Don Anderson, 22nd November, 2010

Let me clarify further. We have been arguing about data, assumptions and interpretations for 40 years. Little progress has been made. Plume "theory" has no theoretical underpinnings and violates well known scaling relations of fluid dynamics and thermodynamics. But this has not ended or slowed the debate. I understand that data mainly controls your thinking, but nevertheless assumptions and deductions are involved in all of the arguments for plumes (and different assumptions can reverse the conclusions). So, let's look at the assumptions.

I suggest that we switch to logic and the identification of fallacies. All sides can agree that the principles of deductive logic and mathematical proof are neutral. So let's analyze the rationale behind plumes as one would analyze premises and conclusions. Forget the data, for the time being, and just list why you, or others, believe in plumes or otherwise.

• Very few geochemists challenge the views that high ³He/⁴He and rapid eruption rates signify lower mantle components and plume heads, respectively. Why do they believe that?
• Circular reasoning. There is no theoretical basis for presuming that an accreting planet will not melt or that it will retain cold ³He-rich domains. The idea that that high ³He/⁴He was a primordial mantle or plume diagnostic can be traced back to the observation that Hawaii and Iceland had higher than MORB ratios and that these are "known plumes". Likewise, the idea that plume heads drain their magmas in 1 Ma or less is not predicted by plume head theory. It can be traced back to the observation that CFB eruptions are brief and these are "known to be due to plume heads". The idea that high ³He/⁴He means high ³He follows from the "knowledge" that the lower mantle is undegassed and therefore a high ³He content will keep the ratio high.

The plume hypothesis has become a paradigm in the sense that the same underlying assumptions are adopted by workers in various disciplines. The main ones are that mantle plumes exist, depleted MORBs represent ambient mantle and that they originate in the upper mantle, magma volumes and seismic velocities are proxies for mantle temperatures, and high ³He/⁴He ratios (higher than MORB) are evidence for a deep undegassed plume reservoir. It is these assumptions, rather than data, that are responsible for the longevity of the hypothesis. If logical underpinnings of the hypothesis are essentially non-existent, this will become evident if we look at them in this light. We also need to discuss proxies (magma volumes, seismic velocities, ³He/⁴He, eruption rates, etc.). Are these really reliable proxies for temperature and plumes?

One can debate the geological, geochemical, seismological and petrological evidence but thermodynamic and logical inconsistencies and errors are more fundamental. Among these are substituting relative plate velocities, seismic velocities, travel times and temperatures for absolute values, confusing high ratios with high numerators, ruling out upper mantle sources because MORBs are from the upper mantle, and assuming that thermodynamic variables can vary independently with T, P and depth.

Possibly True Statements:

The upper mantle is the reservoir for MORBs
Hawaiian magmas are hotter than MORB
Large igneous provinces are of short duration
Fluids heated from below develop plumes
High temperatures cause low seismic velocities
Low velocity features exist in the lower mantle
Some OIB differ chemically from MORB
Island chains and intraplate volcanoes exist
Half the seismic arrivals to Hawaii are slower than the other half

…but to follow up these true statements with the assertion that therefore mantle plumes exist is to commit a logical fallacy.

Common Assertions and hidden assumptions:

…the lower mantle is the reservoir for intraplate magmatism (e.g., O’Nions & Tolstikhin, 1996; Hofmann 1997; Kellogg et al., 1999; Albarede & van der Hilst, 1999);
…the potential temperature of the upper part of the deep boundary layer exceeds any temperatures in the upper mantle by at least 200°C;
…temperatures in the upper mantle do not exceed temperatures recorded at midocean ridges;
…if A/B >C/D, then A>C, where A,B etc. are chemical or isotopic concentrations such as ³He and ⁴He;
…if X implies Y, then not-X implies not-Y, where X are mantle components such as MORB and OIB and Y are hypothetical reservoirs such as upper mantle;
…homogenous products imply homogeneous sources;
…geochemical ratios that exceed the MORB mean are from a lower mantle reservoir;
…plume head magmatism is of short duration because large igneous provinces are of short duration (the Texas Sharpshooter Fallacy);
…one should look for anomalous mantle only where it is available, e.g. oceanic island basalts and OIB xenoliths; unsampled regions are MORB-like (the lamppost fallacy);
...let us look at only those volcanic features where a plume mechanism actually
makes geological sense;
…Evidence from small seamounts seems completely irrelevant to this debate. [begging the question].

