Activity-composition relationship in Tschermak's substituted Fe biotites at 700°C, 2 kbar

The reaction-displacement technique was applied to the end-member reaction annite = sanidine + magnetite + H₂ in order to determine the activity of the annite component (a _Ann) in iron biotites with variable degrees of the Tschermak's substitution (^[6]Fe + ^[4]Si = ^[6]Al + ^[4]Al). Based on the simplified relation a _Ann = f _{H 2}/foH₂ (foH₂ = hydrogen fugacity of the end-member reaction at P, T), two types of experiments were performed at 700°C / 2 kbar: Type I used Fe-Al biotites of known starting composition together with sanidine + magnetite + H₂O. This assemblage was exposed to various f _{H 2} conditions (f _{H 2} < foH₂) produced in the pressure vessel either by using different ratios of water/oil as pressure medium (f _{H 2} in this case was measured by the hydrogen sensor technique), or by the Ni′NiO buffer. The composition of the Fe-Al biotites changed through incorporation or release of the annite component in response to the externally imposed f _{H 2}. By using opposite biotite starting compositions, the equilibrium composition as a function of f _H2 was bracketed. For type II, f _{H 2} in equilibrium with a specific combination of fine-grained Fe-Al biotite (+ sanidine + magnetite + H₂O) was measured internally by application of the hydrogen sensor technique. Both type I and type II experiments yield consistent results demonstrating that a fine-grained assemblage of Fe-Al biotite (+ sanidine + magnetite + H₂O) is able to act as a sliding-scale buffer. The final chemical composition of the Fe-Al biotite after the experiments was determined by electron microprobe and Mössbauer spectroscopy. The ^[4]Al and ^[6]Al in the biotites are coupled according to the Tschermak's substitution. In the tetrahedral sheet 0.1 Al-atoms per formula unit are present in excess to the amount required to balance ^[6]Al, and all Fe-Al biotites contain 8–10% Fe³⁺. Therefore, they are not members of the pure annite - siderophyllite join, but have an almost constant amount (15 Mol%) of two additional Fe³⁺-bearing components (ferri-siderophyllite and a vacancy end-member). The volume - composition relationship obtained does not indicate excess molar volumes of mixing for the annite (Ann) - siderophyllite (Sid) binary. The data are consistent with a molar volume of annite of 15.46 ± 0.02 Jbar^–1 and of 15.06 ± 0.02 Jbar^–1 for siderophyllite. The experimentally determined activity - composition relation shows that biotites on the join annite - siderophyllite deviate negatively from ideality. A symmetric interaction parameter W_AnnSid is sufficient to represent the data within error. This was constrained as: W _AnnSid = –29 ± 4 kJmol^–1. This is in contradiction to empirical interaction parameters derived from natural assemblages for this binary that predict positive deviation from ideality. Reasons for this discrepancy are discussed.     

Annite stability revised. 1. Hydrogen-sensor data for the reaction annite = sanidine + magnetite + H2

In P - T - logfO₂ space, the stability of annite (ideally KFe ₃ ²⁺ (OH)₂AlSi₃O₁₀) at high fO₂ (low fH₂) is limited by the reaction: annite = sanidine + magnetite + H₂. Using the hydrogen-sensor technique, the equilibrium fH₂ of this reaction was measured between 500 and 800° C at 2.8 kbar in 50° C intervals. Microbrobe analyses of the reacted annite+sanidine+magnetite mixtures show that tetrahedral positions of annite have a lower Si/Al ratio than the ideal value of 3/1. Silicon decreases from 2.9 per formula unit at low temperatures to 2.76 at high temperatures. As determined by Mössbauer spectroscopy in three experimental runs, the Fe³⁺ content of annite in the equilibrium assemblage is 11%±3. A least squares fit to the hydrogensensor data gives H _R ⁰ = 50.269 ± 3.987 kJ and S _R ⁰ = 83.01 ± 4.35 J/K for equilibrium (1). The hydrogene-sensor data are consistent with temperature half brackets determined in the classical way along the nickel-nickel oxide (NNO) and quartz-fayalite-magnetite (QFM) buffers with a mixture of annite+sanidine+magnetite for control. Compared to published oxygen buffer reversals, agreement is only found at high temperature and possible reasons for that discrepancy are discussed. The resulting slope of equilibrium (1) in logfO₂ – T dimensions is considerably steeper than previously determined and between 400 and 800°C only intersects with the QFM buffer curve. Based on the hydrogen-sensor data and on the thermodynamic dataset of Berman (1988, and TWEEQ data base) for sanidine, magnetite and H₂, the deduced standard-state properties of annite are: H _f ⁰ =-5127.376±5.279 kJ and S ⁰=422.84±5.29 J/(mol K). From the recently published unit cell refinements of annites and their Fe³⁺ contents, determined by Mössbauer spectroscopy (Redhammer et al. 1993), the molar volume of pure annite was constrained as 15.568±0.030 J/bar. A revised stability field for annite is presented, calculated between 400 and 800°C.     

