Hafnium systematics of the Mariana arc: Evidence for sediment melt and residual phases

Abstract

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.