Category: Geochemistry, mineralogy, petrology and volcanology

Potted history of atmospheric oxygen

The most likely hallmark of an inhabited planet is an atmosphere that contains oxygen; a simple rule of thumb made popular by James Lovelock.  By assembling complex molecules based on carbon, life increases the degree of chemical reduction in its environment.  Effectively it draws in electrons, and the counterpart of that must be that some other component loses them through oxidation.  On Earth the source of electrons needed to make organic molecules through the action of photosynthesis is predominantly the oxygen atoms locked in molecules of water and carbon dioxide.  By losing 4 electrons, 2 oxygens bonded in those two simple compounds are oxidised to become the gas O₂, which itself has become the commonest and most active acceptor of electrons from reduced ions and compounds.  Oxygen gives its name to oxidation, which is the inevitable fate of most organisms, thereby reversing the process of photosynthesis.   A planet whose surface topography is continually changing, because more radioactive energy is produced in its mantle than can be lost to space by simple conduction, generates physical conditions that continually bury and store some unoxidised carbon compounds.  Carbon burial together with continued living processes keeps the photosynthetic chemical equation weighted in favour of free oxygen.

Since the domain of living things to which we and all advanced organisms belong, the Eukarya, is almost wholly one to which oxygen is vital in metabolism, there can be few more important geoscientific topics than how and when oxygen emerged as a free element.  There have been major recent developments in addressing these questions, so it is useful and fascinating to find an up-to-date and easily read review (Kerr, R.A. 2005.  The story of O₂.  Science, v. 308, p. 1730-1732).  Among its highlights is evidence that although cyanobacteria (the most primitive oxygenic photosynthesisers) were definitely around at 2.7 Ga, they may not have produced oxygen until about 300 Ma later, when the first signs of free environmental oxygen appear.  Photosynthetic release of oxygen during life’s early period was not the only reduction-oxidation regime adopted by organisms.  Another of huge importance was generation of methane, which can rise to the limits of the atmosphere unlike the other major hydrogen-bearing gas, water, which is condensed out at quite low altitudes.  Photochemical breakdown of methane at the limits of outer space would release hydrogen to leak away from the Earth, removing a reductant gas that would otherwise consume highly reactive oxygen: without this process, modelling suggests that Earth’s atmosphere would never have accumulated free oxygen, even had primitive life emerged.

Once free oxygen appeared, about 2.4 Ga ago, it took almost 2 billion years for enough to accumulate so that complicated, multicelled Eukarya could use its potential (see The Malnourished Earth hypothesis – evolutionary stasis in the mid-Proterozoic in EPN of September 2002). What kept the levels down?  Quite probably it was oxidation of sulfide minerals on exposed land.  That supplied sulfate ions to a still reducing ocean, so that sulfide ions formed again to become metal sulfide precipitates, which drew from ocean water several essential nutrients for Eukarya.  Oxygen-producing Eukarya (algae) would not be able to bloom because of this ‘starvation’.  Nonetheless, about 600 Ma ago, surface oxidation potential soared to almost modern levels, sufficient for large organisms to appear and evolve, to lead to life as we know it. Another series of questions surrounds this tremendous event, but they remain to be answered convincingly.

At a rough estimate the material that has pushed back the oldest direct dating of supposedly continental material is about the size of a pinch of salt.  It consists of detrital zircon grains contained in Archaean sedimentary quartzites from Western Australia, the oldest of which give U-Pb ages of 4.4 Ga, 400 Ma older than the earliest rocks of the continents.  Arguably, the zircons are products of repeatedly recycled debris from the earliest silica-rich magmas formed in the Hadean: zircon is hard and not affected by sedimentary processes.  Any subduction processes in the early Earth might well have produced silicic magmas by a variety of petrogenetic processes: modern ocean crust contains tiny amounts of plagiogranites.  Minute inclusions of quartz, mica and feldspar in the zircons suggest that such igneous rocks may have formed by partial melting of the clay-rich sedimentary veneer on Hadean oceanic crust  when it descended.  So, the only surprise in a chronological sense is that a few grains have been found among those formed in the 1.4 Ga until the deposition of the 3 Ga old Jack Hills quartzite in which they found a resting place.  The zircons are controversial for another reason.  They contain high concentrations of ¹⁸O that indicate a role for water in their formation.

