Anorthite Diopside Research Articles

Structure of hydrous aluminosilicate glasses along the diopside–anorthite join: A comprehensive one- and two-dimensional 1H and 27Al NMR study

We have taken a systematic approach utilizing advanced solid-state NMR techniques to gain new insights into the controversial issue concerning the dissolution mechanisms of water in aluminosilicate melts (glasses). A series of quenched anhydrous and hydrous (∼2 wt% H 2O) glass samples along the diopside (Di, CaMgSi 2O 6)—anorthite (An, CaAl 2Si 2O 8) join with varying An components (0, 20, 38, 60, 80, and 100 mol %) have been studied. A variety of NMR techniques, including one-dimensional (1D) 1H and 27Al MAS NMR, and 27Al → 1H cross-polarization (CP) MAS NMR, as well as two-dimensional (2D) 1H double-quantum (DQ) MAS NMR, 27Al triple-quantum (3Q) MAS NMR, and 27Al → 1H heteronuclear correlation NMR (HETCOR) and 3QMAS/HETCOR NMR, have been applied. These data revealed the presence of SiOH, free OH ((Ca,Mg)OH) and AlOH species in the hydrous glasses, with the last mostly interconnected with Si and residing in the more polymerized parts of the structure. Thus, there are no fundamental differences in water dissolution mechanisms for Al-free and Al-bearing silicate melts (glasses), both involving two competing processes: the formation of SiOH/AlOH that is accompanied by the depolymerization of the network structure, and the formation of free OH that has an opposite effect. The latter is more important for depolymerized compositions corresponding to mafic and ultramafic magmas. Aluminum is dominantly present in four coordination (Al IV), but a small amount of five-coordinate Al (Al V) is also observed in all the anhydrous and hydrous glasses. Furthermore, six-coordinate Al (Al VI) is also present in most of the hydrous glasses. As Al of higher coordinations are favored by high pressure, Al VIOH and Al VOH may become major water species at higher pressures corresponding to those of the Earth’s mantle.

Low temperature production of glass ceramics in the anorthite–diopside system via sintering and crystallization of glass powder compacts

The production of glass ceramics (GCs) with theoretical anorthite–diopside (An–Di) weight ratios of 60/40, 50/50 and 45/55 via sintering and crystallization of glass powder compacts was investigated at different temperatures between 800 and 950 °C. The investigated compositions are located in the cross-section of the ternary fluorapatite–An–Di system close to An–Di binary joint, with constant fluorapatite content of 4.8 wt.%. Two different groups of glass powders, with mean particle size of 2 and 10 μm, were used. The experimental results showed that sintering is almost complete at 800 °C, preceding crystallization, which takes place via surface crystallization mechanism. The properties values of the produced GCs, which are the best for the composition close to An–Di eutectic line, are discussed with respect to the evolution of crystalline phases and the microstructure over increasing firing temperature. Under the technology perspective, the investigated processing route is significantly superior in comparison to the attempts reported in earlier studies.

Creep and strain-dependent microstructures of synthetic anorthite–diopside aggregates

We investigated plastic deformation of fine-grained synthetic diopside and two-phase anorthite–diopside aggregates in triaxial compression and torsion (up to γ ∼ 5). Temperature, confining pressure and stress ranged between 950–1180°C, 200–400 MPa and 5–500 MPa, respectively. Water content of samples ranged between ∼0.005 ± 0.002 and 0.075 ± 0.025 wt% H 2O. All samples deformed in linear-viscous creep with a stress exponent of 1.0 ± 0.2. The activation energy ranged between 571 ± 53 and 290 ± 28 kJ/mol, depending on mineralogy and water content. Sample strength depended on water fugacity with an exponent of 1.55 ± 0.25. Samples deformed in torsion and coaxial compression gave similar flow laws, in spite of significant differences in the corresponding microstructures. Scanning and transmission electron microscopy of two-phase samples deformed in torsion showed phase mixing, cavitation and dislocation processes. We suggest that linear-viscous creep of fine-grained two-phase aggregates involved grain boundary sliding accommodated by grain boundary diffusion and significant dislocation accommodation at high strains. We also observed that cavity coalescence and microcracking led to sample failure. Hence, dynamic instabilities may exist in high-strain shear zones accommodating viscous deformation in the lower continental crust.

