Heat capacity and phase equilibria of hollandite polymorph of KAlSi3O8

The low-temperature heat capacity (C _p ) of KAlSi₃O₈ with a hollandite structure was measured over the range of 5–303 K with a physical properties measurement system. The standard entropy of KAlSi₃O₈ hollandite is 166.2±0.2 J mol⁻¹ K⁻¹, including an 18.7 J mol⁻¹ K⁻¹ contribution from the configurational entropy due to disorder of Al and Si in the octahedral sites. The entropy of K₂Si₄O₉ with a wadeite structure (Si-wadeite) was also estimated to facilitate calculation of phase equilibria in the system K₂O–Al₂O₃–SiO₂. The calculated phase equilibria obtained using Perple_x are in general agreement with experimental studies. Calculated phase relations in the system K₂O–Al₂O₃–SiO₂ confirm a substantial stability field for kyanite–stishovite/coesite–Si-wadeite intervening between KAlSi₃O₈ hollandite and sanidine. The upper stability of kyanite is bounded by the reaction kyanite (Al₂SiO₅) = corundum (Al₂O₃) + stishovite (SiO₂), which is located at 13–14 GPa for 1,100–1,400 K. The entropy and enthalpy of formation for K-cymrite (KAlSi₃O₈·H₂O) were modified to better fit global best-fit compilations of thermodynamic data and experimental studies. Thermodynamic calculations were undertaken on the reaction of K-cymrite to KAlSi₃O₈ hollandite + H₂O, which is located at 8.3–10.0 GPa for the temperature range 800–1,600 K, well inside the stability field of stishovite. The reaction of muscovite to KAlSi₃O₈ hollandite + corundum + H₂O is placed at 10.0–10.6 GPa for the temperature range 900–1,500 K, in reasonable agreement with some but not all experiments on this reaction.     

High temperature stability limit of phase egg, AlSiO3(OH)

The stability relations of phase egg, AlSiO₃(OH), have been investigated at pressures from 7 to 20 GPa, and temperatures from 900 to 1700 °C in a multi-anvil apparatus. At the lower pressures phase egg breaks down according to the univariant reaction, phase egg = stishovite + topaz-OH, which extends from 1100 °C at 11 GPa to 1400 °C at 13 GPa where it terminates at an invariant point involving corundum. At pressures above the invariant point, the stability of phase egg is limited by the breakdown reaction, phase egg = stishovite + corundum + fluid, which extends from the invariant point to 1700 °C at 20 GPa. Stishovite crystallized in the Al₂O₃-SiO₂-H₂O system contains Al₂O₃, and the amount of Al₂O₃ increases with increasing temperature. It is inferred that the Al₂O₃ content is controlled by the charge-balanced substitution of Si⁴⁺ by Al³⁺ and H⁺. Aluminum-bearing stishovite coexisting with an H₂O-rich fluid may contain a certain amount of water. Therefore, phase egg and stishovite in a subducting slab could transport some H₂O into the deep Earth. Received: 14 October 1998 / Accepted: 19 May 1999     

Heat capacity and phase equilibria of wadeite-type K2Si4O9

The low-temperature heat capacity (C _p) of Si-wadeite (K₂Si₄O₉) synthesized with a piston cylinder device was measured over the range of 5–303 K using the heat capacity option of a physical properties measurement system. The entropy of Si-wadeite at standard temperature and pressure calculated from the measured heat capacity data is 253.8 ± 0.6 J mol⁻¹ K⁻¹, which is considerably larger than some of the previous estimated values. The calculated phase transition boundaries in the system K₂O–Al₂O₃–SiO₂ are generally consistent with previous experimental results. Together with our calculated phase boundaries, seven multi-anvil experiments at 1,400 K and 6.0–7.7 GPa suggest that no equilibrium stability field of kalsilite + coesite intervenes between the stability field of sanidine and that of coesite + kyanite + Si-wadeite, in contrast to previous predictions. First-order approximations were undertaken to calculate the phase diagram in the system K₂Si₄O₉ at lower pressure and temperature. Large discrepancies were shown between the calculated diagram compared with previously published versions, suggesting that further experimental or/and calorimetric work is needed to better constrain the low-pressure phase relations of the K₂Si₄O₉ polymorphs. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Mantle melting in equilibrium with an Iron–Wüstite–Graphite buffered COH-fluid

