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