David Dolejs | Charles University, Prague (original) (raw)

Papers by David Dolejs

Ceramics International, 2009

ZnO doped with Bi 2 O 3 and Sb 2 O 3 (ZBS), is the basic system for ceramic varistors. Phase form... more ZnO doped with Bi 2 O 3 and Sb 2 O 3 (ZBS), is the basic system for ceramic varistors. Phase formation during sintering of ZBS was measured in situ, using 1 mm thick samples and synchrotron X-rays. Sintering shrinkage was measured in different atmospheres by an optical method. Thermodynamic calculations were performed to explain phase formation, composition, stability of additive oxides and influence of the oxygen fugacity on sintering. Sb 2 O 4 , pyrochlore, trirutile and spinel were formed at temperatures of 500-800 8C. The oxidation of antimony was controlled by the oxygen partial pressure and affected both, phase formation and sintering kinetics, in the ZBS system. #

Applied Clay Science, 2010

Sodium fluorohectorite Na 0.6 [Mg 2.4 Li 0.6 ]Si 4 O 10 F 2 was synthesized from the melt in an o... more Sodium fluorohectorite Na 0.6 [Mg 2.4 Li 0.6 ]Si 4 O 10 F 2 was synthesized from the melt in an open glassy carbon crucible at 1265°C under argon flow. A glass (Na 2 O-Li 2 O-SiO 2 ) precursor was used as a fluxing agent in order to maintain a low vapor pressure of volatile fluorides and sustain a low silica activity, which inhibits the formation of silicon fluoride gases and promotes the fluorine solubility in the melt. To minimize the loss of volatile fluorine compounds the pre-synthesized alkali silicate glass was heated together with additional raw materials needed in a high frequency induction furnace rapidly from 800°C up to 1265°C and then kept at this temperature for a short period of time (15 min). The synthesis method can easily be scaled to batches larger than 1 kg. The Na 0.6 -fluorohectorite obtained stands out by (i) phase purity as checked by X-ray powder diffraction (PXRD), (ii) a superb homogeneity of the charge density as demonstrated by stepwise hydration behavior followed by in-situ PXRD in a humidity chamber and the Lagaly method with alkylammonium exchange , (iii) a high cation exchange capacity (CEC) of 136 meq/100 g as determined by the copper complex ([Cu(trien)] 2+ ) method, and (iv) extreme lateral extensions with a median value of the particle size of 45 μm as measured by static laser light scattering (SLS) which was confirmed by scanning electron microscopy (SEM).

Magnetite-rich skarns in the Bohemian Massif (central Europe) frequently occur in supracrustal vo... more Magnetite-rich skarns in the Bohemian Massif (central Europe) frequently occur in supracrustal volcanosedimentary sequences, metamorphosed up to greenschist, amphibolite or eclogite facies. Their magnetites are very pure (95.5-99.3 mol. % Fe 3 O 4 ) and minor and trace element concetrations are very low, defined by hercynite and ulvöspinel substitution trends. These patterns are consistent with derivation of skarns by carbonate or calc-silicate replacement, and exclude their origin in other settings or by involvement exhalative or hydrogeneous components. Significant correlations at trace levels exist between ore-forming element pairs (e.g., Zn-Sn), various divalent pairs (e.g., Zn-Mn) as well as immobile couples (e.g., Al-Ti). Negatively correlated homovalent pairs (e.g., Mg-Fe 2+ , Mn-Fe 2+ , Al-V 3+ ) have the largest potential for reflecting the environmental conditions during magnetite crystallization, whereas the positively correlated Al-Ti pair reflects inheritance from the skarn precursor and element partitioning between coexisting phases. The Al 2 O 3 /TiO 2 ratios in magnetites (5.4-11.6 by weight) are substantially lower than those in the bulk magnetite-rich skarns (18.3-22.6), which is due to aluminum partitioning into garnet and/or clinopyroxene. These observations suggest that trace elements in magnetites are subject to redistribution and reequilibration during superimposed metamorphic or hydrothermal events, in addition to their solubilities in the spinel structure being temperature-dependent as well.

