Neon isotopic compositions in the mantle

The following general characteristics have been noted for mantle Ne isotopes. Exceptions and counterarguments are discussed subsequently. 

MORE °Ne/ Ne and Ne/ Ne ratios generally are correlated (Sarda et al. 1988 Moreira et al. 1998), likely due to mixing of variable amounts of atmospheric contamination with relatively uniform mantle Ne (Fig. 1). The corrected or mantle Ne/ Ne ratio of each sample can be obtained by projecting away from the atmosphere to a solar °Ne/ Ne ratio. These Ne/ Ne ratios of -0.074 are higher than the solar value (0.033) due to additions of nucleogenic Ne. 

Distribution of Terrestrial Noble Gases, Evolution of the Atmosphere 421 

The MORE He and Ne isotopic compositions can be used to calculate the He/ Ne ratio of the source region prior to any recent fractionations created during transport 

Using a corrected MORE value of Ne/ Ne = 0.074 and Ne/ He = 4.5 x 10, then He/ Ne =11. This is greater than the most recent estimate of 1.9 for the solar nebula (see discussion in Porcelli and Pepin 2000). 

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This transfer most plausibly occurs by advection of mantle material (Kellogg and Wasserburg 1990) and so includes all the noble gases. The possibility that transfer of He from the lower mantle occurs by diffusion (Allegre et al. 1986), so that the principles of the residual mantle model can still be applied to the other noble gases, has not been shown to be reasonable for maintaining the present global flux. Also, this would result in substantial mantle He/Ne fractionations, which are not observed (see Neon isotopic compositions in the mantle section). 

The model predicts that the He/ Ne ratio of the two mantle reservoirs is identical. It has been argued that the upper mantle ratio may be -70% greater (see Neon isotopic compositions in the mantle section). This is an important test of the model, although it is possible that other processes (e.g., subduction of Ne) might be incorporated into a revised model to explain any differences. 

Among the five noble gases, Ne and Xe deserve special attention because their isotopic compositions are unique (as far as we know) to the Earth, suggesting that their evolution processes are fundamentally related to some specific processes of Earth evolution. In Section 7.4, we will discuss Ne in that a key issue is to understand the distinct difference between mantle neon and atmospheric neon isotopic compositions. In Section 7.5, we discuss a long-standing missing Xe problem. 

He, Ne, Ar, C and N between cosmochemical potential precursors (PSN and chondrites) and terrestrial reservoirs (the atmosphere and the mantle source of MORB) is given in Figure 2. Volatile abundances are normalized to Ne and the Sun, which, in this figure, results in a flat pattern for the solar abundance. Neon is used for normalization as its isotopic (non-radiogenic) composition in the mantle is clearly different from that of the atmosphere (see below). 

Neon isotope data from Samoan lavas show elevated e/ Ne relative to atmosphere. The data are not consistent with a model of mixing between a degassed MORE mantle with high e/ Ne and a deeper, undegassed plume source with atmospheric ONe/ Ne. The similarity of neon isotopes between the Samoan plume-Uke source and MORE supports the idea that neon isotopes in the mantle as a whole more closely resemble the solar composition than that of the atmosphere (see Chapter 2.06). Plume-like neon isotopic signatures have been identified in an apatite from a southeastern Australian spinel Iherzolite xenolith (Matsumoto et al, 1997). 

In the solid Earth, production of nucleogenic Ne is coupled to that of radiogenic " He. This is because production of Ne is directly proportional to the a-particle production ratio from the uranium and thorium series. The Ne/ He production ratio is constant and has been estimated at a value of 4.5 X 10 (Yatsevich and Honda, 1997). In this way, if the Earth accreted with solar helium and neon and initial ratios were modified by production of Ne and " He in a fixed proportion then the present-day He/ He and Ne/ Ne ratios in the mantle should be correlated. Honda et al. (1993) noted a strong correlation between OIB helium and neon isotopes such that steeper trajectories in three-isotope neon space were characterized by samples with high He/ He ratios. Indeed, they showed that it was possible to estimate the He/ He ratio of a suite of OIBs based solely on measurements of the neon isotope composition. 

Unfortunately, it is not possible to similarly calculate the abundance pattern of OIB source regions, since the heavy element isotope ratios are not constrained. A survey of helium and neon isotope compositions of OIB and MORB has allowed calculations of the He/ Ne ratios for the source region of each type, with He/ Ne = 6.0 1.4 for Hawaii and 10.2 1.6 for MORB, with a mantle average of 7.7 (Honda and McDougall, 1998). This approach suggests that there may be a systematic difference between the MORB and OIB sources. In contrast, it has been argued that Loihi and gas-rich MORB samples have similar He/ Ne ratios (Moreira and Allegre, 1998), and so the issue remains open. 

Fig. 2. Comparison of volatile abundance data in protosolar nebula (PSN), primitive chondrites, terrestrial mantle and atmosphere (atmosphere sensu stricto, crust, sediments, oceans). Data are normalized to Ne and PSN, so that the PSN pattern is flat. The choice of Ne as a normalizing isotope is based on the observation that recycling of atmospheric neon in the mantle is limited as indicated by its isotopic composition. Data sources Mazor et al. (1970), Marty Jambon (1987), Anders Grevesse (1989), Pepin (1991), Moreira et al. (1998), Ozima et al. (1998), Marty Zimmermann (1999).

Early neon isotope measurements indicated that the Ne isotope composition of the Earth s mantle differs from that of the atmosphere (Craig and Lupton 1976 Poreda and Radicati di Brozolo 1984), but confidence in the early results was limited by the low analytical precision. Once some of the analytical obstacles were overcome, the work by Honda et al. (1987) and Ozima and Zashu (1988) on diamonds, and by Sarda et al. (1988) on submarine volcanic glasses from ridges and islands, showed conclusively that the mantle is characterized by elevated ratios of e/ Ne and Ne/ Ne compared to the atmosphere. The relatively high Ne Ne in MORBs and OIBs indicates that the connection between the Earth s atmosphere and mantle is not a simple, complementary relationship. The different correlations between e/ Ne and Ne/ Ne in MORBs and OIBs each pass through the composition of air (Fig. 11), due to the ubiquitous presence of an atmospheric component. Key questions arise from the Ne isotope data, especially about the origin of the atmosphere. The chapters by Porcelli and Ballentine (2002) and Pepin and Porcelli (2002) discuss these questions in more detail. Of special note is the... 

Moreira M, Kunz J, Allegre CJ (1998) Rare gas systematics in popping rock isotopic and elemental compositions in the upper mantle. Science 279 1178-1181 Moreira M, Breddam K, Curtice J, Kurz MD (2001) Solar neon in the Icelandic mantle new evidence for an undegassed lower mantle. Earth Planet Sci Lett 185 15-23 Morrison P, Pine J (1955) Radiogetuc origin of the helium isotopes in rock. Ann NY Acad Sci 62 69-92 Muramatsu YW, Wedepohl KH (1998) The distribution of iodine in the earth s crast. Chem Geol 147 201-216... 

The noble gas geochemistry of natural waters, including formation waters in sedimentary basins, has been used to determine paleotemperatures in the recharge areas, to evaluate water washing of hydrocarbons, and to identify mantle-derived volatiles (Pinti and Marty, 2000). The dissolved noble gases, helium, neon, argon, krypton, and xenon in sedimentary waters, have four principal sources the atmosphere, in situ radiogenic production, the deep crust, and the mantle. These sources have characteristic chemical and isotopic compositions (Ozima and Podosek, 1983 Kennedy et al., 1997). 

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