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1.
Gadolinite, REE2FeBe2Si2O10, is commonly metamict. 57Fe Mössbauer annealing studies of fully metamict gadolinite from Ytterby, Sweden, have been completed in argon atmosphere from 873 to 1473 K. This technique has rarely been employed in studies of metamict minerals. Changes in the experimental parameters of Mössbauer spectra are sensitive indicators of the thermal recrystallization process of metamict gadolinite and revealed two stages of the structural recovery: a major stage from 873 to 1073 K and a slower recovery stage from 1133 to 1473 K. These observations are confirmed by X-ray powder diffraction. In relation to the first stage, the exponential behaviour of the changes in the Mössbauer parameters can be used for deriving the activation energy E a of the recrystallization process. The calculated value E a =1.97 eV in argon atmosphere explains the common occurrence of gadolinite in the fully or partially metamict state. Results of Mössbauer spectroscopy suggest that the recrystallization of metamict gadolinite is a displacive transition that involves rotation and translation of SiO4 and BeO4 to their normal positions associated with removal of OH groups from the structure. 相似文献
2.
We report a near-infrared Fourier-transform (FT) Raman spectroscopic method to characterize the electronic transitions of U ions and the alpha-decay damage in natural zircon. The application is demonstrated by analyzing metamict and annealed zircons from Sri Lanka. The data from crystalline zircon reveal a relatively sharp spectral feature appearing near 2733 cm–1 in Stokes spectra with a laser excitation of 1064 nm. The feature is assigned as signals related to the previously reported U5+ absorption near 6668 cm–1. With increasing self-irradiation dose, the feature shows a systematic decrease in intensity, accompanied by a gradual development of a broad feature between 3000 and 3400 cm–1. On heating for 1 h, the U5+ feature shows an increase in intensity starting near ~700 K for partially metamict zircon, whereas for highly damaged zircon the first recovery of the feature takes place near 1000 K, accompanied by a decrease in the radiation-induced broad band. The changes observed in the present study reflect the variations of local environments of U ions in natural zircon during metamictization and thermal annealing. 相似文献
3.
G. C. Capitani H. Leroux J. C. Doukhan S. Ríos M. Zhang E. K. H. Salje 《Physics and Chemistry of Minerals》2000,27(8):545-556
The nature of the amorphous regions and their recovery processes in two natural metamict zircon samples from Sri Lanka have
been studied by high resolution and analytical transmission electron microscopy. Samples untreated and annealed at different
temperatures were investigated. Nanoprobe analyses on untreated samples and samples annealed at 1000 K show that within experimental
uncertainties, no chemical segregation occurred. In samples annealed at higher temperatures (≥1100 K) recovery occurs in a
two-stage process and leads to different microstructures, which depended on the initial amount of metamictization. In highly
amorphized samples, recrystallization starts at 1200 K. Randomly oriented ZrO2 grains embedded in a silica-rich matrix are detected. At higher temperature (16 h at 1600 K), the assemblage transforms into
a polygonal texture of small zircon grains. Some untransformed zirconia grains and pockets of silica-rich glass are still
present, however. In partially metamict samples, recovery starts at 1100 K. The small surviving oriented zircon domains grow
at the expense of the surrounding amorphous material. At 1200 K, new zirconia grains nucleate with random orientations. After
1 h annealing at 1400 K, the zircon structure is restored and the microstructure coarse-grained. The proportion of crystalline
zirconia and silica-rich glass has dramatically decreased.
Received: 15 November 1999 / Accepted: 1 March 2000 相似文献
4.
