Phase equilibria in the MgO–SiO<Subscript>2</Subscript>–ZrO<Subscript>2</Subscript> system

The results of study of phase equilibria in the MgO–SiO₂–ZrO₂ system at 1450–1550°C are reported. The studied system contains two eutectic points and six fields: (I) MgSiO₃ + SiO₂; (II) MgSiO₃ + ZrO₂; (III) ZrSiO₄ + SiO₂; (IV) MgSiO₃ + Mg₂SiO₄; (V) ZrO₂ + MgO; (VI) ZrSiO₄ + ZrO₂. The presence of fields (II) and (III) on the diagram shows that zircon in equilibrium with olivine and pyroxene crystallizes at very low concentrations of ZrO₂ in the system. This provides a solution for one of the most important problems in zirconology of dunites: the probability of the formation and preservation of zircon in the course of the formation and evolution of dunite.     

Mineral-fluid equilibria in the system CaO–MgO–SiO<Subscript>2</Subscript>–H<Subscript>2</Subscript>O–CO<Subscript>2</Subscript>–NaCl and the record of reactive fluid flow in contact metamorphic aureoles

Phase equilibria in the system CaO–MgO–SiO₂–CO₂–H₂O–NaCl are calculated to illustrate phase relations in metacarbonates over a wide-range of P–T–X[H₂O–CO₂–NaCl] conditions. Calculations are performed using the equation of state of Duan et al. (Geochim Cosmochim Acta 59:2869–2882, 1995) for H₂O–CO₂–NaCl fluids and the internally consistent data set of Gottschalk (Eur J Mineral 9:175–223, 1997) for thermodynamic properties of solids. Results are presented in isothermal-isobarical plots showing stable mineral assemblages as a function of fluid composition. It is shown that in contact-metamorphic P–T regimes the presence of very small concentrations of NaCl in the fluid causes almost all decarbonation reactions to proceed within the two fluid solvus of the H₂O–CO₂–NaCl system. Substantial flow of magma-derived fluids into marbles has been documented for many contact aureoles by shifts in stable isotope geochemistry of the host rocks and by the progress of volatile-producing mineral reactions controlled by fluid compositions. Time-integrated fluid fluxes have been estimated by combining fluid advection/dispersion models with the spatial arrangement of mineral reactions and isotopic resetting. All existing models assume that minerals react in the presence of a single phase H₂O–CO₂ fluid and do not allow for the effect that fluid immiscibility has on the flow patterns. It is shown that fluids emanating from calc-alkaline melts that crystallize at shallow depths are brines. Their salinity may vary depending mainly on pressure and fraction of crystallized melt. Infiltration-driven decarbonation reactions in the host rocks inevitably proceed at the boundaries of the two fluid solvus where the produced CO₂ is immiscible and may separate from the brine as a low salinity, low density H₂O–CO₂ fluid. Most parameters of fluid–rock interaction in contact aureoles that are derived from progress of mineral reactions and stable isotope resetting are probably incorrect because fluid phase separation is disregarded.     

Growth of multilayered polycrystalline reaction rims in the MgO–SiO<Subscript>2</Subscript> system,part I: experiments

Growth of transport-controlled reaction layers between single crystals of periclase and quartz, and forsterite and quartz was investigated experimentally at 1.5 GPa, 1100°C to 1400°C, 5 min to 72 h under dry and melt-free conditions using a piston-cylinder apparatus. Starting assemblies consisting of Per | Qtz | Fo sandwiches produced polycrystalline double layers of forsterite and enstatite between periclase and quartz, and enstatite single layers between forsterite and quartz. The position of inert Pt-markers initially deposited at the interface of the reactants and inspection of mass balance confirmed that both layer-producing reactions are controlled by MgO diffusion, while SiO₂ is relatively immobile. BSE and TEM imaging revealed thicknesses from 0.6 μm to 14 μm for double layers and from 0 to 6.8 μm for single layers. Both single and double layers displayed non-parabolic growth together with pronounced grain coarsening. Textural evolution and growth rates for each reaction are directly comparable. Forsterite–enstatite double layers are always wider than enstatite single layers, and the growth of enstatite in the double layer is slower than that in the single layer. In double layers, the enstatite/forsterite layer thickness ratio significantly increases with temperature, reflecting different MgO mobilities as temperature varies. Thus, thickness ratios in multilayered reaction zones may contain a record of temperature, but also that of any physico-chemical parameter that modifies the mobilities of the chemical components between the various layers. This potential is largely unexplored in geologically relevant systems, which calls for further experimental studies of multilayered reaction zones.     

