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1.
Samples of enstatite and forsterite were crystallized in the presence of a hydrous fluid at 15 kbar and 1100 °C. Water contents
in quenched samples were measured by 1H MAS NMR and by FTIR. If the samples were prepared in the same way, similar water concentrations were obtained by both methods.
There is no evidence that one or the other method would severely over or underestimate water contents in nominally anhydrous
minerals. However, measured water contents vary by orders of magnitude depending on sample preparation. The lowest water contents
are measured by polarized FTIR spectroscopy on clear, inclusion-free single crystals. These water contents probably reflect
the real point defect solubility in the crystals. Polycrystalline material shows much higher total water concentrations, presumably
due to hydrous species on grain boundaries, growth defects, and in submicroscopic fluid inclusions. Grinding the sample in
air further increases water concentration. This effect is even more pronounced if the sample is ground in water and subsequently
dried at 150 °C. Polarized FTIR measurements on clear single crystals of enstatite saturated at 15 kbar and 1100 °C give 199 ± 25 ppm
by weight of water. The spectra show sharp and strongly polarized bands. These bands are also present in spectra measured
through turbid, polycrystalline aggregates of enstatite. However, in these spectra, they are superimposed on much broader,
nearly isotropic bands resulting from hydrous species in grain boundaries, growth defects, and submicroscopic fluid or melt
inclusions. Total water contents for these polycrystalline aggregates are between 2000 and 4000 ppm. Water contents measured
by FTIR on enstatite powders are 5300 ppm after grinding in air and 12 600 ppm after grinding under water und subsequent drying
at 150 °C.
Received: 25 June 1999 / Revised, accepted: 4 October 1999 相似文献
2.
K. Kuroda T. Irifune T. Inoue N. Nishiyama M. Miyashita K. Funakoshi W. Utsumi 《Physics and Chemistry of Minerals》2000,27(8):523-532
Determination of the phase boundary between ilmenite and perovskite structures in MgSiO3 has been made at pressures between 18 and 24 GPa and temperatures up to 2000 °C by in situ X-ray diffraction measurements
using synchrotron radiation and quench experiments. It was difficult to precisely define the phase boundary by the present
in situ X-ray observations, because the grain growth of ilmenite hindered the estimation of relative abundances of these phases.
Moreover, the slow reaction kinetics between these two phases made it difficult to determine the phase boundary by changing
pressure and temperature conditions during in situ X-ray diffraction measurements. Nevertheless, the phase boundary was well
constrained by quench method with a pressure calibration based on the spinel-postspinel boundary of Mg2SiO4 determined by in situ X-ray experiments. This yielded the ilmenite-perovskite phase boundary of P (GPa) = 25.0 (±0.2) – 0.003
T (°C) for a temperature range of 1200–1800 °C, which is generally consistent with the results of the present in situ X-ray
diffraction measurements within the uncertainty of ∼±0.5 GPa. The phase boundary thus determined between ilmenite and perovskite
phases in MgSiO3 is slightly (∼0.5 GPa) lower than that of the spinel-postspinel transformation in Mg2SiO4.
Received: 19 May 1999 / Accepted: 21 March 2000 相似文献
3.
