Subsolidus and melting relationships for calcite,magnesite and the join CaCO3-MgCO3 36 kb

Subsolidus and melting relations for the CaCO₃-MgCO₃ join at 30 kb have been determined using piston-cylinder apparatus. Data are also presented for the melting curve of CaCO₃ to 30 kb, the decomposition and melting curves of MgCO₃ to 36 kb, and the calcite-aragonite transition at 800°C, 950°C and 1100°C. At 30kb, the melting loop for the CaCO₃-MgCO₃ join extends from 1610°C (CaCO₃) to 1585°C (MgCO₃) through a liquidus minimum at 1290°C (near 42 mole% MgCO₃). The dolomite-magnesite solvus barely intersects the 30 kb melting loop to produce a peritectic reaction at 1385°C. Integration of the new experimental data with other published data permits construction of a complete P-T projection and a sequence of isobars for the CaCO₃-MgCO₃ join for pressures between 5 and 30 kb. The phase relations for this join provide part of the essential framework of the model peridotite system CaO-MgO-SiO₈-CO₂-H₂O, which has particular application to the origin of carbonatitic and kimberlitic magmas. In light of the accumulating evidence for CO₂ in various forms within the upper mantle and of its effect on magmatic processes, analysis of the melting relations in this system is of considerable importance.     

Melting relationships in CaO-CO2 and MgO-CO2 to 36 kilobars with comments on CO2 in the mantle

The melting curves of CaCO₃ and MgCO₃ have been extended to pressures of 36 kb by experiments in piston-cylinder apparatus. At 30 kb, the melting temperatures of calcite and magnesite are 1610°C and 1585°C, respectively. New data for the magnesite dissociation reaction permit the location of an invariant point for the assemblage magnesite + periclase + liquid + vapor near 26 kb-1550°C. New data are also presented for the calcite-aragonite transition at 800°C, 950°C and 1100°C. At pressures above 36–50 kb, calcite and magnesite melt at temperatures lower than the solidus of dry mantle peridotite. Natural and experimental evidence suggests that carbon dioxide in the Earth's mantle could be present in a variety of forms: (a) a free vapor phase, (b) vapor dissolved in silicate magma, (c) crystalline carbonate, (d) carbonatite liquid, (e) carbon-bearing silicate analogs, or (f) carbonato-silicates (such as scapolite, spurrite, tilleyite, and related compounds).     

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

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

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

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

Carbonation and melting reactions in the system CaO–MgO–SiO2–CO2 at mantle pressures with geophysical and petrological applications

Bowen's petrogenetic grid was based initially on a series of decarbonation reactions in the system CaO-MgO-SiO₂-CO₂ with starting assemblages including calcite, dolomite, magnesite and quartz, and products including enstatite, forsterite, diopside and wollastonite. We review the positions of 14 decarbonation reactions, experimentally determined or estimated, extending the grid to mantle pressures to evaluate the effect of CO₂ on model mantle peridotite composed of forsterite(Fo)+orthopyroxene(Opx)+clinopyroxene(Cpx). Each reaction terminates at an invariant point involving a liquid, CO₂, carbonates, and silicates. The fusion curves for the mantle mineral assemblages in the presence of excess CO₂ also terminate at these invariant points. The points are connected by a series of reactions involving liquidus relationships among the carbonates and mantle silicates, at temperatures lower (1,100–1,300° C) than the silicate-CO₂ melting reactions (1,400–1,600° C). Review of experimental data in the bounding ternary systems together with preliminary data for the system CaO-MgO-SiO₂-CO₂ permits construction of a partly schematic framework for decarbonation and melting reactions at upper mantle pressures. The key to several problems in the peridotite-CO₂ subsystem is the intersection of a subsolidus carbonation reaction with a melting reaction at an invariant point near 24 kb and 1,200°C. There is an intricate series of reactions between 25 kb and 35 kb involving changes in silicate and carbonate phase fields on the CO₂-saturated liquidus surfaces. Conclusions include the following: (1) Peridotite Fo+Opx+Cpx can be carbonated with increasing pressure, or decreasing temperature, to yield Fo+Opx+Cpx+Cd (Cd=calcic dolomite), Fo+Opx+Cd, Fo+Opx+Cm (Cm=calcic magnesite), and finally Qz+Cm. (2) Free CO₂ cannot exist in subsolidus mantle peridotite with normal temperature distributions; it is stored as carbonate, Cd. (3) The CO₂ bubbles in peridotite nodules do not represent free CO₂ in mantle peridotite along normal geotherms. (4) CO₂ is as effective as H₂O in causing incipient melting, our preferred explanation for the low-velocity zone. (5) Fusion of peridotite with CO₂ at depths shallower than 80 km produces basic magmas, becoming more SiO₂-undersaturated with depth. (6) The solubility of CO₂ in mantle magmas is less than about 5 wt% at depths to 80 km, increasing abruptly to about 40 wt% at 80 km and deeper. (7) Deeper than 80 km, the first liquids produced are carbonatitic, changing towards kimberlitic and eventually, at considerably higher temperatures, to basic magmas. (8) Kimberlite and carbonatite magmas rising from the asthenosphere must evolve CO₂ at depths 100-80 km, which contributes to their explosive emplacement. (9) Fractional crystallization of CO₂-bearing SiO₂-undersaturated basic magmas at most pressures can yield residual kimberlite and carbonatite magmas.     

