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Many chapters in this volume emphasize that water plays a crucial role in determining the properties and behavior of the mantle. One of the most important examples of the effect of water on mantle behavior is its dramatic effect in reducing melting temperatures, and, in the case of decompression melting, increasing the depth of initiation of melting. Even the small amount of water which could be contained within nominally anhydrous minerals (NAMs) is sufficient to play an important role in generation of MORBs (Hirth and Kohlstedt 1996; Asimow and Langmuir 2003; Asimow et al. 2004). Figure 1⇓ illustrates how, according to the model of Aubaud et al. (2004), small amounts of water affect the depth of initiation of melting in upwelling mantle. Furthermore, in regions of the mantle where large concentrations of water could be stored (e.g., the transition zone), phase changes could release water and generate melt in previously unexpected environments (Bercovici and Karato 2003; Hirschmann 2006). The effect of water on mantle melting is one of many reasons why knowledge of the spatial and temporal distribution of water in the mantle is crucial to understanding the evolution of our planet.
It is also possible to look at this problem from the other direction, because the products of partial melting of the mantle provide an opportunity to assess the water concentrations in the source regions of these melts. The amount of water in different regions of the mantle can, in …