Isotopic constraints on the nature and circulation of deep mantle C-H-O-N fluids: Carbon and nitrogen systematics within ultra-deep diamonds from Kankan (Guinea)

The metaperidotitic parageneses and the slightly negative δ13C signatures for both KK-204 and KK-207 are consistent with their formation in the lower mantle by fluids that originate either from mantle or subducted carbon sources. A carbon flux from subducted oceanic lithospheric mantle may be important in the latter case. The strictly positive δ15N signatures found both in KK-200B (δ13C > 0‰) and KK-204 (δ13C slightly negative) illustrate that surficial carbon and nitrogen are potentially recycled as deep as the asthenosphere/transition zone and the lower mantle. Calculations of the diffusive relaxation of carbon isotope heterogeneity indicate that these ultra-deep diamonds may have a relatively young age and/or experienced rapid vertical movement to shallower mantle conditions, possibly by plume-related mantle.