MgO Content Research Articles

Assessment The Mechanical and Color Changes Properties of Denture Base Material After Reinforcement with Nanoparticles Material

Although PMMA is widely used in prosthetic dentistry, but broken dentures are commonly known clinical events that remain as unsolved problems. The aim of this study was to evaluate the effect of incorporation Magnesium oxides nanoparticles with different concentrations on hardness and color changes o f heat cure acrylic resin. A total of 40-disc shape specimens with a diameter of (12±0.1) mm and a thickness of (2±0.1) mm were prepared from heat cure acrylic resin. They were grouped into 4 groups according to nanoparticles concentrations (10) specimens for each group, then they are subjected to hardness and color changes tests. Control group (A): (10 specimens) without addition. Group (B): 10 specimens of acrylic resin with (2%) concentration of MgO. Group (C): 10 specimens of acrylic resin with (4%) concentration of MgO. Group (D): 10 specimens of acrylic resin with (6%) concentration of MgO. The results showed there is high significant difference for MgO (2%, 4%) when compared with the controls. While there is no significant variation for MgO (6%). color changes showed there is a non-significant difference for all groups except for (6%) group which revealed there is high significant difference. Adding limited concentration of MgO (2%, 4%) have a positive effect on hardness of surfaces and color changes of heat-cured acrylic resins. any Increase in Concentration of MgO will effect adversely on hardness of surfaces and color stabiliti

Crustal basement terranes under the Taupō Volcanic Zone, New Zealand: Context for hydrothermal and magmatic processes

Continental magmatic arcs commonly have heterogeneous and anisotropic metamorphic basement foundations hosting their plutonic roots and associated geothermal systems. Quaternary volcanic and volcaniclastic rocks of New Zealand's Taupō Volcanic Zone (TVZ) lie above and between the Torlesse and Waipapa Composite Terranes, two of New Zealand's major basement geological units. Deep geothermal and magma systems beneath TVZ interact with and/or are hosted in these Mesozoic metasedimentary crustal basement terranes rather than in Quaternary volcanic and volcaniclastic fill. New mineralogical, whole rock geochemical, O, Sr, Nd and Pb isotopic, and U-Pb zircon age data from lava-hosted xenoliths and ignimbrite-hosted lithics imply that the terrane boundary is not sub-vertical as previously thought, but instead dips gently west. Most of the non-igneous crust under the TVZ is thus composed of Torlesse Composite Terrane metasedimentary rocks, which have higher quartz, mica and SiO2 abundances, higher 87Sr/86Sr, 206Pb/204Pb and δ18O ratios, lower chlorite and MgO contents and lower 143Nd/144Nd ratios than their Waipapa Composite Terrane counterparts. This new interpretation provides important new constraints to refocus geodynamic understanding, petrological modelling, and future deep geothermal drilling.

Petrogenesis of Syn-Collisional Adakitic Granitoids and Their Copper Mineralization Potential in the North Qilian Suture Zone

The petrogenesis of late Ordovician–early Silurian adakitic plutons in the North Qilian suture zone (NQSZ) and their copper mineralization potential remain poorly understood. Here we present a detailed study of the Heishishan (HSS) granodiorite–granite pluton, spatially associated with Cu–Au mineralization in the eastern section of the NQSZ. Zircon U–Pb dating confirms that the granodiorite–granite were formed at ca. 438–435 Ma, in association with a continental collision. Geochemically, the granitoids resemble low-Mg adakitic rocks featured by elevated Sr/Y and (La/Yb)N ratios with depleted MgO, Cr, and Ni concentrations, suggesting minimal mantle contribution. They are sodium rich with K2O/Na2O < 1, and have higher and more varied Sr/Y, but lower La/Yb than those from the continental lower crust. The εHf(t) values of zircon grains are positive and vary in a wide range of +2.0–12.7, indicating a heterogeneous source rather than a single arc basaltic source. They show moderately radiogenic Sr and Nd isotope compositions with initial 87Sr/86Sr ratios of 0.705101–0.706312 and εNd(t) values of +0.5–1.0, most likely a mixed source of the oceanic basaltic crust plus ca. 15–20% overlying sediments. The magmatic oxygen fugacity was relatively low as indicated by zircon Ce(IV)/Ce(III) ratios of 32–156, which is unfavorable for a large copper mineralization.

