Allanite in Mantle Eclogite Xenoliths.

This article focuses on the discovery and characterization of allanite, a rare-earth element (REE)–bearing mineral, in three eclogite xenoliths from the Udachnaya kimberlite pipe in the Siberian craton. Detailed petrographic, chemical, and isotopic analyses indicate that allanite occurs as discrete grains at grain boundaries and within secondary mineral aggregates, formed likely by metasomatic reactions between Al- and Ca-rich eclogite minerals (garnet and omphacite) and REE- and alkali-rich carbonatite/kimberlite melts during xenolith transport. The presence of 0.005–0.008 wt % allanite reconciles discrepancies in light REE (LREE) and Sm–Nd budgets between reconstructed and measured bulk rock compositions, suggesting that allanite precipitation, rather than simple bulk kimberlite melt addition, accounts for LREE enrichment in these mantle rocks. Pressure–temperature estimates place allanite formation within its stability field at 3.6–4.8 GPa and 1025–1080°C, and Nd isotopic data support a metasomatic origin linked to kimberlite interaction. This finding implies that allanite may be a more common but previously overlooked host of incompatible elements in mantle eclogites, with implications for interpreting trace element systematics and metasomatic processes in subducted oceanic crust and cratonic lithosphere.