Iron isotope fractionation during silicate‑carbonatite liquid immiscibility processes

Abstract

Liquid immiscibility is one of the viable genetic models to generate carbonatites. Experimental studies have demonstrated that lighter Fe isotopes are enriched in carbonatite melts, whereas heavier Fe isotopes preferentially enter silicate melts during liquid immiscibility. However, this observation has not been substantiated by natural samples, and the mechanism behind Fe isotope fractionation during silicate–carbonatite immiscibility remains unclear. Here, we present high-precision Fe isotope data, combined with petrography, whole-rock elemental and Sr–Nd isotopic compositions, for ultramafic lamprophyres (UML) and carbonatites from the Alnö complex in central Sweden, to elucidate the Fe isotope fractionation during silicate–carbonatite immiscibility processes. The presence of various carbonate spherules in UML, coupled with enrichments in Sr and Ba and depletion in high field strength elements in carbonatites, as well as their overlapping Sr–Nd isotope compositions, supports a petrogenetic relationship involving liquid immiscibility between the UML and carbonatites. The mean δ⁵⁷Fe of UML (0.16 ± 0.08 ‰) is higher than that of carbonatites (0.03 ± 0.04 ‰), with ∆⁵⁷Fe_sil−carb of 0.13 ‰ (± 0.05, 2SD). By excluding the effects of low temperature alteration and magmatic processes, we conclude that silicate–carbonatite immiscibility imparts significant Fe isotope fractionation. This fractionation may be influenced by different Fe bond strengths provided by the distinct polymer networks of silicate and carbonatite melts, as well as the varying degrees of Fe enrichment in minerals and melts. This leads to light Fe isotopes being preferentially enriched in the carbonate melt, while heavy Fe isotopes become enriched in the coexisting silicate melt.