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Thread: Do mantle plumes exist?

  1. #1 Do mantle plumes exist? 
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    This is a nice current issue in the Earth sciences so I thought I'd bring it up. From what I've read there are some serious doubts as to the validity of the mantle plume hypothesis. It was initially proposed to explain "hot spots" like Hawaii, where there is volcanism that is not (at first glance) explained by plate tectonics.

    Some seismologists have claimed they have imaged the plumes, but others have discarded their claims; the data is noisy and depends upon certain estimated values for some variables. As far as I am aware the most current data from the geochemists is now begining to mount up against the plume theory.

    Having said that, the plume theory is quite simple and to me at least seems intuitively elegant enough that it could just be right, especially when viewed in conjunction with the experimental evidence of fluid mechanics showing that thermal and compositional convection can lead to plume structures developed at interfaces.


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    An alternative theory would also have to explain why 'hot spot' islands are often found in a line, corresponding to the direction of plate movement.

    Btw, do you know whether there are acceptable alternatives to the mantle plume hypothesis?


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    Check this out

    http://bowfell.geol.ucl.ac.uk/%7Elid.../frederik6.pdf

    It links a chain of volcanoes to lithospheric fracturing in response to plate flexure during subduction.
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    Mantle plumes do exist. I'm almost certain of it. The seismic tomography is pretty good according to the people who know what they're talking about although it would be nice if the data sets were more determined. Plus if the spin state of iron in perovskite is high - which it is unless you get really deep (and it might even be high at the very base of the mantle in the post-perovskites) - convection is the dominant mode of heat transfer. If you've got convection then you've got plumes right?
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    Have you been to www.mantleplumes.org ? Its a pretty good resourse in presenting the case against their existance. Some very interesting reading.
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    My undergrad 4th year project was on this subject, really around the time this was all starting (2004). I have occasionally looked back into it.... so on that note, from memory (I appologise for an inaccuracies), many of the images which were being carried out around that time did show little shallow structures closer to the surface, nothing like the large plumes from the deeper mantle which are the 'typical image'.

    I do not know what has been determind since I wrote my paper however, I do recall that there was one set of images that were being carried out around the time of my paper (Hawaii I think), and so these results would obviously have some impact on more recent findings on this issue

    I shall try to get hold of my original report again as I would like to follow up my initial research on this issue.

    Is this uncertainty of plumes being brought up in university lectures now?
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    I'm doing an undergrad project on it too! Still in progress though, so I'm reserving any judgement 'til the end.
    Look up Montelli et al and 'finite frequency tomography' (I'm thinking 2004, but I'm not sure) for a paper which claims to have imaged numerous plumes down to great depths. If you can't find it let me know and I'll get you the reference when my notes are nearby.
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    i graduated in 2004.... so I only was able to use resources until approximately the march (though I'm really not sure about this!).... I can send you my references as I know there were many including images of Iceland as well as other 'typical mantle plumes'. I think it was Foulgar who had either written about this or done imaging but please do not quote me on that!

    I have a slight plan on trying to collate more recent data to add to my exising report (2004) and seeing what happens with it... although I really have not kept up with the debate so it may not happen!

    Thanks for the reference... if you need any help give me a shout (I will try to track down my paper/research!)
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    More details here... :-D
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    Quote Originally Posted by lynette_mac
    i graduated in 2004.... so I only was able to use resources until approximately the march (though I'm really not sure about this!).... I can send you my references as I know there were many including images of Iceland as well as other 'typical mantle plumes'. I think it was Foulgar who had either written about this or done imaging but please do not quote me on that!

    I have a slight plan on trying to collate more recent data to add to my exising report (2004) and seeing what happens with it... although I really have not kept up with the debate so it may not happen!