Partial melting experiments on a San Carlos peridotite were done in a Walker type multi-anvil press at pressures from 5 to 12.5 GPa. Experiments were done in the presence of a COH-fluid and at oxygen fugacity controlled by the Fe–FeO buffer. Olivine, clinopyroxene, garnet and orthopyroxene are stable in all but the highest temperature 10 GPa experiments where olivine and garnet coexist, and the highest temperature 5 GPa experiments where olivine is the single crystalline phase. The solidus at 5 GPa was found to be at approximately 1,200°C and the liquidus was estimated to be at 1,325°C, which is ∼500°C lower than has been reported for dry melting of peridotite. The aluminum concentration of the melts decreases with increasing melt fraction and decreases also with increasing pressure. At 5 GPa the melts have a CaO/Al₂O₃-ratio of 0.85–1.0, which is similar to that of undepleted komatiites; major element concentrations are also identical to those of undepleted komatiites such as the Munro komatiites. At 10 and 12.5 GPa the partial melts have CaO/Al₂O₃-ratios above 1.5 and major element composition almost identical to aluminum depleted komatiites such as the Barberton komatiites. We therefore conclude that in the presence of a reducing COH-fluid both aluminum-depleted and -undepleted komatiites could have formed at temperatures much lower than generally accepted.     

Geobarometry for Peridotites: Experiments in Simple and Natural Systems from 6 to 10 GPa

Experiments with peridotite minerals in simple (MgO–Al₂O₃–SiO₂,CaO–MgO–SiO₂ and CaO–MgO–Al₂O₃–SiO₂)and natural systems were conducted at 1300–1500°Cand 6–10 GPa using a multi-anvil apparatus. The experimentsin simple systems demonstrated consistency with previous lowerpressure experiments in belt and piston–cylinder set-ups.The analysis of spatial variations in pyroxene compositionswithin experimental samples was used to demonstrate that pressureand temperature variations within the samples were less than0·4 GPa and 50°C. Olivine capsules were used in natural-systemexperiments with two mineral mixtures: SC1 (olivine + high-Alorthopyroxene + high-Al clinopyroxene + spinel) and J4 (olivine+ low-Al orthopyroxene + low-Al clinopyroxene + garnet). Theexperiments produced olivine + orthopyroxene + garnet ±clinopyroxene assemblages, occasionally with magnesite and carbonate-richmelt. Equilibrium compositions were derived by the analysisof grain rims and evaluation of mineral zoning. They were comparedwith our previous experiments with the same starting mixturesat 2·8–6·0 GPa and the results from simplesystems. The compositions of minerals from experiments withnatural mixtures show smooth pressure and temperature dependencesup to a pressure of 8 GPa. The experiments at 9 and 10 GPa producedandradite-rich garnets and pyroxene compositions deviating fromthe trends defined by the lower pressure experiments (e.g. higherAl in orthopyroxene and Ca in clinopyroxene). This discrepancyis attributed to a higher degree of oxidation in the high-pressureexperiments and an orthopyroxene–high-P clinopyroxenephase transition at 9 GPa. Based on new and previous resultsin simple and natural systems, a new version of the Al-in-orthopyroxenebarometer is presented. The new barometer adequately reproducesexperimental pressures up to 8 GPa. KEY WORDS: garnet; mineral equilibrium; multi-anvil apparatus; orthopyroxene; geobarometry     

Dissociation of CAS phase in the uppermost lower mantle

The high-pressure stability limit of calcium aluminosilicate (CAS) phase has been examined in its end-member CaAl₄Si₂O₁₁ composition at 18–39 GPa and 1,670–2,300 K in a laser-heated diamond-anvil cell (LHDAC). The in-situ synchrotron X-ray diffraction measurements revealed that the CAS phase decomposes into three-phase assemblage of cubic Al-bearing CaSiO₃ perovskite, Al₂O₃ corundum, and SiO₂ stishovite above 30 GPa and 2,000 K with a positive pressure–temperature slope. Present results have important implications for the subsolidus mineral assemblage of subducted sediment and the melting phase relation of basalt in the lower mantle.     

