共查询到20条相似文献，搜索用时 31 毫秒
1.
We investigate the use of a ductile material with temperature-sensitive viscosity for thermomechanical modelling of the lithosphere. First, we consider the scaling of mechanical and thermal properties. For a normal field of gravity, the balance of stresses and body forces sets the stress scale, in proportion to the linear dimensions and the densities. The equation of thermal conduction sets the time scale. The activation enthalpy for creep sets the temperature scale; but the thermal expansivity provides an additional constraint on this temperature scale. Gum rosin appears to be a suitable material for lithospheric modelling. We have measured its flow properties, at various temperatures, in a specially designed rotary viscometer with unusually low machine friction. The rosin is almost Newtonian. Strain rate depends upon stress to the power n, where 1.0 <n < 1.14. The viscosity varies over 5 orders of magnitude, from about 102 Pa s at 80°C, to about 107 Pa s at 40°C. The activation enthalphy is thus about 250 kJ/mol. Measured with a needle probe, the thermal conductivity is 0.113 ± 0.001 W m−1K−1; the thermal diffusivity, (6±3) ×10−7 m2 s−1. Calculated from X-ray profiles, the thermal expansivity is about 3 × 10−4 K−1. These thermal and mechanical properties make gum rosin suitable for thermomechanical models, where linear dimensions scale down by a factor of 106; time, by 1011; viscosity, by 1017; and temperature change, by 101. 相似文献
2.
Several shales and oils ranging in age from 3 million to 2·7 billion years have been investigated for their hydrocarbon content using gas chromatography and mass spectrometry as primary analytical tools. From the Soudan Shale from Minnesota (2·7 × 10 9yr) the C 18, C 19, C 20 and C 21 isoprenoid-alkanes were obtained. The Antrim shale from Michigan (about 265 × 10 6 yr) yielded the C 16, C 18, C 19, C 20 and C 21 isoprenoids, as well as a C 16 iso-alkane and the C 18 and C 19 cyclohexyl n-alkanes. The San Joaquin Oil (30 × 10 6 yr) and the Abbott Rock Oil (3 × 10 6 yr) contained the C 16, C 18, C 19, C 20 and the C 18, C 19, C 20 and C 21 isoprenoids respectively. In addition, a series of iso-alkanes (C 16−C 18), anteiso-alkanes (C 16−C 18) and n-alkylcyclohexanes (C 16−C 19) as well as a C 21 isoprenoid were obtained from the Nonesuch Seep Oil (1 × 10 9 yr). This analysis provides a comprehensive picture of the types of biogenic hydrocarbons found in oils and shales of widely differing ages, and in particular, the finding of isoprenoid alkanes in the Soudan Shale furnishes evidence for life processes at that period of geological time. 相似文献
3.
The production rate of 38Ar in meteorites—P(38)—has been determined, as a function of the sample's chemical composition, from 81Kr-Kr exposure ages of four eucrite falls. The cosmogenic 78Kr/ 83Kr ratio is used to estimate the shielding dependence of P(38). From the “true” 38Ar exposure ages and the apparent 81Kr-Kr exposure ages of nine Antarctic eucrite finds, terrestrial ages are calculated. They range from about 3 × 105 a (Pecora Escarpment 82502) to very recent falls (Thiel Mountains 82502). Polymict eucrites from the Allan Hills (A78132, A79017 and A81009) have within the limits of error the same exposure age (15.2 × 106 a) and the same terrestrial age (1.1 × 105 a). This is taken as strong evidence that these meteorites are fragments of the same fall. A similar case are the Elephant Moraine polymict eucrites A79005, A79006 and 82600 with an exposure age of 26 × 106 a and a terrestrial age of 1.8 × 105 a. EETA79004 may be different from this group because its exposure age and terrestrial age are 21 × 106 a and 2.5 × 105 a, respectively. The distribution of terrestrial ages of Allan Hills meteorites is discussed. Meteorites from this blue ice field have two sources: Directly deposited falls and meteorites transported to the Allan Hills inside the moving Antarctic ice sheet. During the surface residence time meteorites decompose due to weathering processes. The weathering “half-life” is about 1.6 × 105 a. From the different age distributions of Allan Hills and Yamato meteorites, it is concluded that meteorite concentrations of different Antarctic ice fields need different explanations. 相似文献
4.