    Thanks for the reference... if you need any help give me a shout (I will try to track down my paper/research!)
    Some references would be appreciated, thanks! If you like, I'll gather some recent ones for you.
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    -i heard somewhere (a theory) that a mantle plume was the reason for the end permian mass extinction, consisting of flood basalt. Does that make sense?
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    Mantle plumes have been hypothesised they have even been demostrated in 'hot oil tanks'. I think there is a good chance they do.
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    Quote Originally Posted by Megabrain
    Mantle plumes have been hypothesised they have even been demostrated in 'hot oil tanks'. I think there is a good chance they do.
    Demonstrating that convection occurs is hardly groundbreaking news.
    The Deccan traps are often cited as a typical product of a mantle plume. How do you account for the observations of Hetu Sheth who, noting the absence of any pre-volcanic regional doming; a normal lithospheric thickness; and a major post volcanic uplift, asserts that the Deccan traps, and associated features, can be accounted for purely by continental rifting?
    Or, how do you account for the absence of most diagnostic features for mantle plumes from most of the alleged instances of mantle plumes?

    Nice to see Matt and lynette involved. Looks like you know this subject area quite well.

    Johnny, the cause(s) of the end Permian mass extinction is still the subject of debate. Eruption of large volumes of plateau lavas with the consequent impact on global climate is one of the principal contenders. Such eruptions are often attributed to mantle plumes (but see my remarks on the Deccan traps, above). So, the idea that a mantle plume was responsible for the end Permian extinction is plausible, perhaps even possible, but almost certainly not probable.
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    Quote Originally Posted by Megabrain
    Mantle plumes have been hypothesised they have even been demostrated in 'hot oil tanks'. I think there is a good chance they do.
    I'm not sure that the fact that hot oil convects counts as proof that the mantle does. These kinds of models don't account for the massive pressure differences between the top and bottom of the mantle for one thing.

    Quote Originally Posted by Ophiolite
    The Deccan traps are often cited as a typical product of a mantle plume. How do you account for the observations of Hetu Sheth who, noting the absence of any pre-volcanic regional doming; a normal lithospheric thickness; and a major post volcanic uplift, asserts that the Deccan traps, and associated features, can be accounted for purely by continental rifting?
    I'd not seen this article yet. Thanks for bringing it to my attention.


    Lynette- here is the article I was talking about if you've not found it already: http://www.sciencemag.org/cgi/reprint/303/5656/338.pdf

    And here's another: http://www.eas.slu.edu/People/KKoper....epsl.2006.pdf
    Check out the figures of the imaged plumes- they're often not narrow, nor vertical, and sometimes barely even hot! Not exactly the features proposed by Morgan eh?
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    -geology is cool 8)
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    Quote Originally Posted by Ophiolite
    Quote Originally Posted by Megabrain
    Mantle plumes have been hypothesised they have even been demostrated in 'hot oil tanks'. I think there is a good chance they do.
    Demonstrating that convection occurs is hardly groundbreaking news.
    The Deccan traps are often cited as a typical product of a mantle plume. How do you account for the observations of Hetu Sheth who, noting the absence of any pre-volcanic regional doming; a normal lithospheric thickness; and a major post volcanic uplift, asserts that the Deccan traps, and associated features, can be accounted for purely by continental rifting?
    Or, how do you account for the absence of most diagnostic features for mantle plumes from most of the alleged instances of mantle plumes?

    Nice to see Matt and lynette involved. Looks like you know this subject area quite well.

    Johnny, the cause(s) of the end Permian mass extinction is still the subject of debate. Eruption of large volumes of plateau lavas with the consequent impact on global climate is one of the principal contenders. Such eruptions are often attributed to mantle plumes (but see my remarks on the Deccan traps, above). So, the idea that a mantle plume was responsible for the end Permian extinction is plausible, perhaps even possible, but almost certainly not probable.
    Your reply seems to suggest I have a definite opinion, if you read the whole of post you will see this is not so.
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    Quote Originally Posted by Megabrain
    Your reply seems to suggest I have a definite opinion, if you read the whole of post you will see this is not so.
    I am sorry if I gave you that impression. I was seeking to point out two things:
    1) Citing lab experiments that show us convection is a real process, when we all learn this in Physics 101, is fatuous in the extreme.
    2) You stated
    "Mantle plumes have been hypothesised they have even been demostrated in 'hot oil tanks'. I think there is a good chance they do."
    If you think there is such a good chance, how do you account for the points I raised.
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    THe experiment I refer to showed clearly distinct hexagonal columns of oil each apparently self contained. This was sufficiently different from my understanding of convection for me to note.