The upper thermal stability of chlorite + quartz: an experimental study in the system MgO-Al2O3-SiO2-H2O

Experiments up to water pressures of 21 kbar have been undertaken to bracket the reactions chlorite + quartz = talc + kyanite + H₂O, chlorite + quartz = talc + cordierite + H₂O, and talc + kyanite + quartz = cordierite ± H₂O by reversed runs in the system MgO-Al₂O₃-SiO₂-H₂O (MASH). These reaction curves intersect at an invariant point (IP1) at PH₂O = 6.4 ± 0.2 kbar and a temperature of 624 ± 4°C. The curve of the chlorite + quartz breakdown to talc + kyanite + H₂O at water pressures above 6.4 kbar shows a negative dP/dT, with the slope decreasing with rising pressure, whereas the slope of the breakdown curve to talc + cordierite + H₂O at water pressures is clearly positive. The composition of the chlorite solid solution reacting with quartz has been estimated to be approximately Mg_4.85Al_1.15[Al_1.15Si_2.85O₁₀](OH)₈ over the entire pressure range investigated. The composition of the talc solid solution forming by the breakdown of chlorite + quartz appears to be Mg_2.94Al_0.06[Al_0.06Si_3.94O₁₀](OH)₂ at PH₂O = 2kbar. With increasing pressure, the Al content of talc decreases, reaching a value of about 0.06 atoms per formula unit at P,H₂O = 21 kbar. As a consequence of the new experimental data, the existing phase topologies of the MASH-system and K₂O-MASH-system have been revised. For example, the invariant point IP1 and the univariant reaction curve kyanite + talc + H₂O = chlorite + cordierite are stable. For this reason, the development of medium- to high-temperature metamorphic rocks compositionally approximating the MASH-system must be reconsidered. The whiteschists from Sar e Sang, Afghanistan, are treated as an example. The application of the present experimental data to metamorphic rocks of more normal composition requires the examination of the influence of further components. This leads to the conclusion that the introduction of Fe²⁺ into magnesian chlorite extends its stability field in the presence of quartz by 10°-15°C in comparison with pure Mg-chlorite.     

Determination of the first ionization constant of silicic acid from quartz solubility in borate buffer solutions to 350°C

The solubility of quartz has been determined in borax buffer solutions having total boron concentrations of 0.10, 0.20, 0.40 and 0.60 mol kg^?1 and over the temperature range 130–350°C at the saturated vapour pressure of the system. The first ionization constant of silicic acid was calculated from the solubility data and varied from ?logK₁ = 8.88 (± 0.15) at 130°C to ?logK₁ = 10.06 (± 0.20) at 350°C. The solubility of quartz in these solutions was due to the presence of the three species, H₄SiO₄, H₃SiO₄^? and NaH₃SiO₄°. The equilibrium constant for the reaction, Na⁺ + H₃SiO₄^? = NaH₃SiO₄° extended from log K_as = 1.18?1.40 (± 0.20) over the temperature interval 135–301°C. The formation of NaH₃SiO₄° ion pairs was concluded to contribute significantly to the solubility of quartz in alkaline hydrothermal solutions when pH > 8 and sodium concentration exceeds 0.10 mol kg^?1.     

Stability of phlogopite-quartz and sanidine-quartz: A model for melting in the lower crust

The melting of phlogopite-quartz and sanidine-quartz under vapor-absent conditions and in the presence of H₂O-CO₂ vapor have been determined from 5–20 kbar. In the lower crust (P=6–10 kbar), phlogopite + quartz melts incongruently to enstatite + liquid at temperatures as low as 710° C in the presence of H₂O. When the activity of water is sufficiently reduced by addition of CO₂, phlogopite + quartz undergoes a dehydration reaction to enstatite + sanidine + vapor, for example at 790±10° C, 5 kbar, with \(X_{H_2 O}^V\) =0.35. In the absence of vapor, phlogopite + quartz is stable up to a maximum temperature of 900° C in the crust; at higher temperatures this assemblage melts incongruently to enstatite + sanidine + liquid. The melting of sanidine-quartz in the presence of H₂O-CO₂ vapor shows marked topological differences from melting in the system albite-H₂O-CO₂, and as a result, apparent activity coefficients for water calculated from sanidine-quartz H₂O-CO₂ are less than those calculated from albite-H₂O-CO₂ by up to a factor of five. These data shed light on anatexis in the lower crust, but uncertainties related to ordering of Al and Si in natural and synthetic micas forestall a more rigorous analysis. Nevertheless, maximum temperatures for some granulite terranes can be established.     

