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1.
The role of the grain boundary at chemical and isotopic fronts in marble during contact metamorphism
Hideki Wada Takamaru Ando Masayuki Suzuki 《Contributions to Mineralogy and Petrology》1998,132(4):309-320
Carbon and oxygen isotopic profiles around a low pressure metasomatic wollastonite reaction front in a marble of the Hida
metamorphic terrain, central Japan, display typical metamorphic fluid-enhanced isotopic zonations. Isotopic profiles obtained
from detailed microscale analyses perpendicular to the chemical reaction front in calcite marble show that diffusion-enhanced
isotopic exchange may control these profiles. Carbon and oxygen isotopic behaviour in grain boundaries is remarkably different.
Oxygen isotopic troughs (18O depleted rims) around the calcite-grain boundaries are widely observed in this contact aureole, demonstrating that diffusion
of oxygen in calcite grain boundary dominates over lattice diffusion in calcite. In contrast, no difference is observed in
carbon isotopic profiles obtained from grain cores and rims. There is thus no specific role of the grain boundary for diffusion
of carbonic species in the metamorphic fluid during transportation. Carbon chemical species such as CO2 and CO3 ions in metamorphic fluid migrate mainly through lattice diffusion. The carbon and oxygen isotope profiles may be modelled
by diffusion into a semi-infinite medium. Empirically lattice diffusion of oxygen isotopes is almost six times faster than
that of carbon isotopes, and oxygen grain-boundary diffusion is ten times faster than oxygen lattice diffusion. Oxygen isotopic
results around the wollastonite vein indicate that migration of the metamorphic fluid into calcite marble was small and was
parallel to the aquifer. From the stability of wollastonite and the attainment of oxygen isotopic equilibrium, we suggest
that diffusion of oxygen occurred through an aqueous fluid phase. The timescale of formation of the oxygen isotopic profile
around the wollastonite vein is calculated to be about 0.76 × 106 years using the experimentally determined diffusion constant.
Received: 14 January 1997 / Accepted: 23 April 1998 相似文献
2.
Colin M. Graham John W. Valley John M. Eiler Hideki Wada 《Contributions to Mineralogy and Petrology》1998,132(4):371-389
Using a recently developed ion microprobe technique, a detailed oxygen isotope map of calcite grains in a coarse-grained
marble has been constructed, supported by trace element (Mn, Sr, Fe) analysis and cathodoluminescence (CL) imaging, in order
to constrain scales of oxygen isotope equilibrium, timescales and mechanisms of metamorphic fluid infiltration, and fluid
sources and pathways. Results are compared with a previous study of this sample (Wada 1988) carried out using a cryo-microtome
technique and conventional oxygen isotope analysis. The marble, from the high temperature/low pressure Hida metamorphic belt
in north-central Japan, underwent granulite facies followed by amphibolite facies metamorphic events, the latter associated
with regional granite intrusion. The CL imaging indicates two types of calcite, a yellow luminescing (YLC) and a purple luminescing
(PLC) variety. The YLC, which occupies grain boundaries, fractures, replacement patches, and most of the abundant deformation
twin lamellae, post-dates the dominant PLC calcite and maps out fluid pathways. Systematic relationships were established
between oxygen isotope and trace element composition, calcite type and texture, based on 74 18O/16O and 17 trace element analyses with 20–30 μ m spatial resolution. The YLC is enriched in Mn and Fe, and depleted in 18O and Sr compared to PLC, and is much more 18O depleted than is indicated from conventional analyses. Results are interpreted to indicate infiltration of 18O-depleted (metamorphic or magmatic) fluid (initial δ18O = 9‰–10.5‰) along grain boundaries, fractures and deformation twin lamellae, depleting calcite grains in Sr and enriching
them in Mn and Fe. The sample is characterised by gross isotopic and elemental disequilibrium, with important implications
for the application of chromatographic theory to constrain fluid fluxes in metacarbonate rocks.
Areas of PLC unaffected by “short-circuiting” fluid pathways contain oxygen diffusion profiles of ∼10‰/∼200 μm in grain boundary
regions or adjacent to fractures/patches. When correction is made for estimated grain boundary/fracture and profile orientation
in 3D, profiles are indistinguishable within error. Modelling of these profiles gives consistent estimates of Dt (where D is the diffusion coefficient and t is time) of ∼0.8 × 10−8 m2, from which, using experimental data for oxygen diffusion in calcite, timescales of fluid transport along grain boundaries
at amphibolite facies temperatures of ∼103 to ∼104 years are obtained. These short timescales, which are much shorter than plausible durations of metamorphism, imply that
rock permeabilities may be transiently much higher during fluid flow than those calculated from time integrated fluid fluxes
or predicted from laboratory measurements. The preservation of 18O/16O profiles requires either rapid cooling rates (∼100–600 °C/million years), or, more plausibly, loss of grain boundary fluid
such that a dry cooling history followed the transient passage of fluid. The δ18O/trace element correlations are also consistent with volume diffusion-controlled transport in the PLC. Fluid transport and
element exchange occurred by two inter-related mechanisms on short timescales and on different lengthscales – long-distance
flow along cracks, grain boundaries and twin lamellae coupled to ∼200 μm-scale volume diffusion of oxygen.
Received: 8 December 1997 / Accepted: 18 May 1998 相似文献
3.
