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1.
Threshold velocity for wind erosion: the effects of porous fences   总被引：3，自引：0，他引：3  
Porous fence is a kind of artificial windbreak that has many practical applications. The threshold wind velocities at different distances downwind from porous fences were measured and the corresponding characteristics of particle movement observed to assess their shelter effect. It is found that the fence’s porosity is the key factor that determines the resulting shelter effect. The area near a fence can be typically classified into five regions, each with a different mode of particle movement. Dense fences, and especially solid fences, favor the accumulation of sand upwind of the fences. Fences with porosities of 0.3–0.4 produce the maximum threshold wind velocity; those with porosities of 0.3–0.6 (depending on the fence height) provide the maximum effective shelter distance. It is confirmed that the fence porosities of 0.3–0.4 that have been proposed for practical application in previous research are the most effective for abating wind erosion.  相似文献   

2.
Using detailed measurements of the instantaneous velocity fields around fences with different heights and porosities, the pressure fields around the fences were calculated using the Reynolds equations. Based on the results of calculation, the relationships among the pressure field, fence porosity (i.e., the β coefficient), fence height, and free-stream wind velocity were examined. For all fences, a high-pressure region exists upwind of the fence, and a low-pressure region exists downwind of the fence, the change of mean pressure is gradually less to a long distance from the fence. The pressure value at the center of the low-pressure area downwind of the fence decreases with increasing fence height and free-stream wind velocity. The impact of the fence’s porosity on pressure is large, when β < 0.2, the mean pressure upwind of the fence is relatively small, and the center value of the low-pressure area downwind of the fence increases with increasing fence porosity; when β > 0.3, the mean pressure increases around the fence.  相似文献   

3.
The trap efficiency of a catcher in wind erosion measurements plays a significant role, and in many cases suspension trap efficiencies at high wind velocities are still unknown. The sediment trap efficiency generally changes with particles size and with wind speed. In this study, the efficiency of Vaseline Slide (VS) and Modified Wilson and Cooke (MWAC) catchers were determined with different sand particle sizes (<50, <75, 50–75, 200–400, and 400–500 μm) at a fixed wind speed (13.3 ms−1) and with different soil textures at different wind velocities (10.3, 12.3, and 14.3 ms−1) in the wind tunnel of the International Center for Eremology (ICE), Ghent University, Belgium. The traps were placed at different heights (4, 6.5, 13, 20, 120, and 192 cm for VS and 1.5, 3, 5, 8, 11, and 30 cm for MWAC) to catch saltating and suspended sediments in a 12-m long, 1.2-m wide and 3.2-m high working section of the wind tunnel. In the sand particle experiments, the efficiency of the VS catcher was 92% for particles smaller than 50 μm and decreased with increasing particles size, falling to 2.2% for 400–500 μm particle size at 13.4 ms−1. However, the MWAC’s efficiency was 0% for particles smaller than 50 μm and increased with increasing particle size to 69.5% at 400–500 μm. In the experiments with different soil textures, the efficiency of each catcher significantly changed with soil and with wind speed. It also considerably varied with the catchers: for instance, for sand (S), the MWAC efficiency was very high (67.4, 113.4, and 90.5% at 10.3, 12.3, and 14.4 ms−1, respectively) while the efficiency of VS was relatively very low (5.2, 4.4, and 1.9% at 10.3, 12.3, and 14.4 ms−1, respectively). Results indicated that the efficiency depends critically on the particle size, type of catcher, and wind speed, and these could be helpful to increase the robustness of wind erosion measurements.  相似文献   

4.
The structure and kinematics of the ISM in an extended vicinity of the star WR 139 is analyzed using the results of original Hα interferometric observations together with radio and infrared data. A CO cavity with a size of up to 40′ has been detected around the star at velocities of V LSR ∼ 2.5–10 km/s; the cavity is bounded to the North by a shell radiating in the optical. Ionized hydrogen emits at the systematic velocities V LSR ∼ 6–14 km/s toward the CO cavity, and at V LSR ≃ 4–11 km/s toward the shell. High-velocity motions of ionized hydrogen inside the cavity testify to the probable expansion of gas that has been swept out by the stellar wind of WR 139 at velocities of up to 60–80 km/s.  相似文献   