Mechanisms and rates of quartz dissolution in melts in the CMAS (CaO–MgO–Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–SiO<Subscript>2</Subscript>) system

The dissolution rate of minerals in silicate melts is generally assumed to be a function of the rate of mass transport of the released cations in the solvent. While this appears to be the case in moderately to highly viscous solvents, there is some evidence that the rate-controlling step may be different in very fluid, highly silica undersaturated melts such as basanites. In this study, convection-free experiments using solvent melts with silica activity from 0.185–0.56 and viscosity from 0.03–4.6 Pa s show that the dissolution rate is strongly dependent on the degree of superheating, silica activity and the viscosity of the solvent. Dissolution rates increase with increasing melt temperature and decreasing silica activity and viscosity. Quartz dissolution in melts with viscosity <0.59–1.9 Pa s and silica activity <0.47 is controlled by the rate of interface reaction as shown by the absence of steady state composition and silica saturation in the interface melts. Only in the most viscous melt with the highest silica activity is quartz dissolution controlled by the rate of diffusion in the melt and only after a long initiation time. The results of this study indicate that although a diffusion-based model may be applicable to dissolution in viscous magmas, a different approach that combines the interplay between the degree of undersaturation of the melt and its viscosity is required in very fluid melts.This revised version was published online September 2004 with a correction to Figure 8.     

Hydrosilicate liquids in the system rare-metal granite–Na<Subscript>2</Subscript>O–SiO<Subscript>2</Subscript>–H<Subscript>2</Subscript>O as accumulators of ore components at high pressure and temperature

Experimental investigations in the system rare-metal granite–Na₂O–SiO₂–H₂O with the addition of aqueous solutions containing Rb, Cs, Sn, W, Mo, and Zn at 600°C and 1.5 kbar showed that the typical elements of rare-metal granites (Li, Rb, Cs, Be, Nb, and Ta) are preferentially concentrated in hydrosilicate liquids coexisting with aqueous fluid. The same behavior is characteristic of Zn and Sn, the minerals of which are usually formed under hydrothermal conditions. In contrast, Mo and W are weakly extracted by hydrosilicate liquids and almost equally distributed between them and aqueous fluids. Liquids similar to those described in this paper are formed during the final stages of magmatic crystallization in granite and granitepegmatite systems. The formation of hydrosilicate liquids in late magmatic and postmagmatic processes will be an important factor controlling the redistribution of metal components between residual magmatic melts, minerals, and aqueous fluids and, consequently, the mobility of these components in fluid-saturated magmatic systems enriched in rare metals.     

Trace element partitioning between orthopyroxene and anhydrous silicate melt on the lherzolite solidus from 1.1 to 3.2 GPa and 1,230 to 1,535°C in the model system Na<Subscript>2</Subscript>O–CaO–MgO–Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–SiO<Subscript>2</Subscript>