H. MIZUOCHI M. SATISH‐KUMAR Y. MOTOYOSHI K. MICHIBAYASHI 《Journal of Metamorphic Geology》2010,28(5):509-526
Calcite–dolomite solvus geothermometry is a versatile method for the estimation of metamorphic temperature because of its simplicity. However, in medium‐ to high‐grade metamorphic rocks the accuracy of estimating temperature by the integration of unmixed dolomite and calcite is hampered by the heterogeneous distribution of unmixed dolomite, difficulties in distinguishing between preexisting and exsolved dolomite and demarcating grain boundaries. In this study, it is shown that calcite–dolomite solvus thermometry can be applied to calcite inclusions in forsterite and spinel for the estimation of peak metamorphic temperature in granulite facies marbles from Skallevikshalsen, East Antarctica. The marbles are comprised of a granoblastic mineral assemblage of calcite + dolomite + forsterite + diopside + spinel + phlogopite ± apatite, characteristic of granulite facies metamorphic conditions. Forsterite, spinel and apatite frequently contain ‘negative crystal’ inclusions of carbonates that display homogeneously distributed dolomite lamellae. On the basis of narrow ranges of temperature (850–870 °C) recorded from carbonate inclusions compared with the range from matrix carbonate it is regarded that the inclusion carbonates represent a closed system. Furthermore, this estimate is consistent with dolomite–graphite carbon isotope geothermometry, and is considered to be the best estimate of peak metamorphic temperature for this region. Matrix calcite records different stages of retrograde metamorphism and re‐equilibration of calcite that continued until Mg diffusion ceased at ~460 °C. Electron backscattered diffraction (EBSD) results together with morphological features of unmixed coarse tabular dolomite suggest anisotropic diffusion and mineral growth are influenced by crystallographic orientation. Identification of sub‐grain boundaries and formation of fine‐grained unmixing in calcite rims suggest the presence of grain boundary fluids in the late retrograde stages of metamorphic evolution. These results, thus, demonstrate the usefulness of carbonate inclusion geothermometry in estimating the peak metamorphic temperatures of high‐grade terranes and the application of EBSD in understanding the unmixing behaviour of minerals with solid solutions. 相似文献
4.
Richard A. Yund 《Contributions to Mineralogy and Petrology》1997,126(3):224-236
The growth rates of enstatite rims produced by reaction of Fo92 and SiO2 were determined at 250–1500 MPa and 900–1100°C for a wide range of water contents. Growth rates were also determined for
forsterite rims between MgO and Mg2Si2O6 and between MgO and SiO2. Rim growth rates are parabolic indicating diffusion-controlled growth of the polycrystalline rims which are composed of ˜ 2
μm diameter grains. Rim growth rates were used to calculate the product of the grain boundary diffusion coefficient (D'A) times the effective grain boundary thickness (δ) assuming in turn that MgO, SiO2, and Mg2Si−1 are the diffusing components (coupled diffusion of a cation and oxygen or interdiffusion of Mg and Si). The values for D'MgOδ, D', and D' for enstatite at 1000°C and 700 MPa confining pressure with about 0.1 wt % water are about five times larger than the corresponding
D'Aδ values for samples initially vacuum dried at 250°C. Most of the increase in D'Aδ occurs with the first 0.1 wt % water. The activation energy for diffusion through the enstatite rims (1100–950°C) is 162 ± 30
kJ/mole. The diffusion rate through enstatite rims is essentially unchanged for confining pressures from 210–1400 MPa, but
the nucleation rate is greatly reduced at low confining pressure (for ≤ 1.0 wt % water present) and limits the conditions
at which rim growth can be measured. The corresponding values for D'Aδ through forsterite rims are essentially identical for the two forsterite-producing reactions when 0.1 wt % water is added
and similar to the D'Aδ values for enstatite at the same conditions. The D'Aδ values for forsterite are ˜ 28 times larger for samples starting with 0.1 wt % water compared to samples that were first
vacuum dried. Thus water enhances these grain boundary diffusion rates by a factor of 5–30 depending on the mineralogy, but
the total range in D'Aδ is only slightly more than an order of magnitude for as wide a range of water contents as expected for most crustal conditions.
Received: 1 July 1995 / Accepted: 1 August 1996 相似文献
5.
N. E. Davis J. Newman P. B. Wheelock A. K. Kronenberg 《Physics and Chemistry of Minerals》2011,38(2):123-138
The rates of grain growth of stoichiometric dolomite [CaMg(CO3)2] and magnesite (MgCO3) have been measured at temperatures T of 700–800°C at a confining pressure P
c of 300 MPa, and compared with growth rates of calcite (CaCO3). Dry, fine-grained aggregates of the three carbonates were synthesized from high purity powders by hot isostatic pressing
(HIP); initial mean grain sizes of HIP-synthesized carbonates were 1.4, 1.1, and 17 μm, respectively, for CaMg(CO3)2, MgCO3, and CaCO3, with porosities of 2, 28, and 0.04% by volume. Grain sizes of all carbonates coarsened during subsequent isostatic annealing,
with mean values reaching 3.9, 5.1, and 27 μm for CaMg(CO3)2, MgCO3, and CaCO3, respectively, in 1 week. Grain growth of dolomite is much slower than the growth rates of magnesite or calcite; assuming
normal grain growth and n = 3 for all three carbonates, the rate constant K for dolomite (≃5 × 10−5 μm3/s) at T = 800°C is less than that for magnesite by a factor of ~30 and less than that for calcite by three orders of magnitude. Variations
in carbonate grain growth may be affected by differences in cation composition and densities of pores at grain boundaries
that decrease grain boundary mobility. However, rates of coarsening correlate best with the extent of solid solution; K is the largest for calcite with extensive Mg substitution for Ca, while K is the smallest for dolomite with negligible solid solution. Secondary phases may nucleate at advancing dolomite grain boundaries,
with implications for deformation processes, rheology, and reaction kinetics of carbonates. 相似文献
6.
