Melting and subsolidus reactions in the system K2O-CaO-Al2O3-SiO2-H2O: corrections and additional experimental data

Melting relationships in the system K₂O-CaO-Al₂O₃-SiO₂-H₂O have been reinvestigated using Schreinemakers analysis and hydrothermal experiments. The reaction sanidine+muscovite+zoisite+quartz+vapor =melt has been bracketed at 10, 15, and 20 kbars and 670–680, 680–690, and 690–700° C, respectively and it marks the lowest solidus temperatures in the system investigated.Below 10 kbars, experimental data on the beginning of melting in zoisite- or muscovite-bearing anorthite+sanidine assemblages have been obtained, which are not showing any differences and therefore point to melt compositions close to the feldspar-quartz join.     

Beginning of melting in the granite system Qz-Or-Ab-An-H2O

The beginning of melting in the system Qz-Or-Ab-An-H₂ O was experimentally reversed in the pressure range kbar using starting materials made up of mixtures of quartz and synthetic feldspars. With increasing pressure the melting temperature decreases from 690° C at 2 kbar to 630° C at 17 kbar in the An-free alkalifeldspar granite system Qz-Or-Ab-H₂O. In the granite system Qz-Or-Ab-An-H₂O the increase of the solidus temperature with increasing An-content is only very small. In comparison to the alkalifeldspar granite system the solidus temperature increases by 3° C (7° C) if albite is replaced by plagioclase An 20 (An 40). The difference between the solidus temperatures of the alkalifeldspar granite system and of quartz — anorthite — sanidine assemblages (system Qz-Or-An-H₂O) is approximately 50° C. With increasing water pressures plagioclase and plagioclase-alkalifeldspar assemblages become unstable and are replaced by zoisite+kyanite+quartz and zoisite+muscovite-paragonite_ss +quartz, respectively. The pressure stability limits of these assemblages are found to lie between 6 and 16 kbar at 600° C. At high water pressures (10–18 kbar) zoisite — muscovite — quartz assemblages are stable up to 700 and 720° C. The solidus curve of this assemblage is 10–20° C above the beginning of melting of sanidine — zoisite — muscovite — quartz mixtures. The amount of water necessary to produce sufficient amounts of melt to change a metamorphic rock into a magmatic looking one is only small. In case of layered migmatites it is shown that 1 % of water (or even less) is sufficient to transform portions of a gneiss into (magmatic looking) leucosomes. High grade metamorphic rocks were probably relatively dry, and anatectic magmas of granitic or granodioritic composition are usually not saturated with water.     

Experimental studies on the formation of inclusions in plagioclases from metatonalites,Hohe Tauern,Austria (lower temperature stability limit of the paragenesis anorthite plus potash feldspar)

New experimental data are presented at stability conditions of paragenesis in the system K₂O-CaO-Al₂O₃-SiO₂-H₂O. These results are used to estimate the pressure temperature conditions under which minute inclusions, mostly consisting of zoisite/clinozoisite and muscovite, have crystallized in calcic plagioclases from metatonalites and metadiorites (Hohe Tauern, Austria). In the pressure region 1.5–8 kb the following reactions were observed: zoisite+muscovite+quartz=anorthite+potash feldspar+water (1) grossularite+muscovite+quartz=anorthite+potash feldspar+water (2) zoisite+quartz=anorthite+grossularite+water (3) natural plagioclase with its inclusions (zoisite/clinozoisite and muscovite) (4) =more basic plagioclase without inclusions.In order to determine the curves of reaction (1), (2) and (3), runs were made in hydrothermal bombs using synthetic phases crystallized from gels as starting materials. The reaction curves (1), (2) and (3) intersect at an invariant point at 7.25±0.5 kb and 685±20° C. In runs to define the reaction (4), it could be demonstrated that the inclusion minerals zoisite/ clinozoisite and muscovite became instable at slightly lower temperatures than those occurring in reaction (1). These facts illustrate that the reaction curve (1), found in the pure system, gives possible information about the pressure temperature conditions during the formation of the inclusions.     