We present a model of the subducting Cocos slab beneath Central Mexico, that provides an explanation for stresses causing the occurrence of the majority of the intraslab earthquakes which are concentrated in a long flat segment. Based on the recently developed thermal models for the Central Mexico subduction zone, the thermal stresses due to non-uniform temperature contrast in the subducting slab are calculated using a finite element approach. The slab is considered purely elastic but due to high temperature at its bottom the behavior is considered as ductile creep. The calculation results show a 20 km slab core characterized by a tensional state of stress with stresses up to 70 MPa. On the other hand, the top of the slab experiences high compressive thermal stresses up to 110 MPa, depending on the elastic constants used and location along the flat part of the subducting plate. These compressive stresses at the top of the slab are not consistent with the exclusive normal fault intraslab earthquakes, and two different sources of stress are proposed. The trenchward migration of the Mexican volcanic arc for the last 7 Ma indicates an increase of the slab dip through time. This observation suggests that the gravity torque might exceed the suction torque. Considering the flat slab as an embedded plate subject to an applied clockwise net torque of 0.5 × 1016–1.5 × 1016 N m, the upper half would exhibit tensional stresses of 40–110 MPa that can actually balance the compressive thermally induced stresses. An alternative stress source might come from the slab pull force caused by the slab positive density anomaly. Based on our density anomaly estimations (75 ± 20 kg/m3), a 350 km slab length, dipping at 20° into the asthenosphere, induces a slab pull force of 1.7 × 1012–4.6 × 1012 N m. This force produces a tensional stress of 41–114 MPa, sufficient to balance the compressive thermal stresses at the top of the flat slab. The linear superposition of the thermally and torque or slab pull induced stresses shows tensile stresses up to 60–180 MPa inside the flat slab core. Also, our results suggest that the majority of the intraslab earthquakes inside the flat slab are situated where the resultant stresses are larger than 40–80 MPa. This study provides a reasonable explanation for the existence of exclusively normal fault intraslab earthquakes in the flat slab beneath Central Mexico, and also it shows that thermal stresses due to non-uniform reheating of subducting slabs play a considerable role in the total stress field. 相似文献
5.
The Orust dykes and their regional correlatives are an important time marker in the polymetamorphic Östfold-Marstrand belt of S.E. Norway and S.W. Sweden. They were deformed and metamorphosed in the amphibolite facies during the Sveconorwegian (= Grenville) orogeny. Three narrow strongly deformed dykes yield Rb---Sr whole-rock ages (1106 ± 52 m.y., 1062 ± 90 m.y., 1000 ± 153 m.y.). The weighted mean age (1087 m.y.) is interpreted as the age of the first post-dyke deformation and the accompanying amphibolite facies metamorphism. High initial 87Sr/ 86Sr ratios (0.7074 ± 9, 0.710 ± 2, 0.713 ± 3, respectively) and other geochemical evidence point to considerable exchange of mobile elements between the dykes and the country rocks. The emplacement age is not known precisely. However, the dykes may be correlated with widespread mafic magnatism which accompanied crustal instability in the N. Atlantic region early in the Sveconorwegian (= Grenville) orogeny about 1200 m.y. ago. 相似文献
6.
