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Thread: The Origin and Evolution of Continental Crust

  1. #1 The Origin and Evolution of Continental Crust 
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    The division of the Earth's crust into two distinct types has been long established:
    • Oceanic crust: young, thin, basic composition, simple structure, primarily igneous
    • Continental crust: old, thick, acidic composition, complext structure, igenous, metamorphic and sedimentary
    Debate and research about how and when continental crust was formed and how it has changed over time is very active. The ability to make precise measurements of isotope ratios of individual minerals, or even layers within minerals has provided the raw data to revolutionise our understanding of the processes involved in crustal genesis.

    This thread is opened as a place to discuss some of the current hypotheses and uncertainties in the field. I shall post some overviews in the next few day, but in the meantime if anyone has any specific questions I shall do my best to answer them.


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    Some continental crust is very old so obviously formed early on in the Earth's evolution.
    There is still some continental crust being created, however the rate of creation today is quite slow.

    This is a subject where the uniformitarian principle doesn't apply. The present is NOT the key to the past.


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    So we stopped our discussion at the zircons do they really represent stones with granitic or better granitoidic composition? If you take a look at the greenstone belts, there are very basic stones, this is logical because of earth mantle higher temperature you get higher degrees of partial melting. Also we don't exactly know how subduction processes looked like in the archaean, the oldest found eclogite peaces are not older than 3,x Ga depending on which part on earth you are there are much older in greenland than in south africa so the mantle cooled not very similar down.
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    Quote Originally Posted by The Geographer View Post
    So we stopped our discussion at the zircons do they really represent stones with granitic or better granitoidic composition? If you take a look at the greenstone belts, there are very basic stones, this is logical because of earth mantle higher temperature you get higher degrees of partial melting. Also we don't exactly know how subduction processes looked like in the archaean, the oldest found eclogite peaces are not older than 3,x Ga depending on which part on earth you are there are much older in greenland than in south africa so the mantle cooled not very similar down.
    Could you start from the beginning?

    I'm sure this is relevant, but jumping into the discussion with a post like this makes the thread almost impossible to follow.

    I guess you're talking about the Jack Hills zircons, any particular paper? As for the rest of your post, I don't know where it's come from, far less where it's going.
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    Sorry, we already discussed here Where's all this Top Soil coming from? about melting up earth mantle, bowen's series...
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    Currently I am reading a new article in SCIENCE, which suggests the creation rate of continents is pretty steasy. What has changed (circa 3 GY ago) was caused by sebduction driven plate tectonics reworking the crust. The rate for was about 3 cu km/yr, after about 0.8 cu km/yr. This not being my area of expertise ("dammit Jim, I'm a meteoricist, not a geologist" ) I will have to finish reading it and rinse and repeat a few times to get as much as I can out of it.

    Science Volume 335 PP 1334-1336, Dhuime, et.al.

    Rats, took a long time for me to look up the reference in the "reading room", this is in reply to post #2
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    The picture I am starting from is one of an Earth with a "magma ocean", the magma ocean differentiates and in the process forms a crust.

    I guess a good starting question might be:

    Why does the crust come in two types?
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    Quote Originally Posted by billiards View Post
    The picture I am starting from is one of an Earth with a "magma ocean", the magma ocean differentiates and in the process forms a crust.
    I guess you are wrong the isotopic ratios of found zircons tell us that there had been even water on earth surface. From later days (perhaps 4,0 Ga I don't remember exactly) we can even say that the geothermic gradient wasn't that different from today... so maybe the crust was thicker. Another paper I hust found says "
    They further suggest that Earth had settled into a pattern of crust formation, erosion, and sediment recycling as early as 4.35 Ga.
    " Science 6 May 2005: Vol. 308 no. 5723 pp. 841-844 DOI: 10.1126/science.1110873
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    Yes, but I referenced an article from SCIENCE from 16 March 2012, 7 years later, including 6972 zircon analyses. It does not include the one you listed though. What's a billion years among friends?
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    I see the Geographer is challenging billiards on the conventional view that felsic crust could not form prior to ~4.0 Ga. I would agree that that position has been undermined. We have much contrary and ambiguous evidence, but there seems some possibility that the Earth was not as hellish as was thought in its very early history. I have a range of intersting paper that shed some light in this area. I'll try to pull a summary together in the next day or two.
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    Quote Originally Posted by MeteorWayne View Post
    Yes, but I referenced an article from SCIENCE from 16 March 2012, 7 years later, including 6972 zircon analyses. It does not include the one you listed though. What's a billion years among friends?
    where do I contradict the paper?

