Mantle source composition beneath the Mid-Atlantic Ridge: controls on the development of E-MORB segments and oceanic core complexes

The thickness and continuity of volcanic cover at slow-spreading ridges is variable. Magmatically ‘robust’ regions are interspersed with areas which experience little volcanism. Near the Fifteen-Twenty Fracture Zone (FTFZ), (MAR), volcanism appears exceptionally limited. Here, spreading at numerous sites is instead accommodated by extension on low-angle detachment faults, exposing peridotite to form oceanic core complexes (OCCs). Little is known about the mechanisms controlling the transition between spreading states. This study investigates these mechanisms in detail (MAR, 12º6’N-14°N).Basalts around the FTFZ are E-MORBs with a HIMU-FOZO flavour. There is no evidence that melt production is currently lower at OCC spreading sites than the surrounding magmatic areas. The geochemistry of dykes (OCC surface, 13º19’N,-44º95W) shows that melt here is diverted along the detachment fault, resulting in reduced volcanism in the axis.The mantle source in this region was extensively depleted during a previous melting episode(s). This study finds that the source was also variably veined with refertilising material of HIMU-FOZO character. Basalts erupted immediately prior to OCC formation, and now located off-axis, (13º19’N,-44º95W) are geochemically bimodal, and different to the on-axis melts. Trace element and geochemical modelling data shows that ‘M1’ (LREE & volatile-rich, CPX-bearing E-MORBs) are high-pressure, garnet-phase, low melt fractions, whilst ‘M2’ (less enriched, flatter REE profiles) are low melt fractions formed by spinel-phasemelting.In areas of exceptional bulk source depletion, melt production is limited by the geochemistry of the source.’ M1’ basalts result from rapid melting and extraction of a small volume of enriched vein material. This locally cools and dehydrates the source. Low melt fractions ‘M2’ are then generated from this depleted source. Due to the lack of fusible components, melt production remains insufficient tosustain magmatic spreading, and an OCC forms. Thus, periods of low melt production, resulting directly from mantle compositional characteristics, drive the transition from magmatic to ‘fault-dominated’ spreading