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 delta Fe-56=0.01 parts per thousand and are significantly lighter than the basalts (average delta Fe-56 0.11 parts per thousand). Furthermore, the peridotites display a negative correlation of delta 56 Fe with Mg# indicating a link between delta 56 Fe and degrees of melt extraction. Taken together, these findings imply that Fe isotopes fractionate during partial melting, with heavy isotopes preferentially entering the melt. The slope of depletion trends (delta 56 Fe versus Mg#) of the peridotites was used to model Fe isotope fractionation during partial melting, resulting in alpha(mantle-metal) approximate to 1.0001- 1.0003 or ln alpha(mantle- metal) approximate to 0.1-0.3 parts per thousand. In contrast to most other peridotites investigated in this study, spinet Iherzolites 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 In alpha(mantle-metal) approximate to 0.3 parts per thousand. This modelled value implies Fe isotope fractionation between residual mantle and mantle-derived melts corresponding to Delta Fe-56(mantle-basalt) approximate to 0.2-0.3 parts per thousand, i.e. significantly higher than the observed difference between averages for all the peridotites and the basalts in this study (corresponding to Delta Fe-56(mantle-blast) approximate to 0.1 parts per thousand). Either disequilibrium melting increased the modelled alpha(mantle-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 delta Fe-56-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, delta Fe-56 approximate to 0.02 +/- 0.03 parts per thousand). Besides 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) delta 56 Fe 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 (delta Fe-56 = - 0.42 parts per thousand) to heavy (delta(56) Fe = + 0.17 parts per thousand). 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. (c) 2007 Published by Elsevier B.V.