    Edit: I also said in my previous post something about that fundamental change in earth tectonic at 3,0 Ga

    Quote Originally Posted by John Galt View Post
    I see the Geographer is challenging billiards on the conventional view that felsic crust could not form prior to ~4.0 Ga. I would agree that that position has been undermined.
    not exactly there is a difference between saying there are existing zircons which are as old as 4,5 Ga and saying there was felsic crust as early as +4 Ga, as I already said the early continents should have been imho less felsic like the greenstone belts.

    Quote Originally Posted by John Galt View Post
    We have much contrary and ambiguous evidence, but there seems some possibility that the Earth was not as hellish as was thought in its very early history. I have a range of intersting paper that shed some light in this area. I'll try to pull a summary together in the next day or two.
    but here I agree
    Last edited by The Geographer; March 26th, 2012 at 02:47 AM.
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    Actually, rather than trying to summarise some of the research there is an excellent review paper from 2008.

    Harrison, T.M. The Hadean Crust: Evidence from >4Ga Zircons Annu.Rev.EarthPlanet.Sci.2009.37:479–505

    I have it on my hard drive, but I'll try to locate where I donwloaded it from. Harrison is very active in this field, but seems to come from one end of the spectrum on what zircons tell us. That said, while he makes his position clear in the review he also fully explores the alternatives.
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    Quote Originally Posted by The Geographer View Post
    Quote Originally Posted by billiards View Post
    The picture I am starting from is one of an Earth with a "magma ocean", the magma ocean differentiates and in the process forms a crust.
    I guess you are wrong the isotopic ratios of found zircons tell us that there had been even water on earth surface. From later days (perhaps 4,0 Ga I don't remember exactly) we can even say that the geothermic gradient wasn't that different from today... so maybe the crust was thicker. Another paper I hust found says "
    They further suggest that Earth had settled into a pattern of crust formation, erosion, and sediment recycling as early as 4.35 Ga.
    " Science 6 May 2005: Vol. 308 no. 5723 pp. 841-844 DOI: 10.1126/science.1110873
    So let's start further back. The planet accreted from a cloud of dust. It would have been very hot because it was being constantly bombarded by (in some cases very large) meteorites. So we have a great big ball of hot material. Are you disagreeing with me that it would have differentiated to form layers? Or is it the magma ocean you have a problem with?
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    Quote Originally Posted by MeteorWayne View Post
    Yes, but I referenced an article from SCIENCE from 16 March 2012, 7 years later, including 6972 zircon analyses. It does not include the one you listed though. What's a billion years among friends?
    Thanks for the paper by the way. I downloaded it last night but haven't found time to look at it yet, however it's by Dhuime who works with Hawkesworth who I believe are prominent figures in this science (Hawkesworth just picked up the prestigious Wollaston medal of the Royal Geological Society), whose work I am a little familiar with.

    I think wires may be getting crossed. I thought the Geographer was replying to me (not you) and so your post may be a stray bullet so to speak.
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    Quote Originally Posted by billiards View Post
    Quote Originally Posted by The Geographer View Post
    Quote Originally Posted by billiards View Post
    The picture I am starting from is one of an Earth with a "magma ocean", the magma ocean differentiates and in the process forms a crust.
    I guess you are wrong the isotopic ratios of found zircons tell us that there had been even water on earth surface. From later days (perhaps 4,0 Ga I don't remember exactly) we can even say that the geothermic gradient wasn't that different from today... so maybe the crust was thicker. Another paper I hust found says "
    They further suggest that Earth had settled into a pattern of crust formation, erosion, and sediment recycling as early as 4.35 Ga.
    " Science 6 May 2005: Vol. 308 no. 5723 pp. 841-844 DOI: 10.1126/science.1110873
    So let's start further back. The planet accreted from a cloud of dust. It would have been very hot because it was being constantly bombarded by (in some cases very large) meteorites. So we have a great big ball of hot material. Are you disagreeing with me that it would have differentiated to form layers? Or is it the magma ocean you have a problem with?
    ok if you go so far back you're right I thought we would rather speak about the time period between 4,5-3,0 Ga, (when imho most of the differentiation happened, but thats depend on the point of view, somebody like me says there had been nearly all layers evolved to something very similar like today)

