Stable chromium isotope fractionation during melt percolation: Implications for chromium isotopic heterogeneity in the mantle

Abstract

To investigate the stable chromium (Cr) isotope variations during melt percolation in the mantle, we analyzed the Cr isotopic compositions of fresh ultramafic rocks from the Balmuccia and Baldissero peridotite massifs located in the Italian Alps. These massifs represent fragments of the subcontinental lithospheric mantle. The samples collected included lherzolites, harzburgites, dunites, and pyroxenites. Lherzolites, formed through 5%–15% fractional melting of a primitive mantle source, exhibited δ⁵³Cr values ranging from −0.13‰ ± 0.03‰ to −0.03‰ ± 0.03‰. These values correlated negatively with Al₂O₃ content, suggesting that partial melting induces Cr isotopic fractionation between the melts and residual peridotites. Harzburgites and dunites, influenced by the silicate melt percolation, displayed distinct δ⁵³Cr values. Notably, dunites not spatially associated with the pyroxenite veins exhibited slightly elevated δ⁵³Cr values (−0.05‰ ± 0.03‰ to 0.10‰ ± 0.03‰) relative to lherzolites. This difference likely resulted from pyroxene dissolution and olivine precipitation during melt percolation processes. However, one dunite sample in direct contact with pyroxenite veins showed lower δ⁵³Cr values (−0.26‰ ± 0.03‰), possibly owing to the kinetic effects during silicate melt percolation. Pyroxenites are formed through the interaction of basaltic melts with the surrounding peridotite via a metasomatic reaction or crystallization in a vein. Most of their δ⁵³Cr values (−0.26‰ ± 0.03‰ to −0.13‰ ± 0.03‰) are positively correlated with MgO contents, suggesting that they were influenced by magmatic differentiation. However, two subsamples from a single clinopyroxenite vein exhibit anomalously low δ⁵³Cr values (−0.30‰ ± 0.03‰ and −0.43‰ ± 0.03‰), which are attributed to kinetic isotopic fractionation during melt-percolation processes. Our findings suggest that melt percolation processes in the mantle contribute to the Cr isotopic heterogeneity observed within the Earth’s mantle.