首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
1.
Heats of mixing of synthetic C2/m fluortremolite-fluoredenite amphiboles measured at 985 K show a systematic deviation from ideal mixing consistent with a subregular solution model. The deviations from ideal mixing are interpreted in terms of Na ordering in the A-site and Na-Al interactions in edenite-poor compositions. Enthalpies of edenite substitution reactions in amphiboles and in SiO2-NaAlO2 glasses and framework silicates are comparable. Gibbs free energies of formation of fluortremolite and fluoredenite at 298K are -2,821.07±3.34 kcal mol–1 and -2,889.59±2.40 kcal mol–1 respectively. The former value is in good agreement with values calculated from both F-OH exchange experiments and from a natural fluortremolite-bearing metamorphic rock. Least-squares fitted sub-regular heat-of-mixing parameters are poorly constrained and unrealistically high, but estimated subregular mixing parameters consistent with 95% confidence interval uncertainties in the calorimetric data and with TEM constraints give activity-composition relations in good agreement with the A-site compositions of natural metamorphic and igneous hornblendes. These relations predict unmixing in edenite-rich compositions over a wide range of temperature, but lend no support to the existence of a hornblende-actinolite miscibility gap. Calibration of the reaction tremolite+ albite=edenite+4 quartz as a function ofP,T andX ed amph indicates negativedP/dT slopes and a limited range of X ed amph (0.3 to 0.5) in equilibrium with plagioclase and quartz over a wide range of pressure and temperature, consistent with metamorphic hornblende-plagioclase assemblages. The energetics of this reaction suggest, however, that amphibole-plagioclase disequilibrium may be common.  相似文献   

2.
The composition and structure of synthetic (Na,K)-richterites have been characterized by EMP, HRTEM, XRD and FTIR methods. Despite the fact that the syntheses were done on bulk compositions along the richterite-K-richterite binary, EMP analyses and FTIR spectra indicate that the amphiboles are not simple solid solutions of the two richterite endmembers richterite and K-richterite alone, but tremolite and Mg-cummingtonite components are also present in considerable amounts. HRTEM observations show that the amphiboles are structurally well ordered. Only a very few chain multiplicity faults are present. XRD examination reveals lattice parameters of 9.9055 Å, 17.9844 Å, 5.2689 Å and 104.212° for richterite and 10.0787 Å, 17.9877 Å, 5.2715 Å and 104.878° for K-richterite endmembers. The unit cell volumes are 909.90 Å3 and 923.61 Å3 for richterite and K-richterite, respectively. The lattice parameters a and β for K-richterite are considerably larger than those published previously implying that those were not determined for pure K-richterite. The positions of the characteristic OH-stretching vibrations in the IR for sodium-potassium (3729.8–3734.8 cm?1) and vacancies (3671.1–3675.4 cm?1) on the A-site are in agreement with earlier determinations. Using synthetic tremolite as a standard the vacancy concentration on the A-site of the synthetic (Na,K)-richterites was determined quantitatively by FTIR-spectroscopy. The OH-stretching vibration of this synthetic tremolite is at 3674.5 cm?1. It is assigned to a local coordination with 3 Mg (2 M1+M3) as nearest neighbors and with 2 Ca (M4) as next nearest neighbors. A well resolved band with a smaller intensity is located at 3669.2 cm?1, which is attributed to a configuration including Ca+Mg on M4 instead of only Ca.  相似文献   

3.
The synthetic amphibole Na0.95(Li0.95Mg1.05)Mg5Si8O22(OH)2 was studied in situ at high-T, using IR OH-stretching spectroscopy and synchrotron X-ray powder diffraction. At room-T the sample has P21 /m symmetry, as shown by the FTIR spectrum. It shows in the OH region two well-defined and intense absorptions at 3,748 and 3,712 cm−1, respectively, and two minor bands at 3,667 and 3,687 cm−1. The main bands are assigned to the two independent O–H groups in the primitive structure. The two minor bands evidencing the presence of small amount of vacant A-site (A□0.05). With increasing T, these bands shift continuously and merge into a unique absorption at high temperature. A change as a function of increasing T is revealed by the evolution of the refined unit-cell parameters, whose trend shows a transition to C2/m at about 320–330°C. The spontaneous scalar strain, fitted with a tricritical 2–6 Landau potential, gives a T c of 325(10)°C (β parameter = 0.27). Comparison with the second-order P21 /m ⇔ C2/m phase transition at 255°C for synthetic amphibole ANa0.8B(Na0.8Mg1.2)CMg5Si8O22(OH)2 indicates that the substitution of Na with Li at the B-sites strongly affects the thermodynamic character and the T c of the phase transition. The comparison of LNMSH amphiboles with cummingtonitic ones shows that the high-T thermodynamic behaviour is affected by A-site occupancy.  相似文献   