Summary Supra-solidus phase relations at temperatures and pressures ranging from 800 to 1700 °C and 2 to 6.4 GPa have been determined
experimentally for three silica-rich lamproites: hyalo-leucite phlogopite lamproite (Oscar, West Kimberley); sanidine richterite
lamproite (Cancarix, Murcia-Almeria); and phlogopite transitional madupitic lamproite (Middle Table Mountain, Wyoming). All
samples have extended melting intervals (500–600 °C). Bulk composition has a significant control on the nature of the initial
liquidus phases, with orthopyroxene occurring at low pressures (<4 GPa) in the relatively calcium-poor Oscar and Cancarix
lamproites. At higher pressure (>6 GPa) orthopyroxene is replaced by garnet plus clinopyroxene as near-liquidus phases in
the Oscar lamproite and by orthopyroxene plus clinopyroxene in the Cancarix sample. Clinopyroxene is a near-liquidus phase
at all pressures in the Middle Table Mountain lamproite. Near-solidus phase assemblages at high pressure (>5 GPa) are: clinopyroxene + phlogopite + coesite + rutile + garnet
(Oscar); clinopyroxene + garnet + coesite + K–Ti-silicate (Cancarix); clinopyroxene + phlogopite + apatite + K–Ti-silicate
(Middle Table Mountain). In all compositions olivine is never found as a liquidus phase at any of the temperatures or pressures
studied here. The phase relationships are interpreted to suggest that silica-rich lamproites cannot be derived by the partial
melting of lherzolitic sources. Their genesis is considered to involve high degrees of partial melting of ancient metasomatic
veins within a harzburgitic-lherzolitic lithospheric substrate mantle. The veins are considered in their mineralogy to be
similar to the experimentally-observed, high pressure, near-solidus phase assemblages. The composition of silica-rich primary
lamproite magmas differs between cratons as a consequence of differing mineralogical modes of the source veins and different
relative contributions from the veins and wall-rocks to the partial melts.
Received February 21, 2000; revised version accepted July 3, 2001 相似文献
4.
High-temperature creep behavior in Ni2GeO4 spinel was investigated using synthetic polycrystalline aggregates with average grain sizes ranging from submicron to 7.4
microns. Cylindrical samples were deformed at constant load in a gas-medium apparatus at temperatures ranging from 1223 to
1523 K and stresses ranging from 40 to 320 MPa. Two deformation mechanisms were identified, characterized by the following
flow laws:
where σ is in MPa, d is in μm and T is in Kelvin. These flow laws suggest that deformation was accommodated by dislocation creep and grain-boundary diffusion
(Coble) creep, respectively. A comparison with other spinels shows that an isomechanical group can be defined for spinels
although some differences between normal and inverse spinels can be identified. When creep data for olivine and spinel are
normalized and extrapolated to Earth-like conditions, spinel (ringwoodite) has a strength similar to olivine in the dislocation
creep regime and is considerably stronger than olivine in the diffusion creep regime at coarse grain size. However, when grain-size
reduction occurs, spinel can become weaker than olivine due to its high grain-size sensitivity (Coble creep behavior). Analysis
of normalized diffusion creep data for olivine and spinel indicate that spinel is weaker than olivine at grain sizes less
than 2 μm.
Received: 18 June 2000 / Accepted: 3 April 2001 相似文献
5.
We present Raman and infrared spectra of gypsum to 21 GPa at 300 K. Our measurements encompass the internal modes of the
(SO4)−4 group that lie between 400 and 1150 cm−1, hydroxyl-stretching vibrations between 3200 and 3600 cm−1, and a libration and bending vibrations of the molecular H2O group. All vibrations of the sulfate group have positive pressure shifts, while the hydroxyl-stretching and -bending vibrations
have a mixture of positive and negative pressure shifts: the effect of pressure on the hydrogen bonding of the water molecule
thus appears to be complex. Near 5 GPa, the two infrared-active bending vibrations of the water molecule coalesce, and the
morphology of the hydroxyl-stretching region of the spectrum shifts dramatically. This behavior is consistent with a pressure-induced
phase transition in gypsum in the vicinity of 5–6 GPa, which is observed to be reversible on decompression to zero pressure.
The spectral observations are consistent with the onset of increased disorder in the position of the water molecule in gypsum:
the sulfate vibrations are largely unaffected by this transition. The Raman-active symmetric stretch of the sulfate group
undergoes an apparent splitting near 4 GPa, which is interpreted to be produced by Fermi resonance with an overtone of the
symmetric bending vibration. The average mode Grüneisen parameter of the 20 vibrational modes we sample is less than 0.05,
in contrast to the bulk thermal Grüneisen parameter of 1.20. Accordingly, the vibrations of both water and sulfate units within
gypsum are highly insensitive to volumetric compaction. Therefore, in spite of the changes in the bonding of the water unit
near 5 GPa, metastably compressed gypsum maintains strongly bound molecular-like units to over 20 GPa at 300 K.
