New Hf isotope and trace element results for submarine basalts from the Kasuga seamounts, Mariana Northern Seamount Province, are used to address the cause of a common geochemical feature of arc magmas: negative Hf concentration anomalies. Northern Seamount Province lavas are characterized by 176Hf/177Hf and 143Nd/144Nd that extend to significantly lower values than in arc-front lavas of the Central Island Province, allowing the sediment end-member mixing component to be uniquely identified. The 176Hf/177Hf ratio correlates positively with Hf anomalies in both Northern Seamount and Central Island Province lavas. Rocks from fluid-dominated Central Island Province volcanoes are characterized by higher 176Hf/177Hf and little or no Hf anomalies, whereas rocks from sediment-dominated volcanoes have lower 176Hf/177Hf and more negative Hf anomalies. Both reach their extremity in Kasuga basalts. Results require mixing between depleted mantle and a partial melt of subducted metasediment saturated with trace quantities of rutile, zircon, and monazite. Central Island Province and especially Northern Seamount Province lavas require a sediment component more enriched in ocean island basalt–derived volcaniclastics than the average subducting sediment.