Helium xenoliths

Valbracht, P. I, Honda, M., Matsumoto, T., Mattielli, N., McDougall, I., Ragettli, R., Weis, D. (1996) Helium, neon and argon isotope systematics in Kerguelen ultramafic xenoliths Implications for mantle source signatures. Earth Planet. Sci. Lett., 138, 29-38. 

An important exception to the general trend in the xenolith data are exceptionally high He/" He ratios that have been observed in samples that have very low helium concentrations and have been exposed for long periods of time at the surface. Exposure to cosmic rays can result in the production of cosmogenic He (Craig and Poreda, 1986 Kurz, 1986), and this can dominate over the mantle He contained in the samples (Porcelli et al, 1987). Subsequent studies have endeavored to avoid any samples that may be susceptible to such alterations. 

Extensive volcanism can be found in eastern Africa associated with the East African rift system. Helium isotope ratios measured in basalts from the Ethiopian Rift Valley and Afar, near the triple junction of Red Sea, Aden, and Ethiopian rift systems, are 6-17/ a (Marty et al., 1993, 1996 Scarsi and Craig, 1996). These values include MORB ratios as well as indications of both contributions from radiogenic He and high He/ He hotspots. To the south, the rift system divides into eastern and western branches. Xenoliths from Tanzanian vents in the eastern branch... 

Porcelli D., O Nions R. K., and O Reilly S. Y. (1986a) Helium and strontium isotopes in ultramaflc xenoliths. Chem. Geol. 54, 237-249. 

Tolstikhin I. N., MaMain B. A., Khabarin L. B., and Erlikh E. N. (1974) Isotope composition of helium in ultrabasic xenoliths from volcanic rocks of Kamchatka. Earth Planet. Sci. Lett. 22, 73-84. 

Poreda RJ, Farley KA (1992) Rare gases in Samoan xenoliths. Earth Planet Sci Lett 113 129-144 Poreda RJ, Schilling J-G, Craig H (1986) Helium and hydrogen isotopes in ocean-ridge basalts north and south of Iceland. Earth Planet Sci Lett 78 1-17... 

Dodson A, Brandon AD (1999) Radiogenic helium in xenoliths from Simcoe, Washington, USA Implications for metasomatic processes in the mantle wedge above subduction zones. Chem Geol 160 371-385... 

Figure 1. Helium concentrations in different host minerals from the same xenolith sample. Note that olivines tend to have the lowest concentrations, while pyroxenes have the highest. References 1 Massif Central (Dunai and Baur 1995) 2 Eifel (Dunai and Baur 1995) Bullenmerri (Porcelli et al.

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