Author: zooks777

Former Senior Lecturer at British Open University, research in remote sensing of arid lands, groundwater exploration, Precambrian tectonics and geochemistry

Evidence for slab break-off in subduction zones

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The detachment of lithospheric masses and their falling-off into the mantle, either by delamination of deep lithosphere beneath continents or the breaking of a subducted slab, have become popular means of explaining a variety of unusual phenomena in mountain belts.  In the Himalaya and Tibetan Plateau, such models have been evoked for the formation of odd K-rich basalts in the Eocene and Miocene, and the crustal melting that generated leucogranites around 20 Ma ago along the entire length of the Greater Himalaya.  Taking all the oddities of the Indo-Asian collision zone together does seem to support such a model (Kohn, M.J. & Parkinson, C.D. 2002.  Petrologic case for Eocene slab breakoff during the Indo-Asian collision.  Geology, v. 30, p. 591-594).  However, there is still no tangible direct evidence beneath the region.

Using seismograms for deep-Earth tomography appears to be able to resolve a range of proposed variants of tectonics, as well as the gross behaviour of the deep mantle. The site where two plates are being subducted on the west side of the North Pacific, marked by the Kamchatka peninsula, is pretty odd as well.  Although rates of subduction of both plates are high, the part of Kamchatka at one boundary no longer has active volcanoes, whereas the other does.  In fact one of the volcanoes there holds the world record for magma output.  Up to 5 Ma ago, the whole of Kamchatka was actively volcanic.  An explanation for the sudden halt to volcanism is that the dehydrating slab which provides the essential watery fluid for partial melting of the overlying mantle wedge – the source of subduction-zone magmas – broke away from the subduction zone and “fell” into the mantle 5 Ma ago.  That would have removed the source of hydrous fluid at a stroke.  Seismic tomography now seems to be capable of resolving just such a foundered slab (Levin, V. et al. 2002.  Seismic evidence for catastrophic slab loss beneath Kamchatka.  Nature, v. 418, p. 763-767).  There is no slab beneath the presently inactive volcanoes, whereas it is intact beneath the active ones.  The authors also claim that the seismic structure reveals a more recently foundered piece of lithosphere, whose rapid loss of hydrous fluid helps explain the phenomenally high magma production of the Klyuchevskoy volcano.  Such slab break-off is clearly a potential engine for enormous changes in magmatism, and the first seismic evidence for it is bound to spur a search for more examples.

The Malnourished Earth hypothesis – evolutionary stasis in the mid-Proterozoic

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Proterozoic

Accepted biogeochemical wisdom suggests that about 2000 Ma ago, the terrestrial environment changed from one in which oxygen was a rare free element to an increasingly oxygenated world.  One line of support for this involves the first appearances around that time of redbeds and lateritic palaeosols, that signify a surge in the O2 content of the atmosphere.  The other pointer is the disappearance of banded iron formations (BIFs), suggesting that soluble iron-2 was no longer available in the oceans due to its oxidation near its main source at mid-ocean ridges. The first unambiguous microfossils of eukaryotes, which need oxygen for their metabolism, also appeared some two billion years ago.

There is, however, a different view; that there was a transition between the anoxic world of the Archaean and Early Palaeoproterozoic and that marked by pervasion of atmosphere and hydrosphere by oxygen.  It stems from studies of sulphur isotopes in Proterozoic marine sediments by Donald Canfield of Odense University Denmark (Canfield, D.E., 1998.  A new model for Proterozoic ocean chemistry.  Nature, v. 396, p. 450-453).  Canfield found evidence for steadily increasing sulphate ions in seawater from 2300 Ma, which he suggested would have led to increasing production of hydrogen sulphide in the deep oceans by sulphate-reducing bacteria.  He proposed that it was combination with deep-ocean sulphide ions that shut off the supply of soluble iron-2, essential for the production of shallow-water BIFs.  Today, sulphide precipitation is restricted to hydrothermal vents and most iron is removed by combination with oxygen in sediments on the main ocean floors.  In short, Canfield proposed a transitional ocean akin to the Black Sea, with an oxic near-surface zone but anoxic at depth.  Not only iron would have been removed in sedimentary sulphides, but many other metals, leading to their depletion in seawater.  Ariel Anbar of the University of Rochester and Andrew Knoll of Harvard examine the biological repercussions of this transitional ocean (Anbar, A.D. & Knoll, A.H. 2002.  Proterozoic ocean chemistry and evolution: a bioinorganic bridge?  Science, v. 297, p. 1137-1142).

