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1.
Aeolian dune dimensions and migration rates are analysed along the Ceará coast, north-east Brazil. Dunes that are currently mobile along the Ceará coast are composed of barchans and sand sheets. The results show that barchans maintain an equilibrium form, which can be characterized by values of dimensionless shape parameters H/W and W/L , where H is the dune height, W is the wing-to-wing width and L is the dune length. Dunes are highly mobile, with average migration rates of 17·5 m year−1 for barchans and 10 m year−1 for sand sheets. The calculated migration rates were found to depend strongly on dune dimensions for both barchans and sand sheets, i.e. the larger the dune is, the lower the migration rate will be. This size dependence was associated with the existence of a representative common transport rate along the dune fields, which induces a different dune migration rate dependent on dune size. Finally, from the observed dune evolution, an aggregated scale aeolian sediment transport was inferred. This bulk transport rate, of the order of 90–100 m3 m−1 year−1, is only valid for a timescale of years to decades, which is the timescale used in dune evolution analysis.  相似文献   

2.
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3.
Oxygen bulk diffusion rates were experimentally determined in a natural ultramylonite sample ( c . 5   μ m grain size; 15–20% biotite, 20% quartz, 60–65% feldspars, and minor Fe-oxides) from the Gerrish Island shear zone, SE Maine, USA. The diffusion experiments were performed at 250–550  °C and 100  MPa water pressure. Oxygen bulk diffusion rates were determined both parallel and perpendicular to the strong foliation of the sample. The Arrhenius parameters for transport parallel to the foliation are: D bulk0=2.0×10−11 m2 s−1 and Q =30±6 kJ mol−1. The bulk diffusivity perpendicular to the foliation is about a factor of 3.5 less than that parallel to the foliation with the same activation energy. The values of bulk diffusivity and activation energy obtained are consistent with ionic diffusion through a static aqueous fluid, suggesting that an interconnected fluid exists in the ultramylonite even under hydrostatic conditions. The microstructure of the ultramylonite was characterized using transmission electron microscopy (TEM). The nature and distribution of the interconnected fluid cannot be completely resolved from the TEM analysis; however, the low percentage of three-grain channels and open grain/interphase boundaries suggests that the fluid resides as a thin film on the grain surfaces. The results of this study have direct applications in many important geological settings and provide valuable insights into the observed rapid diffusion rates, strong lithological control and pervasive nature of fluid transport in mica-bearing rocks.  相似文献   

4.
The south-east Reynolds Range, central Australia, is cut by steep north-west-trending Alice Springs age ( c. 334  Ma) shear zones that are up to hundreds of metres wide and several kilometres long with reverse senses of movement. Amphibolite facies (550–600  °C, 500–600  MPa) shear zones cut metapelites, while greenschist facies shear zones (420–535  °C, 400–650  MPa) cut metagranites. The sheared rocks commonly underwent metasomatism implying that the shear zones were the pathways of significant fluid flow. Altered granites within greenschist facies shear zones have gained Si and K but lost Ca and Na relative to their unsheared counterparts, suggesting that the fluid flowed down-temperature (and hence probably upward) through the shear zones. Time-integrated fluid fluxes calculated from silica addition are up to 2.1×1010 mol  m−2 ( c. 4.2×105  m3  m−2). Similar time-integrated fluid fluxes are also estimated from changes in K and Na. The sheared granitic rocks locally have δ18O values as low as 0 which is much lower than the δ18O values of the adjacent unsheared granites (7 to 9), implying that the fluid which flowed through these shear zones was derived from the surface. For the estimated time-integrated fluid fluxes, the fluids would be able to retain their isotopic signature for many tens to hundreds of kilometres. The flow of surface-derived fluids into the ductile middle crust, with subsequent expulsion upwards through the shear zones, may have been driven by seismic activity accompanying the Alice Springs deformation.  相似文献   

