共查询到20条相似文献,搜索用时 203 毫秒
1.
M. Okuno B. Reynard Y. Shimada Y. Syono C. Willaime 《Physics and Chemistry of Minerals》1999,26(4):304-311
The densification processes in SiO2 glass induced by shock-wave compression up to 43.4 GPa are investigated by Raman spectroscopy. At first, densification increases
with increasing shock pressure. A maximum densification of 11% is obtained for a shock pressure of 26.3 GPa. This densification
is attributed to the reduction of the average Si−O−Si angle, which occurs first by the collapse of the largest ring cavities,
then by further reduction of the average ring size. For higher shock pressures, a different structural modification is observed,
resulting in decreasing densification with increasing shock pressure. Indeed, the recovered densification becomes very small,
with values of 1.8 and 0.5% at 32 and 43.4 GPa, respectively. This is attributed to partial annealing of the samples due to
high after shock residual temperatures. The study of the annealing process of the most densified glass by in situ high temperature
Raman spectroscopy confirms that relaxation of the Si−O−Si angle starts at a lower temperature (about 800 K) than that of
the siloxane rings (about 1000 K), thus explaining the high intensity of the siloxane defect bands in the samples schocked
at compressions of 32 and 43.4 GPa. The large intensity of the siloxane bands in the nearly undensified samples shocked by
compressions above 30 GPa may be explained by the relaxation during decompression of five- and six-fold coordinated silicon
species formed at high pressure and high temperature during the shock event.
Received: March 30, 1998 / Revised, accepted: August 21, 1998 相似文献
2.
Shock-recovery experiments for obsidian and its fused glass have been carried out with pressure up to 35 GPa. Structural evolution accompanying the shock compression was investigated using X-ray diffraction technique, Raman and infrared spectroscopy. The densities of obsidian and its fused glass increased with applied shock pressure up to 25 GPa. Densification reached a maximum of 4.7 and 3.6% for obsidian and its fused glass, respectively. The densification mechanism is attributed to reduction of the T–O–T angle, and changes in ring statistics in the structure. Density reduction observed at greater than 25 GPa of applied shock pressure is due to partial annealing of the high-density glass structures brought by high post-shock residual temperature. The density of fused glass is almost equal to its original value at 35 GPa while the shocked obsidian has a slightly lower value than its original value. Amorphization of crystallites present in the obsidian due to shock compression is probably the cause of the density decrease. The structural evolution observed in shock-compressed obsidian and its fused glass can be explained by densification resulting from average T–O–T angle reduction and increase of small rings, and subsequent structural relaxation by high post-shock temperature at applied shock compression above 25 GPa. 相似文献
3.
Y. Shimada M. Okuno Y. Syono M. Kikuchi K. Fukuoka N. Ishizawa 《Physics and Chemistry of Minerals》2002,29(4):233-239
The densification and structural changes in SiO2 glass compressed up to 43.4 GPa by shock experiments are investigated quantitatively by the X-ray diffraction technique.
Direct structural data (average Si–O and Si–Si distances and Si–O–Si angles, coordination number of the Si atom) of these
shock-densified SiO2 glasses have been obtained by analyzing the radial distribution function curves, RDF(r), calculated with X-ray diffraction data. The coordination number of all densified glasses is about 4 and shows almost no
pressure variation. The SiO2 glass has shown density increase of 11% at a shock compression of 26.3 GPa. This density evolution could not be explained
by the coordination change. The reduction of the average Si–O–Si angle (144° at 0 GPa to 136° at 26.3 GPa) obtained from RDF(r) data may account for this density increase. This Si–O–Si angle change may be caused by shrinkage of the network structure
and the increase of small rings of SiO4 tetrahedra. For higher shock pressure, a decrease in the Si–O–Si angle to 140° was observed. This is consistent with the
decrease in density at 32.0 and 43.2 GPa. This decrease in the Si–O–Si angle and density could be attributed to an annealing
effect due to high after-shock residual temperature. This pressure dependence of average Si–O–Si angles in shock-densified
SiO2 glass agrees with the results of our previous Raman spectroscopic study. On the other hand, the pressure variation for the
first sharp diffraction peak (FSDP) was analyzed to estimate the evolution of intermediate range structures. It is suggested
that the mean d value (d
m
) obtained from the position of FSDP strongly depends on the shock and residual temperature, as well as shock pressure.
