Changing shape of mantle heterogeneity by melt migration beneath mid-ocean ridges

• We show the patterns of deformation, mixing, and decoupling of chemical heterogeneities marked by Nd-Hf isotope ratios in the residue at mid-ocean ridges. • The observed melt fraction (∼1%) beneath fast-spreading ridge would produce strong vertical dispersion of chemical heterogeneity. • If the melt fraction is less than 0.2%, probably in the case of ultraslow-spreading ridge, heterogeneities larger than 2 km in diameter would experience negligible dispersion. • A previous melting event could produce the residual peridotite with extremely depleted Hf isotope ratio that eventually gets recycled into the asthenosphere. The common assumption that residual peridotites retain the Nd-Hf isotope ratios in the mantle source is debated because melt and solid of different isotopic compositions could undergo chemical exchange during melt migration, altering the isotopic signature of the source. By modeling the transport of chemical heterogeneities in the melting region beneath a mid-ocean ridge, we show that the shape of a chemical heterogeneity marked by Nd or Hf isotope ratio changes systematically through subvertical dispersion, stretching, compression, and shearing. The isotope ratios inside the chemical heterogeneity decay toward the values of background mantle. The amount of decay depends on the strength of dispersion, which itself is strongly dependent on the melt fraction in the melting region. When the maximum melt fraction is greater than 1%, buoyancy-driven melt flow relative to the solid causes subvertical dispersion of isotopic signals in the solid. Differential flows of the melt and solid also produce chromatography fractionation of Nd with respect to Hf, causing their isotope ratios to decouple. Compositions of the residue in Nd-Hf isotope ratio diagram do not record the endmembers in the source, instead they represent an area that covers part of the binary mixing line between the background mantle and the original heterogeneity. In the case of small melt fraction (<0.2%), the low permeability results in sluggish melt flow, weak dispersion, and negligible chromatography fractionation. Consequently, Nd and Hf isotope ratios in the residue remain coupled, representing the endmember isotope ratios in the source. The ridge model with larger melt fraction may correspond to the fast-spreading ridge, while the model with smaller melt fraction may correspond to the ultraslow-spreading ridge. The present study underscores the importance of melt migration processes beneath mid-ocean ridges on the deformation, mixing and decoupling of Nd-Hf isotope ratios in residual peridotites.