Bruce Watson and Mark Harrison of the Rensselaer Polytechnic Institute, New York and the Australian National University have devised a way of establishing the temperatures at which the zircon formed, from their content of titanium (Watson, E.B & Harrison, T.M. 2005.  Zircon thermometer reveals minimum melting conditions on earliest Earth.  Science, v. 308, p. 841-844).  Their results from 54 zircons aged from 4.0 to 4.35 Ga cluster around 700°C, which is what would be expected had their parent magmas formed at the minimum temperature for partial melting of sediments to form granite-like magmas in the presence of a water-rich fluid (the “wet-granite minimum”): they look very similar to modern zircons.  This confirms the results from earlier oxygen-isotope studies.  Because the oldest of the Jack Hills zircons are only 75 Ma younger than the mighty thermal effect of the Earth’s collision with a smaller planetary body that excavated matter that formed the Moon, the influence of water in the zircons’ formation has been interpreted as having monumental significance for the effectively vanished 400 Ma-long Hadean Eon.  It has been taken as support for oceans at the Earth’s surface, as well as “normal” plate tectonic processes that can generate continental crust, but also that conditions amenable to pre-biotic chemistry and even the origin of life existed.

The Earth could not have escaped the massive Hadean bombardment of the lunar surface by planetesimals that climaxed between 4.0 and 3.8 Ga.  Rocks from the lunar highlands preserve ages back to 4.45 Ga, close to the time of its origin, and at that time the Moon must have had a solid crust below about 400°C for radiogenic isotopes to accumulate in minerals.  The Earth equally must have had at least a surface veneer of relative cool rock at that time.  So, since the Apollo samples yielded these dates in the 1970’s, the popular image of a long-lived magma ocean has been insupportable, even though it probably existed shortly after the cataclysm of the formation of the Earth-Moon system.  In that sense, evidence in ancient zircons for plate-like processes is not a surprise, although an interesting confirmation of long-held beliefs.  Nor does their showing the influence of water come as a shock.  The Earth is tectonically active partly through it not having been thoroughly dried by Moon formation; lunar rocks are a great deal drier and the Moon is as dead as a doorknob.  At 700°C water cannot exist as a liquid, so its influence in partial melting is not evidence for surface water.  However, the most efficient means of heat loss from any heated body is by radiation to space, and simple calculations show that it would be highly unlikely for Earth not to have had liquid surface water about 100 Ma after Moon formation.  That in itself indicates that there would have been a water-rich atmosphere too. No matter how much  “shock and awe” might colour our view of repeated bombardment during the Hadean, no sane impact theorist has suggested that sufficient energy was delivered to recreate a global magma ocean.  Water may have been boiled off to the atmosphere by the biggest, but only to fall again as rain between major impacts.  Given favourable chemical conditions and liquid water, the route to life might well have opened up in the Hadean itself: some have suggested that it happened again and again only to be snuffed out by high powered impacts, until the Inner Solar System became a safer place after 3.8Ga.  The real mystery of the aged zircons concerns the rocks in which they crystallised: where on Earth are they?  Four decades of radiometric dating of actual rocks has failed to break the 4.0 Ga barrier, so if relics do remain they are either buried or have been reduced to sediments, as the Jack Hills quartzite so nicely demonstrates.

See also: Reich, E.S. 2005.  What the hell…?  New Scientist 14 May 2005, p. 41-43.