5.
A series of laboratory experiments has been conducted in order to elucidate the sediment-induced mixing processes accompanying riverine outflows; specifically, the discharge of a warm, fresh, particle-laden fluid over a relatively dense, cool brine. In a parameter regime analogous to recently acquired field measurements, hypopycnal (surface) plumes were subject to a convective instability driven by some combination of heat diffusing out of the warm, fresh, sediment-laden plume and particle settling within it. Convection was robust in the presence or absence of intense turbulence, at sediment concentrations as low as 1 kg m−3, and took the form of millimetre-scale, sediment-laden fingers descending from the base of the surface plume. A consequence of the convective instability of the original hypopycnal plume is the generation of a hyperpycnal (bottom-riding) flow. The experiments presented here indicate that natural river outflows may thus generate hyperpycnal plumes when sediment concentrations are 40 times less than those required to render the outflow heavy relative to the oceanic ambient. The resulting hyperpycnal plumes may play an important role in transporting substantial quantities of sediment to the continental slope and beyond.  相似文献   

6.
High-pressure metamorphic rocks exposed in the Bantimala area, c . 40  km north-east of Ujung Pandang, were formed as a Cretaceous subduction complex with fault-bounded slices of melange, chert, basalt, turbidite, shallow-marine sedimentary rocks and ultrabasic rocks. Eclogites, garnet–glaucophane rocks and schists of the Bantimala complex have estimated peak temperatures of T  =580–630 °C at 18  kbar and T  =590–640 °C at 24  kbar, using the garnet–clinopyroxene geothermometer. The garnet–omphacite–phengite equilibrium is used to estimate pressures. The distribution coefficient K D1=[( X pyr)3( X grs)6/( X di)6]/[(Al/Mg)M2,wm (Al/Si)T2,wm]3 among omphacite, garnet and phengite is a good index for metamorphic pressures. The K D1values of the Bantimala eclogites were compared with those of eclogites with reliable P–T  estimates. This comparison suggests that peak pressures of the Bantimala eclogites were P =18–24  kbar at T  =580–640 °C. These results are consistent with the P–T  range calculated using garnet–rutile–epidote–quartz and lawsonite–omphacite–glaucophane–epidote equilibria.  相似文献   

7.
Annealing experiments on agate, quartz schist and metachert at 800–1000 °C, confining pressures of 400 and 800  MPa, and annealing times of 6.0×10–3.6×105s revealed normal grain growth of quartz in the agate, grain-size increasing with time, but the rate of grain growth decreasing with increasing grain size.
The boundaries of agate with quartz schist and metachert migrated into the agate at the expense of fine-grained quartz in the agate. The driving force for the migration appears to be the reduction of surface energy associated with removal of fine-grained quartz in the agate. Assuming the activation energy as Q m =11  kcal mol−1, a general expression for the relationship between velocity of boundary migration ( V   ) and driving force ( P ) is where γ is the specific surface energy of quartz, R is the gas constant and T  is the absolute temperature. The velocity is relatively fast at high temperatures on a geological time-scale. The expression assists a quantitative understanding of the microstructural development of quartz at metamorphic conditions.  相似文献   

8.
The pressure dependence of the zirconium-in-rutile thermometer   总被引：19，自引：1，他引：19  
The solubility of ZrO2 in rutile is strongly temperature-dependent and has been identified as a potentially powerful thermometer when the rutile coexists with an appropriate buffer assemblage, e.g. zircon + quartz. In combination with experimental data at 10 kbar, previous consideration of data on natural rutile has not identified a pressure dependence for the thermometer. However, the expected volume change as a result of substitution of the larger Zr4+ cation for Ti4+ suggests that the Zr content of rutile should decrease with increasing pressure. To investigate the pressure dependence of the thermometer, piston cylinder (at 10, 20 & 30 kbar) and 1 atm furnace experiments were performed in the system ZrO2-TiO2-SiO2. The solubility of ZrO2 in rutile, in the presence of zircon and quartz was reversed at each pressure value. From these experiments, the thermodynamics of the end-member reaction ZrSiO4 = SiO2 + ZrO2 (in rutile) have been determined. There is a secondary pressure effect accompanying the primary temperature dependence of the Zr content of rutile. New thermometer equations are, in the α -quartz field: in the β -quartz field and in the coesite field in which φ is ppm Zr, P is in kbar and R is the gas constant, 0.0083144 kJ K−1. Thermometric results using these equations are shown for a range of geological settings.  相似文献   

