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101.
102.
Effects of diffusional modification of garnet growth zoning on P-T path calculations 总被引:2,自引:0,他引:2
The effect of intragranular diffusion on chemical zoning in garnet and on P-T paths calculated from that zoning was evaluated using a numerical model of multicomponent diffusion in combination with simulations of garnet growth. Syn-and post-growth diffusion of Mg–Fe–Mn–Ca species in garnet was calculated for a model pelitic assemblage over a range of temperatures from 485 to 635°C. Compositions from zoned garnet, as modified by diffusion, hypothetical inclusions of plagioclase within garnet and matrix phases were used to reconstruct pressure-temperature (P-T) paths from isobaric and polybaric model histories. P-T path calculations, based on numerical simulations conducted over an input isobaric heating path that reached peak temperatures between 585 and 635°C, show that relaxation of garnet compositional gradients by diffusion can induce modest to appreciable curvature in the inferred paths. Retrieved paths also indicated somewhat smaller overall temperature changes relative to the actual temperature difference of the input path. The magnitude of these distortions is shown to depend upon the heating and cooling rate and garnet crystal size as well as the actual peak temperature condition. The effect of diffusion on path trajectories in simulations with thermal histories that also included cooling were comparable to heating-only models that reached peak temperatures approximately 15–30°C higher. Compositions of garnets with radii less than 1 mm, that reached actual peak temperatures of 605°C along temperature-time histories characteristic of regional metamorphism, experienced sufficient diffusional relaxation to introduce errors of hundreds of bars to in excess of one kilobar in path trajectories. Path distortions were significant at heating/cooling rates less than 10°C/Ma, but rapidly diminished for rates faster than this. In polybaric simulations diffusion effects were least noticeable when the actual pressure-temperature conditions changed in a clockwise sense (i.e., convex to higher P and higher T), but apprecciable modification was seen in path models that underwent counterclockwise changes in P and T. Reequilibration of garnet rim compositions occurred during cooling on all paths, and temperature maxima obtained from garnet-biotite geothermometry underestimated actual peak conditions by 40 to 70°C. Calculations suggest that P-T path trajectories calculated from garnets of at least 1 mm size, and that experienced actual thermal maxima below 585°C, are not likely to be distorted by diffusional effects during regional metamorphism. However, P-T path reconstructions based on garnet zonation with smaller grains or higher temperatures may lead to misinterpretation of crystallization history. The partitioning record of peak metamorphic temperatures may be destroyed by diffusional reequilibration of garnet rim compositions during cooling, seriously complicating the task of quantitatively estimating diffusion effects on path calculations. 相似文献
103.
The mulching of agricultural fields and gardens with stones, pebbles, volcanic ash and similar lithic materials is a variant agricultural strategy uniquely suited to the constraints of a dryland environment. While the agricultural literature contains reports on limited experiments with stone or gravel mulch, data regarding the employment of lithic mulch are lacking. This paper works to bridge this gap by comparing empirical data collected from prehistoric pebble-mulch gardens in the Galisteo Basin (New Mexico) to contemporary agricultural experiments with lithic mulch. Such past-present and experimental-real world comparisons allow us to more fully address the effectiveness of lithic mulching as an agricultural strategy. It is affirmed that lithic mulch is applied to garden plots, especially during periods of drought, in order to reduce soil erosion from wind and water, increase soil temperature to extend the growing season, increase water infiltration, and reduce the evaporative loss of water from wind and sun. The cumulative effect of this soil and water conserving technique is an increase in crop biomass and crop yield. 相似文献
104.
Frank J Millero 《Geochimica et cosmochimica acta》1985,49(2):547-553
The effect of ionic interactions of the major components of natural waters on the oxidation of Cu(I) and Fe(II) has been examined. The various ion pairs of these metals have been shown to have different rates of oxidation. For Fe(II), the chloride and sulfate ion pairs are not easily oxidized. The measured decrease in the rate constant at a fixed pH in chloride and sulfate solutions agrees very well with the values predicted. The effect of pH (6 to 8) on the oxidation of Fe(II) in water and seawater have been shown to follow the rate equation where k1 and k2 are the pseudo first order rate constants, β1 and β2 are the hydrolysis constants for Fe(OH)+ and Fe(OH)0. The value of αFE is the fraction of free Fe2+. The value of k1 (2.0 ±0.5 min?1) in water and seawater are similar within experimental error. The value of k2 (1.2 × 105 min?1) in seawater is 28% of its value in water in reasonable agreement with predictions using an ion pairing model.For the oxidation of Cu(I) a rate equation of the form was found where k0 (14.1 sec?1) and k1 (3.9 sec?1) are the pseudo first order rate constants for the oxidation of Cu+ and CuCl0, β1 is the formation constant for CuCl0 and αCu is the fraction of free Cu+. Thus, unlike the results for Fe(II), Cu(I) chloride complexes have measurable rates of oxidation. 相似文献
105.