4.
OH in zoned amphiboles of eclogite from the western Tianshan,NW-China   总被引：1，自引：0，他引：1  
Chemically-zoned amphibole porphyroblast grains in an eclogite (sample ws24-7) from the western Tianshan (NW-China) have been analyzed by electron microprobe (EMP), micro Fourier-transform infrared (micro-FTIR) and micro-Raman spectroscopy in the OH-stretching region. The EMP data reveal zoned amphibole compositions clustering around two predominant compositions: a glaucophane end-member ( B Na2 C M2+ 3 M3+ 2 T Si8(OH)2) in the cores, whereas the mantle to rim of the samples has an intermediate amphibole composition ( A 0.5 B Ca1.5Na0.5 C M 2+ 4.5 M 0.53+ T Si7.5Al0.5(OH)2) (A = Na and/or K; M 2+ = Mg and Fe2+; M 3+ = Fe3+ and/or Al) between winchite (and ferro-winchite) and katophorite (and Mg-katophorite). Furthermore, we observed complicated FTIR and Raman spectra with OH-stretching absorption bands varying systematically from core to rim. The FTIR/Raman spectra of the core amphibole show three lower-frequency components (at 3,633, 3,649–3,651 and 3,660–3,663 cm−1) which can be attributed to a local O(3)-H dipole surrounded by M(1) M(3)Mg3, M(1) M(3)Mg2Fe2+ and M(1) M(3) Fe2+ 3, respectively, an empty A site and T Si8 environments. On the other hand, bands at higher frequencies (3,672–3,673, 3,691–3,697 and 3,708 cm−1) are observable in the rims of the amphiboles, and they indicate the presence of an occupied A site. The FTIR and Raman data from the OH-stretching region allow us to calculate the site occupancy of the A, M(1)–M(3), T sites with confidence when combined with EPM data. By contrast M(2)- and M(4) site occupancies are more difficult to evaluate. We use these samples to highlight on the opportunities and limitations of FTIR OH-stretching spectroscopy applied to natural high pressure amphibole phases. The much more detailed cation site occupancy of the zoned amphibole from the western Tianshan have been obtained by comparing data from micro-chemical and FTIR and/or Raman in the OH-stretching data. We find the following characteristic substitutions Si(T-site) (Mg, Fe)[M(1)–M(3)-site] → Al(T-site) Al[M(1)–M(3)-site] (tschermakite), Ca(M4-site)□ (A-site) → Na(M4-site) Na + K(A-site) (richterite), and Ca(M4-site) (Mg, Fe) [M(1)–M(3)-site] → Na(M4-site) Al[M(1)–M(3)-site] (glaucophane) from the configurations observed during metamorphism.  相似文献   

5.
The cation exchange equilibrium has been investigated by hydrothermal experiments at 700 and 800°C at 200 MPa. To avoid equilibration problems of conventional exchange experiments, we synthesized amphiboles with an excess fluid allowing exchange between solid and fluid during the experiment. The exchangeable cations Na and K were provided as excess 1 to 2n chloridic solution. These exchange syntheses can be described by the reaction equation with (aq) for hydroxides and chlorides in aqueous solutions and ( s ) and ( p )?=?start and product fluid. The amphiboles grew in presence of the exchange fluid and adjusted their stoichiometry in equilibrium with the fluid phase. The solid products consist of more than 99% amphibole (Na,K-richteritess) with traces of diopside and quartz. The amphiboles are up to 1?mm long and often ≈ 40 μm thick. Detailed EMP- and HRTEM-observations show that they are chemically homogeneous and structurally wellordered. The experimental results give consistent phase relations in the reciprocal ternary system Na-richterite–K-richterite–NaCl–KCl. We analysed the product fluid with AAS- and ICP-methods. The Na-K distribution coefficients between fluid and amphiboles of the richterite–K-richterite join are close to unity at 700°C and 800°C at 200 MPa. Small systematic deviations are explained by a symmetric solution model for the A-position of the amphiboles. Using ideal mixing for H2O-NaCl-KCl fluids, a mixing model for the system richterite–K-richterite is presented. We suggest that the composition of richterite solid solutions can be used as a sensor for NaCl/KCl-ratios in metamorphic fluids.  相似文献   

