[B-PT23_30PM2] 地球史解読：冥王代から現代まで

[BPT23-23] In-situ iron isotope analysis of pyrite in ca. 3.8 Ga metasediments from Isua supracrustal belt, Greenland

*吉屋 一美¹、澤木 佑介²、小宮 剛³、平田 岳史⁴、丸山 茂徳¹ (1.東工大 地球生命研究所、2.東工大 地球惑星科学、3.東京大学 広域システム科学、4.京都大学 地球惑星科学)

キーワード：Early archean, Isua supracrustal belt (ISB), iron isotope ratio, pyrite, microbial dissimilatory iron reduction (DIR)

The timing of emergence of life still remains one of the unresolved questions in the early Earth. Early life could be identified and characterized by its metabolic processes, which must be deposited and preserved in the old rocks. The oldest (ca. 3.8Ga) sedimentary rocks on Earth occur in the Isua supracrustal belt (ISB), southern West Greenland. These rocks have been subjected to until amphibolite facies metamorphism (Nutman, 1986; Hayashi et al., 2000). Despite the contribution of the intense thermal metamorphism, carbon isotope compositions from the Isua metasediments suggested the evidence for biological carbon fixation. Microbial dissimilatory iron reduction (DIR) is also considered to be one of the earliest metabolisms on Earth. σ⁵⁶Fe value of Fe²⁺_aq generated by DIR is expected to have lower value, whereas negative σ⁵⁶Fe values lower than -1 ‰ are not found in the sedimentary record prior to 2.9Ga. Here, we report the in-situ iron isotope analysis of pyrite in sedimentary rocks from the ISB, using femtosecond laser ablation multi-collector ICP-MS technique (fs-LA-MC-ICP-MS). We obtained a large variation of iron isotope data from -2.41 to +2.35 ‰ in σ⁵⁶Fe values, from 212 points of pyrite grains in 15 rock specimens, including metachert, muddy metachert, BIF, carbonate rock and conglomerate. The distribution of σ⁵⁶Fe values varies depending on the lithologies and depth gradient, whereas no correlation could be found between σ⁵⁶Fe values and the metamorphic zone.Low σ¹³C values of graphite in ISB muddy metachert suggested the existence of biological carbon fixation(e.g., Schidlowski et al.,1979). σ⁵⁶Fe values of pyrite grains from the shallow water samples show lower σ⁵⁶Fe values, which suggested the occurrence of microbial DIR in the Early Archean.