Potassium isotope constraints on the mechanism of K enrichment in the mantle source of Italian ultrapotassic magmatic rocks

Investigating the origin of ultrapotassic rocks provides critical insights into the recycling process of crustal material into the mantle and the formation mechanism of mantle heterogeneity. However, the nature of metasomatic agents responsible for the K enrichment and the origin of geochemical differences between silica saturated and silica undersaturated ultrapotassic suites remain debated. In this contribution, we present high-precision potassium (K) isotope data for both groups of ultrapotassic rocks from Italy. These rocks exhibit a broad range of δ⁴¹K values ranging from -1.33‰ to -0.13‰. Silica saturated ultrapotassic rocks (lamproites and post-leucitites) display homogeneous and mantle-like δ⁴¹K values (-0.50‰ to -0.40‰), whereas silica undersaturated ultrapotassic rocks (leucitites and kamafugites) show lower and more variable δ⁴¹K signatures (-1.33‰ to -0.13‰). The observed isotopic variability cannot be explained by shallow-level magmatic processes, but instead it reflects source heterogeneity within the metasomatized subcontinental lithospheric mantle (SCLM). The low δ⁴¹K values argue against slab-derived fluids as the primary control, but instead point to subducted sediments as the primary source of isotopically light K. The extremely low δ⁴¹K values in silica undersaturated rocks further require the involvement of K-rich metasomatic phases, most likely phlogopite-bearing metasomes, which act as efficient reservoirs and transfer media for sediment-derived K. The observed δ⁴¹K contrasts between these compositional groups reflect distinct metasomatic regimes within the metasomatized SCLM. Silica saturated rocks reflect relatively homogeneous SCLM domains modified by silicate-dominated sediments, whereas silica undersaturated rocks record stronger and more heterogeneous contributions from carbonate-rich sediment components, mediated by K-rich metasomatic assemblages. Collectively, our data indicate that K isotopes can effectively distinguish metasomatic agents and constrain mantle source heterogeneity within the metasomatized SCLM, highlighting their potential for resolving the origin of ultrapotassic magmatism in subduction-related settings.