    I have seen on sevral occasions the plume theory put forward and would not be surprised if something along those lines was eventually found. If you have evidence that mantle plumes are impossible perhaps you might share it.
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    Demonstrating that convection can occur in an oil doesn't demonstrate that it does occur in the mantle. If you have any evidence to prove that it does occur, perhaps you might share that too.
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    Matt,

    I have no evidence for or against, but like all theories one hears of you assess it against that which you know, from this you decide if it seems plausible, to me it does, at least for the moment. I neither support nor deny it.
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    Fair enough. It seemed from your post that perhaps you'd made your mind up, but I'm glad to hear you're keeping an open mind to other explanations.
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    The Deccan traps are often cited as a typical product of a mantle plume
    Deccan traps consist of flood basalt, and we have alot of that on earth, right? Anyways they said according to wiki that floodbasalts have been confirmed on the moon.
    http://en.wikipedia.org/wiki/Flood_basalt

    thats kinda cool, mabye mantle plumes were existent on the moon when it was geologically active :-D
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    Can anyone recommend some literature for my project? I'm finding no shortage of material supporting plate tectonic processes (lithospheric cracking, mantle fertility etc) as a cause of hotspots, but I've read a lot less (recent) material supporting plumes, and I'm afraid this will result in quite a bit of accidental bias. I've been relying quite heavily on Coutillot et al (2003) 'Three distinct types of hotspots in the earth's mantle', but I'd quite like to read some other views, as I'm not sure how representative their ideas are of current thinking in terms of plumes. Can anyone recommend some reading that will provide an up to date review of the theory of mantle plumes or any other key (recent) literature?
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    Since I made this thread what seems like years ago I have sided strongly with the pro-plume people. I believe that they can originat from the D'', or the cor-mantle boundary (CMB), but they may also start at shallower depths too. #

    I've seen some good geophysical work on constraining earth's electrical conductivity profile (a proxy for temp), cuta long story short, it seems that the RAyleigh number down there should be high enough for convection to occur (I'm assuming all the work done by those 'dead 19th Century physicsts' still stands). Plus the seismology, Montelli et al. yes, althouggh I'm biased as I'm friends with a seismologist at Princeton.

    The geochemists (I hate anything chemistry reelated so apologize if this is wrong) tell us that the MORB basalts are depleted in incompatible elements such as Th, U so they must be derived from a shallow reservoir (so maybe there aren't plumes beneath mi-ocean ridges?). Whilst OIB are enriched, so perhaps they came from deeper down (looks like there's no argument against plumes here). Looks to me like the mantle is just ambiently rising beneath the the mid-ocean ridge to conserve the mass balance as the crust is subducted... But then how come the shallow reservoir hasn't been used up?

    Bercovici & Karato suggested a filter, that removed incompatible elements from upwelling mantle to keep the reservoir full up. That's a very interesting idea (HInt to the person who needed references:PM me if you want to know more), becasue it can tie together geophysical and geochemical constraints, the problem is that it's probably wrong...
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    Careful Billiards (welcome back) - I got lambasted for suggesting I was 'leaning towards' this as a possibility.
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    Quote Originally Posted by billiards
    I've seen some good geophysical work on constraining earth's electrical conductivity profile (a proxy for temp),
    I don't suppose you have a reference handy do you? Sounds interesting!

    The geochemists (I hate anything chemistry reelated so apologize if this is wrong) tell us that the MORB basalts are depleted in incompatible elements such as Th, U so they must be derived from a shallow reservoir (so maybe there aren't plumes beneath mi-ocean ridges?). Whilst OIB are enriched, so perhaps they came from deeper down (looks like there's no argument against plumes here).
    I'm not too hot on geochemistry either. Only just do i figure out the helium ratios arguement, and I find this: click me. Looks like there are alternative geochemical models too.
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    Quote Originally Posted by johnny
    The Deccan traps are often cited as a typical product of a mantle plume
    Deccan traps consist of flood basalt, and we have alot of that on earth, right? Anyways they said according to wiki that floodbasalts have been confirmed on the moon.
    http://en.wikipedia.org/wiki/Flood_basalt

    thats kinda cool, mabye mantle plumes were existent on the moon when it was geologically active :-D
    Or maybe plumes can be caused by giant meteorite impacts. This is certainly an idea, and could work on the moon as well as the earth.

