Investigating Geochemical Diversity of Seamounts Adjacent to the Main Hawaiian-Emperor Seamount Chain: Insights into the Petrogenesis of the Naifeh and Un-named Seamounts.

The Naifeh and Unnamed seamounts are two clusters of submarine volcanoes ~220km north of the Northwest Hawaiian Ridge that were recently mapped and sampled by the E/V Nautilus expedition NA101. The origin of these seamounts is unknown, but their unique structure and location imply several possible mechanisms of formation. They are located on the north Hawaiian flexural arch at the end of a long fracture zone, align roughly parallel to the Hawaiian seamount chain and include several flat topped guyots, suggesting they once breached the oceans surface. Three possible mechanisms of formation include: 1) arch volcanism, 2) intraplate extension and deformation, and 3) Cretaceous crust building. Investigating the origin of these seamount clusters will better constrain the composition of the underlying mantle, as well as the depth and extent of melting required to generate these large off-axis volcanic structures and provide insight into the interaction between mantle plumes, the upper mantle, and surrounding lithosphere. Here we present major and trace element compositions of 46 samples from 10 seamounts. Major element data indicate that the lavas are alkalic and have compositions of Hawaiites, Mugearites and Benmoreites based on total alkalis versus silica contents. These lavas have lower MgO contents (0.62-2.31 wt%) compared to lavas from the north and south arch, which have only been observed near the active end of the Hawaiian plume. They have enriched incompatible trace element compositions (La/SmN 3.3), indicating their source is not typical depleted upper mantle. Thus, it is not likely these were formed from a mantle similar to the nearby Musician seamounts. Instead, these lavas have compositions similar to Mugearites collected from Pukapuka, a volcanic ridge near the South Pacific Superswell, which have been explained by diffuse lithospheric extension of heterogeneous mantle composed of an enriched component and a depleted MORB source.