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1.
Authigenic carbonates and seep biota are archives of seepage history and record paleo-environmental conditions at seep sites. We obtained the timing of past methane release events at the northeastern slope of the South China Sea based on U/Th dating of seep carbonates and seep bivalve fragments from three sites located at 22°02′–22°09′N, 118°43′–118°52′E (water depths from 473 to 785 m). Also, we were able to reconstruct the paleo-bottom water temperatures by calculating the equilibrium temperature using the ages, the corresponding past δ18O of seawater (δ18Osw) and the δ18O of the selected samples formed in contact with bottom seawater with negligible deep fluid influence. A criterion consists of mineralogy, redox-sensitive trace elements and U/Th-isotope systematics is proposed to identify whether the samples were formed from pore water or have been influenced by deep fluid. Our results show that all methane release events occurred between 11.5 ± 0.2 and 144.5 ± 12.7 ka, when sea level was about 62–104 m lower than today. Enhanced methane release during low sea-level stands seems to be modulated by reduced hydrostatic pressure, increased incision of canyons and increased sediment loads. The calculated past bottom water temperature at one site (Site 3; water depth: 767–771 m) during low sea-level stands 11.5 and 65 ka ago ranges from 3.3 to 4.0 °C, i.e., 1.3 to 2.2 °C colder than at present. The reliability of δ18O of seep carbonates and bivalve shells as a proxy for bottom water temperatures is critically assessed in light of 18O-enriched fluids that might be emitted from gas hydrate and/or clay dehydration. Our approach provides for the first time an independent estimate of past bottom water temperatures of the upper continental slope of the South China Sea.  相似文献   

2.
Despite the gently dipping slopes (ca 1°), large-scale submarine slope failures have occurred on the mid-Norwegian continental margin (Storegga, Sklinnadjupet, Traenadjupet), suggesting the presence of special conditions predisposing to failure in this formerly glaciated margin. With a volume estimated between 2,400 and 3,200 km3 and an affected area of approximately 95,000 km2, the Storegga slide represents one of the largest and best-studied submarine slides of Holocene age known worldwide. Finite element modeling of slope failure indicates that a large (6.5 < Ms < 7.0) seismic triggering mechanism would not be sufficient to cause failure at more than 110 m below the seabed as observed for the slip planes at Storegga (northern sidewall). This implies that other factors (e.g., liquefaction, strain softening, gas charging, rapid burial) are needed to explain the occurrence of the Storegga slide with a deep surface of failure. In this paper, we discuss the importance of the compaction effect of rapidly accumulated sediments in the slide area. During compaction, sediment grains reorganize themselves, thereby, expelling pore water. Consequently, depending on sedimentation rate and permeability, excess pore pressures might result beneath less permeable sediments. Our modeling and cross-checking illustrate how excess pore pressure generation due to high sedimentation rate could explain the development of layers of weakness, and thus, how such a large slide might have been initiated in deep sediments. Using the highest sedimentation rate estimated in the area (36 and 27 m/kyr between 16.2 and 15 kyr BP), 1D modeling shows excess pore pressure values of around 200 kPa at a depth of 100 m below the seafloor 15 kyr BP and 60 kPa at a depth of 100 m at the time of the slide (8 kyr BP). Excess pore pressure apparently drastically reduced the resistance of the sediment (incomplete consolidation). In addition, 2D modeling shows that permeability anisotropies can significantly affect the lateral extent of excess pore pressure dissipation, affecting, that way, normally consolidated sediments far from the excess pore pressure initiation area.  相似文献   