Phase relations of a simplified Marly rock system with application to the Western Hohe Tauern (Austria)

The regional distribution of metamorphic mineral assemblages in Mesozoic carbonate rocks of the Western Hohe Tauern allows the mapping of isograds based on the appearance of biotite+calcite and biotite+zoisite+calcite. The latter isograd corresponds approximately to the thermal maximum of the alpidic metamorphism in the central part of this area. An estimate of P, T, X _fluid conditions can be obtained from phase relations among muscovite, biotite, chlorite, margarite, tremolite, zoisite, anorthite, quartz, calcite, and dolomite in the system K₂O-CaO-MgO-Al₂O₃-SiO₂-H₂O-CO₂ which approximates the composition of marls. Calculations based on various experimental and thermodynamic data have been made with emphasis on phase relations pertinent to a group of carbonate rocks with very low Fe and Na contents in non-opaque minerals. Significant and opposite deviations from the phase relations for stochiometric end member mineral compositions are due to the substitutions F-OH and Mg+Si-2Al. Consistency of observed and calculated phase relations is favoured by high F-contents. For the majority of carbonate rocks in the high metamorphic zone, maximum temperatures around 550° C, minimum pressures of 4–6 kb, and relatively low XCO₂ values within the stability field of zoisite and of biotite+calcite+quartz are indicated.     

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].     

The influence of the fluid phase composition on the solidus temperatures in the haplogranite system NaAlSi3O8-KAlSi3O8-SiO2-H2O-CO2

The solidus temperatures in the haplogranite-system NaAlSi₃O₈-KAlSi₃O₈-SiO₂-H₂O-CO₂ have been determined up to 15 kbar for a constant molar ratio of sodium to potassium of 11 and for fluid compositions ranging from pure water to pure carbon dioxide. The data for the water-saturated solidus are virtually identical with those of previous studies. At constant pressure, the solidus curve as a function of the fluid phase composition exhibits a point of inflection in the range of the water-rich compositions. This phenomenon is attributed to chemical interactions between the CO₂ and the H₂O in the silicate melt. The point of inflection disappears if the CO₂ in the gas phase is replaced by molecular nitrogen. The CO₂-saturated solidi have been measured at 2 and 5 kbars. The data at 5 kbar indicate a melting point depression in the order of 40° C compared to the dry solidus of Huang and Wyllie (1975). The experimental data can be used to estimate the melting temperatures of common quartz and feldspar bearing crustal rocks under the conditions of granulite facies metamorphism. Since for most fluid phase compositions, the solidus curves are very steep in the P, T-diagram, the beginning of melting is nearly exclusively determined by the fluid composition and almost independent of pressure between about 2 and more than 10 kbar. Therefore, the onset of partial melting in quartz and feldspar containing rocks under granulite facies conditions can be used to estimate the composition of a coexisting H₂O-CO₂ fluid phase if geothermometric data are available. The temperature range between the beginning of granulite facies metamorphism and the initiation of melting expands with increasing carbon dioxide content in the H₂O-CO₂ fluid phase. At a CO₂ molar fraction of 0.9, this range extends from about 600° C to 900° C and is almost independent of pressure.     

P CO2 in low-grade metamorphism; zeolite,carbonate, clay mineral,prehnite relations in the system CaO-Al2O3-SiO2-CO2-H2O

Equilibria for several reactions in the system CaO-Al₂O₃-SiO₂-CO₂-H₂O have been calculated from the reactions calcite+quartz=wollastonite+CO₂ (5) and calcite+Al₂SiO₅+quartz=anorthite+CO₂ (19) and other published experimental studies of equilibria in the systems Al₂O₃-SiO₂-H₂O and CaO-Al₂O₃-SiO₂-H₂O.The calculations indicate that the reactions laumontite+CO₂=calcite+kaolinite+2 quartz+2H₂O (1) and laumontite+calcite=prehnite+quartz+3H₂O+CO₂ (3) in the system CaO-Al₂O₃-SiO₂-CO₂-H₂O, are in equilibrium with an H₂O-CO₂ fluid phase having -0.0075 for P _fluid=P _total=2000 bars.These calculations limit the stability of zeolite assemblages to low p CO₂.Using the above reactions as model equilibria, several probelms of p CO₂ in low grade metamorphism are discussed. (a) the problem of producing zeolitic minerals from metasedimentary assemblages of carbonate, clay mineral, quartz. (b) the significance of calcite (or aragonite) associated with zeolite (or lawsonite) in low grade metamorphism and hydrothermal alteration. (c) the reaction of zeolites (or lawsonite) with calcite (or aragonite) to produce dense Ca-Al-hydrosilicates (eg. prehnite, zoisite, grossular).     