François Farges 《Physics and Chemistry of Minerals》1994,20(7):504-514
The thermal annealing (300–1700 K) of two metamict zircons (Ampagabe, Madagascar and Näegy, Japan) has been studied using X-Ray Diffraction (XRD) and Extended X-ray Absorption Fine Structure spectroscopy (EXAFS) at Zr K-edge. Two stages of thermal annealing within the aperiodic zircon are evidenced between 293 and 1700 K. The first stage (up to 600° C) shows a decrease of the a
0-cell parameter from 6.674 (at 300° C) to 6.610 (at 600° C)± 0.005 Å. In that temperature range, the average local environment around Zr (presence of VIIZr and d(Zr-Zr) 3.3–3.6 Å) shows a weak, but significant increase of the Zr-Zr correlations located at 3.3–3.4 Å, undetectable by XRD. At temperatures up to 700° C (stage 2), the XRD-Bragg component arising from crystalline zircon increases in magnitude, whereas, Zr-K EXAFS analysis indicates a progressive VIIZrVIIIZr transition, associated with a recovery of the crystalline zircon medium-range environment. For both techniques, the zircon structure is fully recovered at annealing temperatures up to 900° C.Electrostatic modelings suggest that the VIIIZrVIIZr transition observed in zircon with increasing alpha-decay damage creates significantly overbonded oxygen atoms around Zr. With increasing temperature, those oxygen atoms are better bonded to VIIZr, due to the thermal expansion of the Zr-O bond. The congruent recovery of the zircon structure should therefore be favoured with increasing temperature. On the other hand, the metamict network can be also partially reorganized around 400–500° C, with the creation of Zr-rich domains, as measured by EXAFS. However, the growth of these domains after 3 hours annealing affects only minor portions of the aperiodic network. This model is corroborated by a similar thermal behaviour observed for a synthetic sol-gel of ZrO2 · SiO2 composition. 相似文献
5.
Zircon reaction and stability of the U-Pb isotope system during interaction with carbonate fluid: experimental hydrothermal study 总被引:4,自引:0,他引:4
N. G. Rizvanova O. A. Levchenkov A. E. Belous N. I. Bezmen A. V. Maslenikov A. N. Komarov A. F. Makeev L. K. Levskiy 《Contributions to Mineralogy and Petrology》2000,139(1):101-114
The interpretation of metamorphically induced U-Pb isotopic discordance requires a thorough understanding of zircon-fluid
interactions. With this aim we have studied the behaviour of metamict and crystalline zircon phases and their U-Pb systems
by cathodoluminescence after treatment by 2M Na2CO3 solution at T = 200–800 °C and P = 1–5 kbar for 3–14 days, X-ray diffraction, microprobe and isotope dilution analysis. The data indicate that zircon transformation
under hydrothermal conditions depends on the experimental conditions and the degree of structural damage. Reconstitution of
defective and impurity-enriched zones of metamict zircon (homogenization of impure element concentrations and increase of
crystallinity) was observed at 400 °C and P = 1 kbar. Considerable lead and uranium loss occurred under these conditions. As a result of zircon dissolution, newly formed
baddeleyite accommodating U from 2M Na2CO3 solution and Zr-Na-silicate were recognized. This process intensified with increasing pressure. Study of crystalline zircon
indicates that migration of U and Pb took place only during dissolution of zircon at T above 650 °C. In the presence of carbonate-ions essential U and Pb amounts are lost from metamict zircon at a lower P-T than is typical for greenschist facies metamorphism.
Received: 4 October 1997 / Accepted: 6 December 1999 相似文献
6.
Lutz Nasdala Dieter Grambole Jens Götze Ulf Kempe Tamás Váczi 《Contributions to Mineralogy and Petrology》2011,161(5):777-789
Synthetic ZrSiO4 and (mildly to strongly radiation-damaged) natural zircon samples were irradiated with 8.8 MeV 4He2+ ions (fluences in the range 1 × 1013–5 × 1016 ions/cm2). For comparison, an additional irradiation experiment was done with 30 MeV 16O6+ ions (fluence 1 × 1015 ions/cm2). The light-ion irradiation resulted in the generation of new (synthetic ZrSiO4) or additional (mildly to strongly metamict natural samples) damage. The maximum extent of the damage is observed in a shallow
depth range approximately 32–33 μm (8.8 MeV He) and ~12 μm (30 MeV O) below the sample surface, i.e. near the end of the ion
trajectories. These depth values, and the observed damage distribution, correspond well to defect distribution patterns as
predicted by Monte Carlo simulations. The irradiation damage is recognised from the notable broadening of Raman-active vibrational
modes, lowered interference colours (i.e. decreased birefringence), and changes in the optical activity (i.e. luminescence
emission). At very low damage levels, a broad-band yellow emission centre is generated whereas at elevated damage levels,
this centre is suppressed and samples experience a general decrease in their emission intensity. Most remarkably, there is
no indication of notable structural recovery in pre-damaged natural zircon as induced by the light-ion irradiation, which
questions the relevance of alpha-assisted annealing of radiation damage in natural zircon. 相似文献
7.