Received: 29 June 2001 / Accepted: 14 November 2001 相似文献
4.
Raman microprobe spectra were made on three post shock, diaplectic plagioclase feldspars. Optical and X-ray diffraction studies indicated that feldspars maintained a partially or totally crystalline state after having passed through the mixed phase zone of Hugoniot response to shock waves (15–38 GPa). The appearance of uniquely glass-type spectra occurs at different shock pressures for each specimen according to its atomic structural arrangement, below 38 GPa for mosaic structured labradorite, near 40 GPa for anorthite and above 50 GPa for the highly ordered low albite. The diaplectic anorthite and labradorite glasses give spectra which indicate the presence of two glass types. Shifts in the band envelope frequencies compared to spectra of fused glass and statically pressure densified glass suggest that these glasses have specific structural arrangements. These differences suggest that the shock and fusion glass-forming processes are not exactly identical. The results from material shocked in the mixed phase region of Hugoniot response show that the phase transitions are effected at different pressures depending upon the feldspar structural type. 相似文献
5.
Burkhard O. Dressler Virgil L. Sharpton Benjamin C. Schuraytz 《Contributions to Mineralogy and Petrology》1998,130(3-4):275-287
The Slate Islands archipelago is believed to represent the central uplifted portion of a complex impact structure. Planar
microstructures in quartz and feldspars and shock vitrification of rocks are the most common shock metamorphic features encountered.
No diaplectic quartz was identified in the exposed rocks, but minor maskelynite is present. Shatter cones occur on all islands
of the archipelago suggesting minimum pressures of 4 ± 2 GPa. The relative frequency of low index planar microstructures of
specific, optically determined crystallographic orientations in quartz are correlated with results from shock barometric experiments
to estimate peak shock pressures experienced by the exposed target rocks. In general, there is a decrease in shock pressure
recorded in the target rocks from about 20–25 GPa in east-central Patterson Island to about 5–10 GPa at the western shore
of this island and on Mortimer Island. The shock attenuation gradient is ∼4.5 GPa/km across this section of the island group.
However, the shock attenuation has a roughly concentric plan only over the western part of the archipelago. There is no distinct
shock center and there are other deviations from circularity. This is probably the result of: (1) the shock wave not having
expanded from a point or spherical source because of the ∼1. 0 to 1.5 km size of the impactor; (2) differential movement of
large target rock blocks during the central uplift and crater modification phases of the impact process. The orientation of
planar deformation features in quartz appears to be independent of the shock wave direction suggesting that crystal structure
exerts the primary control on microstructure development. Based on the results of XRD analyses, residual, post-impact temperatures
were high enough to cause annealing of submicroscopic damage in shocked quartz.
Received: 15 July 1997 / Accepted: 6 October 1997 相似文献
6.
Samples of synthetic diaplectic anorthite glass (38 GPa shock pressure), thermal glass and synthetic anorthite crystals were investigated using infrared spectral methods at one atmosphere and high pressures (near 4 GPa). Band positions and pressure derivatives for the Si-O asymmetric modes in the region 1,300–900 cm?1 indicate that the diaplectic glass has more structural similarities with the crystalline material than with thermal glass even though the overall infrared spectral characteristics suggest a glassy state. 相似文献
7.
A. K. Kleppe A. P. Jephcoat H. Olijnyk A. E. Slesinger S. C. Kohn B. J. Wood 《Physics and Chemistry of Minerals》2001,28(4):232-241
Raman spectra of hydrous β-Mg2SiO4 (1.65 wt% H2O) have been measured in a diamond-anvil cell with helium as a pressure-transmitting medium at room temperature to 50 GPa.