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
7.
Fast diffusion along mobile grain boundaries in calcite 总被引:1,自引:0,他引:1
Andrew McCaig Stephen J. Covey-Crump Walid Ben Ismaïl Geoffrey E. Lloyd 《Contributions to Mineralogy and Petrology》2007,153(2):159-175
Experimental measurements of grain boundary diffusion are usually conducted on static boundaries, despite the fact that grain
boundaries deep in the Earth are frequently mobile. In order to explore the possible effect of boundary mobility on grain
boundary diffusion rates we have measured the uptake of 44Ca from a layer of 44Ca-enriched calcite powder during the static recrystallization of a single crystal of calcite at 900°C. A region about 500 μm
wide adjacent to the powder layer is heterogeneously enriched in 44Ca, and complex zoning patterns, including sharp steps in composition and continuous increases and decreases in 44Ca content, are developed. In metamorphic rocks, these would normally be interpreted in terms of changes in pressure or temperature,
Rayleigh fractionation, or episodic fluid infiltration. These explanations cannot apply to our experiments, and instead the
zoning patterns are interpreted as being due to variations in grain boundary migration rate. We have applied an analytical
model which allows the product of grain boundary diffusion coefficient and grain boundary width (D
GB
δ) to be calculated from the grain boundary migration rate and the compositional gradient away from the powder layer. The value
of D
GB
δ in the mobile grain boundaries is at least five orders of magnitude greater than the published value for static boundaries
under the same conditions. In order to allow the scale of chemical equilibrium (and hence textural evolution) to be predicted
under both experimental and geological conditions, we present quantitative diffusion-regime maps for static and mobile boundaries
in calcite, using both published values and our new values for grain boundary diffusion in mobile boundaries. Enhanced diffusion
in mobile boundaries has wide implications for the high temperature rheology of Earth materials, for geochronology, and for
interpretations of the length- and time-scales of chemical mass-transport. Moreover, zones of anomalously high electrical
conductivity in the crust and mantle could be regions undergoing recrystallization such as active shear zones, rather than
regions of anomalous mineralogy, water- or melt-content as is generally suggested. 相似文献
8.
The system CaO-MgO-SiO2-CO2 at 1 GPa, metasomatic wehrlites, and primary carbonatite magmas 总被引:1,自引:0,他引:1
New experimental data in CaO-MgO-SiO2-CO2 at 1 GPa define the vapor-saturated silicate-carbonate liquidus field boundary involving primary minerals calcite, forsterite
and diopside. The eutectic reaction for melting of model calcite (1% MC)-wehrlite at 1 GPa is at 1100 °C, with liquid composition
(by weight) 72% CaCO3 (CC), 9% MgCO3 (MC), and 18% CaMgSi2O6 (Di). These data combined with previous results permit construction of the isotherm-contoured vapor-saturated liquidus surface
for the calcite/dolomite field, and part of the adjacent forsterite and diopside fields. Nearly pure calcite crystals in mantle
xenoliths cannot represent equilibrium liquids. We recently determined the complete vapor-saturated liquidus surface between
carbonates and model peridotites at 2.7 GPa; the peritectic reaction for dolomite (25% MC)-wehrlite at 2.7 GPa occurs at 1300 °C,
with liquid composition 60% CC, 29% MC, and 11% Di. The liquidus field boundaries on these two surfaces provide the road-map
for interpretation of magmatic processes in various peridotite-CO2 systems at depths between the Moho and about 100 km. Relationships among kimberlites, melilitites, carbonatites and the liquidus
phase boundaries are discussed. Experimental data for carbonatite liquid protected by metasomatic wehrlite have been reported.
The liquid trends directly from dolomitic towards CaCO3 with decreasing pressure. The 1.5 GPa liquid contains 87% CC and 4% Di, much lower in silicate components than our phase
boundary. However, the liquids contain approximately the same CaCO3 (90 ± 1 wt%) in terms of only carbonate components. For CO2-bearing mantle, all magmas at depth must pass through initial dolomitic compositions. Rising dolomitic carbonatite melt will
vesiculate and may erupt as primary magmas through cracks from about ˜70 km. If it percolates through metasomatic wehrlite
from 70 km toward the Moho at 35–40 km, primary calcic siliceous carbonatite magma can be generated with silicate content
at least 11–18% (70–40 km) on the silicate-carbonate boundary.
Received: 22 June 1998 / Accepted: 7 July 1999 相似文献
9.
