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1.
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 ofDNiol-andesite melt is greater than that ofDNiol-basalt melt, for example.  相似文献   

2.
The melting curves of the structural analogues SiO 2, BeF 2 and GeO 2 have been studied at pressures ?40 kbar in a piston-cylinder apparatus. The initial slopes dTm/dP of the β-quartz-liquid boundaries for SiO 2 and BeF 2 are ~35° while the slope of the rutile-liquid boundary for GeO 2 is approximately 32°C/kbar. These large values of dT/dP reflect the unusually low entropies of fusion for these compounds in which strong structural similarities exist between the crystalline phases and the melt. Implications for the extended phase diagram of silica are discussed and it is concluded that either: (1) a maximum exists on the coesite melting curve, or (2) estimates of the melting temperature of stishovite need to be revised upwards.  相似文献   

3.
The anionic structure of magmatic liquids has been estimated at 1 atm and at pressures corresponding to those of the upper mantle. These estimates are based predominantly on spectroscopic data on binary metal oxide-silica and ternary metal oxide-silica-alumina melts. Structural information on melt compositions in aluminate-silica joins has been used to provide detailed information on the role of Al3+ in natural magma at atmospheric and high pressure.Regardless of pressure, andesitic melts may be described as combinations of chain, sheet, and three-dimensional network units. Nearly all Al3+ in the magmatic liquid resides in the three-dimensional network units. This Al3+ is locally charge-balanced with Na+, K+, Ca2+, and Mg2+. In the latter two cases, Al3+ and Si4+ are ordered, whereas for Na+ and K+, Si4+ and Al3+ are randomly mixed. Solution of water in natural magma results in the formation of new nonbridging oxygens in addition to OH groups attached to Si4+ and metal cations.On the basis of determined solution mechanisms of CO2 and H2O in silicate melts, thermodynamic properties of HO+CO2, fluids and hydrous silicate melts and melting phase relations in peridotite-H2O-CO2, systems, it is found that natural andesitic magma in equilibrium with spinel Iherzolite in the upper mantle (10–20 kbar) must contain at least 5–7 wt.% H2O. Andesitic magma with 5–7 wt.% H2O in solution may be described as a mixture of Al-free three-dimensional units, sheets, and chains with a small proportion (less than 10%) of monomers.  相似文献   

4.
Bubble growth in rhyolitic melts: experimental and numerical investigation   总被引:2,自引:0,他引:2  
 Bubble growth controlled by mass transfer of water from hydrated rhyolitic melts at high pressures and temperatures was studied experimentally and simulated numerically. Rhyolitic melts were hydrated at 150 MPa, 780–850  °C to uniform water content of 5.5–5.3 wt%. The pressure was then dropped and held constant at 15–145 MPa. Upon the drop bubbles nucleated and were allowed to grow for various periods of time before final, rapid quenching of the samples. The size and number density of bubbles in the quenched glasses were recorded. Where number densities were low and run duration short, bubble sizes were in accord with the growth model of Scriven (1959) for solitary bubbles. However, most results did not fit this simple model because of interaction between neighboring bubbles. Hence, the growth model of Proussevitch et al. (1993), which accounts for finite separation between bubbles, was further developed and used to simulate bubble growth. The good agreement between experimental data, numerical simulation, and analytical solutions enables accurate and reliable examination of bubble growth from a limited volume of supersaturated melt. At modest supersaturations bubble growth in hydrated silicic melts (3–6 wt% water, viscosity 104–106 Pa·s) is diffusion controlled. Water diffusion is fast enough to maintain steady-state concentration gradient in the melt. Viscous resistance is important only at the very early stage of growth (t<1 s). Under the above conditions growth is nearly parabolic, R2=2Dtρm(C0–Cf)/ρg until the bubble approaches its final size. In melts with low water content, viscosity is higher and maintains pressure gradients in the melt. Growth may be delayed for longer times, comparable to time scales of melt ascent during eruptions. At high levels of supersaturation, advection of hydrated melt towards the growing bubble becomes significant. Our results indicate that equilibrium degassing is a good approximation for modeling vesiculation in melts with high water concentrations (C0>3 wt%) in the region above the nucleation level. When the melt accelerates and water content decreases, equilibrium can no longer be maintained between bubbles and melt. Supersaturation develops in melt pockets away from bubbles and new bubbles may nucleate. Further acceleration and increase in viscosity cause buildup of internal pressure in the bubbles and may eventually lead to fragmentation of the melt. Received: 19 June 1995 / Accepted: 27 December 1995  相似文献   