It is actually fun to decompose an argument into a logical statement; premisses and conclusions, such as

MORB is from upper mantle
Therefore, OIB is from lower mantle

MORB is colder than Hawaii
Therefore Hawaii is from a thermal plume

Hawaiian basalts are not like MORB
Therefore Hawaii is a plume

Heating from below makes plumes
Therefore mantle plumes cause hotspots

Lower than average seismic velocities occur in the lower mantle
Therefore plumes exist

You can play this game yourself with your favorite arguments. The above all appear in published, peer-reviewed papers by eminent scientists. If you can demonstrate a logical fallacy or inconsistency, this should end the debate on that isssue. If your theory violates the second law of thermodynamics, you should hang your head in shame and admit defeat.

Dean Presnall, 22nd November, 2010

My J. Petrol. paper (still in the mill) shows that Hawaii samples normal temperature conditions and that MORBs are the result of a perturbed geotherm with (cool) melt extraction within the thermal boundary layer. I think i have a very solid argument, but it will be interesting to see what others think.

Yaoling Niu, 22nd November, 2010

Great discussion! May I add some "confusion"?

My assertion and reasoning:

Hotspots must be hot indeed as they are manifested by volcanoes. Volcanoes must be hot as you can feel their hotness. I remember that I was wearing "fully insulated" clothes, but it still took me more than 10 attempts to get some hot lavas with my geohammer near Pohoiki (?) on Big Island of Hawaii in 1990.

So, my understanding is:

1. Hotspots are hot; they do exist as we can see and feel.
2. However, are they surface expressions of deep-rooted mantle plumes? I don't know because I neither see nor feel mantle plumes.
3. But we can ASSUME these hotspots are surface expressions of deep-rooted mantle plumes. Then we can use the geochemistry (make some more assumptions) and geophysical means (with still more assumptions) to test if mantle plumes exist.

The result is clear to some, but not to others, so one has to choose:

1. Mantle plumes DO exist: from some thermal boundary layers in the Earth, most likely at the CMB or perhaps from the transition zone, or perhaps some compositional anomalies (chemical plumes) or in combination.
2. Mantle plumes DO NOT exist because we don't see and seismology cannot yet reliably resolve them even if they do exist

There are statements in the literature, something like:

1. Geochemical evidence for the presence of mantle plumes (large scale upwelling) - the evidence becomes less evident if we remove some of the assumptions
2. Seismic evidence for the presence of mantle plumes (large scale mantle upwelling) - likewise, the evidence becomes less evident if we are more careful about assumptions
3. Nevertheless, "The lack of evidence is not the evidence of absence"

So, where are we now? I look forward to reading Plates vs Plumes: A Geological Controversy by Gillian R. Foulger

Gillian R. Foulger, 22nd November, 2010

Don: Thank you for your discussion here below on whether or not "hot spots" are hot. I hope you will come along to the session and contribute more of your thoughts in person there. I look forward to attending your own invited talk.

The question of whether/which/if any "hot spots" arise from hot sources is a subject of considerable debate. Some people assume they are hot without question and don't bother to test this assumption. Others assume they are and process data in whatever way is necessary to get that answer. Other people process data in an open-minded way, and get a frosty reception from their colleagues when they fail to find unequivocal evidence for high temperatures at most places. Still others follow the scientific method and try to falsify the conventional hypothesis that "hot spots" are hot.