The alleged kimberlite-carbonatite relationship: evidence from ilmenite and spinel from Premier and Wesselton mines and the Benfontein sill,South Africa

Carbonate-rich, SiO₂-poor residua are developed in some kimberlites solidifying as ocelli, layers, or discrete dikes which satisfy petrographic definitions of carbonatite. Arguments that these rocks have mineralogies, antecedents, and comagmatic rocks differing from those of the carbonatites in alkaline rock complexes, including the specific observation that kimberlites and carbonatites contain ilmenites and spinels of different composition, have been used to refute the alleged kimberlite-carbonatite relationship. New microprobe analyses of ilmenites and spinels from carbonate-rich rocks associated with kimberlites in three South African localities correspond to spinels and ilmenites of carbonatites from alkalic complexes, or have characteristics intermediate between those of carbonatites and kimberlites. The ilmenites are distinguished from kimberlite ilmenites by higher MnO, FeTiO₃, and Nb₂O₅, and by negligible Cr₂O₃. The spinels are distinguished from kimberlite spinels by their Al₂O₃ and Cr₂O₃ contents. There is clearly a genetic relationship between the kimberlites and the carbonate-rich rocks, despite the observation that their ilmenites and spinels are distinctly different, which indicates that the same observation is not a valid argument against a petrogenetic relationship between kimberlites and carbonatites. These rocks are among the diverse products from mantle processes influenced by CO₂, and we believe that the petrogenetic links among them are forged in the upper mantle. We see insufficient justification to deny the name carbonatite to carbonate-rich rocks associated with kimberlites if they satisfy the petrographic definition in terms of major mineralogy.     

Water-saturated and undersaturated melting relations of a granite to 35 kilobars

Biotite granite from the Sierra Nevada batholith was reacted, with known water contents in sealed platinum capsules, in a piston-cylinder apparatus between 10 and 35 kb. With the liquid just over-saturated with respect to water, temperatures for solidus and liquidus (quartz/coesite-out curve), respectively, are: 2 kb, 680°C, 715°C; 10 kb, 620°C, 725°C; 25 kb, 655°C, 800°C; 35 kb, 700°C, 850°C. The temperature interval is 35°C at 2 kb, 105°C at 10 kb, and 150°C at 35 kb, indicating that granite departs from a eutectic composition at depths greater than about 40–50 km. We conclude that crystal-liquid equilibria are not likely to yield primary rhyolite or granite magmas by partial fusion of oceanic crust in subduction zones. The solubility of water in granite liquids, in wt%, is 22.5 ± 2.5 at 25 kb and 810°C and 27 ± 2.5 at 35 kb and 850°C. These results indicate that a miscibility gap persists between water-saturated silicate magmas and aqueous vapor phase at least to pressures corresponding to 100 km depth in the mantle. The formation of kyanite near the liquidus of water over-saturated granite indicates that the aqueous vapor phase is enriched in alkalis and possibly silica, relative to the condensed phases.     

Eutectic between wollastonite II and calcite contrasted with thermal barrier in MgO-SiO2-CO2 at 30 kilobars,with applications to kimberlite-carbonatite petrogenesis

In the join CaCO₃-CaSiO₃ at 30 kbars, calcite melts at 1615°C, wollastonite II at 1600°C, and a binary eutectic occurs at 1365°C with liquid composition 43 wt.% CaCO₃, 57 wt.% CaSiO₃. The eutectic liquid quenches to a glass with few quench crystals. In the join MgCO₃-MgSiO₃ at 30 kbars, magnesite melts at 1590°C, enstatite at 1837°C, and the fields for the primary crystallization of magnesite and enstatite are separated by a thermal barrier near 1900°C for the melting of forsterite in the presence of CO₂. Only about 10 wt.% MgSiO₃ dissolves in the carbonate liquid. These data, are considered together with incomplete results for joins CaMgSi₂O₆-CaMg(CO₃)₂, CaMgSi₂O₆-MgCO₃, CaMgSi₂O₆-CaCO₃, and other published data in the system CaO-MgO-SiO₂-CO₂. A thermal barrier separates the silicate and carbonate liquids in MgO-SiO₂-CO₂ but, in the quaternary system, silicate liquids with dissolved CO₂ can follow fractionation paths around the forsterite field to the fields for the primary crystallization of carbonates. This suggests that fractional crystallization of CO₂-bearing ultrabasic magma at 100 km depth can produce residual carbonatite magma.     

