17:15 〜 18:45
[HRE13-P05] 南アフリカ・バーバトン緑色片岩帯Moodies縞状鉄鉱層における炭酸塩鉱物の形成プロセス
キーワード：地球史、同位体、堆積岩、微生物、鉄、酸素
Carbonate occurrence and isotopic signatures in Banded Iron Formations (BIFs) have the potential to provide insights into the ocean chemistry and evolution of the marine biosphere on the early Earth since they may preserve their biological or abiological formation pathway [1, 2]. Previous researches on iron (Fe) isotope of BIFs primarily focused on the bulk scale analysis due to concerns about iron isotope fractionation by partial extraction and difficulty on dissolution of a single mineral phase. However, a recent research by Rego et al. [3] demonstrate that acetic acid extract only carbonate mineral without isotope fractionation. Therefore, the objective of this study was to understand the formation processes of carbonate minerals in BIFs from the Moodies Group of the Barberton Greenstone Belt in South Africa, which formed in a shallow ocean about 3.2 Ga, based on petrographic observation and Fe isotope analysis of the carbonate minerals by applying the methodology [3]. The analyzed samples were collected from underground mines to avoid the effects of weathering and are valuable samples with remaining carbonate minerals.
The carbonate minerals in the samples are divided into two types: ankerite (Ca0.471(Fe0.214,Mg0.295)(CO3)2) and siderite ((Fe0.617,Mg0.376)CO3). Thin section observations showed different occurrences of carbonate minerals in each microband, mainly ankerite in the sand laminae (with particle diameters >30 µm) and mainly siderite in the mud laminae (<30 µm). Additionally, chemical zoning structures were observed in each type with Scanning Electron Microscope (SEM) images mainly due to difference in Fe and Mg contents. No Ca contents variations were observed in the chemical zoning. Whereas some ankerite have a dolomite nucleus with Fe-rich rims, siderite tends to have Mg-rich rims. These observations suggest that ankerite was formed as dolomite in the sand layer in a shallow sedimentary environments under strong influence of seawater. On the other hand, siderite likely formed when iron was reduced in the mud layer and then Mg-rich rim was formed during late diagenesis when Fe is depleted. Therefore, the microscopic observations imply that the two carbonate minerals were distinct for their timing and processes of the formation. The contribution of seawater was also suggested by the carbon isotope ratios of the carbonate minerals (δ¹³Ccarb =−3.74 –−5.32‰), which were close to the values of the equilibrium with seawater carbonate (δ¹³C carb=0‰)
Regarding Fe isotope analysis of the carbonate minerals, we first compared the δ56Fe values with different amounts of extracted elements using acids of different strengths (HAc 10%, 20% and 0.4M HCl). The result shows the similar iron isotope compositions (Δ56Fe <0.18‰; n=3) regardless of the different degree of partial extraction, indicating there is also no significant isotope fractionation with our samples. Al below detection limit observed during the acetic acid extraction, supporting no dissolution of silicate minerals. The δ56Fe values of the carbonate minerals (δ56Fe carbonate=−0.11–−0.93‰; n=13) showed small differences from the bulk (Δ56Fe bulk-carbonate=−0.17–−0.61‰) , suggesting that abiotic process in a semi-closed system likely governed the iron reduction and carbonate mineral formation in the Moodies BIFs. This further suggests that the bulk δ56Fe isotope data preserve the signal of the primary precipitation.
[1] Heimann, A., Johnson, C. M., Beard, B. L., Valley, J. W., Roden, E. E., Spicuzza, M. J., and Beukes, N. J. (2010) Earth Planet. Sci. Lett. 294, 8-18.
[2] Halama, M., Swanner, E. D., Konhauser, K. O., and Kappler, A. (2016) Earth Planet. Sci. Lett. 450, 243-253.
[3] Rego, E. S., Busigny, V., and Philippot, P. (2022) Chem. Geol. 611, 121120
The carbonate minerals in the samples are divided into two types: ankerite (Ca0.471(Fe0.214,Mg0.295)(CO3)2) and siderite ((Fe0.617,Mg0.376)CO3). Thin section observations showed different occurrences of carbonate minerals in each microband, mainly ankerite in the sand laminae (with particle diameters >30 µm) and mainly siderite in the mud laminae (<30 µm). Additionally, chemical zoning structures were observed in each type with Scanning Electron Microscope (SEM) images mainly due to difference in Fe and Mg contents. No Ca contents variations were observed in the chemical zoning. Whereas some ankerite have a dolomite nucleus with Fe-rich rims, siderite tends to have Mg-rich rims. These observations suggest that ankerite was formed as dolomite in the sand layer in a shallow sedimentary environments under strong influence of seawater. On the other hand, siderite likely formed when iron was reduced in the mud layer and then Mg-rich rim was formed during late diagenesis when Fe is depleted. Therefore, the microscopic observations imply that the two carbonate minerals were distinct for their timing and processes of the formation. The contribution of seawater was also suggested by the carbon isotope ratios of the carbonate minerals (δ¹³Ccarb =−3.74 –−5.32‰), which were close to the values of the equilibrium with seawater carbonate (δ¹³C carb=0‰)
Regarding Fe isotope analysis of the carbonate minerals, we first compared the δ56Fe values with different amounts of extracted elements using acids of different strengths (HAc 10%, 20% and 0.4M HCl). The result shows the similar iron isotope compositions (Δ56Fe <0.18‰; n=3) regardless of the different degree of partial extraction, indicating there is also no significant isotope fractionation with our samples. Al below detection limit observed during the acetic acid extraction, supporting no dissolution of silicate minerals. The δ56Fe values of the carbonate minerals (δ56Fe carbonate=−0.11–−0.93‰; n=13) showed small differences from the bulk (Δ56Fe bulk-carbonate=−0.17–−0.61‰) , suggesting that abiotic process in a semi-closed system likely governed the iron reduction and carbonate mineral formation in the Moodies BIFs. This further suggests that the bulk δ56Fe isotope data preserve the signal of the primary precipitation.
[1] Heimann, A., Johnson, C. M., Beard, B. L., Valley, J. W., Roden, E. E., Spicuzza, M. J., and Beukes, N. J. (2010) Earth Planet. Sci. Lett. 294, 8-18.
[2] Halama, M., Swanner, E. D., Konhauser, K. O., and Kappler, A. (2016) Earth Planet. Sci. Lett. 450, 243-253.
[3] Rego, E. S., Busigny, V., and Philippot, P. (2022) Chem. Geol. 611, 121120