Iron and molybdenum are crucial elements for eukaryotes, albeit only in small quantities, because they are central to the enzymes that fix nitrogen.  Insufficient quantities would put early eukaryotes at an evolutionary disadvantage to prokaryote life.  Moreover it would reduce ocean productivity.  This, they propose, can help explain the lack of evolution among eukaryotes until the late Proterozoic.  The carbon isotope record of seawater (derived from limestones) shows a strange pattern that supports a period of biological stasis from 2000 to about 1200 Ma.  From the end of the Archaean until 2 billion years ago, there are huge fluctuations (to highly positive and negative values) in the proportion of heavy 13C, and so too in the Neoproterozoic.  The period in between shows no significant carbon-isotope fluctuation, d13C remaining at around zero, which Anbar and Knoll attribute to very low biological productivity.  In their model, it was the release of massive amounts of metals by continental erosion during the “Snowball Earth” glacial periods of the Neoproterozoic that was able to kick start life, especially that of the eukaryotes.  Emergence of the efficient, multicelled algal photosynthesizers drove up oxygen levels, eventually to oxygenate the deep oceans.

A cautionary note needs to be thrown in, however, especially when using analogies with the modern Black Sea (see Analogue of Archaean carbon cycle in Black Sea reefs).  Biogenic carbonates on the Black Sea bed show huge negative excursions in their d13C, because organisms that formed them metabolized methane, thereby incorporating methane’s strong depletion in heavy carbon.  As well as there being little direct evidence for Anwar and Knoll’s idea, the methane part of the carbon cycle needs to be factored into interpretations of the carbon-isotope record.

See: Kerr, R.A. 2002.  Could poor nutrition have held life back?  Science, v. 297, p. 1104-1105.

Isotopic evidence for early life may be from metamorphic processes

Controversy has surrounded reports of carbon-isotope evidence from the oldest recognisable sedimentary rocks that can be interpreted as signs of life 3800 Ma ago.  The problem is that the data came from carbon trapped in resistant minerals, such as apatite, in the metamorphosed Isua supracrustal rocks of west Greenland.  A detailed study of carbon in various forms in the Isua metasediments (van Zullen, M.A. et al. 2002.  Reassessing the evidence for the earliest traces of life.  Nature, v. 418, p. 627-630) strongly suggests that the isotopic evidence for life is flawed.  It seems likely that both graphite and carbonates in the Isua rocks originated by chemical reactions that took place during metamorphism; they are probably metasomatic in origin.  The wide range of d13C values found in both graphites and carbonates could have formed by isotopic exchange between graphite and carbonate during metamorphism.  Graphite inclusions in apatite, the source of carbon isotopes claimed to reflect the earliest biological activity, are petrographically no different from inclusions in other minerals.  Indeed, the sample originally used to suggest the isotopic influence of early life is of metasomatic origin.

All is not lost, however, for graphite that is highly depleted in heavy carbon-13 (a sign, albeit ambiguous, for organic processes) also occurs in turbidites that show graded bedding.  These rocks show no petrographic signs of metasomatism, and may contain signs of life.  Ominously, the US, Norwegian and Estonian co-workers, having looked in detail at carbon found in low concentration within BIFs and cherts from Isua, conclude that at least some is recent organic matter that groundwater flow has carried into the rocks.

Bizarre impact structure beneath North Sea

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The increasing use of finely-resolving 3-D seismic surveys in offshore exploration for hydrocarbons reveals exquisite detail of structure in strata beneath the sea floor.  So it is no surprise that oil-company geophysicists are able to image features that would otherwise remain hidden to researchers in universities.  If such discoveries are of little interest commercially, their finders are free to publish.  During routine surveys in the southern North Sea, an array of seismic profiles gradually built up a picture of something more reminiscent of the surface of an icy moon of Jupiter than a sequence of basinal sediments (Stewart, S.A. & Allen, P.J.  2002.  A 20-km-diameter multi-ringed impact structure in the North Sea.  Nature, v. 418, p. 520-523).  The circular feature found in strata at the top of the Cretaceous, might have been passed off as the product of deeper rise of salt diapirs from the widespread Permian evaporites of the North Sea basin, but for several features.  The surveys revealed no signs of the low-density Permian salt having bulged upwards below the structure, and disruption stops at depth.

The feature consists of at least 10 concentric rings extending to 20 km diameter, and at its centre is a bowl-shaped depression around a clear peak.  Not only is it an impact structure, but one of a particular class known as multi-ringed basins.  Those known from the Moon, are vastly bigger and are thought to have formed by such immense energy that the lunar surface rippled to fail along large concentric faults.  Lunar and terrestrial craters of the size of the North Sea structure usually have no concentric structure, being circular pits with rims and occasionally a central peak cause by rebound of the crust after impact.  The only similar features known are from moons of the Giant Planets that are made mostly of ice.  It is surprising that the North Sea example closely resembles them.  Modelling of such craters on Callisto suggests that they form when surface materials are underlain at depth by weaker ones; possibly an ice-liquid slush on ice moons.  The North Sea impact was into the Upper Cretaceous Chalk, whose upper strata are more homogeneous than those at deeper stratigraphic levels, which contain layers of mudstone.  Had impact occurred while the strata were not completely lithified, then the clays would have allowed inward movement to fill the crater excavated by impact, the more rigid upper Chalk having fractured during this movement.

