Wasser im ozeanischem Mantel am schnell spreizenden ostpazifischen Rücken; Infrarot-Spektroskopie an nominell wasserfreiem Orthopyroxen

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Water that is present in the Earth's mantle in nominally anhydrous minerals (NAM's) has a significant influence on mantle dynamics. Orthopyroxene is a very important container for water. It may incorporate several hundred wt.-ppm H2O in the uppermost mantle and even higher amounts in deeper parts, because its storage capacity is pressure-enhanced. Any previous studies on water in mantle orthopyroxene were restricted to peridotite xenoliths from the sub-continental mantle. The first samples from the sub-oceanic mantle were recently studied in the course of an own, DFG-financed project. These first data reveal contrasting water contents in peridotite from the slow spreading Mid-Atlantic Ridge, from levels equal to water saturation in orthopyroxene at ODP-Leg 153 to contents that consistently are one order of magnitude lower at ODP-Leg 209. This contrast implies different mantle residence times after melt depletion and/or different uplift/exhumation rates. This preliminary investigation not only provides us with a better knowledge of the water contents in the mantle but also puts constraints concerning dynamic processes below spreading centers. To develop a more general model and to test the preliminary hypotheses, it is absolutely necessary to study peridotite from a fastspreading ridge. The project suggested is aimed at closing a fundamental gap of knowledge by investigating peridotitic orthopyroxene (water, major, minor and trace element contents, equilibrium conditions) from the Hess Deep, ODP-Leg 147.