Iron Isotope Analyses for the Origin of Fe-Bearing Minerals in the 3.46 Ga Marble Bar Chert, Pilbara Craton, Western Australia, Recovered by Archean Biosphere Drilling Project (ABDP)

In 2003, "Archean Biosphere Drilling Project" recovered 3.46 billion-years-old modern-weathering free banded chert of the Marble Bar Chert Member of the Towers Formation of the Warrawoona Group in the East Pilbara Granite-Greenstone Terrane, Western Australia. The chert samples contain variable abundance of Fe-bearing minerals such as hematite, magnetite, siderite and pyrite. Hematite bands of mm or sub-mm order in thickness are composed of micro-crystals of hematite and cut by several generations of quartz veins. Pyrite crystallization is found both in the quartz veins and the hematite bands. These observations suggest that the hematite bands, probably syn-depositional, were formed earlier than the pyrite crystals. Iron isotope analyses were performed on individual Fe-bearing minerals separated by micro-drilling of the chert samples. Such data, when combined with published data for Fe isotope fractionation between minerals and fluids in natural environments and laboratory experimental system, are expected to give information on (1) the Fe isotope compositions of the fluid or precursor Fe-bearing minerals from which the Fe-bearing minerals were formed, (2) the degree of involvement of microbiological processes such as bacterial Fe reduction / oxidation in the mineralization of Fe-bearing minerals, and (3) redox state of the Early Archean deep marine environments. If hematite mineralization was primary and precipitation of Fe-(oxy)hydroxide as its precursor occurred during sedimentation by oxidizing dissolved ferrous Fe by oxygen, or if microbes were involved in the formation of Fe-bearing minerals, then far-reaching implications will emerge for the redox evolution of the atmosphere and oceans and the evolution of biosphere in the Early Archean.