Received: 31 July 2000 / Accepted: 5 April 2001 相似文献
6.
Radon in groundwater as a tracer to assess flow velocities: two test cases from Israel 总被引:1,自引:1,他引:1
U. Kafri 《Environmental Geology》2001,40(3):392-398
Two test cases from Israel are presented herein employing the decay rate of radon along the flow path to assess groundwater
flow velocities. Groundwater flow reaching the fault zone emerges in several places along the rift fault zone as thermal springs
because of deep water confinement. The high water temperature of the surface is indicative of high vertical flow velocities,
which maintains the original high temperatures. Knowing the Rn content at a source point and at a given down-gradient, and
assuming no Rn addition from the water itself or along the flow path, one can calculate the flow velocity based on the Rn
half-life time. The decay of Rn in western Galilee was found to be ∼570–150 pCi/l, and in the Dead Sea area from 5000–2000 pCi/l
along a respective flow path of 1000 and 200 m, Based on the above, the calculated flow velocities were compared with those
obtained from pumping tests in the study area. The method is applicable, because of the short Rn half-life, to cases of high
Rn contents, short distances and high flow velocities.
Received: 18 January 2000 · Accepted: 21 March 2000 相似文献
7.
Summary From the early Tertiary K?lvegletscher ultramafic cumulate complex, emplaced into the Archaean basement on the west side
of the Kangerlussuaq Fjord, East Greenland, we present geochemical and isotopic data from an outcrop of finely layered dunitic
adcumulates. The dunites were deposited in a trough structure, interpreted to be a fossilized feeder channel to the K?lvegletscher
magma chamber. Detailed geochemical sampling of the trough reveals subtle cryptic compositional variations of olivines (Fo = 86.5–89.3;
Ni = 2000–2700 ppm) and chromites (Cr# = 66–80) in a stratigraphical profile perpendicular to the layering as well as relatively
large simultaneous variations of whole-rock 87Sr/86Sr ratios. The dunites are separable into sequences of normal and reverse cryptic zonations which are interpreted as resulting
from fractional crystallisation and magma chamber recharge, respectively. Up to 20% of magma chamber replenishment by high-Mg
melts is suggested. Sr-isotope compositions of the cumulates correlate with olivine compositions and suggest mixing of fractionated
and unfractionated parental melts assimilating up to 8% local basement. We propose the existence of a vigorous volcanic system
at the K?lvegletscher site in early Tertiary times, where plume-derived magmas incorporated minor amounts of local basement
and underwent fractionation and mixing in crustal chambers. Estimates of processed magma volumes during deposition of the
trough cumulates are in the range of 100–130 km3.
Received January 18, 2000;/revised version accepted December 19, 2000 相似文献
8.
Sedimentary rock-hosted gold mineralization at Zalaa Uul, Khentii Range, northeastern Mongolia 总被引:2,自引:0,他引:2
The recently discovered Zalaa Uul occurrence exhibits gold concentrations averaging about 1 ppm in silicified breccias as
wide as 100 m. Most mineralization is hosted in brecciated siltstone, shale, and calcareous sandstone of the Permian Ulz Formation
that exhibits multiple stages of silicification. Rock geochemistry indicates: (1) gold is strongly associated with arsenic
and silver; (2) antimony, tellurium and thallium are locally anomalous but poorly correlated with gold; (3) mercury is spatially
correlated with copper; and (4) Ag:Au ratios are low (≤3). A low-level Cu–(Hg + Sb, ±Au + As) anomaly occurs over an hypothesized
feeder breccia. The feeder breccia occupies a major northwest-dipping reverse fault zone between dominantly greenschist-facies
phyllite and schist of the Upper Proterozoic Toshint Formation and unmetamorphosed marine clastic rocks of the permian Ulz
Formation. Ground magnetometer surveys identified a magnetic body, thought to represent part of an intrusive complex at depth,
within the reverse fault zone, down-plunge from the ∼70° northwest-dipping feeder breccia. Altered rhyolite dikes crop out
in the vicinity of the feeder breccia. The potentially economic gold grades are 2 to 3 km outboard of the feeder breccia and
may represent the distal Au + As zone of an intrusion-related mineralizing system. Alteration, regional structural and geophysical
setting, host rocks and trace element geochemistry, and finely disseminated nature of gold particles are similar to Carlin-type
gold systems in the Great Basin of the western USA, but local geology, magnetically mapped intrusive bodies, and trace element
zonation suggest affinity with some intrusion-related gold systems.
