Paul Dennis edited discussion_fluids.tex  over 8 years ago

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The low temperature end member has an isotopic composition that is typical of meteoric groundwaters being depleted in ^{18}O with respect to ocean water. We suggest that the range of minimum temperatures (30-50$^{\circ}$C) is characteristic of the depth of burial at the time of mineralization and that the near surface hydrogeology of the platform carbonates was dominated by meteoric recharge. With an elevated geothermal gradient of between 40 to 50$^{\circ}$C.km^{-1} this suggests that mineralization occurred at a depth of 0.8 to 1km. Such an estimate is in good agreement with previous studies (references).  The high temperature end member has a temperature greater than 100$^{\circ}$C and a $\delta$^{18}O value of +5‰_{VSMOW} that is typical of sedimentary basin and oil field brines (Sheppard, 1986). It is most likely sourced from depth within the Visean-Namurian shales of the Bowland-Hodder unit in the Edale Gulf. Likely minimum source depths lie between 2 to 3 km. An interesting comparison can be made between the derived formation water isotope composition of +5‰_{VSMOW} and that expected for a fluid in isotopic equilibrium at 100$^{\circ}C 100$^{\circ}$C  with kaolinite. We are reassured that the derived fluid $\delta$^{18}O values are consistent with our understanding of the isotopic compositions of naturally occurring fluids and suggests that the clumped isotope temperatures are robust. For example were the calcite temperatures to have been as high as the maximum reported fluid inclusion homogenization temperatures of 240$^{\circ}$C then the derived fluid $\delta$^{18}O values would need to have been in excess of 18 - 20‰_{VSMOW}. Such values are in excess of any reported data for modern geothermal and formation waters \citep{Sheppard:1986ta}.