    As for plumes in general, I don't think there is any major debate about their existance (there are a few nut cases who will swear against them), afterall we know we have convection (at least in the asthenosphere) so it kind of follows that we have plumes. The debate is whether we have layered or whole-mantle convection; I personally think we have a bit of both. OIB (ocean island basalts) come from a different reservoir than MORB (mid-ocean ridge basalts), at least that's what geochemistry infers; I think the OIBs (such as the Hawaii basalts) are sourced from the lower mantle, which is good enough evidence for me to be happy to say that there must at least be some whole mantle convection. Of course, if it turned out that there were heterogeneous pockets in the upper mantle, which act as individual reservoirs then I'd be wrong. However, such a hypothesis brings with it various problems; and anyway, you still need some kind of plume to convect the shallow material to the surface. I am yet to see a convincing argument against plumes.
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    I've finished by project now, so I'll add a few thoughts.

    3He:4He ratios do not seem to be a proxy for depth. A number of MORB samples show ratios well into the levels deemed evidence for a deep mantle origin, and quite a lot of hotspots show low or MORB-like ratios. There doesn't seem to be a correlation between the starting depth of the anomalies found by Montelli et al and helium ratios. A number of other problems have been pointed out, but these (especially the last) seem to be the most significant.

    Flood basalts seem too readily connected to present hotspots. The Large Igneous Provinces Commission deem volcanism 100,000 km^2 or km^3 in extent enough evidence to confidently associate a LIP with a plume head, even if no associated volcanic track exists. There is also a problem with the lack of uplift and swelling associated with LIPs. Neither the Deccan nor the Ontong-Java show any, where surely if a plume were to blame, uplift/swelling to the same or to a greater extent as on the volcanic chains would occur.

    The tomographic images of Montelli et al are of high enough resolution to image plume conduits and yet have found no plume heads, even in rising plumes that have not yet reached the surface. Either their method is flawed (in which case, how reliable are their conclusions? Does this constitute evidence for plumes?), or plumes do lack heads. If the latter is the case, firstly, how do LIPs form, and secondly how reliable are all the computer models of plumes that clearly show the formation of plume heads during upwelling?

    The images also show plumes forking at depth. The Azores, Canary and Cape Verde hotspots are all fed by the same anomaly if their images are right. As far as I'm aware this has not been predicted by the plume model, so how can this occur?

    I still find it hard to reconcile the lack of any heatflow anomaly at many hotspots, particularly where some geothermometers suggest much cooler temperatures than assumed.


    I don't believe that there is enough evidence either way to confirm the existence or non-existence of plumes, but I think that we really need to question what exactly a plume is, and what would constitute evidence for them.
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    Matt. Thanks for the update. It's an interesting topic (at least to me.)
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    Quote Originally Posted by The Matt
    3He:4He ratios do not seem to be a proxy for depth. A number of MORB samples show ratios well into the levels deemed evidence for a deep mantle origin, and quite a lot of hotspots show low or MORB-like ratios. There doesn't seem to be a correlation between the starting depth of the anomalies found by Montelli et al and helium ratios. A number of other problems have been pointed out, but these (especially the last) seem to be the most significant.
    I don't see why this has to be a problem. Sure it's a problem if you're trying to work out why the 3He:4He ratio doesn't match the depth of the plume, but it doesn't put the actual existence of plumes into question.

    Flood basalts seem too readily connected to present hotspots. The Large Igneous Provinces Commission deem volcanism 100,000 km^2 or km^3 in extent enough evidence to confidently associate a LIP with a plume head, even if no associated volcanic track exists. There is also a problem with the lack of uplift and swelling associated with LIPs. Neither the Deccan nor the Ontong-Java show any, where surely if a plume were to blame, uplift/swelling to the same or to a greater extent as on the volcanic chains would occur.
    Well who says all LIPs must be associated with plumes? There is some good work done by people like Prof. Adrain Jones arguing that meteorite impacts may have caused some LIPs. In fact the very case studies they present are the Deccan Traps and the Ontong Java Plateau.

