Alluvial diamonds from iron-saturated mantle beneath northeastern margin of Siberian craton V. Shatsky 1,2 , A. Ragozin 1,2, A. Logvinova.1,2, R. Wirth 3, V. Kalinina 1, N. Sobolev1,2 1V.S.Sobolev Institute of Geology and Mineralogy, Koptyuga ave., Novosibirsk, 630090, Russia. shatsky@igm.nsc.ru 2Novosibirsk State University, 2 Pirogova st., Novosibirsk, 630090. Russia 3Helmholtz Center, GFZ, 3.5 Surface Geochemistry, Telegrafenberg, Potsdam, 14473 Germany  

Diamond placer deposits of the northeastern region of the Siberian Platform are situated within the Paleoproterozoic Khapchan fold belt of the Olenek tectonic province Known kimberlites from this region are low-grade or non-diamondiferous. Geological scheme of the Siberian craton (modified after ​Gladkochub et al., 2006, Rosen et al., 1994, Shatsky et al., 2018). Green lines delineate boundaries of the Siberian craton and its major units. Red stars represent kimberlite fields: 1 – Kuoyka field (Obnazhennaya pipe); 2 – Muna field (Novinka and Zapolyarnaya pipes); 3 – Daldyn field (Udachnaya, Leningradskaya and Zarnitsa pipes); 4 – Alakit field (Komsomolskaya pipe); 5. Nakyn field (Nyurbinskaya, Botuobinskaya pipes); 6 – Kharamai kimberlite field; brown star- alluvial diamond deposits.

The carbon isotope composition in the Carbon isotope values and nitrogen abundances for diamonds from alluvial placers in the north-eastern region of the Siberian Craton compared to worldwide data summarised by Cartigny et al. (2001). (A) Diamonds of different groups: 1 – group 1; 2 – group 2 (Zedgenizov et al., 2011); 3 – group 3. (B) Diamonds showing marked local variations within individual diamonds. The arrows show rim ward changes. The field of metamorphic diamonds is outlined by data presented by Cartigny et al. (2001b) and Sitnikova and Shatsky (2009). Many placer diamonds plot outside the sector for kimberlitic and lamproitic diamonds as defined by the upper levels of nitrogen for a given d13C (Cartigny et al., 2001a). The diamonds of group 3 (variety V) differ from the majority of diamonds worldwide due to their high N content and light C isotopic composition. The carbon isotope composition in the rims of many diamonds with contrasting core-rim δ13С values reach typical mantle value. This carbon apparently first originate in recycled crystal materials and then was supplied by a mantle reservoir during the final stage of diamond growth. Shatsky et al., 2014

1mm We have found that in most cases black inclusions are not sulphides or graphite.   A significant portion of diamonds contains black inclusions usually interpreted as graphite or sulphides

Po 1mm iron carbides Microphotograph (a), catodoluminescence image of polished plate (b) and composite inclusions of pyrrothite and iron carbide from diamond Hi-185 Iron carbides comprise a part of the inclusions and according to TEM formed an nanocrystalline aggregate of euhedral grains. According to electron-diffraction data, two phases were identified among iron carbide grains, Fe3C and Fe7C3. HAADE image of a part of inclusions in diamond Hi-185 composed of iron carbides, native iron and graphite (a); TEM bright-field image of polycrystalline iron carbide (b) EDS spectra and indexed electro- diffraction pattern (FFT) from HREM images of iron carbide grains High-resolution TEM image showing a diamond in iron carbides. The diamond tip is coated with thin (4-8 nm) layer of graphite.

1m b a Fe-oxide iron carbides c Fe-oxide iron carbides d Microphotography (a); catodoluminescence image (b); metallic inclusions (c,d) from diamond 17-3 e iron carbide Mag f g h h As shown by studies of this inclusion by the TEM method, it consists of iron carbides which are replaced by magnetite. Olv was found in this cryastall HAADE image of part of inclusions in diamond composed of iron carbides, magnetite and indexed electron-diffraction patterns (FFT) from HREM images of magnetite (e); EDX spectra of iron carbides and magnetite (f); TEM (BF) images of nanocrystalline carbides (g); and diamonds (h)

1mm 200m You can see pseudo-secondary inclusions formed in healed fracture in diamond during original diamond growth. Compositional map give us evidence that iron carbides are replaced by magnetite. Kfsp was found in this ctystall BSE images(a,e), scanning electron microscopy x-ray compositional map for O (c,g,), Fe (d,h) and compositional maps for color superposition (b,f) of metallic inclusions in diamond Hi-76

a b c Microphotograph (a), catodoluminescence image of polished plate (b) and fractures filled with iron carbide from diamond Hi-138 The central part of these crystal contains fractures filled with iron carbides. rim core rim Nitrogen characteristic and carbon isotopic composition in diamond Hi-138

We demonstrate for the first time the presence of iron carbides in alluvial diamonds. We report the second finding of Fe7C3 in terrestrial samples. Earlier, Kaminsky and Wirth (2017) reported Fe7C3 carbide as lamella within the inclusion of carbonitride grain in lower mantle diamond. Iron carbides inclusions give as evidence that diamonds was formed under conditions were metallic iron and iron carbides are thermodynamically stable.   The fragments of diamonds in melt inclusions indicate that diamonds was in equilibrium with the carbon-saturated iron melt. According to experimental data, alloy of iron and carbon of such composition is observed at temperatures corresponding to the transition zone. Take into account that majority of the diamonds contain inclusions of eclogitic paragenesis and iron carbides are characterized by a lower content or absence of Ni impurities, we believe that the inclusions of Fe-C melts is best explained by the interaction of the subducted slabs with ambient reduced mantle (below the IW buffer).