Thermodynamic properties of compounds in the PdO-H2O system at 25°C

Literature thermodynamic data on species and particles existing in the heterogeneous PdO-H₂O system were checked for consistency, and the equilibrium constants for dissolution of palladium oxide and hydroxide in water and for Pd²⁺ (aq) hydrolysis were recommended. Δ _f G _298.15 ^° obtained in this work for Pd²⁺(aq) sharply differs (no less than by 6 kJ/mol) from values that are reported in fundamental thermodynamic reference books and based on experimentally measured palladium electrode potential at 25°C. Detailed examination of literature data on the thermodynamic properties of compounds in the Cl-Pd(aq) system is required to account for revealed inconsistency.     

Diamond formation in the system MgO–SiO2–H2O–C at 7.5 GPa and 1,600°C

Diamond crystallization has been studied in the SiO₂–H₂O–С, Mg₂SiO₄–H₂O–С and H₂O–С subsystems at 7.5 GPa and 1,600°C. We found that dissolution of initial graphite is followed by spontaneous nucleation of diamond and growth of diamond on seed crystals. In 15-h runs, the degree of graphite to diamond transformation [α = M_Dm/(M_Dm + M_Gr)100, where M_Dm is mass of obtained diamond and M_Gr mass of residual graphite] reached 100% in H₂O-rich fluids but was only 35–50% in water-saturated silicate melts. In 40-h runs, an abrupt decrease of α has been established at the weight ratio H₂O/(H₂O + SiO₂) ≤ 0.16 or H₂O/(H₂O + Mg₂SiO₄) ≤ 0.15. Our results indicate that α is a function of the concentration of water, which controls both the kinetics of diamond nucleation and the intensity of carbon mass transfer in the systems. The most favorable conditions for diamond crystallization in the mantle silicate environment at reliable PT-parameters occur in the fluid phase with low concentration of silicates solute. In H₂O-poor silicate melts diamond formation is questionable.     

Formation and properties of hydrosilicate liquids in the systems Na2O-Al2O3-SiO2-H2O and granite-Na2O-SiO2-H2O at 600°C and 1.5 kbar

In order to determine the mechanisms of formation and properties of natural hydrosilicate liquids (HSLs), which are formed during the transition from magmatic to hydrothermal mineral formation in granitic pegmatites and rare-metal granites, the formation of HSLs was experimentally studied in the Na₂O-SiO₂-H₂O, Na₂O-Al₂O₃-SiO₂-H₂O, and Na₂O-K₂O-Li₂O-Al₂O₃-SiO₂-H₂O systems at 600°C and 1.5 kbar. It was shown that the sequential extension of composition does not suppress HSL formation in the systems and expands the stability field of this phase. However, HSLs formed in extended chemical systems have different structure and properties: the addition of alumina induces some compression of the structure of the silicate framework of HSLs, which results in a decrease in water content in this phase and probably hinders the reversibility of its dehydration. It was demonstrated that HSL can be formed by the coagulation of silica present in a silica-oversaturated alkaline aqueous fluid. It was supposed that the HSL formed during this process has a finely dispersed structure. It was argued that anomalous enrichment in some elements in natural HSLs can be due to their sorption by the extensively developed surface of HSL at the moment of its formation.     

Eutectic temperatures and melting relations in the Fe–O–S system at high pressures and temperatures

We have investigated melting relations in the Fe–O–S ternary system in the pressure range of 15–27 GPa and 1873 K. Subsolidus phase relations are Fe, Fe₃S₂, and FeO up to 17 GPa and Fe, Fe₃S, and FeO above this pressure. The eutectic temperature slightly decreases from ambient pressure to 17 GPa, whereas increases above this pressure. The eutectic temperature in this study is 100 K lower than that in the Fe–S binary system. The oxygen content in the Fe–O–S eutectic liquid drops when the coexisting solid phases changes from FeS to Fe₃S₂. The cotectic lines in the ternary phase diagram lie close to the Fe–FeS binary axis. The isothermal sections indicate that oxygen solubility in the Fe–O–S liquid increases with increasing temperature, and with increasing sulfur content. The solubility of sulfur in the solid Fe has a maximum value at the eutectic temperature, and decreases with increasing temperature. Our results could have important implications for formation and composition of the Martian core.     

