Petrogenesis of Fast Spreading Crust at Hess Deep

The fast spreading mid-ocean ridges (MORs), associated with Hess Deep Rift (HDR) and further west at the East Pacific Rise (EPR), are comprised at their base of primitive layered plutonic rocks. Integrated Ocean Drilling Program Expedition 345 (IODP 345) recovered a relatively difficult to access layer of lower plutonic rocks and to test whether these rocks do indeed show the lower crustal characteristics also seen in ophiolites. The lower plutonic rocks from Hess Deep, along with more shallow plutonic rocks, show a crystallization sequence predicted by existing models. However, the Hess Deep lower crust samples also showed evidence of early crystallized orthopyroxene (OPX), that alphaMELTs computer models do not predict. Petrography completed on these rocks reveal pristine OPX in multiple thin sections, with the layered textures expected in the lower crustal gabbros clearly visible as well. EPMA analysis, of the geochemistry of these rocks on the primary phases of plagioclase, clinopyroxene (CPX), and OPX was performed. Comparison of these samples’ pyroxene Mg content, Na content, and plagioclase anorthite (An) content to that of global and modeled results has revealed two surprising conclusions. First, the examination of the degree of partial melting that produced the primitive magmas these samples were derived from, reveals a significantly thicker predicted crust at HDR in comparison to regionally determined total seismic crustal thicknesses. This suggests that the assumed Na content of the mantle source may be need to be revised. Second, by examining main phase geochemistry, the crystallization trends of these samples, when compared to modeled and global values, does not match modeled results. This strongly indicates there may have been multiple melt accretion events in the upper mantle and lower crust that led to the crystallization of these in situ lower crustal OPX bearing rocks. Mantle hybridization may explain the presence of the high Mg# CPX and early OPX.Earth and Atmospheric Sciences, Department o