Phase relation of CaSO<Subscript>4</Subscript> at high pressure and temperature up to 90 GPa and 2300 K

Calcium sulfate (CaSO₄), one of the major sulfate minerals in the Earth’s crust, is expected to play a major role in sulfur recycling into the deep mantle. Here, we investigated the crystal structure and phase relation of CaSO₄ up to ~90 GPa and 2300 K through a series of high-pressure experiments combined with in situ X-ray diffraction. CaSO₄ forms three thermodynamically stable polymorphs: anhydrite (stable below 3 GPa), monazite-type phase (stable between 3 and ~13 GPa) and barite-type phase (stable up to at least 93 GPa). Anhydrite to monazite-type phase transition is induced by pressure even at room temperature, while monazite- to barite-type transition requires heating at least to 1500 K at ~20 GPa. The barite-type phase cannot always be quenched from high temperature and is distorted to metastable AgMnO₄-type structure or another modified barite structure depending on pressure. We obtained the pressure–volume data and density of anhydrite, monazite- and barite-type phases and found that their densities are lower than those calculated from the PREM model in the studied P–T conditions. This suggests that CaSO₄ is gravitationally unstable in the mantle and fluid/melt phase into which sulfur dissolves and/or sulfate–sulfide speciation may play a major role in the sulfur recycling into the deep Earth.     

The high-pressure phase transformations of PbO2: An in-situ X-ray diffraction study

Using a diamond-anvil high-pressure cell, an in situ X-ray diffraction study of PbO₂ to about 240 kbar at room temperature has revealed the following phase transformations: rutile → αPbO₂ → tetragonal fluorite → cubic fluorite with increasing pressure. The volume change for the transition rutile → αPbO₂ is about ?2% and for the transition αPbO₂ → tetragonal fluorite is about ?6%, nearly constant within the pressure range of investigation. The volume difference between the tetragonal and the cubic fluorite-type phases is negligibly small. Both the tetragonal and the cubic fluorite-type phases cannot be preserved after removal of the pressure, even after heating by the laser. Both phases have been found to revert to αPbO₂ at one atmospheric pressure. Optically, the colour of αPbO₂ changes from an opaque black to transparent red with increasing pressure. The tetragonal fluorite-type phase, which exists in the pressure range between about 90 and 180 kbar at toom temperature, is slightly transparent with very dark red or brown colour, and the cubic fluorite-type phase is dark red and transparent.     

Deep Fractionation of Clinopyroxene in the East Pacific Rise 13°N: Evidence from High MgO MORB and Melt Inclusions

Mid-ocean ridge basalts (MORBs) from East Pacific Rise (EPR) 13°N are analysed for major and trace elements, both of which show a continuous evolving trend. Positive MgO–Al2O3 and negative MgO–Sc relationships manifest the cotectic crystallization of plagioclase and olivine, which exist with the presence of plagioclase and olivine phenocrysts and the absence of clinopyroxene phenocrysts. However, the fractionation of clinopyroxene is proven by the positive correlation of MgO and CaO. Thus, MORB samples are believed to show a “clinopyroxene paradox”. The highest magnesium-bearing MORB sample E13-3B (MgO=9.52%) is modelled for isobaric crystallization with COMAGMAT at different pressures. Observed CaO/Al2O3 ratios can be derived from E13-3B only by fractional crystallization at pressure >4 ±1 kbar, which necessitates clinopyroxene crystallization and is not consistent with cotectic crystallization of olivine plus plagioclase in the magma chamber (at pressure ~1 kbar). The initial compositions of the melt inclusions, which could represent potential parental magmas, are reconstructed by correcting for post-entrapment crystallization (PEC). The simulated crystallization of initial melt inclusions also produce observed CaO/Al2O3 ratios only at >4±1 kbar, in which clinopyroxene takes part in crystallization. It is suggested that MORB magmas have experienced clinopyroxene fractionation in the lower crust, in and below the Moho transition zone. The MORB magmas have experienced transition from clinopyroxene+plagioclase+olivine crystallization at >4±1 kbar to mainly olivine+plagioclase crystallization at <1 kbar, which contributes to the explanation of the “clinopyroxene paradox”.     

