[B-CG09] 地球史解読：冥王代から現代まで

[BCG09-P05] コマチアイトの硫黄同位体異常を用いた太古代マントル物質循環の研究

*久保田 勇祐¹、上野 雄一郎^1,2,3、清水 健二² (1.東京工業大学理学院地球惑星科学系、2.海洋研究開発機構、3.東京工業大学地球生命研究所)

キーワード：硫黄同位体、太古代、マントル物質循環

Multiple sulfur isotopes of a 2.7 Ga komatiite and related volcanics were measured to study recycled crustal material in Archean mantle. The measured komatiites are from Belingwe Greenstone Belt, Zimbabwe, which represent one of the best-preserved Archean komatiite in the world. The Belingwe volcanics has been classified into four groups: komatiite, komatiitic basalt, D-basalt and E-basalt. The results show that some komatiites and D-basalts exhibit clear S-MIF (−0.204‰ < Δ³³S < −0.115‰), whereas all the other samples do not show S-MIF (−0.025‰ < Δ³³S < +0.035‰). Based on petrological observation of the S-MIF-bearing samples, the sulfide minerals were crystallized at the time of eruption, but not introduced after the emplacement. The S-MIF signature only occurred in depleted rocks (i.e., komatiite and D-basalt), but not in enriched rocks (i.e., komatiitic basalt and E-basalt), suggesting that the S-MIF source were not assimilated from surrounding Archean crust at the time of eruption, but derived from the source mantle. The S-MIF bearing rocks also show lower initial ²³⁸U/²⁰⁴Pb, higher εNd and lower εSr values than non-S-MIF bearing rocks. All these characteristics support that the source mantle contained the MIF sulfur materials. The S-MIF signature may have been partly erased during the crustal contamination process or represent heterogeneous source in the Archean mantle. In either case, the results of this study indicate that Archean mantle contained S-MIF bearing materials, which probably represent subducted crustal material. Irrespective to tectonic style, transfer of surface material into the mantle has already been operated at least older than 2.7 Ga.