Super-deep diamond from Central African Republic

Key-words: Super-deep diamond, Central African Republic, hydrous ringwoodite, Insitu C- and N- isotope composition, subduction, N aggregation state.

Diamonds and their inclusions are key geological materials that provide a unique opportunity to directly investigate the deepest regions of our planet.

Based on their formation depth, diamonds are classified in lithospheric, which formed between about 120 and 220 km depth and represent about 99% of worldwide diamond population, and sub-lithospheric or super-deep diamonds, extremely rare samples which crystallized from about 300 to more than 800 km depth (Stachel et al., 2008).

Here, we have investigated a 1.3 carats diamond, Type IaAB (determined by FTIR), from an alluvial deposit located in Central African Republic, close to the Ubangy River. As far as we know, this is the first study dedicated to inclusions in diamonds from this country.

The investigated diamond contains the second world finding of hydrous ringwoodite after the one found within a Brazilian diamond by Pearson et al. (2014). This finding indicates that our diamond is certainly a super-deep diamond coming from the lower part of the transition zone (between 525 and 660 km depth). Carbon isotope composition of the host diamond (δ¹³C_mean = -2.2 ± 0.3 ‰, n=16, analytical error = 0.3‰ (2σ)) is significantly enriched in heavy isotope when compare to the canonical mantle value (δ¹³C = -5‰). It is nitrogen poor (N < 44 ± 23 at.p.p.m., mean = 15 at p.p.m.) and partially aggregated (%B= 88.5 %). For N content greater that our analytical precision (23 p.p.m.) we performed N-isotope measurement and the values, although associated to large analytical uncertainties, are all positive, (d¹⁵N = 3.48 ± 3.5 ‰) and significantly enriched in heavy isotope compare with the mantle values (-5‰). These geochemical signatures are similar with those previously found in super-deep diamonds (Stachel et al., 2002). These data are consistent with a diamond forming fluid originating from a N-poor subducted source, such as carbonates, (e.g. Walter et al., 2011), in agreement with studies reporting transition-zone and lower-mantle diamonds (Nestola et al., 2018).

 

References

 

Nestola F., Korolev N., Kopylova M., Rotiroti N., Pearson D.G., Pamato M.G., Alvaro M., Peruzzo L., Gurney J.J., Moore A.M. & Davidson J. 2018. CaSiO₃ perovskite in diamond indicates the recycling of oceanic crust into the lower mantle. Nature, 555, 237-241.

Pearson D.G., Brenker F.E., Nestola F., McNeill J., Nasdala L., Hutchison M.T., Matveev S., Mather K., Silversmith G., Schmitz S., Vekemans B. & Vincze L. 2014. Hydrous mantle transition zone indicated by ringwoodite included within diamond. Nature, 507, 221-224.

Stachel T., Harris J.W., Aulbach S. & Deines P. 2002. Kankan diamonds (Guinea) III: δ¹³C and nitrogen characteristics of deep diamonds. Contrib. Mineral. Petrol., 142, 465-475.

Stachel T. & Harris J.W. 2008. The origin of cratonic diamonds -Constraints from mineral inclusions. Ore Geol. Rev., 34, 5-32.

Walter M.J., Kohn S.C., Araujo D., Bulanova J.P., Smith C.B., Gaillou E., Wang J., Steele A. & Shirey S.B. 2011. Deep Mantle Cycling of Oceanic Crust: Evidence from Diamonds and Their Mineral Inclusions. Science, 334, 54-57.