[B-PT23_30PM2] 地球史解読：冥王代から現代まで

[BPT23-24] カナダ,ラブラドル地域の太古代初期のNain 岩体に含まれる炭質物の起源

*田代 貴志¹、堀 真子²、石田 章純²、伊規須 素子³、佐野 有司²、小宮 剛⁴ (1.東京大学大学院理学系研究科地球惑星科学専攻、2.東京大学大気海洋研究所、3.独立行政法人海洋研究開発機構、4.東京大学大学院総合文化研究科広域科学専攻)

キーワード：炭質物, ラブラドル, 初期生命, 炭素同位体比

Presence of early life in the Early Archean is still controversial, and it is a key issue to find evidence for early life from the Early Archean rocks. Carbon isotope ratio (δ¹³C_org) of carbonaceous matter (CM) is widely used as an indicator of existence of life (Schidlowski, 2001). CM in the 3.80 Ga metasediments of the Isua Supracrustal Belt (ISB), southern West Greenland has low δ¹³C values, interpreted as evidence for organism in the Early Archean (Rosing, 1999). Recently, Ohtomo et al (2013) showed the nano-scale microstructure of the CM, evident for originating from organisms. In contrast, it is presumed that CM in the Nuvvuagittuq Supracrustal Belt (~3.75Ga) has a secondary metamorphic origin because the crystallization temperature (~380 ℃) of the CM estimated from LA-Raman spectrums is much lower than than metamorphic temperature (~640 ℃) (Papineau et al., 2011). Moreover, a putative banded iron formation in the Akilia Island (~3.83 Ga) including apatites with carbonaceous inclusions with the low δ¹³C provides another evidence for the life, but the precursor is still controversial (Fedo and Whitehouse, 2002). Thus, there is no obvious evidence for presence of life in the Early Archean except for that from ISB.Shimojo et al. (2013) showed that >3.96Ga metasediments exist in the Nain Complex, northern Labrador, Canada. The Nain Complex is ca. 100 million years older than the Akilia association, which has the oldest supracrustal rocks in the world. The purpose of this research is to reveal the origin of the CM in the sedimentary rocks in the Nain Complex.We selected pelitic gneisses (n=70), conglomerates (n=14), carbonate rocks (n=39), cherts (n=30), chert nodules in carbonate rocks (n=3) and amphibolites (n=5) from over 2000 samples over the Nain Complex based on the metamorphic grade, geography, their field occurrence and degree of alteration. Among the metasedimentary rocks (n=156), 54 specimens including pelitic gneisses (n=21), conglomerates (n=4), carbonate rocks (n=26) and chert nodules in carbonate rocks (n=3) contain CM. Seven CM-bearing rock samples were selected for δ¹³C_org analysis: pelitic gneisses (n=4), conglomerates (n=1), carbonate rocks (n=1) and chert nodules (n=1), and 3 carbonate rock samples for δ¹³C_carb analysis, respectively. Metamorphic grade was estimated for mineral paragenesis and garnet-biotite thermometry. Among the seven CM-bearing rock samples, the six samples were metamorphosed under up to the amphibolite facies condition, and a sample under the lower granulite facies condition, respectively. The metamorphic temperatures are consistent with the estimated crystallization temperature of the CM calculated by Raman spectral parameters. δ¹³C_carb values range from -3.75 to -2.63 ‰. Because it is well known that secondary alteration and metamorphism decrease a δ¹³C_carb value (Schidlowski et al., 1979), a primary δ¹³C_carb value was estimated to be higher than -2.63 ‰. As a result, the δ¹³C_carb value of marine bicarbonate was at least -2.63 ‰ in the Early Archean. δ¹³C_org values of pelitic gneisses range from -28.86 to -14.07 ‰. The δ¹³C_org values of conglomerate, carbonate rock and chert nodule are -17.52, -5.72 and -10.60 ‰, respectively. Metamorphism, generally speaking, increases a δ¹³C_org value of CM due to partial thermal decomposition, especially methane degassing, suggesting that the variation in the δ¹³C_org values is due to secondary thermal decomposition. The correlation of the δ¹³C_org values with distribution of organic matter under microscopic observation also supports the partial decomposition and consequent increase of the δ¹³C_carb values. As a result, the lowest δ13Corg value is a maximum estimate of the δ¹³C_org