    Edit: and yes it was the magma ocean I had a problem with
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    Quote Originally Posted by John Galt View Post
    I see the Geographer is challenging billiards on the conventional view that felsic crust could not form prior to ~4.0 Ga. I would agree that that position has been undermined. We have much contrary and ambiguous evidence, but there seems some possibility that the Earth was not as hellish as was thought in its very early history. I have a range of intersting paper that shed some light in this area. I'll try to pull a summary together in the next day or two.
    Was he? I didn't mention the composition of the crust at all. Perhaps everything is just going way over my head.
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    Quote Originally Posted by John Galt View Post
    Actually, rather than trying to summarise some of the research there is an excellent review paper from 2008.

    Harrison, T.M. The Hadean Crust: Evidence from >4Ga Zircons Annu.Rev.EarthPlanet.Sci.2009.37:479–505

    I have it on my hard drive, but I'll try to locate where I donwloaded it from. Harrison is very active in this field, but seems to come from one end of the spectrum on what zircons tell us. That said, while he makes his position clear in the review he also fully explores the alternatives.
    for those who have access to science Heterogeneous Hadean Hafnium: Evidence of Continental Crust at 4.4 to 4.5 Ga
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    Quote Originally Posted by The Geographer View Post
    Quote Originally Posted by billiards View Post
    Quote Originally Posted by The Geographer View Post
    Quote Originally Posted by billiards View Post
    The picture I am starting from is one of an Earth with a "magma ocean", the magma ocean differentiates and in the process forms a crust.
    I guess you are wrong the isotopic ratios of found zircons tell us that there had been even water on earth surface. From later days (perhaps 4,0 Ga I don't remember exactly) we can even say that the geothermic gradient wasn't that different from today... so maybe the crust was thicker. Another paper I hust found says "
    They further suggest that Earth had settled into a pattern of crust formation, erosion, and sediment recycling as early as 4.35 Ga.
    " Science 6 May 2005: Vol. 308 no. 5723 pp. 841-844 DOI: 10.1126/science.1110873
    So let's start further back. The planet accreted from a cloud of dust. It would have been very hot because it was being constantly bombarded by (in some cases very large) meteorites. So we have a great big ball of hot material. Are you disagreeing with me that it would have differentiated to form layers? Or is it the magma ocean you have a problem with?
    ok if you go so far back you're right I thought we would rather speak about the time period between 4,5-3,0 Ga, (when imho most of the differentiation happened, but thats depend on the point of view, somebody like me says there had been nearly all layers evolved to something very similar like today)

    Edit: and yes it was the magma ocean I had a problem with

    OK now we are clear that we were talking about slightly separate things.


    I realise this thread is about Continental Crust however I think if we are talking about the origins of continental crust we cannot avoid looking at early Earth differentiation.
    If we do not agree how the crust originally formed [i.e. did it crystallise out from a molten upper mantle? (Perhaps the picture is complicated by later stage magmatism, but I believe that is a less a question of origins and more a question of evolution.)] then we are likely to be at cross purposes from the very beginning and a very scrambled and difficult to follow sequence of ideas is likely to follow.

    I am for example surprised that the Geographer has a problem with the Magma Ocean stage in planetary evolution. I was not aware that it was controversial. Perhaps The Geographer does not believe that the moon was formed by a giant impactor? (which would have almost certainly made the earth a giant ball of melt)

    As a starting point Can everybody access this paper?

