Darren L. Tollstrup

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

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.

Across‐arc geochemical trends in the Izu‐Bonin arc: Contributions from the subducting slab, revisited

New Sr, Nd, Hf, and Pb isotope and trace element data are presented for basalts erupted in the Izu back arc. We propose that across-arc differences in the geochemistry of Izu-Bonin arc basalts are controlled by the addition of aqueous slab fluids to the volcanic front and hydrous partial melt of the slab to the back arc. The volcanic front has the lowest concentrations of incompatible elements, the strongest relative enrichments of fluid-mobile elements, and the most radiogenic Sr, Nd, Hf, and Pb, suggesting the volcanic front is the result of high degrees of partial melting of a previously depleted mantle source caused by an aqueous fluid flux from the slab. Relative to the volcanic front, the back arc has higher concentrations of incompatible elements and elevated La/Yb and Nb/Zr, suggesting lower degrees of partial melting of a less depleted or even enriched mantle source. Positive linear correlations between fluid-immobile element concentrations and the estimated degree of mantle melting suggest the slab contribution added to the mantle wedge in the Izu back arc is a supercritical melt. Pb, Nd, and Hf isotopes and Th/La systematics of back-arc basalts are consistent with a slab melt composed of >90% altered oceanic crust and <10% sediment; that is, altered oceanic crust, not subducted sediment, dominates the slab contribution. High field strength element systematics require supercritical melts to be in equilibrium with residual rutile and zircon.

Sr isotope disequilibrium in Columbia River flood basalts: Evidence for rapid shallow-level open-system processes

Geochemical variability among Columbia River Basalt Group flood lavas has been attributed to two different origins: derivation from heterogeneous mantle and modification of mantle-derived magmas by open-system processes involving continental crust. We present in situ analyses of Sr isotopes from core-to-rim transects of plagioclase phenocrysts and groundmass from each major Columbia River Basalt Group formation and show that plagioclase crystals are usually internally zoned in 87 Sr/ 86 Sr and are in 87 Sr/ 86 Sr disequilibrium with their host groundmass. These data unequivocally demonstrate that Columbia River basalt magmas, regardless of the nature of mantle sources, were modified by opensystem processes operating at crustal depths. One-dimensional diffusion modeling indicates that observed isotopic heterogeneities cannot have existed at magmatic temperatures for more than a few years or decades. In general, results indicate that these flood basalt magmas were erupted while still in the process of assembly. A typical Columbia River flood basalt magma (melt plus phenocrysts) therefore only attains its final geochemical identity just before or during eruption, a fact that is generally obscured when evaluating conventional whole-rock isotope analyses.