Hydrogen and oxygen isotopes of water from inclusions in minerals: design of a new crushing system and on-line continuous-flow isotope ratio mass spectrometric analysis

摘要

An analytical line for stable isotope analyses of water recovered from fluid inclusions in minerals was built and successfully tested. The line is based on the principle of continuous-flow analysis of water via high-temperature reduction on glassy carbon. It includes a custom-designed set of high-efficiency crushers and a cryo-focusing cell. This paper provides details of the line design and discusses strategies for line conditioning and mitigation of memory effects. The line allows measurements of hydrogen and oxygen isotopes during a single acquisition. The precision of the analyses depends on the amount of water released from the inclusions. The best results are obtained for samples containing at least 0.1-0.2 mu L (0.06-0.11 mu mol) H2O. For such samples precision is better than 1.5 parts per thousand for delta D and 0.5 parts per thousand for delta 18O (1 alpha). Smaller amounts of water can be measured but at lower precision. Analyses of modern calcite formed under stable conditions in a deep cave allowed assessment of the accuracy of the analyses. The delta D values measured in fluid inclusions of this working standard match the delta D value of the parent water, and the oxygen isotope values agree within ca. 0.5 parts per thousand. This indicates that fluid inclusions trapped in calcite at near-ambient temperatures (e.g. speleothems and low-temperatures phreatic calcite) faithfully preserve the original isotopic composition of the parent waters.