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學(xué)術(shù)活動(dòng):Highly precise 182W-184W of ocean island and LIP basalts and komatiite derived from deep mantle

撰稿: 發(fā)布時(shí)間:2019-09-23

  各位老師和同學(xué):

  應李杰研究員邀請,日本國立海洋研究開(kāi)發(fā)機構(Japan Agency for Marine-Earth Science and Technology)鈴木勝彥博士訪(fǎng)問(wèn)我所,并將于本周六上午做“同位素室學(xué)術(shù)報告”2019年第25次報告。歡迎大家參加并積極參與討論!

  Topic: Highly precise 182W/184W of ocean island and LIP basalts and komatiite derived from deep mantle: implications for core-mantle interaction

  Speaker: Dr. Katsuhiko SUZUKI (JAMSTEC, Japan)

  Time: 10:00 AM, Sep. 21st (Saturday)

  Place:

  Conference Room 701 of Main Building

  Introduction of the Speaker:

  Dr. Katsuhiko SUZUKI, the Deputy Director and Principal Scientist of Research and Development Center for Submarine Resources, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), got his PhD degree from University of Tokyo in 1993, and was awarded as Excellent Young Geochemist by Geochemical Society of Japan for “Development of Re-Os dating method for sulfide minerals” in 1999. He has been working in JAMSTEC since 2003, and mainly engaged in the study of Isotope Geochemistry. He has published more than 100 papers in Nature, Nature-C, Geology, EPSL, GCA, and so on.

  Abstract:

  182W is a product of b-decay of 182Hf with the short half-life of 8.9 m.y. As Hf and W are lithophile and siderophile, respectively, the Hf-W could be fractionated between metallic core and silicate mantle. Such fractionation during core segregation prior to 182Hf extinction in the very early Earth leads to high 182W/184W of primitive mantle and low 182W/184W of core. Recent improvements of analytical techniques of W isotope analyses through TIMS and MC-ICP-MS allow to obtain highly precise 182W/184W ratios of terrestrial rocks, which led to findings of m182W anomalies (mostly positive) in old komatiites (2.4 – 3.8 Ga) and young volcanic rocks with positive anomalies of 12 Ma Ontong Java Plateau (OJP) and 6 Ma Baffin Bay basalts (Rizo et al., 2016) and with negative anomalies of the basalts from the Loihi and Samoa islands (Mundl et al., 2017). Recently, Kruijer and Kleine (2018) claimed no W isotope anomaly for OJP basalts and proposed that the 182W excesses for an OJP sample may have been resulted from defect of 183W caused by the nuclear field shift effect in analyses because the NTIMS analyses utilized a double normalization involving the 183W/184W ratio. Such 183W defect was also observed in our study.

  High-precision W isotope ratio measurement with MC-ICP-MS (Thermo co. Ltd., NEPTUNE PLUS) has been developed here. We have corrected the measured W isotope ratios of samples with the isotope ratio of the standard solution (bracketing) processed by the same method as that of the samples. This technique led to the precise W isotopic compositions of the reference material JB-2 with high reproducibility of several ppm. The Ontong Java Plateau basalts were analysed, yielding no 182W excess, inconsistent with those of Rizo and co-workers and consistent with those of Kruijer and Kleine. We also obtained negative μ182W for the basalts with the high 3He/4He isotopic composition from the Loihi, Hawaii, which is consistent with that of Mundl et al., (2017). We have applied our method to the hot spot basalt with FOZO isotope signature and other plume-related rocks and obtained the W isotope anomalies for the modern Ethiopian basalts and the Aden Bay MORB associated with the Afar plume. This result indicates some of the plume-related rocks may contain a component with a signature of core-mantle interaction. I will also talk about the results obtained for the Indian komatiite with 3.2 Ga and will discuss the results for the basalts collected from the Samoa island with relation to the seismic deep mantle structure.

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