Hybrid Nanocomposite Solid Electrolytes (n-C4H9)4NBF4-MgO.

Hybrid nanocomposite materials Bu4NBF4-MgO were obtained using a nanocrystalline MgO with a specific surface area of 324 m2/g and the grains size of 5.1 nm. As a result of the strong adhesion, the salt transforms into an interface-stabilized amorphous state within the thin layer near the interface. The analysis of the DSC data allowed one to estimate the concentration and the thickness of this amorphous layer as 4.8 nm. The amorphous interface phase has an enhanced ionic conductivity. As a result, conductivity of the nanocomposite increases with the concentration of the amorphous phase and reaches 1.1 × 10-3 S/cm at 150 °C at a concentration of the MgO additive x = 0.90 corresponding to the maximum content of the amorphous phase. The conductivity of the nanocomposite is by three orders of magnitude higher than the conductivity of pure Bu4NBF4. The nanocomposites are electrochemically stable up to 2.5 V. At high concentrations of MgO when the total volume of the salt is small the composites become nano- and mesoporous.

Biotite Geochemistry and Its Implication for the Difference in Mineralization in the Xiongcun Porphyry Cu–Au Ore District, Tibet

The Xiongcun Cu–Au ore district is in the southern middle Gangdese Metallogenic Belt, Tibet, and formed during Neo-Tethyan oceanic subduction. The Xiongcun ore district mainly comprises two deposits, the No. I and No. II deposits, which were formed by two individual mineralization events according to deposit geology and Re–Os isotopic dating of molybdenite. The No. I deposit is similar to a reduced porphyry copper–gold deposit, given the widespread occurrence of primary and/or hydrothermal pyrrhotite and common CH4-rich and rare N2-rich fluid inclusions. The No. II deposit, similar to classic oxidized porphyry copper–gold deposits, contains highly oxidized minerals, including magnetite, anhydrite, and hematite. The halogen chemistry of the ore-forming fluid from the No. I and No. II deposits is still unclear. Biotite geochemistry with halogen contents was used to investigate the differences in ore-forming fluid between the No. I and No. II deposits. Hydrothermal biotite from the No. I deposit, usually intergrown with sphalerite, is Mg-rich and classified as phlogopite and Mg-biotite, and hydrothermal biotite from the No. II deposit is classified as Mg-biotite. Hydrothermal biotite from the No. I deposit has significantly higher SiO2, MnO, MgO, F, Li, Sc, Zn, Rb, Tl, and Pb contents and lower Al2O3, FeOtot, Cl, Ba, Cr, V, Co, Ni, Y, Sr, Zr, Th, and Cu contents than the biotite from the No. II deposit. Hydrothermal biotites from the No. I and No. II deposits yield temperatures ranging from 230 °C to 593 °C and 212 °C to 306 °C, respectively. The calculated oxygen fugacity and fugacity ratios indicate that the hydrothermal fluid of the No. I deposit has a higher F content, oxygen fugacity, and log(fHF/fHCl) value and a lower log(fH2O/fHF) value than the hydrothermal fluid from the No. II deposit. The biotite geochemistry shows that the No. I and No. II deposits formed from different hydrothermal fluids. The hydrothermal fluid of the No. I deposit was mixed with meteoric waters containing organic matter, resulting in a decrease in oxygen fugacity and more efficient precipitation of gold. The No. I and No. II deposits were formed by a Cl-rich hydrothermal system conducive to transporting Cu and Au. The decreasing Cl, oxygen fugacity, and temperature may be the key factors in Cu and Au precipitation. Biotite geochemistry allows a more detailed evaluation of the halogen chemistry of hydrothermal fluids and their evolution within porphyry Cu systems.

