Acta Geochimica最新文献

A set of low-grade clastic metamorphic and carbonate rocks, and greenschists outcropping in the southwestern (SW) margin of the North China Craton (NCC), was originally classified as the Paleoproterozoic Xiong'er Group according to stratigraphic correlation. To verify the age, this paper carried out detrital zircon U–Pb LA-ICP-MS dating of low-grade clastic metamorphic rocks exposed in the Changqing area at the SW margin of the Ordos Block in the SW part of the NCC. Results from detrital zircon dating indicate that the metamorphic and carbonate rocks can be classified into the Neoproterozoic Nanhua System, which is the only Nanhua System stratum in this block so far, and it probably could provide new clues to Rodinia break-up and Snowball Earth of the NCC. The nine peak ages of the low-grade clastic metamorphic rocks reflected its relatively complex provenance, and almost all major geological events experienced by the NCC basement since the Neoarchean, but some age peaks were difficult to correspond to that of the NCC, indicating that the southwestern part of the Ordos Block was also affected by the Qinling and Qiliang orogenic belts during Nanhua System of Neoproterozoic. Combined with provenance analysis, it was revealed that the current southwest boundary of the Ordos Block was the previous southwest boundary of the Ordos Block during the Qingbaikou-Nanhua Period of the Neoproterozoic.

The Ghansura Rhyolite Dome of Bathani volcano-sedimentary sequence, eastern India, represents a subvolcanic felsic magma chamber that was invaded by crystal-rich mafic magma during its cooling phase to form an assortment of hybrid rocks. A prominent solidified portion of the magma reservoir was embedded in the intruding mafic magma as fragments or clasts that produced mafic rocks with felsic clasts. Two distinct compositional zones could be identified in the mafic rocks containing felsic clasts- (a) medium-grained mafic zones that are dominated by amphiboles, and (b) fine-grained felsic zones consisting primarily of quartz and feldspar. Amphiboles occur in most of the felsic clasts suggesting the mechanical transfer of crystals from the mafic to the felsic zones. Compositions of amphiboles were determined from both the mafic and felsic zones that show linear compositional variation from actinolite to ferro-hornblende through magnesio-hornblende, suggesting the interplay of complex substitutions in individual amphibole sites. Cationic schemes have confirmed the role of pargasite (Pg)-type substitution, which is a combination of edenite (Ed)- and tschermakite (Ts)-type substitutions. Moreover, amphibole has been extensively replaced by titanite in the studied rock. Titanite produced in the mafic zones due to the destabilization of amphiboles was observed migrating from the mafic to the felsic zones through mineral-transporting veins. Compositions of titanite were determined from grains that occur in association with amphiboles and those which are present as individual entities in the felsic zones. Similar to amphiboles, titanite also displays cationic substitutions in the studied rock. From the results presented in this work, we infer that extensive replacement of amphibole by titanite and cationic substitutions in amphiboles, and also titanite, may be considered important petrogenetic indicators to decipher magma mixing events.

Numerous mantle xenoliths 6–12 cm in size and sub-angular to rounded in shape occur within Mio-Pliocene basanite lavas of the monogenic volcano of Hosséré Sédé in the Adamawa plateau. Xenoliths of spinel lherzolite exhibit protogranular, equigranular or porphyroclastic texture. Microprobe chemical analyses show that olivine is highly magnesian (Fo_88-90), clinopyroxene crystals are diopside and augite (Wo_41.6–49.6 En_45.3–53.7 Fs_4.2–6.2), orthopyroxene crystals are enstatite (Wo_1.4–1.5 En_88.6–89.0 Fs_9.6–9.9) and spinel crystals are mainly Al-spinel associated to minor Cr-spinel. Estimated temperatures and pressures through empirical formulas show that Hosséré Sédé xenoliths have equilibrated between 1085 and 1204 °C and 1.08 to 1.57 GPa, corresponding to sampling depths of 36 and 52 km. Detailed petrographical and mineral chemistry of Hosséré Sédé xenoliths evidences the complex nature and composition of the subcontinental lithosphere under the Adamawa plateau. This may involve a probable uplift of the whole area after a limited extensional event, possible metasomatism through melt infiltration during shearing of the lithospheric mantle along the Pan African strike-slip fault system.

As a new promising detection technology in the terahertz research field, the terahertz time-domain spectroscopy (THz-TDS) has very broad application potential in many fields because its advantage on the characteristic spectrum, wide spectrum and non-destructive analysis of interested substances. In this paper, the terahertz absorption spectra of gases mixed with ¹²CO and ¹³CO in the spectrum range of 0.5–2.5 THz are measured by terahertz time-domain spectroscopy for the first time. Several isotopologues can be clearly distinguished based on the difference in their rotational energies and the consequent terahertz spectrum. The experimental results show that ¹²CO and ¹³CO have obvious characteristic absorption peaks in the spectrum range of 0.5–2.5 THz due to the difference in rotational energy, and the rotational constant B can be calculated according to the experimental values to distinguish the two gaseous isotopologues. The frequency positions of the characteristic absorption peak measured by this experiment and the rotation constant B calculated according to the experimental values are compared with those previous theoretical calculations and experimental results, and they are in good agreement. This result lays a foundation for developing more sophisticated terahertz instruments to the detection of different isotopologues.