    Walter and Tronnes. Early Earth differentiation. Earth Planet Sc Lett (2004) vol. 225 (3-4) pp. 253-269

    http://www.norvol.hi.is/pdf/EPSL04-W...nesEE-diff.pdf

    3. Mantle differentiation in the Hadean
    Large-scale melting of Earth is unavoidable in the standard model of planetary accretion. Deposition of
    impact energy when a planetary embryo swallows up planetesimals during late-stage accretion would result in magma ocean formation. Repeated impacts could sustain a magma ocean especially in the presence of a blanketing atmosphere [54]. A giant collision of a dMars-sizedT impactor with proto-Earth is the prevail- ing theory for the origin of the Moon [9,11,12], and such an impact could deliver sufficient energy to melt the entire planet [13].
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    Quote Originally Posted by billiards View Post
    Quote Originally Posted by The Geographer View Post
    Quote Originally Posted by billiards View Post
    Quote Originally Posted by The Geographer View Post
    Quote Originally Posted by billiards View Post
    The picture I am starting from is one of an Earth with a "magma ocean", the magma ocean differentiates and in the process forms a crust.
    I guess you are wrong the isotopic ratios of found zircons tell us that there had been even water on earth surface. From later days (perhaps 4,0 Ga I don't remember exactly) we can even say that the geothermic gradient wasn't that different from today... so maybe the crust was thicker. Another paper I hust found says "
    They further suggest that Earth had settled into a pattern of crust formation, erosion, and sediment recycling as early as 4.35 Ga.
    " Science 6 May 2005: Vol. 308 no. 5723 pp. 841-844 DOI: 10.1126/science.1110873
    So let's start further back. The planet accreted from a cloud of dust. It would have been very hot because it was being constantly bombarded by (in some cases very large) meteorites. So we have a great big ball of hot material. Are you disagreeing with me that it would have differentiated to form layers? Or is it the magma ocean you have a problem with?
    ok if you go so far back you're right I thought we would rather speak about the time period between 4,5-3,0 Ga, (when imho most of the differentiation happened, but thats depend on the point of view, somebody like me says there had been nearly all layers evolved to something very similar like today)

    Edit: and yes it was the magma ocean I had a problem with

    OK now we are clear that we were talking about slightly separate things.


    I realise this thread is about Continental Crust however I think if we are talking about the origins of continental crust we cannot avoid looking at early Earth differentiation.
    If we do not agree how the crust originally formed [i.e. did it crystallise out from a molten upper mantle? (Perhaps the picture is complicated by later stage magmatism, but I believe that is a less a question of origins and more a question of evolution.)] then we are likely to be at cross purposes from the very beginning and a very scrambled and difficult to follow sequence of ideas is likely to follow.

    I am for example surprised that the Geographer has a problem with the Magma Ocean stage in planetary evolution. I was not aware that it was controversial. Perhaps The Geographer does not believe that the moon was formed by a giant impactor? (which would have almost certainly made the earth a giant ball of melt)
    no I just thought of a paper where was supposed that there had been oceans of water at 4,4 Ga I unfortunately found after a short search for this paper just found one were was mentioned that ths isotope ratios could be also gerenerated by a hydrothermal genesis of zircon, so there woulb be no ocean of water needed...

    now I found something ANU - OVC - MEDIA - MEDIA RELEASES - 2005 - NOVEMBER - 181105HARRISONCONTINENTS
    "“A new picture of early Earth is emerging,” Professor Harrison said. “We have evidence that the Earth’s early surface supported water – the key ingredient in making our planet habitable. We have evidence that this water interacted with continent-forming magmas throughout the Hadean."

    Harrison again...
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    there is a very huge research on the origin and the evolution of the constituent crust...i also discuss the theory with you that is The theory of plate tectonics is nowadays more or less universally accepted by geologists, and I have mentioned the basic idea briefly at the beginning of this class. The basic thought is, that instead of being permanent fixtures of the earth's surface, the continents and ocean basins undergo continuous change. Both are parts of lithosphere plates that move against each other, and in the process new crust is created at midoceanic ridges (spreading centers), and old crust is consumed at convergent plate boundaries.
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    Quote Originally Posted by einfopedia View Post
    ...and in the process new crust is created at midoceanic ridges (spreading centers), and old crust is consumed at convergent plate boundaries.
    both processes do not fit to continental crust, which is our focus, and like I allready mentioned nobody knows how subduction looked like before 3,x Ga
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    Quote Originally Posted by The Geographer View Post
    ...and like I allready mentioned nobody knows how subduction looked like before 3,x Ga
    what would it take to find out ?
    "Reality is that which, when you stop believing in it, doesn't go away." (Philip K. Dick)
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    The 3 billion year old man?
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    Quote Originally Posted by marnixR View Post
    Quote Originally Posted by The Geographer View Post
    ...and like I allready mentioned nobody knows how subduction looked like before 3,x Ga
    what would it take to find out ?
    a geological structure where we can see how it looked like (that would be the simplest way)
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    Very interesting discussion gents!

