Anoxygenic photosynthesis linked to Neoarchean iron formations in Carajás (Brazil)

Abstract

The geological record preserves extensive deposits of iron-rich chemical sediments, referred to as iron formations (IFs). These rocks are characterized by a high content (>15%-20%) of iron-bearing minerals, typically layered or massive, interbedded with silica and/or carbonate-rich layers that were deposited throughout the Precambrian (James, 1983;Klein, 2005). Over the past few decades, studies have employed the chemical and isotopic composition of IFs to constrain past environmental conditions (e.g., Planavsky et al., 2014;Satkoski et al., 2015) and to understand how the microbial iron cycle evolved alongside redox conditions through time (Heard & Dauphas, 2020;

Chemical extraction of iron from carbonate in banded iron formations for isotope analysis

“…The studied samples consisting of IF, Fe-rich and Fe-poor carbonates from Carajás basin have been previously described in Rego et al (2021) and the mineralogical assemblages were similar to those shown by Justo et al (2020). Carbonates in IF facies in Carajás occur commonly as ankerite [Ca(Fe 2+ ,Mg,Mn)CO 3 ] disseminated in a microcrystalline quartz (chert) matrix (Fig.…”

“…As explained above, weak acid leaching protocols were tested on a siderite (FeCO 3 ) standard (Lebeau et al, 2014) and in Neoarchean (~2.74 Ga) iron formation and carbonate samples from Carajás, Brazil (Rego et al, 2021). The results of the leaching experiments and whole rock sample dissolution are reported in Tables 2 and 3.…”

“…The three samples analyzed in this study comprise an iron formation, and Fe-rich and Fe-poor carbonates from the 2.7 Ga-old Carajás Formation, Brazil, and were recently described in Rego et al (2021). Additionally, a siderite powder (SID) from the deposits of La Mûre, France (Lebeau et al, 2014), was selected for our experiments.…”

“…This was complemented by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) on carbon-coated samples, utilizing a Zeiss EVO MA10 SEM for semiquantitative SEM-EDS analysis at the Institut de Physique du Globe de Paris (IPGP). Standard operating conditions for SEM imaging and EDS analysis were 15 kV accelerating voltage, working distance of 12 mm, and 2-3 nA electron beam current (see detailed conditions in Rego et al, 2021).…”

“…5A,B). Other Fe-mineral phases in IF include abundant euhedral magnetite crystals, microscopic spheroidal hematite, minor Fe-silicates, and very few pyrites (Rego et al, 2021). Iron-rich and iron-poor carbonate rock samples show higher carbonate content, but differ from each other particularly by the occurrence of calcite [CaCO 3 ] and framboidal pyrites [FeS 2 ] within Fe-poor facies (Figs.…”