Dehydration-melting of solid amphibolite at 10 kbar: Textural development,liquid interconnectivity and applications to the segregation of magmas

Summary Anisotropic crystal structures and rock texture control liquid morphology and distribution during dehydration-melting at 10 kbar in solid cylinders of lineated amphibolite (mode: hornblende 70%, plagioclase 30%), sealed in gold capsules, in piston-cylinder runs ranging from 21 days at 850 °C to 4 days at 1000 °C. The shapes of most liquid pockets are crystallographically-controlled, with many corners having angles greater than 60°. Few crystal/liquid triple junctions develop the interfacial energy-controlled dihedral angles (), which form in experiments using finely-ground powders of minerals with poor cleavage. Liquid interconnectivity probably is attained at 875 °C with only 2% liquid, indicating that dihedral angles less than 60° may not be necessary to achieve interconnectivity in partially melted metamorphic rocks. The surfaces between elongated grains in lineated rocks can become pathways for the migration of liquid or the diffusion of components. By 850 °C, hornblende begins to dehydrate at internal nucleation sites, producing a texture of hornblende rims and clinopyroxene cores (generally attributed to hydration of clinopyroxene). Within the temperature interval of 850–900 °C, transient vapor generates layers of low viscosity, H₂O-saturated, granitoid liquid between hornblende and plagiocase crystal faces, potentially capable of segregation if time-temperature relationships are suitable. At higher temperatures the increased liquid fraction is H₂O-undersaturated, with viscosity too high to permit segregation. There is a prospect that segregation of initially hydrous liquids could contribute to the dehydration of low-potassium amphibolites and effectively remove incompatible trace elements during the transition from amphibolite-facies to granulite-facies. Further experiments are needed to study the effects of time and temperature on textures in anisotropic rocks, particularly lineated amphibolites.

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Dehydrations-Schmelzen von Amphiboliten bei 10 kbar: Texturelle Entwicklung, Interkonnektivität der Schmelze und Anwendungen auf die Segregation von Magmen

Zusammenfassung Die texturelle Entwicklung von festen Zylindern von Amphibolit (Hornblende 70%, Plagioklas 30%) in Goldkapseln versiegelt, wurde w:rend Dehydrations-Schmelzen bei 10 kbar in einem Piston-Zylinder-Apparat bei Temperaturen von 850°C bis 1000°C für 21 bis 4 Tage untersucht. Die anisotropen Mineralstrukturen und die Gesteinstextur kontrollieren die Morphologie und Verteilung der Schmelze. Diese Parameter sowie der Anteil an Schmelze, bestimmen die Interkonnektivität der Schmelze. Im Gegensatz zu Experimenten, die fein gemahlene Pulver von fast isotropen Mineralen (z.B. Olivin oder Quarz) benützen, scheinen hier die Energieverhältnisse der Kristallstruktur die Energiebeziehungen zwischen den Kristall-Schmelzoberflächen während der texturellen Entwicklung der amphibolitischen Gesteine zu dominieren. Wenige Kristall-Schmelze Triple-Junetions entwickeln zwischen Flächen energie-kontrollierte dihedrale Winkel (). Die Formen der meisten Schmelzeinschlüsse sind kristallographisch kontrolliert und viele Ecken zeigen Winkel, die größer als 60° sind. Die Interkonnektivität der Schmelze wird jedoch eindeutig bei 875° C mit nur 2% Schmelze erreicht und könnte möglicherweise auch bei niedrigeren Temperaturen zustande kommen. Das Vorkommen von dihedralen Winkeln, die kleiner als 60° sind, muß nicht notwendig sein, um Interkonnektivität in teilweis aufgeschmolzenen metamorphen Gesteinen zu erzeugen. Die Oberflächen zwischen gelängten Körnern in Amphiboliten mit Lineation können Wege für die Migration von Schmelzen oder für die Diffussion von Komponenten während teilweisen Aufschmelzens werden. Bei 850° C begann die Dehydration der Hornblende an internen Nukleations-Stellen, unabhängig vom Rest des Gesteins. Zwischen 850° C und 900 °C entsteht so eine Textur von Klinopyroxenen mit Hornblenderändern. Die nicht im Gleichgewicht befindliche Dampfphase, die dabei entsteht, führt zur Bildung von Lagen von wassergesättigter granitoider Schmelze zwischen Hornblende und Plagioklasflächen, mit einer berechneten Viskosität, die gerade niedrig genug ist, um Segregation durch Kompaktion zu ermöglichen. Bei höheren Temperaturen und während längerer Zeiten, wobei mehr Schmelze entsteht, löst sich die Dampfphase in wasseruntersättigter Schmelze, mit einer Viskosität, die zu hoch ist um Segregation in geologisch realistischen Zeiten zu ermöglichen. Die Entwässerung von kalium-armen Gesteinen durch Segregation von ursprünglich wässrigen Schmelzen, die sich in dieser Weise gebildet haben, dürfte beim Amphiboht-Granulit-Übergang eine Rolle spielen.[/ p]

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With 10 Figures