A petrogenetic grid for pelitic schists in the system SiO2-Al2O3-FeO-MgO-K2O-H2O

A quantitative petrogenetic grid for pelitic schists in the system KFMASH that includes the phases garnet, chlorite, biotite, chloritoid, cordierite, staurolite, talc, kyanite, andalusite, sillimanite, and pyrophyllite (with quartz, H₂O and muscovite or K-feldspar in excess) is presented. The grid is based on thermodynamic data of Berman et al. (1985) and Berman (1988) for endmember KFASH and KMASH equilibria and natural Fe-Mg partitioning for the KFMASH system. Calculation of P-T slopes and the change in Fe/(Fe+Mg) along reactions in the KFMASH system were made using the Gibbs method. In addition, the effect on the grid of MnO and CaO is evaluated quantitatively. The resulting grid is consistent with typical Buchan and Barrovian parageneses at medium to high grades. At low grades, the grid predicts an extensive stability field for the paragenesis chloritoid+biotite which arises because of the unusual facing of the reaction chloritoid+biotite + quartz+H₂O = garnet+chlorite+muscovite, which proceeds to the right with increasing T in the KFMASH system. However, the reaction proceeds to the left with increasing T in the MnKFASH system so the assemblage chloritoid + biotite is restricted to bulk compositions with high Fe/(Fe+Mg+Mn). Typical metapelites will therefore contain garnet+chlorite at low grades rather than chloritoid + biotite.     

Compressibility of P2O5-Al2O3-Na2SiO3 melts

The effect of composition and temperature on the relaxed adiabatic bulk modulus of melts in the P₂O₅-Al₂O ₃-Na₂SiO₃ system have been investigated in the temperature range of 1140 to 1450 °C using ultrasonic interferometric methods at frequencies of 3, 5 and 7 MHz. The density of these melts was determined using Pt-double-bob Archimedean densitometry techiques. P₂O₅ is known to dramatically affect the structure and the chemical and physical properties of granitic and pegmatitic melts as a function of the peralkalinity of the melt. The physical results of the structural changes occurring in Na₂O-Al₂O₃-SiO₂ melt upon the addition of P₂O₅ are observed by variations in the properties such as density and compressibility. For the present peralkaline melts, the bulk modulus and density decrease with addition of 15 mol% P₂O₅, and increase with the addition of 15 mol% Al₂O₃. The addition of P₂O₅ to the present melts results in a larger increase in melt compressibility than that observed with increasing polymerization between Na₂SiO₃ and Na₂Si₂O₅ melts. This would suggest that not only is the polymerization of the melt increasing with the addition of P₂O₅ (Mysen et al. 1981; Nelson and Tallant 1984; Gan and Hess 1992), but that the tetrahedrally co-ordinated phosphorus complexes are influencing the bond lengths and energies within the melt structure; resulting in the structure becoming more compressible than expected, although incompressible (Vaughan and Weidner 1987) tetrahedral P₂O₅ polyhedra (Mysen et al. 1981; Gan and Hess 1992; Toplis et al. 1994) are being added to the melt structure.     

Sorption of uranyl and arsenate on SiO2, Al2O3, TiO2 and FeOOH

Migration of uranium and arsenic in aquatic environments is often controlled by sorption on minerals present along the water flow path. To investigate the sorption behaviour, batch experiments were conducted for uranium and arsenic as single components and also solutions containing both uranium and arsenic in the presence of SiO₂, Al₂O₃, TiO₂ and FeOOH at a pH ranging from 3 to 9. In solutions containing only U(VI) or As(V) with the minerals, the sorption of U(VI) was low at acidic pH range and increases with increasing pH, whereas As(V) showed opposite sorption behaviour to Al₂O₃, TiO₂ and FeOOH from acidic pH range to alkaline condition. For the As(V)–SiO₂ system, the sorption was low for almost all pH. Sorption of U(VI) and As(V) on SiO₂ and FeOOH is almost similar in solutions containing either U(VI) or As(V) separately, or both together. In the U(VI)–As(V)–Al₂O₃ system, a significant retardation in uranyl sorption and an enhancement in arsenate sorption on Al₂O₃ were observed for a wide range of pH. The sorption behaviour of U(VI) and As(V) was changed when Al₂O₃ was replaced by TiO₂, where an increase in sorption was observed for both elements. The sorption behaviour of uranyl and arsenate in the U(VI)–As(V)–TiO₂ system gives evidence for the formation of uranyl–arsenate complexes. The change in sorption retardation/enhancement of U(VI) and As(V) could be explained by the formation of uranyl–arsenate complexes or due to the competitive sorption between uranyl and arsenate species.     

