Submarine volcanism in the western Arctic Ocean, known as Amerasia Basin, is attributed to a mantle plume based on geophysics and meager geochemical evidence. Basaltic samples dredged from Chukchi Borderland within the basin have produced minimum ⁴⁰Ar/³⁹Ar ages for eruption at ca. 118‐112 Ma, ca. 105‐100 Ma and ca. 90‐70 Ma, which we use to constrain tectonic models for basin opening. Major‐oxide and trace‐element concentrations, and Sr, Nd, and Hf isotopic ratios of the lavas show that the ca. 118‐112 Ma samples from Northwind Ridge are tholeiites (low‐Ti tholeiite I) with low degrees of rare‐earth‐element (REE) fractionation, high overall HREE and Mg#, which suggests magma derivation from a garnet‐free source followed by minor crystal fractionation. Strontium, Nd and Hf isotope systematics for these lavas, and ratios of highly incompatible trace elements, point toward a lithospheric source. Eruptions at ca. 105‐100 Ma and ca. 90‐70 Ma, both at Healy Spur, produced two types of lavas: low‐Ti tholeiite II – which are generally older than high‐Ti tholeiite – both common in continental flood basalt (CFB) provinces, and both with trace‐element abundance patterns typifying a garnet‐free source, and significant crystal fractionation for the high‐Ti tholeiite. The isotope characteristics for both groups are common features of asthenospheric sources. Composition‐time relationships for the lavas suggest inception of melting in the sub‐continental lithospheric mantle (SCLM) – probably due to introduction of a heat source by a plume – followed later (at ca. 105‐100 Ma and ca. 90‐70 Ma) by asthenospheric melting possibly triggered by plume rise.