Geochemistry International最新文献

Focused on the significant scientific question regarding the chemical reaction pathway of methane in high maturity gas-pools generation from C₂–C₄ hydrocarbons, the fluid inclusions trapped within hydrothermal quartz which developed in Dengying Formation were analyzed. The paleo-oil and gas pools of the Dengying Formation exhibit high maturity, with the crude oil having transformed into methane-dominated gas pools completely. The selected fluid inclusion assemblage (FIA) consisted of one pyrobitumen inclusion and less than 20 methane inclusions. This suggests that the FIA was formed through the cracking of precursor oil-H₂O immiscible inclusions. Importantly, the FIA track was confined to the interior of the host mineral quartz, indicating that the thermal cracking of oil occurred within a closed system inside the quartz. The composition of the methane inclusions primarily consisted of CH₄, with trace amounts of CO₂, H₂S, and aromatic hydrocarbons. The absence of pyrobitumen in methane inclusions refutes the notion that methane in highly mature gas pools is directly generated by the fracture of carbon–carbon bonds through C₂–C₄ hydrocarbons. The presence of aromatic hydrocarbons in the methane inclusions suggests that methane in highly mature gas pools may be generated through the demethylation of aromatic ring systems.

The paper reports new geological, geochemical, and U–Pb zircon ages on the northern flank of the Kukasozero structure. It has been established that biotite and amphibole–biotite gneisses of the northeastern framing of Lake Kukas were formed after calc-alkaline andesites and dacites and are complete analogues of greenstone rocks of the adjacent Neoarchean Chelozero structure. The studied volcanic sequence was formed in the Neoarchean at the stage of 2775–2715 Ma, under conditions close to the modern island-arc settings. Sm–Nd isotope-geochemical data indicate an increase of subduction contribution at the final stages of volcanic activity. The central and western parts of the northern shore of Lake Kukas are composed of alternating schists of different mineral composition. Amphibole and biotite–amphibole schists correspond to the calc-alkaline intermediate volcanics formed in island-arc settings. They are comagmatic to the gabbro-diorite bodies with an age of 2739 ± 6 Ma cutting across these sections. The origin of two-mica (muscovite–biotite) schists is controversial: one group is petrogeochemically close to graywackes, while the other, most likely, was produced by metasomatic reworking of greenstone rocks. It has been suggested that the studied two-mica schists previously attributed to the Paleoproterozoic rocks may be Neoarchean (one of the segments of the Chelozero greenstone structure). They were reworked by strong tectonic-metamorphic processes at the Paleoproterozoic collisional stage, when the island arc was accreted to the Karelian craton margin.

This paper presents results of geochemical studies of source-rock, reservoir-rock, and oil samples from oilfields in the Udmurt Republic. The molecular and isotopic composition of the oils and bitumens shows their strong genetic relations to a single source of liquid hydrocarbons: the Domanik (Semiluki) source rocks (of Late Devonian to Early Tournaisian age, D₃f₂). At the same time, features of the molecular and isotopic composition of the hydrocarbons suggest the participation of several interrelated kitchens of same genetic type for the different parts of this territory. This information will be useful for verifying basin modeling results and planning geological explorations at the territory.

The Laptev Sea belongs to the polar climate region, characterized by minimal influence on geochemical weathering processes. The dominant role in the formation of sedimentary rocks in high latitudes is played by mechanical sorting of sedimentary material. Using 15 sediment cores located along 120°‒130° E from the Lena Delta to the continental slope and in the Sadko Trough as an example, a close relationship between the grain-size and geochemical composition of marine sediments was revealed. The results of the study showed that shelf sedimentation occurs in at least two stages: (i) accumulation of sediments in coastal area, the area directly adjacent to the sources of sediments, and (ii) the subsequent mechanical sorting of sedimentary material on the shelf itself. At the first stage, the finest clay fraction is partially separated from the primary terrestrial sedimentary material. At the second stage, as one move away from the coastal area, the sand and silt fractions are separated. This sedimentation pattern is determined by the combination of the Lena Delta morphology and the Siberian Coastal Current. In conditions of an unstable hydrodynamic regime of the shallow shelf, predominantly sandy sediments (SiO₂/Al₂O₃ > 4.5) accumulate at a rate of <0.5 g cm^‒2 yr^‒1. These sediments differ from the sediments of the terrestrial area by a higher content of Si and a lower content of Ti, Mg, Fe and Ni. In conditions of a calm hydrodynamic environment of the Sadko Trough, predominantly silty sediments (SiO₂/Al₂O₃ < 4.5) accumulate at a rate of >0.5 g cm^‒2 yr^‒1, characterized by a higher content of Ti, Mg, Fe and Ni and a lower content of Si. The contents of K, Rb, Cr and Ca in shelf sediments do not show a clear correlation with grain-size distribution and are thus not informative for reconstructing sedimentation patterns.

