Silicate Hydrate Research Articles

COMPORTAMENTO TÉRMICO DE 25°C A 1100°C DO MINÉRIO DE NÍQUEL DA MINA ONÇA-PUMA EM CARAJÁS

Ferronickel alloys are produced by pyrometallurgical processes from nickel-bearing hydrous magnesium silicates occurring in the saprolite in laterite deposit of the Serra de Onça-Puma/Pará-Brazil. The ore is treated by RKEF process. It consists of drying, calcination and reduction, however, the processing of lateritic nickel ore is quite difficult since the Ni-bearing minerals are finely disseminated in the ore bodies. In this work was studied the influence of thermal treatment between 25°C and 1100°C in nickel lateritic ore, evaluated through thermal analysis (Tg/DTA) and x-ray diffraction (XRD). The XRD analyses of raw samples without thermal treatment showed that they are mixture of serpentine, smectite and chlorite. After starting thermal treatment, the curves of Tg/DTA revealed to have two endothermic peaks at 60°C to180°C and 594°C to 613°C due to release the adsorbed and crystalline water, respectively, followed by an exothermic peak at 820ºC which can be attributed to the change in crystal structure and consequent formation of a new mineral phases. Weight differences can be observed when the adsorbed water removed, without phase changes in the range of 60°C to 180°C, indicated stability of nickel ore. Phase changes and weight loss were observed at 594°C to 613°C with clear crystalline breaking of the serpentines and smectite forming amorphous substances. Above this temperature new phases are crystallized until 820°C, when is formed forsterite (trevorite), enstatite and hematite, remaining unchanged quartz. On the analyzed conditions might infer that the pyrometallurgical to iron nickel production through this type of laterite ore effectively corresponds to RKEF process.

Structure and compressibility of Fe-bearing Al-phase D

Abstract Due to its large thermal stability, Al-phase D, the (Al,Fe3+)2SiO6H2 member of the dense hydrous magnesium silicate (DHMS) phase D, may survive along hot subduction geotherms or even at ambient mantle temperatures in the Earth’s transition zone and lower mantle, therefore potentially playing a major role as a water reservoir and carrier in the Earth’s interior. We have investigated the crystal structure and high-pressure behavior of Fe-bearing Al-phase D with a composition of Al1.53(2)Fe0.22(1) Si0.86(1)O6H3.33(9) by means of single-crystal X-ray diffraction. While the structure of pure Al-phase D (Al2SiO6H2) has space group P63/mcm and consists of equally populated and half-occupied (Al,Si)O6 octahedra, Fe-incorporation in Al-phase D seems to induce partial ordering of the cations over the octahedral sites, resulting in a change of the space group from P63/mcm to P6322 and in well-resolved diffuse scattering streaks observed in X-ray images. The evolution of the unit-cell volume of Fe-bearing Al-phase D between room pressure and 38 GPa, determined by means of synchrotron X-ray diffraction in a diamond anvil cell, is well described by a third-order Birch-Murnaghan equation of state having an isothermal bulk modulus KT0 = 166.3(15) GPa and first pressure derivative KT0′ = 4.46(12). Above 38 GPa, a change in the compression behavior is observed, likely related to the high-to-low spin crossover of octahedrally coordinated Fe3+. The evolution of the unit-cell volume across the spin crossover was modeled using a recently proposed formalism based on crystal-field theory, which shows that the spin crossover region extends from approximately 30 to 65 GPa. Given the absence of abrupt changes in the compression mechanism of Fe-bearing Al-phase D before the spin crossover, we show that the strength of H-bonds and likely their symmetrization do not greatly affect the elastic properties of phase D solid solutions, independently of their compositions.

Single‐Crystal Elasticity of Phase E at High Pressure and Temperature: Implications for the Low‐Velocity Layer Atop the 410‐km Depth

