Determining which petrological processes build the mantle-crust dunitic transition zone (DTZ) in oceanic spreading settings has a direct impact on our understanding of thermal and chemical transfers on Earth. We report on understated but widespread mineral assemblages present in the DTZ at the top of a mantle diapir (Oman ophiolite), including pargasite, grossular, and pyroxenes of peculiar composition. These minerals are present interstitially between olivines and as inclusions in the disseminated chromite grains, indicating that they are early, high-temperature features. They call for hybridization between the mid-oceanic ridge basalt melts that fed the crustal section and supercritical water saturated with silica. Our synoptic survey (∼300 samples collected along 11 cross sections) demonstrates that the DTZ was pervasively infiltrated by such hybrid melts and that the abundance of their crystallization products increases upsection, likely in response to increasing supply of water and decreasing temperature. This indicates that water is involved in the reaction leading to the transformation of mantle harzburgite into dunite in the DTZ. On the basis of field evidence, a hydrothermal origin of the water is a reasonable hypothesis.

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