Melting of some alkaline-earth and transition-metal fluorides and alkali fluoroberyllates at elevated pressures: A search for melting systematics

The melting curves of the fluorides ZnF₂ and NiF₂ (rutile structure), CaF₂, SrF₂ and BaF₂ (fluorite structure), and of the fluoroberyllates Na₂BeF₄ and Li₂BeF₄ have been studied at pressures ? 40 kbar by differential thermal analysis in a piston-cylinder high-pressure device. The initial slopes (dT_m/dP)₀ of these melting curves are respectively 7.2, 5.8, 16.7, 15.2, 15.7, 15.1 and <0°C/kbar. A new Li₂BeF₄ polymorph, apparently of the olivine structure type, is stable at pressures > 10 kbar and its melting curve has an average slope of ～6.7°C/kbar. These new data and those for SiO₂, BeF₂, GeO₂, LiF and MgF₂, recently studied by Jackson, are combined with existing data for elements, ionic compounds and silicates to assess the influence of crystal structure, molar volume and the nature of interatomic bonding on the initial slopes of melting curves. It is found that the entropy of fusion (ΔS_m) is primarily a function of crystal structure while the volume change on fusion (ΔV_m) is controlled by crystal molar volume within each isostructural series. Such systematics have recently facilitated estimation of the initial slopes of the melting curves of periclase and stishovite. New and existing melting data for silicates and their analogues have been analysed and a systematic dependence of (dT_m/dP)₀ on SiO₂ concentration has been demonstrated. Possible implications of this trend for partial melting of upper-mantle garnet lherzolite are illustrated. Finally, the use of the traditional silicate-germanate and oxide-fluoride modelling schemes is reviewed in the light of information from this present study.     

High-pressure phase transformations and compressions of ilmenite and rutile,I. Experimental results

Natural ilmenite (Fe,Mg)TiO₃ has been found to transform to the perovskite structure and then to disproportionate into its component oxides, (Fe,Mg)O plus a cubic phase of TiO₂, at loading pressures of 140 and 250 kbar respectively, and at temperatures of 1,400 to 1,800°C. Samples were compressed in a diamond-anvil press and heated by irradiation with a YAG laser. The lattice parameters of the perovskite phase of (Fe,Mg)TiO₃ at room temperature and 1 bar are $\text{a}{}_0\text{= 4.471 ± 0.004, b}{}_0\text{= 5.753 ± 0.005,}\text{and}\text{c}{}_0\text{= 7.429 ± 0.006}\text{A}\text{?}$ with 4 molecules per cell. The zero-pressure volume change is 8.0% for the ilmenite-perovskite transition, 13.3% for the perovskite-mixed-oxides transition, and 20.2% for the ilmenite-mixed-oxides transition. The cubic phase of TiO₂ can be indexed on the basis of space group Fm3m with $\text{Z = 4}\text{and}\text{a}{}_0\text{= 4.455 ± 0.008}\text{A}\text{?}$ at room temperature and 1 bar, which corresponds to a decrease in zero-pressure volume of 29.2% for the rutile-cubic-phase transition. An isentropic bulk modulus at zero pressure of 5.75 ± 0.30 Mbar and a pressure derivative greater than 8 were calculated for the high-pressure cubic phase. The calculated bulk modulus for the mixture of (Fe,Mg)O and cubic TiO₂ is 2.48 ± 0.25 Mbar. All the phase transformations, the calculated lattice parameters, and the bulk moduli observed in this study are in good agreement with published shock-Hugoniot data for ilmenite and rutile.     

The role of sphene as an accessory phase in the high-pressure partial melting of hydrous mafic compositions

In a high-pressure experimental study of reactions and possible melt products occurring in the deep continental crust or in subducted oceanic crust, sphene has been identified over a pressure range of 10–18 kbar and to temperatures of 1020°C. Sphene may be a refractory phase with up to 60% partial melting for hydrous mafic compositions. Sphene breaks down at lower pressure than the maximum pressure stability of amphibole in hydrous mafic compositions, and rutile rather than sphene is the important Ti-bearing accessory phase at pressures greater than 16–18 kbar. Sphene and rutile coexist to pressures as low as 14 kbar. This implies that amphibole eclogites containing primary sphene and no rutile have most likely formed at depths less than 45 km.The presence of minor sphene as a residual phase in equilibrium with low-Ti silicic liquids may have a marked effect on the REE distribution in derivative liquids. Thus melts in equilibrium with a garnet and sphene-bearing residuum may have less light-REE-enriched patterns than those predicted when garnet is a residual phase without coexisting sphene. This effect is modelled using REE patterns for sphenes from high-grade metamorphic terrains of western Norway.Both the new REE data and the experimental study have important implications for the genesis of low-Ti magmas formed in continental margins and island arcs.     

