Sulfide differentiation in slow-spreading ridge magmas

Abstract

Slow-spread oceanic crust is thinner and more heterogeneous than fast-spread oceanic crust. These features of the slow-spread oceanic crust affect how sulfides differentiate during magmatic processes and how metals are transported between the mantle and the ocean floor. In order to better understand sulfide differentiation in slow-spreading ridges we investigated igneous rocks from oceanic core complexes (OCC). There, not only volcanic but also plutonic rocks are exposed on the ocean floor. We selected two of the most extensively sampled OCCs to compare between settings with relatively high (Atlantis Bank OCC located at 57°E along the Southwest Indian Ridge) and relatively low (Kane Megamullion OCC located at 23°N along the Mid-Atlantic Ridge) magma supply. Due to high magma supply in Atlantis Bank, large gabbroic bodies can develop and sulfides differentiate mostly through fractional crystallization. Sulfides fractionate early and thus tend to accumulate at the lower part of gabbroic bodies. The lower parts of gabbroic bodies are by ~50% enriched in Cu and by ~100% enriched in S with respect to the upper parts. In addition, gabbro bodies located deeper in the crust are enriched in chalcophile elements with respect to shallower gabbro bodies. Both the facts are consistent with MORBs having sulfur concentrations above sulfide saturation during their ascent through the slow-spread lower crust with high magma supply. In contrast where magma supply is low as is the case for Kane Megamullion, sulfides typically differentiate through melt-mantle reaction. This process may be global, but it becomes increasingly significant at ridge segments with low magma supply and thin crust, where the melt-mantle reaction can proceed to very shallow depths. Melt-mantle reaction leads to high sulfide enrichment at the contacts of gabbro and peridotite. The crust-mantle transition zone exposed in the Kane Megamullion OCC is highly enriched in chalcophile elements. Most seafloor massive sulfides, especially with the highest Cu-grades, occur along slow-spread oceanic lithosphere with relatively low magma supply. The peculiar distribution of the seafloor massive sulfide seems to reflect the style of magmatic differentiation determined by magma supply.