5.
Oxygen self diffusion rates were determined in quartz samples exchanged with18O-enriched CO2 between 745 and 900°C and various pressures, and the diffusion profiles were measured using an ion microprobe. The activation energy (Q) and preexponential factor (D0) at P(CO2) = P(tot) = 100 bar, for diffusion parallel to the c-axis are 159 ( ± 13) kJ/g atom and 2.10 (+0.75/ −0.55) × 10−8 cm2/s. This rate is approximately 100 times slower than that obtained from hydrothermal experiments and 100 times faster than a previous 1-bar quartz-O2 exchange experiment. The oxygen diffusion rate measured at 0.6 bar, 888°C, and at 900°C in vacuum is in agreement with the previous 1-bar exchange experiments with18O2. The effect of higher CO2 pressures is small. At 900°C, the diffusion rate exchanged with CO2 is = 2.35 × 10−15 cm2/s at 100 bar, 2.24 × 10−15 cm2/s at 3.45 kbar and 8.13 × 10−15 cm2/s at 7.2 kbar.There is probably a diffusing species, other than oxygen, that enhances the oxygen diffusion rate in these quartz-CO2 systems, relative to that occurring at very low pressures or in a vacuum. The effect of this diffusing species, however, is not as strong as that associated with H2O. Preserved oxygen isotope fractionations between coexisting minerals in a slowly cooled, high-grade metamorphic terrane will vary depending upon whether a water-rich phase was present or not. Closure temperatures will be approximately 100°C higher in rocks where no water-rich phase was present during cooling. The measured fractionations between coexisting minerals in metamorphic rocks may potentially be used as a sensor of water presence during retrogression.  相似文献   

6.
High-pressure phase transformations of albite,jadeite and nepheline   总被引:1,自引:0,他引:1  
Phase behaviors of albite, jadeite and nepheline have been studied in the diamond-anvil press employing YAG laser heating from about 100 to 280 kbar and at about 1000°C. Incorporating earlier work, the sequences of phase transformations with increasing pressure are as follows:
where the percentages are the decreases in zero-pressure volume from one to the other. Both NaAlSi3O8 hollandite and NaAlSiO4 (CaFe2O4-type) are new sodium aluminosilicates. The latter is the most likely host for sodium in the earth's lower mantle. High-pressure phases or phase assemblages revealed in albite and jadeite by static experiments are poorly defined by shock-wave studies of albitite and jadeite to about 900 and 1200 kbar respectively.  相似文献   

7.
The viscosity of basalts (quartz and olivine tholeiite) was studied under pressure in dry conditions and in the presence of water. In dry conditions at 1400°C when pressure increases to 20 kbars the viscosity reduces by a factor of 2. In conditions of water saturation of basalt melts at 5 kbar the viscosity is smaller by a factor of ~ 50 than that in dry conditions. In the water undersaturated conditions when water content is fixed (3.3% H2O) in melt the viscosity considerably decreases with pressure and takes intermediate value between those in dry and water saturated conditions. Experimental data recently obtained permit us to consider the peculiarities of physical properties of magma in the presence of water on a new base. Ascending magma can reach critical velocities of transition to the turbulent regime under negligible pressure drop, as a result of low viscosity. It is known at present that water influences on the viscosity of acidic melt under pressure of 1–8 kbars and at temperatures between 800–1200°C. Various authors gave physico-chemical evaluation of the dynamics of granite melts on the basis of these data. The viscosity of basalt melts and their dynamics under normal pressure is also well-known. The known new experimental data of basaltic melt viscosity under pressure in dry conditions (Kushiro et al., 1976;Khitarov et al., 1978) and in the presence of water (Khitarov et al., 1976) embrace broader intervals of physico-chemical conditions as on the pressure (up to 20–30 kbar) as well on the content of water (from 3% up to 12 %). These data permitted to evaluate on a new base the dynamics of magmatic melts under pressure.  相似文献   

