Experimental Constraints on Fe Isotope Fractionation in Carbonatite Melt Systems

Abstract:

Iron isotope data from carbonatite rocks show the largest variability found in igneous rocks to date [1]. Thus, stable Fe isotopes are promising tracers for the interaction of carbonate and silicate magmas in the mantle, particularly because their fractionation is controlled by oxidation state and bonding environment. The interpretation of Fe isotope data from carbonatite rocks remains hampered, since Fe isotope fractionation factors between silicate and carbonate melts are unknown and inter-mineral fractionation can currently only be assessed by theoretical calculations [1;2].

We present results from equilibration experiments in three natrocarbonatite systems between immiscible silicate and carbonate melts, performed at 1200°C and 0.7 GPa in an internally heated gas pressure vessel at intrinsic redox conditions. The Fe isotope compositions of the silicate melt (sil.m.), quenched to a glass, and the carbonate melt (carb.m.), forming fine-grained quench crystals, were analysed by solution MC-ICP-MS.

Our first data indicate a remarkable fractionation of Δ⁵⁶Fe_{sil.m.‒carb.m.}= 0.29 ±0.07 ‰ near equilibrium. At short run durations, even stronger fractionation up to Δ⁵⁶Fe_{sil.m.‒carb.m.} = 0.41 ±0.07 ‰ occurs, due to kinetic effects. Additionally, Δ⁵⁶Fe_{sil.m.‒carb.m.} changes with bulk chemical composition, likely reflecting considerable differences between the studied systems in terms of the Fe³⁺/Fe²⁺-ratios in the two immiscible liquids. Our findings provide experimental support for a carbonatite genesis model, in which extremely negative δ⁵⁶Fe values in carbonatites result from differentiation processes, such as liquid immiscibility [1]. This effect can be enhanced by disequilibrium during fast ascent of carbonatite magmas. Their sensitivity to chemical and redox composition makes Fe isotopes a potential tool for constraining the original compositions of carbonatite magmas.

[1] Johnson et al. (2010) Miner. Petrol. 98, 91-110. [2] Polyakov & Mineev (2000) Geochim. Cosmochim. Acta 64, 849-865.