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1.
Pb diffusion in rutile 总被引:16,自引:0,他引:16
D. J. Cherniak 《Contributions to Mineralogy and Petrology》2000,139(2):198-207
Diffusion of Pb was measured in natural and synthetic rutile under dry, 1 atmosphere conditions, using mixtures of Pb titanate
or Pb sulfide and TiO2 as the sources of diffusant. Pb depth profiles were then measured with Rutherford Backscattering Spectrometry (RBS). Over
the temperature range 700–1100 °C, the following Arrhenius relation was obtained for the synthetic rutile: D=3.9 × 10−10exp(−250 ± 12 kJ mol−1/RT) m2s−1. Results for diffusion in natural and synthetic rutile were quite similar, despite significant differences in trace element
compositions. Mean closure temperatures calculated from the diffusion parameters are around 600 °C for rutile grains of ∼100 μm
size. This is about 100 °C higher than rutile closure temperature determinations from past field-based studies, suggesting
that rutile is more resistant to Pb loss through volume diffusion than previously thought.
Received: 28 June 1999 / Accepted: 29 December 1999 相似文献
2.
Dagma Derdau Robert Freer Kate Wright 《Contributions to Mineralogy and Petrology》1998,133(1-2):199-204
Oxygen exchange experiments have been performed between single crystals of sanidine feldspar and oxygen gas enriched in 18O, at temperatures in the range 869–1053 °C, total pressure 1 atmosphere, for times up to 28 days. Oxygen isotope diffusion
profiles in a direction perpendicular to (001) were determined with an ion microprobe. The experimental data obey a single
Arrhenius relationship of the form D = 8.4 × 10−11 exp. (−245 ± 15 kJ mol−1/RT) m2s−1. The results indicate that oxygen diffusion in anhydrous sanidine feldspar is marginally slower than oxygen diffusion in
anhydrous anorthite. Comparison with published atomistic simulation studies suggests that oxygen transport in feldspar is
by an interstitial mechanism.
Received: 17 October 1997 / Accepted: 6 July 1998 相似文献
3.
The expansivity of supercooled diopside liquid has been determined using techniques of container-based dilatometry. Two thermal
strategies have been employed, one in which the sample is brought to volumetric equilibrium by long-duration dwells at low
temperatures (817 °C) and one in which scanning dilatometry of the sample has been performed at somewhat higher temperatures
(890–913 °C). The results of both experiments yield a supercooled liquid expansivity for diopside liquid in the temperature
range of 817–913 °C of 84.4 ± 2.8 × 10−4 cm3/mol K. The expansivity is 65% higher than that obtained for diopside melt obtained at superliquidus temperatures using the
double bob Archimedean method. Combined fitting of the new low temperature, volume–temperature data from the present study
and the superliquidus data from the literature has been performed. The combined fit yields the following equations for the
volume–temperature relationship of diopside liquid (T=temperature in °C):
The standard error of the fit using both equations reproduces the volume–temperature data for diopside liquid within experimental
error. This result reconciles the disparate values of expansivity measured at low temperatures in the supercooled state and
at superliquidus temperatures and confirms the temperature-dependence of the expansivity of diopside liquid. Comparison with
previous low temperature estimates of melt volume and expansivity are discussed in light of these new results.
Received: 18 November 1999 / Accepted: 24 January 2000 相似文献
4.
Epithermal mineralization and ore controls of the Shasta Au-Ag deposit, Toodoggone District, British Columbia, Canada 总被引:1,自引:0,他引:1
The Shasta gold-silver deposit, British Columbia, Canada, is an adularia-sericite-type epithermal deposit in which deposition
of precious metals coincided with the transition of quartz- to calcite-dominant gangue. Mineralization is associated with
stockwork-breccia zones in potassically altered dacitic lapilli tuffs and flows, and consists of pyrite, sphalerite, chalcopyrite,
galena, acanthite, electrum and native silver. Pre- and post-ore veins consist solely of quartz and calcite, respectively.
Fluid inclusion microthermometry indicates that ore minerals were deposited between 280 ° and 225 °C, from a relatively dilute
hydrothermal fluid (˜1.5 wt.% NaCl equivalent). Abundant vapor-rich inclusions in ore-stage calcite are consistent with boiling.
