Geochemistry International最新文献

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.

Thermodynamic properties of shosanbetsuite (Ag₃Sn) are first determined in the Ag–Sn system in a galvanic cell (–) Pt|Gr|Ag|RbAg₄I₅|Ag₃Sn, Sn|Gr|Pt (+) within the temperature range of 327–427 K in vacuum. Analysis of the data makes it possible to calculate the standard (298.15 K, 10⁵ Pa) Δ_fG°, Δ_fH°, and S° of Ag₃Sn: –21238, –18763 J mol^–1, and 187.5 J K^–1 mol^–1, respectively.

The formation of nanophase metallic iron (npFe⁰) in lunar regoliths, which is observed in the condensate films on the surface of mineral grains and in agglutinate glasses, is one of the signs of “space weathering” on the Moon under the influence of solar wind and micrometeorite bombardment. The paper presents the results of laser experiments simulating micrometeorite “impact” on basalt, olivine, pyroxene and some other types of targets. Numerous iron nanospherules that are often arranged into chains and clusters were found in the molten products of the “impact.” The experiments showed that npFe⁰ can be formed without the participation of implanted solar wind ions (hydrogen ions) as a reducing agent, as well as without iron condensation from shock-formed vapor. Similar clusters of nanophase metallic iron and chain structures are observed in the impact glasses of the lunar regolith and asteroid particles.

The paper presents analysis of the crystal morphology of two similarly shaped interpenetration twins of cubic crystals: the Chinese Lantern diamond (Arkhangelsk diamond mining province, provided for this study by courtesy of the ALROSA company) and a diamond crystal from Brazil, which has been studied in much detail by A.E. Fersman. Comparison of data on these diamonds, which were obtained using original methodological techniques for interpreting morphological indications of crystal growth and dissolution, revealed a number of crystal morphological features of crystals characteristic of regular accretions of this type. In addition to tetragonal pits characteristic of the cuboids, ribbon-shaped, jagged, and drop-shaped relief features typical of surface dissolution, a new type of symmetrical sawtooth microrelief associated with etching of octahedral layers of cuboid growth was discovered on the diamond from the Arkhangelsk province. In accordance with the height and direction of the steps of the relief pattern, the development sequence of the aforementioned types of microrelief is determined according to the increase in the depth of dissolution and the degree of change in the curvature of the surface of the tetrahexahedron on twin cuboids. The genesis of the characteristic equatorial sharp ridge of octahedron faces encircling the twinning plane was revealed. It is demonstrated to has been formed in relation to an early stage of the systematic coalescence of fibrous diamond cuboids and to be a determining condition of its development, first as a interpenetration twin of flat-faced octahedral crystals. Crystal morphological similarities between indications of dissolution on curved tetrahexahedra from kimberlites of the Arkhangelsk pipes and placer sources in Brazil and the north of the Yakutian diamond-bearing province is discussed.

The results of geochemical studies of soils and anthropogenic grounds within the territory of the large Pioneer gold ore deposit developed in the Amur Region are presented. Using the enrichment indices for the soil and ground cover of the Pioneer deposit territory, the following pollutant elements were determined: As, Sb, Mo, Bi, W, S, Cd, and Pb. Using the methods of mathematical statistics, the background contents of As, Sb, Bi, Mo, W, S, Cd, and Pb in the technogenically transformed territory at the sampling time accounted for 63, 8.84, 0.69, 3.54, 4.19, 529, 0.11, and 36.5 mg/kg, respectively. The exceedance of background values for As, Sb, Bi, Mo, W, and S is caused by the natural metallogenic features of the territory. It has been established that the sources of metal emissions into the environment are mining facilities and structures of the mining complex: quarries, waste dumps, heap leaching areas, gold extraction plant, and tailings dumps.