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.     

Pyroxene-garnet solid-solution equilibria in the systems Mg4Si4O12Mg3Al2Si3O12 and Fe4Si4O12Fe3Al2Si3O12 at high pressures and temperatures

Pyroxene-garnet solid-solution equilibria have been studied in the pressure range 41–200 kbar and over the temperature range 850–1,450°C for the system Mg₄Si₄O₁₂Mg₃Al₂Si₃O₁₂, and in the pressure range 30–105 kbar and over the temperature range 1,000–1,300°C for the system Fe₄Si₄O₁₂Fe₃Al₂Si₃O₁₂. At 1,000°C, the solid solubility of enstatite (MgSiO₃) in pyrope (Mg₃Al₂Si₃O₁₂) increases gradually to 140 kbar and then increases suddenly in the pressure range 140–175 kbar, resulting in the formation of a homogeneous garnet with composition Mg₃(Al_0.8Mg_0.6Si_0.6)Si₃O₁₂. In the MgSiO₃-rich field, the three-phase assemblage of β- or γ-Mg₂SiO₄, stishovite and a garnet solid solution is stable at pressures above 175 kbar at 1,000°C. The system Fe₄Si₄O₁₂Fe₃Al₂Si₃O₁₂ shows a similar trend of high-pressure transformations: the maximum solubility of ferrosilite (FeSiO₃) in almandine (Fe₃Al₂Si₃O₁₂) forming a homogeneous garnet solid solution is 40 mol% at 93 kbar and 1,000°C.If a pyrolite mantle is assumed, from the present results, the following transformation scheme is suggested for the pyroxene-garnet assemblage in the mantle. Pyroxenes begin to react with the already present pyrope-rich garnet at depths around 150 km. Although the pyroxene-garnet transformation is spread over more than 400 km in depth, the most effective transition to a complex garnet solid solution takes place at depths between 450 and 540 km. The complex garnet solid solution is expected to be stable at depths between 540 and 590 km. At greater depths, it will decompose to a mixture of modified spinel or spinel, stishovite and garnet solid solutions with smaller amounts of a pyroxene component in solution.     

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 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).     

Phase relations of titan-phlogopite,K2Mg4TiAl2Si6O20(OH)4: a refractory phase in the upper mantle?

Experimental data on the stability of titan-phlogopite [K₂Mg₄TiAl₂Si₆O₂₀(OH)₄] are presented which show it to be stable to substantially higher temperatures than normal phlogopite [K₂Mg₆Al₂Si₆O₂₀(OH)₄]. A qualitative model to explain the role of titan-phlogopite during magma generation is put forward. Breakdown of titan-phlogopite during melting at depth (> 150km) on subducted lithospheric slabs is believed responsible for the concomitant increase of K and Ti observed in magmas erupted during orogenic volcanism. At lower pressures (up to about 10 kbar) beneath mid-oceanic ridges, titan-phlogopite is predicted to behave as a refractory phase during partial melting in the mantle, especially if H₂O-excess conditions pertain, although at higher pressures in this environment it would almost certainly behave as a low-melting component.     

Garnet-ilmenite-perovskite transitions in the system Mg4Si4O12-Mg3Al2Si3O12 at high pressures and high temperatures: phase equilibria, calorimetry and implications for mantle structure

