Month: February 2004

A “Whoops” moment for geochemists?

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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 (KD).  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 KDs, 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 KDs 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 KDs 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 KDs 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.

The creators of worlds

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Inverting Robert Oppenheimer’s memory of the line in the Bhagavad Gita, “I am become Death, the destroyers of worlds”, during his Road-to-Damascus moment when the first atomic weapon was tested, may seem an odd headline for an article on geochemistry.  But geochemists sometimes do give the air of being on the verge of solving the “Big Question”.  Alex Halliday of ETH in Zurich is one of them (Halliday, A.N. 2004,  Mixing, volatile loss and compositional change during impact-driven accretion of the Earth.  Nature, v. 427, p. 505-509). It is now well accepted that Earth’s early evolution was one of repeated big impacts during planetary accretion.  It probably culminated in a collision with a Mars-sized planet that not only created the Moon from the debris splattered from both bodies, but set the Earth’s chemistry for all subsequent time; a sort of geochemists’ Year Zero.  When that happened and what ensued has all manner of connotations (see Geoscience consensus challenged in EPN for January 2004).  Halliday reviews evidence from several isotopic systems (Pb, Xe, Sr, W) that are reckoned to be appropriate “fingerprints” for the environments in which planets accreted.  His treatment takes the data as a whole, rather than separated into one or another isotopic system. He begins with the assumption in most accretion models that metallic cores form continuously and in equilibrium with the silicate outer mantle of rocky planets.  That is important in using W isotopes to model the “when”, since tungsten is likely to enter iron-rich metal rather than silicates (see Mantle and core do not mix in EPN February 2004).  In fact estimates for the time taken for the Earth to gather 2/3 of its mass based on W isotopes (~11 Ma) are a lot faster than those based on other isotopes (between 15 to 40Ma).  Halliday’s explanation is the seemingly sound one that when big things form from smaller ones (whatever contributed to core and mantle), the chances of them mixing and reaching equilibrium, before they definitively separate into the inner and outer Earth, are not good.  Reviewing the somewhat bewildering permissiveness of isotopic data from Earth and Moon that bear on “Year Zero” he concludes that the massive loss of xenon (and other “volatile” elements) that characterises Earth, by comparison with what is known about the Solar System’s pre-planetary composition, was 50 to 80 Ma after the “start of the Solar System”.  The Moon has provided insufficient data for its age of formation to be tied down isotopically.  Although its Hf-W age might be >44 Ma relative to the Earth’s beginning, there again, perhaps >54 Ma, and it may have formed even later.  Eventually we reach modelling (read “speculation”?) that takes us to the putative composition of the culprit for Year Zero, “Theia” (a Titan and the product of incestuous liaison between Uranus and his mother Gaia).

What seems odd to me is that some of the parent isotopes for those used in fingerprinting (e.g. 182Hf for 182W, and plutonium for a Xe isotope) can only form in supernovae events, and are so short-lived that the balance between their formation and their influence on partitioning of their daughters in planets is pretty delicate in terms of timing.  Indeed all radioactive isotopes, and every element with greater atomic mass than iron, in the Solar System have this origin, because it is impossible for a star the size of the Sun to form them.  Massive stars that become supernovas are common enough, and when they “go off” and what blend of heavy elements they produce depend on how big they were and when they formed.  Interstellar material is surely a mix of debris from a number of such events of different ages, and new stars and planetary systems form from that.  Maybe they are triggered by nearby supernovas, but that also contributes to the isotopic mix that has evolved since a galaxy formed.  Just suppose that the mix for the Solar System was heterogeneous, with differently aged uranium, thorium, rubidium, hafnium and other elements heavier than can be formed inside small stars like the Sun, and must have formed in big ones that eventually blasted their products into interstellar space.  If the Earth accreted as an open, non-equilibrated system, then what of the Solar System itself?  Bit early to say, really….

Kennewick Man may not be re-interred

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Seven and a half years after the discovery of a 9300-year old human skeleton in Columbia River alluvium in Washington state, USA, researchers may finally be able to study the remains.  So-called Kennewick Man caused a storm when first unearthed, for his skull was very different from that of any other early American colonist.  Indeed, partial studies suggested close resemblance to Europeans.  Four Native American tribes in the Pacific Northwest claimed the skeleton for reburial, under the Native American Graves Protection and Repatriation Act.  The move was not entirely connected with respect for sacred rites.  Evidence that the area might have been first colonised by people who were not related to the tribes living there just before European occupation in the 19th century could undermine claims for mineral and other land rights by native people.  On 4 February 2004 a San Francisco court ruled that the remains were so different from any North American indigenous people, that the claimants had no rights over them.  Studies of a skull cast of Kennewick Man since he was placed under lock and key now suggest a possible origin from Asian hunter-gatherers similar to the Ainu people of modern Japan.  However, modern techniques of genetic analysis and isotopic studies of tooth enamel that could settle the issue of origin and relatedness require the original material.  Interestingly, a spear point is lodged in the pelvis, so, like the famous Ice Man of the Italian-Austrian Alps, Kennewick Man may have been the victim of either a deadly dispute or ritual killing.

