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

Author(s)

Rego, Eric Siciliano ; Busigny, Vincent ; Philippot, Pascal

• Document type: Journal article
• Source: CHEMICAL GEOLOGY; v. 611, p. 13-pg., 2022-11-20.

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. similar to 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. (AU)

• FAPESP's process: 18/05892-0 - Element and isotope geochemistry from iron formations and black shales: new insights into paleoredox conditions during the Paleo- and Neoproterozoic in North and Midwestern Brazil
• Grantee: Eric Siciliano Rego
• Support Opportunities: Scholarships in Brazil - Doctorate
• FAPESP's process: 19/16271-0 - Element and isotope geochemistry from iron formations, carbonates and black shales: new insights into paleoredox conditions during the Neoarchean, Paleo- and Neoproterozoic in Brazil
• Grantee: Eric Siciliano Rego
• Support Opportunities: Scholarships abroad - Research Internship - Doctorate
• FAPESP's process: 15/16235-2 - The co-evolution of life and oxygen on early Earth: a South American perspective
• Grantee: Pascal Andre Marie Philippot
• Support Opportunities: Research Projects - SPEC Program