Equilibrium silicon isotope fractionation in eclogites and granites constrained by single crystal X-ray diffraction and the force constants approach

Using our recently developed force constants approach, we have determined the equilibrium Si isotope fractionation between omphacite/garnet, quartz/kyanite, and quartz/zircon at temperatures relevant to the petrogenesis. We find that Na strongly affects the Si isotope fractionation between omphacite and garnet. Our results have suggested that the omphacite and garnet in eclogite collected in the Dabie Mountain, as well as the kyanite and its host quartz veins, are isotopically in equilibrium, which further suggests that the Dabie Mountain eclogites and its host veins underwent the same high pressure-temperature condition during their formation. The Si isotope fractionation determined by our methods, together with published mass spectroscopy measurements on natural granites and DFT-CIPW calculations, have suggested that the Si isotope fraction between zircon and whole rock “saturates” at ~0.45‰ at 1000 K when the SiO2 content in the felsic granite is above ~70 wt%.