We observe three OH-stretching modes, a doublet with components at 3329 and 3373 cm−1, which decrease linearly with pressure, and a single mode at 3586 cm−1, which remains nearly constant up to 24 GPa before decreasing at higher pressures. Assessment of the mode frequencies and
their pressure dependence, together with previous results from X-ray and IR data, are consistent with protonation of the O1
site in agreement with previous studies. Strict assignment of Raman activity awaits detailed structural models. The nature
of the protonation in wadsleyite may require more specific experimental probes for full solution of the hydrogen-site problem.
Received: 18 July 2000 / Accepted: 22 November 2000 相似文献
8.
Summary Supra-solidus phase relations at temperatures and pressures ranging from 800 to 1700 °C and 2 to 6.4 GPa have been determined
experimentally for three silica-rich lamproites: hyalo-leucite phlogopite lamproite (Oscar, West Kimberley); sanidine richterite
lamproite (Cancarix, Murcia-Almeria); and phlogopite transitional madupitic lamproite (Middle Table Mountain, Wyoming). All
samples have extended melting intervals (500–600 °C). Bulk composition has a significant control on the nature of the initial
liquidus phases, with orthopyroxene occurring at low pressures (<4 GPa) in the relatively calcium-poor Oscar and Cancarix
lamproites. At higher pressure (>6 GPa) orthopyroxene is replaced by garnet plus clinopyroxene as near-liquidus phases in
the Oscar lamproite and by orthopyroxene plus clinopyroxene in the Cancarix sample. Clinopyroxene is a near-liquidus phase
at all pressures in the Middle Table Mountain lamproite. Near-solidus phase assemblages at high pressure (>5 GPa) are: clinopyroxene + phlogopite + coesite + rutile + garnet
(Oscar); clinopyroxene + garnet + coesite + K–Ti-silicate (Cancarix); clinopyroxene + phlogopite + apatite + K–Ti-silicate
(Middle Table Mountain). In all compositions olivine is never found as a liquidus phase at any of the temperatures or pressures
studied here. The phase relationships are interpreted to suggest that silica-rich lamproites cannot be derived by the partial
melting of lherzolitic sources. Their genesis is considered to involve high degrees of partial melting of ancient metasomatic
veins within a harzburgitic-lherzolitic lithospheric substrate mantle. The veins are considered in their mineralogy to be
similar to the experimentally-observed, high pressure, near-solidus phase assemblages. The composition of silica-rich primary
lamproite magmas differs between cratons as a consequence of differing mineralogical modes of the source veins and different
relative contributions from the veins and wall-rocks to the partial melts.
Received February 21, 2000; revised version accepted July 3, 2001 相似文献
9.
Raman spectra of a single-crystal fragment of hydrous γ-Mg2SiO4, synthesized in a multianvil press, have been measured in a diamond-anvil cell with helium as pressure-transmitting medium
to 56.5 GPa at room temperature. All five characteristic spinel Raman modes shift continuously up to the highest pressure,
showing no evidence for a major change in the crystal structure despite compression well beyond the stability field of ringwoodite
in terms of pressure. At pressures above ∼30 GPa a new mode on the low-frequency site of the two silicate-stretching modes
is clearly identifiable, indicating a modification in the spinel structure which is reversible on pressure release. The frequency
of the new mode (802 cm−1 extrapolated to 1 bar) suggests the presence of Si–O–Si linkages and/or a partial increase in the coordination of Si. Direct
determination of the subtle structural change causing the new Raman mode would require high-pressure, single-crystal synchrotron
X-ray diffraction experiments. The Raman modes of hydrous and anhydrous Mg-end-member ringwoodite are nearly identical up
to 20 GPa, suggesting that protonation has only minor effect on the lattice dynamics over the entire pressure stability range
for ringwoodite in the mantle.
Received: 7 December 2001 / Accepted: 16 April 2002 相似文献
10.
