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Geochemical Journal
Geochemical Journal An open access journal for geochemistry
Published for geochemistry community from Geochemical Society of Japan.

Precise determination of hydrogen and oxygen isotope ratios of water in fluid inclusions of quartz and halite

Takeshi Ohba, Sadao Matsuo
Geochemical Journal, Vol. 22, No. 2, P. 55-68, 1988


A vacuum ball mill was devised to extract volatiles from fluid inclusions in minerals. A special feature of the crushing mode is the horizontal oscillation of the mill, which enhances crushing efficiency and overcomes the fragility of the mill, which is made of Pyrex glass. This ball mill is equipped with a cold finger trap cooled with liquid nitrogen to reduce the influence of adsorption of once extracted volatiles on to mineral powder during the crushing process. Amounts and isotope ratios were determined for water extracted from the fluid inclusins of quartz and halite. The determinations were made for CO2 as well in fluid inclusions of quartz. The influence of adsorption on amounts and isotopic ratios of volatiles was evaluated by comparing the results of repeated extractions with and without the liquid nitrogen trap during the crushing process. In a single run of extraction, two kinds of water were obtained: one was water trapped in the cold finger trap during the crushing, the other was desorbed water given by heating of the sample after crushing. A simple summation of the isotope ratios of the desorbd and the trapped waters gives an erratic result in comparison with the original isotope ratio. As for the fluid inclusions in hydrothermal quartz, the original δ18O and δD values for water in the inclusions can be estimated by applying Rayleigh's equation to the adsorption process. In the case of halite samples, however, the estimation of the original δ18O and δD values is much more difficult because of the formation of hydrate salts. However, in the special case where the brine in inclusions is a pure NaCl solution, the original inclusion water is simply the sum of the desorbed and trapped waters. For CO2 analysis of inclusion fluids, the adsorption during the crushing process is negligibly small, not affecting appreciably the results on δ13C and δ18O values.

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