5.
 The equilibrium water content of cordierite has been measured for 31 samples synthesized at pressures of 1000 and 2000 bars and temperatures from 600 to 750° C using the cold-seal hydrothermal technique. Ten data points are presented for pure magnesian cordierite, 11 data points for intermediate iron/magnesium ratios from 0.25 to 0.65 and 10 data points for pure iron cordierite. By representing the contribution of H2O to the heat capacity of cordierite as steam at the same temperature and pressure, it is possible to calculate a standard enthalpy and entropy of reaction at 298.18° K and 1 bar for, (Mg,Fe)2Al4Si5O18+H2O ⇄ (Fe,Mg)2Al4Si5O18.H2O Combining the 31 new data points with 89 previously published experimental measurements gives: ΔH ° r =–37141±3520 J and ΔS °  r =–99.2±4 J/degree. This enthalpy of reaction is within experimental uncertainty of calorimetric data. The enthalpy and entropy of hydration derived separately for magnesian cordierite (–34400±3016 J, –96.5±3.4 J/degree) and iron cordierite (–39613±2475, –99.5±2.5 J/degree) cannot be distinguished within the present experimental uncertainty. The water content as a function of temperature, T(K), and water fugacity, f(bars), is given by n H2O=1/[1+1/(K ⋅ f H2O)] where the equilibrium constant for the hydration reaction as written above is, ln K=4466.4/T–11.906 with the standard state for H2O as the gas at 1 bar and T, and for cordierite components, the hydrous and anhydrous endmembers at P and T. Received: 2 August 1994/Accepted: 7 February 1996  相似文献   

6.
A study has been carried out by comparing the extreme wind speeds estimated based on NCEP/NCAR reanalysis data for 100 years return period using Fischer Tippet-1 (commonly known as Gumbel) and Weibull distributions for three locations (off Goa, Visakhapatnam and Machilipatnam) in the north Indian Ocean. The wind dataset for Goa is compared with that from ERA-40 data. For higher wind speeds (12–20m s−1), NCEP wind speed has higher percentage of occurrence than that of ERA-40. Analysis has shown slight upward trend in the annual maximum wind for location off Machilipatnam with an increase of 1.2 cm s−1 per year and a decreasing trend of −1.3 cm s−1 per year in the case of Goa. The Weibull distribution with shape parameter 2 fits the annual maximum wind data better than FT-1 distribution.  相似文献   

7.
The H2O content of wadsleyite were measured in a wide pressure (13–20 GPa) and temperature range (1,200–1,900°C) using FTIR method. We confirmed significant decrease of the H2O content of wadsleyite with increasing temperature and reported first systematic data for temperature interval of 1,400–1,900°C. Wadsleyite contains 0.37–0.55 wt% H2O at 1,600°C, which may be close to its water storage capacity along average mantle geotherm in the transition zone. Accordingly, water storage capacity of the average mantle in the transition zone may be estimated as 0.2–0.3 wt% H2O. The H2O contents of wadsleyite at 1,800–1,900°C are 0.22–0.39 wt%, indicating that it can store significant amount of water even under the hot mantle environments. Temperature dependence of the H2O content of wadsleyite can be described by exponential equation C\textH2 \textO = 6 3 7.0 7 \texte - 0.00 4 8T , C_{{{\text{H}}_{2} {\text{O}}}} = 6 3 7.0 7 {\text{e}}^{ - 0.00 4 8T} , where T is in °C. This equation is valid for temperature range 1,200–2,100°C with the coefficient of determination R 2 = 0.954. Temperature dependence of H2O partition coefficient between wadsleyite and forsterite (D wd/fo) is complex. According to our data apparent Dwd/fo decreases with increasing temperature from D wd/fo = 4–5 at 1,200°C, reaches a minimum of D wd/fo = 2.0 at 1,400–1,500°C, and then again increases to D wd/fo = 4–6 at 1,700–1,900°C.  相似文献   