We determined experimentally the Nernst distribution coefficient between orthopyroxene and anhydrous silicate melt for trace elements i in the system Na₂O–CaO–MgO–Al₂O₃–SiO₂ (NCMAS) along the dry model lherzolite solidus from 1.1 GPa/1,230°C up to 3.2 GPa/1,535°C in a piston cylinder apparatus. Major and trace element composition of melt and orthopyroxene were determined with a combination of electron microprobe and ion probe analyses. We provide partitioning data for trace elements Li, Be, B, K, Sc, Ti, V, Cr, Co, Ni, Rb, Sr, Y, Zr, Nb, Cs, Ba, La, Ce, Sm, Nd, Yb, Lu, Hf, Ta, Pb, U, and Th. The melts were chosen to be boninitic at 1.1 and 2.0 GPa, picritic at 2.3 GPa and komatiitic at 2.7 and 3.2 GPa. Orthopyroxene is Tschermakitic with 8 mol% Mg-Tschermaks MgAl[AlSiO₆] at 1.1 GPa while at higher pressure it has 18–20 mol%. The rare earth elements show a continuous, significant increase in compatibility with decreasing ionic radius from D _La^opx−melt ∼ 0.0008 to D _Lu^opx−melt ∼ 0.15. For the high-field-strength elements compatibility increases from D _Th^opx−melt ∼ 0.001 through D _Nb^opx−melt ∼ 0.0015, D _U^opx−melt ∼ 0.002, D _Ta^opx−melt ∼ 0.005, D _Zr^opx−melt ∼ 0.02 and D _Hf^opx−melt ∼ 0.04 to D _Ti^opx−melt ∼ 0.14. From mathematical and graphical fits we determined best-fit values for D ₀^M1, D ₀^M2, r ₀^M1, r ₀^M2, E ₀^M1, and E ₀^M2 for the two different M sites in orthopyroxene according to the lattice strain model and calculated the intracrystalline distribution between M1 and M2. Our data indicate extreme intracrystalline fractionation for most elements in orthopyroxene; for the divalent cations D _i ^M2−M1 varies by three orders of magnitude between D _Co^M2−M1 = 0.00098–0.00919 and D _Ba^M2−M1 = 2.3–28. Trivalent cations Al and Cr almost exclusively substitute on M1 while the other trivalent cations substitute on M2; D _La^M2−M1 reaches extreme values between 6.5 × 10⁷ and 1.4 × 10¹⁶. Tetravalent cations Ti, Hf, and Zr almost exclusively substitute on M1 while U and Th exclusively substitute on M2. Our new comprehensive data set can be used for polybaric-polythermal melting models along the Earth’s mantle solidus. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Titanium solubility in olivine in the system TiO<Subscript>2</Subscript>–MgO–SiO<Subscript>2</Subscript>: no evidence for an ultra-deep origin of Ti-bearing olivine

The finding of ilmenite rods in olivine from orogenic peridotites has sparked a discussion about the processes of incorporation and exsolution of titanium in olivine. We have experimentally investigated the solubility of Ti in olivine as a function of composition, temperature and pressure in the synthetic TiO₂–MgO–SiO₂ system. Experiments at atmospheric pressure in the temperature range 1,200–1,500°C showed that the highest concentration of TiO₂ is obtained when olivine coexists with spinel (Mg₂TiO₄). The amount of TiO₂ in olivine in the assemblages olivine + spinel + periclase and olivine + spinel + ilmenite at 1,500°C was 1.25 wt.%. Changes in the coexisting phases and decreasing temperature result in a significant reduction of the Ti solubility. Olivine coexisting with pseudobrookite (MgTi₂O₅) and a Ti–Si-rich melt at 1,500°C displays a fourfold lower TiO₂ content than when buffered with spinel. A similar decrease in solubility is obtained by a decrease in temperature to 1,200°C. There is a negative correlation between Ti and Si and no correlation between Ti and Mg in Ti-bearing olivine. Together with the established phase relations this suggests that there is a direct substitution of Ti for Si at these temperatures, such that the substituting component has the stoichiometry Mg₂TiO₄. The unit cell volume of olivine increases systematically with increasing TiO₂ content demonstrating that the measured TiO₂ contents in olivine are not caused by micro-inclusions but by incorporation of Ti in the olivine structure. Least squares fitting of 20 olivine unit cell volumes against the Ti content yield the relation: V (ų)=290.12(1) + 23.67(85) N_Ti. The partial molar volume of end-member Mg₂TiO₄ olivine (N_Ti=1) is thus 47.24±0.13 cm³. The change of the Ti solubilty in olivine coexistent with rutile and orthopyroxene with pressure was investigated by piston cylinder experiments at 1,400°C from 15 to 55 kbar. There is no increase in TiO₂ contents with pressure and in all the experiments olivine contains ~0.2 wt.% TiO₂. Moreover, a thermodynamic analysis indicates that Ti contents of olivine coexisting with rutile and orthopyroxene should decrease rather than increase with increasing pressure. These data indicate that the ilmenite exsolution observed in some natural olivine does not signify an ultra-deep origin of peridotite massifs.     

