Unusual δ56Fe values in Samoan rejuvenated lavas generated in the mantle

- Samoan rejuvenated lavas have elevated δ56Fe values.
- Magmatic processes alone cannot explain all Samoan δ56Fe values.
- An unusual mantle source composition is required.

Several magmatic processes contribute to the Fe isotope composition of igneous rocks. Most basalts fall within a limited range of δ56Fe (+0.10±0.05‰), although more differentiated lavas trend towards slightly elevated values (up to +0.3‰). New data for basalts and olivine crystals from the Samoan Islands show higher δ56Fe 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 δ56Fe values (∼+0.3‰) in rejuvenated Samoan lavas. Instead, a mantle source with an elevated δ56Fe 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 δ56Fe values in lavas comes from: (1) High fO2 values of Samoan lavas and their likely sources affecting Fe isotope fractionation during melting; (2) Metasomatism that caused elevated δ56Fe 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 δ56Fe values. Two models are presented to explain the elevated δ56Fe values in Samoan lavas: a metasomatized source (∼+0.07‰) or the presence of a pyroxenite source component (∼+0.12‰). Both models subsequently elevate δ56Fe values with both partial melting (∼+0.14‰) and fractional crystallization (∼+0.1‰). These processes may be related to an upwelling mantle plume with a pyroxenite component, or melting of previously metasomatized upper mantle.