Temperature independent thermal expansivities of calcium aluminosilicate melts between 1150 and 1973 K in the system anorthite–wollastonite–gehlenite (An–Wo–Geh): A density model

The thermal expansivities of 10 compositions from within the anorthite–wollastonite–gehlenite (An–Wo–Geh) compatibility triangle have been investigated using a combination of calorimetry and dilatometry on the glassy and liquid samples. The volumes at room temperature were derived from densities measured using the Archimedean buoyancy method. For each sample, density was measured at 298 K using glass that had a cooling-heating history of 10–10 K min −1. The thermal expansion coefficient of the glass from 298 K to the glass transition interval was measured by a dilatometer and the heat capacity was measured using a differential scanning calorimeter from 298 to 1135 K. The thermal expansion coefficient and the heat flow were determined at a heating rate of 10 K min −1 on glasses which were previously cooled at 10 K min −1. Supercooled liquid density, molar volume and molar thermal expansivities were indirectly determined by combining differential scanning calorimetric and dilatometric measurements assuming that the kinetics of enthalpy and shear relaxation are equivalent. The data obtained on supercooled liquids were compared to high-temperature predictions from the models of (Lange, R.A., Carmichael, I.S.E., 1987. Densities of Na 2O–K 2O–CaO–MgO–FeO–Fe 2O 3–Al 2O 3–TiO 2–SiO 2 liquids: New measurements and derived partial molar properties. Geochim. Cosmochim. Acta 51, 2931–2946; Courtial, P., Dingwell, D.B., 1995. Nonlinear composition dependence of molar volume of melts in the CaO–Al 2O 3–SiO 2 system. Geochim. Cosmochim. Acta 59 (18), 3685–3695; Lange, R.A., 1997. A revised model for the density and thermal expansivity of K 2O–Na 2O–CaO–MgO–Al 2O 3–SiO 2 liquids from 700 to 1900 K: extension to crustal magmatic temperatures. Contrib. Mineral. Petrol. 130, 1–11). The best linear fit combines the supercooled liquid data presented in this study and the high temperature data calculated using the Courtial and Dingwell (1995) model. This dilatometric/calorimetric method of determining supercooled liquid molar thermal expansivity greatly increases the temperature range accessible for thermal expansion. It represents a substantial increase in precision and understanding of the thermodynamics of calcium aluminosilicate melts. This enhanced precision demonstrates clearly the temperature independence of the melt expansions in the An–Wo–Geh system. This contrasts strongly with observations for neighboring system such as anorthite–diopside and raises the question of the compositional/structural origins of temperature dependence of thermal expansivity in multicomponent silicate melts.

Temperature-dependent thermal expansivities of multicomponent natural melts between 993 and 1803 K

