[1] 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.
We present a concerted international effort to cross‐calibrate five synthetic Th isotope reference materials (UCSC Th “A”, OU Th “U”, WUN, IRMM‐35 and IRMM‐36), and six rock reference materials (UCSC TML, Icelandic ATHO, USGS BCR‐2, USGS W‐2, USGS BHVO‐2, LV18) using multi‐collector inductively coupled plasma‐mass spectrometry (MC‐ICP‐MS). We then compare our new values with a compilation of literature mass spectrometric data for these reference materials and derive recommended “consensus”230Th/232Th values for each. We also present isotope dilution U and Th concentration data for four rock reference materials (UCSC TML, Icelandic ATHO, USGS BCR‐2, USGS W‐2).
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.
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