17:15 〜 18:45
[BCG06-P02] 西グリーンランド南部のイスア表成岩帯に産する縞状鉄鉱層中の炭質物の地質学的産状と結晶学的特徴
The origin and evolution of life are among most important issues in the history of the Earth. Previous studies have proposed that the origin and early evolution of life occurred in hydrothermal environments on the Earth because hydrothermal vents supplied electron donors such as H2 and Fe²+, necessary for organism (e.g., Martin et al., 2008). Thus, for instance, carbonaceous material in sedimentary rocks that deposited in hydrothermal environments such as banded iron formation (BIF) in early earth possibly suggests traces of various species of lives in the early Earth. In this study, we observed geologic occurrence and crystallinity of carbonaceous material in BIFs in the 3.7-3.8 Ga Isua Supracrustal Belt (ISB) and found likely biogenic and syngenetic carbonaceous material in order to avoid measure δ13C values of abiogenic and secondary carbonaceous material and find various species of organisms in hydrothermal environments in the early earth.
The Isua Supracrustal Belt is located approximately 150 km northeast of Nuuk, Greenland, and is a part of the Itsaq Gneiss Complex. It consists of metasedimentary rocks such as clastic sedimentary rocks, chert, carbonate rocks, and BIF, as well as basaltic and ultramafic rocks. The northeastern part is divided into four metamorphic zones: Zone A, Zone B, Zone C, and Zone D based on their mineral paragenesis and compositions (Komiya et al., 1999; Hayashi et al., 2000). We collected forty BIF samples in Zone A (Greenschist facies, 360-400 °C), two BIF samples in Zone B (Epidote-amphibolite facies, 400-500 °C), and ten BIF samples in Zone D (Amphibolite facies, 500-600 °C).
We found one grain of carbonaceous material in a BIF sample in Zone A and more than one hundred gains of carbonaceous material in nine BIFs samples in Zone D. The former occurs along quartz and amphibole grain boundaries, while the latter occurs within various kinds of minerals. In six carbonate-rich BIFs, gains of carbonaceous material are included within siderite and cummingtonite grains. In two cummingtonite-bearing BIFs, gains of carbonaceous material are included within cummingtonite grains. In a chlorite-bearing BIF, gains of carbonaceous material are included within only chlorite grains.
We estimated crystallization temperature of one carbonaceous material in Zone A and 103 grains of carbonaceous material in Zone D using the Raman spectroscopy. According to a calculation formula (Kouketsu et al., 2014), the former's temperature is estimated at up to 340 °C, lower than the metamorphic grade in this area. According to a calculation formula (Beyssac et al., 2002), the latter's temperatures are estimated at up to 641 ± 50°C and consistent with metamorphic grade in this area.
Based on geologic occurrence and crystallinity of carbonaceous material, we estimated origin of carbonaceous material in BIFs. Based on crystallization temperature of the carbonaceous material in Zone A, we estimated that it isn't traces of early life in the 3.8Ga because probably it was formed after metamorphic age (ca. 2.7 Ga; Nutman and Collerson, 1991). We estimate that grains of carbonaceous material in carbonate-bearing BIFs and cummingtonite-bearing BIFs in Zone D were formed by thermal decomposition of siderite because they associate with siderite and/or cummingtonite. On the other hands, these in chlorite-bearing BIFs in Zone D associate with neither siderite nor cummingtonite. Furthermore, the high crystallization temperature suggests that they were formed before the metamorphism. Thus, we conclude that these in chlorite-bearing BIFs are possibly biogenic and primitive.
In this study, we reported possibly traces of life in the early earth in a BIF sample in ISB. We will measure δ13C compositions of the carbonaceous material by in situ analyses in order to estimate their species in the Eoarchean.
The Isua Supracrustal Belt is located approximately 150 km northeast of Nuuk, Greenland, and is a part of the Itsaq Gneiss Complex. It consists of metasedimentary rocks such as clastic sedimentary rocks, chert, carbonate rocks, and BIF, as well as basaltic and ultramafic rocks. The northeastern part is divided into four metamorphic zones: Zone A, Zone B, Zone C, and Zone D based on their mineral paragenesis and compositions (Komiya et al., 1999; Hayashi et al., 2000). We collected forty BIF samples in Zone A (Greenschist facies, 360-400 °C), two BIF samples in Zone B (Epidote-amphibolite facies, 400-500 °C), and ten BIF samples in Zone D (Amphibolite facies, 500-600 °C).
We found one grain of carbonaceous material in a BIF sample in Zone A and more than one hundred gains of carbonaceous material in nine BIFs samples in Zone D. The former occurs along quartz and amphibole grain boundaries, while the latter occurs within various kinds of minerals. In six carbonate-rich BIFs, gains of carbonaceous material are included within siderite and cummingtonite grains. In two cummingtonite-bearing BIFs, gains of carbonaceous material are included within cummingtonite grains. In a chlorite-bearing BIF, gains of carbonaceous material are included within only chlorite grains.
We estimated crystallization temperature of one carbonaceous material in Zone A and 103 grains of carbonaceous material in Zone D using the Raman spectroscopy. According to a calculation formula (Kouketsu et al., 2014), the former's temperature is estimated at up to 340 °C, lower than the metamorphic grade in this area. According to a calculation formula (Beyssac et al., 2002), the latter's temperatures are estimated at up to 641 ± 50°C and consistent with metamorphic grade in this area.
Based on geologic occurrence and crystallinity of carbonaceous material, we estimated origin of carbonaceous material in BIFs. Based on crystallization temperature of the carbonaceous material in Zone A, we estimated that it isn't traces of early life in the 3.8Ga because probably it was formed after metamorphic age (ca. 2.7 Ga; Nutman and Collerson, 1991). We estimate that grains of carbonaceous material in carbonate-bearing BIFs and cummingtonite-bearing BIFs in Zone D were formed by thermal decomposition of siderite because they associate with siderite and/or cummingtonite. On the other hands, these in chlorite-bearing BIFs in Zone D associate with neither siderite nor cummingtonite. Furthermore, the high crystallization temperature suggests that they were formed before the metamorphism. Thus, we conclude that these in chlorite-bearing BIFs are possibly biogenic and primitive.
In this study, we reported possibly traces of life in the early earth in a BIF sample in ISB. We will measure δ13C compositions of the carbonaceous material by in situ analyses in order to estimate their species in the Eoarchean.