摘要:

A review of the sources of analytical data for the chondritic meteorites is given, and the latest data, particularly for the minor elements, are tabulated. Though the composition of the chondrites is remarkably constant as compared with terrestrial rocks, important variations in the concentrations of certain major and minor constituents occur, which make it impossible to conclude that any one type of chondrite could have been produced from another by simple physical processes. This is true for the carbonaceous, enstatitic, and the high‐ and low‐iron‐group chondrites. The origin of these chondrites is discussed mostly with reference to the author’s suggestions about the origin of the solar system. Only very complicated processes can account for the detailed characteristics of these objects. The possibility that the carbonaceous chondrites and particularly Wiik's type 1 of this group are the primitive material from which the meteorites and planets originated is considered, and it is concluded that these objects as a group cannot be such material (a conclusion reached by the author some ten years ago). The case for the type 1 carbonaceous chondrites' being this material is better, but serious arguments against this conclusion can be advanced; it is concluded that even this material has undergone some extensive chemical pro

ISSN:8755-1209    

DOI:10.1029/RG002i001p00001

年代:1964

数据来源: WILEY

摘要:

Major regions of inhomogeneity are present in the mantle at depths less than 1000 km. The thermal gradient also greatly exceeds its adiabatic value at relatively shallow depths. Hence the Williamson‐Adams equation cannot be used in this part of the earth to derive the density variation from seismic data. In this paper the density in the upper mantle is obtained by explicitly introducing the constitution of the material there. In the lower mantle the extended Williamson‐Adams equation is used, and the constitution of this region is deduced from the density curve.Recent seismic results for the upper mantle, particularly those relating to low‐velocity zones, are examined. Significant regional differences are present. Beneath the oceans there is a definite LV zone forS, and possibly one forPas well. Beneath Precambrian shields the LV zone forSis less pronounced, and the LV zone forPseems to be absent. Other continental regions are intermediate between these zones. The LV zone is considered to be due to high thermal gradients with mineralogical and chemical heterogeneity superimposed. Differences between the behavior ofPandSare largely due to different temperature coefficients of the two velocities. Regional contrasts arise from regional differences in thermal gradients and mineralogy.Consideration of temperature‐depth relations leads to the conclusion that the mantle is hottest beneath the oceans because of the absence of a thick, radioactive crust, and coolest beneath Precambrian shields because of low heat flow. A self‐consistent model of the mantle requires that the thermal flux at a depth of 400 km be about 0.5 μcal/cm² sec, because geochemical evidence indicates that the K/U ratio in the upper mantle is much smaller than in chondrites. The self‐consistent model requires very high thermal conductivity at high temperatures, such as would be provided by radiative transfer or possibly by movement of material.Petrological models of the upper mantle are constructed on the assumptions of an over‐all pyrolite (ultrabasic) composition and an eclogitic composition. Densities in the mantle are then calculated from known densities, thermal expansions, and compressibilities of minerals inferred to be present. Corrections for the effect of pressure on thermal expansion and compressibility are made from results of the theory of finite strain.The transition zone, at depths between 400 and 1000 km, is the site of a series of major phase transformations leading to close‐packed structures with silicon in sixfold coordination. The density curve in this region is approximated by a linear increase in density with depth. The lower mantle, between 1000 and 2900 km, is considered to be homogeneous, and the density is computed from the Williamson‐Adams equation, modified in some cases to take account of a superadiabatic thermal gradient. The magnitude of the density increase in the transition zone is adjusted to satisfy the restrictions imposed by the total mass and moment of inertia of the earth. A complete density curve for the earth is given for each petrological model of the upper mantle. In constructing them it is assumed that the outer core is homogeneous and adiabatic, and that the inner core is of uniform density.A density curve derived from the third‐order theory of finite strain is fitted to the densities calculated in the lower mantle by least squares. The density of the lower mantle at low temperature and pressure can then be calculated. The results at 20°C and atmospheric pressure are 4.2–4.3 g/cm³ for the adiabatic pyrolite model, and 4.0–4.1 g/cm³ for the adiabatic eclogite model. These values are in good agreement with estimates for the density of the lower mantle based upon recent investigations of phase transformation in olivines and pyroxenes at very high pressures. A superadiabatic gradient of 1°C/km in the lower mantle produces results inconsistent with a plausible constitution; therefore we conclude that the thermal gradient in this part of the earth is smaller than 1°C/km.Our models imply that important differences in density persist to depths of 400 km, and it is inferred that isostatic compensation is not complete before that depth. This conclusion is consistent with gravity data, and it leads to crustal densities more plausible, in terms of observed seismic velocities, than those obtained by assuming compensation at the continental M discontinuity. Because of these deep‐seated density contrasts, the position of the present pole of rotation may well be in equilibrium with the present distribution of continents and oceans.The relatively low temperature beneath the shields could explain their behaving like rigid blocks i