Heat capacities of Fe2O3-bearing silicate liquids

Direct measurements of liquid heat capacity, using a Setaram HT1500 calorimeter in step-scanning mode, have been made in air on six compositions in the Na₂O-FeO-Fe₂O₃-SiO₂ system, two in the CaO-FeO-Fe₂O₃-SiO₂ system and four of natural composition (basanite, andesite, dacite, and peralkaline rhyolite). The fitted standard deviations on our heat capacity measurements range from 0.6 to 3.6%. Step-scanning calorimetry is particularly useful when applied to iron-bearing silicate liquids because: (1) measurements are made over a small temperature interval (10K) through which the ferric-ferrous ratio of the liquid remains essentially constant during a single measurement; (2) the sample is held in equilibrium with an atmosphere that can be controlled; (3) heat capacity is measured directly and not derived from the slope of enthalpy measurements with temperature. Liquid compositions in the sodic and calcic systems were chosen because they contain large concentrations of Fe₂O₃ (up to 19 mol%), and their equilibrium ferric-ferrous ratios were known at every temperature of measurement. These measurement have been combined with heat capacity (Cp) data in the literature on iron-free silicate liquids to fit Cp as a function of composition. A model assuming no excess heat capacity (linear combination of partial molar heat capacities of oxide components) reproduces the liquid data within error (±2.2% on average). The derived partial molar heat capacity of the Fe₂O₃ component is 240.9 ±7.9 J/g.f.w.-K, with a standard error reduced by more than a factor of two from that in earlier studies. The model equation, based primarily on simple, synthetic compositions, predicts the heat capacity of the four magmatic liquids within 1.8% on average.     

The disproportionation of andalusite (Al2SiO5) to Al2O3 and SiO2 under shock compression

Shock-recovery experiments have been carried out on andalusite single crystals of gem quality in a pressure range from 300 up to 575 kbar. Infrared spectroscopic investigations indicate a progressive shock-induced transformation of andalusite into short-range-ordered Al₂O₃ and SiO₂ phases within a pressure interval from ～360 to ～575 kbar. Exposure to dynamic pressures of about 575 kbar results in andalusite breaking down into incoherently crystallized γ-Al₂O₃, well-crystallized α-Al₂O₃ and X-ray amorphous SiO₂. The shock disproportionation of andalusite is presumed to take place in three separate stages of reaction. The comparison of shock-induced reactions with results from static experiments on kyanite indicates significant differences in the transformation pressures and in the mechanism of the high pressure decomposition.     

Experiments on silicate melt immiscibility in the system Fe2SiO4–KAlSi3O8–SiO2–CaO–MgO–TiO2–P2O5 and implications for natural magmas

The effect of CaO and MgO, with or without TiO₂ and P₂O₅, on the two-melt field in the simplified system Fe₂SiO₄–KAlSi₃O₈–SiO₂ has been experimentally determined at 1,050°–1,240°C, 400 MPa. Despite the suppressing effect of MgO, CaO, and pressure on silicate melt immiscibility, our experiments show that this process is still viable at mid-crustal pressures when small amounts (0.6–2.0 wt%) of P₂O₅ and TiO₂ are present. Our data stress that the major element partition coefficients between the two melts are highly correlated with the degree of polymerisation (nbo/t) of the SiO₂-rich melt, whatever temperature, pressure, or exact composition. Experimental immiscible melt compositions in natural systems at 0.1 MPa from the literature (lunar and tholeiitic basalts) plot on similar but distinct curves compared to the simplified system. These relations between melt polymerisation and partition coefficients, which hold for a large range of compositions and fO₂, are extended to various volcanic and plutonic rocks. This analysis strengthens the proposal that silicate melt immiscibility can be important in volcanic rocks of various compositions (from tholeiitic basalts to lamprophyres). However, the majority of proposed immiscible compositions in plutonic rocks are at least not coexisting melts, but may have suffered accumulation of early crystallized minerals.     