Received: 28 February 1999 / Accepted: 3 March 2000 相似文献
9.
Summary
Melting of a spinel lherzolite with a spinel clinopyroxenite layer was investigated experimentally from 3.5 to 20 kbar and
from 1200 to 1450 °C. The melt fraction in the spinel pyroxenite layer increases rapidly, and clinopyroxene disappears leaving
olivine-spinel residua according to the reaction Cpx + Sp = Ol + Liq. The melt in the pyroxenite layer reacts with the surrounding
lherzolite resulting in the formation of an essentially monomineral (olivine) zone with interstitial melt near the former
pyroxenite. Melt compositions in the central melt pool are similar to those produced by other authors in melting experiments
with peridotites similar to the bulk compositions of our samples. It is suggested that similar small-scale mantle heterogeneities
(i.e. thin pyroxenite layers in lherzolite) may exert significant influence on mantle rheology and melt segregation, whereas
melt compositions are not strongly affected and controlled by the dominating lherzolite lithology.
Received October 10, 2000; revised version accepted August 31, 2001 相似文献
10.
A thermoelastic model for calculating the high-pressure and high-temperature properties of isotropic solids is presented
by extending the formalism by Thomsen and combining the resulting one with the Vinet model for static lattice and the Debye
model for lattice vibration. Applying it to polycrystalline corundum, we have shown that the calculated values of entropy
and heat capacity at constant pressure are in agreement with literature values to 2325 K at zero pressure and that the calculated
values of thermal expansivity agree reasonably with experimental data to 1100 K at zero pressure. The model reproduces experimental
data of sound velocities v
p
and v
s
of compressional and shear waves to 1825 K at zero pressure and those to 62 GPa at room temperature, and it reproduces also
experimental shock-wave equation of state to 150 GPa. The velocity correlation (∂ln v
s
/∂ln v
p
)
S
was found to have weak pressure and temperature dependences and the results under lower mantle conditions are compared with
those of magnesian and calcium silicate perovskites and magnesiowüstite, and the PREM values of the Earth's lower mantle.
Received: 12 February 2000 / Accepted: 15 July 2000 相似文献
11.
High-salinity, Na–Ca–Cl-rich fluids (˜20 wt% salts) in inclusions in gangue and ore minerals from Mesozoic Mississippi Valley-type
(MVT) deposits in the Verviers Synclinorium (eastern Belgium) and in Cretaceous vein calcites at the Variscan front were investigated
by microthermometric and crush-leach analysis. The MVT deposits formed at temperatures of ˜110 °C while the Cretaceous vein
calcites were precipitated at temperatures <50 °C. Their Cl–Br content (Cl/Br ratio between 246 and 458) suggests that the
fluids probably originated by the evaporation of seawater during basin development at the southern margin of the Caledonian
Brabant Massif in the Late Palaeozoic. The Na–Ca–K content (Na: 29,700–49,600 ppm, Ca: 25,700–46,200 ppm, K: 1,000–5,620 ppm)
is similar to that of the mineralising fluids in other Pb–Zn districts, interpreted to be of evaporative origin (e.g. Newfoundland,
East Tennessee, Polaris). Furthermore, comparison of the Na–Ca–K content of the fluids with that of an evolved evaporitic
brine enables the recognition of major water–rock interactions that modified the fluid composition. It indicates that the
ambient fluids participated in the early diagenetic dolomitisation of Upper Palaeozoic carbonates and also in the albitisation
of plagioclase in Lower Palaeozoic siliciclastics of the Caledonian basement. Illitisation of smectites or dissolution of
K-feldspar probably controlled the K-content of the fluids. A model is proposed where the bittern brines migrated down into
the deep subsurface because of their density during extension. After the Variscan orogeny, these fluids were finally expelled
along extensional faults, resulting in the formation of Zn–Pb deposits.
