Origin, Occurrence, and Mobility of He and Ar Isotopes in Meliphanite

The U, Th, and ⁴He contents of meliphanite correspond to an age of metamorphism (1800 Ma) indicating good preservation of radiogenic ⁴He in mineral but contradicting the experience of U–Th–⁴He dating. For the analysis of this result, we studied preservation of ³He, which was found to be very low (~2%). The ^3,4He isotopes occur in radiation tracks, the intersection of which with cleavage planes provide for their migration from the mineral. Thus, ⁴He originated from U and Th decay should be completely lost. Meliphanite entraps He, which probably occurs in specific volumes of its crystal structure. Extraction of He isotopes from meliphanite upon vacuum heating supports this conclusion. Application of only one U–Th–⁴He system for dating (as often applied in thermochronology) may yield erroneous results. Identification of the origin of He in minerals requires using both isotopes (³He and ⁴He).