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To help us better understand the possible coupling between faulting and fluid flow in the Peak District we have used clumped isotope thermometry to determine the temperature at which a Variscan hydrothermal calcite vein precipitated. Clumped isotope thermometry is based on the fact that the rare, heavy isotopes of carbon (^{13}C) and oxygen (^{18}O) are ordered in the carbonate lattice. This is a result of the greater stability of the ^{13}C-^{18}O bond compared to bonds involving either no, or a single isotopic substitution. The degree of ordering is an inverse function of temperature. As temperature increases the isotopes tend towards a more random or stochastic distribution \citep{Eiler:2007vua}. Measurement of the degree of ordering allows us to estimate the temperature at which the distribution of isotopes in the calcite structure are locked in \citep{Ghosh:2006cn}. This is analogous to the concept of a closure temperature for radiogenic isotopes or for cation ordering in minerals. A key advantage of the method is that the temperature estimate is based on the distribution of carbon and oxygen isotopes within a single phase and not on the partitioning of oxygen isotopes between calcite and it’s parent fluid as in the conventional oxygen isotope geothermometer. Thus determination of the mineral precipitation temperature is decoupled from knowledge of the parent fluid oxygen isotope composition. Combining the clumped isotope temperature (T($\Delta$_{47})) with the bulk oxygen isotope composition of the carbonate represented by it's $\delta^{13}$C value we can constrain the isotopic composition of the parent fluid. Recently two studies have demonstrated the use of clumped isotopes to constrain the temperature and isotopic composiiton of fluids associated with faulting and fractures in the upper crust \citep{Swanson:2012gw, Bergman:2013ip}.  Using these techniques we find: We find that:  (i) the calcite precipitated at temperatures between 40° and 100°C.