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1.
 Electrical conductivity of lherzolite (65% olivine), measured as a function of time after changes in the oxygen fugacity (f o2) of the surrounding CO2/CO atmosphere, is used to infer the diffusivity of the point defects responsible for conduction in olivine. A total of 63 equilibration runs at temperatures of 900, 1000, 1100, and 1200 ∘C were fit using nonlinear parameter estimation to recover time constants (directly related to diffusivity) and conductivity steps. An observed f o2 dependence in the time constants associated with re-equilibration implies two defect species of fixed diffusivity but with f o2-dependent concentrations. Although the rate-limiting step may not necessarily be associated with a conducting defect, when time constants are converted to diffusivities, the magnitudes and activation energies agree extremely well with the model for magnesium vacancies (the slower species) and small polarons (holes localized on Fe3+) derived by Constable and Roberts (1997). This earlier study used an independent method of simultaneous modeling of thermopower and electrical conductivity as a function of f o2 and temperature, on data from a different type of sample (a dunite). We observe that at high f o2 where polarons dominate over magnesium vacancies in the defect population, re-equilibration is dominated by magnesium vacancy diffusion, and vice versa (at low f o2 magnesium vacancies dominate and re-equilibration proceeds at the faster rate associated with polaron mobility). We interpret this to suggest association between the cation vacancies and polarons, as has been suggested by Tsai and Dieckmann (1997), making the concentration of the minority defect the rate-limiting step in the oxidation/reduction reactions. Received: 18 October 2000 / Accepted: 7 May 2002  相似文献   

2.
The electrical conduction in synthetic, dry polycrystalline, iron-bearing olivine (Fo90) was investigated as a first-order approach to the electrical conductivity in the upper mantle. This fundamental study is of great importance to better understand the charge-transport mechanisms seen in olivine. Conduction processes in synthetic samples are not influenced by a complex geological history in contrast to conductivity in natural olivine. The experiments show that the apparent activation energy for conductivity for Fo90 is 230 kJ mol−1. In currently accepted defect modeling, natural and synthetic olivine requires a mechanism involving small polaron formation (Fe· Mg and magnesium vacancies (V Mg) as the dominant diffusing species to explain a fO21/6 relation to electrical conduction. Here, Fo90 shows no contribution of small polarons to conductivity at temperatures between 1,000 and 1,200°C and almost no dependence on fO2. Instead, under reducing conditions magnesium vacancies (and electrons) appear to be the major charge carriers.  相似文献   

3.
San Carlos olivine samples enclosed in soft iron capsules were annealed in an uniaxial split-sphere apparatus, at pressures ranging from 4.6 to 9.0 GPa and temperature ranging from 1310o to 1595 oC. We estimated the annealing fO2, theoretically controlled by the olivine/Fe-metal equilibrium, to be 1 to 2 log units above the fO2 of the iron/wustite buffer. Samples were investigated by analytical transmission electron microscopy (ATEM) in order to verify that olivine and Fe capsule did equilibrate during the annealings. TEM imaging of the olivine bulk shows a and c dislocations confined in the (010) plane, and small (0.5 μm) spatially coupled precipitates of (1) Al-rich spinel and (2) enstatite (volumic proportion of precipitates ≃60 ppm). These coupled precipitates are surrounded by split c dislocation loops. Olivine composition profiles, determined by ATEM near the Fe-capsule/olivine contact, reveal a weak loss of Ni from the olivine matrix toward the capsule, as expected in such reducing conditions. These profiles also reveal a marked incorporation of Fe from the capsule into the olivine matrix. These observations, and their interpretation in terms of olivine point defect chemistry, lead to the following conclusions: (1) the starting olivine contained a high concentration of vacancies on octahedral sites (≥1000 ppm per site); such a high vacancy concentration is expected in San Carlos olivine which equilibrated in nature at relatively high fO2; (2) the olivine/Fe-metal equilibrium did control fO2 during the annealings, that resulted in a rapid re-equilibration of olivine at the beginning of the runs to the lower fO2 imposed by the Fe capsule; this led to a strong decrease of the octahedral vacancy concentration in olivine. (3) Such a fO2 decrease promoted in olivine the coupled precipitation of both types of Al-rich spinel and enstatite precipitates. These observations show that the use of Fe-capsule in high pressure experiments is an efficient method for controlling fO2 when studying olivine, and more generally Fe-bearing silicates. Received: 5 November 1997 / Accepted: 5 May 1998  相似文献   

