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921.
The abundances of 18O and deuterium in the present and past hydrologic cycle have proven to be an important tool in Earth systems science. In contrast, the abundance of 17O in precipitation has thus far been assumed to carry no additional information to that of 18O. Here, we demonstrate, using known constraints on oxygen isotope abundances from the O2 cycle and existing data about the natural abundance of 17O in water, that the relationship between the discrimination against 17O and 18O in water may vary. This relationship, presented here as θ = ln (17α)/ln (18α), is found to be 0.511 ± 0.005 for kinetic transport effects and 0.526 ± 0.001 for equilibrium effects, with very low temperature sensitivity. As a result, the 17Δ of precipitation is controlled primarily by kinetic effects during evaporation of the initial vapor and, in contrast to the deuterium excess, is independent of the temperature at the evaporation (and condensation) site. This makes 17Δ a unique tracer that complements 18O and deuterium, and may allow for a decoupling of changes in the temperature of the ocean, that serves as the vapor source, from changes in the relative humidity above it. In addition, the 17Δ of ice caps is influenced by the kinetic effects in ice formation, and therefore measurement of ice 17Δ can be used as an additional constraint for better understanding and parameterization of these effects. 相似文献
922.
The behavior of chalcophile metals in volcanic environments is important for a variety of economic and environmental applications, and for understanding large-scale processes such as crustal recycling into the mantle. In order to better define the behavior of chalcophile metals in ocean island volcanoes, we measured the concentrations of Re, Cd, Bi, Cu, Pb, Zn, Pt, S, and a suite of major elements and lithophile trace elements in moderately evolved (6-7% MgO) tholeiitic glasses from Ko’olau and Moloka’i volcanoes. Correlated variations in the Re, Cd, and S contents of these glasses are consistent with loss of these elements as volatile species during magmatic outgassing. Bismuth also shows a good correlation with S in the Ko’olau glasses, but undegassed glasses from Moloka’i have unexpectedly low Bi contents. Rhenium appears to have been more volatile than either Cd or Bi in these magmas.Undegassed glasses with 880-1400 ppm S have 1.2-1.5 ppb Re and 130-145 ppb Cd. In contrast, outgassed melts with low S (<200 ppm) are depleted in these elements by factors of 2-5. Key ratios such as Re/Yb and Cu/Re are fractionated significantly from mantle values. Copper, Pb, and Pt contents of these glasses show no correlation with S, ruling out segregation of an immiscible magmatic sulfide phase as the cause of these variations. Undegassed Hawaiian tholeiites have Re/Yb ratios significantly higher than those of MORB, and extend to values greater than that of the primitive mantle. Loss of Re during outgassing of ocean island volcanoes, may help resolve the apparent paradox of low Re/Os ratios in ocean island basalts with radiogenic Os isotopic compositions. Plume source regions with Re/Yb ratios greater than that of the primitive mantle may provide at least a partial solution to the “missing Re” problem in which one or more reservoirs with high Re/Yb are required to balance the low Re/Yb of MORB.Lithophile trace element compositions of most Ko’olau and Moloka’i tholeiites are consistent with variable degrees of melting of fertile mantle peridotite. However, light rare earth element (LREE)-enriched glasses have trace element compositions more consistent with a garnet-rich source having a distinctive trace element composition. This provides additional evidence for a unique source component possibly related to recycled oceanic crust contributing to Ko’olau tholeiites. 相似文献
923.
Ab initio, molecular orbital (MO) calculations were performed on model systems of SiO2, NaAlSi3O8 (albite), H2O-SiO2 and H2O-NaAlSi3O8 glasses. Model nuclear magnetic resonance (NMR) isotropic chemical shifts (δiso) for 1H, 17O, 27Al and 29Si are consistent with experimental data for the SiO2, NaAlSi3O8, H2O-SiO2 systems where structural interpretations of the NMR peak assignments are accepted. For H2O-NaSi3AlO8 glass, controversy has surrounded the interpretation of NMR and infrared (IR) spectra. Calculated δiso1H, δiso17O, δiso27Al and δiso29Si are consistent with the interpretation of Kohn et al. (1992) that Si-(OH)-Al linkages are responsible for the observed peaks in hydrous Na-aluminosilicate glasses. In addition, a theoretical vibrational frequency associated with the Kohn et al. (1992) model agrees well with the observed shoulder near 900 cm−1 in the IR and Raman spectra of hydrous albite glasses. MO calculations suggest that breaking this Si-(OH)-Al linkage requires ∼+56 to +82 kJ/mol which is comparable to the activation energies for viscous flow in hydrous aluminosilicate melts. 相似文献
924.
A Physical and Numerical Investigation on the Stability of Shallow Tunnels in Strain Softening Media 总被引:11,自引:0,他引:11
Summary The behaviour up to failure of shallow underground openings is discussed on the basis of some laboratory, small-scale model tests and of finite element simulation. The experimental results are first illustrated. They were obtained from two-dimensional (plane strain) and three-dimensional tunnel models tested under standard gravity conditions. Then, the phenomenon of strain localisation that characterizes the medium surrounding the model tunnels is discussed, recalling two alternative approaches for its numerical interpretation. On this basis, a finite element procedure for strain softening analyses is outlined and applied to the simulation of the tests in both two- and three-dimensional conditions. The comparison between experimental and numerical results leads to some conclusions on the influence of strain localisation on the overall behaviour of shallow tunnels and on the stability of their headings. 相似文献
925.