    The tomographic images of Montelli et al are of high enough resolution to image plume conduits and yet have found no plume heads, even in rising plumes that have not yet reached the surface. Either their method is flawed (in which case, how reliable are their conclusions? Does this constitute evidence for plumes?), or plumes do lack heads. If the latter is the case, firstly, how do LIPs form, and secondly how reliable are all the computer models of plumes that clearly show the formation of plume heads during upwelling?
    Well the shallowest data presented shows a concentric slice through the mantle at 1000 km depth (from what I remember). I cannot imagine you'd have plume heads stretching down quite that deeply, I don't really know, but I'd imagine plume heads would occupy LVZ depths (~ 50 - 200 km).

    The images also show plumes forking at depth. The Azores, Canary and Cape Verde hotspots are all fed by the same anomaly if their images are right. As far as I'm aware this has not been predicted by the plume model, so how can this occur?
    Probably just an artifact of the data.

    I still find it hard to reconcile the lack of any heatflow anomaly at many hotspots, particularly where some geothermometers suggest much cooler temperatures than assumed.
    Code:
    Hotspots are characterized by higher temperature,
    topographic swells, and recent volcanism
    with isotopic signatures distinct from those that
    characterize mid-ocean ridge or andesitic basalts.
    That's the first sentence of the Montelli et al. paper, are they lying?

    I don't believe that there is enough evidence either way to confirm the existence or non-existence of plumes, but I think that we really need to question what exactly a plume is, and what would constitute evidence for them.
    I reckon that seismic tomography is very convincing, plumes have been imaged, the Hawaii plume which is clearly seen coming all the way from the lowermost mantle
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    Quote Originally Posted by billiards
    Quote Originally Posted by The Matt
    3He:4He ratios do not seem to be a proxy for depth. A number of MORB samples show ratios well into the levels deemed evidence for a deep mantle origin, and quite a lot of hotspots show low or MORB-like ratios. There doesn't seem to be a correlation between the starting depth of the anomalies found by Montelli et al and helium ratios. A number of other problems have been pointed out, but these (especially the last) seem to be the most significant.
    I don't see why this has to be a problem. Sure it's a problem if you're trying to work out why the 3He:4He ratio doesn't match the depth of the plume, but it doesn't put the actual existence of plumes into question.
    You're inferring meaning that wasn't there. I never said that this counted as evidence against mantle plumes. I merely think that authors should not count this as evidence for them like Courtillot et al (2003)* do.

    *http://www.mantleplumes.org/WebDocum...tillot2003.pdf
    Flood basalts seem too readily connected to present hotspots. The Large Igneous Provinces Commission deem volcanism 100,000 km^2 or km^3 in extent enough evidence to confidently associate a LIP with a plume head, even if no associated volcanic track exists. There is also a problem with the lack of uplift and swelling associated with LIPs. Neither the Deccan nor the Ontong-Java show any, where surely if a plume were to blame, uplift/swelling to the same or to a greater extent as on the volcanic chains would occur.
    Well who says all LIPs must be associated with plumes? There is some good work done by people like Prof. Adrain Jones arguing that meteorite impacts may have caused some LIPs. In fact the very case studies they present are the Deccan Traps and the Ontong Java Plateau.
    Thats precisely my point. It seems to be the commonly held belief* that LIPs are formed by plume heads despite the fact that there are other alternative models.
    *http://www.earth.cf.ac.uk/people/summaries/116.LIP.htm
    *http://www.ldeo.columbia.edu/~polsen/nbcp/lipmc.html


    The tomographic images of Montelli et al are of high enough resolution to image plume conduits and yet have found no plume heads, even in rising plumes that have not yet reached the surface. Either their method is flawed (in which case, how reliable are their conclusions? Does this constitute evidence for plumes?), or plumes do lack heads. If the latter is the case, firstly, how do LIPs form, and secondly how reliable are all the computer models of plumes that clearly show the formation of plume heads during upwelling?
    Well the shallowest data presented shows a concentric slice through the mantle at 1000 km depth (from what I remember). I cannot imagine you'd have plume heads stretching down quite that deeply, I don't really know, but I'd imagine plume heads would occupy LVZ depths (~ 50 - 200 km).
    They reported new plumes rising through the mantle that were still at depth which also didn't show heads. I'd also figured that the stacked cross sections were just an easy method of displaying the data. I've seen another study using the same techniques that show vertical cross sections, so I see no reason why montelli et al's data would only be available to them at the increments shown.