4.
To investigate the point defect chemistry and the kinetic properties of manganese olivine Mn2SiO4, the point defect relaxation time (τ) characterizing the rate of re-equilibration of electrical conductivity following a change in oxygen fugacity was measured for single crystals oriented for electrical conduction along the [010] direction. The experiments were carried out at temperatures T = 1173–1473 K and oxygen fugacities with the MnSiO3 activity controlled at unity. The value of τ, which ranges from 130 to 1463 s, increases with decreasing temperature. At 1273 K, the value of τ in the regime is a factor of ∼1.8 smaller than that in the regime. The point defect relaxation time was used to calculate the chemical diffusivity (). Values of lie in the range 2.2× 10− 10 − 2.5×10− 9 m2/s. For the regime, a semi-log plot of vs 1/T yields a concave downward curve. Based on these results combined with those from Part I of this work for the point defect structure and electrical conductivity in Mn-olivine, it is proposed that the relaxation rate of electrical conduction is determined by the coupled diffusion of manganese vacancies and electron holes which rate-control the relaxation process at low and high temperatures, respectively. Deconvolution of the curve into two straight-line segments yields values for the mobilities and the diffusivities for Mn vacancies and electron holes. These results, combined with the measured electrical conductivity data, were used to calculate the concentrations of Mn vacancies and electron holes. These results in conjunction with those published for other transition-metal silicate olivines reveal the following: The mobility of electron holes in Mn-olivine is about two orders of magnitude smaller than that in Fe-olivine and is somewhat larger than that in (Mg0.9Fe0.1)-olivine. The mobilities of metal vacancies in these three olivines are similar both in magnitude and in temperature dependence. The concentrations of the majority point defects are the highest in Fe-olivine crystals and the lowest in Co-olivine crystals, while those in Mn-olivine and (Mg0.9Fe0.1)-olivine crystals lie in between. The deviation from stoichiometry in the cation sublattice for a transition-metal silicate olivine is about two orders of magnitude lower than that for the corresponding transition-metal oxide. Received: 10 January 1996 / Revised, accepted: 29 May 1997  相似文献   

5.
 The solubility of hydroxyl in the α, β and γ phases of (Mg,Fe)2SiO4 was investigated by hydrothermally annealing single crystals of San Carlos olivine. Experiments were performed at a temperature of 1000° or 1100 °C under a confining pressure of 2.5 to 19.5 GPa in a multianvil apparatus with the oxygen fugacity buffered by the Ni:NiO solid-state reaction. Hydroxyl solubilities were determined from infrared spectra obtained of polished thin sections in crack-free regions ≤100 μm in diameter. In the α-stability field, hydroxyl solubility increases systematically with increasing confining pressure, reaching a value of ∼20,000 H/106Si (1200 wt ppm H2O) at the α-β phase boundary near 13 GPa and 1100 °C. In the β field, the hydroxyl content is ∼400,000 H/106Si (24,000 wt ppm H2O) at 14–15 GPa and 1100 °C. In the γ field, the solubility is ∼450,000 H/106Si (27,000 wt ppm H2O) at 19.5 GPa and 1100 °C. The observed dependence of hydroxyl solubility with increasing confining pressure in the α phase reflects an increase in water fugacity with increasing pressure moderated by a molar volume term associated with the incorporation of hydroxyl ions into the olivine structure. Combined with published results on the dependence of hydroxyl solubility on water fugacity, the present results for the α phase can be summarized by the relation C OH = A(T)f nH2Oexp(−PΔV/RT), where A(T) = 1.1 H/106Si/MPa at 1100 °C, n = 1, and ΔV = 10.6×10–6 m3/mol. These data demonstrate that the entire present-day water content of the upper mantle could be incorporated in the mineral olivine alone; therefore, a free hydrous fluid phase cannot be stable in those regions of the upper mantle with a normal concentration of hydrogen. Free hydrous fluids are restricted to special tectonic environments, such as the mantle wedge above a subduction zone. Received: 10 February 1995 / Accepted: 23 October 1995  相似文献   