9.
Determinations of the absolute age of cleavage formation can provide fundamental information about the evolution of orogenic belts. However, when applied to cleavages in slates and phyllites, conventional dating methods are complicated by problems related to mineral separation and the presence of multiple cleavage generations. In situ high-spatial-resolution 40Ar/39Ar laser microprobe geochronology and microstructural observations indicate that the age of cleavage formation in slates and phyllites can be constrained by analysing zones of tightly packed cleavage domains. Three regionally developed cleavages (S2, S3, and S4) are present in the northern Taconic Allochthon of Vermont and New York. Representative samples were studied from a variety of localities where these cleavages, which are defined by white micas, are well developed. In the suite of samples, only S3 and S4 are expressed as domains that are sufficiently wide and spatially isolated in thin section to permit quantitative 40Ar/39Ar geochronology. Mean 40Ar/39Ar laser microprobe ages for these domains are 370.7 ± 1.0 Myr for S3 and 345.5 ± 1.7 Myr for S4. Because estimates of the Ar closure temperature for white micas are substantially higher than the inferred growth temperatures of the micas defining S3 and S4, these values are interpreted as periods since cleavage formation. This interpretation is consistent with independent geochronological constraints on the age of the Acadian orogeny in the region.  相似文献   

10.
Diffusion modelling is applied to layered garnet–pyroxene–quartz coronas, formed by a pressure-induced reaction between plagioclase and primary pyroxene in a metabasic granulite. The reconstructed reaction involves some change in composition of reactant minerals. The distribution of minerals between layers is satisfactorily explained by diffusion-controlled reaction with local equilibrium, in which the diffusion coefficient for Al was smaller than those for Fe, Mg and Ca by a factor of approximately four. Diffusion of Mg towards plagioclase implies a chemical-potential gradient for MgO component in a direction opposite to the changing Mg content of garnet; this is explained by the influence of Al2O3 on the chemical potential of the pyrope end-member. Grain-boundary diffusion is suggested to have operated, possibly with composition gradients different from those in the bulk minerals. Chemical-potential differences across the corona are estimated from the variation in garnet composition, enabling affinity (the free energy change driving the reaction) to be estimated as 6.9±1.8  kJ per 24-oxygen mole of garnet produced. This implies that the pressure for equilibrium among the minerals was overstepped by 1.4±0.4  kbar. The probable P–T conditions of reaction were in the range 650–790  °C, 8–10  kbar. Assuming a timescale of reaction between 106 and 108 years, estimated diffusion coefficients for Fe, Mg and Ca are in the range 9×10−23 to 5×10−20 m2 s−1. These are consistent with experimental values in the literature for solid-state diffusion, including grain-boundary diffusion.  相似文献   

11.
Most techniques for estimating settling velocities of natural particles have been developed for siliciclastic sediments. Therefore, to understand how these techniques apply to bioclastic environments, measured settling velocities of bioclastic sedimentary deposits sampled from a nearshore fringing reef in Western Australia were compared with settling velocities calculated using results from several common grain‐size analysis techniques (sieve, laser diffraction and image analysis) and established models. The effects of sediment density and shape were also examined using a range of density values and three different models of settling velocity. Sediment density was found to have a significant effect on calculated settling velocity, causing a range in normalized root‐mean‐square error of up to 28%, depending upon settling velocity model and grain‐size method. Accounting for particle shape reduced errors in predicted settling velocity by 3% to 6% and removed any velocity‐dependent bias, which is particularly important for the fastest settling fractions. When shape was accounted for and measured density was used, normalized root‐mean‐square errors were 4%, 10% and 18% for laser diffraction, sieve and image analysis, respectively. The results of this study show that established models of settling velocity that account for particle shape can be used to estimate settling velocity of irregularly shaped, sand‐sized bioclastic sediments from sieve, laser diffraction, or image analysis‐derived measures of grain size with a limited amount of error. Collectively, these findings will allow for grain‐size data measured with different methods to be accurately converted to settling velocity for comparison. This will facilitate greater understanding of the hydraulic properties of bioclastic sediment which can help to increase our general knowledge of sediment dynamics in these environments.  相似文献   