Oxygen and hydrogen isotopic data (δ18Ofluid = −1.5 to −4.1‰; δDfluid = −148 to −171‰) suggest that the fluid had a meteoric origin, but was 18O-enriched by interaction with volcanic wallrocks. Initial (˜280 °C) fluid pH and log f O2 conditions are estimated at 5.3 to 6.0, and −32.5 to −33 bar, respectively; during ore deposition, the fluid became more
alkaline and oxidizing. Ore deposition at Shasta is attributed to localization of meteoric hydrothermal fluids by extensional
faults; mineralization was controlled by boiling in response to hydraulic brecciation. Calcite and base metal sulfides precipitated
due to the increase in pH that accompanied boiling, and the associated decrease in H2S concentration led to precipitation of gold and silver.
Received: 23 February 1995 / Accepted: 16 April 1996 相似文献
5.
Volume and grain boundary diffusion of calcium in natural and hot-pressed calcite aggregates 总被引:3,自引:0,他引:3
Calcium self-diffusion rates in natural calcite single crystals were experimentally determined at 700 to 900° C and 0.1 MPa
in a stream of CO2. Diffusion coefficients (D) were determined from 42Ca concentration profiles measured with an ion microprobe. The Arrhenius parameters yield an activation energy (Q)=382±37 kJ/mol
and pre-exponential factor (D0)=0.13 m2/s, and there is no measurable anisotropy. Calcium grain boundary diffusion rates were experimentally determined in natural
(Solnhofen) limestone and hot-pressed calcite aggregates at 650° to 850° C and 0.1 to 100 MPa pressure. The Solnhofen limestone
was first pre-annealed for 24 h at 700° C and 100 MPa confining pressure under anhydrous conditions to produce an equilibrium
microstructure for the diffusion experiments. Values for the product of the grain boundary diffusion coefficient (D′) and
the effective grain boundary diffusion width (δ) were determined from 42Ca concentration profiles measured with an ion microprobe. The results show that there is no measurable difference between
D′δ values obtained for pre-annealed Solnhofen samples at 0.1 and 100 MPa or between hot-pressed calcite aggregates and pre-annealed
Solnhofen samples. The temperature dependence for calcium grain boundary diffusion in Solnhofen samples annealed at 0.1 MPa
is described by the Arrhenius parameters D′
0δ=1.5×10−9 m3/s and Q=267±47 kJ/mol. Comparison of the results of this study with previously published data show that calcium is the slowest
volume diffusing species in calcite. The calcium diffusivities measured in this study place constraints on several geological
processes that involve diffusive mass transfer including diffusion-accommodated mechanisms in the deformation of calcite rocks.
Received: 19 December 1994/Accepted: 30 June 1995 相似文献
6.
Craig S. Schwandt Randall T. Cygan Henry R. Westrich 《Contributions to Mineralogy and Petrology》1998,130(3-4):390-396
Magnesium self-diffusion coefficients were determined experimentally for diffusion parallel to each of the three crystallographic
directions in natural orthoenstatite (En88Fs12). Experiments were conducted at 1 atm in CO-CO2 gas mixing furnaces, which provided oxygen fugacities equivalent to the iron-wüstite buffer. Diffusion of 25Mg was induced in polished samples of oriented orthoenstatite using a film of isotopically enriched 25MgO as the source material. Very short (<0.15 μm) diffusional penetration profiles were measured by ion microprobe depth profiling.
The diffusion coefficients determined for four temperatures (900, 850, 800, 750 °C) provide the activation energies, E
a
, and frequency factors, D
o, where D = D
o exp (−E
a
/RT) for Mg self-diffusion parallel to each crystallographic direction: a-axis, E
a
= 360 ± 52 kJ/mole and D
o = 1.10 × 10−4 m2/s; b-axis, E
a
= 339 ± 77 kJ/mole and D
o = 6.93 × 10−6 m2/s and c-axis, E
a
= 265 ± 66 kJ/mole and D
o = 4.34 × 10−9 m2/s. In this temperature range, any possible anisotropy of cation diffusion is very small, however the activation energy for
diffusion parallel to the c-axis (001) is the lowest and the activation energies for diffusion parallel to the a-axis (100) and b-axis (010) are higher. Application of these diffusion results to the silicate phases of the Lowicz mesosiderite meteorite
provides cooling rates for the silicate portion of the meteorite (4–11 °C/100 years) that are similar, although slower, to
previous estimates. These silicate cooling rates are still several orders of magnitude faster than the cooling rates (0.1 °C/106 years) for the metal portions.
Received: 22 January 1997 / Accepted: 2 October 1997 相似文献
7.