High-pressure disproportionation of Co2SiO4 spinel and implications for Mg2SiO4 spinel

Co₂SiO₄ spinel has been found to disproportionate into its isochemically mixed oxides with rocksalt and rutile structures at pressures between 170 and 190 kbar and temperatures between 1400 and 1800°C in a diamond-anvil press. The exact disproportionation pressure is not certain due to transient increases in pressure during the local and rapid heating by a continuous YAG laser. The slope of the phase boundary between the spinel phase and the mixed oxides is calculated to be?33 ± 20bar/deg. This negative slope is consistent with the observed anomalously large entropy of CoO (relative to its isostructural oxides) in entropy vs.(MV)^?1/2 systematics, whereM is the formula weight andV the molar volume. The sign of the slope for a phase boundary in the disproportionation of spinel depends on the values of entropy of the rocksalt oxides as well as the inverse character exhibited in the spinel phases. The normal entropy of MgO suggests that the phase boundary for the disproportionation of Mg₂SiO₄ spinel has positive slope.     

High-pressure phase transformations in baddeleyite and zircon,with geophysical implications

Phase transformations in baddeleyite (ZrO₂) and zircon (ZrSiO₄) have been investigated in the pressure range between 100 and 300 kbar at about 1000°C in a diamond-anvil press coupled with laser heating. Baddeleyite has been found to transform to an orthorhombic cotunnite-type structure at pressures greater than 100 kbar, and is the first oxide known to adopt this structure. The lattice parameters of the cotunnite-type ZrO₂ at room temperature and atmospheric pressure area = 3.328 ± 0.001 ,b = 5.565 ± 0.002 , andc = 6.503 ± 0.003A? withZ = 4 , and its volume is 14.3% smaller than baddeleyite and 7.6% smaller than the fluorite-type ZrO₂. It is suggested that all the polymorphic structures of ZrO₂ are possible high-pressure models for the post-rutile phase of SiO₂. The polyhedral coordination in these model structures varies from 7 to “9”, compared with 6 for stishovite. If SiO₂ were to adopt any of these structures in the deep mantle, Birch's hypothesis of a mixed-oxide lower mantle may still be viable, but the primary coordination of silicon would be greater than 6. Zircon has been found to transform to a scheelite-type structure at about 120 kbar as noted earlier. The scheelite-type ZrSiO₄ was found to decompose further into a mixture of ZrO₂ (cotunnite-type) plus SiO₂ (stishovite) in the pressure range 200–250 kbar. As implied by the transitions in zircon, the large cations of U and Th in the earth's deep mantle are most likely to occur in dioxides with structures such as the cotunnite-type, rather than to occur in silicates.     

High-pressure decompositions in cobalt and nickel silicates

High-pressure stability relations in cobalt and nickel silicates have been studied over the pressure range 130–330 kbar employing a double-staged split-sphere-type high-pressure apparatus.γ-Co₂SiO₄ and γ-Ni₂SiO₄ decompose directly into their constituent oxide mixtures (rocksalt plus stishovite) 175 kbar and 280 kbar, respectively. The result that γ-Ni₂SiO₄ has a wider stability field in pressure than γ-Co₂SiO₄, is consistent with simple crystal-field theory.The experimental precision is high enough to show that the decomposition boundary of γ-Co₂SiO₄ has a positive slope (dP/dT > 0) and a preliminary determination of the boundary curve is P(kbar) = 0.065 T (°C) + 110.No positive evidence for the existence of high-pressure forms of CoSiO₃ and NiSiO₃ has been obtained in these quenching experiments, and they finally decompose into constituent oxide mixtures as in the cases of orthosilicates.     

High-pressure phases of Co2GeO4, Ni2GeO4, Mn2GeO4 and MnGeO3: implications for the germanate-silicate modeling scheme and the Earth's mantle

In a diamond-anvil press coupled with YAG laser heating, the spinels of Co₂GeO₄ and Ni₂GeO₄ have been found to disproportionate into their isochemical oxide mixtures at about 250 kbar and 1400–1800°C in the same manner as their silicate analogues. At about the same P-T conditions MnGeO₃ transforms to the orthorhombic perovskite structure (space group Pbnm); the lattice parameters at room temperature and 1 bar are a₀ = 5.084 ± 0.002, b₀ = 5.214 ± 0.002, and c₀ = 7.323 ± 0.003Å with Z = 4 for the perovskite phase. The zero-pressure volume change associated with the ilmenite-perovskite phase transition in MnGeO₃ is ?6.6%. Mn₂GeO₄ disproportionates into a mixture of the perovskite phase of MnGeO₃ plus the rocksalt phase of MnO at P = 250kbar and T = 1400–1800°C. The concept of utilizing germanates as high-pressure models for silicates is valid in general. The results of this study support the previous conclusion that the lower mantle comprises predominantly the orthorhombic perovskite phase of ferromagnesian silicate.     

