Silicate Melts Research Articles

Titanite geochemistry traces extreme differentiation of granitic magma in the collisional orogen

The genesis of granitic magma is important for the growth and evolution of continental crust. However, whether and how granitic magma can undergo efficient crystal fractionation from partial melting at the source to magma emplacement in the shallow crust level are still controversial. To address this problem, a combined study of whole-rock and titanite geochemistry was carried out for syn-exhumation granites from the Sulu ultrahigh-pressure metamorphic belt in eastern China. These high-Si granites were produced by partial melting of the deeply subducted continental crust. Significant geochemical differentiation has been revealed in these granites. Titanite in the granites can be divided into anatectic and magmatic origins, according to the Fe/Al ratios and trace element compositions. Both types of titanite yield similar UPb ages ranging from 209 ± 23 Ma to 233 ± 26 Ma, corresponding to the late collisional stage of the Sulu orogen. These titanite domains generally have consistent εNd(t) values, suggesting that they formed in a nearly closed system without significant magma mixing of different sources. The highly variable Nb/Ta ratios of titanite show good correlations with FeO, Al2O3, Ta, Sr, LREE and (Lu/Gd)N. This interesting feature is ascribed to the extensive crystal fractionation of titanite, plagioclase and allanite/epidote during magma evolution. The results suggest that the highly silicic granitic melts can experience extensive crystal separation from the liquid during granitic magma differentiation. This study shows that accessory minerals such as titanite play a key role in constraining the geochemical differentiation of granitic melts. Furthermore, titanite geochemistry has the potential to decode the complex processes from partial melting at the source to magma differentiation at shallow crust level in collisional orogens.

Nanoscale silicate melt textures determine volcanic ash surface chemistry

Explosive volcanic eruptions produce vast quantities of silicate ash, whose surfaces are subsequently altered during atmospheric transit. These altered surfaces mediate environmental interactions, including atmospheric ice nucleation, and toxic effects in biota. A lack of knowledge of the initial, pre-altered ash surface has required previous studies to assume that the ash surface composition created during magmatic fragmentation is equivalent to the bulk particle assemblage. Here we examine ash particles generated by controlled fragmentation of andesite and find that fragmentation generates ash particles with substantial differences in surface chemistry. We attribute this disparity to observations of nanoscale melt heterogeneities, in which Fe-rich nanophases in the magmatic melt deflect and blunt fractures, thereby focusing fracture propagation within aureoles of single-phase melt formed during diffusion-limited growth of crystals. In this manner, we argue that commonly observed pre-eruptive microtextures caused by disequilibrium crystallisation and/or melt unmixing can modify fracture propagation and generate primary discrepancies in ash surface chemistry, an essential consideration for understanding the cascading consequences of reactive ash surfaces in various environments.

Pressure dependence of metal–silicate partitioning explains the mantle phosphorus abundance

Previous experiments performed below 20 GPa suggested that the metal/silicate partition coefficient of phosphorus (P), DP, extrapolated to typical high-pressure and -temperature conditions of the Earth’s core formation gives too high P concentration in the core unless a large amount of silicon was included in metals. Here we examined DP between liquid metal and coexisting molten silicate at 27–61 GPa and 3820–4760 K, corresponding to conditions of core-forming metal segregation from silicate, by measuring recovered samples using a high-resolution imaging technique coupled with secondary ion mass spectrometry. The results demonstrate that the pressure dependence of DP changes from positive to negative above 15 GPa, likely because of an increase in the coordination number of P5+ in silicate melt. With the present new partitioning data, the observed mantle P abundance may indicate ~ 0.2 wt% P in the core, consistent with the cosmo-/geochemical estimates, based on both single-stage and multi-stage core formation models without involving high amounts of silicon in metals.