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

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

Phase relationships in the system KAlSiO4-Mg2SiO4-SiO2-H2O as a model for hybridization between hydrous siliceous melts and peridotite

The system KAlSiO₄-Mg₂SiO₄-SiO₂-H₂O includes model representatives of (1) hydrous siliceous magma from subducted oceanic crust — the eutectic liquid in KAlSi₃O₈-SiO₂-H₂O, and (2) the overlying mantle peridotite — the assemblage forsterite+enstatite (Fo+En). In a series of partly schematic isobaric isothermal sections, the products of hybridization between the model materials at pressures between 20 and 30 kbar have been determined. The liquid dissolves peridotite components with little change in composition. Hybridization is not a simple mixing process, because of the incongruent melting of peridotitic assemblages with phlogopite (Ph). Hybridization causes solidification of the liquid, with products a sequence of three mineral assemblages: Ph, Ph+quartz (Qz), and Ph+En. The products represent an absolute geochemical separation and local concentration of all potassium from the liquid. Hybridization is accompanied by H₂O-saturation of melts, and evolution of aqueous fluid. Although there are significant differences between the melt composition and that of the magma rising from subducted oceanic slab, and between Fo+En and the mantle rock, extrapolation of the results suggests that the conclusions can probably be extended to mantle conditions with sodium in the melt, and jadeitic clinopyroxene included in the hybrid products.     

Carbon in silicate liquids: the systems NaAlSi3O8-CO2, CaAl2Si2O8-CO2, and KAlSi3O8-CO2

To further our knowledge of the effects of volatile components on phase relationships in aluminosilicate systems, we determined the vapor saturated solidi of albite, anorthite, and sanidine in the presence of CO₂ vapor. The depression of the temperature of the solidus of albite by CO₂ decreases from 30° C at 10 kbar, to 10° C at 20 kbar, to about 0 at 25 kbar, suggesting that the solubility of CO₂ in NaAlSi₃O₈ liquid in equilibrium with solid albite decreases with increasing pressure and temperature. In contrast, CO₂ lowers the temperature of the solidus of anorthite by 30° C at 14 kbar, and by 70dg C at 25 kbar. This contrasting behavior of albite and anorthite is also reflected in the behavior of melting in the absence of volatile components. Whereas albite melts congruently to a liquid of NaAl-Si₃O₈ composition to pressures of 35 kbar, anorthite melts congruently to only about 10 kbar and, at higher pressures, incongruently to corundum plus a liquid that is enriched in SiO₂ and CaO and depleted in Al₂O₃ relative to CaAl₂Si₂O₈.The tendency toward incongruent melting with increasing pressure in albite and anorthite produces an increase in the activity of SiO₂ component in the liquid ( ). We predict that this increases the ratio of molecular CO₂/CO ₃ ^2– in these liquids, but the experimental results from other workers are mutually contradictory. Because of the positive dP/dT of the albite solidus and the negative dP/dT of the anorthite solidus, we propose that a negative temperature derivative of the solubility of molecular CO₂ in plagioclase liquids may partly explain the decrease in solubility of carbon with increasing pressure in near-solidus NaAlSi₃O₈ liquids, which is in contrast to that in CaAl₂Si₂O₈ liquid. Also, reaction of CO₂ with NaAlSi₃O₈ liquid to form CO ₃ ^2– that is complexed with Na⁺ must be accompanied by a change in Al³⁺ from network-former to network-modifier, as Na⁺ is no longer abailable to charge-balance Al³⁺ in a network-forming role. However, when anorthite melts incongruently to corundum plus a CaO-rich liquid, the complexing of CO ₃ ^2– with the excess Ca²⁺ in the liquid does not require a change in the structural role of aluminum, and it may be more energetically favorable.The depression of the temperature of the solidus of sanidine resulting from the addition of CO₂ increases from 50° C at 5 kbar to 170° C at 15 kbar. In marked contrast to the plagioclase feldspars, sanidine melts incongruently to leucite plus a SiO₂-rich liquid up to the singular point at 15 kbar. Above this pressure, sanidine melts congruently, resulting in a decrease in the with increasing pressure in the interval up to 15 kbar. Above this pressure, the congruent melting of sanidine results in a lower and nearly constant relative to those of albite and anorthite, and CO₂ produces a nearly constant freezing-point depression of about 170° C. Because of the low at pressures above the singular point, we infer that most of the carbon dissolves as CO ₃ ^2– , resulting in a low CO₂/ CO ₃ ^2– , but a high total carbon content.The principles derived from the studies of phase equilibria in these chemically simple systems provide some information on the structural and thermal properties of magmas. We propose that the is an important parameter in controlling the speciation of carbon in these feldspathic liquids, but it certainly is not the only factor, and it may be relatively less significant in more complex compositions. In addition, our phase-equilibria approach does not provide direct thermal and structural information as do calorimetry and spectroscopy, but the latter have been used primarily on glasses (quenched liquids) and cannot be used in situ to derive direct information on liquids at elevated pressures, as can our method. Hopefully, the results of all of these approaches can be integrated to yield useful results.Institute of Geophysics and Planetary Physics, Contribution No. 2744     

