Noble gas state of the ancient mantle as deduced from noble gases in coated diamonds

Cores and coats of five coated diamonds, one from Botswana and four from Zaire, were separately analyzed for their noble gases. Noble gases in the diamonds are essentially of a trapped origin, including radio- and nucleogenic components such as 4He, 40Ar, 21Ne excess and excesses in Xe isotopes (129, 131–136). The fairly precise elemental and isotopic abundances allow us to infer the noble gas state in the ancient mantle. 20Ne/ 22Ne ratios are fairly constant (11.8 ± 0.4), and very close to that of SEP (solar energetic particle)-Ne, but distinctly different from the atmospheric ratio. 21Ne/ 22Ne ratios range from 0.028 to 0.06, which is attributed to nucleogenic 21Ne from 18O(α, n) 21Ne and 24Mg( n, α) 21Ne reactions. The difference in 20Ne/ 22Ne between atmosphere and mantle can be attributed to the hydrodynamic escape of hydrogen from the primitive atmosphere during the very early stage in the Earth's history. 38Ar/ 36Ar and Kr isotopic ratios are identical to the atmospheric values within 1%. After correction for 238U- or 244Pu-fission Xe, the 131–136Xe abundance ratios are indistinguishable from atmospheric ratios. Lighter Xe isotopes ( 124–128Xe) are also likely to be atmospheric, but a final conclusion must wait until better data are obtained. In a 136Xe/ 130Xe− 129Xe/ 130Xe diagram, diamond data lie on the same line as defined for MORB. The observed identical correlation for both diamonds and MORB's appears to suggest that the progenitor of the excess 131–136Xe is 244Pu, but not 238U, though the direct Xe isotopic measurements was not precies enough to decide unanimously the progenitor.