Kepler Seminars: Formation of Archean continental crust

Since Dan, he has worked in many places around the world, but in 1998 he started to growl and I just saw that he became an exceptional class teacher, so he sounds good in French, something exceptional. Professor, he is now an emirite, but he is not. active uh he is a petrologist and geochemist with a very broad interest and I just included the habitat of life on the early Earth uh Nick is a member of the European Academy elected fellow of the Gemica Society he is very well cited I think there are more than 30 thousand for those who are aware of H indices, a colleague of mine com y Gana told me that if you have an AG that is older than your age, that is very exceptional and the aunt is 91 and that is way above her age still today and since one's title changed.

If I had talked about when and how the

continental

crust

formed, then the floor is yours, Nick, so thank you very much Tron for the presentation and for inviting me to Oslo. I came on Monday afternoon and since then I have been having a wonderful time talking to Raa with many different people at the center about all kinds of things, magma oceans, AR Indian

crust

, passage of granite through the crust , etc., and I think this will continue. Today I'm going to talk about the

formation

of the

continental

crust and what this tells us about how the Earth operated three and four billion years ago.

Now, after I retired, I spent four years in a small company and I got it called Vactive in French, which is in the active life that they call if you work with the industry, this is active life, this is over and now I'm back to life passive in the Academy. I restarted the research and when I started reading again I came across yes, this statement, there is a broad consensus that a and granitoids formed by partial melting of hydrated maic rocks in a stagnant lid geodynamic regime and I said that is not right, it cannot be good.

I didn't like the idea. I don't think it's correct, so this is what I did. I'm going to try to convince you that it's not that there wasn't a passive, there wasn't a stagnant immobile lid that covered the entire globe through most of the aen. I'm going to ask questions like these, when did the beginning of plate tectonics occur? a major change in the geodynamic regime at some point across Aran, indicating that previously there was another geodynamic Reg related to this, but not quite the same question is when did St subduction occur and can we talk about the difference between subduction and plate tectonics and whether they are one and the same or can they be separated and to address these questions I am going to consider how the granitic material was formed, how the granitic material is extracted from the mantle to form the continental crust because I think that if we understand this process we can get a better idea of ​​the geodynamic regime that operated through Aran, so if you look at the literature you will find many diagrams like this one here, many geoscientists support this idea of ​​the general consensus that arcane geodynamics was very different from modern geodynamics and they talk about stagnation. cap a stationary cap that covered most of the globe under which the mantle convected perhaps with mantle plumes and so on and there was a transition at some point here, they put it in 1 million years between this stagnant cap regime and a plate tectonics regime and you find a lot of terms in the literature like this a stagnant cap they talk about trickle tectonics they talk about sagd duction heat pipes instead of subduction heat pipes it's something else and the question was there was no plate tectonics plates in early Aran now, why so many geoscientists? uh, I think there was no PL tectonics.

There are several approaches to this question. One is shown here. This is Bob Stern, who said what the criteria are for modern plate tectonics and identified a number of blue ultra-high temperature metamorphism features that we associate with modern subduction zones and discovered that these features are not We actually see it in rocks much older than 600 million years during the Paleozoic, but not before arguing that plate tectonics only started somewhere around there, I don't think. This is the case mainly because there is no reason to believe that subduction, if it operated at Aran, would be the same as it is today, conditions were different and subduction would probably be different and these features are not preserved.

Another line of argument to say that there maybe no plates there uh the plate tectonics in the are comes from the comparison with other planets Stephanie knows about this in um the idea shown here is that today contemporary Venus seems to have a material stagnant motionless lid magic we are not sure of the composition but it looks like the lid seems to have a lid, it didn't move, there were no rigid plates moving across the planet, what is the situation on Earth so the idea is that at the beginning Venus and Earth were similar and but they evolved in different ways, the Earth to play tectonics, Venus was left with this stagnant lid situation, but, and there are a lot of other changes that seem to have happened around here between 3 billion years and 2.5 billion years that is also said. to indicate a change in the geodynamic regime, for example, these are Sh and Richardson, who noted that eite inclusions in diamonds are present after 3 billion years, not before these eite inclusions are interpreted as a subducted oceanic crust that eventually becomes trapped in the diamond.

