The local preservation of depleted Nd-Hf and Os isotopic compositions in abyssal peridotites contrasts with the lack of such extreme isotopic signatures in erupted MORB . This is likely a consequence of different preservation potentials of melts from enriched and depleted portions of mantle . Indeed, fertile and geochemically enriched components produce melts hosting the majority of incompatible elements, concealing the chemical fingerprint of the depleted counterparts . However, Mid Ocean Ridges are dynamic environments, where geodynamic responses to far field forces may cause change in ridge-geometry and/or re-organization of the ridge axis. This contribution discusses how this process affects melting regimes, reducing the effect of mixing [5,6]. I will consider study cases from large-offset oceanic transforms from the Mid Atlantic Ridge (Doldrums and Charlie Gibbs Fracture Zones), and from the highly asymmetric Knipovich Ridge, in the Arctic Ocean. Variations in chemistry of basalt and associated peridotites are used to show the coupled effect of cooling at the ridge-transform intersections and/or re-melting the sub-oceanic mantle. Changes in melting regime may cause preferential melting or enriched or depleted mantle domains, uncovering an intrinsic heterogeneity otherwise difficult to assess.
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