About 2.3 billion years ago, ancient soils begin to reveal that Earth, or more precisely life upon it had developed an atmosphere that contained oxygen, albeit at quite low levels.  One of the most interesting events during the Proterozoic Aeon was the world-wide disappearance of vast deposits of iron oxides known as banded iron formations or BIFs, at about 1.8 billion years.  Many authorities view that as the time when sufficient oxygen was dissolved in seawater to have removed soluble Fe-2 at its source, on the ocean floor near hydrothermal vents – BIFs formed in shallow water, and that requires Fe-2 to have permeated the entire oceans.  There is another possibility.  The presence of atmospheric oxygen would have ensured the oxidation of iron sulphide exposed at the land surface, thereby adding sulphate ions to river water, and eventually seawater.  Another line of evidence for atmospheric oxygen is the disappearance of detrital sulphide grains from sedimentary rocks younger than 2.3 billion years, so a build-up of sulphate ions in later seawater is quite plausible.  Should deep-ocean chemistry have been reducing, it is possible that sulphide ions would form there.  The insolubility of iron sulphides would then remove Fe-2 from seawater equally as efficiently as would oxygen.  Danish and Canadian geochemists have investigated this possibility using data from sediments in Canada that mark the last phase of major BIF deposition around 1.8 billion years (Poulton, S.W. et al. 2004.  The transition to a sulphidic ocean ~1.84 billion years ago.  Nature, v. 431, p. 173-177).  They found that conditions changed from one in which seawater contained dissolved Fe-2 at the time of the last BIF deposition to one dominated by sulphide ions, similar to that found in modern anoxic waters such as those in the Black Sea.  That would have sequestered any available Fe-2 to pyrite in sediments, a feature typical of many later Proterozoic sediments.  Since seawater during the Phanerozoic was dominated by sulphate ions, except in periods of ocean anoxia, it looks likely that late Precambrian sulphidic oceans gave way to more modern sulphur chemistry following a rapid rise in atmospheric oxygen at the end of the Proterozoic.  One consequence of highly-reducing deep ocean water would have been very efficient burial of dead organic matter while it lasted, because anaerobic bacteria do not fully convert organic molecules back to water and carbon dioxide.  During the Neoproterozoic d¹³C in seawater underwent rapid swings from highly negative to highly positive, on which all kinds of connotations have been placed.  Another explanation for the carbon hiccups might be that periodically there were short-lived increases in oxygenation of deep ocean water.

The Himalaya and Tibet are known for their huge granite batholiths that show geochemical signs of having formed by partial melting of the continental crust.  They also show signs of ductile zones in the deep crust.  Whether or not this ductility is associated with incipient melting cannot be judged easily, as there are no examples of active felsic volcanism.  However, it is possible to predict theoretically where crustal temperatures exceed the solidus of the crust, whose paths in pressure-temperature space for various amounts of water content is well known.  The problem is knowing the way in which temperature increases with depth.  That is usually estimated from the surface heat flow and modelling the likely thermal conductivity of different crustal layers, but it isn’t suitably precise.  German, US and Chinese geophysicists have tried a clever means of estimating crustal temperature using seismic data (Mechie, J. et al. 2004.  Precise temperature estimation in the Tibetan crust from seismic detection of the a-b quartz transition.  Geology, v. 32, p. 601-604).  Experiments show that quartz in its low-temperature a form transforms to b quartz above 575ºC at atmospheric pressure, and at higher temperatures with increasing pressure.  The P-T change in the transition is well known, so if b quartz can somehow be detected in the deep crust, its depth gives the crustal temperature.  As luck would have it, the transition results in a significant change in the elastic properties of quartz that should effect the speed at which seismic waves travel through rocks rich in b quartz.  More precisely, the P-wave speed should increase abruptly by a detectable amount.  Mechie and colleagues have indeed found the depth of this transition below a seismic profile running across part of the Tibetan Plateau NNW of Lhasa.  Its depth varies between about 20 to 15 km coinciding with the upper-middle crustal boundary.  At its shallowest levels, the transition is directly above a large zone of high electrical conductivity, discovered by magnetotelluric surveys, which has been suggested to be due either to a high content of aqueous fluids or crustal melts.  The geotherm (about 40ºC km^-1) associated with the shallow a-b quartz transition crosses the wet granite solidus about 5 km beneath it, so the lower crust itself is likely to be generating granitic magmas.  Although the deepest levels of the  a-b quartz transition also predict likely conditions for wet melting in the lower and middle crust, below those zones there is no evidence that it is happening.  One possibility is that water content varies considerably in the sub-Tibetan crust.  Where melts or fluids are moving in the crust, heat transfer is not purely by conduction, and steep geothermal gradients can stem from heat being transported upwards with moving fluids.

