Acta Geochimica最新文献

Neoproterozoic island arc assemblage of the Arabian–Nubian Shield (ANS) in the Eastern Desert (ED) of Egypt comprises juvenile suites of metavolcanics (MV), large amounts of meta-sedimentary rocks (MS), and voluminous metagabbros-diorites (MGD) and syn-tectonic intrusions of older granitoids (OG). We report here the updates of these four rock units in terms of classification, distribution, chemical characteristics, geodynamic evolution, metamorphism, and ages. In addition, we discuss these integrated data to elucidate a reasonable and reliable model for crustal evolution in the ANS. The main features of these rock units indicate their relation to each other and the geodynamic environment dominated by early immature oceanic island arcs to primitive continental arcs. Integrated information of the island arc metavolcanic and plutonic rocks (gabbros, diorites, tonalites, and granodiorites) furnish evidence of the genetic relationships. These include proximity and a coeval nature in the field; all protolith magmas are subalkaline in nature following calc-alkaline series with minor tholeiitic affinities; common geochemical signature of the arc rocks and subduction-related magmatism; their similar enrichment in LREEs; and similar major element compositions with mafic melts derived from metasomatized mantle wedge. The volcano-sedimentary and the OG rocks underwent multiphase deformation events whereas the MGD complexes deformed slightly. Based on the magmatic, sedimentological, and metamorphic evolutions constrained by geochronological data as well as the progressive evolutionary trend from extensional to compressional regimes, a possible gradual decrease in the subducted slab dip angle is the most influential in any geodynamic model for arc assemblage in the ED of Egypt.

The Moon has been divided into three terranes: Procellarum KREEP Terrane (PKT), Feldspathic Highland Terrane (FHT), and South Pole-Aitken Terrane (SPAT), using globally measured Th and FeO. Many lunar evolution models have predicted that a lunar magma ocean will produce a residual layer enriched in incompatible elements such as K, REE, and P (i.e., KREEP) in the late age of crystallization; and that the distribution of thorium can be used as a proxy for determining the global distribution of KREEP. The thorium distribution in these three terranes is inhomogeneous. The highest concentration of thorium is in PKT, the medium concentration of thorium is in SPAT, and almost none in FHT. Then what is the specific distribution in each of the terrane and what enlightenment can it tell us? Here we present and describe the detailed thorium distribution in PKT, SPAT, and FHT and provide some information for the origin of asymmetries on the lunar surface.

Ten rock samples consisting of one pyroclastic density current (PDC1) deposit, seven lava flows (LF1–7), and two summit lava domes (LD1, 2) were studied to understand the petrogenesis and magma dynamics at Mt. Sumbing. The stratigraphy is arranged as LF1, PDC1, LF2, LF3, LF4, LF5, LF6, LF7, LD1, and LD2; furthermore, these rocks were divided into two types. Type I, observed in the oldest (LF1) sample, has poor MgO and high Ba/Nb, Th/Yb and Sr. The remaining samples (PDC1–LD2) represent type II, characterized by high MgO and low Ba/Nb, Th/Yb and Sr values. We suggest that type I is derived from AOC (altered oceanic crust)-rich melts that underwent significant crustal assimilation, while type II originates from mantle-rich melts with less significant crustal assimilation. The early stage of type II magma (PDC1–LF3) was considered a closed system, evolving basaltic andesite into andesite (55.0–60.2 wt% SiO₂) with a progressively increasing phenocryst (0.30–0.48 (upphi_{PC})) and decreasing crystal size distribution (CSD) slope (from − 3.9 to − 2.9). The evidence of fluctuating silica and phenocryst contents (between 55.9–59.7 wt% and 0.25–0.41 (upphi_{PC}), respectively), coupled with the kinked and steep (from − 5.0 to − 3.3) CSD curves imply the interchanging condition between open (i.e., magma mixing) and closed magmatic systems during the middle stage (LF4–LF6). Finally, it underwent to closed system again during the final stage (LF7–LD2) because the magma reached dacitic composition (at most 68.9 wt% SiO₂) with abundant phenocryst (0.38–0.45 (upphi_{PC})) and gentle CSD slope (from − 4.1 to − 1.2).

