The dynamics of subduction throughout the Earth's historyJeroen van HunenDurham University, UKThanks to: Jon Davidson (Durham) Jean-Francois Moyen (St. Etienne) Arie van den Berg (Utrecht) Taras Gerya (ETH)

In this talkSubduction and Earth evolutionSince when did subduction operate?TheoriesObservablesDid subduction style change over time?

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

Todays surface heat flux of 80 mW/m2:50% = from radiogenic heat production50% = Earth coolingTo have Earth cooling in Archaean, we need a cooling mechanism more efficient than plate tectonics (PT)Consequences of more radiogenic heat(Sleep, 2000; Turcotte and Schubert, 2002)

Archaean mantle was 100-300 K hotterSignificantly 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)

todayearly Earth?Consequences of a hotter mantleMore melting at mid-ocean ridgesthicker oceanic crustthicker harzburgitic melt residue layer

Weaker plate and mantle material:h = exp (T)~1 order of magnitude for every 100 K Effect of dehydration strengthening?

(van Thienen & al., 2004)

Consequences of more meltingmore meltingthick crust/harzburgitelow average density rno slab pull?no subduction? (Ontong Java) no plate tectonics?(Davies, 1992)

Effect of basalt-eclogite transition?(Cloos, 1993; Hacker, et al., 2003)Costa Rica subduction zoneMeta-stable basalttransition gradual

Stronger Archaean plates? harzburgite = dry = strongplate bending more difficult?slower Archaean plate motion?fits with supercontinent ages

(Korenaga, 2006)But:plate strength in cold top partplates bending induces faulting + rehydration

(Faccenda et al., 2008)

Weaker Archaean plates?DTmantle = 0oC 100oC 200oC 300oCcolors = viscosity

black = basalt

white = eclogiteviscositytime(van Hunen & van den Berg, 2008)

colors = viscosity

black = basalt

white = eclogitetime(van Hunen & van den Berg, 2008)viscosityFor low Tmantle subduction looks like todaysWeaker Archaean plates?

Weaker Archaean plates?colors = viscosity

black = basalt

white = eclogitetime(van Hunen & van den Berg, 2008)viscosityFor higher Tmantle frequent slab break-off occurs

Weaker Archaean plates?colors = viscosity

black = basalt

white = eclogitetime(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

Are these subduction velocities enough to cool early Earth?Possible parameterizations of vsubd

Thermal evolution the EarthHeat productionSurface heat flow(Reymer and Schubert, 1984; Sleep, 2000; van Thienen et al., 2005)(Herzberg et al., 2010)?

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 catastropheModel 2: Rapid plate tectonics efficient cooling thermal catastrophe

Peak in vsubd: Recent rapid cooling since Proterozoic

Model 3: Inefficient subduction hotter Archaean mantle

ObservationsOnly very few old rocks are preserved.Those rocks are often very much reworked: metamorphosed, altered, and deformedPilbaraJack HillsBarbertonIsuaSlave

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

Oldest ophiolitesOldest 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)

Seismic observationsHorizontal vs. vertical motionSub-horizontal dipping reflectors suggest fossil subduction?(Calvert et al., 1995)

(ONeill et al., 2007; Silver and Behn, 2008)Plate tectonics in Archaean?Paleo-magnetismPaleo-latitudes of old continents varied over timeOnly during supercontinent (formation/breakup)Data sparse!

Episodic early plate tectonics?

Subduction as site for crust formationBulk continental crust: Today: andesitesFormed in subduction zoneMantle wedge hydration and -meltingArchaean: 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; Bdard, 2006)

Arc signature in Archaean TTGs?(JF Moyen, pers. comm.)fluid immobileelementsrare-Earthelement (REE)pattern

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

Key characteristics of plate tectonics

(Stern, 2008)No subduction in Precambrian?

PhanerozoicArchaeansubduction ofcontinental crustgives UHPMno subductionof continental crust: absenceof UHPMAbsence of UHPM by slab break-off?(USGS website; Wortel and Spakman, 2000; van Hunen and Allen, 2010, subm.)