Phase relations in the system Mg₄Si₄O₁₂-Mg₃Al₂Si₃O₁₂ were examined at pressures of 19-27 GPa and relatively low temperatures of 800-1000 °C using a multianvil apparatus to clarify phase transitions of pyroxene-garnet assemblages in the mantle. Both of glass and crystalline starting materials were used for the experiments. At 1000 °C, garnet solid solution (s.s.) transforms to aluminous ilmenite s.s. at 20-26 GPa which is stable in the whole compositional range in the system. In Mg₄Si₄O₁₂-rich composition, ilmenite s.s. transforms to a single-phase aluminous perovskite s.s., while Mg₃Al₂Si₃O₁₂-rich ilmenite s.s. dissociates into perovskite s.s. and corundum s.s. These newly determined phase relations at 1000 °C supersede preliminary phase relations determined at about 900 °C in the previous study. The phase relations at 1000 °C are quite different from those reported previously at 1600 °C where garnet s.s. transforms directly to perovskite s.s. and ilmenite is stable only very close to Mg₄Si₄O₁₂. The stability field of Mg₃Al₂Si₃O₁₂ ilmenite was determined at 800-1000 °C and 25-27 GPa by reversed phase boundaries. In ilmenite s.s., the a-axis slightly increases but the c-axis and molar volume decrease substantially with increasing Al₂O₃ content. Enthalpies of ilmenite s.s. were measured by differential drop-solution calorimetry method using a high-temperature calorimeter. The excess enthalpy of mixing of ilmenite s.s. was almost zero within the errors. The measured enthalpies of garnet-ilmenite and ilmenite-perovskite transitions at 298 K were 105.2±10.4 and 168.6±8.2 kJ/mol, respectively, for Mg₄Si₄O₁₂, and 150.2±15.9 and 98.7±27.3 kJ/mol, respectively, for Mg₃Al₂Si₃O₁₂. Thermodynamic calculations using these data give rise to phase relations in the system Mg₄Si₄O₁₂-Mg₃Al₂Si₃O₁₂ at 1000 and 1600 °C that are generally consistent with those determined experimentally, and confirm that the single-phase field of ilmenite expands from Mg₄Si₄O₁₂ to Mg₃Al₂Si₃O₁₂ with decreasing temperature. The earlier mentioned phase relations in the simplified system as well as those in the Mg₂SiO₄-Fe₂SiO₄ system are applied to estimate mineral proportions in pyrolite as a function of depth along two different geotherms: one is a horizontally-averaged temperature distribution in a normal mantle, and the other being 600 °C lower than the former as a possible representative geotherm in subducting slabs. Based on the previously described estimated mineral proportions versus depth along the two geotherms, density and compressional and shear wave velocities are calculated as functions of depth, using available mineral physics data. Along a normal mantle geotherm, jumps of density and velocities at about 660 km corresponding to the post-spinel transition are followed by steep gradients due to the garnet-perovskite transition between 660 and 710 km. In contrast, along a low-temperature geotherm, the first steep gradients of density and velocities are due to the garnet-ilmenite transition between 610 and 690 km. This is followed by abrupt jumps at about 690 km for the post-spinel transition, and steep gradients between 700 and 740 km that correspond to the ilmenite-perovskite transition. In the latter profile along the low-temperature geotherm, density and velocity increases for garnet-ilmenite and ilmenite-perovskite transitions are similar in magnitude to those for the post-spinel transition. The likely presence of ilmenite in cooler regions of subducting slabs is suggested by the fact that the calculated velocity profiles along the low-temperature geotherm are compatible with recent seismic observations indicating three discontinuities or steep velocity gradients at around 600-750 km depth in the regions of subducting slabs.     

The system enstatite-pyrope at high pressures and temperatures and the mineralogy of the earth's mantle

In a diamond-anvil pressure cell coupled with laser heating, the system enstatite (MgSiO₃)-pyrope (3 MgSiO₃ · Al₂O₃) has been studied in the pressure region between about 100 and 300 kbar at about 1000°C using glass starting materials. The high-pressure phase behavior of the intermediate compositions of the system contrasts greatly with that of the two end-members. Differences between MgSiO₃ and 95% MgSiO₃ · 5% Al₂O₃ are especially remarkable. The phase assemblages β-Mg₂SiO₄ + stishovite and γ-Mg₂SiO₄ (spinel) + stishovite displayed by MgSiO₃ were not observed in 95% MgSiO₃ · 5% Al₂O₃, and the garnet phase, which was observed in 95% MgSiO₃ · 5% Al₂O₃ at high pressure, was not detected in MgSiO₃. These results suggest that the high-pressure phase transformations found in pure MgSiO₃ would be inhibited under mantle conditions by the presence even of small amounts of Al₂O₃ (?4% by weight). On the other hand, pyrope displays a wide stability field, finally transforming at 240–250 kbar directly to an ilmenite-type modification of the same stoichiometry. The two-phase region, within which orthopyroxene and garnet solid solutions coexist, is very broad. The structure of the earth's mantle is discussed in terms of the phase transformations to be expected in a simple mixture of 90% MgSiO₃ · 10% Al₂O₃ and Mg₂SiO₄. The seismic discontinuity at a depth of 400 km in the earth's mantle is probably due entirely to the olivine → β-phase transition in Mg₂SiO₄, with the progressive solution of pyroxene in garnet (displayed in 90% MgSiO₃ · 10% Al₂O₃) occurring at shallower depths. The inferred discontinuity at 650 km is due to the combination of the phase changes spinel → perovskite + rocksalt in Mg₂SiO₄ and garnet → ilmenite in 90% MgSiO₃ · 10% Al₂O₃. The 650-km discontinuity is thus characterized by an increase in the primary coordination of silicon from 4 to 6. A further discontinuity in the density and seismic wave velocities at greater depth associated with the ilmenite-perovskite phase transformation in 90% MgSiO₃ · 10% Al₂O₃ is expected.     