Several of us have been discussing privately the issue that the source of "hot spot" lavas may come from different depths in the surface conduction layer, and I have illustrated this in Figure 6.3 of my recent book "Plates vs. Plumes". This hypothesis predicts variable source temperatures, hotter if the lavas originate from deeper (and we are talking about the upper couple of hundred kilometers here, not the deep mantle).

What the temperatures of their sources are is a question relevant to their origin (e.g., depth, in the above model) and I think it is a valuable line of enquiry. It may end up telling us more about the (variable shallow) origin of the lavas than about whether plumes exist or not. To me, that might be more important than chasing the debate about whether plumes exist or not. The plume hypothesis is a belief system that can't be disproved and is thus deserving of only limited attention.

I thus agree with you that determining source temperatures is not likely to solve the problem of whether plumes exist or not, if people assume that finding that one "hot spot" come from a hotter source than another (or MORs) proves that they do. However, I disagree that the question is not relevant or important.

I hope this will also be a useful forum for discussion the validity of some widely used but dubious methods for deducing temperature, e.g. applying olivine-control theory to cumulate rocks, or quoting single, isolated, unrepeated ocean-bottom heat-flow measurements without taking heed of local context.

Jim Natland, 21st November, 2010

As I have done my best to explain, the olivine-liquid FeO-MgO backtrack procedure is fraught with difficulties and almost certainly leads to higher estimated potential temperatures and parental melt MgO than are either necessary or demonstrated to occur in nature. If you wish, you can ignore this and say that potential temperature doesn't matter. Then I could pack up my bag and go home. However, I would think that appropriate estimates of temperature would be useful, even to your model. Putirka said that differences in temperature between hot spots and ridges must be on the order of 200°C if plumes are to exist. Using backtracking but not taking mixing into account, he concluded that plumes do indeed exist. By including mixing, I conclude that the 200°C difference is neither demonstrated nor likely, even at Hawaii, and that we consequently have a reason to doubt that plumes exist. Is this not relevant?

Don Anderson, 21st November, 2010

I notice that there isan AGU Session entitled "Are hotspots hot?". I maintain that this question is not relevant to the plume debate and has diverted attention away from important issues. Of course hotspots are hot and Hawaii magmas may be hotter than MORB. The real questions are "Do midplate volcanoes sample ambient subplate mantle?" and "If Hawaii is hotter than MORB does this prove the existence of mantle plumes?".

I realize that, to many, and in particular, the editors of Science and Nature, if recent papers and commentaries are examples, that low relative seismic velocities and mantle hotter than MORB mantle are considered to be sufficient conditions to declare a plume sighting. But this goes back to the old cooling plate paper by McKenzie & Bickle (1988) who simply asserted than MORB is ambient subplate mantle. This created more paradoxes; anomalous subsidence and seafloor flattening, and absence of the predicted low velocity halo (e.g., Priestley & Tilmann, 1999). The cooling plate model itself, as others admit, has no physical underpinning and is inconsistent with observed lid thickening. As usual, paradoxes are due to bad assumptions. Reverse the assumptions and the paradoxes disappear!

The absence of high heatflow around Hawaii and the absence of low absolute seismic velocities around Hawaii and absence of slow absolute travel times to Hawaii, plus the anomalous bathymetry and subsidence throughout the Pacific suggests that midplate volcanoes sample ambient mantle in and below the boundary layer. This turns the debate on its head. Why are midocean ridge magmas so cold? This is the subject of my invited talk in V25 at the forthcoming AGU meeting, on the Monday.

Hawaii can be "hot" and this is not inconsistent with the lack of any thermal or absolute velocity anomaly around Hawaii. It is the assumption that is wrong or at least debatable. Recent tomographic studies published in Science and Nature have just established that half of the seismic anomalies to Hawaii are later than the other half. The arrivals are not late in any absolute sense.

