The role of CO2 in the genesis of olivine melilitite

The nature of the near-liquidus phases for a mantle-derived olivine melilitite composition have been determined at high pressure under dry conditions and with various water contents. Olivine and clinopyroxene occur on or near the liquidus and there are no conditions where orthopyroxene crystallizes in equilibrium with the olivine melilitite. We have determined the effect on the liquidus temperature and liquidus phases of substituting CO₂ for H₂O on a mole for mole basis at 30 kb, using olivine melilitite + 20 wt% H₂O at = 0 and = (CO₂)/(H₂+CO₂) (mole fraction) = 0.25, 0.5, 0.75 and 1.0 (i.e. olivine melilitite + 38 wt% CO₂). Experiments were buffered by the MH or NNO buffers. At 30 kb, CO₂ is only slightly less soluble than water for <0.5 as judged by the slight increase in liquidus temperature on mole-for-mole substitution of CO₂ for H₂O and at 30 kb, 1200° C, = = 0.5 the olivine melilitite contains 8.8 wt% H₂O and 21 wt% CO₂ in solution. For 1 the CO₂ saturated liquidus is depressed 70 ° C below the anhydrous liquidus and the magma dissolves approx. 17% CO₂ at 30kb, 1400 ° C, 1, 0. Infrared spectra of quenched glasses have absorption bands characteristic of CO ₃ ⁼ and OH- molecules and no evidence for HCO ₃ ^- . The effect of CO ₃ ⁼ molecules dissolved in the olivine melilitite at high pressure is to suppress the near-liquidus crystallization of olivine and clinopyroxene and bring orthopyroxene and garnet on to the liquidus. We infer that olivine melilitite magmas may be derived by equilibrium partial melting (<5%) of pyrolite at 30 kb, 1150–1200 ° C, provided that both H₂O and CO₂ are present in the source region in minor amounts. Preferred conditions are 0< <0.5, 0.5< <1, and at low oxygen fugacities (相似文献   

Carbon isotopic fractionation between CO2 vapour,silicate and carbonate melts: an experimental study to 30 kbar

The carbon isotopic fractionation between CO₂ vapour and sodamelilite (NaCaAlSi₂O₇) melt over a range of pressures and temperatures has been investigated using solid-media piston-cylinder high pressure apparatus. Ag₂C₂O₄ was the source of CO₂ and experimental oxygen fugacity was buffered at hematite-magnetite by the double capsule technique. The abundance and isotopic composition of carbon dissolved in sodamelilite (SM) glass were determined by stepped heating and the ¹³C of coexisting vapour was determined directly by capsule piercing. CO₂ solubility in SM displays a complex behavior with temperature. At pressures up to 10 kbars CO₂ dissolves in SM to form carbonate ion complexes and the solubility data suggest slight negative temperature dependence. Above 20 kbars CO₂ reacts with SM to form immiscible Na-rich silicate and Ca-rich carbonate melts and CO₂ solubility in Na-enriched silicate melt rises with increasing temperature above the liquidus. Measured values for carbon isotopic fractionation between CO₂ vapour and carbonate ions dissoived in sodamelilite melt at 1200°–1400° C and 5–30 kbars average 2.4±0.2, favouring¹³C enrichment in CO₂ vapour. The results are maxima and are independent of pressure and temperature. Similar values of 2 are obtained for the carbon isotopic fractionation between CO₂ vapour and carbonate melts at 1300°–1400° C and 20–30 kbars.     

The system: Two alkali feldspars-KCl-NaCl-H2O at moderate to high temperatures and low pressures

Because of frequent discrepancies between the available experimental data and the measured composition of alkali chloride aqueous solutions coexisting with two alkali feldspars in high temperatures-low pressures natural systems, a systematic investigation of the system KAlSi₃O₈-NaAlSi₃O₈-KCl-NaCl-H₂O has been undertaken.Experiments have been carried out at temperatures from 300 °C to 660 °C, pressures from 0.2 to 2 kbar and total chloride concentrations ranging from 0.05 to 14 moles/kg H₂O.No effect of pressure on the feldspars solvus could be detected. Smoothing the experimental data on the basis of the regular assymetric solid solution model yields a critical temperature of 661°C and a critical composition of Or_0.36Ab_0.64.The equilibrium constant C = m _KCl/m _NaCl does not depend on total chloride molality, as long as the aqueous solution is homogeneous. But, in the miscibility gap (liquid+vapour) of the fluid, C is always lower in the vapour than in the liquid. The higher the temperature and the lower the pressure, the more striking this effect. For instance, at 500 ° C C _vaqour/C _liquid = 1 above 1 kb, 0.9 at 600 bars, 0.8 at 500 bars, 0.7 at 400–450 bars.The effect of pressure can be neglected in homogeneous fluids and in the liquid phase of unmixed fluids, but it is very important in the vapour phase (dilute solutions at low pressure).The selected values of C _max are (±0.01) 300 ° C0.083; 400 ° C0.139; 500 ° C0.200; 600 ° C0.264; 650 ° C0.298Such a behaviour of the fluid at low pressures explains the abnormally low values of m _KCl/m _NaCl measured in many natural hydrothermal systems. A new mechanism of alkali metasomatism (especially potassic alterations) is also proposed, taking into account the unmixing of alkali chloride aqueous solutions. This model seems particularly interesting in late magmatic hydrothermal processes, such as those occuring in porphyry type deposits.     