6.
 Amphiboles were synthesized from bulk compositions prepared along the join Ca1.8Mg5.2Si8O22(OH)2–Ca1.8Mg3Ga4Si6O22(OH)2 hydrothermally at 750–850 °C and 1.0–1.8 GPa, and along the join Ca2Mg5Si8O22F2–Ca2Mg3Ga4Si6O22F2, anhydrously at 1000 °C and 0.7 GPa to document how closely the tschermak-type substitution is obeyed in these analogues of aluminous amphiboles. Electron-microprobe analyses and Rietveld X-ray diffraction structure refinements were performed to determine cation site occupancies. The extent of Ga substitution was found to be limited in both joins, but with the fluorine series having about twice the Ga content (0.6 atoms per formula unit, apfu) of the hydroxyl-series amphiboles (0.3 apfu). The tschermak-type substitution was followed very closely in the hydroxyl series with essentially equal partitioning of Ga between tetrahedral and octahedral sites. The fluorine-series amphiboles deviated significantly from the tschermak-type substitution and, instead, appeared to follow a substitution that is close to a Ca-pargasite substitution of the type: [6]Ga3++2[4]Ga3++1/2[A] Ca2+ = [6]Mg2++2[4]Si4++1/2[A]□. Infrared spectroscopy revealed an inverse correlation between the intensity of the OH-stretching bands and the Ga content for the hydroxyl- and fluorine-series amphiboles. The direct correlation between the Ga and F content and inverse relationship between the Ga and OH content may be a general phenomenon present in other minerals and suggests, for example, that high F contents in titanite are controlled by the Al content of the host rock and that there may be similar direct Al–F correlations in tschermakitic amphiboles. Evidence for the possibility that Al (Ga) might substitute onto only a subset of the tetrahedral sites in tschermakitic amphiboles was sought but not observed in this study. Received: 5 March 2001 / Accepted: 31 July 2001  相似文献   

7.
Equilibria between plagioclase, calcic amphibole and quartz can be described, in part, by the relation among mineral components: NaAlSi3O8+Ca2Mg5Si8O22(OH)2 = NaCa2Mg5AlSi7O22(OH)2+4SiO2; this relation governs the partitioning of Na between plagioclase and the A-site of coexisting amphibole. Data from natural amphibolites reveal that this partitioning is systematic and sensitive to metamorphic grade. The ideal portion of the equilibrium constant (K id = X Na, A/X, A · X Ab) derived from natural samples is sensitive to bulk composition, inasmuch as both plagioclase and amphibole are highly non-ideal. Samples from a single outcrop have values ranging from 0.5 (X Ab=0.74) to 4.1 (X Ab=0.10). The continuous reaction, NaAlSi3O8+Ca2Mg5Si8O22(OH)2 = NaCa2Mg5AlSi7O22(OH)2+4SiO2, proceeds to the right with increasing grade of metamorphism and for a given bulk composition, K id increases with increasing temperature. Two related discontinuous reactions, actinolite+albite=hornblende+oligoclase+quartz and actinolite+oligoclase=hornblende+anorthite+quartz, also proceed to the right with increasing metamorphic grade and result in changes in the topology of a phase diagram that describes the partitioning of Na between plagioclase and amphibole A-site. A Schreinemakers' net is presented that is consistent with natural occurrences. The results of this study should aid in the delineation of metamorphic facies within amphibolites.  相似文献   

8.
Mica nodules from the Dutoitspan and Wesselton mines, Kimberley can be divided into two groups: diopside-phlogopite nodules and potassic richterite-phlogopite nodules. The latter is characterized by the presence of abundant potassic richterite (less than 50% by volume) and large size of the crystals (up to 1 cm in length). It seems this type nodule is the first finding. Phlogopites and potassic richterites show a rather wide variations of major elements they have low Al2O3 and high Fe2O3, with Fe+3 in addition to Al+3 in the tetrahedral sites. It is believed that the mica nodules would be produced from kimberlitic and related magmas under moderate temperature and pressure conditions.  相似文献   

9.
The high-pressure behavior of three synthetic amphiboles crystallized with space group P21/m at room conditions in the system Li2O–Na2O–MgO–SiO2–H2O has been studied by in situ synchrotron infrared absorption spectroscopy. The amphiboles have compositions ANa B(Na x Li1 − x Mg1) CMg5 Si8 O22(OH)2 with x = 0.6, 0.2 and 0.0, respectively. The high-P experiments up to 32 GPa were carried out on the U2A beamline at Brookhaven National Laboratory (NY, USA) using a diamond anvil cell under non-hydrostatic or quasi-hydrostatic conditions. The two most intense absorption bands in the OH-stretching infrared spectra can be assigned to two non-equivalent O–H dipoles in the P21/m structure, bonded to the same local environment M1M3Mg3–OH–ANa, and pointing toward two differently kinked tetrahedral rings. In all samples these bands progressively merge to give a unique symmetrical absorption with increasing pressure, suggesting a change in symmetry from P21/m to C2/m. The pressure at which the transition occurs appears to be linearly correlated to the aggregate B-site dimension. The infrared spectra collected for amphibole B(Na0.2Li0.8Mg1) in the frequency range 50 to 1,400 cm−1 also show a series of changes with increasing pressure. The data reported here support the inference of Iezzi et al. (Am Miner 91:479–482, 2006a) regarding a new high-pressure amphibole polymorph.  相似文献   

