δD values of fluid inclusion water in quartz and calcite ejecta from active geothermal systems: do values reflect those of original hydrothermal water?

Abstract

One of the most common methods for estimating paleohydrothermal δ D values in epithermal quartz veins is hydrogen isotope analysis of H 2 O extracted from fluid inclusions in quartz by thermal decrepitation. The validity of the method is questioned. The δ D values of water extracted from fluid inclusions in clear euhedral quartz from active geothermal systems are up to 30 per mil more negative than the δ D value (–42‰) of the geothermal water. Measured δ D values of the fluid inclusion water are dependent on the temperature at which the water in the quartz was extracted. Water extracted at 800°C has δ D values about 10 to15 per mil lower than fluid inclusion water extracted at 500°C (–53 to –61‰). Fluid inclusion water in calcite from an active geothermal well has δ D values that match those of the geothermal water. Calcite is thus potentially a more suitable mineral for estimating paleohydrothermal δ D values.