Composition Of Glass Matrix Research Articles

Timing, magnitude and geochemistry of major Southeast Asian volcanic eruptions: identifying tephrochronologic markers

ABSTRACTWe review the current knowledge about Southeast Asian volcanoes and their eruption histories, and focus on identifying tephrochronologic markers representing major explosive eruptions in order to further future palaeoclimate and volcanological studies. Forty‐one volcanic edifices in Southeast Asia have been classified as large calderas by Whelleyet al. (2015) and thus have, or are likely to have, produced large explosive eruptions with a Volcanic Explosivity Index (VEI) of 6–8. Unfortunately, only 20 such eruptions have known ages, spanning from 1.2 Ma to 1991ad, and fewer have geochemical data that can be used for tephrostratigraphic correlations. Volcanic products from different geodynamic regions and different sources can generally be distinguished on major element plots (e.g. K2O versus CaO) of matrix glass composition. However, the distinction of multiple eruptions from the same source often requires additional data such as trace element compositions of matrix glass and/or mineral compositions. Biotite, but also magnetite compositions (MgO and TiO2content in particular) appear to be very discriminating. Up to nine tuffs in addition to the three to four Toba tuffs can be utilised as widespread tephrochronologic markers and span a range from 1.2 to 1.6 Ma to recent. As only a few Holocene major eruptions have been well characterised and dated, many large calderas are still unstudied, and many distal tephra layers are still lacking a source, more tephrochronologic markers can certainly be defined in the future.

Broadband near-IR photoluminescence in Ni2+ doped gallium silicate glass–ceramics

Broadband and tunable near-infrared (NIR) emission of Ni2+ doped glass–ceramics (GCs) is highly attractive due to their potential to address the challenge of broadband optical amplification in the optical communication band. However, optical activity of Ni2+ in different glass matrix as well as nucleation and crystallization processes in relevant glasses have not been understood fully. Here, broadband NIR photoluminescence was realized through precipitation of LiGa5O8:Ni2+ nanocrystals (NCs) within an alkali gallium-silicate glass matrix by melt-quenching and successive heat treatment. Upon exciting by a 980 nm laser diode, we observed NIR photoluminescence band centered at ~ 1310 nm with full width at half maximum of wider than 300 nm, which was originated from 3T2g(3F) → 3A2g(3F) electronic transition of octahedral coordinated Ni2+ in LiGa5O8 NCs embedded in the GCs. Controlled precipitation of NCs, LiGa5O8:Ni2+ and Ga2O3:Ni2+ were achieved by tailoring the composition of alkali gallium-silicate glass matrix. ab initio molecular dynamics simulation was carried out to clarify the formation of nanophases in the glass system. We confirmed that optical properties of transparent GCs containing Ni2+ NCs can be realized by changing molar percentages of Ga2O3. Our results offer a new insight into the precipitation of NCs in oxide glasses and Ni2+ doped GCs, which may be applicable in the photonic fields, such as optical amplifier and laser.

Eu and Dy doped borophosphosilicate glass-ceramics for near ultraviolet based light-emitting diode applications

The Eu and Dy doped borophosphosilicate glass-ceramics prepared using melt quenching technique are investigated for light-emitting diode applications. The X-ray diffraction results show that the precipitation of AlPO4 crystals in glass matrix occurred during the melt quenching process, leading to the formation of glass-ceramics. The content of AlPO4 crystals depends on the B2O3/P2O5 relative content in glass matrix composition. The spontaneous conversion of Eu3+ to Eu2+ takes place in the as-obtained glass-ceramics without using reducing atmosphere during the preparation process. The conversion extent of Eu3+ to Eu2+, optical properties and structure of the as-obtained materials are related to the B2O3/P2O5 relative content. The Eu/Dy co-doped borophosphosilicate glass-ceramics display the emission bands in the blue, yellow, and red light regions and their combinations generate near white light-emission. In addition, the tunable light emission can be realized by changing the concentration of Eu and B2O3/P2O5 relative content. The as-developed Eu and Dy doped borophosphosilicate glass-ceramics show promising prospects in light-emitting diode applications.

