Impacts « multi-échelle » d'un échange (Mg,Ca)–Pb et ses conséquences sur l'augmentation de la perméabilité d'une bentonite

The article addresses the structural effects of solutions of lead nitrate on a suspended or compacted bentonite. A permeability increase is observed on compacted clay. Investigating the composition of output solution, using X-Rays Diffusion at Small Angles and Scanning Electron Microscopy, this permeability increase is explained from structural variations at nanometric (reduction of particle size) and micrometric scales (microfissuration of aggregates). To cite this article: N. Jozja et al., C. R. Geoscience 335 (2003).     

Heave Studies on Fly Ash-Stabilised Expansive Clay Liners

Clay liners or compacted earthen barriers are important barrier materials used for preventing contaminant transport through soils. A low hydraulic conductivity (k) is a significant parameter that governs the design and construction of clay liners. Compacted expansive clays, which are montmorillonite clays, also have a very low hydraulic conductivity (k). When expansive clays are blended with fly ash, an industrial waste, the hydraulic conductivity (k) further reduces as the ash-clay blends result in increased dry densities at increased fly ash contents. Hence, fly ash-stabilised expansive clay can also be proposed as a unique clay liner material. As expansive clays undergo heave when they come into contact with water, it is necessary to study the heave behaviour of fly ash-stabilised expansive clay liners. This paper presents heave studies on fly ash-stabilised expansive clay liners. Fly ash in different contents by dry weight of the expansive clay was added to the clay, and the ash-clay blend was compacted as a liner overlying a natural field soil layer. Compacted lateritic clay was used for simulating the natural field soil into which contaminants migrate. Calcium chloride (CaCl₂) solution of varying concentration (5, 10, 20, 50, 100 and 500 mM) was used as the permeating fluid in the heave studies. The rate of heave and the amount of heave of the fly ash-stabilised expansive clay liners were monitored. Deionised water (DIW) was also used as inundating fluid for comparative study. Heave (mm) decreased with increase in solute concentration for all fly ash contents. For a given solute concentration, heave decreased up to a fly ash content of 20 % and thereafter it increased when the fly ash content was increased to 30 %. Heave of the fly ash-stabilised expansive clay liners was correlated with their permeability, liquid limit (LL) and free swell index (FSI) pertaining to the respective fly ash content and CaCl₂ concentration.     

Dissolution kinetics of synthetic Na-smectite. An integrated experimental approach

The effect of pH and Gibbs energy on the dissolution rate of a synthetic Na-montmorillonite was investigated by means of flow-through experiments at 25 and 80 °C at pH of 7 and 9. The dissolution reaction took place stoichiometrically at 80 °C, whereas at 25 °C preferential release of Mg over Si and Al was observed. The TEM-EDX analyses (transmission electronic microscopy with quantitative chemical analysis) of the dissolved synthetic phase at 25 °C showed the presence of newly formed Si-rich phases, which accounts for the Si deficit. At low temperature, depletion of Si concentration was attributed to incongruent clay dissolution with the formation of detached Si tetrahedral sheets (i.e., alteration product) whereas the Al behaviour remains uncertain (e.g., possible incorporation into Al-rich phases). Hence, steady-state rates were based on the release of Mg. Ex situ AFM measurements were used to investigate the variations in reactive surface area. Accordingly, steady-state rates were normalized to the initial edge surface area (11.2 m² g⁻¹) and used to propose the dissolution rate law for the dissolution reactions as a function of ΔG_r at 25 °C and pH∼9:

     

The coprecipitation of Sr into calcite precipitates induced by bacterial ureolysis in artificial groundwater: Temperature and kinetic dependence