Spinel-garnet lherzolite transition in the system CaO-MgO-Al2O3-SiO2 revisited: an in situ X-ray study

Large discrepancies are reported for the near-solidus, pressure-temperature location of the spinel to garnet lherzolite univariant curve in the system CaO-MgO-Al₂O₃-SiO₂ (CMAS). Experimental data obtained previously from the piston-cylinder apparatus indicate interlaboratory pressure differences of up to 30% relative. To investigate this disparity—and because this reaction is pivotal for understanding upper mantle petrology—the phase boundary was located by means of an independent method. The reaction was studied via in situ X-ray diffraction techniques in a 6-8 type multianvil press. Pressure is determined by using MgO as an internal standard and is calculated from measured unit cell volume by using a newly developed high-temperature equation of state for MgO. Combinations of real-time and quenched-sample observations are used to bracket the phase transition. The transition between 1350 and 1500°C was reversed, and the reaction was further constrained from 1207 to 1545°C. Within this temperature range, the transition has an average dT/dP slope of ∼40 ± 10°C/kbar, consistent with several previous piston-cylinder studies. Extrapolation of our curve to 1575°C, an established temperature of the P-T invariant point, yields a pressure of 25.1 ± 1.2 kbar. We also obtained a real-time reversal of the quartz-coesite transition at 30.5 ± 2.3 kbar at 1357°C, which is about 2 to 4 kbar lower in pressure than previously determined in the piston-cylinder apparatus.     

Isothermal compression of low-cordierite to 30 kbar (25° C)

The compression of cordierite (Mg, Fe)₂Al₄Si₅O₁₈·n (H₂O, CO₂; Na⁺, K⁺) has been studied up to 30 kbar (25° C) by volumetric measurements with a piston cylinder apparatus and by X-ray measurements with a diamond-anvil cell. Natural cordierite of intermediate Mg-Fe composition and synthetic Mg-cordierite served as samples. Two discontinuities at 2.2±0.3 and 9.0±0.6 kbar which are correlated with very small volume changes (0.3?0.05%) have been found. The X-ray data indicate, however, no symmetry change of the crystal structure. The two discontinuities are interpreted as phase transitions. The two discontinuities establish three pressure dependent phases referred to as low-pressure (LP)-, first high-pressure (HP1)- and second high-pressure (HP2)-phase. The gross compressibility of cordierite decreases from 1.1 Mbar^?1 at low pressure to 0.7 Mbar^?1 at 30 kbar for the intermediate Mg-Fe cordierite, and to 0.4 Mbar^?1 for Mg-cordierite. Depending on the pressure transmitting medium used in the two different compression techniques, two kinds of compression behavior are observed for cordierite. The measurements with the piston cylinder apparatus where lead is used as quasihydrostatic pressure medium indicate normal compression properties. The X-ray data, however, obtained with the diamond anvil cell where a methanol-ethanol mixture provides hydrostatic pressure conditions yield, e.g. for the HP1-phase a dramatic decrease in compressibility to almost zero. IR-spectra from samples of augmenting experiments with methanol, deuteromethanol and D₂O as pressure media indicate that pressure media of which the molecule size is comparable with the dimensions of the cordierite channels may be incorporated in the structure. This suggests that under such hydrostatic conditions the compression of cordierite is modified by a structure internal component which is acting via the channel system.     

Structure and crystal chemistry of perovskite-type MgSiO3

Synthetic clinoenstatite (MgSiO₃) has been converted to a single phase with the perovskite structure in complete reactions at approx. 300 kbar in experiments that utilize the laser-heated diamond-anvil pressure apparatus. The structure of this phase after quenching was determined by powder X-ray diffraction intensity measurement to be similar to that of the distorted rare-earth, orthoferrite-type, orthorhombic perovskites, but it is suggested that such distortion from ideal cubic perovskite would diminish at high pressure. The unit cell dimensions and density of perovskite-type MgSiO₃ at ambient conditions (1 bar, 25°C) are a=4.780(1) Å, b=4.933(1) Å, c=6.902(1) Å, V=162.75 ų, and ρ=4.098(1) g/cm³. This phase is 3.1% denser than the isochemical oxide mixture [periclase (MgO)+stishovite (SiO₂)]. The small crystal-field stabilization energy of the cation site in the perovskite structure may play an important role in limiting the high-pressure stability field of perovskites that contain transition metal cations. Approximate calculations of the crystal-field effects indicate that perovskite of pure FeSiO₃ composition may become stable at 400–600 kbar; pressures greater than 800 kbar would be required to stabilize CoSiO₃ or NiSiO₃ perovskite.     