If there are none that we know of that are older than three billion years, Sh Right Richardson interpreted it to mean there is no tectonics beforehand. There are also a large number of changes in various geochemical ratios. There is some kind of break here at 2.5, for example, the potassium content. pomegranates became much higher than the potassium content, this sodium-potassium ratio potassium pomegranates only begin to appear in large quantities after about 2.5. There are disruptions in the geochemical ratios that can be documented and are interpreted as there were no plate tonics beforehand, there was no subduction beforehand. subduction and plate tonics next, then there are the geochemical models shown, for example, calculated based on isotopic data and I'll just point out this one here from Bruno deim in his group, they calculated Rel, this shows the growth rate of the crust continental to its current volume here from something zero to a relatively rapid growth of 4.5 with a kink here that they interpreted as the beginning of recycling of the continental crust which they equated with subduction, so they distinguish between what they call a regime of presubduction and a regime in which there was subduction, so the various lines of evidence that analyze the geological characteristics analyze the models that support the idea that there was no small plate tectonics before about 3 billion years and that the tectonics of plates started after that, but perhaps the most compelling argument for a different difference. geodynamic region in the early AR and comes from estimates of temperature in the mantle.

There are a number of theoretical lines of argument that would say that due to increased radioactive heat production due to heat loss from the core, heat loss due to accretion temperatures on Aran. The mantle was significantly higher than those of the modern mantle. This is a diagram of Claude Herzberg's age and potential temperature, which is the temperature of the sources of various types of balsams, we just focus on the black squares that are called nonar basalts. They are basalts that were interpreted to come directly from the mantle. He has a way of calculating temperature and showed in this diagram that the temperatures of the sources of these basalts were perhaps 250 higher than the current salt of the mid-ocean ridge B, so Aki and the mantle can have been much hotter and why this is important for two reasons first.

I show in this diagram this is the temperature, pressure and depth for the peridotite of the mantle, the most solid, the liquid, the beginning of the fusion, the end of the fusion and I show two paths of ascent, one is the path of ascent of modern mantle material that rises beneath a mid-ocean ridge, strikes the solidus at relatively shallow depths, and undergoes melt paths to reach the surface with approximately 15% partial melting. Compare this to the rise path of the hot arcane mantle that hits the solidus, much deeper, follows a longer milk path and reaches the surface with 40° 40% partial melting, so a large volume is produced here of magma, a small volume here, if this magma reaches the surface directly at a ridge in the middle of the ocean, the oceanic crust is formed the modern, fresh mantle the fusion path cuts a small volume of melt produces a thin oceanic crust the Hot Aran mantle a thousand large volume produces a thick oceanic crust perhaps 30 kilometers thick, so that's the idea probably across most of Aran the oceanic crust was much thicker is composed of plag CLS, which is a low density mineral, so we have a thick, floating crust that probably resisted subduction.

This kind of thinking leads to models of the type I show above, numerical modeling done by Heron van Hunan's group. Darham runs numerical models of mantle convection and what we see here are a series of panels with different delta T, which is the temperature between the temperature difference between the modern mantle and a mantle that is 300° hotter. We see that, for a modern mantle, it is able to form something that looks like subduction the crust is subducting and it descends to the base of the uh it descends deeply to the transition zone as the temperature increases the subduction becomes less obvious and becomes less discontinuous is not intended to penetrate deeply, so according to this type of model, subduction is said to have been difficult or impossible across Aran, this then leads to alternative models for the geodynamic regime within Aran, e.g. here There is the terasa group in Zorich. and they start with a quite different situation, they start with what is called the stagnant lid model, since for Venus they assume that there was a thick layer of basaltic crust on the surface that turns into eite at high temperatures at the base of this crust , now it's eite. the high pressure equivalent of the bassal is dense, so the presence of this dense eite drags down the lower part of the crust into the mantle, they talk about eite drips and partially melts to produce yes, this is the type of uh stagnant cap regime that they consider to operate through the Aran, so here we see the contrast between, on the left, according to these authors, a stagnant cap regime with sinking duction that trickles from the base of the crust towards the mantle as opposed to plate tectonics. and subduction and the same type of transition is shown here now.

How do we distinguish between these two situations? Plate tectonics in which rigid plates move towards the mantle and act that way from the alternative idea that there is a stagnant lid at the base that drips down to the mantle. I am going to focus on the

formation

of the continental crust. Formation of the continental crust, which is essentially a process by which material of phallic composition is extracted from the mantle and defines its path to the surface. To ask the question, when did the continent begin to grow and by what mechanism try to distinguish between stagnant cap tectonics versus plag tectonics, so we must now consider models of continental growth at what rate did the continent begin to grow? continental crust?

