Acta Geochimica最新文献

We studied an Archean mafic dike in the Trans-North China Orogen of the North China Craton, which has a magmatic age of 2701 ± 83 Ma and is currently the oldest mafic dike in the North China Craton. Such an old dike is extremely rare in the world. The presence of mafic dikes indicates that the North China Craton was in a tensional tectonic environment at 2.7 Ga. Geochemical characteristics reveal that this mafic dike belongs to continental tholeiitic basalt. Results from Hf isotope analysis reveal that the mafic dike originates from a depleted mantle. The plate assembly in the North China landmass was realized during the Archean era (2.7 Ga), and a thick and stable continental crust was formed. Therefore, the first cratonization of the North China Craton was completed before 2.7 Ga. The intrusion of the 2.7-Ga-old mafic dike from the deep lithospheric mantle of the continent indicates that the North China Craton has undergone a period of extensional tectonic activity. This event marks a significant extensional event that occurred after the cratonization of the North China Craton.

The (ultra-)mafic mine tailings pond revealed a weathering discrepancy in the tailings profile, which provided a valuable analog to assess the role of carbonate and silicate weathering of the basalt. In this study, drill-cores samples were selected from the Wanniangou V–Ti–Fe mine tailings pond (Sichuan province, China) to investigate the mineralogicand geochemical characteristics in the tailings profile. The results reveal (1) the tailings pond profile consist of upper and lower layers. The upper layer is composed of carbonate weathering (1.4 %), which was formed in the initial stages of tailings exposure and represented a minimal weathering degree. (2) The lower layer was primarily observed at the aquifer zone of the tailings pond, and was consistent with 0.45 % carbonate weathering and 48.4 % silicate weathering. (3) The weathering discrepancy in the tailings profile could be due to the sulfide oxidation and aerobic/flowing aquifer, which facilitate the water-tailings reactions. The tailings profile provides an analog to studying basalt weathering, as it spans both carbonate and silicate weathering. This research reinforces the idea that silicate weathering is predominant in basaltic areas and plays a crucial role in regulating atmospheric CO₂ (carbon dioxide) levels on Earth.

The Proterozoic metasedimentary rocks of the Yaounde Group on the northern edge of the Congo Shield in Central Africa were investigated to understand their provenance and depositional environment. Petrography, geochemistry, and field evidence helped to subdivide the metasediments into paragneiss, mica schist, chlorite schist, and quartzite which were derived from greywacke, shale, quartz arenite, litharenite protoliths. They are immature with some mature samples, moderately weathered and reworked Neo- and Post-Archean metasediments. Rare earth element signatures (Chondrite Eu/Eu* ≤ 1), enrichment of light rare earth elements over the heavy ones, and the La/Sc ratio (> 0.7) are compatible with those of the intermediate and felsic sources from the upper continental crust. These metasediments were deposited in the continental arc setting and have evolved during Proterozoic times according to the Wilson cycle to form the West Gondwana including NE Brazil.

Aptian is characterized by widespread deposition of organic-rich sediment. The Aptian bitumen limestone horizon, which is thin decimetre-thick sequences, locally crops out in the Kırcaova area, Eastern Black Sea Region (Eastern Pontides). They are well correlated with Aptian bitumen limestone in the other Tethys Reams. They are proposed as episodes of increased organic matter. However, background factors controlling organic matter enrichment are poorly known. In this study, we present new inorganic geochemistry, including trace elements, rare earth elements (REE), redox-sensitive elements (RSE), stable-isotopes (δ¹⁸O and δ¹³C), and total organic carbon (TOC). We integrated new geochemical data with existing stratigraphy, paleontology, and organic chemistry data to provide new insight into the depositional environment and paleoclimate conditions during Aptian. The lacustrine bitumen limestone (LBL) samples have varied δ¹³C (ave. -1.45‰) and δ¹⁸O (ave.-4.50‰). They possess distinct REE patterns, with an average of REE (ave. 14.45 ppm) and Y/Ho (ave. 35) ratios. In addition, they have variable Nd/Yb_N (0.28–0.81; ave. 0.56) and Ce/Ce* (0.68–0.97; ave. 0.86), and relatively high Eu*/Eu (1.23–1.53; ave. 1.35). They display seawater signatures with reduced oxygen conditions. The enrichment in RSE (Mo, Cu, Ni, and Zn) and the low Mo/TOC (0.70–3.69; ave. 2.41) support a certain degree of water restriction. The high Sr/Ba, Sr/Cu, Ga/Rb, and K/Al records of the LBL facies suggest hot house climatic conditions. The sedimentary environment was probably an isolated basin that is transformed from the marine basin. In addition to depositional conditions, the regional parameters such as the climate, increased run-off period, nutrient levels, alkalinity level, and dominant carbonate producers favored the enrichment in organic matter of LBL facies. Thus, extreme greenhouse palaeoclimate conditions have an important role in organic matter enrichment in the isolated basin. Our results are conformable with the published data from marine, semi-restricted basin, and lacustrine settings in the different parts of the Tethys margin. Thus, this approach provides the first insight into the Aptian greenhouse paleo-climate conditions of the Eastern Black Sea Region, NE Turkey.