Experimental evidence for the coexistence of two liquids in the H2O-SiO2-NaF-Na2SO4 System at T = 700°C and P = 2 kbar

The phase state of fluid in the H₂O-NaF-Na₂SO₄ system in the presence of silicates (quartz and albite) was experimentally explored using the method of synthetic fluid inclusions in quartz at 700°C and pressures of 1 and 2 kbar. Parallel experiments were conducted under identical conditions with either two silicates (quartz and albite) or quartz only. The presence of albite affects heterogeneous fluid equilibria both at different pressures and at different solution compositions. This indicates high solubilities of silicates in a saltwater fluid containing NaF and Na₂SO₄. The absence of inclusions homogenizing to a gas phase in the experimental products provides compelling evidence that liquid-liquid rather than liquid-vapor equilibria are characteristic of the H₂O-SiO₂-NaF-Na₂SO₄ and H₂O-SiO₂-NaF-Na₂SO₄-NaAlSi₃O₂ systems in the heterogeneous region. It can be concluded that critical equilibria in saturated solutions can exist in these systems. In addition, it was shown that the phase diagrams of these systems are complicated by the formation of immiscible liquids in the presence of vapor. This allowed us to conclude that there are two critical curves describing equilibria with two different salts. Fluids containing two salts (NaF and Na₂SO₄) are similar to fluids containing only one of these salts: (a) two liquids are in equilibrium under the parameters of the upper heterogeneous region, (b) each of them can in turn undergo unmixing at decreasing temperature and pressure, and (c) owing to chemical interaction between silicate and fluid components, a glassy phase can be formed and trapped in inclusions.     

Phase relations in the system fayalite–magnetite at high pressures and temperatures

The phase relations in the Fe₂SiO₄–Fe₃O₄ binary system have been determined between 900 and 1200 °C and from 2.0 to 9.0 GPa. At low to moderate pressures magnetite can accommodate significant Si, reaching X_Fe2SiO₄=0.1 and 0.2 at 3.0 and 5.0 GPa respectively, with temperature having only a secondary influence. At pressures below 3.5 GPa at 900 °C and 2.6 GPa at 1100 °C magnetite-rich spinel coexists with pure fayalite. This assemblage becomes unstable at higher pressures with respect to three intermediate phases that are spinelloid polytypes isostructural to spinelloids II, III and V in the Ni-aluminosilicate system. The phase relations between the spinelloid phases are complex. At pressures above ≈8.0 GPa at 1100 °C, the spinelloid phases give way to a complete spinel solid solution between Fe₃O₄ and Fe₂SiO₄. The presence of small amounts of Fe³⁺ stabilises the spinel structure to lower pressures compared to the Fe₂SiO₄ end member. This means that the fayalite–γ-spinel transition is generally unsuitable as a pressure calibration point for experimental apparatuses. The molar volumes of the spinel solid solutions vary nearly linearly with composition, having a small negative deviation from ideal behaviour described by W_v=−0.15(6) cm³. Extrapolation yields V°₍₂₉₈₎ = 41.981(14) cm³ for the Fe₂SiO₄-spinel end member. The cell parameters and molar volumes of the three spinelloid polytypes vary systematically with composition. Cation disorder is an important factor in stabilising the spinelloid polytypes. Each polytype exhibits a particular solid solution range that is directly influenced by the interplay between its structure and the cation distributions that are energetically favourable. In the FeO–FeO_1.5–SiO₂ ternary system Fe₇SiO₁₀ (“iscorite”) coexists with the spinelloid phases at intermediate pressures on the SiO₂-poor, or Fe³⁺-poor side of the Fe₂SiO₄–Fe₃O₄ join. On the SiO₂ and Fe³⁺-rich side of the join, orthopyroxene or high-P clinopyroxene coexists with the spinelloids and spinel solid solutions. The assemblage pyroxene+spinel+SiO₂ is stable over a wide range of bulk composition. The stability of spinelloid III is of particular petrologic interest since this phase has the same structure as (Mg,Fe)₂SiO₄–wadsleyite, indicating that Fe³⁺ can be easily incorporated in this important phase in the Earth's transition zone, in addition to silicate spinel. This has important implications for the redox state of the Earth's transition zone and for the depth at which the olivine to spinel transition occurs in the mantle, potentially leading to a shift in the “410 km” seismic discontinuity to shallower depths depending on the prevailing redox state. In addition, a coupled tetrahedral substitution of Fe³⁺+OH for Si+O could provide a further mechanism for the incorporation of H₂O in wadsleyite. Received: 10 January 2000 / Accepted: 12 May 2000     