To form, do we see evidence that continental crust formed early? I'll talk first about the rate of formation of the continental crust and then I'll talk about how it might have formed, but first, when did it form? and we see that there is in this this diagram here the time versus the fraction of the continental crust. I showed you a curve like this that showed rapid continental crust growth early and then late, so later on there are some models that talk about rapid continental crust either at the beginning. Aran or Late Aran and another set of models in which there was very rapid formation of continental crust early in Earth's history.

Richard Armstrong developed this idea in the 1980s, he said that very early in the hadan the continental crust was extracted from the mantle and continued to accumulate thereafter this idea has been taken up by the group of people working with Jun Kaga two articles here A model that they have very this is the one that shows the accumulated volume of the continental crust and this is the rate of the continental crust, but in both cases continental crust grew rapidly from the beginning and, from then on, the volume of the The crust remains fairly constant because growth is counteracted by recycling back into the mantle.

I do not accept these ideas because of the information that comes from zircons. You may know that first in Australia and now almost. Everywhere, in many other places, including India, we have people who have found zircons with ages that correspond to hadan with ages ofmore than 4 billion years, so here what we see in this diagram is the age and these are the determined age of the zircons, the only minerals that we get from the If we had seen in the hadan which is also shown in this diagram, the isotopic compositions of hafnium are represented as eum hafnium, now the way we interpret this diagram is to say that the material in the mantle will evolve along a curve like this, it is depleted and because of that the Epsilon haum ratio increases The material of the entire continental crust will evolve along a line like this.

We see the comparison. Here we see the zer compositions that lie between these two curves, with very little almost no data coming from here until about 3.8. billion years and suddenly we began to see these zircons with compositions like those that come from the mantle, for me this is very significant because this general trend of decreasing Epsilon hnum with age corresponds to the repeated melting of granitic material, phallic material , so it seems that phallic material was formed. Here and material of the same composition is repelled repeatedly melted in a closed system with no mantle entry started only here, um, so I interpret this to indicate the persistence time of a stagnant cap enriched with magical material that was on the surface and which melted repeatedly until this stage when there was a change in the gamic regime of From here on, in my interpretation, this is when plate tectonic subduction started somewhere around here, so like, this is a ziron jackal 4 billion years old, an image of a ziron, this is the model proposed by Tony Kemp and his coworkers, they represent a stagnation.

Composite bassal cap is very hot because it had a high concentration of radioactive heat-producing elements that melted internally to produce the granitic melts from which the zircons crystallized so early that there was a stagnant cap, a sort of stagnant cap that disappeared. around 3.8 billion years. so I think we can eliminate this first idea of ​​an early subduction, there was something different then, but how do we distinguish between these other two alternatives? The idea that around three billion years ago we saw a transition from what they call sacerdotal subduction to an early subduction regime. Let's distinguish between those two alternatives and now I will focus on granitoids?

How were granitoids formed? Because before we talk about the growth rate of the continental crust, I think no, there wasn't much until about 3.8 and from then on the continental crust. The crust began to form, but how was it formed? What type of geodynamic regime in early Aran formed granitoids so that we can contrast the two Al? Two alternatives plate tectonic subduction subduction fusion in the mantle wedge and so on, what happens now with the tectonic drip prior to the plates Tectonic regime in which you have a thick layer quite similar to the one I imagine for the hadan, but a situation in which the bottom of the mic layer destabilizes, drips into the mantle and partially melts to form felsic magmas that form the continental crust, in other words.

Granite formation in a stagnant eyelid regime. The main one perhaps (I am the main proponent of this type of model) is Jean Bed in Ontario, who produced this subsidence diagram. He uses the term subsidence duction, which is a process in which the base of an oceanic plateau or a thick oceanic crust is composed of basalt, this basalt becomes dense with high pressure, sinks into the mantle, heats up and triggers the formation of granite. Is this a viable process? This is what I will talk about now, for example. Here's another article by Coward Atel showing a thick stack of microphones.

