Volcanism helium isotopes

Poreda, R., Craig, H. (1989) Helium isotope ratios in circum-Pacific volcanic arcs. Nature, 338, 473-8. 

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... 

Marty B., Jambon A., and Sano Y. (1989) Helium isotopes and CO2 in volcanic gases of Japan. Chem. Geol. 76, 25-40. 

In southeast Australia, this conclusion has been supported by recent osmium-isotopic studies of the Quaternary Newer Volcanic Province, which reveal that sodic alkali basalts there have osmium-isotopic compositions indistinguishable from OIB, again supporting a sublithospheric origin for their parental magmas (McBride et al, 2001). Helium-isotope data are available primarily from sodic basalts in the northern half of Africa (Cameroon Line, Sudan). These basalts have R/Ri, values (—5.1-7.5) consistent with a derivation from an HIMU-type plume source (Barfod et al., 1999 Franz et al, 1999). 

Figure 1 Helium isotope data from various mantle-derived volcanics. The upper axis is the He/ He ratio R) normalized to the atmospheric ratio Rj f. As indicated by the data for selected segments of the MORB away from ocean islands falls almost entirely within the range of (7-9)Ra- While there are hotspot basalts that are characterized by high U/Pb ratios and low He/ He ratios (HIMU), many major oceanic hotspots, as well as continental hotspots, have high He/" He ratios (source Porcelli and Ballentine, 2002).

Helium isotope measurements in ocean ridge and island basalts provide some of the most basic geochemical information on mantle source reservoirs. More helium isotope analyses have been performed for oceanic volcanic rocks than for other noble gas species, and helium isotopes have played a leading role in the study of mantle heterogeneity. Helium isotope analyses are readily performed by modern mass spectrometers because there is a general absence of atmospheric contamination in samples due to the low concentration of helium in air (5.24 parts per million by volume at standard temperature and pressure). There are 2 naturally occurring isotopes of helium. He is much less abundant than " He for example, the atmospheric He/" He ratio (Ra) is 1.39x10 (Mamyrin et al. 1970 Clarke et al. 1976). Nearly all of the terrestrial " He has been produced as a-particles from the radioactive decay of U, U and Th over... 

Craig H, Lupton JE, Welhan JA, Poreda R (1978) Helium isotope ratios in Yellowstone and Lassen Park volcanic gases. Geophys Res Lett 5 897-900... 

Marty B, Pik R, Yirgu G (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 Marty B, Upton BGJ, Ellam RM (1998) Helium isotopes in early Tertiary basalts, northeast Greenland evidence for 58 Ma plume activity in the North Atlantic-Iceland volcanic province. Geology 26 407-410... 

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.

Southern Italy. There is a pronounced northward decrease in He/ He ratios between the Aeolian Islands of southern Italy and the region of Mt. Vesuvius known as the Campanian Magmatic Province or the Neopolitan volcanic region. Mt. Etna on Sicily can be included in this trend as recent work (e.g., Schiano et al. 2001) indicates a progressive transition from a plume-related to a typical island-arc source for this volcano. The trend or step function in the helium isotope systematics of southern Italy was first pointed out by Sano et al. (1989) and ascribed to shallow crustal contamination effects. However, subsequent work (Marty et al. 1994) has shown the combined He-O-Sr-C isotope systematics are consistent with the progressive involvement of Africa continental crust which has been subducted beneath the southern Tyrrhenian Sea. The influence of the African plate increases in a northerly direction (Marty et al. 1994)—towards Mt. Vesuvius (see data of Graham et al. 1993) and possibly as far north as the Roman and South Tuscany volcanic provinces (see Tedesco 1997). 

