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)

Sublithospheric diamonds that sample the transition zone and uppermost lower mantle provide a unique view into the deep Earth. In order to investigate the origin of diamond-forming C-H-O-N fluids within the deep mantle, within the framework of the terrestrial deep volatile cycle, we conducted a delta C-13-delta N-15-[N] micro-analytical study, by secondary ion mass spectrometry, of five Kankan diamonds from the asthenosphere/transition zone and the lower mantle. Abrupt and large changes in delta C-13 within KK-99 (up to 10.2 parts per thousand) and KK-200A (up to 6.9 parts per thousand) illustrate distinct episodes of diamond growth, involving different fluids, possibly during transport of diamond to deeper mantle depths from the asthenosphere/transition zone into the lower mantle. Despite limited variability of delta C-13 within individual samples, diamonds KK-200B, KK-203, KK-204 and KK-207 display systematic delta C-13-delta N-15-[N] co-variations which can be modelled as a single diamond growth episode in a Rayleigh process from fluids/melts. These data constrain the carbon isotopic fractionation factors to be both negative (Delta(C) = 0.9 parts per thousand for KK-200B and 2.0 parts per thousand for both KK-203 and 207) and positive (Delta(C) = +1.0 parts per thousand for KK-204), consistent with equilibrium between diamond and oxidised (CO2 or carbonate) and reduced (CH4 or carbide) fluids respectively. The modelling of delta N-15-[N] systematics suggests that the diamonds are depleted by similar to 4 parts per thousand (KK-200B) and similar to 0 parts per thousand (KK-204) relative to the oxidised and reduced sources respectively. Modelling the co-variation indicates a compatible behaviour of nitrogen in diamond relative to the growth medium (K-N = 4-16), independent of the redox state. The parental fluids to the ultra-deep diamonds exhibit geochemical characteristics (delta C-13-delta N-15-[N]-K-N-Delta(C)-Delta(N)) comparable to fluids thought to form lithospheric diamonds, suggesting a common mechanism of diamond genesis. The metaperidotitic parageneses and the slightly negative delta C-13 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 delta N-15 signatures found both in KK-200B (delta C-13 > 0 parts per thousand) and KK-204 (delta C-13 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. (C) 2014 Elsevier Ltd. All rights reserved.