Wetting behavior of partial melts during crustal anatexis: the distribution of hydrous silicic melts in polycrystalline aggregates of quartz

The equilibrium distribution of hydrous silicic melts in polycrystalline aggregates of quartz was characterized in a series of partial melting and melt distribution experiments in the systems quartz-albite-orthoclase-H₂O and quartz-anorthite-H₂O, at 650 to 1000 MPa and 800 to 900° C. Near-equilibrium textures in these experiments are characterized by very low quartz-quartz-melt wetting angles, and by a substantial number of thin melt films along grain boundaries. Wetting angles in the H₂O-saturated experiments are as follows: 18° at 800° C-1000 MPa, and 12° at 900° C-1000 MPa in the granitic system; 18° at 850° C-650 MPa, 15° at 900° C-650 MPa, and 15° at 900° C-1000 MPa in the quartzanorthite system. In the granitic system at 900° C-1000 MPa, a decrease of H₂O content in melt from 17 wt% (at saturation) to 6 wt%, results in a slight increase of wetting angle from 12° to 16°. These low wetting angles — and the observation that many grain boundaries are wetted by melt films-indicate that the ratio of quartz-quartz to quartz-melt interfacial energies (_ss/_s1) is high: 2. Secondary electron imaging of fracture surfaces of melt-poor samples provided a three-dimensional insight into the geometry of melt; at low melt fraction, melt forms an interconnected network of channels along grain edges, as predicted for isotropic systems with wetting angles below 60°. This high-permeability geometry suggests that the segregation of granitic melts is not as sluggish as previously anticipated; simple compaction calculations for a permeability range of 10^-12 to 10^-9 m² indicate that segregation may operate at low to moderate melt fractions (below 30 vol. %), within relatively short time-scales, i.e., 10⁵ to 10⁶ years. Quartzmelt textures show significant deviations from the equilibrium geometries predicted for isotropic partially molten systems. The most consistent deviation is the pervasive development of crystallographically-controlled, planar faces of quartz; these faces provide definitive evidence for non-isotropic quartz-melt surface energy. For most silicates other than quartz, the grain-scale distribution of partial melts deviates even more significantly from equilibrium distributions in isotropic systems; accordingly, in order to describe adequately melt distributions in most natural source regions, the equilibrium model should be modified to account for anisotropy of solid-liquid interfacial energy.Contribution CNRS-INSU-DBT n^o 651     

Stable isotope and fluid inclusion studies of gem-bearing granitic pegmatite-aplite dikes,San Diego Co., California

Late Cretaceous, granitic pegmatite-aplite dikes in southern California have been known for gem-quality minerals and as a commercial source of lithium. Minerals, whole-rock samples, and inclusion fluids from nine of these dikes and from associated wall rocks have been analyzed for their oxygen, hydrogen, and carbon isotope compositions to ascertain the origins and thermal histories of the dikes. Oxygen isotope geothermometry used in combination with thermometric data from primary fluid inclusions enabled the determination of the pressure regime during crystallization.Two groups of dikes are evident from their oxygen isotope compositions (¹⁸O_qtz+10.5 in Group A, and +8.5 in Group B). Prior to the end of crystallization, Group A pegmatites had already extensively exchanged oxygen with their wall rocks, while Group B dikes may represent a closer approximation to the original isotopic composition of the pegmatite melts. Oxygen isotope fractionations between minerals are similar in all dikes and indicate that the pegmatites were emplaced at temperatures of about 730 ° to 700 ° C. Supersolidus crystallization began with the basal aplite zone and ended with formation of quench aplite in the pocket zone, nearly to 565 ° C. Subsolidus formation of gem-bearing pockets took place over a relatively narrow temperature range of about 40 ° C (approximately 565–525 ° C). Nearly closed-system crystallization is indicated.Hornblende in gabbroic and noritic wall rocks (D_w.r. = –90 to –130) in the Mesa Grande district crystallized in the presence of, or exchanged hydrogen with, meteoric water (D –90) prior to the emplacement of the pegmatite dikes. Magmatic water was subsequently added to the wall rocks adjacent to the pegmatites.Groups A and B pegmatites cannot be distinguished on the basis of their hydrogen isotope compositions. A decrease in D of muscovite inward from the walls of the dikes reflects a decrease in temperature. D values of H₂O from fluid inclusions are: –50 to –73 (aplite and pegmatite zones); –62 to –75 (pocket quartz: Tourmaline Queen and Stewart dikes); and –50 ± 4 (pocket quartz from many dikes). The average ¹³C of juvenile CO₂ in fluid inclusions in Group B pegmatites is –7.9. In Group A pegmatities, ¹³C of CO₂ is more negative (–10 to –15.6), due to exchange of C with wall rocks and/or loss of ¹³C-enriched CO₂ to an exsolving vapor phase.Pressures during crystallization of the pockets were on the order of 2,100 bars, and may have increased slightly during pocket growth. A depth of formation of at least 6.8 km (sp. gr. of over burden = 3.0, and P _fiuid=P _load) is indicated, and a rate of uplift of 0.07 cm/yr. follows from available geochronologic data.     

