Does δD from fluid inclusion in quartz reflect the original hydrothermal fluid?

Hydrogen isotope investigations on hydrothermal quartz reveal two H-reservoirs: (i) trapped fluid inclusions, and (ii) structurally bound water in homogenously distributed small clusters or bubbles. Varying mixing ratios of the two reservoirs are sampled by means of mechanical and thermal decrepitation applied to different grain size fractions. A two-component mixing calculation results in an isotopic characterisation of the two H-reservoirs, which fractionate hydrogen isotopes close to the known MOH–H2O system, with water being enriched in deuterium. Temperature controls both the internal fractionation as well as the abundance ratio of inclusion water to bonded water. At high temperatures, fractionation is small but bound water becomes more abundant, comprising a significant amount of water from both thermal and mechanical extraction techniques. Hence, the isotope composition of the extracted water does not reflect the original hydrogen isotope composition of the hydrothermal fluid especially at temperatures higher than 200°C. Previously reported δD data of fluid inclusion, which were used to elucidate the origin of the hydrothermal fluid, tend to be too low.