker plates, more break-off Different continental crust Less regassing However, No widespread shallow flat subduction Plate speed higher/lower? Archaean Present Alternative or additional tectonics possible / required: diapir/plume/delamination dynamics

39 Conclusions Episodic 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?

40 Thank you.