This study analyzed the isotopic signatures of hydrothermal calcite veins from limestone in the Peak District, UK and Clare Basin, Ireland to determine the temperature and fluid composition during vein formation. Clumped isotope thermometry revealed the veins precipitated from mixtures of a hot, evolved fluid (>160°C, δ18O >+12‰) and cooler meteoric water (<40°C, δ18O < -5‰) between 30-160°C. Veins plot along mixing lines, implying precipitation from evolving fluid mixtures. Rapid mixing of the two fluid end-members in seismic fracture networks led to carbonate supersaturation and precipitation over short timescales of 10s-100s of years, challenging