8.
Phase assemblages for five selected compositions in the system CaSiO3-Al2O3 have been investigated in the pressure range 100–300 kbar and at about 1000°C in a diamond-anvil press coupled with laser heating. At pressures below about 250 kbar, the assemblage of grossularite plus corundum is stable for compositions containing more than 25 mole% Al2O3. Above about 250 kbar, phase assemblages for the latter compositions are truncated by those in the join CaAl2O4-SiO2. Garnet solid solutions are stable between about 10 and 25 mole% Al2O3. Grossularite transforms to a new tetragonal form at pressures greater than about 250 kbar, but the stability field for the garnet solid solutions extends to pressures up to about 300 kbar. The perovskite modification appears to be stable at pressures above about 150 kbar, but is probably limited to nearly pure CaSiO3 composition. Phase behaviour for calcium-bearing silicates or aluminosilicates in the lower mantle are apparently more complicated than was suggested earlier.  相似文献   

9.
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.  相似文献   

10.
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 (dTm/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.  相似文献   

11.
A 3D velocity model of the Earth’s crust beneath the Klyuchevskoy volcanic group has been constructed using the seismic tomography method. Anomalies of the velocity parameters related to the zones of magma supply to active volcanoes have been distinguished. Petrological data on the composition, temperature, and pressure of generation and crystallization of parental melts of Klyuchevskoy volcano magnesian basalts have been obtained. The parental melt corresponds to picrite (MgO = 13–14 wt %) with an ultimate saturation of SiO2 (49–50 wt %), a high H2O content (2.2–2.9%), and incompatible elements (Sr, Rb, Ba). This melt is formed at pressures of 15–20 kbar and temperatures of 1280–1320°C. Its further crystallization proceeds in intermediate magma chambers at two discrete pressure levels (i.e., greater than 6, and 1–2 kbar). The results of the petrological studies are in good agreement with the seismotomographic model.  相似文献   

12.
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).  相似文献   

13.
Tholeiitic basalt glasses from the FAMOUS area of the Mid-Atlantic Ridge are among the most primitive basaltic liquids reported from the ocean basins. One of the more primitive of these[Mg/(Mg+Fe2+) = 0.68;Ni= 232ppm;TiO2 = 0.61] glasses (572-1-1) was selected for an experimental investigation. This study found olivine to be the liquidus phase from 1 atm to 10.5 kbar where it is replaced by clinopyroxene. The sequence of appearance of phases at 1 atm pressure is olivine (1268°C), plagioclase (1235°C) and clinopyroxene (1135°C). The sample is multiply saturated at 10.5 kbar with olivine (Fo88), clinopyroxene (Wo32En60Fs9), and orthopyroxene (Wo5En83Fs12). From the 1-atm data we have measured (FeO/MgO) olivine/(FeO*/MgO) liquid (K′D) for olivine-melt pairs equilibrated at 12 temperatures in the range 1268–1205°C.K′D varies from 0.30 at 1205°C to 0.27 at 1268°C. Analysis of high-pressure olivine melt pairs indicates a systematic increase inK′D with pressure.Evaluation of the 1-atm experiments reveals that fractionation of olivine followed by olivine + plagioclase can generate much of the variation in major element chemistry observed in the FAMOUS basalt glasses. However, it cannot account for the entire spectrum of glass compositions — particularly with respect to TiO2 and Na2O. The variations in these components are such as to require different primary liquids.Comparison of clinopyroxene microphenocrysts/xenocrysts found in oceanic tholeiites with experimental clinopyroxenes reveal that the majority of those in the tholeiites may have crystallized from the magma at pressures greater than ~ 10 kbar and are not accidental xenocrysts. Clinopyroxene fractionation at high pressures may be a viable mechanism for fractionating basaltic magmas.The major and minor element mineral/meltK′d's from our experiments have been used to model the source region residual mineralogy for given percentages of partial melting. These data suggest that ~20% partial melting of a lherzolite source containing 0–10% clinopyroxene can generate the major and minor element concentrations in the parental magmas of the Project FAMOUS basalt glasses.  相似文献   

14.
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 Vφ?ρ relation but the αβ transition does not.  相似文献   