The second-order elastic constants of CaF2 (fluorite) have been determined by Brillouin scattering to 9.3 GPa at 300 K. Acoustic velocities have been measured in the
(111) plane and inverted to simultaneously obtain the elastic constants and the orientation of the crystal. A notable feature
of the present inversion is that only the density at ambient condition was used in the inversion. We obtain high-pressure
densities directly from Brillouin data by conversion to isothermal conditions and iterative integration of the compression
curve. The pressure derivative of the isentropic bulk modulus and of the shear modulus determined in this study are 4.78 ± 0.13
and 1.08 ± 0.07, which differ from previous low-pressure ultrasonic elasticity measurements. The pressure derivative of the
isothermal bulk modulus is 4.83 ± 0.13, 8% lower than the value from static compression, and its uncertainty is lower by a
factor of 3. The elastic constants of fluorite increase almost linearly with pressure over the whole investigated pressure
range. However, at P ≥ 9 GPa, C
11 and C
12 show a subtle structure in their pressure dependence while C
44 does not. The behavior of the elastic constants of fluorite in the 9–9.3 GPa pressure range is probably affected by the onset
of a high-pressure structural transition to a lower symmetry phase (α-PbCl2 type). A single-crystal Raman scattering experiment performed in parallel to the Brillouin measurements shows the appearance
of new features at 8.7 GPa. The new features are continuously observed to 49.2 GPa, confirming that the orthorhombic high-pressure
phase is stable along the whole investigated pressure range, in agreement with a previous X-ray diffraction study of CaF2 to 45 GPa. The high-pressure elasticity data in combination with room-pressure values from previous studies allowed us to
determine an independent room-temperature compression curve of fluorite. The new compression curve yields a maximum discrepancy
of 0.05 GPa at 9.5 GPa with respect to that derived from static compression by Angel (1993). This comparison suggests that
the accuracy of the fluorite pressure scale is better than 1% over the 0–9 GPa pressure range.
Received: 10 July 2001 / Accepted: 7 March 2002 相似文献
11.
The crystal structure of MgFe2O4 was investigated by in situ X-ray diffraction at high pressure, using YAG laser annealing in a diamond anvil cell. Magnesioferrite
undergoes a phase transformation at about 25 GPa, which leads to a CaMn2O4-type polymorph about 8% denser, as determined using Rietveld analysis. The consequences of the occurrence of this dense MgFe2O4 form on the high-pressure phase transformations in the (MgSi)0.75(FeIII)0.5O3 system were investigated. After laser annealing at about 20 GPa, we observe decomposition to two phases: stishovite and a
spinel-derived structure with orthorhombic symmetry and probably intermediate composition between MgFe2O4 and Mg2SiO4. At pressures above 35 GPa, we observe recombination of these products to a single phase with Pbnm perovskite structure.
We thus conclude for the formation of Mg3Fe2Si3O12 perovskite.
Received: 27 March 2000 / Accepted: 1 October 2000 相似文献
12.
C. C. Lin 《Physics and Chemistry of Minerals》2001,28(4):249-257
The Raman spectra of synthetic α-Co2SiO4 and α-Ni2SiO4 olivines have been studied at room temperature and various pressures. All the Raman frequencies of the two olivines increase
with increasing pressure, and most of the frequency–pressure plots obtained under both quasi- and nonhydrostatic conditions
are nonlinear. It has been found that the average pressure derivative of Raman frequencies of the lattice modes in both Co-
and Ni-olivines is smaller than that of the internal modes of SiO4, indicating that the distortion of SiO4 tetrahedra under static compression may be more severe than that of MO6 octahedra. In addition, four new Raman bands were observed in Ni-olivine under nonhydrostatic compression and above 30 GPa.
This result suggests that a new phase of Ni-olivine should be formed at 30 GPa or amorphization may occur at still higher
pressure.
Received: 11 July 2000 / Accepted: 19 December 2000 相似文献
13.
Lin-gun Liu C.-C. Lin Y. J. Yung T. P. Mernagh T. Irifune 《Physics and Chemistry of Minerals》2009,36(3):143-149
K-lingunite is a high-pressure modification of K-feldspar that possesses the tetragonal hollandite structure. Variations of
the Raman spectra of K-lingunite were studied up to ~31.5 GPa at room temperature, and in the range 79–823 K at atmospheric
pressure. The Raman frequencies of all bands were observed to increase with increasing pressure, and decrease with increasing
temperature for K-lingunite. This behavior is in line with those observed for most of other materials. New sharp Raman bands
appear at pressures greater than 13–15 GPa, suggesting a phase transition in K-lingunite with increasing pressure. The transition
is reversible when pressure was released. The appearance of these new Raman bands may correspond to the phase transition revealed
earlier at around 20 GPa by X-ray diffraction studies. Instead of transforming back to its stable minerals, such as orthoclase,
microcline or sanidine, K-lingunite became amorphous in the temperature range 803–823 K at atmospheric pressure. 相似文献
14.