Inverted high-temperature quartz

Fifty-two samples of inverted high-temperature quartz from volcanic rocks were investigated by Guinier-Jago powder diffractometry and differential scanning calorimetry (DSC). Quartz megacrysts from Clear Lake and Cinder Cone, California show a variability of ?2.5 ° K in their α-β transition temperature (T _α-β). Quartz phenocrysts and quartz from crystalline rocks give a range of 0.5 ° K in T _α-β. Neutron activation analysis of single crystals demonstrates that Al is the principal impurity (17–380 ppm). Its concentration is inversely correlated with T _α-β. A very small variation was found in the a and c lattice parameters among the specimens of volcanic quartz studied. This variation does not correlate with Al content or transition temperature. Mean values at 22 ° C (a=4.1934±0.0004 Å, c=5.4046±0.0006 Å) are similar to those of quartz grown at low temperatures. Enthalpy of the α-β transition (ΔH _α-β), obtained over 9.0 ° from DSC runs, is dependent upon sample grain size and for a crushed powder with zero hysteresis (T _α-β on heating=T _α-β on cooling) is 92.0 ±1.4 cal/mol. In contrast, a single piece of quartz requires ΔH _α-β be 107.7±1.4 cal/mol and has a T _α-β hysteresis of 1.1 ° K. Regression of published data provides equations for the variation of the molar volume (cc/mol) of quartz with v. These equations imply a ΔV _α-β of 0.205±0.031 cc/- mol. Expressions are also provided for the temperature dependence of the thermal coefficient of expansion, α, the compressibility, β, and (?/gb/?T)_p (which is identically -(?α/?P) _T ). DSC heat capacity measurements over the range 400 to 900 ° K were fitted to extended Maier-Kelley type expressions to give: $$\begin{gathered} C_P = 10.31 + 9.116 \times 10^{ - 3} T - \frac{{1.812 \times 10^5 }}{{T^2 }} \hfill \\ - {\text{5}}{\text{.630}} \times 10^{ - 2} {\text{ }}\frac{T}{{(T - 848)}} - 0.3553\frac{T}{{(T - 848)^2 }} \hfill \\ - 0.9011\frac{T}{{\left( {T - 848} \right)^3 }} \hfill \\ (400{\text{ to 842}}^ \circ {\text{K), and}} \hfill \\ C_P = - 318.8 + 0.2532T \hfill \\ {\text{ + }}\frac{{8.687 \times 10^7 }}{{T^2 }} + 0.1603\frac{T}{{\left( {T - 848} \right)^4 }} \hfill \\ \end{gathered} $$ (851 to 900 ° K), which together with the values of ΔH _α?β measured over the range 842–851° K give 7875.3 cal/mol for H₉₀₀-H₄₀₀. The behavior of α, β, and C _p as a function of T emphasizes that structural changes which occur at the α?β transition do so over a broad temperature interval.     

Experimental investigation and application of garnet granulite equilibria

Two mineralogic geobarometers based on the assemblages olivine-plagioclase-garnet and orthopyroxeneplagioclase-garnet-quartz have been calibrated from the reaction (1) fayalite+anorthite?garnet (Gr₁Alm₂). The reaction boundary has been determined to within 0.2 kbar using piston-cylinder apparatus. It is located at 4.7, 5.1, 5.5, 5.8, 6.2, 6.6, and 7.0 kbar at 750, 800, 850, 900, 950, 1,000, and 1,050° C, respectively. Summation of ΔG for reaction (1) and fayalite +quartz?ferrosilite locates to within 0.3 kbar the following model garnet-forming reaction for quartz-saturated granulites: (2) ferrosilite+ anorthite?garnet(Gr₁ Alm₂) + quartz. Geobarometers based on (1) and (2) are widely applicable in granulite terranes and yield precise pressures that are in agreement with other well-calibrated barometers. Pressures of 7–10 kbar are inferred for many granulite terranes requiring the widespread development of 60–70 km thick continental crust by mid-Proterozoic.     

Internally consistent recalibrations of mineral equilibria for geothermobarometry involving garnet–orthopyroxene–plagioclase–quartz assemblages and their application to the South Indian granulites