Synthesis of a perovskite-type polymorph of CaSiO3

Synthetic crystalline (wollastonite) and glass forms of CaSiO₃ have been compressed to loading pressures above 160 kbar and heated to about 1500° C by a laser in a diamond-anvil cell. After cooling, an X-ray diffraction study carried out whilst the sample was maintained at high pressure revealed that it had transformed to a cubic perovskite-type 3olymorph with a = 3.485 ± 0.008A?. After release of pressure, however, the sample showed a mixture of glass plus a few weak lines corresponding to ε-CaSiO₃ which is thus interpreted as a retrogressive transition product. The density of the perovskite polymorph of CaSiO₃ is about 9.2% greater than that of an isochemical mixture of CaO + SiO₂ (stishovite) at about 160 kbar.     

High-pressure X-ray diffraction studies on β- and γ-Mg2SiO4

Pressure effects on the lattice parameters of β- and γ-Mg₂SiO₄ have been measured at room temperature and at pressures up to 100 kbar using a multi-anvil high-pressure X-ray diffraction apparatus. The volume changes (ΔV/V₀) at 90 kbar are 5.4 · 10^?2 and 4.2 · 10^?2 for β- and γ-Mg₂SiO₄, respectively. Isothermal bulk moduli at zero pressure have been calculated from least-square fits of the data to straight lines. They turn out to be 1.66 ± 0.4 and 2.13 ± 0.1 Mbar for β- and γ-Mg₂SiO₄, respectively. The α → γ transition obeys Wang's linear V_φ?ρ relation but the α → β transition does not.     

Phase relations in the system LiFMgF2 at elevated pressures: Implications for the proposed mixed-oxide zone of the earth's mantle

Solvi and liquidi for various LiFMgF₂ mixtures have been determined at pressures up to 40 kbar by differential-thermal-analysis in a piston-cylinder high-pressure device. The melting curves of pure LiF and MgF₂ were also studied and the initial slopes (dT_m/dP)_{P = 0} were found to be 11.2 and 8.3°C/kbar, respectively. The eutectic composition (LiF)_0.64(MgF₂)_0.36 is independent of pressure to 35 kbar and the eutectic temperature rises approximately 6.3°C per kbar. Initial slopes of 11°C/kbar and 35°C/kbar are inferred for the melting curves of MgO and SiO₂ (stishovite) respectively, on the basis of data for their structural analogue compounds. The observed solid solution of LiF in MgF₂ and other evidence suggest the possibility of solid solution in the system (Mg,Fe)OSiO₂ (stishovite) under mantle conditions which may have important consequences for the elastic properties of a “mixed-oxide” zone of the earth's mantle.     

Compression and phase behavior of solid CO2 to half a megabar

CO₂ has been investigated up to 514 kbar at23 ± 2°C by both optical and in situ X-ray diffraction studies using a diamond-anvil pressure cell. CO₂ solidifies in an unknown structure in the pressure range 5 to 23 kbar, and transforms to ordinary dry-ice structure above 23 kbar at room temperature. Isothermal compression data for dry ice have been obtained above about 24 kbar. These appear to be the first data at room temperature known in the literature. The data fitted to the Birch equation of state yieldK₀ = 29.3 ± 1.0kbar andK₀^′ = 7.8 assuming the volume of the hypothetical dry ice at zero-pressure and room temperature is 31.4 ± 0.2 cm³/mole. The isothermal bulk modulus(K₀) thus derived is consistent with the compression data and compressibilities for dry ice obtained at low temperatures using dilatometry and ultrasonic techniques, respectively, reported in the literature. By comparing shock-wave data for relevant materials, it is suggested that CO₂ is not likely to transform to one of the crystalline forms of SiO₂ which is otherwise expected from empirical grounds, but may instead decompose into C (diamond) + O₂, at high pressures.     