A suite of experiments was performed to investigate the partitioning of Sr²⁺ (to mimic the radionuclide ⁹⁰Sr) between calcite and artificial groundwater in response to the hydrolysis of urea (ureolysis) by Bacillus pasteurii under simulated in situ aquifer conditions. Experiments were performed at 10, 15, and 20°C over 7 days in microcosms inoculated with B. pasteurii ATCC 11859, containing an artificial groundwater and urea (AGW) or an AGW including a Sr contaminant treatment. During the experiments, the concentration of ammonium generated by bacterial ureolysis increased asymptotically, and derived rate constants (k_urea) that were between 13 and 10 times greater at 20°C than at 15 and 10°C. Calcite precipitation was initiated after similar amounts of urea had been hydrolyzed (∼ 4.0 mmol L^-1) and a similar critical saturation state (mean S_critical = 53, variation = 20%) had been reached, independent of temperature and Sr treatment. Because of the positive relationship between the rate of ureolysis and temperature, precipitation began by the end of day 1 at 20°C, and between days 1 and 2 at 15 and 10°C. The rate of calcite precipitation increased with, and was fundamentally controlled by calcite saturation state (S), irrespective of temperature. The presence of Sr slightly slowed calcite precipitation rates at equivalent values of S, which may reflect the screening of active nucleation and crystal growth sites by Sr. Homogeneous partitioning coefficients (D_Sr) exhibited a positive association with calcite precipitation rates, but were greater at higher experimental temperatures at equivalent precipitation rates (20°C mean = 0.46; 15°C mean = 0.24; 10°C mean = 0.29).     

Environmental geological and geotechnical investigations related to the potential use of Ankara clay as a compacted landfill liner material, Turkey

Clay-bearing Upper Pliocene red clastics and Quaternary alluvial deposits occupy the Ankara basin. The clayey levels of the Upper Pliocene deposits, referred to as Ankara clay, is considered as a source for compacted clay liners due to their low coefficients of permeability and widespread distributions throughout Ankara. This study investigates the geological, geotechnical and mineralogical properties of the founding clayey soils at two sites of the Ankara region. The geotechnical index properties along with the hydraulic conductivities of the clayey soil samples collected from these sites were determined. A mathematical relation between the clay mineralogical content and hydraulic conductivity was established. The results of this investigation show that, from a geotechnical point of view, Ankara clay may be regarded as a highly suitable material for a compacted clay landfill liner given that its mineralogical compatibility with leachate is confirmed.     

Stability of pyrope-quartz in the system MgO-Al2O3-SiO2

Pyrope and quartz are stable with respect to aluminous enstatite and sillimanite at 1400 °C, 20 kb and at 1100 °C, 16 kb. The phase boundary limiting the coexistence of pyrope and quartz towards lower pressures is probably slightly curved. A slope of 15 bars/°C at 1400° and of 10 bars/°C at 1000 °C has been estimated from the experimental data. Between 1050 and 1100 °C the curve is intersected by the kyanite-sillimanite phase boundary. The calculated slope of the reaction aluminous enstatite + kyanite pyrope + quartz is negative (ca. 18–25 bars/°C). The existence of a negative slope has been demonstrated experimentally. Experimental evidence indicates that the assemblage aluminous enstatite and sillimanite is metastable with respect to sapphirine + quartz at high temperature. The invariant point involving the phases pyrope-sapphirine-aluminous enstatite-sillimanite-quartz is estimated to occur at 1125°±25 °C and 16±1 kb. A model phase diagram for the silicasaturated part of the system MgO-Al₂O₃-SiO₂ has been constructed. The position of three invariant points in this system has been estimated on the basis of presently available data.     

Experimental determination of the spinel peridotite to garnet peridotite reaction in the system MgO-Al2O3-SiO2 in the range 900 °–1100 °C and Al2O3 isopleths of enstatite in the spinel field