Received: 26 April 2000 / Accepted: 22 November 2000 相似文献
12.
Equilibrium volumes and expansivities of three liquids in the system anorthite (CaAl2Si2O8)–diopside (CaMgSi2O6) have been derived from dilatometric measurements of the equilibrium length of samples in the glass transition range. The
typical temperature range of 40 K for the measurements is limited at low temperature by the very long times necessary to reach
structural equilibrium and at high temperature by the penetration of the rod used to measure sample dilatation. Despite such
narrow intervals, the expansivities are determined to better than 3% thanks to the high precision with which length changes
are measured. The coefficient of volume thermal expansion (1/V dV/dT) of the fully relaxed liquid just above the glass transition
is found to decrease linearly from diopside composition (139 ± 4 × 10−6 K−1) to anorthite composition (59 ± 2 × 10−6 K−1). These values are greater than those determined for the same liquids at superliquidus temperatures, demonstrating that expansivities
of silicate melts may decrease markedly with increasing temperature. A predictive model based upon partial molar volumes which
vary as a linear function of the logarithm of temperature is proposed.
Received: 25 February 2000 / Accepted: 29 May 2000 相似文献
13.
The effects of variable bulk composition on the melting systematics of fertile peridotitic assemblages 总被引:2,自引:1,他引:1
Experiments were conducted at 1 GPa on four starting materials to investigate the effects of variable mineral proportions
on the melting systematics of compositionally fertile peridotitic assemblages. Starting materials were constructed by recombining
Kilbourne Hole xenolith mineral separates by weight into four mixtures with mineral proportions olivine (Ol): orthopyroxene
(Opx): clinopyroxene (Cpx): spinel (Sp) of 0.50:0.07:0.40:0.03 (FER-B), 0.50:0.46:0.01:0.03 (FER-C), 0.50:0.30:0.10:0.10 (FER-D),
and 0.50:0.235:0.235: 0.03 (FER-E). Experiments were performed on a 1.27-cm (0.5 in.) piston-cylinder apparatus over the temperature
interval 1270–1390 °C, using a variation of the diamond aggregate melt extraction technique employing vitreous carbon spheres
in place of diamonds as the melt extraction layer. The solidus temperatures are similar for all the starting materials, with
an average value of 1250 °C. In FER-D and -E, the near-solidus melting reaction for a lherzolite assemblage was determined
to be of the form Cpx + Opx + Sp → melt + Ol. A subsequent reaction of the form Opx + Sp → melt + Ol was determined for FER-D
after the exhaustion of Cpx. Over the entire temperature interval investigated for FER-B and -C, reactions were determined
to be of the form Cpx + Sp → melt + Ol and Opx + Sp → melt + Ol, respectively. Melt percent (F) vs temperature (T) curves
are concave up for all starting materials, demonstrating that isobaric melt productivity increases with progressive batch
melting. At any given melt fraction, (dF/dT)P increases with increasing amount of Cpx in the starting material, indicating that the modal proportion of Cpx is one of the
primary controls on isobaric melt productivity of upwelling peridotite. The concave up melt productivity functions for peridotitic
assemblages determined in this study suggest that assuming linear or concave down melt productivity functions for modeling
mantle melting may not be appropriate.
Received: 2 August 1999 / Accepted: 7 June 2000 相似文献
14.
