Water in mantle orthopyroxene - no visible change in defect water during serpentinization

Abstract

Earlier studies on xenolithic mantle rocks imply that the water contents originally present in the minerals at mantle depth may survive despite uplift and decompression. This study addresses the question if mantle minerals in slowly exhumed peridotites that are partially replaced by secondary hydrous phases still retain the original defect water contents that were present at mantle depths prior to alteration. Orthopyroxene from oceanic spinel peridotite was chosen for this investigation because (i) it has variable defect water contents and (ii) it is partially serpentinized. Infrared spectroscopy reveals that defect water is homogeneously distributed in coarse orthopyroxene grains. In contrast, the amount of secondary serpentine is highly variable. The intensity of defect water peaks stemming from hydroxyl defects in the orthopyroxene structure is constant over a crystal and shows no decrease or increase in areas where serpentine is present. This suggests that secondary hydration of mantle minerals ( i.e. partial replacement by hydrous phases) at low temperature either did not influence the defect water contents or, alternatively, completely re-homogenized water in the crystals. However, hydrogen diffusion in orthopyroxene at serpentinization temperatures of ≤300 °C, corresponding to lizardite serpentinization, is too slow to reset the water contents in mm-sized crystals. Therefore, the measured values of defect water in the tectonically exhumed oceanic peridotites may be used to infer the water contents of the upper (sub-oceanic) mantle. If serpentinization temperature exceeds 400 °C (= antigorite serpentinization), as in many Alpine-type peridotites, water in mantle minerals may be modified to a greater extent by secondary hydrothermal processes.