Fe and Si isotope variations at Cedar Butte volcano; insight into magmatic differentiation

- We measured Fe and Si isotope ratios in a single sample suite from Cedar Butte volcano.
- Both isotope systems show an increase in heavy isotope along differentiation.
- Crystal fractionation (CF) and fluid exsolution cannot explain alone the results.
- We suggest that thermal diffusion could have fractionated Fe and Si isotopes.
- The process involves a top-down sill injection and a vertical T° gradient.

This study presents the stable isotopic variations of both Si and Fe recorded in a single well-characterized magmatic suite from Cedar Butte volcano (ID, USA), as well as a sill with progressive compositional change within Finland granophyre (Duluth Complex, MN, USA). Both isotopic systems show a significant enrichment in heavy isotopes in the more differentiated materials, in agreement with previous studies. In addition, the Finland granophyre sill shows a strong dependence between the isotopic composition and the sampling depth, suggesting the isotopic compositions follow a temperature gradient in which the cold part systematically enriches in heavy isotopes.From these results it appears that at Cedar Butte, neither crystal fractionation, nor crustal contamination, nor late stage fluid exsolution can likely explain the isotopic variations we observe for both Fe and Si. We rather attribute these isotopic fractionations to a thermal migration process involving a top-down sill injection during which the isotopic distribution mostly follows a vertical temperature gradient.