Suites of granitic and metamorphic rocks form a perfect geological environment in which to investigate the source of metals related to gold deposits. In the East Province of Cameroon, the source of metals-related gold (grade ~ 4.6 g/t) that occurs within metamorphic and granitic rocks remains ambiguous. The host rocks were subjected to XRF and ICP-MS whole-rock geochemistry and BSE/EDS mineral analyses to investigate the source(s) of metals related to gold in the Province. Petrographically, chalcopyrite-pyrite and magnetite-ilmenite are the main sulfides and oxides of granites, respectively. The low-grade metamorphic rocks are sulfide-rich consisting of arsenopyrite, chalcopyrite, pyrite, and sphalerite, and oxides include magnetite, ilmenite, hematite, and rutile. Cu, Ni, Co, Sb, Sc, and Zn concentrations depleted in high-grade metamorphic rocks and granites compared to low-grade metamorphic rocks. As well, Bi, Cd, Co, Cu, Ni, Sb, V, W, and Zn concentrations decrease with increasing temperatures from low-grade rocks to high-grade rocks and granites. The suite of depleted elements in high-grade rocks and granites is almost identical to those enriched in gold occurrences. We conclude that metals (Bi, Cd, Co, Cu, Ni, Sb, and Zn) related to gold mineralization in the East Province of Cameroon likely originated from metal-rich low-grade metamorphic rocks during prograde metamorphic processes prior to partial melting.

The quantitative estimation of key parameters of paleotemperature and paleoprecipitation is crucial for paleoclimate reconstruction. Geochemical data from modern sediments are highly consistent with climate data, and their relationship can provide an important reference for the quantitative reconstruction of the paleoclimate. In this study, detailed inorganic geochemical analysis was carried out using high-precision sampling of the Paleogene (LFD-1 well) Guchengzi, Jijuntun and Xiloutian Formations in the Fushun Basin located in the mid-latitudes of the Northern Hemisphere. The Eocene Guchengzi Formation (54.51–47.8 Ma) and Jijuntun Formation (47.8–41.2 Ma) in the Fushun Basin were found to have been deposited under a humid climate. The lower (41.2–40.1 Ma) and upper (40.1–37.8 Ma) parts of the Xiloutian Formation were characterized by semiarid and semihumid–semiarid climates, respectively, which is very similar to the paleoclimatic information reflected by organic carbon isotopes. The Eocene Thermal Maximum 2 (ETM2, ~ 53.7 Ma), Early Eocene Climatic Optimum (EECO, ~ 53.1–46.5 Ma, Eocene Thermal Maximum 3 (ETM 3, ~ 52.8 Ma), and Middle Eocene Climatic Optimum (MECO, ~ 40.7–40.1 Ma) events significantly enhanced chemical weathering during these periods. The rapid increase in pCO₂ concentration leads to an increase in temperature, precipitation, and surface runoff, exhibiting strong chemical weathering. The mean annual temperature (MATa) and mean annual precipitation (MAPa, MAPb, and MAPc) were estimated using parameters, such as the corrosion index without potassium (CIA-K), CaO/Al₂O₃, and (Na₂O + K₂O)/Al₂O₃. Comparing MAPa, MAPb, and MAPc with the MAP estimated using pollen data, MAPa and MAPb were found to be more sensitive to the climate during high precipitation periods (precipitation > 1000 mm, Guchengzi Formation), and the recovered average precipitation was similar to MAP. In contrast, MAPc was more sensitive to the climate during low precipitation periods (precipitation < 1000 mm, Jijuntun, and Xiloutian Formations), with higher accuracy. To fully consider the influence of soluble inorganic salts Ca²⁺ and Na⁺, multivariate linear equations of CIA-K and CaO/Al₂O₃ with CIA, and CIA-K and CaO/Al₂O₃ with MAP were constructed, namely MAPd and MAPe. The results show that MAPe has the highest performance and can be effectively used to estimate the change of paleoprecipitation in Northeast Asia.

