Melt/rock interaction and fractional crystallization in the lower crust at Hess Deep Rift, East Pacific Rise: A combined analytical and experimental study

Drilling studies of the lower oceanic crust in the framework of IODP (Integrated Ocean Discovery Program) have the objective of understanding the construction processes of the ocean crust. At the Hess Deep Rift in the equatorial Pacific at the East Pacific Rise (EPR), IODP Expedition 345 drilled, for the first time in the history of IODP, coherent cores of the deeper part of the lower, plutonic crust from a fast-spreading ridge, with the potential to answer long-debated questions on the evolution and accretion of fast-spread lower oceanic crust. The drilled cores show spectacular modal layering, validating for the first time the use of the ophiolite model for interpreting EPR crust. The bulk rock compositions of these primitive lower plutonics, in combination with previous geochemical data for shallow-level plutonics and basalts, provides the first reliable estimate of the average composition of crust formed at a fast-spreading ridge, corresponding to that parental melt from which the upper (basaltic) and lower (gabbroic) crust differentiate. With these novel findings obtained by Expedition 345, and with the availability of the unique samples recovered by Expedition 345, this project is aimed to understand the exact mechanism of the magmatic evolution of the lower crust from fast-spreading ridges, focusing both on the formation of the primary crystal cumulate framework and its modification by melt/rock interactions occurring in the shallow mantle and in the crust. This projects follows a multimethodical approach: Theme (1) focusses on the petrology/geochemistry of the natural rocks by using microanalytical tools to be applied to the mineral phases of the gabbroic rocks drilled at Hess Deep, with focus on the role of orthopyroxene and on the role of melt/rock interaction. The latter is indicated by our preliminary microanalytical work on clinopyroxene oikocrysts occurring in primitive gabbros in which we observed a significant zoning in some trace elements which cannot be explained by fractional crystallization. Theme (2) is an experimental project with the approach to simulate both melt/rock interaction as well as equilibrium crystallization and near-perfect fractional crystallization in a system corresponding to Hess Deep bulk crust average, which can be regarded as the parental melt for the crust at Hess Deep. The comparison of the experimental data with the natural Hess Deep gabbros will enable us to constrain the magmatic evolution of the Hess Deep crust, the exact mechanism of differentiation/fractionation, and the role of melt/rock interaction. The implications of the combined results have the potential to verify/reject long-lasting hypotheses on crustal accretionary processes at fast-spreading oceanic ridges and will advance our understanding how melts are transported from the mantle through the lower crust.