Rare gas systematics (He, Ne, Ar) as a tool in the study of volcanic eruption precursors

Rare gases offer the unique advantage, among all the other chemical species of the volcanic fumaroles, of being chemically inert and of having some isotopes generated by radioactive decay; this fact gives the rare gases an isotopic signature that has different values for the atmospheric, the crustal and the mantle components that make up the fumarolic gases. It is supposed that in a quiescent volcano a steady-state value characterized the ³He/²He ratio; changes due to atmospheric contamination can be monitored by the He/Ne ratio and possibly by the ??Ar/³?Ar one; increasing of the mantle component in the 3 components mixture will generally increase the ³He/?He ratio, suggesting a possible magma uprising.