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 bulk silicate Earth (BSE). Both values determined primarily by basalts formed via partial melting mantle. Basaltic melts experience igneous differentiation before reaching surface, which may alter their compared to sources. This study investigates isotope fractionation during solidification Kilauea Iki lava lake, Hawaii, crystallization thermal histories well documented. High-precision ratios (δ41K) measured in 13 samples that cover its complete history, ranging from olivine-rich, high-MgO cumulates increasingly differentiated, MgO-depleted samples. The limited δ41K range −0.42 −0.37‰ all but one sample reveals no analytically resolvable across diverse compositions, even though MgO contents varied 26.9 2.37 wt.%. lack is consistent with an absence K-rich minerals crystallizing assemblage, where only plagioclase can accommodate a small amount K. A highly differentiated vein displays lowest −0.47‰ among suite, our modeling calculations suggest measurable at more advanced magmatic stages. Combining new results literature data, we propose average ± 0.08‰ (2SD) pristine 0.07‰ BSE.