Abstract Three reactions are calibrated as geothermobarometers for garnet–orthopyroxene–plagioclase–quartz assemblages, namely: 1/2 ferrosilite + 1/3 pyrope ± 1/2 enstatite + 1/3 almandine (A): ferrosilite + anorthite ± 2/3 almandine + 1/3 grossularite + quartz (B); and enstatite + anorthite ± 2/3 pyrope + 1/3 grossularite + quartz (C). The internally consistent geothermobarometers based on reactions (A), (B) and (C) are calibrated from experimental data only. The thermodynamic parameters of reaction (A) are derived from published experimental data in the FMAS system (n= 104) in the range 700–1400°C and 5–50 kbar, while those for reaction (B) are derived by summation of the existing reversed experimental data of the mineral equilibria: ferrosilite ± fayalite + quartz (D) and anorthite + fayalite ± 2/3 almandine + 1/3 grossularite (E). The retrieved thermodynamic parameters for reactions (A), (B) and (C) are, respectively: (ΔH⁰, cal) -3367 ± 209, -2749 ± 350 and +3985 ± 545; (ΔS⁰, cal K^?1) -1.634 ± 0.163, -8.644 ± 0.298 and -5.376 ± 0.391; and (ΔV⁰_1,298, cal bar^?1) -0.024, -0.60946 and -0.5614. On a one-cation basis, the derived Margules parameters of the ternary Ca–Fe–Mg in garnet are: W_Fe–Mg= -1256 + 1.0 (～0.23) T(K), W_Mg–Fe= 2880 -1.7 (～0.13) T(K), W_Ca–Mg= 4047 (～77) -1.5 T(K), W_Mg–Ca= 1000 (～77) -1.5 T(K), W_Ca–Fe= -723 + 0.332 (～0.02) T(K), W_Fe–Ca= 1090, (cal) and the ternary constant C₁₂₃= -4498 + 1.516 (～0.265) T(K) cal (subregular solution model of non-ideal mixing); and Fe–Mg–Al in orthopyroxene: W_Fe–Mg= 948 (～200) -0.34 (～0.10) T(K), W_Fe–Al= -1950 (～500) and W_Mg–Al= 0 (cal) (regular solution model of non-ideal mixing). The anorthite activity in plagioclase is calculated by the ‘Al-avoidance’model of subregular Ca–Na mixing commonly used for geobarometry based on reactions (B) and (C). When the geothermobarometers are applied to garnet–orthopyroxene–plagioclase–quartz assemblages (n= 45) of wide compositional range from the Precambrian South Indian granulites, temperature ranges of 690–860°C (X= 760 ± 45°C) and pressure ranges of 5–10 kbar were obtained. The P–T values were estimated simultaneously and there is no difference in the pressure calculated from P_Mg (reaction C) and P_Fe (reaction B). In the existing calibrations this difference is 1 kbar or more. Furthermore, there is no compositional dependence of the ln K of the experimental data in the FMAS (n= 104) and the CFMAS (n= 78) systems at different temperatures and the estimated temperatures of the South Indian granulites.     

Phase relations to 3 kbar in the systems edenite + H2O and edenite + excess quartz + H2O

Amphiboles approached edenite (NaCa₂Mg₅Si₇AlO₂₂(OH)₂), richterite (Na₂CaMg₅Si₈O₂₂(OH)₂), tremolite (□Ca₂Mg₅Si₈O₂₂(OH)₂) solid solutions were studied by conventional hydrothermal techniques employing the bulk compositions edenite, and edenite + additional quartz, all with excess H₂O. For the stoichiometric edenite bulk composition + excess H₂O, the equilibrium phase assemblage is diopside + Na-phlogopite + forsterite + fluid at, and just above the amphibole high-temperature limit at 850 ± 5°C, 500 bar, and 880 ± 5°C, 1000 bar. The breakdown temperature of sodic phlogopite is 855 ± 3°C at 500 bar, and 890 ± 5°C at 700 bar, producing nepheline + plagioclase (or melt), additional forsterite and fluid. Diopside and Na-phlogopite solid solution coexist over a broad P_fluid-T region, even within the amphibole field, where they are associated with an edenite-richterite (-tremolite) solid solution of approximate composition Ed₃₅Rc₅₀Tr₁₅.In the system edenite + 4 quartz + excess H₂O, nearly pure tremolite and albite coexist stably between 670° and 830°C at 1000 bar and give way to the possibly metastable assemblage diopside + talc + albite below 670°C. In the presence of albite, tremolite reacts to produce diopside + quartz + enstatite + fluid above 830°C at 1000 bar. For the investigated silica-rich bulk composition, amphibole P_fluid-T stability is divided by the albite melting curve into a tremolite + albite field, and a tremolite + aqueous melt field. Substantial equilibrium solid solution of tremolite towards edenite or richterite was not observed for silica-excess bulk compositions. Metastable edenite-rich amphiboles initially synthesized change to tremolite with increasing run length in the presence of free SiO₂.Edenitic amphibole is stable only over a very limited temperature range in silica-undersaturated environments, thus accounting for its rarity in nature. Na-phlogopite solid solutions are also disfavored by high a_SiO2; even for nepheline-normative lithologies, a hypothesized rapid low-temperature conversion to vermiculite or smectite could partly explain the scarcity of sodic phlogopite in rocks.     

Separate or shared metamorphic histories of eclogites and surrounding rocks? An example from the Bohemian Massif