The lattice constants of paragonite-2M1, NaAl2(AlSi3)O10(OH)2, were determined to 800 °C by the single-crystal diffraction method. Mean thermal expansion coefficients, in the range 25–600 °C,
were: αa = 1.51(8) × 10−5, αb = 1.94(6) × 10−5, αc = 2.15(7) × 10−5 °C−1, and αV = 5.9(2) × 10−5 °C−1. At T higher than 600 °C, cell parameters showed a change in expansion rate due to a dehydroxylation process. The structural refinements
of natural paragonite, carried out at 25, 210, 450 and 600 °C, before dehydroxylation, showed that the larger thermal expansion
along the c parameter was mainly due to interlayer thickness dilatation. In the 25–600 °C range, Si,Al tetrahedra remained quite unchanged,
whereas the other polyhedra expanded linearly with expansion rate proportional to their volume. The polyhedron around the
interlayer cation Na became more regular with temperature. Tetrahedral rotation angle α changed from 16.2 to 12.9°. The structure
of the new phase, nominally NaAl2 (AlSi3)O11, obtained as a consequence of dehydroxylation, had a cell volume 4.2% larger than that of paragonite. It was refined at room
temperature and its expansion coefficients determined in the range 25–800 °C. The most significant structural difference from
paragonite was the presence of Al in fivefold coordination, according to a distorted trigonal bipyramid. Results confirm the
structural effects of the dehydration mechanism of micas and dioctahedral 2:1 layer silicates. By combining thermal expansion
and compressibility data, the following approximate equation of state in the PTV space was obtained for paragonite: V/V
0 = 1 + 5.9(2) × 10−5
T(°C) − 0.00153(4) P(kbar).
Received: 12 July 1999 / Revised, accepted: 7 December 1999 相似文献
8.
Andreas Tennie Radegund Hoffbauer Stephan Hoernes 《Contributions to Mineralogy and Petrology》1998,133(4):346-355
The oxygen isotope fractionation between kyanite and calcium carbonate has been investigated experimentally at four temperatures
in the range between 625 and 775 °C at 13 kbar. Because of low exchange rates, the isotopic reaction was enhanced by polymorphic
transformation of andalusite to kyanite. With this experimental modification a close approach to equilibrium was reached in
all runs. The temperature dependence of the equilibrium fractionation is described by the equation 1000 ln ky-cc=−2.62×106/T
2. Application of the experimental results to natural quartz-kyanite-garnet assemblages indicates the preservation of the oxygen
isotope composition of kyanite acquired during its formation, reflecting its extremely low oxygen diffusivity. This refractory
behaviour restricts the use of kyanite for thermometry but opens the possibility to use its O-isotope composition as an indicator
for recognition of polymetamorphic rock histories and reconstruction of the prograde evolution of a metamorphic sequence.
Received: 8 June 1998 / Accepted: 24 August 1998 相似文献
9.
M. Liu J. C. Peterson Richard A. Yund 《Contributions to Mineralogy and Petrology》1997,126(3):217-223
The growth rates of albite and pyroxene (enstatite + diopside + spinel) reaction rims were measured at 1000°C and ˜700 MPa
and found to be parabolic indicating diffusion-controlled growth. The parabolic rate constants for the pyroxene (+ spinel)
rims in samples with 0.5 wt% H2O added or initially vacuum dried at 25°C and 250°C are 1.68 ± 0.09, 0.54 ± 0.05 and 0.25 ± 0.06 μm2/h, respectively. The values for albite rim growth in samples initially dried at 60°C and with 0.1 wt% H2O added are 0.25 ± 0.04 and 0.33 ± 0.03 μm2/h, respectively. The latter values were used to derive the product of the grain boundary diffusion coefficient D′A, where A = SiO2, NaAlO2, or NaAlSi−1, and the grain boundary thickness δ in albite. The calculated D′SIO2δ in the albite aggregate for the situations of two different water contents are about 9.9 × 10−23 and 1.4 × 10−22 m3 s−1, respectively. Both the rate constants and the calculated D′Aδ demonstrate that the effect of water content on the grain boundary diffusion rate in monomineralic albite and polymineralic
pyroxene (+ spinel) aggregates is small, consistent with recent studies of monomineralic enstatite and forsterite rims.
Received: 1 July 1995 / Accepted: 1 August 1996 相似文献
10.