A thick pile of magic material almost 30 km thick in which granitic magmas form as P melts at the base of this crust, producing magmas that rise into the upper crust and become. gradually convert the crust into felsic material felsic continental material now to evaluate this I give you my recipe for granite which is very simple, you start with basalt material, you need water, this is essential, you need heat and you mix the two and you get a granite, but I Emphasize this, you need both, you need basaltic material, which can be solid B salt that is heated and partially melted or it can be hydris magma.

Both work and I think probably hydrated magma is more important, you need water, you need maic material if you add heat. you can get Granite, but what's the situation? In what situations can this occur? My spell checker doesn't know the term microphone and keeps saying: I really need to talk about magic and I think you're right because it's thick. Oceanic crust doesn't look like that, you need magical oceanic crust if you want to produce granite within it. Melting models of the Earth's crust do not work for two main reasons: the first is that thick piles of basalt do not really exist in nature.

If a large amount of basalt material is extracted from the mantle, it does not accumulate as a thick pile with composition uniform basalt from top to bottom differentiates and I will tell you what it means to differentiate and second the lower portions of a thick pile of Basalt are not not hydrated there is no water there so I think these are important messages and I will explain why I think this is the case. Firstly, we have many examples at different scales of piles of material with ultra-basaltic compositions from one to 1 meter thick Karate flow in which the bottom is accumulated olive, the top is liquid, this is the complex of Bushville, 9 kilometers thick with basaltic material on the top, the bottom having an ultramic composition, builds up and this is the Java onong. plateau in which the surface basalts are normal basalts, but the seismic data show high seismic velocities that are best interpreted as ultra accumulations.

There are strong petrological reasons to say that B's thick stacks of magical alter mic material do not have this compatible composition everywhere. A different argument is that if we look at the composition of the Baltics that erupt in Java or in the Iñaki and greenstone belts, they have low MGO contents, but we know from experimental petrology that if the mantle melts at 60 kilometers deep, at 100 kilometers deep, the composition of the melt that occurs is much more magnesium, so a primary mantle melt has 15 to 20% MGO, the P at the surface has 5 to 7 % MGO. From here to here it goes by fractional crystallization and accumulation of a large amount of Parx orthop. of a large amount of olivine plus orthop parine the second argument is that we know systematically in all contexts if it is a snatched island margin whatever only a fraction of the material reaches the surface only a fraction of the mantle magma reaches to the surface and a large proportion of 50 to 80% is introduced into the crust, so if we take into account that the primary selection of PR Rite, which is dense, will accumulate here, this adds to the idea of ​​the accumulation, differentiation of these, the mantle is different, the thick stack differentiates into Bassel at the top Infertile Ultra mic accumulates at the base, so melting within the thick mic crust can produce voluminous granitic magmas.

I say no, I just go back, you know, and also the water is here, the water in the crust comes from the interaction with the sea water, but it is on the surface, it is not the base, the fusion within a thick crust cannot produce voluminous granitoids because the fertile material containing pests, which is said to be converted into eite to drag the material down, the mantle does not exist, is absent in the lower part of the crust and water. It's confined to lower, shallow levels, so that's the situation if you have a layer of crust, the lowest material is at the base and it's dry, but there's a chance that maybe the bottom crust is denser than the mantle, maybe it's richer in iron and if you have a dense layer of iron that builds up rich in iron, this could destabilize and form a diaper that will drag the material from the crust into the mantle where it could melt, so we look at this .

This is um uh Alberto Roman is a post in Paris. and he was working on this and we asked him to numerically model if this was possible if you had a crust that looks like this with a dense bottom layer, could the stabilization of this bottom layer to form a diaper pulled deep into the mantle? the mantle material from the top of the crust, so he made the models, so we have the time dimensionist, the depth dimensionist, this shows the clue of what happens with the lower crust; it descends through the mantle initially, dragging some of the crust down but then it uncouples and the material stays on the surface as expected and I was very happy that his model showed it reinforced my prejudices.

He did something else that is also interesting and this has to do with the water content. Could there be water in the b at the base? of a thick pile of magical material, so in the literature there is the idea called the heat pipe model. Heap heat pipe model, the idea that if lava flows erupted one after another Rel very quickly, the first lava flow would hydrate on top and then if another one arrived immediately after, it would bury the previous ones and, therefore, we see a lower temperature profile here. The temperature in the lower ones would stay quite low if the eruption were fast enough, so this works if you had a situation like This, the temperature in the lower hydrated lava flows could be relatively low and water could be retained, but If you take into account the fact that a large amount of magma is intruded, the magma is intruded adds heat to the lava to the pile of magic material and the temperatures inevitably rise so this is the situation as I see it. stagnant lid model does not work because the crust is differentiated the water is close to the surface the bottom is Olivine peroxin accumulates if some water penetrated one way or another deep into this pile perhaps along fractures or something like that, if this material was dragged deep into the mantle, what would happen?