There has been speculation, particularly by Thomas Gold in his book The Deep Hot Biosphere, that methane can form without the intervention of organisms.  In Gold’s case, he proposed an origin in the mantle that supported a thriving organic community at great depth in the crust, and that such abiogenic methane is the source of all hydrocarbon and coal deposits.  Not many people believe Gold.  However, there are chemically feasible means of generating simple hydrocarbons in the upper earth, notably the Fischer-Tropsch catalytic process that has been used to synthesise artificial fuels.  The Fischer-Tropsch process hydrogenates a carbon-bearing gas, such as carbon dioxide, and commercially has used chromium oxide as a catalyst.  In hydrothermal systems that permeate olivine- and orthopyroxene-rich ultramafic rocks, those minerals breakdown to serpentines, talc and magnetite, and the reactions generate hydrogen, which is often found dissolved in samples of oceanic hydrothermal fluids and occasionally in onshore springs, where mantle rocks in ophiolites are being weathered.  So there is no shortage of hydrogen for potential reactions in sea-floor hydrothermal systems, and they contain lots of dissolved carbon dioxide.  Ultramafic rocks are rich in chromium generally in the form of Fe-Cr oxide or chromite.  Geochemists from the University of Minnesota simulated a hydrogen-carbon dioxide-chromite hydrothermal system to see if the Fischer-Tropsch process would work (Foustoukos, D.I. & Seyfried, W.E. 2004.  Hydrocarbons in hydrothermal vent fluids: The role of chromium-bearing catalysts.  Science, v. 304, p. 1002-1005).  It did, producing methane, ethane and propane under simulated conditions of sea-floor vents.  They conclude that these simple hydrocarbons help support thriving bacterial communities in “black smokers”.  Their results also support the possibility of such vents having produced “feedstock” for processes that led to the origin of life, but also lend a cautionary note to claims for ancient organic matter (see Early biomarkers in South African pillow lavas in May 2004 EPN)

