Gen Shimoda

Setouchi high-Mg andesites revisited : geochemical evidence for melting of subducting sediments

Abstract In order to evaluate the mechanism of production of unusual high-Mg andesite (HMA) magmas, Pb–Nd–Sr isotopic compositions were determined for HMAs and basalts from the Miocene Setouchi volcanic belt in the SW Japan arc. The isotopic compositions of Setouchi rocks form mixing lines between local oceanic sediments and Japan Sea backarc basin basalts, suggesting a significant contribution of the subducting sediment component to the HMA magma generation. Mixing calculations using compositions of an inferred original mantle and local oceanic sediments suggest that a sediment-derived melt, neither an H 2 O-rich fluid nor an amphibolite/eclogite-derived melt, could have been produced first and served as a plausible metasomatic agent for the HMA magma source. The unusual tectonic setting, including subduction of a newly-borne hence hot plate, may be responsible for melting of subducting sediments.

High μ (HIMU) ocean island basalts in southern Polynesia: New evidence for whole mantle scale recycling of subducted oceanic crust

Major elements, trace elements, and Pb isotopic compositions were determined for ocean island basalts (OIBs) from Polynesia in the southern Pacific in order to document the chemical characteristics of OIB sources and to understand their origin. High μ (HIMU: μ=238U/204Pb) basalts, which have distinctly high Pb isotopic ratios, have systematically different compositions from non-HIMU basalts; HIMU basalts are more enriched in Fe2O3*, MnO, and CaO and more depleted in SiO2, K2O, P2O5, Ni and incompatible trace elements than non-HIMU, except for Nb. Major element characteristics of HIMU basalts suggest that the HIMU source is more fertile, i.e., more enriched in a basaltic component, than non-HIMU sources. This is consistent with the suggestion that subducted oceanic crust may contribute to the formation of the HIMU reservoir. Relative depletion of incompatible trace elements in HIMU is consistent with involvement of sedimentary components in non-HIMU sources. However, enrichment of Nb relative to other incompatible elements in HIMU cannot be explained by simple addition of the crustal component nor partial melting processes in the upper mantle, implying that lower mantle processes may contribute to the formation of the HIMU source.

Quantitative determination of gold and the platinum-group elements in geological samples using improved NiS fire-assay and tellurium coprecipitation with inductively coupled plasma-mass spectrometry (ICP-MS)

Abstract New fire-assay and tellurium coprecipitation methods were developed for precise determination of Ru, Rh, Pd, Ir, Pt, and Au in geological samples by using ICP-MS. High recovery (>97%) of those elements was obtained by technical improvements including (1) duplicate sample fusion under reduced conditions, and (2) duplicate Te-coprecipitation at an adequate temperature (210°C) and for adequate duration (75 min). Total blanks were less than 1 ng except for Ru and Au, and detection limits were within 2–53 ppt on the rock sample base for all elements. The present procedures are relatively simple and can be applied with high precision and reproducibility to the analysis of geological samples with wide ranges of concentrations.