    The images also show plumes forking at depth. The Azores, Canary and Cape Verde hotspots are all fed by the same anomaly if their images are right. As far as I'm aware this has not been predicted by the plume model, so how can this occur?
    Probably just an artifact of the data.
    Based on? They mention issues with resolution with another anomaly that appears to show this behavior, but seem pretty confident about this one...

    Code:
    Hotspots are characterized by higher temperature,
    topographic swells, and recent volcanism
    with isotopic signatures distinct from those that
    characterize mid-ocean ridge or andesitic basalts.
    That's the first sentence of the Montelli et al. paper, are they lying?
    Or maybe mistaken.
    Try reading the following:
    Von Herzen, R.P., Cordery, M.J., Detrick, R.S. & Fang, C. (1989) Heat Flow and the thermal origin of hotspot swells: The Hawaiian swell revisited. Journal of Geophysical Research 94 B10 pp. 13783-13799

    Stein, C. & Stein, S. (2003) Mantle plumes: Heat flow near Iceland. Astronomy & Geophysics. 44 pp. 1.8-1.10

    Bonatti, E. (1990) Not so hot “hot spots” in oceanic mantle. Science 250 pp.107-111

    Anderson, D.L., (2000) The statistics and distribution of helium in the mantle. International Geology Review, 42, pp. 289-311.

    Meibom, A., Anderson, D.L., Sleep, N.H., Frei, R., Chamberlain, C.P., Hren, M.T., & Wooden, J.L. (2003) Are high 3He/4He ratios in oceanic basalts an indicator of deep-mantle plume components?, Earth and planetary Science Letters. 208, pp.197-204
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    Quote Originally Posted by Ophiolite
    Matt. Thanks for the update. It's an interesting topic (at least to me.)
    It is for me too. I'm going to try to keep an eye on how it progresses even though the obligation is now over.
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    The Matt, I think we might have our wires crossed, I am aware of the scope of the debate (although I conceed that I don't appear to have researched it quite as thoroughly as your goodself). I am quite firmly pro-plume in my stance, I am willing to argue my case, however I cannot do this if you are only throwing references at me without telling me where you stand on the matter. In my previous post I was simply countering arguments on the assumption that you were challenging the existance of plumes, which it seems you were not.

    Cheers
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    Sorry, you're right- I haven't made my stance clear.

    I'm actually not convinced either way at the moment. Tomography has given some interesting images, though there are some issues with the technique that sadly are beyond my understanding, so I can't really comment further.

    My main problem is the over-simplified statements and generalisations just like the one you quoted from Montelli et al. Hotspots are supposed to be hot, and yet I've not seen a great deal of evidence for this. Neither Hawaii nor Iceland exhibit high heat flow, and geothermometers suggest no anomalously high temperature at the Azores. Work has also been done which seems to suggest that isotope ratios (particularly of helium) are not significantly different from MORB distributions, and are largely a result of degassing processes rather than original composition of mantle. Despite this people still view hotspots as "characterized by higher temperature, topographic swells, and recent volcanism with isotopic signatures distinct from those that characterize mid-ocean ridge or andesitic basalts"


    I believe that we really need to question what constitutes evidence of a plume, and what constitutes evidence of a lack, as people do seem to think plumes exist based to an extent on things that are either not backed up by the data (ie. helium isotopes, heat flow) or by things that could easily be explained by other mechanisms (ie. flood basalts, age progressive tracks). I've come up with a few criteria that i believe could be used, however there seems to be insufficient data to apply these to many hotspots.