Phase relations in the system diopside-jadeite at high pressures and high temperatures

Phase behaviour in the system diopside-jadeite (CaMgSi₂O₆NaAlSi₂O₆) have been investigated in the pressure region 100–300 kbar at about 1000°C in a diamond-anvil press coupled with laser heating. The omphacite solid solution extends from 30 to at least 200 kbar for the entire system. Omphacites, ranging in composition from pure diopside to more than 40 mole % jadeite, transform to diopside (II) at pressures greater than 230 kbar. Diopside (II), which probably possesses a perovskite-type structure, cannot be preserved when experiments are quenched to ambient conditions. Jadeite-rich omphacites were found to decompose into an assemblage of NaAlSiO₄(CaFe₂O₄-type structure) + stishovite + diopside (II) (?) at pressures greater than about 260 kbar. These results suggest that an eclogitic model mantle would not display the 400-km seismic discontinuity. Moreover, sodium in the transition zone and lower mantle would most likely be accommodated in phases of omphacite and diopside (II).     

Viscosity and structural changes of albite (NaAlSi3O8) melt at high pressures

Viscosity of anhydrous albite melt, determined by the falling-sphere method in the solid-media, piston-cylinder apparatus, decreases with increasing pressure from 1.13 × 10⁵ P at 1 atm to 1.8 × 10⁴ P at 20 kbar at 1400°C. The rate of decrease in viscosity is larger between 12 and 15 kbar than in other pressure ranges examined. The density of the quenched albite melt increases with increasing pressure of quenching from 2.38 g/cm³ at 1 atm to 2.53 g/cm³ at 20 kbar. The rate of increase in density is largest at pressures between 15 and 20 kbar. The melting curve of albite shows an inflexion at about 16 kbar. These observations strongly suggest that structural changes of albite melt would take place effectively at pressures near 15 kbar. Melt of jadeite (NaAlSi₂O₆) composition shows very similar changes in viscosity and density and a melting curve inflexion at pressures near 10 kbar. Difference in pressure for the suggested effective structural changes of albite and jadeite melts is 5–6 kbar, which is nearly the same as that between the subsolidus reaction curves nepheline + albite= 2jadeite and albite=jadeite + quartz. The structural changes of the melts are, however, continuous and begin to take place at pressures lower than those of the crystalline phases.     

Experimental investigation on forsterite-grossularite incompatibility in presence of excess water

A mixture containing equal amounts of forsterite and grossularite by weight (Fo₅₀Gr₅₀) has been studied at temperatures between 750 and 1400°C under pressures ranging from 6 to 25 kbar in presence of excess water. The assemblages noted under low pressure (<8 kbar) are as follows: Diopside_ss+forsterite_ss+monticellite_ss+vapor and Diopside_ss+forsterite_ss+monticellite_ss+liquid+vapor. (ss denotes solid solution) Under intermediate pressures between 8 and 24 kbar following assemblages were noted in the order of increasing temperature: Diopside_ss+forsterite_ss+spinel+vapor, Diopside_ss+forsterite_ss+spinel+liquid+vapor, Diopside_ss+forsterite_ss+liquid+vapor, and Forsterite_ss+liquid+vapor. At pressures above 24 kbar the assemblages are as follows: Diopside_ss+forsterite_ss+garnet+vapor, Diopside_ss+forsterite_ss+garnet+liquid+vapor, Diopside_ss+forsterite_ss+liquid+vapor, and Forsterite_ss+liquid+vapor. Electron microprobe analyses of diopside and forsterite crystallized at 1050°C and 23 kbar, show that the former contains 6 to 6.5 wt % of Al₂O₃ as solid solution whereas the latter incorporates 1.3 wt % of monticellite in solid solution. The monticellite content of forsterite increases at low pressures at a given temperature to about 6 wt % at 1050°C and 6 kbar. The study indicates that forsteritic olivine does not coexist with pure grossularite in the studied temperature and pressure ranges, although the former is in equilibrium with pyrope-rich garnet, containing 23 mole % grossularite. The study supports the conclusion ofWerner andLuth (1973) that the solubility of monticellite in forsterite decreases with increasing pressure at a given temperature. The results of the investigation are also in agreement with the findings ofKushiro andYoder (1966), who noted that spinel peridotites found in folded belts and in alkalic basalts are produced under intermediate pressures, whereas garnet peridotite xenoliths found in kimberlite and in orogenic belts are formed at high pressures.     

