GEOCHEMICAL EVOLUTION OF THE COBB HOTSPOT

The Cobb seamount chain is part of a large group of seamounts in the northeast Pacific Basin. The Cobb chain is discontinuous and extends about 1800 km from Axial Seamount, the current active locus of the Cobb hotspot on the Juan de Fuca mid-ocean ridge, to the Marchand and Chirikof Seamounts near the Aleutian Trench. We analyzed major and trace elements and Sr-Nd-Pb isotopes for samples recovered from along the length of the Cobb chain to characterize and model the geochemical evolution of Cobb hotspot magmatism over the past 29 Ma. Samples from the youngest seamounts (<7 Ma) in the southeastern portion of the chain (Axial, Cobb, Sloth, Lust, Gluttony, Pipe, Eickelberg, Warwick, and Thompson Seamounts) formed near the Juan de Fuca Ridge on crust <2 Ma older than the seamounts. These are mostly tholeiitic with modestly enriched incompatible trace element compositions relative to eastern Pacific MORB, and have MORB-like Sr-Nd-Pb isotopic characteristics as well. The older seamounts (>25 Ma) in the northwestern portion of the chain (Miller, Murray, and Patton Seamounts) formed on crust >11 Ma older than the seamounts. These are mostly alkalic trachytes, trachydacites, hawaiities, and mugearites that generally have progressively more enriched trace element compositions with increasing age, although a few tholeiites have also been recovered. These older lavas have Pb isotopic compositions that are distinctly more enriched than MORB. In this study, we model the effects of decreasing lithospheric thickness and the influence of the encroaching Juan de Fuca Ridge magmatic system over time on the chemistry of Cobb lavas, and interpret the Cobb chain in the context of the group of northeast Pacific seamounts.