8.
Studies were conducted to characterize soil humin by acid hydrolysis.Two humin samples collected from two different types of soil,namely chernozem and laterite,which are widespread over a vast area from the north to south of China,were hyrolyzed under reflux with 0.5M H2SO4or 3M H2SO4for 4h.The results showed that 25%-29% of organic carbon and 46%-54%of organic nitrogen could be hydrolyzed by 0.5M H2SO4;36%-40%of organic carbon and 93%-97% of organic nitrogen hydrolyzed by 3M H2SO4.The C/N ration in hydrolyzed organic matter is lower than that in soil humin and that in organic matter hydrolyzed by 3M H2SO4 is lower than that in organic matter hydrolyzed by 0.5M H2SO4.The proportion of nitrogen hydrolyzed from humin is markedly larger than that from the original soil and also markedly larger than that from humic acid fraction.Only 3%-7% of nitrogen in humin exists in a relatively stable from,which is not easy to hydrolyze.There in little nitrogen that occurs in the form of heterocyclic rings in humin.Incubation experiments showed that the newly formed organic matter can be hydrolyzed more easily.  相似文献   

9.
SOHO/LASCO C2 and C3 data have been used to carry out a detailed study of the perturbed zone and shock that form as a coronal mass ejection (CME) moves away from the Sun, as a result of its interaction with the ambient solar wind. The event of January 4, 2002 is used as an example. The perturbed zone is most extensive along the direction of propagation of the CME, decreases away from this direction, and reaches its minimum values perpendicular to this direction. The mass of the perturbed zone is ≥0.1 of the total mass of the CME. The condition for the formation of a shock preceding the CME (in the direction of propagation of the CME) is V − V SW > V A , where V, V SW , and V A are the CME, solar wind, and Alfvén velocities, respectively. Perpendicular to the CME axis, at distances of ≈4–6R ⊙ fromthe center of the Sun, the condition for the formation of shock is V/2 > V A .  相似文献   

10.
The infrared (IR) spectra of gem-quality baryte crystals from different occurrences are characterized by relatively weak but strongly pleochroic absorption bands at 3,280, 3,220, 3,155, and 3,115 cm−1. These bands are assigned to anti-symmetric and symmetric OH stretching vibrations of two types of H2O molecules localized on vacant Ba sites. The H–H axis of the H2O I molecule is slightly tilted from the a-axis direction, its twofold axis being nearly parallel to the b-axis, thus defining the plane of the H2O molecule practically parallel to (001). The H2O II molecule has its H–H axis parallel to the b-axis direction, with its plane lying approximately parallel to (101). The values of the total water contents of the baryte crystals, calculated on the basis of IR spectroscopic data, are ranging from about 1.7–3.8 wt.ppm. The possible presence of H3O+ ions is also discussed.  相似文献   

11.
The residual (time-average) salinity and circulation in a numerical ocean model of the Tampa Bay estuary are shown to experience significant temporal variation under realistic forcing conditions. A version of the Estuarine Coastal Ocean Model developed for Tampa Bay with 70 by 100 horizontal grid points and 11 sigma levels is examined for the years 2001–2003. Model output variables are averaged over the entire time of the simulation to generate long-term residual fields. The residual axial current is found to be dominated by the buoyancy-driven baroclinic circulation with an outflow (southwestward) at the surface and to the sides of the shipping channel, and an inflow (northeastward) usually occurring subsurface within or above the shipping channel. Averages over 30 d are used to examine variations in the residual fields. During the simulation the average surface salinity near the head of Tampa Bay varies with the freshwater inflow, from 12‰ to 33%. At the bay mouth salinity varies from 30%. to 36%.. A localized measure of the baroclinic circulation in the shipping channel indicates the residual circulation can vary strongly, attaining a magnitude triple the long-term mean value. The baroclinic circulation can be disrupted, going to near zero or even reversing, when the buoyancy-driven flow is weak and the surface winds are to the northeast. Three time periods, representing different environmental conditions, are chosen to examine these results in detail. A scaling argument indicates the relative strength of buoyancy versus wind as ΔρgH2(LC Dω2)−1, where δρ is head-to-mouth density difference across the bay,g is gravitational acceleration,H is depth,L is bay length,C D is the surface wind drag coefficient, andw is wind speed. Tampa Bay is usually in the buoyancy dominated regime. The importance of winds in the weak-buoyancy case is demonstrated in an additional simulation without wind stress.  相似文献   