“Everyone knows” that free atmospheric oxygen appeared about 2300 million years ago, thanks to the waste products of blue-green bacterial photosynthesis.  At least the land surface became an oxidising environment and a progressively redder place, as Fe-2 was oxidised to Fe-3 which forms insoluble oxides and hydroxides.  Paradoxically, the shallow sea floor of earlier times was redder than anything since, because of exactly the same oxygen-containing, ferric minerals.  It hosted the largest build-up of any metal concentration in Earth’s history; the banded iron formations (BIFs) that have for a century or more been the source of industrial iron.  A simple, and probably accurate explanation for BIFs is that iron dissolved in ocean water that lacked oxygen as Fe-2, and was supplied by sea-floor volcanism.  Once blue-green bacteria began pumping out oxygen, an oxidising reaction dumped both elements as slimy red sediment where the two met.  Dissolved iron consumed oxygen – just as well, because to most prokaryote life it is a poison – yet as oxygen productivity rose (and perhaps sea-floor spreading slowed) dissolved iron was increasingly removed by oxyidation from sea water.  The tipping point, when air contained oxygen and sea water became starved of iron (a vital micronutrient for phytoplankton) is difficult to address since the two chemical environments are so different and interact in complicated ways.  BIFs continued to form for about half a billion years after the first sign of atmospheric oxygen, then they disappear from the geological record at 1800 Ma ago.  There were minor reappearences in the Neoproterozoic, at the time of “Snowball Earth” events, and that is a fascinating topic in its own right.  Clearly, there was a long period of transition to what we can regard as a thoroughly modern world.  Studies that use sulphur isotopes suggest that in the Mesoproterozoic the upper ocean was oxygenated while bottom waters were perpetually akin to those of the Black Sea today.  Conditions in them may have been highly conducive to burial of dead organic matter – rapid drawdown of atmospheric CO­­­₂, but allowing the massive production of methane by anaerobic bacteria.  Methane is a far more potent greenhouse gas than carbon dioxide, so controls over climate may have been very different from today’s.  Molybdenum offers an independent and potentially useful means of testing hypotheses about ocean chemistry.  It enters the sea in river water, which in post 2300 Ma times would have been oxygenated, allowing the formation of the soluble and very stable molybdate ion.  In anoxic ocean floor conditions, bacteria that generate hydrogen sulphide remove molybdenum as the sulphide, which is why modern Mo concentrations remain stable – it ends up in a very small percentage of ocean floor sediments.  The stable isotopes of molybdenum (⁹⁷Mo and ⁹⁵Mo) fractionate during precipitation of the element, the heavier one being preferentially removed during sulphide precipitation, to give high ⁹⁷Mo/⁹⁵Mo ratios in sediments.  The opposite seems to occur if precipitation is in the oxide form, as in sea-floor manganese nodules.  Geochemists from the Universities of Rochester and Missouri, USA have compared Mo isotopes from apparently anoxic Mesoproterozoic sediments with those in modern euxinic basins (Arnold, G.L. et al. 2004.  Molybdenum isotope evidence for widespread anoxia in mid-Proterozoic oceans.  Science v. 304, p. 87-90).  The Precambrian results are isotopically much lighter than modern ones, suggesting that ⁹⁷Mo did not become enriched in seawater as a result of oxide precipitation in the equivalent of modern manganese nodules.  They estimate that 10 times more of the ocean floor was anoxic than today or since about 1300 Ma ago.  So far no comparable work has been done of the extremely abundant black shales and schists of the Neoproterozoic, that link with “Snowball Earth” events.  Whether or not “modern” redox conditions emerged 1300 Ma ago, with probably a big impact on climate controls, the oddest time climatically was between about 750 and 600 Ma ago.  Not only were there several dramatic coolings and warmings, but the main indicator of organic carbon burial, d¹³C, went haywire.  As did the BIFs, did ocean anoxic conditions once more get footholds.  Molybdenum isotope data seem likely to shed some light on  those strange times.

A great deal of effort and innumerable theses and papers have gone into modelling the derivation of magmas from their parent rocks, especially the mantle, over the last three decades.  Most is based on the division of trace elements into “compatible” and “incompatible”, the first being those which tend to remain in minerals that make up the residuum during magmagenesis, and the second those that favour melts.  Most incompatible elements have large ionic radii. The modelling centres on the degree to which elements remain in solids, the appropriate parameter being an element’s mineral-melt partition coefficient (K_D).  Partition coefficients are usually deduced from an element’s abundance in phenocrysts that are in contact (and supposed equilibrium) with an igneous rock’s groundmass material, which is assumed to have formed from magma, and its concentration in that once liquid phase.  Models for partial melting and fractional crystallisation, plus several variants, all involve K_Ds, for olivines, pyroxenes, feldspars, garnet, amphiboles and so on.  For the generation of basaltic magmas, the first step is partial melting in the mantle itself, for which direct estimation of K_Ds is not possible.  Instead they are assumed from mineral-melt chemistries in crustal igneous rocks, with some allowance for elevated temperatures and pressures and other conditions.  Each mineral has its own distinctive suite of K_Ds for many elements, and the chemistry of an igneous rock has often been traced back to which suite of minerals was present in a residue, i.e. the source rock itself, as well as the degree to which one or other process proceeded.  The 19 February 2004 issue of Nature included an ominous article (Hiraga, T, et al. 2004.  Grain boundaries as reservoirs of incompatible elements in the Earth’s mantle.  Nature, v. 427, p. 699-703). 

