Iron oceanic mantle

The planets nearest the Sun have a high-temperature surface while those further away have a low temperature. The temperature depends on the closeness to the Sun, but it also depends on the chemical composition and zone structures of the individual planets and their sizes. In this respect Earth is a somewhat peculiar planet, we do not know whether it is unique or not in that its core has remained very hot, mainly due to gravitic compression and radioactive decay of some unstable isotopes, and loss of core heat has been restricted by a poorly conducting mainly oxide mantle. This heat still contributes very considerably to the overall temperature of the Earth s surface. The hot core, some of it solid, is composed of metals, mainly iron, while the mantle is largely of molten oxidic rocks until the thin surface of solid rocks of many different compositions, such as silicates, sulfides and carbonates, occurs. This is usually called the crust, below the oceans, and forms the continents of today. Water and the atmosphere are reached in further outward succession. We shall describe the relevant chemistry in more detail later here, we are concerned first with the temperature gradient from the interior to the surface (Figure 1.2). The Earth s surface, i.e. the crust, the sea and the atmosphere, is of... 

Williams H, Lee D-C, Levasseur S, Teutsch N, Poitrasson R, Halliday AN (2002) Iron isotope composition of mid-ocean ridge basalts and mantle peridotites. Geochim Cosmochim Acta 66 A838... 

The special events of this planet s accretion (Newsom and Jones, 1990 Weatherill, 1990 Ahrens, 1990 Taylor, 2001) were crucial in making Earth habitable over bilhons of years. Segregation of the core physically separated reduction power in the iron-rich center of the Earth, from a more oxidized mantle. Simultaneously, the early events controlling the surface environment made possible the development of a habitable ocean/atmosphere system. 

During the thermally driven differentiation of the Earth into core-mantle-crust, numerous reactions would have produced oxidized forms of iron, sulfur and carbon. These would have contributed to the redox chemistry in the early planet development. Volcanic and hydrothermal emission of sulfur dioxide, SO2, delivered oxidants to the oceans and atmosphere. Photodissociation of water vapor in the atmosphere have undoubtedly provided a small but significant source of molecular oxygen. Furthermore, UV-driven ferrous iron oxidation could have been coupled to the reduction of a variety of reactants, for instance, CO2 (Figure 16). 

In this model the mantle is differentiated, into an upper layer (1,600-2,000 km thick), and a compositionally distinct lower layer, enriched in iron and heat producing elements. This lower layer, making up 20-30% of the mantle, is thought to have originated either during the early differentiation of the Earth or by the burial of subducted oceanic crust. 

Earth is unique among the planets in our solar system primarily because of its vast oceans of water. Formed over billions of years, water was produced during high-temperature interactions between atmospheric hydrocarbons and the silicate and iron oxides in the Earth s mantle. Moisture reached the planet s surface as steam emitted during volcanic eruptions. Oceans formed as the steam condensed and fell back to Earth as rain. This first rain may have lasted more than 60,000 years. 

We can estimate how much water was dissociated by considering the oxidation of the mantle. If the Earth started out with the oxidation state of chondrites (although not necessarily of total chondritic composition), it would take the oxygen from about one present-day ocean volume to reach the oxidation state of the mantle inferred from mantle-derived rocks. Alternatively, if we start with a more reduced ensemble, suggested by condensation calculations from a solar nebula, we will have primarily enstatite (MgSiOs) and metallic iron as the original source to be oxidized. The oxidation reaction would then be as shown in eqn [1]. 

The second problem is cause and effect. The continental crust and mantle are more plausibly oxidized by the surface environments than the other way around. For example, arc volcanics contain surface water and ferric iron from altered oceanic cmst. 

Still, the Earth s mantle is more oxidized than the other solar system bodies for which we have information from samples, the asteroid Vesta, the Moon, and Mars. The Earth s mantle most likely became oxidized during its accretion (for review see Ref. [52]). The climax was the collision of a Mars-sized body with the Protoearth that left the present Earth-Moon system in its wake. During this event (and after subsequent asteroid impacts) metallic iron mixed into the mantle and consumed hydrogen equivalent to that in the present oceans. A comparable amount of hydrogen was consumed making ferric iron in the mantle. 

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