In situ X-ray diffraction experiments at high pressure were carried out up to 8.9 GPa and 1100 °C to study phase transformations
of iron and two iron-silicon alloys Fe0.91Si0.09 and Fe0.83Si0.17. For iron, the transformation from the bcc phase to the fcc phase was observed at pressures 3.8–8.2 GPa and temperatures
that are consistent with previous in situ X-ray diffraction studies. Reversal of the transformation of iron was found to be
sensitive to temperature; hysteresis of the transformation increased from 25 °C at 3.8 GPa to 100 °C at 7.0 GPa, primarily
because the bcc-fcc phase boundary has a negative Clayperon slope. In the binary system Fe-Si, the observations of the present
study indicate that the ferrite (bcc phase)-stabilizing behavior of silicon persists at high pressures and that the maximum
solubility of silicon in the fcc phase increases with increasing pressure: (1) the transformation from the bcc phase to the
fcc phase was observed in Fe0.91Si0.09 at 6.0, 7.4 and 8.9 GPa and the temperatures measured at the onset of the transformations were 300 °C higher than those in
iron at similar pressures, (2) the transformation rate in Fe0.91Si0.09 was extremely sluggish compared to that of iron, and (3) the bcc-fcc phase transformation was not observed in Fe0.91Si0.09 at 4.7 GPa up to 1000 °C and in Fe0.83Si0.17 at 8.2 GPa and 1100 °C.
Received: 1 June 1998 / Revised, accepted: 9 October 1998 相似文献
15.
Equilibrium volumes and expansivities of three liquids in the system anorthite (CaAl2Si2O8)–diopside (CaMgSi2O6) have been derived from dilatometric measurements of the equilibrium length of samples in the glass transition range. The
typical temperature range of 40 K for the measurements is limited at low temperature by the very long times necessary to reach
structural equilibrium and at high temperature by the penetration of the rod used to measure sample dilatation. Despite such
narrow intervals, the expansivities are determined to better than 3% thanks to the high precision with which length changes
are measured. The coefficient of volume thermal expansion (1/V dV/dT) of the fully relaxed liquid just above the glass transition
is found to decrease linearly from diopside composition (139 ± 4 × 10−6 K−1) to anorthite composition (59 ± 2 × 10−6 K−1). These values are greater than those determined for the same liquids at superliquidus temperatures, demonstrating that expansivities
of silicate melts may decrease markedly with increasing temperature. A predictive model based upon partial molar volumes which
vary as a linear function of the logarithm of temperature is proposed.
Received: 25 February 2000 / Accepted: 29 May 2000 相似文献
16.
The breakdown of potassium feldspar at high water pressures 总被引:1,自引:0,他引:1
Pauline Thompson Ian Parsons Colin M. Graham Brian Jackson 《Contributions to Mineralogy and Petrology》1998,130(2):176-186
The equilibrium position of the reaction between sanidine and water to form “sanidine hydrate” has been determined by reversal
experiments on well characterised synthetic starting materials in a piston cylinder apparatus. The reaction was found to lie
between four reversed brackets of 2.35 and 2.50 GPa at 450 °C, 2.40 and 2.59 GPa at 550 °C, 2.67 and 2.74 GPa at 650 °C, and
2.70 and 2.72 GPa at 680 °C. Infrared spectroscopy showed that the dominant water species in sanidine hydrate was structural
H2O. The minimum quantity of this structural H2O, measured by thermogravimetric analysis, varied between 4.42 and 5.85 wt% over the pressure range of 2.7 to 3.2 GPa and
the temperature range of 450 to 680 °C. Systematic variation in water content with pressure and temperature was not clearly
established. The maximum value was below 6.07 wt%, the equivalent of 1 molecule of H2O per formula unit. The water could be removed entirely by heating at atmospheric pressure to produce a metastable, anhydrous,
hexagonal KAlSi3O8 phase (“hexasanidine”) implying that the structural H2O content of sanidine hydrate can vary. The unit cell parameters for sanidine hydrate, measured by powder X-ray diffraction,
were a = 0.53366 (±0.00022) nm and c = 0.77141 (±0.00052) nm, and those for hexasanidine were a = 0.52893 (±0.00016) nm and c = 0.78185 (±0.00036) nm. The behaviour and properties of sanidine hydrate appear to be analogous to those of the hydrate
phase cymrite in the equivalent barium system. The occurrence of sanidine hydrate in the Earth would be limited to high pressure
but very low temperature conditions and hence it could be a potential reservoir for water in cold subduction zones. However,
sanidine hydrate would probably be constrained to granitic rock compositions at these pressures and temperatures.