Effect of Magnesian-Expansive Components in Steel Slag on the Volume Stability of Cement-Based Materials.

Millimeter-scale magnesian refractory granules were found to be a unique magnesian-expansive component in steel slag. To systematically study the effects of these granular magnesian-expansive components on the volume stability of cement-based materials containing steel slag, an investigation of their existing forms and influence on the volume stability was conducted in this paper. The various-sizing waste-magnesium-chromate-based refractory brick (Mg-Cr brick) granules and different (FeO + MnO)/MgO ratios' synthetic MgO·xFeO·yMnO ternary solid solutions granules were adopted to simulate magnesian-expansive granules by partially replacing manufactured sand in mortar. The 100 °C-3 h boiling and 213 °C-2 MPa-3 h autoclaving treatments were adopted as volume stability testing methods. The results indicated that whether Mg-Cr brick or MgO·xFeO·yMnO solid solution, the concentration of expansive stress and the anisotropy expansion came with the granular size rising weakening the volume stability of cement-based materials which contained magnesian-expansive granules, significantly. Meanwhile, this phenomenon resulted in the ineffectiveness of the single linear expansion rate when assessing the qualification of volume stability. Furthermore, it also changed the mortars' failure mode from "muddy damage" to "break into blocks". Especially, there is no volume stability issue when the MgO·xFeO·yMnO satisfied (FeO + MnO)/MgO ≥ 1.00. Considering the significant effect of the granular magnesian-expansive components on the volume stability of cement-based materials containing steel slag, it is imperative to enhance the detection of both MgO content and mineral existing forms in steel slag in practical applications. For recommendation, the threshold value of conducting autoclaved volume stability testing on steel slag should be set at MgO ≥ 3%. Furthermore, the qualification cannot be judged by the single linear expansion rate; the specimens' appearance integrity and strength loss should also be noted.

Investigation into the Affect of Chemometrics and Spectral Data Preprocessing Approaches upon Laser-Induced Breakdown Spectroscopy Quantification Accuracy Based on MarSCoDe Laboratory Model and MarSDEEP Equipment

As part of China’s Tianwen-1 Mars mission, the Mars Surface Composition Detector (MarSCoDe) instrument on the Zhurong rover adopts laser-induced breakdown spectroscopy (LIBS) to perform chemical component detection of the materials on the Martian surface. However, it has always been a challenging issue to achieve high accuracy in LIBS quantification. This study investigated the effect of chemometrics and spectral data preprocessing approaches on LIBS quantification accuracy based on different chemometrics algorithms and diverse preprocessing methods. A total of 2340 LIBS spectra were collected from 39 kinds of geochemical samples by a laboratory duplicate model of the MarSCoDe instrument. The samples and the MarSCoDe laboratory model were placed in a simulated Martian atmosphere environment based on equipment called the Mars-Simulated Detection Environment Experiment Platform (MarSDEEP). To quantify the concentration of MgO in the samples, we employed two common LIBS chemometrics; i.e., partial least squares (PLS) and a back-propagation neural network (BPNN). Meanwhile, in addition to necessary routine preprocessing such as dark subtraction, we used five specific preprocessing approaches, namely intensity normalization, baseline removal, Mg-peak wavelength correction, Mg-peak feature engineering, and concentration range reduction. The results indicated that the performance of the BPNN was better than that of the PLS and that the preprocessing of Mg-peak wavelength correction had the most prominent effect to improve the quantification accuracy. The results of this study are expected to provide inspiration for the processing and analysis of the in situ LIBS data acquired by MarSCoDe on Mars.

