title: Global electromagnetic induction constraints on transition-zone water content variations.
abstract: Small amounts of water can significantly affect the physical properties of mantle materials, including lowering of the solidus, and reducing effective viscosity and seismic velocity. The amount and distribution of water within the mantle thus has profound implications for the dynamics and geochemical evolution of the Earth. Electrical conductivity is also highly sensitive to the presence of hydrogen in mantle minerals. The mantle transition zone minerals wadsleyite and ringwoodite in particular have high water solubility, and recent high pressure experiments show that the electrical conductivity of these minerals is very sensitive to water content. 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 view that at least some of the water in the transition zone has been carried into that region by cold subducting slabs.
authors.affiliation: College of Oceanic & Atmospheric Sciences, Oregon State University, Corvallis, Oregon 97331-5503, USA.
authors.forename: Anna
authors.initials: A
authors.lastname: Kelbert
authors.forename: Adam
authors.initials: A
authors.lastname: Schultz
authors.forename: Gary
authors.initials: G
authors.lastname: Egbert
references.citation: Science. 2007 Jun 8;316(5830):1468-71
references.pmid: 17556582
references.citation: Nature. 2008 Jan 17;451(7176):326-9
references.pmid: 18202656
references.citation: Science. 2008 Nov 28;322(5906):1363-5
references.pmid: 19039132
references.citation: Nature. 2005 Sep 8;437(7056):249-52
references.pmid: 16148932
references.citation: Nature. 2007 Jun 21;447(7147):991-4
references.pmid: 17581582
references.citation: Nature. 2005 Apr 7;434(7034):746-9
references.pmid: 15815625
references.citation: Nature. 2004 May 27;429(6990):399-403
references.pmid: 15164059
references.citation: Science. 1999 Dec 3;286(5446):1928-1931
references.pmid: 10583950
references.citation: Nature. 2003 Sep 4;425(6953):39-44
references.pmid: 12955133