The stable K and Rb isotope composition of Icelandic basalts: Implications for mantle heterogeneity

Abstract

Basalts erupted in Iceland show varying contributions from the depleted, recycled and primordial components present in the underlying heterogeneous mantle plume. While many Icelandic basalts have a substantial contribution from recycled mantle components, some do not, preserving instead magmas derived from depleted and primordial mantle components. Iceland provides an opportunity for characterising the compositions of different mantle domains and, in particular, to deconvolve the contributions of recycled and primordial components. In order to characterise the stable K and Rb isotope compositions in different mantle domains, we present K and Rb stable isotope data for 31 basaltic glasses, tholeiitic through transitional to alkalic in composition, sampled from Iceland’s active neovolcanic zones. Our results show that δ⁴¹K values vary between −0.299 ± 0.025‰ and − 0.499 ± 0.021‰ (2SD) and δ⁸⁷Rb values vary from −0.15 ± 0.04‰ to 0.02 ± 0.06‰ (2SD), in Icelandic basalts, extending outside of the range previously identified for the bulk silicate Earth (BSE, −0.42 ± 0.07‰ 2SD and − 0.13 ± 0.06‰ 2SD, respectively). While the δ⁴¹K values demonstrate no mantle-derived variation, the δ⁸⁷Rb values show correlations with conventional mantle source proxies (e.g., radiogenic isotopes) indicating a mantle source control. The highest δ⁸⁷Rb values, those furthest removed from BSE, are from two primitive glasses from southwest Iceland whose radiogenic and noble gas isotope ratios indicate minimal contributions from recycled material, but significant contributions from primordial material. These high δ⁸⁷Rb values may represent δ⁸⁷Rb values of the primordial component sampled by the mantle plume beneath Iceland, and therefore a vestige of Earth’s earliest mantle.

DOI

10.1016/j.chemgeo.2026.123384

Reference

Merrill, H., Matthews, S., Halldórsson, S. A., Bali, E., Wang, B., Dai, W., & Moynier, F. (2026). The stable K and Rb isotope composition of Icelandic basalts: Implications for mantle heterogeneity. Chemical Geology, 123384.