The univariant high-pressure reaction of aluminous enstatite and spinel to pyrope and forsterite in the MgO-Al₂O₃-SiO₂ system has been determined in the temperature range 900 °–1100 °C by hydrothermal reversals in the piston-cylinder apparatus using the low-friction NaCl pressure medium. A mixture of synthetic minerals, including an enstatite with 6 wt% Al₂O₃, with product and reactant assemblages in nearly equal amounts, was the starting material. The equilibrium pressure of 19.3±0.3 kbar at 1000 ° C and average dP/dT slope of 8.0 bars/ ° C confirm the strong curvature of the equilibrium below 1200 ° C deduced by Obata (1976) from a theoretical study of experimental Al₂O₃ isopleths of enstatite in the garnet field. His prediction of an absolute minimum pressure near 18 kbar of the garnet peridotite assemblage in the ternary system is undoubtedly correct.Three reversed determinations of the equilibrium Al₂O₃ content of enstatite in the presence of spinel +forsterite were made at points adjacent to the univariant curve. The points are 5.5 wt% Al₂O₃ at 950 ° C and 20 kbar, 6.2 wt% at 1000 ° C and 20 kbar and 7.2 wt% at 1080 ° C and 20 kbar. These values are somewhat higher than given by the MacGregor (1974) isopleth set and quite close to those predicted by Fujii (1976) from experimental synthesis data at higher temperatures, using the Wood and Banno (1973) model of ideal solution of the Mg₂Si₂O₆ and MgAl₂SiO₆ components in enstatite to reduce the data.All of the available spinel-field isopleth data can be systematized with the use of the ideal solution model. A value of H ⁰ of 9000 cal fits the reduced data well, and is in agreement with the calorimetrically determined value of 8500±1900 calories. An accurate calculation of the dP/dT slope of the univariant equilibrium at 1000 ° C based on calorimetry gives 7±2bars/ °C, also in good agreement with experiment. Thus, all of the available experimental and calorimetric data are consistent with the ideal-solution aluminous enstatite model.The dP/dT slopes of the spinel-field isopleths are too large to permit their use as an accurate geobarometric scale. They do have considerable potential as a thermometric indicator for certain natural peridotites, however. The southwestern Oregon overthrust peridotite masses of Cretaceous age have enstatite of 5.6 wt% Al₂O₃ with spinel of nearly 80 mole% MgAl₂O₄. The present reduced isopleth data directly give 930 ° C for the equilibration, assuming 12 kbar pressure. A first order correction based on ideal solution departures from the ternary system, as suggested by Stroh (1976) gives 1000 ° C. Thus, the high temperatures deduced by Medaris (1972) are confirmed. The pressure cannot be deduced independently from the pyroxene Al₂O₃ contents.     

Modification of physical, mechanical and hydraulic properties of bentonite by thermo-hydraulic gradients

The conditions of the bentonite in an engineered barrier for high-level radioactive waste disposal have been simulated in a series of tests. Cylindrical cells with an inner length of 60 cm and a diameter of 7 cm have been constructed. Inside the cells, six blocks of FEBEX bentonite have been piled-up, giving rise to a total length similar to the thickness of the clay barrier in a repository according to the Spanish concept. To obtain the blocks, the clay with its hygroscopic water content has been uniaxially compacted at a dry density of 1.65 g/cm³. The bottom surface of the material was heated at 100 °C and the top surface was injected with granitic water. The duration of the tests was 6, 12 and 24 months. The temperatures inside the clay and the water intake were measured during the tests and, at the end, the cells were dismounted and the dry density, water content and some hydro-mechanical properties of the clay (permeability, swelling pressure and swelling under load) were measured at different positions. The values obtained are compared to those of the untreated FEBEX bentonite. The injection of water provokes in the vicinity of the hydration surface an increase of the water content and a decrease of the dry density due to the swelling of the clay, while heating gives rise to an increase of the dry density and a reduction of the water content in the 18 cm closest to the heater, even after 2 years of thermo-hydraulic (TH) treatment. The swelling capacity and the hydraulic conductivity after TH treatment are mainly related to the dry density and water content attained during it. No major irreversible modifications of these properties with respect to those of the untreated clay have been detected.     

Experimental growth of quartz in petroleum environment. Part I: Procedures and Fluid Trapping

The quartz-water-oil-gas system has been experimentally studied with the objective of investigating the trapping of petroleum and aqueous inclusions in quartz at different water/oil (W/O) ratios (0/100, 5/95, 10/90, 20/80, 50/50, 100/0). Experiments were carried out in both a gas-pressure autoclave (GPA) under CH₄ pressure control, up to 250°C and 212 bar, and in a fluid-pressure autoclave (FPA) up to 350°C and 400 bar. High p-T conditions have notably allowed the growth of quartz at high oil saturation levels (W/O ratios from 10/90 to 50/50). Petroleum inclusions have been synthesised inside quartz microfractures (W/O ratios from 0/100 to 50/50; 209-350°C; 175-400 bar), and also inside quartz overgrowths (W/O ratios from 10/90 to 50/50; 289-350°C; 350-400 bar). Aqueous inclusions have been synthesised in presence of oil inside quartz microfractures from 185°C-163 bar up to 400°C-400 bar, and inside quartz overgrowth from 277°C-330 bar. Synthesised petroleum inclusions are representative of the parent oil up to 250°C. At 350°C, evidence of a cracking process has been observed with the consequent formation of methane. The segregation of the oil/gas/water column inside the GPA autoclave may also have prevented methane diffusion into the water phase when oil is present. This experimental approach shows that the trapping of fluid inclusions and the formation of quartz cement, under conditions of high oil saturation, have not been suppressed or prevented.     