The influence of magnesium compounds on the properties of silicon nitride ceramics

AbstractSilicon nitride (Si3N4) ceramics using four compound sintering aids Er2O3–MgO, Er2O3–MgF3, Er2O3–Mg2Si, and Er2O3–Mg3N2. The effects of binary sintering aid types and temperatures on the microstructure, mechanical properties, and thermal conductivity of Si3N4 ceramics were investigated. The depolymerization effect of fluorine atoms introduced by MgF2 sintering additives on the silicate melt structure reduced the viscosity of the liquid phase. However, the existence of volatile phase leads to low density of Si3N4 ceramics. While the additional Si and N atoms increase the concentration of solute atoms, which was conducive to sintering densification of Si3N4 ceramics. Although Mg2Si and Mg3N2 sintering aids produced higher viscosity liquid phase compared with MgF2, denser Si3N4 ceramics were obtained at 1900°C. The Si3N4 ceramics doped with 5 mol% Er2O3 and 2 mol% Mg3N2 at 1900°C showed the excellent properties: including the highest Vickers hardness (16.8 GPa), flexural strength (912 MPa), and fracture toughness (8.1 MPa m1/2), as well as the desirable thermal conductivity (83 W/(m K)).

Two styles of melt-peridotite interactions in the upper mantle revealed by mantle xenoliths from northeastern China

To better constrain the mechanisms of interactions between different types of melts and mantle peridotites in the upper mantle of the big mantle wedge, we studied petrology, mineral chemistry, and temperatures recorded by major element and rare earth element (REE) for mantle xenoliths from the Wangqing area in the eastern part of northeastern China, and compared these data with those for xenoliths from the adjacent Jiaohe and Shuangliao areas. The Wangqing xenoliths, including olivine clinopyroxenite, wehrlite, orthopyroxenite, and websterite, generally contain clinopyroxene that is surrounding or replacing olivine. Mineral compositions of the xenoliths from northeastern China vary along melt–peridotite reaction trends, where the Wangqing xenoliths have lower mineral Mg#’s (77–85 in olivine and 81–87 in clinopyroxene) than the Jiaohe and Shuangliao peridotite and pyroxenite xenoliths. In addition, clinopyroxenes in the Wangqing samples have moderately elevated rare earth element contents and (La/Yb)N and 87Sr/86Sr ratios (0.7035–0.7046), within the range of those for clinopyroxenes in the Jiaohe and Shuangliao samples. The REE temperatures of the Wangqing samples (1024–1253 °C) are higher than those of the Jiaohe and Shuangliao samples (844–1120 °C). Based on the differences in mineral chemistry and temperature, we conclude that the Jiaohe and Shuangliao peridotites and pyroxenites were mainly derived from lithospheric mantle accreted from carbonated regions of the asthenosphere, whereas the Wangqing wehrlites and pyroxenites were formed by reaction of lithospheric mantle peridotites with hot carbonated silicate melts. These results provide evidence for two different types interactions between of carbonate-related melts and mantle peridotite, which are important for understanding the chemical evolution of the upper mantle in the big mantle wedge of northeastern Asia.

Silver Behavior During Magmatic and Magmatic-Hydrothermal Evolution of a Highly Evolved Reduced Granitic System Related to the Giant Shuangjianzishan Ag-Pb-Zn-(Sn) Epithermal Deposit, Northeast China

Abstract Magmatic-related epithermal silver-rich polymetallic deposits are among the most important sources of Ag in the world, and they are found associated with magmatic systems with striking differences. Most of the time, they are associated either with I-type oxidized (magnetite-series granite) intermediate to evolved intrusions or with S-type/A-type reduced (ilmenite-series granite) highly evolved intrusions. To better understand these associations, the Ag evolution has been tracked during the magmatic differentiation and the magmatic-hydrothermal transition stage of A-type highly evolved porphyritic granites associated with the giant Shuangjianzishan Ag-Pb-Zn-(Sn) epithermal deposit, the largest known Ag deposit of Asia (145 million tonnes at 128.5 g/t Ag and 2.2 wt % Pb + Zn) located in the largest known metallogenic province for Ag in China (the southern Great Xing’an Range). At the Shuangjianzishan deposit, the porphyritic granite complex consists of three temporally distinct intrusions—a coarse-grained monzogranite porphyry, a fine-grained syenogranite porphyry, and a fine-grained syenogranite—having crystallized at ~2 kbar and ~750°C and recording a continuous magmatic differentiation trend. The silicate melt that generated the last highly differentiated intrusion (fine-grained syenogranite) is interpreted as the source of the mineralizing fluids forming the Shuangjianzishan Ag-Pb-Zn-(Sn) epithermal deposit, as it is the only intrusive unit that reached fluid saturation, as indicated by cotrapped fluid and melt inclusions in quartz phenocrysts and by the occurrences of unidirectional solidification textures (USTs). Silver evolution in the different porphyritic granite facies was reconstructed with laser ablation-inductively coupled plasma-mass spectrometry analyses of quartz-hosted silicate melt inclusions, amphibole-hosted magmatic sulfide inclusions, and chemical modeling. The silicate melt forming the porphyritic granite complex was sulfide saturated during the first crystallization stage, as shown by the occurrence of Ag-rich monosulfide solid solution (MSS) inclusions hosted in amphibole phenocrysts from the coarse-grained monzogranite porphyry and from mafic microgranular enclaves hosted in the coarse-grained monzogranite porphyry. However, these Ag-rich MSSs had only a minimal impact on the Ag budget of the magmatic system, as shown by the increase of the Ag concentration (~100–1,000 ppb) in quartz-hosted silicate melt inclusions during the further evolution of the system until fluid exsolution was reached. These results combined with mass balance modeling suggest that Ag and Sn are efficiently transferred to the evolving residual melt during crystallization and crystal-melt segregation. The results of this study indicate that highly Ag endowed epithermal polymetallic deposits can be formed from the exsolution of Ag-rich mineralizing fluids from relatively low volume, highly evolved, reduced melts, similar to those responsible for the formation of Sn-rich greisen deposits.

