Large exchange through the 660km discontinuity: evidence from C- and N- isotopes in super-deep diamonds

Diamond provides a unique opportunity to sample parts of the mantle that remain unaccessible by any other means. Some mineral associations in diamond, such as majoritic garnet, calcic and magnesian perovskite and manganoan ilmenite with ferropericlase have been recognised as originated from the transition zone down to the lower mantle (Stachel et al. [1]; Kaminsky et al. [2]). In addition, nitrogen is potentially a good tracer for mantle geodynamics. Exchanges between an inner reservoir (characterised by negative δ15N) via degassing at oceanic ridges with an outer reservoir (characterised by positive δ15N) via recycling at a subduction zones can lead to
nitrogen isotopic contrast in a stratified mantle.

Taking advantage of the rather common occurence superdeep mineral inclusion assemblages in diamonds from Juina (Brazil) and Kankan (Guinea), we carried out a detailed study of C and N-isotopes. There are broadly similar ranges of δ15N between upper (UM) and lower (LM) mantle diamonds from +3.8‰ down to -8.8‰ and from +9.6‰ down to -39.4‰ for repectively Juina and Kankan diamonds. Both sets of results suggests extensive material-isotopic exchange through the
660km discontinuity, this contrast with the existence of isolated lower mantle. Most of !15N values are negative and in the present upper mantle range with the exception of 3 Kankan diamond values down to -39.4‰. These very low δ15N are compatible with primitive material and provide evidence that mantle has kept heterogeneities.

Three zoned super-deep diamonds demonstrate large δ13C variations with parts which are typical of their local transition zone δ13C value. Most likely these diamonds have initiated their growth in the LM and following slow uplift in a convective mantle have equilibrated in the TZ, and in doing so
show an evolution in carbon isotopic composition.

Therefore the C- and N- independent isotopic tracers provide evidences that a significant amounts of material are exchanged across the 660km discontinuity.