High-temperature kinetic isotope fractionation of calcium in epidosites from modern and ancient seafloor hydrothermal systems

•Epidosites from ophiolites have lower δ44Ca than hydrothermal fluids.•The fractionation of −0.2 to −0.5 is most likely a kinetic isotope effect.•Epidote can form from oversaturated solutions at T>250°C.•Hydrothermal silicates may commonly form under non-equilibrium conditions.