10.
Summary. ?Ca-tourmaline has been synthesized hydrothermally in the presence of Ca(OH)2 and CaCl2-bearing solutions of different concentration at T = 300–700 °C at a constant fluid pressure of 200 MPa in the system CaO-MgO-Al2O3-SiO2-B2O3-H2O-HCl. Synthesis of tourmaline was possible at 400 °C, but only above 500 °C considerable amounts of tourmaline formed. Electron microprobe analysis and X-ray powder data indicate that the synthetic tourmalines are essentially solid solutions between oxy-uvite, CaMg3- Al6(Si6O18)(BO3)3(OH)3O, and oxy-Mg-foitite, □(MgAl2)Al6(Si6O18)(BO3)3(OH)3O. The amount of Ca ranges from 0.36 to 0.88 Ca pfu and increases with synthesis temperature as well as with bulk Ca-concentration in the starting mixture. No hydroxy-uvite, CaMg3(MgAl5)(Si6O18)(BO3)3(OH)3(OH), could be synthesized. All tourmalines have < 3 Mg and > 6 Al pfu. The Al/(Al + Mg)-ratio decreases from 0.80 to 0.70 with increasing Ca content. Al is coupled with Mg and Ca via the substitutions Al2□Mg−2Ca−1 and AlMg−1H−1. No single phase tourmaline could be synthesized. Anorthite ( + quartz in most runs) has been found coexisting with tourmaline. Other phases are chlorite, tremolite, enstatite or cordierite. Between solid and fluid, Ca is strongly fractionated into tourmaline ( + anorthite). The concentration ratio D = Ca(fluid)/Ca(tur) increases from 0.20 at 500 °C up to 0.31 at 700 °C. For the assemblage turmaline + anorthite + quartz + chlorite or tremolite or cordierite, the relationship between Ca content in tourmaline and in fluid with temperature can be described by the equation (whereby T = temperature in °C, Ca(tur) = amount of Ca on the X-site in tourmaline, Ca( fluid) = concentration of Ca2+ in the fluid in mol/l). The investigations may serve as a first guideline to evaluate the possibility to use tourmaline as an indicator for the fluid composition.
Zusammenfassung. ?Synthese von Ca-Turmelin im System CaO-MgO-Al 2 O 3 -SiO 2 -B 2 O 3 -H 2 O-HCl Im System CaO-MgO-Al2O3-SiO2-B2O3-H2O-HCl wurde Ca-Turmalin hydrothermal aus Ca(OH)2 and CaCl2-haltigen L?sungen bei T = 300–700 °C und einem konstanten Fluiddruck von 200 MPa synthetisiert. Die Synthese von Turmalin war m?glich ab 400 °C, aber nur oberhalb von 500 °C bildeten sich deutliche Mengen an Turmalin. Elektronenstrahl-Mikrosondenanalysen und R?ntgenpulveraufnahmen zeigen, da? Mischkristalle der Reihe Oxy-Uvit, CaMg3Al6(Si6O18)(BO3)3(OH)3O, und Oxy-Mg-Foitit, □(MgAl2)Al6(Si6O18)(BO3)3(OH)3O gebildet wurden. Der Anteil an Ca variiert zwischen 0.36 und 0.88 Ca pfu und nimmt mit zunehmender Synthesetemperatur und zunehmender Ca-Konzentration im System zu. Hydroxy-Uvit, CaMg3(MgAl5) (Si6O18)(BO3)3(OH)3(OH), konnte nicht synthetisiert werden. Alle Turmaline haben < 3 Mg und > 6 Al pfu. Dabei nimmt das Al/(Al + Mg)- Verh?ltnis mit zunehmendem Ca-Gehalt von 0.80 auf 0.70 ab. Al ist gekoppelt mit Mg und Ca über die Substitutionen Al2□Mg−2Ca−1 und AlMg−1H−1. Einphasiger Turmalin konnte nicht synthetisiert werden. Anorthit (+ Quarz in den meisten F?llen) koexistiert mit Turmalin. Andere Phasen sind Chlorit, Tremolit, Enstatit oder Cordierit. Ca zeigt eine deutliche Fraktionierung in den Festk?rpern Turmalin (+ Anorthit). Das Konzentrationsverh?ltnis D = Ca(fluid)/Ca(tur) nimmt von 0.20 bei 500 °C auf 0.31 bei 700 °C zu. Für die Paragenese Turmalin + Anorthit + Quarz mit Chlorit oder Tremolit oder Cordierit gilt folgende Beziehung zwischen Ca-Gehalt in Turmalin und Fluid und der Temperatur: (wobei T = Temperatur in °C, Ca(tur) = Anteil an Ca auf der X-Position in Turmalin, Ca(fluid) = Konzentration von Ca2+ im Fluid in mol/l). Die Untersuchungen dienen zur ersten Absch?tzung, ob Turmalin als Fluidindikator petrologisch nutzbar ist.