Synthesis and crystal-chemical characterization of MgSiO3 perovskite

The orthorhombic MgSiO₃ perovskite has been synthesized with the aid of a double-stage split-sphere-type high-pressure apparatus at about 280 kbar and 1000°C. The unit cell dimensions are: a = 4.7754(3)Å, b = 4.9292(4)Å and c = 6.8969(5)Å with the probable space group Pbnm. Calculated density is 4.108 g cm^?3. Crystal structure determination has been carried out by means of both the geometrical simulation (DLS) technique and the ordinary powder X-ray analysis. The results indicate that the MgSiO₃ perovskite is closer to the ideal perovskite than ScAlO₃ perovskite.     

The system enstatite-wollastonite at high pressures and temperatures,with emphasis on diopside

High-pressure phase transformations for three intermediate compositions (including diopside) in the system enstatite (MgSiO₃)-wollastonite (CaSiO₃) were investigated in the pressure range 100–300 kbar at about 1000°C in a diamond-anvil press coupled with laser heating. The phase behaviour of the two end components (enstatite and wollastonite) at high pressure has been reported earlier. The results of this study reveal that there is very limited solid solution of diopside (CaMgSi₂O₆) in the various high-pressure phase assemblages of enstatite. At pressures greater than about 200 kbar, diopside and a composition between diopside and wollastonite were found to transform into non-quenchable phases, as does wollastonite. It is thought probable that diopside and wollastonite form solid solutions with the perovskite structure at high pressure, but that on release of pressure it is not possible to preserve the high-pressure modification.     

Disproportionation of Ni2SiO4 to stishovite plus bunsenite at high pressures and temperatures

Samples of Ni₂SiO₄ in both olivine and spinel phases have been compressed to pressures above 140 kbar in a diamond-anvil cell and heated to temperatures of 1400–1800°C using a continuous YAG laser. After quenching and releasing pressure, X-ray diffraction examination indicates that the samples disproportionate to a mixture of stishovite (SiO₂) and bunsenite (NiO) at pressures between 140 and 190 kbar. The exact disproportionation pressure is not certain due to transient increases in pressure during the local and rapid heating. However, thermodynamic calculations suggest that the transition pressure is about 192 ± 4 kbar at 1545°C and that the equation of the spinel-mixed oxides phase boundary isP(kbar) = 121 + (0.046 ± 0.020) T (°C).     

High-pressure decomposition of Ca2GeO4 and Ca2MnO4 with the K2NiF4-type structures

By using the diamond-anvil pressure cell coupled with laser heating, Ca₂GeO₄ in the K₂NiF₄-type structure has been found to decompose into the mixture Ca₃Ge₂O₇ plus CaO at pressures greater than 200 kbar and at about 1000°C, and the same type of structure for Ca₂MnO₄ has been found to decompose into the mixture CaMnO₃ (perovskite) plus CaO at pressures greater than 100 kbar and at about 1400°C. The decomposition product of Ca₃Ge₂O₇ is a new compound which is isostructural with Sr₃Ti₂O₇ and has the lattice parameters of a = 3.72 ± 0.01 and $\text{c = 19.32 ± 0.05}\text{A}\text{?}$ at room temperature and 1 bar pressure. The results of the study of Ca₂GeO₄ and Ca₂MnO₄ (both with the K₂NiF₄-type structure) strongly support the view that compounds possessing the K₂NiF₄-type structure are unstable relative to corresponding mixtures possessing the perovskite and rocksalt structures. It is concluded that, in the earth's mantle, the K₂NiF₄-type Ca₂SiO₄ would ultimately decompose into the mixture CaSiO₃ (perovskite) + CaO or would otherwise transform to other as-yet-unknown phase(s), and that the mixture of MgSiO₃ (perovskite) + MgO (the post-spinel phase of Mg₂SiO₄) would not adopt the K₂NiF₄-type structure.     

Disproportionation of kyanite to corundum plus stishovite at high pressure and temperature

Natural kyanite (Al₂SiO₅) has been found to disproportionate into a mixture of its component oxides, corundum and stishovite, at a loading pressure of about 160 kbar and temperature between 1000–1400°C in a diamond-anvil press. The exact transition pressure is not certain due to transient increases in pressure during the local and rapid heating by a continuous YAG laser. The phase boundary, however, has been estimated to be P(kbar) = (138 ～ 174) + 0.011 T (°C) on the basis of the available thermodynamic data. The shock-wave Hugoniot data above 650 kbar for andalusite (Al₂SiO₅) and sillimanite (Al₂SiO₅) as starting materials are consistent with the present results.     

