17:15 〜 18:30
[BCG04-P06] 32億年前の海洋における一次生産者に対する制約, ピルバラおよびバーバートン地域の縞状鉄鉱層微細有機物に対する地質学的, 地球化学的比較研究
キーワード：縞状鉄鉱層、バーバートン、ピルバラ、太古代、有機物
The role of microbial activities has been discussed for deposition of Precambrian banded iron formations (BIFs). On the other hand, most BIFs do not contain organic matter (OM), and it has been difficult to constrain which microbial activities were responsible for the BIF depositions. Here we report the discovery of organic matter (OM) in the ca. 3.2 Ga banded iron formation from the Fig Tree Group (FT) in Barberton. Detailed study on such organic matter will constrain the contemporary microbial activities during BIFs deposition. Besides the FT BIFs, ca.3.2 to 3.1 Ga BIFs from the Gorge Creek Group (GC) in Pilbara were studied to compare the spatial differences in the similar geological age.
Concentrations of Al2O3 in the FT BIFs samples were 0.2 to 3.4 wt% (N=37) and those in the GC BIFs samples were 0.1 to 1.1 wt% (N=8), correlated to Ti and Zr concentrations. This suggests that the FT BIFs contain more detrital materials than the GC BIFs, and were most likely formed at relatively shallow environments. On the other hand, the GC BIFs deposited in deep ocean environments, where fewer detritus were supplied
Iron-rich layers in the FT BIFs mainly consisted of hematite (up to 100μm in diameter) and a matrix of microcrystalline quartz. Fine particles of OM, in approximately 1μm in diameter, were commonly observed in the microcrystalline quartz among iron minerals by the thin section observation. Such occurrence of OM in BIFs have never been reported in previous studies. The metamorphic temperature up to 350 °C was estimated by the Raman spectroscopic analyses on those OM, which is consistent with the lower-green schist facies of metamorphic grade suggested by previous studies. Occurrences of those OM suggested that they were syndepositional with BIFs, and most likely represent the OM of primary producers. The GC BIFs consisted of similar assemblages of minerals with the FT BIF samples, such as hematite and a matrix of microcrystalline quartz. Nevertheless, no visible OM was obtained on thin section samples even by the SEM observation. Instead of in-situ analysis of OM on the thin section, isolated kerogen from the GC BIF samples were analyzed by Raman spectroscopy, suggesting that metamorphic temperature was 325 °C.
Average concentrations of organic carbon in the FT BIF samples were 0.05 wt% C (N=32) and 0.03 wt% C (N=8)for the GC BIFs. The carbon isotope compositions of kerogen (=δ13Ckerogen)values of the FT BIFs samples were ranged from -30.2 to -22.9 ‰ (VPDB) (N=30) and those of the GC BIFs were ranged from -39.2 to -28.0 ‰ (VPDB) (N=6). The differences in bulk chemistry correspond to the difference in δ13C values of OM in two BIF samples. These results suggest that kerogen from the FT BIFs, which deposited relatively shallower environment, has information of shallower biosphere, whereas the GC BIF has those of deeper one. The δ13C values of OM from the FT BIFs consistent with photosynthetic microorganisms that use the Calvin-Benson cycle as a carbon fixation metabolism. On the other hand, δ13C values of the GC BIFs represent more negative than the FT BIFs, suggesting the contribution of additional, possibly anaerobic, microbial communities in the deeper sediments.
Concentrations of Al2O3 in the FT BIFs samples were 0.2 to 3.4 wt% (N=37) and those in the GC BIFs samples were 0.1 to 1.1 wt% (N=8), correlated to Ti and Zr concentrations. This suggests that the FT BIFs contain more detrital materials than the GC BIFs, and were most likely formed at relatively shallow environments. On the other hand, the GC BIFs deposited in deep ocean environments, where fewer detritus were supplied
Iron-rich layers in the FT BIFs mainly consisted of hematite (up to 100μm in diameter) and a matrix of microcrystalline quartz. Fine particles of OM, in approximately 1μm in diameter, were commonly observed in the microcrystalline quartz among iron minerals by the thin section observation. Such occurrence of OM in BIFs have never been reported in previous studies. The metamorphic temperature up to 350 °C was estimated by the Raman spectroscopic analyses on those OM, which is consistent with the lower-green schist facies of metamorphic grade suggested by previous studies. Occurrences of those OM suggested that they were syndepositional with BIFs, and most likely represent the OM of primary producers. The GC BIFs consisted of similar assemblages of minerals with the FT BIF samples, such as hematite and a matrix of microcrystalline quartz. Nevertheless, no visible OM was obtained on thin section samples even by the SEM observation. Instead of in-situ analysis of OM on the thin section, isolated kerogen from the GC BIF samples were analyzed by Raman spectroscopy, suggesting that metamorphic temperature was 325 °C.
Average concentrations of organic carbon in the FT BIF samples were 0.05 wt% C (N=32) and 0.03 wt% C (N=8)for the GC BIFs. The carbon isotope compositions of kerogen (=δ13Ckerogen)values of the FT BIFs samples were ranged from -30.2 to -22.9 ‰ (VPDB) (N=30) and those of the GC BIFs were ranged from -39.2 to -28.0 ‰ (VPDB) (N=6). The differences in bulk chemistry correspond to the difference in δ13C values of OM in two BIF samples. These results suggest that kerogen from the FT BIFs, which deposited relatively shallower environment, has information of shallower biosphere, whereas the GC BIF has those of deeper one. The δ13C values of OM from the FT BIFs consistent with photosynthetic microorganisms that use the Calvin-Benson cycle as a carbon fixation metabolism. On the other hand, δ13C values of the GC BIFs represent more negative than the FT BIFs, suggesting the contribution of additional, possibly anaerobic, microbial communities in the deeper sediments.