Solid Earth Sciences最新文献

The investigation of glacial sediments has not received a lot of attention, but the processes in place on the surface of glaciers are quite interesting and multidirectional. In this article, we focused on glacial sediments material sampled from the surface of the Bellingshausen Ice Dome (King George Island, South Shetland Islands, Antarctica). These sediments have different genesis: material from cryoconite holes, denuded layers of volcanic ash, flushed ash and soils and soil-like bodies formed in the glacial zone. Chemical analysis of the samples showed that the bulk composition of sediments is as follows: SiO₂ > Fe₂O₃ > Al₂O₃ > SO₃ > CaO > MgO > Na₂O > TiO₂ > K₂O > P₂O₅ > MnO (with SiO₂ content 50–55%, Fe₂O₃ – 17–23%, Al₂O₃ – 6–12%). The loss on ignition was maximum (10–11%) for samples taken at the top of the dome. Calculation of geochemical indexes showed that the mineral part of the samples is a product of erosion and sedimentation processes and is less chemically weathered in the lower part of the ice dome. The weathering type is fersiallitic. The maximum content of organogenic compounds (Total organic carbon – up to 5%, ammonium nitrogen – up to 116 mg/kg and mobile potassium – up to 373 mg/kg) also recorded at the top of the glacial dome; this may be associated with microbiological activity in cryoconite holes and the penetration of organic material from bird fauna. The low content of organogenic compounds on the slopes is caused by the processes of their washout with melt water. The content of trace metals Zn, Ni, and Pb found at higher elevations and distribution of Cu and Cd have more a local character associated with tourism activities and anthropogenic influence from year-round scientific stations. Thus, a number of multidirectional processes take place in the glacial sediments on the Bellingshausen Ice Dome, these are not simple denuded layers of ancient volcanic ash. The glacier cover can be considered as a “living” biogeochemical shell, accumulating products of microbiological and anthropogenic activity, products of erosion and sedimentation and organic matter of ornithogenic origin.

A new procedure has been developed for high precision and high accuracy Mg isotope analysis on multiple-collector inductively coupled plasma mass spectrometry using a critical mixture double spike technique. Errors arising from improper preparation of the critical double spike solutions are able to be corrected against the regression on a set of over- and under-spiked standards. Accuracy is ensured by correcting mass bias offset based on Monte Carlo calculations. Doping experiments indicate that the double spike method is robust to non-spectrum matrix effects. A long-term precision and accuracy of ±0.03‰ (2SD) was demonstrated for δ²⁶Mg by replicate analyses of well-characterized in-house pure Mg solutions and synthetic samples passed through the column chemistry, provided that each sample was measured four times. The robustness of the method was further assessed by replicate analyses of fifteen geological reference materials ranging from peridotite, basalt, granodiorite, carbonatite to seawater. Rock standards yielded consistently higher δ²⁶Mg by 0.076 ± 0.052‰ (2SD, N = 12) compared to data previously reported by standard-sample-bracketing from the same lab. This discrepancy might result from the difficulty in matching the matrices of natural samples exactly the same to the bracketing standards, even after purification, suggesting a careful evaluation on residual matrix effect for the standard-sampling-bracketing method. Our new data for geological reference materials serve as a reference for quality assessment and inter-laboratory comparison in future studies.

The shales of the Mamfe basin were geochemically analyzed to determine their paleosalinity and hydrothermal characteristics which existed at the time of their deposition. Elemental ratios such as boron/gallium (B/Ga), strontium/barium (Sr/Ba), and total sulphur/total organic carbon (S/TOC) and TOC/S were applied for paleosalinity reconstruction. The shales of the Mamfe basin show B/Ga is <3, Sr/Ba <0.2, and S/TOC is <0.1 and TOC/S >6 the values of the elemental ratio show that the Mamfe shales were deposited in a freshwater environment. For hydrothermal activities ratio, Sc/Cr was used and Binary diagrams TOC vs P and Co/Zn vs TOC were constructed to discriminate between normal water, mixed and hydrothermal influence. The Sc/Cr ratio was greater than 0.14 indicating that the shales in the Mamfe basin were deposited in a normal water environment with no influence from hydrothermal processes. The binary discriminant diagram of TOC vs P and Co/Zn vs TOC shows that all the shales were not influenced by any hydrothermal processes during the time of their deposition.