The progressive enrichment in volatiles and light incompatible elements observed during upper-cru... more The progressive enrichment in volatiles and light incompatible elements observed during upper-crustal differentiation of granitic and rhyolitic magmas leads to significant changes in melt physical-chemical properties and has important implications for ore deposition and volcanic devolatization. Thermodynamic calculations and experimental studies of melting equilibria in the Na 2O-K2O-Al2O3-SiO2-F 2O-1-H2O system are used to evaluate mineral stabilities, fluid compositions, the extent of fluoride-silicate liquid-liquid immiscibility, fluorine and water solubility limits and differentiation paths of natural fluorine-bearing silicic magmas. The interaction of fluorine with rock-forming aluminosilicates corresponds to progressive fluorination by the thermodynamic component F2O-1. Formation of fluorine-bearing minerals first occurs in peralkaline and silica-undersaturated systems that buffer fluorine concentrations at very low levels (villiaumite, fluorite). The highest concentrations of fluorine are achieved in peraluminous silica-oversaturated systems, saturated with fluorite or topaz. Thermodynamic models of fluorosilicate melts indicate clustering of silicate tetrahedra in the Na2O-SiO 2-F2O-1 system, whereas initial NaAl-F short-range order evolves into partial O-F disorder in the albite-cryolite system. Experiments performed at 520-1100°C and 0.1-100 MPa completely describe liquidus relations and differentiation paths of fluorine-bearing felsic magmas. Coordination differences and short-range order effects between [NaAl]-F, Na-F vs. Si-O lead to the fluoride-silicate liquid immiscibility, which extends from the silica-cryolite binary through the peralkaline albite-silica-cryolite ternary and closes in multicomponent, topaz-bearing systems owing to the destabilizing effect of increasing peraluminosity. Liquidus relations indicate that fluoride-silicate liquid-liquid immiscibility is inaccessible to quartz-feldspar-saturated granitic melts. Differentiation paths of Ca-poor granitic melts with fluorine reach the cryolite or topaz saturation surface, respectively, and, depending on aluminosity buffering by rock-forming silicates, may evolve to the high fluorine concentrations of the haplogranite-topaz-cryolite-H2O eutectic at max. 5.9 wt. % F, 540°C, 100 MPa and H2O saturation. In contrast, the potential of Ca-rich silicic magmas for fluorine enrichment is severely limited by fluorite crystallization. Fluorite solubility is determined by individual concentrations of CaO and F2O-1 in the melt and exhibits a minimum at subaluminous compositions due to the short-range ordering of Ca-Al and alkali-F. These results provide a framework for differentiation of natural fluorine-bearing magmas and have applications in electrolytic and flux metallurgy.

Earth and Environmental Science Transactions of the Royal Society of Edinburgh, 2004

The prominent felsic granulites in the southern part of the Bohemian Massif (Gföhl Unit, Moldanub... more The prominent felsic granulites in the southern part of the Bohemian Massif (Gföhl Unit, Moldanubian Zone), with the Variscan (w340 Ma) high-pressure and high-temperature assemblage garnet+quartz+hypersolvus feldspar kyanite, correspond geochemically to slightly peraluminous, fractionated granitic rocks. Compared to the average upper crust and most granites, the U, Th and Cs concentrations are strongly depleted, probably because of the fluid and/or slight melt loss during the high-grade metamorphism (900-1050(C, 1·5-2·0 GPa). However, the rest of the trace-element contents and variation trends, such as decreasing Sr, Ba, Eu, LREE and Zr with increasing SiO 2 and Rb, can be explained by fractional crystallisation of a granitic magma. Low Zr and LREE contents yield w750(C zircon and monazite saturation temperatures and suggest relatively low-temperature crystallisation. The granulites contain radiogenic Sr ( 87 Sr/ 86 Sr 340 = 0·7106-0·7706) and unradiogenic Nd ( 340 Nd = 4·2 to 7·5), indicating derivation from an old crustal source. The whole-rock Rb-Sr isotopic system preserves the memory of an earlier, probably Ordovician, isotopic equilibrium.