The Ca-Eskola component in eclogitic clinopyroxene as a function of pressure, temperature and bulk composition: an experimental study to 15 GPa with possible implications for the formation of oriented SiO2-inclusions in omphacite

Experiments have been conducted in the P-T range 2.5–15 GPa and 850–1,500°C using bulk compositions in the systems SiO₂–TiO₂–Al₂O₃–Fe₂O₃–FeO–MnO–MgO–CaO–Na₂O–K₂O–P₂O₅ and SiO₂–TiO₂–Al₂O₃–MgO–CaO–Na₂O to investigate the Ca-Eskola (CaEs Ca_0.5□_0.5AlSi₂O₆) content of clinopyroxene in eclogitic assemblages containing garnet + clinopyroxene + SiO₂ ± TiO₂ ± kyanite as a function of P, T, and bulk composition. The results show that CaEs_ss in clinopyroxene increases with increasing T and is strongly bulk composition dependent whereby high CaEs-contents are favoured by bulk compositions with high normative anorthite and low diopside contents. In this study, a maximum of 18 mol% CaEs_ss was found at 6 GPa and 1,350°C in a kyanite-eclogite assemblage garnet + clinopyroxene + kyanite + rutile + coesite. By comparison, no significant increase in CaEs_ss with increasing P could be observed. If the formation of oriented SiO₂-rods frequently observed in eclogititc clinopyroxenes is due to the retrogressive breakdown of a CaEs-component then these textures are a cooling rather than a decompression phenomenon and are most likely to be found in kyanite-bearing eclogites cooled from temperatures ≥750°C. The presence of clinopyroxene with approx. 4 mol% CaEs_ss in an experiment conducted at 2.5 GPa/850°C confirms earlier suggestions based on field data that vacancy-rich clinopyroxenes are not necessarily restricted to ultrahigh pressure metamorphic conditions. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

High pressure transitions in the system KAlSi3O8-NaAlSi3O8

Phase relations in the system KAlSi₃O₈-NaAlSi ₃O₈ have been examined at pressures of 5–23 GPa and temperatures of 700–1200° C. KAlSi₃O₈ sanidine first dissociates into a mixture of wadeite-type K₂Si₄O₉, kyanite and coesite at 6–7 GPa, which further recombines into KAlSi₃O₈ hollandite at 9–10 GPa. In contrast, NaAlSi₃O₈ hollandite is not stable at 800–1200° C near 23 GPa, where the mixture of jadeite plus stishovite directly changes into the assemblage of calcium ferrite-type NaAlSiO₄ plus stishovite. Phase relations in the system KAlSi₃O₈-NaAlSi₃O₈ at 1000° C show that NaAlSi₃O₈ component gradually dissolves into hollandite with increasing pressure. The maximum solubility of NaAlSi₃O₈ in hollandite at 1000° C was about 40 mol% at 22.5 GPa, above which it decreases with pressure. Unit cell volume of the hollandite solid solution decreases with increasing NaAlSi₃O₈ component. The hollandite solid solution in this system may be an important candidate as a host mineral of K and Na in the uppermost lower mantle     

Phase equilibria for the Fe2SiO4–Fe3O4 system under high pressure

High pressure phase relation of the system Fe₂SiO₄–Fe₃O₄ was investigated by synthesis experiments using multi-anvil high pressure apparatus. A complete solid solution with spinel structure along Fe₂SiO₄–Fe₃O₄ join occurs above 9 GPa at 1200 °C. Lattice constants of the solid solution show almost linear variation with composition. A spinelloid phase is stable for intermediate compositions in the pressure range from 3 to 9 GPa. the synthesized spinelloid phase is successfully indexed assuming nickel aluminosilicate V type structure. Received: October 16, 1995 / Revised, accepted: March 19, 1997     

