Geochemistry International最新文献

Studies of molecular and hydrocarbon composition of marine sediments provide important information on sedimentary organic matter (OM). In this work, the distribution of hydrocarbons and molecular markers with depth in the sedimentary cover of the linear depression of the West Kara stage was studied. To do this, two sediment cores were sampled during 89^th cruise of the R/V Akademik Mstislav Keldysh at site 7444 and at background site 7441. The distribution of n-alkanes was measured by GC-MS analysis, and the distribution patterns of the biomarker indices of sedimentary OM in the marine sediments with depth were determined. It was shown that higher terrestrial vegetation was the dominant source to the sedimentary OM of marine sediments at sites 7444 and 7441. The OM is of low maturity, as follows from its Ts/(Ts + Tm) ratio. According to the values of H31-S/H31-(S + R) and NAR indices, a small constant inflow of oil hydrocarbons occurs in the study area, and a much lower inflow takes place at background site 7441. The distribution of the CPI25–33, TAR, and NAR suggest that hydrocarbons of anthropogenic origin occur in the near-surface layers of 0–15 cm sediment at background site 7441.

This paper reports the first comprehensive study of urban dust from Krasnoyarsk. The elemental composition of the dust was determined, and several natural and anthropogenic sources of its formation were distinguished. Dust samples (n = 68) were collected in different functional regions of the city. The contents of 70 elements were measured in the dust samples. The (CaO + Na₂O)–Al₂O₃–K₂O and (CaO + Na₂O + K₂O)–Al₂O₃–(Fe₂O₃ + MgO) diagrams showed that the mineral component of most of the Krasnoyarsk dust samples is dominated by plagioclases. The calculation of enrichment factors and geoaccumulation indices showed that the Krasnoyarsk dust is enriched by Co, Sn, Ni, Pb, Mo, Cu, As, Zn, Cd, W, Ag, and Sb. The characteristics of trace element accumulation depending on sampling locality were evaluated. Antimony is accumulated mainly in samples from major highways of Krasnoyarsk, whereas elevated As, Cu, W, and Zn contents were usually found in samples collected in industrial zones of the city. Using principal component analysis and the Pearson correlation matrix, six groups of elements were distinguished in the dust samples, and their natural and anthropogenic sources were determined: Al, Ti, Cr, Fe, and Ni were supplied by soil and rock weathering; Fe, Co, Ni, Cu, and As, emissions from metallurgical plants and coal-burning thermal power plants; W, Bi, Zn, and Mo, heavy metallurgical and mechanical engineering industries; Cu, Cd, Sn, and Pb, emissions of motor and rail transport, as well as wear of metal parts and structures; Sn and Sb, wear of brake pads and tires and industrial emissions; and Hg, soil and rock weathering.

Unique compositional diversity of volcanic rocks was recently discovered in an active fault zone of the Sredinny Range (Kamchatka) located in the zone of monogenetic volcanism near the Tigil and Oxi volcanoes. The lavas in the zone represent common for SR medium-K, high-K and high-Ti, and also unique for Kamchatka high-LREE and high-Mg varieties. In this contribution, we present new Sr-Nd isotope data for a representative set of lava samples from this area. The isotopic and geochemical characteristics of the majority of selected samples can be explained by the differences in the amount of fluid, fluid sources and melting conditions, proposed earlier for the SR. The high-LREE picritic basalts, however, differ significantly from the other Kamchatka volcanic rocks—their highly unusual high-LREE, high-Li and low-LILE geochemical characteristics combine with the highest ⁸⁷Sr/⁸⁶Sr (0.70365) and second-highest εNd (9.9) values ever reported for lavas from the SR. We suggest that they represent the product of melting of the specific SR lithosphere domain that has been metasomatized by melts derived from a strongly degassed slab.