Dehydration would release water that would simply move upward from hot rock to cold rock, from hot rock to cold rock and does not cause melting, so in my opinion any idea about the melting process or any SD duction model does not work, it cannot produce granitic magmas in volume, it cannot produce continental crust, on the other hand, contrast this with the process that we know well, which is subduction, subduction in which the ocean crust partially hydrates at the mid-ocean ridge , but predominantly when the slab bends and fractures as it descends the Sho, here it is then dehydrated.

Vol, large quantities of water move towards the mantal wedge, where melting is triggered to form the parent magmas. of the continental crust today and I think the same thing happened in Aran, but what about these arguments that I talked about before? The argument that the mantle was too hot to allow efficient subduction. This argument here. More recently, other people have looked at this question. This is a more recent paper by Gun and Fang that looks at the same type of rocks that Claude Herzberg looks at using the same type of method and they found numbers like this, potential temperatures that are not much higher than the modern mantle.

So this notion that the Aan mantle was significantly hotter than the modern mantle is not necessarily correct, but even if it were correct, I want to go back to this diagram here, the Fen Hunan modeling, and there is something important here: there seems to be modeling a There's a nice subduction zone here, but even at the highest temperature we seem to see the oceanic crust sinking towards the mantle not for long, not persistently, maybe not permanently, but transiently , hydrated oceanic crust sinks maybe 200 kilometers and that's all you need because Magma production in a modern arc occurs in the upper 100 kilometers or so, so that's how I see it.

The subduction may have been shallow, short and transient, but it is the only process that can produce large volumes of CR granitic material that forms the continental crust. This does not mean that there was plate tectonics. I have been a little hesitant when talking about sexual subduction and plate tectonics, they are not necessarily the same, the existence of a transient, shallow and impermanent subduction zone does not mean that there was a Much there was a large there was not a largely immobile lid that covered the rest of the planet. We could have had some kind of transitional Pro Zone, but the main point is that to form the continental crust the production must have been fine, so now we can draw preliminary conclusions.

I gave this talk atime elsewhere and when they saw the preliminary there was an audible groan from the audience saying, "There's more to come and I'm afraid there's a little more to come, but we'll talk quickly." above the main subduction, subduction cannot produce voluminous granite, most of the continental crust in the are now formed in subduction. There are concerns that subduction has coexisted with a partially immobile lid across the Aran, then, then, then, tront, I'm supposed to ask, check the time, about 40 minutes and in fact, it's exactly 40 minutes W. What's up with these changes? And what about the changes that seem to have occurred around 2.5 2.8 billion years ago?

John Dewey how he made this nice statement: The temporal distribution of almost everything shows a rapid change from 3 points in the period since 3 to 2 billion years ago, what happened then? Does it indicate a change in the geodynamic regime from the subduction regime to a previous subduction regime? I would say no, I would say that these changes are the consequence of several ongoing processes, continuous processes that simply reached some kind of Nexus at this time, the changes that took place in mantle temperature probably evolved along a curve like this one that, without scale, the volume of the ocean probably changed, the oceans where there is more water on the surface of the Aran Land than now simply because as the mantle cooled, the transport of hydrated material in the plates towards the mantle slowed down. becomes more efficient, so there are probably two now, there is more water in the mantle now than there was then, which means there was less water on the surface at the same time as the continent.

I would maintain that the crust was growing by subduction and the land surface was rising as the ocean level fell as the continents grew, the land had more and more land emerging, so these processes can explain most of the changes we saw, for example, the growth of continental crust takes progressively more granitic material at the surface at the site where the new crust grows, so if the new crust grows in a subduction environment, the Magnums of the mantle will assimilate a certain amount of pre-existing felic material and this increases potassium. The content of the later Granite, this assimilation process changes a lot, changes these proportions and explains many of the discontinuities that we see in the profiles, decreasing the volume of the ocean.