Whether or not the impact accompanied the Chicxulub crater, implicated in the end-Cretaceous mass extinction, is not certain, although it does seem to predate Tertiary sedimentation in the North Sea.  There are probably many more impact structures on the sea floor, buried by marine sediments, but only in hydrocarbon-rich basins are they likely to be unmasked by seismic surveys.

Evidence builds for major impacts in Early Archaean

Following the discovery that anomalous tungsten isotope compositions of some Early Archaean rocks suggest a major component of extraterrestrial material in them (See Earth Pages News, August 2002, Tungsten and Archaean heavy bombardment), geochemists from Louisiana State and Stanford universities report evidence of debris from very large impacts in the same period (Byerly, G.R. et al. 2002.  An Archean impact layer from the Pilbara and Kaapvaal cratons.  Science, v. 297, p. 1325-1327).  Their case rests on the occurrence of layers of rock containing spherules of what formed as molten silicate droplets, in Early Archaean greenstone belts of the Barberton and Warrawoona areas of South Africa and Australia.  Zircons from a single layer in both areas yield identical ages of 3470 Ma, suggesting that the layers formed during a single impact event.  The authors speculate that a major unconformity in the Archaean of the Pilbara province in Australia, which is around the same age, may be the result of tsunamis induced by the impact.  It seems as if the responsible impact had a global effect, and may have released 1 to 2 orders of magnitude more energy than that responsible for the K/T event.  Judging by the lunar cratering record, this and previous finds help confirm expectations of similar bombardment on Earth during the Early Archaean.

Very early differentiation of planetary bodies

The radioactive decay of 182hafnium to 182tungsten seems likely to resolve the influence of impacts on the Earth ‘s evolution (See Earth Pages News, August 2002, Tungsten and Archaean heavy bombardment).  It is even more useful in refining ideas about the evolutionary pace of the parent bodies of meteorites.  The half-life of 182Hf is only 9 million years (all of it has decayed away in the Solar System by now), so the amount of radiogenic 182W associated with hafnium in a meteorite is a guide to pervasive geochemical processes early in the history of their parent bodies.  Hafnium has an affinity for silicates, whereas tungsten is siderophile and likely to enter planetary cores, should they form.  Because 182Hf decays so quickly, it is not easy to work out its original abundance, relative to stable 180Hf, in the source material for the Solar System.  That is a prerequisite for estimating when the hafnium-tungsten differentiation took place in a planetary body.  Two papers in the final August 2002 issue of Nature agree on this initial ratio (Yin, Q. et al. 2002.  A short timescale for terrestrial planet formation from Hf-W chronometry of meteorites.  Nature, v. 418, p. 949-952.  Kleine, T. et al. 2002.  Rapid accretion and early core formation on asteroids and the terrestrial planets from Hf-W chronometry.  Nature, v. 418, p. 952-955), which has important connotations; it is less than half the previously assumed value.  They determined this initial ratio using Hf-W data from independently dated carbonaceous-chondrite meteorites, whose parent bodies were never fractionated.

The two research groups, from Harvard University and the French Laboratoire des Sciences de la Terre, and the universities of Münster and Köln, Germany, respectively, use the new initial ratio to estimate the age of core formation from a range of meteorites.  Their estimates dramatically shorten the time between original accretion and core formation in a variety of bodies whose Hf-W isotopes have been studied previously.  The parent of the eucrite class of meteorites, probably the asteroid Vesta, differentiated within only 3 to 4 Ma, whereas the cores of the Earth and Mars took a little longer – about 29 and 13 Ma respectively.  In geological terms, accretion and core formation probably accompanied one another.  Of course, such estimates based on isotopic decay systems assume that the initial ratios existed at the time of accretion.  That may not be valid if the pre-Solar nebula took millions of years to evolve to the stage of self-collapse under gravity, which is the prerequisite for the formation of a planetary system.  However, there is evidence from short-lived decay systems involving other radioactive isotopes, such as 26Al, in meteorites, that points to the influence of a nearby supernova that triggered the formation of our Solar System.  Such an event is required to synthesize short-lived isotopes anyway.  Moreover, the shock from a supernova could accelerate collapse to mere few tens of thousand years.

See: Cameron, A.G.W. 2002.  Birth of a Solar System.  Nature, v. 418, p. 924-925.