In this study, 30 sediment samples were collected from the lower reaches of the Shichuanhe River in Xi'an, Shaanxi Province, China, to test the distribution of heavy metal elements in this area and for an analysis of the pollution levels of this area, hope to provide guidance on agricultural production activities in this region. The results show that the heavy metal elements in this area are mainly concentrated at the Qinghe River and Shichuanhe River confluences. Furthermore, the element contents are higher than that of the background levels of the continental crust (UCC) and close to the background levels of the soil from Shaanxi Province; the two most enriched elements are Cd and As, with contents of 0.79 and 22.7 mg·kg⁻¹, respectively, and their contents are 3.8 and 1.72 times higher than that of the background values. Herein, the heavy metal pollution assessment methods applied indicated that Cd and As are the two most abundant pollutant elements in the area’s soils. As has a peak geo-accumulation index value of 3, and the pollution level is high, while Cd exhibits high potential ecological risks due to its high toxicity (potential risk index of 143) and an active fraction of more than 64%. In addition, a principal component analysis and hierarchical cluster analysis study showed that there are two sources of the heavy metals in this area. The Zn, As, Ni, Cu, Pb, and Cr are mainly from natural sources, and the Cd likely comes from a discharge of untreated agricultural wastewater in the region. The Cd which poses a high potential risk and mainly results from human activities, needs to be further monitored.

The thermal state of the early Earth’s interior and its way of cooling are crucial for its subsequent evolution. Earth is initially hot as it acquired enormous heat in response to violent processes during its formation, e.g., the Moon-forming giant impact, the segregation and formation of its metallic core, the tidal interaction with the early Moon, and the decay of radioactive elements, etc. In the meantime, the cooling mechanisms of early Earth’s mantle remain elusive despite their importance, and the previously proposed cooling models of the mantle are controversial. In this paper, we first reviewed several prevalent parameterized thermal evolution models of the early mantle. The models give unrealistic predictions since they were established solely based on a single tectonic regime, such as the stagnant-lid regime, or relied on the disputable existence of the plate tectonics prior to ~ 3.5 Ga. Then we argue that the mantle should have started to cool down from a very hot state after the solidification of the ferocious magma ocean. Instead of using one single scaling law to describe a single-stage model, we suggest that an episodic multi-stage cooling model (EMCM) of the early mantle could be more plausible to account for the mantle’s early cooling process. The model reconciles with the fact that the mantle cools down from a hot state prior to ~ 3.5 Ga and can also explain the well-constrained post-3.5 Ga thermal history of the mantle.

The coal metamorphism in Central Hunan provides valuable information about hydrothermal activity and water/rock reactions. Learning how to collect age data on hydrothermal fluid systems is necessary for understanding the history and genetic mechanisms of large-scale coal-generated graphite deposits. The Shihangli graphite deposit, formed by significant siliceous hydrothermal alteration, is the most distinctive in Central Hunan. Re–Os dating of pyrite from the Shihangli graphite deposit demonstrates that the coal-generated graphite mineralization age is ~ 127.6 ± 3.8 Ma. Based on in-situ mineral analysis, the hydrothermal pyrite in the Shihangli graphite deposit is mostly enriched in Sb, As, Au, W, Ag, Cu, Pb, and Zn. Based on the pyrite Re–Os isochron, the initial (¹⁸⁷Os/¹⁸⁸Os) values of pyrite were 1.03 ± 0.24 and the Os(t) values varied from 571.8 to 755.1. Pyrite from the Shihangli graphite deposit comprises a Pb isotope composition similar to that of the Madiyi Formation bulk rock and stibnite from the Xikuangshan Sb deposit. Based on the Re–Os, Sr, S, and Pb isotopic compositions of sulfides in the graphite and Sb deposits in Central Hunan, the Madiyi Formation was likely the primary source of ore-forming elements (Sb, Au, and As). The Re–Os and Pb isotope compositions of pyrite most likely reflect when large-scale fluid migration and coal-generated graphite mineralization occurred in Central Hunan.