6.
Inelastic magnetic neutron scattering has been used to determine the energy of the 4 A 2→4 T 2 transition in CoAl2O4 spinel and the δ1 transition in Co2[Al4Si5]O18 cordierite. The observed crystal field splitting in Co-spinel is 485 meV (3900 cm−1), which corresponds to a crystal field stabilization energy of 56.2 kJmol−1. The transition energy of the δ1 transition in Co-cordierite has been determined to be 21 meV (170 cm−1). The present data demonstrate that magnetic neutron scattering can be used to measure crystal field transitions at energies of interest in the study of 3d-containing silicates. It may be used to measure transition energies when the use of optical spectroscopy is inappropriate. Received: 30 January 1997 / Accepted: 5 July 1997  相似文献   

7.
 Phase A, Mg7Si2O8(OH)6, is a dense hydrous magnesium silicate whose importance as a host of H2O in the Earth’s mantle is a subject of debate. We have investigated the low-pressure stability of phase A in experiments on the reaction phase A=brucite+forsterite. Experiments were conducted in piston-cylinder and multi-anvil apparatus, using mixtures of synthetic phase A, brucite and forsterite. The reaction was bracketed between 2.60 and 2.75 GPa at 500° C, between 3.25 and 3.48 GPa at 600° C and between 3.75 and 3.95 GPa at 650° C. These pressures are much lower than observed in the synthesis experiments of Yamamoto and Akimoto (1977). At 750° C the stability field of brucite + chondrodite was entered. The enthalpy of formation and entropy of phase A at 1 bar (105 Pa), 298 K, were derived from the experimental brackets on the reaction phase A=brucite+forsterite using a modified version of the thermodynamic dataset THERMOCALC of Holland and Powell (1990), which includes a new equation of state of H2O derived from the molecular dynamics simulations of Brodholt and Wood (1993). The data for phase A are: ΔH o f =−7126±8 kJ mol-1, S o=351 J K-1 mol-1. Incorporating these data into THERMOCALC allows the positions of other reactions involving phase A to be calculated, for example the reaction phase A + enstatite=forsterite+vapour, which limits the stability of phase A in equilibrium with enstatite. The calculated position of this reaction (753° C at 7 GPa to 937° C at 10 GPa) is in excellent agreement with the experimental brackets of Luth (1995) between 7 and 10 GPa, supporting the choice of equation of state of H2O used in THERMOCALC. Comparison of our results with calculated P-T paths of subducting slabs (Peacock et al. 1994) suggests that, in the system MgO–SiO2–H2O, phase A could crystallise in compositions with Mg/Si>2 at pressures as low as 3 GPa. In less Mg rich compositions phase A could crystallise at pressures above approximately 6 GPa. Received: 3 July 1995/Accepted: 14 December 1995  相似文献   

8.
 In Madagascar, hibonite occurs as a rather frequent mineral within thorianite-bearing skarns which are widespread in the Pan African granulitic formations constituting the S-E part of the Island (Tranomaro area). In these skarns, leucocratic segregations made up of CO3-scapolite to meionite (Anequivalent=89–95% which implies T≥850° C), spinel and corundum were formed at stage 1 of metasomatism in a titanite-bearing matrix consisting of scapolite (Aneq=77–88) and aluminous diopside. During stage 2 of metasomatism, scapolite from the lenses were altered to anorthite+calcite while the less calcic scapolite remained stable which indicates T≈800° C. Hibonite crystallized at the expense of corundum and spinel. Expressed as mol% of the CaAl12O19/Ca(Al10TiR2+)O19/REE(Al11R2+)O19 [+Th (Al10R2+ 2)O19] end-members (R 2+=Mg, Fe2+, Zn2+; Al=Al, Fe3+; Ti=Ti, Si), its composition varies from 26/72/2 to 50/23/27. The ideal activity of the CaAl12O19 component is about 0.25. Fluid inclusions in corundum, hibonite and anorthite are composed of nearly pure CO2. In corundum, the isochores for primary inclusions are in agreement with the P-T estimates for regional metamorphism and stage 1 metasomatism (T≈850° C, P≈5 kbar). Inclusions with the highest density in hibonite and anorthite constrain P to about 3–3.5 kbar for T=800° C. Thermodynamic calculations indicate that, in addition to a low activity of CaAl12O19, stability of hibonite in equilibrium with anorthite and calcite implies an extremely low activity of silica (below the zircon-baddeleyite buffer). By contrast the activity of CO2 may be high, in agreement with the observed fluid compositions. These results are corroborated by a short comparison with the other granulite occurrences of hibonite in Tanzania and South India. Received: 18 August 1994 / Accepted: 12 October 1995  相似文献   