Holger Stünitz 《Contributions to Mineralogy and Petrology》1998,131(2-3):219-236
Dynamic recrystallization in the strict sense of the term is the reconstitution of crystalline material without a change
in chemical composition, driven by strain energy in the form of dislocations. Driving potentials additional to internal strain
energy may contribute to the recrystallization of naturally deformed minerals, which form solid solutions such as feldspar,
amphiboles and pyroxenes, if they change their composition during recrystallization. To estimate the relative importance of
these driving potentials, the chemical composition of porphyroclasts and recrystallized grains of plagioclase, clinopyroxene
and hornblende have been investigated in samples from a high grade shear zone of the Ivrea Zone, Italy. The plagioclases show
two different recrystallization microstructures: bulging recrystallization at grain boundaries and discrete zones of recrystallized
grains across porphyroclasts probably involving fracturing. Deformation took place under amphibolite facies conditions on
a retrograde P,T-path. Porphyroclast and recrystallized compositions from bulging recrystallization microstructures differ only in their Or-content
and yield a ΔG between mean host grain and mean recrystallized grain composition at fixed P,T-conditions of approximately 5 Joules/10−4 m3. Extreme compositional variations yield approximately 60 J/10−4 m3. The increase of free energy due to dislocations calculated for common glide systems in plagioclase are on the order of 100
Joules/10−4 m3 for high values of dislocation densities of 1014 m−2. Thus, the effect of chemically induced driving energies on grain boundary velocity appears small for mean compositions but
may be as great as that of deformational energies for larger chemical differences. In the other type of microstructure, porphyroclasts
and recrystallized grains in discrete zones differ in their anorthite content. The maximum ΔG induced by the compositional disequilibrium is on the order of 100 J/10−4 m3. This maximum value is of the same magnitude as the ΔG derived from high dislocation densities of 1014 m−2. The resulting combined ΔG is approximately twice as high as for deformational ΔG alone, and heterogeneous nucleation may become a feasible recrystallization mechanism which is evident from the microstructures.
The recrystallization mechanism depends on the nature of the driving potential. Grain boundary migration (GBM) and heterogeneous
nucleation can release Gibbs free energy induced by compositional disequilibrium, whereas this is not likely for subgrain
rotation. Therefore, only GBM and heterogeneous nucleation may link metamorphism and deformation, so that syndeformational
recrystallization may represent a transitional process ranging from dynamic recrystallization to metamorphic reaction.
Received: 8 July 1996 / Accepted: 17 November 1997 相似文献
8.
Chalcedony is a spatial arrangement of hydroxylated nanometre-sized α-quartz (SiO2) crystallites that are often found in association with the silica mineral moganite (SiO2). A supplementary Raman band at 501 cm−1 in the chalcedony spectrum, attributed to moganite, has been used for the evaluation of the quartz/moganite ratio in silica
rocks. Its frequency lies at 503 cm−1 in sedimentary chalcedony, representing a 2 cm−1 difference with its position in pure moganite. We present a study of the 503 cm−1 band’s behaviour upon heat treatment, showing its gradual disappearance upon heating to temperatures above 300 °C. Infrared
spectroscopic measurements of the silanole (SiOH) content in the samples as a function of annealing temperature show a good
correlation between the disappearance of the 503 cm−1 Raman band and the decrease of structural hydroxyl. Thermogravimetric analyses reveal a significant weight loss that can
be correlated with the decreasing of this Raman band. X-ray powder diffraction data suggest the moganite content in the samples
to remain stable. We propose therefore the existence of a hitherto unknown Raman band at 503 cm−1 in chalcedony, assigned to ‘free’ Si–O vibrations of non-bridging Si–OH that oscillate with a higher natural frequency than
bridging Si–O–Si (at 464 cm−1). A similar phenomenon was recently observed in the infrared spectra of chalcedony. The position of this Si–OH-related band
is nearly the same as the Raman moganite band and the two bands may interfere. The actually observed Raman band in silica
rocks might therefore be a convolution of a silanole and a moganite vibration. These findings have broad implications for
future Raman spectroscopic studies of moganite, for the assessment of the quartz/moganite ratio, using this band, must take
into account the contribution from silanole that are present in chalcedony and moganite. 相似文献
9.