The join grossularite-calcite through the system CaO—Al2O3—SiO2—CO2 at 30 kilobars: Crystallization range of silicates and carbonates on the liquidus

At 30 kbar, calcite melts congruently at 1615°C, and grossularite melts incongruently to liquid + gehlenite (tentative identification) at 1535°C. The assemblage calcite + grossularite melts at 1450°C to produce liquid + vapor, with piercing point at about 49 wt.% CaCO₃. Vapor phase is present in all hypersolidus phase fields except for those with less than about 7% CaCO₃ or 8% Ca₃Al₂Si₃O₁₂. These results, together with known liquidus data for CaO—SiO₂—CO₂ and inferred results for CaO—Al₂O₃—CO₂ and Al₂O₃—SiO₂—CO₂, permit construction of the position of the CO₂- saturated liquidus surface in the quaternary system, and estimation of the positions of liquidus field boundaries separating some of the primary crystallization fields on this surface. The field of calcite is separated from those for grossularite and quartz by a field boundary with about 50% dissolved CaCO₃. Crystallization paths of silicate liquids in the range Ca₂SiO₄—Ca₃Al₂Si₃O₁₂—SiO₂, with some dissolved CO₂, will terminate at a quaternary eutectic on this field boundary, with the precipitation of calcite together with grossularite and quartz, at a temperature below 1450°C. Addition of Al₂O₃ to CaO—SiO₂—CO₂ in amounts sufficient to stabilize garnet thus causes little change in the general liquidus pattern as far as carbonates and silicates are concerned. With addition of MgO, we anticipate that silicate liquids with dissolved CO₂ will also follow liquidus paths to fields for the precipitation of carbonates; we conclude that similar paths link kimberlite and some carnbonatite magmas.     

Elasticity of the ilmenite - perovskite phase transformation in CdTiO3

Ultrasonic data for the velocities of the ilmenite and perovskite polymorphs of CdTiO₃ have been determined as a function of pressure to 7.5 kbar at room temperature for polycrystalline specimens hot-pressed at pressures up to 25 kbar. This transition is characterized by the following velocity (ν)-density (?) relationships: (1) the changes in compressional (ν_p) and bulk sound (ν_?) velocities are comparable in percentage magnitude to the density jump, while the shear (ν_s) velocity jump is three times greater than that for ?; (2) (ν_p/ν_s) decreases across the transition from the low- to high-pressure phase; and (3) low slopes (linear or logarithmic) on ν-? diagrams. The (ν_p/ν_s) behaviour for the ilmenite-perovskite transformation is unusual for the transitions studied in our laboratory. The observed relationships (1) and (2) are typical of the elasticity behaviour across phase transformations which involve increases in cation-anion co-ordination and in nearest-neighbour interatomic distances, such as those exhibited by CdTiO₃ in transforming from the ilmenite to the perovskite phase. Elasticity systematics for isostructural sequences are used to estimate the bulk moduli of the perovskite polymorphs of CaSiO₃ (2.7 Mbar) and MgSiO₃ (2.8 Mbar).     

