X-ray diffraction pattern and Fe-isotope of Banded Iron Formation to constrain the fluctuating redox State of Archean Ocean: A Case Study from Banded Iron Formation from Bundelkhand Craton, North Central India

Banded Iron Formations (BIFs), generally containing magnetite (Fe₃O₄), hematite (Fe₂O₃), and quartz (SiO₂) in Archean oceans, have important implications for the primitive-life and sea-water condition of the primitive Earth. It is generally agreed that BIFs were deposited within marine sediments on stable continental margins (Superior-type) or associated with volcanic rocks (Algoma-type). The oxidation of Fe with free O₂ is challenging due to the lack of knowledge on the extent and amount of the free oxygen available in oceanic water. Iron can precipitate in two oxidation states, i.e., +2 and +3, in magnetite and hematite, respectively. In addition, the magnetite crystallizes in the isometric crystal system, whereas hematite crystallizes in the trigonal crystal system. Since Electron Probe microanalysis cannot differentiate the oxidation states of Fe, we use X-ray diffraction and δ⁵⁶ Fe values to identify the mineral phases in BIFs and utilize the EH–pH diagram to constrain the pH and redox state of Archean oceans.

The Banded Iron Formation occurs in two east-west trending linear belts in the Bundelkhand Craton. The northern belt, extending from Babina in the west to Mauranipur in the east, is an Algoma-type BIF. In contrast, the least explored southern belt, near Girar, is a Superior-type BIF. Powder X-ray diffraction patterns of the northern BIF belt indicate hematite and quartz without carbonates. On the contrary, the X-ray diffraction pattern of BIFs from Girar suggests the presence of quartz, hematite, and siderite. In addition, the result of Fe-isotope data indicates that the δ⁵⁶ Fe values of Northern BIFs range from 0.205⁰/₀₀ to 0.255⁰/₀₀, while the δ⁵⁶ Fe values from the Southern BIFs range from 1.350⁰/₀₀ to 2.126⁰/₀₀.

The high positive δ⁵⁶ Fe from the Fe-rich bands indicates that Fe (III) delivery was the primary mechanism driving iron enrichment in the Southern BIFs. The Southern BIF's substantial Fe isotope fractionations indicate initial partial Fe (II) oxidation in low-Eh conditions as siderite, the primary mineral for hematite, was deposited between pH and Eh of 7.5 to 10 and -0.3 to -0.2 volts. With an increase in the Eh to -0.2 to -0.1, magnetite becomes the stable phase. Still, with an increase in the Eh (0.1 to 0.3 Volts), hematite becomes stable in phases with the same pH range as the aqueous medium. Since an increasing Eh with a similar pH range indicates oxidation, we suggest that southern Bundelkhand Craton BIFs materials were deposited in more oxidized conditions. This evidence suggests that microbial Fe (III) reduction in Southern BIFs occurred during the Earth's Late Neoarchean – Early Paleoproterozoic transition. The increased oxidation condition, i.e., excess oxygen in the Archean Sea in Bundelkhand Craton, could probably relate to photochemical or microbial oxidation.