EARLY STAGE ORTHOPYROXENE BEARING GABBROS FROM HESS DEEP: THE INTEGRATED STORY OF AN ULTRA FAST SPREADING CENTER

The fast spreading ridges, associated with Hess Deep and further west at the East Pacific Rise, consist of primitive layered lower plutonic rocks. A sequence of layered lower crustal rocks is observed in ophiolite complexes abducted onto continental margins world wide, and are understood to be representative of the crust generated at fast spreading ridges in general. IODP Expedition 345 was designed to sample relatively hard to access lower plutonic rocks and to test whether these rocks do indeed show the layering that is also seen in ophiolites. The coupled study of the lower plutonic crust at fast spreading ridges like Hess Deep, with layered ophiolites as analogs has been pivotal in developing our understanding of lower crust generation at MOR's.

These rocks have strongly layered parallel foliations, which are believed to have formed early, insinuating that the lower oceanic crust formed at a fast spreading ridge. These lower plutonic rocks, along with more shallow plutonic rocks, sheeted dykes, and lavas were analyzed to obtain a bulk rock composition of the local fast spreading oceanic crust. Although models accurately predicted this bulk composition, the lower crust showed evidence of early crystallized OPX, which the models do not predict.

Petrography, completed on the rocks recovered by IODP Expedition 345, revealed pristine OPX in multiple thin sections, with the layered texture expected in the lower crustal gabbros clearly visible. Individual pyroxene grains exhibit strongly layered parallel foliations, which are believed to have formed early. Preliminary EPMA analysis on CPX and Plagioclase grains result in a range of Mg#'s and An values which plots a predictable liquid line of descent. In comparing this predictive liquid line of descent to the Mg# of the OPX and its associated Plagioclase An value, the OPX does not appear related to the CPX's liquid line of descent.

The first order petrologic and geochemical data gathered from these rocks show that they were derived from significantly different parent melts. Comparison of Mg#'s between CPX and OPX grains, along with their associated Plagioclase An values, show compositions suggesting multiple crystallization paths. The simplest way to explain the presence of this OPX involves crustal accretion and multifaceted melt differentiation within the lower crust.