Accretion of the lowermost oceanic crust at fast-spreading ridges: Insights from the East Pacific Rise (Hess Deep, IODP Leg 345)

At mid-ocean ridges, melts formed during adiabatic melting of a heterogeneous mantle migrate upwards and ultimately crystallize the oceanic crust. In this context, 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 Earths 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 lowermost portion of the fast-spread oceanic crust drilled during IODP Leg 345 at the East Pacific Rise (IODP Site U1415), together with the processes involved in crustal accretion. The recovered gabbroic rocks are primitive in composition and range from olivine-rich troctolites to troctolites, 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. Yet, the occurrence of primitive orthopyroxene in most of the olivine-bearing samples indicates that reactive processes allowed for its local saturation within the percolating MORB-type melt. 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 lowermost gabbros from magmatically productive spreading centres. Oceanic accretion thereby seems driven by in situ crystallization in the lowermost gabbroic layers, followed by upward reactive percolation of melts towards shallower sections. In addition, we here furnish a first estimate of the trace element composition of the parental melts that led to the accretion of the lower crust at Hess Deep, Atlantis Massif and Atlantis Bank; we show 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.