Eclogite boudins occur within an orthogneiss sheet enclosed in a Barrovian metapelite‐dominated volcano‐sedimentary sequence within the Velké Vrbno unit, NE Bohemian Massif. A metamorphic and lithological break defines the base of the eclogite‐bearing orthogneiss nappe, with a structurally lower sequence without eclogite exposed in a tectonic window. The typical assemblage of the structurally upper metapelites is garnet–staurolite–kyanite–biotite–plagioclase–muscovite–quartz–ilmenite ± rutile ± silli‐manite and prograde‐zoned garnet includes chloritoid–chlorite–paragonite–margarite, staurolite–chlorite–paragonite–margarite and kyanite–chlorite–rutile. In pseudosection modelling in the system Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O (NCKFMASH) using THERMOCALC, the prograde path crosses the discontinuous reaction chloritoid + margarite = chlorite + garnet + staurolite + paragonite (with muscovite + quartz + H₂O) at 9.5 kbar and 570 °C and the metamorphic peak is reached at 11 kbar and 640 °C. Decompression through about 7 kbar is indicated by sillimanite and biotite growing at the expense of garnet. In the tectonic window, the structurally lower metapelites (garnet–staurolite–biotite–muscovite–quartz ± plagioclase ± sillimanite ± kyanite) and amphibolites (garnet–amphibole–plagioclase ± epidote) indicate a metamorphic peak of 10 kbar at 620 °C and 11 kbar and 610–660 °C, respectively, that is consistent with the other metapelites. The eclogites are composed of garnet, omphacite relicts (jadeite = 33%) within plagioclase–clinopyroxene symplectites, epidote and late amphibole–plagioclase domains. Garnet commonly includes rutile–quartz–epidote ± clinopyroxene (jadeite = 43%) ± magnetite ± amphibole and its growth zoning is compatible in the pseudosection with burial under H₂O‐undersaturated conditions to 18 kbar and 680 °C. Plagioclase + amphibole replaces garnet within foliated boudin margins and results in the assemblage epidote–amphibole–plagioclase indicating that decompression occurred under decreasing temperature into garnet‐free epidote–amphibolite facies conditions. The prograde path of eclogites and metapelites up to the metamorphic peak cannot be shared, being along different geothermal gradients, of about 11 and 17 °C km^?1, respectively, to metamorphic pressure peaks that are 6–7 kbar apart. The eclogite–orthogneiss sheet docked with metapelites at about 11 kbar and 650 °C, and from this depth the exhumation of the pile is shared.     

Dehydration melting of tonalites. Part I. Beginning of melting

 The beginning of dehydration melting in the tonalite system (biotite-plagioclase-quartz) is investigated in the pressure range of 2–12 kbar. A special method consisting of surrounding a crystal of natural plagioclase (An₄₅) with a biotite-quartz mixture, and observing reactions at the plagioclase margin was employed for precise determination of the solidus for dehydration melting. The beginning of dehydration melting was worked out at 5 kbar for a range of compositions of biotite varying from iron-free phlogopite to iron-rich Ann₇₀, with and without titanium, fluorine and extra aluminium in the biotite. The dehydration melting of phlogopite + plagioclase (An₄₅) + quartz begins between 750 and 770°C at pressures of 2 and 5 kbar, at approximately 740°C at 8 kbar and between 700 and 730°C at 10 kbar. At 12 kbar, the first melts are observed at temperatures as low as 700°C. The data indicate an almost vertical dehydration melting solidus curve at low pressures which bends backward to lower temperatures at higher pressures (> 5 kbar). The new phases observed at pressures ≤ 10 kbar are melt + enstatite + clinopyroxene + potassium feldspar ± amphibole. In addition to these, zoisite was also observed at 12 kbar. With increasing temperature, phlogopite becomes enriched in aluminium and deficient in potassium. Substitution of octahedral magnesium by aluminium and titanium in the phlogopite, as well as substitution of hydroxyl by fluorine, have little effect on the beginning of dehydration melting temperatures in this system. The dehydration melting of biotite (Ann₅₀) + plagioclase (An₄₅) + quartz begins 50°C below that of phlogopite bearing starting composition. Solid reaction products are orthopyroxene + clinopyroxene + potassium feldspar ± amphibole. Epidote was also observed above 8 kbar, and garnet at 12 kbar (750°C). The experiments on the iron-bearing system performed at ≤ 5 kbar were buffered with NiNiO. The f _{O 2} in high pressure runs lies close to CoCoO. With the substitution of octahedral magnesium and iron by aluminium and titanium, and replacement of hydroxyl by fluorine in biotite, the beginning of dehydration melting temperatures in this system increase up to 780°C at 5 kbar, which is 70°C above the beginning of dehydration melting of the assemblage containing biotite (Ann₅₀) of ideal composition. The dehydration melting at 5 kbar in the more iron-rich Ann₇₀-bearing starting composition begins at 730°C, and in the Ann₂₅-bearing assemblage at 710°C. This indicates that quartz-biotite-plagioclase assemblages with intermediate compositions of biotite (Ann₂₅ and Ann₅₀) melt at lower temperatures as compared to those containing Fe-richer or Mg-richer biotites. This study shows that the dehydration melting of tonalites may begin at considerably lower temperatures than previously thought, especially at high pressures (>5 kbar). Received: 27 December 1995 / Accepted: 7 May 1996     

Solubility and complexing of Ni in the system NiO-H2O-HCl

The solubility of bunsenite (NiO) in Cl-bearing fluids in the range of 450°–700°C, 1–2 kbar was determined using the Ag + AgCl acid buffer technique. Based on the results of the experiments, it is concluded that the associated NiCl⁰₂ complex is the dominant Ni species in the fluid over the entire temperature-pressure range investigated. The temperature dependence of the equilibrium constant for the reaction NiO(s) + 2HCl⁰(aq) = NiCl⁰₂(aq) + H₂O is given by logK = ?4.17(±0.55) + 4629(±464)/T(K) at 1 kbar, and logK = ?4.75(±0.91) + 5933(±756)/T(K) at 2 kbar. The calculated difference in standard state Gibbs free energy of formation between NiCl⁰₂ and 2HCl⁰ in kcal is G⁰(NiCl⁰₂) ? 2G⁰(HCl⁰) = ?20.77(±2.22) + 0.03264(±0.0026)T(K), at 1 kbar and G⁰(NiCl⁰₂) ? 2G⁰(HCl⁰) = ?25.01(±1.35) + 0.03264(±0.0016)T(K) at 2 kbar. Comparison of the solubilities of Ni end-member minerals with those of Ca, Mn, Fe, and Mg indicates that nickel minerals generally are the least soluble at a given temperature and pressure. The relatively low solubility of Ni end-member minerals, combined with the relatively low concentration of Ni in most rocks, should result in a quite low mobility of Ni in hydrothermal fluids.     

