The history of science shows that what is widely agreed is generally wrong. Yet, there is more than the temptation of cosiness, and the ease of publication that goes with it, that induces even the most imaginative scientists rarely to stick their necks out. In their overthrow of the geocentric view of the cosmos, both Copernicus and Kepler felt ideological pressures that we can only guess at. Colleagues of Copernicus had been burnt at the stake, so he hid himself for the 40 years of his life and only dared publish his ideas so late that the galleys arrived at his deathbed. Kepler, a Protestant in the Holy Roman Empire, kept one step ahead of trouble by networking that would done many a modern scientist proud, and a sort of Bowdlerisation of his ideas so that they merged almost seamlessly with the prevailing ideology of both sides of European Christianity. Even the bravest, most honest and gifted scientists generally agree with their peers, simply because they rarely know any better. If they do, they either keep or are kept quiet. There is very little, if any objectivity in the science of any age… because it is scientists who do it! Kepler cuddled up to Tycho de Brahe, he of the gold and silver nose (fitted after student duelling), in order to gain access to Tycho’s observational data when the old feller died. He got them alright, and began to turn the universe back on its feet, thereby opening an avenue for Newton. Neither Kepler, an unstable hypochondiac who was good at geometry, but not much else, nor Tycho, an anal retentive maker of revolutionising instruments and the founder of empirical science, but devoid of ideas, would have been celebrated for four centuries if the one had not worked with the other. The evolution of science has been marked by the influence of non-conformists, but few worked in isolation against the mainstream.
One modern geoscientist who seems rarely to conform is Warren Hamilton of the Colorado School of Mines, and now he has gone for it big time (Hamilton, W.B. 2003. An alternative Earth. GSA Today, v. 13(11), p. 4-12). His starting point is to challenge the consensus among geophysicists and geochemists that the mantle has a still-unfractionated lower part beneath depleted upper mantle which has sourced oceanic and continental lithosphere progressively over time. Linked to that is the notion of easy circulation of material from top to bottom through descending, subducted slabs and plumes rising from the core-mantle boundary. Hamilton says that neither exists, and that upper and lower mantle are decoupled. His challenge stems from the certainty that the Earth accreted “hot, fast and violently”, and the strong likelihood that its Moon originated after a titanic collision of Earth with a Mars-sized planet less than 100 Ma after accretion. Chances are it became wholly molten and suffered massive loss of volatiles. Such a body would have fractionated rapidly, to produce a lower mantle very unlike that imagined by most geochemists and geophysicists. Moreover, it would have remained so, partly due to its likely perovskite mineralogy, highly fractionated nature and phase-change barriers to transfer of matter – the 630, 1000 and 2000 km discontinuities. Such an early scenario would have transferred most potassium, uranium and thorium into the outermost Earth, where the generation of radiogenic heat would have concentrated. This is very similar to models proposed in the 1960s and early 70’s by J.V. Smith and others, when lunar geochemistry, particularly that of the anorthositic highlands, set in motion ideas about a planet-wide magma ocean and global fractionation as it cooled. Like Smith and others, Hamilton considers continental crust to have formed rapidly, sequestering a large proportion of the elements that make mantle rocks “fertile”. But only traces remain in the form of a small pinch of pre-4 Ga zircons, that could easily be lost in a single sneeze. Much of this early sial returned swiftly to the upper mantle to make it increasingly heterogeneous – fertile parts and some not so petrogenetically prone.
The current consensus has its roots, according to Hamilton, in much older ideas about the early phases of Earth’s evolution. Harold Urey and others in the 1950s and early 60s considered the planet to have formed by slow, cold accretion of the most primitive meteoritic materials, chondrites, particularly those containing carbonaceous materials. They are petrogenetically highly fertile, and the radioactive heating of a chondritic Earth, plus that from core formation, would involve a continual, slow fractionation of the mantle that would probably still be going on today. That this fundamental set of assumptions still dominates, though is rarely mentioned, is down to the rapidly increasing number of mantle profiles based on seismic tomography, that are claimed to have imaged seismic-speed anomalies that could be explained by both slabs and plumes extending to the core-mantle boundary. Hamilton makes the reasonable point that the very irregular distribution of earthquakes in the top 600 km of the Earth leaves large volumes of the mantle in blind spots, and that the majority that are used are subduction related. That, he suggests, predestines tomograph images to create artifacts that just “look” like deep penetration of descending slabs. Moreover, stunning as they look in publications, there is much graphic sleight of hand that assigns primary colours to lower mantle anomalies that have an order of magnitude lower amplitude than those at shallower depths, as well as filling unimaged areas with average or interpolated values, placement of sections to look most plausible, and a great deal of data filtering. There is a “fudge factor” that hypes the hoped-for, and avoids alternative data analysis – you can’t do this kind of thing on a PC. The plume hypothesis is falsified exactly where it ought not to be – in the Emperor-Hawaiian seamount chain (see Wandering hot spots in the September issue of EPN). There the great bend dated at 45 Ma is not matched by any known change in the direction of Pacific sea-floor spreading. The magma source for the chain might well be a restricted volume of mantle, but it didn’t stay still as a plume must. Seismic tomography, at the time Hamilton’s essay went to press, had not verified a single plume sourced in the lower mantle – there are many cases of volcanic hotspots without any plume, and tomographically inferred hot mantle doesn’t always have a volcanic expression.
Hamilton’s essay is worth reading in its entirety, as it reviews the whole of Earth’s tectonic and magmatic evolution. I have just tried to pick out the critical aspects here.
More, or less plumes
In view of Warren Hamilton’s questioning the existence of mantle plumes (Geoscience consensus challenged), in the same month as his essay appeared a team of seismologists from the universities of Princeton, California, Colorado and the National Taiwan University used a new approach to seismic tomography to seek evidence for plumes (Montelli, R. et al, 2003. Finite-frequency tomography reveals a variety of plumes in the mantle. Science Express http://www.sciencexpress.org, 4 December 2003, p, 1-10). They present evidence for 32 suspected plumes. Some have a seismic expression at shallower depths than 650 km in the mantle, such as beneath Iceland and the Galapagos. Others seem to reach as deep as the core-mantle boundary, as beneath Hawaii and the Kerguelen Plateau. In fact most of the classic volcanic hotspots that have associated chains appear to have plumes beneath them, with the exception of Yellowstone. An apparent duality of shallow and deep plumes suggests to the authors a two-tier division in vertically moving mantle, above and below the 660 km discontinuity. The long-suspected major plumes beneath Africa and the Pacific also appear to spawn lesser plumes, that in turn sometimes split
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