Phase relations in the carbon-saturated C–Mg–Fe–Si–O system and C and Si solubility in liquid Fe at high pressure and temperature: implications for planetary interiors

The phase and melting relations of the C-saturated C–Mg–Fe–Si–O system were investigated at high pressure and temperature to understand the role of carbon in the structure of the Earth, terrestrial planets, and carbon-enriched extraterrestrial planets. The phase relations were studied using two types of experiments at 4 GPa: analyses of recovered samples and in situ X-ray diffractions. Our experiments revealed that the composition of metallic iron melts changes from a C-rich composition with up to about 5 wt.% C under oxidizing conditions (ΔIW = ?1.7 to ?1.2, where ΔIW is the deviation of the oxygen fugacity (fO₂) from an iron-wüstite (IW) buffer) to a C-depleted composition with 21 wt.% Si under reducing conditions (ΔIW < ?3.3) at 4 GPa and 1,873 K. SiC grains also coexisted with the Fe–Si melt under the most reducing conditions. The solubility of C in liquid Fe increased with increasing fO₂, whereas the solubility of Si decreased with increasing fO₂. The carbon-bearing phases were graphite, Fe₃C, SiC, and Fe alloy melt (Fe–C or Fe–Si–C melts) under the redox conditions applied at 4 GPa, but carbonate was not observed under our experimental conditions. The phase relations observed in this study can be applicable to the Earth and other planets. In hypothetical reducing carbon planets (ΔIW < ?6.2), graphite/diamond and/or SiC exist in the mantle, whereas the core would be an Fe–Si alloy containing very small amount of C even in the carbon-enriched planets. The mutually exclusive nature of C and Si may be important also for considering the light elements of the Earth’s core.     

The solubility of gold and silver in the system AuAgSO2H2O at 25°C and 1 atm.

During supergene alteration of auriferous carbonate ore, the weathering fluids formed are likely to be alkaline and therefore unsuitable as a medium for gold transport as a chloride complex. Secondary gold remobilization in such deposits can often be attributed instead to gold complexing by sulphur-bearing ligands. Gold and silver solubility in the systems AuSO₂H₂O and AgSO₂H₂O respectively, from the thermodynamic data available, is due to complex formation with thiosulphate and bisulphide ligands. The most stable gold complexes, Au(S₂O₃)₂³⁻ (at φO₂ > 10⁻⁶⁰) and Au(HS)⁻₂ (atφO₂ < 10⁻⁶⁰), exist in neutral or alkaline solutions. Like gold, silver forms a stable thiosulphate complex, Ag(S₂O₃)³⁻₂ in moderately oxidizing, and bisulphide complexes, AgHS⁰ and Ag(HS)⁻₂ in reducing, alkaline media. Silver solubility in highly oxidized, neutral or acid solutions is increased by formation of AgS₂O₃⁻, Ag⁺ and AgSO₄⁻ complexes.Colloidal, crystalline and alloyed gold and silver reacted with 0.1 M Na₂S₂O₃ do not, however, demonstrate independent solubility. The rate of gold solubility in 0.1 M Na₂S₂O₃, for example, is increased both by the presence of silver-thiosulphate complexes and alloyed silver. It is possible that such behaviour is due to the formation a mixed metal complex of the type (Au, Ag)(S₂O₃)₂³⁻.The nature and mineral association of secondary gold in the oxidized zone of carbonate ore at Wau. in Papua New Guinea, is consistent with prior remobilization as a thiosulphate complex. Here the secondary gold is coarsely crystalline, alloyed with 50–75 at% Ag and enriched at the watertable and with manganese dioxide in the oxidized zone.     