The paper presents data on the characteristics of organic matter in bottom sediment samples collected during the 2016 expedition in the Laptev Sea (northern polygon, outer shelf zone and south-eastern part of the sea). The distribution of total organic carbon over the study area is shown. Samples were analysed by Rock-Eval analytical pyrolysis, ¹³C isotope analysis and chromatography-mass spectrometry. The organic matter composition included hopanoids similar to those found in the 2011 expedition samples taken from the first discovered mega-seepage area. The hopanoids detected are thought to be derived from the activity of methanotrophic microorganisms. The peculiar distribution of n-alkanes, hopanoids and traces of steranes found in samples confined to rift zones may indicate the migration of petroleum fluids.

Clarify the petrogenesis and dynamics mechanism of Triassic magmatic rock in the South China Block (SCB) is crucial for understanding the geological evolution and mineralization of South China. A series of metal deposits (including lead, zinc, tungsten, tin, uranium, etc.) have been discovered inside and near the Yajiangqiao plutonic rock, which is important ore-forming granite in the eastern Hunan region. However, its petrogenesis and tectonic setting are still controversial. Here, we present new zircon U–Pb ages and trace elements, and whole-rock major- and trace- element compositions, and in-situ Hf isotope data obtained from the Yajiangqiao granites in central SCB. LA-ICP-MS zircon U–Pb dating reveals that the granites were emplaced in the late Triassic (227 ± 2 and 226 ± 2 Ma). The geochemical characteristics of zircon and whole-rock trace elements indicate that the Yajiangqiao granites are S-type granite, which originated from the partial melting of crustal material mainly composed of metapelitic sources, with a small amount of mantle-derived material added and undergoing strong fractional crystallization. In combination with regional geological data, we suggest that the late Triassic granites in the central SCB likely were the response to the underplating of mantle-derived magmas in a post-collision setting.

Based on 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 layerwise precipitation 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 SiO₂ movement from the host basalt to the agate cavities, the causes of SiO₂ precipitation, the role of phase transformations, and the mechanism of banding formation. To clarify these questions, 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 precipitated in the pores and on the sample surface 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-concentration regions and its precipitation in high-concentration regions, and also explains the agate banding by fluctuations in supply and consumption of dissolved silica in precipitation and phase transformation reactions.

The Kaalamo early orogenic Paleoproterozoic massif is located in the southeastern part of the Raahe–Ladoga zone, a junction zone between the Archean Karelian craton and the Proterozoic Svecofennian orogen. The massif consists of three intrusive phases: the first phase is peridotites, olivine clinopyroxenites, and gabbro; the second one is gabbro-norites and gabbro-diorites; and the third phase is diorites, tonalites, and plagiogranites. The paper presents newly obtained data on the petrochemistry and geochemistry of metaperidotites, metapyroxenites, and gabbroids from the Kaalamo complex and on the compositions of their rock-forming minerals. These data were used in the COMAGMAT-3.75 program for thermodynamic calculations of the equilibrium crystallization trajectories of representative rocks and average compositions of the first and second intrusive phases. The calculation results were processed using the geochemical thermometry method, which made it possible to estimate the temperature (∼1220°C) and the probable composition of the initial high-Mg melt (∼9.5 wt % MgO, olivine with 84 mol % forsterite). Comparison of this primitive melt with the model compositions of derivatives and petro- and geochemical characteristics confirms that the rocks of the first and second intrusive phases were derived from a single source, which is consistent with the REE patterns of these rocks and data on their Sm–Nd isotope system and indicate that the rocks have a common mantle source. It was found out that the calculated crystallization sequences of the rocks are in good agreement with the abundance of peridotites and olivine pyroxenites, indicating that the primitive melt was in equilibrium with clinopyroxene and orthopyroxene, which are in peritectic relationships with the olivine. The most differentiated rocks of the first phase are characterized by the crystallization of cotectic plagioclase containing about 80 mol % anorthite. The rocks of the second phase are more differentiated material, corresponding to the gabbronorite association of cumulus phases without olivine and with the late crystallization of titanomagnetite. Comparison of the model and real mineral compositions shows a systematic shift of the natural compositions of the olivine and pyroxenes toward the iron richer from early to late phases, which is a consequence of the interaction of the early cumulus minerals with residual intercumulus melt. The rocks are similar to intrusions of the nickel-bearing belt of Finland, which opens prospects for correlations of magmatic events on an interregional scale.

