Discussion“Recycled” volatiles in mantle-derived diamonds- Evidence from nitrogen and noble gas isotopic data

Noble gas isotopic data from diamonds are commonly interpreted as a two component mixture of gases from the mantle source of mid oceanic ridge basalt (MORB) and air. The air-like component in diamonds is generally considered to have been acquired secondarily through atmospheric contamination. In a recent study [C. Gautheron, P. Cartigny, M. Moreira, J.W. Harris, C.J. Allègre, Evidence for a mantle component shown by rare gases, C and N isotopes in polycrystalline diamonds from Orapa (Botswana), Earth Planet. Sci. Lett. 240(2005) 559-572.] that involved analyses of noble gases, carbon and nitrogen, such interpretation based on noble gases is used to constrain the sources of carbon and nitrogen in diamond to be solely from the mantle.With the help of heavy noble gas and nitrogen isotopic data from diamonds, we show here that the air-like noble gas component in diamonds is actually a mixture of contributions derived from recycled materials and (minor) air contamination. This, in turn, suggests that some of the carbon in diamonds may also have a recycled origin.