Effect of Main Composition on the Viscosity and Thermal Stability of BaO-Containing Slag

The authors of this study systematically investigate the influence of the main components of BaO-containing slag on its viscosity and thermal stability. The results indicate that the viscosity of slag significantly increases with the mass fraction of the crystalline phase. Increasing the slag basicity from 1.00 to 1.10 and the MgO content from 5.0% to 9.5% is an effective way to weaken the influence of crystallization on slag viscosity and improve the stability of BaO-containing slag. Al2O3 content over 11% is necessary to maintain a higher slag temperature, which enhances the ability of BaO-containing slag to resist the influence of crystallization on relative viscosity. When the heat of BaO slag is greatly reduced, increasing the slag basicity from 1.05 to 1.15 and MgO content to over 6.5% can help maintain the thermal stability of slag. The effect of Al2O3 content on the viscosity of slag under conditions of fixed heat is greater than that under constant temperature conditions. As the heat decreases, the Al2O3 content increase from 9% to 13% has a more significant effect on the viscosity and temperature of slag. A lower Al2O3 content in slag is advantageous for reducing the influence of heat fluctuation on slag viscosity.

Radiation attenuation properties of chemically prepared MgO nanoparticles/HDPE composites

Sheets of high-density polyethylene (HDPE) loaded with magnesium oxide in micro and nano were synthesized with different weight percentages of micro-MgO (0,5,10,20 and 30% by weight) and nano-MgO (5 and 30%) and shaped in form of disc and dog bone shape. The morphological, mechanical, and attenuation characteristics of each concentration were determined. The linear attenuation coefficients (LAC) of the prepared discs were calculated using a well-calibrated scintillation detector and five standard gamma-ray point sources (241Am, 133Ba, 137Cs, 60Co and 152Eu). The LAC was theoretically calculated for HDPE/micro-MgO composites using XCOM software. A good agreement between the theoretical and experimental results was observed. The comparison between micro and nano-MgO as a filler in HDPE was evaluated. The results proved that the loaded nano-MgO in different proportions of HDPE produced greater attenuation coefficients than its micro counterpart. The addition of nano MgO with different weight percentage led to a significant improvement in the mechanical properties of HDPE, the ultimate force and ultimate stress increased as the concentration of nano MgO increased, and the young modulus of HDPE also increased with increasing concentration of micro and nano MgO.

Neoproterozoic tectonic transition from subduction to back-arc extension along the western Yangtze Block, South China: Petrological evidence of Nb-enriched basalts and arc-type intrusive rocks

Identifying the tectonic transition process from persistent slab subduction to back-arc extension is vital for the systematical study of arc magmatic evolution in a temporal perspective. Although Neoproterozoic subduction setting along the western Yangtze Block has been widely documented, the detailed tectonic transition from subduction to back-arc extension was not systematically constrained. The coexistence of Nb-enriched rocks and normal arc-type rocks can generally provide unique insights into back-arc extension background under subduction setting. Herein we therefore present combined data of petrology, mineralogy, zircon U-Pb geochronology, whole-rock major and trace elements, and Sr-Nd isotopes as well as zircon Lu-Hf isotopes for the Neoproterozoic Nb-enriched basalts and arc-type intrusive rocks (i.e., gabbros and diorites) from the Luding area along the western Yangtze Block (South China), to characterize back-arc extension setting of Neoproterozoic continental magmatic arc. Zircon U-Pb ages suggest that the Nb-enriched basalts and arc-type gabbros-diorites formed coevally at ca. 780 Ma. The Nb-enriched basalts have much clinopyroxene phases and display higher TiO2, P2O5, Nb, Nb/La, and Nb/U values than normal arc volcanics. They are weakly enriched in Rb and Ba, and slightly depleted in Nb, Ta, Zr, and Hf, and show decoupled Nd-Hf isotopes with highly positive whole-rocks εNd(t) (+5.79 ∼ +7.39) and relatively low zircon εHf(t) (+1.09 ∼ +9.05) values, indicating an origination of subduction slab melts-metasomatized mantle wedge source, resembling with the genesis of those typical Nb-enriched mafic rocks in a back-arc setting. Compared with Nb-enriched basalts, the arc-type gabbros and diorites contain more hornblende minerals, and are characterized by variably high MgO (3.10–5.71 wt%) contents and Mg# values (51.5–57.2), and enriched LREEs and LILEs (e.g., Rb, Ba, Sr, and K) as well as depleted Nb, Ta, and Ti. Considering their relatively depleted and coupled Nd-Hf isotopes, these arc-type gabbros and diorites were formed by partial melting of hydrous mantle wedge modified by subduction fluids. The co-occurrence of Nb-enriched basalts and arc-type gabbros-diorites studied here therefore reveal Neoproterozoic back-arc extension background under the subduction setting. Combined with ca. 810–780 Ma mafic rocks showing Nb-enriched signatures and ca. 860–740 Ma normal arc-type mafic-intermediate rocks in the western Yangtze Block, we support that Neoproterozoic geodynamic transition from continuing subduction to back-arc extension initially occurred at ca. 820–810 Ma. The Neoproterozoic back-arc extension is thus significant for geochemical changes of subduction-related metasomatized agents and arc magmatic types.