Measurements of ‘quadrature’ susceptibility (Xq) at low and high frequency on seabottom sediments of the Gulf of Elefsis supplement the results derived from the study of magnetic susceptibility (X) and saturation isothermal remanent magnetization (SIRM) (Scoullos et al., 1979) and make possible the distinction between the background soil erosional fraction and that of anthropogenic combustion origin. New devices speed up the on-site appraisal of the impact of discharges on marine sediment quality. 相似文献
106.
Recognition that the cooling of the core is accomplished by conduction of heat into a thermal boundary layer (D″) at the base of the mantle, partly decouples calculations of the thermal histories of the core and mantle. Both are controlled by the temperature-dependent rheology of the mantle, but in different ways. Thermal parameters of the Earth are more tightly constrained than hitherto by demanding that they satisfy both core and mantle histories. We require evolution from an early state, in which the temperatures of the top of the core and the base of the mantle were both very close to the mantle solidus, to the present state in which a temperature increment, estimated to be ~ 800 K, has developed across D″. The thermal history is not very dependent upon the assumption of Newtonian or non-Newtonian mantle rheology. The thermal boundary layer at the base of the mantle (i.e., D″) developed within the first few hundred million years and the temperature increment across it is still increasing slowly. In our preferred model the present temperature at the top of the core is 3800 K and the mantle temperature, extrapolated to the core boundary without the thermal boundary layer, is 3000 K. The mantle solidus is 3860 K. These temperatures could be varied within quite wide limits without seriously affecting our conclusions. Core gravitational energy release is found to have been remarkably constant at ~ 3 × 1011 W. nearly 20% of the core heat flux, for the past 3 × 109 y, although the total terrestrial heat flux has decreased by a factor of 2 or 3 in that time. This gravitational energy can power the “chemical” dynamo in spite of a core heat flux that is less than that required by conduction down an adiabatic gradient in the outer core; part of the gravitational energy is used to redistribute the excess heat back into the core, leaving 1.8 × 1011 W to drive the dynamo. At no time was the dynamo thermally driven and the present radioactive heating in the core is negligibly small. The dynamo can persist indefinitely into the future; available power 1010 y from now is estimated to be 0.3 × 1011 W if linear mantle rheology is assumed or more if mantle rheology is non-linear. The assumption that the gravitational constant decreases with time imposes an implausible rate of decrease in dynamo energy. With conventional thermodynamics it also requires radiogenic heating of the mantle considerably in excess of the likely content of radioactive elements. 相似文献
107.
The spectrophotometric measurements of chloro complexes of lead in aqueous HCl, NaCl, MgCl2 and CaCl2 solutions at 25°C have been analyzed using Pitzer's specific interaction equations. Parameters for activity coefficients of the complexes PbCl+, PbCl20 and PbCl3? have been determined for the various media. Values of , and were obtained for the cumulative formation constants. []. These values are in reasonable agreement with literature data. The Pitzer parameters for the PbCl ion pairs in various media were used to calculate the speciation of Pb2+ in an artificial seawater solution. 相似文献
108.