Biofilms and BIFs

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Biomineralization is a growing topic that ranges from life’s influence on the production of economic deposits of metal ores to even the suspicion that it might play a role in Alzheimer’s syndrome.  The most common, and enduring evidence of the influence of micro-organisms in making rocks are stromatolites made of carbonates that blue-green bacteria have secreted, perhaps from as early as 3500 Ma ago.  Something similar, though it involves eukaryotic algae, is the formation of tufa or travertine where springs emerge from limestones.  Many a child, including my young self, consigned a cuddly toy to “petrifying” springs, such as Mother Shipton’s Well in Knaresborough, Yorkshire.  Few retrieved them, which is why there aren’t many rock-like Teddies around..  Another childhood memory, that bears on biomineralization, is a spring surrounded by orange and brown slime that we supposed was so deadly that only bathing in helicopter fuel would ward off a dreadful end brought on by the faintest splash of the loathsome gunk.  It is a great surprise to learn that such ochreous springs, common where coal mines drain to the surface, might hold a key to the formation of Precambrian banded iron formations (BIFs) (Brake, S.S. et al. 2002.  Eukaryotic stromatolite builders in acid mine drainage: implications for Precambrian iron formations and oxygenation of the atmosphere.  Geology, v. 30, p. 599-602).

Groundwater that has passed through iron-sulphide bearing rocks, becomes both acid and charged with iron-2 after oxidation of pyrite.  It is high acidity and low Eh that dissolves toxic heavy metals and arsenic, rather than their iron content, that make springs of such waters so hazardous to small boys bent on careers as hydraulic engineers (check their shins and fingers for the lingering water blisters that are a sure sign of the onset of arsenic poisoning).  It seems that Euglena, a common “animalcule” in such springs that is easily seen with a cheap microscope, is an ochre (iron-3 hydroxides and sulphates) forming agent.  It is an acid-tolerant, oxygenic photosynthesizer that builds slimy mats.  Given time and substantial supplies of dissolved iron, Euglena actually builds hard structures reminiscent of stromatolites.  Brake and colleagues from Indiana State and Kansas universities, and the Colorado School of Mines, studied Euglena from coal-mine drainages under lab conditions, and provide details of their metabolism.  The modern iron-stromatolites are so like some variants of BIFs from the Archaean and Palaeoproterozoic, when they were at their acme, that the authors suspect their origins in biofilms formed by prokaryotic organisms with similar metabolism to the more complex Euglena.  Until their work, most geologists regarded BIFs as products of inorganic precipitation of iron-3 compounds and silica when iron-2 rich seawater met oxygen produced by photosynthesizing cyanobacteria.  Indeed they speculate that the biofilm makers could have been early eukaryotes, despite the first unambiguous evidence for nucleus-bearing organisms being no older than 2100 Ma.  If they are correct, then such communities would have needed free oxygen, and would themselves have contributed to oxygen build-up in the early atmosphere.

Bonanza time for Bonzo

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The big news of July was without doubt from the palaeoanthropologists; a report on finds at the 1.75 Ma Dmanisi site in Georgia (Vekua, A. and 11 others 2002.  A new skull from Dmanisi, Georgia.  Science, v. 297, p. 85-89.), and the unveiling of a hominid-like skull from Chad dated at 7 Ma (Brunet, M. and 37 others 2002.  A new hominid from the Upper Miocene of Chad, central Africa.  Nature, v.  4418, p. 145-151).  Both threw the issues of human origins, evolution and migration back into the arena of debate.

Time and Newsweek, and once upon a time Life magazine often figure celebrities of the week or month on their covers.  Nature entered the celebrity cult on 11 July with a front-page photo of the magnificent cranium of Sahelanthropus tchadensis’ holotype found and analysed by a vast team from France, Chad, USA, Switzerland and Spain.  The skull is from Upper Miocene sediments around Lake Chad, dated from their varied fauna which is very like that of similar sediments in Kenya.  Its hominid credentials stem from the skull’s face, jaw and teeth, but it is odd.  From the back, it resembles a chimp, and so does the capacity of its brain case.  From the front, it bears close resemblance to an advanced Australopithecine.  Yet no limb bones have been recovered so far, and the attachment point of the skull to its backbone is not mentioned.  Both features would be needed to prove upright gait.  Undeterred, the authors and many commentators are convinced that it is the oldest human ancestor, from the very limit in time at which modern genetic analyses suggest that the human “bush” of descent parted from that which led to modern chimpanzees.  Bernard Wood of George Washington University (Wood, B. 2002.  Hominid revelations from Chad.  Nature, v.  418, p. 133-135) discusses Sahelanthropus’ significance to human evolution, implying that it poses problems for both the linear model of descent from a single emergence of basic human anatomy and the “untidy” model, to which he subscribes – adaptive radiation to changed circumstances that occurred more than once.  In the “untidy” model, even an excellent-looking candidate for the first in the line may not have been ancestral to us.