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.     

Solid-Fluid Equilibria in the System KAlSi3O8-NaAlSi3O8-Al2SiO5-SiO2-H2O-HCl

Activity diagrams in the system KAlSi₃O₈-NaAlSi₃O₈-Al₂SiO₅-SiO₂-H₂O-HClhave been calculated in terms of a_K+/a_H+ and a_N+/a_H+ from existingexperimental data. They show the effect of temperature, pressure,and a_H2O on the stability fields of the alkali feldspars, micas,and aluminium silicate. These activity diagrams are useful in revealing the bufferingcapacity of mineral assemblages and the chemical potential gradientsestablished by changes in T, P, a_H2O, and mineral assemblage.An analysis of mineral paragenesis in terms of these diagramssuggests that mosaic equilibrium, allowing limited metasomatismand internal buffering of chemical potentials, best describemetamorphic systems. Thus the dehydration reaction: muscovite+quartz=K-feldspar+Al₂SiO₅+H₂O which is most important in closed systems, probably fails todescribe in detail the mechanism of natural muscovite decomposition.Rather the decomposition of muscovite is more likely representedby ionic reactions. The replacement of muscovite by feldspar: muscovite+6 SiO₂+2 K⁺=3 K-feldspar+2 H⁺ muscovite+6 SiO₂+3 Na⁺=3 Albite+K⁺+2 H⁺ is favored at high temperature and low pressure, and may accountfor the crystallization of some feldspars in metamorphic rocks.The reaction involving aluminium silicate replacement of muscovite: 2 muscovite+2 H⁺=3 Al₂SiO₅+3 SiO₂+3 H₂O+2 K⁺ is favored at high temperature and pressure and low a_H2O, andcould contribute to the development of the aluminium silicates.It is concluded that both activity diagrams and AKNa projectionsshould be used together to more completely evaluate mineralparagenesis in terms of mosaic equilibria.     

The restricted stability of osumilite under hydrous conditions in the system K2O-MgO–Al2O3–SiO2–H2O

Osumilite_ss was synthesized as a single phase product in the model system K₂O-MgO-Al₂O₃-SiO₂ at 800° C/ 0.5 Kbar water pressure and at 800° to 840° C/1.0 Kbar total pressure with 0.3 in the gas phase. The experimentally determined solid solubility range of synthetic osumilites can be expressed by the formula KMg₂(Al_3-xMg_x) (Al_2–xSi_10+x)O₃₀ with 0x0.4. A survey of sixteen chemical analyses of natural osumilites from eleven occurrences shows a solid solubility characterized by 0x0.6. Reversed stability experiments for the synthetic osumilite KMg₂(Al_2.75Mg_0.25)(Al_1.75Si_10.25)O₃₀ determined at water pressure equal to total pressure demonstrate its restriction to water pressures below 0.8 Kbar (at 0.5 Kbar, the stability range is between 765° and 800° C). At the lower thermal stability limit osumilite+H₂O vapor break down to cordierite+K feldspar+phlogopite_ss+quartz, at the higher one to cordierite+K feldspar+phlogopite+liquid. Reduction of water fugacity will expand the stability field largely by shifting the lower and higher thermal stability limits to lower and higher temperatures, respectively. The dependence of osumilite stability on water fugacity makes osumilite a sensitive indicator mineral for dry conditions in rocks formed at total pressures higher than about 0.8 Kbar.     