In situ determination of crystal structure for high pressure phase of ZrO2 using a diamond anvil and single crystal X-ray diffraction method

Combining a miniature diamond-anvil pressure cell with a single crystal four-circle diffractometer, the crystal structure of a synthetic ZrO₂ has been studied in situ up to 51 kbar at room temperature. The space group of the unquenchable orthorhombic high pressure phase is Pbcm. The directions of the b and c axes are preserved through the transition and the transformation is displacive. The coordination configurations of the Zr atoms and oxygen atoms are the same in the high pressure and low pressure phases. The orthorhombic high pressure phase has a higher entropy than that of low pressure monoclinic phase.     

The gypsum-anhydrite equilibrium by solubility measurements

Solubility measurements have been used to establish the gypsum-anhydrite equilibrium in the CaSO₄-H₂O system at atmospheric pressure. The saturation equilibrium has been approached both from undersaturated and supersaturated solutions. The invariant point temperature has been found to be 49.5 ± 2.5°C.     

The influence of H2O-H2 fluids and redox conditions on melting temperatures in the haplogranite system

Solidus temperatures of quartz–alkali feldspar assemblages in the haplogranite system (Qz-Ab-Or) and subsystems in the presence of H₂O-H₂ fluids have been determined at 1, 2, 5 and 8 kbar vapour pressure to constrain the effects of redox conditions on phase relations in quartzofeldspathic assemblages. The hydrogen fugacity (f _H2) in the fluid phase has been controlled using the Shaw membrane technique for moderately reducing conditions (f _H2 < 60 bars) at 1 and 2 kbar total pressure. Solid oxygen buffer assemblages in double capsule experiments have been used to obtain more reducing conditions at 1 and 2 kbar and for all investigations at 5 and 8 kbar. The systems Qz-Or-H₂O-H₂ and Qz-Ab-H₂O-H₂ have only been investigated at moderately reducing conditions (1 and 5 kbar) and the system Qz-Ab-Or-H₂O-H₂ has been investigated at redox conditions down to IW (1 to 8 kbar). The results obtained for the water saturated solidi are in good agreement with those of previous studies. At a given pressure, the solidus temperature is found to be constant (within the experimental precision of ± 5°C) in the f _H2 range of 0–75 bars. At higher f _H2, generated by the oxygen buffers FeO-Fe₃O₄ (WM) and Fe-FeO (IW), the solidus temperatures increase with increasing H₂ content in the vapour phase. The solidus curves obtained at 2 and 5 kbar have similar shapes to those determined for the same quartz - alkali feldspar assemblages with H₂O-CO₂- or H₂O-N₂-bearing systems. This suggests that H₂ has the behaviour of an inert diluent of the fluid phase and that H₂ solubility in aluminosilicate melts is very low. The application of the results to geological relevant conditions [HM (hematite-magnetite) > f _O2 > WM] shows that increasing f _H2 produces a slight increase of the solidus temperatures (up to 30 °C) of quartz–alkali feldspar assemblages in the presence of H₂O-H₂ fluids between 1 and 5 kbar total pressure. Received: 4 March 1996 / Accepted: 22 August 1996     

Liquidus phase relations on the join diopside-forsterite-anorthite from 1 atm to 20 kbar: Their bearing on the generation and crystallization of basaltic magma

