Accretion of the Lower Oceanic Crust at Fast-Spreading Ridges: Insights from Hess Deep (East Pacific Rise, IODP Expedition 345)

At mid-ocean ridges, melts that formed during adiabatic melting of a heterogeneous mantle migrate upwards and ultimately crystallize the oceanic crust. The lower crustal gabbros represent the first crystallization products of these melts and the processes involved in the accretion of the lowermost crust drive the chemical evolution of the magmas forming two thirds of Earth's surface. At fast-spreading ridges, elevated melt supply leads to the formation of a '6-km-thick layered oceanic crust. Here, we provide a detailed petrochemical characterization of the lower portion of the fast-spread oceanic crust drilled during IODP Expedition 345 at the East Pacific Rise (IODP Holes U1415), together with the processes involved in crustal accretion. The recovered gabbroic rocks are primitive in composition and range from troctolites to olivine gabbros, olivine gabbronorites and gabbros. Although textural evidence of dissolution-precipitation processes is widespread within this gabbroic section, only the most interstitial phases record chemical compositions driven by melt-mush interaction processes during closure of the magmatic system. Comparing mineral compositions from this lower crustal section with its slow-spreading counterparts, we propose that the impact of reactive processes on the chemical evolution of the parental melts is dampened in the lower gabbros from magmatically productive spreading centres. Oceanic accretion thereby seems driven by fractional crystallization in the lower gabbroic layers, followed by upward reactive percolation of melts towards shallower sections. Using the composition of clinopyroxene from these primitive, nearly unmodified gabbros, we estimate the parental melt trace element compositions of Hess Deep, showing that the primary melts of the East Pacific Rise are more depleted in incompatible trace elements compared to those formed at slower spreading rates, as a result of higher melting degrees of the underlying mantle.