Ian Cartwright Ian S. Buick Roland Maas 《Contributions to Mineralogy and Petrology》1997,128(4):335-351
The Jervois region of the Arunta Inlier, central Australia, contains para- and orthogneisses that underwent low-pressure
amphibolite facies metamorphism (P = 200–300 MPa, T = 520–600 °C). Marble layers cut by metre-wide quartz + garnet ± epidote veins comprise calcite, quartz, epidote, clinopyroxene,
grandite garnet, and locally wollastonite. The marbles also contain locally discordant decimetre-thick garnet and epidote
skarn layers. The mineral assemblages imply that the rocks were infiltrated by water-rich fluids (XCO2 = 0.1–0.3) at ∼600 °C. The fluids were probably derived from the quartz-garnet vein systems that represent conduits for fluids
exsolved from crystallizing pegmatites emplaced close to the metamorphic peak. At one locality, the marble has calcite (Cc)
δ18O values of 9–18‰ and garnet (Gnt) δ18O values of 10–14‰. The δ18O(Gnt) values are only poorly correlated with δ18O(Cc), and the δ18O values of some garnet cores are higher than the rims. The isotopic disequilibrium indicates that garnet grew before the
δ18O values of the rock were reset. The marbles contain ≤15% garnet and, for water-rich fluids, garnet-forming reactions are
predicted to propagate faster than O-isotopes are reset. The Sm-Nd and Pb-Pb ages of garnets imply that fluid flow occurred
at 1750–1720 Ma. There are no significant age differences between garnet cores and rims, suggesting that fluid flow was relatively
rapid. Texturally late epidote has δ18O values of 1.5–6.2‰ implying δ18O(H2O) values of 2–7‰. Waters with such low-δ18O values are probably at least partly meteoric in origin, and the epidote may be recording the late influx of meteoric water
into a cooling hydrothermal system.
Received: 29 April 1996 / Accepted: 12 March 1997 相似文献
10.
Stephen J. Covey-Crump 《Contributions to Mineralogy and Petrology》1997,129(2-3):239-254
The normal grain growth behaviour of four different, but all nominally pure, calcite powders (99%+ analytic grade calcite,
99.7% chalk, 99.97% crushed Iceland Spar, 99.95%+ chelometric grade calcite) has been investigated as a function of temperature
(550, 600, 650, 700 °C) and confining pressure (100, 190 MPa) under both “dry” and hydrostatic (P
fluid = P
total) conditions. The initial particle size of both the analytic grade and chelometric grade calcite was about 5 μm, and that
of the chalk was about 3 μm, while the experiments on the Iceland Spar were conducted on powders of three different initial
particle sizes (3.4, 7.5, 38.5 μm). On each material, at each pressure/temperature condition 6 to 15 experiments, equally
spaced in log time from 15 minutes to 50 days, were conducted. Under dry conditions all four materials recrystallized to aggregates
which contained less than 2% porosity and which had a grain size of between 4 and 20 μm (depending on the initial particle
size). Subsequently the aggregates coarsened by normal grain growth, with the kinetics of the growth process being controlled
by the rate at which the grain boundaries could drag the residual pores with them as they migrated. Under nominally identical
conditions both the mechanism and rates of pore drag differed greatly for the different materials, implying that this process
is highly sensitive to trace solute impurity concentrations. This sensitivity renders the task of providing a systematic account
of dry calcite grain growth kinetics highly problematic. Under hydrostatic conditions all the powders followed the same normal
grain growth kinetics in which the growth process was rate-controlled by diffusion through the pore fluid on the grain boundaries.
An activation enthalpy of 162.6 kJ mol−1 and an activation volume of 34.35 cm3 mol−1 was obtained for this process.
Received: 23 May 1996 / Accepted: 8 July 1997 相似文献
11.
Martin R. Lee Kim A. Waldron I. Parsons William L. Brown 《Contributions to Mineralogy and Petrology》1997,127(3):291-304
Braid microperthitic alkali feldspars in the Klokken, South Greenland and Coldwell, Ontario syenite intrusions have bulk-compositional
variations along grain boundaries called pleated rims. These, together with vein microperthites in aplites which cross-cut the syenites, have been investigated by SEM and TEM.
We distinguish two main types of pleated rims, “arched ” and “parallel-sided ”, consisting of alternating Ab- and Or-rich areas on (001), which are 0.5–300 μm in length normal to (010) and 0.2–20 μm
in width along (010). The smallest pleats, which occur on intracrystalline boundaries in Klokken feldspars, are fully coherent
and composed of low albite and low microcline. Above the heads of some of the coarser pleats, braid microperthite grades into
a film crypto- and micro-perthite and antiperthite microtexture called a “transitional zone” containing roughly planar lamellae of low albite and tweed orthoclase. During pleat development, local alternating volumes
form in which the proportions of the phases differ ( phase separation) and the morphology of the intergrowths changes from braided to straight in response to this change in local bulk composition.
Straightening is also accompanied by transformation of low microcline to tweed orthoclase. The coarsest pleats, which occur
along grain boundaries in feldspars from the Coldwell syenite, are semi- or in-coherent and have a thick coherent and semicoherent
transitional zone. Coarsening of pleats and development of the transitional zone has been facilitated by diffusion of “water”
into grain interiors. In many cases, pleated rims have suffered deuteric alteration, by dissolution–reprecipitation processes,
through the action of a water-rich fluid from the grain boundary, in which tweed orthoclase was transformed into irregular
microcline and micropores developed. Vein microperthites in aplites from Klokken, and by extension the vein microperthites
almost universal in most alkali granites, are interpreted to have formed by propagation of pleat heads across entire crystals
during pervasive interaction with water.
Received: 10 June 1996 / Accepted: 12 December 1996 相似文献
12.