3.
We present rock mechanical test results and analytical calculations which demonstrate that a negative surface charge, resulting from sulfate adsorption from the pore water, impacts the rock mechanical behavior of high-porosity chalk. Na2SO4 brine flooded into chalk cores at 130 °C results in significantly reduced bulk modulus and yield point compared with that of NaCl brine at the same conditions. The experimental results have been interpreted using a surface complexation model combined with the Gouy-Chapman theory to describe the double layer. The calculated sulfate adsorption agrees well with the measured data. A sulfate adsorption of about 0.3 μmol/m2 and 0.7–1 μmol/m2 was measured at 50 and 130 °C, respectively. Relative to a total sites of 5 sites/nm2 these values correspond to an occupation of 4 % and 8–13 % which sufficiently explains the negative charging of the calcite surfaces. The interaction between charged surfaces specifically in the weak overlaps of electrical double layer gives rise to the total disjoining pressure in granular contacts. The net repulsive forces act as normal forces in the grains vicinity, counteracting the cohesive forces and enhance pore collapse failure during isotropic loading, which we argue to account for the reduced yield and bulk modulus of chalk cores. The effect of disjoining pressure is also assessed at different sulfate concentrations in aqueous solution, temperatures, as well as ionic strength of solution; all together remarkably reproduce similar trends as observed in the mechanical properties.  相似文献   

4.
Layers from one manganese nodule dredged from the Philippine Sea(16°56'N, 129°48'E; water depth, 5700 m) and 45 bulk nodules from offshore Minami-Torishima Island, Japan(23°3'N, 153°22'E; water depth, 1200 m) were analyzed chemically and their origin is discussed based on geochemical constraints. In general, Cu, Ni, Zn and Mo tend to increase with increasing Mn content, while Co, Pb, Ba, V, Sc, Th, and the rare earth elements(REEs) show less variation with increasing Mn content. Nodule 42 H from the Philippine Sea has an average Mn/Fe ratio close to 1 and shows a positive Ce anomaly, suggesting a predominant hydrogenous origin. Profiles of 230Th230 ex and Thex/232 Th ratios in the outer ~0.3 mm of nodule 42 H indicate a steady growth rate of ~1.7 mm/Myr. Nodule E30 from offshore Minami-Torishima is characterized by lower Mn, Fe, Mn/Fe(0.53) and Mo/V(0.2) ratios but higher P and Cu/Ni(0.31) ratio relative to other nodules from that area. The Ce content of E30 is unusually low(82 ppm) when compared with other nodules from the area and it is the only nodule analyzed with a negative Ce anomaly(-0.64). Based on the geochemical data we suggest that most nodules from offshore Minami-Torishima are primarily of hydrogenous origin except E30, which is dominated by hydrothermal input, and E45, which has about a 35% hydrothermal contribution.  相似文献   

5.
Cyclic mobility is a mechanism of ground failure due to lateral spreading of soils during an earthquake that usually occurs in soft or medium stiff saturated soils. The simplified procedures developed by the researchers give a factor of safety for judging the cyclic mobility potential. However, the simplified procedures do not take into account the uncertainty in the parameters required to estimate the cyclic stresses in the soil. In this study, a reliability framework based on the simplified procedure, considering the parameter uncertainty, has been proposed for computing the probability of cyclic mobility of clay deposits for a metro city of India, i.e., Mumbai city (latitudes 18°53′N–19°19′N and longitudes 72°47′E–72°58′E). Extensive geotechnical borehole data from 1028 boreholes across 50 locations in the city area of 390 km2 and laboratory test data are collected and analyzed thoroughly. A correlation between undrained shear strength (Su) and other parameters such as natural water content (w), SPT N value, liquid limit (LL) and plasticity index (PI) has been established for Mumbai city and has been used in the proposed approach. The sensitivity analysis of the proposed approach predicts that Su has significant influence in the evaluation of the cyclic mobility. Cyclic mobility hazard maps are prepared using the geo-statistical analysis tool in GIS, and it shows that the clayey soils at few locations have a 60–90 % probability of cyclic mobility for a moment magnitude (M w) of an earthquake of 7.5. These hazard maps can be used by the geotechnical engineers for the cyclic mobility hazard assessment of Mumbai city.  相似文献   

