Volcanism noble gases

We summarize noble gas amounts in deep-sea and subaerial sediments in Figure 5.1. From the data displayed here, we calculated median values which are shown in Table 5.1. Both Figure 5.1 and Table 5.1 show that even though there is little difference in the lighter noble gas concentration between subaerial and deep-sea sediments (He, Ne, and Ar), heavier noble gases are much more abundant in subaerial sediments than in deep-sea sediments. As in volcanic rocks (cf. Section 6.6), most sediments, either deep-sea or subaerial, show fractionation toward the heavier ones relative to air noble gas, although the mechanism for the fractionation may be different. Figure 5.2 shows noble a gas elemental abundance pattern relative to the air abundance subaerial sediments show much more severe fractionation. 

To obtain information on the noble gas state in the mantle, it is necessary to analyze mantle-derived materials that have trapped mantle noble gases. Accessible samples include volcanic rocks, volcanic gases, mantle xenoliths, and diamonds. Among various mantle-derived materials, submarine volcanic rocks are particularly useful because of their wide occurrence and their relatively large (for mantle samples) amounts of trapped noble gases. So far, information has been obtained mainly from... 

Recently, precise noble gas isotopic analyses of diamonds have also become available. Diamonds are derived from the mantle. However, diamonds are unique in that most known diamonds are of Precambrian age hence, we can hope for noble gas records of the very ancient mantle, complementary to the constraints on the present mantle provided by recent volcanics. 

A number of noble gas studies have focused on xenoliths from the Pliocene to Recent alkali basalts of the Newer Volcanics in SE Australia... 

Sasaki S. and Tajika E. (1995) Degassing history and evolution of volcanic activities of terrestrial planets based on radiogenic noble gas degassing models. In Volatiles in the Earth and Solar System, AlP Conf. Proc. 341 (ed. K. A. Farley). AIP Press, New York, pp. 186-199. 

Crust-mantle chemical mass-balance models offer important constraints on compositional variations in the mantle, but their constraints on the size of the various reservoirs involved depend critically on uncertainties in the estimates of the bulk composition of the continental crust, the degree of depletion of the complementary depleted mantle, and the existence of enriched reservoirs in Earth s interior, for example, possibly significant volumes of subducted oceanic crust. This last item was left out of the mass-balance models that suggested that the upper and lower mantle are chemically distinct. Chapter 2.03 makes it clear that much of the chemical and isotopic heterogeneity observed in oceanic volcanic rocks reflects various mixtures of depleted mantle with different types of recycled subducted crust. With this realization, and excepting the noble gas evidence for undegassed mantle, some of the characteristics of what was once labeled... 

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... 

Ozima M, Podosek FA (2002) Noble Gas Geochemistiy. Cambridge University Press. Cambridge, UK Ozima M, Zashu S (1983) Noble gases in submarine pillow volcanic glasses. Earth Planet Sci Lett 62 24-40... 

Mafic minerals contained in volcanic rocks (phenocrysts) or in xenoliths are widely exploited in noble gas studies as they frequently contain fluid and/or melt inclusions that trap noble gases. Olivines and pyroxenes are the most commonly utilized minerals. Whole-rock samples are first crushed to 0.5 to 2 mm or larger, depending on the size of the crystals. The olivines and pyroxenes are then separated using a Frantz-Isodynamic magnet separator, followed by hand-picking under a binocular microscope to remove any... 

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. 

Relative noble gas abundance systematics of arc-related volcanism... 

There have been numerous attempts at estimating volatile fluxes associated with arc-related volcanism. In this respect, the noble gas isotope that has received most attention is He as its primordial origin makes it an unambiguous tracer of mantle-derived volatiles. Therefore, if the arc flux of He can be established, it would lead to the derivation of other volatile fluxes by simple measurement of the ratio x/He, where Xi is any volatile species discharging from volcanoes (CO2, SO2, H2S, HCl, N2, etc.). Two distinct approaches have been taken to estimate the He flux from arc volcanoes ... 

Hofmann AW (1997) Mantle geochemistry The message from oceanic volcanism. Nature 385 219-229 Honda M, McDougall I, Patterson DB, Donlgeris A, Clagne DA (1991) Possible solar noble-gas component in Hawaiian basalts. Natnre 349 149-151... 

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