Deciphering Melt Percolation through Mantle Microstructures: Evidence from Oman Harzburgites

Abstract Reactive porous flow and melt–rock interaction are fundamental processes in the evolution of the oceanic lithospheric mantle. Melt migration through reactive porous flow in the Oman ophiolite has been extensively studied in the context of focused melt flow, leading to the formation of dunite channels and the Moho Transition Zone. On the other hand, diffuse melt percolation within the Oman mantle remains poorly constrained. In this study, we investigate the uppermost mantle section exposed in the Wadi Tayin Massif, documenting microstructural and mineral modal changes that allow to reconstruct melt–rock interaction in the newly formed oceanic lithospheric mantle. We document pervasive percolation of an olivine-saturated melt causing pyroxene consumption and olivine crystallization in harzburgites. Newly formed olivine is fine-grained, occurs interstitially rimming orthopyroxene crystals, and presents an axial-[010] Crystallographic Preferred Orientation, different from that of the residual porphyroclastic olivine, which displays orthorhombic Crystallographic Preferred Orientation. The transition of the bulk olivine Crystallographic Preferred Orientation from orthorhombic to axial-[010] is correlated to an increase of modal proportion of newly formed interstitial olivine and reflects the increasing melt–rock ratio integrated over time. Fe–Mg partitioning between the residual harzburgite minerals and the interstitial olivine indicates chemical equilibrium. However, variations in Mn contents among olivines support the reactive origin of interstitial crystals. The observed mantle microstructural history points to a magmatic overprinting during the progressive cooling and lithospheric accretion of the Wadi Tayin upper mantle, in turn indicating a mantle high-temperature deformation history partially overprinted by a melt-assisted deformation event.