    A drilling rig on the Moon would give more insight.
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  27. #26 New paper, new insights 
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    There are new papers about the genesis of the first continental crust and about the moon, I summarized it:
    News about the moon:

    - The moon seems to be younger OR not to have been molten to 100%
    - The stones on the moon are have the same isotope ratios than those on earth, so maybe Theia was just some ice

    News about the first crust:

    The first crust seems to be a product of melting up MORB to 10-30%, the paper evidences that this could have happened, because of the hotter mantle in only 30-40 km depth, so no deep subduction in the hadean (perhaps even no subduction thinking of the missing hadean eclogites).

    Sources:
    Geology, 2012; DOI: 10.1130/G32729.1
    Nature Geoscience, 2012; doi:10.1038/ngeo1429
    Nature, 2011; DOI:10.1038/nature10328
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    Quote Originally Posted by The Geographer View Post
    There are new papers about the genesis of the first continental crust and about the moon, I summarized it:
    News about the moon:

    - The moon seems to be younger OR not to have been molten to 100%
    - The stones on the moon are have the same isotope ratios than those on earth, so maybe Theia was just some ice

    In the light of all evidence, Theia is an unnecessary hypothesis.

    Quote Originally Posted by The Geographer View Post
    The first crust seems to be a product of melting up MORB to 10-30%, ...
    This claim is contradicted by the paper you cite (Geology, 2012; DOI: 10.1130/G32729.1).
    They claim in the abstract that the first crust is a product of melting of mafic island arc crust, not MORB.


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    Quote Originally Posted by florian View Post
    Quote Originally Posted by The Geographer View Post
    There are new papers about the genesis of the first continental crust and about the moon, I summarized it:
    News about the moon:

    - The moon seems to be younger OR not to have been molten to 100%
    - The stones on the moon are have the same isotope ratios than those on earth, so maybe Theia was just some ice

    In the light of all evidence, Theia is an unnecessary hypothesis.

    How can the genesis of the moon be declared in your eyes?

    Quote Originally Posted by The Geographer View Post
    The first crust seems to be a product of melting up MORB to 10-30%, ...
    This claim is contradicted by the paper you cite (Geology, 2012; DOI: 10.1130/G32729.1).
    They claim in the abstract that the first crust is a product of melting of mafic island arc crust, not MORB.


    You are right, I just read a german popular scientific summary of this paper, respectively older papers (perhaps you noticed this "TTGs are widely seen as originating from melting of hydrated oceanic crust in subduction zones."). Though this answer isn't very satisfying cause it can't declare why there is no eclogite before 3.2 Ga while with melting up the subducted crust this would have been no problem.
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    Quote Originally Posted by The Geographer View Post
    How can the genesis of the moon be declared in your eyes?
    It could be a twin system, like Charon and Pluton, with the two partners evolving at different pace.
    Quote Originally Posted by The Geographer View Post
    Quote Originally Posted by The Geographer View Post
    The first crust seems to be a product of melting up MORB to 10-30%, ...
    This claim is contradicted by the paper you cite (Geology, 2012; DOI: 10.1130/G32729.1).
    They claim in the abstract that the first crust is a product of melting of mafic island arc crust, not MORB.
    You are right, I just read a german popular scientific summary of this paper, respectively older papers (perhaps you noticed this "TTGs are widely seen as originating from melting of hydrated oceanic crust in subduction zones."). Though this answer isn't very satisfying cause it can't declare why there is no eclogite before 3.2 Ga while with melting up the subducted crust this would have been no problem.
    Is "complete melting of overducted crust" and "arc magmatism", really incompatible? I don't think so.
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    Quote Originally Posted by florian View Post
    It could be a twin system, like Charon and Pluton, with the two partners evolving at different pace.
    possible

    Quote Originally Posted by florian View Post
    Is "complete melting of overducted crust" and "arc magmatism", really incompatible? I don't think so.
    Well I don't know what's exactly the difference but there seem to be differences, cause they are saying:

    TTGs are widely seen as originating from melting of hydrated oceanic crust in subduction zones. Alternative models argue that they may have formed by melting within thickened mafic oceanic protocrust...
    they are also saying melting up "arc tholeiits" also very strange cause tholeiits are not typical for arcs today
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    Quote Originally Posted by The Geographer View Post
    Quote Originally Posted by florian View Post
    Is "complete melting of overducted crust" and "arc magmatism", really incompatible? I don't think so.
    Well I don't know what's exactly the difference but there seem to be differences, cause they are saying:

    TTGs are widely seen as originating from melting of hydrated oceanic crust in subduction zones. Alternative models argue that they may have formed by melting within thickened mafic oceanic protocrust...
    they are also saying melting up "arc tholeiits" also very strange cause tholeiits are not typical for arcs today
    May be not today, but they used to.

    I understand that in the alternative model, the fluids released by the slab melted tholeiit basalts of the arc. Don't you?
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    Quote Originally Posted by florian View Post
    I understand that in the alternative model, the fluids released by the slab melted tholeiit basalts of the arc. Don't you?
    Yes but this is even happening today but look at greenstone belts and modern arcs there is a big difference (Ok you can say different tempretures different arcs, but in my eyes it's totally different). And it also doesn't fit to our eclogite evidence, you know if at first a relative normal (to modern circumstances) created arc would have been generated (with subduction) there should have been eclogites.
    Last edited by The Geographer; July 9th, 2012 at 04:55 PM.
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    Quote Originally Posted by The Geographer View Post
    And it also doesn't fit to our eclogite evidence, you know if at first a relative normal (to modern circumstances) created arc would have been generated (with subduction) there should have been eclogites.
    Eclogites must be brought back to the surface to be observed. May be they never came back to the surface at this time. Better mixing in a hotter mantle?
    Last edited by florian; April 30th, 2012 at 04:15 PM.
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    [QUOTE=florian;322328]
    Quote Originally Posted by The Geographer View Post
    Quote Originally Posted by florian View Post
    And it also doesn't fit to our eclogite evidence, you know if at first a relative normal (to modern circumstances) created arc would have been generated (with subduction) there should have been eclogites.
    Eclogites must be brought back to the surface to be observed. May be they never came back to the surface at this time. Better mixing in a hotter mantle?
    1. In a more homogenous mantle should be lesser convection, shouldn't be?
    2. I think it's very unlikely that there really existed eclogites cause even the eclogites from 3,2 Ga we have only persisted in diamonds.
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    Quote Originally Posted by The Geographer View Post
    2. I think it's very unlikely that there really existed eclogites cause even the eclogites from 3,2 Ga we have only persisted in diamonds.
    Do you mean that we should have evidence of >3.2 Ga eclogites in diamonds?
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    Quote Originally Posted by florian View Post
    Quote Originally Posted by The Geographer View Post
    2. I think it's very unlikely that there really existed eclogites cause even the eclogites from 3,2 Ga we have only persisted in diamonds.
    Do you mean that we should have evidence of >3.2 Ga eclogites in diamonds?
    yes, if there were eclogites we should have evidence
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    Quote Originally Posted by The Geographer View Post
    Quote Originally Posted by florian View Post
    Quote Originally Posted by The Geographer View Post
    2. I think it's very unlikely that there really existed eclogites cause even the eclogites from 3,2 Ga we have only persisted in diamonds.
    Do you mean that we should have evidence of >3.2 Ga eclogites in diamonds?
    yes, if there were eclogites we should have evidence
    I'm not sure it is always possible to show that something existed in the past. You have to be lucky to find evidence, especially knowing that samples become sparse going back in time.
    Last edited by florian; May 1st, 2012 at 06:28 AM.
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    Quote Originally Posted by florian View Post
    Quote Originally Posted by The Geographer View Post
    Quote Originally Posted by florian View Post
    Quote Originally Posted by The Geographer View Post
    2. I think it's very unlikely that there really existed eclogites cause even the eclogites from 3,2 Ga we have only persisted in diamonds.
    Do you mean that we should have evidence of >3.2 Ga eclogites in diamonds?
    yes, if there were eclogites we should have evidence
    I'm not sure it is always possible to show that something existed in the past. You have to be lucky to find evidence, especially knowing that samples become sparse going back in time.
    You forget that we really have samplings of the older mantle but there is peridotite in those diamonds not eclogite
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    Quote Originally Posted by The Geographer View Post
    Quote Originally Posted by florian View Post
    Quote Originally Posted by The Geographer View Post
    Quote Originally Posted by florian View Post
    Quote Originally Posted by The Geographer View Post
    2. I think it's very unlikely that there really existed eclogites cause even the eclogites from 3,2 Ga we have only persisted in diamonds.
    Do you mean that we should have evidence of >3.2 Ga eclogites in diamonds?
    yes, if there were eclogites we should have evidence
    I'm not sure it is always possible to show that something existed in the past. You have to be lucky to find evidence, especially knowing that samples become sparse going back in time.
    You forget that we really have samplings of the older mantle but there is peridotite in those diamonds not eclogite