The dissolution behavior of carbonates in subduction zone fluids has not been well constrained. In this study we investigated the solubility of magnesite in pure water and NaCl solutions (with up to 19 wt% NaCl) in situ with a hydrothermal diamond anvil cell, and determined carbon speciation in the fluid by Raman spectroscopy. The solubility of magnesite in pure water falls in the range of 0.01–0.05 mol/kg at 0.7–2.4 GPa and 635–940 °C, enhanced strongly by temperature. Least squares fitting of experimental data leads to the following empirical expression for magnesite solubility in pure water: $C_{{\text{MgCO}}_3}=(964510\pm 41362)\exp \left(\frac{-6953\pm 690}{T}\right)\exp \left[\frac{0.0200\pm 0.0098}{T}(P-1)\right]$ where $C_{{\text{MgCO}}_3}$ is given in μg/g, T is the temperature in K, and P is the pressure in bar, respectively. The solubility of MgCO₃ is about an order of magnitude lower than that of CaCO₃ at 0.7–2.4 GPa, 640–940 °C. The solubility enhancement factor by NaCl (m/m° with m and m° being magnesite solubility in NaCl solution and water, respectively) presents as a parabolic trend with the mole fraction of NaCl in the range of 0–0.07, with a maximum amplification of 5.2 at X_NaCl = 0.035, which is different from the continuously increase of solubility with salinity increasing at high salinity conditions in previous studies and suggests the dissolution reaction of magnesite in dilute NaCl solution is different. Despite slight contamination of CH₄ formed by the reaction of the diamond anvils, we were able to identify CO₃²⁻ and HCO₃^- to be the aqueous carbon species, HCO₃^- was predominant over CO₃²⁻ in the range of 200–800 °C and 1.9–3.8 GPa and its proportion was affected by temperature, but not affected by pressure at 400–600 °C. Our experimental data suggest that in the absence of melting, only a small amount of magnesite can be mobilized by the slab-released fluid at subarc depths.

Identifying magmatic rock associations in a subduction zone is substantial for understanding the related geodynamic evolution. The final closure time of the Paleo-Asian Ocean and the exact subduction processes have continuously been controversial, hindering our recognition of the tectonic evolution of the Central Asian Orogenic Belt (CAOB). Here we investigated the Early Permian gabbros-granodiorites from Xi Ujimqin in the northern Inner Mongolian region. The gabbros have slightly depleted light rare-earth elements [(La/Yb)_N = 0.8] and flat heavy rare earth elements (HREE) [(Gd/Yb)_N = 1.1] that are comparable to those of N-MORB. They show depletion in Nb and Ta, positive whole-rock ε_Nd(t) values of +7.7 to +8.7 and zircon ε_Hf(t) values of +8.7 to +10.6. These mafic rocks are interpreted to be products of partial melting of depleted mantle that had been metasomatized by slab-derived fluids, with subsequent fractional crystallization and minor crustal contamination. These granodiorites chemically resemble typical adakites and have MORB-like whole-rock SrNd isotopic compositions (⁸⁷Sr/⁸⁶Sr_i = 0.7028–0.7029; ε_Nd(t) = +8.2–+8.5) and positive zircon ε_Hf(t) (+13.4–+15.9) values, suggesting that they originated from the subducted slab-melts and reacted with mantle wedge peridotite. UPb zircon dating shows emplacement of N-MORB-type gabbros and adakitic granodiorites at ∼297 Ma and ∼290 Ma, respectively. Our new data indicate the presence coeval slab-derived adakites and slab fluid-metasomatized asthenosphere-derived N-MORB-type rocks, indicating that subduction lasted until at least the Early Permian. Such an association along with the extension-related magmatism in northern Mongolia recorded the northward ridge subduction of the Paleo-Asian Ocean during the Late Carboniferous–Early Permian. This model is consistent with the spatial distribution and the ages of magmatic activity with positive ε_Nd(t)–ε_Hf(t) values from this region. Ridge subduction and induced slab windows probably played a key role in Paleozoic crustal growth of CAOB, and by inference in the other accretionary orogens throughout Earth history.