Identification of metasomatic components in lithospheric mantle under multiple tectonic regimes: Insights from Early Cretaceous mafic rocks in the northeastern North China Craton

Identification of mantle metasomatic components under multiple tectonic regimes is crucial for reconstructing the subduction history of multiple plates. Representative Early Cretaceous (125–122 Ma) gabbroic diabase from the northeastern North China Craton (NCC), provides a geochemical record to explore the genetic relationship between mantle metasomatic agents and tectonic evolution. Samples from gabbroic diabase exhibit low SiO2 and high MgO contents as well as arc-like trace element patterns, suggesting a sub-continental lithospheric mantle (SCLM) origin. Their inherited zircons and isotope variations indicate the existence of two crustal components with different tectonic affinities that were recycled into the SCLM: Paleo-Asian oceanic crust and Yangtze Craton continental crust. The metasomatic signatures of the two crustal components are previously imprinted on the geochemical compositions of the studied mafic rocks. Subsequently, the melting of the metasomatic SCLM is triggered by the subduction of the Paleo-Pacific Plate. Based on the identification of mantle metasomatic components in this study and previous geological evidence, the spatial extents of the subduction influence of the Paleo-Asian oceanic plate and Yangtze Craton on the SCLM beneath the northeastern NCC are broader than previously considered. These new findings signify a critical role of the mantle metasomatic components that can effectively identify the geological relics of the previous plate subduction. Thus we believe that the former mantle metasomatism and latter mantle melting for mafic rocks suggest two geological implications.

Effect of MgO on wetting and corrosion behaviour of corundum substrate by CaO–SiO2–MgO (–Al2O3) slags

ABSTRACT Effect of MgO content on wetting and corrosion behaviour of corundum substrate by CaO–SiO2–MgO (−15%Al2O3) molten slags was investigated at 1450°C via joint application of a sessile drop technique, SEM–EDS detection and FactSage thermodynamic software. All slags exhibit good wettability on the substrate, and the contact angle values are small. Increasing MgO content from 7% to 9% in CaO-SiO2-MgO slags can increase the contact angle between the droplet and the substrate, while the substrate dissolution thickness and the slag penetration depth tend to decrease. Increasing MgO content from 8% to 15% in CaO-SiO2-MgO-15% Al2O3 slags can also increase the contact angle, but significantly decrease the slag penetration depth. The indirect dissolution thickness increases instead due to the formation of a large amount of MgAl2O4 by the interface reaction. The wettability can give an indication of the slag penetration depth in the substrate but not the dissolution thickness of the substrate.