Samples of allanite and gadolinite with a range of alpha-recoil damage 0.1 to 3.0 dpa, were annealed in Ar and analysed by X-ray powder diffraction (XRD), high resolution transmission electron microscopy (HRTEM), infrared (IR) and differential thermal analysis (DTA). Samples that were fully metamict, and also amorphous regions of partially metamict samples, annealed according to the Ostwald step rule. After annealing, X-ray crystalline material still showed significantly damaged regions under transmission electron microscopy (TEM). Hydrothermal annealing of fully metamict gadolinite at 710° C and PH2O=2.3 kbar resulted in direct recrystallization. Direct recrystallization, by heterogeneous nucleation, occurred also in samples with significant amount of relict crystalline material. Of two exotherms observed on DTA curves for fully metamict gadolinite only one, at 840° C, resulted from recrystallization. The second exotherm at 895° C was related to the transformation of a transitional, high-temperature γ-phase into gadolinite. The activation energy of recrystallization of partially metamict gadolinite is 0.58 eV. The same annealing path for fully metamict gadolinite and for the amorphous component of partially metamict allanite is consistent with the model of an aperiodic random network structure of metamict minerals. 相似文献
10.
Interpretation of discordant U-Pb zircon ages: An evaluation 总被引:36,自引:0,他引:36
The most widely used technique for the determination of high precision mineral growth ages in igneous and metamorphic rocks is dating of zircons with the U-Pb method. The interpretation of these ages, particularly in metamorphic settings, is hampered by an incomplete understanding of the common phenomenon of partial Pb-loss in zircon. In principle, this Pb-loss may occur in four very different ways: diffusion in metamict zircon, diffusion in pristine zircon, leaching from metamict zircon and recrystallization of metamict zircon. Here it is argued that, under conditions common in the continental crust, Pb-loss is only possible in partially to strongly metamict zircons. Pb-diffusion in the pristine zircon lattice is insignificant up to temperatures of at least 1000 °C. Pb-loss is only possible if the zircons experienced a time interval below their annealing temperature of about 600–650 °C, because only below this temperature can the lattice damage through α-decay and spontaneous fission accumulate. Zircons that remain above this temperature do not lose Pb by diffusion and will stay closed systems. Complete resetting of the U-Pb system in zircon under crustal conditions is only possible through dissolution and reprecipitation of zircon. Partial resetting results from recrystallization, leaching or diffusion in metamict zircon. As a consequence, special care has to be taken to interpret lower intercepts on concordia diagrams defined by discordant U-Pb data. Lower intercept ages may be significant only if they are defined by zircons with low U-content (<100 p.p.m.) or if confirmed by other geochronological methods. In addition, the accuracy of the lower intercept should be confirmed by abrading the zircon fractions that define the discordia. 相似文献
11.
The structural behavior of synthetic gahnite (ZnAl2O4) has been investigated by X-ray powder diffraction at high pressure (0–43 GPa) and room temperature, on the ID9 beamline
at ESRF. The equation of state of gahnite has been derived using the models of Birch–Murnaghan, Vinet and Poirier–Tarantola,
and the results have been mutually compared (the elastic bulk modulus and its derivatives versus P determined by the third-order Birch–Murnaghan equation of state are K
0=201.7(±0.9) GPa, K
′
0=7.62(±0.09) and K
″
0=−0.1022 GPa−1 (implied value). The compressibilities of the tetrahedral and octahedral bond lengths [0.00188(8) and 0.00142(5) GPa−1 at P=0, respectively], and the␣polyhedral volume compressibilities of the four-␣and␣sixfold coordination sites [0.0057(2) and
0.0041(2) GPa−1 at P=0, respectively] are discussed.
Received: 15 January 2001 / Accepted: 23 April 2001 相似文献
12.
D. J. Cherniak 《Physics and Chemistry of Minerals》2008,35(4):179-187
Self-diffusion of Si under anhydrous conditions at 1 atm has been measured in natural zircon. The source of diffusant for
experiments was a mixture of ZrO2 and 30Si-enriched SiO2 in 1:1 molar proportions; experiments were run in crimped Pt capsules in 1-atm furnaces. 30Si profiles were measured with both Rutherford backscattering spectrometry (RBS) and nuclear reaction analysis with the resonant
nuclear reaction 30Si(p,γ)31P. For Si diffusion normal to c over the temperature range 1,350–1,550°C, we obtain an Arrhenius relation D = 5.8 exp(−702 ± 54 kJ mol−1/RT) m2 s−1 for the NRA measurements, which agrees within uncertainty with an Arrhenius relation determined from the RBS measurements
[62 exp(−738 ± 61 kJ mol−1/RT) m2 s−1]. Diffusion of Si parallel to c appears slightly faster, but agrees within experimental uncertainty at most temperatures
with diffusivities for Si normal to c. Diffusion of Si in zircon is similar to that of Ti, but about an order of magnitude faster than diffusion of Hf and two
orders of magnitude faster than diffusion of U and Th. Si diffusion is, however, many orders of magnitude slower than oxygen
diffusion under both dry and hydrothermal conditions, with the difference increasing with decreasing temperature because of
the larger activation energy for Si diffusion. If we consider Hf as a proxy for Zr, given its similar charge and size, we
can rank the diffusivities of the major constituents in zircon as follows: D
Zr < D
Si << D
O, dry < D
O, ‘wet’. 相似文献
13.