9.
This paper reports measurements of electrical conductivity in Co2SiO4 as a function of temperature in the range from 780 to 1540 K and oxygen fugacity in the range from 5 × 10–8 to 2.6 × 10–5 MPa at 1540 K. From the present data and available literature data on defect concentrations in Co2SiO4, the mobility of the defect responsible for conduction is determined to be 8.6 × 10–7 m2/V-s at 1373 K. On the basis of these data and the observed dependence of conduction on oxygen fugacity, it is concluded that small polarons associated with oxidized cobalt ions is the likely mechanism controlling conduction. The mobility of small polarons in iron-bearing olivine under similar conditions is likely to be of the same order of magnitude.  相似文献   

10.
 Picritic units of the Miocene shield volcanics on Gran Canaria, Canary Islands, contain olivine and clinopyroxene phenocrysts with abundant primary melt, crystal and fluid inclusions. Composition and crystallization conditions of primary magmas in equilibrium with olivine Fo90-92 were inferred from high-temperature microthermometric quench experiments, low-temperature microthermometry of fluid inclusions and simulation of the reverse path of olivine fractional crystallization based on major element composition of melt inclusions. Primary magmas parental for the Miocene shield basalts range from transitional to alkaline picrites (14.7–19.3 wt% MgO, 43.2–45.7 wt% SiO2). Crystallization of these primary magmas is believed to have occurred over the temperature range 1490–1150° C at pressures ≈5 kbar producing olivine of Fo80.6-90.2, high-Ti chrome spinel [Mg/ (Mg+Fe2+)=0.32–0.56, Cr/(Cr+Al)=0.50–0.78, 2.52–8.58 wt% TiO2], and clinopyroxene [Mg/(Mg+Fe)=0.79–0.88, Wo44.1-45.3, En43.9-48.0, Fs6.8-11.0] which appeared on the liquidus together with olivine≈Fo86. Redox conditions evolved from intermediate between the QFM and WM buffers to late-stage conditions of NNO+1 to NNO+2. The primary magmas crystallized in the presence of an essentially pure CO2 fluid. The primary magmas originated at pressures >30 kbar and temperatures of 1500–1600° C, assuming equilibrium with mantle peridotite. This implies melting of the mantle source at a depth of ≈100 km within the garnet stability field followed by migration of melts into magma reservoirs located at the boundary between the upper mantle and lower crust. The temperatures and pressures of primary magma generation suggest that the Canarian plume originated in the lower mantle at depth ≈900 km that supports the plume concept of origin of the Canary Islands. Received: 23 October 1995/Accepted: 21 February 1996  相似文献   

11.
Experiments have been carried out to determine the temperature, oxygen fugacity (fO2) and compositional dependence of the tracer diffusion coefficient (D) of calcium in olivine. These data constrain the diffusion coefficient over the temperature range 900 to 1500°C for the three principal crystallographic axes. Well constrained linear relationships between the reciprocal of the absolute temperature and log(D) exist at any given oxygen fugacity. There is a strong dependence of the diffusion coefficient on oxygen fugacity with D ∝ fO2(1/3). This makes a knowledge of the T-fO2 path followed by geological samples a prerequisite for modelling Ca diffusion in olivine. The best fitting preexponential factor (Do) and activation energy (E) to the Arrhenius equation log (D) = log [Do exp(−E/RT)] + 0.31Δ log fO2 for Ca diffusion in olivine at a given oxygen fugacity (fO2*) are given by:diffusion along [100]: log [Do (m2/s)] = −10.78 ± 0.43; E = 193 ± 11 kJ/moldiffusion along [010]: log [Do (m2/s)] = −10.46 ± 0.37; E = 201 ± 10 kJ/moldiffusion along [001]: log [Do (m2/s)] = −10.02 ± 0.29; E = 207 ± 8 kJ/molwhere Δ log fO2 = log[fO2*] − log[10−12] with fO2* in units of bars. There is no measurable compositional dependence of the diffusion coefficient between Fo83 and Fo92. Diffusion in Fo100 has a much higher activation energy than in Fe-bearing olivine and has a weaker fO2 dependence.  相似文献   