The high-pressure stability of chlorite and other hydrates in subduction mélanges: experiments in the system Cr<Subscript>2</Subscript>O<Subscript>3</Subscript>–MgO–Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–SiO<Subscript>2</Subscript>–H<Subscript>2</Subscript>O

The solubility of chromium in chlorite as a function of pressure, temperature, and bulk composition was investigated in the system Cr₂O₃–MgO–Al₂O₃–SiO₂–H₂O, and its effect on phase relations evaluated. Three different compositions with X _Cr = Cr/(Cr + Al) = 0.075, 0.25, and 0.5 respectively, were investigated at 1.5–6.5 GPa, 650–900 °C. Cr-chlorite only occurs in the bulk composition with X _Cr = 0.075; otherwise, spinel and garnet are the major aluminous phases. In the experiments, Cr-chlorite coexists with enstatite up to 3.5 GPa, 800–850 °C, and with forsterite, pyrope, and spinel at higher pressure. At P > 5 GPa other hydrates occur: a Cr-bearing phase-HAPY (Mg_2.2Al_1.5Cr_0.1Si_1.1O₆(OH)₂) is stable in assemblage with pyrope, forsterite, and spinel; Mg-sursassite coexists at 6.0 GPa, 650 °C with forsterite and spinel and a new Cr-bearing phase, named 11.5 Å phase (Mg:Al:Si = 6.3:1.2:2.4) after the first diffraction peak observed in high-resolution X-ray diffraction pattern. Cr affects the stability of chlorite by shifting its breakdown reactions toward higher temperature, but Cr solubility at high pressure is reduced compared with the solubility observed in low-pressure occurrences in hydrothermal environments. Chromium partitions generally according to \(X_{\text{Cr}}^{\text{spinel}}\) ? \(X_{\text{Cr}}^{\text{opx}}\) > \(X_{\text{Cr}}^{\text{chlorite}}\) ≥ \(X_{\text{Cr}}^{\text{HAPY}}\) > \(X_{\text{Cr}}^{\text{garnet}}\). At 5 GPa, 750 °C (bulk with X _Cr = 0.075) equilibrium values are \(X_{\text{Cr}}^{\text{spinel}}\) = 0.27, \(X_{\text{Cr}}^{\text{chlorite}}\) = 0.08, \(X_{\text{Cr}}^{\text{garnet}}\) = 0.05; at 5.4 GPa, 720 °C \(X_{\text{Cr}}^{\text{spinel}}\) = 0.33, \(X_{\text{Cr}}^{\text{HAPY}}\) = 0.06, and \(X_{\text{Cr}}^{\text{garnet}}\) = 0.04; and at 3.5 GPa, 850 °C \(X_{\text{Cr}}^{\text{opx}}\) = 0.12 and \(X_{\text{Cr}}^{\text{chlorite}}\) = 0.07. Results on Cr–Al partitioning between spinel and garnet suggest that at low temperature the spinel- to garnet-peridotite transition has a negative slope of 0.5 GPa/100 °C. The formation of phase-HAPY, in assemblage with garnet and spinel, at pressures above chlorite breakdown, provides a viable mechanism to promote H₂O transport in metasomatized ultramafic mélanges of subduction channels.     