The kinetics of hydrocarbon formation in the sedimentary cover of a subducting plate was numerically simulated using published estimates for the temperature of the subducting plate surface, subsidence rate, and the kinetic spectrum of aquatic organic matter. It was shown that the peak oil window occurs in a subduction environment at depths of 12.6–23.3 km and temperatures of 147.6–179.4°C and requires 0.1–6.4 million years. The scatter in the estimates of depth, temperature, and time required to reach the peak oil window is related to variations in subduction angle and velocity and temperature gradient in the published models. The estimates of depths and temperatures for the formation of hydrocarbons in the sedimentary cover of a subducting plate are higher than those for Cenozoic, Mesozoic, and Paleozoic sedimentary basins, whereas the time required for oil formation is shorter. It can be suggested that accumulations of liquid hydrocarbons could hardly be produced by the organic matter of the sedimentary cover of a subduction plate, but there are prospects for the formation of dry gas fields.

This paper reports the thermodynamic analysis of Ni_xFe_1-x oxidation for awaruite, Ni₃Fe, which is widespread in serpentinized ultrabasic rocks, and other Ni–Fe phases over a wide range of temperatures (400–1873 K) and crustal pressures up to 2 kbar. It was found that the equilibrium γ(Ni_xFe_{1 – x})–Fe oxides constrains oxygen fugacity (fO₂) at the NNO–IW range. For γ(Ni_xFe_{1 – x}) with an iron mole fraction x ≥ 0.5, fO₂ approaches the IW buffer. The reaction of kamacite, α(Ni_0.05Fe_0.95), with oxygen is close to the IW–IM buffer reactions. The fO₂ values of awaruite preservation are no higher than ∆QFM = –(7.8–5.2) at T = 400–600 K and increase to ∆QFM = –(2.7–2.0) at T > 1000–1200 K. The obtained approximations of the temperature dependence of fO₂ can be used to estimate the redox conditions in low-sulfide systems containing Ni_xFe_{1 – x} under the conditions of the Earth’s crust.

The paper presents data on the composition and phase heterogeneity of calzirtite Ca₂Zr₅Ti₂O₁₆ and perovskite CaTiO₃, which are HFSE oxides that crystallized during the early stages of formation of the carbonatite rock series of the Guli alkaline–ultramafic complex in Polar Siberia. The composition of HFSE minerals systematically changed during the evolution of the carbonatite melt from phoscorites to carbonatites. The calzirtite enriched up to 6 wt % Nb₂O₅, and the perovskite enriched up to 15 wt % Nb₂O₅, 7.7 wt % ZrO₂, and 6 wt % LREE₂O₃ in the phoscorites and early calcite carbonatites. Perovskite with low concentrations of admixtures crystallized in the late calcite carbonatites in association with U-, Th-, Ta-rich fluorcalciopyrochlore, thorianite, zirconolite, and baddeleyite. The composition of perovskite-group minerals evolved according to the following of isomorphic exchange schemes: Nb⁵⁺ + Fe³⁺ ↔ Ti⁴⁺ + Zr⁴⁺ and 2Ca²⁺ ↔ Na⁺ + REE³⁺. The enrichment of the early calzirtite and perovskite generations in HFSE is explained by the high Nb, Zr, and LREE partition coefficients in carbonatite melt–mineral equilibria. During the crystallization of the carbonatite melt, the activity of alkaline elements decreased, which is confirmed by a decrease in sodium content in the perovskite and a change in the composition of the solid inclusions. The early generations of perovskite and calzirtite from the phoscorites commonly host numerous polyphase inclusions of Ca, Na, K, Ba, and Sr carbonates, halides, and alkali metal sulfides, whereas calcite, fluorapatite, pyrophanite, and barite are found in the late generations of these minerals. It is shown that the crystallization of the phoscorites have crystallized from anhydrous melt that contained no water, and this was favorable for the preservation of alkaline carbonates as solid inclusions in minerals.