Strain-controlled cyclic triaxial tests were performed on a one-size silica (Ottawa) sand artificially frozen into 71.1-mm-diameter cylindrical samples. Ice-saturated samples with three different sand contents were tested under the following conditions: axial strains ranging from 3 × 10 −3 to 3 X 10 −2%, confining pressures from zero to 1.378 MPa, frequencies of 0.05–5.0 cps and temperatures from −1 to −10 °C. Test equipment included (1) an MTS electrohydraulic closed-loop testing system which applies the load to the sample, (2) a triaxial cell completely immersed in a low-temperature coolant for temperature control, (3) a refrigeration unit for control of the coolant temperature and constant coolant circulation and (4) measuring devices including an LVDT and load cell, together with recording devices such as a digital multimeter, an oscilloscope, a strip-chart recorder and a minicomputer. Test results indicate that the dynamic Young's modulus increases with increasing frequency, confining pressure and sand content, but decreases with increasing strain and temperature. The damping ratio decreases with increasing frequency, sand content and lower temperatures. The influence of confining pressures and axial strain on the damping ratio are less explicit for the ranges considered. The experimental results are compared with data from other sources. 相似文献
7.
We determine seismic strain rate of tectonic earthquakes along the Central America Volcanic Arc. We then compare this result to those obtained from earthquakes related to the convergence of the Cocos and Caribbean plates and to earthquakes in the back-arc region of northern Central America. The seismic strain-rate tensor for shallow-focus earthquakes along the Central America volcanic arc since 1700, has a compressive eigenvector with a magnitude of 0.7 × 10−8 year−1, and oriented in a 357° azimuth. The extensive eigenvector is oriented in a 86° azimuth, with a magnitude of 0.82 × 10−8 year−1. When only Centroid Moment-tensor solutions (CMT) are considered, the respective eigenvectors are 1.2 × 10−8 year−1 and 1.0 × 10−8 year−1. The compressive eigenvector from the seismic strain-rate tensor for earthquakes along the Cocos-Caribbean convergent margin is 2.0 × 10−8 year−1, plunging at 25°, and oriented in a 29° azimuth. Its magnitude and direction are similar to those of the compressive eigenvector for earthquakes along the volcanic arc. The extensive eigenvector along the convergent margin, on the other hand, has a large vertical component. The compressive and extensive eigevenvectors are 4.9 × 10−8 year−1 and 4.6 × 10−8 year−1, using only CMTs as the database. Earthquakes along the grabens of northern Central America yield a seismic strain-rate tensor whose extensive eigenvector has a magnitude of 2.4 × 10−8 year−1, oriented in a 109° azimuth. Magnitude and direction are similar to those of the extensive eigenvector for earthquakes along the volcanic arc. The compressive eigenvector along the grabens is practically vertical. Similarities in magnitudes and directions for compressive and extensive eigenvectors suggest to us that the strain field along the Central America volcanic arc is the result of compression along the convergent Cocos-Caribbean margin, and extension in the back-arc region, along the grabens of northern Central America. This field is resolved as strike-slip faulting along the arc. 相似文献
8.
Along the Japan trench where some Mw8 class interplate earthquakes occurred in the past century such as the 1896 Sanriku tsunami earthquake (M6.8, Mt8.6, 12×10 20 N m) and the 1968 Tokachi-oki earthquake (Mw8.2, 28×10 20 N m), the Pacific plate is subducting under northeast Japan at a rate of around 8 cm/year. The seismic coupling coefficient in this region has been estimated to be 20–40%. In the past decade, three ultra-slow earthquakes have occurred in the Sanriku-oki region (39°N–42°N): the 1989 Sanriku-oki (Mw7.4), the 1992 Sanriku-oki (Mw6.9), and the 1994 Sanriku-oki (Mw7.7) earthquakes. Integrating their interplate moments released both seismically and aseismically, we have the following conclusions. (1) The sum of the seismic moments of the three ultra-slow earthquakes was (4.8–6.6)×10 20 N m, which was 20–35% of the accumulated moment (18.6–23.0)×10 20 N m, in the region (39°N–40.6°N, 142°E–144°E) for the 21–26 years since the 1968 Mw8.2 Tokachi-oki earthquake. This is consistent with the previous estimates of the seismic coupling coefficient of 20–40%. On the other hand, the sum of the interplate moments including aseismic faulting is (11–16)×10 20 N m, leading to a “seismo-geodetic coupling coefficient” of 50–85%, which is an extension of the seismic coupling coefficient to include slow events. (2) The time constants showed a large range from 1 min (10 2 s) for the 1968 Tokachi-oki earthquake to 10–20 min (10 3 s) for the 1896 Sanriku tsunami earthquake, to one day (10 5 s) for the 1992 Sanriku-oki ultra-slow earthquake, to on the order of one year (10 7 s) for the 1994 Sanriku-oki ultra-slow earthquakes. (3) Based on the space–time distribution, three “gaps of moment release,” (40.6°N–42°N, 142°E–144°E) 39°N–40°N, 142°E–143°E) and (39°N–40°N, 142°E–144°E), are identified, instead of the gaps of seismicity. 相似文献
9.