    This links back to the plate tectonics thread. This work links the eclogite diamond evidence to the start of plate tectonics at 3 Ga ago. Further evidence for this is the change in chemistry and rate of formation of continental crust at 3 Ga.

    Shirey and Richardson. Start of the Wilson Cycle at 3 Ga Shown by Diamonds from Subcontinental Mantle. Science (2011) vol. 333 (6041) pp. 434-436

    The Mesoarchean assembly of the Kaapvaal craton, which hosts the majority of the diamonds reported here, resulted from the collision of two separate continental blocks—the Witwatersrand and Kimberley blocks—at 3.1 to 2.9 Ga (15, 16). This collision of two blocks of continental litho- sphere via the closure of an ocean basin, evident from the separate tectonothermal history of each block (16), is a good example of the latter stages (i.e., stages 5 and 6) of Wilson’s original defi- nition. Small percentages of Mesoarchean eclo- gite are widespread in the SCLM of the Kaapvaal craton (17), as are eclogitic silicate and sulfide inclusions in diamonds (Fig. 1) (17, 18). This provides compelling evidence that capture and incorporation of eclogite in the SCLM occurs via subduction and continental collision, and is linked to the deep subduction of fluids neces- sary for diamond formation.
    Don't bother visiting my Earth Sciences forum, it died a death due to lack of love
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    Quote Originally Posted by billiards View Post



    This links back to the plate tectonics thread. This work links the eclogite diamond evidence to the start of plate tectonics at 3 Ga ago. Further evidence for this is the change in chemistry and rate of formation of continental crust at 3 Ga.

    Shirey and Richardson. Start of the Wilson Cycle at 3 Ga Shown by Diamonds from Subcontinental Mantle. Science (2011) vol. 333 (6041) pp. 434-436

    The Mesoarchean assembly of the Kaapvaal craton, which hosts the majority of the diamonds reported here, resulted from the collision of two separate continental blocks—the Witwatersrand and Kimberley blocks—at 3.1 to 2.9 Ga (15, 16). This collision of two blocks of continental litho- sphere via the closure of an ocean basin, evident from the separate tectonothermal history of each block (16), is a good example of the latter stages (i.e., stages 5 and 6) of Wilson’s original defi- nition. Small percentages of Mesoarchean eclo- gite are widespread in the SCLM of the Kaapvaal craton (17), as are eclogitic silicate and sulfide inclusions in diamonds (Fig. 1) (17, 18). This provides compelling evidence that capture and incorporation of eclogite in the SCLM occurs via subduction and continental collision, and is linked to the deep subduction of fluids neces- sary for diamond formation.
    I remember that paper and the edito by van Kranendonk in the same issue. But what is true for the Kaapvaal craton is not necessary representative of the whole planet. What about the other cratons? I remember that paper by Furnes et al supporting arc-magmatism and overduction at 3.8 Ga in the Isua supracrustal belt.
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    Quote Originally Posted by florian View Post
    Quote Originally Posted by billiards View Post



    This links back to the plate tectonics thread. This work links the eclogite diamond evidence to the start of plate tectonics at 3 Ga ago. Further evidence for this is the change in chemistry and rate of formation of continental crust at 3 Ga.