AbstractDense hydrous magnesium silicate (DHMS) phase E is a potential water carrier in subducting slabs that can transport water to the Earth's deep mantle between the bottom of the upper mantle and the uppermost transition zone. Therefore, knowledge on the high pressure‒temperature (P‒T) full elastic moduli of phase E at relevant mantle conditions is important in deciphering the existence of DHMS phases and their influences on seismic profiles in the region; however, the high P‒T elasticity data of phase E still remains lacking. In this work, we determined the combined effect of P‒T on the single‐crystal elasticity of phase E up to 24 GPa and 900 K by in situ X‐ray diffraction and Brillouin scattering measurements in externally‐heated diamond anvil cells. The aggregate elastic moduli and compressional‐wave (VP) and shear‐wave (VS) velocities of phase E are then derived by analyzing the single‐crystal elasticity and density data using the third‐order finite‐strain equations. We found that phase E exhibits much lower bulk and shear moduli and acoustic velocities than the most abundant constituent minerals in the upper mantle and transition zone, such as olivine, clinopyroxene, garnet, and wadsleyite. The modeled results using the obtained elasticity results show that the existence of phase E in a hydrated pyrolite model can result in relatively lower Vp and Vs profiles and negative velocity anomalies in seismic observations. The existence of phase E with relatively lower velocity profiles could be a possible origin of the low‐velocity layers atop the 410‐km discontinuity in some cold and highly‐hydrated regions.

Structures and transport properties of supercritical SiO2-H2O and NaAlSi3O8-H2O fluids

Abstract Speciation and transport properties of supercritical fluids is critical for understanding their behavior in the Earth’s interior. Here, we report a systematic first principles molecular dynamics simulation study of the structure, speciation, self-diffusivity (D), and viscosity (η) of SiO2 melt, NaAlSi3O8 melt, SiO2-H2O and NaAlSi3O8-H2O fluids at 2000–3500 K with 0–70 wt% H2O. Our calculations show that as the water content increases, the proportion of Q0 species (Qn species, where n is the number of bridging oxygens in an individual Si/Al-O polyhedra) increases while Q4 decreases. The proportions of Q1, Q2, and Q3 species first increase and then decrease with increasing water content. The diffusivity sequence for the supercritical SiO2-H2O fluids is DH >DO >DSi, and for the supercritical NaAlSi3O8-H2O fluids, on the whole, is DNa ≈ DH >DO >DAl ≈ DSi. The viscosities of the two systems decrease drastically at the beginning of the increase in water content, and then decrease slowly. We demonstrate that the exponential decrease in the viscosity of polymerized silicate melt with increasing water content is due to a sharp decrease in the proportion of Q4 species and increase in Si-O-H. The typical structural feature of supercritical fluid is that it contains a large amount of easy-to-flow partially polymerized or depolymerized protonated silicate units, which leads to a low viscosity while being enriched in silicate. This feature provides supercritical fluids the potential to transport elements that are hard to migrate in aqueous fluids or hydrous silicate melts, such as high field strength elements.

Polarons in Rock-Forming Minerals: Physical Implications

The existence of thermally-activated quasiparticles in amphiboles is an important issue, as amphiboles are among the main hydrous complex silicate minerals in the Earth’s lithosphere. The amphibole structure consists of stripes of 6-membered TO4-rings sandwiching MO6 octahedral slabs. To elucidate the atomistic origin of the anomalous rock conductivity in subduction-wedge regions, we studied several Fe-containing amphiboles with diverse chemistry by using in situ, temperature-dependent, polarised Raman spectroscopy. The occurrence of resonance Raman scattering at high temperatures unambiguously reveal temperature-activated small polarons arising from the coupling between polar optical phonons and electron transitions within Fe2+O6 octahedra, independently of the amphibole chemical composition. The FeO6-related polarons coexist with delocalised H+; that is, at elevated temperatures Fe-bearing amphiboles are conductive and exhibit two types of charge carriers: electronic polarons with highly anisotropic mobility and H+ cations. The results from density-functional-theory calculations on the electron band structure for a selected amphibole compound with a relatively simple composition are in full agreement with experimental data. The polaron activation temperature, mobility, and polaron-dipole magnitude and alignment can be controlled by varying the mineral composition, which makes amphiboles attractive “geo-stripes” that can serve as mineral-inspired technology to design thermally-stable smart materials with anisotropic properties.

The Winchcombe meteorite, a unique and pristine witness from the outer solar system.