A. Pavese V. Diella V. Pischedda M. Merli R. Bocchio M. Mezouar 《Physics and Chemistry of Minerals》2001,28(4):242-248
The thermoelastic parameters of natural andradite and grossular have been investigated by high-pressure and -temperature
synchrotron X-ray powder diffraction, at ESRF, on the ID30 beamline. The P–V–T data have been fitted by Birch-Murnaghan-like EOSs, using both the approximated and the general form. We have obtained for
andradite K
0=158.0(±1.5) GPa, (dK/dT )0=−0.020(3) GPa K−1 and α0=31.6(2) 10−6 K−1, and for grossular K
0=168.2(±1.7) GPa, (dK/dT)0=−0.016(3) GPa K−1 and α0=27.8(2) 10−6 K−1. Comparisons between the present issues and thermoelastic properties of garnets earlier determined are carried out.
Received: 7 July 2000 / Accepted: 20 October 2000 相似文献
11.
Laurence Hopkinson Stephen Roberts Richard Herrington Jamie Wilkinson 《Contributions to Mineralogy and Petrology》1999,137(4):342-350
Silica occurs in abundance in a variety of hydrothermal samples from the Trans-Atlantic Geotraverse (TAG) hydrothermal mound,
26°N Mid-Atlantic Ridge. The water content, trace element chemistry, and mineralogy of crystalline silica from 15 different
samples have been examined by vibrational spectroscopy and probe microanalysis. The samples are from: shallow subsurface ferric
iron oxyhydroxide silica deposits (n=4), a fragment of an active white smoker chimney (n=1), anhydrite bearing hydrothermal breccias (n=2), pyrite silica breccias (n=3), and silicified wall rock breccias (n=5). Length-fast chalcedony occurs in association with variable quantities of ferric iron oxyhydroxides in hydrothermal breccias
from the mound flanks, within shallower subsurface chert samples, and within white smoker chimney walls. Samples from the
anhydrite zone contain textures which are suggestive of an origin involving replacement of anhydrite. Samples taken from TAG
1 and 5 from below the anhydrite zone contain no chalcedony. Instead they contain subhedral quartz crystals which show oscillatory
zoning in aluminium. Two types of crystalline silica namely, type A and type B quartz, are defined on the basis of the infrared
spectra in the OH region from 3200 cm−1 to 3600 cm−1. The type A quartz occurs beneath the anhydrite zone at TAG 1 and 5. We propose a model that relates specific varieties of
crystalline silica to different thermal and chemical environments within the mound interior. Length-fast chalcedony occurs
in an outer low temperature envelope across the top and sides of the mound. The common association between length-fast chalcedony
and ferric iron oxyhydroxides suggests that chalcedony crystallization is favoured where catalysis by ferric iron can occur.
The apparent suppression of fibrous silica at the expense of single quartz crystals with increasing depth is attributed to
differing growth rates and degrees of supersaturation of silica-bearing solutions with increasing temperature within the mound.
The transition from type A to type B single crystal growth is interpreted to occur at temperatures approaching ˜360 °C due
to decreasing solubility of aluminium in quartz, so that aluminium is rendered unavailable for type A valence compensation.
Received: 10 September 1998 / Accepted: 6 July 1999 相似文献
12.
Ca-(Fe,Mg) interdiffusion experiments between natural single crystals of grossular (Ca2.74Mg0.15 Fe0.23Al1.76Cr0.04Si3.05O12) and almandine (Ca0.21Mg0.40 Fe2.23Mn0.13Al2.00Cr0.08Si2.99O12 or Ca0.43Mg0.36Fe2.11 Al1.95Si3.04O12), were undertaken at 900–1100 °C and 30 kbar, and pressures of 15.0–32.5 kbar at 1000 °C. Samples were buffered by Fe/FeO
in most cases. Diffusion profiles were determined by electron microprobe. Across the experimental couples the interdiffusion
coefficients (D˜) were almost independent of composition. The diffusion rates in an unbuffered sample were significantly faster than in buffered
samples. The temperature dependence of the D˜ (Ca-Fe,Mg) interdiffusion coefficients may be described by
at 30 kbar and 900–1100 °C. This activation energy is marginally higher than previous experimental studies involving Ca-free
garnets; the interdiffusion coefficients are higher than previous studies for Fe-Mg and Fe-Mn exchange in garnet. The pressure
dependence of D˜ (Ca-Fe,Mg) at 1000 °C yielded an activation volume of 11.2 cm3 mol−1, which is higher than previous results from studies involving garnet and olivine. Comparison with simulation studies suggests
a vacancy mechanism for divalent ion migration in garnet, with extrinsic processes being dominant up to very high temperatures.
