The isotope mass effect on chlorine diffusion in dacite melt, with implications for fractionation during bubble growth

•Volatile element isotopes can fractionate during chemical diffusion.•Chlorine isotopes have a diffusivity ratio, DCl37/DCl35=0.995±0.001 (β=0.09±0.02).•No temperature effect on the diffusivity ratio between 1200 and 1350 °C.•Bubble growth can fractionate 37Cl and 35Cl by up to about 5‰ in dacitic melts.