Onshore gas hydrate reserves close to recovery

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The Mackenzie delta in Arctic Canada has been an area of conventional hydrocarbon exploration for decades.  In 1972 methane-ice mixtures in the permanently frozen ground were discovered in one well at a depth of about a kilometre during exploratory drilling.  They are rich, with up to 90% of the pore spaces in alluvial gravels being full of the white gas hydrate.  Being associated with conventional gas at greater depths, there is a good chance that combined production could make the considerable reserves economic.  On their own, gas hydrates are not yet economic, even onshore, since they would need heating to break down the peculiar compound, and natural gas prices are currently at a low level.  Economics also depend on a conventional gas pipeline being extended to the area  Tests and computer simulations suggest that production of deeper conventional gas can lower the pressure on the gas hydrate inducing it to break down and add to the flow from a well.  In maybe 10 to 20 years production could begin.  The likely origin of the Canadian reserves and those in the North Slope of Alaska is from methane leaking from deeper reserves to “freeze” in the colder conditions at shallow depths.

Arctic North America could eventually produce up to one sixth of current US natural gas consumption from onshore gas hydrate.  Of course, vastly greater gas-hydrate potential exists offshore – between 10 000 to 42 000 trillion cubic metres (tcm) world-wide, compared with 370 tcm of estimated conventional gas reserves.  Methane (CH4) burns to produce less carbon dioxide per unit of heat energy than more carbonic natural gas, so is a means of easing “greenhouse” gas emissions.  Potentially it could be feedstock for CO2-free hydrogen production.  Pressures on the economy of Japan, which has very few natural energy resources, have prompted Japanese researchers to begin exploratory offshore drilling into the Nankai trough offshore of SE Japan, where there are potentially rich reserves of gas hydrate in sands.  This may produce commercially in 10 to 15 years.  The thorniest problem with many gas hydrate deposits is that they are in “tight”, fine-grained sediments.

Source:  Kerr, R.A. 2004.  Gas hydrate resource: smaller but sooner.  Science, v. 303, p. 946-947

Quantifying motions inside continents

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If you are a member of the Geological Society of America you will either have heard or read the 2003 Address of its President (Burchfiel, B.C. 2004.  New technology; new geological challenges.  GSA Today, v. 14, p. 4-10).  If not, get the February 2004 issue of GSA Today, if only for the wonderful illustrations in Burchfiel’s paper.  His topic is how the use of ever-increasing precision of satellite global positioning (GPS) has revolutionised continental neotectonics, since it began to be used by geoscientists in the late-1980s.  The illustrations have a backdrop of what I suspect to be the 90m resolution Shuttle Radar Topography Mission (SRTM) digital elevation model (DEM), and show the fine topographic detail that stems very much from active tectonic movements.  Superimposed on them are estimates of the speed at which points on the surface are moving and the directions of motion, gathered using GPS technology.  Measured in mm per year, these velocities stem from the most precise positional measurements, with the degradation built into the GPS satellite signals for US military reasons (turned off in 2001) removed using differential processing.  They are averages representing motions over the last 17 years or so.  The most dramatic example covers the Tibetan Plateau and areas to the east of it, based on extensive work by Chinese scientists..  In general it shows a sort of clockwise swirling away of expelled crust east of the Eastern Himalayan Syntaxis (the “big bend” at the eastern termination of the Himalaya) in the ranges through which the headwaters of the Irrawaddy, Salween and Mekong rivers flow, rather than the eastward expulsion towards the China Sea first postulated by Tapponier in the early 1980s.  Field studies suggest that this kind of motion has been going on for at least the last 4-6 Ma.  Another conflict with expectation lies in the area of the Longmen Shan mountains and the huge Sichuan Basin of western China.  A simple model of crust being expelled from the zone of the India-Asia collision suggests that Tibetan crust would be moving eastwards here to throw up the steep front of the Longmen Shan above the Sichuan Basin.  There is in fact very little sideways movement at the surface.  Explaining this requires deep crust from Tibet moving in a ductile manner far below, thereby “inflating” the Longmen Shan where entirely different kinds of crust are juxtaposed..  Many of the motions in East Asia can only be explained in terms of differential movements at different levels in the lithosphere, and the influence of subduction systems, such as the Indo-Burman and West Pacific, as well as the long-suspected expulsion of over-thickened crust in Tibet due to increased gravitational potential there.