The observed anomalies around Hawaii can be due to heterogeneity and anisotropy above 220 km depth, which is ignored in the Princeton and Carnegie studies. There are better ways to study Hawaii than with the relative travel times of near-vertical teleseismic waves.

Dean Presnall, 18th November, 2010

Jim Natland, 17th November, 2010

I don't think that any picrite or meimechite is a liquid composition, but that they have a lot of accumulated olivine. Nothing I've seen so far goes beyond Kilauea mineralogy, and the highest MgO in glass there has 15% MgO and an eruptive T of about 1350°C (Beattie geothermometer). The glass accounts almost perfectly for the spinel with highest Cr# and highest Mg# in the suite. So who needs more, or hotter?
That's still plenty warm. Spinel from Karoo looks like Kilauea spinel, and doesn't need more than 1350°C. Add what you want and however you want to do it to get it up to a potential T from eruptive T. Samoa is a lot cooler. MORB tops out at about 1225°C.

Who can tell with komatiites? Gorgona spinel has slightly higher Cr# and Mg# than Kilauea spinel, but is more oxidized. So there's a temperature trade off.

Ever since Langmuir came up with temperature as the explanation for different parental MORB, there has been a widespread assumption that everything can be explained by T variations. But it's heterogeneity, not difference in T, that is the key. The tendency is to explain as much as possible with T, especially invoking plumes. But I'd say there are two numbers to think about, no more. One is 1225°C for MORB; the other is 1350°C for Kilauea picrite glass. Those are the extremal bounds. Plenty of places are cooler than Kilauea and nothing so far is hotter, although Baffin-West Greenland and Karoo approach Hawaii.

Dean Presnall, 17th November, 2010

I am skeptical about assigning temperature to anything that is not a glass and cannot be compared to experimental data for similar compositions. If picrites and komatiites can be shown to have been melts, then they would indicate very high temperatures. Many (all?) picrites are mixtures of melt and crystals. Komatiies are more difficult because they are old, may have once been melts, and may represent temperatures that existed only in the past. I think komatiites would carry more weight as high temperature magmas than picrites.

Don Anderson, 10th November, 2010

Plume theory predicts that plume basalts have "high" ³He contents, "high" temperatures and differ geochemically from MORB. Most hotspot magmas have extraordinarily low ³He contents, much lower than MORB. The depleted character and isotopic similarity between the highest temperature terrestrial magmas (komatiites, ferropicrites, meimechites) and MORB are not predicted by plume theory. The highest ³He magmas (popping rock) and the highest ³He/⁴He magmas (Greenland and Baffin picrites) are indistinguishable from MORB for Pb, Os, Sr and Nd isotopes. The most common isotopic component of basalt is C- or FOZO, the component bearing high ³He/⁴He ratios. These observations should give pause to anyone attributing these to mantle plumes or the lower mantle. All the heavy isotopes imply ambient upper mantle compositions. These components all appear to come from the shallow mantle, and often appear at the onset of rifting or at the early stages of magmatism. A newly fractured continent will surely expose mantle that is hotter than under mature spreading ridges.

Potential temperatures as high as ~1600°C are considered to be "excess" because they are higher than MORB temperatures, at least MORB from mature ridges. There is no reason other than convention why hotspot magmas cannot sample ambient shallow mantle and why they cannot be hotter than MORB. The actual temperatures of OIB, CFB, komatiites, picrites etc. may be higher than MORB but this does not imply that they come from localized hotspots, or plumes, in the mantle. One has to defend the proposition that MORB temperatures are the maximum allowable temperatures in the upper mantle, and that temperatures of ~1600°C cannot exist in the surface boundary layer.

If the thermal boundary layer is ~220 km thick, as implied by seismology, and conduction gradients are 6-10 °C/km, then the base of the boundary layer can be 1320-2200°C, and much hotter when the T dependence of conductivity is taken into account. If one accepts the 1300°C at 100 km of the Cambridge cooling plate model, then the potential temperature at 220 km depth can be >2000°C! A more rigorous derivation gives Tp of 1600±200 C as a plausible ambient temperature range at the base of the upper mantle boundary layer. MORB come from shallower depths or colder mantle or both. If there is radioactive heating in the mantle, the Tp at the top of the lower mantle thermal boundary layer will be less than at the base of the upper mantle boundary layer.