Stable Isotope Fractionation during Experimental Formation of Norsethite (BaMg[CO3]2): A Mineral Analogue of Dolomite

Stable oxygen and carbon isotopefractionation during the experimental formation ofordered norsethite (BaMg[CO₃]₂) from thereaction of anhydrous BaCO₃ (witherite) withrelatively low concentrated sodium-magnesiumbicarbonate solutions has been studied between20° and 135 °C. In the investigatedtemperature range, ¹⁸O and ¹³C are enrichedin norsethite with respect to water and gaseous carbondioxide, respectively. Whereas ¹⁸O/¹⁶Opartitioning is intermediate between those of theBaCO₃–H₂O and MgCO₃–H₂O systems,¹³C/¹²C partitioning is more similar to thatfor BaCO₃–CO₂. Between 20° and90°C, the temperature dependences of the¹⁸O/¹⁶O and ¹³C/¹²C fractionationfactors are represented by the equations (T in °K):10³ ln _BaMg[CO₃]₂-H_2O = 2.83 10⁶T^--2.85, and 10³ln_BaMg[CO₃]₂-CO₂(gas) = 1.78 10⁶T^--10.16. The later equation considers carbon isotope fractionationbetween the dissolved carbonate ion and carbon dioxide measured by Halaset al. (1997). Under standard state conditions (25 °C) the fractionation factors in the system BaMg[CO₃]₂-CO₂-H₂O are: Oxygen isotopes: _BaMg(CO₃)₂-H_2O = 1.02941, _BaMg(CO₃)₂-OH-(aq) = 1.07059,_BaMg(CO₃)₂-CO₂(gas) = 0.98868, and_BaMg(CO₃)₂-H₂CO₃ ^* = 0.98843; carbon isotopes:_BaMg(CO₃)₂-CO₂(gas) = 1.00992,_BaMg(CO₃)₂-H₂CO₃ ^* = 1.01099,_BaMg(CO₃)₂-HCO₃ ^- = 1.00194,_BaMg(CO₃)₂-CO₃ ^2- = 1.00491 or 1.00150.The spontaneous precipitation of aBaMg[CO₃]₂ gel at 20 °C,followed by the alteration of the products at20° or 60°C for 31 days,demonstrated isotope exchange reactions betweensolids and mother solutions dueto recrystallization. Isotope equilibrium, wasnot reached within run time.     

Experiments on the phase transition calcite+wollastonite+ +epidote=grossular-andraditess+CO2+H2O

The reaction 2 epidote+2 calcite+3 wollastonite3 grossular-andradite_ss+ 2 CO₂+1 H₂O has been explored by hydrothermal experiments at a total fluid pressure of 1000 bars. For a grossular-andradite_ss of andradite ₂₅ composition, the isobaric univariant curve passes through the points 458°C: X_CO2=0.00; 521°C: X_CO2=0.026; 523°C: X_CO2=0.052; 526°C: 0.088; 528°C: X_CO2=0.104. This curve intersects the isobaric univariant curve of the reaction calcite+quartz+[H₂O] wollastonite+CO₂+[H₂O] at the isobaric invariant point around 528°C and X_CO2=0.12. At higher values of X_CO2, this reaction is replaced by another one, namely: 2 epidote+5 calcite+3 quartz3 grossular-andradite_ss+5 CO₂+ 1 H₂O. It is demonstrated that both the reactions do actually take place during the metamorphism of calcareous rocks. The petrologic significance of contrasted sequence of reactions within this system observed by various workers is also discussed.     

Die Kristallstruktur von Warikahnit,Zn3[(H2O)2|(AsO4)2]

Zusammenfassung Die Kristallstruktur des neuen Minerals Warikahnit, Zn₃[(H₂O)₂|(AsO₄)₂], wurde mit Diffraktometerdaten bestimmt und bis zuR=0,038 für 3428 unabhängige Reflexe verfeinert.Warikahnit ist triklin, , mita=6,710(1),b=8,989(2),c=14,533(2) Å, =105,59(1), =93,44(1), =108,68(1)°,Z=4.Die Kristallstruktur des Warikahnits enthält sechs unterschiedliche Koordinationspolyeder des Zinks mit den Koordinationszahlen 6, 5 und 4 und mit fünf verschiedenen Ligandenkombinationen. Die Wasserstoffbrückenbindungen werden mit Hilfe der Ladungsbilanz und des IR-Spektrums diskutiert.