South China developed large-scale Mesozoic magmatism and polymetallic mineralization, especially Jurassic W-Sn and Mo-W mineralization. Compared with W-Sn deposits, Mo-W deposits in South China have received less attention. The recently discovered Gangmei Mo-W deposit, which occurred in Guangdong Province, is the southernmost Jurassic Mo-W mineralization in South China. Here, we report the molybdenite Re-Os isotopic age of the Gangmei deposit and compile the Mo-W- and W-Sn-associated deposits in South China and their Re concentrations in molybdenites, aiming at constraining the ore-forming age of the Gangmei deposit, the scope of Mo-W mineralization in South China and the possible controlling factors of different mineralization types. Four molybdenite samples from the Gangmei deposit were chosen for Re-Os isotopic dating. The Re-Os model ages vary from 162.6 ± 1.6 Ma to 164.1 ± 1.6 Ma with a weighted mean age of 163.1 ± 1.4 Ma and yield an isochron age of 162.2 ± 4.1 Ma (MSWD = 1.01), consistent with the emplacement age of the Gangmei intrusion, indicating a genetic relationship between magmatic activity and mineralization. This age also agrees well with the large-scale Jurassic Mo-W- and W-Sn-associated mineralization in South China, suggesting that the scope of Jurassic Mo-W-associated mineralization can reach the southernmost part of South China. Rhenium concentrations in molybdenites from Jurassic Mo-W-associated and W-Sn-associated deposits in South China suggest that their magma sources are different, and Mo-W-associated deposits may involve more mantle-derived materials. In addition, oxygen fugacity may be another factor controlling different types of mineralization in South China.

The Physico-chemical and mineralogical properties of the Cretaceous shales of Calabar Flank southeastern Nigeria were examined for their Provenance, Paleo-weathering Conditions and Industrial Applications. Twenty samples of shale from Ekekpon and Nkporo Formations were analysed using X-Ray Fluorescence, X-ray diffraction (XRD), scanning electron microscopy (SEM) and geotechnical techniques. Results from the major oxides of elements revealed that the Nkporo Shale is enriched with SiO₂ > Al₂O₃ > LOI > Fe₂O₃ > SO₃ while the Ekepkon shale is also enriched in SiO₂ > Al₂O₃ > LOI > Fe₂O₃ > CaO > K₂O. The XRD revealed kaolinite as the major clay mineral, while other non-clay minerals are quartz, vallerite, mica, nacaphite, sodalite. The SEM shows that the Ekepkon Shale is enriched in calcium. The correlation matrix revealed SiO₂ resides more in the quartz phase of the Nkporo Shales, while in the Ekekpon Shale the SiO₂ is associated with the clay. The element ratios and discrimination diagrams revealed that the provenance of the shales is of intermediate to mafic igneous rocks and a tectonic setting of continental to oceanic arcs. The presence of kaolinite in the Nkporo Shale with the chemical index of alteration (CIA) values ranging from 83.58 to 96.93, indicated that the shale is derived from intensive chemical weathering in the source area. The low CIA values (37.90–76.96) in Ekenkpon Shale is attributed to CaO and calcium enrichment in the shale and the shale contains high quartz and kaolinite therefore the shale is suggested to have been derived from an intermediate to intense weathering activities in the source area. The geotechnical properties revealed that both shales are suitable for ceramic and refractory industries. The Ekenkpon Shale will be suitable for use as filling material and as raw material in cement industries. However, the shales do not meet the specifications for paper and rubber production due to the high contents of iron oxide_, lime, magnesia, potash and soda.

The Sanmenxia Basin, located on the southeastern margin of the Shanxi rift and filled with Cretaceous-Paleogene fluvial and lacustrine sediments, is a faulted basin bounded by a series of normal strike-slip faults. This provides a valuable opportunity to investigate the early tectonic deformation in the Fenwei graben. In this study, we report an integrated rock magnetism and AMS analysis of two sections from the Sanmenxia Basin spanning an interval from the Late Cretaceous to the Eocene. The rock magnetic results demonstrate that magnetite and hematite are the main magnetic carriers of remanence, and paramagnetic minerals and hematite are major contributors to the AMS in both sections of the Sanmenxia Basin. Together with the relatively low-corrected anisotropy values, the tightly grouped minimum principal axes are almost perpendicular to the bedding plane, and the well-defined magnetic lineation is generally parallel to the bedding trend, indicating that the primary sedimentary fabrics in the Sanmenxia Basin were overprinted by the initial deformation. The NW–SE magnetic lineation denotes the NW–SE stretching during the Late Cretaceous-Eocene. The stretching process may have been controlled by the Indian-Eurasian convergence and/or the subduction of the western Pacific plate.