Masako Yoshikawa

Mantle wedge infiltrated with saline fluids from dehydration and decarbonation of subducting slab

Slab-derived fluids play an important role in heat and material transfer in subduction zones. Dehydration and decarbonation reactions of minerals in the subducting slab have been investigated using phase equilibria and modeling of fluid flow. Nevertheless, direct observations of the fluid chemistry and pressure–temperature conditions of fluids are few. This report describes CO2-bearing saline fluid inclusions in spinel-harzburgite xenoliths collected from the 1991 Pinatubo pumice deposits. The fluid inclusions are filled with saline solutions with 5.1 ± 1.0% (wt) NaCl-equivalent magnesite crystals, CO2-bearing vapor bubbles, and a talc and/or chrysotile layer on the walls. The xenoliths contain tremolite amphibole, which is stable in temperatures lower than 830 °C at the uppermost mantle. The Pinatubo volcano is located at the volcanic front of the Luzon arc associated with subduction of warm oceanic plate. The present observation suggests hydration of forearc mantle and the uppermost mantle by slab-derived CO2-bearing saline fluids. Dehydration and decarbonation take place, and seawater-like saline fluids migrate from the subducting slab to the mantle wedge. The presence of saline fluids is important because they can dissolve more metals than pure H2O and affect the chemical evolution of the mantle wedge.

Geochemical and Sr–Nd isotopic characteristics and pressure–temperature estimates of mantle xenoliths from the French Massif Central: evidence for melting and multiple metasomatism by silicate-rich carbonatite and asthenospheric melts

Abstract Ultramafic xenoliths from Mont Briançon, Ray Pic and Puy Beaunit in the French Massif Central show variable mineral compositions that indicate a residual origin after various degrees of partial melting of a fertile peridotite. Furthermore, trace element and Sr–Nd isotopic variations of clinopyroxenes indicate mixing processes between depleted mantle and enriched components such as asthenospheric melt and silicate carbonatite melt. Pyroxene geothermometer and CO2 geobarometer estimates are 860–1060 °C at 0.92–1.10 GPa for Mont Briançon, 930–980 °C at 0.89–1.04 GPa for Ray Pic and 840–940 °C at 0.59–0.71 GPa for Puy Beaunit. From south to north, the xenoliths show the following trends: (1) deeper to shallower origin; (2) more depleted mineral compositions, suggesting higher degrees of partial melting; and (3) more enriched isotopes and trace elements, indicating a mixing process with a silicate-rich carbonatite melt characterized by high H2O and K2O, possibly during Variscan subduction.

Diopsidites and Rodingites: Serpentinisation and Ca-Metasomatism in the Oman Ophiolite Mantle

Diopsidites and rodingites are two specific kind of dyke cropping out in the mantle section of the Oman ophiolite. Diopsidites show a diopside monomineralic modal composition, associated in some exceptional cases with anorthite. Typical rodingites mineralogical assemblages include grossular, chlorite, tremolite, zoisite/clinozoisite, epidote, prehnite, diopside, magnetite and other accessory minerals in various proportions. Both are result from the alteration in Ca-rich environment of the mantle or pre-existing gabbroic dykes. In spite of this apparent common origin, significant divergences are observed between these two lithologies and their host, allowing us to think that they are two different kind of rock which have been generated by two different processes: rodingites are the result of the interaction between gabbroic dykes and serpentinisation-issued fluids, while diopsidites formed by genuine Ca-metasomatism of the mantle with an external source of calcium. A precise petrographical study of the diopsidites and rodingites, their characteristics and their interaction with their host show that these two lithologies are related to two different type of alteration in the mantle.

Along-arc geochemical variations in Quaternary magmas of northern Kyushu Island, Japan

Abstract We analysed major and trace elements and Sr–Nd–Pb isotope ratios of Himeshima, Futagoyama and Aso volcanoes, northern Kyushu, Japan. The purpose of the study was to investigate the petrogenesis of Quaternary arc magmas of the northern Kyushu arc, Japan, through along-strike variations. The northern Kyushu arc is underlain by the Philippine Sea Plate, which is divided by the Kyushu–Palau Ridge into the Palaeogene West Philippine Basin segment (60–40 Ma) and the Miocene Shikoku Basin segment (26–15 Ma). The Sr/Y ratios of the arc lavas decrease from north to south along the volcanic front. Mixing relations in Sr–Nd–Pb isotope space suggest recycling of the subducted slab materials from the Philippine Sea Plate to the arc. The 87Sr/86Sr ratios decrease with increasing Sr/Y and SiO2, which argues against a genetic link of fractional crystallization between adakites and basaltic magmas. The observations further suggest that partial melting of the hot and young Shikoku Basin slab produces the high Sr/Y component visible in the arc magmas in the north, whereas dehydration of the older West Philippine Basin slab produces the low Sr/Y arc magmas in the south.

Application of Mass Spectrometry for Analysis of Cesium and Strontium in Environmental Samples Obtained in Fukushima Prefecture

For the assessment of Fukushima Daiichi Nuclear Power Plant accident, the applicability of the thermal ionization mass spectrometry (TIMS), which is a type of mass spectrometry, was studied. For the study of the recovery/analysis method of cesium and strontium, at first, the radioactive cesium and strontium were generated by the irradiation of natural uranium at KUR. After this study, the applicability of this method to the environmental samples obtained in Fukushima prefecture was verified.