K. Honegger V. Dietrich W. Frank A. Gansser M. Thöni V. Trommsdorff 《Earth and Planetary Science Letters》1982,60(2):253-292
Ladakh (India) provides a complete geological section through the northwestern part of the Himalayas from Kashmir to Tibet. Within this section the magmatic, metamorphic and geotectonic evolution of the northern Himalayan orogeny has been studied using petrographic, geochemical and isotope analytical techniques.The beginning of the Himalayan cycle was marked by large basaltic extrusions (Panjal Trap) of Permian to Lower Triassic age at the “northern” margin of the Gondwana continent (Indian Shield). These continental type tholeiitic basalts were followed by a more alkaline volcanism within the Triassic to Jurassic Lamayuru unit of the Gondwana continental margin.Lower Jurassic to Cretaceous oceanic crust and sediments (ophiolitic mélange s.s.) accompany the Triassic to Cretaceous flysch deposits within the Indus-Tsangpo suture zone, the major structural divide between the Indian Shield (High Himalaya) and the Tibetan Platform. So far, no relic of Paleozoic oceanic crust has been found.Subduction of the Tethyan oceanic crust during Upper Jurassic and Cretaceous time produced an island arc represented by tholeiitic and calc-alkaline volcanic rock series (Dras volcanics) and related intrusives accompanied by volcaniclastic flysch deposits towards the Tibetan continental margin.Subsequent to the subduction of oceanic crust, large volumes of calc-alkaline plutons (Trans-Himalayan or Kangdese plutons) intruded the Tibetan continental margin over a distance of 2000 km and partly the Dras island arc in the Ladakh region.The collision of the Indian Shield and Tibetan Platform started during the middle to upper Eocene and caused large-scale, still active intracrustal thrusting as well as the piling up of the Himalayan nappes. The tectonically highest of these nappes is built up of oceanic crust and huge slices of peridotitic oceanic mantle (Spongtang klippe).In the High Himalayas the tectonic activity was accompanied and outlasted by a Barrovian-type metamorphism that affected Triassic sediments of the Kashmir-Nun-Kun synclinorium up to kyanite/staurolite grade and the deeper-seated units up to sillimanite grade. Cooling ages of micas are around 20 m.y. (muscovite) and 13 m.y. (biotite). Towards the Indus-Tsangpo suture zone metamorphism decreases with no obvious discontinuity through greenschist, prehnite-pumpellyite to zeolite grade. Remnants of possibly an Eo-Himalayan blueschist metamorphism have been found within thrust zones accompanying ophiolitic mélange in the suture zone. 相似文献
109.
Any conductive sulphide ore body can generate electrical ore potentials and this can be regarded as a geobattery, with an upper positive cathode and a lower negative anode. Two mechanisms which can occur simultaneously and which appear to explain these potentials are the oxygen concentration cell (OCC) and the sulphide galvanic cell (SGC). When there is a difference of O2 concentration between the cathode and anode, the OCC operates by contributing oxygen to the anode, and ceases when the difference is diminished. However, gradients of pH, temperature or pressure can keep the cell operating. In the SGC the sulphides dissolve at the anode (producing metal ions, sulphur and electrons), while at the cathode, either sulphides dissolve producing metal and sulphur ions, or reaction is as in the OCC. The ore potential measured in the field is the sum of the oxygen concentration cell and at times locally dominant sulphide galvanic cells. 相似文献
110.
The “anomalous” layer in the lowermost mantle, identified as D″ in the notation of K.E. Bullen, appears in the PREM Earth model as a 150 km-thick zone in which the gradient of incompressibility with pressure, , is almost 1.6, instead of 3.2 as in the overlying mantle. Since PREM shows no accompanying change in density or density gradient, we identify D″ as a thermal boundary layer and not as a chemically distinct zone. The anomaly in is related to the temperature gradient by the temperature dependence of Ks, for which we present a thermodynamic identity in terms of accessible quantities. This gives the numerical result (?Ks/?T)P=?1.6×107 Pa K?1 for D″ material. The corresponding temperature increment over the D″ range is 840 K. Such a layer cannot be a static feature, but must be maintained by a downward motion of the lower mantle toward the core-mantle boundary with a strong horizontal flow near the base of D″. Assuming a core heat flux of 1.6 × 1012 W, the downward speed is 0.07 mm y?1 and the temperature profile in D″, scaled to match PREM data, is approximately exponential with a scale height of 73 km. The inferred thermal conductivity is 1.2 W m?1 K?1. Using these values we develop a new analytical model of D″ which is dynamically and thermally consistent. In this model, the lower-mantle material is heated and softened as it moves down into D″ where the strong temperature dependence of viscosity concentrates the horizontal flow in a layer ~ 12 km thick and similarly ensures its removal via narrow plumes. 相似文献