Solid Earth Sciences最新文献

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.

Tectonism often plays an important role in the mineralization process, which is generally thought to be the main controlling factor in the accumulation of economic materials (e.g., gold, coal, oil and gas) through deformation. However, numerous experimental and theoretical studies have suggested that tectonic stress not only causes deformation (physical changes) in rocks and minerals but also promotes their chemical changes by acting directly on chemical bonds and causing bond scission or regeneration, called tectonic stress chemistry (TSC). In recent years, TSC actions caused by tectonic activities have provided new ideas and evidence for explaining the chemical structural evolution of coal, hydrocarbon formation, organic (coal-derived) and inorganic graphitization and hydrothermal mineralization under shear stress. These background studies have provided incentives and insights into how tectonic stress affects the chemical structures of minerals, rocks and even ore-forming fluids in the process of mineralization. In this paper, we briefly review: (1) the concept of TSC; (2) the TSC process in the formation of shear zone type gold deposits from stress concentration, brittle fracturing, sudden reduction of fluid pressure, and flash vaporization to gold precipitation; (3) mechanisms of the macromolecular structural evolution of coal and gas generation under shear stress from deformation experiments and molecular dynamic simulations; (4) coal-derived graphitization caused by preferred orientation and extension of the basic structural units (BSUs) under shear stress; and (5) some preliminary experimental explorations on inorganic graphitization in carbonate-hosted shear zones. In addition, some existing problems and possible solutions for these processes are also discussed. Finally, we propose additional potential TSC processes in extensive geological processes, e.g., the relationship between deformation and metamorphism and trigger mechanisms of slow-slip earthquakes. To further explore these processes, a combination of experiments and molecular dynamic simulations should be undertaken by researchers.

In present study three sections of early Eocene Chorgali Formation were measured from Pail, Wanhar and Sar Kalan area, Central Salt Range, Punjab, Pakistan. At these localities the Chorgali Formation is about 12m, 8.8m and 15.1m thick, respectively. A total number of 53 samples were collected, 18 from Pail section, 18 from Wanhar section and 17 from Sar Kalan section. Eocene Chorgali Formation in this area is consisted of grey to pale grey limestone, greenish grey shale intercalations and argillaceous limestone. At Pail section nine various microfacies were recorded i.e., Mudstone microfacies (CGP1), Bioclastic mudstone microfacies (CGP2), Nummulites-Lockhartia wackestone microfacies (CGP3), Nummulitidae wackestone microfacies (CGP4), Bioclastic wackestone microfacies (CGP5), Alveolina-Nummulites wackestone microfacies (CGP6), Alveolina wackestone microfacies (CGP7), Nummulites-Alveolina wackestone microfacies (CGP8) and Intraclastic-peloidal packstone microfacies (CGP9).At Wanhar section five facies were recorded i.e., Rotaliidae wackestone microfacies (CGW1), Nummulitidae Wackestone microfacies (CGW2), Nummulites- Lockhartia wackestone microfacies (CGW3), Nummulites-Assilina wackestone microfacies (CGW4), Intraclastic-peloidal packstone microfacies (CGW5).At Sar Kalan section total four facies were recorded i.e., Bioclastic wackestone microfacies (CGSK1), Nummulites-Assilina wackestone microfacies (CGSK2), Nummulitidae wackestone microfacies (CGSK3), Intraclastic-peloidal packstone microfacies (CGSK4).The assemblage of larger foraminifera were recorded to describe the biota of the formation and to interpret the paleo-environments with implications of sequence stratigraphy. Field observations and microfacies analysis suggest that the deposition of Chorgali Formation at these localities probably took place in inner shelf conditions. Presence of shallow water benthic larger foraminifer's further support lagoon to bay environment of the genesis of the formation. The formation might have been deposited because of falling stage system tract (FSST), showing a progradational pattern of deposition. The basin ward shift of deposition indicates the regressive sequence.