The crucial role of transient tri-coordinated oxygen in the flow of silicate melts.

The transport properties of high-temperature silicate melts control magma flow and are crucial for a wide variety of industrial processes involving minerals. However, anomalous melt properties have been observed that cannot be explained by the traditional polymerization degree theory, which was derived based on quenched melts. Ab initio molecular dynamics (AIMD) simulations were conducted to investigate the flow mechanism of CaO-Al2O3-SiO2 melts under high temperature atmospheric conditions. By analyzing the dynamic structure of melted silicates and employing molecular orbital theory, we gained a fundamental understanding of the flow mechanism from a chemistry perspective. Transient tri-coordinated oxygen (TO) bonded with one Si and two Al atoms (SiOAl2) was found to be a pivotal intermediate in melt flow and atomic diffusion processes. Frequent chemical transition between TO in SiOAl2 and bridging oxygen (BO) dominated the fluidity of melted silicates. The presence of such transitions is facilitated by the unstable nature of [SiAlO2] 4-membered rings, which are susceptible to instability due to the intense repulsion between the O 2p lone pairs and the excessively bent O-Al-O angle. Additionally, the density of SiOAl2 type TO motif could serve as an indicator to determine the relationship between structure and fluidity. Our results challenge the traditional polymerization degree theory and suggest the need to reassess high-temperature liquid properties that govern processes in the Earth and industry by monitoring transient motifs.

Origin and evolution of lithospheric mantle beneath the juvenile Arabian–Nubian Shield: Evidence from the Natash peridotite xenoliths, Southern Eastern Desert, Egypt

Compared with the Archean cratonic mantle, much less is known about the genesis and composition of the subcontinental lithospheric mantle (SCLM) beneath the juvenile terranes. In this study, we present petrology and mineral chemistry analyses of peridotite xenoliths carried by Cretaceous volcanics from the Natash area in the western Arabian–Nubian Shield (ANS). Combined with mantle xenoliths from the eastern ANS, we aim to constrain the nature, origin, and evolution of SCLM beneath the juvenile terrane. The Natash peridotite xenoliths, including well-preserved clinopyroxene (Cpx) and spinel (Sp), are categorized into two groups. Group A peridotites display exceptionally high Mg# (molar100 × Mg/(Mg + Fetotal)) in Cpx (95.6–96.7) and Cr# (molar100 × Cr/(Cr + Al)) in Sp (45.5–51.7), signifying a high degree of melting and refractory nature. Cpx in Group A demonstrates low Ti/Eu (532–1785) and high (La/Yb)N (5.34–26.29) ratios, indicative of carbonatitic metasomatism. Comparatively, Group B peridotites exhibit relatively high Mg# in Cpx (93.8–95.5) and Cr# in Sp (15.9–26.2), indicating a low degree of melting and fertile nature. Cpx trace element patterns subdivide Group B into three sub-groups. Cpx in Group B1 are characterized by high Ti/Eu (3934–5622) but low (La/Yb)N (0.91–2.98), implying silicate melt metasomatism. Both Groups B2 and B3 Cpx show steep rare earth element (REE) patterns, negative anomalies in high field-strength elements, and high (La/Yb)N (3.62–7.57). However, Cpx in B3 exhibit higher Ti/Eu ratios and water-soluble element contents than those in B2 peridotites. In addition, the spoon-shaped REE patterns are prominently unique to Cpx in Group B3. Hence, it is inferred that Group B2 were influenced by carbonatitic melts, whereas the evolved small-volume CO2-H2O-rich fluids infiltrated into Group B3 samples. Furthermore, the high Sr contents (71–404 ppm) but low 87Sr/86Sr (0.7003–0.7034) ratios of Cpx in Natash peridotites suggest that the various metasomatic agents all originated from the asthenosphere. Collectively, the Natash peridotite xenoliths exhibit similar nature and metasomatism processes with those from Cenozoic volcanics in the eastern ANS, and they all show affinity to continental peridotites. The pMELTS simulation suggests that the refractory peridotites have a high melting pressure (typically >3 GPa), while the fertile samples register a low melting pressure. Consequently, we infer that the SCLM beneath the juvenile ANS was formed through the cooling of upwelling asthenosphere, incorporating the ancient mantle fragments, and subsequently underwent asthenosphere-derived carbonatitic and silicate melt/fluid metasomatisms.

