New Insights into the Diverse Intraplate Volcanism Present within the Howland and Baker Island United States Exclusive Economic Zone

The origin of oceanic volcanism has been attributed to various mechanisms, such as upwelling mantle plumes, lithospheric extension driven decompression melting, and small-scale convective cells in the asthenosphere. Discovering the range of magmatic drivers present in the ocean basins aids in understanding mantle geodynamics and tectonic processes. A recent seafloor exploration campaign within the Howland and Baker Island (HBI) U.S. Exclusive Economic Zone (EEZ; approximately near the equator and the International Date Line) resulted in the collection of multiple lava flow samples, which offer valuable insights into the complex origins and dynamics of intraplate seamounts. According to current absolute plate motion (APM) models, the seamounts and islands that compose part of the HBI, as well as the Tokelau Seamount Chain to the south, was sourced by the long-lived Macdonald hotspot during the Late Cretaceous. This study utilizes new high precision 40Ar/39Ar age determinations, along with whole rock major and trace element analyses from previously unexplored seamounts, which help elucidate the volcanic history of the HBI EEZ during the Cretaceous. Three major age groups, each with their own geodynamic origins, are observed in the HBI EEZ. The new age data confirms a linear age progression at Howland Island (73.75 ± 0.28 Ma) and Titov Ridge (71.41 ± 0.23 Ma) aligning with the projected Macdonald hotspot track. The Unnamed Seamount 2 (85.61 ± 0.14 Ma) west of Howland Island (around 1.0 °N, 179.3 °E), does not fit the expected age or geometry of the Macdonald hotspot track. The origin of this seamount and neighboring Unnamed Seamount 3 remains enigmatic but may represent a pulse of plume melt channelization or similar anomalous short-lived volcanic activity. The oldest pulse of regional volcanism are tephra deposits (124.17 ± 0.12 Ma; 124.22 ± 0.13 Ma) from Unnamed Seamount 1 (around 3.9 °N, 176.3 °W). The deposits do not have chemistry consistent with mantle plume volcanism (i.e., deep melting and enriched reservoir rare earth element ratios) and are likely related either to the emplacement of the massive Ontong Java Plateau large igneous province on the Pacific basin at that time, or to plate deformation resulting from the adherence of the Magellan microplate onto the Pacific. These chemical and chronological findings from previously unexplored seamounts support the notion of the Macdonald Hotspot's long-term activity extending to the Late Cretaceous and provide new perspectives on the variety of intraplate volcanism in oceanic basins.