Received July 24, 1998;/revised version accepted October 21, 1999  相似文献   

11.
Mg-Al-rich rocks from the Palghat-Cauvery Shear Zone System (PCSZ) within the Gondwana suture zone in southern India contain sodicgedrite as one of the prograde to peak phases, stable during T = 900–990°C ultrahigh-temperature metamorphism. Gedrite in these samples is Mg-rich (Mg/[Fe + Mg] = X Mg = 0.69–0.80) and shows wide variation in Na2O content (1.4–2.3 wt.%, NaA = 0.33–0.61 pfu). Gedrite adjacent to kyanite pseudomorph is in part mantled by garnet and cordierite. The gedrite proximal to garnet shows an increase in NaA and AlIV from the core (NaA = 0.40–0.51 pfu, AlIV = 1.6–1.9 pfu) to the rim (NaA = 0.49–0.61 pfu, AlIV = 2.0–2.2 pfu), suggesting the progress of the following dehydration reaction: Ged + Ky → Na-Ged + Grt + Crd + H2O. This reaction suggests that, as the reactants broke down during the prograde stage, the remaining gedrite became enriched in Na to form sodicgedrite, which is regarded as a unique feature of high-grade rocks with Mg-Al-rich and K–Si-poor bulk chemistry. We carried out high-P-T experimental studies on natural sodicgedrite and the results indicate that gedrite and melt are stable phases at 12 kbar and 1,000°C. However, the product gedrite is Na-poor with only <0.13 wt.% Na2O (NaA = 0.015–0.034 pfu). In contrast, the matrix glass contains up to 8.5 wt.% Na2O, suggesting that, with the progressive melting of the starting material, Na was partitioned into the melt rather than gedrite. The results therefore imply that the occurrence of sodicgedrite in the UHT rocks of the PCSZ is probably due to the low H2O activity during peak P-T conditions that restricted extensive partial melting in these rocks, leaving Na partitioned into the solid phase (gedrite). The occurrence of abundant primary CO2-rich fluid inclusions in this rock, which possibly infiltrated along the collisional suture during the final amalgamation of the Gondwana supercontinent, strengthens the inference of low water activity.  相似文献   

12.
The relative incompatibility of Ar and K are fundamental parameters in understanding the degassing history of the mantle. Clinopyroxene is the main host for K in most of the upper mantle, playing an important role in controlling the K/Ar ratio of residual mantle and the subsequent time-integrated evolution of 40Ar/36Ar ratios. Clinopyroxene also contributes to the bulk Ar partition coefficient that controls the Ar degassing rate during mantle melting. The partitioning of Ar and K between clinopyroxene and quenched silicate melt has been experimentally determined from 1 to 8 GPa for the bulk compositions Ab80Di20 (80 mol% albite-20 mol% diopside) and Ab20Di80 with an ultraviolet laser ablation microprobe (UVLAMP) technique for Ar analysis and the ion microprobe for K. Data for Kr (UVLAMP) and Rb (ion probe) have also been determined to evaluate the role of crystal lattice sites in controlling partitioning. By excluding crystal analyses that show evidence of glass contamination, we find relatively constant Ar partition coefficients (DAr) of 2.6 × 10−4 to 3.9 × 10−4 for the Ab80Di20 system at pressures from 2 to 8 GPa. In the Ab20Di80 system, DAr shows similar low values of 7.0 × 10−5 and 3.0 × 10−4 at 1 to 3 GPa. All these values are several orders of magnitude lower than previous measurements on separated crystal-glass pairs.DK is 10 to 50 times greater than DRb for all experiments, and both elements follow parallel trends with increasing pressure, although these trends are significantly different in each system studied. The DK values for clinopyroxene are at least an order of magnitude greater than DAr under all conditions investigated here, but DAr appears to show more consistent behavior between the two systems than K or Rb. The partitioning behavior of K and Rb can be explained in terms of combined pressure, temperature, and crystal chemistry effects that result in changes for the size of the clinopyroxene M2 site. In the Ab20Di80 system, where clinopyroxene is diopside rich at all pressures, DK and DRb increase with pressure (and temperature) in an analogous fashion to the well-documented behavior of Na. For the Ab80Di20 system, the jadeite content of the clinopyroxene increases from 22 to 75 mol% with pressure resulting in a contraction of the M2 site. This has the effect of discriminating against the large K+ and Rb+ ions, thereby countering the effect of increasing pressure. As a consequence DK and DRb do not increase with pressure in this system.In contrast to the alkalis (Na, K, and Rb), DKr values are similar to DAr despite a large difference in atomic radius. This lack of discrimination (and the constant DAr over a range of crystal compositions) is also consistent with incorporation of these heavier noble gases at crystal lattice sites and a predicted consequence of their neutrality or “zero charge.” Combined with published DAr values for olivine, our results confirm that magma generation is an efficient mechanism for the removal of Ar from the uppermost 200 km of the mantle, and that K/Ar ratios in the residuum are controlled by the amount of clinopyroxene. Generally, Ar is more compatible than K during mantle melting because DAr for olivine is similar to DK for clinopyroxene. As a result, residual mantle that has experienced variable amounts of melt extraction may show considerable variability in time-integrated 36Ar/40Ar.  相似文献   