L. M. Keller C. A. Hauzenberger R. Abart 《Contributions to Mineralogy and Petrology》2007,154(2):205-216
In this study we use two dimensional chemical patterns and numerical modeling to estimate the relative rates of chemical transport
along interphase boundaries (ib) and through grain (s) interiors during retrograde Fe–Mg exchange between garnet and biotite
at a garnet–biotite–quartz triple junction. We demonstrate that systematic variations in garnet–rim compositions, which are
frequently observed along garnet–quartz interfaces, and deviations from concentric retrograde zoning patterns start to develop
when chemical transport along the interphase boundaries becomes slow during cooling. The capacities for chemical transport
along an interphase boundary depend on the product D
ib
K
ib/s
a, where D
ib is the diffusion coefficient of the exchangeable components within the interphase boundary medium, K
ib/s is the equilibrium partitioning coefficient between the cation exchange partners and the interphase boundary medium and a is the interphase boundary width. The model is applied to analyze the retrograde zoning patterns in garnets from the Mozambique
belt (SE-Kenya), which cooled from 820°C at a rate of ca. 2°C/my. It is found that non-equilibrated compositions in garnet along
garnet/quartz interphase boundaries started to develop below 700°C due to insufficient rates of chemical transport along these
boundaries. The transport capacities of garnet/quartz interphase boundaries was estimated to have been between about 1E-23 cm3/s (575°C) and 1E-20 cm3/s (700°C) from modeling the observed X
Fe pattern in garnet close to a garnet–quartz–biotite triple junction and relying on published data on the diffusivity of Fe2+ in garnet. Similar transport capacities are obtained; when the interphase boundary is assumed to be filled with a material
that has the transport properties and chemical composition of a free melt in equilibrium with garnet, biotite and quartz at
the respective conditions. In contrast, if the transport properties of the interphase boundary medium are related to the diffusivities
and solubility of Fe2+ and FeOH+ within a free aqueous solution, chemical transport along the interphase boundaries would be much more efficient, and exchange
equilibrium would have been maintained during the entire cooling history of the rocks. The observation of systematic deviations
from local equilibrium along the garnet–quartz interphase boundaries leads us to exclude the presence of an aqueous fluid
along the interphase boundary at any time during cooling. 相似文献
13.
Diffusion-controlled growth of wollastonite rims between quartz and calcite: comparison between nature and experiment 总被引:2,自引:2,他引:2
Growth rates of wollastonite reaction rims between quartz and calcite were experimentally determined at 0.1 and 1 GPa and temperatures from 850 to 1200 °C. Rim growth follows a parabolic rate law indicating that this reaction is diffusion‐controlled. From the rate constants, the D′δ‐values of the rate‐limiting species were derived, i.e. the product of grain boundary diffusion coefficient D′ and the effective grain boundary width, δ. In dry runs at 0.1 GPa, wollastonite grew exclusively on quartz surfaces. From volume considerations it is inferred that (D′CaOδ)/(D′SiO2δ)≥1.33, and that SiO2 diffusion controls rim growth. D′SiO2δ increases from about 10?25 to 10?23 m3 s?1 as temperature increases from 850 to 1000 °C, yielding an apparent activation energy of 330±36 kJ mol?1. In runs at 1 GPa, performed in a piston‐cylinder apparatus, there were always small amounts of water present. Here, wollastonite rims always overgrew calcite. Rims around calcite grains in quartz matrix are porous and their growth rates are controlled by a complex diffusion‐advection mechanism. Rim growth on matrix calcite around quartz grains is controlled by grain boundary diffusion, but it is not clear whether CaO or SiO2 diffusion is rate‐limiting. D′δ increases from about 10?21 to 10?20 m3 s?1 as temperature increases from 1100 to 1200 °C. D′SiO2δ or D′CaOδ in rims on calcite is c. 10 times larger than D′SiO2δ in dry rims at the same temperature. Growth structures of the experimentally produced rims are very similar to contact‐metamorphic wollastonite rims between metachert bands and limestone in the Bufa del Diente aureole, Mexico, whereby noninfiltrated metacherts correspond to dry and brine‐infiltrated metacherts to water‐bearing experiments. However, the observed diffusivities were 4 to 5 orders of magnitude larger during contact‐metamorphism as compared to our experimental results. 相似文献
14.
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
15.