The unit-cell parameters and volumes of geikielite (MgTiO₃) and ilmenite (FeTiO₃) were investigated at high temperatures up to 700 K and ambient pressure, using in-situ angle-dispersive synchrotron X-ray diffraction. No phase transition was detected over the experimental temperature range. Using (Berman in J Petrol 29:445–522, 1988. 10.1093/petrology/29.2.445) equations to fit the temperature-volume data, the volumetric thermal expansion coefficients at ambient conditions (α_V0) of MgTiO₃ and FeTiO₃ were obtained as follows: 2.55 (6) × 10^–5 K⁻¹ and 2.82 (10) × 10^–5 K⁻¹, respectively. We infer that the larger effective ionic radius of Fe²⁺(VI) (0.78 Å) than that of Mg²⁺(VI) (0.72 Å) renders FeTiO₃ has a larger volumetric thermal expansivity than MgTiO₃. Simultaneously, the refined axial thermal expansion coefficients under ambient conditions are α_a0 = 0.74 (3) × 10^–5 K⁻¹ and α_c0 = 1.08 (5) × 10^–5 K⁻¹ for the a-axis and c-axis of MgTiO₃, respectively, and α_a0 = 0.95 (5) × 10^–5 K⁻¹ and α_c0 = 0.92 (12) × 10^–5 K⁻¹ for the a-axis and c-axis of FeTiO₃, respectively. The axial thermal expansivity of MgTiO₃ is anisotropic, but that of FeTiO₃ is nearly isotropic. We infer that the main reason for the different axial thermal expansivity between MgTiO₃ and FeTiO₃ is that the thermal expansion mode of the Mg-O bond in MgTiO₃ is different from that of the Fe–O bonds in FeTiO₃.

The Lake Tana area is located within a complex volcano-tectonic basin on the northwestern Ethiopian plateau. The basin is underlain by a thick succession of Oligocene transitional basalts and sub-alkaline rhyolites overlain in places, particularly south of the lake, by Quaternary alkaline to mildly transitional basalts, and dotted with Oligo-Miocene trachyte domes and plugs. This paper presents the results of integrated field, petrographic, and major and trace element geochemical studies of the Lake Tana area volcanic rocks, with particular emphasis on the Oligocene basalts and rhyolites. The studies reveal a clear petrogenetic link between the Oligocene basalts and rhyolites. The Oligocene basalts are: (1) plagioclase, olivine, and/or pyroxene phyric; (2) show an overall decreasing trend in MgO, Fe₂O₃, and CaO with silica; (3) have relatively low Mg#, Ni and Cr contents and high Nb/La and Nb/Yb ratios; and (4) show LREE enriched and generally flat HREE patterns. All these imply the origin of the Oligocene basalts by shallow-level fractional crystallization of an enriched magma sourced at the asthenospheric mantle. The Oligocene rhyolites: (1) are enriched in incompatible while depleted in compatible trace elements, P and Ti; (2) show a strong negative Eu anomaly; (3) contain appreciable amounts of plagioclase, apatite, and Fe-Ti oxides; and (4) show clear geochemical similarity with well-constrained rhyolites from the Large Igneous Province (LIP) of the northwestern Ethiopian plateau. Low-pressure fractional crystallization of mantle-derived basaltic magma in crustal magma chambers explains the origin of these rhyolites. Our study further shows that the Oligocene basalts and rhyolites are co-genetic and the felsic rocks of the Lake Tana area are related differentiates of the flood basalt volcanism in the northwestern Ethiopian plateau.