Direct links between carbonaceous chondrites and their parent bodies in the solar system are rare. The Winchcombe meteorite is the most accurately recorded carbonaceous chondrite fall. Its pre-atmospheric orbit and cosmic-ray exposure age confirm that it arrived on Earth shortly after ejection from a primitive asteroid. Recovered only hours after falling, the composition of the Winchcombe meteorite is largely unmodified by the terrestrial environment. It contains abundant hydrated silicates formed during fluid-rock reactions, and carbon- and nitrogen-bearing organic matter including soluble protein amino acids. The near-pristine hydrogen isotopic composition of the Winchcombe meteorite is comparable to the terrestrial hydrosphere, providing further evidence that volatile-rich carbonaceous asteroids played an important role in the origin of Earth's water.

Strength optimization and key factors correlation of one-part fly ash/ladle furnace slag (FA/LFS) geopolymer using statistical approach

The utilization of ladle furnace slag (LFS) in one-part geopolymer technology has not been reported. The study of the cause-and-effect relationship between the mixing ratio is therefore important. The present work optimized one-part fly ash (FA)/LFS geopolymer with 33 full factorial design using variance analysis (ANOVA) to predict the key engineering properties of the one-part geopolymer with satisfactory precision. Three factors, which are alkali activator to aluminosilicate sources (AA/AS), sodium metasilicate to sodium hydroxide, Na2SiO3 to NaOH (SM/SH) and water to binder (W/B) ratios, were considered. The AA/AS, W/B and interrelationship between AA/AS and W/B ratios were the most significant factors influencing the key engineering properties. The one-part geopolymer with AA/AS, SM/SH and W/B ratios of 0.20, 5.0 and 0.25 were concluded as an optimal response to achieve a good compressive strength of 38.8 MPa after 28 days. The microstructural and phase analysis indicated that the LFS participated moderately in geopolymerization reaction with the formation of calcium silicate hydrate (C–S–H) and sodium aluminate silicate hydrate (N-A-S-H). The optimized one-part FA/LFS geopolymer met the minimum requirement of ASTM C1157 (>28.0 MPa) for a functional construction binder. The outcome of the paper offers a guideline to the construction industry to maximize the use of LFS to prepare green construction binder.

Effects of PVA fibers on microstructures and hydration products of cementitious composites with and without fly ash

This study explores the effects of poly(vinyl alcohol) (PVA) fibers on the microstructures and hydration products of cementitious composites with and without high-volume fly ash. To this end, the prepared composites are systematically characterized. The results indicate significant differences in the effect of PVA fibers on the cementitious composites with different fly ash contents. In the cementitious composites with high-volume fly ash, the PVA fibers give rise to the decalcification and polymerization of calcium (alumino) silicate hydrate (C(-A)-S-H) gels and a looser gel pore structure. In the composites without fly ash, PVA fibers promote the formation of Ca(OH)2 on the surface of fibers, increase the calcium-to-silicon ratio, and refine the gel pore structure. Furthermore, this study provides new insights into the influence of PVA fibers on the pore structure and molecular structure of the cementitious composites with different fly ash contents associated with their durability and dielectric properties.

High strength insulating polymeric composite based on recycled/virgin polyethylene terephthalate (PET) reinforced with hydrous magnesium silicate (talc)

In the current work, post-consumer polyethylene terephthalate (PET) bottles were reclaimed and reinforced with hydrous magnesium silicate, Mg3Si4O10(OH)2 commonly known as talc and with virgin PET (VPET) for the development of high strength polymeric composites. The composite sheets were prepared using the thermomechanical melt processing method followed by a thermal press technique. VPET was mixed during melt processing of recycled PET (RPET) in a specific ratio to suppress thermal degradation and hydrolysis of developed composite. Various blends of the composite samples of VPET and RPET (RPETx - VPET1-x) with a fixed ratio of talc were prepared and investigated for their strength, conductivity and micro-properties. 45 μm talc particle in ratio 1 to 4 was used. The effect of the talc addition on mechanical properties of the composites at the sub-micron scale was investigated through nanoindentation with quasi-continuous stiffness mode (QCSM) to analyze the nano-mechanical properties as a function of contact displacement. Surface morphology and composition were studied through scanning electron microscopy (SEM) and energy dispersive x-ray analysis (EDX), which showed a uniform dispersion of talc particles with no agglomerates on the surface. Compressive strength was determined with the universal testing machine (UTM) that revealed the highest compressive strength of RPET80 – VPET20 blend with a lowest thermal conductivity gradient value of 0.0213 W.m−1 observed, thus making it preferable material for insulation purposes. Load-displacement curves displayed the harder behavior of the composite sheet containing RPET80 – VPET20/talc as it needed 100 μN load to impose a penetration depth of 7.5 μm only. The elastic modulus and hardness of RPET80–VPET20/talc composite was significantly improved as compared to other samples.