The experimental results of the reactions between an alkaline basaltic melt and mantle orthopyroxenes under high-temperature and high-pressure conditions of 1300–1400 °C and 2.0–3.0 GPa using a six-anvil apparatus are reported in this paper. The reactions are proposed to simulate the interactions between melts from the asthenospheric mantle and the lithospheric mantle. The starting melt in the experiments was made from the alkaline basalt occurring in Fuxin, Liaoning Province, and the orthopyroxenes were separated from the mantle xenoliths in Damaping, Hebei Province. The results show that clinopyroxenes were formed in all the reactions between the alkaline basaltic melt and orthopyroxenes under the studied P–T conditions. The formation of clinopyroxene in the reaction zone is mainly controlled by dissolution–crystallization, and the chemical compositions of the reacted melt are primarily influenced by the diffusion effect. Temperature is the most important parameter controlling the reactions between the melt and orthopyroxenes, which has a direct impact on the melting of orthopyroxenes and the diffusion of chemical components in the melt. Temperature also directly controls the chemical compositions of the newly formed clinopyroxenes in the reaction zone and the reacted melt. The formation of clinopyroxenes from the reactions between the alkaline basaltic melt and orthopyroxenes can result in an increase of CaO and Al₂O₃ contents in the rocks containing this mineral. Therefore, the reactions between the alkaline basaltic melt from the asthenospheric mantle and orthopyroxenes from the lithospheric mantle can lead to the evolution of lithospheric mantle in the North China Craton from refractory to fertile with relatively high CaO and Al₂O₃ contents. In addition, the reacted melts in some runs were transformed from the starting alkaline basaltic into tholeiitic after reactions, indicating that tholeiitic magma could be generated from alkaline basaltic one via reactions between the latter and orthopyroxene.

To explain the presence and spatial distribution of NO₃⁻ and N–NH₃ in the Aquifer of the Metropolitan Area of Mexico City (AMAMC), a hydrogeochemical and isotopic analysis using ¹³C_DIC (as well as the stable isotopes ¹⁸O and ²H) in groundwater was conducted. This aquifer is located in an old closed lacustrine volcano-sedimentary basin; some wells hosted in the semi-confined zone contain high N–NH₃ concentrations, while others present NO₃⁻ contents in the recharge zones (hosted in an oxidizing environment). In this study, a change in the isotopic signature (primarily in ¹⁸O and ²H) was observed from the recharge zones to the basin center in some of the wells with high NO₃⁻ concentrations, this behavior can be attributed to evaporation during the incorporation of recently infiltrated water. In addition, the results for ¹³C (along with ²H) in wells with the highest N–NH₃ concentrations exhibited an atypically broad range of values. Results indicated the occurrence of hydrogeochemical and/or biochemical processes in the aquifer (in an oxidizing or reducing environment), such as organic degradation, bacterial decomposition (primarily in the ancient Lake Texcoco and which acts as a natural sink for carbon, nitrogen, sulfur, and phosphorus), besides rock weathering and dissolution, which may be responsible for a very marked isotopic modification of the ¹³C (and, to a lesser extent, ²H). Methanotrophic bacterial activity and methanogenic activity may be related to N–NH₃ removal processes by oxidation and residual water incorporation respectively, whereas the increase in the NO₃⁻ content in some wells is due to the recent contribution of poor-quality water due to contamination.

By studying the light isotopic compositions of carbon, oxygen, and hydrogen, combined with previous research results on the ore-forming source of the deposit, the authors try to uncover its metallogenic origin. The δ¹⁸O and δ¹³C isotope signatures of dolomite samples vary between 10.2 and 13.0‰, and between −7.2 and −5.2‰, respectively, implying that the carbon derives from the upper mantle. δD and δ¹⁸O of quartz, biotite, and muscovite from different ore veins of the deposit vary between −82 and −59‰, and between 11.6 and 12.4‰, respectively, implying that the metallogenic solutions are mainly magmatic. According to the relevant research results of many isotope geologists, the fractionation degree of hydrogen isotopes increases as the depth to the Earth’s core increases, and the more differentiated the hydrogen isotopes are, the lower their values will be. In other words, mantle-derived solutions can have extremely low hydrogen isotope values. This means that the δD‰ value − 134 of the pyrrhotite sample numbered SD-34 in this article may indicate mantle-derived ore-forming fluid of the deposit. The formation of the Dashuigou tellurium deposit occurred between 91.71 and 80.19 Ma.