Depth profiles of in situ-produced cosmogenic nuclides, including 10Be ( T1/2=1.5×10 6 years) and 26Al ( T1/20.73×10 6 years), in the upper few meters of the Earth's crust may be used to study surficial processes, quantifying denudation and burial rates and elucidating mechanisms involved in landform evolution and soil formations. In this paper, we discuss the fundamentals of the method and apply it to two lateritic sequences located in African tropical forests. 相似文献
10.
The pre-Mesozoic metamorphic belt runs parallel to the Day Nui Con Voi - Red River shear zone in Vietnam to the south. The belt is mainly composed of hornblende gneisses, amphibolite lenses and mica-schists. Five hornblendes from a gneiss and an amphibolite were analyzed chemically and chronologically by Electron Probe Micro Analysis (EPMA) and 40Ar/ 39Ar methods. EPMA analyses show that hornblendes in the gneiss and the amphibolite have significant amount of edenite component and similar average composition. However, the recalculated Fe 3+ content is significantly heterogeneous in a thin section while total Fe is nearly the same among the analyses. The rim of each crystal is higher in Fe 3+/(Fe 3+ + Fe 2+) than the core. These chemical and petrological features suggest that the hornblendes have suffered significant oxidation, in particular, largely in the gneiss. 40Ar/39Ar analyses showed that the gneiss has a significant variation of plateau ages (2089±14, 1977±19 and 1873±13 Ma) among three hornblende grains, whereas the amphibolite gives the same plateau ages (2056±14 and 2044±21 Ma) for two grains. All grains of both samples have excess ages in the first few fractions at low temperatures and partial-loss ages between the excess and plateau spectra. The Ca/K ratios indicate some disturbed phases for the lower temperature spectra but the partial-loss ages are also derived from hornblende phase. These facts suggest that hornblende in the gneiss has experienced partial argon loss by oxidation and/or thermally activated argon diffusion process. However, the gneiss and the associated amphibolite have preserved the early Proterozoic tectono-metamorphic event in the hornblende crystals except for their rims, giving new evidence for the early Proterozoic event within the pre-Mesozoic metamorphic belt (northern Vietnam) south of the Red River shear zone in Indochina. 相似文献
11.
Twiss (1976) has suggested that the “ductile faulting” events observed by Post (1973) during high temperature creep of dunite are due to a transition from creep by dislocation movement to a diffusion accommodated, grain-boundary sliding mechanism following a reduction in grain size by dynamic recrystallization. Similarly, Goetze (1978) has explained both ductile faulting and water weakening of dunite by transition to a “nonlinear Coble” creep mechanism. However, the fundamental assumption made by Twiss (1976) that the stress exponent, n, reduces to unity during ductile faulting events is questionable. If the stress exponent remains high, ( n≥3), then a diffusion-accomodated grain-boundary sliding mechanism is excluded. “Nonlinear Coble” creep would remain a viable alternative; however, this model fails to adequately explain the water weakening phenomenon, and the available data do not constrain us to this model. Assuming that the water-weakening phenomenon can be explained by other models (e.g., Blacic, 1972), it will be shown (by analogy with the behavior of metals) that a third model, also consistent with the available data, also qualitatively explains the observations associated with ductile faulting without appeal to a transition in creep mechanisms. The model is similar to one for metals undergoing deformation by dislocation movement and recovery by dynamic recrystallization, which commonly exhibit behavior virtually identical to that observed in dunite during ductile faulting events without transition to grain-size-sensitive creep mechanisms. 相似文献
12.
