[S-CG56] ICDP オマーン掘削プロジェクト

[SCG56-04] Strontium isotopic compositions of mantle peridotites in the Oman ophiolite

*Eom Jiwon¹、若木 重行²、Akizawa Norikatsu¹、Ishikawa Tsuyoshi²、山岡 香子³、高澤 栄一⁴、川幡 穂高¹ (1.Atmosphere and Ocean Research Institute, The University of Tokyo、2.海洋研究開発機構 高知コア研究所 、3.産業技術総合研究所地質情報研究部門 、4.新潟大学理学部理学科地質科学科プログラム )

キーワード：Oman ophiolite、Strontium isotope

The Oman ophiolite is one of the best exposed and preserved ophiolites on Earth. It preserves complete oceanic stratigraphy with a mid-ocean ridge basalt (MORB) underlain by sheeted dykes, gabbros and peridotites. Although the initial stage of its formation is still controversial (at a mid-ocean ridge or in supra-subduction zone), many geochemical evidences support that the Oman ophiolite was modified by arc magmatism. Here, we report strontium isotopic compositions of mantle peridotites in the Oman ophiolite. The peridotites are harzburgite considering their modal composition (~a few volume % clinopyroxene). We collected the harzburgites from the north to south massifs, covering entire the Oman ophiolite. All the data is age-corrected to 96Ma. The harzburgites from the northern Oman ophiolite are significantly higher in ⁸⁷Sr/⁸⁶Sr values than Cretaceous seawater (0.7076). Particularly, northernmost samples traversing northern Fizh mantle section from basal thrust to the crust/mantle boundary are characterized by homogeneous ⁸⁷Sr/⁸⁶Sr values (0.7078 to 0.7086) indicating the involvement of fluids with high ⁸⁷Sr/⁸⁶Sr in the mantle. On the contrary, lower ⁸⁷Sr/⁸⁶Sr values than Cretaceous seawater are dominant to the south, implying that the mantle was insufficiently modified by the fluids. The regional variation of ⁸⁷Sr/⁸⁶Sr values may be relevant to the contribution of pelagic sediment with elevated ⁸⁷Sr/⁸⁶Sr values derived from the subducting slab. This feature is consistent with the fact that the arc magmatism is only developed in the northern Oman ophiolite, e.g. discordant dunite channels in the mantle section and arc tholeiitic and boninitic extrusive rocks in the crustal section.