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Geology; September 2005; v. 33; no. 9; p. 737-740; DOI: 10.1130/G21639.1
© 2005 Geological Society of America
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Hafnium systematics of the Mariana arc: Evidence for sediment melt and residual phases

Darren L. Tollstrup1 and James B. Gill1

1 Department of Earth Sciences, University of California–Santa Cruz, Santa Cruz, California 95064, USA

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.

Key Words: Mariana arc • Hf isotopes • sediment • rutile • zircon • monazite






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