17:15 〜 18:30
[BCG04-P02] Geology and Geochemistry of Siliceous Rocks in the Eoarchean Nuvvuagittuq Supracrustal Belt: Estimation of Their Origin and Evidence for Life
キーワード：原太古代、炭素同位体、ヌブアギツック表成岩帯
Obtaining evidence from geological samples is essential in research to understand the origin of life. However, early Archean rocks are generally highly metamorphosed and deformed and no body fossil remains, so most such studies rely upon organic carbon isotope (e.g. Mojzsis et al., 1996; Ohtomo et al., 2014; Tashiro et al., 2017), or iron isotope (e.g. Craddock & Dauphas 2011; Yoshiya et al., 2015) signature.
The Nuvvuagittuq Supracrustal Belt (NSB) is a >3.75 Ga supracrustal belt (Cates & Mojzsis, 2007), located in northeastern Quebec, Canada, which possibly dates back to 4.3 Ga (O’Neil et al., 2008), mainly composed of cummingtonite-bearing amphibolite, ultramafic rock, banded iron formation (BIF), and siliceous rock, with intrusive rocks of various compositions. Previous studies argued that texture of hematite (Dodd et al., 2017) and organic carbon isotope (Dodd et al., 2019) of BIF are biological origin.
The protolith of the siliceous rocks of NSB is poorly understood. Some previous studies described it as “silica formation” and interpreted as a large silica phase of BIF (O’Neil et al, 2007; David et al., 2009) based on geological occurrence, while another study (Augland & David, 2015) does not describe it as a bedrock.
In this study, (1) the protolith of siliceous rocks in NSB was estimated by geology and elemental analysis. (2) origins of carbon in the siliceous rocks were constrained by Raman spectroscopy and whole-rock organic carbon isotope analysis.
(1) the siliceous rocks lie conformably on metavolcanic rocks and have mm- to cm-scale layering structure, defined by mineral compositions and orientation, in spite of recrystallization, consistent with its sedimentary protolith. Rare earth element patterns also support its sedimentary origin. (2) the siliceous rocks contain up to 1.3% organic carbon now crystallized to graphite. Their crystallization temperature of >650℃ was estimated by Raman spectroscopy of graphite grains in thin sections, consistent with the maximum metamorphic temperature of NSB. The organic carbon isotope ratios (δ13C) are -10‰ to -25‰, consistent with biological origin. Moreover, the variation of carbon isotope ratios suggests mixing of biological and/or abiological carbon sources with initially different carbon isotope ratio.
The Nuvvuagittuq Supracrustal Belt (NSB) is a >3.75 Ga supracrustal belt (Cates & Mojzsis, 2007), located in northeastern Quebec, Canada, which possibly dates back to 4.3 Ga (O’Neil et al., 2008), mainly composed of cummingtonite-bearing amphibolite, ultramafic rock, banded iron formation (BIF), and siliceous rock, with intrusive rocks of various compositions. Previous studies argued that texture of hematite (Dodd et al., 2017) and organic carbon isotope (Dodd et al., 2019) of BIF are biological origin.
The protolith of the siliceous rocks of NSB is poorly understood. Some previous studies described it as “silica formation” and interpreted as a large silica phase of BIF (O’Neil et al, 2007; David et al., 2009) based on geological occurrence, while another study (Augland & David, 2015) does not describe it as a bedrock.
In this study, (1) the protolith of siliceous rocks in NSB was estimated by geology and elemental analysis. (2) origins of carbon in the siliceous rocks were constrained by Raman spectroscopy and whole-rock organic carbon isotope analysis.
(1) the siliceous rocks lie conformably on metavolcanic rocks and have mm- to cm-scale layering structure, defined by mineral compositions and orientation, in spite of recrystallization, consistent with its sedimentary protolith. Rare earth element patterns also support its sedimentary origin. (2) the siliceous rocks contain up to 1.3% organic carbon now crystallized to graphite. Their crystallization temperature of >650℃ was estimated by Raman spectroscopy of graphite grains in thin sections, consistent with the maximum metamorphic temperature of NSB. The organic carbon isotope ratios (δ13C) are -10‰ to -25‰, consistent with biological origin. Moreover, the variation of carbon isotope ratios suggests mixing of biological and/or abiological carbon sources with initially different carbon isotope ratio.