Effects of compaction moisture content on the shear strength of an unsaturated clay

This paper deals with the results of an experimental study carried out on compacted clay (PL=23%) to investigate the influence of compaction water content on the shear strength of the clay. For the measurement of shear strength parameters, direct shear tests were done on samples compacted at optimum moisture content (i.e. w=24%), at the dry side of optimum (i.e. w=18%, 20% and 22%) and at the wet side of optimum (i.e. w=26% and 28%). Thermocouple psychrometers were used for suction measurements and the suction-moisture content relationship of the soil sample was obtained. The trends for suction, angle of friction and cohesion, which change below and above optimum moisture content, were analyzed. The compacted clay behaves like a granular soil on the dry side of optimum water content; and a reduction in angle of friction and an increase in cohesion are observed as the compaction water contents approach the optimum value. The angle of friction is not affected by soaking, but after soaking cohesion of the sample reduces. This revised version was published online in July 2006 with corrections to the Cover Date.     

The system diopside-nepheline-leucite

The system diopside-nepheline-leucite, representing a join in the undersaturated part of the system nepheline (Ne)-kalsilite (Ks)-CaO-MgO-SiO₂, has been investigated at atmospheric pressure. The system is pseudoternary and cuts the primary phase volumes of forsterite solid solution (Fo_ss), diopside solid solution (Di_ss), nepheline solid solution (Ne_ss), carnegieite solid solution (Cg_ss), and leucite solid solution (Lc_ss). Melilite (Mel) occurs as a subliquidus phase. The phase diagram has two four-phase points: 1. one at 1275±5° C and Di₆₀Ne₈Lc₃₂ where liquid coexists with Fo_ss, Di_ss and Lc_ss, corresponding to olivine (Ol) leucitite; 2. the other at 1194±5° C and Di_27.5Ne_29.5Lc₄₃ where Ne_ss, Fo_ss and Lc_ss coexist with liquid, corresponding to Ol-Ne italite. With decreasing temperature, liquid moves from point (1) to a five-phase assemblage (3) where liquid is in equilibrium with Fo_ss, Di_ss, Mel and Lc_ss (1258±5°C), which is representative of Ol-Mel-leucitite. From point (2) liquid moves to a second five-phase assemblage (4), where Fo_ss, Mel, Ne_ss, Lc_ss and liquid are in equilibrium (1175±5°C, corresponding to a Lc-Ne katungite. The assemblage Fo_ss+Ne_ss+Di_ss+Mel+Lc_ss+ liquid, is reached between 1168° and 1100° C and corresponds to Ol-Mel-Ne leucitite. Fo_ss reacts with liquid and disappears. Near the point (1) it disappears at 1135±10° C, whereas near the point (2) it reacts out at 1060±10° C. Near the join Di-Ne it disappears at 950±10° C. The final assemblage in the system is representative of Mel-Ne leucitite.Presented at the symposium Recent Advances in the Studies of Rocks and Minerals at High Pressures and Temperatures held in Montreal, 1972. Jointly sponsored by the International Mineralogical Association and the Commission on Experimental Petrologie.     

In situ and laboratory investigation of the alteration of Boom Clay (Oligocene) at the air–geological barrier interface within the Mol underground facility (Belgium): Consequences on kerogen and bitumen compositions

