Reappraisal of potassium isotopic composition of the continental lithospheric mantle: Insights from calc-alkaline lamprophyres in the eastern North China Craton

Abstract The continental lithospheric mantle plays a crucial role in the chemical differentiation of the silicate Earth, primarily through mantle melting processes that lead to crustal formation. The continental lithospheric mantle frequently experiences metasomatic enrichment via subduction, which results in substantial chemical and isotopic heterogeneity. This study explores the effects of crustal recycling on the potassium (K) isotopic heterogeneity of the continental lithospheric mantle beneath the eastern North China Craton. We present K isotopic compositions of well-characterized calc-alkaline lamprophyres derived from the continental lithospheric mantle, which display arc-like trace element patterns and crustal Sr-Nd-Pb-Os isotopic signatures that are indicative of recycled continental material. These samples show a relatively narrow δ41K range of −0.52‰ ± 0.07‰ to −0.29‰ ± 0.06‰ (2SD), and no systematic variation with MgO content or melting degree indexes. This suggests negligible K isotopic fractionation during lamprophyre formation, and points to homogeneous K isotopic compositions in their mantle sources (average δ41K of −0.40‰ ± 0.11‰; 2SD; n = 39). Both the average δ41K value and the homogeneity of lamprophyres are similar to those observed in oceanic basalts (average δ41K of −0.43‰ ± 0.17‰), and contrast with the variable K isotopic compositions typically found in arc lavas. Quantitative modeling suggests that this uniform K isotopic composition in lamprophyres is likely due to the limited influence of crustal materials in modifying the K isotopic composition of the continental lithospheric mantle. These findings imply that long-term processes of continental differentiation and recycling since the initiation of plate subduction—which formed both depleted and enriched mantle reservoirs—do not necessarily affect the K isotopic signature of the continental lithospheric mantle. By integrating our results with existing data for oceanic and continental basalts, we propose average δ41K values of −0.41‰ ± 0.12‰ (2SD; n = 49) for the continental lithospheric mantle and −0.42‰ ± 0.13‰ (2SD; n = 144) for Earth's mantle.