Phase relations in Fe–Ni–C system at high pressures and temperatures

We performed comparative study of phase relations in Fe_1−x Ni _x (0.10 ≤ x ≤ 0.22 atomic fraction) and Fe_0.90Ni_0.10−x C _x (0.1 ≤ x ≤ 0.5 atomic fraction) systems at pressures to 45 GPa and temperatures to 2,600 K using laser-heated diamond anvil cell and large-volume press (LVP) techniques. We show that laser heating of Fe,Ni alloys in DAC even to relatively low temperatures can lead to the contamination of the sample with the carbon coming from diamond anvils, which results in the decomposition of the alloy into iron- and nickel-rich phases. Based on the results of LVP experiments with Fe–Ni–C system (at pressures up to 20 GPa and temperatures to 2,300 K) we demonstrate decrease of carbon solubility in Fe,Ni alloy with pressure.     

Reactivity of organic-rich sediment in seawater at 350°c, 500 bars: experimental and theoretical constraints and implications for the guaymas basin hydrothermal system

Hydrothermal alteration of organic-rich diatomaceous sediment by seawater was modelled experimentally at 350°C, 500 bars and seawater/sediment mass ratio of 3. The experiment was performed to assess the effect of organic matter reactivity on solution speciation and sediment alteration processes at an elevated temperature and pressure and provide requisite data to better understand the chemistry of hydrothermal fluids issuing from vents in the Guaymas Basin, Gulf of California.Seawater chemistry changed greatly during the experiment. In particular, Na, Mg and SO₄ decreased, while ∑ CO₂, ∑ NH₃, ∑ H₂S, SiO₂, Ca, K, H₂, CH₄ and heavy and base metals increased. Moreover, owing to the thermal alteration of sediment organic matter, organic acids, phenolic derivatives and phthlate were released to solution. Examination of solid alteration products revealed the effects of extensive dissolution and precipitation processes characterized by total elimination of diatoms and formation of cristobalite, quartz (?), pyrite, pyrrhotite, mixed layer chlorite/smectite and calcite. Plagioclase feldspar (An₄₀) recrystallized to a more albitic form owing to Na fixation and Ca cycling to calcite. A graphitic residue was also present in the products of the experiment.Mg and Na fixation reactions during the experiment generated significant H⁺, although the pH measured at 25°C was approximately 6.2. SO₄ reduction and thermal alteration and dissolution of organics, however, consume H⁺ and are chiefly responsible for the near neutral pH for the overall reaction. Speciation calculations including ammine and acetate protonation reactions give a pH at experimental conditions of approximately 5.1, while mineral solubility relations involving virtually all alteration phases require a pH of 5.57 to 5.94. A near neutral pH at experimental conditions constrains the mobility of Fe, Mn, Zn, Cu and Ni, which existed in solution as chloro-complexes. Dissolved concentrations of Pb and Al, in contrast, covaried with dissolved organics, especially acetate, suggesting organo-metallic complex formation.     