Elasticity of Phase H Under the Mantle Temperatures and Pressures: Implications for Discontinuities and Water Transport in the Mid‐Mantle

AbstractPhase H (MgSiO4H2), one of the lower mantle's dense hydrous magnesium silicates (DHMSs), may form and exist in cold slabs and is crucial in carrying water into the deep mantle. Its sound velocities and density are crucial for inferring the mid‐mantle water cycling via seismic approaches. Here we obtain the elastic and thermodynamic properties of phase H under lower‐mantle conditions using first‐principles calculations and discuss the effect of the Mg‐Si disorder on elasticity. The density of phase H is ∼15% and ∼6% lower than that of bridgmanite and periclase, respectively. The dehydration reaction from phase H to bridgmanite, which may occur at the depth of ∼1,300–1,700 km in cold slabs, will cause an increase of 1.0%, 2.7%, and 15% at 1,500 km on VP, VS, and density, respectively. The dehydration of phase H in subduction zones could produce a seismic VS impedance contrast of ∼17% in the mid‐mantle, which can provide an explanation for some seismic discontinuities detected by previous studies. Meanwhile, phase H has remarkable anisotropies and this may help explain the observed seismic anisotropy within subduction zones. Collectively, our results suggest that some seismic observations in mid‐mantle slabs may be related to the presence of phase H formed via the deep water cycle, further constraining the potential water content in local regions of the subducted slabs.

Petrogenesis of Jian forsterite jade solely composed of end-member forsterite (Fo 99.8): Constrained by trace element and oxygen isotope

The Jian forsterite jade, named for its enrichment of end-member forsterite, is a new type of jade discovered in Ji'an County (Jilin Province, Northeast China). The formation mechanism of this rare end-member forsterite remains unclear. In this study, we present the mineral chemistry and oxygen isotope analysis of end-member forsterite to elucidate the formation mechanism of the Jian forsterite jade. The Jian forsterite jade can be divided into three main types: yellowish-green, dark-green, and black. The Jian forsterite jade consists mainly of forsterite, serpentine, as well as minor brucite, spinel, phlogopite, dolomite, calcite, magnesite, and magnetite. The Fo% content (100 × Mg/[Mg + Fe], mol%) of forsterite varies from 96.69 to 99.82. All tested forsterite samples show low Ni, Mn, Cr, and Ca contents and very high B contents. The δ18O values of forsterite range from 13.79 ‰ to 18.57 ‰, which are significantly higher than those from the mantle and are similar to those from skarn, marble, and calc-silicate rocks. The oxygen isotope values of forsterite, coupled with a distinctive trace element signature, suggest that the genesis of end-member forsterite results from a siliceous fluid-mediated decarbonation of Mg-rich dolomite. The genesis of the Jian forsterite jade can be outlined in two main metamorphic stages: (1) high-temperature prograde stage related to the interaction of the gneiss-derived SiO2-rich fluids with overlying dolomite. High temperature leads to the formation of forsterite, spinel, and calcite mineral assemblage. (2) low-temperature retrograde stage characterized by the infiltration of hydrous fluids. Hydrous silicate minerals, mostly serpentine, phlogopite, and talc, are formed during this stage. Our work highlights and confirms the effectiveness of the trace element geochemistry of forsterite in identifying magnesian skarns, and classifies the forsterite in the Jian forsterite jade as an extreme example of this occurrence.

Glass-forming ability and structural features of melt-quenched and gel-derived SiO2-TiO2 glasses

SiO2-TiO2 glasses produced by aerodynamic levitation coupled to laser heating or by sol-gel spray-drying were compared to highlight their structural differences. Glass formation was possible by melt-quenching up to 10 mol% TiO2, while higher contents led to devitrification. Raman spectroscopy and solid-state 17O and 29Si magic-angle-spinning nuclear magnetic resonance confirmed the clear emergence of Ti-O-Si bonds and a tetrahedral oxygen coordination of Ti4+ leading to full network connectivity, as also substantiated by the synthesis of TiO2-doped cristobalite. In gel-derived glasses, water content induced partial network depolymerization, thereby enhancing the solubility of TiO2 in the hydrous silicate matrix. However, full dehydration during heating proved challenging due to a competing tendency towards devitrification: the glass-forming range in the anhydrous binary SiO2-TiO2 system does not therefore appear to be significantly enlarged by the sol-gel synthesis route.

