Major elements heterogeneities in the Earth’s mantle: perspectives from mantle outcrops and transition metal systematic of basalts

Major elements heterogeneities in the Earth’s mantle may be inferred from peridotite samples, oceanic basalts (MORB and OIB) and seismological data. We focus here on peridotites and oceanic basalts systematics. It is widely accepted that the Earth’s mantle holds heterogeneities, since oceanic basalts are characterized by large isotopic and trace element variability that is difficult to reconcile with partial melting of a peridotite alone. Mantle heterogeneities have been mainly attributed to the presence of recycled subducted crust, rise of outer core material or melt-rock reactions in the mantle. Melt percolation, in particular, can create lithological heterogeneities by refertilizing the lithospheric mantle. Refertilization and percolation events have been observed world-wide in xenoliths and orogenic peridotites. For instance, structural and geochemical data from the type-locality of lherzolites (Lherz massif) show how major element heterogeneities can form at the lithosphere-asthenosphere boundary. However, when no peridotite samples are available, we may rely on oceanic basalts, which sample the Earth’s mantle world-wide. Oceanic basalts may carry the signature of major elements heterogeneities like recycled crust or core contribution. However their chemistry may reflect fractionation during partial melting and/or late differentiation processes. In this work we explore the use of Zn, Mn, and Fe systematic in oceanic basalts as tools for detecting major element heterogeneities in the mantle and we suggest that Zn/Fe ratios are minimally fractionated during partial melting of peridotite and differentiation of primitive basalts. Thus, the Zn/Fe ratios of primitive basalts preserve the Zn/Fe ratio of the primary parental magma, providing insight into the signature of the mantle source region. We also suggest that Zn/Fe ratios in melts are unlikely to be fractionated by modal variations in peridotitic material but are highly fractionated if garnet and/or clinopyroxene are the main phases in the source during melting. Similar Zn/Fe ratios between MORB and average upper mantle confirm the lack of fractionation during peridotite melting. However, high Zn/Fe ratios of some OIB cannot be explained by peridotite melting alone, but instead require the presence of high Zn/Fe lithologies or lithologies that have Zn/Fe rock/melt exchange coefficients < 1. All garnet-bearing or clinopyroxene-bearing lithologies, such as eclogites and garnet pyroxenites, fit the latter requirement. Some pyroxenites and amphibolites fit the former requirement as well. Finally, we explore the possibility of using Zn/Fe as a tracer of Fe redox state in the mantle.