The Role of Subducted Basalt in the Source of Island Arc Magmas: Evidence from Seafloor Lavas of the Western Aleutians

Discovery of seafloor volcanism west of Buldir Volcano, the westernmost emergent volcano in the Aleutian arc, demonstrates that surface expression of active Aleutian volcanism falls below sea level just west of 1759E longitude, but is otherwise continuous from mainland Alaska to Kamchatka. Lavas dredged from newly discovered seafloor volcanoes up to 300 km west of Buldir have end-member geochemical characteristics that provide new insights into the role of subducted basalt as a source component in Aleutian magmas. Western Aleutian seafloor lavas define a highly calc-alkaline series with 50–70 % SiO2. Most samples have Mg-numbers [Mg#Mg/(MgþFe)] greater than 060, with higher MgO and lower FeO * compared with average Aleutian volcanic rocks at all silica contents. Common basalts and basaltic andesites in the series are primitive, with aver-age Mg # values of 067 (6002, n99, 1SD), and have Sr concentrations (4236 29 ppm, n 99) and La/Yb ratios (45604, n 29) that are typical of island arc basaltic lavas. A smaller group of bas-altic samples is more evolved and geochemically more enriched, with higher and more variable Sr and La/Yb (average Mg# 0616 01, n 31; Sr 8826 333 ppm, n 31; La/Yb 916 09, n 16). None of the geochemically enriched basalts or basaltic andesites has low Y (<15 ppm) o