Ree minerals in black shales of the paleoproterozoic Mikhailovka formation (Baikal-Patom Highland, Siberia)

he paper characterizes REE mineralization from carbonaceous metapelites of the Paleoproterozoic Mikhailovka Formation, which is the most ancient gold-bearing horizon of the Lena province (Bodaibo district, Irkutsk region). The conditions of metamorphism of the studied samples do not exceed those of chlorite-muscovite subfacies of greenschist facies (ilmenite-pyrrhotite isograde). The metamorphic allanite is a main REE host, which crystallized before the last stage of plastic deformation and folding. The matter source for its formation is related to REE and Th absorbed on organic matter and clay minerals, as well as the detrital monazite. Late hydrothermal-metasomatic processes resulted in its replacement by hydroxycarbonates (hydroxybastnaesite, ancylite) and hydrous phosphates of light REEs (rhabdophane?), while Th precipitated as a hydrous silicate. Findings of low-temperature metamorphic monazite and xenotime are also described.

Fingerprinting secondary mantle plumes

Many vertical seismic velocity anomalies observed below different parts of the Eurasian plate are rooted in the transition zone between the upper and lower mantle (410–660 km), forming so-called secondary plumes. These anomalies are interpreted as the result of thermal effects of large-scale thermal upwelling (primary plume) in the lower mantle or deep dehydration of fluid-rich subducting oceanic plates. We present the results of thermo-mechanical numerical modelling to investigate the dynamics of such small-scale thermal and chemical (hydrous) anomalies rising from the lower part of the Earth's upper mantle. Our objective is to determine the conditions that allow thermo-chemical secondary plumes of moderate size (initial radius of 50 km) to penetrate the continental lithosphere, as often detected in seismo-tomographic studies. To this end, we examine the effect of the following parameters: (1) the compositional deficit of the plume density due to the presence of water and hydrous silicate melts, (2) the width of the weak zone in the overlying lithosphere formed because of plume-induced magmatic weakening and/or previous tectonic events, and (3) a tectonic regime varied from neutral to extensional. In our models, secondary plumes of purely thermal origin do not penetrate the overlying plate, but flatten at its base, forming “mushroom”-shaped structures at the level of the lithosphere-asthenosphere boundary. On the contrary, plumes with enhanced density contrast due to a chemical (hydrous) component are shown to be able to pass upwards through the lithospheric mantle to shallow depths near the Moho when (1) the compositional density contrast is ≥ 100 kg m−3 and (2) the width of the lithospheric weakness zone above the plume is ≥ 100 km. An extensional tectonic regime facilitates plume penetration into the lithosphere but is not mandatory. Our findings can explain observations that have long remained enigmatic, such as the “arrow”-shaped zone of low seismic velocities below the Tengchong volcano in south-western China and the columnar (“finger”-shaped) anomaly within the lithospheric mantle discovered more than two decades ago beneath the Eifel volcanic fields in north-western Germany. It appears that a chemical component is a characteristic feature not only of conventional hydrous plumes located over presently downgoing oceanic slabs, but also of upper mantle plumes in other tectonic settings.

A bio-inspired, plant-derived admixture for metakaolin blended cement mortars

Partially replacing ordinary Portland cement (OPC) with metakaolin (MK) can refine the pore structure, improve mechanical properties, and enhance the durability of concretes. However, MK tends to agglomerate due to its high surface area and high concentration of hydroxyl groups. To overcome this problem, this study exploits a plant-derived polyphenol, tannic acid (TA), as an eco-friendly admixture to disperse MK. Due to its similar chemical functional groups as the adhesive protein in the mussel byssus, TA can adhere to various surfaces like the mussel’s adhesive protein. An experimental program was carried out to characterize the TA treated MK and investigate the effects of the proposed method on the properties of the cement paste and mortar. After mixing with the MK, TA can be adsorbed on the surface of MK particles, achieving deflocculation and dispersion of mineral grains by electrostatic repulsion forces and steric effect, significantly improving the workability of the mortar. Pore structure analysis suggests that TA dispersion densifies the hydration products, as revealed by the much-reduced gel and interhydrate pores. The nanoindentation results indicated that all low-density calcium hydrate silicate (CSH) was converted to high density by the TA dispersion, leading to a drastic improvement in the compressive strength of the produced mortars. As a result, the 28 days compressive strength of cement mortar prepared with TA dispersed MK was enhanced by up to 97 % compared with the one prepared without MK substitution, which exhibits great potential in reducing the carbon footprint of cement-based construction materials.

