Partial melting and melt percolation in the mantle: The message from Fe isotopes

High precision Fe isotope measurements have been performed on various mantle peridotites (fertile lherzolites, harzburgites, metasomatised Fe-enriched peridotites) and volcanic rocks (mainly oceanic basalts) from different localities and tectonic settings. The peridotites yield an average δ56Fe = 0.01‰ and are significantly lighter than the basalts (average δ56Fe = 0.11‰). Furthermore, the peridotites display a negative correlation of δ56Fe with Mg# indicating a link between δ56Fe and degrees of melt extraction. Taken together, these findings imply that Fe isotopes fractionate during partial melting, with heavy isotopes preferentially entering the melt. ope of depletion trends (δ56Fe versus Mg#) of the peridotites was used to model Fe isotope fractionation during partial melting, resulting in αmantle-melt ≈ 1.0001–1.0003 or lnαmantle-melt ≈ 0.1–0.3‰. In contrast to most other peridotites investigated in this study, spinel lherzolites and harzburgites from three localities (Horoman, Kamchatka and Lherz) are virtually unaffected by metasomatism. These three sites display a particularly good correlation and define an isotope fractionation factor of lnαmantle-melt ≈ 0.3‰. This modelled value implies Fe isotope fractionation between residual mantle and mantle-derived melts corresponding to Δ56Femantle–basalt ≈ 0.2–0.3‰, i.e. significantly higher than the observed difference between averages for all the peridotites and the basalts in this study (corresponding to Δ56Femantle–basalt ≈ 0.1‰). Either disequilibrium melting increased the modelled αmantle-melt for these particular sites or the difference between average peridotite and basalt may be reduced by partial re-equilibration between the isotopically heavy basalts and the isotopically light depleted lithospheric mantle during melt ascent. The slope of the weaker δ56Fe–Mg# trend defined by the combined set of all mantle peridotites from this study is more consistent with the generally observed difference between peridotites and basalts; this slope was used here to estimate the Fe isotope composition of the fertile upper mantle (at Mg# = 0.894, δ56Fe ≈ 0.02 ± 0.03‰). s partial melting, the Fe isotope composition of mantle peridotites can also be significantly modified by metasomatic events, e.g. melt percolation. At two localities (Tok, Siberia and Tariat, Mongolia) δ56Fe correlates with iron contents of the peridotites, which was increased from about 8% to up to 14.5% FeO by post-melting melt percolation. This process produced a range of Fe isotope compositions in the percolation columns, from extremely light (δ56Fe = − 0.42‰) to heavy (δ56Fe = + 0.17‰). We propose reaction with isotopically heavy melts and diffusion (enrichment of light Fe isotopes) as the most likely processes that produced the large isotope variations at these sites. Thus, Fe isotopes might be used as a sensitive tracer to identify such metasomatic processes in the mantle.