High-temperature inter-mineral potassium isotope fractionation in ultrapotassic and granitic rocks: Implications for the potassium isotopic compositions of arc magmas

Abstract

To constrain the mechanism and magnitude of inter-mineral K isotope fractionation during high-temperature magmatic processes, we conducted high-precision K isotope measurements on ultrapotassic and granitic rocks and mineral separates from the Qinling Orogenic Belt, China, as well as first-principles investigations of the corresponding K-bearing minerals. Our results show that the ultrapotassic rocks (−0.33 to −0.14‰) have considerably higher δ41K than the primitive mantle (−0.42 ± 0.08‰), confirming the previous conclusion that slab-derived fluids have contributed heavy K isotopic signatures to their mantle sources. Specifically, the K isotopic fractionation between biotite and microcline (Δ41Kbiotite-microcline = −0.11 to 0.05‰, with biotite and microcline ranging from −0.49 to −0.14‰ and − 0.48 to −0.12‰, respectively) agrees well with the calculated equilibrium isotopic fractionation scale. This evidence, together with the homogeneous mineral chemical compositions, suggests equilibrium K isotopic fractionation between biotite and microcline. Notably, amphiboles (δ41K ranging from −0.41 to 0.48‰) have heterogeneous K isotopic compositions and are generally heavier than coexisting microcline and biotite. In addition, the determined K isotopic fractionation between amphibole/plagioclase and microcline is inconsistent with the calculated results and is likely caused by the kinetic fractionation processes. To reveal the potential controls on the K isotopic compositions of arc magma, we utilized theoretical and measured K isotopic fractionation factors to model K isotopic fractionation behavior during amphibole crystallization. The results show that when Δ41Kamphibole-melt = 0.05‰, the fractional crystallization of amphibole shows negligible effect on the K isotopic composition of the residual melts. Even in the case of Δ41Kamphibole-melt = 0.24‰, fractional crystallization of amphibole exerts an insignificant control on the K isotopic composition of arc magma. Therefore, although amphibole may have heavier K isotopic compositions than the melts, its fractional crystallization may not distinctly decrease the K isotopic composition of residual melts.