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The crystal structure of warikahnite, Zn₃[(H₂O)₂|(AsO₄)₂]

Summary The crystal structure of the new mineral warikahnite, Zn₃[(H₂O)₂|(AsO₄)₂], was determined from diffractometer data and refined toR=0,038 for 3428 observed independent reflections.Warikahnit is triclinic, , witha=6.710(1),b=8.989(2),c=14.533(2) Å, =105.59(1), =93.44(1), =108.68(1)°,Z=4.The crystal structure of warikahnite contains 6 different coordination polyhedra of zinc with the coordination numbers 6,5 and 4 and with 5 different combinations of ligand. The hydrogen bonds are discussed on the basis of charge balance and IR spectra.

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Mit 3 Abbildungen     

The restricted stability of osumilite under hydrous conditions in the system K2O-MgO–Al2O3–SiO2–H2O

Osumilite_ss was synthesized as a single phase product in the model system K₂O-MgO-Al₂O₃-SiO₂ at 800° C/ 0.5 Kbar water pressure and at 800° to 840° C/1.0 Kbar total pressure with 0.3 in the gas phase. The experimentally determined solid solubility range of synthetic osumilites can be expressed by the formula KMg₂(Al_3-xMg_x) (Al_2–xSi_10+x)O₃₀ with 0x0.4. A survey of sixteen chemical analyses of natural osumilites from eleven occurrences shows a solid solubility characterized by 0x0.6. Reversed stability experiments for the synthetic osumilite KMg₂(Al_2.75Mg_0.25)(Al_1.75Si_10.25)O₃₀ determined at water pressure equal to total pressure demonstrate its restriction to water pressures below 0.8 Kbar (at 0.5 Kbar, the stability range is between 765° and 800° C). At the lower thermal stability limit osumilite+H₂O vapor break down to cordierite+K feldspar+phlogopite_ss+quartz, at the higher one to cordierite+K feldspar+phlogopite+liquid. Reduction of water fugacity will expand the stability field largely by shifting the lower and higher thermal stability limits to lower and higher temperatures, respectively. The dependence of osumilite stability on water fugacity makes osumilite a sensitive indicator mineral for dry conditions in rocks formed at total pressures higher than about 0.8 Kbar.     

Stable isotope geochemistry and phase equilibria of coesite-bearing whiteschists,Dora Maira Massif,western Alps

Peak metamorphic temperatures for the coesite-pyrope-bearing whiteschists from the Dora Maira Massif, western Alps were determined with oxygen isotope thermometry. The ¹⁸O(smow) values of the quartz (after coesite) (¹⁸O=8.1 to 8.6, n=6), phengite (6.2 to 6.4, n=3), kyanite (6.1, n=2), garnet (5.5 to 5.8, n=9), ellenbergerite (6.3, n=1) and rutile (3.3 to 3.6, n=3) reflect isotopic equilibrium. Temperature estimates based on quartz-garnet-rutile fractionation are 700–750 °C. Minimum pressures are 31–32 kb based on the pressure-sensitive reaction pyrope + coesite = kyanite + enstatite. In order to stabilize pyrope and coesite by the temperature-sensitive dehydration reaction talc+kyanite=pyrope+coesite+H₂O, the a(H₂O) must be reduced to 0.4–0.75 at 700–750 °C. The reduced a(H₂O) cannot be due to dilution by CO₂, as pyrope is not stable at X(CO₂)>0.02 (T=750 °C; P=30 kb). In the absence of a more exotic fluid diluent (e.g. CH₄ or N₂), a melt phase is required. Granite solidus temperatures are 680 °C/30 kb at a(H₂O)=1.0 and are calculated to be 70°C higher at a(H₂O)=0.7, consistent with this hypothesis. Kyanite-jadeite-quartz bands may represent a relict melt phase. Peak P-T-f(H₂O) estimates for the whiteschist are 34±2 kb, 700–750 °C and 0.4–0.75. The oxygen isotope fractionation between quartz (¹⁸O=11.6) and garnet (¹⁸O=8.7) in the surrounding orthognesiss is identical to that in the coesitebearing unit, suggesting that the two units shared a common, final metamorphic history. Hydrogen isotope measurements were made on primary talc and phengite (D(_SMOW)=-27 to-32), on secondary talc and chlorite rite after pyrope (D=-39 to -44) and on the surrounding biotite (D=-64) and phengite (D=-44) gneiss. All phases appear to be in nearequilibrium. The very high D values for the primary hydrous phases is consistent with an initial oceanicderived/connate fluid source. The fluid source for the retrograde talc+chlorite after pyrope may be fluids evolved locally during retrograde melt crystallization. The similar D, but dissimilar ¹⁸O values of the coesite bearing whiteschists and hosting orthogneiss suggest that the two were in hydrogen isotope equilibrium, but not oxygen isotope equilibrium. The unusual hydrogen and oxygen isotope compositions of the coesite-bearing unit can be explained as the result of metasomatism from slab-derived fluids at depth.     

