Unusual δ56Fe 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 Fe-56 (+0.10 +/- 0.05 parts per thousand), although more differentiated lavas trend towards slightly elevated values (up to +0.3 parts per thousand). New data for basalts and olivine crystals from the Samoan Islands show higher delta Fe-56 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 Fe-56 values (similar to+0.3 parts per thousand) in rejuvenated Samoan lavas. Instead, a mantle source with an elevated delta Fe-56 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 Fe-56 values in lavas comes from: (1) High fO(2) values of Samoan lavas and their likely sources affecting Fe isotope fractionation during melting; (2) Metasomatism that caused elevated delta Fe-56 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 Fe-56 values. Two models are presented to explain the elevated delta Fe-56 values in Samoan lavas: a metasomatized source (similar to +0.07 parts per thousand) or the presence of a pyroxenite source component (similar to +0.12 parts per thousand). Both models subsequently elevate delta Fe-56 values with both partial melting (similar to +0.14 parts per thousand) and fractional crystallization (similar to +0.1 parts per thousand). These processes may be related to an upwelling mantle plume with a pyroxenite component, or melting of, previously metasomatized upper mantle. (C) 2016 Elsevier B.V. All rights reserved.