Insight into differentiation in alkalic systems: Nephelinite-carbonate-water experiments aimed at Ol Doinyo Lengai carbonatite genesis

Ol Doinyo Lengai (ODL, Tanzania, East African Rift) is the only known volcano currently erupting carbonatite on Earth with 30 yr. cycles alternating between quiescent carbonatite effusion and explosive, compositionally-zoned silicate eruptions. We performed isothermal crystallization and thermal gradient experiments involving ODL nephelinite, Na2CO3 and H2O to understand magmatic differentiation in this system using SEM-EDS x-ray analysis, x-ray tomography, SIMS and LA-ICPMS to characterize samples. Isothermal crystallization experiments document that hydrous liquids coexist with nepheline+feldspar; as peralkalinity increases, temperatures decrease. Presence of Na2CO3 increases the solubility of water in the liquid. Experiments placing nephelinite with H2O+ Na2CO3 in a 1,000–350°C thermal gradient show that rapid reaction occurs, resulting in virtually melt-free mineral aggregates having mineral layering reflecting systematic differentiation throughout the capsule. Both types of experiments argue that a continuous interconnected melt exists over a large temperature range in alkalic magmatic systems allowing for differentiation in a reactive mush zone process. Liquid compositions change from carbonate-water bearing nephelinites at high temperature down to hydrous carbonate silicate liquids at <400°C. We propose a model for ODL eruption behavior: 1) nephelinite magmas pond and build a sill complex downward with time; 2) hydrous carbonate melts form in the mush and buoyantly rise, ultimately erupting as natrocarbonatites observed; 3) H2O contents build up in melt at the bottom of the sill complex, eventually leading to water vapor saturation and explosive silicate eruptions. The model accounts for eruption cycling and the unusual compositional zoning of ODL silicate tephras.

Development and characterization of ceramic waste powder-slag blended geopolymer concrete designed using Taguchi method

• Taguchi L16 matrix was used to design the experiments with five factors and four levels. • Taguchi method optimized the mix design of CWP-slag blended geopolymer concrete. • The compressive strength of the optimum mix reached 58.9 MPa. • Mechanical and durability properties were correlated using analytical regression models. • Scanning electron microscopy highlighted the formation of C-A-S-H and N -A-S-H gels. The ability to utilize industrial by-products in the production of cement-free geopolymer concrete is a promising technology. However, the development, characterization, and optimization of geopolymer concrete made with ceramic waste powder (CWP) and ground granulated blast furnace slag (or simply slag) have not been examined yet. This paper investigates the mechanical and durability properties of geopolymer concrete incorporating CWP and slag. Taguchi method was employed to design sixteen CWP-slag blended geopolymer concrete mixes. Five factors with four levels were considered in the design, including binder content, CWP replacement percentage by slag, alkali-activator solution to binder ratio (AAS/Binder), sodium silicate (SS) to sodium hydroxide (SH) ratio (SS/SH), and SH solution molarity. Results showed that geopolymer concrete properties remarkably degraded when CWP served as the sole binding material. However, combining 40 % CWP and 60 % slag yielded superior performance than the control mix made with 100 % slag. Correlations between mechanical and durability properties were developed in the form of regression models. The optimal compressive strength response (58.9 MPa) of CWP geopolymer concrete was attained using 450 kg/m 3 of binder content, 60 % of CWP replacement by slag, 0.5 of AAS/Binder, 1.5 of SS/SH, and 10 M of SH solution molarity. Scanning electron microscope and energy-dispersive X-ray spectroscopy highlighted the formation of calcium aluminum silicate hydrate and calcium silicate hydrate gels. Experimental findings provide evidence of the ability to utilize CWP in the production of structural grade geopolymer concrete.

