摘要:

A model is proposed in which Pb is preferentially partitioned into the melt during partial melting of the mantle resulting in a higher U/Pb ratio in the residual mantle. The degree of increase is suggested to be inversely correlated with the degree of melting. By combining subduction of oceanic crust (µcrust<µsource) into the lower mantle with intra‐crustal fractionation of U/Pb through magmatic and metamorphic differentiation, this model can explain the J‐type Pb in both the upper mantle (i.e., young oceanic volcanics) and the upper continental crust. It obviates the need for an extended period of core formation or recycling of continental materials into the mantle to explain the J‐type leads although it does not preclude the occurrence of either of these mec

ISSN:0094-8276    

DOI:10.1029/GL012i011p00741

年代:1985

数据来源: WILEY

摘要:

Chromium isotopic anomalies appear to be widespread in the Allende inclusions. Clear anomalies are shown in the53Cr/52Cr isotopic ratio. Deviations from the terrestrial value are correlated with the Mn/Cr ratios indicating that53Mn (half life T=3.7 × 106y) existed at the time of formation of the meteorite inclusions. The initial53Cr/52Cr in the inclusions is lower than the terrestrial value.Recent results have shown that chromium isotopic anomalies are frequent on the54Cr isotope (Birck and Allègre, 1984). At the same time our results suggested the possibility of some anomalies on the53Cr isotope which can be produced by the decay of53Mn.The present article presents improved precision measurements on the same inclusions of the Allende meteorite and an extension of the measurements to mineral separates. These results are the first evidence for live53Mn in the early solar system (noted53Mn* in the following

ISSN:0094-8276    

DOI:10.1029/GL012i011p00745

年代:1985

数据来源: WILEY

摘要:

A model of the structure and evolution of cometary rays is described which considers the rays to be the channeled outflow of cometary ions from the ionosphere. Ions in the central tail ray flow down the channel formed by the tailward closing of the draped interplanetary field. Ions in the side rays flow through magnetic flux ropes which become embedded in the cometary ionosphere through Kelvin‐Helmholtz instability of the cometary ionopause. The flux ropes tunnel through the ionosphere, penetrate the ionopause at two points, and extend out into the solar wind on both sides of the comet. As the flux ropes are dragged through the ionosphere by the solar wind, they are continually resupplied with ionospheric plasma through Bohm diffusion driven by ion‐neutral collisions. Differences in observed cometary ray structure are attributed to differences in the radial distance and ecliptic latitude of different comets at the time of observation, as well as to variations in the interplanetary magnetic field direction and the gas production rate of the co

ISSN:0094-8276    

DOI:10.1029/GL012i011p00749

年代:1985

数据来源: WILEY

摘要:

We model the diffusion of ions in a satellite plasma torus by means of a symmetric one dimensional random walk in which the particle source is at O, the particle sink is at N (an integer ⩾ 2), and the scale size of the diffusion cell is unity. We obtain the probability distribution function of the number of steps to exit for an ion or, equivalently, the probability distribution of ion residence times. This distribution is used in a model incorporating ionization by electron impact to derive steady state expressions for the ratio of the numbers of doubly to singly ionized ions, and the total number of ions in the torus. These expressions involve the diffusion time scale τs, the ionization time scale τi, and N. We compare the result for the charge‐state ratio with its corresponding value based on the assumption that all ions reside in the torus for the mean residence time, the latter being the standard assumption. Typically, we find that for given values of τs, τiand N our result is twice that obtained with the standard assumption. We apply the results to the torus of the Jovian satellite Io to predict mean residence times for sulphur and oxygen ions. A factor of two reduction in the time required to produce the observed charge state partitioning gives residence time estimates closer to those found by considering power requi

ISSN:0094-8276    

DOI:10.1029/GL012i011p00753

年代:1985

数据来源: WILEY

摘要:

Observations of ‘supersonic’ O+outflows by DE‐1 in the polar magnetosphere are interpreted as a velocity‐filter effect resulting from the transport of these low‐energy ions from a horizontally narrow source associated with the polar cleft topside ionosphere into the polar cap magnetosphere. Under certain simplifying circumstances it is shown that the ‘Mach number’ for the flow is essentially a geometrically determined quantitity, approximately the ratio of the horizontal distance of the observation location to the source over the horizontal width of the source. A recently developed two‐dimensional kinetic model of ion transport in the polar magnetosphere is used to calculate distribution functions and the associated bulk flow parameters, and these calculated quantities are compared with measured flow param