The Youjiang basin, which flanks the southwest edge of the Yangtze craton in South China, contains many Carlin-type gold deposits and abundant paleo-oil reservoirs. The gold deposits and paleo-oil reservoirs are restricted to the same tectonic units, commonly at the basinal margins and within the intrabasinal isolated platforms and/or bioherms. The gold deposits are hosted by Permian to Triassic carbonate and siliciclastic rocks that typically contain high contents of organic carbon. Paragenetic relationships indicate that most of the deposits exhibit an early stage of barren quartz ± pyrite (stage I), a main stage of auriferous quartz + arsenian pyrite + arsenopyrite + marcasite (stage II), and a late stage of quartz + calcite + realgar ± orpiment ± native arsenic ± stibnite ± cinnabar ± dolomite (stage III). Bitumen in the gold deposits is commonly present as a migrated hydrocarbon product in mineralized host rocks, particularly close to high grade ores, but is absent in barren sedimentary rocks. Bitumen dispersed in the mineralized rocks is closely associated and/or intergrown with the main stage jasperoidal quartz, arsenian pyrite, and arsenopyrite. Bitumen occurring in hydrothermal veins and veinlets is paragenetically associated with stages II and III mineral assemblages. These observations suggest an intimate relationship between bitumen precipitation and gold mineralization. In the paleo-petroleum reservoirs that typically occur in Permian reef limestones, bitumen is most commonly observed in open spaces, either alone or associated with calcite. Where bitumen occurs with calcite, it is typically concentrated along pore/vein centers as well as along the wall of pores and fractures, indicating approximately coeval precipitation. In the gold deposits, aqueous fluid inclusions are dominant in the early stage barren quartz veins (stage I), with a homogenization temperature range typically of 230°C to 270°C and a salinity range of 2.6 to 7.2 wt% NaCl eq. Fluid inclusions in the main and late-stage quartz and calcite are dominated by aqueous inclusions as well as hydrocarbon- and CO2-rich inclusions. The presence of abundant hydrocarbon fluid inclusions in the gold deposits provides evidence that at least during main periods of the hydrothermal activity responsible for gold mineralization, the ore fluids consisted of an aqueous solution and an immiscible hydrocarbon phase. Aqueous inclusions in the main stage quartz associated with gold mineralization (stage II) typically have a homogenization temperature range of 200–230°C and a modal salinity around 5.3 wt% NaCl eq. Homogenization temperatures and salinities of aqueous inclusions in the late-stage drusy quartz and calcite (stage III) typically range from 120°C to 160°C and from 2.0 to 5.6 wt% NaCl eq., respectively. In the paleo-oil reservoirs, aqueous fluid inclusions with an average homogenization temperature of 80°C are dominant in early diagenetic calcite. Fluid inclusions in late diagenetic pore- and fissure-filling calcite associated with bitumen are dominated by liquid C2H6, vapor CH4, CH4–H2O, and aqueous inclusions, with a typical homogenization temperature range of 90°C to 180°C and a salinity range of 2–8 wt% NaCl eq. It is suggested that the hydrocarbons may have been trapped at relatively low temperatures, while the formation of gold deposits could have occurred under a wider and higher range of temperatures. The timing of gold mineralization in the Youjiang basin is still in dispute and a wide range of ages has been reported for individual deposits. Among the limited isotopic data, the Rb–Sr date of 206 ± 12 Ma for Au-bearing hydrothermal sericite at Jinya as well as the Re–Os date of 193 ± 13 Ma on auriferous arsenian pyrite and 40Ar/39Ar date of 194.6 ± 2 Ma on vein-filling sericite at Lannigou may provide the most reliable age constraints on gold mineralization. This age range is comparable with the estimated petroleum charging age range of 238–185 Ma and the Sm–Nd date of 182 ± 21 Ma for the pore- and fissure-filling calcite associated with bitumen at the Shitouzhai paleo-oil reservoir, corresponding to the late Indosinian to early Yanshanian orogenies in South China. The close association of Carlin-type gold deposits and paleo-oil reservoirs, the paragenetic coexistence of bitumens with ore-stage minerals, the presence of abundant hydrocarbons in the ore fluids, and the temporal coincidence of gold mineralization and hydrocarbon accumulation all support a coeval model in which the gold originated, migrated, and precipitated along with the hydrocarbons in an immiscible, gold- and hydrocarbon-bearing, basinal fluid system. 相似文献
16.
Grain boundary diffusion rates of oxygen, potassium and calcium in fine-grained feldspar aggregates were determined experimentally.
The starting materials were a natural albite rock from the Tanco pegmatite and aggregates hot-pressed from fragments of Amelia
albite or Ab, Or and An composition glasses. The technique employed isotopic tracers (18O, 41K, 42Ca) either evaporated onto the surface or in an aqueous solution surrounding the sample, and depth profiling using an ion
microprobe (SIMS). From the depth profiles, the product of the grain boundary diffusion coefficient (D′) and effective boundary
width (δ) was calculated using numerical solutions to the appropriate diffusion equation. The experimental reproducibility
of D′δ is a factor of 3. A separate determination of D′ independent of δ yields an effective grain boundary width of ∼3 nm,
consistent with high resolution TEM observations of a physical grain boundary width <5 nm. Oxygen (as molecular water) grain
boundary diffusion rates were determined in the Ab and Or aggregates at 450°–800° C and 100 MPa (hydrothermal), potassium
rates in Or aggregates at 450°–700° C both at 0.1 MPa (in air) and at 100 MPa (hydrothermal), and calcium rates in An aggregates
at 700°–1100° C and 0.1 MPa (in air). Oxygen grain boundary diffusion rates are similar in all three of the Ab aggregates
and in the Or aggregate. Potassium and oxygen depth profiles measured in the same samples yield different D′δ values, confirming
a diffusional transport mechanism. Potassium diffusion in the Or aggregate has a greater activation energy (216 vs 78 kJ/mol)
than oxygen, and the Arrhenius relations cross at ∼625° C. Potassium D′δ values in Or aggregates are about a factor of five
greater in hydrothermal experiments at 100 MPa than in experiments at 0.1 MPa in air. Calcium grain boundary diffusion rates
in An aggregates are 4 to 5 orders of magnitude slower than potassium in Or and have a greater (291 kJ/mol) activation energy.
This suggests that differences in formal charge and/or size of diffusing species may play an important role in their relative
grain boundary diffusion rates.
Received: 24 December 1993 / Accepted: 16 June 1994 相似文献
17.
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
18.
The Malanjkhand copper ( + molybdenum) deposit, India: mineralization from a low-temperature ore-fluid of granitoid affiliation 总被引:2,自引:0,他引:2
Copper and subordinate molybdenum mineralization at Malanjkhand occurs within a fracture-controlled quartz-reef enclosed
in a pink granitoid body surrounded by grey-granitoids constituting the regional matrix. Sulfide-bearing stringers, pegmatites
with only quartz + microcline and sulfide disseminations, all within the pink-granitoid, represent other minor modes of occurrences.