Experimental evidence for the role of accessory phases in magma genesis

Recent experimental studies have established petrogenetic models based on melting processes involving major phases. The possible residual character of trace-element-enriched accessory phases is not considered for temperatures well above the solidus in these models. In contrast, geochemists, applying trace element data to independently test the experimentally-based models, have concluded that residual (or fractionating) accessory phases may have an essential role in controlling the trace element (especially REE) distributions in magmas.Some recent experimental work provides data on the stability of potentially significant accessories such as sphene, rutile, apatite, zoisite and mica in basaltic compositions at elevated P and T. Sphene is stable to 1000°C with 60% melting of a hydrous tholeiite at 15 kbar. At higher pressure, rutile is the only Ti-rich accessory phase, and is present to at least 1000°C and high degrees of melting. Published REE data on sphene and rutile suggest that these phases may be important in controlling REE distribution in some magmas. For example, island are high-Mg, low-Ca-Ti tholeiites with low REE abundances and U-shaped patterns (Hickey and Frey, 1979) may reflect the role of sphene. In addition to rutile, similar close-packed Ti-rich accessory phases such as priderite, perovskite, crichtonite and loveringite may occur in mantle-derived magmas. These phases readily accommodate the REE but their possible role needs experimental confirmation.Apatite is recorded in hawaiite (1.16% P₂O_s) with 2% H₂O added at 5–6 kbar and 1050°C within 30°C of the liquidus, but at present no other experimental data are available on its high P, T stability, although thermodynamic calculations indicate that F may increase its stability markedly. Apatite is well known in high-pressure inclusions and as a phenocryst phase in rocks of the alkaline and calc-alkaline series.Ilmenite is known as a near-liquidus phase in some mafic magmas at 5–10 kbar, but its stability decreases to near-solidus at 25–30 kbar. Zoisite occurs in hydrous mafic compositions at mantle pressures, but it is confined to temperatures < 780°C. Finally, mica has a wide temperature range of stability at mantle pressures, especially in potassic magmas, and phlogopitic mica is stable to 1040°C at 20–25 kbar in a hydrous, K-rich “tholeiite” (1.6% K₂O).     

Pressure dependence of nickel partitioning between forsterite and aluminous silicate melts

Nickel partitioning between forsterite and aluminosilicate melt of fixed bulk composition has been determined at 1300°C to 20 kbar pressure. The value of the forsterite-liquid nickel partition coefficient is lowered from >20 at pressures equal to or less than 15 kbar to <10 at pressures above 15 kbar.Published data indicate that melts on the join Na₂O-Al₂O₃-SiO₂ become depolymerized in the pressure range 10–20 kbar as a result of Al shifting from four-coordination at low pressure to higher coordination as the pressure is increased. This coordination shift results in a decreasing number of bridging oxygens in the melt. It is suggested that the activity coefficient of nickel decreases with this decrease in the number of bridging oxygens. As a result, the nickel partition coefficient for olivine and liquid is lowered.Magma genesis in the upper mantle occurs in the pressure range where the suggested change in aluminum coordination occurs in silicate melts. It is suggested, therefore, that data on nickel partitioning obtained at low pressure are not applicable to calculation of the nickel distribution between crystals and melts during partial melting in the upper mantle. Application of high-pressure experimental data determined here for Al-rich melts to the partial melting process indicates that the melts would contain about twice as much nickel as indicated by the data for the low-pressure experiments. If, as suggested here, the polymerization with pressure is related to the Al content of the melt, the difference in the crystal-liquid partition coefficient for nickel at low and high pressure is reduced with decreasing Al content of the melt. Consequently, the change ofD_Ni^{ol-andesite melt} is greater than that ofD_Ni^{ol-basalt melt}, for example.     

High-pressure reconnaissance investigation in the system Mg3Al2Si3O12-Fe3Al2Si3O12

Two synthetic end-members and two natural solid solutions of the system Mg₃Al₂Si₃O₁₂-Fe₃Al₂Si₃O₁₂ have been found to display successive phase transformations at increasingly high pressures when they were compressed in a diamond-anvil cell and heated with a YAG laser to temperatures of approximately 1400–1800°C. X-ray diffraction studies of the quenched samples show that the iron-rich garnets apparently first transform to a garnet-related high-pressure phase, then disproportionate into a mixture of magnesiowüstite plus an unknown phase(s). The latter phase(s) may further transform to a still denser unknown phase(s). The ultimate high-pressure phase may be a perovskite-like structure as was previously found for the magnesium-rich garnets. One of the unknown phases may be the high-pressure phase of Al₂O₃ · nSiO₂ compounds. Magnesium-rich garnets display similar phase transformations as do the iron-rich garnets with the exception of the garnet-related high-pressure phase. These results disagree with a previous interpretation for the high-pressure phase of iron-silicate garnets recovered in shock-wave experiments reported by Ahrens and Graham (1972).