On the mechanisms for H and Al incorporation in stishovite

The solubility and incorporation mechanisms of hydrogen in synthetic stishovite as a function of Al₂O₃ content have been investigated. Mechanisms for H incorporation in stishovite are more complex than previously thought. Most H in stishovite is incorporated via the Smyth et al. (Am Mineral 80:454–456, 1995) model, where H docks close to one of the shared O–O edges, giving rise to an OH stretching band in infrared (IR) spectra at 3,111–3,117 cm⁻¹. However, careful examination of IR spectra from Al-stishovite reveals the presence of an additional OH band at 3,157–3,170 cm⁻¹. All H is present on one site, with interstitial H both coupled to Al³⁺ substitutional defects on adjacent octahedral (Si⁴⁺) sites, and decoupled from other defects, giving rise to two distinct absorption bands. Trends in IR data as a function of composition are consistent with a change in Al incorporation mechanism in stishovite, with Al³⁺ substitution for Si⁴⁺ charge-balanced by oxygen vacancies at low bulk Al₂O₃ contents, and coupled substitution of Al³⁺ onto octahedral (Si⁴⁺) and interstitial sites at high bulk Al₂O₃ contents. Trends in OH stretching frequencies as a function of Al₂O₃ content suggest that any such change in Al incorporation mechanism could alter the effect that Al incorporation has on the compressibility of stishovite, as noted by Ono et al. (Am Mineral 87:1486–1489, 2002).     

Tourmaline breakdown in a pelitic system: implications for boron cycling through subduction zones

Pressure–temperature conditions of tourmaline breakdown in a metapelite were determined by high-pressure experiments at 700–900°C and 4–6 GPa. These experiments produced an eclogite–facies assemblage of garnet, clinopyroxene, phengite, coesite, kyanite and rare rutile. The modal proportions of tourmaline clearly decreased between 4.5 and 5 GPa at 700°C, between 4 and 4.5 GPa at 800°C, and between 800 and 850°C at 4 GPa, with tourmaline that survived the higher temperature conditions appearing corroded and thus metastable. Decreases in the modal abundance of tourmaline are accompanied by decreasing modal abundance of coesite, and increasing that of clinopyroxene, garnet and kyanite; the boron content of phengite increases significantly. These changes suggest that, with increasing pressure and temperature, tourmaline reacts with coesite to produce clinopyroxene, garnet, kyanite, and boron-bearing phengite and fluid. Our results suggest that: (1) tourmaline breakdown occurs at lower pressures and temperatures in SiO₂-saturated systems than in SiO₂-undersaturated systems. (2) In even cold subduction zones, subducting sediments should release boron-rich fluids by tourmaline breakdown before reaching depths of 150 km, and (3) even after tourmaline breakdown, a significant amount of boron partitioned into phengite could be stored in deeply subducted sediments.     

First-principles simulation of high-pressure polymorphs in MgAl<Subscript>2</Subscript>O<Subscript>4</Subscript>

We have used density functional theory to investigate the stability of MgAl₂O₄ polymorphs under pressure. Our results can reasonably explain the transition sequence of MgAl₂O₄ polymorphs observed in previous experiments. The spinel phase (stable at ambient conditions) dissociates into periclase and corundum at 14 GPa. With increasing pressure, a phase change from the two oxides to a calcium-ferrite phase occurs, and finally transforms to a calcium-titanate phase at 68 GPa. The calcium-titanate phase is stable up to at least 150 GPa, and we did not observe a stability field for a hexagonal phase or periclase + Rh₂O₃(II)-type Al₂O₃. The bulk moduli of the phases calculated in this study are in good agreement with those measured in high-pressure experiments. Our results differ from those of a previous study using similar methods. We attribute this inconsistency to an incomplete optimization of a cell shape and ionic positions at high pressures in the previous calculations.     