Despite this diversity in mode of occurrence, the mineralogy of ores is quite consistent and conform to a common paragenetic
sequence comprising an early `ferrous' stage of precipitation of magnetite (I) and pyrite (I) and, the main-stage chalcopyrite
mineralization with minor sphalerite, pyrite (II), magnetite (II), molybdenite and hematite. Both stages witnessed continuous
precipitation of quartz ± microcline ± (chlorite, biotite and epidote). The enclosing pink-granitoid and the regional grey-granitoids
display alteration features such as saussuritization of plagioclase, breakdown of hornblende and chloritization of biotite
on a regional scale, indicating interaction with a pervasive fluid. Quartz and microcline precipitation mostly restricted
within the pink granitoid, postdates this alteration. Four types of primary inclusions were encountered in quartz from ore samples: (1) type-I – aqueous-biphase(L + V) inclusions, the commonest variety in all ore types; (2) type-II – aqueous-carbonic(Laq + Lcarb ± Vcarb); (3) type-III – pure-carbonic(Lcarb ± Vcarb) – type-II and III being restricted to stringer and pegmatitic ores, and (4) rare polyphase (Laq + Vaq + calcite/gypsum) inclusions. Quartz in granitoids contain primary type-I inclusions only. Type-I inclusions from ore samples furnish a temperature range (after a rough pressure
correction to the T
H
-maxima of 140–180 °C) of 150–275 °C and a moderately low salinity of 4–12 wt.% NaCl equivalent. This is inferred to represent
the signature of the major component (F2) of the ore fluid. A few type-I inclusions of higher T
M
(up to 380 °C) and low salinity and density represent the other (F1) identifiable component of the ore fluid present in low
proportion. The T
H
-maxima and the total range in salinity of type-I inclusions in quartz from granitoids are strikingly similar to those from the ore samples. Composition of syn-ore chlorites furnished a temperature range of 185–327
°C, which conforms to the fluid inclusion microthermometric data. Pressure estimates using standard fluid inclusion geobarometric
methods, vary from 550 to 1790 bar in the stringer ores. Observed temperature-salinity/density relationships are best explained
by a two-stage evolution model of the ore fluid: the first stage witnessed mixing of the two components, F1 and F2 in unequal
proportion, bringing about mineralization. The second stage of evolution was marked by the separation of a carbonic component
on continued sulfide precipitation and attendant increase in salinity of the fluid. The F1 component emerged as a distinct,
heated and (CO2 + S)-charged entity due to steam-heating and contamination of the early-ingressed F2 fluid at the fracture zone. The pervasive
fluid phase in the surrounding granitoids contributed the F2 component.
Received: (10 August 1994), 15 August 1995 / Accepted: 12 January 1996 相似文献
19.
E. Schmädicke M. Okrusch W. Schubert B. Elwart U. Görke 《Mineralogy and Petrology》2001,72(1-3):77-111
Summary In the Odenwald Crystalline Complex, calc-silicate rocks are concentrated at the margins of the marble layer of Auerbach.
They were presumably formed by metasomatic exchange between the calcite marble and the neighbouring granodioritic and quartz-dioritic
intrusives. The investigated samples contain the characteristic mineral assemblages: garnet + clinopyroxene + epidote/clinozoisite + calcite + quartz ± titanite
(1) and wollastonite + clinopyroxene + garnet + calcite ± quartz ± epidote/clinozoisite ± titanite (2). Microprobe analyses
revealed the following compositional ranges: garnet grs40–98adr2–55alm<5.5sps<5.5pyp<1; clinopyroxene di46–88hed9–47joh0–5cats0–6; epidote/clinozoisite ps20–80. Different phase diagrams were calculated in the system CaO-MgO-Al2O3-TiO2-SiO2-CO2-H2O (CMATSCH) to decipher the P-T-XCO2 parameters of metamorphism: isobaric T-XCO2 sections and a P-T projection with mixed volatiles. The phase diagrams illustrate that the observed assemblages can only
form in the presence of an H2O-rich volatile phase. The assemblages are stable over a large range of temperatures, from 580 °C to < 400 °C (at 4 kbar)
and at XCO2 values of less than 0.055 (at 4 kbar). Higher temperatures can be inferred from reaction textures which indicate that garnet + plagioclase
(T > 580 °C, at 4 kbar) and wollastonite + plagioclase (T > 660 °C, at 4 kbar) coexisted during an early metamorphic stage. A minimum pressure of 3.5 kbar can be inferred for the
early high-temperature stage. Furthermore, on the basis of the calculated phase diagrams, combined with modal abundances in
thin sections, it is possible to evaluate fluid behaviour; in the investigated specimens, infiltration of fluids from an external
reservoir occurred. A minimum fluid:rock ratio of 3.6:1 can be estimated.