Experimental determination of the solubility of the assemblage paragonite,albite, and quartz in supercritical H2O

The concentrations of Na, Al, and Si in an aqueous fluid in equilibrium with natural albite, paragonite, and quartz have been measured between 350°C and 500°C and 1 to 2.5 kbar. Si is the dominant solute in solution and is near values reported for quartz solubility in pure H₂O. At 1 kbar the concentrations of Na and Al remain fairly constant from 350°C to 425°C but then decrease at 450°C. At 2 kbar, Na increases slightly with increasing temperature while Al remains nearly constant. Concentrations of Si, Na, and Al all increase with increasing pressure at constant temperature.The molality of Al is close to that of Na and is nearly a log unit greater than calculated molalities assuming Al(OH)⁰₃ is the dominant Al species. This indicates a Na-Al complex is the dominant Al species in solution as shown by Anderson and Burnham (1983) at higher temperature and pressure. The complex can be written as NaAl(OH)⁰₄ ± nSiO₂ where n is the number of Si atoms in the complex. The value of n is not well constrained but appears to be less than or equal to 3.The results indicate Al can be readily transported in pure H₂O solutions at temperatures and pressures as low as 350°C and 1 kbar.     

Zur Stabilität von Margarit im System CaO-Al2O3-SiO2-H2O

An increasing number of occurrences of margarite have been reported in the last years. However, previous experimental investigations in the system CaO-Al₂O₃-SiO₂-H₂O are limited to the synthesis of margarite and to the upper stability limit according to the reaction (1) 1 margarite?1 anorthite +1 corundum +1 H₂O (Chatterjee, 1971; Velde, 1971). Since margarite often occurs together with quartz, the upper stability limit of margarite in the presence of quartz is of special interest. Therefore, the reactions (5) 1 margarite +1 quartz ?1anorthite +1 kyanite/andalusite +1 H₂O and (6) 4 margarite+3 quartz ? 2 zoisite+5 kyanite+3 H₂O were investigated experimentally using mixtures of natural margarite (from Chester, Mass., USA), quartz, kyanite, andalusite, zoisite, and synthetic anorthite. The indicated equilibrium temperatures at water pressures equal to total pressure are: 515± 25°C at 4 kb, 545 ±15°C at 5 kb, 590±10°C at 7 kb, and 650±10°C at 9 kb for reaction (5), and 651±11°C at 10 kb, 648 ± 8°C at 12.5kb, and 643±13°C at 15kb for reaction (6), respectively. Besides this, additional brackets for equilibrium temperatures were determined for the above cited reaction (1): 520±10°C at 3 kb, 580±10°C at 5 kb, and 640± 20°C at 7 kb. On the basis of these experimentally determined reactions (1), (5), and (6) and of the reactions (3) 2 zoisite +1 kyanite? 4 anorthite +1 corundum +1 H₂O (7) 2 zoisite +1 kyanite +1 quartz ? 4 anorthite +1 H₂O and (10) 1 pyrophyllite ? 1 andalusite/kyanite+3 quartz+1 H₂O for which experimental or, in the case of reaction (3), calculated data were already available, a pressure-temperature diagram with 3 invariant points and 11 univariant reactions was developed using the method of Schreinemakers. This diagram, summarizing both experimental and phase relation studies, allows conclusions about the conditions under which margarite has been formed in nature. Margarite is limited to low grade metamorphism at water pressures up to approximately 3.5 kb; in the presence of quartz, margarite is even limited to low grade metamorphism at water pressures up to 5.5 kb. Only at water pressures higher than the values stated before margarite, and margarite+quartz, respectively, can occur in medium grade metamorphism (as defined by Winkler, 1970 and 1973). For the combined occurrence of margarite+quartz and staurolite as reported by Harder (1956) and Frey (personal communication, 1973) it may be estimated that water pressure has been greater than approximately 5.5 kb, wheras temperature has been in the range from 550 to 650°C. Furthermore, the present study shows that the assemblage zoisite+kyanite (+ H₂O) is an indicator of both pressure [P _{H 2 O}> approximately 9kb]and temperature [T> approximately 640 to 650° Cat water Pressures up to 15 kb].     

