Helium isotopic variations

Several different reasons can be considered for He isotope displacements between the xenoliths and the range of MORE values that now serve as a reference  

Inclusion of in situ produced cosmogenic He. There are some xenoliths that have He/ e ratios that are greater than the MORE field. While the involvement of a 

Distinctive mantle source melt source regions. While many OIB have He characterized by He/ He ratios that are higher than those in MORB, some have been found to have lower ratios (see Graham 2002). As discussed in the Regional studies section, the presence of such a component has implications for the causes of host magma volcanism as well as the sources of lithosphere He. 

Distinctive lithospheric mantle He. As discussed in the Possible xenolith noble gas components section, radiogenic production within the lithosphere can lower the He/ He ratio of He derived from the convecting mantle. Therefore, the He represents a distinctive lithospheric component. This may be created in the xenolith source region, or may involve He that is remobilized in the lithosphere. As discussed below, regional studies often contend with distinguishing between distinctive asthenospheric and lithospheric sources for such He. 

Overall, samples that have been erupted recently and have had minimal surface exposure to cosmic rays therefore can be evaluated for subsurface He components. Outside of areas where subducted crustal material may be important, the sources of He are generally limited by the narrow range in xenolithic He/ He ratios to be the convecting upper mantle as sampled by MORB, in some cases with contributions from radiogenic production within the lithosphere or from a distinctive OIB component. The significance of these components is addressed in the more detailed discussion in the Regional studies section. 

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Marty B., Pik R., and Gezahegn Y. (1996) Helium isotopic variations in Ethiopian plume lavas—nature of magmatic sources and limit on lower mantle contribution. Earth Planet. Sci. Lett. 144, 223-237. 

Reid M. R. and Graham D. W. (1996) Resolving lithospheric and sub-lithospheric contributions to helium isotope variations in basalts from the southwestern US. Earth Planet. Sci. Lett. 144, 213 -222. 

Lupton JE, Baker ET, Massoth GJ (1989) Variable He/heat ratios in submarine hydrothermal systems evidence from two plumes over the Juan de Fuca Ridge. Nature 337 161-164 Lupton JE, Graham DW, Delaney JR, Johnson HP (1993) Helium isotope variations in Juan de Fuca Ridge basalts. Geophys Res Lett 20 1851-1854... 

In this section, we review noble gas systematics of arc-related volcanism worldwide. Helium isotope studies dominate because most arc products are erupted subaerially, and air contamination is a relatively minor (correctable) problem for helium this is not the case for Ne-Ar-Kr-Xe isotope systematics. Consequently, this section is weighted towards reporting observations of helium isotope variations in arc-related minerals and fluids. However, we summarize also the available database for neon, argon and xenon isotopes (todateKr shows only air-like isotopic compositions). Finally, we consider the limited database of the relative abundances of the noble gases in arc-related products. 

Table 1. Summary of helium isotope variations in arc-related volcanics and geothermal fluids worldwide.

Black DC (1972) On the origins of trapped helium, neon and argon isotopic variation in meteorites - II. 

Oxygen isotope signatures of Vesuvio rocks show wide variations (S180 —1-7.0 to +10.0) and are negatively correlated with MgO (Ayuso et al. 1998). Helium isotope studies on clinopyroxene and olivine from historical lavas gave values of R/Ra 2.2 to 2.7, close to ratios found in the fumar-oles of Campanian volcanoes (Tedesco et al. 1990 Graham et al. 1993). 

The isotopes ofHe do notalways occur in all natural samples in their usual proportion. Because helium has only two stable isotopes, variations in their abundance ratio are usually attributed to 3He. But in cases where radioactive alpha decays have enriched 4He, that reason for 4He richness is usually fairly obvious. One example is He in rocks containing uranium. The 4He/3He ratio is about 100 times greater than solar in the Earth s atmosphere because the history of radioactive decay of uranium in the Earth (Rutherford ) and its outgassing has enriched our atmosphere in daughter 4He. 

Studies of 0/ 0 isotope variations in several vascular plant species, mosses and environmental surface bog water from temperate peat bogs (Switzerland) used as climatic archives for paleoclimatic reconstruction were reported by Menot-Combes et 8 0-values in organic material were determined by the online continuous flow method after sample pyrolysis at 1080 °C in the presence of glassy carbon in a Carlo-Erba elemental analyzer. The gases obtained (CO, N2 and H2O) were separated by passing them through a water trap and a GC column in a helium carrier gas. The isotope composition of CO was measured with a VG Prism II isotope ratio mass spectrometer relative to the VSMOW isotope standard. The overall analytical uncertainty is 0.08%o for water 8 0-values." ... 

Fig. 2 highlights the isotopic effect for H2 and D2 by showing the adsorption isotherms measured under both sub- and supercritical conditions (Tcrii(H2>=33.19 K, Tcrit(D2)=38.34 K). The lower part illustrates the evaluation of the experimental data according to the Langmuir equation. It becomes clear that the Langmuir fit works very well for nitrogen, shows an acceptable performance for the hydrogens, but is not applicable to helium. The variation of the results is shown in Table 1. 

Filer J. M., Farley K. A., and Stolper E. M. (1998) Correlated helium and lead isotope variations in Hawaiian lavas. Geochim. Cosmochim. Acta 62, 1977-1984. 

Graham D. W., Furman T. H., Ebinger C. J., Rogers N. W., and Lupton J. E. (1995) Helium, lead, strontium, and neodymium isotope variations in mafic volcanic rocks from the western branch of the East African rift. EOS Trans. Am. Geophys. Union 76, F686. 

Sorey M. L., Kennedy B. M., Evans W. C., Farrar C. D., and Suemnicht G. A. (1993) Helium isotope and gas discharge variations associated with crustal unrest in Long Valley caldera, California. J. Geophys. Res. 98, 15871-15889. 

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