Ar Solubility Research Articles

Influence of dissolved argon or carbon dioxide on the viscosity and surface tension of the imidazolium-based ionic liquids [OMIM][PF6] or [m(PEG2)2IM

The present study investigates the influence of dissolved gases on the viscosity and surface tension of ILs at saturation conditions. As model systems, the imidazolium-based ILs, namely, the hydrophobic 1-methyl-3-octylimidazolium hexafluorophosphate ([OMIM][PF6]) and the hydrophilic 1,3-bis(2-(2-methoxyethoxy)ethyl)imidazolium iodide ([(mPEG2)2IM]I), were studied in combination with argon (Ar) and carbon dioxide (CO2) representing gases of relatively low and high solubility. By a combination of surface light scattering (SLS) and the pendant-drop (PD) method within one setup and sample cell, the surface tension determined by the PD method was used to access the saturated viscosity of the liquid bulk phase via SLS at temperatures of (303 and 323) K and pressures from (0.02 up to 8) MPa. At the same states and within the same setup, the gas solubility in the ILs was estimated on the basis of the evolution of the pressure during transient gas absorption. The addition of either gas causes a decrease in the surface tension of both ILs, which is more pronounced for CO2 at a given pressure and for the lower temperature. This behavior seems to be related to the weakening of intermolecular interactions at the gas–liquid interface by adding gas, facilitated by possible surface enrichment effects of CO2. The viscosity of both ILs is distinctly lowered with increasing CO2 pressure, amounting to a factor of 14 for [OMIM][PF6] at the largest pressure of 5 MPa. In the presence of Ar, the viscosity of [OMIM][PF6] reduces with increasing pressure by 11% at 8 MPa, while that of [(mPEG2)2IM]I increases by 7%. This behavior indicates that the fluidization effect on the liquid phase due to the solvation of Ar is dominant for the hydrophobic IL with higher Ar solubility, whereas it is outweighed by the competing compression effect of the gas for the hydrophobic IL.

Light noble gas dissolution into ring structure-bearing materials and lattice influences on noble gas recycling

Light noble gas (He–Ne–Ar) solubility has been experimentally determined in a range of materials with six-member, tetrahedral ring structures: beryl, cordierite, tourmaline, antigorite, muscovite, F-phlogopite, actinolite, and pargasite. Helium solubility in these materials is relatively high, 4×10−10 to 3×10−7molg−1bar−1, which is ∼100 to 100,000× greater than He solubility in olivine, pyroxene, or spinel. Helium solubility broadly correlates with the topology of ring structures within different minerals. Distinctive He–Ne–Ar solubility patterns are associated with the different ring structure topologies. Combined, these observations suggest ring structures have a strong influence on noble gas solubility in materials and could facilitate the recycling of noble gases, along with other volatiles (i.e., water, chlorine, and fluorine), into the mantle. Measurements of Ne and Ar solubility in antigorite, however, are highly variable and correlated with each other, suggesting multiple factors contribute the solubility of noble gases in serpentine-rich materials.

Ar diffusion and solubility measurements in plagioclases using the ultra-violet laser depth-profiling technique

Abstract We describe the first direct measurements of Ar diffusion and solubility in plagioclases using ultra-violet (UV) laser ablation depth-profiling and noble gas mass spectrometer analyses of experimentally treated (599–1000 °C, 50–200 MPa of Ar) crystal fragments of labradorite and oligoclase. Labradorite 40 Ar gain diffusion profiles were measured, yielding an activation energy of 26.72±4.58 kcal mol −1 (118.0±19.16 kJ mol −1 ) and a frequency factor of 9.77×10 −9 (+8.79×10 −8 , −8.79×10 −9 ) cm 2 s −1 (95% confidence). The Ar solubility in labradorite was measured yielding a value of <0.2 ppb bar −1 , which is similar to or lower than many rock forming minerals. The labradorite diffusion parameters indicate Ar closure temperatures of 211 °C for a spherical diffusion geometry, and 243 °C for a planar diffusion geometry (for 100 µm-diameter grains, with cooling rates of 10 °C Ma −1 ). The data indicate that labradorite is less Ar retentive than K-feldspar at low temperatures, but more Ar retentive than K-feldspar at high temperatures, corroborating previous work on plagioclase. The relatively slow Ar diffusion rates in labradorite at magmatic temperatures may explain the common observation of older ages in large plagioclase grains in acidic volcanic systems. Supplementary material: Details of the UV laser depth profiles obtained from the labradorite and oligoclase samples used in this study are available at www.geolsoc.org.uk/SUP18608 .

