Mantle Melting, Melt Transport, and Delivery Beneath a Slow-Spreading Ridge: The Paleo-MAR from 23 15'N to 23 45'N

Abstract

Kane Megamullion, an oceanic core complex near the Mid-Atlantic Ridge (MAR) abutting the Kane Transform, exposes nearly the full plutonic foundation of the MARK paleo-ridge segment. This provides the first opportunity for a detailed look at the patterns of mantle melting, melt transport and delivery at a slow-spreading ridge. The Kane lower crust and mantle section is heterogeneous, as a result of focused mantle melt flow to different points beneath the ridge segment in time and space, over an $300^400 kyr time scale. The association of residual mantle peridotite, dunite and troctolite with a large $1km thick gabbro section at the Adam Dome Magmatic Center in the southern third of the complex probably represents the crust^mantle transition. This provides direct evidence for local melt accumulation in the shallow mantle near the base of the crust as a result of dilation accompanying corner flow beneath the ridge. Dunite and troctolite with high-Mg Cpx represent melt^rock reaction with the mantle, and suggest that this should be taken into account in modeling the evolution of mid-ocean ridge basalt (MORB). Despite early precipitation of high-Mg Cpx, wehrlites similar to those in many ophiolites were not found. Peridotite modes from the main core complex and transform wall define a depletion trend coincident with that for the SW Indian Ridge projecting toward East Pacific Rise mantle exposed at Hess Deep.The average Kane transform peridotite is a lherzolite with 52% Cpx, whereas that from the main core complex is a harzburgite with only 35%