Lithos, 2010

We investigated several mineral phases and their replacement products which occur as inclusions i... more We investigated several mineral phases and their replacement products which occur as inclusions in garnets from felsic and mafic granulites of the Gföhl Unit in the Moldanubian Zone. The most important mineral inclusions, Ti-rich muscovite and omphacite, were used for the reconstruction of the metamorphic history of granulites. Some inclusions were transformed during high-temperature granulite facies metamorphism, partial melting and decompression to other phases, and so the original mineral can only be deduced from the inclusion morphology and reaction products. These inclusions have columnar shapes and consist of Kfeldspar + kaolinite, albite + Fe-oxide, plagioclase + Fe-oxide, or albite + K-feldspar, respectively. The pseudomorphs with albite/plagioclase occur in a Ca-rich garnet that shows prograde zoning. Pressuretemperature (PT) evolution, derived from mineral assemblages in granulite and based on the inclusions, suggests a prograde metamorphism from amphibolite through eclogite to granulite facies conditions with subsequent amphibolite facies overprint during exhumation. The estimated PT trajectory for the studied granulites, which also host lenses or boudins of eclogites and garnet peridotites, allows reconstruction of the complete clockwise metamorphic path that is consistent with subduction geotherm prior to the tectonic amalgamation within the continental collisional root.

Lithos, 2004

A quartz-monzodioritic dike associated with the porphyry-Cu mineralized stock at Santa Rita, NM, ... more A quartz-monzodioritic dike associated with the porphyry-Cu mineralized stock at Santa Rita, NM, has been studied to constrain physico-chemical factors ( P, T, f O 2 , and volatile content) responsible for mineralization. The dike contains a lowvariance mineral assemblage of amphibole, plagioclase (An 30 -50 ), quartz, biotite, sphene, magnetite, and apatite, plus anhydrite and calcite preserved as primary inclusions within the major phenocryst phases. Petrographic relationships demonstrate that anhydrite originally was abundant in the form of phenocrysts (1 -2 vol.%), but later was replaced by either quartz or calcite. Hornblende -plagioclase thermobarometry suggests that several magmas were involved in the formation of the quartzmonzodiorite, with one magma having ascended directly from z 14 km depth. Rapid magma ascent is supported by the presence of intact calcite inclusions within quartz phenocrysts.

Lithos, 2011

Felsic granulites from the Kutná Hora complex in the Moldanubian zone of central Europe preserve ... more Felsic granulites from the Kutná Hora complex in the Moldanubian zone of central Europe preserve mineral assemblage that records transition from early eclogite to granulite facies conditions, and exhibits leucocratic banding, which is interpreted as an evidence for melt loss during the decompression path. The granulites are layered and consist of variable proportions of quartz, ternary feldspar, garnet, biotite, kyanite, and rutile. In the mesocratic layers, garnet grains show relatively high Ca contents corresponding to 28-41mol% grossular end member. They have remarkably flat compositional profiles in their cores but their rims exhibit an increase in pyrope and a decrease in grossular and almandine components. In contrast, garnets from the leucocratic layers have relatively low Ca contents (15-26mol% grossular) that further decrease towards the rims. In addition to modeling of pressure-temperature pseudosections, compositions of garnet core composition, garnet rim-ternary feldspar-kyanite-quartz equilibrium, ternary feldspar composition, and the garnet-biotite equilibrium provide five constraints that were used to reconstruct the pressure-temperature path from eclogite through the granulite and amphibolite facies. In both layers, garnet cores grew during omphacite breakdown and phengite dehydration melting at 940°C and 2.6 GPa. Subsequent decompression heating to 1020°C and 2.1 GPa produced Ca-and Fepoor garnet rims due to the formation of Ca-bearing ternary feldspar and partial melt. In both the mesocratic and leucocratic layer, the maximum melt productivity was 26 and 18 vol.%, respectively, at peak temperature constrained by the maximum whole-rock H 2 O budget,~1.05-0.75 wt.%, prior to the melting. The preservation of prograde garnet-rich assemblages required nearly complete melt loss (15-25 vol.%), interpreted to have occurred at 1000-1020°C and 2.2-2.4 GPa by garnet mode isopleths, followed by crystallization of small amounts of residual melt at 760°C and 1.0 GPa. Phase formation and melt productivity were independently determined by experiments in the piston-cylinder apparatus at 850-1100°C and 1.7-2.1 GPa. Both the thermodynamic calculations and phase equilibrium experiments suggest that the partial melt was produced by the dehydration melting: muscovite+ quartz = melt+ K-feldspar+ kyanite. The presence of partial melt facilitated attainment of mineral equilibria at peak temperature thus eliminating any potential relics of early high-pressure phases such as phengite or omphacite. By contrast, adjacent mafic granulites and eclogites, which apparently share the same metamorphic path but have not undergone partial melting commonly preserve relics or inclusions of eclogitefacies mineral assemblages.