In the system CaO-MgO-Al₂O₃-SiO₂, the tetrahedron CaMgSi₂O₆(di)-Mg₂SiO₄(fo)-SiO₂-CaAl₂ SiO₆(CaTs) forms a simplified basalt tetrahedron, and within this tetrahedron, the plane di-fo-CaAl₂Si₂O₈(an) separates simplified tholeiitic from alkalic basalts. Liquidus phase relations on this join have been studied at 1 atm and at 7, 10, 15, and 20 kbar. The temperature maximum on the 1 atm isobaric quaternary univariant line along which forsterite, diopside, anorthite, and liquid are in equilibrium lies to the SiO₂-rich side of the join di-fo-an. The isobaric quaternary invariant point at which forsterite, diopside, anorthite, spinel, and liquid are in equilibrium passes, with increasing pressure, from the silica-poor to the silica-rich side of the join di-fo-an, which causes the piercing points on this join to change from forsterite+diopside+anorthite+liquid and forsterite +spinel+anorthite+liquid below 5 kbar to forsterite +diopside+spinel+liquid and diopside +spinel+anorthite+liquid above 5 kbar. As pressure increases, the forsterite and anorthite fields contract and the diopside and corundum fields expand. The anorthite primary phase field disappears entirely from the join di-fo-an between 15 and 20 kbar. Below about 4 kbar, the join di-fo-an represents, in simplified form, a thermal divide between alkalic and tholeiitic basalts. From about 4 to at least 12 kbar, alkalic basalts can produce tholeiitic basalts by fractional crystallization, and at pressures above about 12 kbar, it is possible for alkalic basalt to be produced from oceanite by crystallization of both olivine and orthopyroxene. If alkalic basalts are primary melts from a lherzolite mantle, they must be produced at high pressures, probably greater than about 12 kbar.Department of Geosciences, University of Texas at Dallas Contribution No. 327. Hawaii Institute of Geophysics Contribution No. 814.     

Phase relations in the MgO–P2O5–H2O system and the stability of phosphoellenbergerite: petrological implications

The polymorphic relations for Mg₃(PO₄)₂ and Mg₂PO₄OH have been determined by reversed experiments in the temperature-pressure (T-P) range 500–1100 °C, 2–30 kbar. The phase transition between the low-pressure phase farringtonite and Mg₃(PO₄)₂-II, the Mg analogue of sarcopside, is very pressure dependent and was tightly bracketed between 625 °C, 7 kbar and 850 °C, 9 kbar. The high-temperature, high-pressure polymorph, Mg₃(PO₄)₂-III, is stable above 1050 °C at 10 kbar and above 900 °C at 30 kbar. The low-pressure stability of farringtonite is in keeping with its occurrence in meteorites. The presence of iron stabilizes the sarcopside-type phase towards lower P. From the five Mg₂PO₄OH polymorphs only althausite, holtedahlite, β-Mg₂PO₄OH (the hydroxyl analogue of wagnerite) and ɛ-Mg₂PO₄OH were encountered. Relatively speaking, holtedahlite is the low-temperature phase (<600 °C), ɛ-Mg₂PO₄OH the high-temperature, low-pressure phase and β-Mg₂PO₄OH the high-temperature, high-pressure phase, with an intervening stability field for althausite which extends from about 3 kbar at 500 °C to about 12 kbar at 800 °C. Althausite and holtedahlite are to be expected in F-free natural systems under most geological conditions; however, wagnerite is the most common Mg-phosphate mineral, implying that fluorine has a major effect in stabilizing the wagnerite structure. Coexisting althausite and holtedahlite from Modum, S. Norway, show that minor fluorine is strongly partitioned into althausite (K_D ^F/OH≈ 4) and that holtedahlite may incorporate up to 4 wt% SiO₂. Synthetic phosphoellenbergerite has a composition close to (Mg_0.9□_0.1)₂Mg₁₂P₈O₃₈H_8.4. It is a high-pressure phase, which breaks down to Mg₂PO₄OH + Mg₃(PO₄)₂ + H₂O below 8.5 kbar at 650 °C, 22.5 kbar at 900 °C and 30 kbar at 975 °C. The stability field of the phosphate end-member of the ellenbergerite series extends therefore to much lower P and higher T than that of the silicate end-members (stable above 27 kbar and below ca. 725 °C). Thus the Si/P ratio of intermediate members of the series has a great barometric potential, especially in the Si-buffering assemblage with clinochlore + talc + kyanite + rutile + H₂O. Application to zoned ellenbergerite crystals included in the Dora-Maira pyrope megablasts, western Alps, reveals that growth zoning is preserved at T as high as 700–725 °C. However, the record of attainment of the highest T and/or of decreasing P through P-rich rims (1 to 2 Si pfu) is only possible in the presence of an additional phosphate phase (OH-bearing or even OH-dominant wagnerite in these rocks), otherwise the trace amounts of P in the system remain sequestered in the core of Si-rich crystals (5 to 8 Si pfu) and can no longer react. Received: 7 April 1995 / Accepted: 12 November 1997     