Helium is an indispensable and scarce strategic resource. Both research and exploration targeting geothermal-associated helium resources in geothermal fields on both sides of the Cangdong Fault, China remain entirely lacking. To investigate the helium origin, source rocks, phase partitioning, as well as identify helium accumulations, the research team collected geothermal gases and water samples from 25 sampling sites across different geothermal reservoirs in the study area. The collected samples were subjected to comprehensive geochemical analyses including: helium isotope composition, major gas composition, CH₄ and CO₂ carbon isotope, and hydrochemical parameters. The test results show that the helium in the study area is mainly of crustal origin; the average proportion of mantle-derived helium of sandstone geothermal reservoirs, bedrock geothermal reservoirs, and mixed geothermal reservoirs are: 2.78, 4.63, and 5.89%, respectively; N₂ accounts for the largest proportion of geothermal gases composition; CH₄ is coal-type or oil-type. Quantitative calculations revealed that granite and bauxite constitute the predominant helium source rocks, with free gas phase helium being hosted within geothermal reservoirs. The detection of helium-bearing horizons within bedrock geothermal reservoirs and adjacent regions surrounding sampling site D81 constitutes a critical factor for successful helium exploration.

The mineral ferrodimolybdenite (FeMo₂S₄) and the associated mineral assemblage were identified for the first time in an extraterrestrial environment: in a sulfide–metal veinlet of the Kunya-Urgench (H5) ordinary chondrite. They were studied using optical microscopy, SEM, EPMA, and EBSD. Ferrodimolybdenite was found as an inclusion in troilite in terrestrial pyrometamorphic rocks in 2023. Its synthetic analogue has been known as a semiconductor since 1960. Experimental data and properties of the natural mineral assemblage suggest that ferrodimolybdenite should have crystallized from troilite melt at a temperature close to 1100–1000°C. The quenching of metal–sulfide melt enriched in Mo, Cu, and Mn probably formed the metastable phase FeMo₂S₄ in association with native copper, alabandite, and mercury sulfides. The presence of alabandite can indicate strongly reducing conditions (log fO₂ < –4 IW), which are atypical of the impact melting of ordinary chondrites. The fact that this phenomenon occurs locally suggests that a reducing agent may have been locally involved, which was probably a carbon phase contained in the groundmass of the chondrite or brought from the meteoroid that initiated the impact event with the formation of the veinlet. The anomalously high concentrations of Mo (2 × 10² CI), Mn, Cu, and Hg in the Fe–S melt could not have been reached either during the fractional crystallization of large volumes of Fe–FeS melt or during the recurrent partial melting of metal sulfide and silicates during impact events. The ferrodimolybdenite and associated mineral phases were most likely formed during the impact melting of an foreign sulfide–metal aggregate that had been formed under conditions different from those characteristic of the formation of the chondrite matrix in which carbonaceous chondrites were presumably formed. An alternative explanation is hydrothermal activity on the parent body of H chondrites. Although prerequisites for this activity have been identified, its P–T boundary parameters remain uncertain.

A meteorite of a new type, NWA 13202, was revealed for the first time in the collection of the Russian Academy of Sciences. It was assigned to metal-rich ungrouped chondrites and is paired with the NWA 12379/12273 chondrites. These chondrites consist of, on average, ∼70 vol % Fe–Ni metal and ∼20 vol % chondrules and contain small silicate inclusions embedded in the metal. Similar to other known metal-rich chondrites (G, CH, CBa, and CBb), these is no fine-grained matrix in NWA 13202. The chondrules are mainly of the porphyritic olivine–pyroxene, olivine, and pyroxene varieties (POP, OP, and PP). Nonporphyritic chondrules (BO, SO, CC, RC, and GC) are rare. Olivine has an L-chondrite composition, Fa25.9 ± 3.5 mol %, and low-Ca pyroxene is Fs17.2 ± 5.7 mol %, which resembles more closely H-chondrites. The degree of olivine heterogeneity corresponds to chondrites of petrological types 3–4. Accessory minerals are phosphates and chromite. The metal includes low-Ni kamacite and high-Ni taenite and tetrataenite, and the only sulfide is troilite. The oxygen isotope composition of silicates from the chondrules of these ungrouped chondrites supports their affinity to the oxygen isotope reservoir of LL chondrites (Jansen et al., 2019). The metal underwent partial melting, and was formed ~2.4 My after the formation of Ca–Al-rich inclusions (Liu et al., 2023). Chondrites of this type were probably formed by a catastrophic collision of metal and chondrite bodies. The intensity and conditions during this event were not sufficient to form chondrules with chondrules with quench textures, such as the CC and SO types. After the reaccretion of a new parent body of the metal-rich ungrouped chondrite, the material of NWA 13202 and NWA 12379/12273 was affected by aqueous alteration and metamorphism at a temperature of ∼600°C, which produced phosphates and rims of Fe-rich olivine around low-Ca pyroxene.