Synthetic bayleyite,Mg2[UO2(CO3)3]·18H2O: Thermochemistry,crystallography and crystal structure

Summary Thermochemistry, morphology, optical properties and crystal structure of synthetic bayleyite, Mg₂[UO₂(CO₃)₃]·18H₂O, monoclinic, have been studied. Incongruent melting at 55°, three steps of dehydration and two steps of decarboxylation have been found by thermochemic investigations. Morphology: Prisms along [001] with {100}, {110}, {210}, {001}, {401}, {021}, {211}, {111} and as the most important forms. Optical data:n =1.453,n =1.498,n =1.499, 2V _x =16°,Y=b,X c=11°. Crystal structure: Space groupP2₁/a,a=26.560(3),b=15.256(2),c=6.505(1) Å, =92.90(1)°,Z=4,R=0.029 for 5126 independent reflections measured with MoK -radiation. The structure is built up from isolated Mg(H₂O)₆ octahedra, UO₂(CO₃)₃ units and lattice water molecules, all held together by hydrogen bonds only.

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Synthetischer Bayleyit, Mg₂[UO₂(CO₃)₃]·18H₂O: Thermochemie, Kristallographie und Kristallstruktur

Zuseammenfasung Thermochemie, Morphologie, optische Eigenschaften und Kristallstruktur von Bayleyit, Mg₂[UO₂(CO₃)₃]·18H₂O, monoklin, wurden anhand künstlich hergestellter Kristalle untersucht. Durch thermochemische Untersuchung wurden inkongruentes Schmelzen bei 55°, eine dreistufige Wasserabgabe sowie eine zweistufige CO₂-Abgabe festgestellt. Morphologie: parallel zu [001] gestreckte Prismen mit {100}, {110}, {210}, {001}, {401}, {021}, {211}, {111}, und {311} als wichtigste Formen. Optische Daten:n =1.453,n =1.498,n =1.499, 2V _x =16°,Y=b,X c=11°. Kristallstruktur: RaumgruppeP2₁/a,a=26.560(3),b=15.256(2),c=6.505(1) Å, =92.90(1)°,Z=4;R=0.029 für 5126 unabhängige, mit MoK -Strahlung gemessene Reflexe. Die Struktur enthält isolierte Mg(H₂O)₆-Oktaeder, UO₂(CO₃)₃-Gruppen und freie Wassermoleküle, die ausschließlich durch Wasserstoffbrücken miteinander verknüpft sind.

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With 4 Figures     

Solubility of CO2 in olivine melilitite at high pressures and role of CO2 in the earth's upper mantle

The positions of the liquidi and the near-liquidus phases of olivine-melilitite+CO₂ have been determined under MH-buffered and furnace-buffered conditions up to 40 kb. It is found that CO₂ alone lowers the liquidus compared to dry conditions, yet its influence is minor compared to H₂O. The major role of CO₂ is to favour the growth of orthopyroxene and garnet over that of olivine at least at high pressures. CO₂-contents of glasses from experiments just above the liquidus (MH-buffered) were determined as 5.1 % at 10kb; 7.5 % at 20kb, 9.3 % at 30kb and 10–11 % (estimated) at 40 kb. Experiments on (pyrolite –40 % olivine)+H₂O+CO₂ show that CO₂ occurs under mantle conditions as carbonate under subsolidus conditions and dissolved in a melt above the solidus. At 30kb, the solidus lies between 1,000 ° C and 1,050 ° C for vapour-saturated conditions, at and at .     

APL computer programs for thermodynamic calculations of equilibria in P-T-X CO 2 space

APL computer programs for the thermodynamic calculation of devolatilization and solid-solid equilibria operate using stored values for the molar volume and entropy of solids, the free energies of H₂O and CO₂, and the free energies of formation for 110 geologically-important phases. P-T-X _{CO 2} calculations of devolatilization equilibria can be made at pressures from 0.2 through 10 kb, and temperatures from 200 through 1,000° C. P-T-X calculations of solid-solid equilibria may be accomplished at pressures to 30 kb and temperatures to 1,000° C. Calculations can be extrapolations from experimental points, or direct calculations from thermochemical data alone. Options are available in these programs to consider effects of: real vs. ideal gas mixing, thermal expansion and compressibility, solid solution, fluid pressure differing from solid pressure, and uncertainties in high-temperature entropies.A collection of thermodynamic data programs accompanies the programs for calculating P-T-X _{CO 2} equilibria. Over a wide range of physical conditions, the data functions report free energies, entropies, fugacities of H₂O and CO₂, high temperature entropies of solids, and activities of components in H₂O-CO₂ mixtures.List of Symbols Activity of H₂O and CO₂ - G_f Free energy of formation of a phase from elements - G_r Free energy change of reaction - G _r ^o Standard state free energy change of a reaction - Free energies of pure H₂O and CO₂ - H _r ^o Standard state enthalpy change for a reaction - K Equilibrium constant - R Gas constant - S _r ^o Standard state entropy change of reaction - S _s ^o Standard state entropy change of solids in a reaction - V_s ^o Standard state volume change of a reaction - V_s ^o Standard state volume change of solids in a reaction - Mole fraction of H₂O and CO₂ - Activity coefficient of H₂O and CO₂     