12.
 The speciation of water dissolved in glasses along the join NaAlSi3O8-KAlSi3O8 has been investigated using infrared spectroscopy. Hydrous melts have been hydrothermally synthesized by chemical equilibration of cylinders of bubble-free anhydrous start glasses with water at 1040° C and 2 kbar. These melts have been isobarically and rapidly (200° C/s) “drop”-quenched to room temperature and then subsequently depressurized. The speciation of water in the quenched glasses reflects the state of water speciation at a temperature (the so-called fictive temperature) where the quenched-in structure of the glasses closely corresponds to the melt structure at equilibrium. This fictive temperature is detectable as the macroscopically measureable glass transition temperature of these melt compositions. A separate set of experiments using vesicular samples of the same chemistry has precisely defined the glass transition temperature of these melts (±5° C) on the basis of homogenization temperatures for water-filled fluid inclusions (Romano et al. 1994). The spectroscopic data on the speciation of water in these quenched glasses has been quantified using experimentally determined absorptivities for OH and H2O for each individual melt composition. The knowledge of glass transition temperatures, together with quantitative speciation data permits an analysis of the temperature dependence of the water speciation over the 113° C range of fictive temperatures obtained for these water-saturated melts. The variation of water speciation, cast as the equilibrium constant K where K = [H2O] [O m ]/[OH]2 is plotted versus the fictive temperature of the melt to obtain the temperature dependence of speciation. Such a plot describes a single linear trend of the logarithm of the equilibrium constant versus reciprocal temperature, implying that the exchange of K for Na has little influence on melt speciation of water. The enthalpy derived from temperature dependence is 36.5(±5) kJ/mol. The results indicate a large variation in speciation with temperature and an insensitivity of the speciation to the K–Na exchange. Received: 8 March 1995/Accepted: 6 June 1995  相似文献   

13.
Mineralogical analysis, electrical conductivity and thermopower are reported for monocrystalline heterosite (Fe3+, Mn3+)PO4 with the orthorhombic olivine-type structure. The 57Fe Mössbauer spectrum could be adequately described using two Fe3+ doublets. By impedance spectroscopy (20 Hz–1 MHz) the electrical DC conductivity σDC and AC conductivity σAC were determined parallel (∥) and perpendicular to the [001] direction (space group Pnma) in the range ～160–440 K. The graph log σDC?1/T shows a slightly bent curve in both directions with activation energies of E A ～0.30 and ～0.15 eV in the high and low temperature ranges, respectively. The reduced E A is associated with electronic conduction; σDC ∥ [001] follows Mott’s T 1/4 variable range hopping law at lower temperatures with hopping between localized levels. The values of σAC are increased relative to σDC at high frequencies and low temperatures, obeying Jonscher’s universal dynamic response law; for σAC ∥ [001], the variation with temperature of the frequency exponent is in fair agreement with the model of small polaron hopping. The absolute thermopower Θ is negative and low between ～295 and ～440 K, Θ does hardly vary with temperatures in both directions; the temperature independency of Θ ∥ [001] is consistent with the small polaron hopping model.  相似文献   

