An Experimental Attempt to Determine History of 3He, 4He, and Isotopes of Ne and Ar in the Mantle

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This project will determine the history of He, Ne and Ar isotopes in the mantle, based on studies of diamonds and associated mantle samples. When this study was undertaken, it was believed by some that certain diamonds bore records of pristine helium isotopic composition. It is how seems unlikely that diamonds could carry vestigial records of He composition at genesis of the Earth; most likely diamonds having high 3He/4He ratios acquired their3He during their residence in the alluvium by cosmic ray interactions, or by interactions of radiogenic neutrons with lithium if present in the diamond at concentrations exceeding 1 ppm. Discovery of the cosmogenic 3He component in alluvial diamonds, observations of very large helium diffusivity in diamonds, with a very strange release pattern, and other observations urgently warrant extension of helium studies, including Ne (and Ar) isotopes. The isotopic composition of Ne may indeed have a bearing on the formation of old diamonds. Also it is planned to determine cosmic ray exposure histories of alluvial diamonds using 21Ne and 10Be as tracers. The lab is set up for 3He measurements; low blank systems were built under the previous NSF grant. Neon isotopic measurements can now be carried out with the required precision since the Isotope Laboratory will soon be acquiring a triple collector N2-Ne mass spectrometer under a grant from NSF.