6.
Understanding the pore water conversion characteristics during hydrate formation in porous media is important to study the accumulation mechanism of marine gas hydrate. In this study, low-field NMR was used to study the pore water conversion characteristics during methane hydrate formation in unsaturated sand samples. Results show that the signal intensity of T2 distribution isn’t affected by sediment type and pore pressure, but is affected by temperature. The increase in the pressure of hydrogen-containing gas can cause the increase in the signal intensity of T2 distribution. The heterogeneity of pore structure is aggravated due to the hydrate formation in porous media. The water conversion rate fluctuates during the hydrate formation. The sand size affects the water conversion ratio and rate by affecting the specific surface of sand in unsaturated porous media. For the fine sand sample, the large specific surface causes a large gas-water contact area resulting in a higher water conversion rate, but causes a large water-sand contact area resulting in a low water conversion ratio (Cw=96.2%). The clay can reduce the water conversion rate and ratio, especially montmorillonite (Cw=95.8%). The crystal layer of montmorillonite affects the pore water conversion characteristics by hindering the conversion of interlayer water.©2022 China Geology Editorial Office.  相似文献   

7.
A probabilistic 3-D slope stability analysis model (PTDSSAM) is developed to evaluate the stability of embankment dams and their foundations under conditions of staged construction taking into consideration uncertainty, spatial variabilities and correlations of shear strength parameters, as well as the uncertainties in pore water pressure. The model has the following capabilities: (1) conducting undrained shear strength analysis (USA) and effective stress analysis (ESA) slope stability analysis of staged construction, (2) incorporation of field monitored data of pore water pressure, and (3) incorporation of increase of undrained shear strength with depth, effective stress, and pore water pressure dissipation. The PTDSSAM model is incorporated in a computer program that can analyze slopes located in multilayered deposits, considering the total slope width.

The main outputs of the program are the geometric parameters of the most critical sliding surface (i.e., center of rotation/radius of rotation and critical width of failure), mean 2-D safety factor, mean 3-D safety factor, squared coefficient of variation of resisting moment, and the probability of slope failure. The program is applied to a case study, Karameh dam in Jordan. Monitored data of induced pore water pressure in the dam embankment and soft foundation were gathered during dam construction.

The stability of Karameh dam embankment and foundation was evaluated during staged construction using deterministic and probabilistic analysis. Foundation stability was evaluated based on the monitored data of pore water pressure.

The study showed that the mean values of the corrective factors which account for the discrepancies between the in situ and laboratory-measured values of soil properties and for the modeling errors have significant influence on the 2-D safety factor, 3-D safety factor, slope probability of failure, and on the expected failure width.

The degree of spatial correlation associated with shear strength parameters within a soil deposit also influences the probability of slope failure and the expected failure width. This correlation is quantified by scale of fluctuation. It is found that a larger scale of fluctuation gives an increase in the probability of slope failure and a reduction in the critical failure width.  相似文献   


8.
Extracting the contiguous coal seams under the lowermost aquifer in the unconsolidated Cenozoic alluvium is apt to water and quicksand inrush. By using a series of investigation methods including empirical formulas, numerical simulation, theoretical analysis, etc., the study focused on the fracture and the excess pore water pressure in the overlying strata in the process of extracting no. 8 coal seam firstly and no. 9 coal seam (under no. 8 coal seam) subsequently in no. 8102 working face of Luling coal mine in the north of Anhui Province of China. When no. 8 coal seam was extracted, the water-conducting fractured zone penetrated into the lowermost aquifer and rapid dissipation of excess pore water pressure above the gob occurred, accompanied by relatively high seepage hydraulic gradient over the headgate and the tailgate. When no. 9 coal seam was extracted, failure did not obviously extend upwards and the excess pore water pressure decreased slowly and a relatively high seepage hydraulic gradient transferred downwards from the headgate to the tailgate in the inclined profile. The safe water head (H s) in the lowermost aquifer was confirmed to 15.6 m. Therefore, water and quicksand inrush was avoided effectively in the process of extracting the contiguous coal seams by dewatering, controlling mining height, and laying double resistance nets in the working face.  相似文献   