Liquid immiscibility in the system NaF–H<Subscript>2</Subscript>O and microlite solubility at 800°C

The NaF effect on microlite solubility at 800°C and 170, 200, and 230 MPa is studied experimentally. The immiscibility boundaries and compositions of fluid phases L₁ and L₂ are defined in the system NaF–H₂O at 800°C. It is established that microlite solubility increase in the L₁ phase, as compared with a homogeneous solution, is explained by the appearance in the L₁ phase of “free” HF in an amount of 0.025 ± 0.003 mol kg^–1 H₂O. The model of “acidification” L₁ and “alkalizing” L₂ is supposed.     

An experimental study of phase equilibria in the system H2O-CO2-NaCl at 800 °C and 9 kbar

Phase equilibria in the ternary system H₂O-CO₂-NaCl were studied at 800 °C and 9 kbar in internally heated gas pressure vessels using a modified synthetic fluid inclusion technique. The low rate of quartz overgrowth along the `b' and `a' axes of quartz crystals was used to avoid fluid inclusion formation during heating, prior to attainment of equilibrium run conditions. The density of CO₂ in the synthetic fluid inclusions was calibrated using inclusions in the binary H₂O-CO₂ system synthesised by the same method and measured on the same heating-freezing stage. In the two-phase field, two types of fluid inclusions with different densities of CO₂ were observed. Using mass balance calculations, these inclusions are used to constrain the miscibility gap and the orientation of two-phase tie-lines in the H₂O-CO₂-NaCl system at 800 °C and 9 kbar. The equation of state of Duan et al. (1995) approximately describes the P-T section of the ternary system up to about 40 wt% of NaCl. At higher NaCl concentrations the measured solubility of CO₂ in the brine is much smaller than predicted by the EOS. A “salting out” effect must be added to the equation of state to include coulomb interaction in the model of Anderko and Pitzer (1993) and Pitzer and Jiang (1996). The new experimental data together with published data up to 5 kbar (Shmulovich et al. 1995) encompass practically all subsolidus crustal P-T conditions. A feature of the new experimental results is the large compositional range in the H₂O-CO₂-NaCl system occupied by the stability fields of halite + CO₂-rich fluid ± H₂O-NaCl brine. The prediction of halite stability in equilibrium with CO₂-rich fluid in deep-crustal rocks is supported by recent petrological and fluid inclusion studies of granulites. Received: 29 June 1998 / Accepted: 17 March 1999     

Phase relations in the MgO–P2O5–H2O system and the stability of phosphoellenbergerite: petrological implications

The polymorphic relations for Mg₃(PO₄)₂ and Mg₂PO₄OH have been determined by reversed experiments in the temperature-pressure (T-P) range 500–1100 °C, 2–30 kbar. The phase transition between the low-pressure phase farringtonite and Mg₃(PO₄)₂-II, the Mg analogue of sarcopside, is very pressure dependent and was tightly bracketed between 625 °C, 7 kbar and 850 °C, 9 kbar. The high-temperature, high-pressure polymorph, Mg₃(PO₄)₂-III, is stable above 1050 °C at 10 kbar and above 900 °C at 30 kbar. The low-pressure stability of farringtonite is in keeping with its occurrence in meteorites. The presence of iron stabilizes the sarcopside-type phase towards lower P. From the five Mg₂PO₄OH polymorphs only althausite, holtedahlite, β-Mg₂PO₄OH (the hydroxyl analogue of wagnerite) and ɛ-Mg₂PO₄OH were encountered. Relatively speaking, holtedahlite is the low-temperature phase (<600 °C), ɛ-Mg₂PO₄OH the high-temperature, low-pressure phase and β-Mg₂PO₄OH the high-temperature, high-pressure phase, with an intervening stability field for althausite which extends from about 3 kbar at 500 °C to about 12 kbar at 800 °C. Althausite and holtedahlite are to be expected in F-free natural systems under most geological conditions; however, wagnerite is the most common Mg-phosphate mineral, implying that fluorine has a major effect in stabilizing the wagnerite structure. Coexisting althausite and holtedahlite from Modum, S. Norway, show that minor fluorine is strongly partitioned into althausite (K_D ^F/OH≈ 4) and that holtedahlite may incorporate up to 4 wt% SiO₂. Synthetic phosphoellenbergerite has a composition close to (Mg_0.9□_0.1)₂Mg₁₂P₈O₃₈H_8.4. It is a high-pressure phase, which breaks down to Mg₂PO₄OH + Mg₃(PO₄)₂ + H₂O below 8.5 kbar at 650 °C, 22.5 kbar at 900 °C and 30 kbar at 975 °C. The stability field of the phosphate end-member of the ellenbergerite series extends therefore to much lower P and higher T than that of the silicate end-members (stable above 27 kbar and below ca. 725 °C). Thus the Si/P ratio of intermediate members of the series has a great barometric potential, especially in the Si-buffering assemblage with clinochlore + talc + kyanite + rutile + H₂O. Application to zoned ellenbergerite crystals included in the Dora-Maira pyrope megablasts, western Alps, reveals that growth zoning is preserved at T as high as 700–725 °C. However, the record of attainment of the highest T and/or of decreasing P through P-rich rims (1 to 2 Si pfu) is only possible in the presence of an additional phosphate phase (OH-bearing or even OH-dominant wagnerite in these rocks), otherwise the trace amounts of P in the system remain sequestered in the core of Si-rich crystals (5 to 8 Si pfu) and can no longer react. Received: 7 April 1995 / Accepted: 12 November 1997     