Cadmium (Cd) isotopes in seawater have been proven as an important geochemical tool for tracing ocean Cd circulation in the modern ocean. In this study, we evaluated a new method to separate Cd (~ 60 ng) from seawater using Chelex resin (1.0 g) coupled with AG-MP-1M resin. The results show that the Chelex resin is sufficient to remove Cd from Na and Mg matrix with Cd recoveries at 98.3 ± 3.5 % (2SD, N = 6); while AG-MP-1M resin could separate Cd from the residual Na, Mg, and isobaric inferences. The total Cd recoveries of the method are 96.3 ± 1.5 % (2SD; N = 4) and the salinity of the samples has no significant impacts on Cd recovery. Cd isotope ratios were measured using a Nu Plasma III MC-ICP-MS and ¹¹¹Cd–¹¹⁰Cd double spike technique. By comparing the δ^114/110Cd values (0.00 ± 0.06‰) of synthetic seawaters doped with Cd isotope standard (NIST-3108; treated by Chelex + AG-MP-1M resin) and the reference value (~ 0.00 ‰), no variations were observed. We also analyzed the Cd isotope compositions of three deep seawaters from a column at the Southwest Indian Ocean Ridges (SWIR). The δ^114/110Cd values of the column are decreased from 1.05 ± 0.05 ‰ at 3200 m to 0.36 ± 0.05 ‰ at 2800 m, differing from reported δ^114/110Cd values of deep seawater in other oceans. Considering the spatial distance between the column and active hydrothermal vents in SWIR (~ 13 km), we propose that such positive δ^114/110Cd values of deep seawater were likely contaminated by vent fluids, which could provide heavy Cd isotope to deep seawaters. This study demonstrates that Cd isotope is more sufficient to distinguish the impact of plumes on deep seawater.

In the last decade, North Africa has witnessed significant population growth, particularly those bordering the Mediterranean Sea. This led to increased demand for groundwater, which is an essential source for various water uses such as drinking water supplies and irrigation. Generally, human activities play a crucial role in the different quantitative and qualitative changes in groundwater. Now, climate changes such as a decrease in precipitation have also led to a shortage of water resources and a decline in the groundwater table. This paper presents the impact of climate changes on groundwater resources in the Ain Azel region, Setif, northeastern Algeria. The analysis of long-term spatiotemporal variability in rainfall over 63 years (1958–2021) revealed a significant decline in groundwater recharge, especially after 2013. In contrast, the Pettitt and Mann–Kendall tests show increased temperatures with breaks between 1984 and 1986. A piezometric analysis of the alluvial aquifer demonstrated a significant decline in groundwater levels in the last 20 years. Hydrochemical analysis showed that groundwater in the region is dominated by Ca–Mg–Cl water type, which indicates the presence of water salinity phenomenon. Water Quality Index (WQI) analysis showed the deterioration of groundwater in the area, which may be caused by several factors: brine intrusion from the Salt Lake (Sebkha) in the north; the dissolution of evaporites (Triassic) and/or anthropogenic sources of agricultural and industrial origin. Our findings provide an overview summarizing the state of groundwater, which will help improve groundwater resource management in the region in the coming years.

We carry out a chemical treatment (acidization or basification) of typical rock specimens in-situ, and characterize the emerging pattern resulting from the infiltration-precipitation scenario. Galena and limestone samples were treated with sulfuric acid, while pyrite was reacted with sodium hydroxide. Various infiltration techniques were employed, after selection of the most feasible method for each rock separately. The patterns of anglesite (PbSO₄), anhydrite (CaSO₄) and goethite (FeOOH) deposition presented different alteration modes of the bare rock textures. Among the three deposited minerals, only the anhydrite (CaSO₄) displayed a band stratification. The formation of a Liesegang pattern in the rock of highest porosity indicates a plausible correlation between the band formation and a minimum porosity requirement. A banded rock of compact texture could then be formed by a cementation mechanism, governing the long time evolution of the rock.