Interfacial storage of noble gases and other trace elements in magmatic systems

Trace elements are widely used to unravel magmatic processes and constrain the chemical differentiation of the Earth. Key to their use is understanding the controls on fractionation between solid and liquid, which hinge on the energetics of trace element incorporation into crystals. At equilibrium most trace elements are incompatible in bulk magmatic minerals because of unfavourable incorporation energies. We use computational methods to explore the role that crystal interfaces can play in trace element incorporation in minerals. We demonstrate that differences between bulk and interface incorporation energies can be very large and lead to concentration differences of many orders of magnitude, consistent with experimental evidence for interface enrichment. By emphasising the importance of adsorption/incorporation at interfaces, we account for the competing effects of bulk equilibrium and interface segregation operating during melting and crystallisation. Computational results are presented for divalent cations and Ar in bulk forsterite, at the {010} surface and at the {100}/{010} stepped grain boundary. For all species studied larger than Mn2+, segregation to the interface is highly exothermic. We discuss in particular the take-up of noble gases where, in contrast to earlier work, a recent experimental study concluded that argon is highly compatible in mantle minerals. In contrast our calculations indicate bulk Ar solubility is very small and suggests incorporation at mineral interfaces is overwhelmingly favourable.

Noble gases in high-pressure silicate liquids: A computer simulation study

The test particle method has been used in conjunction with molecular dynamics simulations to evaluate the solubility of noble gases in silicate melts of various composition. At low pressure the calculated solubility constants (the inverse of the Henry’s constant) are in excellent agreement with data of the literature. In particular it is found that the solubility constant (i) decreases when the size of the noble gas increases, (ii) decreases from silica-rich to silica-poor composition of the melt, and (iii) is positively correlated with the temperature. Moreover it is shown that the solubility is governed primarily by the entropic cost of cavity formation for inserting the noble gas into the melt and secondarily by its solvation energy. Interestingly, the behaviour of these two contributions differ from each other as the entropic cost of cavity formation increases strongly with the size of the solute atom to insert whereas large atoms are better solvated than small ones. Considerations of thermodynamics show that the weight fraction of a noble gas in a silicate melt coexisting with its parent fluid at T and P is equal to ngγm/nmγg, where ng and nm are the densities of the two coexisting phases (gas and melt, respectively) and where the solubility parameters γm and γg express the probability of inserting the noble gas atom in the melt and in the parent fluid, respectively. The γm and γg decrease drastically when the pressure is increased and the noble gas solubility at high pressure is the result of a balance between these two quantities. Here again, the pressure behaviour of γm and γg is dominated by the pressure dependence of the entropic cost of cavity formation, the energetic contribution being of minor importance but not negligible at high pressures. With all melt composition investigated here (silica, rhyolite, MORB and olivine), the calculated solubility curves exhibit the same qualitative behaviour with pressure; a steep rise culminating in a broad maximum followed by a gradual decrease of the solubility at higher pressure. At variance with LHDAC experiments (Chamorro-Pérez et al., 1996, 1998; Bouhifd et al., 2006, 2008) where a Ar solubility drop is observed at about 50kbar in silica and molten olivine and in the pressure range ∼100–160kbar with other melt composition, we do not find such a sudden change of the solubility.

