Thorium/uranium ratio of the Earth

  • Allègre C
  • Dupré B
  • Lewin E
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Through the study of komatiites and ophiolites, remnants of the old mantle, we review the evolution of the lead system in the depleted mantle through time. Using the230Th232Th vs.208Pb{black star}206Pb{black star} diagram, (208Pb{black star}206Pb{black star} = "radiogenic" lead ratio), we constrain the Th/U ratio of the bulk silicate Earth to 4.2. This value is also in full agreement with ratios inferred from the study of the various types of meteorites. Using this value, in the [208Pb{black star}206Pb{black star},143Nd144Nd] diagram, bulk-Earth plots on the oceanic basalt correlation line, as previously guessed. A comparison with Sr and Nd data shows that the Th/U fractionation correlates with that of Rb/Sr and Nd/Sm whereas the U/Pb fractionation is independent of other ratios. © 1986.

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  • Claude J. Allègre

  • Bernard Dupré

  • Eric Lewin

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