Enthalpy of formation of Fe3Al2Si3O12 (almandine) by high temperature alkali borate solution calorimetry

A high-temperature solution calorimetric method suitable for thermochemical studies of anhydrous minerals containing Fe²⁺ ions has been developed. The method is based on an oxide melt solvent with 52 wt% LiBO₂ and 48 wt% NaBO₂ maintained at a temperature of 750°C. In a first application of this method the enthalpies of solution of synthetic almandine, fayalite, a mixture of fayalite plus quartz on FeSiO₃ composition, and natural quartz were measured. For the reaction:

the enthalpy change at 1023 K is ?3.82 ± 0.87 kcal, based on fayalite, quartz, corundum and almandine, and ?5.96 ± 0.90 kcal based on the fayalite plus quartz mixture, corundum, and almandine. These values lead to standard molar enthalpies of formation of almandine from the oxides at 1023 K of ?14.10 ± 1.22 kcal and ?16.24 ± 1.74 kcal, respectively. The measured enthalpy of formation of almandine is less negative by several kilocalories than values derived from analysis of the phase equilibrium work of Hsu (1968), but in closer agreement with the phase equilibrium study of O'Neill and Wood (1979) and similar to the phase equilibrium deduction of Froese (1973).The agreement of the present almandine enthalpy of formation with O'Neill and Wood (1979) and Froese (1973) suggests that almandine entropies at 298 K to be obtained from their studies, in the range 79–81 cal/K, are more nearly correct than the several estimates based on oxide sum and volume-entropy systematics, most of which are much lower.     

The thermodynamic properties and phase relations of some minerals in the system CaO-AI2O3-SiO2-H2O

The heat capacities of lawsonite, margante, prehnite and zoisite have been measured from 5 to 350 K with an adiabatic-shield calorimeter and from 320 to 999.9 K with a differential-scanning calorimeter. At 298.15 K, their heat capacities, corrected to end-member compositions, are 66.35, 77.30, 79.13 and 83.84 cal K^?1 mol^?1; their entropies are 54.98, 63.01, 69.97 and 70.71 cal K^?1 mol^?1, respectively. Their high-temperature heat capacities are described by the following equations (in calories, K, mol): Lawsonite (298–600 K): Cp° = 66.28 + 55.95 × 10^?3T ? 15.27 × 10⁵T^?2 Margarite (298–1000 K): Cp° = 101.83 + 24.17 × 10^?3T ? 30.24 × 10⁵T^?2 Prehnite (298–800 K): Cp° = 97.04 + 29.99 × 10^?3T ? 25.02 × 10⁵T^?2 Zoisite (298–730 K): Cp° = 98.92 + 36.36 × 10^?3T ? 24.08 × 10⁵T^?2 Calculated Clapeyron slopes for univariant equilibria in the CaO-Al₂O₃-SiO₂-H₂O system compare well with experimental results in most cases. However, the reaction zoisite + quartz = anorthite + grossular + H₂O and some reactions involving prehnite or margarite show disagreements between the experimentally determined and the calculated slopes which may possibly be due to disorder in experimental run products. A phase diagram, calculated from the measured thermodynamic values in conjunction with selected experimental results places strict limits on the stabilities of prehnite and assemblages such as prehnite + aragonite, grossular + lawsonite, grossular + quartz, zoisite + quartz, and zoisite + kyanite + quartz. The presence of this last assemblage in eclogites indicates that they were formed at moderate to high water pressure.     

Experimental study of subsolidus phase relations and mixing properties of pyroxene in the system CaO-Al2O3-SiO2

Subsolidus phase relations in the system CaO-Al₂O₃-SiO₂ (CAS) were experimentally determined with tight reversals of several univariant curves and with 14 equilibration experiments containing the assemblage pyroxene + anorthite, where pyroxene is a binary solid solution of Ca-Tschermak (CaTs-CaAl₂SiO₆) and Ca-Eskola (CaEs-Ca_0.5AlSi₂O₆) endmembers.Reversals were obtained on the following reactions (bar, °C): 3An = Gr + 2Ky + Q (P = 22T ? 700), 3An + Cor = Gr + 3Ky (P = 21.8T ? 950), 3CaTs= Gr + 2Cor(P = 55T ? 53900), and 6CaTs_{(1 ? x)}CaEs_x = 2(1 ? 2x)Gr + 4(1 ? 2x)Cor + 9xAn. Observed slopes indicate 9.8 J/mol · K of Al-Si disorder in Ca-Tschermak pyroxene and 5.3 J/mol·K of Al-Si disorder in anorthite, at 1300°C. It is suggested that Al-Si disorder in anorthite increases by 1.9 J/mol · K from 700°C to 1300°C.Compositions of CaTs-CaEs pyroxene in equilibrium with anorthite and PbO-rich liquid were experimentally determined at 1400–1430°C and 22.7–30.8 kbar. Microprobe measurements gave compositions which are consistent with an ideal pyroxene solution and the following parameters for the reaction 3An = 2CaTs + 2CaEs (J, bar, K): 2RTln(X_CaTs · X_CaEs) + 60200 + 86.4T ? (5.06 + 13 × 10^?7P)P = 0, resulting in ΔH⁰_j = ?39.8 kJ/mol and S⁰ = 461.8 J/mol · K for the Ca-Eskola endmember at 1300°C. The obtained properties of the Ca-Eskola component are necessary for thermobarometry based on pyroxene bearing assemblages containing plagioclase, quartz, or kyanite.     

