Problematic issues of noble gas analysis

A correlation between measured He/ He ratio and helium content for co-genetic suites of samples may reveal late-stage addition of crustal (radiogenic) helium to helium-poor samples. Hilton et al. (1995) noted that He-poor crystals in some ocean-island phenocrysts had low He/ He ratios consistent with addition of 

this observation is more likely observed in pyroxenes than in olivines. 

A correlation between He/ He ratios and an index of magmatic differentiation. For example, Gasparon et al. (1994) found lower He/ He ratios in more highly evolved phenocrysts (phenocrysts with a lower Mg ) from the Sunda arc, Indonesia. 

The occurrence of isotopic disequilibrium between coexisting phenocryst pairs. Marty et al. (1994) reported higher He/ He ratios in olivine phenocrysts compared to pyroxene or hornblende for Mt. Etna volcano, an observation that is consistent with addition of late-stage, radiogenic or atmospheric helium. The same effect is seen in arc-related phenocrysts from the Lesser Antilles (Van Soest et al. 2002). 

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