12.
Summary A forward model is proposed to reproduce the formation of garnet under conditions of sluggish diffusion transport in the matrix. Starting from a matrix consisting of chlorite and quartz, the amount of garnet growth and the chemical composition was calculated at each P–T increment in the system MnO–FeO–MgO–Al2O3–SiO2–H2O. Sluggish diffusion transport was introduced considering the local equilibrium between garnet surface and the matrix within a given diffusion distance (equilibration volume). Varying the diffusion distance, calculations were performed along the prograde P–T path of the Sambagawa metamorphic belt, Japan. The final size of the garnet grains was largely proportional to the diffusion distance. In contrast to the model without diffusion limitations, a shorter diffusion distance resulted in a rise of the Mg/(Mg + Fe) ratio in garnet before Mn approached zero. These results indicate that the chemical composition trend in zoned garnet from the Sambagawa belt is consistent with growth under sluggish material transport. The calculated amount of garnet growth increases dramatically with temperature. The amount of newly grown natural garnet in the Sambagawa metamorphic rocks was plotted against temperatures, where chemical compositions of garnet were calibrated against temperatures with the Gibbs’ method. This trend was also consistent with the modelled garnet behaviour.  相似文献   

13.
Diamond crystallization has been studied in the SiO2–H2O–С, Mg2SiO4–H2O–С and H2O–С subsystems at 7.5 GPa and 1,600°C. We found that dissolution of initial graphite is followed by spontaneous nucleation of diamond and growth of diamond on seed crystals. In 15-h runs, the degree of graphite to diamond transformation [α = MDm/(MDm + MGr)100, where MDm is mass of obtained diamond and MGr mass of residual graphite] reached 100% in H2O-rich fluids but was only 35–50% in water-saturated silicate melts. In 40-h runs, an abrupt decrease of α has been established at the weight ratio H2O/(H2O + SiO2) ≤ 0.16 or H2O/(H2O + Mg2SiO4) ≤ 0.15. Our results indicate that α is a function of the concentration of water, which controls both the kinetics of diamond nucleation and the intensity of carbon mass transfer in the systems. The most favorable conditions for diamond crystallization in the mantle silicate environment at reliable PT-parameters occur in the fluid phase with low concentration of silicates solute. In H2O-poor silicate melts diamond formation is questionable.  相似文献   

14.
Death tolls from tornadoes in Bangladesh are the highest in the world due to lack of storm warnings, poor communication, weak housing, and lack of shelters from strong winds in tornadoes and nor’westers. Based on surveys of housing types and designs in the Tangail district, a household tornado shelter is proposed to be placed in the elevated storage platform that is common in houses. The shelter is 2 m tall, 1.2 m wide, and 2–4 m long (4.8–9.6 m3 in volume) with the floor of the shelter placed one meter below the floor of the house. Walls are 7–10 cm thick and made of concrete or an earthen wall stabilized with cement or strengthened with bamboo or bricks. A survey of 200 residents of the region found nearly universal acceptance for the shelter design, and residents were eager for installation of the household shelters. The shelter cost is 2,500–10,000 taka (US50 to50 to 200) depending on local material and labor costs but residents were willing to pay an average of only 1,071 taka (US$21) toward the cost of the shelter. Families with greater income and land holdings and families in villages with recent tornado experience were willing to spend more for a shelter. A pilot project to install household tornado shelters in selected villages and monitor their use, along with continued efforts to issue storm warnings, communicate the warnings, and improve education about storm hazards, will prevent injuries and save lives in Bangladesh and reduce the descent into poverty that results from losses in severe local storms.  相似文献   

