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 2.7 x 10¹⁹ atoms) and ~10⁻¹³ to 10⁻¹⁰ cm³ STP/g (Ne, Kr, Xe). Therefore their concentrations and isotopic …