Effect of KCl on melting in the Mg<Subscript>2</Subscript>SiO<Subscript>4</Subscript>–MgSiO<Subscript>3</Subscript>–H<Subscript>2</Subscript>O system at 5 GPa

To examine the effect of KCl-bearing fluids on the melting behavior of the Earth’s mantle, we conducted experiments in the Mg₂SiO₄–MgSiO₃–H₂O and Mg₂SiO₄–MgSiO₃–KCl–H₂O systems at 5 GPa. In the Mg₂SiO₄–MgSiO₃–H₂O system, the temperature of the fluid-saturated solidus is bracketed between 1,200–1,250°C, and both forsterite and enstatite coexist with the liquid under supersolidus conditions. In the Mg₂SiO₄–MgSiO₃–KCl–H₂O systems with molar Cl/(Cl + H₂O) ratios of 0.2, 0.4, and 0.6, the temperatures of the fluid-saturated solidus are bracketed between 1,400–1,450°C, 1,550–1,600°C, and 1,600–1,650°C, respectively, and only forsterite coexists with liquid under supersolidus conditions. This increase in the temperature of the solidus demonstrates the significant effect of KCl on reducing the activity of H₂O in the fluid in the Mg₂SiO₄–MgSiO₃–H₂O system. The change in the melting residues indicates that the incongruent melting of enstatite (enstatite = forsterite + silica-rich melt) could extend to pressures above 5 GPa in KCl-bearing systems, in contrast to the behavior in the KCl-free system.     

Growth of multilayered polycrystalline reaction rims in the MgO–SiO<Subscript>2</Subscript> system,part II: modelling

Part I of this contribution (Gardés et al. in Contrib Mineral Petrol, 2010) reported time- and temperature-dependent experimental growth of polycrystalline forsterite-enstatite double layers between single crystals of periclase and quartz, and enstatite single layers between forsterite and quartz. Both double and single layers displayed growth rates decreasing with time and pronounced grain coarsening. Here, a model is presented for the growth of the layers that couples grain boundary diffusion and grain coarsening to interpret the drop of the growth rates. It results that the growth of the layers is such that (Δx)² ∝ t ^1−1/n , where Δx is the layer thickness and n the grain coarsening exponent, as experimentally observed. It is shown that component transport occurs mainly by grain boundary diffusion and that the contribution of volume diffusion is negligible. Assuming a value of 1 nm for the effective grain boundary width, the following Arrhenius laws for MgO grain boundary diffusion are derived: log D _gb,0^Fo (m²/s) = −2.71 ± 1.03 and E _gb^Fo = 329 ± 30 kJ/mol in forsterite and log D _gb,0^En (m²/s) = 0.13 ± 1.31 and E _gb^En = 417 ± 38 kJ/mol in enstatite. The different activation energies are responsible for the changes in the enstatite/forsterite thickness ratio with varying temperature. We show that significant biases are introduced if grain boundary diffusion-controlled rim growth is modelled assuming constant bulk diffusivities so that differences in activation energies of more than 100 kJ/mol may arise. It is thus important to consider grain coarsening when modelling layered reaction zones because they are usually polycrystalline and controlled by grain boundary transport.     

Thermophysical properties of the α–β–γ polymorphs of Mg<Subscript>2</Subscript>SiO<Subscript>4</Subscript>: a computational study