I mentioned that the emergence of land, once the land arises, you can get sediment, erosion, sedimentation formation. Descending detrital sediments are introduced into the site where pomegranate trees form and this also contributes to changes in composition. There are others that I rate because I don't really want to go into them because I want to focus on this last process that I think is interesting the idea that the continental crust did not grow regularly and instead there were episodes of rapid continental crust growth separated by periods with very little growth the evidence for this comes from again from ziron, not the hadan but the more modern ones, it is a compilation of uranium ages, Leed, ziron, zircon ages from various localities, the red curve shows ziron extracted from pomegranate trees, the green curves ex uh show the zircons in large rivers, total trital zerons in rivers and what we see is that there is no continuous distribution, there are large peaks separated by depressions 2.7 is the largest 2.5 is a real peak, It is common in India and China and is probably as big as this one, so how do we interpret these?

Again with geology there are different alternative schools they interpret different interpretations there is a school that I call creationists and the creationists think that this is a primary process driven by the conviction of the mantle the idea that the mantle does not convince regularly there are peaks in which mental convection accelerates for one reason or another, which directly leads to spikes in the content growth rate and I will explain why this might be the case. The alternative that conservationists point to points to calculations made in a modern subduction environment where the rate of continental addition and continental growth is approximately the same.

Look at these numbers, this is about the same as the rate of destruction of continental crust through sediment subduction or erosion at the base of the crust, so they maintain that in a normal subduction regime there is no net growth of the continental crust. continental crust they say that continental crust is only retained during continent-continent collision such as during the growth of supercontinents where zans are preserved in the interior of continents, so for them the peaks represent periods of enhanced preservation of continental crust , not periods of accelerated growth of the continental crust. The continental crust and its arguments look like this in this outrageous diagram with which they combined the two peaks at 2.5 2 2.7.5 and said that this corresponds to the supposed position of a supercontinent which they call hello canora Matt is Matt here there you are Matt , this is a diagram for you in which for some reason I have to tell you about this because for some reason you said that probably the younger Peaks do not correspond to the times of creation or destruction of the supercontinent, while the Yan ones do. true, I don't think so, well anyway, this is your diagram, the way I interpreted it is that there is no evidence of an arcane supercontinent, there is no evidence of two super continents here and instead another Waltera diagram at another time and these spots represent the periods of greatest continental growth The ages of zuran uranium zuran that don't really correspond to any credible time of supercontinent formation, but they correspond to the times of the great provinces of Ignus, so I think that's it the important association.

I will argue that the uranium peaks LED Zera ages represented in the creationist model accelerated mantle convection in the form of large mantle columns, as we see, for example, in the Cretaceous when onong Java maniki and hikurangi erupted around 120 million of years. coffen and elome did this very nice calculation, they calculated the volume of magma on the plateau they assumed 10% partial melting and they calculated the volume of the source the amount of material in the mantle that must melt to produce this volume of magma at the surface and it is enormous the volume of the mantle that must be involved in the formation of these plateaus extends into the lower mantle, the peaks in the are probably much larger than this, so with andava, she is from Paris, we developed a model supported by his analog experiments uh that said that during these times 2.7 2.5 billion years and so on a lot of material moved from the lower mantle to the upper mantle, kind of like the columns, this material heated the Upper mantle heated the upper mantle causing it to move more rapidly, but more important is the movement of material from the lower mantle. from the mantle to the upper mantle a return flow was required which is subduction, so basically our idea is simply that plumes accelerate subduction; the Aral periodic plumes correspond to peaks in the continental growth rate, so I would redraw this curve simply saying that the continental growth curve was probably something like this with the Exel growth spurt periods corresponding to the zura ages, so that's my final conclusion, subduction and formation of continental crust was episodic, particularly during the period from 2.7 to 1.8 billion years ago, thanks for your attention, okay, thanks Nick for a very very talk interesting.

So are there any questions, comments, agreements, disagreements? Yes, it must be someone Rader, of course, yes of course, so I wonder about this shallow subduction that you favor for the generation of continental crust and older granites. Would it then be the same as Ed, for example? by taros G and and for Venus I mean that it is a kind of subduction initiated by a column, so I think it is very different, there is no water on the surface of Venus, so you don't understand it, yes, even if you, but I want say, if if there was water if there was if there was water and you see the key, the key difference between what they call sagd duction and subduction is the inclination, if something moves down vertically, even if it contains water, the water just moves towards rocks colder and nothing will form.

