Abundance of noble gases in morb glasses: A key to the early history of the earth

Abstract

Cubic diamonds are known to contain fluid in submicrometer inclusions which are thought to originate from the mantle beneath the continental crust. Noble gas elemental and isotopic compositions of sixteen cubic diamonds from Zaire are noticeably different from those in MORB. The 3He/4He ratios observed in the diamonds are (5–12) × 10−6 and lower than the typical MORB value of 1 × 10−5. The 20Ne/22Ne ratios (≤11) and the 21Ne/22Ne ratios (≤0.06) show a rough linear correlation and have a slope that is lower than that defined for MORB. The excess 21Ne responsible for the shallower slope may be produced by nuclear reactions either in the source mantle or in the fluid trapped in the diamonds. A nucleogenic origin for excess 21Ne in the cubic diamonds relative to MORB is consistent with their lower 3He/4He ratios. Positively correlated excesses of 129Xe and 131–136Xe of up to 10% relative to the atmospheric Xe ratios are also observed. They are attributed to the decay of extinct 129I and to spontaneous fission of extinct 244Pu and/or extant 238U, respectively. Xenon isotopic data for the cubic diamonds lie on the same correlation line as that defined for MORB in the three isotope diagram of 136Xe/130Xe vs. 129Xe/130Xe. Very high 40Ar/36Ar ratios of up to 35000 are found and are similar to the expected value for the MORB source mantle. From the 3He/4He and 40Ar/36Ar ratios, some constraints can be made on the origin of cubic diamonds. The observed ratios can be explained if the cubic diamonds crystallized in a fluid which separated from a mantle source previously enriched in incompatible elements. This is consistent with the previously suggested model for the origin of fluid- bearing diamonds. The 38Ar/36Ar ratios in the cubic diamonds are slightly but significantly higher than the atmospheric ratio. This high 38Ar/36Ar ratio may represent the primordial isotopic ratio preserved in the mantle. Alternatively, the observed excess 38Ar may be attributed to the nuclear reaction 35Cl(α,p)38Ar in the fluid phase if the production rate of nucleogenic 38Ar is about ten times higher than the previously reported value and the age of the fluid inclusions is quite old (>109 yr).