Axial seamount located along the central portion of the Juan de Fuca Ridge axis and at the eastern end of the Cobb-Eickelberg Seamount Chain, is the current center of the Cobb Hotspot. The Axial Seamount lavas are transitional between N-type and E-type mid-ocean ridge basalt (MORB), characteristics that they share with lavas along the rest of the Juan de Fuca Ridge. There are, however, subtle, but distinct, differences between the seamount lavas and those of the adjoining ridge segments. These include higher Na2O, CaO, and Sr at a given MgO content and lower silica saturation in the seamount lavas as compared with the ridge lavas. Lava chemistry and bathymetry indicate that Axial Seamount is a discrete volcanic unit, with a more productive shallow magmatic plumbing system separate from the adjacent ridge segments. These high magma supply rates have sustained a continuously replenished, steady state magma reservoir that has erupted remarkably homogeneous lavas over a long time period. Despite this classic association of spreading center and hotspot volcanic activity, there is no evidence in the lavas for geochemical of isotopic enrichment typical of hotspot or mantle plume activity. The difference in composition between the Axial Seamount lavas and the Jaun de Fuca Ridge lavas are attributed to melting processes rather than to any fundamental differences in their mantle source compositions. The higher magma production rates, higher Sr, and lower silica saturation in the seamount lavas relative to the ridge lavas are thought to be a consequence of melt initiation at greater depths. The melting column producing the seamount lavas is thought to be initiated in the stability field of spinel peridotite, whereas the ridge lavas are produced from a melting column initiated at shallower levels, possibly within or close to the stability field of plagioclase peridotite.

Implicit in this interpretation is the conclusion that the Juan de Fuca Ridge lavas, and by analogy most MORB, are generated at shallow mantle levels, mostly within the stability field stability field of plagioclase peridotite. This interpretation also requires that for the upwelling mantle to intersect the solidus at different depths, the mantle supplying Axial Seamount must be hotter than the rest of the Juan de Fuca Ridge. Axial Seamount, therefore, reflects a thermal anomaly in the mantle, rather than a geochemically enriched ocean island basalt type mantle plume. ¿ American Geophysical Union 1990