What subduction works. Subduction works very well because you have this conveyor belt that transfers water deep into the mantle, where it moves up, into the hotter mantle, the hotter material and that below that with that. in that situation you can produce magma, so then essentially you infer a relatively extensive lateral transfer of oranic lithosphere, that is what yes, I think you require an oblique and transoblique transport of hydrated material towards the mantle, which is subduction and that project that produces Granitoid magma, parental magmas, any others there are. I would like to speak a little along the same lines.

You mentioned well the work of the Duram group and this initial subduction at 200 kilometers, if I remember correctly, that is actually their initial configuration. It's not a result of the model that's how they set it up because you need a subduction zone to drive subduction so that's not really a test in itself well back to we don't know what exactly starts that's why I was going to do it We have evidence that this is a system that can work well, although we don't necessarily understand it. No, I think it's you. I think it's important to distinguish between Long Life subduction situations where the oceanic crust goes deep into the mantle and the situation continues for a significant time, which is happening now and that's plate tectonics when you have lateral motion. of rigid eyelids and so on.

I don't think that probably hasn't happened in Aran, but it's not necessary if you have a bunch of shallow, transient subduction zones, that's all you need to form continental crust um um I just wanted to add something to this that I think one of the biggest questions also between suction and subduction is whether you can break the crust and the leosphere at the surface if you can create a brutal deformation because if something is very hot then it will deform rather viscous into a duct formation how fragile, so the question is did we have the conditions in the early Aran to break at the surface to create a deep slab, yes, well, you know, this is my, you know, I try to explain my reasoning, the continental crust is voluminous to to form large amounts of continental crust, you need to transport a lot of water into the mantle and this doesn't, I don't see how this would happen in any of the alternative models, you need this for one, it doesn't have to go down much, but the hydrated material has to go down obliquely towards the mantle to achieve this, no one here.

I have a microphone just to ask for your observations. I mean, you've seen a lot of green Aran stones and granite succession, so just to hear from you, many have contributed to modern plate subduction lines and things like that. You have seen something? as close to that in arcane green stones as anything that has a sheet of anything that looks like or not, no, no, but I don't think this is a problem because you know, remember, I showed you the thickness of the crust now probably in Aran the green stones aan the green the basalts and the green stone belts I think they are oceanic plateaus but they could be the upper part of that oceanic crust.

I know that an opalite is just what the thickest oolites are 6 kilometers thick, so in a modern opalite you can sample through the crust and down to the tectonite, the lithosphere, but cut through the top of a thick ocean de Aran will only sample Bassel and that's what you see, so in your model or creationist perspective is there some kind of secular variation throughout the 3.8 billion years of subduction since you have this apparently very intense period or? Do you see this recurring before and after? Here you know there don't seem to be any prominent peaks. until this first one in 2.7 there's a suggestion of something here, but I think it's just oversampling, you know, people like these old AR and Greenstone belts on Barberon etc, this is probably overrepresented, so no, I do not know.

I don't really have any reasonable explanation for why in my model, in our model, the mantle should have just operated continuously up to this stage and then suddenly started producing these pulses, the pulses that generated the spikes in the populations. of ciron, others, others. there in your recipe for the formation of continental lithosphere, so it needs, like basalt, it needs heat and water, how do we know that in the beginning there was more water than there is now and what are the limitations for the formationof the continental lithosphere? like the amount of water we had in our ear two things, you know, these Jack Hill zircons, the ancient detrital zircons are notable because they exist, we never expected to find anything from the hadyan, but we found them, they are zircons, they have trace element compositions that are Unlike zircons that crystallize from kimlit, etc., their trace element composition is that of a zircon that crystallizes from a felic granitic melt, but also by observing their temperature by using several geothermometers we found that they crystallized at relatively low temperatures. that can only be achieved if there was water at the source, so this argument is taken as evidence that there were already oceans on the surface of the Earth 4.4 billion years ago, so it seemed that there were oceans, then there is the qu This other question is how much water is distributed among the surface of the oceans and what proportion is present in various forms in the mantle and then this leads to the idea that the volume of the oceans has been reducing? decreasing as the temperature in the mantle decreases and as water can be transported more efficiently from the surface to the mantle, okay, I had a billion questions, but we can ask them at the bar later because now I got the signal that we have to stop okay thank you all for coming it was a good crowd and thanks again Nick for a great conference so thank you.