Potassium Isotope Fractionation During Magmatic Differentiation and the Composition of the Mantle

Stable potassium (K) isotopes are emerging as tracers of terrestrial recycling and planetary processes. These applications require well‐defined K isotopic compositions for the mantle and the bulk silicate Earth (BSE). Both values are determined primarily by basalts formed via partial melting of the mantle. Basaltic melts experience igneous differentiation before reaching the surface, which may alter their isotopic compositions compared to their mantle sources. This study investigates K isotope fractionation during the differentiation and solidification of the Kilauea Iki lava lake, Hawaii, for which crystallization and thermal histories are well documented. High‐precision K isotopic ratios ( δ 41 K) are measured in 13 Kilauea Iki samples that cover its complete differentiation history, ranging from olivine‐rich, high‐MgO cumulates to increasingly differentiated, MgO‐depleted samples. The limited δ 41 K range of −0.42 to −0.37‰ in all but one sample reveals no analytically resolvable fractionation across diverse bulk compositions, even though their bulk MgO contents varied from 26.9 to 2.37 wt.%. The lack of K isotopic fractionation is consistent with an absence of K‐rich minerals in the crystallizing assemblage, where only plagioclase can accommodate a small amount of K. A highly differentiated vein displays the lowest δ 41 K of −0.47‰ among the Kilauea Iki suite, which is consistent with our modeling calculations that suggest measurable K isotope fractionation at more advanced magmatic differentiation stages. Combining our new results with literature data, we propose an average δ 41 K of −0.42 ± 0.08‰ (2SD) for the pristine mantle and of −0.42 ± 0.07‰ (2SD) for the BSE.