The thermal expansivities of three multicomponent glasses and liquids have been obtained over a large temperature interval (298 – 1803 K) which combine the results of low and high temperature measurements. The sample compositions investigated were derived from three natural lavas; Vesuvius 1631 eruption, Etna 1992 eruption and an Oligocene–Miocene lava flow from Slapany in the Bohemian massif. The original rocks are tephriphonolite, trachybasalt and basanite, respectively. The density values of the glassy samples were derived from dilatometic measurements of each sample after cooling at 5 K min − 1 at 298 K, followed by measurements of the glass thermal expansion coefficient from 298 K to the samples' respective glass transition interval. Supercooled liquid volumes and thermal molar expansivities were determined by combining scanning calorimetric and dilatometric measurements, assuming that the kinetics of enthalpy and shear relaxation are equivalent [Webb, S.L., 1992. Shear, volume, enthalpy and structural relaxation in silicate melts. Chem. Geol. 96, 449–457.]. Thermal molar expansivity at supercooled liquid temperature varies from 16.86 ± 0.48 × 10 − 4 cm 3 mol − 1 K − 1 for basalt/basanite, to 18.99 ± 0.48 × 10 − 4 cm 3 mol − 1 K − 1 for trachybasalt, and 20.98 ± 0.62 × 10 − 4 cm 3 mol − 1 K − 1 for tephriphonolite. High temperature densities were measured using Pt double bob Archimedean densitometry. Across the super-liquidus temperature interval investigated, the densities range from 2.655 ± 0.002 to 2.708 ± 0.012 g cm − 3 for basalt–basanite, from 2.578 ± 0.003 to 2.601 ± 0.002 g cm − 3 for trachybasalt and 2.458 ± 0.006 from 2.467 ± 0.002 g cm − 3 to tephriphonolite. In addition, the oxidation state of iron was analyzed using a wet chemistry method. The measured high temperature densities have been compared with predicted densities across the same temperature interval calculated using the multicomponent density models of Lange and Carmichael [Lange, R.A., Carmichael, I.S.E., 1987. Densities of Na 2O–K 2O–CaO–MgO–FeO–Fe 2O 3–Al 2O 3–TiO 2–SiO 2 liquids: new measurements and derived partial molar properties. Geochim. Cosmochim. Acta 51, 2931–2946.] and Lange [Lange, R.A., 1997. A revised model for the density and thermal expansivity of K 2O–Na 2O–CaO–MgO–Al 2O 3–SiO 2 liquids from 700 to 1900 K: extension to crustal magmatic temperatures. Contrib. Mineral. Petrol. 130, 1–11]. The resulting data for volumes near glass transition temperature (993 – 1010 K) and at super-liquidus temperature (1512 – 1803 K) are combined to yield temperature-dependent thermal expansivities over the entire supercooled and stable liquid range. These results confirm the observation of Knoche et al. [Knoche, R., Dingwell, D.B., Webb, S.L., 1992a. Non-linear temperature dependence of liquid volumes in the system albite–anorthite–diopside. Contrib. Mineral. Petrol. 111, 61–73], Knoche et al. [Knoche, R., Dingwell, D.B., Wegg, S.L., 1992b. Temperature-dependent thermal expansivities of silicate melts: the system anorthite–diopside. Geochim. Cosmochim. Acta 56, 689–699], Toplis and Richet [Toplis, M.J., Richet, P., 2000. Equilibrium density and expansivity of silicate melts in the glass transition range. Contrib. Mineral. Petrol. 139, 672–683], Gottsmann and Dingwell [Gottsmann, J., Dingwell, D.B., 2002. Thermal expansivities of supercooled haplobasaltic liquids. Geochim. Cosmochim. Acta 66 (12) 2231–2238] of the temperature dependence of thermal expansivity. In the systems investigated here, the temperature dependence of thermal expansivity increases from basalt/basanite to tephriphonolite compositions.

Low temperature synthesis of anorthite based glass-ceramics via sintering and crystallization of glass-powder compacts

Anorthite based glass-ceramics were synthesized. The investigated glass compositions are located close to the anorthite-rich corner of the fluorapatite–anorthite–diopside ternary system. Glass powder compacts with mean particle size of 2 and 10 μm were prepared. Sintering behaviour, crystallization and the properties of glass-ceramics were investigated between 800 and 950 °C. In the case of specimens made from the finer particles, complete densification was achieved at a remarkably low temperature (825 °C) and the highest mechanical strength was obtained at 850 °C, but density significantly decreased at higher temperatures. The samples prepared from the larger particles exhibited higher values of density, shrinkage and bending strength within a wider temperature range (825–900 °C). Anorthite was predominantly crystallized between 850 and 950 °C, along with traces of fluorapatite. Diopside was detected only in the MgO richer compositions.