Thermophysical properties of the various polymorphs (i.e. α-, β- and γ) of Mg₂SiO₄ were computed with the CRYSTAL06 code within the framework of CO-LCAO-GTF approach by using the hybrid B3LYP density functional method. Potential wells were calculated through a symmetry preserving, variable cell-shape structure relaxation procedure. Vibrational frequencies were computed at the long-wavelength limit corresponding to the center of the Brillouin zone (k → 0). Thermodynamic properties were estimated through a semiclassical approach that combines B3LYP vibrational frequencies for optic modes and the Kieffer’s model for the dispersion relation of acoustic modes. All computed values except volume (i.e. electronic energy, zero point energy, optical vibrational modes, thermal corrections to internal energy, standard state enthalpy and Gibbs free energy of reaction, bulk modulus and its P and T derivatives, entropy, C _V, C _P) are consistent with available experimental data and/or reasonable estimates. Volumes are slightly overestimated relative to those determined directly by X-ray diffraction. A set of optimized volumetric properties that are consistent with the other semiclassical properties of the phases α, β and γ have been derived by optimization procedure such that the calculated boundaries for the α/β and β/γ equilibria have the best overall agreement with the experimental data for these transitions. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.

La–SiO<Subscript>2</Subscript> and Yb–SiO<Subscript>2</Subscript> systematics in mid-ocean ridge magmas: implications for the origin of oceanic plagiogranite

Analytical expressions for the variation in D _La and D _Yb with increasing liquid SiO₂ for olivine, plagioclase, augite, hornblende, orthopyroxene, magnetite and ilmenite (Brophy in Contrib Mineral Petrol 2008, online first) have been combined with numerical models of hydrous partial melting, of mid-ocean ridge (MOR) cumulate gabbro melting, and fractional crystallization of slightly hydrous mid-ocean ridge basalt (MORB) magma to assess a melting versus fractionation origin for oceanic plagiogranite. For felsic magmas (>63 wt.% SiO₂) the modeling predicts the following. MOR cumulate gabbro melting should yield constant or decreasing La and constant Yb abundances with increasing liquid SiO₂. The overall abundances should be similar to those in associated mafic magmas. MORB fractional crystallization should yield steadily increasing La and Yb abundances with increasing SiO₂ with overall abundances significantly higher than those in associated mafic magmas. Application to natural occurrences of oceanic plagiogranite indicate that both MOR cumulate gabbro melting and MORB fractionation are responsible. Application of the model results to Icelandic rhyolites strongly support a fractional crystallization rather than a crustal melting origin.     

Measurements and calculations of solid-liquid equilibria in the ternary systems NaBr–Na<Subscript>2</Subscript>SO<Subscript>4</Subscript>–H<Subscript>2</Subscript>O and KBr–K<Subscript>2</Subscript>SO<Subscript>4</Subscript>–H<Subscript>2</Subscript>O at 348 K

According to the compositions of the underground brine resources in the west of Sichuan Basin, solubilities of the ternary systems NaBr–Na₂SO₄–H₂O and KBr–K₂SO₄–H₂O were investigated by isothermal method at 348 K. The equilibrium solid phases, solubilities of salts, and densities of the solutions were determined. On the basis of the experimental data, the phase diagrams and the density-composition diagrams were plotted. In the two ternary systems, the phase diagrams consist of two univariant curves, one invariant point and two crystallization fields. Neither solid solution nor double salts were found. The equilibrium solid phases in the ternary system NaBr–Na₂SO₄–H₂O are NaBr and Na₂SO₄, and those in the ternary system KBr–K₂SO₄–H₂O are KBr and K₂SO₄. Using the solubilities data of the two ternary subsystems at 348 K, mixing ion-interaction parameters of Pitzer’s equation θxxx, Ψxxx and Ψxxx were fitted by multiple linear regression method. Based on the chemical model of Pitzer’s electrolyte solution theory, the solubilities of phase equilibria in the two ternary systems NaBr–Na₂SO₄–H₂O and KBr–K₂SO₄–H₂O were calculated with corresponding parameters. The calculation diagrams were plotted. The results showed that the calculated values have a good agreement with experimental data.     