Sano Y, Wakita H, Giggenbach WF (1987) Island arc tectonics of New Zealand manifested in helium isotope ratios. Geochim Cosmochim Acta 51 1855-1860 Sano Y, Nakamura Y, Notsu K, Wakita H (1988) Influence of volcanic eruptions on helium isotope ratios in hydrothermal systems induced by volcanic eraptions. Geochim Cosmochim Acta 52 1305-1308 Sano Y, Wakita H, Italiano F, Nuccio MP (1989) Helium isotopes and tectonics in southern Italy. Geophys Res Lett 16 511-514... 

Furman T, Graham DW (1999) Erosion of lithospheric mantle beneath the East African Rift system geochemical evidence from the Kivu volcanic province. Lithos 48 237-262 Gasparon M, Hilton DR (1994) Crustal contamination processes traced by helium isotopes examples from the Sunda Arc, Indonesia. Earth Planet Sci Lett 126 15-22 Goes S, Spakman W., Bijwaard H (1999) A lower mantle source for central European volcanism. Science 286 1928-1931... 

Menzies MA, Kempton PD, Dungan M (1985) Interaction of continental lithosphere and asthenospheiic melts below the Geronimo Volcanic Field, Arizona, USA. J Petrol 26 663-693 Moreira M, Kurz MD (2001) Subducted oceanic lithosphere and the origin of the high p basalt helium isotopic signature. Earth Planet Sci Lett 189 49-57... 

Rison W, Craig H (1983) Helium isotopes and mantle volatiles in Loihi Seamount and Hawaiian Island basalts and xenoliths. Earth Planet Sci Lett 66 407-426 Ritter JRR, Jordan M, Christensen UR, Achauer U (2001) A mantle plume below the Eifel volcanic fields, Germany. Earth Planet Sci Lett 186 7-14... 

Hooker PJ, Bertrami R, Lombardi S, O Nions RK, Oxburgh ER (1985b) Helium-3 anomalies and crast mantle interaction in Italy. Geochim Cosmochim Acta 49 2505-2513 Hulston JR, Lupton JE (1996) Helium isotope studies of geothermal fields in the Taupo volcanic zone, New Zealand. J Vole Geotherm Res 73 297-321... 

There are two stable isotopes of helium, He and " He, which vary in their ratio by over three orders of magnitude in terrestrial samples. The Earth s atmosphere is well mixed with respect to helium and contains helium with a uniform isotopic composition of He/" He = 1.39 x 10 . Atmospheric helium is a convenient standard for helium isotope determinations, and terrestrial He/" He ratios are usually normalized to the air ratio and expressed in units of R/Ra, where R = He/ He and Ra = ( He/ He) . In contrast to atmospheric helium R/Ra = 1), the radiogenic helium produced by a-decay of U and Th series isotopes has a much lower ratioof R/Ra0.1, while the volcanic helium that is derived from the Earth s mantle is highly enriched in He R/Ra =5-30). Thus volcanic helium has an isotopic composition distinct from other sources such as atmospheric helium or the helium produced by radioactive decay. This He-rich mantle helium is sometimes called primordial helium, since it is thought to be the remnant of a primitive component... 

In order of decreasing importance, the most abundant forms of helium in sea water are dissolved atmospheric helium, volcanic helium, and to a lesser degree radiogenic helium from sediments. Thereis also an input of pure He into the oceans from tri-tium( H), the radioactive isotope of hydrogen, which decays to He with a half-life of 12.4 years. Because tritium isgenerally found only in the upper ocean, He from tritium decayftritiogenic helium) is only significant at depths less than about 1000 m. 

Although there are only two isotopes of helium, it is still possible to clearly distinguish submarine volcanic helium from the other components because of its high He/ He ratio and because volcanic helium is introduced at mid-depth rather thanat the ocean surface or on the abyssal plain. 

Eor samples highly enriched in helium such as volcanic rocks and hydrothermal vent fluids, the helium isotope ratio is usually expressed in the R/Ra notation described above. However, for the relatively small variations observed in sea water samples, the He/" He variations are usually expressed as i5( He), which is the percentage deviation from the ratio in air, defined as in eqn [1]. 

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