Na–Fe and Na–Ca–Mg–Fe phosphates were found in the impact melt associations of the Chelyabinsk meteorite (Chebarkul fragment). They drastically differ in composition from phosphates of the initial chondrite (chlorapatite and merrillite). Chladniite Na_2.25Ca_2.14Mg_6.47Fe_3.76Mn_0.17(PO₄)₉ and a merrillite-like phase Na_1.32Ca_6.80Mg_2.07Fe_0.98Mn_0.04(PO₄)₇ were found in the silicate part in quenched interstitial groundmass between olivine grains; merrillite and chlorapatite are rare in it. The spongy metal–sulfide aggregate in the large vugs and metal–sulfide blebs in the silicate part contain Na–Fe phosphate globules. They consist of sarcopside and graftonite (Fe²⁺,Mn²⁺)₃(PO₄)₂, galileiite Na(Fe²⁺,Mn²⁺)₄(PO₄)₃, xenophyllite Na₄(Fe²⁺,Mn²⁺)₇(PO₄)₆, an unidentified Na–Fe phosphate Na₂(Fe²⁺,Mn²⁺)₁₇(PO₄)₁₂, and sometimes chromite-2. Dendritic–skeletal growth of crystals (providing evidence of very rapid quenching) is found in all associations of the impact melt (silicate part, vugs, metal–sulfide aggregate, metal–sulfide blebs, and phosphate globules). The following crystallization sequence is revealed in the Na–Fe phosphate globules: chromite-2 → sarcоpside/graftonite → galileiite → xenophyllite. They are thought to have formed due to the separation of Na–Fe phosphate liquid from homogenous Fe–Ni metal–sulfide melt enriched in Na, P, Cr, and O. The Na–Ca–Mg–Fe phosphates crystallized without involvement of any processes of liquid immiscibility, directly from silicate melt. The paper provides data on the chemical composition and Raman spectroscopy of all studied phosphates and the major minerals of the impact melt associations of the Chelyabinsk meteorite.

A melt pocket (MP) found in only one of the silicate inclusions in the Elga iron meteorite was studied using TEM, SEM, EMPA, and Raman spectroscopy methods. The MP demonstrates the liquid immiscibility of the FeCO₃–Fe₃(PO₄)₂–SiO₂–(Fe, Ni)₃P melts, the mineralogical and bulk chemical composition of which is inconsistent with that of the silicate inclusions in the Elga meteorite. Key differences include: (1) The high content of Fe oxide in the MP is inconsistent with the low FeO content (≈3 wt %) in the SiO₂ glass of silicate inclusions; (2) Ca and Mg, the main phase-forming cations of silicate inclusions, are absent in the MP; (3) Siderite and sarcopside, the main oxygen-bearing phases in the MP, were not found in other silicate inclusions of Elga; (4) carbon compounds (aromatized sp² carbon, phenols) identified in the MP were not found in the host silicate substance. These contradictions lead to the conclusion that the melt pocket is a melted fragment of carbonaceous chondrite captured by Elga’s parent body during a collision with carbonaceous asteroid.

Earlier discovery of magnetite in the Chang’E-5 regolith raised the question about a source of oxidized material in young basaltic volcanism area of the landing site. Here we report the find of Fe-oxide microspherule fragment found in the Chang’E-5 sample, which retained its original structure suggesting it could be magnetite polyframboid or dendrite-like microspherule. The size and texture of the object suggest its prolonged formation from a Fe-rich oxidized environment. Shape and the growth morphology observed on the microcrystals surface suggest a possible free growth from gaseous or fluid phase. Volcanic gas/fluid accumulated within erupted lava flow could be an oxidizing agent at the late stage of eruption or during post-eruption fumarolic activity. If fumaroles existed in the volcanic complexes of Oceanus Procellarum, then the products should be reworked during regolith gardening afterwards, having preserved traces of such processes in the regolith.