The study by geochemists at the University of Minnesota and Oak Ridge National Laboratory, USA, concentrated only on the mineral olivine, and a few elements present at trace levels in it.  Their experiments simulated equilibrium conditions under mantle conditions.  Results showed that incompatible elements in olivine, such as Ca and Al, tend to concentrate mainly at boundaries between grains where they are readily available to any melt that starts to form, rather than uniformly throughout the mineral grain.  The finer the grain size of the rock, the greater the area of grain boundaries, and so the more incompatible elements tend to be concentrated at them  The tendency is predictable on thermodynamic grounds, but has only been studied previously in alloys and other artificial materials.  Geochemists have generally regarded grain boundaries as places where impurities in rocks gather.  If the same rock is analysed with and without the crushed powder having been washed in acid, different trace element concentrations result.  This has been attributed to secondary effects, such as the passage of hydrothermal fluids or groundwater.  Since K_Ds that are used widely involve concentrations in whole mineral grains, the basis of geochemical modelling might be compromised.  Melting begins at grain boundaries, so the low degrees involved in generating basalts could be biased by the effect.  Moreover, vapour phases moving through the mantle (supercritical water and CO₂), will follow grain boundaries too, and so may easily pick up and transport incompatible elements.  Their entry into the crust carrying mantle-derived incompatible elements, such as rare-earths, strontium and lead, would lead to metasomatic effects that could play havoc with interpretations of isotopic data based on these elements.  Carbonatites, probably formed from mantle-derived carbonic fluids, are enriched in many incompatible elements.  Similarly worrying data, such as estimates of the incompatible element partitioning into carbonic fluids, have emerged in the past, but so far have been notable only for the silence with which most geochemists greeted them.

Given the growing controversy about whether or not plumes of mantle rock can rise from the core-mantle boundary to source large igneous provinces (see Geoscience consensus challenged in EPN January 2004) the hypothesis has been tested by seeking material in hot-spot lavas that may have crossed from the outer core into the deepest mantle.  Some hot-spot lavas contain traces of Osmium-186 that may have formed by decay of an unstable platinum isotope (¹⁹⁰Pt) that is most likely to be enriched in the core, thereby supporting the hypothesis.  Another isotopic approach is to look at tungsten isotopes (Scherstén, A. et al. 2004.  Tungsten isotope evidence that mantle plumes contain no contributions from the Earth’s core.  Nature, v. 427, p. 234-237).  Tungsten, like osmium, has a strong affinity for iron, and the bulk of terrestrial W is likely to be present in the core.  One isotope ¹⁸²W forms from the decay of an unstable isotope of hafnium ¹⁸²Hf, whose half life is geologically short (about 9 Ma).  As a result all ¹⁸²W in the Earth must have been produced in the first 60 Ma of the planet’s evolution.  Moreover, hafnium is likely to favour the mantle far more than the core, so most ¹⁸²W seems likely to be present in the mantle and the core should be depleted in it.  This is borne out by comparing values in primitive meteorites with those in mantle-derived lavas; the mantle is enriched by comparison.  So, if there was significant chemical exchange between the core and mantle a lot of tungsten with very low ¹⁸²W should contaminate the lower mantle.  If plumes did rise from the core-mantle boundary, then lavas derived from them ought to have anomalously low ¹⁸²W contents. Scherstén and colleagues from the University of Bristol and the Australian National University show that Hawaiian lavas (the same samples used to suggest a mantle-wide plume beneath Hawaii using osmium isotopes) and South African kimberlites do not show this signature, and argue convincingly that the osmium data must represent another source of contamination, probably recycled crustal rocks.  However, that does not rule out a plume rising from the core-mantle boundary, just that the core did not play a significant geochemical role.

