Abstract
The iron isotope composition of iron-bearing carbonates is commonly used to obtain insights into ancientenvironmental conditions. However, it is often challenging to target only Fe‑carbonates (e.g. siderite andankerite) from samples containing a variety of other Fe-bearing minerals, such as observed in Precambrian ironformations. Chemical extraction (i.e. leaching) methods of Fe‑carbonates could be an alternative to in-situmeasurements and/or micro-drilling techniques applied to isotopic studies. Yet, only a few studies havelooked 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 sideritestandard and three natural samples, including an iron formation, Fe-rich and Fe-poor carbonates. We showedthat carbonate mineralogy has a strong control on how much of each mineral phase was being dissolved, and thatvariations 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 partiallydissolved siderite had indistinguishable values within error from the whole-rock composition (i.e. completedissolution) as shown with HAc and HCl attacks. Carbonates from the three natural samples were almostcompletely dissolved under the same protocol with 5 to 20% HAc, while 0.4 M HCl attacks dissolved additionalmineralogical phases, which might contribute to the Fe leachate. Moreover, the iron isotope composition ofcarbonate leachates was preserved without generating anomalous results. Hence, weak chemical leachesrepresent a reliable tool to study Fe isotopic composition of carbonate to understand how the Fe cycle wasoperating throughout Earth’s history.