Calciocarbonatite and magnesiocarbonatite rocks and magmas represented in the system CaO-MgO-CO2-H2O at 0.2 GPa

Summary ?The low-pressure eutectic for the coprecipitation of calcite, portlandite, and periclase/brucite (with H₂O-rich vapor) has served as a model for the existence and crystallization of carbonatite magmas. Attempts to determine conditions for the appearance of dolomite at this eutectic have been unsuccessful. We have discovered a second low-temperature eutectic for more magnesian liquids which excludes portlandite and includes dolomite (all results are vapor-saturated). Addition of Ca(OH)₂-Mg(OH)₂ to CaCO₃-MgCO₃ at 0.2 GPa depresses the liquidus to temperatures below the crest of the calcite-dolomite solvus; the vapor-saturated liquidus surface falls steeply, and the field boundary for liquids coexisting with calcite and periclase reaches a peritectic at 880 °C, where a narrow field for liquidus dolomite begins, extending down to the eutectic at 659 °C for the coprecipitation of calcite, dolomite and periclase (brucite should replace periclase at slightly higher pressures). The calcite liquidus is very large. The field boundary for coexistence of calcite and dolomite extends approximately in the direction from CaMg(CO₃)₂ towards Mg(OH)₂. The results illustrate conditions for the formation of mineral-specific cumulates from variable magma compositions. Hydrous (or sodic) carbonate-rich liquids with compositions from CaCO₃ to CaMg(CO₃)₂ will precipitate calcite-carbonatites first, followed by calcite-dolomite-carbonatites, with the prospect of precipitating dolomite-carbonatite alone through a limited temperature interval, and with periclase joining the assemblage in the closing stages. Periclase in the Fe-free system may represent the ubiquitous occurrence of magnetite in natural carbonatites. The restricted range for the precipitation of dolomite-carbonatites adds credibility to the evidence for primary magnesiocarbonatite (near-dolomite composition) magmas. Magnesiocarbonatite magmas can precipitate much calcite-carbonatite rock.