Interstitial voids in silica melts and implication for argon solubility under high pressures

Molecular dynamics simulations with ab initio potential under various pressures up to 11.58 GPa have been carried out to investigate the relationship between microscopic structure and solubility of Ar in silica melts. The Voronoi diagram method is used to identify interstitial voids in the structure of silica melts. We find that (1) the radius distribution of interstitial voids generally obeys the log-normal probability function; (2) the number of available interstitial voids large enough to accommodate Ar atoms decreases with increasing pressure; (3) Ar solubility increases linearly with pressure up to about 3 GPa, where Ar solubility reaches a maximum value, and then it decreases dramatically when pressure becomes higher than 5–6 GPa. The drop of Ar solubility is a result of structural continuous change but not structural transformation. These results are in good agreement with experimental measurements.

Gas isotopic signatures (He, C, and Ar) in the Lake Kivu region (western branch of the East African rift system): Geodynamic and volcanological implications

On 17 January 2002, the city of Goma was partly destroyed by two of the several lava flows erupted from a roughly N‐S oriented fracture system opened along the southern flank of Mount Nyiragongo (Democratic Republic of Congo), in the western branch of the East African rift system. A humanitarian and scientific response was promptly organized by international, governmental, and nongovernmental agencies coordinated by the United Nations and the European Union. Among the different scientific projects undertaken to study the mechanisms triggering this and possible future eruptions, we focused on the isotopic (He, C, and Ar) analysis of the magmatic‐hydrothermal and cold gas discharges related to the Nyiragongo volcanic system, the Kivu and Virunga region. The studied area includes the Nyiragongo volcano, its surroundings, and peripheral areas inside and outside the rift. They have been subdivided into seven regions characterized by distinct 3He/4He (expressed as R/Rair) ratios and/or δ13C‐CO2 values. The Nyiragongo summit crater fumaroles, whose R/Rair and δ13C‐CO2 values are up to 8.73 and from −3.5‰ to −4.0‰ VPDB, respectively, show a clear mantle, mid‐ocean ridge basalt (MORB)‐like contribution. Similar mantle‐like He isotopic values (6.5–8.3 R/Rair) are also found in CO2‐rich gas emanations (mazukus) along the northern shoreline of Lake Kivu main basin, whereas the 13δC‐CO2 values range from −5.3‰ to −6.8‰ VPDB. The mantle influence progressively decreases in (1) dissolved gases of Lake Kivu (2.6–5.5 R/Rair) and (2) the distal gas discharges within and outside the two sides of the rift (from 0.1 to 1.7 R/Rair). Similarly, δ13C‐CO2 ratios of the peripheral gas emissions are lighter (from −5.9‰ to −11.6‰ VPDB) than those of the crater fumaroles. Therefore, the spatial distribution of He and C signatures in the Lake Kivu region is mainly produced by mixing of mantle‐related (e.g., Nyiragongo crater fumaroles and/or mazukus gases) and crustal‐related (e.g., gas discharges in the Archean craton) fluids. The CO2/3He ratio (up to 10 × 1010) is 1 order of magnitude higher than those found in MORB, and it is due to the increasing solubility of CO2 in the foiditic magma feeding the Nyiragongo volcano. However, the exceptionally high 40Ar*/4He ratio (up to 8.7) of the Nyiragongo crater fumaroles may be related to the difference between He and Ar solubility in the magmatic source. The results of the present investigation suggest that in this area the uprising of mantle‐originated f luids seems strongly controlled by regional tectonics in relation to the geodynamic assessment of the rift. These fluids are mainly localized in a relatively small zone between Lake Kivu and Nyiragongo volcano, with important implications in terms of volcanic activity.