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

Short-term episodicity in subduction?Abitibi, Superior Province(van Hunen & van den Berg, 2008; Moyen and van Hunen, in prep.)

Did subduction style change over time?(Abbott et al., 1994; van Hunen et al., 2004)Evolution flat steep subduction (Abbott et al., 1994)? No, because:

If too buoyant, slabs wont subduct at allA hot, weak mantle is unable to support flat subduction

Archaean water worldToday: regassing > degassingEarly Earth:More volcanism more degassingHotter mantle faster slab dehydration less regassing?Weaker rocks no high mountain ranges?Buoyant oceanic lithosphere shallow ocean basins(Wallmann, 2001; Rpke et al., 2004; Rey & Coltice, 2009; Flament et al., 2008)

Subduction evolution:

Changing dynamics due to changing mantle temperature:Different crustal thickness, plate strengthEpisodic subduction (long / short time scale)?

Observational evidence:Geology/Geophysics: Ophiolites, dipping reflectors, palaeomagnetismGeochemistry: Continental crust, geochemical fingerprintsPetrology: Ultra-high pressure metamorphism, blueschistsConcluding remarks

Computer model simulationsHigh Tmantle thick crust no subduction? But

Thick crust is only sustainable if mantle doesnt deplete, i.e. if crust mixes back in after subduction

Perhaps heavy eclogite settled at bottom of mantle?(Davies, 2006)

Heat flow, cooling, and vsubd through time:parameterized cooling model 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) uncertain46 TW

Subduction in Archaean?Dipping seismic reflectors

Ophiolites (?)

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

Ultra-high pressure metamorphism (UHPM)UHPMOceanic subductionContinental 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.

(after Sleep, 2000; Turcotte & Schubert, 2002; Korenaga, 2005)Tm(t) strongly depends on surface tectonicsconstant coolingcooling of afluidsurface heat flow by plate tectonicsas todayDifferent cooling scenarios

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)

Cooling ability of flood basaltsPlumes transport heat from CMB and gives melting at surfaceDiapir 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)

Did style of PT change over time?(after Davies, 1992)TODAYIN HOTTER EARTHEvolving style of PT?Unsubductable crust to form stacks (?)Mechanism to form greenstone belts (?)

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 formationAbundant melting releases latent heat

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

Archaean sea level and emerged continentsConstant continental freeboard (200 m)Very early ocean presentContinental growth model (?)

DT < 110-210 K unless orogenies were weaker2-3% late Archaean continent emergence Archaean water world

(Harrison et al., 2005; Flament et al., 2008) 1350 1400 1450 1500 T(oC)02542% continental area