Luminescent properties of Pr3+ and Tm3+ doped oxyfluoride silicate glasses for light emitting diode applications

A series of Pr3+ and Tm3+ doped oxyfluoride silicate glasses prepared by melt-quenching method are investigated by X-ray diffraction, differential scanning calorimeter, Fourier transform infrared spectra, and photoluminescence spectra. The luminescence of Pr3+ in the as-prepared glasses mainly show two intensified emission peaks in blue (484 nm) and reddish orange (604 nm) regions corresponding to the transitions 3P0→3H4 and 3P0→3H6 (1D2→3H4) of Pr3+ ions. The relative emission intensity of these two emission peaks can be tailored by varying the glass matrix compositions. For certain base glass compositions, the formation of CaF2 crystals occurred during the melt quenching process, which can change the luminescence performance of the glasses. In addition, we find that the Tm3+ ions play a significant role in modifying the luminescence of Pr3+. The Pr3+ and Tm3+ doped oxyfluoride silicate glasses reported here have broad prospects in light emitting diode applications.

Luminescent properties of binary MO-2SiO2 (M = Ca2+, Sr2+, Ba2+) glasses doped with Ce3+, Tb3+ and Dy3+

Binary alkali-earth silicate glasses doped with Ce3+, Tb3+, and Dy3+ were obtained by melt-quenching technique and their photo- and radio-luminescence properties were investigated. The disordered structure of the glass is responsible for an inhomogeneous broadening of the electronic radiative 5d1 energy level of Ce3+ due to their localization in several positions with different surroundings, possessing a slightly different local ligand field. The influence of glass matrix composition on the Stokes shift as a function of ionic radius of the cation was shown. The radio-luminescence spectral shapes were related to the simultaneous occurrence of emissions from all Ce3+ configurations with relative weights proportional to their abundance in the glasses. The possibility of excitation of Ce3+ in different localizations was demonstrated and confirmed by the systematic shift of emission from excitation. On the contrary, no shift was observed in the Tb3+ and Dy3+ doped glasses, thanks to the lower effect of ligand field on their radiative electronic levels.

Pr3+ ions doped single alkali and mixed alkali fluoro tungsten tellurite glasses for visible red luminescent devices

The present work illustrates the optical absorption and fluorescence properties of Pr3+ ions doped single alkali and mixed alkali fluoro tungsten tellurite glasses prepared by using melt quenching technique. The prepared glasses were characterized by using absorption, excitation and photoluminescence (PL) spectral measurements. The energies of the absorption spectral features called oscillator strengths were calculated using area method and in turn used to evaluate the Judd-Ofelt (J-O) intensity parameters (Ω2, Ω4, Ω6). Such J-O parameters are used to estimate various radiative parameters such as radiative transition probabilities (AR), branching ratios (βR), and radiative lifetimes (τR) for the prominent fluorescent levels of Pr3+ ions in these glasses. The PL spectra of the as prepared glasses show three prominent peaks at wavelengths 488, 646 and 670 nm related to the transitions 3P0 → 3H4, 3P0 → 3F2 and 1D2 → 3H5 respectively for which emission cross-sections and branching ratios were evaluated. The variations in the spectral parameters with the variation of glass matrix composition have been examined in detailed. The decay profiles of the prepared glasses have been recorded to evaluate quantum efficiency of present series of glasses. From the measured branching ratios, emission cross-sections and quantum efficiency, it was concluded that the tungsten tellurite glasses added with potassium fluoride and doped with 1 mol% of Pr3+ions (TeWK glass) are quite suitable to produce visible red emission at 670 nm suitable to fabricate red luminescent devices.