Experimentelle Bestimmung der P-T-Stabilitätsbereiche in der Mischkristallreihe Tremolit-Tschermakit

The join tremolite (Tr)—tschermakite (Ts) was studied at temperatures of 450 to 900° C under water vapour pressure of 2 kbar. Solid solution between the end members is restricted to composition range Tr₁₀₀-Tr₄₅. Reconnaissance runs at 800°C and 10 kbar indicated that no further substitution of Al in the tremolite structure is possible by an increase of pressure. In the composition range Ts₅₅-Ts₁₀₀ tremolite-tschermakite solid solution Tr₄₅Ts₅₅ is formed with anorthite, forsterite and enstatite above 700°C and with anorthite and chlorite below 700° C. No amphibole could be synthezised from a material of composition Ts₁₀₀. Materials of composition Ts₁₀₀ crystallized to anorthite, enstatite and frosterite above 700°C and to anorthite and chlorite below 700°C. The high temperature breakdown curve for tremolite-tschermakite solid solutions drops from 870°C for pure tremolite to 826°C for Tr₄₅Ts₅₅. Additional experiments at 1 and 3 kbar indicate that the pressure effect on breakdown temperatures amounts to about 35°C/kbar. The formation of natural amphiboles belonging to the tremolite-tschermakite series is discussed in the light of the experimental data.     

Coordination of Fe,Ga and Ge in high pressure glasses by Mössbauer,Raman and X-ray absorption spectroscopy,and geological implications

Mössbauer spectra of glasses of NaFeSi₃O₈ and 3NaAlSi₂O₆ · NaFeSiO₄ starting compositions consist of a dominant Fe³⁺ and subordinate Fe²⁺ quadrupole-split doublet, in agreement with previous work. Fe³⁺ is assigned to tetrahedral coordination. Pressure-induced coordination changes are not observed in the pressure range 1 bar to 30 kbar. A gradual increase in isomer shift of the Fe³⁺ doublet with increase in pressure is attributed to steric effects. Raman spectra of GeO₂, NaGaSi₃O₈ and NaGaSiO₄ glasses are dominated by network structure vibrations. There is no detectable change in the nearest-neighbor coordination of Ge⁴⁺ in GeO₂ from 1 bar to 14 kbar, of Ga³⁺ in NaGaSi₃O₈ from 1 bar to 28 kbar and of Ga³⁺ in NaGaSiO₄ from 1 bar to 25 kbar. However, some structural reorganization outside of the first coordination sphere occurs in the high pressure glasses.XANES and EXAFS spectra on powdered samples of 1 bar and 25 kbar NaGaSiO₄ glasses and crystalline NaGaSiO₄ were obtained from K edge absorption spectra at the Stanford Synchrotron Radiation Laboratory using a double crystal monochromator equipped with Si(220) crystals. The XANES spectra indicate that Ga³⁺ has a similar extended coordination geometry in both glasses. The EXAFS spectra reveal that Ga³⁺ is four-coordinated with oxygen in all three samples with a Ga³⁺-O distance of about 1.83 Å. The radial distribution functions of the two glasses are virtually identical. However, both XANES and EXAFS spectra indicate significant structural differences between crystalline NaGaSiO₄ (nepheline-type structure) and vitreous NaGaSiO₄ beyond the first coordination shell of Ga³⁺. Thus, X-ray absorption spectroscopy independently confirms the Raman results on the unchanged coordination of Ga³⁺ in NaGaSiO₄ glasses with pressures up to 25 kbar.Glass compositions were selected in anticipation that larger and/or lower charged cations would exhibit pressure-induced coordination changes at lower pressures than Al³⁺ and Si⁴⁺. The present null result suggests that the stabilizing features of open network structures in the liquid state (large entropy and minimized cation-cation repulsion) more than compensate for large molar volume in the pressure range accessible to experimentation. It appears that network structures in natural magmas should remain stable throughout the upper mantle. Consequently, the densities of magmas at high pressures which are calculated from compressibility data and the appropriate equation of state will be only slightly underestimated, due to the effect of minor structural changes beyond the first coordination sphere.     

