Uranium mantle

Global uranium flux calculations have typically been based on the following two assumptions (a) riverine-end member concentrations of dissolved uranium are relatively constant, and (b) no significant input or removal of uranium occurs in coastal environments. Other sources of uranium to the ocean may include mantle emanations, diffusion through pore waters of deep-sea sediments, leaching of river-borne sediments by seawater," and remobilization through reduction of a Fe-Mn carrier phase. However, there is still considerable debate... 

The uses of Th are at present limited and only a few hundred tonnes are produced annually, about half of this still being devoted to the production of gas mantles (p. 1228). In view of its availability as a by-product of lanthanide and uranium production, output could be increased easily if it were to be used on a large scale as a nuclear fuel (see below). 

These data clearly indicate that U and Th are not fractionated from each other by amphibole. This is supported by a compilation of all available data, which show that DulD-Yh is within error of unity over a range in Du values from 0.004 to 0.034. Tiepolo et al. (2000a) conclude that amphibole plays no role in fractionation of uranium from thorium in magmas or in the mantle. By the same token we predict Ao Djh-... 

Reagan MK, Volpe AM, Cashman KV (1992) and Th-series chronology of phonolite fractionation at Mount Erebus, Antartica. Geochim Cosmochim Acta 56 1401-1407 Reagan MK, Morris JD, Herrstrom EA, Murrell MT. (1994) Uranium series and beryllium isotope evidence for an extended history of subduction modification of the mantle below Nicaragua. Geochim Cosmochim Acta 58 4199-4212... 

O Hara MJ (1968) The bearing of phase equilibria studies in synthetic and natural systems on the origin and evolution of basic and ultrabasic rocks. Earth Sci Rev 4 69-133 O Nions RK, McKenzie D (1993) Estimates of mantle thorium/uranium ratios from Th, U and Pb isotope abundances in basaltic melts. Phil Trans Royal Soc 342 65-77 Oversby V, Gast PW (1968) Lead isotope compositions and uranium decay series disequilibrium in reeent volcanic rocks. Earth Planet Sci Lett 5 199-206... 

Sturm ME, Goldstein SJ, Klein EM, Karson JA, Mnrrell MT (2000) Uranium-series age constraints on lavas from the axial valley of the Mid-Atlantic Ridge, MARK area. Earth Planet Sci Lett 181 61-70 Sun S, McDonongh WF (1989) Chemical and isotopic systematics of ocean basalts implications for mantle composition and processes. In Magmatism in the Ocean Basins. Saunders AD, Norry MJ (eds) Blackwell Scientific Pnbl. Oxford, p 313-345... 

Allegre CJ, Dupre B, Lewin E (1986) Thorium/uranium ratio of the Earth. Chem Geol 56 217-227 Allegre CJ, Turcotte D (1986) Implications of a two-component marble-cake mantle. Nature 323 123-127 Asimow PD, Hirschmann MM, Ghiorso MS, O Hara MJ Stolper EM (1995) The effect of pressure-induced solid-solid phase transitions on decompression melting of the mantle. Geochim Cosmochim Acta 59 4489-4506... 

Blatter DL, Carmichael ISE (1998) Hornblende peridotite xenoliths from central Mexico reveal the highly oxidized nature of subarc upper mantle. Geology 26 1035-1038 Blundy J, Wood B (2003) Mineral-melt partitioning of uranium, thorium and their daughters. Rev Mineral Geochem 52 59-123... 

Galer SJG, O Nions RK (1985) Residence time of thorium, uranium and lead in the mantle with implications for mantle convection. Nature 316 778-782. 

Wendt Jl, Regelous M, Collerson KD, Ewart A (1997) Evidence for a contribution from two mantle plumes to island arc lavas from northern Tonga. Geology 25 611-614 Williams RW, Gill JB (1989) Effects of partial melting on the uranium decay series. Geochim Cosmochim Acta 53 1607-1619... 

Caro G, Bourdon B, Birck JL, Moorbath S (2003) Sm- Nd evidence from Isua metamorphosed sediments for early differentiation of the Earth s mantle. Nature 423 428-432 Chen JH, Wasserburg GJ (1981) The isotopic composition of uranium and lead in Allende inclusions and meteoritic phosphates. Earth Planet Sci Lett 52 1-15... 

At the same time it is quite obvious that most mantle material cannot be the source of uranium, because it was not... 

Ives et al. (79) tended to reject our hypothesis that brown colours of mixed oxides (and in particular less pure NdaOs) are due to traces of praseodymium. However, these authors noted the interesting effect that such dark colours (also of Pro,oaTho.9802) bleach in the reflection spectrum at higher T. It was noted that mantles of NdaOa alone rapidly hydrate to a pinkish powder (carbonate ) in humid air. It is weU-known that -type sesquioxides are far more reactive, and for instance dissolve almost instantaneously in aqueous acid, than cubic C-type samples. Ives et al. 19) also studied the broad continuous spectrum of the orange light emitted from Thi- 11 0 2+2/ where the oxidation state of uranium is rather uncertain. 

O Nions, R.K. and McKenzie, D., 1993. Estimates of mantle thorium/uranium ratios from Th, U and Pb isotope abundances in basaltic melts. Phil. Trans. R. Soc. Lond. A., 342 65-77. 

The imbalance between heat flow and 4He flux can also be seen from the consideration of a uranium inventory in the Earth. O Nions and Oxburgh (1983) pointed out that even though a reasonable geochemical model with 5ppb of U (K/U = 104, Th/U = 3.8) for the upper mantle can approximately account for the observed helium flux, it will yield only 3% of the observed heat flux at ridges. This result indicates that the remaining 97% of the heat flow must come from somewhere other than the upper mantle, namely either from the lower mantle or from the core or from both, whereas little extraneous 4He flux is required. This led O Nions and Oxburgh to conclude that 4He flux from the lower mantle is essentially inhibited. 

In plumes of terrestrial gases from the Earth s mantle, gases with isotopic ratios near 3He/ 4He = 20-30 times the atmospheric ratio are recorded, showing that those mantle gases are ancientand have notmixed with today s atmosphere, which is known to be rich in 4He as a result of uranium decay within the Earth followed by volcanic outgassing. 

Of particular note is that methane, and natural gas, if from great depths, should contain He from radioactive decay of the very dense uranium and thorium in the lower crust and mantle domains. This has been documented in most of the above instances. Natural gas reservoirs producing gas containing over 5%, sometimes as much as 10%, helium are long known, and they provide this purified gas for many industrial uses and lighter-than-air transportation craft. 

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