The dynamics of subduction throughout the Earth's history

1. The dynamics of subduction throughout the Earth's history Jeroen van Hunen Durham University, UK Thanks to: Jon Davidson (Durham) Jean-Francois Moyen (St. Etienne) Arie van den Berg (Utrecht) Taras Gerya (ETH)

2. In this talk • Subduction and Earth evolution • Since when did subduction operate? • Theories • Observables • Did subduction style change over time?

3. How was Earth different in the past? Produced 3x as much radiogenic heat • was 100-300 K hotter (Herzberg et al., 2010)

4. Consequences of more radiogenic heat • Today’s surface heat flux of 80 mW/m2: • 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) (Sleep, 2000; Turcotte and Schubert, 2002)

5. Archaean mantle was 100-300 K hotter Significantly hotter Archaean mantle(Nisbet et al., 1993; Abbott et al., 1994) Wet, slightly hotter Archean mantle (Grove and Parman, 2004) Peak temperature in Archaean? (Herzberg et al., 2010)

6. Consequences of a hotter mantle • More melting at mid-ocean ridges • thicker oceanic crust • thicker harzburgitic melt residue layer • Weaker plate and mantle material: • h = exp (T) • ~1 order of magnitude for every 100 K • Effect of dehydration strengthening? early Earth? today (van Thienen & al., 2004)

7. today Archaean very low r crust very low r low r harzburgite low r lithosphere normal r peridotite normal r Consequences of more melting • more melting • thick crust/harzburgite • low average density r • no slab pull? • no subduction? (Ontong Java) • no plate tectonics? (Davies, 1992)

8. Subduction density eclogitic crust density basaltic crust No subduction Effect of basalt-eclogite transition? Costa Rica subduction zone • Meta-stable basalt • transition gradual (Cloos, 1993; Hacker, et al., 2003)

9. Stronger Archaean plates? • harzburgite = dry = strong • plate bending more difficult? • slower Archaean plate motion? • fits with supercontinent ages • But: • plate strength in cold top part • plates bending induces faulting + rehydration (Faccenda et al., 2008) (Korenaga, 2006)

10. Weaker Archaean plates? colors = viscosity black = basalt white = eclogite time viscosity DTmantle = 0oC 100oC 200oC 300oC (van Hunen & van den Berg, 2008)

11. Weaker Archaean plates? • For low Tmantle subduction looks like today’s colors = viscosity black = basalt white = eclogite time viscosity (van Hunen & van den Berg, 2008)

12. Weaker Archaean plates? • For higher Tmantle frequent slab break-off occurs … colors = viscosity black = basalt white = eclogite time viscosity (van Hunen & van den Berg, 2008)

13. Weaker Archaean plates? • … or subduction completely stops. colors = viscosity black = basalt white = eclogite time viscosity (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 vsubd 2 3 1 Are these subduction velocities enough to cool early Earth?

16. Thermal evolution the Earth Heat production 2 Surface heat flow 3 1 ? (Reymer and Schubert, 1984; Sleep, 2000; van Thienen et al., 2005) (Herzberg et al., 2010)

17. Model 1: subduction for all Tm • Flat vsubd rate: cooling since early Archean • Cooling curve similar to Korenaga,’06 and Labrosse & Jaupart,’07.

18. Model 2: Rapid plate tectonics  efficient cooling  ‘thermal catastrophe’ Increasing vsubd with Tpot: ‘Thermal catastrophe’

19. Model 3: Inefficient subduction  hotter Archaean mantle Peak in vsubd: Recent rapid cooling since Proterozoic

20. Observations Slave Isua • Only very few old rocks are preserved. Barberton Pilbara Jack Hills • Those rocks are often very much reworked: metamorphosed, altered, and deformed

21. Linear features? Abitibi, Superior Province Pilbara, Australia (Calvert et al., 1995, JF Moyen, pers.comm.)

22. Oldest ophiolites • Oldest ophiolite 3.7 Gyrs old? • Oldest generally accepted ophiolites are ~2 Gyrs old (Jormua, Finland; Purtuniq, Canada) • Ophiolites become wide-spread after 1.0 Gyrs ago (Stern, 2005; Furnes et al., 2007)

23. Seismic observations • Horizontal vs. vertical motion • Sub-horizontal dipping reflectors suggest fossil subduction? (Calvert et al., 1995)

24. Plate tectonics in Archaean? Paleo-magnetism • Paleo-latitudes of old continents varied over time • Only during supercontinent (formation/breakup) • Data sparse! • Episodic early plate tectonics? (O’Neill et al., 2007; Silver and Behn, 2008)

25. Subduction as site for crust formation 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: (Defant and Drummond, 1993; Foley et al., 2002, 2003; van Thienen et al., 2004; Bédard, 2006)

26. Arc signature in Archaean TTGs? rare-Earth element (REE) pattern fluid immobile elements (JF Moyen, pers. comm.)

27. “Arc” signature in Icelandic dacites !?! (Willbord et al., 2009; JF Moyen, pers. comm.)