A decrease in temperature (Δ T up to 45.5 °C) and chloride concentration (ΔCl up to 4.65 mol/l) characterises the brine–seawater boundary in the Atlantis-II, Discovery, and Kebrit Deeps of the Red Sea, where redox conditions change from anoxic to oxic over a boundary layer several meters thick. High-resolution (100 cm) profiles of the methane concentration, stable carbon isotope ratio of methane, and redox-sensitive tracers (O 2, Mn 4+/Mn 2+, Fe 3+/Fe 2+, and SO 42−) were measured across the brine–seawater boundary layer to investigate methane fluxes and secondary methane oxidation processes. Substantial amounts of thermogenic hydrocarbons are found in the deep brines (mostly methane, with a maximum concentration up to 4.8×105 nmol/l), and steep methane concentration gradients mainly controlled by diffusive flow characterize the brine–seawater boundary (maximum of 2×105 nmol/l/m in Kebrit Deep). However, locally the actual methane concentration profiles deviate from theoretical diffusion-controlled concentration profiles and extremely positive δ13C–CH4 values can be found (up to +49‰ PDB in the Discovery Deep). Both, the actual CH4 concentration profiles and the carbon-13 enrichment in the residual CH4 of the Atlantis-II and Discovery Deeps indicate consumption (oxidation) of 12C-rich CH4 under suboxic conditions (probably utilizing readily available—up to 2000 μmol/l—Mn(IV)-oxihydroxides as electron acceptor). Thus, a combined diffusion–oxidation model was used to calculate methane fluxes of 0.3–393 kg/year across the brine–seawater boundary layer. Assuming steady-state conditions, this slow loss of methane from the brines into the Red Sea bottom water reflects a low thermogenic hydrocarbon input into the deep brines. 相似文献
13.
Strontium chemical diffusion has been measured in albite and sanidine under dry, 1 atm, and QFM buffered conditions. Strontium oxide-aluminosilicate powdered sources were used to introduce the diffusant and Rutherford Backscattering Spectroscopy (RBS) used to measure diffusion profiles. For the 1 atm experiments, the following Arrhenius relations were obtained: Sanidine (Or61), temperature range 725–1075°C, diffusion normal to (001): D=8.4 exp(−450±13 kJ mol−1/RT) m2s−1. Albite (Or1), temperature range 675–1025°C, diffusion normal to (001): D=2.9 × exp(−224±11 kJ mol−1/RT) m2s−1. The alkali feldspars in this and earlier work display a broad range of activation energies for Sr diffusion, which may be a consequence of the thermodynamic non-ideality of the alkali feldspar system and/or the mixed alkali effect. 相似文献
14.
