Том 69, № 1 (2024)

A petrogeochemical study of basalts (lithophile elements and Sr-Nd-Pb isotopes, compositions of liquidus olivine and spinel) from the transition zone of the Southwest Indian Ridge in the area of the Du Toit and Andrew Bain faults revealed significant differences in their composition. Within the rift valley adjacent to the faults, tholeiites enriched in Na and depleted in Fe (Na-TOR genetic type) are typical. Deep-type basalts (TOR-1) are present in the western side of the Andrew Bain Fault. The outpouring of these types of magmas reflects a possible change in geodynamic regime during this zone formation: from deeper and higher temperature melting to shallower ones (Sushchevskaya et al., 2022).

Differences in the primary melts of tholeiites from the rift valley and the Andrew Bain Transform Fault are also traced in the liquidus olivine compositions. The rift valley olivines are similar to typical Na-TOR olivines with a Mg content of Fo_88–87, low Ni and elevated Mn. On the contrary, tholeiite olivines of the Andrew Bain Fault are enriched in Ni and depleted in Mn, which may indicate pyroxenite included in the primary melt formation. This component is either oceanic lithosphere recycled through the deep mantle or fragments of previously formed oceanic crust, which are subsequently involved in melting during the spreading axes jumping. A similar process is typical for the region of the Bouvet Triple Junction, where a significant heterogeneity of the olivine composition in terms of trace-element contents was revealed.

The isotope characteristics of the Andrew Bain Fault tholeiites differ in Pb and Sr radiogenic composition and are similar to those of enriched magmas from such Indian Ocean rises as Crozet, Marion and Bouvet, but not from the Konrad and Af. Nikitin Rises. The source of such tholeiite melts is close in composition to the model HIMU type (with high U/Pb), possibly with an admixture of mantle material with EMII characteristics (with elevated Rb/Sr).

Influence of the supercritical C-O-H-fluid (7.5 wt.%) onto melting phase relations of the multicomponent multiphase diamond-forming system olivine-jadeite-diopside-(Mg-Fe-Ca-Na-carbonates)-(C-O-H) in experiments at 6 GPa and 700–1200 °C (the upper mantle conditions) has been studied. The peritectic reaction of olivine and jadeite-bearing melt with garnet formation has been retained as a key mechanism of the ultrabasic-basic evolution of diamond-forming melts. The CO₂-fluid and silicate components react forming carbonate phases. The H₂O-fluid together with carbonates has essentially lowered temperatures of the liquidus and solidus boundaries. The phase of supercritical water fluid and water-bearing carbonate nesquehonite (Nes) MgCO₃·3H₂O were identified with the Raman-spectroscopy method after crystallization of the completely mixed silicate-carbonate-(C-O-H-fluid) melt.

The paper is devoted to the Sr-, Nd-, Pb-isotope data obtained for two cores of bottom sediments taken in the Oga and Tsivolki bays of the Severny Island of the Novaya Zemlya archipelago. The studied sequence of sediments from Oga Bay has been accumulated over the last thousand years. The ⁸⁷Sr/⁸⁶Sr ratio decreases from top to bottom down the section from 0.72225 to 0.71995, the value of ε_Nd varies from –6.1 to –5.5. The Pb isotopic composition varies within narrow limits: the ²⁰⁶Pb/²⁰⁴Pb ratio from 19.107 to 19.139, the ²⁰⁷Pb/²⁰⁴Pb ratio from 15.632 to 15.635, and the ²⁰⁸Pb/²⁰⁴Pb ratio from 38.568 to 38.635. A rapid decrease in the ⁸⁷Sr/⁸⁶Sr ratio at a relatively stable neodymium and lead isotope composition indicates a change in the source of the clastogenic material. This can be explained by the fact that the material of the destruction of Permian clay shales, and then the Devonian-Silurian sedimentary carbonates, first entered the area of glacier abrasion and further, respectively, into the sedimentation zone.

The sediment column from the Tsivolki Bay was formed over a little more than 10 thousand years. Based on the Sr, Nd, and Pb isotope ratios, these bottom sediments are divided into lower and upper parts: before and after 150 cm (or ~3500 years). In the lower part of the column, the ⁸⁷Sr/⁸⁶Sr ratio increases from 0.72055 to 0.72580, the value of ε_Nd remains approximately the same and varies around –8.2. In the upper part, the ⁸⁷Sr/⁸⁶Sr ratio drops to 0.72049 in the near-surface layer; at the same time, the value of εNd increases to –6.4. At the boundary of these two units, the ²⁰⁶Pb/²⁰⁴Pb ratio abruptly changes from about 18.0 in the lower part to 19.3 in the upper part and ²⁰⁸Pb/²⁰⁴Pb from about 36.5 in the lower part to 38.7 in the upper part of the section. The change in the Sr, Nd, and Pb isotope characteristics is likely a reflection of changes in the composition of the rocks in the area where the basin was removed, which is now being eroded by the glacier.

Comparison with modern sources supplying clastic material to the Kara Sea showed that the material inputs the Oga and Tsivolki bays only from Novaya Zemlya.

The results of the geochemical study of loose sediments of the catchment basin and bottom sediments of Lake Chistoye, located in the Northern Priokhotye, showed that the lake was formed at the beginning of the early Holocene about 11200 cal. years ago. Terrigenous sedimentation dominates in it, i. e. the geochemical characteristics of sediments are determined by the particle sizes. Thin grain size sediments have low SiO₂, Na₂O, K₂O, CaO, and Sr contents; and are enriched with Al₂O₃, TiO₂, MgO, Fe₂O₃, and V. Changes in the nature of sedimentation may be due to climatic reasons and may be associated with cold Bond events. In the Early Holocene, mostly thin silts were deposited in Lake Chistoye. The impulse of “coarse-grained” sediments (>140 microns) enriched with silica occurred (9760–9650) and 8810 cal. years ago. A noticeable accumulation of relatively coarse-grained sediments occurred at the very beginning of the Middle Holocene 8540–6920 cal. years ago, as well as 6140 and 4450 cal. years ago. For the Late Holocene, the input of detrital material with increased SiO₂ contents was noted in the range of 3470–850 cal. years ago.