    Shirey and Richardson. Start of the Wilson Cycle at 3 Ga Shown by Diamonds from Subcontinental Mantle. Science (2011) vol. 333 (6041) pp. 434-436

    The Mesoarchean assembly of the Kaapvaal craton, which hosts the majority of the diamonds reported here, resulted from the collision of two separate continental blocks—the Witwatersrand and Kimberley blocks—at 3.1 to 2.9 Ga (15, 16). This collision of two blocks of continental litho- sphere via the closure of an ocean basin, evident from the separate tectonothermal history of each block (16), is a good example of the latter stages (i.e., stages 5 and 6) of Wilson’s original defi- nition. Small percentages of Mesoarchean eclo- gite are widespread in the SCLM of the Kaapvaal craton (17), as are eclogitic silicate and sulfide inclusions in diamonds (Fig. 1) (17, 18). This provides compelling evidence that capture and incorporation of eclogite in the SCLM occurs via subduction and continental collision, and is linked to the deep subduction of fluids neces- sary for diamond formation.
    I remember that paper and the edito by van Kranendonk in the same issue. But what is true for the Kaapvaal craton is not necessary representative of the whole planet. What about the other cratons? I remember that paper by Furnes et al supporting arc-magmatism and overduction at 3.8 Ga in the Isua supracrustal belt.
    You're right. There is no doubt that the coming up of eclogite is resulting out of a mantle cooling down, how fast the mantle cooled down is even a matter of convection in the mantle though it's 3.8 Ga at the canadian Craton but 3.0 Ga at the South African.
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    There is a paper on the topic in the current issue of Nature: Hafnium isotope evidence for a transition in the dynamics of continental growth 3.2 Gyr ago

    They identify Different regimes of crust formation before 3.9 Ga, then from 3.9 to 3.5, they a transition to a "modern" regime from 3.5 to 3.2 Ga.
    Last edited by florian; June 13th, 2012 at 04:21 PM. Reason: changed the link
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    edit: your link doesn't run but I found it
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    Quote Originally Posted by florian View Post
    I understand that in the alternative model, the fluids released by the slab melted tholeiit basalts of the arc. Don't you?
    Yes but this is even happening today but look at greenstone belts and modern arcs there is a big difference (Ok you can say different tempretures different arcs, but in my eyes it's totally different). And it also doesn't fit to our eclogite evidence, you know if at first a relative normal (to modern circumstances) created arc would have been generated (with subduction) there should have been eclogites.
    There is a solution for this problem

    Under the higher geothermal gradients of the Archean, tectonically buried ocean crust would have been severely dehydrated before reaching eclogite facies pressures. Because rapid eclogitization is dependent on water as a medium for advective ion transport, the very shallow dehydration in the Archean may have inhibited the formation of eclogite facies minerals. The importance of water in eclogite metamorphism is illustrated by a complex of partly eclogitized mafic granulites in Holsnøy, western Norway, in which reaction progress was limited by the availability of water. When water is scarce or absent, metastable granulite facies mineral assemblages can persist at eclogite facies depths owing to the extremely slow reaction kinetics when diffusion is the only chemical transport mechanism. Such dehydrated but uneclogitized mafic crust would have been very strong and too buoyant to sink into the mantle, and it may have formed the substrate for the first continental lithosphere.

    Source: M.G. Bjørnerud and H. Austrheim: Inhibited eclogite formation: The key to the rapid growth of strong and buoyant Archean continental crust. Geology,September, 2004, v. 32, p. 765-768, doi:10.1130/G20590.1]

    now I understood it
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    Currenty I'm reading Taylor: Planetary crusts he is saying that there are primary crusts like KREEP secondary crusts like oceanic crust (maybe he even calls komatiitic oceanic crust secondary) and tertiary crusts with the only example earths continental crust
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    Quote Originally Posted by florian View Post
    I remember that paper and the edito by van Kranendonk in the same issue. But what is true for the Kaapvaal craton is not necessary representative of the whole planet. What about the other cratons? I remember that paper by Furnes et al supporting arc-magmatism and overduction at 3.8 Ga in the Isua supracrustal belt.
    I just realised that the paper missed my point overthrusting and subduction are different things maybe there had been overthrusting but no deep subduction...just because there is no eclogite.
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