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

This article focuses on the petrochemical characterization and magmatic processes involved in the formation of the lower oceanic crust at fast-spreading mid-ocean ridges, specifically from gabbroic rocks drilled during IODP Expedition 345 at Hess Deep, East Pacific Rise. The study reveals that the lower crustal gabbros, ranging from troctolites to gabbros, represent primitive crystallization products of mantle-derived melts and that oceanic crust accretion at fast-spreading ridges is primarily driven by fractional crystallization in the lower crust, followed by upward reactive melt percolation. Compared to slow-spreading ridge counterparts (Atlantis Bank and Atlantis Massif), the parental melts at Hess Deep are more depleted in incompatible trace elements due to higher degrees of mantle melting, and the lower crustal minerals show limited chemical modification from melt-rock interaction except in interstitial phases. The findings suggest that lower crustal gabbros preserve a more primary signature of mantle melts than erupted basalts, which are often modified by crustal processes, and highlight a consistent depletion in zirconium (Zr) and hafnium (Hf) in primary melts worldwide that contrasts with global mid-ocean ridge basalt (MORB) estimates based on erupted glasses.