Iron isotope compositions of carbonatites record melt generation, crystallization, and late-stage volatile-transport processes

Carbonatites define the largest range in Fe isotope compositions yet measured for igneous rocks, recording significant isotopic fractionations between carbonate, oxide, and silicate minerals during generation in the mantle and subsequent differentiation. In contrast to the relatively restricted range in delta Fe-56 values for mantle-derived basaltic magmas (delta Fe-56 = 0.0 +/- 0.1aEuro degrees), calcite from carbonatites have delta Fe-56 values between -1.0 and +0.8aEuro degrees, similar to the range defined by whole-rock samples of carbonatites. Based on expected carbonate-silicate fractionation factors at igneous or mantle temperatures, carbonatite magmas that have modestly negative delta Fe-56 values of similar to -0.3aEuro degrees or lower can be explained by equilibrium with a silicate mantle. More negative delta Fe-56 values were probably produced by differentiation processes, including crystal fractionation and liquid immiscibility. Positive delta Fe-56 values for carbonatites are, however, unexpected, and such values seem to likely reflect interaction between low-Fe carbonates and Fe3+-rich fluids at igneous or near-igneous temperatures; the expected delta Fe-56 values for Fe2+-bearing fluids are too low to produced the observed positive delta Fe-56 values of some carbonatites, indicating that Fe isotopes may be a valuable tracer of redox conditions in carbonatite complexes. Further evidence for fluid-rock or fluid-magma interactions comes from the common occurrence of Fe isotope disequilibrium among carbonate, oxide, silicate, and sulfide minerals in the majority of the carbonatites studied. The common occurrence of Fe isotope disequilibrium among minerals in carbonatites may also indicate mixing of phenocyrsts from distinct magmas. Expulsion of Fe3+-rich brines into metasomatic aureols that surround carbonatite complexes are expected to produce high-delta Fe-56 fenites, but this has yet to be tested.