The stability and compressibility of MgAl2O4 high-pressure polymorphs

High-pressure and high-temperature experiments using a laser-heated diamond anvil cell (LHDAC) and synchrotron X-ray diffraction have revealed a phase transition in MgAl₂O₄. CaTi₂O₄-type MgAl₂O₄ was found to be stable at pressures between 45 and at least 117 GPa. The transition pressure of CaTi₂O₄-type phase in MgAl₂O₄ is much lower than that in the natural N-type mid-oceanic ridge basalt composition. The Birch–Murnaghan equation of state for CaTi₂O₄-type MgAl₂O₄ was determined from the experimental unit cell parameters with K ₀=219(±6) GPa, K ₀′=4(constrained value), and V ₀=238.9(±9) ų. The observed compressibility was in agreement with the theoretical compressibility calculated in a previous study. ε-MgAl₂O₄ was observed at pressures between 40 and 45 GPa, which has not been reported in natural rock compositions. The gradient (dP/dT slope) of the transition from the ε-type to CaTi₂O₄-type MgAl₂O₄ had a positive value. These results should resolve the dispute regarding the stable high-pressure phase of MgAl₂O₄, which has been reported in earlier studies using both the multi-anvil press and the diamond anvil cell.     

The Mg3Al2Si3O12-Na2MgSi5O12 system at pressures of 7.0 and 8.5 GPa and a temperature of 1300–1800°C: Phase relationships and crystallization of Na-bearing majoritic garnet

The results of an experimental study of the pyrope (Mg₃Al₂Si₃O₁₂)-jadeite (NaAlSi₂O₆) system at P = 7.0 and 8.5 GPa and T = 1300?C1800°C are summarized in this paper. The main phases that were obtained in the experiments are garnet, pyroxene, kyanite (in some cases corundum), and quenched melt. Garnets are characterized by a stable Na₂O admixture (up to 0.6 wt % at 7.0 GPa and up to 0.8 wt % at 8.5 GPa) and the high silicon content (Si = 3.016?C3.166). The maximal sodium concentrations in garnet were found at the solidus of the system, which results from an increase of the coefficient of sodium partitioning between garnet and melt during a temperature decrease.     

Heat capacity,entropy and phase equilibria of stishovite

The low-temperature heat capacity (C _P) of stishovite (SiO₂) synthesized with a multi-anvil device was measured over the range of 5–303 K using the heat capacity option of a physical properties measurement system (PPMS) and around ambient temperature using a differential scanning calorimeter (DSC). The entropy of stishovite at standard temperature and pressure calculated from DSC-corrected PPMS data is 24.94 J mol⁻¹ K⁻¹, which is considerably smaller (by 2.86 J mol⁻¹ K⁻¹) than that determined from adiabatic calorimetry (Holm et al. in Geochimica et Cosmochimica Acta 31:2289–2307, 1967) and about 4% larger than the recently reported value (Akaogi et al. in Am Mineral 96:1325–1330, 2011). The coesite–stishovite phase transition boundary calculated using the newly determined entropy value of stishovite agrees reasonably well with the previous experimental results by Zhang et al. (Phys Chem Miner 23:1–10, 1996). The calculated phase boundary of kyanite decomposition reaction is most comparable with the experimental study by Irifune et al. (Earth Planet Sci Lett 77:245–256, 1995) at low temperatures around 1,400 K, and the calculated slope in this temperature range is mostly consistent with that determined by in situ X-ray diffraction experiments (Ono et al. in Am Mineral 92:1624–1629, 2007).     