Zircon and monazite response to prograde metamorphism in the Reynolds Range, central Australia 总被引:30,自引:1,他引:30
Daniela Rubatto Ian S. Williams Ian S. Buick 《Contributions to Mineralogy and Petrology》2001,140(4):458-468
We report an extensive field-based study of zircon and monazite in the metamorphic sequence of the Reynolds Range (central
Australia), where greenschist- to granulite-facies metamorphism is recorded over a continuous crustal section. Detailed cathodoluminescence
and back-scattered electron imaging, supported by SHRIMP U–Pb dating, has revealed the different behaviours of zircon and
monazite during metamorphism. Monazite first recorded regional metamorphic ages (1576 ± 5 Ma), at amphibolite-facies grade,
at ∼600 °C. Abundant monazite yielding similar ages (1557 ± 2 to 1585 ± 3 Ma) is found at granulite-facies conditions in both
partial melt segregations and restites. New zircon growth occurred between 1562 ± 4 and 1587 ± 4 Ma, but, in contrast to monazite,
is only recorded in granulite-facies rocks where melt was present (≥700 °C). New zircon appears to form at the expense of
pre-existing detrital and inherited cores, which are partly resorbed. The amount of metamorphic growth in both accessory minerals
increases with temperature and metamorphic grade. However, new zircon growth is influenced by rock composition and driven
by partial melting, factors that appear to have little effect on the formation of metamorphic monazite. The growth of these
accessory phases in response to metamorphism extends over the 30 Ma period of melt crystallisation (1557–1587 Ma) in a stable
high geothermal regime. Rare earth element patterns of zircon overgrowths in leucosome and restite indicate that, during the
protracted metamorphism, melt-restite equilibrium was reached. Even in the extreme conditions of long-lasting high temperature
(750–800 °C) metamorphism, Pb inheritance is widely preserved in the detrital zircon cores. A trace of inheritance is found
in monazite, indicating that the closure temperature of the U–Pb system in relatively large monazite crystals can exceed 750–800 °C.
Received: 7 April 2000 / Accepted: 12 August 2000 相似文献
15.
Using a combination of dielectric spectroscopy and atomistic computer simulation techniques, the dynamical behaviour of the
loosely bound (Na+ and K+) channel ions in nepheline has been investigated. The low-frequency dielectric properties of a natural Bancroft nepheline
have been studied from room temperature to 1100 K. At each temperature, the dielectric constant, conductivity and dielectric
loss were determined over a range of frequencies from 100 Hz to 10 MHz. At high temperatures a distinct Debye-type relaxation
in the dielectric loss spectrum was observed; the activation energy for this process was determined to be 1.38 ± 0.02 eV.
Atomistic simulation techniques were used to elucidate the mechanism and energetics of cation migration. A mechanism involving
the hopping of Na+ ions between oval sites and partially occupied hexagonal (K+) sites, via a bottleneck consisting of a distorted sixfold ring of (Al,Si)O4 tetrahedra, was found to give a calculated energy barrier in very good agreement with the experimentally determined activation
energy. These results confirm the nature of the process responsible for the observed dielectric behaviour. Overall, this study
demonstrates the intrinsic, microscopic control of cation diffusion processes in rock-forming minerals. Identifying specific
energy barriers and preferred diffusion pathways is fundamental to the prediction of diffusion energetics.
Received: 8 May 2000 / Accepted: 21 July 2000 相似文献
16.
G. C. Capitani H. Leroux J. C. Doukhan S. Ríos M. Zhang E. K. H. Salje 《Physics and Chemistry of Minerals》2000,27(8):545-556
The nature of the amorphous regions and their recovery processes in two natural metamict zircon samples from Sri Lanka have
been studied by high resolution and analytical transmission electron microscopy. Samples untreated and annealed at different
temperatures were investigated. Nanoprobe analyses on untreated samples and samples annealed at 1000 K show that within experimental
uncertainties, no chemical segregation occurred. In samples annealed at higher temperatures (≥1100 K) recovery occurs in a
two-stage process and leads to different microstructures, which depended on the initial amount of metamictization. In highly
amorphized samples, recrystallization starts at 1200 K. Randomly oriented ZrO2 grains embedded in a silica-rich matrix are detected. At higher temperature (16 h at 1600 K), the assemblage transforms into
a polygonal texture of small zircon grains. Some untransformed zirconia grains and pockets of silica-rich glass are still
present, however. In partially metamict samples, recovery starts at 1100 K. The small surviving oriented zircon domains grow
at the expense of the surrounding amorphous material. At 1200 K, new zirconia grains nucleate with random orientations. After
1 h annealing at 1400 K, the zircon structure is restored and the microstructure coarse-grained. The proportion of crystalline
zirconia and silica-rich glass has dramatically decreased.