The Anorthite–Diopside System: Structural and Devitrification Study. Part II: Crystallinity Analysis by the Rietveld–RIR Method

The crystallization behavior of 10 binary glasses belonging to the CaO–MgO–Al2O3–SiO2 quaternary system and two glasses corresponding to anorthite and diopside composition was investigated by X‐ray diffraction, thermal, and thermomechanical analyses, and scanning electron microscopy. Particular emphasis is laid on the quantitative X‐ray diffraction analysis by the Rietveld–reference intensity ratio combined procedure, which seems to be a useful tool to obtain time–temperature–transformation diagrams. Results showed that to obtain glass–ceramics with a significant crystalline phase presence, it is necessary to treat samples at 1000°C for 4 h or at 1100°C for 1 h.

The Anorthite–Diopside System: Structural and Devitrification Study. Part I: Structural Characterization by Molecular Dynamic Simulations

Molecular dynamic simulations of glasses belonging to the anorthite–diopside system have been performed in order to obtain an atomistic description of the material's structure. The structural parameters obtained by the simulations allow to confirm that the glass materials are characterized by a very similar short‐range environment. The main differences have been observed in the intermediate‐range order of the structure that describe the distribution and the packing of the tetrahedra constituting the three‐dimensional networks. It is shown that the glass materials with composition close to the two extremes, corresponding to the pure glass anorthite or pure glass diopside, display the typical structural features of the tectosilicates and inosilicates subclass of minerals, respectively, to which anorthite and diopside crystals belong.

Experimental and computer simulation study of glasses belonging to diopside–anorthite system

The glasses belonging to the CaO–MgO–Al2O3–SiO2 quaternary system are materials largely used in industrial applications by virtue of their melting and devitrifible properties. A systematic study of several compositions falling in the anorthite (CaO–Al2O32SiO2)-diopside (CaO–MgO–2SiO2) system has been carried out by means of thermal and physical measurements in order to obtain the relationships between the structure and the properties of the glasses. The experimental evidence has been compared to molecular dynamic simulation results. Attention has been devoted to the definition of the structural role in the glass formulation played by Al3+ and Mg2+ which can behave as network formers or modifiers depending on their coordination number in the structure.

Liquidus karrooite stability and composition at reducing conditions

We have determined the stability of rutile and karrooite on the liquidus of pseudobinary silicate melts of anorthite–diopside eutectic composition with, in addition, of up to 32 wt% TiO2 at one atm total pressure and at wide range of oxygen fugacities. At 1,300°C and at an fO2 below 10−11.2 atm rutile (TiO2) is replaced as liquidus phase by the pseudobrookite-type MgTi2O5–Ti3O5 solid solution with some Al in the crystal structure. The composition of karrooite was found to be strongly dependent on oxygen fugacity. Crystalline phases were identified by transmission electron microscopy (TEM). The results obtained in this study are relevant for understanding the chemistry of lunar armalcolites. Using “excess” rutile solubility at reducing conditions, we estimated the Ti4+ /Ti3+ ratio in silicate melts at 1,300°C as function of fO2.

Thermal expansivities of supercooled haplobasaltic liquids

The thermal expansion of supercooled liquids in the haplobasaltic (anorthite–diopside) system have been determined via methods of container-based dilatometry. The expansivity data obtained in this study agree well with estimates provided by previous dilatometric determinations in the system that have relied on alternative experimental strategies. The data have been combined with high-temperature, superliquidus determinations of melt density to obtain expressions for the volume–temperature (V-T) relationships of liquids in the anorthite–diopside system. The V-T data clearly indicate a nonlinear temperature dependence of volume for all melts investigated. The variation is most striking for diopside, where the coefficient of volume thermal expansion decreases ∼56% from temperatures near the glass transition to superliquidus temperatures. With increasing anorthite content, the degree of variation appears to decrease. An 42Di 58 exhibits a decrease of 39% of its coefficient of thermal expansion and An 98Di 02 of 33%, respectively. The expansivities obtained in this study cannot be reproduced by means of published models that are based on linear V-T relationships. They require instead a reanalysis of existing pressure–V-T equations of state models for silicate melts.

