Unusual δ56Fe values in Samoan rejuvenated lavas generated in the mantle

Abstract

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 fO<inf>2</inf> 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 TiO<inf>2</inf> (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.