Petrology and mineralogy of pumice from the Iheya North Knoll, Okinawa Trough: Implications for the differentiation of crystal‐poor and volatile‐rich melts in the magma chamber

The Iheya North Knoll is a submarine hill that is covered with pumice in the Okinawa Trough. The petrology and mineralogy of the crystal‐poor rhyolitic pumice collected from two sampling sites (T3 and T6) on the knoll were analysed in this article. The T6 pumice, which contains some mafic minerals (e.g., clinopyroxene, orthopyroxene), is slightly more crystal‐rich (<5 vol.%) than the T3 pumice (<2 vol.%). However, both types of pumice are high‐potassium rhyolites with similar Nd isotopic ratios and rare earth element patterns, as well as similar glass compositions, which indicates that they may fall along a line of descent of comagmatic evolution. The T6 pumice contains some xenomorphic clinopyroxenes that are not in equilibrium with their whole‐rock composition, which suggests that the crystal‐poor melt was extracted from a more mafic source. The T3 pumice features a flow structure, and its matrix glass records higher Cl concentrations and a lower total mass than the T6 pumice, which implies that the T3 melt has a higher volatile content and lower viscosity. The distinct petrographic characteristics but similar matrix glass compositions of the T3 and T6 pumice suggest that the extracted melt was physically differentiated during ascent, with the more crystal‐poor and volatile‐rich T3 melt reaching a higher lever in the magma chamber. Meanwhile, simulation calculations indicate that the melt experienced near‐adiabatic decompression‐driven chemical differentiation, which led to the growth of sodium‐rich plagioclase but not quartz in the pumice.

Erosion mechanism of YAG:Ce3+ phosphor in bismuth borate glasses

The poor thermal stability of current commercial white light emitting diodes (WLED) fabricated using a blue chip, a yellow YAG:Ce3+ phosphor (YAG) and resins, restricts their application in fields where long service time and high power lighting systems are required. Incorporating YAG into glass offers a possible way to resolve the problem by removing the organic resins in LED encapsulation. In the present study, YAG glass-ceramics with bismuth borate glass matrices were prepared using a rapid melt quenching technique. The YAG erosion mechanism was investigated for the first time in various glass matrix compositions at different co-sintering temperatures. The results demonstrate that neither B2O3 nor Bi2O3 individually erodes YAG particles, but the mixtures of B2O3 and Bi2O3 binary glass systems react with YAG and dissolve it almost completely or generate YAl3(BO3)4 (abbreviated as YAB), and they even erode as-produced YAB again under certain conditions. The production of YAB particles increases with the increase of the nBi/nB ratio at a lower temperature (680°C), while it first increases and then decreases at a higher temperature (800°C). The maximum production of YAB can be obtained at the co-sintering temperature of 680°C with an nBi/nB ratio of 55:45 or at 760°C with an nBi/nB ratio of 35:65. The intermediate Bi4B2O9 phase is found during the transformation of YAG into YAB. A simple but efficient approach is proposed to prevent YAG erosion and increase the luminous properties of YAG glass ceramics to a great extent by simply adding Ca2+ into bismuth borate glass matrices. The degree of YAG erosion decreases with increasing Ca2+ concentration until an impurity phase appears when the Ca2+ concentration exceeds 20mol%. The combination of good white light emission and suitable quantum efficiency as well as higher thermal stability makes the as-prepared YAG glass-ceramics equipped with a blue chip a possible alternative to commercial WLED.

Phase equilibria modelling of residual migmatites and granulites: An evaluation of the melt‐reintegration approach

ABSTRACTSuprasolidus continental crust is prone to loss and redistribution of anatectic melt to shallow crustal levels. These processes ultimately lead to differentiation of the continental crust. The majority of granulite facies rocks worldwide has experienced melt loss and the reintegration of melt is becoming an increasingly popular approach to reconstruct the prograde history of melt‐depleted rocks by means of phase equilibria modelling. It involves the stepwise down‐temperature reintegration of a certain amount of melt into the residual bulk composition along an inferredP–Tpath, and various ways of calculating and reintegrating melt compositions have been developed and applied. Here different melt‐reintegration approaches are tested using El Hoyazo granulitic enclaves (SE Spain), and Mt. Stafford residual migmatites (central Australia). Various sets ofP–Tpseudosections were constructed progressing step by step, to lower temperatures along the inferredP–Tpaths. Melt‐reintegration was done following one‐step and multi‐step procedures proposed in the literature. For El Hoyazo granulites, modelling was also performed reintegrating the measured melt inclusions and matrix glass compositions and considering the melt amounts inferred by mass–balance calculations. The overall topology of phase diagrams is pretty similar, suggesting that, in spite of the different methods adopted, reintegrating a certain amount of melt can be sufficient to reconstruct a plausible prograde history (i.e. melting conditions and reactions, and melt productivity) of residual migmatites and granulites. However, significant underestimations of melt productivity may occur and have to be taken into account when a melt‐reintegration approach is applied to highly residual (SiO2<55 wt%) rocks, or to rocks for which H2O retention from subsolidus conditions is high (such as in the case of rapid crustal melting triggered by mafic magma underplating).