9.
The shear strength of cohesionless soil is reduced as the water pressure inside the pores of the soil mass increases. The mathematical relationship between the shear strength and the pore water pressure was derived using Mohr–Coulomb failure criteria as a function of the confining pressure and the effective angle of friction. Experimentally, a series of consolidated drained triaxial tests with back pore water pressure was run on samples of saturated uniform dense sand. The tests were conducted at different confining pressures in the range of 100–400 kPa with an increment of 100 kPa. At each level of confining pressure, the tests were repeated at different values of back pore water pressure in the range of 0–100 kPa with an increment of 25 kPa. For each test, the initial applied back pore water pressure was kept constant during the test for comparing the results at the same effective confining pressure. This study concludes that the mathematical relationship gives accurate results at any level of confining pressure and/or pore water pressure as a function of the effective angle of friction that can be evaluated using single consolidated drained triaxial test at zero back pore water pressure.  相似文献   

10.
A full-scale landslide-triggering experiment was conducted on a natural sandy slope subjected to an artificial rainfall event, which resulted in mobilisation of 130 m3 of soil mass. Novel slope deformation sensors (SDSs) were applied to monitor the subsurface pre-failure movements and the precursors of the artificially triggered landslide. These fully automated sensors are more flexible than the conventional inclinometers by several orders of magnitude and therefore are able to detect fine movements (<?1 mm) of the soil mass reliably. Data from high-frequency measurements of the external bending work, indicating the transmitted energy from the surrounding soil to these sensors, pore water pressure at various depths, horizontal soil pressure and advanced surface monitoring techniques, contributed to an integrated analysis of the processes that led to triggering of the landslide. Precursors of movements were detected before the failure using the horizontal earth pressure measurements, as well as surface and subsurface movement records. The measurements showed accelerating increases of the horizontal earth pressure in the compression zone of the unstable area and external bending work applied to the slope deformation sensors. These data are compared to the pore water pressure and volumetric water content changes leading to failure.  相似文献   

11.
The stress–strain behaviour of a soil is crucial to the recognition of the mechanism of slope failure. Triaxial tests, composed of isotropically consolidated drained (ICD) and undrained compression (ICU) tests and anisotropically consolidated constant deviatoric stress path (CQD) tests, were carried out with the aim of investigating the stress–strain behaviour of loosely compacted volcanic-derived soils. The fact that the critical states are the same for ICU, ICD and CQD tests may show that the critical state is independent of the above three stress paths. The critical state line, as defined in critical state soil mechanics, is obtained from the e–log p′ and q–p′ plots based on the results of the above tests. The initial state of the consolidated specimens at initiation of failure may be classified as dilative or contractive in the light of the locations of the soil state relative to the critical state line. For contractional soils, the increased pore water pressure generated by rainfall infiltration leads to a contractive failure in a drained manner, giving rise to high excess pore water pressure. The excess pore water pressure caused by contraction cannot be dissipated instantly, resulting in a decrease in the shear resistance of the soil. The failure process is rapid. The failed soil mass is prone to flow after failure under the action of gravity due to its high moisture content and inflow of surface runoff and rainwater. For dilational soils, the increased pore water pressure resulting from infiltration leads to dilation, which reduces pore water pressure and thus increases the shear resistance of the soil. However, continued rainfall infiltration may be able to equilibrate the reduction in pore water pressure caused by dilation and, therefore, the dilation or displacement can continue. In Hong Kong, volcanic-derived soil is characterized by high permeability. Both the high permeability of volcanic-derived soil and a shallow failure surface make it possible for the reduction in pore pressure to equilibrate relatively quickly. Therefore, the failure is also rapid, at least for poorly compacted fill slopes.  相似文献   