12.
Experimental modelling of an aggrading braided river has allowed investigation of the relationship between the frequency of channel avulsion ( A f), the duration of time that the braidplain is occupied by flow, the spatial pattern of braidplain sedimentation and how these respond to a change in sediment supply ( S s). Model results demonstrate a strong, positive relationship between S s and A f and that there is no downstream change in A f over the short braidplain distances ( ca 100 m) modelled herein. Although A f is strongly dependent on S s, the degree of channel switching does not influence the rate, or spatial pattern, of braidplain sedimentation. All experiments used a single, central input for water and sediment, and the channels occupied the centre of the alluvial basin for a longer period of time than the margins for all sediment supply rates and distances downstream. Despite this spatio-temporal pattern in flow occupancy, braidplain sedimentation rates were nearly uniform both downstream and across the basin, and increased at approximately the same rate as increases in S s. As a consequence, less frequent, and possibly short-lived, flows at the margins of the braidplain deposited and preserved more sediment per unit time in comparison with the centre of the basin where flow occupancy was higher. An approximate order of magnitude change in sediment supply resulted in only a factor of two change in bed slope, probably due to both an increase in channelization and adjustment of the channel form that maintained sediment transport through the basin. This result suggests that linear diffusion models are unlikely to be applicable in landscape evolution models that possess aggrading multi-thread rivers, which are capable of self-adjustment in channel number and form.  相似文献   

13.
The threshold of movement of sediment obtained from sandbanks within the Bristol Channel (UK) is investigated under unidirectional, oscillatory and combined flows. The experiments were undertaken in a recirculating, unidirectional laboratory flume containing an oscillating plate to simulate wave action, with movement along the same axis as the unidirectional flows. The sand samples consisted of cohesionless quartz grains with median grain sizes between 0·315 and 0·513 mm. The experiments were performed under flow velocities (measured at 2 cm above the bed) ranging between 0 and 24 cm s–1 and oscillatory currents (wave periods of 5, 12 and 15 s) ranging from 0 to 28 cm s–1. The critical conditions for the initiation of sediment movement were assessed, by visual observation, using the Yalin criterion. The results show that, under unidirectional flow, there is a slight overestimation of the threshold of naturally graded sediments derived on the basis of empirically derived threshold curves for artificially prepared sediments under similar flow conditions. In the case of oscillatory flows, the threshold for the natural sands is found to be higher than that predicted by previously derived empirical curves. Under combined flows, wave period is shown to control threshold conditions, with the unidirectional and oscillatory flow components combining in a linear fashion for long-period (12 s and 15 s) waves. In contrast, in the presence of short-period (5 s) waves, the unidirectional and oscillatory components of the flow appear to 'decouple'. For high orbital velocities, in both cases, the effect of the wave period on threshold diminishes.  相似文献   

14.
A systematic NanoSIMS isotope imaging study of sub-micrometer phases in interplanetary dust particles (IDPs) has led to the discovery of two presolar grain types that previously were observed only in primitive meteorites. A 350 × 600 nm2 Al2O3 grain has a large 17O enrichment and a slight 18O depletion, as well as a 26Mg excess due to the decay of extinct 26Al. Because of its relatively large size and prominent location within the IDP, this presolar Al2O3 grain is well characterized by SEM-EDX analyses. A second, much smaller presolar grain has a diameter of 150 nm and a 13C enrichment of more than 300%. Isotopic anomalies in C are rarely found in IDPs and the magnitude of this anomaly is unprecedented. This grain also has a 15N-rich composition and its isotopic makeup as well as its secondary ion yields identify it as a SiC grain. The discovery of presolar Al2O3 and SiC in IDPs seamlessly complements earlier notions of interplanetary dust particles as the most primitive extraterrestrial material currently available for laboratory analysis. Both Al2O3 and SiC are common presolar grain types in primitive meteorites, but they appeared conspicuously absent from the presolar grain inventory in interplanetary dust particles, which is dominated by silicate stardust. Not finding these presolar grain types in interplanetary dust would have been difficult to explain. Abundance estimates of the new presolar grain types in IDPs are hampered by limited statistics, but both Al2O3 and SiC are less common than presolar silicates which have been found at relatively high abundances in IDPs. The particle in which these presolar grains have been found belongs to the ‘isotopically primitive subgroup’ of IDPs, yet does not contain any presolar silicates.  相似文献   

