The noble gases (or “rare gases”) helium, neon, argon, krypton, and xenon are chemically inert. Due to their volatile nature they have a strong tendency to partition into gas or fluid phases and can be used as tracers for the origin and the transport of fluids. In rocks they are typically present in very low concentrations of ~10-9 to 10-6 cm3 STP/g (He, Ar; 1 cm3 STP is equivalent to 2.7x1019 atoms) and ~10-13 to 10-10 cm3 STP/g (Ne, Kr, Xe). Therefore their concentrations and isotopic compositions may be modified to a measurable extent by nuclear processes such as radioactive decay or natural nuclear reactions. They can thus be used as dating tools (e.g. U/Th-4He, 40K-40Ar, surface exposure dating with cosmogenic nuclides). Over the history of the Earth, such processes have modified the noble gas isotopic compositions in distinct terrestrial reservoirs (mantle, crust, atmosphere). The isotopic signature of noble gases yields therefore important information about the origin and history of a rock or fluid sample.
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