Water in a rock can affect many of its properties—for example, water can lower the melting temperature and reduce the viscosity of the rock so it deforms more easily. One way to infer water content at depth is by measuring electrical conductivity, which can be enhanced by the presence of water or another highly conductive mineral, such as graphite. Most measurements of electrical conductivity have been of continental or oceanic crust, but a few measurements have been made across long arrays of the underlying mantle and can be used to address the variability of the water content of the upper mantle.
Hirth et al. analyze recent measurements of the mantle beneath Archean cratons. These structures represent the oldest continental crust on Earth, and many cratons still appear to be related to their underlying mantle. The data imply that between depths of 150 and 250 kilometers, the mantle beneath the cratons is anomalously dry compared to the oceanic mantle, but that below about 250 km, the water content of the mantle is more uniform. The stability of Archean mantle may derive from this relative lack of water.—BH
Geochem. Geophys. Geosys. 1, 2000GC000048 (2000).