Raman spectroscopy, an ideal technique to quantify the Mg–Al cation distribution on the tetrahedral T-sites and octahedral M-sites of MgAl₂O₄-spinel, was not calibrated before. By performing 16 annealing experiments on some MgAl₂O₄-spinel crystal plates at P from 1 atm to 5.0 GPa and T from 823 to 1873 K, a series of samples with different magnitudes of cation disorder (characterized by the inversion parameter x, i.e., the molar fraction of Al cations on the T-sites) have been generated. Single-crystal X-ray diffraction analyses have been performed to all these samples, and found the x values varying from 0.146 (15) to 0.362 (17). Multiple Raman spectra have been collected and analyzed for every sample. The Raman results show that the Raman scattering capabilities of the Al cations and the Mg cations on the T-sites are different, and their ratios are dependent on the Mg–Al cation distributions (i.e., x). With our extensive single-crystal X-ray diffraction data and Raman data, correlations between the x values and the relative Raman intensities of the ∼766 and ∼722 cm⁻¹ peaks (I_∼766/I_∼722 ratio), respectively caused by the internal vibrational modes of the MgO₄ and AlO₄ groups, have been established: the equations are x = 0.121 × log₁₀²(I_∼766/I_∼722) − 0.344 × log₁₀(I_∼766/I_∼722) + 0.392 (R² = 0.927; Raman peak height data used) and x = 0.069 × log₁₀²(I_∼766/I_∼722) − 0.253 × log₁₀(I_∼766/I_∼722) + 0.379 (R² = 0.928; Raman peak area data used). With this calibration, Raman spectroscopy can now be conveniently used to determine the x values of the MgAl₂O₄-spinel of different geological origins, significantly facilitating the inference of the thermal history of relevant geological bodies.

The evolution of the U-Pb decay system is determined by their initial isotopic composition in the proto-Earth and the subsequent global differentiation. The differentiation is highly complicated because of large-scale evaporation and multi-stage core formation in Earth accretion. We statistically rebuild the accretional history of Earth using a series of N-body simulations. This provides us with an estimation of the amount of silicate melting and thus temperature and pressure at the bottom of the magma oceans driven by continuous planetesimal impacts. We further assumed different evolutionary paths of the redox state and found a reduced process from an oxidized state consistent with the current value of Pb content and μ value (²³⁸U/²⁰⁴Pb) in the bulk silicate Earth. Meanwhile, the fraction of the impactor's core that participates in the re-equilibration is around 0.2–0.7. Our model predicts the final μ value equals the observed value, 8.25, regardless of the minor contribution of the late veneer (0.2). The evolution of μ determines the growth rate of radiogenic Pb isotopes. The episodic increase of μ in multi-stage core formation accelerates the growth of radiogenic Pb isotopes (²⁰⁶Pb and ²⁰⁷Pb) and finally causes a slight deviation of the composition of Pb isotopes (²⁰⁶Pb/²⁰⁴Pb and ²⁰⁷Pb/²⁰⁴Pb) to the right of 4.567-Ga Earth Geochron. A multi-stage evolution model for U–Pb system can explain the modern terrestrial μ value, but has little influence on the puzzle of “the first Pb paradox”.

Combined use of trace elements, rare earth elements (REEs) with hydrogen (δH) and oxygen (δ¹⁸O) stable isotope composition to elucidate origin and paleo-environment-conditions and reconstruction is a contemporary trend in the field of geochemistry. Geochemical investigations were carried out on the shale deposit from five wells within the Asu River Group Formation, exposed at Ikwo, Lower Benue Trough (LBT). Results of trace elements for the shale deposits examined showed averages of Co (19.10 ppm), Th (16.50 ppm), Zn (103.73 ppm), Sr (203.71 ppm) and Zr (292.80 ppm) compared to PAAS and UCC. Observed negative Eu anomalies, enriched LREEs and depleted HREEs patterns have shown that the shales are from rocks of continental origin. This is provided by Al/(Al + Fe + Mn) with a value > 0.2. Plots of Zr/Sc vs Th/Sc, La/Th vs. Hf and Cr/V vs. Y/Ni indicated felsic igneous rock precursors for the shale samples. C-parameter, ratios of Rb/Sr and Sr/Cu signify paleo-climatic conditions of semi-humid to arid, Ba/Al showed low paleo-productivity of the basin during shale deposition, largely within the freshwater setting according to the ratio of Sr/Ba. V/(V + Ni), U/Th, Ni/Co ratios and Ce/Ce∗ anomalies have revealed an oxic depositional environment with Fe/Ti > 20 suggesting hydrothermal activity. The temperature of formation (120 °C–∼195 °C) coupled with δ¹⁸O (+17 to + 23‰) and δH (−46.90 to −38.80‰) is consistent with materials of sedimentary origin from chemically weathered felsic precursors under humid climatic conditions with an influence of hydrothermal activity.