Diffusive fractionation of K isotopes in molten basalts

The K-41/K-39 isotope ratio profiles in diffusion couple experiments have been measured by Secondary Ion Mass Spectrometry (SIMS). One goal of this research is to push the use of SIMS in measuring non-traditional stable isotope ratios. The second, more important goal, is to quantify for the first time diffusive fractionation of K isotopes and its dependence on temperature and different counter-diffusion elements. The data show that the precision in a single day-night session of SIMS measurements can reach 0.2 parts per thousand (1cr hereafter) with effort, and the long-term accuracy without using any isotope ratio standard is about 2.5 parts per thousand. At an initial concentration contrast (ratio of high concentration to low concentration) of about 70 in a diffusion couple, the total K-41/K-39 fractionation (maximum minus minimum) is about 10 parts per thousand. The 41K/39K ratio profiles were initially fit by assuming constant effective binary diffusivity (D) of K2O, which led to minor misfits and more importantly, to large disagreement between diffusivities based on chemical diffusion and isotope diffusion profiles. It was found that D for K2O varies with its concentration. The profiles were then fit by assuming D for K2O increases exponentially with K2O concentration, which resolved the misfits and disagreements. That is, combining concentration and isotope ratio profiles enables distinguishing subtle concentration-dependent diffusivity. For the SiO2-K2O interdiffusion couples, the empirical diffusive isotope fractionation parameter beta increases slightly with temperature from 0.104 +/- 0.003 at 1260 C to 0.116 +/- 0.003 at 1500 C. For the MgO-K2O interdiffusion couples, excluding an outlier point at 1260 C, beta increases from 0.090 +/- 0.005 at 1350 C to 0.100 +/- 0.003 at 1500 C. These beta values are roughly consistent with a diffusion mechanism of NaKO exchanging with SiO2 in the SiO2-K2O interdiffusion couples, or NaKO exchanging with MgO in the MgO-K2O interdiffusion couples. Applying the obtained beta value to model diffusive isotope fractionation in nature, diffusive K isotope fractionation during magma mixing is expected to be large enough to be resolvable by SIMS. When collecting samples for K-Ar or K-Ca dating, it is important to correct for the effect of possible K isotope fractionation by measuring K isotope ratio in the sample. When diffusive and convective diffusive modeling was applied to evaluate isotope fractionation during volatile loss through diffusion and evaporation, it is found that a loss of 80% potassium would lead to an increase of 841K by 6.3 parts per thousand to 8.9 parts per thousand , more than 10 times greater than the enrichment of 841K in the Moon relative to the Earth. Hence, the depletion of K and the associated enrichment of 841K in the Moon relative to the Earth are unlikely diffusion controlled. (c) 2022 Elsevier B.V. All rights reserved.

Automated summary

This study measured K-41/K-39 isotope ratio profiles using SIMS in diffusion couple experiments, with the goal of advancing the use of SIMS in measuring non-traditional stable isotope ratios and quantifying diffusive fractionation of K isotopes. The results demonstrate the precision and accuracy of SIMS measurements and reveal the concentration-dependent diffusivity of K2O. The findings also suggest that the diffusive fractionation of K isotopes during magma mixing can be resolved by SIMS.