10:45 AM - 12:15 PM
[BCG07-P04] Reconstruction of hydrothermal oceanic chert
and banded iron formation in Archean by mineral identification in Pilbara terrane, Western Australia
Banded Iron Formation (BIF) formed by oxidation has been reported at many locations, and there is much debate as to its origin, since in many cases the structure and composition of the deposit was lost due to diagenesis and metamorphic processes. (Holland 2006, Konhauser et al. 2002). For example (1) Fe2+ and SiO2 are supplied from hydrothermal vents and precipitate as divalent iron minerals (siderite (FeCO3), greenalite (Fe2-3Si2O5(OH)4), greenrust (Fe2+4Fe3+2(OH-)12CO32-2H2O), etc.) in the zone deeper than the photic layer. (2) In the photic layer, Fe2+ is oxidized by cyanobacteria and precipitated as ferrihydrite (Fe(OH)3). Then, this ferrihydrite is reduced from Fe3+ to Fe2+ by the action of methane-reducing bacteria on the seafloor and re-precipitated as siderite, which are models. For primary iron minerals in the BIF, assuming that amorphous silica in seawater at the time was saturated, with greenalite as the primary mineral, four patterns of precipitation models have been proposed (Johnson, 2018). (1) Fe2+ becomes saturated in seawater, associates with silica, and precipitates. (2) Fe2+ in the water is catalytically oxidized by biological activity or ultraviolet light, and precipitated as (Fe2+, Fe3+)greenalite. (3) Oxidation of Fe2+ generates greenlust, a metastable mineral, which is replaced by low Fe3+ greenalite during precipitation and precipitated. (4) Oxidation of Fe2+ produces ferrihydrite as a precursor, which is reduced again by iron-oxidizing bacteria and precipitated as low Fe3+ type greenalite.
(Stratigraphy)Dixon Island Formation (DX Fm) and Cleaverville Formation (CL Fm) in the Pilbara Coastal Greenstone Belt, Western Australia including well preserved continues stratigraphic sequence at Mesoarchean oceanic sedimentary section with hydrothermal chert and Iron formation (Kiyokawa et al, 2002). The DX Fm can reconstruct hydrothermal activity 3.2 billion years ago when it transitioned from silicified volcanic rocks to chert (Kiyokawa et al, 2006, Kiyokawa et al, 2019). This formation has a total thickness of more than 400 m, and is composed of three subgroups: The Komatiite-Rhyolite Tuff Member (<250 m thick), the Black Chert Member (7-20 m thick), and the Varicolored Tuff Member (<250 m thick). The CL Fm preserves the 3.1-billion-year-old succession of the transition from black shale to BIF. CL Fm is composed of Black Shale Member and BIF Member. These formations are the least metamorphosed of the BIF Formation and hydrothermal cherts in Mesoarchean, and are likely to have preserved the stratigraphic information at that time.
(Methods)We used samples from shoreline outcrop of the Black Chert Member of the DX Fm and the CL3 drill core of CL Fm, Iron ore of Brockman Iron Formation were used for detailed observation and analysis to identify the minerals remaining in the rocks. The DX and CL3 samples were thinned and observed by microscopy, SEM, and in some cases by TEM.
(Results)SEM observation of the DX Fm revealed that pyrite is concentrated and arranged as layers, and Apatite crystals were found in the black chert layer just above the Biomat layer. TEM observation of the chert in CL Fm revealed strips of crystals similar to those of greenalite. The apatite in the chert of the DX Formation suggests the possibility of life in a hydrothermal environment, which is consistent with the biomat fossils in the formation and bacterial micro fossile (Kiyokawa et al. 2006). It is possible that this formation preserves information about the early sedimentary period.
(Reference)
Kiyokawa et al., 2002, Structural evolution of the middle Archean coastal Pilbara terrane, Western Australia: Tectonics, v. 21, no. 5, p. 8-1-8-24.
