Mapping the Lower Ocean Crust: Evidence for Variable Melt Production and Mantle Composition during Normal Seafloor Spreading

Three major oceanic core complexes have been mapped and two drilled: Atlantis Bank at 57°E on the ultraslow SW Indian Ridge, and Kane Megamullion and Atlantis Massif at 23°N and 30°N on the slow spreading MAR. In theory, magmatism should be most robust at 30°N due the proximity of the Azores Hot Spot at 38°N, and less so at 24°N, while it should be anemic at Atlantis Bank, which, like Kane, is 2300 km from a hotspot but ultraslow spreading. In fact, detachment faulting through the dike-gabbro transition at Atlantis Bank exposed a 400 km2 gabbro massif at least 1.5 km thick, and possibly as much as 5 km thick. By Contrast, faulting at Kane Megamullion, also through the dike-gabbro transition, exposed a discontinuous lower crust of local magmatic centers intruding partially serpentinized mantle. At the midpoint of this paleo-ridge segment, the intact crust consisted at one time of peridotite overlain by dikes and lavas. At 30°N, the full lithologic sequence that characterizes long-lived oceanic detachment faults is missing. There, gabbro-veined serpentinized mantle is exposed on the transform wall, while 5.5 km to the north 1.4 km of troctolite and gabbro were drilled beneath a thin cover of talc-serpentine schist. While such schist characterizes many oceanic core complexes, high-temperature gabbro mylonites often found beneath them are rare. While the mechanism of emplacement accordingly is debated, it would appear to represent a gabbro massif intermediate in size between those at Atlantis Bank and the Kane Megamullion. Thus, it would appear that major element indicators of the extent of mantle melting (e.g. Na8.0), ridge depth, proximity to mantle hotspots, or spreading rate are not reliable indicators of crustal thickness. Rather, other factors, such as the initial mantle composition, may also play a major role in determining the thickness of the crust.