13.
The fractionation of boron isotopes between synthetic dravitic tourmaline and fluid was determined by hydrothermal experiments between 400 and 700°C at 200 MPa and at 500°C, 500 MPa. Tourmaline was crystallized from an oxide mix in presence of water that contained boron in excess. In one series of experiments, [B]fluid/[B]tour was 9 after the run; in another series it was 0.1. All experiments produced tourmaline as the sole boron-bearing solid, along with traces of quartz and talc. Powder XRD and Rietveld refinements revealed no significant amounts of tetrahedrally coordinated boron in tourmaline. 11B always preferentially fractionated into the fluid. For experiments where [B]fluid/[B]tour was 9, a consistent temperature-dependent boron isotope fractionation curve resulted, approximated by Δ11B(tour–fluid) = −4.20 · [1,000/T (K)] + 3.52; R 2 = 0.77, and valid from 400 to 700°C. No pressure dependence was observed. The fractionation (−2.7 ± 0.5‰ at 400°C; and −0.8 ± 0.5‰ at 700°C) is much lower than that previously presented by Palmer et al. (1992). Experiments where [B]fluid/[B]tour was 0.1 showed a significant larger apparent fractionation of up to −4.7‰. In one of these runs, the isotopic composition of handpicked tourmaline crystals of different size varied by 1.3‰. This is interpreted as resulting from fractional crystallization of boron isotopes during tourmaline growth due to the small boron reservoir of the fluid relative to tourmaline, thus indicating larger fractionation than observed at equilibrium. The effect is eliminated or minimized in experiments with very high boron excess in the fluid. We therefore suggest that values given by the above relation represent the true equilibrium fractionations.  相似文献   

14.
The effects of pressure and oxygen fugacity (fO2) on trace element partitioning between pargasitic amphibole and alkali-basaltic melts have been determined at pressures from 1.5 to 2.5 GPa and oxygen fugacities at 2 log units above and below the nickel–nickel oxide buffer. Amphibole crystallization experiments were performed in a piston cylinder apparatus and partition coefficients between amphibole and quenched melt of large-ion-lithophile elements (LILE: Rb, Sr, Ba), high-field-strength elements (HFSE: Zr, Nb, Ta, Hf, U, Th) and rare-earth elements (REE: La to Lu; +Y) were measured with a LASER ablation inductively coupled plasma – mass spectrometer. Increasing pressure from 1.5 to 2.5 GPa at similar temperatures and approximately constant fO2 increases D Rb but decreases D Zr and D Hf and D REE (D La, D Ce, D Pr). An empirical relationship was observed between D Zr and (Ti/Al)M2 in the amphibole, which can be described by:
Increasing the fO2 by ∼4 log units (∼NNO–2.0 to ∼NNO+2.2) at similar temperatures and constant pressure increases D Ba and D Nd but decreases D Ti. An increase in pressure or fO2 decreases the maximum partition coefficient (D o ), the Young's modulus (E) and the optimum ionic radius (r o ) of the A-, M2- and M4-lattice sites. The calculated r o values from the monovalent cations (Na, K, Rb) in the A site and the quadrivalent cations (Ti, Hf, Zr) in the M2 lattice sites suggests that amphiboles crystallized from alkaline basalt material have smaller 〈A-O〉 and 〈M2-O〉, mean bond-lengths than those formed from pargasitic materials at identical pressures and fO2's. The measured partition coefficients were used to calculate trace element concentrations in melts formed by partial melting of amphibole-bearing peridotite. This modeling demonstrates those changes in either the pressure or fO2 of melting can exert a significant effect on Rb/HFSE ratios in the melts and thus help explain the wide variations of these ratios sometimes observed in basaltic rock suites. Received: 7 August 1998 / Accepted: 7 June 2000  相似文献   