Received: 15 December 1996 / Accepted: 3 November 1998 相似文献
13.
Unit-cell dimensions of a natural phlogopite from Pargas, Finland, have been determined in the temperature interval of 27–1050 °C
by X-ray powder diffraction technique. Expansion rates vary discontinuously with temperature with a break at 412 °C. Below
this temperature, the linear expansions (α) for a, b and c axis lengths are 3.74 × 10−5 K−1, 1.09 × 10−5 K−1, and 1.19 × 10−5 K−1, respectively, and above that they are 0.86 × 10−5 K−1, 0.80 × 10−5 K−1, and 1.93 × 10−5 K−1. The volume thermal expansion coefficients are 6.26 × 10−5 K−1 and 3.71 × 10−5 K−1 for low-temperature and high-temperature intervals, respectively. The observed kink in the rate of thermal expansions with
temperature could be due to the different mode of structural changes. Thermogravimetric analysis of the sample indicates the
oxidation of iron in the temperature range of 500–600 °C and dehydroxylation as well as decomposition of phlogopite in the
temperature range of 900–1200 °C.
Received: 8 September 1998 / Accepted: 28 February 2000 相似文献
14.
The Archean Shawmere anorthosite lies within the granulite facies portion of the Kapuskasing Structural Zone (KSZ), Ontario,
and is crosscut by numerous linear alteration veins containing calcite + quartz ± dolomite ± zoisite ± clinozoisite ± margarite ±paragonite ± chlorite.
These veins roughly parallel the trend of the Ivanhoe Lake Cataclastic Zone. Equilibria involving clinozoisite + margarite + quartz ± calcite
± plagioclase show that the vein minerals were stable at T < 600 °C, XCO2 < 0.4 at P ≈ 6 kbar. The stabilities of margarite and paragonite in equilibrium with quartz are also consistent with T < 600 °C and XCO2 < 0.4 at 6 kbar. Additional assemblages consisting of calcite + clinochlore + quartz + talc + margarite indicate T < 500 °C with XCO2 > 0.9. Thus, vein formation, while clearly retrograde, spanned a range of temperatures, and fluid compositions evolved from
H2O-rich to CO2-rich. The calcite in the retrograde veins has δ18O values that range from 8.4 to 11.2‰ (average = +9.7 ± 0.9‰) and δ13C values that range from −3.9 to −1.6‰ (average = −3.1 ± 0.6‰). These values indicate that the fluids from which calcite precipitated
underwent extensive exchange with the anorthosite and other crustal lithologies. The fluids may have been initially derived
either from devolatilization of metamorphic rocks or crystallization of igneous rocks in the adjacent Abitibi subprovince.
Vein quartz contains CO2-rich fluid inclusions (final melting T = −57.0 to −58.7 °C) that range in size from 5 to 17 μm. Measured homogenization temperatures (T h) range from −44.0 to 14.5 °C, however for most inclusions (46 of S1), T h = −44.0 to −21.1 °C (ρCO2 ≈ 1.13 to 1.05 g/cm3). At 400 to 600 °C, these densities correspond to pressures of 3.5 to 7 kbar, which is the best estimate of pressures of
vein formation. It has been argued that some high density CO2-rich fluid inclusions found in the KSZ were formed during peak metamorphism and thus document the presence of a CO2-rich fluid during peak granulite facies metamorphism (Rudnick et al. 1984). The association of high density CO2-rich fluid inclusions with clearly retrograde veins documents the formation of similar composition and density inclusions
after the peak of metamorphism. Thus, the coincidence of entrapment pressures calculated from fluid inclusion density measurements
with peak metamorphic pressures alone should not be considered strong evidence for peak metamorphic inclusion entrapment.
All fluid inclusion results are consistent with an initially semi-isobaric retrograde P–T path.
Received: 2 April 1996 / Accepted: 15 November 1996 相似文献
15.