Journal of Petrology, 2007

relations in the four-component system Na 2 O^Al 2 O 3^S iO 2^F2 O À1 were studied at 0Á1 and 100... more relations in the four-component system Na 2 O^Al 2 O 3^S iO 2^F2 O À1 were studied at 0Á1 and 100 MPa to define the location of fluoride^silicate liquid immiscibility and outline differentiation paths of fluorine-bearing silicic magmas. The fluoride^silicate liquid immiscibility spans the silica^albite^cryolite and silica^topaz^cryolite ternaries and the haplogranite-cryolite binary at greater than 9608C and 0Á1^100 MPa. With increasing Al 2 O 3 in the system and increasing aluminum/alkali cation ratio, the two-liquid gap contracts and migrates from the silica liquidus to the cryolite liquidus. The gap does not extend to subaluminous and peraluminous melt compositions. For all alkali feldspar^quartz-bearing systems, the miscibility gap remains located on the cryolite liquidus and is thus inaccessible to differentiating granitic and rhyolitic melts. In peralkaline systems, the magmatic differentiation is terminated at the albite^quartz^cryolite eutectic at $ 7708C, 100 MPa, 5wt5 wt % F and cation Al/Na ¼ 0Á75. The addition of topaz, however, significantly lowers melting temperatures and allows strong fluorine enrichment in subaluminous compositions. At 100 MPa, the binary topaz^cryolite eutectic is located at 7708C, 39 wt % F, cation Al/Na 5wt 0Á95, and the ternary quartz^topaz^cryolite eutectic is found at 7408C, 32 wt % F, 30 wt % SiO 2 and cation Al/Na $ 0Á95. Such location of both eutectics enables fractionation paths of subaluminous quartz-saturated systems to produce fluorine-rich, SiO 2 -depleted and nepheline-normative residual liquids.

Journal of Petrology, 2011

We identified molybdenite (MoS 2 ) as an accessory magmatic phase in 13 out of 27 felsic magma sy... more We identified molybdenite (MoS 2 ) as an accessory magmatic phase in 13 out of 27 felsic magma systems examined worldwide. The molybdenite occurs as small (520 m) triangular or hexagonal platelets included in quartz phenocrysts. Laser-ablation inductively coupled plasma mass spectrometry analyses of melt inclusions in molybdenite-saturated samples reveal 1^13 ppm Mo in the melt and geochemical signatures that imply a strong link to continental rift basalt^rhyolite associations. In contrast, arc-associated rhyolites are rarely molybdenite-saturated, despite similar Mo concentrations. This systematic dependence on tectonic setting seems to reflect the higher oxidation state of arc magmas compared with within-plate magmas. A thermodynamic model devised to investigate the effects of T, f O 2 and f S 2 on molybdenite solubility reliably predicts measured Mo concentrations in molybdenite-saturated samples if the magmas are assumed to have been saturated also in pyrrhotite. Whereas pyrrhotite microphenocrysts have been observed in some of these samples, they have not been observed from other molybdenitebearing magmas. Based on the strong influence of f S 2 on molybdenite solubility we calculate that also these latter magmas must have been at (or very close to) pyrrhotite saturation. In this case the Mo concentration of molybdenite-saturated melts can be used to constrain both magmatic f O 2 and f S 2 if temperature is known independently (e.g. by zircon saturation thermometry). Our model thus permits evaluation of magmatic f S 2 , which is an important variable but is difficult to estimate otherwise, particularly in slowly cooled rocks.