Fluid evolution of rare-element and muscovite granitic pegmatites from central Galicia, NW Spain

Fluid inclusions have been studied in three pegmatite fields in Galicia, NW Iberian Peninsula. Based on microthermometry and Raman spectroscopy, eight fluid systems have been recognized. The first fluid may be considered to be a pegmatitic fluid which is represented by daughter mineral (silicates)-rich aqueous inclusions. These inclusions are primary and formed above 500 °C (dissolution of daughter minerals). During pegmatite crystallization, this fluid evolved to a low-density, volatile-rich aqueous fluid with low salinity (93% H₂O; 5% CO₂; 0.5% CH₄; 0.2% N₂; 1.3% NaCl) at minimum P–T conditions around 3 ± 0.5 kbar and 420 °C. This fluid is related to rare-metal mineralization. The volatile enrichment may be due to mixing of magmatic fluids and fluids equilibrated with the host rock. A drop in pressure from 3 ± 0.5 to 1 kbar at a temperature above 420 °C, which may be due to the transition from predominantly lithostatic to hydrostatic pressure, is recorded by two-phase, water-rich inclusions with a low-density vapour phase (CO₂, CH₄ and N₂). Another inclusion type is represented by two-phase, vapour-rich inclusions with a low-density vapour phase (CO₂, CH₄ and N₂), indicating a last stage of decreasing temperature (360 °C) and pressure (around 0.5 kbar), probably due to progressive exhumation. Finally, volatile (CO₂)-rich aqueous inclusions, aqueous inclusions (H₂O-NaCl) and mixed-salt aqueous inclusions with low Th, are secondary in charac- ter and represent independent episodes of hydrothermal fluid circulation below 310 °C and 0.5 kbar. Received: 14 October 1999 / Accepted: 5 October 1999     

“Pseudosinhalite”, a new hydrous MgAl-borate: synthesis, phase characterization, crystal structure, and PT-stability

The new synthetic phase Mg₂Al₃O[BO₄]₂(OH) provisionally named “pseudosinhalite” is optically, chemically, and structurally similar to the mineral sinhalite, MgAl[BO₄], isostructural with forsterite. It grows hydrothermally from appropriate bulk compositions in the range 4–40?kbar at temperatures that increase with pressure (～650?→?900?°C), and it breaks down at higher temperatures to sinhalite?+?corundum?+?H₂O. At P?≥?20?kbar single-phase products of euhedral twinned crystals could often be obtained. Pseudosinhalite is monoclinic with a?=?7.455 (1) Å, b?=?4.330 (1) Å, c?=?9.825 (2) Å, β?=?110.68 (1)°, and space group P2₁/c. Crystal structure analysis reveals that pseudosinhalite is also based on hexagonal close packing (hcp) of oxygen atoms with Mg and Al in octahedral and B in tetrahedral coordination. In pseudosinhalite the winged octahedral chains in the plane of hcp are not straight as in sinhalite but have a zigzag, 3-repeat period (Dreierkette), and only 1/10 instead of 1/8 of all tetrahedral sites are filled by boron. Hydrogen is located at a split position between two oxygen atoms O5—O5, which are only 2.550 Å apart and thus generate strong hydrogen bonding. This may be responsible for the absence of an hydroxyl absorption band between 2800?cm^?1 and 3500?cm^?1 in the powder IR spectrum. The equilibrium breakdown curve of pseudosinhalite to form sinhalite, corundum, and water was determined by bracketing experiments to pass through 10?kbar, 745?°C and 35?kbar, 950?°C, giving a slope of about 8?°C/kbar, similar to dehydration curves of some silicates at high pressure. In nature pseudosinhalite could have been misidentified as sinhalite. A possible appearance, like sinhalite in boron-rich skarns, would require more aluminous bulk compositions than for sinhalite at relatively low temperatures. However, pseudosinhalite might also form as a hydrous alteration product of sinhalite at low temperatures, perhaps in association with szaibelyite, MgBO₂(OH).     