12.
《Applied Geochemistry》2001,16(2):161-181
Thermodynamic properties of water, in various families of hydroxides, oxihydroxides and hydrates (chlorides, chlorates, sulfates and sulfites …), have been calculated by using a large number of data available in the literature. A phase diagram of water has been used to find the first complete set of thermodynamic properties at 298 K, 1 bar of 8 ice polymorphs, from Ih (hexagonal ice, the common polymorph) to IX (very low temperature and high pressure polymorph). These results are used to illustrate the concept of ‘ice-like water’ available for a very large number of hydrated phases (noted X.H2O) in which water is attached to the corresponding anhydrous substrate (noted X) within a large spectrum of different enthalpies (ΔfH°) or Gibbs free energies (ΔfG°), but within a relatively small range of others properties. Heat capacity (Cp°), entropy (S°), and volume (V°) of hydration water (X.H2O−X=H2O) appeared to be very close to those characterizing ice polymorphs such as ice II or ice VIII. This concept allows the authors to propose a classification of minerals in terms of affinity for water and to predict the relative stability of hydrated and dehydrated phases under climatic variations.  相似文献   

13.
An analytical solution to 1D coupled water infiltration and deformation in layered soils is derived using a Laplace transformation. Coupling between seepage and deformation, and initial conditions defined by arbitrary continuous pore‐water pressure distributions are considered. The analytical solutions describe the transient pore‐water pressure distributions during 1D, vertical infiltration toward the water table through two‐layer unsaturated soils. The nonlinear coupled formulations are first linearized and transformed into a form that is solvable using a Laplace transformation. The solutions provide a reliable means of comparing the accuracy of various numerical methods. Parameters considered in the coupled analysis include the saturated permeability (ks), desaturation coefficient (α), and saturated volumetric water content (θs) of each soil layer, and antecedent and subsequent rainfall infiltration rates. The analytical solution demonstrates that the coupling of seepage and deformation plays an important role in water infiltration in layered unsaturated soils. A smaller value of α or a smaller absolute value of the elastic modulus of the soil with respect to a change in soil suction (H) for layered unsaturated soils means more marked coupling effect. A smaller absolute value of H of the upper layer soil also tends to cause more marked coupling effect. A large difference between the saturated coefficients of permeability for the top and bottom soil layers leads to reduced rainfall infiltration into the deep soil layer. The initial conditions also play a significant role in the pore‐water pressure redistribution and coupling effect. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

14.
For evaluation of slope stability in materials displaying strain-softening behavior, knowledge concerning the failed state material response is of importance. Here, soft sensitive clay is studied. Such clays behave contractant at failure, which for undrained conditions yields a strain-softening behavior governed by the generation of excess pore water pressure. Strain softening is further linked with material instability and the phenomenon of strain localization. In the case of shear band formation, internal pore pressure gradients are then expected to be present for globally undrained conditions in the sensitive clay due to its low permeability. In the present study, this hypothesis and its implications on the global response and shear band properties are investigated. Utilizing an experimental setup with a modified triaxial cell allowing for shear band formation, the effect of varying the displacement rate is studied. Onset of strain localization is interpreted to occur just before or at the peak shear strength. A strong rate dependency of the softening response is observed. Increasing displacement rates give raised brittleness in terms of the slope of the global softening curve due to accumulating pore pressure. Also, reduced shear band thickness and a shear band inclination approaching 45° are obtained for increasing rates. In the context of slope failure in such materials, the rate dependency in the post-peak state opens up for a large variation in behavior, all depending on time as an important factor.  相似文献   

15.
In 2007, we demonstrated that radiolarians are proxies for a wide range of oceanic physico-chemical properties from the surface to depths of up to 500 m below sea level. In this study, our results are refined and Correspondence Analysis (CA) scores derived from census counts of radiolarian subfossils from southern Indian Ocean core-tops are correlated with the physico-chemical properties of the region obtained from the 2005 World Ocean Database.Calibration and regression techniques are employed to reconstruct palaeoenvironmental conditions spanning the last 40 ka for four Indian Ocean cores MD88-769 [46°04′S 90°06′E], MD88-770 [46°01′S 96°27′E], MD94-102 [43°30′S 79°50′E], and MD94-103 [45°35′S 86°31′E], all from close to the Southeast Indian Ridge. For the first time, reconstructions of temperature, salinity, dissolved oxygen, and the silicate, nitrate, and phosphate concentrations for a range of water depths are proved possible.Changes of the oceanic environment and the movement of water masses over the last 40 ka, as suggested by these reconstructions, are discussed. During Marine Isotope Stages 2 and 3 (MIS-2 and MIS-3), the water column at some of the core sites has similar characteristics to the waters south of the Polar Front today. At the MIS-1/MIS-2 transition, the development of the Subantarctic Mode Water is apparent. Temperature reconstructions include evidence of the Antarctic Cold Reversal and the Holocene Optimum.  相似文献   

