Abundance in the Earth and atmosphere

There have been relatively few systematic descriptions of noble gas concentrations in rocks and minerals. In general, the He content is dependent upon the abundances of Th and U, the crystal density of host minerals and the permeability of the rock. Mantle-derived rocks and minerals seem to be relatively enriched in He, with high He/ He ratios. Thus, minerals and nodules in kimberlites can have He contents up to 2755 cm /g x 10 (phlogopite) and 350 cm /g x 10 (diamond) in contrast, many igneous rocks may contain 0.1 cm /g X 10 (Hoppe and Alexander, 1978). Very high concentrations of He can occur 

The variation of the isotopic abundance of He in nature is by far the largest of any stable isotope ratio known. It spans a range of lO with a minimum He/ He ratio of 1 for iron meteorites and maximum of 10 for certain U minerals (Kamenskiy et al., 1971). 

It has been established that the He/ He ratio has values characteristic of the source of the gas. Deep oceanic waters over rift zones and sea-floor spreading centres are found not only to be enriched in total He, relative to the solubility equilibrium with the atmosphere. 

Crustal rocks exhibit a range of He/ He ratios (10 -10 ). The lower values are the result of degassing during emption and relatively slow cooling, degassing during metamorphism and contamination by radiogenic He. 

Wakita and Sano (1983) have reported that high He/ He ratios, up to 8.65 x 10 occur in methane-rich natural gases from oil and gas wells in northeast Japan. These authors conclude that at least one-third of the gas could therefore be of magmatic rather than biological origin, although it is possible that only the He is magmatic. 

---