Bulkur phreato-magmatic diamond deposits – new incites for origin from the geochemistry and thermobarometry.

The phreatomagmatic Carnian (Upper Triassic) kimberlitic tuff deposit of the Bulkur anticline at the right side of the Lena Mouth has the most high diamond grade in Russia (to 12 crt/t). Comparison of the  thermobarometric reconstructions Ashchepkov ea 2001; Grakhanov ea., 2024) and geochemistry of pyropes (Skuzovatov et al., 2022) and presented here set of the EPMA (560), SEM (980) and LA ICP (140) analyses  from tuffs at Olenek’s duct at  Lena Mouth. Pyropes variations  0<Cr2O3<13% are similar to   (Grakhanov et al., 2024) but TiO2 are higher  to 2%.

 Chromites are  Ti -  Al rich - varieties, compared to (Biller et al., 2017). Micas from Bulkur refer to reaction of peridotite with K- rich melt (3.5-5 % FeO and Cr2O3 to 3%)  similar to range in orangeites or lamproites (Downes ea, 2006).  All   ilmenites ( MgO < 4%)   are not deep-seated. The Cr- diopsides divides to the Cr-Al rich and Fe- enriched types.

The PT diagram show 8 layers and comparing to (Ashchepkov et al., 2017) and (Grakhanov et al., 2024) show straight line P (6.5-2GPa)-Fe# (0.11-0.15) for megacrystic pyropes. The eclogitic inclined P- Fe# trend is less abundant than in previous sets. And Ca -rich layer in middle Eclogite layer 4-5 GPa chromites are Ti to 6% from 6.5-3. GPa

The Geochemistry of almost all pyropes despite on variations in REE from U to S- shaped (dunitic) and HMREE low harzburgitic and rounded lherzolitic varieties almost all show high U-Th-Nb-Ta- levels. There are extreme Zr-Hf types those with elevated Zr-Hf. Even Cr- highest pyropes reveal high HFSE. As well as Cpxs and Amphs with Nb peaks and Chrs – Ta peak. All carbonates in tuffs are magmatic showing high REE gently increasing Yb to La ~ 5000*C1. The mantle column beneath Bulkur was reacted with the HFSE -rich K-rich aillikite melt. The studied phase is late comparing to studied before. Grant RNF 24-27-00411.

Ashchepkov, I. V., Vladykin, N. V., Ivanov, A., Babushkina, S., Vavilov, M., & Medvedev, N. 2021, Ma Problems of mantle structure and compositions of various terranes of Siberian Craton. In Alkaline Rocks, Kimberlites and Carbonatites: Geochemistry and Genesis. pp. 15-48. Cham: Springer International Publishing.

Skuzovatov, S., Shatsky, V. S., Ragozin, A. L., & Smelov, A. P. 2022. The evolution of refertilized lithospheric mantle beneath the northeastern Siberian craton: Links between mantle metasomatism, thermal state and diamond potential. Geoscience Frontiers, 13(6), 101455.

Grakhanov, S. A., Goloburdina, M. N., Ivanov, A. S., & Ashchepkov, I. V. (2024). Mineralogical and petrographic characteristics of diamondiferous formations of the Bulkur anticline, Republic of Sakha (Yakutia). Regional Geology and Metallogeny, (98), 41-63.