15.
The proposed geothermobarometer is based on an empirical calibration which takes account of two equilibria involving the tremolite, edenite, pargasite and hastingsite components in amphiboles. It has applications to assemblages found in metabasic rocks of widely different chemical compositions (magnesian to Fe-rich metabasalts), and for metamorphism ranging from lower greenschist to highest amphibolite facies. Knowing the Si(T1), Aliv, Alvi, Fe3+, Fe2+, Mg, Ca, NaM4, NaA and A vacancy in an amphibole, and the Al3+ and X Mg in coexisting epidote and chlorite, it is possible to calculate two values of In K d for this assemblage. These equilibria involve edenite-tremolite and (pargasite/hastingsite)-tremolite end-members in amphibole (the calculation program is given). For these equilibria, the isopleths (iso-values of K d) have been calculated for 0.27 < X Mg < 0.75 and 0 < X Fe3+= Fe3+/(Fe3++ Alvi) < 0.8. It is then possible to determine pressure and temperature directly when X Mg, X Fe3+, In K d for tremoliteedenite and In K d for (pargasite/hastingsite)-tremolite are known. Application of this geothermobarometer is limited to Ca-free plagioclase assemblages, and complete P–T paths can be drawn only if all the minerals are considered together. Phase relations at successive stages of crystallization can be constrained by studying the relationships between the coexisting minerals, their zoning and the metamorphic fabrics.  相似文献   

16.
The cathodoluminescence analyses of the trigonal carbonates calcite and dolomite have been applied intensively in sedimentary petrology for a long time and the properties of these minerals are well-known, but much less attention has been paid to aragonite. In this study, the cathodoluminescence behaviour and the trace element composition of natural and synthetic aragonite have been studied employing trace element analyses (proton induced X-ray emission) and luminescence spectroscopy. Aragonite doped with Mn2+ has been synthesized in a NH4+–Mg2+–Ca2+–Cl− solution in contact with a CO2–H2O–NH3 atmosphere. The low effective distribution coefficients indicate a rapid growth of the crystals of millimetre size which occurred within hours or days. The natural aragonite samples contain Mn, Fe and Sr in different concentrations. The Mn-bearing aragonites exhibit a bright green luminescence which is caused by a strong emission band at 575 nm with a half-width of about 84 nm. The luminescence intensity shows a strong positive correlation with Mn in aragonite when Fe and Mn do not exceed 2000 p.p.m. The intensity is depressed if the concentration of these elements exceeds the critical values. In the shell of a recent Unio sp., the luminescence intensity deviates from the linear correlation, although the trace element contents are not too high; this is probably an effect of quenching by organic material between the crystallites of the biogenic aragonite.  相似文献   

17.
A method for the selective separation of Ag, Cd, Cr, Cu, Ni, Pb and Zn in traces from solutions of calcite (CaCO3), dolomite (CaMg(CO3)2) and gypsum (CaSO4.2H2O) before their determination by inductively coupled plasma-atomic emission spectrometry (ICP-AES) is presented. The expected interferences of Ca and Mg on intensities of trace analytes were removed by collecting the elements of interest with cobalt(III) hexamethylenedithiocar-bamate, Co(HMDTC)3. The flotation of aqueous solutions (1 l) of calcite, dolomite and gypsum was performed at pH 6.0, by 1.5 mg l−1 Co and 0.6 mmol l−1 HMDTC−. To minimise the effect of the reaction between Ca/Mg, which restrains the function of the surfactant, careful selection of the most suitable foaming reagent was necessary. The accuracy of the method was established by analysing natural alkaline-earth minerals by the standard addition method as well as using the dolomite reference materials GBW 07114 and GSJ JDo-1. The ICP-AES limits of detection following flotation on different minerals were found to be 0.080 μg g−1 for Cd, 0.105 μg g−1 for Ag, 0.142 μg g−1 for Cu, 0.195 μg g−1 for Cr, 0.212 μg g−1 for Ni, 0.235 μg g−1 for Zn and 0.450 μg g−1 for Pb.  相似文献   