Experimental investigation of zoisite–clinozoisite phase equilibria in the system CaO–Fe2O3–Al2O3–SiO2–H2O

The system Ca₂Al₃Si₃O₁₁(O/OH)-Ca₂Al₂FeSi₃O₁₁(O/OH), with emphasis on the Al-rich portion, was investigated by synthesis experiments at 0.5 and 2.0 GPa, 500-800 °C, using the technique of producing overgrowths on natural seed crystals. Electron microprobe analyses of overgrowths up to >100 µm wide have located the phase transition from clinozoisite to zoisite as a function of P-T-X_ps and a miscibility gap in the clinozoisite solid solution. The experiments confirm a narrow, steep zoisite-clinozoisite two-phase loop in T-X_ps section. Maximum and minimum iron contents in coexisting zoisite and clinozoisite are given by X_ps^zo (max) = 1.9*10 ^{- 4} T+ 3.1*10 ^{- 2} P - 5.36*10 ^{- 2}{\rm X}_{{\rm ps}}^{{\rm zo}} {\rm (max) = 1}{\rm .9*10}^{ - 4} T{\rm + 3}{\rm .1*10}^{ - 2} P - {\rm 5}{\rm .36*10}^{ - 2} and X_ps^czo (min) = (4.6 * 10 ^{- 4} - 4 * 10 ^{- 5} P)T + 3.82 * 10 ^{- 2} P - 8.76 * 10 ^{- 2}{\rm X}_{{\rm ps}}^{{\rm czo}} {\rm (min)} = {\rm (4}{\rm .6} * {\rm 10}^{ - {\rm 4}} - 4 * {\rm 10}^{ - {\rm 5}} P{\rm )}T + {\rm 3}{\rm .82} * {\rm 10}^{ - {\rm 2}} P - {\rm 8}{\rm .76} * {\rm 10}^{ - {\rm 2}} (P in GPa, T in °C). The iron-free end member reaction clinozoisite = zoisite has equilibrium temperatures of 185ᇆ °C at 0.5 GPa and 0ᇆ °C at 2.0 GPa, with (H_r⁰=2.8ǃ.3 kJ/mol and (S_r⁰=4.5ǃ.4 J/mol2K. At 0.5 GPa, two clinozoisite modifications exist, which have compositions of clinozoisite I ~0.15 to 0.25 X_ps and clinozoisite II >0.55 X_ps. The upper thermal stability of clinozoisite I at 0.5 GPa lies slightly above 600 °C, whereas Fe-rich clinozoisite II is stable at 650 °C. The schematic phase relations between epidote minerals, grossular-andradite solid solutions and other phases in the system CaO-Al₂O₃-Fe₂O₃-SiO₂-H₂O are shown.     

Synthetic Mn3+-kyanite and viridine, (Al2-xMn x 3+ ) SiO5, in the system Al2O3-MnO-MnO2-SiO2