The effect of variations in the rate of ionization of neutral chemical species by cosmic rays, ζ, on the abundances of some observed molecules in the dense cores of dark molecular clouds is studied. Changes in molecular abundances accompanying an increased (decreased) ionization rate have a single origin: the acceleration (deceleration) of processes that are affected directly or indirectlybychemical reactions with charged species. In addition to affecting the gas-phase chemistry, an increased cosmic-ray flux leads to the more efficient destruction of dust-grain mantles and also accelerates the freezing of some components onto dust. In particular, in a model with an increased ζ, the destruction of the volatile N2 molecule by ionized helium leads to the rapid accumulation of nitrogen atoms in dust-phase ammonia, which has a higher desorption energythan N2. As a result, the gas-phase abundance of NH3 and N2H+ decreases significantly. This mechanism can explain the unusual chemical structures of some dense globules, such as B68, where surprisingly low abundances of nitrogen-bearing molecules are observed together with a central drop in the NH3 and N2H+ column densities. Observations of clouds in HCN and HNC lines can discriminate between the two possible origins of the reduced NH3 and N2H+ abundances: an increased cosmic-ray flux or N2 freezing due to the higher desorption energy of this molecule. 相似文献
926.
Variations in the positions of the intersection points of tangents to ray structures in the polar corona of the Sun during the solar cycle are considered. At first glance, the decrease in the distance q between the tangent intersection point and the center of the solar disk during activity maximum contradicts harmonic analyses that indicate that the relative weight of higher harmonics in the global field increases during this period. Indeed, the higher the harmonic number in an axisymmetric field, the closer the intersection point of the field-line tangents (the magnetic focus) to the solar surface. It is shown that q for a field composed of two harmonics with opposite polarities at the poles can be smaller than q for either of them taken alone. A simple model representing the global field using the third and seventh harmonics is analyzed; this model can reproduce quite satisfactorily the observed dynamics of magnetic foci of the polar field. 相似文献
927.
D. V. Bisikalo A. A. Boyarchuk P. V. Kaigorodov O. A. Kuznetsov T. Matsuda 《Astronomy Reports》2004,48(7):588-596
We suggest a new mechanism for the superoutbursts in SU UMa binaries, in which the increase in the accretion rate resulting in a superoutburst is associated with the formation of a spiral “precessional” wave in the inner parts of the disk, where gas-dynamical perturbations are negligible. The existence of such waves was suggested by us previously. The results of three-dimensional gas-dynamical simulations have shown that a considerable increase in the accretion rate (by up to an order of magnitude) is associated with the formation of the precessional wave. The features of the precessional spiral wave can explain both the energy release in the superoutburst and all its observational manifestations. One distinguishing feature of superoutbursts in SU UMa-type stars is the formation of a “superhump” in the light curve. Our model reproduces well both the formation of a superhump and its observational features, including its period, which is up to 3–7% longer than the orbital period, and the detectability of the superhump independent of the orbital inclination of the binary. 相似文献
928.
A possible mechanism for the formation of near-polar magnetic spots on some stars with convective envelopes is proposed. The mechanism is based on the idea that the maximum of the dynamo waves that are excited in thin convective shells by the dynamo mechanism is shifted appreciably from the maximum of the magnetic-field sources in the direction of motion of the dynamo wave. If there is no region of super-rotation near the equator for some reason (as a consequence of disruption due to tidal interaction with a companion in a binary system, for example) and the wave of stellar activity propagates toward the poles rather than toward the equator, this maximum will be in the near-polar regions. 相似文献
929.
The Shape and Volume of the Skaergaard Intrusion, Greenland: Implications for Mass Balance and Bulk Composition 总被引:6,自引:0,他引:6
Re-examination of the Skaergaard intrusion in the context ofits regional setting, combined with new data from explorationdrilling, has resulted in a revised structural model for theintrusion. It is modelled as an irregular box, c. 11 km fromnorth to south, up to 8 km from east to west, and 3·44km from the lower to the upper contact. The walls of the intrusionare inferred to follow pre-existing and penecontemporaneoussteep faults, and the floor and roof seem largely controlledby bedding planes in the host sediments and lavas, similar toregional sills. The suggested shape and volume are in agreementwith published gravimetric modelling. Crystallization alongall margins of the intrusion concentrated the evolving meltin the upper, central part of the intrusion, best visualizedas an onion-skin structure inside the box. Thetotal volume is estimated to c. 280 ± 23 km3, of which13·7% are referred to the Upper Border Series (UBS),16·4% to the Marginal Border Series (MBS) and 69·9%to the Layered Series (LS). In the LS, the Lower Zone (LZ) isestimated to constitute 66·8%, the Middle Zone (MZ) 13·5%and the Upper Zone (UZ) 19·7%. The new volume relationshipsprovide a mass balance estimate of the major and trace elementbulk composition of the intrusion. The parental magma to theSkaergaard intrusion is similar to high-Ti East Greenland tholeiiticplateau basalts with Mg number c. 0.45. The intrusion representsthe solidification of contemporary plateau basalt magma trappedand crystallized under closed-system conditions in a crustalreservoir at the developing East Greenland continental margin. KEY WORDS: bulk composition; emplacement; mass proportions; Skaergaard intrusion; structure 相似文献
930.
NEKVASIL H.; DONDOLINI A.; HORN J.; FILIBERTO J.; LONG H.; LINDSLEY D. H. 《Journal of Petrology》2004,45(4):693-721
Experimental simulation of incremental crystal fractionationof a hy-normative hawaiite indicates that the spectrum of compositionsfrom mildly alkalic hawaiite to sodic rhyolite found in silica-saturatedalkalic suites of the ocean islands and continental hotspotsand rifts can be produced by fractionation at 9·3 kbarwith bulk water contents > 相似文献