FluidNET International Conference 2024 "Fluid inclusion isotopic signals from Variscan quartz veins using a new CRDS method"

Fluid inclusions of quartz veins are essential for determining the pathways of fluids flow in
the subsurface. In this study, we provide a new method for analysing the isotopic composition
of fluid inclusion water via Cavity Ring-Down Spectroscopy (CRDS). The CRDS is
connected to a mechanical crusher to release fluid inclusion water from the mineral it is
contained in, at a temperature of around 120°C. The water vapour from the pulverised sample
is introduced into a wet background that has a consistent composition and concentration of
oxygen and hydrogen isotopes. The Earth Science Stable Isotope Laboratory at the Vrije
Universiteit Amsterdam (VU) has designed a temperature-controlled evaporation system that
ensures a consistently stable background.
This newly designed configuration offers accurate measurements of the combined oxygen
and hydrogen isotope compositions from quartz grains. In this work, we examined quartz
veins from Germany and Portugal that are associated with the Variscan orogeny. The isotopes
results are consistent with the current Global Meteoric Water Line and indicate the meteoric
impact on this fold-and-thrust belt. The silicon and oxygen isotopic characteristics of quartz
crystals, together with microthermometric studies, provide evidence of the hydrothermal
system undergoing cooling at various phases of the Variscan orogeny, in conjunction with the
isotopic data obtained from fluid inclusions. This technique enables direct measurement of
isotopic compositions of fluid inclusions, offering a quicker and more efficient alternative to
current approaches. Moreover, it may be used to a wide range of minerals and geological
environments.