Received: 6 May 1997 / Accepted: 2 October 1997 相似文献
17.
The infrared spectrum of CaAl2Si2O7 · H2O-lawsonite, has been characterized to pressures of 20 GPa at 300 K. Our results constrain the response to compression of
the silicate tetrahedra, hydroxyl units, and water molecules in this material. The asymmetric and symmetric stretching and
bending vibrations of the Si2O7 groups (at zero pressure frequencies between 600 and 1000 cm−1) increase in frequency with pressure at rates between 3.6 and 5.9 cm−1/GPa. All silicate modes appear to shift continuously with pressure to 20 GPa, although the lowest frequency stretching vibration
becomes unresolvable above 18 GPa, and a splitting of the main bending vibration is observed near this pressure. The O-H stretches
of the hydroxyl units exhibit a discontinuity in their mode shifts at ∼8–9 GPa, which we interpret to be produced by a pressure-induced
change in hydrogen bonding. The stretching and bending vibrations of the water molecule are relatively unaffected by compression
to 20 GPa, thus demonstrating that the structural cavities in which water molecules reside are relatively rigid. Significant
changes in the amplitude of the O-H stretches of the hydroxyl and water units are observed at this pressure as well; nevertheless,
our results demonstrate that the dominant structural units in lawsonite persist metastably at 300 K with only modest structural
modifications well beyond the known stability field of this phase.
Received: 10 July 1998 / Revised, accepted: 23 October 1998 相似文献
18.
S. Ono E. Ito T. Katsura A. Yoneda M. J. Walter S. Urakawa W. Utsumi K. Funakoshi 《Physics and Chemistry of Minerals》2000,27(9):618-622
In situ synchrotron X-ray experiments in the system SnO2 were made at pressures of 4–29 GPa and temperatures of 300–1400 K using sintered diamond anvils in a 6–8 type high-pressure
apparatus. Orthorhombic phase (α-PbO2 structure) underwent a transition to a cubic phase (Pa3ˉ structure) at 18 GPa. This transition was observed at significantly lower pressures in DAC experiments. We obtained the
isothermal bulk modulus of cubic phase K
0 = 252(28) GPa and its pressure derivative K
′=3.5(2.2). The thermal expansion coefficient of cubic phase at 25 GPa up to 1300 K was determined from interpolation of the
P-V-T data obtained, and is 1.7(±0.7) × 10−5 K−1 at 25 GPa.
Received: 7 December 1999 / Accepted: 27 April 2000 相似文献
19.
We present Raman and infrared spectra of gypsum to 21 GPa at 300 K. Our measurements encompass the internal modes of the
(SO4)−4 group that lie between 400 and 1150 cm−1, hydroxyl-stretching vibrations between 3200 and 3600 cm−1, and a libration and bending vibrations of the molecular H2O group. All vibrations of the sulfate group have positive pressure shifts, while the hydroxyl-stretching and -bending vibrations
have a mixture of positive and negative pressure shifts: the effect of pressure on the hydrogen bonding of the water molecule
thus appears to be complex. Near 5 GPa, the two infrared-active bending vibrations of the water molecule coalesce, and the
morphology of the hydroxyl-stretching region of the spectrum shifts dramatically. This behavior is consistent with a pressure-induced
phase transition in gypsum in the vicinity of 5–6 GPa, which is observed to be reversible on decompression to zero pressure.