Low solubility of He and Ar in carbonatitic liquids: Implications for decoupling noble gas and lithophile isotope systems

In order to asses the importance of carbonatitic liquids in transporting noble gases in the mantle, the solubilities of He and Ar in carbonatitic liquids were estimated from analyses of calcium–potassium carbonate glasses that had been synthesized at 1 bar and temperatures between 850 and 950 °C under He or Ar enriched atmospheres. Despite poor reproducibility related to difficulties synthesizing carbonatite glass, we have been able to estimate He and Ar solubilities in carbonatite liquids to be 1 × 10 −8 and 2 × 10 −9 mol g −1 at 1 bar respectively (with ⩾50% uncertainty). Despite the significant uncertainties on these estimates, it is clear that the noble gases are not massively soluble in carbonatite liquids (within error, these solubilities are identical to their equivalent solubilities in tholeiitic melts). Assuming the results of these low pressure experiments can be applied to mantle conditions, it seems unlikely that carbonatite metasomatism per se transports noble gases within the mantle. It is nevertheless possible that partitioning of lithophile trace elements (including the important radioelements, U, K and Th) and noble gases between a carbonatitic melt and a silicate melt could effectively decouple lithophile and noble gas isotope systematics because the carbonatitic melt expressedly does not transport noble gases, yet is known to efficiently transport incompatible trace elements.

Argon solubility drop in silicate melts at high pressures: A review of recent experiments

Controversy surrounding Ar solubility in silicate melts at high pressure (HP) has led us to perform a suite of Ar solubility experiments on silicate melts with a variety of compositions up to ~ 25 GPa. These results of these experiments strongly constrain the structure of the melts at HP, and confirm the features first observed in olivine composition melts, resolving the controversy over the solubility of Ar in silicate melts at the upper mantle conditions. The Ar solubility experiments show that argon contents in aluminosilicate melts reach a maximum that persists to pressures as high as 17 GPa well above that observed for Al-free melts. A subsequent drop in argon solubility correlates well with the expected void loss in the melt structure predicted by molecular dynamics simulations [Nevins, D. and Spera, F.J. (1998). Molecular dynamics simulations of molten CaAl 2Si 2O 8: dependence of structure and properties on pressure, Am. Mineral. 83: 1220–1230]. This discontinuous drop for argon solubility at high pressure should be a universal behaviour of silicate melts, and our experiments show that the onset of pressure for reduced argon solubility has a positive correlation with the Al/(Al + Si) ratio of the melts. Thus the experimental data for Ar solubility are entirely consistent with known natural samples from upper mantle conditions.

Lattice diffusion and solubility of argon in forsterite, enstatite, quartz and corundum

Argon diffusivities and solubilities in single, gem-quality crystals of forsterite, enstatite, quartz and corundum were determined from experiments conducted between 0.5 and ~ 6000 bars Ar pressure and temperatures from 425 °C to 1200 °C. Polished single-crystal slabs and specimens with natural facets were placed in open containers and exposed to an argon atmosphere either in a pressure vessel or in a gas-flow tube furnace at near-atmospheric pressure. Argon atoms from the pressure medium diffused into the crystals to produce near-surface concentration gradients, which were directly profiled using Rutherford backscattering spectrometry. The following Arrhenius relations were obtained for Ar diffusion: F o r s t e r i t e : D A r = 7.2 × 10 - 20 exp ( - 42 k J m o l - 1 / R T ) m 2 s - 1 E n s t a t i t e : D A r = 1.3 × 10 - 20 exp ( - 32 k J m o l - 1 / R T ) m 2 s - 1 Q u a r t z : D A r = 3.1 × 10 - 19 exp ( - 43 k J m o l - 1 / R T ) m 2 s - 1 C o r u n d u m : D A r = 1.2 × 10 - 20 exp ( - 33 k J m o l - 1 / R T ) m 2 s - 1 The reported activation energies probably represent apparent activation energies that are a combination of the effects of lattice diffusion of Ar atoms with trapping in point-defect vacancies. There are no discernible differences in Ar diffusion in different crystallographic directions, and diffusivities do not vary as a function of the intrinsic oxygen fugacity of the experimental vessels. In addition to diffusivities, the uptake gradients also yielded lattice solubilities of argon, as represented by the concentrations at the mineral surfaces. There are small differences between the solubilities of forsterite, quartz and corundum, but all solubilities are high at ~ 2000 ppm (by mass). The Ar solubilities for the iron-bearing minerals enstatite and San Carlos olivine are much higher than for the other minerals. The mean value for enstatite is 30–60% higher than the iron-free minerals, owing, perhaps, to incorporation of Ar in point defects that are more abundant in enstatite due to oxidation of minor FeO. Argon solubilities are independent of Ar pressure at P Ar > ~ 1 bar, but slightly lower at P Ar ≈ 0.5–1 bar. This behavior suggests that all sites capable of accommodating Ar atoms are filled at Ar pressures of ~ 1 bar. In all cases, Ar appears to be compatible in the minerals investigated, although the degree of compatibility will depend upon assumptions concerning the minimum Ar fugacity required to fully populate the available vacancies. A series of validation experiments and surface-sensitive analyses were conducted to confirm that the near-surface regions of crystals used in the experiments were crystalline, unreacted and free of structural damage that could potentially influence diffusion and solubility results.

