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Contrasting behavior of noble-metal elements during magmatic differentiation in basalts from the Cook Islands, Polynesia

¹ Institute for Geothermal Sciences, Kyoto University, Beppu 874-0903, Japan, and Ocean Research Institute, University of Tokyo, Tokyo 164-8639, Japan
² School of Earth Sciences, Kyoto University, Kyoto 606-8501, Japan
³ Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Tokyo 152-8551, Japan
⁴ GBE-CNRS-ISTEEM, Université de Montpellier II Sciences et Techniques,34095 Montpellier Cedex 5, France
⁶ Ocean Research Institute, University of Tokyo, Tokyo 164-8639, Japan

Concentrations of noble metals (Ir, Ru, Rh, Pt, Pd, and Au) in ocean-island basalts from the Cook Islands, Polynesia, were determined by improved fire-assay and tellurium coprecipitation techniques with an inductively-coupled-plasma mass spectrometer. Isotope, major element, and trace element compositions of these basalts indicate that the present samples include distinctive HIMU (high µ = high ²³⁸U/²⁰⁴Pb) and normal non-HIMU basalts. Examination based on Ni-Mg-Fe partitioning between olivine and liquid suggests an only minor effect of accumulation of phenocrysts in governing the compositional variations of the present samples. The fractionation trends obtained show monotonic decrease and increase in noble-metal elements with decreasing MgO content in HIMU and non-HIMU basalts, respectively. These characteristic trends indicate that HIMU magmas are differentiated by fractional crystallization and have higher sulfide/silicate ratios than non-HIMU basalts.

Key Words: noble-metal elements • magmatic fractionation • Pacific hotspots • HIMU basalts • sulfides

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