Water-saturated and water-undersaturated experiments (a
H2
O = 1.0 and 0.5) were performed in the temperature range 780–1040°C at 2 and 5 kbar in order to determine the upper thermal
stability of phlogopite in granitic melts. Starting compositions were: (A) subaluminous mixtures of 20 wt % synthetic phlogopite
and 80 wt % synthetic anhydrous haplogranitic glass; (B) peraluminous mixtures (normative corundum = 4 %) of 20 wt % synthetic
phlogopite and 80 wt % synthetic anhydrous peraluminous haplogranitic glass. The molar quartz: albite: orthoclase ratio of
the glasses of the 2␣kbar runs was 35:39:26 and that of the 5 kbar runs 30:42:28. In the subaluminous system, phlogopite is
stable up to 820°C at a
H2
O = 1.0 and up to 780°C at a
H2
O = 0.5. At higher temperatures, it is replaced by enstatite. In the peraluminous system phlogopite has a remarkably higher
thermal stability (up to 1000°C at 5 kbar and a
H2
O = 1.0) and there is a temperature interval of 80°C at a
H2
O = 1.0, and 90–100°C at a
H2
O = 0.5 between the first appearance of enstatite and the disappearance of phlogopite. In the peraluminous system, phlogopite
is a solid solution (ss) of phlogopite, muscovite, talc and eastonite components. The crystalline product of the phlogopitess breakdown reaction is an aluminous enstatite. The MgO-content of the melt depends on the normative corundum content of the
starting material and the run temperature. It is independent of pressure. In the subaluminous system, the MgO-content ranges
between 0.05 and 0.3 wt % in the temperature interval 780–880°C at both investigated water activities. The MgO-content of
the peraluminous melts at a
H2
O = 1.0 ranges between 0.4 and 1.7 wt % and at a
H2
O = 0.5 between 0.2 and 1.4 wt % in the temperature range 780–980°C.
Received: 28 August 1995 / Accepted: 6 August 1996 相似文献
16.
Calcium self-diffusion in diopside at high temperature: implications for transport properties 总被引:1,自引:0,他引:1
We have investigated 44Ca self-diffusion in natural diopside single crystals (containing ∼2 atomic % Fe) at temperatures up to 1320 °C (i.e. 30 °C
below the nominal melting point). Oxygen fugacity was controlled by gaseous mixtures. Diffusion profiles ranging from ∼50
to 500 nm were analysed by Rutherford Back-Scattering Spectrometry (RBS). The present results are complementary to previous
studies, and show that in both synthetic (Fe-poor) and natural (Fe-rich) diopside, there are two different diffusion regimes
for Ca with a transition at ∼1230±15 °C. Below this temperature diffusion is characterised by an activation enthalpy of ∼284±10
kJ/mol, while at higher temperatures it increases up to ∼1006±75 kJ/mol. These regimes are proposed to be respectively extrinsic
and intrinsic. For the intrinsic regime Ca self-diffusion may involve Ca-Frenkel point defects. These are pairs of a vacancy
on a M2 site and a calcium cation on an interstitial (normally unoccupied) site. The concentration of such point defects depends
only on temperature, and it is especially important at very high temperatures. The activation enthalpy for intrinsic diffusion
may represent the half defect formation enthalpy plus the migration enthalpy for movement through interstitial sites. For
the extrinsic regime we propose Ca self-diffusion to involve extrinsic interstitial point defects with concentration proportional
to ()–0.19±0.03. We suggest that for both regimes, Ca diffusion involves the well known M3 sites in the octahedral layers, as well as sites in the tetrahedral layers, that we call M4. These sites are especially convenient to explain the observed isotropic diffusion. Increasing concentration of Ca-Frenkel
point defects may be related to the onset of premelting, which affects the thermodynamic properties of Fe-“free” diopside
above 1250 °C. In the light of the present results, premelting is also expected to occur in natural Fe-bearing diopside and
it could strongly influence its thermodynamic and transport properties. Subsequently, in deep upper mantle conditions (T≈1250 °C–1300 °C)
where premelting could occur, diffusional cation exchanges with surrounding phases and diffusion controlled creep might be
facilitated. Finally, our diffusion data support a previous suggestion that electrical conductivity may be electronic rather
than ionic.
Received: 17 December 1997 / Revised, accepted: 17 April 1998 相似文献
17.