Journal of Metamorphic Geology, 2009

Exsolution lamellae of garnet in clinopyroxene and orthopyroxene porphyroclasts from garnet pyrox... more Exsolution lamellae of garnet in clinopyroxene and orthopyroxene porphyroclasts from garnet pyroxenites in the Moldanubian zone were studied to elucidate the pressure-temperature conditions of the exsolution process and to reconstruct the burial and exhumation path of ultramafic rocks in the Variscan orogen. The porphyroclasts occur in a fine-grained matrix with metamorphic fabrics, which consists of clinopyroxene and small amounts of garnet, orthopyroxene and amphibole. The clinopyroxene porphyroclasts contain garnet + orthopyroxene lamellae as well as ilmenite rods that have orientation parallel to (100) planes of the porphyroclasts. Orthopyroxene porphyroclasts host garnet and clinopyroxene lamellae, which show the same lattice preferred orientation. In both cases, lamellar orthopyroxene, clinopyroxene and garnet were partially replaced by secondary amphibole. Composition of exsolution phases and that of host pyroxene were reintegrated according to measured modal proportions and demonstrate that the primary pyroxene was enriched in Al and contained 8-11 mol.% Tschermak components. Conventional thermobarometry and thermodynamic modelling on the reintegrated pyroxene indicate that primary clinopyroxene and orthopyroxene megacrysts crystallized at 1300-1400°C and 2.2-2.5 GPa. Unmixing and exsolution of garnet and a second pyroxene phase occurred in response to cooling and pressure increase before the peak pressure of 4.5-5.0 GPa was reached at 1100°C. This scenario is consistent with a burial of hot upper-mantle ultramafics into a cold subcratonic environment and subsequent exhumation through 900°C and 2.2-3.3 GPa, when the pyroxenites would have partially recrystallized during tectonic incorporation into eclogites and felsic granulites.

Geofluids, 2010

We present a thermodynamic model for mineral dissolution in aqueous fluids at elevated temperatur... more We present a thermodynamic model for mineral dissolution in aqueous fluids at elevated temperatures and pressures, based on intrinsic thermal properties and variations of volumetric properties of the aqueous solvent. The standard thermodynamic properties of mineral dissolution into aqueous fluid consist of two contributions: one from the energy of transformation from the solid to the hydrated-species state and the other from the compression of solvent molecules during the formation of a hydration shell. The latter contribution has the dimension of the generalized Krichevskii parameter. This approach describes the energetics of solvation more accurately than does the Born electrostatic theory and can be extended beyond the limits of experimental measurements of the dielectric constant of H 2 O. The new model has been calibrated by experimental solubilities of quartz, corundum, rutile, calcite, apatite, fluorite and portlandite in pure H 2 O at temperatures up to 1100°C and pressures up to 20 kbar. All minerals show a steady increase in solubility along constant geothermal gradients or water isochores. By contrast, isobaric solubilities initially increase with rising temperature but then decline above 200-400°C. This retrograde behavior is caused by variations in the isobaric expansivity of the aqueous solvent, which approaches infinity at its critical point. Oxide minerals predominantly dissolve to neutral species; so, their dissolution energetics involve a relatively small contribution from the solvent volumetric properties and their retrograde solubilities are restricted to a relatively narrow window of temperature and pressure near the critical point of water. By contrast, Ca-bearing minerals dissolve to a variety of charged species; so, the energetics of their dissolution reactions involve a comparatively large contribution from volume changes of the aqueous solvent and their isobaric retrograde solubility spans nearly all metamorphic and magmatic conditions. These features correlate with and can be predicted from the standard partial molar volumes of aqueous species.