    As for alternative theories, many of them are interesting and certainly seem feasible, but some do not seem satisfactory. Propagating cracks and reactivated faults have been used to explain some hotspots in the pacific for example, but I don't see how these could produce a 1200m swell. The impact hypothesis for LIPs has also been criticised, though i can't say on what grounds off the top of my head.
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  36. #35  
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    Quote Originally Posted by The Matt
    My main problem is the over-simplified statements and generalisations just like the one you quoted from Montelli et al. Hotspots are supposed to be hot, and yet I've not seen a great deal of evidence for this. Neither Hawaii nor Iceland exhibit high heat flow, and geothermometers suggest no anomalously high temperature at the Azores.
    Funny that you should pick on Iceland. Not sure we have a plume at Iceland, although the Montelli et al. image a shallow plume-like structure where you can clearly see a broadening -ve wave speed anomaly towards the surface, as would be expected from a plume head type feature. It's not the strongest candidate.

    Hot spots is perhaps a misnoma, I have seen some work by N. Sleep that says that "most of the heat from hotspots is implaced in the lower lithosphere and later subducted." So according to this guy's calculations, plumes do not necessarily have to be associated with +ve heat flow anomalies.

    Work has also been done which seems to suggest that isotope ratios (particularly of helium) are not significantly different from MORB distributions, and are largely a result of degassing processes rather than original composition of mantle. Despite this people still view hotspots as "characterized by higher temperature, topographic swells, and recent volcanism with isotopic signatures distinct from those that characterize mid-ocean ridge or andesitic basalts"
    From my understanding it is the enrichment in incompatible elements which distinguishes the OIB from the MORB (a simple geophysicst's view on things). Topographic swells are hard to refute, they exist.

    As for alternative theories, many of them are interesting and certainly seem feasible, but some do not seem satisfactory. Propagating cracks and reactivated faults have been used to explain some hotspots in the pacific for example, but I don't see how these could produce a 1200m swell. The impact hypothesis for LIPs has also been criticised, though i can't say on what grounds off the top of my head.
    The fact is that these different mechanisms are certainly not mutually exclusive. In that sense they are not alternative theories. The existance of one phenomena does not disprove the existance of another.
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  37. #36  
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    Quote Originally Posted by billiards
    Funny that you should pick on Iceland. Not sure we have a plume at Iceland, although the Montelli et al. image a shallow plume-like structure where you can clearly see a broadening -ve wave speed anomaly towards the surface, as would be expected from a plume head type feature. It's not the strongest candidate.
    I use Iceland because it seems to be the most frequently used example (after Hawaii) of a hotspot formed by a deep mantle plume.


    Hot spots is perhaps a misnoma, I have seen some work by N. Sleep that says that "most of the heat from hotspots is implaced in the lower lithosphere and later subducted." So according to this guy's calculations, plumes do not necessarily have to be associated with +ve heat flow anomalies.
    That makes sense. The only problem is that I have also seen mineralogical data suggesting magma at cooler temperatures than expected melting due to the presence of volatiles lowering the melting point. This was at the Azores, which Montelli et al have imaged a plume beneath. I wonder how accurate these geothermometers are.

    From my understanding it is the enrichment in incompatible elements which distinguishes the OIB from the MORB (a simple geophysicst's view on things). Topographic swells are hard to refute, they exist.
    I'll take your word on the OIB/MORB. Its a bit beyond what I've read so far. I wasn't meaning to imply swells didn't exist. I was focusing on the temperature and isotope statements within the quote. Regarding the swells though- am I right in thinking that they could also be generated by mantle of low density due to chemical differences, rather than high temperatures? This idea was put forward in the paper about the Azores I mentioned above, but I'm not sure how thoroughly it has been explored.


    The fact is that these different mechanisms are certainly not mutually exclusive. In that sense they are not alternative theories. The existance of one phenomena does not disprove the existance of another.
    I wasn't trying to imply that it is an either-or situation.



    What I'd really like to see to settle the debate would be a detailed study of the relationships between transition zone thickness across Montelli et al's anomalies. If they are thermal plumes, we should see a thick transition zone away from the plume, thinning towards the middle where it is hottest. As far as I'm aware this hasn't been studied in enough detail yet though.
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