Liping Wang Eric J. Essene Youxue Zhang 《Contributions to Mineralogy and Petrology》1999,135(2-3):164-178
Mineral inclusions in pyrope crystals from Garnet Ridge in the Navajo Volcanic Field on the Colorado Plateau are investigated
in this study with emphasis on the oxide minerals. Each pyrope crystal is roughly uniform in composition except for diffusion
halos surrounding some inclusions. The pyrope crystals have near constant Ca:Fe:Mg ratios, 0.3 to 5.7 wt% Cr2O3, and 20 to 220 ppm H2O. Thermobarometric calculations show that pyrope crystals with different Cr contents formed at different depths ranging from
50 km (where T ≈ 600 °C and P = 15 kbar) to 95 km (where T ≈ 800 °C and P = 30 kbar) along the local geotherm. In addition to previously reported inclusions of rutile, spinel and ilmenite, we discovered
crichtonite series minerals (AM21O38, where A = Sr, Ca, Ba and LREE, and M mainly includes Ti, Cr, Fe and Zr), srilankite (ZrTi2O6), and a new oxide mineral, carmichaelite (MO2−x(OH)x, where M = Ti, Cr, Fe, Al and Mg). Relatively large rutile inclusions contain a significant Nb (up to 2.7 wt% Nb2O5), Cr (up to ∼6 wt% Cr2O3), and OH (up to ∼0.9 wt% H2O). The Cr and OH contents of rutile inclusions are positively related to those of pyrope hosts, respectively. Needle- and
blade-like oxide inclusions are commonly preferentially oriented. Composite inclusions consisting mainly of carbonate, amphibole,
phlogopite, chlorapatite, spinel and rutile are interpreted to have crystallized from trapped fluid/melt. These minerals in
composite inclusions commonly occur at the boundaries between garnet host and large silicate inclusions of peridotitic origin,
such as olivine, enstatite and diopside. The Ti-rich oxide minerals may constitute a potential repository for high field strength
elements (HFSE), large ion lithophile elements and light rare earth elements (LREE) in the upper mantle. The composite and
exotic oxide inclusions strongly suggest an episode of metasomatism in the depleted upper mantle beneath the Colorado Plateau,
contemporaneous with the formation of pyrope crystals. Our observations show that mantle metasomatism may deplete HFSE in
metasomatic fluids/melts. Such fluids/melts may subsequently contribute substantial trace elements to island arc basalts,
providing a possible mechanism for HFSE depletion in these rocks.
Received: 20 December 1997 / Accepted: 15 October 1998 相似文献
18.
To interpret the degassing of F-bearing felsic magmas, the solubilities of H2O, NaCl, and KCl in topaz rhyolite liquids have been investigated experimentally at 2000, 500, and ≈1 bar and 700° to 975 °C.
Chloride solubility in these liquids increases with decreasing H2O activity, increasing pressure, increasing F content of the liquid from 0.2 to 1.2 wt% F, and increasing the molar ratio
of ((Al + Na + Ca + Mg)/Si). Small quantities of Cl− exert a strong influence on the exsolution of magmatic volatile phases (MVPs) from F-bearing topaz rhyolite melts at shallow
crustal pressures. Water- and chloride-bearing volatile phases, such as vapor, brine, or fluid, exsolve from F-enriched silicate
liquids containing as little as 1 wt% H2O and 0.2 to 0.6 wt% Cl at 2000 bar compared with 5 to 6 wt% H2O required for volatile phase exsolution in chloride-free liquids. The maximum solubility of Cl− in H2O-poor silicate liquids at 500 and 2000 bar is not related to the maximum solubility of H2O in chloride-poor liquids by simple linear and negative relationships; there are strong positive deviations from ideality
in the activities of each volatile in both the silicate liquid and the MVP(s). Plots of H2O versus Cl− in rhyolite liquids, for experiments conducted at 500 bar and 910°–930 °C, show a distinct 90° break-in-slope pattern that
is indicative of coexisting vapor and brine under closed-system conditions. The presence of two MVPs buffers the H2O and Cl− concentrations of the silicate liquids. Comparison of these experimentally-determined volatile solubilities with the pre-eruptive
H2O and Cl− concentrations of five North American topaz and tin rhyolite melts, determined from melt inclusion compositions, provides
evidence for the exsolution of MVPs from felsic magmas. One of these, the Cerro el Lobo magma, appears to have exsolved alkali
chloride-bearing vapor plus brine or a single supercritical fluid phase prior to entrapment of the melt inclusions and prior
to eruption.
Received: 6 November 1995 / Accepted: 29 January 1998 相似文献
19.
The Janggun iron deposits, Republic of␣Korea, occur as lens-shaped magnesian skarn, magnetite and base-metal sulfide orebodies
developed in the Cambrian Janggun Limestone Formation. Mineralization stage of the deposits can be divided into two separate
events. The skarn stage (107 Ma) consists of magnetite, pyrrhotite, base-metal sulfides, carbonates and magnesian skarn minerals.