Solubility of Apatite in Aluminosilicate Melts of the Albite–Diopside, Anorthite–Diopside, and Orthoclase–Diopside Systems

The liquidus surface temperatures, eutectic temperatures and compositions, phase crystallization fields, and phase separation regions are determined for the albite–apatite, anorthite–apatite, orthoclase–apatite, diopside–apatite, albite–diopside–apatite, anorthite–diopside–apatite, and orthoclase–diopside–apatite systems. The influence of the composition of the silicate melt on the solubility of apatite is described in a mathematical form. A program of calculating the limiting solubility of apatite in the silicate melt is developed.

Experimental study of the textural development of igneous rocks in the late stage of crystallization: the importance of interfacial energies under non-equilibrium conditions

Textural development of igneous rocks in the late stage of crystallization was experimentally studied using the diopside–anorthite and diopside-forsterite-anorthite systems. Isothermal, cooling, and heating experiments were performed within the temperature range in which the melt fraction was low (less than 25 vol%). Dihedral angles at solid-melt-solid triple junctions were measured as the indicator of the interfacial energy ratio (solid-melt interfacial energy/solid-solid grain boundary energy). In isothermal experiments, the dihedral angle (i.e. interfacial energy ratio) was large at low temperatures and small at high temperatures. The dihedral angle increased during cooling and decreased during heating compared to the equilibrium value. In cooling, crystal clustering and melt segregation were observed when the dihedral angle increased. The observed clustering threshold (dihedral angle~58°) in cooling experiments almost corresponded to the theoretical threshold (60°) which causes crystal clustering and melt segregation to minimize total interfacial energy under equilibrium conditions. These experimental results suggest that the change of dihedral angle under non-equilibrium conditions is a substantial phenomenon depending on the actual change of the interfacial energy ratio. Many kinds of clustering textures are observed in natural igneous rocks. Glomeroporphyritic texture in volcanic rock and clustering texture in granite were observed as examples of clustering textures in natural igneous rocks. Owing to the analogy between the natural clustering textures and experimental textures, a textural diversification process depending on interfacial energies is suggested.

Experimental determination of partial molar volumes of Ga 2O 3 and GeO 2 in silicate melts: implications for the pressure dependence of metal–silicate partition coefficients

In this study, densities of gallium and germanium bearing silicate liquids were determined using the double bob Archimedian method. The silicate base components of the liquids were sodium disilicate (NS2) and anorthite–diopside eutectic composition (AD). Densities were converted to molar volumes and plotted against mole fractions of Ga 2O 3 or GeO 2. The mixing behavior of volumes is linear in all systems except in the NS2+Ga 2O 3 liquids where a strong non-linearity is observed. In this case, the partial molar volume at infinite dilution was determined by the common tangent method. At 1600 K, the partial molar volume of Ga 2O 3 is 50±8 cm 3/mol in the NS2 liquid and 36.3±0.3 cm 3/mole in the AD liquid. For GeO 2 a partial molar volume of 29.7±0.2 cm 3/mol is found independent of silicate composition. The knowledge of the partial molar volumes of Ga 2O 3 and GeO 2 in silicate melts allows first order predictions of the pressure dependence of metal/silicate partition coefficients. Such data are important in deciding between homogeneous and inhomogeneous accretion models of the Earth. Using the partial molar volumes of Ga 2O 3 and GeO 2 measured in this study combined with literature data on the metallic components, the volume change of the exchange reaction x 2 Fe met + MO x/2 sil = x 2 FeO sil + M met was calculated (M=Ga or Ge). Volume changes for Ga-bearing AD liquids are positive and increase with temperature leading to a decrease in siderophility of Ga relative to Fe with increasing pressure whereas the corresponding change in the Ge-bearing AD liquids is negative implying higher siderophility of Ge with pressure. The decrease of the metal–silicate partition coefficient of Ga with pressure is consistent with models of core formation by global metal–silicate equilibrium at some depth. The increase of the metal/silicate partition coefficient of Ge with pressure is incompatible with global core mantle equilibrium. However, there is some ambiguity regarding the valence state and the coordination of Ge during core formation. Either Ge was present as GeO or as GeO 2 while the experimental results obtained in this study are only applicable for GeO 2. Our extrapolations are only valid as long as the coordination of Ge does not change because the partial molar volumes depend strongly on the coordination number.