Stability of the assemblage orthopyroxene-sillimanite-quartz in the system MgO-FeO-Fe2O3-Al2O3-SiO2-H2O

The stability of coexisting orthopyroxene, sillimanite and quartz and the composition of orthopyroxene in this assemblage has been determined in the system MgO-FeO-Fe₂O₃-Al₂O₃-SiO₂-H₂O as a function of pressure, mainly at 1,000° C, and at oxygen fugacities defined mostly by the hematite-magnetite buffer. The upper stability of the assemblage is terminated at 17 kbars, 1,000° C, by the reaction opx+Al-silicate gar+qz, proceeding toward lower pressures with increasing Fe/(Fe+Mg) ratio in the system. The lower stability is controlled by the reaction opx+sill+qz cord, which occurs at 11 kbars in the iron-free system but is lowered to 9 kbars with increasing Fe/(Fe+Mg). Spinel solid solutions are stabilized, besides quartz, up to 14 kbars in favour of garnet in the iron-rich part of the system (Fe/(Fe+Mg)0.30). Ferric-ferrous ratios in orthopyroxene are increasing with increasing ferro-magnesian ratio. At least part of the generally observed increase in Al content with Fe²⁺ in orthopyroxene is not due to an increased solubility of the MgAlAlSiO₆ component but rather of a MgFe³⁺AlSiO₆ component. The data permit an estimate of oxygen fugacity from the composition of orthopyroxene in coexistence with sillimanite and quartz.     

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.     

Compositions of aqueous fluids in equilibrium with pyroxenes and olivines at mantle pressures and temperatures

Solubility experiments were performed at 30 kbars in the system Mg₂SiO₄-SiO₂-H₂O, and at 20 and 30 kbars on omphacitic pyroxene-water mixtures. They confirm that the solubility of the forsterite component in aqueous fluids remains rather low (up to 5 wt.%), whereas the solubility of the SiO₂ component from solids of appropriate SiO₂-rich compositions in the system Mg₂SiO₄-SiO₂-H₂O increases with temperature up to some 75% at 1,100° C. At this temperature a simplified harzburgite consisting of forsterite and enstatite coexists with a fluid containing about 35% (MgO+SiO₂). Hydrous fluids coexisting with omphacitic clinopyroxenes leach sodium silicate component from the solid leaving less jadeitic pyroxenes behind. Most interestingly, the amount of sodium leached at constant temperature increases with decreasing pressure.Comparison of the results with previous solubility studies in the system K₂O-MgO-Al₂O₃-SiO₂-H₂O indicates that hydrous fluids in the mantle must be alkaline rather than silicanormative. Alkali metasomatism caused by such fluids would lead to potassium enrichment in deeper portions of the upper mantle and to sodium enrichment at shallower levels, where amphiboles become stable. This K/Na fractionation in the upper mantle may explain the generation of K-rich or of Na-rich magmas through partial melting at different depths.     

