Nature 363, 63 - 65 (06 May 1993); doi:10.1038/363063a0

Uranium–thorium disequilibria and partitioning on melting of garnet peridotite

Paul Beattie

Department of Earth Sciences, University of Cambridge, Cambridge CB2 3EQ, UK

THE abundances of isotopes in the ²³⁸U decay series can be used as both tracers and chronometers of magmatic processes. In the subsolidus asthenosphere, the activity of each daughter isotope (defined as the product of its concentration and decay constant, and denoted by parentheses) is assumed to be equal to that of its parent. By contrast, (²³⁰Th/²³⁸U) is greater than unity in most recent mid-ocean-ridge and ocean-island basalts¹, implying that thorium is more incompatible (that is, it is partitioned into the melt phase more strongly) than uranium. Melting of spinel peridotite cannot produce the (²³⁰Th) excesses, because measured partition coefficients for pyroxenes and olivine demonstrate that uranium is more incompatible than thorium for this rock². Here I report garnet–melt partitioning data which show that for this mineral-melt pair thorium does behave more incompatibly than uranium, thus supporting the suggestion that mid-ocean-ridge basalts (MORB) are produced by melting initiated at depths where garnet is stable^3–6. Using these data, I show that the observed (²³⁰Th/²³⁸U) ratios of MORB and most ocean-island basalts can be explained by slow, near-fractional melting initiated in the garnet stability field.

References

1.	Gill, J. B., Pyle, D. M. & Williams, R. W. in Uranium-Series Disequilibrium (eds Ivanovich, M. & Harmon, R. S.) 207−258 (Clarendon, Oxford, 1992).
2.	Beattie, P. D. Earth planet Sci. Lett. (in the press).
3.	Salters, V. J. M. & Hart, S. R. Nature 342, 420−422 (1989). | Article | ISI | ChemPort |
4.	Johnson, K. T. M., Dick, H. J. B. & Shimizu, N. J. geophys. Res. 95, 2661−2678 (1990). | ISI |
5.	McKenzie, D. P. & O'Nions, R. K. J. Petrol. 32, 1021−1091 (1991). | ISI | ChemPort |
6.	White, R. S., McKenzie, D. & O'Nions, R. K. J. geophys. Res. 97, 19683−19715 (1992). | ISI |
7.	Beattie, P. D. Geochim. cosmochim. Acta 57, 47−55 (1993). | Article | ChemPort |
8.	Takahashi, E. J. geophys. Res. 91, 9367−9382 (1986). | ISI | ChemPort |
9.	Burton, K. W. & O'Nions, R. K. Earth planet. Sci. Lett. 107, 649−671 (1991). | Article | ChemPort |
10.	Günther, M. & Jagoutz, E. Fifth Int. Kimberlite Conf. (ext. Abstr.) 122−124 (1991).
11.	Sun, S.-S., Nesbitt, R. W. & Sharaskin, A. Y. Earth planet. Sci. Lett 44, 119−138 (1979). | Article | ISI | ChemPort |
12.	McKenzie, D. P. Earth planet. Sci. Lett. 72, 149−157 (1985). | Article | ChemPort |
13.	Williams, R. W. & Gill, J. B. Geochim. cosmochim. Acta 53, 1607−1619 (1989). | Article | ISI | ChemPort |
14.	Condomines, M., Morand, P. & Allègre, C. J. Earth planet. Sci. Lett. 55, 247−256 (1981). | Article | ISI | ChemPort |
15.	Newman, S., Finkel, R. C. & Macdougall, J. D. Earth Planet. Sci. Lett. 65, 17−33 (1983). | Article | ISI | ChemPort |
16.	Rubin, K. H. & Macdougall, J. D. Nature 335, 158−161 (1988). | Article | ChemPort |
17.	Goldstein, S. J., Murrell, M. T. & Janecky, D. R. Earth Planet. Sci. Lett. 96, 134−146 (1989). | Article | ChemPort |
18.	Reinitz, I. & Turekian, K. K. Earth Planet. Sci. Letts 94, 199−207 (1989). | Article | ChemPort |
19.	Walker, D., Shibata, T. & DeLong, S. E. Contrib. Mineral. Petrol. 70, 111−125 (1979). | ChemPort |
20.	Sparkes, D. W. & Parmentier, E. M. Earth planet Sci. Lett 105, 368−377 (1991). | Article | ISI |
21.	Larson, R. L. & Chase, C. G. Earth planet. Sci. Letts. 7, 425−428 (1970). | Article |
22.	Oversby, V. M. & Gast, P. W. Earth planet. Sci. Lett. 5, 199−206 (1968). | ISI | ChemPort |
23.	Condomines, M., Morand, P., Allègre, C. J. & Sigvaldasson, G. Earth planet Sci. Lett. 55, 393−406 (1981). | Article | ISI | ChemPort |
24.	Krishnaswami, S., Turekian, K. K. & Bennett, J. T. Geochim. cosmochim. Acta 48, 505−511 (1984). | Article | ChemPort |
25.	Newman, S., Finkel, R. C. & Macdougall, J. D. Geochim. cosmochim. Acta 48, 315−324 (1984). | Article | ISI | ChemPort |
26.	Hemond, C. H. et al. Earth planet. Sci. Lett. 87, 273−285 (1988). | Article | ChemPort |
27.	Condomines, M., Bernat, M. & Allègre, C. J. Earth planet. Sci. Lett. 33, 122−125 (1976). | Article | ISI | ChemPort |
28.	Reinitz, I. & Turekian, K. K. Geochim. cosmochim. Acta 55, 3735−3740 (1991). | Article | ChemPort |
29.	Williams, R. W. & Gill, J. B. Geophys. Res. Lett. 19, 139−142 (1992). | ChemPort |
30.	Watson, S. & McKenzie, D. J. Petrol. 32, 501−537 (1991). | ISI | ChemPort |