The circulation of ocean water through new oceanic crust not only cools oceanic lithosphere sufficiently for it to droop and help drive sea-floor spreading.  It also re-emerges as hot submarine springs that today host curious ecosystems, which depend entirely on energy and chemicals that spew out of these “smokers”.  The chemistry of life molecules, particularly the metals in them, reveals a blend that is surprisingly similar to that of hydrothermal fluids.  This, along with other matters, such as the highly primitive genetics of thermophilic bacteria, make sea-floor hydrothermal vents or the crust beneath them excellent candidates for the cradle of life’s origin.  So getting samples of the very earliest such fluids has to be among the most exciting discoveries relevant to palaeobiology.  Jacques Touret of the Free University of Amsterdam, one of the pioneers of fluid inclusion studies, believes that he has found some (Touret , J.L.R. 2003. Remnants of early Archaean hydrothermal methane and brines in pillow-breccia from the Isua Greenstone Belt, West Greenland.  Precambrian Research, v. 126, p. 219-233).  The host rock is an undeformed, but metamorphosed breccia made of basaltic pillows from the famous Isua greenstone belt of West Greenland, which formed about 2.8 billion years ago.  Quartz crystals in amygdales and veins that cement the breccia together contain minute fluid inclusions.  There is little of interest in that fact alone, for most igneous or metamorphic minerals trap samples of the fluids involved in the origin of the host rocks.  What is intriguing abut the Isua fluids is their high content of methane and brine; just as expected from low temperature hydrothermal fluids.  Their chemistry compares well with that of inclusions in altered basalts from modern oceanic crust, in which bacterial activity is implicated.  Metamorphism generally results in carbon dioxide as the main carbon-containing gas in fluid inclusions.  Formation of methane in sea-floor environments can be biologically controlled, but the hydration of deeper ultramafic rocks to serpentine can also generate enough hydrogen to reduce CO₂ to methane abiogenically.  The full association at Isua suggests carbon-dominated hydrothermal activity, which today precipitates carbonates at vents, forming so-called “white smokers”.  [“Black smokers” are sulphur dominated, and take their name from the massive precipitation of metal sulphides when the fluids emerge at the seabed.]  These create alkaline conditions that are well suited to bacterial growth.  Touret does not claim that the inclusions indicate living processes, merely that the right conditions were around in the earliest Archaean for life to thrive.  It would be an immense feat if he subsequently discovers bacterial fossils in the inclusions, but that is highly unlikely.  However, the brines might provide proxy evidence, because living cells uniquely accumulate bromine from sea water.  Anomalous ratios of chlorine to bromine might point strongly towards life having been around during Isua times.

See also:  Hecht, J.  2003.  Droplets may reveal life’s oceanic beginnings.  New Scientist, 13 September 2003, p. 25.

The British press has been awash with speculation that the Prince of Wales is worried about nanotechnology and the slim possibility that the next big threat after Osama and SARS might be minute, self-replicating robots that invade our bodily orifices.  It stemmed from the Prince of Wales’ having asked experts for a briefing, and that may well have been just HRH’s curiosity about a changing world.  There is rarely an issue of the weekly science journals without news of some discovery of phenomena that occur in nanotubes and minuscule cavities; the world at scales less than a micrometre is beginning to seem strange.  Rocks are full of pore spaces and inter-grain boundaries with the dimensions on which new wings of the other sciences are emerging.  So it is no surprise to learn that there will soon be “nanogeochemistry” (Wang, Y. et al. 2003.  Nanogeochemistry: geochemical reactions and mass transfers in nanopores.  Geology, v. 31, p. 387-390).  The use of natural and artificial zeolites as ionic filters has been around for a long time, so this is a branch with a new name, rather than a fundamental breakthrough.  But zeolites are profitable, and only now has “blue-skies” research turned up the magnification.

Typical nanopores and pathways are grain boundaries in crystalline rocks, cleavage planes in phyllosilicates and clay minerals, and pores in fine-grained sediments, such as diatomite and kaolin, and minerals that have been precipitated as amorphous masses rather than discrete crystals, a good example being the iron oxy-hydroxides in soils.    To see these structures requires advanced transmission electron microscopy, and even with them the features are somewhat indistinct.  Nanopores can make up to 40% of a material’s porosity, and having such minute radii they contribute as much as 90% of the internal surface area that is exposed to chemical reactions.  Artificial materials that show nanoporosity have internal surface areas as high as hundreds of square metres per gram. Clearly, such materials in nature must play a major, but largely uncharted role in geochemical change.  Among the oddities discovered by Wang and colleagues at the Sandia National Laboratories and the University of New Mexico, are inclusions of native copper in weathered clay minerals and equally small particles of gold along microfractures in mylonites.  Their experiments with artificial simulants of natural fine-grained materials focussed on two simple phenomena: the electrical charge on small surfaces in relation to acidty; and their ability to absorb trace elements.  The paper is highly technical, but the conclusions are surprising .  Nanopores develop unusually high surface-charge densities that should affect their ability to adsorb ions, and also exert controls on reactions that might seem unlikely in macro-scale simulations of geological conditions.  Indeed, finely porous materials enrich trace elements by an order of magnitude compared with isolated small particles, and encourage precipitation or solution of different compounds when that would be unexpected in more open systems.  As well as bearing on burial of toxic and radioactive wastes, and on mineralising processes, nano-scale processes are probably central to the whole process of weathering.  Interestingly, such small scales exclude even the tiniest bacteria, so that the geochemical processes seem unlikely to impinge on life.  However, spaces in rocks comprise a nested series of dimensions, and changing conditions may well flush material from one scale to another.  In particular, bacteria of various kinds can control pH at the micro-scale, thereby creating the ambient conditions for nano-scale geochemistry.