Magma mush chemistry at subduction zones, revealed by new melt major element inversion from calcic amphiboles

We have used multiple-regression methods to calibrate new, pressure-independent empirical chemometric equations to calculate the major element composition of basanitic to rhyolitic melts in equilibrium with calcic amphibole. The equations are based on amphibole stoichiometric formula components ± temperature from published experimental P-T-X data and avoid some problems of previous studies associated with uncertainties in pressure determination. Compared with the pressure-dependent equations of Ridolfi and Renzulli (2012), tests run using an independent data set indicate that the new equations yield improved precision and accuracy, in particular for SiO2, TiO2, CaO, and K2O. The results are only marginally more precise when temperature is used as a dependent variable, demonstrating that temperature has a relatively minor role in controlling amphibole crystal chemistry compared with melt composition. This allows us to accept a small decrease in precision in excluding temperature from the analysis, which is very convenient for application of the equations to natural amphiboles where temperature is typically unknown. Using the new chemometric equations, reconstructed melt compositions in equilibrium with the rims of amphiboles in pumice clasts of the Ongatiti ignimbrite are in good agreement with coexisting matrix glass compositions, lending support for our analysis. The compositionally variable cores of the amphiboles give predicted melt compositions with large compositional variations from andesitic (63 wt% SiO2) to high-Si rhyolite. These compositional variations in the predicted melt compositions suggest that there may be a range of heterogeneous melts undergoing progressive differentiation within a major crustal magma storage region underneath the volcano. The results support the existence of genuine intermediate composition melts within the storage region. Interaction between these stored melts, disaggregating mush fragments and replenishing magmas gives rise to the chemical complexity observed in erupted magmas. We also used our multiple regression model to predict the compositions of melts that were in equilibrium with amphiboles in plutonic nodules from Grenada lavas. The predicted melts cover a wide range of compositions, perhaps as a result of in situ fractionation, but are consistent with melt inclusions hosted in those cumulates, as reported by Stamper et al. (2014). Overall, our new pressure- and temperature-independent equations resolve issues associated with previous pressure-dependent studies and represent a useful tool for further investigation of crustal processes at subduction zones.

A study of the behaviour of copper in different types of silicate glasses implanted with Cu+ and O+ ions

Glasses containing copper are promising photonic materials for lasing devices and all-optical components. It has already been shown that the oxidation state of the implants depends on many factors. This paper is going to report on one of them, i.e. the influence of the composition of a silicate glass matrix on the behaviour of the implanted Cu ions before and after a subsequent implantation of oxygen ions.Three types of silicate glasses having a different extent of cross-linking were implanted with copper ions with an energy of 330keV and a fluence 1×1016ionscm−2. Then the glasses were implanted with oxygen ions with an energy of 110keV into the same depth as the already implanted Cu ions. The concentration depth profiles of Cu in the glasses were studied by Rutherford Backscattering Spectrometry. After the implantation, the samples were characterised by optical absorption and photoluminescence spectroscopy. The samples were annealed in ambient atmosphere for 1h at 600°C, which is near the transformation temperature of those glasses. The effect of annealing on the distribution of the implants and on the absorption and emission spectra of the as-implanted glasses will be discussed as well.

