Volume 70, Nº 3 (2025)

The Paleoproterozoic Kaalamo massif is located within the southeastern part of the Raahe-Ladoga zone – the junction zone of the Archean Karelian Craton and the Proterozoic Svecofennian Orogen. The massif consists of three phases of intrusion: the 1st phase is represented by peridotites; olivine clinopyroxenites; and gabbro; the 2nd phase is represented by gabbro-norites and gabbro-diorites; the 3rd phase is represented by diorites; tonalites; and plagiogranites. New data on petrochemistry; geochemistry and compositions of rock-forming minerals are presented for metaperidotites; metapyroxenites and gabbroids from the Kaalamo Complex. These data were used in the COMAGMAT-3.75 program for thermodynamic calculations of equilibrium crystallization trajectories of representative rocks and average compositions of the first and second intrusion phases. The calculation results were processed using the geochemical thermometry method; which made it possible to estimate the temperature (~1220 ^оС) and the probable composition of the initial high-magnesian melt (~9.5 wt. % MgO; olivine with 84 mol. % of forsterite). Comparison of this primitive melt with model compositions of differentiates and petrogeochemical characteristics confirms the formation of rocks of the first and second phases of intrusion from a single source; which is consistent with the similar distribution of REE in these rocks; as well as the results of the study of the Sm-Nd isotope system; which indicate a common mantle source of the rocks. It was found that the calculated orders of mineral crystallization are in good agreement with the abundance of peridotites and olivine pyroxenites; indicating the equilibrium of the primitive melt with clinopyroxene and orthopyroxene; which are in peritectic relationships with olivine. The most differentiated rocks of the first phase of intrusion are characterized by the appearance of cotectic plagioclase containing about 80 mol. % of anorthite. Rocks of the second phase of intrusion represent more differentiated material corresponding to the gabbronorite association of cumulus phases without olivine; but with the appearance of titanomagnetite at the late stages of crystallization. Comparison of model and real mineral compositions indicates a systematic shift of the observed compositions of olivine and pyroxenes to the iron-rich region and “desilication” of plagioclase compositions from early to late phases; which is a consequence of the interaction of early cumulus minerals with residual intercumulus melt. The studied rocks are similar to intrusions of the nickel-bearing belt of Finland; which creates prospects for correlations of magmatic events on an interregional scale.

Organic carbon content combined with organic carbon isotope composition have been applied for the study of organic matter transformation in marine sediments during upward gas migration at seep areas of the Laptev Sea. Organic matter extracted from marine sediments was separated into five fractions (hexane, hexane-benzene, benzene, benzene-methanol, asphaltenes) using solvents of increasing polarity. It has been shown that in the seep the destruction of asphaltenes fractions leads to enrichment of benzene-methanol fraction by isotope-light components. δ¹³C values of benzene-methanol fractions were much lower than δ¹³C values of asphaltenes fractions and were associated with the accumulation of bacterial biomass in the sediments core through which the upward methane flow was passed. The organic matter of seep area sediment cores can be classified by two clusters, according to δ¹³C values of benzene fractions of organic matter. The first cluster center was in the surface layer (about 10 cm) of marine sediments. The second cluster center was in a deeper sediment layer. The difference in carbon isotope composition between the cluster centers was 2–3 ‰. The use of carbon isotope type-curves for different horizons of a sediment core has enabled a better understanding of the biological effects related to upward gas migration in seep areas of the Arctic Seas.

Based on the literature data, the main properties of agates and the conditions of their formation are summarized, and a critical analysis of the hypotheses of agate formation is performed. The hypothesis of layered deposition and crystallization of silica extracted from the host rock (in particular, basalt) turned out to be the most adequate. However, difficulties remained in explaining the movement of SiO₂ from the host basalt to the agate cavities, the causes of SiO₂ deposition, the role of phase transformations and the mechanism of banding formation. To clarify these issues, experiments were performed on the dissolution of basalt samples in water for 4 months at 300 °C. The formation of silica was noticeable only in experiments where the basalt sample was half submerged in water. In particular, amorphous silica (opal-A) was deposited in the pores and on the surface of the sample above the water level, which formed agate-like layered textures and was replaced in places by chalcedony. The experimental results are explained within the framework of the distillation hypothesis, which may be suitable for the formation of agates. The hypothesis combines the possibilities of SiO₂ transfer in low concentrations and SiO₂ deposition in high concentrations, and also explains the banding of agate by fluctuations in the balance of supply and consumption of dissolved silica in precipitation and phase transformation reactions.