14.
 An olivine grain from a peridotite nodule 9206 (Udachnaya kimberlite, Siberia) was investigated by TEM methods including AEM, HRTEM, SAED and EELS techniques. A previous study of the 9206 olivine sample revealed OH absorption bands in the IR spectrum and abundant nanometer-sized OH-bearing inclusions, of hexagonal-like or lamellar shape. Inclusions, which are several hundred nm in size, consist of 10 ? phase, talc and serpentine (chrysotile and lizardite). The lamellar (LI) and hexagon-like small inclusions of several ten nm in size (SI) are the topic of the present paper. AEM investigations of the inclusions reveal Mg, Fe and Si as cations only. The Mg/Si and Fe/Si atomic ratios are lower in the inclusions than in the host olivine. The Si concentration in the olivine host and both lamellar inclusions and small inclusions is the same. A pre-peak at 528eV was observed in EEL spectra of LI and SI, which is attributed to OH− or Fe3+. From these data it is concluded that there is a OH−- or Fe3+-bearing cation-deficient olivine-like phase present. HRTEM lattice fringe images of LI and SI exhibit modulated band-like contrasts, which are superimposed onto the olivine lattice. Diffraction patterns (Fourier-transforms) of the HREM images as well as SAED patterns show that the band-like contrasts in HRTEM images of the inclusions are caused by periodic modulations of the olivine lattice. Three kinds of superperiodicity in the olivine structure such as 2a, 3a and 3c, were observed in SAED patterns. The corresponding olivine supercells labelled here as Hy-2a, Hy-3a and Hy-3c were derived. The M1-vacancies located in the (100) and (001) octahedral layers of the olivine lattice are suggested to form ordered arrays of planar defects (PD), which cause the band-like contrasts in HRTEM patterns as well as the superperiodicity in the SAED patterns. The vacancy concentrations as well as the chemical composition of Hy-2a, Hy-3a and Hy-3c olivine supercells were calculated using crystal chemical approaches, assuming either {(OH)< O−V" Me−(OH)< O}↔, or {F e < Fe −H Me ′}↔ or {2F e < Fe −V Me "}↔ point defect associates. The calculated theoretical compositions Mg1.615Fe+2 0.135v0.25SiO4H0.5 (Hy-2a) and Mg1.54Fe2+ 0.12v0.33SiO4H0.66 (Hy-3a and Hy-3c) are in a good agreement with the AEM data on inclusions. Hy-2a, Hy-3a and Hy-3c are considered to be a hydrous olivine with the extended chemical formula (Mg1-yFe2+ y)2−xvxSiO4H2x. The crystal structure of hydrous olivine is proposed to be a modular olivine structure with Mg-vacant modules. The crystal chemical formula of hydrous olivines in terms of a modular structure can be written as [MgSiO4H2] · 3[Mg1.82Fe0.18SiO4] for Hy-2a, [MgSiO4H2] · 2[Mg1.82Fe0.18SiO4] for Hy-3a and Hy-3c. Hydrous olivine is suggested to be exsolved from the olivine 9206, which has been initially saturated by OH-bearing point defects. The olivine 9206 hydration as well as the following exsolution of hydrous olivine inclusions is suggested to occur at high pressure-high temperature conditions of the upper mantle. Received: 15 January 2001 / Accepted: 2 July 2001  相似文献   

15.
A selected set of five different kyanite samples was analysed by electron microprobe and found to contain chromium between <0.001 and 0.055 per formula unit (pfu). Polarized electronic absorption spectroscopy on oriented single crystals, R1, R2-sharp line luminescence and spectra of excitation of λ3- and λ4-components of R1-line of Cr3+-emission had the following results: (1) The Fe2+–Ti4+ charge transfer in c-parallel chains of edge connected M(1) and M(2) octahedra shows up in the electronic absorption spectra as an almost exclusively c(||Z′)-polarized, very strong and broad band at 16000 cm−1 if <, in this case the only band in the spectrum, and at an invariably lower energy of 15400 cm−1 in crystals with  ≥ . The energy difference is explained by an expansion of the Of–Ok, and Ob–Om edges, by which the M(1) and M(2) octahedra are interconnected (Burnham 1963), when Cr3+ substitutes for Al compared to the chromium-free case. (2) The Cr3+ is proven in two greatly differing crystal fields a and b, giving rise to two sets of bands, derived from the well known dd transitions of Cr3+ 4A2g→4T2g(F)(I), →4T1g(F)(II), and →4T1g(P)(III). Band energies in the two sets a and b, as obtained by absorption, A, and excitation, E, agree well: I: 17300(a, A), 17200(a, E), 16000(b, A), 16200(b, E); II: 24800(a, A), 24400(a, E); 22300(b, A), 22200(b, E); III: 28800(b,A) cm−1. Evaluation of crystal field parameters from the bands in the electronic spectra yield Dq(a)=1730 cm−1, Dq(b)=1600 cm−1, B(a)=790 cm−1, B(b)=620 cm−1 (errors ca. ±10 cm−1), again in agreement with values extracted from the λ3, λ4 excitation spectra. The CF-values of set a are close to those typical of Cr3+ substituting for Al in octahedra of other silicate minerals without constitutional OH− as for sapphirine, mantle garnets or beryl, and are, therefore, interpreted as caused by Cr3+ substituting for Al in some or all of the M(1) to M(4) octaheda of the kyanite structure, which are crystallographically different but close in their mean Al–O distances, ranging from 1.896 to 1.919 A (Burnham 1963), and slight degrees of distortion. Hence, band set a originates from substitutive Cr3+ in the kyanite structural matrix. The CF-data of Cr3+ type b, expecially B, resemble those of Cr3+ in oxides, especially of corundum type solid solutions or eskolaite. This may be interpreted by the assumption that a fraction of the total chromium contents might be allocated in a precursor of a corundum type exsolution. Received: 3 January 1997 / Revised, accepted: 2 May 1997  相似文献   

