Ancient Xe

Xe isotope variations have been measured in various diamond samples (Takeoka and Ozima 1978 Honda et al. 1987 McConville and Reynolds 1989 Ozima and Zashu 1991 Wada and Matsuda 1998). Excesses in both Xe and Xe have been found relative to the atmospheric Xe composition, with Xe/ °Xe and Xe/ °Xe ratios corresponding to values found for MOR (Staudacher and Allegre 1982). This indicates that the Xe has not resided in an environment with an usually high Xe/U ratio. Also, Ozima and Zashu (1991) noted that if this Xe was derived from the mantle 3 Ga ago, this constrains the 

As discussed above, there are many effects that can alter He isotope compositions. While these are clearly operative where unusual ratios are found, more subtle differences cannot be unambiguously interpreted. Therefore, while many diamonds have He with MORB He/ He ratios, those with somewhat higher values cannot necessarily be associated with another mantle domain. Diamond Ne isotope compositions have been reported by Ozima and Zashu (1988, 1991) and Honda et al. (1987). All measured values fall on the MORB correlation line, with the highest values of Ne/ Ne = 13.5+0.5 

The major conclusions that can be drawn from xenoliths studies of noble gases are  

He in ultramafic xenoliths and diamonds is located dominantly in fluid inclusions, and was likely introduced along with the C02-rich fluids. The distribution of rare gases in the xenoliths is controlled by the introduction and survival of these inclusions. 

There are relatively uniform He/ He ratios in ultramafic xenoliths, which is remarkable considering the isotopic variations in other elements such as Sr and Nd. This decoupling of He from other isotopic signatures is likely due to relatively recent addition of He to the xenolith in fluids with high concentrations of He relative to other trace elements, either within the mantle or during transport. Any radiogenic He that resided in the lithosphere prior to this addition was overwhelmed. 

---