Abstract
A continuous 260 m core of Marble Bar Chert, Pilbara Craton, Western Australia, had been drilled by Archean Biosphere Drilling Project (ABDP) in 2003. In this study, the cores from 150 m to 200 m which shows rhythmic black and red bands are treated. We have examined the mineral assemblages, textures and chemical compositions, using microscope, SEM-EDX, XGT, mass spectrometer, ESCA and Raman spectrometer, to understand the contribution of biomineralization of the iron minerals in banded chert. Red bands in the rhythmically banded chert contain numerous hematite grains, which are accumulated as spherical grains of about 100 nm in diameter to euhedral grains up to 1 micron in botryoidal quartz with the size of 10-30 µm in diameter. Black bands also contain numerous magnetite grains with the similar occurrence with that of hematite. Both bands contain euhedral grains of siderite of submicron size, in the botryoidal quartz. The carbon isotopic ratios of siderite are from –1 to +2 per mill. Banded cherts contain euhedral syngenetic pyrites. SEM images show bacteria like cells connecting with and enveloping hematite grains in the botroidal quartz. This image is similar to that of iron bacteria, and mineral assemblages in the banded chert support the existence of iron bacteria. The intensity of cathodoluminescence from the botryoidal quartz is heterogeneous, and it is two-dimensional image is associated with bacterial texture. The ESCA detects the photoelectron peak at about 285 eV, suggesting the C-C and C-H bonding of organic matters, from the banded chert. Furthermore, the Raman spectrum from the black band chert shows the peaks at about 1300cm-1 and 1600cm-1, which should be derived from organic carbon. The Raman spectrum image reveals the heterogeneous distribution of organic carbon. The occurrences of mineral and organic carbon suggest that the banded cherts have not been affected by hydrothermal alteration, metamorphism or modern weathering, but suggest that the original texture has remained unchanged. Siderite was produced by the reaction between dissolved Fe2+ and HCO3-1 in seawater. The occurrences suggest further that bacteria had existed in the banded cherts, and that the precipitation of iron oxides was related with biological activity. Furthermore, carbon isotope ratio of black chert in the core ranges from -39 to -23 per mill, suggests that the bacteria related production of iron oxide might live in coexistence with cyano-bacteria, sulphur reducing bacteria and methanotrophs.