Nd3+-doped TeO2–Bi2O3–ZnO transparent glass ceramics for laser application at 1.06 μm

The high crystallinity transparent glass ceramics based on Nd3+-doped 70TeO2–15Bi2O3–15ZnO (TBZ) compositions were successfully prepared by two-step heat treatment process. The effects of Nd2O3 content on the thermal, structural, mechanical, and optical properties of TBZ glass ceramics were studied. The incorporation of Nd2O3 enhanced the crystallization tendency in the matrix glass composition. The crystal phase and morphology of Bi2Te4O11 in the glass ceramics were confirmed by X-ray diffraction and field emission scanning electron microscopy. Due to precipitate more crystal phase, the hardness values increased from 3.21 to 3.66 GPa. Eight absorption peaks were observed from 400 to 900 nm and three emission bands appeared in the range of 850–1400 nm. With the increasing of Nd2O3 content from 0.5 to 2.5 wt%, the intensity of absorption peaks enhanced and the emission intensity increased up to 1.0 wt% and then fell down for further dopant concentration. The fluorescence decay lifetime decreased rapidly starting from 1.5 wt% Nd2O3 content due to the obvious energy migration among Nd3+. According to the extreme strong emission band around 1062 nm and the optimum Nd2O3 content (1.0 wt%), N10 glass ceramic was considered as a potential material for 1.06 μm laser applications.

Near white light emission of Ce3+‐, Ho3+‐, and Sm3+‐doped oxyfluoride silicate glasses

AbstractThe Ce3+‐, Ho3+‐, and Sm3+‐ single and co‐doped oxyfluoride silicate glasses for light emitting diodes are studied. These glasses were prepared by melt quenching method and their optical and structural properties were investigated by absorption spectra, photoluminescence spectra, Commission International de I'Eclairage chromaticity coordinates, X‐ray diffraction, and Fourier transform infrared spectra. It is found that the introduction of Al2O3 in glass composition can improve the emissions of Ho3+ and Sm3+. While the presence of B2O3 has the adverse effect and can suppress the emissions of Ho3+ and Sm3+. With substituting Na2O for CaO in the glass compositions, CaF2 crystals can be formed during the melt quenching process. We find the formation of CaF2 crystals can change the emission behavior of Ho3+ and Sm3+ ions. White light emissions can be achieved in the glasses and the luminescence colors can be tuned by varying the concentrations of the doped rare‐earth ions and the composition of glass matrix. The Ce3+‐, Ho3+‐, and Sm3+‐doped oxyfluoride silicate glasses presented here demonstrate promising applications in the fields of light emitting diodes.

Color-tunable luminescence in Ce-, Dy-, and Eu-doped oxyfluoride aluminoborosilicate glasses

A series of Ce-, Dy-, and Eu doped oxyfluoride aluminoborosilicate glasses prepared by melt-quenching method are studied for light emitting diodes applications. These glasses are investigated via optical absorption, photoluminescence, X-ray diffraction, Fourier transform infrared spectra, Commission International de I’Eclairage (CIE) color coordinates, and correlated color temperatures (CCTs). We find that the emission bands of Ce3+ ions move to the longer wavelengths with increasing excitation wavelengths and the molar ratio of B2O3/SiO2, Al2O3/SiO2, and B2O3/Al2O3 in the glass compositions. The precipitation of CaF2 crystals in glass can also lead to the red shift of the emission of Ce3+. The energy transfer processes among Ce3+, Dy3+, Eu3+, and Eu2+ ions are also investigated. The influence of the variation of glass network structure on the luminescence of Ce/Dy/Eu codoped glasses is studied. The CIE color coordinates and CCTs could be effectively tuned by adjusting the glass matrix compositions and excitation wavelengths. Furthermore, the near-ideal white light emission (color coordinate x=0.32, y=0.31) from the Ce/Dy/Eu codoped glass under near ultraviolet (UV) light excitation is realized.

Characterizing the residual glass in a MgO/Al2O3/SiO2/ZrO2/Y2O3 glass-ceramic.

The non-isochemical crystallization of glasses leads to glass-ceramics in which the chemical composition of the amorphous matrix differs from that of the parent glass. It is challenging to solely analyse the properties of these residual glassy phases because they frequently contain finely dispersed crystals. In this study, the composition of the residual glass matrix after the crystallization of a glass with the mol% composition 50.6 SiO2 · 20.7 MgO · 20.7 Al2O3 · 5.6 ZrO2 · 2.4 Y2O3 is analysed by scanning transmission electron microscopy (STEM) including energy dispersive X-ray analysis (EDXS). A batch of the residual glass with the determined composition is subsequently melted and selected properties are analysed. Furthermore, the crystallization behaviour of this residual glass is studied by X-ray diffraction, scanning electron microscopy including electron backscatter diffraction and STEM-EDXS analyses. The residual glass shows sole surface crystallization of indialite and multiple yttrium silicates while bulk nucleation does not occur. This is in contrast to the crystallization behaviour of the parent glass, in which a predominant bulk nucleation of spinel and ZrO2 is observed. The crystallization of the residual glass probably leads to different crystalline phases when it is in contact to air, rather than when it is enclosed within the microstructure of the parent glass-ceramics.