15.
 Rock and flow parameters of three karstic-fissured-porous aquifers in the Krakow-Silesian Triassic formations were measured using various methods and compared. Though cavern and fissure porosities are shown to be very low (cavern porosity below 0.5% and fracture porosity below 0.2%), they contribute dominantly to the hydraulic conductivity (from about 1.3×10–6 to about 11×10–6 m/s). Matrix porosity (2–11%) is shown to be the main water reservoir for solute transport and the main or significant contributor to the specific yield (<2%). Though the matrix porosity is shown to be much larger than the sum of the cavern and fissure porosities, its contribution to the total hydraulic conductivity is practically negligible (hydraulic conductivity of the matrix is from about 5×10–11 m/s to about 2×10–8 m/s). On the other hand, the matrix porosity (for neglected cavern and fissure porosities) when combined with tracer ages (or mean travel times) is shown to yield proper values of the hydraulic conductivity (K) by applying the following formula: K≅(matrix porosity×mean travel distance)/(mean hydraulic gradient×mean tracer age). Confirming earlier findings of the authors, this equation is shown to be of great practical importance because matrix porosity is easily measured in the laboratory on rock samples, whereas cavern and fracture porosities usually remain unmeasurable. Received: 21 February 1997 · Accepted: 13 May 1997  相似文献   

16.
 H2O activities in concentrated NaCl solutions were measured in the ranges 600°–900° C and 2–15 kbar and at NaCl concentrations up to halite saturation by depression of the brucite (Mg(OH)2) – periclase (MgO) dehydration equilibrium. Experiments were made in internally heated Ar pressure apparatus at 2 and 4.2 kbar and in 1.91-cm-diameter piston-cylinder apparatus with NaCl pressure medium at 4.2, 7, 10 and 15 kbar. Fluid compositions in equilibrium with brucite and periclase were reversed to closures of less than 2 mol% by measuring weight changes after drying of punctured Pt capsules. Brucite-periclase equilibrium in the binary system was redetermined using coarsely crystalline synthetic brucite and periclase to inhibit back-reaction in quenching. These data lead to a linear expression for the standard Gibbs free energy of the brucite dehydration reaction in the experimental temperature range: ΔG° (±120J)=73418–134.95T(K). Using this function as a baseline, the experimental dehydration points in the system MgO−H2O−NaCl lead to a simple systematic relationship of high-temperature H2O activity in NaCl solution. At low pressure and low fluid densities near 2 kbar the H2O activity is closely approximated by its mole fraction. At pressures of 10 kbar and greater, with fluid densities approaching those of condensed H2O, the H2O activity becomes nearly equal to the square of its mole fraction. Isobaric halite saturation points terminating the univariant brucite-periclase curves were determined at each experimental pressure. The five temperature-composition points in the system NaCl−H2O are in close agreement with the halite saturation curves (liquidus curves) given by existing data from differential thermal analysis to 6 kbar. Solubility of MgO in the vapor phase near halite saturation is much less than one mole percent and could not have influenced our determinations. Activity concentration relations in the experimental P-T range may be retrieved for the binary system H2O-NaCl from our brucite-periclase data and from halite liquidus data with minor extrapolation. At two kbar, solutions closely approach an ideal gas mixture, whereas at 10 kbar and above the solutions closely approximate an ideal fused salt mixture, where the activities of H2O and NaCl correspond to an ideal activity formulation. This profound pressure-induced change of state may be characterized by the activity (a) – concentration (X) expression: a H 2O=X H 2O/(1+αX NaCl), and a NaCl=(1+α)(1+α)[X NaCl/(1+αX NaCl)](1+α). The parameter α is determined by regression of the brucite-periclase H2O activity data: α=exp[A–B/ϱH 2O ]-CP/T, where A=4.226, B=2.9605, C=164.984, and P is in kbar, T is in Kelvins, and ϱH 2O is the density of H2O at given P and T in g/cm3. These formulas reproduce both the H2O activity data and the NaCl activity data with a standard deviation of ±0.010. The thermodynamic behavior of concentrated NaCl solutions at high temperature and pressure is thus much simpler than portrayed by extended Debye-Hückel theory. The low H2O activity at high pressures in concentrated supercritical NaCl solutions (or hydrosaline melts) indicates that such solutions should be feasible as chemically active fluids capable of coexisting with solid rocks and silicate liquids (and a CO2-rich vapor) in many processes of deep crustal and upper mantle metamorphism and metasomatism. Received: 1 September 1995 / Accepted: 24 March 1996  相似文献   