高圧下における SiO<SUB>2</SUB> メルトへの希ガス溶解度

Solubility of noble gas in silicate melt under high pressures is important for understanding the origin of the Earth’s atmosphere and evolution of the Earth’s interior. In this article, previous studies of noble gas solubility based on high pressure experiments are reviewed. Then we present new experimental results on Ar solubility in silica melt up to about 12 GPa by using a laser-heated diamond-anvil cell (LHDAC). It was found that solubility of noble gas increased and then saturated with increasing pressure, which is completely different from the results of previous LHDAC experiments. The source of discrepancy and problems of solubility measurements using LHDAC are discussed.

Estimates of Ar diffusion and solubility in leucite and nepheline: Electron microprobe imaging of Ar distribution in a mineral

Leucite is unique among minerals that have been exploited for K-Ar and Ar-Ar dating, in that it exhibits a major phase change at 645-665 degrees C, across which Ar diffusion and solubility can be studied. In addition, well-developed twinning occurs only in the low temperature form, offering further opportunity to examine the effects of crystallographic change upon Ar diffusion. Nepheline was studied to compare the effect of its I-dimensional c-axis oriented lattice channels with the 3-dimensional channels of leucite. The amounts of Ar introduced into leucite were far higher than those observed in feldspars, allowing its analysis by electron microprobe spot traverses and X-ray maps. This has provided the first high spatial resolution 2-dimensional study of Ar distribution in a K-bearing mineral. Extreme changes in Ar solubility were observed across the leucite phase transition, with Ar solubilities jumping from similar to 70 ppm (at 1 kbar) in the tetragonal (low-temperature) form to similar to 750 ppm (at I kbar) in the cubic (high-temperature) form. Argon penetration profiles were complex and estimates of the diffusion rates show that they are more rapid than in K-feldspar. In addition, spikes of high Ar concentration were observed in both forms, suggesting extended defects or micro-inclusions formed argon traps in the structure. No concentration changes could be correlated to leucite twin planes, suggesting that twin planes did not act as fast pathways for Ar movement. Nepheline yielded a much lower Ar solubility of 0.15-0.31 ppm at 1 kbar.