Partitioning of nickel and cobalt between metal and silicate melts: Expanding the oxy-barometer to reducing conditions

Moderately siderophile elements (MSEs) are potential tracers of the thermodynamic conditions prevailing during planetary core formation because their metal–silicate partition coefficients (Dmet/sil) vary as a function of P, T, and oxygen fugacity (fO2). Those properties result in the production of planetary mantles with unique MSE depletion signatures. Among the MSEs, Ni and Co are reliable barometers in magma oceans because their Dmet/sil values are strongly correlated with pressure, decreasing by almost 3 orders of magnitude between 1 bar and 100 GPa. Current pressure-dependent expressions of Dmet/sil were calibrated based on experiments performed under relatively oxidizing conditions, mostly at fO2 slightly below the iron–wüstite Fe–FeO buffer (IW), which is relevant to the mantles of Earth and Mars. However, planets and asteroids formed under a wide range of redox conditions, from Mercury, the most reduced (∼IW − 5.5), to the most oxidized angrite parent body (IW − 1.5 to IW + 1). In this study, we performed and analyzed 38 metal–silicate partitioning experiments over a wide range of pressures (1 bar to 26 GPa) and oxygen fugacities (IW − 6.4 to IW − 1.9) to expand the available Ni and Co Dmet/sil values to reducing conditions. We then parameterized 255 Ni and 194 Co Dmet/sil values as a function of T (1573–5700 K), P (1 bar to 100 GPa), and fO2 (IW − 6.4 to IW + 0.2). We also modeled the evolution of Ni and Co Dmet/sil values along the liquidus of a chondritic mantle at various P and fO2 conditions to investigate the thermodynamic conditions of various planetary bodies’ magma oceans. The P and fO2 conditions we obtained for Earth, Mars, the Moon, and Vesta are consistent with previous studies using similar methods, and the pressure during core formation is strongly correlated to planetary size. Finally, we also applied our model to several achondrite parent bodies; our results indicate a wide variety of objects, from the asteroid-sized, oxidized angrite parent body to the planet-sized, highly reduced aubrite parent body.

Estimates of chlorine isotope fractionation factors using density functional theory: Applications to ore-forming systems

Chloride is the most important anion in ore-forming hydrothermal fluids, and chlorine isotopes are therefore potentially sensitive tracers of the origin and evolution of ore-forming fluids. However, they remain a relatively under-utilized tool in ore deposit geochemistry due to the lack of knowledge of chlorine isotope fractionation during ore-forming processes. Using first-principles density functional theory, this study estimates chlorine isotope fractionation factors for various ore-forming processes. All metal-Cl complexes in hydrothermal fluids are enriched in 37Cl compared to chloride ions, but the change in δ37Cl of the fluid trapped in ore minerals is small after the destabilization of metal-chloride complexes during ore mineral deposition. In the precipitation of evaporite minerals from brine, the sequence of enrichment of the heavy Cl isotope (37Cl) is halite > carnallite > aqueous chloride > kainite > sylvite > bischofite. The results of this study agree with the experimental observations that progressive precipitation of halite from brine lowers the δ37Cl value of the residual fluid until the formation of K-Mg chlorides. In low-temperature deposits, δ37Cl values for fluid inclusions and minerals reflect those of the hydrothermal fluid provenance and the mixing of this fluids with saltwater or basinal brines. In high-temperature magmatic-hydrothermal ore deposits that undergo liquid–vapour phase separation, chlorine isotopes fractionate among phases of silicate melt, vapour and chloride-rich liquid. The considerable range in δ37Cl in fluid inclusions may also reflect fluid mixing and hydrothermal alteration. At ambient temperature, the δ37Cl values may reflect evaporative processes and, in the case of chemical weathering of metallic mineral deposits, may record the supergene enrichment of the metals. This study highlights the use of chlorine isotopes as a new tool for interpreting ore-forming processes.

