I know a fair few of you are aware of the ongoing debate about the existance of mantle plumes, as I've seen it discussed here before. I've just finished a wee mini-dissertation on them, and it seems to me that one of the major problems is that people seem to disagree about exactly what constitutes evidence of their existance. Some authors seem to choose their 'evidence' very poorly, which doesn't help the debate to progress. Courtillot et al (2003) for example uses: an age progressive track, a large igneous province (LIP), a high 3He:4He ratio, a high buoyancy flux, and a tomographic low density anomaly at 500km depth. I'm pretty sure that 3 of these are pretty useless as proof. An age progressive track could be created by a relatively shallow, relatively fixed chemical anomaly or a propagating fracture. LIPs are only questionably associated with hotspots, with a great many with no confident link. The LIPC (no date) only confidently link 6 LIPs to current hotspots for example. Some are lacking evidence of uplift or swelling before/during eruptions, and in some cases may be better explained by other theories, ie. release of ponded melt or decompression melting during rifting. Furthermore recent tomographic studies (Montelli et al, 2004, 2006) have failed to find plume heads anywhere in the mantle, begging the question of how a plume might form a LIP. Helium ratios although good in theory seem to be poorly backed up by data, and it can be argued statistically that MORB and OIB are samples from the same population (Anderson, 2000). The tomographic studies of Montelli et al (2004), being some of the first to identify plume-like anomalies in the deep mantle have failed to find a correlation between depth and helium 3 to 4 ratios, making the evidence even less credible.
Bearing this in mind it is pretty hard to accept the existance of a plume beneath a hotspot based on those criteria. I've got some ideas of what I would count as evidence:
-A deep tomographic anomaly.
-A thin transition zone corresponding to this anomaly
-Evidence for high temperature in the form of heat flow measurements and/or geothermometers.
-A crustal swell, with proven support by thermal expansion and/or dynamic support.
Do you guys think this is reasonable? Am I counting out helium ratios too soon? What would you take as evidence?
References:
Anderson, D.L., (2000) The statistics and distribution of helium in the mantle. International Geology Review, 42, pp. 289-311
Courtillot, V., Daville, A., Besse, J. & Stock, J. (2003) Three distinct types of hotspots in the earth’s mantle. Earth and planetary science letters205 pp.295-308
Montelli, R., Nolet, G., Dahlen, F.A. & Masters, G. (2006) A catalogue of deep mantle plumes: New results from finite frequency tomography. Geochemistry Geophysics Geosystems7, no.11 pp.1-69
Montelli, R., Nolet, G., Dahlen, F.A., Masters, G., Engdahl, E.R. & Hung, S. (2004) Finite frequency tomography reveals a vareity of plumes in the mantle. Science. 303 pp.338-343
Recently I read a special publication on LIPs geology. I think that you can look for more information at Terra Nova´s Journal of one or two months ago.
It was so interesting.
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Lucky man whose destiny is to know the Earth´s secrets. Eurípides (480-405)
Rank: Fluorite
Joined: 20/04/2005
Points: 354
Recently I read a special publication on LIPs geology. I think that you can look for more information at Terra Nova´s Journal of one or two months ago.
It was so interesting.
Lucky man whose destiny is to know the Earth´s secrets. Eurípides (480-405)