Lattice diffusion of Ar in quartz, with constraints on Ar solubility and evidence of nanopores

Diffusion and solubility of Ar in optically clear natural and synthetic quartz crystals were examined at ∼500 to 1200 °C by treating polished specimens in pressurized Ar (1–185 MPa) and characterizing the resulting diffusive-uptake (or subsequent diffusive-loss) profiles using Rutherford backscattering spectroscopy (RBS). Analytical uncertainty leads to significant scatter in the data, but the Ar diffusivity, D, is reasonably well constrained by the following Arrhenius relationship:D = 8.2−4.2+8.8 × 10 −19 exp[(−6150 ± 750)/T(K)] m 2/sNo effects of crystallographic orientation or quartz structural form (α or β) are discernible. Apparent solubilities typically fall between 1000 and 3000 ppm (by mass), with large uncertainties (±50–60% 2σ), but some lower values (∼700 ppm) are observed near the low end of the Ar pressure range investigated. Occasional high-concentration outlier values fall between 5000 ppm and 3.8 wt.% Ar. These do not correlate with Ar pressure, suggesting extrinsic (non-lattice) siting of Ar in some cases. Field-emission SEM images and numerical simulations of the diffusion process document isolated nanopores as the hosts for the occasional very high concentrations of Ar (observable pores range down to ∼10 nm in diameter; indirect evidence points to smaller ones as the more common sinks for Ar). The systematics of the data suggest an actual (lattice) solubility of ∼2000 ppm at 100- to 200-MPa Ar pressure, which is equivalent to a partition coefficient of ∼0.001cm 3STP/g · atm. Using either organic clathrate or fullerene as the Ar sources, 1-GPa experiments in a piston-cylinder apparatus result in similar uptake of Ar into quartz, in this case through partitioning equilibrium with C-O-H fluid (clathrate source) or amorphous carbon (fullerene source). The ability of quartz, relative to other minerals, to incorporate significant amounts of Ar may allow this ubiquitous and abundant mineral to serve as a local sink for Ar in crustal rocks lacking a free fluid phase. The diffusion data permit open-system behavior of Ar in quartz below the closure temperature of biotite and other 40Ar source minerals.

Vesiculation and vesicle loss in mid-ocean ridge basalt glasses: He, Ne, Ar elemental fractionation and pressure influence

Sarda and Graham (1990) proposed that in mid-ocean ridge basalts (MORBs), degassing occurs through equilibrium vesiculation followed by various extents of vesicle loss. This model predicts that in a bulk sample of MORB glass with vesicles, the rare gases represent a binary mixture between a vesicle component and a component dissolved in the melt. As vesiculation is expected to produce very different rare gas concentrations and elemental ratios in gas and melt, binary mixing systematics should be recorded in the MORB rare gas abundance data. Indeed, a large range of 4He/40Ar∗ ratios was known to exist, but these binary mixing systematics remained elusive because helium was used as a proxy for rare gas abundance because helium is not affected by air addition. Here we show that using Ar instead of He, the 4He/40Ar∗ ratio is higher where the Ar concentration is lower, as expected from simple binary mixing systematics.Taking advantage of the growing Ne database, we further show that the predicted binary mixing is recorded by the He-Ar and He-Ne couples, provided He concentration is not used to trace vesicle abundance. This is because a significant part of helium remains in the melt due to its higher solubility. In contrast, Ar or Ne concentrations, which can both be corrected for air addition, clearly trace vesicles and yield binary mixing patterns that hold for ridges worldwide. The model of vesiculation and vesicle loss thereby finds geochemical support in the rare gas abundance data.The He-Ne-Ar concentration data is best explained by assuming the ratio of helium to neon or argon solubility is about 5 to 15 times higher than values measured in 1 bar laboratory experiments, due to higher He and lower Ne and Ar solubilities. We propose that this is a pressure effect, and vesiculation mainly occurs during magma ascent in the mantle after melting.

