Does delta D from fluid inclusion in quartz reflect the original hydrothermal fluid?

Hydrogen isotope investigations on hydrothermal quartz reveal two H-reservo irs: (i) trapped fluid inclusions, and (ii) structurally bound water in hom ogenously distributed small clusters or bubbles. Varying mixing ratios of t he two reservoirs are sampled by means of mechanical and thermal decrepitat ion applied to different grain size fractions. A two-component mixing calcu lation results in an isotopic characterisation of the two H-reservoirs, whi ch fractionate hydrogen isotopes close to the known MOH-H2O system, with wa ter being enriched in deuterium. Temperature controls both the internal fra ctionation as well as the abundance ratio of inclusion water to bonded wate r. At high temperatures, fractionation is small but bound water becomes mor e abundant, comprising a significant amount of water from both thermal and mechanical extraction techniques. Hence, the isotope composition of the ext racted water does not reflect the original hydrogen isotope composition of the hydrothermal fluid especially at temperatures higher than 200 degreesC. Previously reported deltaD data of fluid inclusion, which were used to elu cidate the origin of the hydrothermal fluid, tend to be too low. (C) 2001 E lsevier Science B.V. All rights reserved.