The Dunde iron-zinc polymetallic deposit is one of large iron deposits occurred in the Awulale Metallogenetic Belt, Western Tianshan (NW-China). This study reports new geochronology and geochemistry for granite in the Dunde mining area in order to constrain the tectonic-magmatic activities and metallogenesis of this region. Granites in the southwest of Dunde mining area are mainly syenogranites intruded into volcanics of the Dahalajunshan Formation in the Early Carboniferous, and they are far from the area where ore bodies and mineralized altered rocks are widely developed. LA-ICP-MS U–Pb zircon dating indicates that Dunde syenogranite was at 306.8 ± 1.0 Ma, which could constrain the upper limit of metallogenic age for this deposit. The Dunde granites are high SiO₂ (73.41–80.07 wt%), high differentiation index (D.I. = 89.7–95.0), weakly peraluminous to metaluminous (A/CNK = 0.94–1.08), and they are enriched in LILE and LREE and depleted in Eu, Ba, Sr and P₂O₅, indicating that they belong to highly fractionated I-type granite. Based on ε_Hf values (+ 9.2 to + 10.5) for zircon and high ε_Nd(t) values (+ 4.7 to  + 5.8) for whole-rock, and the two-stage model ages for 601–735 Ma, suggest that the magma source could be the juvenile lower crust. Combined with regional geological setting, the 306.8 Ma Dunde granites are formed in post-collision extensional tectonic setting.

The field and microstructural features coupled with mineral chemical data from microgranular enclave (ME) and host Mesoproterozoic Kanigiri granite (KG) pluton of Nellore Schist Belt (NSB), Southeastern India, have been documented in order to infer the likely processes responsible for the origin and evolution of ME and host KG magma. The ME and host KG bear the same mineral assemblages barring the KG which does not contain amphibole; however, they are modally disequilibrated. The ME in KG is originated due to multiple intrusions of ME magmas into the crystallizing host KG magma chamber. Field and textural features indicate the dynamic magma flow, mingling, and undercooling of the ME against a relatively cooler surface of host KG magma. The presence of NSB country rock xenoliths and its diffuse boundaries suggest the intrusive relation and marginal assimilation by the intruding KG magma. The occasional cumulate texture in the ME appears to have formed by the accumulation of early-formed minerals that crystallized rapidly in the ME magma globules. The ME shows the magmatically deform features developed due to the flowage and erosion by the subsequent intrusions of ME magma pulses into the crystallizing host KG magma chamber. The ME amphiboles show unusual composition as ferro-edenitic hornblende to potassian-hastingsitic hornblende, that crystallized in the subalkaline-alkaline transition, low fO₂(reducing to mildly oxidizing) magma. The unusual extremely low Mg/Mg + Fe^t = 0.015 (avg.) of ME amphiboles may be related to the changing physico-chemical (P, T, fO₂, and H₂O) condition of the ME magma or they might have crystallized in equilibrium with more evolved KG magma. The KG (FeO^t/MgO = 37.04, avg.) and ME (FeO^t/MgO = 77.72, avg.) biotites are siderophyllite, and buffered between QFM and NNO syn-crystallizing in the water undersaturated (H₂O ≈ 3.58 wt.% in KG; ≈3.53wt.% in ME), alkaline anorogenic (A-type) host magmas that were emplaced at mid-crustal (4–5 kbar; ~ 17 km) depth. Field, microtextural and mineral chemical evidences suggest that the alkaline KG magma originated from crustal source and evolved through synchronous fractionation, mixing, and mingling with coeval ME magmas in the KG magma chamber.

Groundwater quality monitoring and geochemical characterization in the phreatic aquifer are critical for ensuring universal and equitable access to clean, reliable, and inexpensive drinking water for all. This research was intended to investigate the hydrogeochemical attributes and mechanisms regulating the chemistry of groundwater as well as to assess spatial variation in groundwater quality in Satna district, India. To accomplish this, the groundwater data comprising 13 physio-chemical parameters from thirty-eight phreatic aquifer locations were analysed for May 2020 by combining entropy-weighted water quality index (EWQI), multivariate statistics, geochemical modelling, and geographical information system. The findings revealed that the groundwater is fresh and slightly alkaline. Hardness was a significant concern as 57.89% of samples were beyond the permissible limit of the World Health Organisation. The dominance of ions were in the order of Ca²⁺  > Na⁺  > Mg²⁺  > K⁺ and HCO₃⁻ > SO₄²⁻ > Cl⁻ > NO₃⁻ > F⁻. Higher concentration of these ions is mainly concentrated in the northeast and eastern regions. Pearson correlation analysis and principal component analysis (PCA) demonstrated that both natural and human factors regulate groundwater chemistry in the region. The analysis of Q-mode agglomerative hierarchical clustering highlighted three significant water clusters. Ca–HCO₃ was the most prevalent hydro-chemical facies in all three clusters. Geochemical modelling through various conventional plots indicated that groundwater chemistry in the research region is influenced by the dissolution of calcite/dolomite, reverse ion exchange, and by silicate and halite weathering. EWQI data of the study area disclosed that 73.69% of the samples were appropriate for drinking. Due to high salinity, Magnesium (Mg²⁺), Nitrate (NO₃⁻), and Bicarbonate (HCO₃⁻) concentrations, the north-central and north-eastern regions are particularly susceptible. The findings of the study may be accomplished by policymakers and groundwater managers to achieve sustainable groundwater development at the regional scale.