Palaeoanthropologists have never been as well-endowed with bones as they are with funds, and one detects hints of the protectiveness that has long plagued the discipline.  The finders of the previous candidate for the first hominid – Brigite Senut and Michael Pickford of the Natural History Museum in Paris (Taking stock of hominid evolution, Earth Pages News, March 2002) who found Orrorin tugenensis, in 5.72 to 5.88 Ma sediments of the Tugen Hills in the Kenyan Rift – claim that Sahelanthropus is merely an ancestral gorilla, citing the creature’s large canines.  Without a pelvis or footbones to back up the hominid claim, they could well be right.  However, the good news is that East Africa has lost its primacy as the source of fossils bearing on human evolution.  Being 1500 km west of the nearest previous site, and unrelated to the East African Rift system.  The new sites in Chad open up a vast area for future searches of potentially fruitful Miocene sediments, that are neither abundant nor complete in the Rift (its formation is post-Miocene).

Georgia in the former Soviet Union has grown in significance since the first reports of very old human remains near Dmanisi, a decade ago.  The site is well preserved, contains abundant mammalian remains, and the containing strata overlie a 1.85 Ma basalt.  With supplementary palaeomagnetic stratigraphy, Abesalom Vekua and his colleagues from several Georgian institutions, the USA, Spain and Switzerland have narrowed the age of the site to 1.75 Ma.  Their new find is a superbly preserved  skull, together with a lower jaw, following earlier discoveries of two other cranial fossils.  The site is well endowed with stone artefacts, similar to those of the Oldowan culture of East Africa. 

The new skull has a smaller brain capacity than co-eval H. ergaster or H. erectus in Africa, and bears some resemblance to the earliest species of human, H. habilis, although the authors prefer not to muddy the waters with yet another species of Homo.  However, had this skull been found first, they might well have gone for H. habilis, and in the paper suggest that it and the others may have descended from habilines that left Africa some time before they were preserved.  As with Sahelanthropus, no limb bones have been found at Dmansi so far.  The three fossils are not identical, and another important possibility is that these humans, like us, were polymorphic, though this needs to be tempered with the possibility of differences between males and females, or that the smallest may have been adolescent.  Others have jumped on the differences to suggest that more than one species are represented.  Here we see the problem of meagre evidence, so that anatomy alone permits either “lumping” or “splitting”.  Jonathan Kingdon, in his book Self made man and his undoing (1993, Simon and Schuster) raised the issue of polymorphism, so characteristic of modern humans, to the consternation of most palaeoanthropologists, who remain largely silent on its implications for the whole issue of human classification.

There is no doubt that early humans with primitive tools were able to expand out of Africa as early as 1.75 Ma ago.  They were not well-endowed with brain power, and they were little people – they did not stride purposefully into the wide, blue yonder.  That they reached Georgia, of all places, is extremely odd, because a direct route from Africa is barred by the Caucasus mountain range, and the deserts of Syria and Iraq.  They might have tramped around the coast of Asia Minor, following the Dardanelles to the Black Sea coast and then into the Georgian plains.  A more extreme possibility is that first they crossed the Straits of Bab el Mandab (closed at the time) and, in Kingdon’s words, “standloped” to east Asia and the backtracked along the northern flanks of the great mountains of Asia to reach the steppes.  Finds of Oldowan artefacts and meagre human remains in China also provide ages around 1.8 Ma.

Water on Mars

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From time to time Earth Pages News has tried to temper the flood of papers that seek every which way to support the notion that Mars is still well-endowed with water.  That is what NASA seeks in order to fuel its bid for the vast funds needed to launch a staffed mission to the Red Planet.  The evidence in each case was ambiguous.  I have always thought that attention and money would be better directed towards the one sixth of the human population who have no access to safe and abundant water supplies.  That remains my view, but the appearance of 10 pages of Science forces me to accept near proof of Martian water in abundance (Feldman, W.C. and 12 others 2002.  Global distribution of neutrons from Mars:results from Mars Odyssey.  Science, v. 297, p. 75-78.  Mitrofanov, I and 11others 2002.  Maps of subsurface hydrogen from the high energy neutron detector, Mars Odyssey.  Science, v. 297, p. 78-81.  Boynton, W.V. and 24 others 2002.  Distribution of hydrogen in the near surface of mars: evidence for subsurface ice deposits.  Science, v.  297, p. 81-85).

The neutron and gamma-ray detectors aboard Mars Odyssey only needed to operate for a month to reveal the abundance of hydrogen across the surface of Mars.  It varies a great deal, the highest levels showing up at high northern and southern latitudes.  Preliminary modelling suggests that these regions have at least several metres of ice-rich debris, containing between 25-35 % water ice.  Quite possibly the modelled ice-rich layer could reach a kilometre in thickness.  High anomalies at lower latitudes are modelled as being due to hydrated minerals in the Martian soil.

More results at higher precision are to come from Mars Odyssey, and experts emphasize that the reported modelling of neutron fluxes and those of gamma rays emitted by neutron-capture reactions is complex and preliminary.  However it does look like NASA scientists will soon by selecting sites for future landings on Mars.  Even more certain, it will have sent a frisson of excitement through those intent on the glory of finding signs of life there.

