Evolution of lithospheric mantle beneath Bulkur diamond orangeitic tuff deposits NE Siberian Craton: Insights from geochemistry and thermobarometry of xenocrysts

• Debris of carbonatites, kimberlites and aillikite, rare lamproites in Bulkur tuffs. • NE margin of Siberian Craton enriched in HFSE and K 2 O by subduction-related melts. • Extreme enrichments in HFSE of pyropes produced by carbonatitic melts. • Progressive reactions accompanied by protomagmatic channels from LAB to spinel facies. • Abundance of diamonds in Bulkur mantle is due to eclogites reacted with peridotites. The xenocrysts of studied Bulkur tuff site 1 (Skuzovatov et al.,2022; Grakhanov et al., 2024) with highest diamond grade are comparing with xenocrysts from new site 2 of phreatomagmatic Carnian (Lower Triassic) tuffs in Tuora Sis ridge. New set includes electron probe microanalyses (EPMA, 420 grains), scanning electron microscope (SEM, 1750 grains) and (laser ablation inductively coupled plasma mass spectrometry) LA ICP MS (180) analyses. Pyropes (0-13 wt.% Cr 2 O 3 ), chromites (Cr-rich and Ti-Al rich varieties), micas (3.5-5% FeO and Cr 2 O 3 to 3%) from site 2 reflect reactions of peridotites with K-rich melts, carbonatites and later protokimberlites which formed Mg rich ilmenites and T-augites. The Cr-diopsides divides to the Cr-Al and Fe- enriched types. The PTXFO2 diagram for mantle lithosphere site 2 show 8 layers similar to received for site 1 and long pyrope megacrysts trend P (6.5-2 GPa)-Fe# (0.11-0.15). The eclogitic inclined P- Fe# trend (Cr-bearing) 6.5-4 GPa is more typical to site 2, it corresponds to AFC reactions of partial eclogitic melts with peridotites going to Ca-rich layer in middle mantle pyroxenite-eclogite layer 4-5 GPa. Chromites show common Al-Cr and also ulvospinel trend starting from lithosphere base (TiO 2 ∼12%). The geochemistry of almost all pyropes show high U-Th-Nb-Ta and extreme Zr-Hf peaks in multicomponent spiderdiagrams (MSD). Chromites reveal Nb and Ta peaks. Cr-phlogopites with peaks of large ion lithophile components show HFSE depressions like Cr-diopsides and megacrystic Cpx. The carbonatite debris in tuffs display high REE gently increasing up to La n ∼ 5000/Cl (chondrite). The glasses in rock debris from tuffs vary from Ca- to Ca-Mg carbonatites to Mg-aillikites, orangeites (rarely lamproites) and kimberlites. The mantle column beneath Bulkur anticline (at Tuora Sis ridge) was reacted at first with silicate and adakite melts created refertilisation of peridotites, then with the K-rich melts created phlogopites and with HFSE rich carbonatite melts progressively increasing Zr, Hf, Nb, Ta. Studied phase (site 2) is later and reflect progressive enrichment of whole mantle column from lithosphere base (highest) in HFSE, 5 times higher then in site 1. The primary contamination of the lithospheric roots in K 2 O and HFSE was a result of the interaction with subduction-related melts contaminated in continental sediments.