The central trough of the Bolivian Altiplano is occupied by two wide salt crusts: the salar of Uyuni, which is probably the largest salt pan in the world (10,000 km 2) and the salar of Coipasa (2,500 km 2). Both crusts are essentially made of porous halite filled with an interstitial brine very rich in Li, K, Mg, B (up to 4.7 g/l Li, 4.3 g/l B, 30 g/l K and 75 g/l Mg). Lithium reserves are the highest known in the world, around 9 × 10 6 tons. Potassium, magnesium and boron reserves in brines are also important (around 194 × 10 6 tons K, 8 × 10 6 tons B and 211 × 10 6 tons Mg). The crusts are the remnant of saline Lake Tauca (13,000–10,000 yr BP). Its salinity was estimated approximately at 80 g/l. Its paleochemistry was derived in two ways: (1) by dissolving the present amounts of all chemical components in the former lake volume, and (2) by simulating the evaporation of the major inflows to the basin. The resulting chemical compositions are quite different. The dissolution-derived one is 5 to 50 times less concentrated in Li, K, Mg, B than the evaporation-simulated ones. However all compositions present the same Na and Cl contents. This suggests either a removal of bittern salts or an enrichment of the former lake water in Na and Cl. The most probable interpretation is that Lake Tauca redissolved a salt crust akin to that existing today. Several older lakes have been detected on the Altiplano. Nevertheless, such an explanation only pushes the problem back. It is likely that the anomaly was transferred from one lake to an other. Three hypotheses may be put forward: (1) bittern seepage through bottom sediments, (2) uptake of the missing components by minerals, and (3) leaching of ancient evaporites from the catchment area at the beginning of the lacustrine history of the basin. The excess halite could have been recycled from lake to lake. This latter process seems to be the most effective to explain the large excess of Na and Cl over the bittern solutes — Li, K, Mg and B. The occurrence of almost pure Na/1bCl saline springs flowing out from a gypsum diapir in the northern Altiplano gives substantial support to this hypothesis. 相似文献
15.
The large landslide, Kostanjek, on the southern slopes of the mountain, Zagreba
ka Gora, in the western suburb of Zagreb was activated in 1963 after some 2.1 × 10 6 m 3 of marl for the cement factory, Croatia, was excavated at the foot of the slope. The geological features (sinclinal structure, faults, hydrogeological conditions, extension of the “Tripoli” marl strata) which had an important role in the formation of the slide, as well as the means of marl exploitation by means of mass blasting, are presented in the paper. The Kostanjek slide involves an urbanized area of some 100 ha. It is estimated that a sliding mass of some 32 × 10 6 m 3 is involved, with a maximum depth of 90 m. Sliding occurs in three levels. The displacements of the surface are 3–6 m. The excavation of marl was stopped in 1988, when a total of 5.3 × 10 6 m 3 of material was excavated. Since then the magnitude of surface displacement per year has decreased. The Kostanjek slide is still active. It is estimated that natural stabilization will take a long time, probably decades. Analyses of possible measures for the stabilization of movements suggested that preventive drainage with continuous observation of the slide is most suitable. 相似文献
16.
As a result of the collapse of a mine tailing dam, a large extension of the Guadiamar valley was covered with a layer of pyritic sludge. Despite the removal of most of the sludge, a small amount remained in the soil, constituting a potential risk of water contamination. The kinetics of the sludge oxidation was studied by means of laboratory flow-through experiments at different pH and oxygen pressures. The sludge is composed mainly of pyrite (76%), together with quartz, gypsum, clays, and sulphides of zinc, copper, and lead. Trace elements, such as arsenic and cadmium, also constitute a potential source of pollution. The sludge is fine grained (median of 12 μm) and exhibits a large surface (BET area of 1.4±0.2 m 2 g −1). The dissolution rate law of sludge obtained is r=10−6.1(±0.3) [O2(aq)]0.41(±0.04) aH+0.09(±0.06) gsludge m−2 s−1 (22 °C, pH=2.5–4.7). The dissolution rate law of pyrite obtained is r=10−7.8(±0.3) [O2(aq)]0.50(±0.04) aH+0.10(±0.08) mol m−2 s−1 (22 °C, pH=2.5–4.7). Under the same experimental conditions, sphalerite dissolved faster than pyrite but chalcopyrite dissolves at a rate similar to that of pyrite. No clear dependence on pH or oxygen pressure was observed. Only galena dissolution seemed to be promoted by proton activity. Arsenic and antimony were released consistently with sulphate, except at low pH conditions under which they were released faster, suggesting that additional sources other than pyrite such as arsenopyrite could be present in the sludge. Cobalt dissolved congruently with pyrite, but Tl and Cd seemed to be related to galena and sphalerite, respectively. A mechanism for pyrite dissolution where the rate-limiting step is the surface oxidation of sulphide to sulphate after the adsorption of O2 onto pyrite surface is proposed. 相似文献
17.