Mechanism of carbonate assimilation by intraplate basaltic magma and liquid immiscibility: example of Wangtian’e volcano (Changbaishan volcanic area, NE China)

The balance of CO2 during abundant basaltic magma production is an important factor of volcanic hazards and climate. In particular, this can be explored based on CO2-rich mantle-derived magmas or carbonate assimilation by basaltic melts. To reconstruct the origin of Fe-rich carbonates hosted by Cenozoic basalts from Wangtian’e volcano (northeast China), we studied elemental compositions of melt, crystalline and fluid inclusions in magmatic minerals as well as the oxygen and carbon isotope compositions of the plagioclase and carbonates from basalts. The crystallization of basaltic magmas occurred in shallow chamber (∼4 km) at temperatures of 1,180°C–1,200°C and a pressure of 0.1 ± 0.01 GPa. Stable Fe-rich carbonates occur in the Wangtian’e tholeiite basalts as groundmass minerals, crystalline inclusions in plagioclase and globules in melt inclusions, which suggests that they crystallized from a ferrocarbonate melt. The values of δ18О and δ13С in the minerals analyzed by laser fluorination method are in line with the sedimentary source of Fe-rich carbonates, indicating assimilation and partial decomposition of carbonate phases. The parent ferrocarbonate melt could be produced during interactions between the basaltic magma and the crustal marbles. The phase diagram and thermodynamic calculations show that the ferrocarbonate melt is stable at a temperature of 1,200°C and a pressure of 0.1 GPa. Our thermodynamic calculations show that carbonate melt containing 73 wt% FeCO3, 24 wt% MgCO3 and 3 wt% CaCO3 is in thermodynamic equilibrium with silicate melt in agreement with our natural observations. The proposed mechanism is crustal carbonate sediment assimilation by the intraplate basaltic magma resulting in the melt immiscibility, production of the ferrocarbonate melt and the following Fe-rich carbonate mineral crystallization during magma residence and cooling.

Mechanisms for concentrating critical metals in granitic complexes: insights from the Mourne Mountains, Northern Ireland

The Tellus stream sediment and deep soil geochemistry data sets for Northern Ireland were used to locate four types of critical metals anomalies in granite bedrocks of the Mourne Mountains. A curvi-linear array of Nb, REE, Th and U soil anomalies across the eastern Mourne Mountains correlated with late-stage and eutectic temperature minerals in the roof zone of the most peralkaline F- and volatile-rich granite body, remobilized on micron to millimetre scales. Li, Be, B, As, Sn, Mn 3+ and Ce 4+ partitioned into pockets of late-stage heterogeneously distributed F-rich silicic residual melts and relatively oxidizing halide-rich magmatic fluids, resulting in drusy mineral and hydrothermal assemblages. Isolated soil anomalies correlated with amorphous Mn 3+ - and Ce 4+ -rich masses infilling drusy cavities, which resulted from short-distance percolation of small volumes of late-stage magmatic fluids. A significant As plume in stream sediments emanated from a greisen that hosted multiple critical and base metals including Sn, from reactions between large volumes of magmatic As + halide-rich fluids and mafic silicate + diverse accessory minerals on the metre- to kilometre-scale along geological structures. Diverse, small-scale REE anomalies in the soil data along structural features in the western Mournes correlate with vein mineralization resulting from episodic migration of hydrous fluids of variable composition, probably with a much smaller magmatic component than elsewhere. The regional geochemical dataset proved useful to develop a multi-stage model for enrichment of critical metals in the Mourne Mountains granites, which is analogous to the petrogenesis of some of the igneous-hosted economic deposits of critical metals. Thematic collection: This article is part of the energy-critical metals for a low carbon transition collection available at: https://www.lyellcollection.org/topic/collections/critical-metals

