Magmatic and tectonic effects of the interaction of the Juan de Fuca mid-ocean ridge with the Cobb hotspot

The Cobb hotspot has had a major influence on the Axial Segment, one of seven segments of the Juan de Fuca Ridge and the one the hotspot is directly interacting with. The Axial Segment has anomalous shallow bathymetry and is dominated by volcanic landforms instead of the tectonic features found on the other segments. Major element, trace element, and isotopic studies of the lavas erupted along the Axial Segment reveal variable mixing of the indigenous mid-ocean ridge basalt source with the Cobb hotspot magmas, with the proportion of the mid-ocean ridge component increasing with distance from the hotspot.; The hotspot has also “captured” the Axial Segment, a process that is also occurring where the Mid-Atlantic Ridge overlies the Iceland hotspot. Ridge capture has caused the neo-volcanic zone to remain fixed over the hotspot for about 0.4 Ma in spite of continued migration of the rest of the ridge system. Ridge capture has led to the exposure of basalts of different ages along the Axial Segment in a spatial framework, allowing for geochemical changes to be detected between younger and older basalts. Younger basalts are more mafic and slightly more enriched in some incompatible trace elements than older basalts, suggesting a recent influx of primitive magmas into the Axial Segment system that have a slightly higher proportion of the Cobb hotspot end-member.; The Cobb hotspot itself is anomalous in that it exhibits none of the radiogenic isotopic enrichments observed at other hotspots, although it is clearly a fixed mantle plume with a source below the mobile mantle. Although the hotspot has major and trace element characteristics that are distinct from the nearby Juan de Fuca ridge, these are within global mid-ocean ridge basalt geochemical ranges. A set of ‘model ages’ calculated for Cobb-Eickelberg basalts suggest that the hotspot is too young to be derived from oceanic crustal material that has undergone subduction to the core-mantle boundary.