Relative noble gas abundance systematics of arc-related volcanism

Relative noble gas abundance systematics of arc-related volcanism 

Patterns of relative noble gas abundances are usually reported in the Fm notation where Fm = ( X/ Ar)/( X/ Ar)atm where X represents a noble gas element X of mass m. Data are available from the following arc segments Japan (Matsubayashi et al. 1978 Nagao et al. 1981), New Zealand (Marty and Giggenbach 1990 Patterson et al. 1994), Central America (Kennedy et al. 1991) and the Aeolian arc (Nakai et al. 1997 Tedesco and Nagao 1996). Irrespective of sampling medium (phenocryst or geothermal fluids) there appear to be a number of common features in the relative noble gas abundance patterns of arc-related samples  

Extreme enrichments in helium (both He and He) compared to air. For example, Patterson et al. (1994) report a maximum F3 value of -22,000 for New Zealand phenocrysts while Kennedy et al. (4991) found a maximum F4 of 107 for Honduras geothermal fluids. Based on the He/ He ratio of the helium, the enrichment is usually attributed to the input of magmatic gas originating from the mantle wedge. 

F22 and p84 values which resemble air or the noble gas abundances of air-saturated water (asw) i.e., F22 1 and p84 1. In the case of geothermal fluids, slight differences from the actual air or asw values have been ascribed to either uncertainties in the recharge temperature of the fluids or to fractionation processes associated with either vapor-phase separation or bubble formation (e.g., Kennedy et al. 1991). In the case of phenocrysts (e.g., from New Zealand), Patterson et al. (1994) explained the variations in F22 and Fg4 about the atmospheric values by solubility-controlled elemental fractionation associated with mass transfer between basaltic melt and a distinct vapor phase. 

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