Chalcophile element degassing at an active continental arc volcano

Arc volcanoes are significant natural sources of trace chalcophile elements to the atmosphere via gas and aerosol plumes. Villarrica volcano, part of the Andean arc, erupts basaltic magmas and is characterised by a persistent volcanic gas plume and therefore presents an opportunity to quantify volcanic chalcophile processing in a subduction zone from slab to surface. Here we present geochemical data for olivine-hosted melt inclusions, as well as for the gas and aerosol plume. We show that melts erupted at Villarrica are enriched (over mid-ocean ridge basalts) in a suite of fluid-mobile elements comprising the large ion lithophiles, including Cs, and chalcophile elements W, Tl, Pb and Sb. Volcanic gas and aerosol samples show that the chalcophile elements, and Cs, are strongly enriched in the gas phase over the silicate melt, 103 to 106 times more so than the non-volatile Rare Earth Elements. Volatilities (the percentage of an element that degasses from a melt on eruption) reach ∼ 45 % for Tl, with Pb, Sn and Mo exhibiting volatilities of up to 0.3 % and Cu up to 0.08 %. Many of the chalcophile elements (e.g. Cu, Ag, Zn) have an affinity for chloride in the gas phase and we observe that the volatility of chloride-speciating trace metals is linked strongly to the availability of chlorine in volcanic plumes globally. Overall, we show that the trace element composition of the volcanic gas—and hence probably also the deeper, denser and more saline fluids in the subsurface—is sensitive to both the availability of chloride in the gas phase and the composition of the melt, which is controlled by the slab flux and may be variable between subduction zones.

Heat treatment of ground surface using an electric arc plasma generator

The article presents a generalization of the results of experimental and theoretical studies on the intensive thermal effect on the surface of foundations from cohesive soils (as opposed to deep heat treatment) using an electric arc generator of low-temperature plasma (plasmatron). A plasmatron with massive consumable electrodes made of borosilicated graphite and a wide "blurred" plasma torch was used for heat treatment. Physical and mathematical modeling of the thermal effect on the surface of clay soil by a moving high-temperature source has been performed. For the first time, a special installation was manufactured and experimental studies were carried out on the heat treatment of ground surfaces on the site with an electric arc plasma torch. It is established that the effective heat flux density reaches 2.5-.3.5 kW/cm2. The main part of the thermal energy for the used plasmatron enters the material due to radiant heat transfer. An analytical mathematical model of the temperature field of a three-dimensional semi-bounded body in the technological process under consideration is given by an essentially nonlinear boundary value problem for a nonlinear heat equation with nonlinear boundary conditions of the second kind. When solving thermophysical problems, the ground semi-bounded space was considered as a quasi-homogeneous medium having a constant initial temperature and initial thermophysical parameters that change in the process of temperature increase. The effective values of the thermophysical characteristics were determined on the basis of experimental data. Calculations and experiments have shown that the temperature boundary leading to significant changes in the structural properties of soils drops to a depth of 4-6 cm from the surface, despite the boiling of the ground melt with a temperature of 2500 ...2800 K on the ground surface. Therefore, a new technological principle of heat treatment is proposed, which consists in building up the ground melt in layers of 4-5 cm up from the initial surface. The impossibility of obtaining a positive effect even with the use of a powerful high-temperature source of exposure in the case of traditional surface heat treatment technology has been confirmed. The new plasma technology of surface heat treatment of ground surfaces to the stage of silicate melt allows to obtain a common layer of the required thickness. At the same time, the efficiency of surface heat treatment is significantly increased. The new technology of layer-by-layer surface deposition reduces cracking in the layer when the melt cools, but does not eliminate this negative process.

