Global electromagnetic induction constraints on transition-zone water content variations

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Abstract

Small amounts of water can significantly affect the physical properties of mantle materials, including lowering of the solidus1, and reducing effective viscosity2 and seismic velocity3. The amount and distribution of water within the mantle thus has profound implications for the dynamics and geochemical evolution of the Earth4,5. Electrical conductivity is also highly sensitive to the presence of hydrogen in mantle minerals6. The mantle transition zone minerals wadsleyite and ringwoodite in particular have high water solubility4, and recent high pressure experiments show that the electrical conductivity of these minerals is very sensitive to water content7,8,9. Thus estimates of the electrical conductivity of the mantle transition zone derived from electromagnetic induction studies have the potential to constrain the water content of this region. Here we invert long period geomagnetic response functions to derive a global-scale three-dimensional model of electrical conductivity variations in the Earth’s mantle, revealing variations in the electrical conductivity of the transition zone of approximately one order of magnitude. Conductivities are high in cold, seismically fast, areas where slabs have subducted into or through the transition zone. Significant variations in water content throughout the transition zone provide a plausible explanation for the observed patterns. Our results support the view10,11 that at least some of the water in the transition zone has been carried into that region by cold subducting slabs.

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Figure 1: Regularized degree and order nine electrical conductivity inverse solution.
Figure 2: Global and regional electrical conductivity profiles, based on the three-dimensional inverse solution presented in Fig. 1 .

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Acknowledgements

We acknowledge support from the US National Science Foundation (grant number EAR-0739111) and from the US National Aeronautics and Space Administration (grant number NNX08AG04G). A. Kuvshinov is thanked for help with the near-surface data correction.

Author Contributions A.S. provided the original forward solver and the data sets. The methods were developed jointly by G.E. and A.K. A.K. implemented the inversion and performed all computational experiments. All authors were involved in the interpretation of the results and creation of this manuscript.

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Correspondence to Anna Kelbert.

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Kelbert, A., Schultz, A. & Egbert, G. Global electromagnetic induction constraints on transition-zone water content variations. Nature 460, 1003–1006 (2009) doi:10.1038/nature08257

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