The Boom Clay formation (Oligocene) is studied as a reference host rock for methodological studies on deep geological disposal of radioactive waste. During excavation of galleries within the Clay formation (HADES underground research facility, Mol, Belgium), the physico-chemical conditions are significantly modified as an air–clay interface is created. In order to study the long-term impact of the air–clay contact on the organic matter contained in the Boom Clay, two types of samples were studied: (1) a reference series of clay samples having been in contact with the atmosphere of the HADES gallery for increasing times up to several years and (2) unaltered clay samples submitted to artificial oxidation in a ventilated oven at 80 °C. The evolution of geochemical data of the two series was compared using Rock-Eval pyrolysis, GC–MS and size exclusion chromatography. The organic matter of the unaltered clays sampled in the HADES galleries is dominated by type III kerogen (terrestrial) with some contribution of type II (marine) and is thermally immature. The evolution of geochemical parameters during air alteration for the two series are very similar. They show progressive oxidation of kerogen accompanied by the release of bitumen enriched in low molecular weight constituents. Molecular analysis evidences the presence of a complex mixture of aliphatic and aromatic O-bearing compounds, inherited from the degradation of kerogen as well as from the clay catalyzed oxidation of the bitumen. These results show that (1) air oxidation is a major process in the in situ alteration of the organic matter of Boom Clay within the HADES galleries, (2) laboratory oxidation experiments at 80 °C yield similar results as in situ air alteration of Boom Clay and (3) artificial air oxidation may be used to assess the long term exposure of the organic matter to air.     

混合型缓冲回填材料膨胀力试验研究

与纯膨润土相比,混合型缓冲回填材料（膨润土与石英砂混合物）能够实现防渗阻隔能力、热传导性能、力学强度和可施工性能的最佳组合。选用高庙子钠基膨润土（GMZ001）为缓冲回填材料的主料,添加不同比例的石英砂,对掺砂比分别为0、10%、20%、30%、40%和50%的膨润土-砂混合物压实试样进行室内试验。结果表明,混合物的液限、塑限随掺砂率的增大而线性降低;膨胀力随时间呈指数增长。初始含水率较大时,最大膨胀力随初始含水率的增大略有降低。掺砂率一定时,最大膨胀力随初始干密度指数增长。提出了有效黏土密度的概念,建立了一定初始含水率条件下,任意掺砂率和初始干密度的高庙子膨润土-砂混合物最大膨胀力归一化模型,为混合型缓冲回填材料膨胀力的预测与控制提供了依据。     

Leachate studies on fly ash-stabilised expansive clay liners

This paper presents leachate studies on fly ash-stabilised expansive clay liners. Fly ash in different contents (0%, 10%, 20% and 30%) by dry weight of the expansive clay was added to the clay, and the ash-clay blend was compacted as a liner overlying a compacted lateritic clay layer. Deionised water (DIW) and calcium chloride (CaCl₂) solutions of varying concentration (5 mM, 10 mM, 20 mM, 50 mM, 100 mM and 500 mM) were used as the permeating fluids in the leachate studies. Chemical analysis of the leachate was performed. For a given CaCl₂ concentration, the concentrations of both calcium ion and chloride ion in the leachate decreased up to a fly ash content of 20%, and thereafter they increased when the fly ash content was increased to 30%. Further, for a given fly ash content, concentrations of calcium ion and chloride ion increased with increasing CaCl₂ concentration.     

Caprock interaction with CO2: A laboratory study of reactivity of shale with supercritical CO2 and brine

Crushed rock from two caprock samples, a carbonate-rich shale and a clay-rich shale, were reacted with a mixture of brine and supercritical CO₂ (CO₂–brine) in a laboratory batch reactor, at different temperature and pressure conditions. The samples were cored from a proposed underground CO₂ storage site near the town of Longyearbyen in Svalbard. The reacting fluid was a mixture of 1 M NaCl solution and CO₂ (110 bar) and the water/rock ratio was 20:1. Carbon dioxide was injected into the reactors after the solution had been bubbled with N₂, in order to mimic O₂-depleted natural storage conditions. A control reaction was also run on the clay-rich shale sample, where the crushed rock was reacted with brine (CO₂-free brine) at the same experimental conditions. A total of 8 batch reaction experiments were run at temperatures ranging from 80 to 250 °C and total pressures of 110 bar (∼40 bar for the control experiment). The experiments lasted 1–5 weeks.Fluid analysis showed that the aqueous concentration of major elements (i.e. Ca, Mg, Fe, K, Al) and SiO₂ increased in all experiments. Release rates of Fe and SiO₂ were more pronounced in solutions reacted with CO₂–brine as compared to those reacted with CO₂-free brine. For samples reacted with the CO₂–brine, lower temperature reactions (80 °C) released much more Fe and SiO₂ than higher temperature reactions (150–250 °C). Analysis by SEM and XRD of reacted solids also revealed changes in mineralogical compositions. The carbonate-rich shale was more reactive at 250 °C, as revealed by the dissolution of plagioclase and clay minerals (illite and chlorite), dissolution and re-precipitation of carbonates, and the formation of smectite. Carbon dioxide was also permanently sequestered as calcite in the same sample. The clay-rich shale reacted with CO₂–brine did not show major mineralogical alteration. However, a significant amount of analcime was formed in the clay-rich shale reacted with CO₂-free brine; while no trace of analcime was observed in either of the samples reacted with CO₂–brine.     