Received July 29, 1999; accepted March 28, 2000 相似文献
Zusammenfassung Phasenbeziehungen in Kalksilikat-Paragenesen des Marmorzuges von Auerbach, Odenwald-Kristallin-Komplex, Deutschland Im kristallinen Odenwald konzentrieren sich kalksilikatische Gesteine in den Randbereichen des Marmorzuges von Auerbach. Die kalksilikatischen Partien wurden vermutlich duch metasomatischen Austausch zwischen dem Calcit-Marmor und benachbarten Granodioriten und Quarzdioriten gebildet. Die untersuchten Proben enthalten die charakteristischen Mineralparagenesen: Granat + Klinopyroxen + Epidot/Klinozoisit + Calcit + Quarz + Titanit (1) und Wollastonit + Klinopyroxen + Granat + Calcit ± Quarz ± Epidot/Klinozoisit ± Titanit (2). Mikrosondenanalysen ergaben folgendes Zusammensetzungsspektrum: Granat grs40–98adr2–55alm<5.5sps<5.5pyp<1; Klinopyroxen di46–88hed9–47joh0–5cats0–6s; Epidot/Klinozoisit ps20–80. Verschiedene Phasendiagramme wurden für das Modellsystem CaO-MgO-Al2O3-TiO2-SiO2-CO2-H2O (CMATSCH) berechnet, um die P-T-XCO2-Parameter der Metamorphose einzugrenzen: Isobare T-XCO2-Schnitte und eine P-T-Projektion mit einer Fluid-Mischphase. Die Phasendiagramme verdeutlichen, da? die beobachteten Paragenesen nur in Anwesenheit eines H2O-reichen Fluids gebildet werden k?nnen. Die Paragenesen sind über einen gro?en Temperaturbereich hinweg stabil, von 580 °C bis < 400 °C (bei 4 kbar) und bei XCO2-Gehalten von < 0.055 (bei 4 kbar). Ursprünglich h?here Temperaturen k?nnen anhand von Reaktions-Texturen rekonstruiert werden, die zeigen, da? Granat + Plagioklas (T > 580 °C, bei 4 kbar) und Wollastonit + Plagioklas (T > 660 °C, bei 4 kbar) w?hrend eines früheren Metamorphosestadiums koexistierten. Ein Minimaldruck von 3.5 kbar kann für dieses frühe Hochtemperatur-Stadium abgeleitet werden. Mit Hilfe der berechneten Phasendiagramme, in Kombination mit beobachteten Modalgehalten, ist es m?glich, das Verhalten der fluiden Phase abzusch?tzen. Die untersuchten Gesteine implizieren Fluidinfiltration, wobei ein minimales Verh?ltnis Fluid:Gestein von 3.6:1 abgesch?tzt werden kann.
Received July 29, 1999; accepted March 28, 2000 相似文献
20.
A detailed study based on textural observations combined with microanalysis [back scattered electron imaging (BSE) and electron
microprobe analysis (EMPA)] and microstructural data transmission electron microscopy (TEM) has been made of K-feldspar micro-veins
along quartz–plagioclase phase and plagioclase–plagioclase grain boundaries in granulite facies, orthopyroxene–garnet-bearing
gneiss's (700–825 °C, 6–8 kbar) from the Val Strona di Omegna, Ivrea–Verbano Zone, northern Italy. The K-feldspar micro-veins
are commonly associated with quartz and plagioclase and are not found in quartz absent regions of the thin section. This association
appears to represent a localised reaction texture resulting from a common high grade dehydration reaction, namely: amphibole + quartz
= orthopyroxene + clinopyroxene + plagioclase + K-feldspar + H2O, which occurred during the granulite facies metamorphism of these rocks. There are a number of lines of evidence for this.
These include abundant Ti-rich biotite, which was apparently stable during granulite facies metamorphism, and total lack of
amphibole, which apparently was not. Disorder between Al and Si in the K-feldspar indicates crystallisation at temperatures
>500 °C. Myrmekite and albitic rim intergrowths in the K-feldspar along the K-feldspar–plagioclase interface could only have
formed at temperatures >500–600 °C. Symplectic intergrowths of albite and Ca-rich plagioclase between these albitic rim intergrowths
and plagioclase suggest a high temperature grain boundary reaction, which most likely occurred at the start of decompression
in conjunction with a fluid phase. Relatively high dislocation densities (>2 × 109 to 3 × 109/cm2) in the K-feldspar suggest plastic deformation at temperatures >500 °C. We propose that this plastic deformation is linked
with the extensional tectonic environment present during the mafic underplating event responsible for the granulite facies
metamorphism in these rocks. Lastly, apparently active garnet grain rims associated with side inclusions of K-feldspar and
quartz and an exterior K-feldspar micro-vein indicate equilibrium temperatures within 20–30 °C of the peak metamorphic temperatures
estimated for the sample (770 °C). Contact between these K-feldspar micro-veins and Fe-Mg silicate minerals, such as garnet,
orthopyroxene, clinopyroxene or biotite along the interface, is observed to be very clean with no signs of melt textures or
alteration to sheet silicates. This lends support to the idea that these micro-veins did not originate from a melt and, if
fluid induced, that the water activity of these fluids must have been relatively low. All of these lines of evidence point
to a high grade origin for the K-feldspar micro-veins and support the hypothesis that they formed during the granulite facies
metamorphism of the metabasite layers in an extensional tectonic environment as the consequence of localised dehydration reactions
involving the breakdown of amphibole in the presence of quartz to orthopyroxene, clinopyroxene, plagioclase, K-feldspar and
H2O. It is proposed that the dehydration of the metabasite layers to an orthopyroxene–garnet-bearing gneiss over a 4-km traverse
in the upper Val Strona during granulite facies metamorphism was a metasomatic event initiated by the presence of a high-grade,
low H2O activity fluid (most likely a NaCl–KCl supercritical brine), related to the magmatic underplating event responsible for
the Mafic Formation; and that this dehydration event did not involve partial melting.
Received: 15 February 2000 / Accepted: 26 June 2000 相似文献