Résumé
Drilling by the Integrated Ocean Drilling Program (IODP) at the Hess Deep Rift recovered young (ca. 1 Ma) primitive gabbroic rocks that formed at the fast-spreading East Pacific Rise. Olivine gabbro and troctolite are the dominant rock types recovered at Site U1415, with minor gabbro, clinopyroxene oikocryst-bearing troctolite, clinopyroxene oikocryst-bearing gabbro, and gabbronorite.Two rock series were identified, a layered series with simple layers to diffuse bands of gabbroic rock types and a troctolite series. Olivine gabbros, gabbros and gabbronorites from the layered series have high Mg-numbers (Mg/Mg+Fe)=79-87), high Ni (130-570 ppm), and low TiO2 (0.1-0.3 wt.%) and incompatible element (e.g., Y <11 ppm) contents. The troctolite series overlaps the gabbroic compositions but are, on average, more primitive with high Mg-numbers (81-89), Ni (260-1500 ppm), and Cr (365-1100 ppm) concentrations, and low TiO2 (<0.1 wt.%) and incompatible element (e.g., Y <3 ppm) contents. The most primitive troctolites sampled have compositions overlapping the field of impregnated mantle peridotites, including those from the Hess Deep Rift. However, these samples are low in Ni relative to their high Mg-number, indicating formation by a dominantly cumulate process. The gabbroic rocks at Site U1415 are far more primitive than the shallow-level gabbros at the Hess Deep Rift and are similar in bulk composition to gabbros from the shallow gabbros from Pito Deep where fast-spreading EPR crust is exposed (Perk et al., 2007). These primitive rock types fall within the range of primitive oceanic gabbros from fast-spreading crust. The preliminary geochemical data are consistent with a petrogenesis as a cumulate sequence from parental mid-ocean ridge basaltic (MORB) melt. However, the occurrence of orthopyroxene in highly primitive rocks challenges current models for melt extraction and MORB crystallization, where orthopyroxene is believed to be only stable within a more evolved MORB crystallization sequence.