The Gangdese belt in Xizang has experienced both Jurassic subduction and Cenozoic continental collision processes, making it a globally renowned region for magmatic rocks and porphyry copper deposits. Numerous Jurassic intrusions have been identified in the belt. Apart from the quartz diorite porphyry in the large Xietongmen deposit, the Cu mineralization potential of other Jurassic intrusions in this belt remains unclear. This study presents zircon U–Pb dating and trace elements, apatite major and trace elements as well as published whole-rock geochemical and isotopic data of the Dongga tonalite in the central part of the Gangdese belt, aiming to reveal the petrogenesis, oxidation state, volatile content, and Cu mineralization potential of this intrusion. The Dongga tonalite has a zircon U–Pb age of 179.4 ± 0.9 Ma. It exhibits high whole-rock V/Sc values (8.76–14.6), relatively low apatite Ce_N/Ce_N^* ratios (1.04–1.28), elevated zircon (Eu/Eu^*)_N values (an average of 0.44), high Ce⁴⁺/Ce³⁺ values (205–1896), and high ∆FMQ values (1.3–3.7), collectively suggesting a high magmatic oxygen fugacity. The Dongga tonalite features amphibole phenocrysts, relatively high whole-rock Sr/Y ratios (20.3–58.9), and lower zircon Ti temperatures (502–740 °C), reflecting a high magmatic water content. Estimation of magmatic sulfur content (0.002–0.024 wt%) based on apatite SO₃ contents indicates an enriched magma sulfur content. Combined with previous studies and the collected Sr–Nd–Hf isotopes, the Dongga tonalite is derived from juvenile lower crust related with subduction of the NeoTethys oceanic slab. When compared with Xietongmen ore-bearing porphyries, the Dongga tonalite exhibits remarkable similarities with the Xietongmen ore-bearing porphyries in terms of magma source, tectonic background, magmatic redox state, and volatile components, which indicates that the Dongga tonalite has a high porphyry Cu mineralization potential, and therefore, provides important guidance for the future mineralization exploration.

Comprehensive research has been implemented to raise the efficiency of the geochemical survey of stream sediments (SSs) that formed under the cryolithogenesis conditions. The authors analysed the composition, structure and specific features of the formation of exogenous anomalous geochemical fields (AGFs) identified through SSs of large river valleys of IV order. In our case, these were the valleys of Maly Ken, Ken and Tap Rivers. These rivers are located in the central and southern parts of the Balygychan–Sugoy trough enclosed in the Magadan region, North–East of Russia. The authors proposed a new technique to sample loose alluvium of SSs in the large river valleys along the profiles. The profiles were located across the valleys. The AGFs of Au, Ag, Pb, Zn, Sn, Bi, Mo and W were studied. Correlations between elements have been established. These elements are the main indicator elements of Au–Ag, Ag–Pb, Sn–Ag, Mo–W and Sn–W mineralization occurring on the sites under study. The results obtained were compared with the results of geochemical surveys of SSs. It is concluded that the AGFs recognized along the profiles reflect the composition and structure of eroded and drained ore zones, uncover completely and precisely the pattern of element distribution in loose sediments of large water flows. The alluvium fraction < 0.25 mm seems to be most significant in a practical sense, as it concentrated numerous ore elements. Sampling of this fraction in the river valleys of IV order does not cause any difficulty, for this kind of material is plentiful. The developed technique of alluvium sampling within large river valleys is efficient in searching for diverse mineralization at all stages of prognostic prospecting. It is applicable for geochemical survey of SSs performed at different scales both in the North–East of Russia, as well as other regions with similar climatic conditions, where the SSs are formed under the cryolithogenesis conditions.