Liquidus phase relationships on the join anorthite-forsterite-quartz at 10 kbar with applications to basalt petrogenesis

Liquidus phase relationships determined on the join CaAl₂Si₂O₈ (anorthite)-Mg₂SiO₄ (forsterite)-SiO₂ (quartz) at 10 kbar show that increasing pressure causes the forsterite and anorthite primary phase fields to shrink and the spinel, enstatite and silica fields to expand. The boundary line between the enstatite and forsterite fields and that between the enstatite and quartz fields both move away from the SiO₂ apex as pressure increases. Therefore, simplified source peridotite would yield simplified basaltic partial melts with decreasing silica as pressure increases, as has been found in other studies. Also, increasing pressure decreases the amount of silica enrichment in residual liquids produced by fractional crystallization. Although anorthite is unstable in simplified peridotite above 9 kbar in the system CaO-MgO-Al₂O₃-SiO₂, it is an important phase in the fractional crystallization of simplified basalts at 10 kbar and probably also in natural basalts.Contribution no. 419, Department of Geosciences, University of Texas at Dallas     

Experimental study of phase relations involving osumilite in the system K2O-FeO-MgO-Al2O3-SiO2-H2O at high pressure and temperature

Abstract Phase relations and mineral chemistry for garnet (Grt), orthopyroxene (Opx), sapphirine (Spr), water-undersaturated cordierite (Crd), osumilite (Osu), sillimanite (Sil), K-feldspar (Kfs), quartz (Qtz) and a water-undersaturated liquid (Liq) have been determined experimentally in the system KFMASH (K₂O-FeO-MgO-Al₂O₃-SiO₂-H₂O) under low P_H2O and f_O2 conditions. Four compositions have been studied with 100 [Mg/(Mg + Fe)] ranging from 65.6 to 89.7. Based on our experimental data, a P-T grid is derived for the KFMASH system in the presence of quartz, orthopyroxene and liquid. Osumilite has been found in various mineral assemblages from 950 to 1100°C and 7.5 to 11 kbar. In the temperature range 1000-1100°C, the pair Os-Grt is stable over a pressure range of about 3kbar. The divariant reaction Os + Opx = Grt + Kfs + Qtz runs to the right with increasing pressure. Because osumilite is the most magnesian phase it is restricted to Mg-rich compositions at high pressure. The reaction defining the upper pressure stability limit of Os-Grt is located around 11 kbar with a nearly flat dP/dT slope over the temperature range 950–100°C. Over the entire temperature range investigated osumilite is not stable beyond 12 kbar. The data imply a restricted pressure range between 11 and 12 kbar for the stability of the assemblage Os-Opx-Sil-Kfs-Qtz. At 1050°C and above, osumilite occurs in various mineral assemblages together with the high-T pair Spr-Qtz. When coexisting with garnet, orthopyroxene or sapphirine, osumilite is always the most magnesian phase. At 1050 and 1100°C, liquid is invariably the most Fe-rich phase in the run product. Our data support a theoretical P-T grid for the KFMAS system in which osumilite is stable outside the field of the high-T assemblage Spr-Qtz. Moreover, our grid indicates that Os-Opx-Sil-Kfs-Qtz has a more restricted pressure and compositional stability domain than Os-Grt, in agreement with natural occurrences. Osumilite is stable over a large pressure range, such that in Mg-rich rocks, and at high temperature, it can occur at any depth in normal thickness continental crust.     