16.
The beginning of melting in the system Qz-Or-Ab-An-H2 O was experimentally reversed in the pressure range kbar using starting materials made up of mixtures of quartz and synthetic feldspars. With increasing pressure the melting temperature decreases from 690° C at 2 kbar to 630° C at 17 kbar in the An-free alkalifeldspar granite system Qz-Or-Ab-H2O. In the granite system Qz-Or-Ab-An-H2O the increase of the solidus temperature with increasing An-content is only very small. In comparison to the alkalifeldspar granite system the solidus temperature increases by 3° C (7° C) if albite is replaced by plagioclase An 20 (An 40). The difference between the solidus temperatures of the alkalifeldspar granite system and of quartz — anorthite — sanidine assemblages (system Qz-Or-An-H2O) is approximately 50° C. With increasing water pressures plagioclase and plagioclase-alkalifeldspar assemblages become unstable and are replaced by zoisite+kyanite+quartz and zoisite+muscovite-paragonitess +quartz, respectively. The pressure stability limits of these assemblages are found to lie between 6 and 16 kbar at 600° C. At high water pressures (10–18 kbar) zoisite — muscovite — quartz assemblages are stable up to 700 and 720° C. The solidus curve of this assemblage is 10–20° C above the beginning of melting of sanidine — zoisite — muscovite — quartz mixtures. The amount of water necessary to produce sufficient amounts of melt to change a metamorphic rock into a magmatic looking one is only small. In case of layered migmatites it is shown that 1 % of water (or even less) is sufficient to transform portions of a gneiss into (magmatic looking) leucosomes. High grade metamorphic rocks were probably relatively dry, and anatectic magmas of granitic or granodioritic composition are usually not saturated with water.  相似文献   

17.
简涛  孔令伟  柏巍  舒荣军 《岩土力学》2023,(8):2238-2248
通过一系列不排水动三轴试验探究了饱和黄土振动液化过程中孔隙水压力和累积耗散能量的演化模式,并讨论了围压、动应力幅值和固结应力比对其演化过程的影响。结果表明：饱和黄土的孔隙水压力和耗散能量随着循环荷载作用逐渐累积。固结围压抑制孔隙水压力增长而消耗更多能量;更大的动应力幅值使得孔隙水压力增长更快而消耗能量更少;等压固结下,孔隙水压力增长至围压从而触发初始液化,而偏压固结下,通常先达到振动液化应变标准而孔隙水压力并没有增长至围压水平,并且固结应力比越大,液化时孔隙水压力越小,消耗能量也更少。归一化孔隙水压力u/σ0’与累积耗散能量W/Wf之间关系受围压、循环应力比和固结应力比影响较小,可统一用双曲线模型表示。  相似文献   

18.
Liquefaction of soils is a natural phenomenon associated with a dramatic loss of the soil shear strength in undrained conditions due to a development of excess pore water pressure. It usually causes extensive damages to buildings and infrastructures during earthquakes. Thus, it is important to evaluate extent of influential parameters on the liquefaction phenomenon of soils in order to clearly understand the different mechanisms leading to its triggering. The soil gradation is one of the most important parameters affecting the liquefaction phenomenon. In this context, a series of undrained compression triaxial tests were carried out on eighteen natural loose (Dr = 25%) sandy samples containing low plastic fines content of 2% (Ip = 5%) considering different extreme sizes (1.6 mm ≤ Dmax ≤ 4 mm and 0.001 mm ≤ Dmin ≤ 0.63 mm) and two mean grain size ranges (0.25 mm ≤ D50 ≤ 1.0 mm) and (1.0 mm ≤ D50 ≤ 2.5 mm). The initial confining pressure for all tests was kept constant (P′c = 100 kPa). The obtained test results indicate that the mean grain size (D50) and extreme grain sizes (Dmax and Dmin) have a significant influence on the undrained shear strength (known as liquefaction resistance) and appear as pertinent factors for the prediction of the undrained shear strength for the soil gradation under study. The undrained shear strength and the excess pore water pressure can be correlated to the extreme grain sizes (Dmax and Dmin) and the mean grain size (D50) of tested wet deposited samples.  相似文献   

