Helium and how the Earth convects

In the last ten years the new technology of seismic tomography that produces ghostly images of high and low density mantle has convinced many geoscientists that two major dynamic features extend to almost to the core mantle boundary (CMB). Dense, high-velocity zones descend from subduction zones, suggesting that the slabs continue to fall through the entire mantle below the ~700 km maximum depth of the earthquakes that Bennioff and Wadati used to define subduction.  Some hotspots seem to be above diffuse zones of low seismic velocity that are supposed to signify hot, low density plumes that rise from the CMB. An inkling of a grand theory of mantle convection might then be that the descending slabs ruck up the deepest and hottest mantle layers to set them rising as narrow diapirs. Yet, other tomographic features appear to be restricted to the uppermost mantle, less than the 660 km depth of a major discontinuity long considered to be due to a mineral phase change at high pressure. A whole-mantle theory of convective heat transfer should transfer some geochemical trace of an exchange between core and silicate mantle. Osmium isotopes from plume-related magmatism suggest that there might be an exchange, but those of tungsten do not (see: Mantle and core do not mix, February 2004 issue of EPN).  The oldest and perhaps most convincing evidence against whole-mantle convection comes from study of helium in volcanic rocks, neatly reviewed by Francis Albarède (Albarède, F., 2005. Helium feels the heat in Earth’s mantle. Science, v. 310, p. 1777-1778).

Helium is generated by the decay of radioactive uranium and thorium isotopes as alpha particles (4He), which generates much of the Earth’s geothermal heat flow. There should be a close correlation between helium and helium, but at mid-ocean ridges the amount of 4He is only 5% of that expected from the associated heat flow. One explanation for this is that somewhere in the mantle there is a barrier to upward movement of helium, yet is allows heat to pass through: a thermally conductive layer that bars convective mass transfer. Albarède cites recent work that uses the flow of heat and helium through groundwater in an aquifer (Castro, M.C. et al., 2005. 2-D numerical simulations of groundwater flow, heat transfer and 4He transport — implications for the He terrestrial budget and the mantle helium–heat imbalance. Earth and Planetary Science Letters, v. 237, p. 893-910) as analogy of mantle processes. There too helium is less than might be expected, the reason being that the aquifer is recharged by rainwater, low in He.  Likewise, ocean-floor basalts are probably affected in the same way by hydrothermal circulation of seawater, thereby diluting the flux of helium from the mantle and perhaps helping to account for anomalously low helium flux. Another widely accepted view that the high 3He/4He ratios of hotspot basalts is evidence for their source in primitive mantle – 3He is probably a product of nucleosynthesis and therefore primordial as far as the Earth is concerned – is challenged by a recent paper that shows that helium is dissolved in mantle minerals (Parman, S.W. et al., 2005. Helium solubility in olivine and implications for high 3He/4He in ocean island basalts. Nature, v. 437, p. 1140-1143).  Parman et al.’s measurements suggest that the high 3He might result from residues of earlier melting in the mantle, rather than coming from parts that have remain in the state they were when the Earth accreted.

Vanished Martian sea or not?

The Mars Rover data from the Opportunity site that showed up masses of sulfate minerals in the large depression that it has roamed for 2 years prompted the notion that they formed as a sizeable body of surface water evaporated. The Rover Opportunity scientists have also speculated on Mars once having had highly acidic ‘weather’, in the form of sulfuric acid rain from SO2 emitted by volcanoes. The sediments at the Opportunity site also show signs of fluid transport in the form of bedding and cross stratification, ascribed to moving water. Most independent-minded scientists confronted by a united front of vast teams of highly focused scientists sometimes feel that there is more than one way of skinning a cat.  Such is the case of Paul Knauth and Donald Burt of Arizona State University and Kenneth Wohletz of the Los Alamos National Laboratory in New Mexico. The visualise the dramatic evidence from Opportunity in an altogether more mundane scenario (Knauth, L.P. et al., 2005.  Impact origin of sediments at the Opportunity landing site on Mars. Nature, v. 438, p. 1123-1128). Their main point of departure is quite simple; acidic water full of hydrogen ions is a powerful means of weathering and the production of clay minerals. Clays are very uncommon on Mars, particularly at the Opportunity site, and have only shown up rarely on hyperspectral remote sensing images.

Layered sediments are evidence for fluid deposition, but not only water produces them. As well as wind transport and deposition, they are also formed by gas-rich base surges from explosive volcanism and meteorite impacts – and also during surface nuclear explosions that mimic impacts, hence the Los Alamos connection. Knauth et al. explain the Opportunity deposits as debris originally made of rock, sulphides brines and ice flung from a massive impact. They explain the sulfates as products of interaction between melted ices and sulfides. The extension of the Opportunity team’s hypothesis of evaporating surface water is that it would have been long-lived, perhaps sufficiently so for the emergence of acid-loving organisms, similar to those that infest groundwater in terrestrial massive sulfide deposits. Should the deposit prove to have formed during an extremely rapid event, such as an impact, the idea of it having hosted primitive life forms becomes extremely unlikely. Gleefully, Knauth et al. almost exactly match the Opportunity image mosaic of layered sediments with a photograph of a New Mexico layered, volcanic surge deposit. Surges from large impacts, and Mars was intensely bombarded in its early history, can extend hundreds of kilometres from the crater rim. Many other examples of layered sequences are being revealed by high-resolution orbital images of Mars, and interpreters regularly ascribe them to wind, flowing water or volcanic processes. Ockham’s Razor demands the most likely and simplest explanation for phenomena, and impacts could have formed the lot. The earliest detection of features that only flowing water could have carved – the sinuous canyons on Mars, originally prompted such a simple explanation, that water was released en masse by early massive impacts. Perhaps there is a much wider link between many Martian features and the most common geological agent in the Solar System.

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