Tungsten and Archaean heavy bombardment

One of the major revelations that arose from the Apollo missions to the Moon is that the vast maria basins, filled with basalt, formed when a series of huge impacts wracked the lunar interior.  Surprisingly, they formed between 4 to 3.8 Ga ago, rather than in the earlier evolution of the Moon, and this “late heavy bombardment” (LHB) spans the period when the oldest rocks were forming on the Earth.  Controversy has raged for 3 decades about whether the LHB had a major influence on early Archaean geology.  The problem was that direct evidence has been hard to find, and difficult to get across to critics of such outlandish notions.  A careful investigation by geochemists from the Universities of Queensland and Oxford seems likely to force some critics to eat hat (Schoenberg, R. et al. 2002.  Tungsten isotope evidence from ~3.8-Gyr metamorphosed sediments for early meteorite bombardment of the Earth.  Nature, v.  418, p. 403-405).

Because stable 182W forms by the decay of 182Hf, with a short (9 Ma) half life, virtually none will have formed since the Earth accreted.  The 182W/183W ratio of objects from different parts of the Solar System should show distinct differences, and so they do.  Different classes of meteorites show tungsten isotopes that are significantly different from one another, and from products of mantle melting on Earth.  Ronny Schoenberg and co-workers analysed tungsten from two early-Archaean sources: the dominant grey gneisses, which are probably calc-alkaline igneous rocks formed at mantle depths, and metasediments from the famous Isua area in Greenland and another around the same age (~3.8 Ga) in Labrador.  The gneisses show no difference from later products of mantle processes, but the metasediments deviate significantly from the terrestrial isotopic composition of tungsten, towards that characteristic of meteorites.  They conclude that the metasediments mix debris formed by weathering and erosion of normal early Archaean crustal rocks with that formed in major blankets of ejecta from meteorite-induced impacts.

Exploration licence lepton by physicists

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The search for hitherto undiscovered and totally hidden hydrocarbon reserves has attracted a bizarre range of patented techniques over the years.  They range from using thermal images of the sea surface to pinpoint stationary cold spots that may mark deep water upwellings driven by rising natural gas bubbles, through helicopter borne hydrocarbon sniffers to fine-resolution aeromagnetic surveys to detect anomalies due to magnetite formed by bacteria that metabolise oil and reduce hematite to magnetite.  Most have a rational scientific basis, but there are a few that defy reason.  Most explorationists have been button-holed by dowsers, but the latest venture seems to have convinced Her Majesty’s Government, to the extent that the Department of Trade and Industry has granted three licences to explore parts of rural England, generally known for their fox-hunting aficionados.

 A company, Technology Investment and Exploration Limited of Guernsey, has invented a device that they call a “microlepton generator”, supposedly based on the Nobel-winning work of Martin Perl of Stanford University, who discovered the subatomic tau lepton in the early 1990s ( http://physicsweb.org/article/news/6/7/1 ).  They claim that their beam of microleptons, highlights areas underlain by hydrocarbon deposits, when used to illuminate satellite images.  They contend that oil generates vast amounts of microleptons that produce subtle effects on such images, but they can only be detected by microlepton beams  TIEL intends to deploy a hand-held microlepton detector from an aircraft overflying areas that they claim have given “tell-tale” signatures using their instrument.  In this respect, they are one up on particle physicists, who have so-far failed to detect microleptons under laboratory conditions.  The smallest known lepton is the electron that is 1000 times more massive than the microleptons claimed by TIEL at the base of their leading-edge technology.  Despite that, it is hardly likely to have escaped discovery by the best-financed branch of science.

Robin Marshall, a particle physicist at Manchester University, discovered that microlepton technology is based on a paper published by a Russian physicist called Anatoly Okhatrin in the journal Doklady in 1989. “He was clearly either mad, drunk or deluded,” says Marshall. “He spun a cone of lead weighing several kilograms in front of a pin-hole camera and claimed to have photographed a ‘glow’ surrounding the cone that was due to microleptons.”   Enough said?  No.  One of TIELs targets is in Charnwood Forest in Leicestershire, well-known to geologists for not being above an oil-prone basin.  Indeed the area is underlain by Neoproterozoic volcanic rocks that bolster the Midland craton of central England, which thwarted extensional basin formation from the Silurian to modern times.  Still, an onshore exploration licence is a handy item for a company’s CV.

TIEL is not the only outfit making these claims.  Another, Alkor International, seems to have a Russian link, and its website (http://www.alkorinternational.com/ ) gives details of the method it uses; “special” photographic processes, computers and software, and is also claimed to locate water resources and gold deposits!