Experimental evidence for high noble gas solubilities in silicate melts under mantle pressures

The solubilities of Ar and Xe in Fe-free synthetic haplogranitic and tholeiitic melts were experimentally determined in the pressure range of 1–11 GPa and at temperatures between 1500 and 2000°C. Experiments were performed in a piston cylinder apparatus (1–3 GPa) and in a multi-anvil apparatus (2–11 GPa). The noble gas concentrations in the quenched glasses were determined with electron microprobe. As a function of pressure, Ar solubility increases linearly up to about 4–5 GPa where it reaches about 4.0 and 0.8 wt% for the haplogranitic and tholeiitic melt, respectively. At higher pressure the amount of dissolved Ar remains constant, suggesting that some threshold concentration is reached. The Xe solubility in tholeiite melt exhibits a very similar pattern. It increases linearly up to about 6 GPa, where a threshold concentration of 0.8 wt% is reached. A further increase of pressure up to 11 GPa does not result in changes in Xe solubility. The leveling off in noble gas solubility at high pressures may imply that the interstitial sites in the melt structure, suitable for the accommodation of noble gas atoms, are fully occupied. Indeed, the experimental data can be successfully reproduced with the Langmuir isotherm, implying a solubility model in which the gas atoms occupy a certain population of interstitial sites. However, the data can be equally well described by a model assuming mixing of the noble gas atoms with the oxygen atoms of the silicate melt. From a thermodynamic point of view, the constant noble gas solubility at high pressures simply implies that the partial molar volumes of the respective noble gas in the fluid and in the melt are equal. Our results differ from those of Chamorro-Perez et al. [Earth Planet. Sci. Lett. 145 (1996) 97–107; Nature 393 (1998) 352–355] who reported an abrupt, order-of-magnitude drop of Ar solubility in silica and olivine melt at around 5 GPa, suggesting that melt densification results in an abrupt decrease of the hole size distribution. A geochemical consequence of our results is that noble gases remain incompatible elements at pressure conditions covering most of the upper mantle. Therefore partial melting remains an efficient process in extracting noble gases and other volatiles from the Earth’s mantle.

Solubilities of Ar in Molten Binary Nitrate Systems.

The solubilities of Ar in molten nitrate mixtures were measured by using a modified gas probe, which injected He into molten nitrate through a nozzle, collected ascending bubbles of He-Ar gas mixture, and determined the concentration of Ar in the He-Ar gas mixture. The solubilities of Ar in molten LiNO3, NaNO3, and KNO3 are determined as a function of the temperature from 463 to 513 K, and those of LiNO3-NaNO3, NaNO3-KNO3, and LiNO3-KNO3 binary systems are determined as a function of composition at 623 K.From the Ar solubility in molten nitrate mixtures, the thermodynamical properties of molten nitrate mixtures and the interaction with Ar are investigated.

Molecular dynamics simulations of molten CaAl2Si2O8; dependence of structure and properties on pressure

Molecular dynamics (MD) simulations were carried out on molten CaAl2Si2O8 at nine pressures from 0.2 to 76 GPa at 4000 K, well above the computed glass transition tem- perature. A simple effective pair-potential with both Coulombic interaction and Born-May- er-Huggins short-range repulsion was used. Simulations of duration 30 ps long were per- formed in the microcanonical ensemble (NEV) utilizing 1300 particles (O1Ca1Si1Al). As pressure increases, profound changes occur in the relative abundance of (4) T, T decreases and (6) T increases). At 5 GPa, the average coordination number (CN) of oxygen about Ca is 8. At 20 GPa, (6) T constitutes more than 50% of the TOn subunit population and the CN of oxygen around Ca is 10. At medium range (3 to 10 A ˚), the most significant change in melt structure as pressure increases is replacement of corner-sharing ring-former TOn polyhedra with edge-sharing polyhedra pairs such as TO5-TO6 and TO6- TO6. The destruction of the ring structure correlates with the maximization of Ar solubility as pressure increases. Ring structure collapse is geometrically captured by variation in the distribution of T-O-T angles with increasing pressure. The low-pressure maximum at 1408 with large variance agrees with X-ray data and follows from the distribution of N-member rings with N ranging from 4 to 8 (or larger). At higher pressure, the T-O-T distribution narrows and shifts toward smaller angles around 1058 consistent with the dominating pres- ence of edge sharing between TO6-TO6 and TO5-TO6 pairs. Tracer diffusivities for O, Ca, Al, and Si increase at low pressure as pressure increases. The diffusivity of oxygen cor- relates with ring collapse and attains a maximum at 5 GPa (Dox 5 6.6 3 10 29 m 2 /s); the ratio of diffusivities for Ca, Al, and Si relative to oxygen are 0.67, 0.62, and 0.51, re- spectively. These data suggest that pentahedrally coordinated T acts as an ''activated com- plex'' for oxygen diffusion. Below 5 GPa, activation volumes ( V*) for all species are large in absolute value and lie in the range 225 to 235 cm

