Micro-Fourier Transform Infrared (FT-IR) and δD value investigation of hydrothermal vein quartz: Interpretation of fluid inclusion δD values in hydrothermal systems

摘要

Some recent studies have suggested that the hydrogen isotopic composition (delta D) of hydrothermal fluids, released in vacuo by thermal decrepitation of quartz, are not always accurately revealed. We report the results of a step-heating delta D value study of vein quartz, hosted by Lower Palaeozoic rocks in SW England, which was analyzed by micro-FT-IR for hydrogen speciation, before and after fluid extraction at temperatures between 750 and 1500 degrees C. The delta D values of individual aliquots of released water vary between -3 parts per thousand and -208 parts per thousand, with the lowest values generally corresponding to the highest temperature fractions and samples of relatively low yield. The data show significant departures from geologically reasonable delta D. Micro-FT-IR analyses show that a variety of OH species are present within the vein quartz, with significant intra and inter sample variation. Typically a broad absorption due to molecular water, in the region 3400 cm(-1) is observed, along with bands attributed to Li-OH and Al-OH. On heating, the broad absorption due to molecular water is reduced, accompanied by a measurable loss of Li-OH species. The latter becomes more pronounced in the higher temperature fractions (> 750 degrees C). These data support earlier studies which indicated that contributions from the contrasting OH reservoirs in quartz can significantly influence the reported delta D values. These new data also suggest that the incorporation of OH released from Li-OH sites in the quartz may be the most important factor in the generation of the anomalous values for these samples. Crown copyright (C) 2008 Published by Elsevier Ltd. All rights reserved.