16.
 This paper uses the geochemistry of primitive mafic lavas from the Rungwe volcanic province (southwestern Tanzania) to infer the source mineralogy and melting history. Post-Miocene mafic lavas from Rungwe include alkali basalts, basanites, nephelinites and picrites with up to 18.9 wt% MgO; nephelinites (>13.5% normative nepheline) are restricted to Kiejo volcano in the southern portion of the province. Rungwe lavas differ from most Western Rift volcanics in that they are not unusually potassic (K2O/Na2O ca. 0.40). Sparsely phyric mafic lavas contain phenocrysts and xenocrysts of plagioclase (An82–90), clinopyroxene (4.5–9.5 wt% Al2O3), and olivine (Fo79–88); one basanite contains a 1 mm xenocryst of apatite included in magnesian clinopyroxene. All samples have high abundances of incompatible elements (e.g., 0.7–2.2 wt% P2O5) and are enriched in REE relative to HFSE (Hf, Zr, Ti, Y), Cs, Ba, and K. Some incompatible element ratios are constant throughout the Rungwe suite (e.g., Zr/Nb, Sr/Ce, K/Rb), but other ratios are extremely variable and exceed the range measured in global Ocean Island Basalts (OIB) (e.g., Ba/Nb, Sm/Zr, La/Nb, Pb/Ce, Nb/U). The range in degree of silica saturation, and its excellent correlation with P2O5/Al2O3, indicate that the Rungwe suite records variable degrees of melting. Variations of individual incompatible trace element abundances in nephelinite and basanite samples suggest that the source contains metasomatic amphibole, ilmenite, apatite, and zircon. The Rungwe suite is interpreted as a series of low-percentage melts of CO2-rich peridotite at pressures that span the garnet-spinel transition. A geochemical comparison of Rungwe samples to lavas from other Western Rift volcanic centers requires that the source mineralogy varies along the rift axis, although each province is underlain by metasomatized peridotite. The incompatible trace element signatures of Western Rift lavas indicate that the source area is typically homogeneous on the scale of individual volcanoes, although lavas from each volcano reflect a range in degree of melting. Significantly, volcanoes with distinct geochemistry are always separated by major rift faults, suggesting that volcanic and tectonic surface features may correspond to metasomatic provinces within the subcontinental lithospheric mantle. Received: 30 May 1994 / Accepted: 5 April 1995  相似文献   

17.
Using impedance spectroscopy, we have measured the electrical properties of two dunites and a single crystal olivine sample from 1000 to 1200° C as a function of oxygen fugacity (f o 2). Two conduction mechanisms with resistances that add in series are observed for the dunites corresponding to grain interior and grain boundary conduction mechanisms. The conductivities for each mechanism were determined by analyzing the data using a complex nonlinear least squares fitting routine and the equivalent circuit approach. The grain interiors display a conductivity dependent on f o 2 to the 1/5.5–1/7 power, consistent with other determinations, and interpreted as indicating small polaron transport (Fe Mg · ). The grain boundaries demonstrate a weaker f o 2 dependence that is dependent on temperature and material. Under certain conditions the f o 2 dependence of the grain boundary conductivity is negative. This result indicates that oxygen ion transport is probably not the dominant grain boundary charge transport mechanism; however, an unequivocal determination of the grain boundary mechanism has not been achieved. In some dunites the grain boundaries are more conductive than the grain interiors; in other dunites they are more resistive than grain interiors. The grain boundaries do not enhance the total conductivity of any of the materials of this study but are the controlling mechanism in some instances. Measurement of the complex electrical response at frequencies as low as 10-4 Hz is required to determine the role of grain boundaries on the overall electrical properties of polycrystalline dunite.  相似文献   