The Solubility of Ar in Liquid Succinonitrile.

Since a gas bubble formation process ahead of a growing dendritic solid/liquid interface is quite complex, the direct observation of gas bubble formation by using the transparent materials is very useful method to study. For works on the gas bubble formation during the solidification, the information of solubility of gas in the melt was required. The first attempt to measure the solubility of Ar in molten succinonitrile was carried out by using a newly developed method which consists of the injection of He into molten salts through a nozzle, collection of ascending bubbles of He–Ar gas mixture, determination of concentration of Ar in the He-Ar gas mixture using a high sensitive mass spectrometer. The solubilities of Ar in molten succinonitrile have been determined as a function of temperature from 333 to 348 K and were best expressed byln[Ar]=–7.64–1630/Twhere the unit of solubility is expressed by mol-Ar/cm3-succinonitrile. From the temperature dependency of Ar solubility, the enthalpy of solution was found to be about 13.5 kJ/mol.

四重極質量分析計による水へのAr溶解度測定

四重極質量分析計による水へのAr溶解度測定

Experimental study of argon sorption in quartz: Evidence for argon incompatibility

We have conducted Ar sorption experiments on two varieties of natural quartz minerals (Q112, Q113) and a synthetic one (QHE37). Runs were performed in an internally heated pressure vessel at 1300°C and pressures of up to 8000 bar for run times between 1 and 12 d, on grain sizes ranging from 11–20 to 60–80 μm. Run products were analysed by gas chromatography (GC), Knudsen cell mass spectroscopy (KMS), electron microprobe (EMP), and scanning electron microscopy (SEM). Release spectra of Ar desorption were monitored by KMS. For sample Q112 and QHE37 two release signals are observed (500–1000°C and 1200–1600°C). When two grain sizes of the same specimen (QHE37) are analysed, the high temperature peak does not vary whereas the low temperature peak is significantly increased with decreasing grain size, suggesting desorption of surface bonded Ar. Argon contents from the high temperature peak indicate an Ar sorption of 28 ppm (QHE37) and 48 ppm (Q112) at 4070 bar. Specimen Q113 does not exhibit low temperature release, nevertheless, its Ar content is higher in smaller grains (11–20 μm: 431 ppm, 60–80 pm: 128 ppm), while increasing the duration of the experiments from ∼ 1 d to ∼ 10 d does not change the Ar content. This apparently erratic behaviour suggests an extrinsic control for Ar sorption. EMP analysis of all samples at the μm scale reveals heterogeneous Ar distribution. A few enriched spots with Ar up to 4000 ppm are observed (e.g., Q113) compared to a background concentration below the detection limit of ∼30 ppm. The average concentration measured by EMP is fairly similar to the high temperature step of bulk analytical methods (KMS or GC). We can conclude that bulk measurements of the sorption of Ar do not document equilibrium dissolution. Assuming the Ar diffusivity to be fast enough to permit saturation of at least the 11–20 μm grain fraction after ∼ 10 d, at 8000 bar and 1300°C, an upper bound of Ar solubility can be given as 30 ppm. In contrast, bulk methods yield variable average Ar concentrations which depend on experimental conditions. This indicates that solubilities measured by bulk methods grossly overestimate the true solubility. A quartz /melt partition coefficient of less than 0.006 can be derived.