A theory of the pressure-induced high-spin—low-spin transition of transition-metal oxides

The pressure-induced high-spin—low-spin transition of the transition-metal ions in octahedral coordination is studied theoretically. The relation between the crossover point and the transition point is discussed and the formula to determine the transition point is given in terms of the crystal-field splitting Δ and the spin-pairing energy Π asΔ = αΠ. The numerical coefficient α is determined by the ratio of the change of the interatomic distances between the transition-metal ion and ligands. It is proved that α is generally less than 1 and takes a value of about 0.95-0.80 for the transition-metal oxides. Based on the discussions on the P–V relation of the low-spin oxides, transition pressures are estimated to be about 700–1300 kbar for MnO, CoO and Fe₂O₃, and about 250–400 kbar for FeO.The magnitude of the shifts of transition pressures due to entropy changes is roughly evaluated to be about 0.1 kbar K^?1 for transition-metal oxides. It is discussed whether the transition at high temperature takes place continuously or not. For MnO, FeO and Fe₂O₃, the high-spin—low-spin transition will take place gradually at temperatures higher than about 2000 K. For CoO, the gradual transition will take place at above 300 K.     

Isotopic composition of lead and strontium in lavas and coarse-grained blocks from Ascension Island,South Atlantic—an addendum

Natural marokite (CaMn₂O₄) has been studied at high pressures and temperatures using a diamond-anvil press coupled with laser heating in the pressure range 100–250 kbar. A mixture of marokite, CaMnO₃ (perovskite) and MnO (rocksalt) has been observed in all runs in the above pressure range by X-ray diffraction study of the quenched samples. It was interpreted that marokite disproportionates into the mixture CaMnO₃ (perovskite) + MnO (rocksalt) at pressures below 100 kbar. A general comparison of the molar volume for all known compounds having the marokite-related structures (including CaFe₂O₄ and CaTi₂O₄) with those for a mixture of perovskite plus rocksalt structures suggested that the mixture is more stable than the marokite-related structures at high pressures, as confirmed by the present experimental result. The CaFe₂O₄-modification of common nepheline (NaAlSiO₄) is also suggested to be unstable relative to the component oxides of α-NaAlO₂ + SiO₂ (stishovite) at high pressures.     

The influence of pressure,temperature and bulk composition on the appearance of garnet in orthogneisses — an example from South Harris,Scotland

Granulite facies metamorphism of the igneous complex of South Harris has produced garnet-clinopyroxene-plagioclase assemblages from olivine-normative rocks and 2 pyroxene-plagioclase-quartz assemblages from quartz-normative rocks. The appearance of garnet can be considered in terms of two complex reactions:Olivine + plagioclase₁ → (Ca, Mg, Fe) garnet + plagioclase₂(olivine-normative) (A)Orthopyroxene + plagioclase₁ → (Ca, Mg, Fe) garnet + plagioclase₂ + quartz (quartz-normative) (B)For bulk compositions of the South Harris rocks the equilibrium pressure for reaction (A) has been exceeded whereas that for reaction (B) was not reached. Estimated physical conditions of metamorphism bracketed by these and other reactions are: 800–860°C and 10–13 kbar. These estimates, based on experimental data on simple systems combined with thermodynamic models of the solid solutions involved are in good agreement with extrapolated pressures for the experimentally determined appearance of garnet in basaltic compositions (Green and Ringwood, 1967). The latter give 9–12 kbar in the temperature range of interest. The calculations are also consistent with the occurrence of kyanite in associated metapelites and with the stability of spinel-lherzolite during the granulite metamorphism.     

High-pressure phase equilibria in a garnet lherzolite,with special reference to Mg2+Fe2+ partitioning among constituent minerals

