Where did all our water come from? The Earth’s large complement of H2O, at the surface, in its crust and even in the mantle, is what sets it apart in many ways from the rest of the rocky Inner Planets. They are largely dry, tectonically torpid and devoid of signs of life. For a long while the standard answer has been that it was delivered by wave after wave of comet impacts during the Hadean, based on the fact that most volatiles were driven to the outermost Solar System, eventually to accrete as the giant planets and the icy worlds and comets of the Kuiper Belt and Oort Cloud, once the Sun sparked its fusion reactions That left its immediate surroundings depleted in them and enriched in more refractory elements and compounds from which the Inner Planets accreted. But that begs another question: how come an early comet ‘storm’ failed to ‘irrigate’ Mercury, Venus and Mars? New geochemical data offer a different scenario, albeit with a link to the early comet-storms paradigm.
Three geochemists from the Institut für Planetologie, University of Münster, Germany, led by Gerrit Budde have been studying the isotopes of the element molybdenum (Mo) in terrestrial rocks and meteorite collections. Molybdenum is a strongly siderophile (‘iron loving’) metal that, along with other transition-group metals, easily dissolves in molten iron. Consequently, when the Earth’s core began to form very early in Earth’s history, available molybdenum was mostly incorporated into it. Yet Mo is not that uncommon in younger rocks that formed by partial melting of the mantle, which implies that there is still plenty of it mantle peridotites. That surprising abundance may be explained by its addition along with other interplanetary material after the core had formed. Using Mo isotopes to investigate pre- and post-core formation events is similar to the use of isotopes of other transition metals, such as tungsten (see Planetary science, May 2016). Continue reading “Earth’s water and the Moon”→
A large number of water-oriented tropes have been applied to Earth for ‘artistic effect’, ranging from Waterworld to the Blue Planet, but from a geoscientific perspective H2O in its many forms – liquid, solid, gas, supercritical fluid and chemically bound – has as much influence over the way the world works as do its internal heat production and transfer. Leaving aside surface processes, the presence of water has dramatic effects on the temperature at which rocks – felsic, mafic and ultramafic – begin to melt and deform and on the rates of important chemical reactions bound up with internal processes.
For a long while many geologists believed that the oceans were the product of water being transferred from the mantle by degassing through volcanoes so that the deep Earth has steadily been desiccated. But now it has become clear that such is the rate at which subduction can shift water back to the mantle that the entire volume of modern ocean water may have been cycled back and forth more than 3 times in Earth history (see Subduction and the water cycle). Besides, it is conceivable that accretion of cometary material up to about 3.8 Ga may have delivered the bulk of it.
An important aspect of the deep part of the water cycle concerns just how far into the mantle subduction can transport this most dominant volatile component of our planet. Ultra high-pressure experimental petrology has reached the stage when conditions at depths more than halfway to the core-mantle boundary (pressures up to 50 GPa) can be sustained using diamond anvils surrounding chemical mixtures that approximate mantle ultramafic materials. Previously, it was thought that serpentinite, the hydrous mineral most likely to be subducted, broke down into magnesium-rich, anhydrous silicates at around 1250 km down. This would prevent the deepest mantle from gaining any subducted water and retaining any that it had since the Earth formed. A team of Japanese geochemists has discovered a hint that hydrous silicates can, through a series of phase changes, achieve stability under the conditions of the deepest mantle (Nishi, M. 2014. Stability of hydrous silicate at high pressures and water transport to the deep lower mantle. Nature Geoscience, v. 7, p. 224-227). Their experiments yielded a yet unnamed mineral (phase H or MgSiH2O4) from approximate mantle composition that could remain stable in subducted slabs down to the core-mantle boundary. This development may help explain why the lowermost mantle is able to participate in plume activity through reduction in viscosity at those depths.
A parallel discovery concerns conditions at the base of the upper mantle; the 410 to 660 km mantle seismic transition zone. It comes from close study of a rare class of Brazilian diamonds that have been swiftly transported to the Earth’s surface from such depths, probably in kimberlite magma pipes, though their actual source rock has yet to be discovered. These ultra-deep diamonds prove to contain inclusions of mantle materials from the transition zone (Pearson, D.G. and 11 others 2014. Hydrous mantle transition zone indicated by ringwoodite included within diamond. Nature, v. 507, p. 221-224). Australian geochemist Ted Ringwood pioneered the idea in the 1950s and 60s that the mantle transition zone might be due to the main mantle mineral olivine ((Mg,Fe)2SiO4) being transformed to structures commensurate with extremely high pressures, including one akin to that of spinel. Such a mineral was first observed in stony meteorites that had undergone shock metamorphism, and was dubbed ringwoodite in honour of its eponymous predictor. Yet ringwoodite had never been found in terrestrial rocks, until it turned up in the Brazilian diamonds thanks to Pearson and colleagues.
Earlier experimental work to synthesise ultra-deep minerals discovered that ringwoodite may contain up to 2% water (actually OH groups) in its molecular lattice: an astonishing thing for material formed under such extreme conditions. The ringwoodite inclusions in diamond show infrared spectra that closely resemble its hydrous form. From this it may be inferred that the 401-660 km transition zone contains a vast amount of water; roughly the same as in all the oceans combined, though the find is yet to be confirmed in a wider selection of diamonds. One of the puzzles about diamondiferous kimberlites is that the magma must have been rich in water and carbon dioxide. That can now be explained by volatile-rich materials at the depths where diamonds form, But that does not necessarily implicate the whole transition zone: there may be pockets ripe for kimberlitic magma formation in a more widely water-poor mantle.
Keppler, H. 2014. Earth’s deep water reservoir. Nature , v. 507, p. 174-175
Because they can be so big, consist mainly of water ice and there are probably a great many lurking in the outer reaches of the solar system impacting comets have long been thought to have delivered the water that makes the Earth so dynamic and, so far as we know, the only place in near-space that hosts complex life. Remote sensing studies of the isotopic composition of water in one comet (Hartley 2) caused great excitement in 2011 by showing that its ratio of deuterium to hydrogen was very similar to that of Earthly ocean water. Other D:H ratios have recently been published from a suite of meteorites gleaned from the surface of Antarctic ice (Alexander, C.M.O’D. et al. 2012. The provenances of asteroids, and their contributions to the volatile inventories of the terrestrial planets. Science, v. 337, p. 721-723). These meteorites are carbonaceous chondrites thought to be the source of much of the solid material in planets of the Inner Solar System. To cut short a long and closely argued argument, it seems that the CI-type chondrites’ water is isotopically quite different from that in analysed comets, knocking another popular hypothesis on the head; that comets and carbonaceous chondrites formed in the same part of the Solar System.
Since hydrocarbons in comets – known from interplanetary dust particles – contain hydrogen with a far richer complement of its heavy isotope deuterium than does cometary water ice, the crashing of entire comets onto planets such as the Earth would not produce the observed terrestrial D:H ratio even though their water ice alone does match it. The US, British and Canadian meteoriticists conclude what seems to be a unifying explanation whereby CI chondritic solids and volatiles alone would have been able to form the Inner Planets and their various complements of water by initial accretion. Comets as a second-stage source, in this account, are relegated to mere curiosities of the Solar System with little role to play other than occasional big impacts that may, or may not, have influenced evolution by the power that they delivered not through their chemistry.