Geochimica et Cosmochimica Acta, 2014

The solubility of molybdenite, MoS 2 , in fluid-saturated, subaluminous to peraluminous granitic ... more The solubility of molybdenite, MoS 2 , in fluid-saturated, subaluminous to peraluminous granitic melts was determined experimentally using rapid-quench cold-seal pressure vessels at 800°C and 100-200 MPa, and analysis by laser-ablation ICP-MS. Molybdenite solubility seems to be independent of pressure, but it shows strong variations with oxygen and sulfur fugacity. At constant log fS 2 = À1.3 it increases from 0.1-0.7 ppm by weight Mo at the Co-CoO buffer to 29-38 ppm by weight Mo near the MnO-Mn 3 O 4 buffer. The solubility isopleths are nearly parallel to the pyrrhotite-magnetite equilibrium, along which the solubility varies only slightly, from 10 ppmw Mo at the quartz-fayalite-magnetite buffer to 29-38 ppmw Mo at the MnO-Mn 3 O 4 buffer. The observed solubility variations are consistent with the equilibrium MoS 2 (s) + 3/2O 2 = MoO 3 (l) + S 2 and thus confirm that molybdenum(VI) oxide is the predominant species in subaluminous silicate melts at log fO 2 = À16 to À11. In addition, the experimental results are well reproduced by a simple thermodynamic model employing the Burnham eight-oxygen formulation for silicate melt species and assuming ideal mixing of dissolved MoO 3 . The thermodynamic calibration can be used to estimate the molybdenum solubility in subaluminous silicic melts or, for pyrrhotite-and molybdenite-saturated assemblages, the oxygen and sulfur fugacities during magma crystallization.

Geochimica et Cosmochimica Acta, 2008

We present the results of thermodynamic modeling of fluid-rock interaction in the system Si-Al-Fe... more We present the results of thermodynamic modeling of fluid-rock interaction in the system Si-Al-Fe-Mg-Ca-Na-H-O-Cl using the GEM-Selektor Gibbs free energy minimization code. Combination of non-ideal mixing properties in solids with multicomponent aqueous fluids represents a substantial improvement and it provides increased accuracy over existing modeling strategies. Application to the 10-component system allows us to link fluid composition and speciation with whole-rock mineralogy, mass and volume changes. We have simulated granite-fluid interaction over a wide range of conditions (200-600°C, 100 MPa, 0-5 m Cl and fluid/rock ratios of 10 À2 -10 4 ) in order to explore composition of magmatic fluids of variable salinity, temperature effects on fluid composition and speciation and to simulate several paths of alteration zoning. At low fluid/rock ratios (f/r) the fluid composition is buffered by the silicate-oxide assemblage and remains close to invariant. This behavior extends to a f/r of 0.1 which exceeds the amount of exsolved magmatic fluids controlled by water solubility in silicate melts. With increasing peraluminosity of the parental granite, the Na-, K-and Fe-bearing fluids become more acidic and the oxidation state increases as a consequence of hydrogen and ferrous iron transfer to the fluid. With decreasing temperature, saline fluids become more Ca-and Na-rich, change from weakly acidic to alkaline, and become significantly more oxidizing. Large variations in Ca/Fe and Ca/Mg ratios in the fluid are a potential geothermometer. The mineral assemblage changes from cordierite-biotite granites through two-mica granites to chlorite-, epidote-and zeolite-bearing rocks. We have carried out three rock-titration simulations:

Geochimica et Cosmochimica Acta, 2012

Solubility experiments of molybdenite in single-phase, NaCl (±HCl)-bearing aqueous fluids were co... more Solubility experiments of molybdenite in single-phase, NaCl (±HCl)-bearing aqueous fluids were conducted at 600-800°C, 200 MPa and various fO 2 -fS 2 conditions imposed by mineral buffers. Small aliquots of fluids were trapped after 1-7 days of equilibration as synthetic fluid inclusions in quartz and subsequently analyzed by laser-ablation ICP MS. Measured Mo concentrations range from 20 to 3000 ppm by weight and increase with increasing temperature, NaCl concentration and oxygen fugacity, but decrease with increasing sulfur fugacity. Our solubility data can be fitted by the following equation:

Geochimica et Cosmochimica Acta, 2005

Fluorine is a common volatile element in magmatic-hydrothermal systems, but its solution mechanis... more Fluorine is a common volatile element in magmatic-hydrothermal systems, but its solution mechanisms and thermodynamic description in highly polymerized silicate melts are poorly known. We have developed a thermodynamic model for fluorosilicate liquids that links experimentally determined phase equilibria and spectroscopic information on melt structure. The model is applicable to crystallization of fluoride minerals, fluoride-silicate immiscibility in natural felsic melts, and metallurgical processes. Configurational properties of fluorosilicate melts are described by mixing on three site levels (sublattices): (1) alkali fluoride, polyhedral aluminofluoride and silicofluoride species and nonbridging terminations of the aluminosilicate network, (2) alkali-aluminate and silicate tetrahedra within the network and (3) bridging oxygen, nonbridging oxygen and terminal fluorine atoms on tetrahedral apices of the network. Abundances of individual chemical species are described by a homogeneous equilibrium representing melt depolymerization:

Geochimica et Cosmochimica Acta, 2013

The depletion of high field strength elements such as Zr, Nb and Ta is a characteristic feature o... more The depletion of high field strength elements such as Zr, Nb and Ta is a characteristic feature of arc magmas and it has been attributed to a low solubility of these elements in slab-derived aqueous fluids. We have determined zircon solubility in aqueous fluids up to 1025°C and 20 kbar by in situ observation of dissolving zircon grains in the hydrothermal diamond anvil cell. Zircon solubilities in H 2 O with silica activity buffered by quartz are very low, from 1.0 to 3.3 ppm Zr, and weakly increase with temperature and pressure. Experimental results were fitted to the following fluid density model: log c ðAE0:10Þ ¼ ð3:45 AE 0:92Þ À ð3803 AE 1098Þ T À1 þ ð1:52 AE 0:63Þ log q where c is the Zr concentration in the fluid (ppm by weight), T is temperature (K) and q is the fluid density (g cm À3 ). An additional experiment with a saline fluid (15 wt.% NaCl) revealed an increase in zircon solubility by a factor of 3 (4.8 ± 1.6 ppm Zr at 890°C and 14 kbar) whereas addition of 4.5 wt.% albite as solute increased solubility by about a factor of 5. The Zr solubility at the forsterite-enstatite silica buffer appears to be slightly higher than that at the quartz buffer and it further increases at baddeleyite saturation (48 ± 15 ppm Zr at 930°C and 16 kbar). These observations are consistent with the stability of zircon relative to ZrO 2 + SiO 2 and suggest that Zr-Si complexes are not abundant in the fluid. During slab dehydration, the Zr content in the aqueous fluid is predicted to be 1-4 ppm. Mass balance calculations imply that the high field strength element concentrations in primary arc melts will slightly decrease due to the dilution effect of infiltrating fluid. By contrast, mobile lithophile elements are predicted to increase their abundances in the melt by orders of magnitude. Our results suggest that the high abundance of large ion lithophile elements relative to high field strength elements in arc magmas is related to different solubilities of these elements in aqueous fluids migrating from the slab to the magma source regions.

Geochemistry, Geophysics, Geosystems, 2007

1] We present a thermodynamic model of high-pressure mineralogy that allows the evaluation of pha... more 1] We present a thermodynamic model of high-pressure mineralogy that allows the evaluation of phase stability and physical properties for the Earth's mantle. The thermodynamic model is built from previous assessments and experiments in the five-component CFMAS system (CaO-FeO-MgO-Al 2 O 3 -SiO 2 ), including mineral phases that occur close to typical chemical models of the mantle and reasonable mantle temperatures. In this system we have performed a system Gibbs free energy minimization, including pure end-member phases and a nonideal formulation for solid solutions. Solid solutions were subdivided into discrete pseudocompounds and treated as stoichiometric phases during computation of chemical equilibrium by the simplex method. We have complemented the thermodynamic model with a model of shear wave properties to obtain a full description of aggregate elastic properties (density, bulk, and shear moduli) that provide a useful basis for the consideration of seismic and geodynamic models of the Earth's mantle. The thermodynamic model described here is made available for research and training purposes through a Web interface (http://www.earthmodel.org). We examine its validity in light of experiments from mineral physics and briefly discuss inferences for mantle structure.