The hydrochemical interaction between groundwater and lakewater influences the composition of water that percolates downward from the surficial aquifer system through the underlying intermediate confining unit and recharges the Upper Floridan aquifer along highlands in Florida. The 87Sr/ 86Sr ratio along with the stable isotopes, D, 18O, and 13C were used as tracers to study the interaction between groundwater, lakewater, and aquifer minerals near Lake Barco, a seepage lake in the mantled karst terrane of northern Florida. Upgradient from the lake, the 87Sr/ 86Sr ratio of groundwater decreases with depth (mean values of 0.71004, 0.70890, and 0.70852 for water from the surficial aquifer system, intermediate confining unit, and Upper Floridan aquifer, respectively), resulting from the interaction of dilute oxygenated recharge water with aquifer minerals that are less radiogenic with depth. The concentrations of Sr 2+ generally increase with depth, and higher concentrations of Sr 2+ in water from the Upper Floridan aquifer (20–35 μg/L), relative to water from the surficial aquifer system and the intermediate confining unit, result from the dissolution of Sr-bearing calcite and dolomite in the Eocene limestone. Dissolution of calcite [δ 13C = −1.6 permil (% o)] is also indicated by an enriched δ 13C DIC(-8.8 to -11.4% o) in water from the Upper Floridan aquifer, relative to the overlying hydrogeologic units (δ 13C DIC < -16% o). Groundwater downgradient from Lake Barco was enriched in 18O and D relative to groundwater upgradient from the lake, indicating mixing of lakewater leakage and groundwater. Downgradient from the lake, the 87Sr/86Sr ratio of groundwater and aquifer material become less radiogenic and the Sr2+ concentrations generally increase with depth. However, Sr2+ concentrations are substantially less than in upgradient groundwaters at similar depths. The lower Sr2+ concentrations result from the influence of anoxic lakewater leakage on the mobility of Sr2+ from clays. Based on results from mass-balance modeling, it is probable that cation exchange plays the dominant role in controlling the 87Sr/86Sr ratio of groundwater, both upgradient and downgradient from Lake Barco. Even though groundwater from the three distinct hydrogeologic units displays considerable variability in Sr concentration and isotopic composition, the dominant processes associated with the mixing of lakewater leakage with groundwater, as well as the effects of mineral-water interaction, can be ascertained by integrating the use of stable and radiogenic isotopic measurements of groundwater, lakewater, and aquifer minerals. 相似文献
18.
Cerro Redondo is an ancient cinder cone now almost completely eroded, sited over a sill that corresponds to a sub-volcanic magma chamber, in Santa Cruz province, Patagonia, Argentina. It is composed of Pliocene-Pleistocene alkaline basalt containing spinel-facies lherzolite and harzburgite mantle xenoliths. Core compositions of pyroxenes indicate temperatures of 823 °C to 1043 °C and pressures of 12.4 kb to 21.4 kb. Based on P– T estimates, petrographic, geochemical, and isotopic characteristics, we propose that Cerro Redondo xenoliths come from a thick homogeneous mantle column (36 km to 63 km depth), and present different degrees of basalt infiltration. A simple mixing model based on Sr isotopes was used to quantify the host basalt infiltration, and contamination values of 0.0%, 0.2%, 3%, and 12% were obtained for samples X-F, X-D, X-C, and X-B, respectively. For unknown reasons, samples X-G and X-E suffered selective isotopic and trace element modification, respectively, associated with 1% of basalt infiltration. Sample X-F best represents the sub-continental lithospheric mantle column, conserving primary equilibrium textures with sharp grain boundaries, and having TiO 2, CaO, Na 2O, K 2O, and P 2O 5 contents lower than average spinel lherzolite, flat chondrite-normalized REE pattern, and 87Sr/ 86Sr and 143Nd/ 144Nd ratios of 0.70519 and 0.51297, respectively. This sample records a decoupling of the Sr–Nd system where Sr ratios increase at constant Nd ratios, possibly