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Zusammenfassung ?Calciokarbonatitische und magnesiokarbonatitische Gesteine und Magmen im System CaO-MgO-CO ₂ -H ₂ O bei 0.2 GPa Das Niedrigdruck-Eutektikum der gemeinsamen Ausscheidung von Calcit, Portlandit und Periklas/Brucit (mit H₂O-reicher Fluidphase) diente als Modell um die Existenz und Kristallisation karbonatitischer Magmen zu erkl?ren. Versuche die Bedingungen des Auftretens von Dolomit an diesem Eutektikum zu bestimmen blieben bisher ergebnislos. Wir entdeckten ein zweites Niedrigtemperatur-Eutektikum für magnesiumreichere Schmelzen, das Portlandit ausschlie?t, aber Dolomit inkludiert (alle Ergebnisse bei Fluids?ttigung). Die Zugabe von Ca(OH)₂-Mg(OH)₂ zu CaCO₃-MgCO₃ bei 0.2 GPa senkt den Liquidus auf Temperaturen unter die Solvus-Schwelle von Calcit-Dolomit. Die fluidges?ttigte Liquidusfl?che verl?uft steil und die Grenzfl?che von Schmelze, die mit Calcit und Periklas koexistiert erreicht ein Peritektikum bei 880 °C. Dort ?ffnet sich ein schmales Feld für Liquidus-Dolomit, das bis zum Eutektikum bei 659 °C reicht, an dem Calcit, Dolomit und Periklas (Brucit sollte Periklas bei geringfügig h?heren Drucken ersetzen) gemeinsam ausgeschieden werden. Der Calcit- Liquidus ist sehr gro?. Die Linie an der Calcit und Dolomit koexistieren erstreckt sich ungef?hr von CaMg(CO₃)₂ zu Mg(OH)₂. Die Ergebnisse zeigen die Bildungsbedingungen für die Bildung mineralspezifischer Kumulate aus unterschiedlichen Magmenzusammensetzungen. Aus w?ssrigen (oder Na-reichen) karbonatreichen Schmelzen mit Zusammensetzungen zwischen CaCO₃ und CaMg(CO₃)₂ werden sich zuerst Calcitkarbonatite und dann Calcit-Dolomitkarbonatite ausscheiden, mit der M?glichkeit Dolomitkarbonatite über ein sehr eingeschr?nktes Temperaturintervall zu bilden und mit Periklas, der zu dieser Vergesellschaftung im Endstadium hinzukommt. Periklas im Fe-freien System k?nnte das weitverbreitete Analog zu Magnetit in natürlichen Karbonatiten sein. Der enge Bereich für die Ausscheidung von Dolomitkarbonatiten untermauert die Existenz prim?rer magnesiokarbonatitischer Magmen (nahe der Zusammensetzung von Dolomit). Magnesiokarbonatitische Magmen k?nnen daher entsprechende Mengen an calcitkarbonatitischen Gesteinen ausscheiden.

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Received July 20, 1998;/revised version accepted August 18, 1999     

Sodium-rich metasomatism in the upper mantle: Implications of experiments on the pyrolite-Na2O-rich fluid system at 950°C, 20 kbar

Sodium-rich metasomatism in the upper levels of the mantle has been modelled by reacting pyrolite with alkali-bearing H₂O fluids containing minor CO₂ and concentrations of Na₂O and Na₂O + K₂O (K/K + Na = 0.1 ) up to 4.0 g alkalies/10 g H₂O at 20 kbar and 950°C. With increasing alkali concentration, the amounts of amphibole (pargasite-edenite) and olivine increase as orthopyroxene and clinopyroxene decrease. Amphiboles show progressive increases in Na (and K) and Si concentrations and decreases in Al and Ca concentrations suggesting the dominant substitution mechanism is (Na, K) + SiAl + Ca. These results and least squares mass balance calculations suggest the reaction of clinopyroxene + orthopyroxene + spinel produces amphibole + olivine.In nature, upper mantle spinel lherzolite is commonly veined by a variety of rock types which may contain Ti-pargasite as a magmatic crystallization product. Pargasite-edenite occurs interstitially in spinel Iherzolite, often spatially related to Ti-pargasite and may be produced by hydrous fluids evolved during late stage crystallization of the veined rocks. This is supported by the close compositional correlation between the natural pargasite-edenite amphiboles and those produced in this study.The present study suggests that up to 43 wt.% amphibole may be accommodated in pyrolite in the presence of Na₂O-rich H₂O-CO₂ fluids. This represents 0.8 wt.% H₂O and 1.7 wt.% Na₂O in the hydrated pyrolite composition and indicates the importance of sodium in determining the extent of metasomatism. Sodium also lowers the solidus temperature of pyrolite by more than 50°C over the H₂O-saturated pyrolite system at 20 kbar.     