Zircon Hf-O and apatite Sr-O isotopes jointly reveal the origin and mineralization of the Miocene adakitic porphyries in the Gangdese belt

The Gangdese belt is a significant belt for exploring the dynamic processes of continental collision orogeny and mineralization. The impact of petrogenesis and chemical heterogeneity of the E-trending Miocene adakitic rocks in Gangdese porphyry deposit belt on mineralization mechanisms has always been controversial. New whole-rock Sr-Nd isotope, zircon U-Pb dating and Hf-O isotope data, especially integrating apatite volatile (Cl and S) and Sr-O isotope compositions, of the Miocene mineralized porphyries from Lakange deposit can provide the development of metallogenic system origin, as well as magmatic and volatile process. The Lakange granites show adakitic geochemical composition affinity with high potassic calc-alkaline. They have high SiO2 content, low MgO and Ni contents, positive εNd(t) value (0.1 ∼ 0.8), and high (87Sr/86Sr)i ratios (0.705088 ∼ 0.705269), positive zircon εHf(t) (7.6 ∼ 11.2), medium δ18O (4.33‰∼7.27‰) values, combining with apatite Sr-O isotopes (87Sr/86Sr = 0.7040 ∼ 0.7169; δ18O = 7.0‰∼8.2‰), implying that the granites are originated from the partial melting of the juvenile lower crust beneath the southern Lhasa subterrane. The Miocene adakitic rocks of the Gangdese metallogenic belt exhibit spatially isotopically heterogeneity in a west to east direction (87°E-93°E). These variations are attributed to the lower crust modification by the oxidized mantle-derived melts/fluids from metasomatic subcontinental lithospheric mantle in Miocene, resulting in the formation of juvenile lower crust, that thickened the crust and shortened the lithospheric mantle. In-situ Sr-O isotope of apatites identified the oceanic components in the volatile sources, indicating that volatiles existed beneath the Lhasa terrane during Neo-Tethyan oceanic slab subduction prior to the Miocene partial melting of the lower crust. The high volatile concentrations (Cl and S) aid in the transport of metals that produce the Gangdese Miocene porphyry deposits.

Recycling MgO from the waste magnesium oxychloride cement (MOC): Properties, CO2 footprint and reuse in MOC

Recycled magnesia was prepared via using magnesium oxychloride cement (MOC)-based waste as the raw material instead of magnesite through calcination under the temperatures from 600 °C to 1000 °C. The effects of calcination temperature on the components, microstructures, and properties of recycled MgO, and its influence on the performance of MOC containing the recycled MgO were investigated. Results indicated that the reactivity of recycled MgO depended on the calcination conditions, the active MgO content approached 80 wt% through the calcination of MOC-based waste in the temperature range of 700 °C–900 °C. The MOC paste was prepared by using 30 wt% MgO recycled under 800 °C exhibited higher mechanical performance, denser microstructure and more stable volume. The carbon emissions of recycled MgO are 0.49 t/t of magnesia, which is only 31.2% of that emitted by the calcination of magnesite via conventional dry route. This study provides an efficient approach to reducing the consumption of non-renewable natural resources and the generation of construction waste to realize cleaner and sustainable production of reactive MgO.

Microstructure and mechanical properties of SiO2-Al2O3-CaO-ZrO2 based glazes containing zircon and diopside phases

ABSTRACT MgO was added to the SiO2-Al2O3-CaO-ZrO2-based glaze to produce a high-hardness glaze by forming a composite crystal phase. The effects of MgO addition on the surface properties of the glaze were analyzed through crystal phase and microstructural investigations, and the whiteness, roughness, gloss, wear rate, and hardness were measured. When the amount of added MgO increased by 2.20 wt% or more, the diopside (MgCaSi2O6) crystal phase was observed via X-ray diffraction (XRD) and scanning electron microscopy (SEM), in addition to the zircon (ZrSiO4) crystal phase. All samples exhibited high whiteness and the surface roughness increased with increasing MgO content. At the same time, the gloss decreased significantly, resulting in a matte. The Vickers hardness test results of the glazed surface indicated high hardness values of 6.23–7.07 GPa with increased amounts of MgO.