Phase equilibria in a natural garnet lherzolite nodule (PHN 1611) from Lesotho kimberlite and its chemical analogue have been studied in the pressure range 45–205 kbar and in the temperature range 1050–1200°C. Partition of elements, particularly Mg²⁺Fe²⁺, among coexisting minerals at varying pressures has also been examined. High-pressure transformations of olivine(α) to spinel(γ) through modified spinel(β) were confirmed in the garnet lherzolite. The transformation behavior is quite consistent with the information previously accumulated for the simple system Mg₂SiO₄Fe₂SiO₄. At pressures of 50–150 kbar, a continuous increase in the solid solubility of the pyroxene component in garnet was demonstrated in the lherzolite system by means of microprobe analyses. At 45–75 kbar and 1200°C, the Fe²⁺/(Mg + Fe²⁺) value becomes greater in the ascending order orthopyroxene, Ca-rich clinopyroxene, olivine and garnet. At 144–146 kbar and 1200°C, garnet exhibits the highest Fe²⁺/(Mg + Fe²⁺) value; modified spinel(β) and Ca-poor clinopyroxene follow it. When the modified spinel(β)-spinel(γ) transformation occurred, a higher concentration of Fe²⁺ was found in spinel(γ) rather than in garnet. As a result of the change in the Mg²⁺Fe²⁺ partition relation among coexisting minerals, an increase of about 1% in the Fe₂SiO₄ component in (Mg,Fe)₂SiO₄ modified spinel and spinel was observed compared with olivine.These experimental results strongly suggest that the olivine(α)-modified spinel(β) transformation is responsible for the seismic discontinuity at depths of 380–410 km in the mantle. They also support the idea that the minor seismic discontinuity around 520 km is due to the superposition effect of two types of phase transformation, i.e. the modified spinel(β)-spinel(γ) transformation and the pyroxene-garnet transformation. Mineral assemblages in the upper mantle and the upper half of the transition zone are given as a function of depth for the following regions: 100–150, 150–380, 380–410, 410–500, 500–600 and 600–650 km.     

Petrology and genetic significance of an ultramafic xenolith suite from Tahiti

A suite of ultramafic xenoliths 2–10 cm in size occurs in basanite near Papeete, Tahiti, and consists of spinel lherzolites with minor dunites and wehrlites. Petrographic examination of xenoliths reveals that they are typically coarse grained with well-developed annealed textures. Microprobe analyses of constituent minerals in 11 xenoliths indicate that bulk compositions of xenoliths are magnesian but with significant variability from xenolith to xenolith especially in Fe/Mg and Cr/Al ratios and in absolute amounts of Al₂O₃ and Cr₂O₃. Within any single xenolith, however, coexisting minerals are homogeneous and appear to be compositionally equilibrated. Geothermometry of coexisting orthopyroxene and augite indicates temperatures of equilibration of about 1100°C but there is considerable uncertainty in this estimate due to significant non-quadrilateral pyroxene substitutions. There is no accurate way to determine pressures, but the ubiquity of Cr-poor spinel and absence of garnet imply pressures less than about 15–20 kbar.The margins of most xenoliths show significant alteration through reaction with enclosing alkaline magma. Principal reaction features include zoning of spinels and olivines toward compositions in equilibrium with the magma, and reaction-melting of orthopyroxene to a symplectite of olivine plus silica-rich glass. Glass composition profiles across the symplectites indicate that alkalis, titanium and aluminum diffused into the symplectite from the magma and that silica diffused into the magma. All glass analyses show very low iron, magnesium and calcium.Xenolith mineral assemblages and chemistry indicate their origin in the upper mantle at relatively shallow depths. They are therefore not related genetically to the enclosing basanite magma which came from deeper in the mantle, but rather are accidental fragments of country rock picked up by magma on its way to the surface. The details of the reaction features strongly imply that the magma had partially crystallized by the time it reacted with xenoliths, possibly while still in the mantle.     

Phase transformations in MSiO4 compounds at high pressures and their geophysical implications

The phase behaviour of MSiO₄ compounds (MHf, Zr, U and Th0 has been investigated at high pressures and temperatures in a diamond-anvil press coupled with laser heating. All of these compounds have been found to undergo two or perhaps three phase transformations at pressures below 300 kbar. The high-pressure phase transformations of these compounds differ from one another, with the exception of HfSiO₄ and ZrSiO₄, which undergo identical phase transformations. The ultimate phase assemblages of these compounds are of dense component dioxides (although this is yet to be confirmed in ThSiO₄). It is suggested that the heat-producing elements U and Th would exist as dioxide solid solutions rather than silicates in the deep interior of the earth. Moreover, the densities of these dioxides are more than twice as great as mantle silicates and even slightly greater than pure iron under similar P, T conditions. Gravitational separation due to mandle convection may transport these dioxides to the deep interior of the earth to form deep heat sources. It is also suggested, however, that these deep heat sources are located in the inner-outer core boundary instead of in the lower mantle.     

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).     

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.