Japan Geoscience Union Meeting 2018

Presentation information

[EE] Poster

S (Solid Earth Sciences) » S-IT Science of the Earth's Interior & Tectonophysics

[S-IT22] Interaction and Coevolution of the Core and Mantle in the Earth and Planets

Tue. May 22, 2018 10:45 AM - 12:15 PM Poster Hall (International Exhibition Hall7, Makuhari Messe)

convener:Tsuyoshi Iizuka(University of Tokyo), Hidetoshi Shibuya(Department of Earth and Environmental Sciences, Faculty of Advanced Science and Technology, Kumamoto University), Taku Tsuchiya(愛媛大学地球深部ダイナミクス研究センター, 共同), Kenji Ohta(Department of Earth and Planetary Sciences, Tokyo Institute of Technology)

[SIT22-P30] High precision 182W/183W isotopic compositions of terrestrial samples

Asako Takamasa1, Yusuke Fukami1, *Katsuhiko Suzuki1,2 (1.Research and Development Center for Submarine Resources, Japan Agency for Marine-Earth Science and Technology, 2.Department of Earth Sciences, Graduate School of Science, Tohoku University)

Keywords:core-mantle interaction, W isotope, Ocean island basalt, high-precision isotope analysis

Tungsten (W) has five isotopes (M = 180, 182, 183, 184, 186), and 182W isotope is a beta-decay product of derived from 182Hf with the short half life of 8.9 m.y. Both Hf and W are highly refractory elements and are accumulated in the early stage of the proto-earth. As Hf and W are a lithophile and is a siderophile elements, respectively, 182Hf-182W system could give constraints on metal-silicate (core-mantle) differentiation especially core segregation in the very early Earth system because of its large fractionation between metal-silicate and the short half life of 182Hf. Improvement of analytical techniques of W isotope analyses allows us to obtain highly precise 182W/183W ratios of vocanic rocks, which leads to findings of W isotope anomalies (mostly positive) in old komatiites (2.4 – 3.8 Ga) and young volcanic rocks with positive anomalies of 12 Ma Ontong Java Plateau and 6 Ma Baffin Bay (Rizo et al., 2016) and with negative anamalies of those such as the Loihi basalt.

In our study, high-precision W isotope ratio measurement with MC-ICP-MS (Thermo co. Ltd., NEPTUNE PLUS) has been developed. We have measured W standard solution (SRM 3163) and obtained the isotopic compositions with a enough high precision of ± 5ppm. However, the standard solution, which separated by cation or anion exchange resin, has systematical 183W/184W drift of -5ppm. These phenomena was also reported by Willbold et al. (2011). Therefore, we corrected the measured W isotope ratios of samples with the standard solution processed by the same method as that of the samples. This technique leads to the reproducible W isotopic compositions with reproducibility of several ppm. We have obtained the negative 182W/183W for the basalts with the high 3He/4He isotopic composition from the Loihi, Hawaii, through the developed analytical method. This result is consitent with that of Mundl et al., (2017). As negative anomaly of 182W/183W could be created by the early earth core segregation, it is probably a signature of core-mantle interaction.