Thermodynamic properties of Pd chloride complexes and the Pd2+(aq) ion in aqueous solutions

Based on the expert review of literature data on the thermodynamic properties of species in the Cl-Pd system, stepwise and overall stability constants are recommended for species of the composition [PdCl _n ]^{2 ? n} , and the standard electrode potential of the half-cell PdCl ₄ ^2? /Pd(c) is evaluated at E _298,15° = 0.646 ± 0.007 V, which corresponds to Δ _f G _298.15° = ?400.4 ± 1.4 kJ/mol for the ion PdCl ₄ ^2? (aq). Derived from calorimetric data, Δ _f H _298.15° PdCl ₄ ^2? (aq) = ?524.6 ± 1.6 kJ/mol and Δ _f H _298.15° Pd²⁺(aq) = 189.7 ± 2.6 kJ/mol. The assumed values of the overall stability constant of the PdCl ₄ ^2? ion and the standard electrode potential of the PdCl ₄ ^2? /Pd(c) half-cell correspond to Δ _f G _298.15° = 190.1 ± 1.4 kJ/mol and S _298.15° = ?94.2 ± 10 J/(mol K) for the Pd²⁺(aq) ion.     

Experimental determination of portlandite and brucite solubilities in supercritical H2O

Experimentally reversed portlandite and brucite solubilities were determined between 300° and 600°C and 1 to 3 kbar. In the portlandite runs the molality of Ca decreases with increasing pressure at constant temperature. For instance, at 2 kbar log molalities at 300°, 400°, 500° and 600°C give values of −2.34, −2.71, −3.18 and −4.18, respectively. At 500°C, pressures of 1, 2 and 3 kbar yield values of −4.40, −3.18 and −2.65. Distribution of species in solution can be calculated with the aid of data from Helgeson and co-workers assuming Ca⁺⁺ is the dominant Ca species. These calculated Ca concentrations are within ± 0.2 log units of experimental values in most cases. The solubility reaction is, in all likelihood: 2H⁺ + Ca(OH)₂ ↔a3 Ca⁺⁺ + 2H₂O.Although the computed pH's are close to 2 units greater than neutral, the solutions apparently contained no significant Ca(OH)⁺ or Ca(OH)_2sq.Concentrations of Mg in the brucite runs show a sigmoidal behavior at 2 kbar as a function of temperature with log molalities of Mg of −4.00, −4.28, −4.14 and −4.60 at 350°, 450°, 550° and 600°C, respectively. Values at 1 kbar are lower and decrease monotonically from 350° to 550°C. Based on available thermodynamic data for Mg⁺⁺ it appears that Mg(OH)⁺ is the dominant Mg species in solution. The solubility reaction is proposed to be: H⁺ + Mg(OH)₂↔a3 Mg(OH)⁺ + H₂O.With the aid of data of Helgeson and co-workers values of the equilibrium constant for H₂O + Mg⁺⁺ ↔a3 Mg(OH)⁺ + H⁺ necessary to account for the measured solution compositions can be calculated. These calculations indicate Mg(OH)⁺ becomes dominant at temperatures above 450°C at 2 kbar and above 360°C at 1 kbar at neutral pH.     

Metamorphic equilibria in the siliceous dolomite system: 6 kbar experimental data and geologic implications

Hydrothermal experiments with H₂O-CO₂ fluids at P_fluid = 6 kbar yielded the following quilibrium conditions for reactions important in metamorphosed siliceous dolomites (T = °C; X = X_co2): (3) dolomite + 2 quartz = diopside + 2 CO₂T = 620 ± 8X = 0.73 ± 0.03 (5) 5 dolomite + 8 quartz + H₂O = tremolite + 3 calcite + 7 CO₂T = 600 ± 5 550 ±5 540±5 500±5X = 0.66 ± 0.03 0.21 ± 0.03 0.21 ± 0.04 0.06 ± 0.02 (7) 3 dolomite + 4 quartz + H₂O = talc + 3 calcite + 3 CO₂T = 550±5 500±5 450 ±5X = 0.25 ± 0.05 0.07 ± 0.02 0.03 ± 0.02 (8) 2 dolomite + talc + 4 quartz = tremolite + 4 CO₂T = 550 ± 5 540 ±5 500 ± 5X = 0.22 ± 0.03 0.21 ± 0.02 0.06 ± 0.02 A thermodynamically self-consistent 6 kbar T-X_CO2, topology results by extrapolating equilibria from experimental brackets using a modified Redlich-Kwong equation for activities in H₂O-CO₂ mixtures. This topology restricts the assemblage talc + calcite to a narrow stability band in T-X_CO2 space at X_CO2 < 0.55 and T < 590°C. Accordingly, the occurrence of talc + calcite in pure siliceous dolomites metamorphosed at P_fluid = 6 kbar implies correspondingly water-rich fluids.