18.
ABSTRACT
The mineralogy and isotope geochemistry of carbonate minerals in the Coorong area are determined by the water chemistry of different depositional environments ranging from seawater to evaporitically modified continental water. The different isotopic compositions of coexisting calcite and dolomite suggest that each of the above two minerals was formed from water of composition and origin unique to that specific mineral. In addition, the dolomite was not formed by simple solid state cation exchange.
The occurrence of two types of dolomite was shown by isotope analysis and SEM observations. The dolomite, which is isotopically light (δ13C = -1 to -2% 0 ; δ18O=+3 to +5%0) and of fine grain size (˜ 0·5 μm) probably precipitated under the influence of evaporitically modified continental water. Coarser grained dolomite (up to 4 μm) is isotopically heavier (δ13C=+3 to +4%0; δ18O=+5 to + 6%0) contains Mg in excess of Ca and was formed in or close to equilibrium with atmospheric CO2 probably by the dolomitization of aragonite.  相似文献   

19.
Garnet from a kinzigite, a high-grade gneiss from the central Black Forest (Germany), displays a prominent and regular retrograde diffusion zoning in Fe, Mn and particularly Mg. The Mg diffusion profiles are suitable to derive cooling rates using recent datasets for cation diffusion in garnet. This information, together with textural relationships, thermobarometry and thermochronology, is used to constrain the pressure–temperature–time history of the high-grade gneisses. The garnet–biotite thermometer indicates peak metamorphic temperatures for the garnet cores of 730–810  °C. The temperatures for the outer rims are 600–650  °C. Garnet–Al2SiO5–plagioclase–quartz (GASP) barometry, garnet–rutile–Al2SiO5–ilmenite (GRAIL) and garnet–rutile–ilmenite–plagioclase–quartz (GRIPS) barometry yield pressures from 6–9  kbar. U–Pb ages of monazite of 341±2  Ma date the low- P high- T metamorphism in the central Black Forest. A Rb/Sr biotite–whole rock pair defines a cooling age of 321±2  Ma. The two mineral ages yield a cooling rate of about 15±2  °C Ma−1. The petrologic cooling rates, with particular consideration of the f O2 conditions for modelling retrograde diffusion profiles, agree with the geochronological cooling rate. The oldest sediments overlying the crystalline basement indicate a minimum cooling rate of 10  °C Ma−1.  相似文献   

20.
Abstract Deformed quartz veins in garnet-zone schist adjacent to the active Alpine Fault, New Zealand, have fluid inclusions trapped along quartz grain boundaries. Textures suggest that the inclusions formed in their present shapes during annealing of the deformed veins. Many of the inclusions are empty, but some contain carbon dioxide with densities that range from 0.16 to 0.80 g cm−3. No water, nitrogen or methane was detected. The inclusions are considerably more CO2-rich than either the primary metamorphic fluid (<5% CO2) or fluids trapped in fracture-related situations in the same, or related, rocks (<50% CO2). Enrichment of CO2 is inferred to have resulted from selective migration (wicking) of saline water from the inclusions along water-wet grain boundaries after cooling-induced immiscibility of a water-CO2 mixture. Inclusion volumes changed after loss of water. Non-wetting CO2 remained trapped in the inclusions until further percolation progressively removed CO2 in solution. This mechanism of fluid migration dominated in ductile quartz-rich rocks near, but below, the brittle-ductile transition. At deeper levels, hydraulic fracturing is also an important mechanism for fluid migration, whereas at shallower levels advection through open fractures dominates the fluid flow regime.  相似文献   

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