Experiments on the join Al₂SiO₅-“Mn₂SiO₅” of the system Al₂O₃-SiO₂-MnO-MnO₂ in the pressure/temperature range 10–20 kb/900–1050° C with gem quality andalusite, Mn₂O₃, and high purity SiO₂ as starting materials and using /_O2-buffer techniques to preserve the Mn³⁺ oxidation state had following results: At 20 kb/1000°C orange-yellow kyanite mixed crystals are formed. The kyanite solid solubility is limited at about (Al_1.88Mn _0.12 ³⁺ )SiO₅ and, thus, equals approximately that on the join Al₂SiO₅-“Fe₂SiO₅” (Langer and Frentrup, 1973) indicating that there is no Jahn-Teller stabilisation of Mn³⁺ in the kyanite matrix. 5 mole % substitution causes the kyanite lattice constants a _o, b _o, c _o, and V _o to increase by 0.015, 0.009, 0.014 Å, and 1.6 ų, resp., while α, β, γ, remain unchanged. Between 10 and 18 kb/900°C, Mn³⁺-substituted, strongly pleochroitic (emeraldgreen-yellow) andalusite_ss (viridine) was obtained. At 15 kb/900°C, the viridine compositional range is about (Al_1.86Mn _0.14 ³⁺ )SiO₅-(Al_1.56Mn _0,44 ³⁺ )SiO₅. Thus, Al→Mn³⁺ substitutional degrees are appreciably higher in andalusite than in kyanite, proving a strong Jahn-Teller effect of Mn³⁺ in the andalusite structure, which stabilises this structure type at the expense of kyanite and sillimanite and, thus, enlarges its PT-stability range extremely. 17 mole % substitution cause the andalusite constants a _o, b _o, c _o, and V _o to increase by 0.118, 0.029, 0.047 Å and 9.4 ų, resp. At “Mn₂SiO₅”-contents smaller than about 7 mole %, viridine coexists with Mn-poor kyanite. At “Mn₂SiO₅”-concentrations higher than the maximum kyanite or viridine miscibility, braunite (tetragonal, ideal formula Mn²⁺Mn³⁺[O₈/Si0₄]), pyrolusite and SiO₂ were found to coexist with the Mn³⁺-saturated ky _ss or and _ss, respectively. In both cases, braunites were Al-substituted (about 1 Al for 1 Mn³⁺). Pure synthetic braunites had the lattice constants a _o 9.425, c _o, 18.700 Å, V _o 1661.1 ų (ideal compn.) and a _o 9.374, c _o 18.593 ų, V _o 1633.6 ų (1 Al for 1 Mn³⁺). Stable coexistence of the Mn²⁺-bearing phase braunite with the Mn⁴⁺-bearing phase pyrolusite was proved by runs in the limiting system MnO-MnO₂-SiO₂.     

High temperature structural investigation of Na2O·0.5Fe2O3·3SiO2 and Na2O·FeO·3SiO2 melts and glasses

The structure of glasses and melts of Na₂O· 0.5Fe₂O₃·3SiO₂ and Na₂O·FeO·3SiO₂ compositions have been measured using high temperature Raman spectroscopy. For the oxidized sample it has been demonstrated that there is a close structural relationship between melt and glass. No coordination changes of Fe³⁺ with temperature and no new anionic species have been observed in the oxidized melt. The Raman spectra of the reduced sample clearly show a decrease in the degree of polymerization, as determined by the observation of the polarization character of the spectra and the details of the change of the Raman intensities during heating in hydrogen. Mössbauer spectra suggest that Fe³⁺ is tetrahedrally coordinated in the oxidized glass and part of the Fe²⁺ is tetrahedrally coordinated in the reduced glass.     

The discrete association of water with Na2O and SiO2 in NaAl silicate melts

The solubility of water in several NaAl silicate melts has been determined up to 8 kbars. The results are shown on a (fugacity)^1/2 versus mole percent solubility diagram and data points for any composition can be joined by 2 or more straight lines. It is suggested that each straight line segment represents a different water-solubility mechanism.     

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     

Forsterite-diopside-spinel-liquid equilibria in the system CaO−MgO−Al2O3−SiO2 at 20 kbar and petrological applications

