| Summary: | This paper presents new data on geochemistry of rare-earth and high-field-strength elements in the upper mantle peridotite (spinel lherzolites) and pyroxenite (amphibole clinopyroxenites, garnet clinopyroxenites, websterites, and garnet websterites) xenoliths collected from Quaternary volcanoes of the north-western part of Spitsbergen Island, Svalbard Archipelago. The fractionating of HFSE in the peridotites and pyroxenites, and the effect of temperature on the equilibration distribution of these elements between minerals of spinel lherzolites have been investigated based on geochemical features of rocks and minerals, obtained by ICP MS and SIMS. It has been concluded that fractionation of Nb, Ta, Zr, Hf, and Ti occurs in the upper part of the continental lithospheric mantle under the conditions that exist at depth corresponding to the Moho discontinuity (at the temperature from 730 to 1180 °C and pressures from 1.3 to 2.7 GPa). These conditions correspond to those of the phase transition of spinel peridotites into garnet peridotites in the CMAS systems. It is most likely that the degree of partial melting and the thermodynamic conditions of this process are the main factors regulating fractionation of HFSE. The fractionation of these elements in the mantle of north-west Spitsbergen is noticeably different than in the mantle beneath Early Precambrian cratons.
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