Preparation and characterization of new dental porcelains, using K-feldspar and quartz raw materials. Effect of B2O3 additions on sintering and mechanical properties

The aim of this work was to determine the effect of temperature and boric oxide (B2O3) addition on sintering and mechanical properties of a newly developed dental porcelain (DP) prepared from local Algerian raw materials. Based on a preliminary work, the new selected composition was 75wt.% feldspar, 20wt.% quartz and 5wt.% kaolin. It was prepared by sintering the mixture at different temperatures (1100–1250°C). The optimum sintering conditions gave a relatively higher density (2.47g/cm3) and excellent mechanical properties. The three point flexural strength (3PFS) and Martens micro-hardness of dental porcelains were 149MPa and 2600MPa, respectively. This obtained 3PFS value is more than four times greater than that of hydroxyapatite (HA) value (about 37MPa) sintered under the same conditions. However, the sintering temperature was lowered by about 25 and 50°C for 3 and 5wt.% B2O3 additions, respectively. But, it did not improve furthermore the samples density and their mechanical properties. It has also been found that B2O3 additions provoke a glass matrix composition variation which delays the leucite formation during sintering.

Synthesis and characterization of a BaGdF5:Tb glass ceramic as a nanocomposite scintillator for x-ray imaging

Transparent glass ceramics with embedded light-emitting nanocrystals show great potential as low-cost nanocomposite scintillators in comparison to single crystal and transparent ceramic scintillators. In this study, cubic structure BaGdF5:Tb nanocrystals embedded in an aluminosilicate glass matrix are reported for potential high performance MeV imaging applications. Scintillator samples with systematically varied compositions were prepared by a simple conventional melt-quenching method followed by annealing. Optical, structural and scintillation properties were characterized to guide the design and optimization of selected material systems, aiming at the development of a system with higher crystal volume and larger crystal size for improved luminosity. It is observed that enhanced scintillation performance was achieved by tuning the glass matrix composition and using GdF3 in the raw materials, which served as a nucleation agent. A 26% improvement in light output was observed from a BaGdF5:Tb glass ceramic with addition of GdF3.

Composition-induced tunable white emission in Ce/Tb/Eu co-doped lithium–barium borophosphate glasses

Ce/Tb/Eu co-doped lithium–barium borophosphate glasses were successfully synthesized by melt quenching method. The structure and luminescence properties of glasses were characterized by Fourier transform infrared spectroscopy, photoluminescence spectra spectroscopy and colorimetric analysis. The influence of matrix composition on glass structure and photoluminescent properties was investigated in detail. The partial replacement of BaO for B2O3 changed the ratios of Ce3+/Ce4+ and Eu2+/Eu3+, and altered the glass structure, which affected the physical properties of the glass samples. The emission intensity of Ce3+, Eu2+ and Tb3+ decreased with increasing BaO content, but the intensity of Eu3+ increased continuously. Energy was transferred from Ce3+, Tb3+and Eu2+ to Eu3+ ions, which resulted in the enhancement of Eu3+ luminescence intensity. Meanwhile, the colors of Ce/Tb/Eu co-doped luminescent glasses changed from cold white to warm white by changing glass matrix composition. Possible mechanisms of the reduction for valence-variable rare earth ions were also discussed.