Liquidus phase relationships in the CaAlAl–SiO₆–Mg₂SiO₄–CaMgSi₂O₆–CaAlSi₂O₈ portion of the simplified basalt tetrahedron in the CaO–MgO–SiO₂–Al₂O₃ system have been experimentally determined at 20 kbar pressure. The fo+di _ss+sp+li univariant curve, that pierces the fo-di-an join and meets the fo+di _ss+ en_ss+sp+li invariant point in the basalt tetrahedron, extends all the way to and pierces the di-fo-CaTs join, the limit of the simplified basalt tetrahedron toward the silica undersaturated portion.An algebraic method, relying on compositions of two successive liquids on a univariant curve and those of the crystalline phases in equilibrium with the respective liquids, is developed to identify the type of reaction that takes place along an isobarically univariant curve and to detect whether there is a temperature maximum on that curve. Use of this method for the di _ss+fo+sp+li univariant equilibria shows that a temperature maximum exists on this curve at the composition Fo₁₁Di₅₆An₃CaTs₃₀, very close to and slighthly to the SiO₂-rich side of the fo-di-CaTs join. The temperature along the univariant curve continuously decreases from the temperature maximum (1500°C) to the invariant point (1475°C) where the univariant curve is terminated by the appearance of e _ss as a member of the equilibrium assemblage. Along this part of the curve, a reaction relationship occurs according to the equation fo+li=di _ss+ sp. Compositions of di _ss in equilibrium with the liquids from the temperature maximum to the fo+di _ss+en_ss+ sp+li invariant point range from Di₆₆En₉CaTs₂₅ to Di₃₆En₄₀CaTs₂₄. Because of the reaction relationship of forsterite with liquid, fractional crystallization of a model alkalic basaltic liquid would cause liquids to move off the fo-di _ss-sp-li univariant curve onto the sp-di _ss divariant surface. Crystallization of di _ss and sp would then lead to silica enrichment of residual liquids. Thus at pressures below 30 kbar, at which pressure the Al₂O₃–CaSiO₃–MgSiO₃ plane becomes a new thermal divide cutting through both the tholeiitic and alkalic volumes, alkalic liquids will fractionate toward tholeiitic compositions without crossing a thermal divide. This relationship would be expected to persist at pressures down to about 4 kbar where a maximum on the fo-di-an-li boundary line causes a thermal divide near the fo-di-an plane. Strongly SiO₂-undersaturated liquids (e.g. nephelinites, basanites), on the other hand, cannot be derived from SiO₂-undersaturated basalts (e.g. alkali olivine basalt) by fractional crystallization at 20 kbar. We also found that no gt primary phase volume cuts the wo-en-Al₂O₃ join at 20 kbar pressure. The wehrlite, the olivine clinopyroxenite, and the Al-augite group lherzolite xenoliths, containing highly aluminous clinopyroxenes (enriched in Ca-Tschermak), can be interpreted as crystal cumulates from alkalic basalts in the light of this experimental study. This is consistent with the mode of origin of these xenoliths proposed from petrographic, mineralogic, and geochemical studies.Abbreviations and notations di CaMgSi₂O₆ - fo Mg₂SiO₄ - an CaAl₂Si₂O₈ - CaTs CaAlAlSiO₆ - sp MgAl₂O₄ - en MgSiO₃ - wo CaSiO₃ - gt Ca₃Al₂Si₃O₁₂–Mg₃Al₂Si₃O₁₂ - qz SiO₂ - li Liquid - gl glass - ss Solid Solution - A An mxn matrix - X A column vector - kbar kilobar     

The effect of TiO2 and Fe2O3 on metapelitic assemblages at greenschist and amphibolite facies conditions: mineral equilibria calculations in the system K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3

Mineral equilibria calculations in the system K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–Fe₂O₃ (KFMASHTO) using thermocalc and its internally consistent thermodynamic dataset constrain the effect of TiO₂ and Fe₂O₃ on greenschist and amphibolite facies mineral equilibria in metapelites. The end‐member data and activity–composition relationships for biotite and chloritoid, calibrated with natural rock data, and activity–composition data for garnet, calibrated using experimental data, provide new constraints on the effects of TiO₂ and Fe₂O₃ on the stability of these minerals. Thermodynamic models for ilmenite–hematite and magnetite–ulvospinel solid solutions accounting for order–disorder in these phases allow the distribution of TiO₂ and Fe₂O₃ between oxide minerals and silicate minerals to be calculated. The calculations indicate that small to moderate amounts of TiO₂ and Fe₂O₃ in typical metapelitic bulk compositions have little effect on silicate mineral equilibria in metapelites at greenschist to amphibolite facies, compared with those calculated in KFMASH. The addition of large amounts of TiO₂ to typical pelitic bulk compositions has little effect on the stability of silicate assemblages; in contrast, rocks rich in Fe₂O₃ develop a markedly different metamorphic succession from that of common Barrovian sequences. In particular, Fe₂O₃‐rich metapelites show a marked reduction in the stability fields of staurolite and garnet to higher pressures, in comparison to those predicted by KFMASH grids.     

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.     