Stability range and decomposition of potassic richterite and phlogopite end members at 5–15 GPa

Summary The phase relations of K-richterite, KNaCaMg₅Si₈O₂₂(OH)₂, and phlogopite, K₃Mg₆ Al₂Si₆O₂₀(OH)₂, have been investigated at pressures of 5–15 GPa and temperatures of 1000–1500 °C. K-richterite is stable to about 1450 °C at 9–10 GPa, where the dp/dT-slope of the decomposition curve changes from positive to negative. At 1000 °C the alkali-rich, low-Al amphibole is stable to more than 14 GPa. Phlogopite has a more limited stability range with a maximum thermal stability limit of 1350 °C at 4–5 GPa and a pressure stability limit of 9–10 GPa at 1000 °C. The high-pressure decomposition reactions for both of the phases produce relatively small amounts of highly alkaline water-dominated fluids, in combination with mineral assemblages that are relatively close to the decomposing hydrous phase in bulk composition. In contrast, the incongruent melting of K-richterite and phlogopite in the 1–3 GPa range involves a larger proportion of hydrous silicate melts. The K-richterite breakdown produces high-Ca pyroxene and orthoenstatite or clinoenstatite at all pressures above 4 GPa. At higher pressures additional phases are: wadeite-structured K₂Si^VISi^IV ₃O₉ at 10 GPa and 1500 °C, wadeite-structured K₂Si^VISi^IV ₃O₉ and phase X at 15 GPa and 1500 °C, and stishovite at 15 GPa and 1100 °C. The solid breakdown phases of phlogopite are dominated by pyrope and forsterite. At 9–10 GPa and 1100–1400 °C phase X is an additional phase, partly accompanied by clinoenstatite close to the decomposition curve. Phase X has variable composition. In the KCMSH-system (K₂CaMg₅Si₈O₂₂(OH)₂) investigated by Inoue et al. (1998) and in the KMASH-system investigated in this report the compositions are approximately K₄Mg₈Si₈O₂₅(OH)₂ and K_3.7Mg_7.4Al_0.6Si_8.0O₂₅(OH)₂, respectively. Observations from natural compositions and from the phlogopite-diopside system indicate that phlogopite-clinopyroxene assemblages are stable along common geothermal gradients (including subduction zones) to 8–9 GPa and are replaced by K-richterite at higher pressures. The stability relations of the pure end member phases of K-richterite and phlogopite are consistent with these observations, suggesting that K-richterite may be stable into the mantle transition zone, at least along colder slab geotherms. The breakdown of moderate proportions of K-richterite in peridotite in the upper part of the transition zone may be accompanied by the formation of the potassic and hydrous phase X. Additional hydrogen released by this breakdown may dissolve in wadsleyite. Therefore, very small amounts of hydrous fluids may be released during such a decomposition. Received April 10, 2000; revised version accepted November 6, 2000     

The stability of hercynite at high pressures and temperatures

The stability of hercynite (FeAl₂O₄) has been investigated experimentally between 7 and 24 GPa and 900 and 1,700°C. Hercynite breaks down to its constituent oxides at 7–8.5 GPa and temperatures >1,000°C. The incorporation of a small magnetite component in the hercynite necessitated a small correction to fix the location of the endmember reaction: FeAl₂O₄  = Al₂O₃ + FeO in P–T space. After making this correction, the position of the phase boundary was used to evaluate thermodynamic data for hercynite. Our results support a relatively large S ₂₉₈^° for hercynite, on the order of 115 J mol⁻¹ K⁻¹. Experiments up to 24 GPa and 1,400°C failed to detect any high-pressure polymorph of FeAl₂O₄; only corundum + wüstite were detected. This behaviour contrasts with that observed for the analogous MgAl₂O₄ system where the constituent oxides recombine at high pressure to produce “post-spinel” phases with CaFe₂O₄-type and CaTi₂O₄-type structures.     

Decarbonation reaction of magnesite in subducting slabs at the lower mantle

High-pressure and temperature experiments (28–62 GPa, and 1,490–2,000 K, corresponding to approximately 770–1,500 km depth in the mantle) have been conducted on a MgCO₃ + SiO₂ mixture using a laser-heated diamond anvil cell combined with analytical transmission electron microscope observation of the product phases to constrain the fate of carbonates carried on the subducting basalt into the lower mantle. At these conditions, the decarbonation reaction MgCO₃ (magnesite) + SiO₂ (stishovite) → MgSiO₃ (perovskite) + CO₂ (solid) has been recognized. This indicates that above reaction takes place as a candidate for decarbonation of the carbonated subducting mid ocean ridge basalts in the Earth’s lower mantle.     

