Morphological and geochemical evolution of the eruptive activity along axial volcanic ridges in the Northern section of the Reykjanes ridge.

The Reykjanes Ridge is a segment of the slow-spreading Mid-Atlantic Ridge interacting with the Iceland plume. The 900 km long segment consists in “en echelon” axial volcanic ridges. They are typically 3-6 km wide, 20-30 km long, 200-500 m high, and overlap with each other over a distance of, on average, 1/3 of their length.  The Reykjanes ridge AVRs have been the subject of several studies and are the base of numerous models of AVRs evolution. However, most of these studies are based on bathymetry with a resolution > 20 m and sidescan data > 5 m, with no geochemical component. Thus, small temporal variations of the accretionary processes, especially changes in eruptive activity and magma composition, are still not well constrained. We here retrace the development of AVRs using high-resolution data combined with lava flow composition. During the MSM75 expedition in 2018, four AVRs between 62.95ºN and 63.20ºN were mapped at the resolution of 5 m. At the 63.08ºN AVR, bathymetric and backscatter data are combined with side-scan sonar data (with a 50 cm resolution) acquired with an autonomous underwater vehicle (AUV Abyss from GEOMAR) and near-bottom video from six remotely operated vehicle dives (ROV Phoca from GEOMAR) to: 1) delineate individual lava flows and tectonic structures, 2) determine flow morphologies (i.e., lobate flows, hummocky flows, hummocky ridges, seamounts), 3) locate extrusion sources, and 4) determine the chronology of the geological events. In addition, the composition of samples collected via ROV and wax corer is used to determine the geochemical evolution of the AVR. Around 200 flow units with distinct morphologies and stages of sedimentation were delineated. Our study reveals that major changes in the flow morphology at 63.08ºN is correlated with changes in flow composition. The AVR development appears to have initiated with the emplacement of seamounts aligned along an eruptive fissure. This was followed by a period of relatively high-extrusion rate / low viscosity eruptions leading to the emplacement of lobate flows. A decrease in extrusion rate and/or increase in viscosity results in the transition from lobate to hummocky morphology. In the last stage, the volcanic activity focuses along numerous narrow hummocky ridges. The similarity of the morphology distribution on several neighboring AVRs in this region indicates comparable evolutions.