Cookeite LiAl4(Si3Al)O10(OH)8: Experimental study and thermodynamical analysis of its compatibility relations in the Li2O−Al2O3−SiO2−H2O system

Three reactions limiting the stability field of the di-trioctahedral chlorite cookeite in the presence of quartz, in the system Li₂O−Al₂O₃−SiO₂−H₂O (LASH) have been reversed in the range 290–480°C, 0.8–14 kbar, using natural material close to the end member composition. Combining our results with known and estimated thermodynamic properties of the other minerals belonging to the LASH system, the enthalpy (-8512200 J/mol) and the entropy (504.8 J/mol^*K) of cookeite are calculated by a feasible solution space approach. The knowledge of these values allowed us to draw the first P−T phase diagram involving both the hydrated Li-aluminosilicates cookeite and bikitaite, which is applicable to a large variety of natural rock systems. The low thermal extent of the stability field of cookeite+quartz (260–480°C) makes cookeite a valuable indicator of low temperature conditions within a wide range of pressure (1–14 kbar).     

The stability of sapphirine-quartz and hypersthene-sillimanite-quartz assemblages: an experimental investigation in the system FeO−MgO−Al2O3−SiO2 under H2O and CO2 conditions

Phase relations and mineral chemistry involving the phases garnet (Gt), spinel (Sp), hypersthene (Hy), sapphirine (Sa), cordierite (Cd), sillimanite (Sil) and quartz (Qz) have been experimentally determined in the system FMAS (FeO−MgO−Al₂O₂−SiO₂) under low fO₂ and for various H₂O/CO₂ conditions. Several compositions were studied with 100 (Mg/Mg+Fe) ratio ranging from 64 to 87 with excess quartz and sillimanite. Our data do not show any differences in Gt−Cd stability and composition as a function of H₂O, CO₂ and H₂O−CO₂ (±CH₄) content, in good agreement with a previous experimental study at lower temperature (Aranovich and Podlesskii 1983). At 1,000° C and 11 kbar, under CO₂-saturated conditions, cordierite grew from a crystalline mix unseeded with cordierite. Thus, under water-absent conditions, cordierite will have a high-P stability field in the presence of CO₂. If water has a pressure stabilizing effect on cordierite, then our results would indicate that the effects of H₂O and CO₂ are of the same magnitude at high temperature. Our data support the theoretical P-T grid proposed by Hensen (1986) for high-T metapelites and are largely consistent with the high-temperature experimental data of Hensen and Green (1973). The univariant boundary Gt+Cd=Hy+Sil+Qz, which marks the disappearance of Hy−Sil−Qz assemblages, has a negative dP/dT slope above 1,000° C and a positive one below this temperature. Extrapolation of our data to iron-free systems shows that the high-P breakdown limit of Mg-cordierite has a negative slope in the range 1,025–1,300° C and probably positive below 1,000° C. This indicates a maximum of stability for Mg-cordierite at around 1,000° C and 13 kbar. Because of the curvature of the univariant reactions En+Sil=Py+Qz, Mg−Cd=En+Sil+Qz and Gt+Cd=Hy+Sil+Qz, the iron-free invariant point involving the phases Py, En, Cd, Sil and Qz probably does not exist. Sapphirine—Qz-bearing assemblages are stable only at temperatures above 1,050° C. At 1,075° C, the joint Gt−Sa is stable up to 11 kbar. At higher pressure, garnet, sapphirine and quartz react according to the reaction Gt+Sa+Qz=Hy+Sil. Reequilibrated sapphirines are more aluminous than the theoretical endmember Mg₂Al₄SiO₁₀ due to AlAl=MgSi substitutions [100(Al₂O₃/Al₂O₃+FeO+MgO) in experimental sapphirines ranges from 50.5 to 52.2]. Sapphirine in the assemblage Sa−Cd−Sil−Qz shows a decrease in Al content with decreasing temperature and pressure, such that the alumina isopleths for sapphirine have a slight negative dP/dT slope. A similar decrease in Al content of sapphirine with temperature is also observed in Sa−Sil−Qz assemblages.