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.

Keywords

Cr isotopes / Ultramafic rocks / Peridotite massifs / Melt percolation

CRediT authorship contribution statement

Haibo Ma: Writing - original draft, Software, Methodology, Investigation, Formal analysis, Data curation, Conceptualization, Writing - review & editing, Visualization. Li-Juan Xu: Writing - review & editing, Supervision, Funding acquisition, Formal analy-sis, Data curation, Conceptualization, Writing - original draft. Yu-Wen Su: Supervision, Methodology, Investigation, Visualization, Writing - review & editing. Chunyang Liu: Methodology, Formal analysis, Software, Writing - review & editing. Sheng-Ao Liu: Writing - review & editing, Formal analysis, Supervision. Jia Liu: Writing - review & editing, Formal analysis, Conceptualization, Supervision. Zezhou Wang: Writing - review & editing, Formal analysis, Validation. Guochun Zhao: Writing - review & editing.

Declaration of competing interest

The authors declare that they have no known competing finan-cial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

We are grateful to Xuanxue Mo (Editor-in-Chief) and Kristoffer Szilas (AE) for handing and constructive comments, and to two anonymous reviewers for their careful comments and suggestions that helped to improve the manuscript. We also thank Prof. Ger-hard Wörner and Yuan-Ru Qu for their valuable suggestions for language editing. We would like to express our gratitude to Wei Jiang for the laboratory analyses. This study was financially sup-ported by National Natural Science Foundation of China (Grant No. 42473017), Hong Kong RGC grants (JLFS/P-702/24 and 17308023), China Geological Survey project (Grant No. DD20242037).

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.gsf.2025.102049.

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