28. Key characteristics of plate tectonics No subduction in Precambrian? (Stern, 2008)

29. UHPM UHPM Absence of UHPM by slab break-off? Archaean Phanerozoic no subduction of continental crust: absence of UHPM subduction of continental crust gives UHPM (USGS website; Wortel and Spakman, 2000; van Hunen and Allen, 2010, subm.)

30. Long-term episodicity in subduction? ? (McCulloch and Bennett, 1994; Davies, 1995; O’Neill et al., 2007)

31. Short-term episodicity in subduction? Abitibi, Superior Province Clastic Time (Ma)  Calc-alkaline Tholeitic (van Hunen & van den Berg, 2008; Moyen and van Hunen, in prep.)

32. Did subduction style change over time? Evolution flat  steep subduction (Abbott et al., 1994)?  No, because: If too buoyant, slabs won’t subduct at all A hot, weak mantle is unable to support flat subduction (Abbott et al., 1994; van Hunen et al., 2004)

33. ‘Archaean water world’ • Today: regassing > degassing • Early Earth: • More volcanism  more degassing • Hotter mantle  faster slab dehydration  less regassing? • Weaker rocks  no high mountain ranges? • Buoyant oceanic lithosphere  shallow ocean basins (Wallmann, 2001; Rüpke et al., 2004; Rey & Coltice, 2009; Flament et al., 2008)

34. Concluding remarks Subduction evolution: • Changing dynamics due to changing mantle temperature: • Different crustal thickness, plate strength • Episodic subduction (long / short time scale)? • Observational evidence: • Geology/Geophysics: Ophiolites, dipping reflectors, palaeomagnetism • Geochemistry: Continental crust, geochemical fingerprints • Petrology: Ultra-high pressure metamorphism, blueschists

36. Computer model simulations • High Tmantle thick crust  no subduction? But … • Thick crust is only sustainable if mantle doesn’t ‘deplete’, i.e. if crust mixes back in after subduction • Perhaps heavy eclogite settled at bottom of mantle? (Davies, 2006)

37. Heat flow, cooling, and vsubd through time: parameterized coolingmodel assumptions • today: qoc,0 =32 TW • qcont,0 =14 TW (Jaupart et al., 2007) • Present-day cooling rate (118 K/Gyrs, Jaupart et al., 2007) • extrapolated. Too high? Steady state reasonable? • only plate-tectonic cooling, no alternative mechanisms • (e.g. magma ocean, flood basalts) • Urey ratio (ratio of internal heating to surface heat flow, ~0.33) • uncertain 46 TW

38. Subduction in Archaean? Dipping seismic reflectors Ophiolites (?) (Calvert et al., 1995; Furnes et al., 2007)

39. Ultra-high pressure metamorphism (UHPM) • Oceanic subduction • Continental collision, UHPM rocks form (gneisses, eclogites). • Slab break-off, and ‘rebound’ of continent, UHPM to surface • UHPM is typical for modern PT, but is absent in rock record older than 600 Myrs. Why? • No subduction. • Different slab temperature, so different UHPM rocks formed. • No continent subduction, so no UHPM rocks formed. UHPM

40. Different cooling scenarios cooling of a fluid constant cooling (after Sleep, 2000; Turcotte & Schubert, 2002; Korenaga, 2005) surface heat flow by plate tectonics as today Tm(t) strongly depends on surface tectonics

41. 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)

42. Cooling ability of flood basalts • Plumes transport heat from CMB and gives melting at surface • Diapir tectonics results in large-scale melting • Latent heat can be important cooling agent: enough to cool Archaean Earth if 100x more vigorous than in Phanerozoic (van Thienen et al., 2005)

43. Did style of PT change over time? Evolving style of PT? TODAY • Unsubductable crust to form stacks (?) • Mechanism to form greenstone belts (?) IN HOTTER EARTH (after Davies, 1992)

44. 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)

45. 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)

46. Archaean sea level and emerged continents • Constant continental freeboard (±200 m) • Very early ocean present • Continental growth model (?) • DT < 110-210 K unless orogenies were weaker • 2-3% late Archaean continent emergence ‘Archaean water world’ 42 25 % continental area 0 • 1350 1400 1450 1500 • T(oC) (Harrison et al., 2005; Flament et al., 2008)