Frontiers Water distribution across the mantle transition zone and its implications for global material circulation Shun-ichiro Karato Yale University, Department of Geology and Geophysics, New Haven, CT 06520, USA abstract article info Article history: Accepted 23 November 2010 Available online 13 December 2010 Editor: R.W. Carlson Keywords: water hydrogen seismology electrical conductivity partial melting Various methods for inferring the water distribution in Earth's mantle are reviewed including geochemical and geophysical methods. The geochemical approach using the water contents of basalts shows that the water content in the source regions of ocean island basalt is generally larger than that of the source region of mid- ocean ridge basalt, but the location of the source regions of ocean island basalts is poorly constrained. Geophysical approaches have potential of providing constraints on the spatial distribution of water but their usefulness depends critically on the sensitivity of geophysical observations to water content relative to other factors, in addition to the resolution of geophysical observations. Existing experimental data on the influence of water on seismologically observable properties and on electrical conductivity are reviewed. Frequently used seismological observations such as the anomalies in seismic wave velocities and of the topography on the mantle discontinuities are only weakly sensitive to water content but more sensitive to other factors such as the major element chemistry and temperature for a typical range of water contents. In contrast, electrical conductivity is highly sensitive to water content and only modestly sensitive to other factors such as temperature, oxygen fugacity and major element chemistry. Models of electrical conductivity–depth profiles are constructed where the influence of hydrogen and iron partitioning among coexisting minerals and of the depth variation in oxygen fugacity are incorporated. It is shown (i) that the electrical conductivity varies more than two orders of magnitude for a plausible range of water content in the mantle (~ 10 ppm wt to ~ 1 wt.%) and (ii) that if water content is constant with depth, there will be a drop in electrical conductivity at ~ 410-km. Although the resolution is not as high as seismological observations, geophysically inferred electrical conductivity distributions generally show higher conductivity in the mantle transition zone than the upper mantle, suggesting that the water content in the transition zone is higher than that in the upper mantle with some lateral variations. Implications of inferred water distribution are discussed including the possible partial melting near 410-km and its role in global water circulation. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Water (hydrogen) is a unique component in terrestrial planets. Its abundance is small compared to other components (SiO 2 , MgO, CaO, etc.), yet even a small amount of water can modify the melting relationships (e.g., Inoue, 1994) and rheological properties (e.g., Karato and Jung, 2003; Mei and Kohlstedt, 2000) drastically. Consequently, the distribution of water has important influence on the dynamics and evolution of terrestrial planets. In most of the previous studies on water in Earth's interior, focus has been placed on the solubility limit and/or the solubility mechanisms of water (hydrogen) in minerals particularly in nomi- nally anhydrous minerals (e.g., Bolfan-Casanova, 2005; Inoue et al., 2010). Although there is still a major discrepancy about the water solubility in lower mantle minerals, it is clear that the total amount of maximum water content in these minerals far exceeds the amount of ocean water. In particular, the observed high solubility of water in minerals in the mantle transition zone (the MTZ) suggests an important role of this layer in controlling the global water circulation. However, although these studies form an important basis for all studies on water in Earth, they are not sufficient to constrain the actual water distribution in Earth's interior. Actual water distribution in Earth's interior is only indirectly controlled by the solubility limit, and for example, even regions with high water solubility (e.g., the MTZ) could have little water (Richard et al., 2002). Because diffusion is inefficient (diffusion distance is only ~10 km for one billion years calculated from the experimental results on chemical diffusion of water (Kohlstedt and Mackwell, 1998)), distribution of water in the real Earth is controlled largely by the location and degree of partial melting and by the large-scale material transport (e.g., Iwamori, 2007; Richard et al., 2006). However, in addition to partial melting that created the continental crust, the only well-recognized partial melting and resultant chemical segregation Earth and Planetary Science Letters 301 (2011) 413–423 E-mail address: shun-ichiro.karato@yale.edu. 0012-821X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2010.11.038 Contents lists available at ScienceDirect Earth and Planetary Science Letters journal homepage: www.elsevier.com/locate/epsl