Neon isotopes in mantle

Honda M., McDougall I., and Patterson D. (1993) Solar noble gases in the Earth the systematics of helium-neon isotopes in mantle derived samples. Lithos 30, 257-265. 

Figure 5. Neon isotopes in mantle xenoliths from locations worldwide. Many samples show values with emichments in °Ne and Ne and fall on the MORE correlation line. Some plot to further to the right, due to nucleogenic or cosmogenic inputs of Ne. A few samples fall to the left, toward the hotspot correlation line. Gray triangles, Cameroon Line (Barfod et al. 1999) black circles, SE-Australia (Matsmnoto et al. 1998 2000) white squares, Emope (Dunai and Baur 1995). MORB and mass discrimination line (mdl) are from Sarda et al. (1988). Loihi-Kilauea Line (L-K) from Honda et al. (1991). The gray arrow indicates the direction of possible shifts if samples were exposed to cosmic rays.

Figure 3 The distribution of neon isotopes in mantle-derived rocks, indicating the presence of an atmospheric component, a radiogenic component adding Ne (produced by neutrons from uranium fission acting on oxygen and magnesium), and a solar component. It is this latter that indicates that gases in the mantle were derived from the capture of solar material in the early history of the Earth. M = MORB (midocean ridge basalts) P = plume or ocean island basalts (OIB) A = atmosphere. Solar neon is represented by the horizontal line at Ne/ Ne = 12.5 MFL is the mass fractionation line. The presence of solar neon in ocean basalts was first identified by Craig and Lupton (Craig H and Lupton JE (1976) Earth and Planetary Science Letters 31 369-385). (Reprinted with permission from Farley and Poreda (1993).

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). 

Dodson A, Keimedy BM, DePaolo DJ (1997) Helium and neon isotopes in the Imnaha Basalt, Columbia River Basalt Group evidence for a Yellowstone plume source. Earth Planet Sci Lett 150 443-451 Dodson A, DePaolo DJ, Keimedy BM (1998) Helium isotopes in lithospheric mantle Evidence from Tertiaiy basalts of the western USA. Geochim Cosmochim Acta 62 3775-3787. 

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. 

Niedermann, S., Bach, W., Erzinger, J. (1997) Noble gas evidence for a lower mantle in MORBs from the southern East Pacific Rise Decoupling of helium and neon isotope systematics. Geochim. Cosmochim. Acta, 61, 2697-715. 

With the exception of Davies, who favored whole-mantle convection all along, the above authors concluded that it was only the upper mantle above the 660 km seismic discontinuity that was needed to balance the continental crust. The corollary conclusion was that the deeper mantle must be in an essentially primitive, nearly undepleted state, and consequently convection in the mantle had to occur in two layers with only little exchange between these layers. These conclusions were strongly reinforced by noble gas data, especially He/ He ratios and, more recently, neon isotope data. These indicated that hotspots such as Hawaii are derived from a deep-mantle source with a more primordial, high He/" He ratio, whereas MORBs are derived from a more degassed, upper-mantle reservoir with lower He/ He ratios. The noble-gas aspects are treated in Chapter 2.06. In the present context, two points must be mentioned. Essentially all quantitative evolution models dealing with the noble gas evidence concluded that, although plumes carry... 

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. 

There are several observations that require that this type of mantle model be modified. The relationship between the upper mantle and the atmosphere cannot be simply related. The difference between MORB and air Ne/ Ne ratios indicates either that neon isotopes were not initially uniformly distributed in the Earth or that neon in the atmosphere has been modified by losses to space after degassing from the mantle. Regardless of the reason, the assumption that the atmosphere and upper mantle together form a closed system does not hold. Also, the higher °Ne/ Ne and Ne/Ar ratios of the upper mantle limit the contribution that presently degassing volatiles can have made to the atmosphere (Marty and Alle, 1994). These observations suggest that neon has been lost by fractionating processes from the early atmosphere, a feature that can be appended to the model of mantle stmcture. 

There are three neon isotopes. The more abundant Ne and Ne are both essentially all primordial, as there is no significant global production of these isotopes. In contrast, Ne is produced by nuclear reactions. In mantle-derived materials, measured Ne/ Ne ratios are greater than that of the atmosphere of 9.8, and extend toward the values of the solar wind (13.8) or... 

OlBs). Poreda and Farley (1992) found values of " °Ar/ Ar < 1.2 X 10" in Samoan xenoliths that have intermediate He/" He ratios (9-20/ a)- Kola Peninsula carbonatites with high Ne/ Ne ratios were used to calculate a mantle " Ar/ Ar value of 5,000 (Marty et al., 1998). Other attempts to remove the effects of air contamination have used associated neon isotopes and the debatable assumption that the contaminant Ne/Ar ratio is constant, and have also found Ar/ Ar values substantially lower than in MOREs (Sarda et al., 2000). Overall, it appears that " Ar/ Ar ratios in the high He/ He OIB source are >3,000 but probably <10, and so lower than that of the MORE source (see also Matsuda and Marty, 1995). 

An outstanding question is how much of the mantle still maintains high volatile concentrations. This involves resolution of the nature of the high He/" He OIB-source region. Most models equate this with undepleted, undegassed mantle, although some models invoke depletion mechanisms. However, none of these has matched the end-member components seen in OIB lithophile isotope correlations. It remains to be demonstrated that a primitive component is present and so can dominate the helium and neon isotope signatures in OIB. The heavy-noble-gas characteristics in OIB must still be documented. It is not known to what extent major volatiles are stored in the deep Earth and associated with these noble gas components. 

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