To investigate the behavior of Cr isotope during partial melting and melt percolation in the mantle, we present bull-rock Cr isotopic compositions of fresh mantle ultramafic rocks from the Baldissero and Balmuccia peridotite massifs in Ivrea-Verbano Zone, Italian Alps. Our samples include lherzolites, harzburgites, dunites, and pyroxenites. Lherzolites, which formed by 5–15% fractional melting of a primitive mantle source, displayed δ⁵³Cr values ranging from −0.16‰ to −0.05‰ (n = 14), showing a negative correlation with indicators of melt extraction such as Al₂O₃ (R² = 0.56). This indicates that partial melting causes measurable Cr isotope fractionation in the mantle residual peridotites. The harzburgites and dunites studied originated from partial melting of the lherzolites induced by silicate melt percolation at varying extent melt/rock ratios. Harzburgites and dunites (−0.13‰ to −0.01‰; n = 4) far way with pyroxenite veins had the Cr isotopic compositions compatible with those of lherzolites in the study. However, dunites in contact with veins exhibited heterogeneous δ⁵³Cr values (−0.28‰ to 0.08‰; n = 2) relative to those of lherzolites. This is attributed to kinetic effects during silicate melt percolation, where ⁵²Cr preferentially diffuses. A diffusion model suggests that the heterogeneous Cr isotopic composition in dunites can be explained by an empirical β_Cr (i.e., β_Cr-exponent in D_53Cr/D_52Cr = (m_52Cr/m_53Cr)^βCr) of 0.05–0.07 in spinel. These pyroxenites formed by melt‐peridotite reaction and mineral accumulation from basaltic melt. The δ⁵³Cr values of the pyroxenites ranged from −0.45‰ to −0.15‰ (n = 8), and two distinct groups were identified based on the correlation between δ⁵³Cr with Mg^#. The δ⁵³Cr of group Ⅰ (−0.23‰ to −0.14‰; n = 6) were similar with the previously reported value of the MORBs (δ⁵³Cr = −0.27‰ to −0.07‰), which positively correlated with their Mg^# and Al₂O₃ contents. This indicates that Cr isotopes were fractionated during magmatic differentiation. The group Ⅱ pyroxenites (−0.45‰ to −0.32‰; n = 2), which were from a single spinel clinopyroxenite vein, displayed extreme low δ⁵³Cr values than the range known for MORBs, indicated light Cr isotopes preferentially diffuses from peridotites to silicate melts during melt percolation, resulting in the rocks inheriting the low δ⁵³Cr values from melt. In addition, the variation of δ⁵³Cr in group Ⅱ pyroxenites was controlled by different spinel phase modes. Our results indicate that the process of melt percolation is an important mechanism leading to the heterogeneity of Cr isotope composition in the mantle.
 

铬同位素，超基性岩，橄榄岩地体，熔体渗滤