Fe isotope systematics of coexisting amphibole and pyroxene in the alkaline igneous rock suite of the Ilimaussa Complex, South Greenland

The Ilimaussaq intrusion, South Greenland, provides an exceptional test case for investigating the changes of stable Fe isotope fractionation of solidus phases with changes in the Fe3+/Sigma Fe ratio of an evolving melt. The intrusion comprises a sequence Of four melt batches that were fed from the same parental alkali basaltic magma. Differentiation produced cumulate rocks that range from augite syenite (phase I) over peralkaline granite (phase II) to agpaitic syenites (phases IIIa and IIIb). Fe3+/Sigma Fe ratios in amphiboles increase substantially from phase I to phase II and III rocks and mark a major change in the parental magma Composition from augite syenites to peralkaline granites and agpaitic syenites. Before this transition, olivine, clinopyroxene, and amphibole in augite syenite, the most primitive rock type in the Ilimaussaq Complex, have a uniform Fe isotope composition that is identical to that of the bulk of igneous crustal rocks and approximated by the average isotopic composition of basalts (delta Fe-56/54(IRMM-014)= 0.072 +/- 0.046 parts per thousand). After the transition, amphiboles in the peralkaline granites and agpaitic syenites yield significantly heavier Fe isotope compositions with delta(56/54) FeIRMM-014 values ranging from 0.123 to 0.237 parts per thousand. Contamination of the llimaussaq magma by ongoing crustal assimilation as Cause for this increase can be excluded on the grounds of Nd isotope data. Large-scale metasomatics overprint with all external fluid call also be dismissed based on amphibole O and Li isotope systematics. Rather, the increase towards heavy Fe isotope compositions most likely reflects the change in chemical compositions of amphiboles (calcic ill augite syenite to sodic in the agpaitic syenites) and their Fe3+/Sigma Fe ratios that mirror changes in the chemical composition of the melt and its oxygen fugacity. A sensitive adjustment of equilibrium Fe isotope fractionation factors to amphibole ferric/ferrous ratios is also supported by beta-factors Calculated from Mossbauer spetroscopy data. Comparison of the measured isotope fractionation between clinopyroxene and amphibole with that predicted from Mossbauer data reveal Fe isotope systematics close to equilibrium in augite syenites but Fe isotopic disequilibrium between these two phases in phase IIIa agpaitic syenites. These results are in agreement with 0 and Li isotope systematics. While amphiboles in all Ilimaussaq lithologies crystallized at temperatures between 650 and 850 degrees C, textural evidence reveals later clinopyroxene crystallization at temperatures as low as 300-400 degrees C. Therefore, isotopic equilibrium at crystallization conditions between these two phases can not be expected, but importantly, subsolidus reequilibration can also be dismissed. (C) 2008 Elsevier B.V. All rights reserved.