Crystallizing phases from multi-component silicate glasses in the system K 2O–CaO–MgO–Al 2O 3–SiO 2

The sequence of crystallization of phases in some glasses based on anorthite–diopside–forsterite containing 10–40% (wt.%) K-feldspar was studied. Phase association, transformation and extent of solid solutions development have been followed by X-ray diffraction, polarizing microscope; indentation micro-hardness was also measured. Diopside and anorthite were the primary crystalline phases and small amounts of microcline in K-feldspar-poor glasses. With increasing soaking time and temperature of heat-treatment, forsterite was developed meanwhile microcline was transformed into orthoclase. With an increase of K-feldspar to 40%, the anorthite phase disappeared and forsterite was transformed into enstatite while sluggish transformations of microcline into orthoclase were observed. The sequence of crystallizing phases is found to be as follows: diopside, anorthite, forsterite, microcline, then the orthoclase.

Phase Equilibrium in the Fluorapatite–Anorthite–Diopside System

The binary systems Ca5[PO4]3F–CaAl2Si2O8 and Ca5[PO4]3F–CaMgSi2O6 have been investigated, via annealing and quenching in the experimental method, direct observation of the melting behavior of the samples, X‐ray diffraction analysis, petrography, and transmission electron microscopy. Phase equilibrium in the ternary system Ca5[PO4]3F–CaAl2Si2O8–CaMgSi2O6 was determined by combining information from the structure of the binary boundary systems and additional experimental data that were obtained from ternary compositions. The glass‐formation region of the fluorapatite–anorthite–diopside system was studied, and the glass compositions for the development of glass‐ceramics for technical and medical applications were identified.

Directional solidification and microstructural evolution between diopside and anorthite in the system CaO-MgO-Al 2 O 3 -SiO 2

Silicate mixtures close to the diopside–anorthite tie line have been unidirectionally solidified by float–zone melting in an arc image furnace and the microstructures examined by optical microscopy, scanning electron microscopy, electron microprobe analysis and from electron backscatter patterns. The structures which evolved have been interpreted by reference to available phase diagram data for the anorthite–diopside eutectic surface within the four–component oxide system, CaO–MgO–Al2O3–SiO2. It is demonstrated how the microstructures and segregation paths can be described in a visually explicit way within the quaternary system. The morphologies of primary diopside, anorthite and the eutectic mixture are described and discussed. In this silicate system the low liquid diffusion rates lead to glass formation at rapid cooling rates and at temperatures close to the freezing point, so that local equilibrium is only partially realized. The consequences are interpreted with reference to the quaternary phase diagram.

Liquidus Phase Relations in the System CaO–MgO–Al2O3–SiO2 at 2·0 GPa: Applications to Basalt Fractionation, Eclogites, and Igneous Sapphirine

with suggestions that fractionation of tholeiitic basalt in this pressure To model magmatic crystallization processes for mafic to intermediate range yields an alkalic basalt. Both trends may occur for natural compositions at high pressure, liquidus phase relations in the basalts depending on the Na2O content of the parental magma. forsterite–anorthite–diopside–silica (FADS) tetrahedron within the Also, the data show that the minimum pressure for the formation CaO–MgO–Al2O3–SiO2 system have been determined at 2·0 GPa. of cumulate eclogites and garnet pyroxenites is about 1·8–1·9 GPa. Compositions of five liquidus invariant points have been determined The lower limit of pressure at which sapphirine can crystallize and the approximate compositions of five others have been inferred. from a liquid in the FADS tetrahedron is estimated to be 1·1–1·5 These involve primary phase volumes for forsterite (fo), enstatite GPa and the upper limit is >3 GPa. Sapphirine crystallizes from (en), diopside (di), high quartz (qz), spinel (sp), sapphirine (sa), magmas intermediate in composition between basalt and andesite. garnet (gt), anorthite (an), and corundum (cor). The determined Probable igneous sapphirine in mafic associations is rare, but it (with wt % coefficients) and inferred reactions (without coefficients) occurs as part of a pyroxenite xenolith from Delegate, Australia, that define each isobaric invariant point are as follows: that we suggest is a cumulate assemblage and in a sapphirine norite 23 en + 68 di + 9 sp = 84 liq + 16 fo at Wilson Lake, Labrador, Canada. 37 di + 63 sa = 47 liq + 40 sp + 13 en 100 gt = 21 liq + 27 sa + 55 en + 18 di 1 di + 59 en + 41 an = 43 liq + 57 gt 18 di + 21 qz + 15 en + 47 an = 100 liq