Abin Das 《Geochimica et cosmochimica acta》2010,74(18):5333-5348
Electron backscatter imaging, Raman spectroscopy and U-Pb geochronology have been applied to Precambrian zircon grains that were annealed at 1000 and 1450 °C for various times, then leached with HF to constrain the conditions for healing radiation damage and attaining primary U-Pb zircon ages using the chemical abrasion (CA-TIMS) method. SEM images reveal a variety of textures for ZrO2 overgrowths on 1450 °C annealed and leached zircon surfaces that depend on the degree of radiation damage and annealing history. Highly damaged zircon produces finer textures than zircon with less damage.Raman spectroscopy indicates that crystals with different levels of radiation damage are only partially restored by annealing at 1000 °C for 2-3 days. Longer annealing periods of 20 days are not noticeably more effective. Annealing at 1450 °C for 1 h results in partial breakdown of zircon but restores Raman peak widths and wave numbers to values characteristic of undamaged zircon after ZrO2 overgrowths are removed by HF. Raman spectra are much less sensitive to polarization angle for annealed highly damaged grains than for weakly damaged zircon.U-Pb isotopic analyses of low to moderately damaged zircon (alpha fluence ranging up to 1019/g corresponding to an amorphization volume fraction of 80% or more) yield almost concordant data (0.3-0.5% discordance) after high-temperature annealing at 1450 °C followed by HF leaching at 195 °C. Analyses of cracked zircon annealed at 1450 °C and leached may remain discordant but those of uncracked grains are concordant. Most analyses show primary 207Pb/206Pb ages although cracked grains annealed at 1450 °C may produce discordant data with 207Pb/206Pb ages that are too young after leaching. The solubility of highly damaged, very disordered zircon (amorphization level of 99%) is only slightly reduced by annealing, and analyses of leach residues are strongly discordant although primary 207Pb/206Pb ages are obtained.Annealing of highly damaged zircon under any conditions apparently results in a mass of randomly oriented micro-crystals that pseudomorph the original grain. This could explain the fine-scale pattern observed on etched crystal surfaces, reduced anisotropy at the 5 μm scale of the Raman laser beam and high solubility in HF. It may be impossible to restore primary U-Pb isotopic ages in such cases but precise ages can still potentially be determined from 207Pb/206Pb ratios or by application of the air abrasion method. 相似文献
14.
Tantalite,occurring as intergranular tabular crystals,was reported for the first time in the Suzhou granite.Electron microprobe analyses show that it is rich in W and Ti,with a Ta/(Ta Nb) ratio ranging from 0.5 to 0.73 and a Mn(Mn Fe) ratio between 0.20 and 0.40.It is structurally distinct from isomorphic tapiolite by a remarked Ag Raman peak at 880cm^-1.The associated zircon is striking by significant enrichment of Hf,with the HfO2 content amounting up to 35%-40%,The discovery of tantalite suggests that the Suzhou granite should be classified as a S-type granite instead of A-type as considered previously. 相似文献
15.