Potassium in the core

It might seem impossible for planetary cores dominated by iron-nickel alloys to contain any source of heat generation.  The main three elements (uranium, thorium and potassium) with long-lived radioactive isotopes and sufficient abundance to produce substantial heat energy are all highly concentrated in the Earth’s crust.  That is because they are incompatible with the minerals in mantle rocks, and so readily enter magmas that contribute to continental growth.  However, the only natural materials that bear any resemblance to geoscientists’ notions of core materials, metallic meteorites, contain abundant sulphur.  Theoretically, potassium can enter sulphide minerals.  So, since as long ago as the 1970s there has been debate about whether motion in the core was driven entirely by residual heat from Earth’s accretion and the formation of the core, or that it contained its own heat source in the form of ⁴⁰K.  If the first was true, then the self-exciting dynamo responsible for the Earth’s magnetic field has been running down over geological time, because heat is transferred across the core-mantle boundary, eventually to reach the surface by convection.  The existence of a solid inner core might result from such cooling, though its formation would release latent heat of crystallization and prolong inner motion.  However, some calculations suggest that core motion and so geomagnetism ought to have vanished long ago, through loss of core heat to the surface.  Substantial potassium in the core would demand considerable revision of ideas about the bulk evolution of the Earth, and other rocky planets.  Experiments to prove that iron-sulphur alloys can contain abundant potassium have had a chequered history.  Research at the University of Minnesota and the Carnegie Institute of Washington has discovered why there were such ambiguous results (Murthy, V.R. et al, 2003.  Experimental evidence that potassium is a substantial radioactive heat source in planetary cores.  Nature, v. 423, p. 163-165).  The problem was in the preparation of samples for analysis.  Rama Murthy and colleagues found that the oils used in polishing samples for electron-microprobe analysis actually leach potassium from the sulphides in them, nearly all disappearing in a few days of contact.  With great care, they repeated experiments on mixtures of metallic iron, iron sulphide and potassium bearing glass held at high temperature under pressures between 5 and 10 % of those experienced in the core.  Their results show that potassium can indeed enter core materials with high sulphur contents.  The higher the temperature the more gets in, and their most extreme run saw almost 4 % K in the quenched sulphide.  Plan are afoot to discover if uranium and thorium might also be in core materials.

Incidentally, in the week that the film The Matrix: Reloaded was premiered in the USA, a proposal to send a probe to the core-mantle boundary also appeared (Stephenson, D.J. 2003.  Mission to Earth’s core – a modest proposal.  Nature, v. 423, p. 239). David Stephenson, of the California Institute of Technology, builds on the notion of the “China Syndrome”, in which meltdown of the core of a nuclear reactor would lead to superdense molten uranium melting its way through the mantle.  In his proposal, ruggedised instruments in a capsule the size of a grapefruit would make the journey, along with about 10 million tons of molten iron, by propagating a large crack started by a 10 Mt nuclear explosion.  Data is to be transmitted by modulated acoustic signals in the kHz range.  The article helps to demonstrate the delays in publication, even in a prestigious weekly journal; it should have appeared 6 weeks earlier….