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Abstract Amphibole and pyroxenes are the main reservoirs of rare earth elements (REEs) in the lithospheric mantle that has been affected by hydrous metasomatism. In this study, we developed semi-empirical models for REE partitioning between orthopyroxene and amphibole and between clinopyroxene and amphibole. These models were formulated on the basis of parameterized lattice strain models of mineral-melt REE partitioning for orthopyroxene, clinopyroxene, and amphibole, and they were calibrated using major element and REE data of amphibole and pyroxenes in natural mantle samples from intraplate settings. The mineral-melt REE partitioning models suggest that amphibole is not in equilibrium with coexisting pyroxenes in the mantle samples and that the amphibole crystallized at a lower temperature than that of the pyroxenes. We estimated the apparent amphibole crystallization temperature using major element compositions of the amphibole and established temperature- and composition-dependent models that can be used to predict apparent pyroxene-amphibole REE partition coefficients for amphibole-bearing peridotite and pyroxenite from intraplate lithospheric mantle. Apparent pyroxene-amphibole REE partition coefficients predicted by the models can be used to infer REE contents of amphibole from REE contents of coexisting pyroxenes. This is especially useful when the grain size of amphibole is too small for trace element analysis. 

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. 

Understanding the petrological and geochemical processes shaping the Moho transition zone (MTZ) is crucial for advancing our knowledge of thermal and chemical exchanges between the oceanic crust and the residual upper mantle. In this study, we systematically investigate the MTZ outcropped within the Zedong ophiolite, located in the eastern part of the Yarlung-Tsangpo Suture Zone (YTSZ), with the aim of at reconstructing the magmatic processes responsible for generating the petrological Moho. The Zedong MTZ comprises a sequence of dunite, wehrlite, pyroxenite, and gabbro, with frequent occurrences of clinopyroxene-rich lithologies. Cyclicity within the MTZ sequences is characterized by the recurrence of olivine-rich intervals and the presence of zig-zag patterns in both major and trace elements of clinopyroxenes. Zircon Usingle bondPb dating on the Zedong gabbros supports the coeval formation of the Zedong ophiolite with other YTSZ ophiolites. Clinopyroxene in the Zedong MTZ follows a differentiation sequence characterized by an increase in contents of Al2O3 and TiO2, coupled with a decrease in Mg#. This differentiation sequence along with frequent occurrences of amphibole suggest the evolution of a primitive hydrous melt depleted in Al2O3, TiO2, and Na2O. The depleted Ndsingle bondHf isotopes and rare earth element patterns of the MTZ rocks indicate that their parental magmas originated from fluid-enhanced re-melting of a previously depleted mantle. Additionally, we proposed that the initiation of a new subduction zone results in the re-melting of the mantle peridotite, leading to the formation of primitive hydrous basaltic melts. The variable lithologies observed in the Zedong MTZ arise from fractional crystallization and repeated replenishment of hydrous melts.