Gold Dissolution in Dry Salt Melts in the Presence of SiO<Subscript>2</Subscript> as a Function of Р(O<Subscript>2</Subscript>)

The solubility of gold was measured in dry NaCl salt melt at 860°С in closed systems with SiO₂ (silica glass). The reactions do not occur in a closed system without oxidizer. Reaction of SiO₂ with salt in the presence of an oxidizer (KClO₄) results in the formation of water-soluble sodium silicates (a mixture of meta-, ortho-, and pyrosilicates). Gold mobilization by a salt melt is limited by the diffusion of Na in SiO₂. In a closed system with the addition of a strong oxidizer (dry KClO₄ salt), the solubility of gold increase with increasing amount of KClO₄ and the saturation level is estimated to be ~3 wt % Au. For ampoule configurations used in our experiments, 5.5 g of gold dissolved per 1 g of KClO₄. Only cheap, non-toxic reagents were used in our model experiments on gold dissolution in a salt melt, which did not require elevated pressures. The solubility of 30 g Au per 1 kg NaCl will eliminate geochemical problems associated with the compact leaching of gold ores using cyanide.     

Phase relations of potassium-bearing clinopyroxene in the system CaMgSi<Subscript>2</Subscript>O<Subscript>6</Subscript>-KAlSi<Subscript>2</Subscript>O<Subscript>6</Subscript> at 7 GPa

The pseudo-binary system CaMgSi₂O₆-KAlSi₂O₆, modeling the potassium-bearing clinopyroxene (KCpx) solid solution, has been studied at 7 GPa and 1,100–1,650 °C. The KCpx is a liquidus phase of the system up to 60 mol% of KAlSi₂O₆. At higher content of KAlSi₂O₆ in the system, grossular-rich garnet becomes a liquidus phase. Above 75 mol% of KAlSi₂O₆ in the system, KCpx is unstable at the solidus as well, and garnet coexists with kalsilite, Si-wadeite and kyanite. No coexistence of KCpx with kyanite was observed. Above the solidus, KAlSi₂O₆ content of the KCpx coexisting with melt increases with decreasing temperature. Near the solidus of the system (about 1,250 °C) KCpx contains up to 5.6 wt% of K₂O, i.e. about 22–26 mol% of KAlSi₂O₆. Such high concentration of potassium in KCpx is presumably the maximal content of KAlSi₂O₆ in the Fe-free clinopyroxene at 7 GPa. In addition to the major substitution Mg^M1C²Al¹K², the KCpx solid solution contains Ca-Eskola and only minor Ca-Tschermack components. Our experimental results indicate that the natural assemblage KCpx+grossular-rich garnet might be a product of crystallization of the ultra-potassic SiO₂-rich alumino-silicate mantle melts (>200 km).Editorial responsibility: J. Hoefs     

Solubility and partitioning of water in synthetic forsterite and enstatite in the system MgO–SiO2–H2O±Al2O3

The solubility of water in coexisting enstatite and forsterite was investigated by simultaneously synthesizing the two phases in a series of high pressure and temperature piston cylinder experiments. Experiments were performed at 1.0 and 2.0 GPa at temperatures between 1,100 and 1,420°C. Integrated OH absorbances were determined using polarized infrared spectroscopy on orientated single crystals of each phase. Phase water contents were estimated using the calibration of Libowitzky and Rossman (Am Mineral 82:1111–1115, 1997). Enstatite crystals, synthesized in equilibrium with forsterite and an aqueous phase at 1,350°C and 2.0 GPa, contain 114 ppm H₂O. This is reduced to 59 ppm at 1,100°C, under otherwise identical conditions, suggesting a strong temperature dependence. At 1,350°C and 1.0 GPa water solubility in enstatite is 89 ppm, significantly lower than that at 2.0 GPa. In contrast water solubility in forsterite is essentially constant, being in the range 36–41 ppm for all conditions studied. These data give partition coefficients in the range 2.28–3.31 for all experiments at 1,350°C and 1.34 for one experiment at 1,100°C. The incorporation of Al₂O₃ in enstatite modifies the OH stretching spectrum in a systematic way, and slightly increases the water solubility.