17.
Calculated phase equilibria involving minerals and H2O–CO2–NaCl fluid lead to predictions of how infiltration of rock by H2O–NaCl fluids with X NaCl in the range 0–0.3 (0–58 wt% NaCl) drives the reactions calcite + quartz = wollastonite + CO2 and dolomite = periclase + calcite + CO2. Calculations focus on metamorphism in four aureoles that together are representative of the normal P–T conditions and processes of infiltration-driven contact metamorphic reactions. The effect of salinity on the spatial extent of oxygen isotope alteration was also computed. The time-integrated input fluid flux (q°) that displaces the mineral reaction front an increment of distance along the flow path always increases with increasing X NaCl. For input fluids with salinity up to approximately five times that of seawater (X NaCl ≤ 0.05), values of q° required to explain the spatial extent of decarbonation reaction are no more than 1.1–1.5 times that computed assuming the input fluid was pure H2O. For more saline fluids, values of q° may be up to 1.4–7.9 times that for pure H2O. Except for reaction in the presence of halite and vapor (V), infiltration of H2O–NaCl fluids expands the region of oxygen isotope alteration relative to the size of the region of mineral reaction. The expansion is significant only for saline fluids with X NaCl ≥ ~0.1. Immiscible fluid phase separation and differential loss of the liquid (L) or V phase from the mineral reaction site increases the amount of reactive fluid required to advance the mineral reaction front compared to conditions under which equilibration of minerals and fluid is attained with no loss of L or V. Decarbonation reactions driven by infiltration of fluids with even modest seawater-like salinity can explain the occurrence of salt-saturated fluid and solid halide inclusions in contact metamorphosed carbonate rocks.  相似文献   

18.
This paper presents data on burovaite-Ca, the first Ti-dominant member of the labuntsovite group with a calcium D-octahedron. The idealized formula of burovaite-Ca is (K,Na)4Ca2(Ti,Nb)8[Si4O12]4(OH,O)8 · 12H2O. The mineral has been found in the hydrothermal zone of aegirine-microcline pegmatite located in khibinite at Mt. Khibinpakhkchorr, the Khibiny pluton, Kola Peninsula, Russia. Radiaxial intergrowths of burovaite-Ca and labuntsovite-Mn associated with lemmleynite-Ba, analcime, and apophyllite have been identified in caverns within microcline. The mean composition of the mineral is as follows, wt %: 3.72 Na2O, 2.76 K2O, 4.22 CaO, 0.47 SrO, 0.23 BaO, 0.01 MnO, 0.30 Fe2O3, 0.14 Al2O3, 42.02 SiO2, 17.30 TiO2, 15.21 Nb2O5, 12.60 H2O (measured); the total is 98.98. Its empirical formula has been calculated on the basis of [(Si,Al)16O48]: {(Na3.10K1.07Ca0.37Sr0.04Ba0.04)4.62}(Ca1.28Zn0.01)1.29(Ti4.97Nb2.56Fe0.08Ta0.02)7.63(Si15.93Al0.07)16O48(OH6.70O0.93)7.63 · 12H2O. The strongest lines in the X-ray powder diffraction pattern of burovaite-Ca (I-d ?] are as follows: 70–7.08, 40–6.39, 40–4.97, 30–3.92, 40–3.57, 100–3.25, 70–3.11, 50–2.61, 70–2.49, 40–2.15, 50–2.05, 70–1.712, 70–1.577, and 70–1.444. The structure of burovaite-Ca was solved by A.A. Zolotarev, Jr. The mineral is monoclinic, space group C2/m. The unit-cell dimensions are a = 14.529(3), b = 14.203(3), c = 7.899(1), β = 117.37(1)°, V = 1447.57 ?3. Burovaite-Ca is an isostructural Ti-dominant analogue of karupm?llerite-Ca and gjerdingenite-Ca. Two stages of mineral formation—pegmatite proper and hydrothermal—have been recognized in the host pegmatite. The hydrothermal stage included K-Ba-Na, Na-K-Ca, and Na-Sr substages. Burovaite-Ca is related to the intermediate Na-K-Ca substage. At the first substage, labuntsovite-Mn and lemmleynite-Ba were formed, and tsepinite-Na, paratsepinite-Nd, and tsepinite-Sr were formed at the final substage. Thus, the sequence of crystallization of labuntsovite-group minerals is characterized by the replacement of the potassium regime by the sodium regime of alkaline solutions in the evolved host pegmatite.  相似文献   