Received: 15 November 1999 / Accepted: 1 March 2000 相似文献
17.
The monoclinic titanite-like high-pressure form of calcium disilicate has been synthesized and quenched to ambient conditions
to form the triclinic low-pressure phase containing silicon in four-, five- and sixfold coordination. The enthalpy of formation
of the quench product has been measured by high-temperature oxide melt calorimetry. The value obtained from samples from a
series of several synthesis experiments is ΔH
f
= (−26.32 ± 4.27) kJ mol−1 for the formation from the component oxides, or ΔH
f
= (−2482.81 ± 4.59) kJ mol−1 for the formation from the elements. The result is identical within experimental error to available estimates, although the
previously predicted energy difference between the monoclinic and triclinic phases could not be verified.
Received: 16 February 2000 / Accepted: 14 July 2000 相似文献
18.
Alfons M. van den Kerkhof Geoffrey H. Grantham 《Contributions to Mineralogy and Petrology》1999,137(1-2):115-132
In the Port Edward area of southern Kwa-Zulu Natal, South Africa, charnockitic aureoles up to 10 m in width in the normally
garnetiferous Nicholson's Point Granite, are developed adjacent to intrusive contacts with the Port Edward Enderbite and anhydrous
pegmatitic veins. Mineralogical differences between the country rock and charnockitic aureole suggest that the dehydration
reaction Bt + Qtz → Opx + Kfs + H2O and the reaction of Grt + Qtz → Opx + Pl were responsible for the charnockitization. The compositions of fluid inclusions
show systematic variation with: (1) the Port Edward Enderbite being dominated by CO2 and N2 fluid inclusions; (2) the non-charnockitized granite by saline aqueous inclusions with 18–23 EqWt% NaCl; (3) the charnockitic
aureoles by low-salinity and pure water inclusions (<7 EqWt% NaCl); (4) the pegmatites by aqueous inclusions of various salinity
with minor CO2. As a result of the thermal event the homogenization temperatures of the inclusions in charnockite show a much larger range
(up to 390 °C) compared to the fluid inclusions in granite (mostly <250 °C). Contrary to fluid-controlled charnockitization
(brines, CO2) which may have taken place along shear zones away from the intrusive body, the present “proximal” charnockitized granite
formed directly at the contact with enderbite. The inclusions indicate contact metamorphism induced by the intrusion of “dry”
enderbitic magma into “wet” granite resulting in local dehydration. This was confirmed by cathodoluminescence microscopy showing
textures indicative for the local reduction of structural water in the charnockite quartz. Two-pyroxene thermometry on the
Port Edward Enderbite suggests intrusion at temperatures of ∼1000–1050 °C into country rock with temperature of <700 °C. The
temperature of aureole formation must have been between ∼700 °C (breakdown of pyrite to form pyrrhotite) and ∼1000 °C. Charnockitization
was probably controlled largely by heat related to anhydrous intrusions causing dehydration reactions and resulting in the
release and subsequent trapping of dehydration fluids. The salinity of the metamorphic fluid in the contact zones is supposed
to have been higher at an early stage of contact metamorphism, but it has lost its salt content by K-metasomatic reactions
and/or the preferential migration of the saline fluids out of the contact zones towards the enderbite. The low water activity
inhibited the localized melting of the granite. Mineral thermobarometry suggests that after charnockite aureole genesis, an
isobaric cooling path was followed during which reequilibration of most of the aqueous inclusions occurred.
Received: 8 November 1998 / Accepted: 21 June 1999 相似文献
19.