An experimental study of the effect of water and chlorine on plagioclase nucleation and growth in mafic magmas: application to mafic pegmatites

Abstract. In this study, the effects of H2O and Cl on the grain size and nucleation delay of plagioclase in basaltic magma were investigated using dynamic and equilibrium experiments at 1150 ∘C, 300 MPa, and oxygen fugacity between FMQ − 1.65 and FMQ + 0.05 (fayalite–magnetite–quartz). Each experiment consisted of five samples of basaltic composition (from the Hamn intrusion in Northern Norway) containing varying amounts of H2O (up to 2 wt %) and Cl (up to 1 wt %). The equilibrium experiments were used as a reference frame for the phase assemblage, geochemical composition, and liquidus temperatures and were compared to thermodynamic models using MELTS software. Experimental phase abundances and plagioclase compositions are in good agreement with the predictions of MELTS. The dynamic experiments were initially heated above the liquidus temperature to destroy crystal nuclei and then kept at 1150 ∘C for 100, 250, or 1800 min. These experiments show that as the concentration of H2O in the melt increases, plagioclase nucleation is delayed, plagioclase abundance decreases, but its size increases. Therefore, the addition of H2O seems to favor plagioclase growth at the expense of nucleation. Thermodynamic and kinetic calculations corroborate an increase in the nucleation delay of plagioclase with increasing H2O content dissolved in the melt, suggesting that H2O decreases the undercooling of the silicate melt. The addition of Cl also seems to delay plagioclase nucleation, although this is not supported by kinetic calculations. Increasing the Cl content decreases plagioclase abundance but does not significantly affect its size. The homogeneous pegmatitic pockets of the mafic–ultramafic Hamn intrusion exhibit several petrological and geochemical features, suggesting that H2O and Cl enrichment in the silicate melt was the origin of the pegmatitic texture. The experimental results presented here indicate that H2O, rather than Cl, may have played an important role in the formation of the pegmatitic texture.

Submicroscopic iron-rich grains throughout impact glasses in Chang'E-5 regolith

In lunar regolith returned by Chang'E-5, Apollo and Luna missions, submicron-sized iron-rich grains that are dominated by reduced iron and with sizes of about 10 s to 100 s nanometers are widespread on surfaces of impact glasses, such as glass spherules and glass that welds agglutinates. With flat or mound-like tops, such submicroscopic iron-rich grains typically occur in chains, nets and/or mists, and they were interpreted to be surface adherents that are mainly composed of native iron. Processes such as cooling of iron melt veneer, condensation of iron vapor, and in situ thermal reduction of regolith particles have been separately invoked to explain their origin. Meanwhile, some iron-rich grains have lower hemispheres being embedded in their host glasses, and spherical iron-rich grains with similar sizes, compositions and spatial distributions are also widespread in interiors of their host glasses. It is possible that both the surface and interior iron-rich grains were formed simultaneously during cooling of the host impact melt. However, the precise composition and crystallography of the surface and interior iron-rich grains have not been adequately constrained. In this study, we prepared focused ion beam (FIB) sections for chains of iron-rich mounds that occur on surfaces of impact glasses in the Chang'E-5 lunar regolith. Several FIB sections also exposed interior iron-rich grains that exhibit similar sizes and regular distribution patterns. High-resolution elemental mapping shows that both the surface and interior iron-rich grains are mainly composed of iron and sulfur, and crystallographic inspections reveal that the grains are similarly composed of α-Fe and troilite with varying volume proportions. As the glass spherules and agglutinate glasses are mainly melt products of local regolith materials, both the surface and interior iron-rich grains are most consistent with being formed due to liquid immiscibility between FeS and silicate melt. Troilite preferentially occurs along margins of the iron-rich grains due to heterogeneous distribution of S in the iron-rich melt and the relatively greater compatibility between troilite and silicate melt. The varying morphology of the surface iron-rich grains (e.g., flat tops, mounds) is likely caused by different surface tensions of FeS melt droplets and different wettability between the solidifying Fe-dominating melt and FeS melt with silicate melt. This interpretation predicts that iron-rich grains formed during cooling of impact melt may be dispersed in melt strips and even detached from the melt body during ejecta motion, and they could be subsequently adhered to surfaces of other lunar regolith particles. For reflectance spectra of lunar regolith at visible to near-infrared wavelength, the long-noticed darkening effect caused by submicron-sized iron may be caused by such two-phase iron-rich grains (i.e., α-Fe plus troilite).

