Hydrogen isotope determination of fluid inclusion water from epithermal vein quartz : do values accurately reflect water of deposition?

Fine-grained quartz and adularia in the Hishikari (Japan) crustiform epithermal veins, contain up to ∼ 1 wt.%, excluding adsorbed and fluid inclusion water. Water extracted from the veins is predominantly from sub-micron size fluid inclusions in quartz which are the remnants of water from precursor hydrous silica polymorphs. Quartz-rich parts of veins have total water contents >0.5 wt.%, and constant δD values (-75 per thousand). These δD values are lower than present day meteoric water (-50 per thousand). Hydrogen isotope analyses of amorphous silica and water from geothermal power plants and natural springs suggest that a 20 to 0 per thousand fractionation occurs between the precursor silica polymorphs and water at epithermal temperatures (150 degrees C to 200 degrees C). Extraction of water from fine-grained epithermal quartz is not considered to be a suitable technique for determining the δDH2O values of palaeo-geothermal waters. (author). 16 refs., 7 figs