Christopher J. Yeats Ernst A. Kohler Neal J. McNaughton Luke J. Tkatchyk 《Mineralium Deposita》2001,36(2):125-136
The 7 million oz. Jundee–Nimary lode-gold deposit occurs in the northern portion of the Yandal greenstone belt in the northeastern
part of the Archean Yilgarn Craton of Western Australia. Gold mineralization at Jundee–Nimary is similar in structural style,
mineralogy, geochemistry and relative timing with respect to deformation and metamorphism, to other Western Australian Archean
greenstone-hosted gold deposits, but is unusual in the fact that mineralized structures are crosscut by structurally late
intermediate to felsic dykes. Within the Deakin South open cut, gold mineralization is hosted in brittle–ductile shear zones
primarily developed within the dacitic Mitchell Porphyry. The Moore Porphyry, a broad dyke of porphyritic granodiorite, intrudes
the Mitchell Porphyry, crosscutting and post-dating gold mineralization. Analytically indistinguishable SHRIMP U–Pb zircon
ages of 2678 ± 5 Ma for the Mitchell Porphyry and 2669 ± 7 Ma for the Moore Porphyry require that gold mineralization at Jundee–Nimary
occurred at ca. 2680–2660 Ma, approximately 40 million years earlier than the majority of published robust ages for gold mineralization
in the Yilgarn Craton, which mostly overlap at ca. 2640–2630 Ma. The close spatial and temporal relationship between gold
mineralization and felsic to intermediate magmatism at Jundee–Nimary also raises the possibility of a genetic link between
hydrothermal and igneous activity. However, additional work is required to establish a firm connection. Current research on
lode-gold mineralization in Archean, Paleozoic and Phanerozoic terranes suggests a model which postulates that these deposits
formed during transpressional to compressional deformation in accretionary and collisional orogens and that their formation
is intimately related to orogenic processes. Consequently, mineralization and regional metamorphism are expected to be diachronous,
as terranes are accreted and the front of orogenesis migrates. Consideration of the new data presented in this paper in conjunction
with previously published dates supports the hypothesis that gold mineralization, along with regional metamorphism, was generally
diachronous from northeast to southwest across the Yilgarn Craton, over a period of approximately 40 million years from ca.
2680–2660 Ma to ca. 2640–2630 Ma. This is directly analogous to the accepted model for the timing of orogenic lode-gold mineralization
in other provinces and therefore provides further support for a unified model for this style of deposit through geological
time.
Received: 17 March 2000 / Accepted: 8 September 2000 相似文献
20.
Summary
A suite of clinopyroxene and amphibole megacrysts and mafic–ultramafic xenoliths are present in ignimbritic rocks of trachybasaltic–andesitic
composition from the Sirwa volcanic district, Morocco. The stumpy prismatic and sometimes euhedral clinopyroxene megacrysts
are Ti–Al-rich diopsides with mg values in the range 0.82–0.87 and Ca/(Ca + Mg) ratios in the range 0.53–0.54. The prismatic, elongated amphibole megacrysts
are calcic kaersutites–kaersutites with a narrow mg range (0.66–0.68). The xenoliths are represented by gabbroic and pyroxenitic types. In the gabbroic xenoliths two distinct
textural types can be distinguished: medium-sized granular and banded. The granular type is characterized by the mineral assemblage
Pl + Amph + Spl + Ilm + Ap. The banded type is distinct for the absence of Ilm and the presence of Cpx and Opx and shows alternating
bands enriched in Pl and Amph, respectively. The megacrysts and, probably, the xenoliths are considered not cognate with the
present host rocks since the calculated liquids in equilibrium with clinopyroxene and amphibole megacrysts over a wide range
of physical conditions have different trace and rare earth element contents. The observed phase relations and thermobarometric
calculations indicate that the megacrysts and xenoliths crystallized from their parent melts at P ≥ 10 kbar and T ≤ 1160 °C, i.e., in the upper mantle or near the crust-mantle boundary. A deep ( ≥ 30 km) magmatic chamber, where the megacrysts
and xenoliths originated, and a shallow volcanic chamber, energetically activated up to explosive conditions by injection
of deep-originated melts, is suggested to explain the occurrence of high-pressure megacrysts and xenoliths in the Sirwa volcanic
explosive products.
Received October 8, 2000; revised version accepted September 9, 2001 相似文献