Seismic tomography and the African superplume

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Analysis of travel paths taken by many S waves that travelled beneath the African continent, largely by geophysicists at the California Institute of Technology, shows that beneath it is a large zone of anomalously low wave speeds.  Part of the zone dips down obliquely from the rough location at shallow depths of the Afar plume beneath Ethiopia/Yemen to the core-mantle boundary between the surface locations of Africa and South America.  The structure is well placed for seismic tomography, by virtue of its good match with useful earthquakes and the world-wide network of seismometers.  More advanced analysis (Ni, S et al. 2002.  Sharp sides to the African superplume.  Science, v. 296, p. 1850-1862) shows up a strangely sharp-sided part of the plume that rises from the core-mantle boundary for about 1500 km below southern Africa.  There its boundary with more normal mantle is little more than 50 km wide.  Modelling suggests that the upward flow has caught up a dense layer with possibly different chemistry, which would result in a tilt towards the direction of movement so that instead of rising vertically, the plume would have an oblique trajectory.  The tilt also fits with Africa’s north-eastwards drift (in an absolute frame of reference, relative to other hotspots) since 100 Ma ago.

Whatever its origin, a rising, hot mantle zone beneath Africa is consistent with the continent’s high overall topography, which has encouraged the lithosphere to rift.  This extension has resulted in the East African Rift, which further encouraged partial melting in the underlying mantle and the resulting volcanism.  By far the most important aspect of Africa’s recent volcanic activity has been the Eocene to Oligocene flood-basalt event of the Ethiopian Plateau and the current activity in the Afar part of the Rift.

Subduction metamorphism and earthquakes

The recently commissioned Hi-net array of 600 digital seismometers in Japan paid dividends in an unexpected way during 2001, by picking up long-lived vibrations rather than discrete seismic events Obara, K. 2002.  Nonvolcanic seep tremor associated with subduction in southwest Japan.  Science, v. 296, p. 1679-1681).  The tremors occurred in a part of Japan where there are no active volcanoes, with which protracted vibrations are usually associated.  Their epicentres define a clear zone, at about the depth of the Moho and on the Wadati-Benioff zone where the Philippine Plate is being subducted.  This region is where dehydration reactions that convert cold, wet oceanic crust to dense eclogite, the driving force for plate tectonics through slab pull, are predicted to occur by thermodynamics.  Kazushige Obara, of Japan’s National Research Institute for Earth Science and Disaster Prevention, suggests that this correlation might fit with the release and rise of hydrothermal fluids released by dehydration of the slab.  Part of his evidence is that such tremors seem not to occur where the much older (and therefore cooler) Pacific Plate is being subducted beneath the northwest of Japan.  It probably does not undergo such reactions until it has reached about 100 km depth, where temperature would be sufficient to enter the field of eclogite stability.  Detecting fluid motion at 3 times the depth of that beneath southwest Japan might emerge with more specialized procesing.

See also:  Julian, B.  2002.  Seismological detection of slab metamorphism.  Science, v. 296, p. 1625-1626.

Continental roots

Crustal shortening and thickening in collisional orogeny produces mountain belts with a root of crust beneath them.  This truism is central to isostasy, where the mass of uplifted mountains is balanced by a compensating mass of low-density root material beneath that penetrates the mantle lithosphere.  The classic story of the reduction of mountain belts to a peneplain involves continuous isostatic uplift as the topography is eroded away.  Finally, no root remains and the exposed rocks reflect in their high-grade metamorphism a steady upward passage from the root.  Later cover rests with profound unconformity upon this peneplain.  Yet this essentially simple theory does not hold in many cases, especially for older collisional orogens.  As Karen Fischer of Brown University, USA has shown (Fischer, K.M. 2002.  Waning buoyancy in the crustal roots of old mountains.  Nature, v. 417, p. 933-936), there is a crude correlation between the age of orogens and their ratio of elevation to root thickness.  The ratio decreases from around 0.15 (root about 7 times thicker than surface elevation) in active orogens to zero before 1 Ga ago, when peneplained orogens still have a substantial root.

In order for this to happen, either the roots’ buoyancy must somehow decline with age or the mantle lithosphere which it penetrates becomes too rigid to allow isostatic uplift to occur.  Resolving which has most effect depends on analysing the gravity anomalies above orogens.  It is no easy task to model the two processes, and this is what Fischer has achieved.  She finds that mantle viscosity is not responsible, and that the cause is variation in root density.  This is probably a result of slow decline in heat flow, and the resulting mineralogical equilibria in the root.  For mafic granulite roots, a change from heat flow values of 70mWm-2 to around 40 mWm-2 could increase their density by 100-150 kg m-3, by an increase in the proportion of garnet, perhaps to the extent of producing eclogites at the deepest levels.  Eclogites would be seismically very similar to mantle lithosphere, so that even thicker, hidden roots may be present.  Reduction in buoyancy by this means could take as little as 20 Ma, before which the elevation to root thickness ratio has declined below that in active orogens.