Diffusion of tetravalent cations in zircon 总被引:18,自引:1,他引:18
D. J. Cherniak J. M. Hanchar E. B. Watson 《Contributions to Mineralogy and Petrology》1997,127(4):383-390
Diffusion rates for the three tetravalent cations U, Th and Hf have been measured in synthetic zircon. Diffusant sources
included oxide powders and ground pre-synthesized silicates. Rutherford backscattering spectrometry (RBS) was used to measure
depth profiles. Over the temperature range 1400–1650 °C, the following Arrhenius relations were obtained (diffusion coefficients
in m2sec−1):
log D
Th = (1.936 ± 0.9820) + (− 792 ± 34 kJ mol−1 /2.303 RT)
log D
U = (0.212 ± 2.440) + (− 726 ± 83 kJ mol−1 /2.303 RT)
log D
Hf = (3.206 ± 1.592) + (− 812 ± 54 kJ mol−1 /2.303 RT)
The data show a systematic increase in diffusivity with decreasing ionic radius (i.e., faster diffusion rates for Hf than
for U or Th), a trend also observed in our earlier study of rare earth diffusion in zircon. Diffusive fractionation may be
a factor in the Lu-Hf system given the much slower diffusion rates of tetravalent cations when compared with the trivalent
rare earths. The very slow diffusion rates measured for these tetravalent cations suggest that they are essentially immobile
under most geologic conditions, permitting the preservation of fine-scale chemical zoning and isotopic signatures of inherited
cores.
Received: 12 July 1996 / Accepted: 2 December 1996 相似文献
16.
Fluid-assisted zircon and monazite growth within a shear zone: a case study from Finnmark,Arctic Norway 总被引:1,自引:0,他引:1
Christopher L. Kirkland Martin J. Whitehouse Trond Slagstad 《Contributions to Mineralogy and Petrology》2009,158(5):637-657
The U–Pb ages, REE content, and oxygen isotopic composition of zircon rims developed within a major shear zone in the Kalak
Nappe Complex (KNC), Arctic Norway have been determined along with the age of monazite crystals. Different generations of
granitic veins have been distinguished based on both field criteria and monazite ages of 446 ± 3 and 424 ± 3 Ma. Within each
of these veins, inherited zircon cores are mantled by homogeneous low CL-response zircon rims which yield a range of concordant
U–Pb dates of ca. 470–360 Ma. Significant numbers of zircon rims coincide with the timing of monazite crystallization. The
zircon rims have moderate light REE enrichment compared to cores, distinctive (Sm/La)
n
values of less than 12, and La between 0.3 and 10 ppm. This indicates free elemental exchange between newly formed zircon
rims and the surrounding matrix. The rims have calculated accumulated alpha-radiation dosages corresponding with a crystalline
structure and δ18O values of 1‰. This implies rim crystallization directly from a zirconium-saturated hydrothermal fluid which was modified
by some silicate melt. Growth of the zircon rims was prolonged and locally variable due to preferential fluid flow. A third
type of zircon can be recognized, forming both rims and cores, with high alpha-radiation doses, and significant enrichment
in La, Pr, and Eu. These are interpreted as low-temperature hydrothermally altered metamict zircons. The high volatile input
and partial melting in the shear zone favoured prolonged zircon rim growth due to its ability to easily nucleate on inherited
seeds. On the other hand, monazite, susceptible to dissolution and re-growth, crystallized in brief episodes, as has been
predicted from theoretical phase diagrams. From a regional perspective, these results elucidate cryptic Ar–Ar cooling ages,
providing the first record of a Late Ordovician heating and cooling phase within the KNC prior to the climactic Scandian collision. 相似文献
17.
The niobium and zirconium L III-absorption spectra in some niobates and zircons were obtained with a vacuum focusing crystal spectrometer. The effective charges of Nb and Zr in these minerals were derived from the X-ray absorption spectra. The fine structure of the absorption spectra and effective charges Nb and Zr in metamict, partly-metamict minerals and crystalline analogues made it possible to draw a conclusion as to the nature of the first coordination sphere of Nb and Zr during metamict decay and subsequent recrystallization under annealing of these minerals. 相似文献
18.
Diopside (CaMgSi2O6) and pseudowollastonite (CaSiO3) have been studied by X-ray powder diffraction and Raman spectroscopy up to their respective melting points. In agreement
with previous unit-cell parameters determinations below 1100 K, thermal expansion of diopside along the a and c axis is much smaller than along the b axis. For pseudowollastonite, the axis expansivity increases slightly in the order b>a>c. For both minerals, the change in unit-cell angles is very small and there are no anomalous variations of the other unit-cell
parameters near the melting point. With increasing temperatures, the main changes observed in the Raman spectra are strong
increases of the linewidths for those bands which mainly represent Si−O−Si bending (near 600 cm−1) or involve Ca−O or Mg−O stretching, in the range 270–500 cm−1 for diopside, and 240–450 cm−1 for pseudowollastonite. At temperatures near the onset of calorimetric premelting effects, this extensive band widening results
in a broad Raman feature that can no longer be deconvoluted into its individual components. No significant changes affect
the Si−O streching modes. For both diopside and pseudowollastonite, premelting appears to be associated with enhanced dynamics
of the alkaline-earth elements. This conclusion contrasts markedly with that drawn for sodium metasilicate in which weaker
bonding of sodium allows the silicate framework to distort and deform in such a way as to prefigure the silicate entities
present in the melt.