Phase relations and formation of sodium-rich majoritic garnet in the system Mg3Al2Si3O12–Na2MgSi5O12 at 7.0 and 8.5 GPa

The pseudo-binary system Mg₃Al₂Si₃O₁₂–Na₂MgSi₅O₁₂ modelling the sodium-bearing garnet solid solutions has been studied at 7 and 8.5 GPa and 1,500–1,950°C. The Na-bearing garnet is a liquidus phase of the system up to 60 mol% Na₂MgSi₅O₁₂ (NaGrt). At higher content of NaGrt in the system, enstatite (up to ∼80 mol%) and then coesite are observed as liquidus phases. Our experiments provided evidence for a stable sodium incorporation in garnet (0.3–0.6 wt% Na₂O) and its control by temperature and pressure. The highest sodium contents were obtained in experiments at P = 8.5 GPa. Near the liquidus (T = 1,840°C), the equilibrium concentration of Na₂O in garnet is 0.7–0.8 wt% (∼6 mol% Na₂MgSi₅O₁₂). With the temperature decrease, Na concentration in Grt increases, and the maximal Na₂MgSi₅O₁₂ content of ∼12 mol% (1.52 wt% Na₂O) is gained at the solidus of the system (T = 1,760°С). The data obtained show that most of natural diamonds, with inclusions of Na-bearing garnets usually containing <0.4 wt% Na₂O, could be formed from sodium-rich melts at pressures lower than 7 GPa. Majoritic garnets with higher sodium concentrations (>1 wt% Na₂O) may crystallize at a pressure range of 7.0–8.5 GPa. However the upper pressure limit for the formation of naturally occurring Na-bearing garnets is restricted by the eclogite/garnetite bulk composition.     

Experimental study of uraninite solubility in aqueous HCl solutions at 500°C and 1 kbar

Uraninite solubility in 0.001–2.0 m HCl solutions was experimentally studied at 500°C, 1000 bar, and hydrogen fugacity corresponding to the Ni/NiO buffer. It was shown that the following U(IV) species dominate in the aqueous solution: U(OH)₄⁰, U(OH)₂Cl₂⁰, and UOH Cl₃⁰ Using the results of uraninite solubility measurement, the Gibbs free energies of U(IV) species at 500°C and 1000 bar were calculated (kJ/mol): −9865.55 for UO₂(aq), −1374.57 for U(OH)₂ Cl₂⁰, and −1265.49 for UOH Cl₃⁰, and the equilibrium constants of uraninite dissolution in water and aqueous HCl solutions were estimated: UO₂(cr) = UO₂(aq), pK ₀ = 6.64; UO₂(cr) + 2HCl⁰ = U(OH)₂ Cl₂⁰, pK ₂ = 3.56; and UO₂(cr) + 3HCl⁰ = UOHcl₃⁰ + H₂O, pK ₃ = 3.05. The value pK ₁ ≈ 5.0 was obtained as a first approximation for the equilibrium UO₂(cr) + H₂O + HCl⁰ = U(OH)₃Cl⁰. The constant of the reaction UO₂(cr) + 4HCl⁰ = UCl₄⁰ + 2H₂O (pK ₄ = 7.02) was calculated taking into account the ionization constants of U Cl₄⁰ and U(OH)₄⁰, obtained by extrapolation from 25 to 500°C at 1000 bar using the BR model. Intense dissolution and redeposition of gold (material of experimental capsules) was observed in our experiments. The analysis and modeling of this phenomenon suggested that the UO_{2 + x} /UO₂ redox pair oxidized Au(cr) to Au⁺(aq), which was then reduced under the influence of stronger reducers.