One implication of this process is that orogenic collapse by lateral extension of highly elevated crust, which might lead to rapid root thinning, is not the general process that many structural geologists believe.  If it was, orogenic roots would be removed relatively quickly.  Decrease in root buoyancy is also a plausible explanation for the creation of cratons, where quite low-grade metamorphic rocks, formed at shallow crustal levels occupy vast areas of low-lying shields.

Flood basalts of Siberian Traps doubled at a stroke

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Erupted at the time of the Palaeozoic-Mesozoic boundary, and coinciding with the largest mass extinction during the Phanerozoic, the Siberian Traps are by far the biggest example of flood-basalt volcanism known.  They blanket a huge area of the Siberian Platform.  To the east of their outcrops is a large extensional downwarp, known as the West Siberian Basin, where recent deep drilling has cut through up to 1 km of flood basalts.  Dating samples from 15 boreholes proves that these too are members of the Siberian Trap suite (Reichow, M.K. et al. 2002.  40Ar/39Ar dates from the West Siberian Basin: Siberian flood basalt province doubled.  Science, v. 296, p. 1846-1849).  Combined, the two zones of Siberian Traps represent eruption of around 2.3 million km3 of plume-derived magma at around 250 Ma ago, possibly within 2 or 3 Ma.  Gas release from such a stupendous event is implicated in the Permian-Triassic mass extinction, either through climate change associated with CO2 and SO2, or toxic effects of hydrofluoric acid.  Unlike the end-Triassic and K-T extinctions, no clear evidence has emerged for coincident flood volcanism and major impact at the end of the Palaeozoic Era.  However, the use of tungsten isotopes as “fingerprints” for extraterrestrial debris in boundary sediments may help resolve the issue of whether an impact accompanied the Siberian Traps (see Tungsten and Archaean heavy bombardment, this issue)

Early Argentines did not witness a meteorite impact

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Ten years ago, planetary scientist Peter Schultz and Argentine pilot Ruben Lianza observed several depressions shaped like tear drops while flying over the Pampa. Because they also found meteorites and tektite glass when they examined the structures on the ground, it seemed certain that the depressions had formed by the impact of bodies travelling almost parallel to the Earth’s surface.  The structures were clearly no more than a few thousand years old, and the discovery encouraged lurid artistic impressions of terrified native South Americans cowering from an extraterrestrial firestorm.  The Rio Cuarto structures were a godsend for those who fear social and economic disaster from Earth-bound NEOs (near-Earth objects), and have been lobbying for a sky watch for impending doom.

In reality, the Pampas of northern Argentina has hundreds of similar structures over an area of more than 50 thousand square kilometres, and their long axes parallel the prevailing wind direction (Bland, P.A. and 10 others 2002.  A possible tektite strewn field in the Argentinian Pampa. Science, v. 296, p, 1109-1111).  They are “blow-outs” developed in the fine loess soils of the Pampa, and much the same structures affect most loess plains.  Being formed of wind-blown silica and clay dust, loess is not well known for its content of objects above a millimetre in size, so any larger objects found on wind-deposited plains stand a high chance of having arrived by some extraterrestrial process.  Meteorites and tektites are rare, but ablation concentrates them in wind-blown depressions as they are too heavy to be blown away.  That is the likely origin of the objects that Schultz and Lianza used in support of their hypothesis of impact devastation wrought on early South Americans.  Phil Bland of the Open University, and his colleagues from Brazil, the USA, Australia, Russia, Argentine and Britain, were able to date organic matter in the Rio Cuarto structures using the C-14 method at 4000 years.  Yet Ar-Ar ages of the meteorites range from 52 to 36 thousand years, so the two are unconnected.  The glassy tektite fragments provided yet another age of 57 thousand years.  Along with similar glasses at a couple of other sites in Argentina, these support melting of the homogeneous loess by an impact around that time, although no crater from which they might have been ejected is known.  The search is on for the source of a hitherto unknown field of strewn tektites, although it seems strange that in the featureless plains of southern South America one hasn’t shown up long before now.

The mantle’s breath and Earth’s early evolution

Many lavas contain bubbles, which form when gases dissolved under pressure in magma froth out at low pressures.  For the most part the gas is water vapour, carbon dioxide and sulphur dioxide.  It comes from mantle peridotite, and represents the volatile fraction of the deep Earth.  But there are traces of other gases, the most revealing of which are the noble gases helium, neon, argon, krypton and xenon, because some of their isotopes originate from radioactive decay of other elements (mainly potassium, uranium and thorium.  Noble gases in basalts offer important insights into how the mantle has evolved since the origin of the Earth.  Chris Ballentine of the University of Manchester, reviews how such trace-gas isotopes in basalts help resolve some otherwise intangible challenges (Ballentine, C.J. 2002.  Tiny tracers tell tall tales.  Science, v. 296, p. 1247-1248).