15.
A series of amphiboles along the magnesioriebeckite—Na2Mg3Fe3+ 2Si8O22(OH)2– ferri-clinoholmquistite—Li2Mg3Fe3+ 2Si8O22(OH)2 - join, defined by the BLiB Na–1 exchange vector, were hydrothermally synthesized at 700°C, 0.4 GPa, NNO + 1 redox conditions. Powder XRD and SEM-EDAX showed a very high (> 90%) amphibole yield for all samples. X-ray patterns were indexed in the C2/m space group; refined cell-parameters show a linear decrease of a and as a function of chemistry. IR spectra in the OH-stretching region show four main and rather sharp bands; these are assigned to Mg and Fe2+ at M(1,3), and indicate that the obtained amphiboles depart from the nominal octahedral composition (M1,3Mg3). The IR spectra also show that there is an increasing filling-up of the A-site for increasing Na in the system (increasing solid-solution toward, arfvedsonite). Mössbauer spectra show four well-defined quadrupole doublets which are assigned to Fe3+ at M2 and to Fe2+ at M1, M3 and M4, respectively. The Fe3+/Fe2+ content derived from fitted peak areas show variable Fe3+ concentration along the series. Mössbauer spectra also show a distinct alteration of 57Fe hyperfine parameters with changing Na–Li at M4. The most evident variation is observed for the quadrupole splitting of Fe3+ at M2, which increases by 50% from ferri-clinoholmquistite to magnesio-riebeckite; this suggest that the M2 octahedron in ferri-clinoholmquistite is much closer to the ideal geometry than the M2 octahedron in magnesio-riebeckite. Mössbauer spectra show also a well-defined increase in the Fe2+ quadrupole splitting of the M1 and M3 octahedra, which is attributed to the Na–Li distribution at the B-sites.  相似文献   

16.
Summary Elevated P contents of up to 0.086 apfu (1.21 wt.% P2O5) were found in garnet from leucocratic granitic rocks (orthogneisses, granites, barren to highly evolved pegmatites) in the Moldanubicum and Silesicum, Czech Republic, and in complex granitic pegmatites from southern California, USA, and Australia. Minor concentrations (0.15–0.55 wt.% P2O5) appear ubiquitous in garnet from leucocratic granitic rocks of different origins and degrees of fractionation. Concentrations of P are not related to Mn/(Mn + Fe) that vary from 0.12–0.86 and to textural types of garnet (i.e., isolated anhedral to euhedral grains and nodules, graphic and random garnet–quartz aggregates, subsolidus veins of fine-grained garnet). Garnet compositions exhibit negative correlations for P/Si and P/R2+ where R2+ = Fe + Mn + Mg + Ca, while Al is constant at ∼2.05 apfu. Concentrations of Na are largely below 0.02 apfu but positively correlate with P. The main substitution may involve A-site vacancy and/or the presence of some light element(s) in the crystal structure. The substitution □P2 R2+ −1Si−2 and/or alluaudite-type Na□P3 R2+ −1Si−3 seem the most likely P-incorporating mechanisms. The partitioning of P among garnet and associated minerals in granitic systems remains unclear; however, it directly affects the distribution of Y and REEs.  相似文献   

17.
Rare-element pegmatites in the Voron’i tundras, Kola Peninsula, Russia, contain late abundant Rb mineralization. Individual Rb minerals are Rb-dominant feldspars and micas that form continuous solid solution series with K analoques. The feldspars contain from 17 to 86 mol % RbAlSi3O8 (5.5–25 wt % Rb2O) and 1–5 mol % CsAlSi3O8, and the muscovite contains 2.6–9.4 wt % Rb2O. The Li micas are members of the lepidolite-polylithionite series and their Rb-dominant analogues. They form a continuous series of solid solutions with the Rb concentrations varying from 0.09 to 0.54 apfu., K concentrations varying from 0.82 to 0.33 apfu, and Cs concentrations varying from 0.02 to 0.18 apfu The maximum Rb2O concentration in the newly found mineral voloshinite, an Rb analogue of lepidolite, is 12.2 wt %. The Rb-rich feldspars and micas sometimes crystallized directly or were formed via cation exchange with the young fluid. The Rb minerals are spatially and genetically closely associated with pollucite. It is supposed that initially Rb was contained in a high-temperature pollucite solid solution and was released from it at decreasing temperature as a result of a reaction with the aqueous fluid and notably enriched the latter. It is shown that Rb mineralization is generally typical of pollucite-bearing pegmatites.  相似文献   