Kiyokawa et al. 2006, Middle Archean volcano-hydrothermal sequence: Bacterial microfossilbearing 3.2 Ga Dixon Island Formation, coastal Pilbara terrane, Australia, GSA Bulletin; January/February 2006; v. 118; no. 1/2; p. 3-22
Kiyokawa et al., 2019, Timing and development of sedimentation of the Cleaverville Formation and a post-accretion pull-apart system in the Cleaverville area, coastal Pilbara Terrane, Pilbara, Western Australia, Island Arc; 2019;28:e12324.
Holland, H.D., 2006. The oxygenation of the atmosphere and oceans, Philos Trans R Soc Lond B Biol Sci. 2006 Jun 29; 361(1470): 903-915.
Konhauser et al., 2002. Could bacteria have formed the Precambrian banded iron
formations? Geology 30, 1079-1082.
Johnson et al., 2018, Low-Fe3+ greenalite was a primary mineral from Neoarchean oceans. Geophys Res Lett 45.
(Stratigraphy)Dixon Island Formation (DX Fm) and Cleaverville Formation (CL Fm) in the Pilbara Coastal Greenstone Belt, Western Australia including well preserved continues stratigraphic sequence at Mesoarchean oceanic sedimentary section with hydrothermal chert and Iron formation (Kiyokawa et al, 2002). The DX Fm can reconstruct hydrothermal activity 3.2 billion years ago when it transitioned from silicified volcanic rocks to chert (Kiyokawa et al, 2006, Kiyokawa et al, 2019). This formation has a total thickness of more than 400 m, and is composed of three subgroups: The Komatiite-Rhyolite Tuff Member (<250 m thick), the Black Chert Member (7-20 m thick), and the Varicolored Tuff Member (<250 m thick). The CL Fm preserves the 3.1-billion-year-old succession of the transition from black shale to BIF. CL Fm is composed of Black Shale Member and BIF Member. These formations are the least metamorphosed of the BIF Formation and hydrothermal cherts in Mesoarchean, and are likely to have preserved the stratigraphic information at that time.
(Methods)We used samples from shoreline outcrop of the Black Chert Member of the DX Fm and the CL3 drill core of CL Fm, Iron ore of Brockman Iron Formation were used for detailed observation and analysis to identify the minerals remaining in the rocks. The DX and CL3 samples were thinned and observed by microscopy, SEM, and in some cases by TEM.
(Results)SEM observation of the DX Fm revealed that pyrite is concentrated and arranged as layers, and Apatite crystals were found in the black chert layer just above the Biomat layer. TEM observation of the chert in CL Fm revealed strips of crystals similar to those of greenalite. The apatite in the chert of the DX Formation suggests the possibility of life in a hydrothermal environment, which is consistent with the biomat fossils in the formation and bacterial micro fossile (Kiyokawa et al. 2006). It is possible that this formation preserves information about the early sedimentary period.
(Reference)
Kiyokawa et al., 2002, Structural evolution of the middle Archean coastal Pilbara terrane, Western Australia: Tectonics, v. 21, no. 5, p. 8-1-8-24.
Kiyokawa et al. 2006, Middle Archean volcano-hydrothermal sequence: Bacterial microfossilbearing 3.2 Ga Dixon Island Formation, coastal Pilbara terrane, Australia, GSA Bulletin; January/February 2006; v. 118; no. 1/2; p. 3-22
Kiyokawa et al., 2019, Timing and development of sedimentation of the Cleaverville Formation and a post-accretion pull-apart system in the Cleaverville area, coastal Pilbara Terrane, Pilbara, Western Australia, Island Arc; 2019;28:e12324.
Holland, H.D., 2006. The oxygenation of the atmosphere and oceans, Philos Trans R Soc Lond B Biol Sci. 2006 Jun 29; 361(1470): 903-915.
Konhauser et al., 2002. Could bacteria have formed the Precambrian banded iron
formations? Geology 30, 1079-1082.
Johnson et al., 2018, Low-Fe3+ greenalite was a primary mineral from Neoarchean oceans. Geophys Res Lett 45.