Experimental phase and melting relations of metapelite in the upper mantle: implications for the petrogenesis of intraplate magmas

We performed a series of piston-cylinder experiments on a synthetic pelite starting material over a pressure and temperature range of 3.0–5.0 GPa and 1,100–1,600°C, respectively, to examine the melting behaviour and phase relations of sedimentary rocks at upper mantle conditions. The anhydrous pelite solidus is between 1,150 and 1,200°C at 3.0 GPa and close to 1,250°C at 5.0 GPa, whereas the liquidus is likely to be at 1,600°C or higher at all investigated pressures, giving a large melting interval of over 400°C. The subsolidus paragenesis consists of quartz/coesite, feldspar, garnet, kyanite, rutile, ±clinopyroxene ±apatite. Feldspar, rutile and apatite are rapidly melted out above the solidus, whereas garnet and kyanite are stable to high melt fractions (>70%). Clinopyroxene stability increases with increasing pressure, and quartz/coesite is the sole liquidus phase at all pressures. Feldspars are relatively Na-rich [K/(K + Na) = 0.4–0.5] at 3.0 GPa, but are nearly pure K-feldspar at 5.0 GPa. Clinopyroxenes are jadeite and Ca-eskolaite rich, with jadeite contents increasing with pressure. All supersolidus experiments produced alkaline dacitic melts with relatively constant SiO₂ and Al₂O₃ contents. At 3.0 GPa, initial melting is controlled almost exclusively by feldspar and quartz, giving melts with K₂O/Na₂O ~1. At 4.0 and 5.0 GPa, low-fraction melting is controlled by jadeite-rich clinopyroxene and K-rich feldspar, which leads to compatible behaviour of Na and melts with K₂O/Na₂O ≫ 1. Our results indicate that sedimentary protoliths entrained in upwelling heterogeneous mantle domains may undergo melting at greater depths than mafic lithologies to produce ultrapotassic dacitic melts. Such melts are expected to react with and metasomatise the surrounding peridotite, which may subsequently undergo melting at shallower levels to produce compositionally distinct magma types. This scenario may account for many of the distinctive geochemical characteristics of EM-type ocean island magma suites. Moreover, unmelted or partially melted sedimentary rocks in the mantle may contribute to some seismic discontinuities that have been observed beneath intraplate and island-arc volcanic regions.     

Study on Al2O3 content and phase stability of aluminous-CaSiO3 perovskite at high pressure and temperature

 In order to clarify Al₂O₃ content and phase stability of aluminous CaSiO₃-perovskite, high-pressure and high-temperature transformations of Ca₃Al₂Si₃O₁₂ garnet (grossular) were studied using a MA8-type high-pressure apparatus combined with synchrotron radiation. Recovered samples were examined by analytical transmission electron microscopy. At pressures of 23–25 GPa and temperatures of 1000–1600 K, grossular garnet decomposed into a mixture of aluminum-bearing Ca-perovskite and corundum, although a metastable perovskite with grossular composition was formed when the heating duration was not long enough at 1000 K. On release of pressure, this aluminum-bearing CaSiO₃-perovskite transformed to the “LiNbO₃-type phase” and/or amorphous phase depending on its Al₂O₃ content. The structure of this LiNbO₃-type phase is very similar to that of LiNbO₃ but is not identical. CaSiO₃-perovskite with 8 to 25 mol% Al₂O₃ was quenched to alternating lamellae of amorphous layer and LiNbO₃-type phase. On the other hand, a quenched product from CaSiO₃-perovskite with less than 6 mol% consisted only of amorphous phase. Most of the inconsistencies amongst previous studies could be explained by the formation of perovskite with grossular composition, amorphous phase, and the LiNbO₃-type phase. Received: 11 April 2001 / Accepted: 5 July 2002