Mantle isotopic reservoirs

Graham D. W. (2002) Noble gas isotope geochemistry of midocean ridge and ocean island basalts characterization of mantle source reservoirs. Rev. Mineral. Geochem. 47, 247-317. 

Dixon et al. (2002) take this approach one step further, and use new data on water contents, trace element and isotope geochemistry of Atlantic basalts to argue that mantle endmember reservoirs or components, previously defined on the basis of their isotope geochemistry, have different water contents. They propose that depleted MORE mantle (DMM) has 100 ppm H2O, FOZO ( focus zone component of Hart et al., 1992) has 750 ppm H2O, and enriched mantle (EM) has approximately half the H2O concentration as FOZO. 

It is clear that the Earth s mantle has at least two Os-isotopic reservoirs - a plume-related isotopically enriched reservoir and a chondritic upper mantle reservoir. Both have long histories (Fig. 3.32). The variations in composition within the upper mantle reservoir reflect Re-depletion and enrichment related to melt extraction. The isotopically enriched plume reservoir represents chemically isolated, rhenium-enriched, recycled oceanic lithosphere. There is some evidence to suggest that this enriched reservoir may have been in existence since the early Archaean (Walker Nisbet, 2002) and was the source of some Archaean komatiites and the 3.81 Ga Itsaq Gneiss chromitites. If this is true, then basaltic crust was being created and recycled even before 4.0 Ga. Estimates of the present size of this high Re/Os basaltic reservoir vary from 5% to >10% of the whole mantle (Bennett et al., 2002 Walker et al., 2002). 

The simplest way of thinking about the relationship between the continental crust and mantle is to use a three reservoir box model, in which an initial primitive mantle composition (reservoir 1) is progressively differentiated through time into a depleted mantle reservoir (reservoir 2) and the continental crust (reservoir 3). This simple approach provides a good explanation of the Nd- and Sr-isotope compositions of the mantle and crust. On a Nd-Sr isotope plot, crustal compositions are complementary to the mantle isotopic array (Fig. 4.21a) relative to the composition of the primitive mantle. Expressing the Nd-isotope data in a slightly different manner, an eNd versus time... 

Particularly powerful in this respect are Hf (hafnium)-isotopes, for when these data are expressed as eHf, they are more sensitive than their geochemical twin eNd to the processes of mantle differentiation. Bizzarro et al. (2000) demonstrated this for carbonatite melts and showed that some carbonatites have a Hf-iso-tope signature which indicates that they are derived from an ancient, enriched mantle reservoir, with an entirely different character from that of depleted mantle. This reservoir has subchondritic Lu/Hf and suprachondritic Sm/Nd and is thought to represent an ancient mafic component. The existence of such an unradiogenic-Hf reservoir would also explain the mismatch between the Nd-Hf composition of modern depleted mantle and the BSE (Fig. 3.28). Bizzarro et al. (2000) suggest that it could account for up to 10-15% of the total mass of the silicate Earth. Support for such a model has been equivocal. 

Noble Gas Isotope Geoehemistry of Mid-Oeean Ridge and Oeean Island Basalts Charaeterization of Mantle Souree Reservoirs... 

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... 

McDonough WF, McCulloch MT (1987) The southeast Australian lithospheric mantle isotopic and geochemical constraints on its growth and evolution. Earth Planet Sci Lett 86 327-340 McKenzie D, O Nions RK (1983) Mantle reservoirs and ocean island basalts. Nature 301 229-231... 

In terms of oxygen isotopes, on a planetary scale, the isotopic (and chemical) composition of the ocean is dominated by the competition between two major processes exchange between mantle-derived reservoirs at midocean ridges and the exchange with silicate rocks at low temperature due to chemical weathering. The change in the isotopic composition of the ocean can be described by a differential eqiration of the form ... 

Barling, J. Goldstein, S. L. (1989). Extreme isotopic variations in Heard Island lavas and the nature of mantle reservoirs. Nature, 348, 59-62. 

The mantle, although Li-poor relative to the continents (5-6 ppm in normal MORE and c. 1 ppm in depleted peridotites Ryan and Langmuir 1987 Eggins et al. 1998), is a significant reservoir due to its large volume (discussed below under, Significance of lithium isotopes in the bulk Earth ). 

Figure 9. Plots of Li and radiogenic isotopes for mantle rocks, (a) 5 Li vs. Sr/ Sr (b) 5 Li vs. Nd/ Nd (c) "Sr/ Sr vs. Pb/ Pb (d) 5"Li vs. Pb/ Pb (Nishio et al. 2003, 2004). Symbols + = south Pacific island basalts (six islands) O = Iherzolite xenolith, Bullenmerri, Australia = Iherzolite xenolith, Sikhote-Alin, Russia (three localities) A = dunite-peridotite-pyroxenite xenolith, Kyushu, Japan (two localities) V = Iherzolite xenolith, Ichinomegata, Japan. The ocean island data are from bulk rocks, the xenolith data are clinopyroxene separates. For explanations of the derivation of radiogenic isotope fields (DM, EMI, EM2, HIMU), see Zindler and Hart (1986). The estimate for Li isotopes in DM is based on MORE. The Li isotopic ranges for the other mantle reservoirs are based on Nishio et al. (2004) and Nishio et al. (2003), but these will require further examination (hence the use of question marks).

Measurements of terrestrial Mg isotope ratios on a plot of A Mg vs. 5 Mg are all within the region bounded by the equilibrium and kinetic mass fractionation laws given expected uncertainties (Fig. 5). Apparently, all of the terrestrial reservoirs represented by the data thus far are related to the primitive chondrite/mantle reservoir by relatively simple fractionation histories. Adherence of the data to the regions accessible by simple mass fractionation processes in Figure 5 (the shaded regions in Fig. 3) is testimony to the veracity of the fractionation laws since there is no reason to suspect that Mg could be affected by any processes other than purely mass-dependent fractionation on Earth. 

The analysis of fractionation law exponents quantifies the impression from the A -5 plots that aqueous Mg is related to primitive mantle and average crustal Mg by kinetic processes while carbonates precipitated from waters approach isotopic equilibrium with aqueous Mg. In any case, the positive A Mg values of carbonates relative to the primitive chondrite/mantle reservoir and crust is a robust feature of the data and requires a component of kinetic Mg isotope fractionation prior to carbonate formation, as illustrated schematically in Figure 3. 

The high precision with which Mg isotope ratios can be measured using MC-ICPMS opens up new opportunities for using Mg as a tracer in both terrestrial and extraterrestrial materials. A key advance is the ability to resolve kinetic from equilibrium mass-dependent fractionation processes. From these new data it appears that Mg in waters is related to mantle and crustal reservoirs of Mg by kinetic fractionation while Mg in carbonates is related in turn to the waters by equilibrium processes. Resolution of different fractionation laws is only possible for measurements of Mg in solution at present laser ablation combined with MC-ICPMS (LA-MC-ICPMS) is not yet sufficiently precise to measure different fractionation laws. 

In order to develop a complete understanding of the distribution of chlorine-isotopes in Earth s exogenic (oceans, crust, and atmosphere) and mantle reservoirs it is necessary to... 

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