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)

Abstract

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 δ13C-δ15N-[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 δ13C within KK-99 (up to 10.2‰) and KK-200A (up to 6.9‰) 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 δ13C within individual samples, diamonds KK-200B, KK-203, KK-204 and KK-207 display systematic δ13C–δ15N–[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 (ΔC=−0.9‰ for KK-200B and −2.0‰ for both KK-203 and −207) and positive (ΔC=+1.0‰ for KK-204), consistent with equilibrium between diamond and oxidised (CO2 or carbonate) and reduced (CH4 or carbide) fluids respectively. The modelling of δ15N–[N] systematics suggests that the diamonds are depleted by ∼4‰ (KK-200B) and ∼0‰ (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 (KN=4–16), independent of the redox state. The parental fluids to the ultra-deep diamonds exhibit geochemical characteristics (δ13C–δ15N–[N]–KN–ΔC–ΔN) comparable to fluids thought to form lithospheric diamonds, suggesting a common mechanism of diamond genesis.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.