ISSN:0094-8276    

DOI:10.1029/GL012i011p00757

年代:1985

数据来源: WILEY

摘要:

Nearly all ionospheric heaters operate at vertical incidence. Oblique waves cannot satisfy frequency‐matching conditions necessary to excite the parametric decay instability, and they are weakened by geometric spreading below the ionosphere. Those spreading losses are mitigated, however, by focussing near caustics. This paper calculates fields near the caustics of oblique waves and estimates the corresponding increases in electron temperature. It finds that a transmitter having a power‐gain product of 5 MW can raise the temperature of F‐layer electrons by a hundred degrees or more. Those temperature increases are initially concentrated in narrow regions near caustics, but spread by heat conduction over tens of kilom

ISSN:0094-8276    

DOI:10.1029/GL012i011p00761

年代:1985

数据来源: WILEY

摘要:

The linear theory of internal gravity waves predicts an exponential growth of the velocity amplitudes with altitude. At high enough altitudes, the wave amplitudes become so large that nonlinear effects and turbulent breakdown prevent the velocities from increasing further with height. When this happens, the wave is said to be saturated. In this paper, it is shown that the wave amplitude and amplitude growth factor can be accurately computed from measurements of the sodium layer spatial power spectrum. Lidar measurements are presented which show gravity wave growth lengths that are nearly twice the unsaturated value of 12 km.

ISSN:0094-8276    

DOI:10.1029/GL012i011p00765

年代:1985

数据来源: WILEY

摘要:

Formate and acetate as well as other organic and inorganic anions have been measured in precipitation collected in Colorado at Niwot Ridge (a rural site) and Boulder (a suburban site). At both sites, the organic anion concentration is usually at least 20% of the nitrate concentration and occasionally exceeds the nitrate. Formate is the dominant organic anion measured with concentrations as large as 9 × 10−5M occurring in summer rain showers. A variety of dicarboxylic anions are observed but their concentration is generally much less than formate. Correlation with nitrate suggests that the large values found for the organic anions results from anthropogenic polluti

ISSN:0094-8276    

DOI:10.1029/GL012i011p00769

年代:1985

数据来源: WILEY

摘要:

DSDP data for the Barbados Ridge complex show the existence of high pore pressures in sediments both oceanward and arcward of oceanic trenches. We model the evolution of pore pressure by using a finite element procedure. Three consecutive stages are recognized: (1) In the oceanic basin, sedimentation rate is low; there is little excess pore pressure. (2) Approaching the trench but before being incorporated into the accretionary prism, sediments are subjected to increasing tectonic compression as well as greater sediment overburden. Thus, high pore pressure can be created. An important result is that the lowest effective stress occurs within the sedimentary layer instead of at its base and a decollement may develop along this weakened zone. (3) Arcward of the trench, sediments above the decollement are offscraped while those beneath it are subducted. Fast sedimentation combined with tectonic compression can create pore pressures so high as to cause porosity to increase with depth. This facilitates the formation of mud volcanoes, even oceanward of the deformation front.

ISSN:0094-8276    

DOI:10.1029/GL012i011p00773

年代:1985

数据来源: WILEY

摘要:

The behavior of the geomagnetic field during polarity transitions has been studied by modelling Parker‐Levy cyclonic convection cells with radial dipole sources located at the core/mantle boundary. The polarity transitions are generated by inverting the dipoles in an ever‐increasing spherical cap. Initial results for observation sites at northern mid‐latitudes demonstrate the dependence of the observed behavior of the geomagnetic field on the location of the center of the spherical cap. Although the modelling reproduces certain features of earlier models, it also accounts for observed phenomena not predicted by the other models. By initiating the reversal at 70°S, 30°W, we obtain a good fit to the available data for the Brunhes‐Matuyama transition as observed at northern‐lat

ISSN:0094-8276    

DOI:10.1029/GL012i011p00777

年代:1985

数据来源: WILEY