The transition from pyroxene granulite facies to garnet clinopyroxene granulite facies. Experiments in the system CaO-MgO-Al2O3-SiO2

The pressure-temperature curve for the equilibrium anorthite+2enstatite=pyrope+diopside+quartz has been determined in the system CaO-MgO-Al₂O₂-SiO₂ to be between 13.4 and 14.0 kbars at 900° C. The slope up to 1,240° C is 8.5 bar/K. The entropy change at 1,200 K is 20 kJ. These data, combined with data from the literature, lead to a geobarometer equation which, when applied to rocks from the Agto area (West Greenland), gives pressure estimates of 6–10 kbars at 800° C. The results are consistent for rocks of differing Fe/Mg ratios and are consistent with independent pressure estimates.     

The origin of the fumaroles of La Solfatara (Campi Flegrei, South Italy)

The analysis of gaseous compositions from Solfatara (Campi Flegrei, South Italy) fumaroles since the early 1980s, clearly reveals a double thermobarometric signature. A first signature at temperatures of about 360 °C was inferred by methane-based chemical-isotopic geoindicators and by the H₂/Ar geothermometer. These high temperatures, close to the critical point of water, are representative of a deep zone where magmatic gases flash the hydrothermal liquid, forming a gas plume. A second signature was found to be at around 200-240 °C. At these temperatures, the kinetically fast reactive species (H₂ and CO) re-equilibrate in a pure vapor phase during the rise of the plume. A combination of these observations with an original interpretation of the oxygen isotopic composition of the two dominant species, i.e. H₂O and CO₂, shed light on the origin of fumarolic fluids by showing that effluents are mixture between fluids degassed from a magma body and the vapor generated at about 360 °C by the vaporization of hydrothermal liquids. A typical ‘andesitic’ water type (δD ∼ −20‰, δ¹⁸O ∼10‰) and a CO₂-rich composition (X_CO2∼0.4) has been inferred for the magmatic fluids, while for the hydrothermal component a meteoric origin and a CO₂ fugacity fixed by fluid-rock reaction at high temperatures have been estimated. In the time the fraction of magmatic fluids in the fumaroles increased (up to ∼0.5) at each seismic and ground uplift crisis (bradyseism) which occurred at Campi Flegrei, suggesting that bradyseismic crises are triggered by periodic injections of CO₂-rich magmatic fluids at the bottom of the hydrothermal system.     

The impact of salinity on the Mg/Ca and Sr/Ca ratio in the benthic foraminifera Ammonia tepida: Results from culture experiments

Over the last decade, sea surface temperature (SST) reconstructed from the Mg/Ca ratio of foraminiferal calcite has increasingly been used, in combination with the δ¹⁸O signal measured on the same material, to calculate the δ¹⁸O_w, a proxy for sea surface salinity (SSS). A number of studies, however, have shown that the Mg/Ca ratio is also sensitive to other parameters, such as pH or , and salinity. To increase the reliability of foraminiferal Mg/Ca ratios as temperature proxies, these effects should be quantified in isolation. Individuals of the benthic foraminifera Ammonia tepida were cultured at three different salinities (20, 33 and 40 psu) and two temperatures (10-15 °C). The Mg/Ca and Sr/Ca ratios of newly formed calcite were analyzed by Laser Ablation ICP-MS and demonstrate that the Mg concentration in A. tepida is overall relatively low (mean value per experimental condition between 0.5 and 1.3 mmol/mol) when compared to other foraminiferal species, Sr being similar to other foraminiferal species. The Mg and Sr incorporation are both enhanced with increasing temperatures. However, the temperature dependency for Sr disappears when the distribution factor D_Sr is plotted as a function of calcite saturation state (Ω). This suggests that a kinetic process related to Ω is responsible for the observed dependency of Sr incorporation on sea water temperature. The inferred relative increase in D_Mg per unit salinity is 2.8% at 10 °C and 3.3% at 15 °C, for the salinity interval 20-40 psu. This implies that a salinity increase of 2 psu results in enhanced Mg incorporation equivalent to 1 °C temperature increase. The D_Sr increase per unit salinity is 0.8% at 10 °C and 1.3% at 15 °C, for the salinity interval 20-40 psu.     

