Undepleted mantle

He with high He/ He ratios is often assumed to be stored in a mantle reservoir that has evolved approximately as a closed system for noble gases and has BSE parent nuclide concentrations. Since this reservoir must be isolated from degassing at mid-ocean ridges and subduction zones, it is often placed within the deeper, or lower mantle. Assigning the highest OIB He/ He ratios to this reservoir, a comparison between the total production of He and the shift in He/ He from the initial terrestrial value to the present value provides an estimate of the He concentration in this reservoir  

02 x 10 atoms He/g (see Radiogenic He section), an Iceland value of He/ He = 37 Ra and an initial value of He/ He = 120 Ra (see Mantle Noble Gas Characteristics Section), then the reservoir has 7.6 x 10 ° atoms He/g. The concentration of another noble gas is required for comparison of lower mantle noble gas abundances with the atmosphere. Using He/ Ne = 11, a concentration in a closed system lower mantle of 7x10 atoms Ne/g is obtained. A benchmark for comparison is the atmospheric Ne abundance divided by the mass of the upper mantle (1 x lO g) of 1.8 x 10 atoms Ne/g, which is much higher, and might be taken to indicate that the atmosphere source 

Nonradiogenic Ar and Xe isotope concentrations of such a lower mantle reservoir cannot be directly calculated without assuming either lower mantle Ar/Ne and Xe/Ne ratios or Ar and Xe isotopic compositions. For example, a closed system lower mantle 

Note that these closed system considerations do not require assumptions regarding the size of the reservoir, and can involve a portion of a stratified mantle or material that is distributed as heterogeneities within another mantle reservoir. It should be emphasized that while an undepleted, undegassed mantle reservoir is a component in some models, there is no direct evidence that such a reservoir does exist. 

There have been various ideas regarding the relative behaviors of the noble gases. 

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Fisher, D. E. (1985a) Noble gas data from oceanic island basalts do not require an undepleted mantle source. Nature, 316, 716-18. 

Matsuda, J., Marty, B. (1995) The 40Ar/36Ar ratio of the undepleted mantle A reevaluation. Geophys. Res. Lett., 22, 1937-40. 

The high He/ He and Ne/ Ne ratios found in Loihi are used to calculate He and Ne isotope concentrations (see Undepleted mantle section). 

Uranium is strongly concentrated in the crust (1.26-1.8 ppm ) with respect to the mantle. Estimates of U concentrations in the mantle range from 0.013ppm for the undepleted mantle to 0.032 ppm for the present mantle. ... 

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... 

Figure 24 Covariation of ) ttrium and zirconium in mantle garnets showing fields ascribed to different mantle protoUths (undepleted, depleted) and processes (high-T melt metasomatism, low-T phlogopite metasomatism) and zonation patterns from cores to rims of garnets (after Griffin et al., 1999c).

Fan J. and Kerrich R. (1997) Geochemical characteristics of aluminum depleted and undepleted komatiites and HREE-enriched low-Ti tholeiites, western Abitibi greenstone belt a heterogeneous mantle plume convergent margin environment. Geochim. Cosmochim. Acta 61, 4723—4744. 

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. 

Doin, M.-P., Fleitout, L. Christensen, U. 1997. Mantle convection and stability of depleted and undepleted continental lithosphere. Journal of Geophysical Research, 102, 2771-2787. 

Calculations of this type can, however, be turned on their head and used to argue that since the volume of mantle required to make the continental crust is inconsistent with the volume of the depleted mantle the mantle cannot be chemically layered. In that case we arrive at a model for the mantle in which there is a depleted portion, from which the continental crust has been extracted, and an undepleted portion, but the two are not confined to discrete layers. In addition trace element constraints outlined below will show that other reservoirs are also present. 

Although many models call upon a mantle with undepleted and undegassed (i.e., primitive) mantle characteristics, such as with a chondritic Th/U ratio (Galer and O Nions 1985 Turcotte et al. 2001) or undegassed noble gas concentrations (e.g., Hart et al. 1979 Kurz et al. 1982 Farley et al. 1992 Kellogg et al. 1999), there is no direct evidence that a component with all the expected associated geochemical characteristics exists. For example, OIB and MORE generally contain Nb/U and Ce/Pb ratios that are both non-chondritic and uniform. The observed ratios can only be produced by arc subduction or crust formation processes, and it appears that there are no mantle reservoirs that have not been impacted by these processes (Hofmann et al. 1986 Newsom et al. 1986 Hofmann... 

Accordingly, it is difficult to find undepleted or primitive mantle samples, in regions from which accessible material is available. This remains as a principal... 

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