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How rusty is the Earth?

June 10, 2004

An iron object lying around outside quickly turns rusty. Iron metal always has to be combined with some other elements or coated with paint to stop it corroding. The reason for this is that iron metal is unstable in the presence of the oxygen in the Earth's atmosphere. It reacts with the iron to produce ferric iron, a form of iron that exists in iron oxide molecules that are particularly rich in oxygen. In a similar way iron metal does not occur naturally in the earth's crust. It is too unstable to survive over geological time. Iron metal has to be manufactured by converting iron compounds, like iron oxides, that can be easily mined and are in plentiful supply. The Iron Age, which transformed civilisation across Europe, started with the discovery of how to do just this.

Metallic core presents massive puzzle

Such chemical reactions between iron and oxygen have had a huge effect on our planet but present scientists with a massive puzzle. Decades ago it was realised that the Earth's metallic core that produces our magnetic field cannot have formed if there was a lot of extra oxygen around because like the iron we use in everyday life it should have been changed to iron oxide. It is thought that the core must have segregated by gravity as a dense metallic liquid from the mantle. The upper mantle and crust of the Earth are oxidised with a relatively large proportion of ferric iron. How could a core form if metallic iron is not stable in these oxygen-rich rocks that make up the outer Earth?

Addition of an outer veneer of oxidised material

One idea, proposed many years ago, is that the Earth must have had a very different atmosphere to start with, maybe one dominated by elements like hydrogen and methane, and the metallic core segregated under such conditions. The "extra" oxygen we have today would have been added later. Of course the oxygen in the present day atmosphere is thought to have originated by photosynthesis and therefore its increase is a side effect of billions of years of biological evolution. However, that oxygen must have come from the Earth's mantle. Biology moves the oxygen around but on its own it does not explain why the Earth has a metallic core when at the same time it has an oxidised upper mantle.

The most likely explanation is that the outer portion of the Earth became oxidised after the core formed as a result of the addition of an outer veneer of oxidised material. The Earth's water may have originated in the same way. Therefore, it may be that the amount of oxygen is different deeper in the Earth. The mantle extends to 2900 km and it is very little known about its composition or properties at great depth. One possibility is that the oxidation state of iron in the mantle (or proportion of ferric iron) is not uniform and that it is higher in places where ocean water is being taken back down into the mantle. This happens in subduction zones where Earth's surface is being dragged down into the interior.

Studying the level of oxidation of the mantle

Solving this puzzle and getting information on how the Earth's oxygen varies with depth requires the discovery of some measurable feature of a rock or mineral that is sensitive to just this one thing. ETH researchers in collaboration with the University of Houston and the University of Bayreuth have used the exciting new technique of multiple collector inductively coupled plasma mass spectrometry (MC-ICPMS) to measure variations in the proportions of different isotopes of iron and have shown that this varies in mantle rocks depending on how oxidised the iron is. By measuring iron isotopes it should be possible to study rocks from different depths and of different ages to get some idea of how the level of oxidation of the mantle has changed over time. ETH has the world's largest facility for MC-ICPMS measurements.

ETH Zuerich