MELT-FILLED HYBRID FRACTURES IN THE SUB-OCEANIC MANTLE OF THE OMAN OPHIOLITE

Magmatic accretion at mid-ocean ridges is the dominant process responsible for the formation of oceanic crust. Studies of exposed ophiolites have previously suggested that melt transport in the mantle beneath mid-ocean ridges occurs: 1) as dikes in tensile fractures, 2) via porous flow, 3) in porous dunite channels and 4) in shear zones. Our recent observations from the sub-oceanic mantle beneath a ridge axis in the Oman ophiolite show that gabbro dikes commonly fill hybrid fractures that show both shear and extensional components of strain. The magnitudes of shear strain recorded by the dikes are significant and comparable to the longitudinal extensions across the dikes. We suggest that hybrid dikes form as a result of the interactions of shear deformation and pressurized melt in regions of diapiric upwelling along the axes of mid-ocean ridges. Furthermore, the hybrid dikes may play an important role in localizing melt transport at ridge axes, as they require less melt pressure to form than purely tensile dikes, and thus are able to tap melt reservoirs that are under-pressurized with respect to lithostatic pressure in the upper mantle.