19.
A refined thermodynamic model of H2O and CO2 bearing cordierite based on recent data on volatile incorporation into cordierite (Thompson et al. in Contrib Mineral Petrol 142:107–118, 2001; Harley and Carrington in J Petrol 42:1595–1620, 2001) reflects non-ideality of channel H2O and CO2 mixing. The dependence of cordierite H2O and CO2 contents on P, T and equilibrium fluid composition has been calculated for the range 600–800°C and 200–800 MPa. It has been used for establishing thermodynamic conditions of cordierite formation and the following retrograde P–T paths of cordierite rocks from many localities. Estimates of the H2O and CO2 activities have shown that cordierites in granites, pegmatites and high-pressure granulites were formed in fluid-saturated conditions and wide range of H2O/CO2 relations. Very low cordierite H2O contents in many migmatites may be caused not only by fluid-undersaturated conditions at rock formation and H2O leakage on retrograde P–T paths but also by the presence of additional volatile components like CH4 and N2. The pressure dependence of cordierite-bearing mineral equilibria on fluid H2O/CO2 relations has been evaluated.  相似文献   

20.
We report methane-dominant hydrocarbon (fluid) inclusions (CH4±C2H6–C2H2, C3H8) coexisting with primary brine inclusions and secondary halide melt (solid NaCl) inclusions in Au–Pt-rich quartz-sulfide-epidote alteration veins associated with the footwall-style Cu–PGE (platinum-group element)–Au deposits at the Fraser Mine (North Range of the Sudbury Igneous Complex). Evidence for coentrapment of immiscible hydrocarbon–brine, and hydrocarbon–halide melt mixtures is demonstrated. A primary CH4–brine assemblage was trapped during quartz growth at relatively low T (min. T trapping∼145–315°C) and P (max. P trapping∼500 bar), prior to the crystallization of sulfide minerals in the veins. Secondary inclusions contain solid halite and a mixture of CH4, C2H6–C2H2 and C3H8 and were trapped at a minimum T of ∼710°C. The halite inclusions may represent halide melt that exsolved from crystallizing sulfide ores that texturally postdate (by replacement) early alteration quartz hosting the primary, lower T brine–CH4 assemblage. Laser ablation ICP-MS analyses show that the brine, hydrocarbon and halide melt inclusions contain significant concentrations of Cu (0.1–1 wt% range), Au, Bi, Ag and Pt (all 0.1–10 ppm range). Cu:Pt and Cu:Au ratios in the inclusions are significantly (up to 4 log units) lower than in the host alteration veins and adjacent massive sulfide ore veins, suggesting either (1) early Cu loss from the volatiles by chalcopyrite precipitation or (2) enhanced Au and Pt solubilities relative to Cu at the temperatures of entrapment. Concentration ratios between coexisting brine and CH4 inclusions are lower for Cu, Au, Bi and Ag than for other elements (Na, Ca, Fe, Mn, Zn, Pb) indicating that during interaction with the brine, the hydrocarbon phase was enriched in ore metals. The high concentrations of ore metals in hydrocarbon, brine and halide melt phases confirm that both aqueous and non-aqueous volatiles were carriers of precious metals in the Sudbury environment over a wide range of temperatures. Volatile evolution and magmatic sulfide differentiation were clearly part of a single, continuous process in the Sudbury footwall. The exsolution of H2O-poor volatiles from fractionated sulfide liquid may have been a principal mechanism controlling the final distribution of PGE and Au in the footwall ore systems. The study reports the first measurements of precious metal concentrations in fluid inclusions from a magmatic Ni–Cu–PGE environment (the Sudbury district). Electronic Supplementary Material Supplementary material is available for this article at  相似文献   

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