Ore-forming environment of Pb−Zn mineralization related to granite porphyry at Huangshaping skarn deposit, Nanling Range, South China

Multiple metallogenic types (skarn-type and vein-type) related to hypabyssal granites are found at the Huangshaping polymetallic deposit in the Nanling Range, South China. To constrain the crystallization and mineralization processes of skarn formation, three generations of magnetite and pyrrhotite from the hydrous silicate stage, oxide stage, early quartz–sulfide stage, and late quartz–sulfide stage were distinguished. The geochemical compositions of magnetite and pyrrhotite were obtained by electron probe microanalyzer (EPMA) and in-situ ablation−inductively coupled plasma−mass spectrometry (LA-ICP-MS). The results show that there may be silicate inclusions in magnetite and interaction of wall rock occurred in the mineralization process. The geochemical trends recorded in pyrrhotite show the influence of limestone during the crystallization of pyrrhotite. The re-equilibration temperatures of Po I, Po II, and Po III are 420.46, 380.45, and 341.81 °C, respectively, which suggests a continuous evolution following the high-temperature W–Sn mineralized system. The content change of Ni and V reflects a gradual decrease of oxygen fugacity from Mag I to Mag III, while the sulfur fugacity calculated from pyrrhotite gradually decreases. This continuous skarn mineralization evolution process helps us to better understand the change of metallogenic environment in the retrograde stage of the Huangshaping deposit.

Intramolecular hydrogen isotope exchange inside silicate melts – The effect of deuterium concentration

Tracing the deep geological water cycle requires knowledge of the hydrogen isotope systematics between and within hydrous materials. For quenched hydrous alkali-silicate melts, hydrogen NMR reveals a distinct heterogeneity in the distribution of stable hydrogen isotopes (D, H) within the silicate tetrahedral network, where deuterons concentrate strongly in network regions that are associated with alkali cations. Previous hydrogen NMR studies performed in the sodium tetrasilicate system (Na2O x 4SiO2, NS4) with a 1:1 D2O/H2O ratio showed on average 1300 ‰ deuterium enrichment in the alkali-associated network, but the effect on varying bulk D2O/H2O ratios on this intramolecular isotope effect remained unconstrained. Experiments in the hydrous sodium tetrasilicate system with 8 wt% bulk water and varying bulk D2O/H2O ratios were performed at 1400 °C and 1.5 GPa. It is found that both hydrogen isotopes preferably partition into the silicate network that is associated with alkali ions. The partitioning is always stronger for the deuterated than for the protonated hydrous species. The relative enrichment of deuterium over protium in the alkali-associated network, i.e., the intramolecular isotope effect, correlates positively with the D2O/H2O bulk ratio of the hydrous NS4 system. Modeled for natural deuterium abundance (D/H near 1.56 × 10−4), a 1.4-fold (c. 340 ‰) deuterium enrichment in the alkali-associated silicate network is predicted. The partitioning model further predicts a positive correlation between the bulk water content of the silicate melt and the intramolecular deuterium partitioning into the alkali-associated silicate network. Such heterogeneities may explain the magnitude and direction of hydrogen isotope fractionation in hydrous silicate melts coexisting with silicate-saturated fluids. As such, this intramolecular isotope effect appears to be an effective mechanism for deuterium separation, particularly in hydrous magmatic settings, such as subduction zones.

Phase Stability of Al‐Bearing Dense Hydrous Magnesium Silicates at Topmost Lower Mantle Conditions: Implication for Water Transport in the Mantle

AbstractIn this study, we investigated the phase stability of Al‐free and Al‐bearing superhydrous phase B (shy‐B) up to 55 GPa and 2500 K. In comparison with Al‐free shy‐B, the incorporation of 11.7 wt.% Al2O3 in shy‐B expands the stability by ∼400–800 K at 20–30 GPa. The determined dehydration boundary for Al‐bearing phase D indicates that it could be present even at normal mantle geotherm conditions at 30–40 GPa. Up to 23.8 mol.% Al2O3 can be dissolved into the structures of akimotoite and bridgmanite as a result of the decomposition reactions of Al‐bearing shy‐B and phase D between 20 and 40 GPa. Results of further experiments indicate that δ‐AlOOH is the stable hydrous phase coexisting with Al‐depleted bridgmanite at pressures above 52 GPa. This study shows that the incorporation of Al in dense hydrous magnesium silicates can have a profound impact on our picture of the water cycle in the deep Earth.