Mafic–felsic igneous rocks in the north‐eastern North China Craton, eastern Jilin, NE China: Petrogenesis and implications for regional geodynamic evolution during the Cretaceous

We present zircon U–Pb ages, major and trace element analyses and zircon Hf isotope data on the Cretaceous mafic–felsic igneous rocks in the north‐eastern North China Craton (NCC) of NE China. These data have been used to discuss the petrogenesis of the mafic–felsic igneous rocks in order to constrain the geodynamical and magmatic evolution of the region. Zircon U–Pb dating results indicate that these igneous rocks crystallized during the Cretaceous (ca. 123–90 Ma). These mafic igneous rocks display low SiO2 contents, high MgO contents and marked enrichments in large‐ion lithophile elements (LILEs) and depletions in high‐field‐strength elements (HFSEs), combined with their εHf(t) values and La/Nb versus La/Ba diagram, suggesting they were derived from lithospheric mantle that experienced gradually increasing fluid metasomatism from 123 to 90 Ma. The felsic igneous rocks with high SiO2 values are metaluminous to weakly peraluminous and belong to the high–K calc–alkaline series. These felsic igneous rocks are all enriched in light rare earth elements (LREEs) and LILEs and relatively depleted in HFSEs, with barely negative Eu anomalies, combined with their εHf(t) values, these felsic igneous rocks were derived from partial melting of a lower crust. A synthesis of these new data with previous researches shows that the subduction and rollback of the Palaeo‐Pacific slab played a major role in the destruction of the north‐eastern NCC. The diagenetic temperatures of igneous rocks indicate that the mechanism of destruction of the NCC may be heterogeneous, most likely a thermal erosion mechanism in eastern Jilin area and a delamination mechanism in Liaodong Peninsula.

Long-lived subduction retreating led to continental rifting along the northern Gondwana: Insights from Devonian igneous rocks and ophiolite in the Beishan orogenic collage

The Beishan orogenic collage is an ideal setting for investigating the temporal and spatial relationships between extensional tectonics in the Central Asian Orogenic Belt and the rifting of Gondwana. This study presents new geochemical and geochronological data for Devonian igneous rocks and the Zhangfangshan ophiolitic mélange in the southern Beishan region. The Zhangfangshan ophiolitic mélange contains a Late Devonian gabbro block (ca. 369 Ma) with extremely depleted zircon Hf isotopic compositions (εHf[t] = +16.5 to +22.4). The Lucaogou diorites (ca. 367 Ma) have low MgO and compatible element contents, low Sr/Y and La/Yb ratios, and positive zircon εHf(t) values, indicative of derivation by dehydration melting of Mesoproterozoic mafic igneous rocks in a high-temperature and medium- to low-pressure environment. The Lucaogou K-feldspar granites (ca. 374 Ma) and Dundunshan silicic volcanic rocks (ca. 375 Ma) have A-type granite affinities, and were probably derived from magmas formed by mixing between Neoproterozoic orthogneiss-derived melts and depleted mantle-derived mafic magmas in a high-temperature shallow-crustal environment. Compiled geochronological data for subduction-related magmatism and supra-subduction zone-type ophiolites are consistent with protracted subduction–accretion until the early Carboniferous along the northern margin of the Dunhuang Block. Slab retreat in this long-lived subduction zone led to the generation of oceanic lithosphere in the southern Beishan region. Furthermore, Devonian extensional tectonics in the southern Beishan area coincided with the opening of the Paleo-Tethys Ocean, indicating that protracted subduction retreat caused the separation of the Dunhuang Block from East Gondwana.