Partitioning of F between H2O and CO2 fluids and topaz rhyolite melt

Fluid/melt distribution coefficients for F have been determined in experiments conducted with peraluminous topaz rhyolite melts and fluids consisting of H₂O and H₂O+CO₂ at pressures of 0.5 to 5 kbar, temperatures of 775°–1000°C, and concentrations of F in the melt ranging from 0.5 to 6.9 wt%. The major element, F, and Cl concentrations of the starting material and run product glasses were determined by electron microprobe, and the concentration of F in the fluid was calculated by mass balance. The H₂O concentrations of some run product glasses were determined by ion microprobe (SIMS). The solubility of melt in the fluid phase increases with increasing F in the system; the solubility of H₂O in the melt is independent of the F concentration of the system with up to 6.3 wt% F in the melt. No evidence of immiscible silica- and fluoriderich liquids was detected in the hydrous but water-undersaturated starting material glasses (8.5 wt% F in melt) or in the water-saturated run product glasses. F concentrates in topaz rhyolite melts relative to coexisting fluids at most conditions studied; however, D_F (wt% F in fluid/wt% F in melt) increases strongly with increasing F in the system. Maximum values of D_F in this study are significantly larger than those previously reported in the literature. Linear extrapolation of the data suggests that D_F is greater than one for water-saturated, peraluminous granitic melts containing 8 wt% F at 800° C and 2 kbar. D_F increases as temperature and as (H₂O/H₂O+CO₂) of the fluid increase. For topaz rhyolite melts containing 1 wt% F and with H₂O-rich fluids, D_F is independent of changes in pressure from 2 to 5 kbar at 800° C; for melts containing 1 wt% F and in equilibrium with CO₂-bearing fluids the concentrations of F in fluid increases with increasing pressure. F-and lithophile element-enriched granites may evolve to compositions containing extreme concentrations of F during the final stages of crystallization. If F in the melt exceeds 8 wt%, D_F is greater than one and the associated magmatic-hydrothermal fluid contains >4 molal F. Such F-enriched fluids may be important in the mass transport of ore constituents, i.e., F, Mo, W, Sn, Li, Be, Rb, Cs, U, Th, Nb, Ta, and B, from the magma.     

Deep crustal oxygen isotope variations: the Wawa-Kapuskasing crustal transect,Ontario

We have studied the oxygen isotopic composition of rocks from a 100 km transect through the central Superior province of Ontario, representing progressively the shallower terrains of the Kapuskasing structural zone (KSZ), the Wawa gneiss terrane (WGT), and the Michipicoten greenstone belt (MGB). These rocks range in age from 2.76 to 2.60 Ga, and correspond to a section through approximately 20 km of crustal thickness. Equivalent lithologic types have similar range of ¹⁸O values at each crustal level: tonalitic to granodioritic rocks: 6.4 to 9.5; dioritic and anorthositic rocks: 5.5 to 7.6; mafic gneisses: group 1 (majority): 5.7 to 7.1; group 2: 8.1 to 9.5. ¹⁸O values exhibit a remarkable correlation with SiO₂ values, similar to that observed in unaltered plutonic rocks of equivalent composition. Paragneisses have significantly higher ¹⁸O values: 9.3 to 12.2. Low-grade metavolcanic and metasedimentary rocks of the MGB are ¹⁸O-enriched compared to their high-grade equivalents in the KSZ and WGT: 7.4 to 13.3 for mafic to felsic metavolcanic rocks; 11.4 to 14.7 for clastic metasediments. Coexisting minerals exhibit ¹⁸O-fractionation consistent with equilibrium, but corresponding to uniform isotopic temperatures about 553 to 584°C across the entire transect, lower than the inferred metamorphic temperatures in the highest-grade (KSZ) terrane. The lack of distinctive isotopic differences between equivalent rock types in the KSZ, WGT and MGB suggests that there is no significant gradient in ¹⁸O with depth in the crust. The majority of mafic gneisses (group 1) appear to have been emplaced either as subaerial extrusives, intrusive sills, or, less likely, as submarine extrusives that were hydrothermally altered at high temperatures. The less abundant group 2 mafic rocks have the ¹⁸O values typical of greenstones that were altered at low temperature by seawater, and isotopically resemble low-grade rocks in the Michipicoten and Abitibi belts. In general, no major changes in whole-rock isotopic composition appear to have occurred during granulite facies metamorphism, implying limited flux of water or CO₂. The continuous linear gradient in ¹⁸O versus SiO₂ in the high-grade rocks cannot be due to differentiation of a mafic source magma. A model involving an association between mantle-derived mafic magma and ¹⁸O-enriched crustal materials is more consistent with the oxygen isotopic and the REE data.McMaster Isotopic, Nuclear and Geochemical Studies Group Publication 163; LITHOPROBE Publication 168.     

