Vol 33, No 2 (2025)

Zhokhov Island beongs to the De-Long Archipelago located in the Eastern sector of the Russian continental shelf within the Arctic Basin. The Island is a young volcanic center and is composed of lava flowsalkaline olivine-porphyry basalts and subordanate limburgites. The study was aimed at identifying possible regional and geodynamic factors influencing the specifics of the partial melting process and mineral transformations in mantle xenoliths of Zhokhov Island. Five xenoliths selected from samples of alkali basalts on Zhokhov Island were studied using a scanning electron microscope. The data obtained allowed us to conclude that the formation of high-sodium glasses in the mantle xenoliths of Zhokhov Island is associated with the interaction between Spinel Lherzolites and parental for host Olivine Basalts magmatic melt. At the same time, high-potassium glasses inside mantle xenoliths were formed in situ during the melting of a primary potassium-containing pgase which may have been Phlogopite. The formation of two distinct contrasting in composition zones of recrystallization in contact between mantle xenoliths and host basalt is due to the evolution of composition of the alkaline silicate melt carried xenoliths. Signs of activation of young intraplate magmatism facilitating the transport of fragments of metasomatized shallow mantle are established in a large area of the Arctic Basin.

The paper presents the first data on the petrography, mineralogy, and geochemistry of jadeitites from the El’denyr massif, Chukotka, Russia, as well as host metalherzolites and amphibolite inclusions in the jadeitites. The jadeitite is composed of an association of jadeite, omphacite, analcime, and pectolite with a Ba-Ti-Si accessory mineral. The host metalherzolite is made of an association of olivine, antigorite, diopside, chlorite, ferrite-chromite, chromium magnetite, and accessory awaruite, heazlewoodite, and pentlandite. The jadeitite contains inclusions with a relict coarse-grained hypidiomorphic-granular texture, which are considered to be relics of the metasomatized protolith of the jadeitite. This protolith was probably high-temperature hydrothermal diopsidite. The inclusions show local recrystallization of primary diopside to aegirine-augite and pseudomorphic development of a fine-grained aggregate of amphiboles (several generations of richterite, actinolite, magnesiokatophorite, K-richterite, and eckermannite), omphacite, pectolite, analcime, phlogopite, accessory maucherite and heazlewoodite after diopside/aegirine-augite and an associated unidentified mineral. The protolith was transformed in several stages before the onset of jadeite crystallization, and these transformations included metasomatic recrystallization and a complete change in its texture. During the last stage, crystallization of the euhedral concentrically zoned jadeite with analcime and pectolite from fluid was accompanied by the recrystallization and dissolution of the last reworked relics of the protolith represented by high-calcium omphacite in microgranular omphacite-jadeite aggregates of jadeitite. The formation of jadeitites and the accompanying metamorphism of the host lherzolites occurred at 500°C and 8.5 kbar, which corresponds to P–T conditions typical of the metamorphism of mantle wedge peridotites in the "warm" subduction regime. The presence of jadeites in the El'denyr massif and high-pressure metamorphic rocks in the Ust’-Belaya massif, which were studied previously, allows us to consider the Ust’-Belaya terrane as a mélange of a subduction zone active in the Early–Middle Triassic that was deformed and disintegrated during its subsequent exhumation in the Cretaceous.

New results have been obtained from experimental studies of the characteristics of counter chemical diffusion of the main petrogenic components (SiO₂, Al₂O₃, Na₂O, CaO, MgO, FeO) and the CO₃²⁻ anion during the interaction of basalt and kimberlite melts at mantle pressures. The studies were carried out by the diffusion pair method using an original high-pressure installation of the “BARS” type at a pressure of 5.5 GPa and a temperature 1850°C. It has been shown that the rate of counter chemical diffusion of all the main components of melts (SiO₂, Al₂O₃, Na₂O, CaO, MgO, FeO) and the CO₃²⁻ anion is almost identical in the interaction of model basalt and kimberlite carbonate-containing melts, and is approximately 1 order of magnitude greater than the diffusion rate these components during the interaction of such melts at moderate pressures (100 MPa). The equal rates of diffusion of CaO and the CO₃²⁻ anion indicate that the diffusion of carbonate CaCO₃ from the kimberlite into the basaltic (model and natural) melt remains of the minal nature even at such a high pressure. The diffusion pattern changes significantly during the interaction of a melt of natural magnesian basalt and model kimberlite, as was the case during the interaction of such melts at moderate pressures. In this case, along with the minal diffusion of calcite into magnesian basalt, the diffusion rates of the remaining components of the melts become significantly higher. A weak exponential concentration dependence of all diffusing components has been established, which is close to D_i = const, and which occurred during the interaction of such melts at moderate pressures.