Unusual delta 56Fe values in Samoan rejuvenated lavas generated in the mantle

Several magmatic processes contribute to the Fe isotope composition of igneous rocks. Most basalts fall within a limited range of delta 56Fe (), although more differentiated lavas trend towards slightly elevated values (up to ). New data for basalts and olivine crystals from the Samoan Islands show higher delta 56Fe values than have previously been reported for basalts worldwide. Common magmatic processes - from partial melting of average mantle to subsequent differentiation of melts - cannot sufficiently fractionate the Fe isotopes to explain the elevated delta 56Fe values () in rejuvenated Samoan lavas. Instead, a mantle source with an elevated delta 56Fe value - in conjunction with effects due to common magmatic processes - is required. The Samoan mantle source is known to be unique in its radiogenic isotope composition and indications that melting of the Samoan mantle source can generate elevated delta 56Fe values in lavas comes from: (1) High values of Samoan lavas and their likely sources affecting Fe isotope fractionation during melting; (2) Metasomatism that caused elevated delta 56Fe in the Samoan mantle, as observed in xenoliths; and (3) Involvement of a pyroxenite source lithology, based on the Zn/Fe ratios and TiO2 (and other high field-strength element) abundances of the lavas, that can generate melts with elevated delta 56Fe values. Two models are presented to explain the elevated delta 56Fe values in Samoan lavas: a metasomatized source () or the presence of a pyroxenite source component (). Both models subsequently elevate delta 56Fe values with both partial melting () and fractional crystallization (). These processes may be related to an upwelling mantle plume with a pyroxenite component, or melting of previously metasomatized upper mantle.