Super-deep" diamonds are thought to have a sub-lithospheric origin (i.e., below ~300 km depth) because some of the mineral phases entrapped within them as inclusions are considered to be the products of retrograde transformation from lower-mantle or transition-zone precursors. CaSiO 3 -walstromite, the most abundant Ca-bearing mineral inclusion found in super-deep diamonds, is believed to derive from CaSiO 3 -perovskite, which is stable only below ~600 km depth, although its real depth of origin is controversial. The remnant pressure (P inc ) retained by an inclusion, combined with the thermoelastic parameters of the mineral inclusion and the diamond host, allows calculation of the entrapment pressure of the diamond-inclusion pair. Raman spectroscopy, together with X-ray diffraction, is the most commonly used method for measuring the P inc without damaging the diamond host.In the present study we provide, for the first time, a calibration curve to determine the P inc of a CaSiO 3 -walstromite inclusion by means of Raman spectroscopy without breaking the diamond. To do so, we performed high-pressure micro-Raman investigations on a CaSiO 3 -walstromite crystal under hydrostatic stress conditions within a diamond-anvil cell. We additionally calculated the Raman spectrum of CaSiO 3 -walstromite by ab initio methods both under hydrostatic and non-hydrostatic stress conditions to avoid misinterpretation of the results caused by the possible presence of deviatoric stresses causing anomalous shift of CaSiO 3 -walstromite Raman peaks. Last, we applied single-inclusion elastic barometry to estimate the minimum entrapment pressure of a CaSiO 3 -walstromite inclusion trapped in a natural diamond, which is ~9 GPa (~260 km) at 1800 K. These results suggest that the diamond investigated is certainly sub-lithospheric and endorse the hypothesis that the presence of CaSiO 3 -walstromite is a strong indication of super-deep origin.

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Natural diamonds and their inclusions provide unique glimpses of mantle processes from as deep as ~800 km and dating back to 3.5 G.y. Once formed, diamonds are commonly interpreted to travel upward, either slowly within mantle upwellings or rapidly within explosive, carbonate-rich magmas erupting at the surface. Although global tectonics induce subduction of material from shallow depths into the deep mantle, mineralogical evidence for downward movements of diamonds has never been reported. We report the finding of an unusual composite inclusion consisting of ringwoodite (the second finding to date), tetragonal zirconia, and coesite within an alluvial super-deep diamond from the Central African Republic. We interpret zirconia + coesite and ringwoodite as prograde transformation products after zircon or reidite (ZrSiO4) and olivine or wadsleyite, respectively. This inclusion assemblage can be explained if the diamond traveled downward after entrapping olivine/wadsleyite + zircon/reidite, dragged down by a subducting slab, before being delivered to the surface. This indicates that the commonly assumed view that diamonds form at, and capture material from, a specific mantle level and then travel upward is probably too simplistic.

<p><strong>Key-words: </strong>Super-deep diamond, Central African Republic, hydrous ringwoodite, Insitu C- and N- isotope composition, subduction, N aggregation state.</p><p>Diamonds and their inclusions are key geological materials that provide a unique opportunity to directly investigate the deepest regions of our planet.</p><p>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).</p><p>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.</p><p>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 (δ<sup>13</sup>C<sub>mean</sub> = -2.2 ± 0.3 ‰, n=16, analytical error = 0.3‰ (2σ)) is significantly enriched in heavy isotope when compare to the canonical mantle value (δ<sup>13</sup>C = -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, (d<sup>15</sup>N = 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).</p><p> </p><p><strong>References</strong></p><p> </p><p>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. CaSiO<sub>3</sub> perovskite in diamond indicates the recycling of oceanic crust into the lower mantle. Nature, 555, 237-241.</p><p>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.</p><p>Stachel T., Harris J.W., Aulbach S. & Deines P. 2002. Kankan diamonds (Guinea) III: δ<sup>13</sup>C and nitrogen characteristics of deep diamonds. Contrib. Mineral. Petrol., 142, 465-475.</p><p>Stachel T. & Harris J.W. 2008. The origin of cratonic diamonds -Constraints from mineral inclusions. Ore Geol. Rev., 34, 5-32.</p><p>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.</p><p> </p><p> </p>