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

Highlights

• Weak acid leachates as tools to target Fe-carbonates in natural samples.

• Complete dissolution of natural samples with 5 to 20% acetic acid attacks.

• Iron isotope composition of carbonate leachates was preserved without generating anomalous results.

Abstract

The iron isotope composition of iron-bearing carbonates is commonly used to obtain insights into ancient environmental conditions. However, it is often challenging to target only Fe‑carbonates (e.g. siderite and ankerite) from samples containing a variety of other Fe-bearing minerals, such as observed in Precambrian iron formations. Chemical extraction (i.e. leaching) methods of Fe‑carbonates could be an alternative to in-situ measurements and/or micro-drilling techniques applied to isotopic studies. Yet, only a few studies have looked at the effects of leaching carbonates (e.g. partial and/or total dissolution) on their Fe isotope composition. Here, we tested several leaching protocols, using 5 to 20% acetic acid (HAc) and 0.4 M HCl, on a siderite standard and three natural samples, including an iron formation, Fe-rich and Fe-poor carbonates. We showed that carbonate mineralogy has a strong control on how much of each mineral phase was being dissolved, and that variations in HAc concentration from 5% to 20% are less likely to change how much siderite dissolves (e.g. ∼30% dissolution) under a 12 h period at room temperature. Importantly, the Fe isotope composition of partially dissolved siderite had indistinguishable values within error from the whole-rock composition (i.e. complete dissolution) as shown with HAc and HCl attacks. Carbonates from the three natural samples were almost completely dissolved under the same protocol with 5 to 20% HAc, while 0.4 M HCl attacks dissolved additional mineralogical phases, which might contribute to the Fe leachate. Moreover, the iron isotope composition of carbonate leachates was preserved without generating anomalous results. Hence, weak chemical leaches represent a reliable tool to study Fe isotopic composition of carbonate to understand how the Fe cycle was operating throughout Earth's history.

Introduction

Sedimentary iron-bearing minerals are ancient archives largely used to track changes in the chemical evolution of Earth's oceans through time (Konhauser et al., 2017). There is a general view that iron-bearing carbonates (e.g. siderite and ankerite) and associated iron formations reflect environmental conditions in which they formed, specifically related to anoxic and ferruginous seawater chemistry prior to the Great Oxidation Event (Holland, 2002). Siderite [FeCO₃] and ankerite [Ca(Fe²⁺, Mg, Mn,)CO₃] represent the dominant and best-preserved carbonate minerals in iron formations (Fe-dolomite and calcite being less common), and have provided key insights into the biogeochemical cycles of iron and carbon throughout Earth's history (Beukes and Klein, 1990; Holland, 1984; James, 1954; Kaufman et al., 1990; Klein, 2005; Klein and Beukes, 1989; Konhauser et al., 2005, Konhauser et al., 2017; Raiswell and Canfield, 2012). Significant contributions towards the origin of Fe‑carbonates in Precambrian rocks have been made, which rely to some extent in measuring the carbon and iron isotope compositions of these rocks.

Carbon and iron (Fe) isotope signatures are widely used as tools for understanding the processes of Fe‑carbonate formation, such as the remineralization of organic matter to produce a depleted ¹³C inorganic carbon source (Beukes et al., 1990; Fischer et al., 2009; Heimann et al., 2010; Johnson et al., 2013), the identification of biological reduction of Fe(III) minerals leading to the enrichment of light Fe isotope (i.e. ⁵⁴Fe) in the reduced Fe phase (Craddock and Dauphas, 2011a; Johnson et al., 2008a; McCoy et al., 2017), and the distinction of kinetic effects associated to mineral precipitation (Jiang et al., 2022; Jiang and Tosca, 2019). Carbon isotope composition in carbonates is easily measured in iron formations through gas extraction via reaction of whole-rock powders with orthophosphoric acid (Becker and Clayton, 1972; Busigny et al., 2013; Fischer et al., 2009; Garcia et al., 2021; Kaufman et al., 1991; Lebeau et al., 2014). In contrast, the analysis of Fe isotope composition of distinct Fe-mineral phases such as carbonates, as well as oxides (e.g. magnetite, hematite), sulfides (e.g. locally pyrites), and minor phyllosilicates (e.g. stilpnomelane, riebeckite), is often more challenging. Previous studies have tackled this issue using micro-drilling techniques (Heimann et al., 2010), in-situ analysis (Marin-Carbonne et al., 2020; Steinhoefel et al., 2010), and chemical leaching procedures to target distinct minerals (Frost et al., 2007; Hyslop et al., 2008; von Blanckenburg et al., 2008). In the case of carbonates, this might be difficult using only micro-drilling techniques, particularly in IFs, where siderite and/or ankerite can be disseminated in chert matrixes closely associated and surrounded by Fe-oxides with varying sizes (e.g. nm to cm) (e.g. Kaufman et al., 1990).

A simple method used for isotope analysis of carbonates is weak acetic acid extractions, as illustrated for Sr isotopes measurement (Bayon et al., 2002). However, only a few studies have reported the effect of leaching carbonates on their Fe isotope composition (Hyslop et al., 2008; von Blanckenburg et al., 2008). Therefore, it is not clear if acetic acid can be efficiently applied for Fe isotope analysis of Fe-bearing carbonates. Importantly, when targeting only carbonates, all of the other mineral phases should be avoided. The sequential extraction of Fe‑carbonates with sodium acetate has shown to be an efficient technique dissolving completely the carbonates contained in sediments (PC method; Poulton and Canfield, 2005, Oonk et al., 2017). However, the PC method has also proven to be challenging particularly in carbonates exposed to higher metamorphic grades (Slotznick et al., 2018), although no Fe isotopic work has been done examining specifically the Fe speciation proxy in Precambrian rocks (only modern sediments were studied; Henkel et al., 2016). The present contribution aims at filling this gap by testing the effect of a simple weak acetic acid leaching protocol on iron isotope fractionation in a siderite standard and three natural rock samples. The selected natural samples contain various mixtures of Fe minerals with different shapes and sizes, and include one Precambrian iron formation (Fe-oxide facies), as well as one Fe-rich and one Fe-poor carbonate. The protocols are shown to be reproducible, reliable and accurate, and can therefore be used as a tool for targeting carbonates in the ancient rock record without significantly dissolving other minerals. More importantly, this method provides a means to identify Fe isotope signatures that could be masked when utilizing other mineral separation and/or dissolution methods.