Received 16 July 1997 / Revised, accepted: 6 March 1998 相似文献
19.
Shuan-Hong Zhang Yue Zhao Alfred Kröner Xiao-Ming Liu Lie-Wen Xie Fu-Kun Chen 《International Journal of Earth Sciences》2009,98(6):1441-1467
Recent zircon dating identified several late Carboniferous to early Permian hornblende gabbro–diorite–quartz diorite–granodiorite–tonalite–granite
plutons in lithological assemblages at the northern margin of the North China Block (NCB) that were previously regarded as
Archaean to Palaeoproterozoic. Our geochronological results indicate that emplacement of these plutons was a continuous process
during the late Carboniferous to early Permian, from 324 ± 6 to 274 ± 6 Ma, and lasted for at least 50 Ma. In this paper,
the early Permian components with compositions from gabbro to granite within the intrusive complex were studied. The early
Permian plutons exhibit calc-alkaline or high-K calc-alkaline, metaluminous geochemical features and highly variable SiO2 contents. They have no significant Eu anomaly in their REE patterns, and in primitive-mantle-normalized spidergrams they
display depletion in Th, U, Nb, Ta, P and Ti, and enrichment in Ba, K, Pb and Sr. The granitoid bodies within these plutons
display I-type and adakitic geochemical signatures. The early Permian rocks exhibit low whole-rock initial 87Sr/86Sr ratios from 0.70520 to 0.70615 and have negative whole-rock ε
Nd(t) values ranging from −17.4 to −9.3 and zircon ε
Hf(t) values of −23.2 to −10.5. The gabbros exhibit higher ε
Nd(t) values from −11.1 to −9.3 and ε
Hf(t) values from −16.5 to −10.5, and one granodiorite exhibits an even lower ε
Nd(t) value of −17.4 and zircon ε
Hf(t) values of −23.2 to −15.1. Geochemical, Sr–Nd and in situ zircon Hf isotopic compositions suggest that the hornblende gabbros
were derived from a metasomatized lithospheric mantle, and the diorite and quartz diorite were generated from a gabbroic magma
by fractional crystallization, coupled with differential assimilation of ancient lower crustal material. The granodiorite
was likely derived from partial melting of ancient lower crust with involvement of some mantle components. Involvement of
both lithospheric mantle and ancient lower crust in the generation of the early Permian plutons indicates strong crust–mantle
interaction in the northern NCB. Petrological associations as well as geochemical and Sr–Nd–Hf isotopic results show that
the early Permian plutons were emplaced along an Andean-type active continental margin during southward subduction of the
Palaeo-Asian oceanic plate beneath the NCB. Integration of our results with previously published data for late Carboniferous
and late Permian to middle Triassic intrusions suggests that the continental arc on the northern margin of the NCB existed
for at least 50 Ma during the late Palaeozoic, and final amalgamation of the Mongolian arc terranes with the northern NCB
likely occurred during a period from ~270 to ~250 Ma, i.e, in the late Permian to earliest Triassic. 相似文献
20.
Raman spectra of diopside were collected from atmospheric pressure to 71 GPa. The pressure dependences of 22 modes were determined.
Changes occurred in the spectra at three different pressures. First, at approximately 10 GPa, the two Raman modes at 356 and
875 cm−1 disappeared, while the mode at 324 cm−1 split into two modes, diverging at this pressure with significantly different pressure shifts; second, at approximately 15
GPa, a small (1 to 2 cm−1) drop in several of the frequencies was observed accompanied by changes in the pressure dependency of some of the modes;
and third, above 55 GPa, the modes characteristic of chains of tetrahedrally coordinated silicon disappeared, while those
for octahedrally coordinated silicon appeared. The first change at 10 GPa appears to be a C2/c to C2/c transition involving a change in the Ca coordination. The third change above 55 GPa appears to be a change in the silicon
coordination. At 15 GPa, it is suggested that a change in compressional mechanism takes place.
Received: 14 November 2000 / Accepted: 9 January 2002 相似文献