**What I plan to discuss: *So what was different in Archaean? Earth is a very strong heater: it produces a lot of heat insight from decay of radiogenic elements. This decay is going on for 4.5 Ga, suggests thats why is was higher in past: about 3x in the Archean. This is fairly well constrained, apart from some uncertainty about the original composition of the EarthsAlso Earth has been cooling: as I said today 50-100 K/Grs, and geological record of MORBs gives indication of mantle T in past: depending on a few assumptions 100-300 hotter in Archean.*This additional heat production can be a serious problem for PT:Today, about 80 mW/m2 of heat flows from Earth, so football field produces enough heat energy to run a microwave oven on. This is mostly coming out of oceans and is the result of PT: without PT there would be much less heat escaping. is produced by radioactive heating, other half is primordial heat: heat that is still there from Earth formation, and is slowly released (rough numbers, if Earth is chondritic, radiogenic component is larger than 50%).In past, more radiogenic heat. So if we want to avoid heat accumulation, and we assume PT in past, then PT must have been capable to remove this extra heat as well. So PT must have been faster/more efficient. If not, the Earth was not cooling as fast, or perhaps not cooling at all, but heating up. Same arguments for any other kind of tectonics.*Some debate about how much hotter. Main evidence comes from komatiites, which suggest large degrees of mantle melting.Depending on how this is interpreted, this suggests a mantle of 100-300 K hotter. Lower bound if mantle was also wetter, upper bound if extra melting comes mainly from the higher mantle temperatures*Hotter Earth also had consequences: Most materials (including rocks) become weaker when heated up: rule of thumb: 1 oom per 100 degrees.Mantle melts close to surface, 7 km crust. In hotter Earth more melting, thicker crust, perhaps 20 km instead of 7.Plot shows expected crusal thickness. Btw, Tpot is mantle T extrapolated to Earths surface. *This has significant dynamical consequences: crust (basalt) is buoyant: its density is lower than mantle density. So it floats. Today this is not hugely important, and doesnt prevent plate from subducting. But if crust is 3x as thick, it IS a problem: no way this material is going to go down.Today we see this at Ontong Java, has anomalously thick crust, and is approaching the subduction zone. This socalled oceanic plateau is not going to subduct.*Not full story, and there might be a way out. B->EEclogite is denser, even a little denser than mantle material, so more of it would only help subduction.There is one problem: transition only once material at 40 km depth. So how to get it there? Chicken&egg problem.So subduction might benefit from this transition, but the fact remains that crust has to be brought to 40 km depth first, and this requires force.*Adding this transition and other complexities to the problem, it quickly becomes too large for a back-of-the-envelope calculation. I did computer modelling to look at subduction dynamics in hotter Earth, when crust is thicker. Explain plots: colours, axes, columns, rows.*Today and 100 K hotter are similar. Quantitative differences, but qualitatively same: PT works in a similar way.*Increasing T more gives problems: thick crust starts to produce large buoyancy stresses, and at same time slab becomes weak, because it is hotter. This leads to more and more slab breakoff. Consequence: broken-off slab offers no slab pull anymore: subduction slows down *And may even completely stop.So for too hot mantle, PT is not viable anymore.*Summary of all results in terms of average subduction speed vs. Tpot. Almost all models show speedup for DT=0100K Most models have peak vsubd at DT=100 or 200 K and start to decrease afterwards again. So what kind of cooling history would those models give?*To answer this, I cleaned up this spaghetti plot, and parameterized the lines into simple functions, looking at 3 different endmember scenarios. Simply assume vsubd didnt change vsubd keeps on increasing with incresaing mantle temperature. Vsubd increased at first (weaker mantle), but then decreased (resistance from buoyant crust and breakoff)*We want to calculate the temperature of the Earth T.Translating T to heat energy requires heat capacity C (in J/K): total heat of the Earth = C*TWhat we know is the sources and sinks of the heat, so we know what causes T to change over time, see eqn in slide.H=source, Q=sink.

*Main plot here is on right: T and v versus time.

Top-left shows again vsubd as function of Tpot, and bottom-left shows resulting heat flow.Flat vsubd almost constant qs, but since early Earth produced more heat, it couldnt get rid of it all, and heated up in beginning. But we have to be careful of any plots like these for Hadean: highly speculative

Results look a lot like those suggested by Korenaga,06 and Labrosse and Jaupart (06)*2nd scenario: hotter weaker Earth gives faster plate tectonics. This scenario almost always leads to a thermal catastrophe: an incredibly hot Archaean Earth:Positive feedback: hotter Earth faster tectonics faster cooling Earth started off even hotter, etc. *Model 3 gives more reasonable results: Until Arch-Prot. Earth was in downgoing branch of curve in top-left: hotter Earth gives slower vsubd, and therefore less surface heat release, and so even hotter Earth. However, heat production diminishes more rapidly than surface heatflow, so heating up of Earth reduces and even stops at 2.5 Ga, after which Earth rapidly cools down. This curve suggests that PT was most effective around 1 Ga. *Clearly, theories and models have their uncertainties. Can we find evidence for PT in the rock record?