Compositional dependence of REE partitioning between diopside and melt at 1 atmosphere

The effects of composition on pyroxene-melt partitioning of several REE (rare earth elements), Y, and Sr were experimentally evaluated. Using the synthetic model systems anorthite–diopside, diopside–titanite and anorthite–diopside–titanite different diopsides were grown at atmospheric conditions in a double-ellipsoid mirror furnace. The single samples were melted and crystallised in a Pt/Au crucible with compositions corresponding to the invariant points of these systems. Rotational motion with approximately 25 rpm around the longitudinal axis of the crucible increases the prevailing convection flows. By this means, the exclusively diffusional transport of assembly groups onto the growing crystals is avoided. Quenching is achieved by dropping the crucible into water. Crystals up to 2 mm were obtained and analysed by electron microprobe. No inhomogeneities or compositional zonation, either in the diopsides or in the coexisting melts, were observed within the analytical uncertainty of the electron microprobe. The crystallised diopsides occur as both euhedral single crystals and large symplectitic lamellar intergrowths with anorthite or titanite. The chemical homogeneity and the texture indicate near-equilibrium conditions. The analyses show strong positive correlations between DREE and tetrahedrally coordinated Al in diopside but are not affected by octahedral Al or Ti-concentration. By means of correlations and mass balances the incorporation of REE can be described by 2 different coupled substitutions: $$$$ $$$$

Liquidus Phase Relations in the CaO-MgO-Al2O3-SiO2 System at 3.0 GPa: the Aluminous Pyroxene Thermal Divide and High-Pressure Fractionation of Picritic and Komatiitic Magmas

We present liquidus phase equilibrium data at 3.0 GPa for the model tholeiitic basalt tetrahedron, diopside–anorthite–fosterite–quartz, in the CaO–MgO–Al2O3–SiO2 system. This pressure coincides with the invariant point (1568°C) on the simplified model lherzolite solidus that marks the transition between spinel lherzolite and garnet lherzolite (fo + en + di + sp + gt + liq). The composition of the liquid at the invariant point (46.4 An, 16.0 Di, 33.5 Fo, 4.2 Qz, wt %) is a model olivine-rich basalt that lies slightly (0.2% excess fo) to the SiO2-poor side of the aluminous pyroxene plane, MgSiO3–CaSiO3–Al2O3. A large garnet primary phase volume is bordered by primary phase volumes for forsterite, spinel, sapphirine, corundum, enstatite, diopside, quartz, and kyanite. The observed absence of enstatite at the solidus of lherzolite at pressures above ∼3.3 GPa is readily understood from the phase relations in this system. During melting at these high pressures, enstatite first forms at a temperature somewhat above the solidus and then dissolves before complete melting. As pressure increases above 3.0 GPa, the aluminous pyroxene plane, MgSiO3–CaSiO3–Al2O3, becomes increasingly effective as a thermal divide that causes picritic and komatiitic melts lying on the silica-poor side of the plane to fractionate toward alkalic picritic compositions. However, if the rate of ascent of these melts is sufficiently rapid, expansion of the olivine primary phase volume as pressure decreases produces a fractionation trend dominated by olivine crystallization and the thermal divide is ignored.