The nature of the Mush Zone beneath fast-spreading ocean ridges: insights from the lower crust of the Oman ophiolite

Quantifying the timescales associated with magma emplacement and eruption beneath mid ocean ridges is critical to our understanding of the lower crustal magmatic plumbing system at oceanic spreading centers. Questions as to how magmas are emplaced and crystallized, as well as how long these magmas and their crystals remain in a semi-molten mush-like state, are traditionally difficult to constrain due to inaccessibility. Previous attempts to quantify the longevity of the Mush Zone, both in duration and distance off-axis, have largely used seismic methods. Here we present a new method for constraining the nature of the Mush Zone beneath the ridge axis using geochemical methods. The exposed lower gabbroic crust from the Wadi Tayin massif of the Samail ophiolite provides unparalleled access to samples throughout the entire solidified lower crust formed at a fast-spreading ridge. Cotectic crystallization temperatures constrained by clinopyroxene Mg#, and sub-solidus temperatures estimated from the closure temperature of Ca diffusion in olivine, are paired with cooling rates from the Ca-in-olivine geospeedometer measured on individual samples. Our method allows for a determination of the maximum amount of time each gabbro sample spent in a semi-molten state beneath the spreading center. Results show that lower crustal gabbros spend, on average, 10,000 years above the solidus and there is no systematic change with depth in the crust. These results suggest that the Mush Zone at fast- spreading ridges remains partially molten for only 0.5 1.5 km away from the ridge axis. This revised estimate of the Mush Zone is slightly narrower than, but in overall agreement with, that previously inferred from seismic data from the East Pacific Rise (EPR). The results imply that the lavas erupted to the surface during a single event are mixtures of new mantle-derived melt as well as significantly older melt fractionated within the lower crust.