15.
The melting curve of forsterite has been studied by static experiment up to a pressure of 15 GPa. Forsterite melts congruently at least up to 12.7 GPa. The congruent melting temperature is expressed by the Kraut-Kennedy equation in the following form: Tm(K)=2163 (1+3.0(V0 ? V)/V0), where the volume change with pressure was calculated by the Birch-Managhan equation of state with the isothermal bulk modulus K0 = 125.4 GPa and its pressure derivative K′ = 5.33. The triple point of forsterite-β-Mg2SiO4-liquid will be located at about 2600°C and 20 GPa, assuming that congruent melting persists up to the limit of the stability field of forsterite. The extrapolation of the previous melting data on enstatite and periclase indicates that the eutectic composition of the forsterite-enstatite system should shift toward the forsterite component with increasing pressure, and there is a possibility of incongruent melting of forsterite into periclase and liquid at higher pressure, although no evidence on incongruent melting has been obtained in the present experiment.  相似文献   

16.
The solubility of fluorapatite in a wide variety of basic magmatic liquids was experimentally determined over a range of upper mantle P-T conditions (8–25 kbar, 1275–1350°C). Fluorapatite is stable over the entire range of conditions investigated, but its solubility in melts is variable, depending negatively on SiO2 content of the melt and positively upon temperature, with relatively little sensitivity to pressure above 8 kbar. At upper mantle pressures and a temperature of 1250°C, molten basalt (50% SiO2) will dissolve 3–4 wt.% P2O5 before saturation in apatite is reached. For a magma 100°C cooler or containing 10 wt.% more SiO2, apatite saturation occurs at less than 2 wt.% dissolved P2O5. The observed high solubility of apatite in basic magmas at their normal near-liquidus temperatures virtually precludes the occurrence of residual apatite in mantle source regions. If relatively low-temperature melting conditions prevail (e.g., 1100°C), as might be possible in H2O-bearing regions of the upper mantle, apatite could remain in the residue, but only in amounts too small to have significant effects on the rare earth patterns of the liquids.Because of the high solubility of apatite in basic magmas, phosphorus can be confidently treated as an incompatible element in peridotite melting models. Such models, in combination with observed characteristics of basic lavas, indicate that the upper mantle contains ~200 ppm of phosphorus, much less than the chondritic abundance of ~900 ppm.  相似文献   

17.
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.  相似文献   

18.
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.  相似文献   

19.
Although the bulk moduli (KT0) of silicate melts have a relatively narrow range of values, the pressure derivatives of the isothermal bulk modulus (KT0) can assume a broad range of values and have an important influence on the compositional dependence of the melt compressibility at high pressure. Based on the melt density data from sink/float experiments at high pressures in the literature, we calculate KT0 using an isothermal equation of state (EOS) (e.g., Birch–Murnaghan EOS and Vinet EOS) with the previously determined values of room-pressure density (ρ0) and room-pressure bulk modulus (KT0). The results show that best estimates of KT0 vary considerably from ~ 3 to ~ 7 for different compositions. KT0 is nearly independent of Mg # (molar Mg/(Mg + Fe)), but decreases with SiO2 content. Hydrous melts have anomalously small KT0 leading to a high degree of compression at high pressures. For anhydrous melts, KT0 is ~ 7 for peridotitic melts, ~ 6 for picritic melts, ~ 5 for komatiitic melts, and ~ 4 for basaltic melts.  相似文献   

20.
Ultrasonic compressional wave velocity Vp and quality factor Qp have been measured in alkali basalt, olivine basalt and basic andesite melts in the frequency range of 3.4–22 MHz and in the temperature range of 1100–1400°C. Velocity and attenuation of the melts depend on frequency and temperature, showing that there are relaxation mechanisms in the melts. Complex moduli are calculated from the ultrasonic data. The results fit well a complex modulus of Arrhenius temperature dependence with log-normal Gaussian distribution in relaxation times of attenuation. The analysis yields average relaxation time, its activation energy, relaxed modulus, unrelaxed modulus and width of Gaussian distribution in relaxation times. Relaxed modulus is smaller (17.5 GPa) for basic andesite melt of high silica and high alumina contents than for the other two basalt melts (18.1–18.4 GPa). The most probable relaxation times decrease from ~ 3 × 10?10 s for basic andesite to ~ 10?11 s for alkali basalt at 1400°C. Activation energies of attenuation, ranging from 270 to 340 kJ mol?1 in the three melts, are highest in basic andesite. Longitudinal viscosity values and their temperature dependences are also calculated from Vp and Qp data. The volume viscosity values are estimated from the data using the shear viscosity values. Longitudinal, volume and shear viscosities and their activation energies are highest in the basic andesite melt of the most polymerized structure.  相似文献   

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