19.
This paper examines the potential of relevance vector machine (RVM) in slope stability analysis. The nonlinear relationship between slope stability and its influence factors is presented by the relevance vector learning mechanism based on a kernel‐based Bayesian framework. The six input variables used for the RVM for the prediction of stability slope are density (γ), friction angle (C), friction coefficient (?), slope angle (?r), slope height (H), and pore water pressure (ru). Comparison of RVM with some other methods is also presented. RVM has been used to compute the error bar. The results presented in this paper clearly highlight that the RVM is a robust tool for the prediction of slope stability. The experimental results show the effectiveness of the proposed approach. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

20.
An increasing number of occurrences of margarite have been reported in the last years. However, previous experimental investigations in the system CaO-Al2O3-SiO2-H2O are limited to the synthesis of margarite and to the upper stability limit according to the reaction (1) 1 margarite?1 anorthite +1 corundum +1 H2O (Chatterjee, 1971; Velde, 1971). Since margarite often occurs together with quartz, the upper stability limit of margarite in the presence of quartz is of special interest. Therefore, the reactions (5) 1 margarite +1 quartz ?1anorthite +1 kyanite/andalusite +1 H2O and (6) 4 margarite+3 quartz ? 2 zoisite+5 kyanite+3 H2O were investigated experimentally using mixtures of natural margarite (from Chester, Mass., USA), quartz, kyanite, andalusite, zoisite, and synthetic anorthite. The indicated equilibrium temperatures at water pressures equal to total pressure are: 515± 25°C at 4 kb, 545 ±15°C at 5 kb, 590±10°C at 7 kb, and 650±10°C at 9 kb for reaction (5), and 651±11°C at 10 kb, 648 ± 8°C at 12.5kb, and 643±13°C at 15kb for reaction (6), respectively. Besides this, additional brackets for equilibrium temperatures were determined for the above cited reaction (1): 520±10°C at 3 kb, 580±10°C at 5 kb, and 640± 20°C at 7 kb. On the basis of these experimentally determined reactions (1), (5), and (6) and of the reactions (3) 2 zoisite +1 kyanite? 4 anorthite +1 corundum +1 H2O (7) 2 zoisite +1 kyanite +1 quartz ? 4 anorthite +1 H2O and (10) 1 pyrophyllite ? 1 andalusite/kyanite+3 quartz+1 H2O for which experimental or, in the case of reaction (3), calculated data were already available, a pressure-temperature diagram with 3 invariant points and 11 univariant reactions was developed using the method of Schreinemakers. This diagram, summarizing both experimental and phase relation studies, allows conclusions about the conditions under which margarite has been formed in nature. Margarite is limited to low grade metamorphism at water pressures up to approximately 3.5 kb; in the presence of quartz, margarite is even limited to low grade metamorphism at water pressures up to 5.5 kb. Only at water pressures higher than the values stated before margarite, and margarite+quartz, respectively, can occur in medium grade metamorphism (as defined by Winkler, 1970 and 1973). For the combined occurrence of margarite+quartz and staurolite as reported by Harder (1956) and Frey (personal communication, 1973) it may be estimated that water pressure has been greater than approximately 5.5 kb, wheras temperature has been in the range from 550 to 650°C. Furthermore, the present study shows that the assemblage zoisite+kyanite (+ H2O) is an indicator of both pressure [P H 2 O> approximately 9kb]and temperature [T> approximately 640 to 650° Cat water Pressures up to 15 kb].  相似文献   

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