5:15 PM - 6:30 PM
[BPT05-P06] Carbon isotope and chemical compositions of the metasedimentary rocks from Saglek Block (>3.95 Ga), Labrador, Canada: Discovery of the oldest life and its habitat environment
Keywords:The oldest evidence for organism, Eoarchean, Saglek Block in Labrador, Carbonate rock
We obtained carbon isotope compositions of graphite (δ13Corg) from -28.2 to -11.0‰ in pelitic rocks, from -27.6 to -20.8‰ in conglomerates, from -9.9 to -6.9‰ in carbonate rocks and from -10.3 to -9.9‰ in chert nodules, respectively. The maximum δ13Corg values of the graphite in pelitic rocks of each locality increase with increasing metamorphic grade from amphibolite to granulite facies, indicating that the variation of the δ13Corg values is due to later metamorphism so that a primary δ13Corg value is lower than the minimum δ13Corg value. The crystallization temperature of the graphite, estimated from Raman spectroscopic analyses, is consistent with metamorphic temperature of the host rocks except for chert nodules, suggesting that the graphite does not originate from later contamination. On the other hand, the carbon isotope compositions of carbonates range from -3.8 to -2.6‰. The large fractionation (δ13Ccarb - δ13Corg), up to 25‰, implies the presence of autotroph utilizing the reductive acetyl-CoA pathway or Calvin cycle at least 3.95 Ga, ca. 110 Ma earlier than previous records.
We analyzed major element compositions of the carbonate rocks, pelitic rocks, conglomerates, chert nodules and ultramafic rocks and their trace element compositions except for conglomerates and chert nodules are reported. The origins of the carbonate rocks in the Eoarchean metamorphic terrains are always controversial because of severe later carbonate metasomatism and presumption of acidic seawater condition (so-called a soda ocean model) due to quite high CO2 atmosphere. The rare earth element + yttrium (REE + Y) patterns of some carbonate rocks are obtained in order to reveal the origins of the carbonate rocks, namely metasomatized mafic rock or chemical sedimentary rock. They are disrupted by input of crustal detritus or post-depositional disturbance. However, the carbonate rocks, which preserve seawater-like REE + Y patterns, still exist in all of our studied areas, indicating the chemical sedimentary origin of the carbonate rocks. All carbonate rocks in Saglek Block have no Ce anomalies, supporting the reduced condition of the Eoarchean ocean. The combination of carbon isotope values of the graphite and REE patterns of the carbonate rocks suggest the presence of the autotroph using the reductive acetyl-CoA pathway or Calvin cycle except for cyanobacteria at least 3.95 Ga.