Résumé
Vanadium (V) isotopes were used to trace magmatic processes during planetary differentiation, however, whether and how they could be fractionated in the terrestrial mantle remains unclear. Here, we report V isotopic compositions for samples from the Balmuccia (BM) and Baldissero (BD) massifs in the Italian Alps, and Zhimafang in Dabie-Sulu orogen of China, as well as for xenoliths in basalts from Tariat (Mongolia), Tok (Siberia), and Panshishan and Tashan in eastern China. These samples are grouped, according to their origin, as follows: Group 1 combines 20 fertile to moderately melt-depleted, unmetasomatised lherzolites from massifs and xenoliths; Group 2a combines five strongly melt-depleted peridotites (harzburgites, dunites) and 12 associated vein pyroxenites from the Italian Alps; Group 2b samples are three composite xenoliths (pyroxenite veins in peridotites) from Tariat. Group 3 comprises strongly metasomatized peridotites (two garnet lherzolites and a wehrlite) from Tok and Zhimafang. The δ51V values in Group 1 peridotites range from −1.14‰ to −0.82‰ and show broad covariations with melt-extraction indices (e.g., Al2O3). Melting models indicate that V isotopes may fractionate during partial melting with △51Vresidue-melt of −0.15‰ to −0.10‰. The δ51V values in Group 2a peridotites range from −1.19‰ to −0.98‰, possibly involving minor redistribution of V between residual peridotites and parental melts of pyroxenite veins that show δ51V from −1.09‰ to −0.92‰. Group 2b composite xenoliths show contrasted δ51V values between peridotite hosts (−1.05‰ to −0.85‰) and pyroxenite veins (−1.32‰ to −0.99‰). Host-vein δ51V differences in the xenoliths range from 0.05 to 0.27‰, likely reflecting diffusion-driven kinetic V isotope fractionation during melt intrusion and the formation of the pyroxenites. The δ51V values in Group 3 peridotites (−1.04‰ to −0.92‰) are close to the BSE estimate (−0.91 ± 0.09‰). Fe-Ca-V-rich melt metasomatism may slightly lower δ51V, while fluid metasomatism preserves earlier magmatic values.
We report first δ51V data for mineral separates from mantle rocks (Groups 1 and 3). They show narrow, BSE-like ranges in spinel (−0.89‰ to −0.85‰), garnet (−0.85‰ to −0.80‰), orthopyroxene (−0.99‰ to −0.95‰) and phlogopite (−1.02‰ to −0.96‰), but a greater δ51V range (−1.13‰ to −0.77‰) in clinopyroxene. Clinopyroxene from Group 1 samples from Tashan and Panshishan displays a spongy texture, while clinopyroxene from Group 3 exhibits textural and trace element evidence for metasomatism. The lower δ51V values in Group 3 clinopyroxene may reflect isotope fractionation during solidification of evolved interstitial liquids. However, mass balance calculations indicate that mineral-scale variations have a negligible impact on whole-rock V isotopic compositions. Overall, V isotopes can be fractionated in the mantle with δ51V from −1.32‰ to −0.82‰ by a combination of partial melting and melt-solid interactions.