Melting Processes of Pelitic Rocks in Combustion Metamorphic Complexes of Mongolia: Mineral Chemistry, Raman Spectroscopy, Formation Conditions of Mullite, Silicate Spinel, Silica Polymorphs, and Cordierite-Group Minerals

Melted rocks (clinkers and paralavas) of the Mongolian combustion metamorphic (CM) complexes were formed during modern and ancient (since the Quaternary) wild-fires of brown coal layers in the sedimentary strata of the Early Cretaceous Dzunbain Formation. According to XRD, Raman spectroscopy, and SEM-EDS data, cordierite, sekaninaite, indialite, ferroindialite, silica polymorphs, mullite, Fe-mullite, anhydrous Al-Fe-Mg silicate spinel (presumably new mineral), and other phases were identified. It has been established that isomorphic impurity of potassium in the cordierite-group minerals does not correlate with their crystal structure (hexagonal or orthorhombic). Indialite and ferroindialite retained their hexagonal structure in some fragments of the CM rocks, possibly due to the very fast cooling of local zones of sedimentary strata and the quenching of high-temperature K-rich peraluminous melt. Clinkers, tridymite–sekaninaite, and cristobalite–fayalite ferroan paralavas were produced by partial melting of Fe-enriched pelitic rocks (mudstone, siltstone, and silty sandstone) in a wide temperature range. The formation of mullite, Fe-mullite, and Al-Fe-Mg silicate spinel in clinkers developed from dehydration–dehydroxylation and incongruent partial melting of Fe-enriched pelitic matter (Al-Mg-Fe-rich phyllosilicates, ‘meta-kaolinite’, and ‘meta-illite’). Large-scale crystallization of these minerals in the K-rich peraluminous melts occurred, presumably, in the range of T > 850–900 °C. The subsurface combustion of coal layers heated the overburden pelitic rocks from sedimentary strata to T > 1050 °C (judging by the formation of cordierite-group minerals) or locally till the melting point of detrital quartz grains at T > 1300 °C and, possibly, till the stability field of stable β-cristobalite at T > 1470 °C. Ferroan paralavas were formed during the rapid crystallization of Fe-rich silicate melts under various redox conditions. From the analysis of the liquidus surface in the Al2O3–FeO–Fe2O3–SiO2 major-oxide system, it follows that the least high-temperature (<1250 °C) and the most oxidizing conditions occurred during the crystallization of mineral assemblages in the most-enriched iron silicate melts parental for cristobalite–fayalite paralava.

Геохимия органического вещества природных битумов в разрезе р. Кожвы (Тимано-Печорская провинция)

The objects of study are diaplectic and melt silicate glasses from vein melt impactites of the Kara astrobleme. Microscopic and spectroscopic characteristics of two glasses with fundamentally different formation mechanisms were obtained, which allowed comparing their structural features. We found that silicate diaplectic and melt impact glasses from high-pressure/high-temperature vein bodies of the Kara astrobleme were characterized by similar structural features — a high degree of polymerization, the presence of four-membered and polymembered SiO4 rings, which was characteristic of all glasses were the SiO2 composition. Diaplectic glasses are characterized by the constant presence of three-membered SiO4 rings; melt glasses were characterized by the absence of this feature, which was determined by more extreme conditions for the glass from melt.

Сравнительная характеристика диаплектовых и расплавных силикатных стёкол Карской астроблемы

The objects of study are diaplectic and melt silicate glasses from vein melt impactites of the Kara astrobleme. Microscopic and spectroscopic characteristics of two glasses with fundamentally different formation mechanisms were obtained, which allowed comparing their structural features. We found that silicate diaplectic and melt impact glasses from high-pressure/high-temperature vein bodies of the Kara astrobleme were characterized by similar structural features — a high degree of polymerization, the presence of four-membered and polymembered SiO4 rings, which was characteristic of all glasses were the SiO2 composition. Diaplectic glasses are characterized by the constant presence of three-membered SiO4 rings; melt glasses were characterized by the absence of this feature, which was determined by more extreme conditions for the glass from melt.

Corrosion behavior of Fe-containing silicate over ytterbium hafnate ceramics with different HfO2 content

High-temperature molten silicate deposit corrosion has become a critical factor for the failure of thermal barrier coatings. In this work, promising ytterbium hafnate ceramics with different HfO2 content were prepared by solid-phase reaction, and the effect of HfO2 content on Fe-containing silicate corrosion behavior of ytterbium hafnate ceramics was investigated. The evolution of HfO2 content showed negligible influences on reaction products and corrosion mechanism. The attack of Fe-containing silicate melts on the ceramic led to the formation of a reaction layer consisting of apatite, fluorite, and garnet. This layer plays a critical role in preventing further penetration of the melts. However, ytterbium hafnate ceramics with lower HfO2 content showed better resistance to melt attack.