Linking seismic tomography to chemical mantle heterogeneity

Analysis of historic, global seismograph records using sophisticated software allows far more than the detection of various discontinuities in the deep mantle and core that figure in most textbooks.  Essentially, it maps parts of the mantle where P and S waves travel faster or slower than expected from the depth.  Up to now, most results have been interpreted in simple terms of cold (fast) and hot (slow) patches, which have been linked to gross tectonic features such as signs of descending slabs far below the earthquake belts associated with subduction, and possible zones of rising mantle that might (or might not) be plumes.  That leaves a lot unsaid about the mantle, for rising and falling of material is linked to density, and that can be due to temperature anomalies, and also to compositional variations involving either bulk chemistry or different assemblages of minerals in mantle rock.  A difference in seismic wave speed can be an ambiguous indicator of possible motion.  Making the connections between wave speed, temperature and composition is an order of magnitude or more computationally taxing than the tomography itself, but it has been shown to be possible, given supercomputer power and plenty of free time (Trampert, J. et al. 2004.  Probabilistic tomographic maps chemical heterogeneities throughout the lower mantle.  Science, v. 306, p. 853-856).  Trampert and colleagues from the Netherlands and the US factored in mineral physics and temperature data, and were able to calculate the probabilities of tomographic features having a thermal or compositional origin.  Their results will worry some of the earlier workers on seismic tomography who used a simplistic connection with temperature and thus slow = hot = low density and rising, while fast = cool = high density and sinking.  Some zones of low wave speed can as well be connected with high-density mantle as with hot, buoyant material.  That plays havoc with concepts of plumes rising from the core-mantle boundary, that have been all the rage since moderately well resolving tomograms appeared.  Trampert et al’r results, which superficially look just the same as other tomographic renderings of the same seismic data, include statistical evaluations of the likelihoods of wave-speed shifts being either thermal or compositional in origin.  They reveal that many of the slow zones are probably chemical and mineralogical heterogeneities, especially in the deepest mantle levels.  One of the largest slow zones known rises obliquely from the core-mantle boundary around southern Africa towards the surface in NE Africa.  It was leapt on as a reputed superplume, perhaps connected to the last outpouring of flood basalts in Ethiopia and the Yemen around 30 Ma ago, and still active beneath the Afar Depression.  Chances are, from the new work, that it is denser than average and not especially hot.  Mantle geochemists will probably be gleeful at the new look at deep mantle, because they have long been wrangling ideas about gross lateral variations in the source chemistry of basaltic magmas.  Some enthusiastic geotectonic speculators might remain very silent, in the hope that the Dutch-US team’s work is not duplicated, and fades away…

See also:  van der Hilst, R.D. 2004.  Changing views on Earth’s deep mantle.  Science, v. 306, p. 817-818

Bedout end-Permian “impact” hammered

The claim that a large circular feature beneath the sea bed between Australia and New Guinea is linked to the end-Permian mass extinction (Becker, L. et al. 2004. Bedout: A possible end-Permian impact crater offshore of northwestern Australia.  Science Express 14 May 2004 –  (See Crater linked to end-Permian extinction, June 2004 EPN) has met with a flurry of sceptical comment in letters to the editor of Science(2004, v. 306, p. 609-613).  Becker and colleagues have published several articles on the P-Tr boundary, including data on noble gases from the boundary in China, which are alleged to be consistent with an extraterrestrial influence, a meteorite from Antarctica which they consider to be a fragment of the impacting body and this year the claim for shocked minerals and impact glass in sedimentary core over the Bedout structure.  There have been unsuccessful attempts to duplicate the results on the noble gas analyses, the Antarctic meteorite is regarded as being insufficiently altered to be as old as 250 Ma, and as regards the Bedout material, the authors of the letters to Science consider none of the evidence to stand up to proper scrutiny.  One letter from specialists in the US, Russia, South Africa, Austria and the UK (Renne. P.R. and 7 others 2004.  Is Bedout an impact crater?  Take 2.  Science, v. 306, p. 610-611) also claims that the 250 Ma argon-isotope age for Bedout samples is misconceived and without objective basis.  One of the authors, Jay Melosh of the University of Arizona, is reported to have said that the Becker group, “..have deeply muddied the waters about what is going on at the Permian/Triassic boundary”.  These and material in the other letters are tough words indeed.  Becker’s group is funded by NASA, and when the flurry of letters hit home earlier in October, NASA sent a team of three scientists, including Becker, to resample the Chinese P-Tr boundary section.  Ten geochemistry laboratories will receive splits of the material to settle the issue of noble-gas evidence for an end-Permian impact.  But it looks very much as if a major scandal may break when the multi-lab analyses are published next year.  That is not to imply that there are no other skeletons lurking in cupboards along with impact-related materials.  A few years ago, editors of a major journal were asked to withdraw or refute a paper that used analyses of impact-related materials that had found there way to several laboratories without the permission of their originators or their names being mentioned.  The kudos associated with publishing on extraterrestrial influences on biological extinction patterns seems hard to resist…..

See also:  Dalton, R 2004.  Comet impact theory faces repeat analysis.  Nature, v. 431, p. 1027.


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