Experimental determination of pargasite stability relations in the presence of orthopyroxene

The stability and partial melting of synthetic pargasite in the presence of enstatitic orthopyroxene (opx), forsterite, diopsidic clinopyroxene (cpx), plagioclase (An₅₀), and water has been studied in the range of 0.4–6.0 kb and 750–1000°C in the system Na₂O-CaO-MgO-Al₂O₃-SiO₂-H₂O with a fixed bulk composition of pargasite+5 opx. The addition of orthopyroxene effectively reduces the stability field of pargasite by approximately 200°C at 1 kb. The invariant point involving pargasite coexisting with water-saturated liquid and anhydrous phase shifts from about 0.85 kb and 1025°C to 2.5±0.5 kb and 925±25°C with the addition of opx. Based on the solidus mineral assemblage and direct chemical analysis of quenched glass, the vapor-saturated liquid has a composition close to that of intermediate plagioclase. A layered silicate, interpreted to be Na-phlogopite, has an upper-thermal stability that nearly equals that of pargasite in the field of partial melting and coexists with liquid, pargasite, cpx, and forsterite at 6 kb, 1000°C. These results support the hypothesis that mantle metasomatism could involve formation of pargasitic amphibole from a silicate melt at depths as shallow as 8–10 km.     

Melting relations of muscovite-granite to 35 kbar as a model for fusion of metamorphosed subducted oceanic sediments

Muscovite-granite was reacted in cold-seal pressure vessels at 2 kbar and in pistoncylinder apparatus between 10 and 35 kbar, with just 0.6 weight per cent water structurally bound in 14 modal per cent muscovite, and with additional water contents varying to 50 weight per cent. Phase relationships are presented through the melting interval with excess water, and with no free water added. Selected reactions above 10 kbars have been successfully reversed. An isobar at 15 kbar shows the effect of varying water contents on the mineral phase boundaries for vapor-present and vapor-absent conditions. For the dry rock, temperatures for the solidus and liquidas (quartz-out) curves, respectively, are 10 kbar-760° C, 1160° C; 15 kbar-810° C, 1220° C; 25 kbar-880° C; 1340° C; 35 kbar-1040° C, 1460° C. The solidus curve corresponds to the melting of muscovite + quartz. With water vapor present, the solidus is considerably lower, 15 kbar-610° C, 25 kbar-665° C. Water solubility in the liquid at 15 kbar is 24±3 weight per cent. Maximum temperatures for quartz and feldspars in the vapor-absent region decrease considerably with increasing water content. Temperatures for the quartz-out curve at 15 kbars are 0.6 % H₂O-1230° C; 24 % H₂O-760° C. At 15 kbars for low water contents, water-undersaturated liquid coexists with quartz and feldspars through hundreds of degrees. Subducted pelagic sediments which metamorphosed to muscovitebearing quartzo-feldspathic rocks would undergo two episodes of melting, beginning at different depths: (1) the first liquid dissolves all pore fluid, and transports it away when it escapes from the crystalline host, (2) reaction of muscovite yields a second liquid, with less dissolved water. According to two published thermal models for a lithosphere slab dipping at 45°, the depths would be (a) 60 km and 92 km, or (b) 17 km and 21 km. Magmas generated by partial fusion in subducted oceanic crust are cooler than the overlying crustal layers and the mantle above the slab by as much as 200° C to 300° C. This must lead to intrusion of relatively cool magma into hot rock. Consequent heating of the magma increases its prospects of reaching high levels in the upper mantle or crust before it solidifies by crossing the solidus curve.