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Estimates of the depth of burial at the time of vein formation are poorly constrained. Gillespie et al. (2001) have attempted to estimate the burial depth by: (i) reconstructing the missing Silesian overburden prior to the culmination of Variscan activity, and; (ii) using the mechanical characteristics of veins crossing discontinuities between beds of similar elastic properties. They determine a likely minimum and maximum depth of burial of 1.25km and 2km respectively. Previous fluid inclusion homogenisation temperatures obtained from fluorite in the veins range from 85 to 200°C (O’Connor et al., 1993) .  We sampled hydrothermal calcite from fractures in the Slievenaglasha Limestones that are exposed on the coastal section to both the north and south of Fisherstreet (GR).  \section{Methods}  (i) Sample preparation and mass spectrometry.  Samples of vein calcite were prepared for clumped isotope analysis by either gently crushing small crystal fragments in an agate pestle and mortar, or by drilling using a dental bit in a Dremel drill and avoiding undue use of pressure so as to minimise frictional heating of the sample. Analyte CO2 is produced by reacting 6-8mg of each sample with 102% ortho-phosphoric acid in vacuo at 25° for a period of 12 hours. The evolved CO2 is then dried, collected by cryo-distillation into a calibrated volume manometer to check reaction yields and then stripped of potential hydrocarbon contaminants before collection in a valved glass gas tube. The drying stage involves freezing the CO2 into a glass spiral trap at liquid nitrogen temperatures before sublimation at -120°C, passing the gas through a second trap at -120°C, and freezing with liquid nitrogen into the manometer. We strip any potential hydrocarbon contaminants from the CO2 by cryo-distillation into a gas tube via a 20cm glass tube packed with porapak Q ion exchange resin held at a temperature of -20°C.  The sample gases were analysed for their clumped isotope values, δ45 - δ49 on the UEA MIRA dual-inlet isotope ratio mass spectrometer (Dennis, 2015). Samples were analysed at a major beam (m/z=44) intensity of 7.5×10-8A and data collected for each cardinal mass of the CO2 molecule (m/z = 44 - 49). Each sample measurement consists of 4 acquisitions each of 20 reference-sample gas pairs. Before analysis and between each acquisition the sample and reference gas volumes and signal strengths are balanced to within 1%. Each sample or reference cycle consists of a 10s ‘dead time’ after switching of the changeover valve followed by a 20s integration period. The total measurement time, including sample and reference gas balancing is approximately 90 minutes. The total integration time is 1600s each for the sample and reference gas. Internal precisions (±1σ) for δ45 and δ46 are better than 0.001‰, for δ47 better than 0.008‰, for δ48 better than 0.03‰ and for δ49 better than 10‰.  The reference gas used in MIRA is CO2 produced by reaction of BDH marble chips with 85% ortho-phosphoric acid and subsequently equilibrated with water at 20°C for a period of 1 month. This is to ensure that the Δ47 value of the reference gas is in equilibrium at the laboratory temperature. The nominal composition of the reference gas is: δ13C = 2.007‰VPDB; δ18O = 34.899‰VSMOW, and Δ47 = 0.94URF. To ensure a robust calibration of scale compression and transfer function between the local reference frame for Δ47 and the universal reference frame (URF) both 1000°C heated and 20°C water equilibrated reference gas samples are measured on a daily basis (Dennis et al., 2011). Data quality and long term stability of measured values is monitored by daily measurement of two laboratory standards that bracket the range of Δ47 values for samples in this study: UEACMST (Δ47 = 0.39±0.01, n= ) and UEAHTC (Δ47 = 0.56±0.01, n=).  MIRA is inherently linear with no variation in the calculated Δ47 and Δ48 values of samples as a function of their bulk isotopic composition as represented by the δ47 and δ48 values of samples (Huntington et al., 2010). Not-with-standing this we regularly check for linearity by measurement of 1000°C heated cylinder CO2 (BOC) that is depleted in δ47 with respect to the reference gas by approximately 65‰. We have not recorded any change in the linear behaviour of MIRA over a four year period since commissioning.  (ii) Data handling and calculation of Δ values.  