Contributions to Mineralogy and Petrology, 2004

Thermodynamic analysis of the system Na 2 O-K 2 O-CaO-Al 2 O 3 -SiO 2 -H 2 O-F 2 O )1 provides ph... more Thermodynamic analysis of the system Na 2 O-K 2 O-CaO-Al 2 O 3 -SiO 2 -H 2 O-F 2 O )1 provides phase equilibria and solidus compatibilities of rock-forming silicates and fluorides in evolved granitic systems and associated hydrothermal processes. The interaction of fluorine with aluminosilicate melts and solids corresponds to progressive fluorination of their constituent oxides by the thermodynamic component F 2 O )1 . The chemical potential l(F 2 O )1 ) buffered by reaction of the type: MO n/2 (s)+n/2 [F 2 O )1 ]=MF n (s, g) where M=K, Na, Ca, Al, Si, explains the sequential formation of fluorides: carobbiite, villiaumite, fluorite, AlF 3 , SiF 4 as well as the common coexistence of alkali-and alkaliearth fluorides with rock-forming aluminosilicates. Formation of fluorine-bearing minerals first starts in peralkaline silica-undersaturated, proceeds in peraluminous silica-oversaturated compositions and causes progressive destabilization of nepheline, albite and quartz, in favour of villiaumite, cryolite, topaz, chiolite. Additionally, it implies the increase of buffered fluorine solubilities in silicate melts or aqueous fluids from peralkaline silica-undersaturated to peraluminous silicaoversaturated environments. Subsolidus equilibria reveal several incompatibilities: (i) topaz is unstable with nepheline or villiaumite; (ii) chiolite is not compatible with albite because it only occurs only at very high F 2 O )1 levels. The stability of topaz, fluorite, cryolite and villiaumite in natural felsic systems is related to their peralkalinity (peraluminosity), calcia and silica activity, and linked by corresponding chemical potentials to rock-forming mineral buffers. Villiaumite is stable in strongly peralkaline and Ca-poor compositions (An <0.001 ). Similarly, cryolite stability requires coexistence with nearly-pure albite (An <2 ). Granitic rocks with Ca-bearing plagioclase (An >5 ) saturate with topaz or fluorite. Crystallization of topaz is restricted to peraluminous conditions, consistent with the presence of Li-micas or anhydrous aluminosilicates (cordierite, garnet, andalusite). Fluorite is predicted to be stable in peraluminous biotite granites, amphibole-, clinopyroxene-or titanite-bearing calc-alkaline suites as well as in peralkaline granitic and syenitic rocks. Fluorine concentrations in felsic melts buffered by the coexistence of F-bearing minerals and feldspars increase from peralkaline through metaluminous to mildly peraluminous compositions. At low-temperature conditions, the hydrothermal evolution of peraluminous granitic and greisen systems is controlled by white mica-feldsparfluoride equilibria. With decreasing temperature, topaz gradually breaks down via: (i) (OH)F )1 substitution and fluorine transfer to fluorite by decalcification of plagioclase below 600°C, (ii) formation of muscovite and additional fluorite at 475-315°C, and (iii) formation of paragonite and cryolite, consuming F-rich topaz and albite below 315°C. These equilibria explain the absence of magmatic fluorite in Ca-bearing topaz granitic rocks; its abundance in hydrothermal rocks is due to: (i) closedsystem defluorination of topaz, (ii) open-system decalcification of plagioclase or (iii) hydrolytic alteration. These results provide a complete framework for the investigation of fluorine-bearing mineral stabilities in felsic igneous suites.