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

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

Synchrotron radiation study on the high-pressure and high-temperature phase relations of KAlSi3O8

In situ X-ray diffraction study on KAlSi₃O₈ has been performed using the cubic type high pressure apparatus, MAX90, combined with synchrotron radiation. We determined the phase relations of sanidine, the wadeite-type K₂Si₄O₉+kyanite (Al₂SiO₅)+coesite (SiO₂) assemblage, and hollandite-type KAlSi₃O₈, including melting temperatures of potassic phases, up to 11 GPa. Our data on subsolidus phase boundaries are close to the recent data of Yagi and Akaogi (1991). Melting relations of sanidine are consistent with the low pressure data of Lindsley (1966). The breakdown of sanidine into three phases reduces melting temperature, and wadeite-type K₂Si₄O₉ melts first around 1500° C in three phase coexisting region. Melting point of hollandite-type KAlSi₃O₈ is between 1700° C and 1800° C at 11 GPa. If these potassic phases host potassium in the earth's mantle, the true mantle solidus temperature will be much lower than the reported dry solidus temperature of peridotite.     

Melting and subsolidus reactions in the system K2O−CaO−MgO−Al2O3−SiO2−H2O: experiments and petrologic application

Experiments with synthetic starting materials of muscovite, phlogopite, zoisite, kyanite and quartz were performed in the pressure temperature range 10–25 kbar, 640–780° C under water excess conditions. The reaction muscovite+zoisite+quartz+vapor=liquid+kyanite was bracketed at 10.5 kbar/689–700° C, 15.5 kbar/709–731° C and 20 kbar/734–745° C. The equivalent reaction in the Mg-bearing system muscovite_ss +zoisite+quartz+vapor=liquid+kyanite+phlogopite_ss lies at the same temperature around 10 kbar and approximately 10° C higher around 20 kbar, compared with the Mg-free reaction. At slightly higher temperatures formation of melt and tremolite_ss was reversibly observed from the assemblage phlogopite_ss+zoisite +kyanite+quartz around 10.5 kbar/690–710° C, 15.5 kbar/720–750° C and 20.5 kbar/745–760° C. In the subsolidus region, the reaction muscovite_ss+talc_ss+ tremolite_ss=phlogopite_ss+zoisite+quartz+vapor were located in the range 700° C/16.7–19.0 kbar and 740° C/19.7–20.8 kbar. From these data, a wedge shaped stability field of phlogopite_ss+zoisite+quartz appears with a high P, T termination around 21 kbar/755° C. Muscovite+tremolite+talc or kyanite comes in at higher pressures. These phase relations are in qualitative accord with petrographic observations from high pressure metamorphic areas. Formation and crystallization of melts in rocks of a wide compositional range involving zoisite/epidote has been ascribed to relatively high pressures and is consistent with experimentally determined stability fields in the simplified KCMASH system.     

Theoretical phase relations involving cordierite and garnet in the system MgO-FeO-Al2O3-SiO2

Theoretical stability relations have been derived between the phases cordierite (Cd), garnet (Ga), hypersthene (Hy), olivine (Ol), sapphirine (Sa), spinel (Sp), sillimanite (Si) and quartz (Qz) in the system MgO-FeO-Al₂O₃-SiO₂. Natural rock data and experimental evidence suggest that the Mg/Mg+Fe²⁺ ratio (X) of coexisting ferromagnesian phases decreases as follows: X _Cd>X _Sa>X _Hy>X _Ol>X _Sp>X _Ga. By use of this information four stable invariant points are proposed involving the phases: Cd, Hy, Sa, Ga, Si, Qz; Cd, Sa, Ga, Sp, Si, Qz; Cd, Hy, Sa, Ga, Sp, Qz; Cd, Ga, Hy, Ol, Sp, Qz. All univariant curves in the system are nonterminal, representing the breakdown of a join rather than the stability limit of an individual phase. A detailed treatment of divariant equilibria involving two and three ferromagnesian solid solutions illustrates the potential of these equilibria as Pressure-Temperature indicators. Interactions between solid-solid reactions and dehydration reactions involving biotite in the system MgO-FeO-Al₂O₃-SiO₂-K₂O-H₂O have been graphically analysed. The addition of biotite to anhydrous divariant assemblages does not affect the composition of coexisting phases at constant P and T but can affect their relative proportions.     

