Petrologic and Geochemical Complexities Revealed in Gabbros From Fast-Spread Lower Oceanic Crust at Pito Deep

Abstract

The Pito Deep Rift (23°S, 112°W) provides a tectonic window through lavas, dikes and the uppermost gabbros of fast-spread oceanic crust accreted at the East Pacific Rise (EPR).During R/V Atlantis Cruise AT37-08 (2017), >300 gabbroic samples were collected from a ~1.5km (spreading parallel) by 1.3km (depth) exposure. Within this area, there are two compositionally and spatially distinct magmatic suites: the Southern Magmatic Suite (SMS) and Northern Magmatic Suite (NMS).

This work focuses on the SMS, which is dominated by troctolite and olivine gabbro with primitive average mineral compositions (Fo₈₅, An₈₃, and cpx Mg#87). The SMS is more primitive than the NMS, which is comprised of gabbro, olivine gabbro, and Fe-Ti oxide-bearing gabbro with less primitive mineral compositions (Fo₇₆, An₆₅, and cpx Mg#80; Gess, 2020). The uppermost ~100m of the SMS is a chemically, modally, and texturally distinct series, characterized by elongate and poikilitic clinopyroxene, Fe-Ti oxides (0.1-0.5%), and more evolved average mineral compositions (Fo₇₃, An₇₂, and cpx Mg#82). Below the upper ~100m is a complicated sequence of texturally variable olivine gabbro and troctolite, without recognizable stratigraphy. However, there is a gentle trend of upward evolving mineral compositions. High Cr₂O₃ with high TiO₂ in low-volume (marginal, interstitial) and poikilitic clinopyroxene implies reaction with late-migrating melts played a significant role in formation of the SMS. The accretion of the uppermost gabbroic crust at Pito Deep is most consistent with a gabbro flow model. Preserved temporal variation from more primitive SMS to more evolved NMS indicates multi-dimensional complexity in EPR fast-spread crust.