18.
 Agali–Coimbatore dolerite dykes constitute an important Proterozoic magmatic event that affected the south Indian shield. Rb-Sr whole rock isotope data yield an “errorchron” of 2369±400 Ma (2σ error) which is within error of the reported 2030±65 Ma K-Ar age. The dyke magmas were evolved Fe-rich tholeiitic melts produced by fractionation of clinopyroxene, orthopyroxene and olivine in the initial stages. Plagioclase became a fractionation phase during the latter stages of crystallization. The dykes characteristically have high 87Sr/86Sri (0.703–0.706) and are enriched in large-ion lithophile and light rare earth elements relative to primordial mantle values and show negative Nb anomalies. These compositional characteristics are interpreted as source mantle characteristics whereas some crustal effects are visible in some samples with high initial 87Sr/86Sr. Peridotite with minor hydrous metasomatic phases like amphibole (and phlogopite) within the shallow lithospheric mantle could be a potential source material for the dykes. However, at this stage we cannot convincingly differentiate whether the source of the parent magmas is solely lithospheric or a product of asthenosphere-lithosphere mixing. The δ18O values of the dykes range from +5.2 to +7.2 per mil (vs standard mean oceanic water). Initial Nd isotope values at the time of dyke intrusion (ɛNd at t=2.0 Ga) range from −2.3 to −4.8. Whole rocks define a correlation on an Sm-Nd isochron plot with a slope equivalent to an age of 3.15±0.53 Ga (2σ error); Sm-Nd crustal residence ages average at 2.87 Ga. The isochron age does not appear to be the result of systematic mixing with an older crustal component. These results together with trace element geochemistry suggest that the south Indian mantle lithosphere developed by addition of enriched melts/fluids at about 3.0 Ga synchronously with major crustal gene- ration in the south Indian shield. Received 20 June 1994/Accepted: 17 May 1995  相似文献   

19.
 Experiments were performed in the three phase system high-silica rhyolite melt+low-salinity aqueous vapor+hydrosaline brine, to investigate the partitioning equilibria for copper in magmatic-hydrothermal systems at 800° C and 1 kbar, and 850° C and 0.5 kbar. Daqm/mlt Cu and apparent equilibrium constants, Kaqm/mlt Cu,Na, between the aqueous mixture (aqm=quenched vapor+brine) and the silicate melt (mlt) are calculated. Daqm/mlt Cu increases with increasing aqueous chloride concentration and is a function of pressure. Kaqm/mlt Cu,Na=215(±73) at 1 kbar and 800° C and Kaqm/mlt Cu,Na=11(±6) at 0.5 kbar and 850°C. Decreasing pressure from 1 to 0.5 kbar lowers Kaqm/mlt Cu,Na by a factor of approximately 20. Data revealed no difference in Kaqm/mlt Cu,Na or Daqm/mlt Cu as a function of the melt aluminium saturation index. Within the 2-phase field the Kaqm/mlt Cu,Na show no variation with total aqueous chloride, indicating that copper-sodium exchange between the vapor, brine and silicate melt is independent of the mass proportion of vapor and brine. Model copper-sodium apparent equilibrium constants for the hydrosaline brine and the silicate melt revealed a negative dependence on pressure. Model apparent equilibrium constants for copper-sodium exchange between the brine and vapor were close to unity at 1 kbar and 800° C. Received: 27 June 1994/Accepted: 30 March 1995  相似文献   

20.
 The melting reaction: albite(solid)+ H2O(fluid) =albite-H2O(melt) has been determined in the presence of H2O–NaCl fluids at 5 and 9.2 kbar, and results compared with those obtained in presence of H2O–CO2 fluids. To a good approximation, albite melts congruently at 9 kbar, indicating that the melting temperature at constant pressure is principally determined by water activity. At 5 kbar, the temperature (T)- mole fraction (X (H2O) ) melting relations in the two systems are almost coincident. By contrast, H2O–NaCl mixing at 9 kbar is quite non-ideal; albite melts ∼70 °C higher in H2O–NaCl brines than in H2O–CO2 fluids for X (H2O) =0.8 and ∼100 °C higher for X (H2O) =0.5. The melting temperature of albite in H2O–NaCl fluids of X (H2O)=0.8 is ∼100 °C higher than in pure water. The P–T curves for albite melting at constant H2O–NaCl show a temperature minimum at about 5 kbar. Water activities in H2O–NaCl fluids calculated from these results, from new experimental data on the dehydration of brucite in presence of H2O–NaCl fluid at 9 kbar, and from previously published experimental data, indicate a large decrease with increasing fluid pressure at pressures up to 10 kbar. Aqueous brines with dissolved chloride salt contents comparable to those of real crustal fluids provide a mechanism for reducing water activities, buffering and limiting crustal melting, and generating anhydrous mineral assemblages during deep crustal metamorphism in the granulite facies and in subduction-related metamorphism. Low water activity in high pressure-temperature metamorphic mineral assemblages is not necessarily a criterion of fluid absence or melting, but may be due to the presence of low a (H2O) brines. Received: 17 March 1995/Accepted: 9 April 1996  相似文献   

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