The evolution and storage of primitive melts in the Eastern Volcanic Zone of Iceland: the 10 ka Grímsvötn tephra series (i.e. the Saksunarvatn ash)

Major, trace and volatile elements were measured in a suite of primitive macrocrysts and melt inclusions from the thickest layer of the 10 ka Grímsvötn tephra series (i.e. Saksunarvatn ash) at Lake Hvítárvatn in central Iceland. In the absence of primitive tholeiitic eruptions (MgO > 7 wt%) within the Eastern Volcanic Zone (EVZ) of Iceland, these crystal and inclusion compositions provide an important insight into magmatic processes in this volcanically productive region. Matrix glass compositions show strong similarities with glass compositions from the AD 1783–1784 Laki eruption, confirming the affinity of the tephra series with the Grímsvötn volcanic system. Macrocrysts can be divided into a primitive assemblage of zoned macrocryst cores (An78–An92, Mg#cpx = 82–87, Fo79.5–Fo87) and an evolved assemblage consisting of unzoned macrocrysts and the rims of zoned macrocrysts (An60–An68, Mg#cpx = 71–78, Fo70–Fo76). Although the evolved assemblage is close to being in equilibrium with the matrix glass, trace element disequilibrium between primitive and evolved assemblages indicates that they were derived from different distributions of mantle melt compositions. Juxtaposition of disequilibrium assemblages probably occurred during disaggregation of incompatible trace element-depleted mushes (mean La/Ybmelt = 2.1) into aphyric and incompatible trace element-enriched liquids (La/Ybmelt = 3.6) shortly before the growth of the evolved macrocryst assemblage. Post-entrapment modification of plagioclase-hosted melt inclusions has been minimal and high-Mg# inclusions record differentiation and mixing of compositionally variable mantle melts that are amongst the most primitive liquids known from the EVZ. Coupled high-field strength element (HFSE) depletion and incompatible trace element enrichment in a subset of primitive plagioclase-hosted melt inclusions can be accounted for by inclusion formation following plagioclase dissolution driven by interaction with plagioclase-undersaturated melts. Thermobarometric calculations indicate that final crystal–melt equilibration within the evolved assemblage occurred at ~1140 °C and 0.0–1.5 kbar. Considering the large volume of the erupted tephra and textural evidence for rapid crystallisation of the evolved assemblage, 0.0–1.5 kbar is considered unlikely to represent a pressure of long-term magma accumulation and storage. Multiple thermometers indicate that the primitive assemblage crystallised at high temperatures of 1240–1300 °C. Different barometers, however, return markedly different crystallisation depth estimates. Raw clinopyroxene–melt pressures of 5.5–7.5 kbar conflict with apparent melt inclusion entrapment pressures of 1.4 kbar. After applying a correction derived from published experimental data, clinopyroxene–melt equilibria return mid-crustal pressures of 4 ± 1.5 kbar, which are consistent with pressures estimated from the major element content of primitive melt inclusions. Long-term storage of primitive magmas in the mid-crust implies that low CO2 concentrations measured in primitive plagioclase-hosted inclusions (262–800 ppm) result from post-entrapment CO2 loss during transport through the shallow crust. In order to reconstruct basaltic plumbing system geometries from petrological data with greater confidence, mineral–melt equilibrium models require refinement at pressures of magma storage in Iceland. Further basalt phase equilibria experiments are thus needed within the crucial 1–7 kbar range.

Peralkaline felsic magmatism at the Nemrut volcano, Turkey: impact of volcanism on the evolution of Lake Van (Anatolia) IV

Nemrut volcano, adjacent to Lake Van (Turkey), is one of the most important peralkaline silicic centres in the world, where magmatism for ~570,000 years has been dominated by peralkaline trachytes and rhyolites. Using onshore and Lake Van drill site tephra samples, we document the phenocryst and glass matrix compositions, confirming a complete spectrum from very rare mafic to dominantly silicic magmas. Magma mixing has been common and, along with the multi-lineage nature of the magmas, indicates that Nemrut has been a very open system where, nevertheless, compositionally zoned caps developed during periods of relative eruptive quiescence. Geothermometry suggests that the intermediate-silicic magmas evolved in an upper crustal magma reservoir at temperatures between 1100 and 750 °C, at fO2 close to the FMQ buffer. The silicic magmas either were halogen poor or exsolved a halogen-rich phase prior to or during eruption. An unusual Pb-rich phase, with up to 98.78 wt% PbO, is interpreted as having exsolved from the intermediate-rhyolitic magmas.