18.
 A synthesis technique is described which results in >99% pure NH4-phlogopite (NH4) (Mg3) [AlSi3O10] (OH)2 and its deuterium analogue ND4-phlogopite (ND4) (Mg3) [AlSi3O10] (OD)2. Both phases are characterised using both IR spectroscopy at 298 and 77 K as well as Rietveld refinement of their X-ray powder diffraction pattern. Both NH4 + and ND4 + are found to occupy the interlayer site in the phlogopite structure. Absorption bands in the IR caused by either NH4 + or ND4 + can be explained to a good approximation using Td symmetry as a basis. Rietveld refinement indicates that either phlogopite synthesis contains several mica polytypes. The principle polytype is the one-layer monoclinic polytype (1M), which possesses the space group symmetry C2/m. The next most common polytype is the two-layer polytype (2M 1 ) with space group symmetry C2/c. Minor amounts of the trigonal polytype 3T with the space group symmetry P3112 were found only in the synthesis run for the ND4-phlogopite. Electron microprobe analyses indicate that NH4-phlogopite deviates from the ideal phlogopite composition with respect to variable Si/Al and Mg/Al on both the tetrahedral and octahedral sites, respectively, due to the Tschermaks substitution VIMg2++IVSi4+↔VIAl3++IVAl3+ and with respect to vacancies on the interlayer site due to the exchange vector XII(NH4)++IVAl3+↔XII□+IVSi4+. Received: 30 August 1999 / Accepted: 2 October 2000  相似文献   

19.
Four pairs of associated calcic and sodic amphiboles from blueschistfacies metamorphic rocks were analyzed with the electron microprobeand studied by single-crystal X-ray diffraction techniques.Except for ranges in the ratios Mg/(Mg+Fe) and Fe3+/(Fe3++Al+Ti),the sodic amphiboles are similar in chemical composition. Theamount of calcium in the M(4)-site ranges only from 0·18to 0·21 ion per formula unit. The calcic amphiboles,in addition to a range in Mg/(Mg+Fe), vary in Na/(Na+Ca) ratio(0·29–0·48). Three of the calcic amphibolescontain less than 1·5 calcium ions per formula unit,indicating a significant solid solution of sodic amphibole componentsin the calcic amphibole phase. The a and b unit-cell parametersof the calcic amphiboles decrease with increased content ofthe sodic component.  相似文献   

20.
Sodic amphiboles in high pressure and ultra-high pressure (UHP) metamorphic rocks are complex solid solutions in the system Na2O–MgO–Al2O3–SiO2–H2O (NMASH) whose compositions vary with pressure and temperature. We conducted piston-cylinder experiments at 20–30?kbar and 700–800?°C to investigate the stability and compositional variations of sodic amphiboles, based on the reaction glaucophane=2jadeite+talc, by using the starting assemblage of natural glaucophane, talc and quartz, with synthetic jadeite. A close approach to equilibrium was achieved by performing compositional reversals, by evaluating compositional changes with time, and by suppressing the formation of Na-phyllosilicates. STEM observations show that the abundance of wide-chain structures in the synthetic amphiboles is low. An important feature of sodic amphibole in the NMASH system is that the assemblage jadeite–talc?±?quartz does not fix its composition at glaucophane. This is because other amphibole species such as cummingtonite (Cm), nyböite (Nyb), Al–Na-cummingtonite (Al–Na-Cm) and sodium anthophyllite (Na-Anth) are also buffered via the model reactions: 3cummingtonite?+?4quartz?+?4H2O=7talc, nyböite?+?3quartz=3jadeite?+?talc, 3Al–Na-cummingtonite + 11quartz + 2H2O=6jadeite + 5talc, and 3 sodium anthophyllite?+?13quartz?+?4H2O=3 jadeite + 7talc. We observed that at all pressures and temperatures investigated, the compositions of newly grown amphiboles deviate significantly from stoichiometric glaucophane due to varying substitutions of AlIV for Si, Mg on the M(4) site, and Na on the A-site. The deviation can be described chiefly by two compositional vectors: [NaAAlIV]<=>[□ASi] (edenite) toward nyböite, and [Na(M4)AlVI]<=>[Mg(M4)MgVI] toward cummingtonite. The extent of nyböite and cummingtonite substitution increases with temperature and decreases with pressure in the experiments. Similar compositional variations occur in sodic amphiboles from UHP rocks. The experimentally calibrated compositional changes therefore may prove useful for thermobarometric applications.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号