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Detailed backscattered electron (BSE) and cathodoluminescence (CL) observations in an olivine gabbro from the Wadi Rajmi of the Fizh Block of the Oman Ophiolite revealed that this gabbro represents a perfect example of a partial melting reaction. The observed microtextures imply that water-rich fluids migrated on grain boundaries in a ductile regime, causing an incongruent partial melting reaction. All grain boundaries between primary plagioclases are characterized by c. 50 µm thick haloes of Anorthite-rich (An) plagioclase, with An contents which are c. 10 mol higher compared to the host plagioclase composition. Where these An-rich zones are in contact with the mafic primary minerals (olivine, clinopyroxene) in places, reactive rims of orthopyroxene and pargasitic amphibole around the primary mafic silicates have been formed. In situ analyses of 87Sr/86Sr isotope ratios on primary plagioclases and on An-enriched zones along grain boundaries revealed no significant difference, implying that the water/rock ratio was very low during the fluid–rock interaction. The observed features imply a model of hydrothermal activity proceeding on grain boundaries within the deep oceanic crust at very high temperatures (900–1000 °C) without any crack system, a prerequisite in current models to enable hydrothermal circulation. Interconnected porosity is generated by coupled dissolution–reprecipitation which allows pervasive fluid transport through the minerals at the grain boundaries, establishing a novel type of hydrothermal circulation maintaining water–rock interaction at very high temperatures in the ductile regime. The presence of isolated product phases as inclusions within the parent minerals emphasizes the importance of an interconnecting fluid establishing local equilibrium between parent and product phases of the incongruent melting reaction.

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