Geochemical analysis of diachronous V‐shaped ridges and troughs that flank the Reykjanes Ridge south of Iceland

It is recognized that mantle plumes play a direct role in generating regional uplift and producing immense volumes of basaltic magmatism, both of which can influence paleoclimate. The Icelandic Plume, beneath the North Atlantic Ocean, is of particular importance due to its size and position at a significant paleoceanographic gateway. It is transected by a mid-oceanic ridge system, which has generated a series of V-shaped ridges and troughs that flank the Reykjanes Ridge south of Iceland. The origin of these diachronous features is debated—do they reflect thermal fluctuations within the plume head or have they formed as a result of compositional variations within the buoyant convecting mantle? To address these and other hypotheses, the International Ocean Discovery Program (IODP) carried out three drilling expeditions, which recovered basalt cores from a sequence of V-shaped ridges and troughs. Here, we show that the petrology and geochemistry of fifty whole-rock samples taken from boreholes that penetrate different ridges and troughs reveal systematic differences in major, trace and rare earth element concentrations. By combining forward and inverse modeling based upon polybaric fractional melting, we show that these geochemical variations can be explained by varying melt fraction as a function of depth for plausible mantle source compositions. Our results suggest that temperature differences of 25–30C between cooler troughs and hotter ridges play a dominant role. We conclude that the drilled basaltic rocks reveal a chronology of resolvable temperature perturbations that should help to elucidate the fluid dynamics of flow within this major plume.