Effect of magnesium oxide in multiphase additive on the formation process and morphology of β-Si3N4 whisker

ABSTRACT β-Si3N4 whiskers were synthesized by using a moderately low-temperature heat-treatment process, with the aid of a low weight fraction of multiple aids (Y2O3-MgO). The synthesis conditions and aspect ratio of the whiskers were adjusted by changing the MgO content in the composite additive system, heat treatment temperature and holding time. The phase composition and morphology of the whiskers were analyzed by XRD and SEM, respectively. The phase transformation could be completed at 1700°C by introducing only 0.5 wt% MgO. The heat-treated temperature was successfully further reduced to 1650°C by extending the holding time to 5 h. The introduction of MgO can make the whiskers form rapidly at a lower temperature. By introducing a small amount of MgO and appropriately extending the holding time at a lower treatment temperature has been proved to be an effective method to obtain β-Si3N4 whiskers with higher aspect ratio under mild conditions.

Influence of BaO and MgO on viscosity, crystalline phase, and structure of CaO–Al2O3–10%SiO2–20%CaF2-based system

In the current study, the low SiO2 mold flux for the continuous casting of the transformation-induced plasticity steel was studied to avoid the interface reaction between the dissolved aluminum in the molten steel and silica in the mold flux. The viscometer, Raman spectra, and X-ray disproportion were employed to study the viscosity, melt structure, and crystalline phase of the mold flux, respectively. The viscosity of the mold flux decreased with the replacement of MgO by BaO due to the balance of the melt structure and the precipitation phase. The aluminosilicate and silicate structures were depolymerized to simpler units with the replacement of MgO by BaO in the mold flux since a preference that Ba2+ tended to act as a charge compensator. The viscosity of the mold flux roughly increased with the addition of the total BaO and MgO content due to the formation of the MgAl2O4 phase in the pure liquid phase. The substantial precipitation of the MgAl2O4 phase in the liquid melt caused a sharp increase in the viscosity of the mold flux, which should be avoided by lowering the MgO content to less than 3 wt%.

Genesis of Pyroxenite Veins in the Zedang Ophiolite, Southern Tibetan Plateau

AbstractUnderstanding the nature of parental melts for pyroxenite veins in supra‐subduction zone (SSZ) ophiolites provides vibrant constraints on melt infiltration processes operating in subduction zones. The Zedang ophiolitic massif in the eastern Yarlung–Zangbo suture zone in Tibet consists of mantle peridotites and a crustal section of gabbro, diabase, and basalt. Veins of two pyroxenite varieties cut the southern part of the Zedang massif. These pyroxenite rocks have different geochemical characteristics, where the first variety (type‐I) has relatively higher contents of SiO2 (51.82–53.08 wt%), MgO (20.08–23.23 wt%), and ΣPGE (3.42–13.97 ppb), and lower Al2O3 (1.59–2.28 wt%) and ΣREE (1.63–2.94 ppm). The second pyroxenite variety (type‐II) is characterized by SiO2 (45.44–49.61 wt%), MgO (16.68–19.78 wt%), Al2O3 (4.24–8.77 wt%), ΣPGE (14.46–322.06 ppb), and ΣREE (5.82–7.44 ppm). Pyroxenite type‐I shows N‐MORB‐like chondrite‐normalized REE patterns. Zircon U‐Pb ages of pyroxenite type‐I (194 ± 10 Ma), associated ophiolitic gabbro (135.3 ± 2.0 Ma), and plagiogranite (124.2 ± 2.3 Ma) evidently imply episodic evolution of the Zedang ophiolites. The mineralogical and geochemical characteristics of the investigated pyroxenites can be explained by subduction‐initiated hydrous melting of metasomatized sub‐arc mantle, later overprinted by sub‐slab mantle melting triggered by upwelling asthenosphere during the Jurassic–Early Cretaceous times. The geochemical variations in pyroxenite vein composition, coupled with age differences amongst the other ophiolite units, may correspond to intermittent emplacement of pyroxenite dikes and isotropic gabbroic intrusions where the geodynamic setting progressed from arc maturation and slab rollback to slab tearing and delamination.