Japan Geoscience Union Meeting 2019

Presentation information

[E] Poster

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

[S-IT21] Interaction and Coevolution of the Core and Mantle in the Earth and Planets

Mon. May 27, 2019 3:30 PM - 5:00 PM Poster Hall (International Exhibition Hall8, Makuhari Messe)

convener:Kenji Kawai(Department of Earth and Planetary Science, School of Science, University of Tokyo), Tsuyoshi Iizuka(University of Tokyo), Kenji Ohta(Department of Earth and Planetary Sciences, Tokyo Institute of Technology), Taku Tsuchiya(Geodynamics Research Center, Ehime University)

[SIT21-P17] Ancient, highly depleted mantle constrained by Re-Os isotope and highly-siderophile element compositions of French Polynesian xenoliths

*Norikatsu Akizawa1, Akira Ishikawa2, Katsuhiko Suzuki3, Tetsu Kogiso4 (1.Atmosphere and Ocean Research Institute, The University of Tokyo, 2.Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 3.Research and Development Center for Submarine Resources, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 4.Graduate School of Human and Environmental Studies, Kyoto University)

Keywords:Harzburgite, Dunite, PGE, TIMS, ICP-MS, Whole-rock chemical composition

Mantle xenoliths, which are fragmented mantle material trapped by ascending magma, can provide lithological and geochemical information of the inaccessible mantle. In order to better understand the lithological and geochemical characteristics of the mantle, we conducted extensive sampling of mantle xenoliths throughout French Polynesia archipelago: Tahiti, Moorea, and Rurutu islands. We collected a total of 42 mantle xenoliths: 1 lherzolite, 14 harzburgites, 22 dunites, 3 wehrlites, and 2 orthopyroxenites. Here, we present whole-rock rhenium-osmium isotope and highly-siderophile element (Os, Ir, Ru, Pt, Pd, and Re) compositions in addition to whole-rock major-element compositions. Osmium isotope ratio (187Os/188Os) of the mantle xenoliths ranges from 0.1172 to 0.1464, where the harzburgites show a statistical bimodal distribution with unradiogenic (187Os/188Os < 0.125) and radiogenic (187Os/188Os > 0.125) compositions. The harzburgites with unradiogenic 187Os/188Os are depleted with regards to whole-rock major-element compositions (< 1.22 wt% Al2O3), and chondrite-normalized patterns of highly-siderophile elements. Considering the lithological variation of the mantle xenoliths used herein, the mantle is considerably heterogeneous beneath French Polynesia archipelago. In particular, the contrasting isotopic compositions of the harzburgites suggest different mantle reservoirs with unradiogenic and radiogenic 187Os/188Os compositions. We propose a possibility that the harzburgites with unradiogenic 187Os/188Os are impinged mantle material beneath normal oceanic lithosphere due probably to ascending plumes.