The clumped isotope Δi value of a sample is defined as:   (1)  where Ri is the measured ratio of isotopologue i to the non-isotopically substituted isotopologue and Ri* is the expected ratio assuming a stochastic distribution of all isotopes over all possible sites in the lattice (reference). For CO2 we are largely concerned with the isotopologue 13C18O16O (i = 47) but also determine Δ values for 13C18O17O (i = 48) and 13C18O18O (i = 49).  The ratios Ri and Ri* are determined from the measured δi, δ13C and δ18O values of the sample CO2. For R47:   (2)  where R47wrg is the 47/44 ratio of the working reference gas (wrg) and is determined as:   (3)  Note that implicit in this treatment is an assumption that the mass spectrometer working reference gas has a stochastic distribution of isotopes. It is self evident that this is incorrect since the working reference gas has been equilibrated with water at the laboratory temperature. However, since the Δ values are <1‰ we can make this assumption and carry out a later linear transformation of the data to take account of the actual reference gas R47 value without introducing any significant errors.  The ratios R13, R17 and R18 are determined from the δ13CVPDB, δ17OVSMOW and δ18OVSMOW values of the working reference gas:   (4)  and similar equations for R17 and R18. The reference gas ratios are R13VPDB = 0.0112372 (reference), R17VSMOW = 0.0004023261 (reference) and R18VSMOW = 0.0020052 (reference).  Similarly the R47* ratio for a sample corresponding to a stochastic distribution of the isotopes is given by:   (5)  with R13, R17 and R18 determined using equation 4 and the measured δ13C, δ17O and δ18O values for the sample.  Substitution of R47 (equation 2) and R47* (equation 5) into equation 1 allows determination of Δ47. Evaluation of Δ48 and Δ49 follows the same steps as above. The complete data reduction algorithm and it’s implementation in a Mathematica program for data obtained on the MIRA mass spectrometer is included in the supplementary information.  Finally, using the method outlined by Dennis et al. (2011) and the heated and water equilibrated gas standards we determine a transfer function between measurements made on the local scale with reference to the mass spectrometer working reference gas the absolute reference frame (ARF). All the results are reported with respect to the ARF. The full data set including results reported on both the local and ARF scales are included with the supplementary information.  (iii) Temperature estimation using Δ47  Using the clumped isotope composition of carbonate minerals as a geothermometer is a relatively immature technique. Of particular importance is the lack of agreement on the calibration between the deviation from a stochastic distribution of the isotopes and temperature. There exists a large number of calibrations with varying degrees of sensitivity. Moreover, most of these calibrations have been made over a restricted range of temperatures based on the collection of biogenic carbonates from environments with known temperature (e.g. references). More recently calibrations have become available for inorganically precipitated calcites. However these have not served to resolve the origin of the differences.  What is clear from these studies is that differences between calibrations are laboratory dependent. i.e. measurements of unknown samples when converted to a temperature only make geological sense when the local Δ47 - T calibration is used. If another laboratories calibration is used then unrealistic temperatures are often estimated. This points to significant methodological issues associated with sample preparation and mass spectrometry that have yet to be resolved. We don’t discuss these here. For this study we have used the UEA determined calibration to translate Δ47 values for carbonates into temperature.    (6)  This calibration is based on (i) biogenic carbonates (bivalves and foraminifera) collected from environments with known environmental temperature; (ii) natural inorganic calcites from travertines precipitated at spring heads with known temperature, and (iii) experimental inorganic carbonates precipitated or recrystallised at known temperatures. A single calibration is consistent across the temperature range covered by the calibration from 0 - 600°C. We note that within measurement error this calibration is indistinguishable with the theoretical line of Guo et al. (2009).