Water linked to mantle oxidation

US scientists have used an emerging technique to analyse minuscule samples of magma derived from the Earth’s mantle in different tectonic environments and discovered a direct link between water content and the oxidation state of iron within the sample. The finding helps explain why the Earth’s mantle is more oxidised at subduction zones (where tectonic plates merge) than at mid-ocean ridges (where tectonic plates move apart) which is important in understanding how the mantle and the Earth’s surface interact and evolve over time.

Scientists have known for decades that the lavas at subduction zones are more oxidised than at mid-ocean ridges and it was suspected that water contained in the subducted slab material might be the reason. However, knowing for sure would require analysis of mantle-derived melt samples that had not degassed during eruption (i.e. not lost their dissolved water), which are found trapped in crystals that grow in magmas - like an insect in amber. The problem was that these trapped samples are so small - just 30 to 50 microns in size - that no technique existed to determine their iron oxidation states.

Now, however, Katherine Kelley at the University of Rhode Island, Narragansett, and Elizabeth Cottrell at the US National Museum of Natural History have managed to extract the required information from the tiny magma samples and provided the first quantitative evidence to support the theory that water in subducted slab material is linked to the oxidation state of the mantle. ’It was really seeing the opportunity to apply a new technique to a long standing problem,’ says Cottrell. 

The team obtained the oxidation state of iron in the samples by using a synchrotron-based method of micro x-ray absorption near edge structure (?-XANES) spectroscopy. ’There’s been no technique with the spatial resolution and the spectroscopic capability to determine redox states of something as small as these trapped bubbles of magma,’ says Andrew Berry, an experimental petrologist at Imperial College London, UK. ’The technique they used has been around for about five years but it’s only really being applied just now to the earth sciences,’ he adds. To determine the water content of the samples Kelley and Cottrell used standard infra-red spectroscopy.  

’From our work it appears that the area where melting is taking place beneath subduction zones is more oxidised due to the slab inputs than at mid-ocean ridges, and that has been a matter of debate,’ explains Cottrell. ’It’s one of the first steps in understanding if the mantle oxidation state is evolving over time, for example due to the presence of life in an oxygen rich atmosphere on the surface of the earth.  We basically want to understand where the earth is coming from and where it’s going,’ she adds. 

James Urquhart

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