Seismic waves that bounce off the surface (SS) in the Pacific show that Hawaii is not anomalous; it is just like other places in the Pacific and is faster than many.

Jussi Heinonen, 10th November, 2010

It is great that our recent webpage has fueled discussion on Karoo origins.

As Dr. Natland mentions in his comment below, we are looking at mixed magmas. Importantly, however, all the evidence points to these magmas being very primitive (i.e. olivine-controlled). Thus we can deduce the parental magma compositions by adding olivine until Fe-Mg equilibrium is reached. There is no geochemical, mineralogical or petrographical evidence for mixing with cpx- or plag-saturated (and thus Fe-enriched) magmas. In any case, the predominance of forsteritic olivine phenocrysts (up to Fo₉₂) in the meimechites indicates that (a) mixing was minor and (b) that the olivines had to have crystallized from highly magnesian parental magmas. Importantly, the calculated parental melt compositions are in equilibrium with depleted upper mantle peridotite – a source also indicated by the isotope compositions.

Concerning the calculated temperatures, we took the presence of water into account in the calculations. The composition of amphibole and its abundance indicates that the water contents in the melt were not likely to be higher than 2 wt. %. Equivalent dry magmas would have temperatures around 1700°C. One additional factor that was mentioned by Dr. Ivanov and could lower the calculated temperatures is the CO₂ content of the parental magma. The overall subalkaline character of the meimechites (in fact, the major element compositions are more reminiscent of komatiites than Siberian meimechites) and the absence of associated carbonatites do not indicate marked mantle CO₂ influence so far in the case of Vestfjella, however.

I would also like to add that although the ferropicritic and meimechitic magmas were not parental to Karoo CFBs in sensu stricto, their source is likely to represent a significant sublithospheric end-member for Karoo magmatism. Parental magmas of some Karoo CFBs formed from this same source at lower pressures and at higher degrees of melting and are thus not compositionally equivalent to ferropicrites/meimechites.

All of these issues are more thoroughly discussed in Heinonen & Luttinen (2010), Heinonen et al. (2010), and in my nearly complete PhD Thesis (Heinonen, in prep).

James Natland, 5th November, 2010

With reference to the new webpage by Heinonen & Luttinen, I have been pondering these very same meimichites and ferropicrites in connection with temperatures. Some of this will be in my forthcoming AGU poster. First of all, they are manifestly mixed rocks, based on both geochemical and mineralogical grounds. So they are not all that hot. Second, they are not parental to the bulk of Karoo tholeiites, therefore they are irrelevant to consideration of temperatures of those rocks. The rest of the web page is a measured consideration of geochemistry. Some of it reminds me of Iceland, but at the core of it, everything is depleted. if it were not for the (erroneous, in my opinion) interpretation of high T, there would be nothing at all for plumes.

Alexei Ivanov, 5th November, 2010

The finding of meimechites shows that Karoo is very similar to the Siberian traps - (1) located in a back-arc subduction system, (2) the dominant type of magma shows 'subduction' trace element signatures, which are usually interpreted as evidence of lithospheric involvement, and (3) high-Mg magma. Probably findings of carbonatites will follow. What is high-Mg magma? It is usally interpreted as evidence of high-T. However, both Siberian and Karoo meimechites show mineralogic evidence of water in the source (magmatic amphibole and mica). It was shown by Elkins-Tanton et al. (2007) [Cont. Min. Pet., 153, 191-209] that both water and CO₂ lower the melting point for meimechite sources. So the temperature may not be so high. Meimechites = depleted MORB-type source in terms of radiogenic isotopes, depth and water.

last updated 10th November, 2010