The problem there is: there is not so much left to take the evidence from. Continents are the only old pieces on Earth, so we have to look for evidence on continents. Very few parts are old enough. And those available are often very much reworked*Seismic observations: dipping reflectors suggest remnants of fossil subduction zones?*Oldest in Greenland, Isua. Not everyone agrees that this is a proper one: parts too distributed.\Oldest accepted ones: 2 Gyrs, and these provide pretty solid evidence for oceanic lithosphere 2 Ga, and therefore PT, since it is hard to come up with how oceanic lithosphere would form without PT.However, for ages < 1 Ga, the number of ophiolites is increasing significantly. Presenrvation problem, or different PT mechanism?*Seismic observations: dipping reflectors suggest remnants of fossil subduction zones?*Looking at magnetic record of rocks of different ages in same continent might tell us how this continent was travelling N-S. This travelling would be evidence for PT if different continents travel with different speeds.Here are results for the few leftover old cratons.They suggest rapid motion around when supercontinents existed.Only then PT? Episodic?

*Continental crust might give us another clue. Today, andesites are thought to form primarily in subduction settings, so how far back in time can we trace this? Archaean continental crust (TTG), however, TTGs are not the same geochemically: this complicates things. BUT (abundant) TTG looks a lot like (rare) modern adakites: slab melts. Makes sense for hotter Earth. There are some problems with this model however: no MW influence (unlike for adakites)

Alternative model base-of-lithosphere has its own problems: TTGs are thought to be wet melting products, and the base of the lithosphere is most likely not wet at all. Also low T (900-1100 degC, high p (>12-15 kbar) conditions are more in line with a cold subduction geotherm than a normal continental geotherm.How to explain this then? Well, it is thought that remelting from mafic lower crust and fractional crystallisation to upper felsic crust can produce this signature as well. This doesnt mean that a slab melt scenario is not likely anymore (it still is the favourite model), but that subduction is not required to create this geochemical fingerprint, and that melting at the base of the crust is still a viable model. **A compilation of these hallmarks of PT is given by Bob Stern. Many typical PT features (passive margins, UHPM, ophiolites) are absent in early Prot. and Archaean. He takes this as evidence that PT didnt start before late Proterozoic.

Ill come back to this interpretation in a few slides. *To explain: UHPM associated with cont.coll: continent dragged down breakoff rebound = UHPMPerhaps Archaean continents not dragged down, and therefore no UHPM?*CC provides another interesting piece of information. When CC is plotted in a histogram against age, it is clear that most of the crust is formed episodically, in peak periods, at 3.3, 2.7, 1.9, and 1.1 Gyrs.To explain this, we probably have to look into the dynamics of the mantle over time, and this is the next topic I would like to go through briefly.

*A suggested change in PT style is flatsteep subduction. Initially based on buoyancy arguments: buoyant slabs dont want to sink and will float in mantle. However, if slab too buoyant to subduct, subduction and PT will stop altogether. Later used to explain geochemical characteristics in CC (next slide)

Furthermore, flat subduction (as observed today) requires some component of overriding continent motion (S.Am moves westward rapidly flat subduction where buoyant plateaus subduct). In that case a flat slab occurs only when the slab is overridden faster than it is sinking in the mantle, and sinking rates are larger in a weak mantle. But an Archaean mantle was probably to weak to support those flat slabs.

*Water is one of the volatiles: it enters ocean/atmosphere during MOR/OI volcanism, but can also re-enter mantle at subduction zones (most of water that enters subduction zone comes out again with arc volcanism, but some makes it to deep earth).De/regassing rates quite unconstrained, but regassing seems more important than regassing today, so ocean shrinks? Observations suggest a very early (Hadean) ocean present. In hotter Earth this was probably different, as slabs would dehydrate fasater during subduction, and less water made it to deep mantle.*Other people did computer calculations that focus on other aspects. Today, plates subduct and mix with mantle again. After long time of mixing, material homogeneous again, and can melt again. But in past, mantle hotter, so weaker, and perhaps mixing of heavy eclogite not so easy. Maybe all eclogite sank to bottom of mantle, and stayed there?Then mantle material becomes more depleted over time. This means it cannot melt as much anymore. Material can only melt once, and wont melt again unless you enrich it again with the melt products (mixing). Then crust might have been thinner than today, and subduction wasnt a problem in past. *From these results: peak T in early ArcheanNB1: Present-day cooling rate (118 deg/Ga, Jaupart et al., 2007) extrapolated. Too high?NB2: only PT cooling, no alternative mechanisms (magma ocean, flood basalts)NB3: Urey ratio (ratio of internal heating to surface heat flow, ~0.33) uncertain*These models can show insight, but are poorly constrained. What they do show is that for some models, PT is viable back to early Archaean, for others it isnt.