The pressure and temperature stability limits of lawsonite: implications for H2O recycling in subduction zones

The stability relations of lawsonite, CaAl₂Si₂O₇(OH)₂H₂O, have been investigated at pressures of 6 to 14 GPa and temperatures of 740 to 1150°C in a multi-anvil apparatus. Experiments used the bulk composition lawsonite+H₂O to determine the maximum stability of lawsonite. Lawsonite is stable on its own bulk composition to a pressure of 13.5 GPa at 800°C, and between 6.5 and 12 GPa at 1000°C. Its composition does not change with pressure or temperature. All lawsonite reactions have grossular, vapour and two other phases in the system Al₂O₃-SiO₂-H₂O (ASH) on their high-temperature side. A Schreinemakers analysis of the ASH phases was used to relate the reactions to each other. At the lowest pressures studied lawsonite breaks down to grossular+kyanite+coesite+vapour in a reaction passing through 980°C at 6 GPa and 1070°C at 9 GPa. Above 9 GPa the reactions coesite=stishovite and kyanite+vapour=topaz-OH are crossed. The maximum thermal stability of lawsonite is at 1080°C, at 9.4 GPa. At higher pressures the lawsonite breakdown reactions have negative slopes. The reaction lawsonite=grossular+topaz-OH+stishovite+vapour passes through 1070°C at 10 GPa and 1010°C at 12 GPa. At 14 GPa, 740–840°C, lawsonite is unstable relative to the assemblage grossular+diaspore+vapour+a hydrous phase with an Al:Si ratio of 1:1. Oxide totals in electron microprobe analyses suggest that the composition of this phase is AlSiO₃(OH). Two experiments on the bulk composition lawsonite+pyrope [Mg₃Al₂Si₃O₁₂] show that at 10 GPa the reaction lawsonite=Gr-Py_ss+topaz-OH+stishovite+vapour is displaced down temperature from the end-member reaction by 200°C for a garnet composition of Gr₂₀Py₈₀. Calculations suggest similar temperature displacements for reaction between lawsonite and Gr-Py-Alm garnets of compositions likely to occur in high-pressure eclogites. Temperatures in subduction zones remain relatively low to considerable depth, and therefore slab P-T paths can be within the stability field of lawsonite from the conditions of its crystallisation in blueschists and eclogites, up to pressures of at least 10 GPa. Lawsonite contains 11.5 wt% H₂O, which when released may trigger partial melting of the slab or mantle, or be incorporated in hydrous phases such as the aluminosilicates synthesised here. These phases may then transport H₂O to an even greater depth in the mantle.     

The crystal structure of trigonite,Pb3Mn(AsO3)2(AsO2OH)

Summary The mineral trigonite crystallizes in the monoclinic space groupPn–C _s ² witha ₀=7.26,b ₀=6.78,c ₀=11.09Å; =91.5°,Z=2. The structure was determined from 1250 X-ray intensities collected on an automatic two circle Weissenberg-type diffractometer. The final residual isR=6.5% using anisotropic temperature factors for Pb, Mn and As, and isotropic temperature factors for O.The structure consists of MnO₆ octahedra, sharing all six oxygens with arsenite groups to form a framework. The Pb atoms are attached to this framework with Pb–O distances2.23Å. One oxygen, bound only to an As atom, is interpreted as the donor for a hydrogen bond of 2.75Å.

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Die Kristallstruktur des Trigonits, Pb₃Mn(AsO₃)₂(AsO₂OH)

Zusammenfassung Das Mineral Trigonit kristallisiert monoklin, RaumgruppePn–C _s ² ,a ₀=7,26,b ₀=6,78,c ₀=11,09Å; =91,5°;Z=2. Die Strukturermittlung erfolgte anhand von 1250 Röntgenintensitäten, die auf einem automatischen Zweikreis-Weissenbergdiffraktometer gesammelt wurden. Mit anisotropen Temperaturfaktoren für Pb, Mn und As sowie isotropen für die O-Atome ergibt sich einR-Wert von 6,5%.Die MnO₆-Oktaeder werden über die sechs Sauerstoffe mit Arsenitgruppen zu einem dreidimensionalen Gerüst verknüpft. Über Pb-O-Abstände2,23 Å sind die Pb-Atome in dieses Gerüst eingebaut. Ein Sauerstoff, nur an ein As-Atom gebunden, wird als Donator einer H-Brücke von 2,75 Å interpretiert.

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Magmatic CO2 and saline melts from the Sybille Monzosyenite,Laramie Anorthosite Complex,Wyoming

There are three populations of fluid inclusions in quartz from the Sybille Monzosyenite: early CO₂, secondary CO₂, and rare secondary brines. The oldest consist of low density CO₂ (0.70) inclusions that appear to be co-magmatic. The densities of these inclusions are consistent with the inferred crystallization conditions of the Sybille Monzosyenite, namely 3 kilobars and 950–1000° C. The other types of inclusions are secondary; they contain CO₂ (0.50) and secondary brine inclusions that form trains radiating out from a decrepitated inclusion. The sites of these decrepitated inclusions are now marked by irregularly shaped fluid inclusions and solid inclusions of salt and carbonate. Rather than fluid inclusions, feldspar contain abundant solid inclusions. These consist of magmatic minerals, hedenbergite, hornblende, ilmenite, apatite, and graphite, intimately associated with K, Na chlorides. We interpret these relations as follows: The Sybille Monzosyenite formed from a magma that contained immiscible droplets of a halide-rich melt along with a CO₂ vapor phase. The salt was trapped along with the other obvious magmatic minerals during growth of the feldspars. CO₂ may have also been included in the feldspars but it probably leaked later during exsolution of the feldspars and was not preserved. Both the saline melt and the CO₂ vapor were trapped in the quartz. The melt inclusions in the quartz later decrepitated, perhaps due to progressive exsolution of fluids, to produce the secondary H₂O and CO₂ inclusions. These observations indicate that the Sybille Monzosyenite, which is a markedly anhydrous rock, was actually vapor-saturated. Rather than being H₂O, however, the vapor was CO₂-rich and possibly related to an immiscible chloride-rich melt.     

