Orphan Crust, The Dike, Lower Crust and Mantle Transitions: Ridge Architecture at High, Medium, and Low Magma Supply

The fundamental boundary defining the architecture of the ocean crust is that separating lavas erupted on the seafloor and the dike melt-transport zone from the underlying melt storage region and mantle. It is key to understanding how the architecture and composition of the ocean crust vary with spreading rate and magma flux. The dike-gabbro transition is believed to generally coincide with the transition from advective to conductive cooling: the limit to which hydrothermal fluid can circulate freely through cracks, removing heat as it circulates back to the ocean. Similar dynamics may exist at the dike-mantle interface. Although fundamental differences in the nature of the ocean crust have long been implied by the presence of relatively smooth seafloor and an axial rise at fast-spreading ridges, and the rough terrain and deep rift valleys at slow-spreading ridges, the transition was long regarded as showing little variation. This is not the case. Whereas the dike-gabbro transition in old EPR crust at Site 1256D, Hess and Pito Deeps, and the Oman ophiolite analog (fast-spreading), have high-level gabbros that erode, stope, and assimilate dikes , at slow and ultraslow-spreading ridges dikes consistently cut and stope the gabbros and, laterally often interface directly with the mantle beneath a lava carapace. Intercalated with screens of mantle peridotite they also do not constitute an easily identifiable seismic boundary. Overall, at slow and ultraslow spreading ridges it has become increasingly clear that the axial melt storage region (melt lens, crystal mush zone, magma chamber) is discontinuous, and that gabbroic lower crust is largely missing beneath at least 50% of the crust. At the same time at the ultraslow spreading SW Indian Ridge gabbroic crust is largely missing and crust is entirely missing over ~23.1% of the seafloor along the ridge.