Seismic dipping reflectors occur in many old cratons, and are mostly interpreted as fossil subduction zones.Ophiolites are one of the main characteristics of todays PT, and one of the few remnants of oceanic lithosphere older than 180 Ma. Phanerozoic ophiolites occur widespread, Proterozoic ones are often debated, and I think only one Archaean one is reported in Greenland, although the essential components of the ophiolite are not found as one continuous sequence, but rather a scattered set of components. *Another indicator is UHPM: eclogites and gneisses that have been buried to large depth (>>100km), and somehow made it back to surface.We think we understand how this works today: - subduction brings eclogites down to large depth- continental collision stops this process, because CC is too buoyant. - slabs might break off, which causes the bit of CC that was dragged down to bounce back to surface (gneisses), pulling some eclogites with it.

Now UHPM doesnt occur prior to 600 Ma. Why?- no PT, no subduction- different pT conditions, rocks might have looked different, and we cant recognize them- my favourite: weaker slabs couldnt drag CC down as much as they can today, so CC never makes it to depths where UHPM is formed.*What determines the cooling rate of the Earth: Heatr production Relase of heat through surface, i.e. vigour of PT.

Some scenarios: Green: If Earth simply cooled the way it does now green line. No physical basis for constant cooling, but it is a simple assumption.

Red=thermal catastrophe: has a fluid-dynamical basis: (explain: hotter mantle is weaker, convects faster, and therefore cools faster. That means a faster drop of Tm through time, so a steeper line when going back in time. This eventually leads to thermal catastrophe).

Blue=constant surface heatflow (explain: assume pl.tect. Always operated as it does today, but radiogenic hp was larger in past): has a physical basis.

So 3 totally different mantle cooling curves. We are looking for a curve like the green one (the one without a physical basis), but two first attempts show something totally different. Apparently, the physical processes responsible for the cooling are quite important. And apparently these models are too rough to determine Earths cooling history.So either theres something seriously wrong with at least one of these theories, or plate tectonics only operated recently, and we cannot use the older parts of the blue/red curves. So this is where my work will hopefully fit in: I modelled pl.tect. in hotter Earth.*Illustrated how UM and LM Ts can evolve.This would also have sign. Consequences for PT, as the larger amount of melting would give a thicker crust that suddenly doesnt want to subduct anymore?*Also possible if crust was too buoyant to subduct, it might stack up at surface. This could explain presence of greenstone belts, but a problem is that if this stacking up occurred on large scale, wed expect a lot more Archaean oceanic crust, which we dont find.*One of the more popular models is that of diapir tectonics (or delamination tectonics).*Last slide:Related to ocean volume is a useful observation that Archaean cratons have been around for Gyrs without significant erosion/sedimentation, suggesting roughly constant elevation of continents above sealevel (+-200 m), or constant continental freeboard.

Given changed amount of continental area, buoyancy of oceanic lithosphere, and all other potential changes that we saw before, this is quite remarkable.E.g., hotter mantle gives thicker oceanic crust, that would be buoyant, and would sink less deep than todays oceanic crust, and would lead to shallower oceans. If ocean volume was roughly the same, this means that part of that ocean water was flooding the continents.

Constant freeboard argues that that was not happening, and this limits early mantle T to at most 110-210 K hotter than today, which is fairly low. In plot (continental area vs Tm), gray area is constant freeboard area, purple area is likely Archaean situation.The authors of this paper suggested that continents were quite weak, and therefore high mountains didnt form, which relaxes freeboard constraint somewhat.They argue that perhaps during Archaean only 2-3% of Earths surface was above sea level as continental area.