Unusual noble gas compositions in polycrystalline diamonds: preliminary results from the Jwaneng kimberlite, Botswana

Abstract

We have undertaken noble gas analyses of four polycrystalline framesite diamonds from the Jwaneng kimberlite pipe, Botswana. These samples yielded complex, multiple noble gas components (crustal, atmospheric and in situ radiogenic/fissiogenic), which were successfully deconvoluted by combining vacuum crushing and step-heating experiments and examining a full suite of noble gas isotope and elemental abundances. The most striking observation is the presence of crustal nucleogenic neon, released on graphitisation of the framesites. Neon of this composition can only have been produced in the crust and subsequently incorporated during formation of the framesites in the mantle. This may indicate that noble gases produced in the crust, such as nucleogenic neon and possibly xenon, and some radiogenic4He and40Ar and fissiogenic xenon, were introduced into the subcontinental mantle source during ancient subduction-related processes. If correct, then some parts of the mantle may contain significant quantities of crustal noble gases.