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where T is in Kelvins. As with most previous studies the calibration has been made over a restricted temperature range (0-56$^{\circ}$C). For the most part this range is below the temperature of most of the samples reported in this study. We have confidence, however, that the calibration can be extrapolated to higher temperatures. In Figure 4 we have plotted data obtained in our laboratory for the $\Delta_{47}$ value of a Carrara marble sample that had been crushed and experimentally re-crytallised at 600$^{\circ}$C and 1000MPa in a solid media apparatus before rapidly quenching(Bernasconi, 2014 pers. comm.) Our extrapolate calibration passes very close to the mean of these high temperature data. Also, our calibration lies very close to the theoretical calculations of the temperature dependence of heavy isotope clumping in calcite \citep{Guo:2009fy}. These observations suggest that the UEA calibration is robust over the range of temperatures expected in the upper crust.   \section{Results}