The spectral observations are consistent with the onset of increased disorder in the position of the water molecule in gypsum:
the sulfate vibrations are largely unaffected by this transition. The Raman-active symmetric stretch of the sulfate group
undergoes an apparent splitting near 4 GPa, which is interpreted to be produced by Fermi resonance with an overtone of the
symmetric bending vibration. The average mode Grüneisen parameter of the 20 vibrational modes we sample is less than 0.05,
in contrast to the bulk thermal Grüneisen parameter of 1.20. Accordingly, the vibrations of both water and sulfate units within
gypsum are highly insensitive to volumetric compaction. Therefore, in spite of the changes in the bonding of the water unit
near 5 GPa, metastably compressed gypsum maintains strongly bound molecular-like units to over 20 GPa at 300 K.
Received: 31 July 2000 / Accepted: 5 April 2001 相似文献
20.
Summary Mantle-derived xenoliths from Baarley in the Quaternary West Eifel volcanic field contain six distinct varieties of glass
in veins, selvages and pools. 1) Silica-undersaturated glass rich in zoned clinopyroxene microlites that forms jackets around
and veins within the xenoliths. This glass is compositionally similar to groundmass glass in the host basanite. 2) Silica-undersaturated
alkaline glass that contains microlites of Cr-diopside, olivine and spinel associated with amphibole in peridotites. This
glass locally contains corroded primary spinel and phlogopite. 3) Silica-undersaturated glass associated with diopside, spinel ± olivine
and rh?nite microlites in partly to completely broken down amphibole grains in clinopyroxenites. 4) Silica-undersaturated
to silica-saturated, potassic glass in microlite-rich fringes around phlogopite grains in peridotite. 5) Silica-undersaturated
potassic glass in glimmerite xenoliths. 6) Silica-rich glass around partly dissolved orthopyroxene crystals in peridotites.
Geothermometry of orthopyroxene–clinopyroxene pairs (P = 1.5 GPa) gives temperatures of ∼ 850 °C for unveined xenoliths to
950–1020 °C for veined xenoliths. Clinopyroxene – melt thermobarometry shows that Cr-diopside – type 2 glass pairs in harzburgite
formed at 1.4 to 1.1 GPa and ∼ 1250 °C whereas Cr-diopside – type 2 glass pairs in wehrlite formed at 0.9 to 0.7 GPa and 1120–1200 °C.
This bimodal distribution in pressure and temperature suggests that harzburgite xenoliths may have been entrained at greater
depth than wehrlite xenoliths.
Glass in the Baarley xenoliths has three different origins: infiltration of an early host melt different in composition from
the erupted host basanite; partial melting of amphibole; reaction of either of these melts with xenolith minerals. The composition
of type 1 glass suggests that jackets are accumulations of relatively evolved host magma. Mass balance modelling of the type
2 glass and its microlites indicates that it results from breakdown of disseminated amphibole and reaction of the melt with
the surrounding xenolith minerals. Type 3 glass in clinopyroxenite xenoliths is the result of breakdown of amphibole at low
pressure. Type 4 and 5 glass formed by reaction between phlogopite and type 2 melt or jacket melt. Type 6 glass associated
with orthopyroxene is due to the incongruent dissolution of orthopyroxene by any of the above mentioned melts.
Compositional gradients in xenolith olivine adjacent to type 2 glass pools and jacket glass can be modelled as Fe–Mg interdiffusion
profiles that indicate melt – olivine contact times between 0.5 and 58 days. Together with the clinopyroxene – melt thermobarometry
calculations these data suggest that the glass (melt) formed over a short time due to decompression melting of amphibole and
infiltration of evolved host melt. None of the glass in these xenoliths can be directly related to metasomatism or any other
process that occurred insitu in the mantle.
Received November 23, 1999; revised version accepted September 5, 2001 相似文献