Mineralogy and geochemistry of the xenocrysts from Aykhal kimberlite pipe, Yakutia: Mantle reconstructions and comparison of phases

• In Aykhal pipe, later tuffisitic breccia shows wide mineral compositions. • Deeper material in the earlier breccia but higher metasomatism in the later phase. • The highest diamond grade in the Daldyn-Alakite region. • Mineral geochemistry evidence for subduction-related metasomatism in early stages. • Aykhal and Komsomolskaya pipes evidences for plume-related and H₂O metasomatism. Minerals separated from heavy mineral concentrates from two phases of the Aykhal kimberlite pipe, Yakutia, were analyzed with the EPMA, SEM, and ICP-MS. They were used to reconstruct the mantle sections and their evolution and to determine the features of the protokimberlite melts and melt/fluid metasomatic agents responsible for the geochemistry of the minerals. The clinopyroxenes, as well as amphiboles, show mostly Mg-rich compositions and high variation of Al, Cr, Ti, and Na. Micas refer mainly to Ti-biotites derived from protokimberlites. The ilmenites and chromites also show long trends with domination of Mg- and Cr-rich compositions. The mantle section of subcratonic lithospheric mantle (SCLM) determined for autholitic kimberlite breccia (AKB) reveals a long range of PT estimates for garnets from 8 GPa to Moho heated at the deeper part. It shows in the P-Fe# plot sharp layering compiled by 6 thick layers (divided into 2 sub-layers each) visible by high Mg deviations and Ca fluctuations for peridotitic and eclogitic garnets and grouping of PT points for other minerals. The lithosphere-asthenosphere boundary (LAB) is marked by the ilmenite trend going from LAB to the middle layer (4.5-3.5 and sporadically to 2.3 GPa), traced also by the Ti-augite and pyrope megacrysts. The minerals from the next phase, represented by tuffisitic kimberlitic breccia (TKB), show a similar division of the mantle section. The amount of low-pressure pyrope and eclogite garnets is much higher in TKB, and the pycroilmenite and accompanied Ti-augite trends rise to the garnet-spinel boundary. The geochemistry of lherzolitic garnets having rounded curves of depletion in light rare earth elements (LREE) in the REE diagram allows them to be subdivided into the enriched, depleted, and common lherzolitic types according to the level of the REE. The megacrystic and low-crust garnets show higher HREE levels. The dunitic garnets reveal S-shaped and harzburgitic depressions in heavy-middle REE and curved patterns. All peridotitic types demonstrate U, Nb, and small Zr enrichment in a multicomponent spider diagram (MSD). The Cr-diopsides show small U enrichment, and pyroxenitic clinopyroxenes show higher Th peaks and Pb and Ba depressions. Ilmenites display very high Ta-Nb and Zr-Hf peaks and very low REE levels except for two samples. The Cr-spinelides demonstrate Ta peaks on the MSD. The phlogopites reveal Eu peaks and W-shaped REE distributions and a high LILE in MSD. Diamonds show low REE levels and Pb peaks. The differences in TKB and AKB geochemistry of garnets and diopsides are in the higher level of the Th-Nb and Zr-Hf levels, showing the influence of the carbonate and H₂O-bearing melts that accompanied the interactions with the protokimberlite melts. Reconstructed with partition coefficients, parental melts reveal highly inclined lines up to 1000/PM (primitive mantle). Peridotites show U-Ba enrichment typical for subduction-related melts and also high Nb due to superplume melt influence. Cr-diopsides and pyroxenites show dominating Th enrichment due to interaction with the carbonatite melt. The high diamond grade of the Aykhal pipe is correlating with high contentment with dunitic garnets found also as diamond inclusions. The diamonds were formed by interaction of subduction-related Na-Mn-U and peridotitic high Mg-Cr components with Ti-Nb-Th plume and hybrid melts and interaction with the peridotitic and eclogitic material.