The crystal structures of CuSO4 · H2O and CuSeO4 · H2O,and their relationships to kieserite

Summary The crystal structures of hydrothermally grown CuSO₄ · H₂O and CuSeO₄ · H₂O were determined by single crystal X-ray methods [Space group ,a = 5.037 (1), 5.129 (1) Å,b = 5.170(1), 5.527(1)Å,c = 7.578(2), 7.469(2)Å, = 108.62(1), 103.98(1)°, = 108.39(1), 106.52(1)°, = 90.93(1), 97.19(1)°; Z = 2; R_w = 0.026, 0.030 for 2065, 2235 reflections with sin / 0.90 Å^–1]. The Cu atoms are [4 + 2]-coordinated to O atoms. These elongated octahedra are corner connected via the H₂O molecule to form chains. The formal units ¹ [Cu₂O₈(H₂O)2]^12- are interconnected by [XO₄]^2- groups (X=S,Se) and hydrogen bonds (bond lengths 2.72–2.83 Å). The crystal structures show pseudomonoclinic symmetry and are strongly related to the structure type of kieserite.[/p]

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Die Kristallstrukturen von CuSO₄ · H₂O und CuSeO₄ · H₂O und ihre Beziehungen zum Kieserit

Zusammenfassung Die Kristallstrukturen von hydrothermal gezüchtetem CUSO₄ · H₂O und CuSeO₄. H₂O wurden an Einkristallen mittels Röntgenbeugung bestimmt [Raumgruppe ;a = 5.037(1), 5.129(1)Å,b = 5.170(1), 5.527(1)Å,c = 7.578(2), 7.469 (2) Å, = 108.62(1), 103.98(1)°, = 108.39(1), 106.52(1)°, = 90.93(1), 97.19(1)°; Z = 2; R_W = 0.026, 0.030 für 2065, 2235 Reflexe mit sin / 0.90)Å^–1]. Die Cu-Atome werden durch O-Atome [4+2]-koordiniert. Diese gestreckten /lOktaeder sind miteinander über Ecken durch die H₂O-Moleküle zu Ketten verknüpft. Die formalen Einheiten ¹ [CU₂O₈(H₂O)₂]^12– werden durch [XO₄]^2–-Gruppen (X = S, Se) und Wasserstoffbrücken (Bindungslängen 2.72–2.83Å) miteinander verbunden. Die Kristallstrukturen zeigen pseudomonokline Symmetrie und sind sehr nahe mit dem Strukturtyp des Kieserits verwandt.

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Synthesis and upper stability of paragonite

The mineral paragonite, NaAl₂[AlSi₃O₁₀ (OH)]₂, has been synthesized on its own composition starting from a variety of different materials. Indexed powder data and refined cell parameters are given for both the 1M and 2M₁ polymorphs obtained. The upper stability limit of paragonite is marked by its breakdown to albite + corundum + vapour. The univariant equilibria pertaining to this reaction have been established by reversing the reaction at six different pressures, the equilibrium curve running through the following intervals: 1 kb: 530°–550° C 2 kb: 555°–575° C 3 kb: 580°–600° C 5kb: 625°–640° C 6 kb: 620°–650° C 7 kb: 650°–670° C.Comparison with the upper stability limit of muscovite (Velde, 1966) shows that paragonite has a notably lower thermal stability thus explaining the field observation that paragonite is absent in many higher grade metamorphic rocks in which muscovite is still stable.The enthalpy and entropy of the paragonite breakdown reaction have been estimated. Since intermediate albites of varying structural states are in equilibrium with paragonite, corundum and H₂O along the univariant equilibrium curve, two sets of data pertaining to the entropy of paragonite (S ₂₉₈ ⁰ ) as well as the enthalpy ( H _f,298 ⁰ ) and Gibbs free energy ( G _f,298 ⁰ ) of its formation were computed, assuming (1) high albite and (2) low albite as the equilibrium phase. The values are: (1) (2) S ₂₉₈ ⁰ 67.8±3.9 cal deg^–1 gfw^–1 63.7±3.9 cal deg^–1 gfw^–1 H _f,298 ⁰ –1417.9±2.7 kcal gfw^–1 –1420.2±2.6 kcal gfw^–1 G _f,298 ⁰ –1327.4±4.0 kcal gfw^–1 –1328.5±4.0 kcal gfw^–1.Adapted from a part of the author's Habilitationsschrift accepted by the Ruhr University, Bochum (Chatterjee, 1968).