NOBLE GAS MANTLE MODELS

It has been argued (Ozima et al. 1993) that upper mantle Xe must be Pu-derived, both because a closed system could not be dominated by U-derived fission, and because diamonds that may be >1 Ga old have similar Xe/ Xe ratios to those of MORE, a ratio which might be expected to change due to continuing Xe production. However, there are models that can explain the association of Xe with dominantly U-derived Xe (see Noble Gas Mantle Models section). Further, not only is a fractional change in Xe production over 1 Ga not discounted by the data given current error limits on both diamond and MORE data, but also it is not clear when diamonds acquired their noble gases (see Dunai and Porcelli 2002, this volume). 

How these processes affect the degassing of mantle reservoirs is discussed in Noble Gas Mantle Models. 

Numerical models, geochemistry and the zero-paradox noble-gas mantle. Phil. Trans. Roy. Soc. London A360, 2611-2631. 

BaUentine C. J., van Keken P. E., Porcelli D., and Hauri E. K. (2002) Numerical models, geochemistry and the zero-paradox noble-gas mantle. Phil. Trans. Roy. Soc. London A 360, 2611-2631. 

The specific model for Earth formation is the crucial assumption in this absorption scenario. If Earth formation proceeded in nebula-free space as was originally postulated by Safronov (1969) and henceforth advocated by various researchers (e.g., Levy Lunine, 1993), the absorption scenario must be abandoned. Also, the Mizuno model predicts that Ne as well as other noble gases in the mantle should have solar noble gas isotopic compositions. However, as we discussed in Chapter 5, the currently available observational data on mantle noble gases show nonsolar isotopic compositions, arguing against the Mizuno model. 

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... 

There are several possible descriptions of a layered mantle. The possibilities that have been incorporated into noble gas models include a boundary layer at the 670 km seismic discontinuity, a deeper layer of variable thickness, and a boundary layer at the core-mantle boundary. 

The first mantle degassing models that incorporated considerations of mantle structure were developed with the discovery of mantle noble gas isotope heterogeneities (Allegre et al, 1986, 1983 Hart et al., 1979 Kurz et al., 1982). These models, following earher interpretations of htho-phile element isotope composition variations... 

DePaolo, 1979 O Nions et al., 1979), divide the mantle into two convectively isolated layers with a boundary at 670 km. Such models incorporate the degassing of the upper mantle reservoir to the atmosphere. In order to explain the high OIB He/ He ratios, the underlying gas-rich reservoir is isolated from the degassing upper mantle. Therefore, these layered mantle models can be considered to incorporate two separate systems the upper mantle-atmosphere and the lower mantle. There is no interaction between these two systems, and the lower mantle is completely isolated except for a minor flux to OIB that marks its existence. It is further assumed that the mantle was initially uniform in noble gas and parent isotope concentrations, so that both systems had the same starting conditions. Note that various modifications to this basic scheme have been proposed, and are discussed below (Allegre et a/., 1983, 1986). 

Overall, using the basic principles of this model, the basic relationships between the isotopic compositions of the atmosphere and upper mantle cannot be explained. However, the preservation of noble gas isotope heterogeneities in convectively isolated mantle layers remains appealing. 

Overall, if a deep mantle layer is found to be geophysically viable, it must be incorporated into a comprehensive noble gas model. This will... 

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