We present the first water concentration data for oceanic mantle peridotite. The rocks come from the Mid-Atlantic Ridge and were sampled by drilling during Leg 153 of the Ocean Drilling Program. Earlier studies of peridotite xenoliths suggest that orthopyroxene is the most important host of water in the upper mantle. However, an unresolved question is whether the measured water content in orthopyroxene is equal to its original concentration in the mantle or possibly lower because of decompression-related water loss. Our investigations of oceanic mantle rocks yield water contents in orthopyroxene in the range between 160 and 270 ppm by weight (wt). These new data, compared with those from xenoliths, experimental studies, and geophysical investigations, imply that the measured values reflect the original mantle contents. The preservation of mantle water contents is ascribed to an exhumation mechanism that provides for cooling during ascent of the suboceanic mantle slices.

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