caused by chromatographic processes. Its 206Pb/ 204Pb, 207Pb/ 204Pb, and 208Pb/ 204Pb ratios are 17.987, 15.556, and 37.959, respectively. As the interaction with the host basalt increases, xenoliths show a gradual increase of disequilibrium textures such as reaction rims and exsolution lamellae in orthopyroxene and clinopyroxene, and increase of TiO 2, CaO, Al 2O 3, Na 2O, K 2O, P 2O 5, LREE, and incompatible element concentrations. The Sr–Nd system shows an unusual positive trend from the unmodified sample X-F toward the host basalt isotope composition with 87Sr/ 86Sr and 143Nd/ 144Nd ratios of 0.70447 and 0.51279, respectively, while 206Pb/ 204Pb, 207Pb/ 204Pb, and 208Pb/ 204Pb ratios tend to increase toward those of the host basalt (18.424, 15.648, and 38.728, respectively) as the xenolith–basalt interaction increases. The basalt–xenolith reaction probably started during the transport of the xenoliths to the surface, and continued during the residence of xenoliths in the sub-volcanic magma chamber of Cerro Redondo. 相似文献
19.
We present a database and a graphical analysis of published experimental results for dissolution rates of olivine, quartz plagioclase, clinopyroxene, orthopyroxene, spinel, and garnet in basaltic and andesitic melts covering a range of experimental temperatures (1100–1500°C) and pressures (10 5 Pa-3.0 GPa). The published datasets of Donaldson (1985, 1990) and Brearly and Scarfe (1986) are the most complete. Experimental dissolution rates from all datasets are recalculated and normalized to a constant oxygen basis to allow for direct comparison of dissolution rates between different minerals. Dissolution rates (ν) range from 5·10 −10 oxygen equivalent moles (o.e.m.) cm −2 s −1 for olivine in a basaltic melt to 1.3·10 −5 o.e.m. cm −2 s −1 for garnet in a basaltic melt. Values of ln ν are Arthenian for the experiments examined and activation energies range from 118 to 1800 kJ/o.e.m. for quartz and clinopyroxene, respectively. The relationship between calculated A/RT for the dissolution reactions, where A is the thermodynamic potential affinity, and values of ν is linear for olivine, plagioclase, and quartz. We interpret this as strong evidence in support of using calculated A as a predictor of ν for, at least, superliquidus melt conditions. 相似文献
20.
For the territory of Northern Eurasia (6°E–165°W; 30–75°N) the distribution of anomalous masses in the lithosphere has been estimated in accordance with the lithosphere isostatic model. The method of model construction is based on the admittance technique. The experimental admittance presents a relation between the part of the outer load uncompensated by the Moho undulations and the residual gravity field and is used to select the best model. The 1 × 1° averaged values of topography elevations, basement and Moho depths, sedimentary cover density and gravity anomalies have been used as initial data. According to the correlation equation relating the outer load and Moho depths, the mean density contrast between the lower crust and the subcrustal lithosphere is 0.43 g/cm 3, but the Moho undulation can not provide complete isostatic equilibrium. In some areas, the part of the outer load uncompensated by Moho undulations may be as large as 10 7 kg/m 2 and the residual gravity field is as intensive as + 260 mGal. Assuming that for loads of wavelength > 200 km, local isostatic compensation is valid, in accordance with the admittance analysis, the anomalous masses compensating for the part of the outer load, which is not compensated by Moho undulations, have to be located partly in the lower crust and in the subcrustal layer. The regional trend of anomalous compensating masses is negative under Western Europe, the Mediterranean, Eastern Asia and adjacent marginal seas, and positive under the East European Platform and Western and Central Asia. The local compensating masses correspond to particular tectonic units. The isostatic gravity anomalies of Northern Eurasia have been determined and the long-wave component of the field reflecting anomalous masses under the isostatic compensation level has been evaluated. 相似文献
|
