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 He-4, Ar-40, Ne-21excess 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. Ne-20/Ne-22 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. Ne-21/Ne-22 ratios range from 0.028 to 0.06, which is attributed to nucleogenic Ne-21 from O-18(alpha, n)Ne-21 and Mg-24(n, alpha)Ne-21 reactions. The difference in Ne-20/Ne-22 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. Ar-38/Ar-36 and Kr isotopic ratios are identical to the atmospheric values within 1%. After correction for U-238- or Pu-244-fission Xe, the Xe-131-136 abundance ratios are indistinguishable from atmospheric ratios. Lighter Xe isotopes (Xe-124-128) are also likely to be atmospheric, but a final conclusion must wait until better data are obtained. In a Xe-136/Xe-130-Xe-129/Xe-130 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 Xe-131-136 is Pu-244, but not U-238, though the direct Xe isotopic measurements was not precise enough to decide unanimously the progenitor.