Time to Go Deeper: Drilling the Lower Crust at a Magmatically Robust Ultraslow Spreading Segment

Abstract:

Atlantis Bank, in the rift mountains of the SW Indian Ridge exposes a 660-km² gabbro batholith representing lower crustal accretion at an ultraslow spreading ridge. This occurred by intrusion of numerous small melt batches which, as they crystallized, were transported upward by diapiric crystal-mush flow and hyper and sub-solidus crystal-plastic deformation. It shares characteristics with lower crust formed at low-melt supply at Atlantis Massif on the MAR, and at high-melt supply in EPR lower crust at Hess Deep, as well as features unique to moderate melt supply. Most notably the large extent to which tectonic extension was accommodated by crystal-plastic deformation, as opposed to direct intrusion of new melt, or brittle deformation. Like EPR lower crust, intrusion was frequent enough to maintain a long-lived crystal mush zone. Thus, km-scale permeable melt transport and melt rock reaction substantially modified the ascending gabbros. With more intermittent intrusion and rapid cooling at Atlantis Massif, most extension was accommodated by brittle deformation and direct intrusion. However, unlike the EPR, there was no melt lens at the dike-gabbro transition. Thus, Atlantis Bank is the key link between high and low-melt flux regimes for slow and fast-spread crust, and conditions that likely dominate over much of the spectrum of accretion at ocean ridges. The Atlantis Bank gabbros are highly evolved, and are not the residues of fractional crystallization required to mass balance MORB back to a parental mantle melt. Thus, there should be a large mass of primitive cumulates at the center of the batholith that can be reached by drilling a clean hole to 1500 m, and then coring ahead to 3-km at Site 735. This would test our hypotheses, and determine if there are mantle screens or enclaves of hybridized mantle within the gabbros deeper in the hole similar to those at higher levels at Atlantis massif. Given the emerging pattern of diversity, and the rapidly evolving hypotheses for lower ocean crust, getting deeper at Atlantis Bank is key to inform future drilling plans at in different tectonic environments, and varying melt flux at slow, intermediate and fast-spreading rate ridges. For this reason, it is critical to get a deeper hole at Atlantis Bank as soon as possible.