Deep-earth methane and mantle dynamics: Insights from northern Israel, Southern Tibet and Kamchatka

The oxidation state of fluids in Earth's mantle affects processes ranging from volcanism and the formation of the crust, to the generation of many types of ore deposits. In general, the lowest oxidation state of the mantle (and hence its fluids) is defined by the buffer reaction 2FeO ! 2Fe + O2. However, unusual mineral assemblages that require far more reducing conditions are found in volcanic rocks from a variety of geotectonic settings, raising questions about how such conditions can be generated in the mantle. Examples from northern Israel, Tibet and Kamchatka suggest that interaction between magmas and methane-hydrogen fluids derived from the deep Earth have generated highly reducing conditions within some volcanic plumbing systems. Such systems appear to be related to the margins of tectonic plates, including zones of continent-continent collision and/or deep oceanic subduction, and transform faults extending deep (up to 200 km) into the Earth's mantle. This represents an important but previously unrecognized fluid-transfer process within the mantle.