ISOTOPIC CONSTRAINTS ON THE NATURE AND CIRCULATION OF DEEP MANTLE C-H-O-N FLUIDS: CARBON AND NITROGEN SYSTEMATICS WITHIN SUPER DEEP DIAMONDS FROM KANKAN (GUINEA)

Diamond, as the deepest sample available for study, provides a unique opportunity to sample and examine parts of the Earth’s mantle not directly accessible. The vast majority of diamonds are derived from near the base of the lithosphere, only a small fraction originating within the asthenosphere/transition zone (300-660 km depth) and lower mantle (>660 km depth) – the so-called “ultra-deep diamonds”. The origin and the transport of these ultra-deep diamonds are still a matter of debate.

Here we focus on a suite of diamonds from the Kankan locality, Guinea (West Africa), because the alluvial deposits in Kankan represent one of the major occurrences for super deep diamonds and several studies have focused on the isotopic characterisation of the diamonds and the crystal structure and chemistry of their mineral inclusions. In order to investigate the origin of diamond-forming C-H-O-N fluids within the deep mantle, we conducted a carbon and nitrogen micro-analytical study (δ¹³C‑δ¹⁵N‑[N]), 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 and sources of diamond growth, possibly during transport of diamond to deeper mantle depths from the asthenosphere/transition zone into the lower mantle. The 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 oxidised (CO₂ or carbonate) and reduced (CH₄ or carbide) fluids/melts. The parental fluids to the ultra-deep diamonds exhibit geochemical characteristics (δ¹³C‑δ¹⁵N‑[N]-K_N-Δ_C-Δ_N) comparable to fluids thought to form lithospheric diamonds, suggesting a common mechanism for diamond genesis.

The strictly positive δ¹⁵N signatures found both in KK-200B and KK-204 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 in diamonds indicate that these deep diamonds may have a relatively young age and/or experienced rapid vertical movement to shallower mantle conditions, possibly by plume-related mantle.