Beginning of melting and composition of first melts in the system Qz-Ab-Or-H2O-CO2

Solidus temperatures have been determined for minimum melt compositions in the system Qz(SiO₂)-Ab(NaAlSi₃O₈)-Or(KAlSi₃O₈) at P(fluid)=2,5 and 10 kbar and at various water activities. The dry solidus was investigated in a dry argon environment. Water activities (aH₂O) between 0.0 and 1.0 were obtained by using H₂O-CO₂ mixtures. The Or/Ab+Or ratio of first melts increases considerably with decreasing water activity. At 10 kbar it is 0.28 in the water-saturated system and 0.56 at water activity 0.1. The Qz-content does not change with changing water activities. The Ab-content of minimum melts formed at high pressures and low aH₂O may remain almost constant in ascending magmas that are cooling and crystallizing. Qz-content increases at the expense of the Or-component. Solidus temperatures decrease considerably when aH₂O increases slightly from zero. At 10 kbar, the temperature difference between dry melting and the solidus for aH₂O=0.1 is 120°C. The influence of pure CO₂ on the solidus is very limited in the investigated P-T range. The solidus is approximatively 760°C at aH₂O=0.5 between 2 and 10 kbar and approximatively 830°C at aH₂O=0.3. This means that melting of quartz-feldspar assemblages may induce dehydration reactions at P-T conditions of the granulite facies.     

Metastable melting in the granite system Qz-Or-Ab-An-H2O

Time studies were performed in the quinary system Qz-Or-Ab-An-H₂O at kbars and T=665 ° and 660 ° C. Starting material was a mixture of quartz, alkali feldspar Or₈₀ and plagioclase An₃₁. The compositions of plagioclases of run products were determined and compared with the plagioclase of stable solidus conditions.The solidus of the granite system was fixed at P _HäO=5 kbars using various plagioclase — and appropriate alkali feldspar — compositions besides quartz in the starting mixture (Fig. 1).The results of time studies (Table 3 and Fig. 3) reveal metastable melting in the granite system Qz-Or-Ab-An-H₂O. Plagioclase melts almost stoichiometrically. The new plagioclase compositions formed during melting of cotectic compositions approach the theoretically expected stable plagioclase compositions only extremely slowly. An extrapolation of the data achieved in run times of 5–1,500 h indicates attainment of equilibrium after 10¹⁴ years. Metastable melting of granitic compositions is not only considered as an experimental problem but also as a rock forming process in nature.     

Ephesite,Na(LiAl2) [Al2Si2O10] (OH)2: II. Thermodynamic analysis of its stability and compatibility relations,and its geological occurrences

The trioctahedral mica ephesite, Na(LiAl₂) [Al₂Si₂O₁₀] (OH)₂, has a large -T stability field in the quaternary system NaAlSiO₄-LiAlSiO₄-Al₂O₃-H₂O. At temperatures below 400–500° C it coexists with diaspore, while at higher temperatures it occurs with corundum, until it decomposes to nepheline +eucryptite+corundum+H₂O at 600–800° C (Fig. 1). Nature faithfully reflects these phase relations; ephesite is found to coexist with diaspore or corundum in silicadeficient metamorphosed rocks or in hydrothermally altered nepheline-syenite pegmatite.Thermodynamic analysis of phase relations of ephesite in the silica saturated portion of the quinary system NaAlSiO₄-LiAlSiO₄-Al₂O₃-SiO₂-H₂O shows that the assemblage quartz+ephesite is always metastable with respect to paragonite+spodumene or paragonite+petalite at temperatures down to approximately 300° C (Fig. 3). At lower temperatures, a number of other phases like bikitaite, cookeite, Na-montmorillonite, and analcime are stabilized. Stability and compatibility relations involving these phases are presently not amenable to thermodynamic treatment due to lack of suitable data. Nevertheless, the absence of the assemblage quartz+ephesite in nature seems to vindicate our conclusion that it is metastable down to at least 300° C.The frequently encountered assemblage quartzspodumene (or petalite)-microcline-albite of some lithium pegmatites contains muscovite (±lepidolite), rather than paragonite. The absence of paragonite in such rocks is best explained by the inherent metastability of the phase-pair paragonite+microcline with respect to muscovite+albite. The pegmatite bulk compositions plot in the four-phase field spodumene (petalite)-microcline-muscovite-albite, cutting out paragonite from the observed assemblage Thus, absence of paragonite-spodumene or paragonitepetalite in nature reflects lack of suitable bulk compositions in rocks.