The hydrothermal stage (70 Ma) consists of base-metal sulfides, native bismuth, bismuthinite, tetrahedrite, boulangerite,
bournonite and stannite. Mineral assemblages, chemical compositions and thermodynamic considerations indicate that formation
temperatures, −log fs2 and −log fo2 values of ore fluids from the skarn stage were 433 to 345 °C, 8.1 to 9.7 bar and 29.4 to 31.6 bar, and the hydrothermal stage
was 245 to 315 °C, 10.4 to 13.2 bar and 33.6 to 35.4 bar, respectively. Thermochemical considerations indicate that the XCO2 during magnesian skarnization ranged from 0.06 to 0.09, and the activity of H+ presumably decreased when the fluids equilibrated with host dolomitic limestone which resulted in a pH change from about
6.1 to 7.8, and decreases in fo2 and fs2. The δ34S values of ore sulfides have a wide range from 3.2 to 11.6 ‰ (CDT). Calculated 34SH2
S values of ore fluids are 2.9 to 5.4 ‰ (skarn stage) and 8.7 to 13.5 ‰ (hydrothermal stage). These are interpreted to represent
an initial deep-seated, igneous source of sulfur which gave way to influence of oxidized sedimentary sulfur to hydrothermal
stage. The δ13C values of carbonates in ores range from −4.6 to −2.5 ‰ (PDB). It is likely that carbon in the ore fluids was a mixture of
deep-seated magmatic carbon and dissolved carbon of dolomitic limestone. The δ18OH2
O and δD values (SMOW) of water in the ore fluids were 14.7 to 1.8 and −85 to −73 ‰ during the skarn stage and 11.1 to −0.2
and −87 to −80 ‰ in the hydrothermal stage.
Received: 5 March 1997 / Accepted: 28 August 1997 相似文献
20.
H. A. Gilg A. Lima R. Somma H. E. Belkin B. De Vivo R. A. Ayuso 《Mineralogy and Petrology》2001,73(1-3):145-176
Summary We present new mineral chemistry, fluid inclusion, stable carbon and oxygen, as well as Pb, Sr, and Nd isotope data of Ca-Mg-silicate-rich
ejecta (skarns) and associated cognate and xenolithic nodules from the Mt. Somma-Vesuvius volcanic complex, Italy. The typically
zoned skarn ejecta consist mainly of diopsidic and hedenbergitic, sometimes “fassaitic” clinopyroxene, Mg-rich and Ti-poor
phlogopite, F-bearing vesuvianite, wollastonite, gehlenite, meionite, forsterite, clinohumite, anorthite and Mg-poor calcite
with accessory apatite, spinell, magnetite, perovskite, baddeleyite, and various REE-, U-, Th-, Zr- and Ti-rich minerals.
Four major types of fluid inclusions were observed in wollastonite, vesuvianite, gehlenite, clinopyroxene and calcite: a)
primary silicate melt inclusions (THOM = 1000–1050 °C), b) CO2 ± H2S-rich fluid inclusions (THOM = 20–31.3 °C into the vapor phase), c) multiphase aqueous brine inclusions (THOM = 720–820 °C) with mainly sylvite and halite daughter minerals, and d) complex chloride-carbonate-sulfate-fluoride-silicate-bearing
saline-melt inclusions (THOM = 870–890 °C). The last inclusion type shows evidence for immiscibility between several fluids (silicate melt – aqueous chloride-rich
liquid – carbonate/sulfate melt?) during heating and cooling below 870 °C. There is no evidence for fluid circulation below
700 °C and participation of externally derived meteoric fluids in skarn formation. Skarns have considerably variable 206Pb/204Pb (19.047–19.202), 207Pb/204Pb (15.655–15.670), and 208Pb/204Pb (38.915–39.069) and relatively low 143Nd/144Nd (0.51211–0.51244) ratios. The carbon and oxygen isotope compositions of skarn calcites (δ13CV-PDB = −5.4 to −1.1‰; δ18OV-SMOW = 11.7 to 16.4‰) indicate formation from a 18O- and 13C-enriched fluid. The isotope composition of skarns and the presence of silicate melt inclusion-bearing wollastonite nodules
suggests assimilation of carbonate wall rocks by the alkaline magma at moderate depths (< 5 km) and consequent exsolution
of CO2-rich vapor and complex saline melts from the contaminated magma that reacted with the carbonate rocks to form skarns.
Received March 1, 2000; revised version accepted November 2, 2000 相似文献