P-T-X effects on equilibrium carbonate-H2O-CO2-NaCl dihedral angles: constraints on carbonate permeability and the role of deformation during fluid infiltration

Fluid-solid-solid dihedral angles in the NaCl-H₂O-CO₂-calcite-dolomite-magnesite system have been determined at pressures ranging from 0.5 to 7 kbar and temperatures from 450°C to 750°C. At 1 kbar and 650°C, both dolomite and magnesite exhibit a dihedral angle minimum for intermediate H₂O-CO₂ fluids similar to that previously determined by the present authors for calcite, but the depth of the minimum is smaller, being above the critical value of 60° for both dolomite and magnesite for all fluid compositions. Calcite-calcite-brine dihedral angles at 650°C have been determined in the pressure range 1–5 kbar. Angles decrease with increasing salt content of the fluid, tending towards a constant value of about 65° for strong brines at pressures above 2 kbar. There is a general increase of angle with increasing pressure which is most marked for strong brines. A positive correlation of angle with pressure is also observed in calcite-H₂O-CO₂ fluids, the position of the minimum moving towards higher angles and towards H₂O-rich fluids with increasing pressure. The permeability window previously observed by the present authors at 1 kbar and intermediate fluid compositions closes at about 1.5 kbar. The results demonstrate that the permeability of carbonates to grain edge fluid flow is only possible at low pressures and for fluids of restricted H₂O-CO₂-NaCl compositions. However, geochemical evidence from metamorphic terrains suggests that pervasive infiltration does occur under conditions where impermeability is predicted. From examination of published studies of infiltrated carbonates we conclude that deformation plays a critical role in enhancing carbonate permeability. Possible mechanisms for this include shear-enhanced dilatancy (micro-cracking), fluid inclusion drag by deformation-controlled grain boundary migration, and dynamically maintained transient grain boundary fluid films.     

Raman spectra of analcime under pressure

Raman spectra of natural analcime have been recorded at atmospheric pressure and up to 9.4 kbar. The basic Si, Al-O network vibrations are little affected by pressure even though significant volume changes and a minor phase transition take place. However, the 3,557 cm^?1 OH-stretch mode is modified in that band splitting takes place indicating at least two O-OH hydrogen bond distances. Thus there are at least three sites of hydrogen bonding in analcime. The bonded water (H₂O) in analcime appears to remain in the mineral at high pressure. The bulk volume change, determined previously by cell dimension measurements, can be traced to reduction of the size of the “voids” in the structure. This is deduced from the fact that Si-Al-O vibrations are little affected by pressure but O-H vibrations of water molecules found in the voids are strongly pressure-dependent.     

Phase equilibria of phlogopite lamprophyres from western Mexico: biotite-liquid equilibria and P-T estimates for biotite-bearing igneous rocks

 Olivine and augite minette powders have been equilibrated from one bar to nearly 2.0 kbar (water-saturated), and from 900 to 1300° C, and then quenched rapidly, at oxygen fugacities controlled between the nickel-nickel oxide (NNO) and hematite-magnetite (HM) oxygen buffers. The liquidus of both samples is suppressed ∼100° C at water-saturated conditions and 1500 bar. Both lavas contained between 3 and 4 wt% water at the stage of phenocryst precipitation. The partitioning of ferric and ferrous iron between phlogopite and liquid has been determined on eight samples across 3 log f _O2 units; when these determinations are combined with previous studies, Fe₂O₃/(Σ FeO total) of Mg-rich biotite can be calculated knowing log f _O2, T, and X _Fe. Thermodynamic modelling of biotite-liquid equilibria results in two expressions for calculating activity coefficients (γ) for annite and phlogopite in natural biotites. Based on the partitioning of BaO and TiO₂ between biotite and liquid, we have formulated a thermometer and barometer. Over the range of 400° C, TiO₂ partitioning between phlogopite and liquid is a function of temperature (±50° C), and is insensitive to pressure and H₂O and O₂ activities. BaO partitioning between phlogopite and liquid is a function of both temperature and pressure (±4 kbar), the latter being most important. Applying the TiO₂ and BaO partitioning expressions to lamprophyre and lamproite suites shows that Mexican minettes equilibrated at low pressures (5 to 15 kbar;±4 kbar) and temperatures (1090 to 1160° C; ±50° C), while Australian lamproites equilibrated at higher P (up to 30 kbar; ±4 kbar) and T (1125 to 1400° C; ±50° C). Experimental glass compositions and phenocryst fractionation calculations, together with the BaO- and TiO₂- based pressure calculations indicate that felsic minettes from the Mexican suite of lavas can be generated by simple fractionation of a more mafic parent minette at mid to lower crustal pressures. Received: 1 August 1994/Accepted: 30 June 1995