ISSN:8755-1209    

DOI:10.1029/RG002i001p00035

年代:1964

数据来源: WILEY

摘要:

The linear stability problem for a number of models of the mantle of the earth is considered. For appropriate values of the physical parameters of the mantle it seems likely that the Rayleigh number for mantle‐wide convection is far in excess of the value necessary for marginal instability. For very high Rayleigh numbers the velocities in the models can be derived from solutions for turbulent convection. But even for very high Rayleigh numbers the inhomogeneity in Bullen's region C is amply strong enough to prevent mantle‐wide convection from occurring, whether the inhomogeneity involves a phase transition or represents a chemical inhomogeneity. Convection on a smaller scale is also considered. Convection in the upper mantle may occur. These events are not widespread and are of small scale, having dimensions of about 1200–1500 km in lateral extent and depths of the order of 400 km. Large Rayleigh numbers and associated turbulent convection are not ruled out for the lower mantle. The conclusions depend crucially on the assumptions of the values of the viscosity and of the strength of the mantle. The model of turbulent convection in the lower mantle is consistent with localizing a material of high strength and high viscosity in the upper mantle and with the observation that earthquakes are not observed to occur in the lower m

ISSN:8755-1209    

DOI:10.1029/RG002i001p00089

年代:1964

数据来源: WILEY

摘要:

By using a technique first applied to seismological problems by Haskell, it is a simple matter to obtain the secular function for any given elastic waveguide, when the waveguide is composed of homogeneous layers with plane parallel boundaries. Roots of the secular function are usually exhibited as dispersion curves. In this paper the independent variable is dimensionless wave number,K, and the dependent variable is dimensionless frequency,F. A plot of complexFversusKyields the diagnostic (F, K) diagram—a dispersion curve. The procedure employed is to expand the secular function aboutK= 0 and then to trace the behavior of each root,F(K), for increasing values ofK. In each of the specific problems treated the initial position of each root,F(0), admits of a simple physical description. For example, in a simple continental waveguide (solid layer/solid half‐space), there are three sets of roots: the Lamb roots, consisting of andmodes; the shear ‘Organ pipe’ roots; and the compressional ‘organ pipe’ roots. There is an infinite number of the two kinds of organ pipe roots. The low‐frequencyPLwave arises from one of the roots, and the normal shear modes arise from transitions of all three kinds of roots. In a simple oceanic waveguide (fluid layer/solid half‐space) the shear organ pipe roots are absent. The low‐frequencyPLwave again arises from one of the roots, and the normal modes arise from transitions of the Lamb roots and the compressional organ pipe roots. In a simple acoustic waveguide (fluid layer/fluid half‐space) only the compressional organ pipe roots are present, and the normal modes arise from transitions of these roots. ThePLwave is absent. Its disappearance is clearly traced as the half‐space of the oceanic waveguide approaches a Poisson ratio of 0.5.The treatment of waveguides composed of more than one layer offers only the additional difficulty of finding the initial positions of the organ pipe roots. When these positions have been found, the analysis proceeds in a manner similar to that for simple waveguides. The initial positions of the organ pipe roots are complex, a situation that may be interpreted physically as radiation into the half‐space of the waveguide. It is the presence of such radiation that leads one to speak of leaking modes. The roots also have complex initial positions, if the half‐space is soft enough. In addition to being complex, the dispersion curves for the leaking modes sometimes have regions of negative group velocity. If one equates group velocity to velocity of energy transport along the waveguide, then one must conclude that there is an inward energy flux when the group velocity is negative. But when the group velocity is demonstrably not the velocity of energy transport, a simple physical picture of negative group velocity is sometimes unavailable. Plots of group velocity versusFshow a banded structure in some cases but are generally quite complex. The situation can be clarified somewhat by rejecting those modes with weak excitation functions, large decay parameters, or both. Still, the (F, K) diagram is clearer. Now that seismic array processing procedures are being developed, one hopes that experimental (F, K) diagrams will become a standard tool in seismic analysis, leading to a clearer picture of seismic dispersion and pr

ISSN:8755-1209    

DOI:10.1029/RG002i001p00123

年代:1964

数据来源: WILEY

摘要:

On the assumptions of a perfect gas, adiabatic motion, gravitational potential proportional to altitude, small Rossby number, and large Richardson number, the equations governing obliquely traveling, small disturbances relative to a zonal wind are reduced to a singular Sturm‐Liouville problem in which the latitude appears only as an implicit parameter. The structure of this problem is examined with the aid of an arbitrarily weighted quadratic integral and by transformation to an integral equation. Generalizations of Rayleigh's flex point and Howard's semicircle and rate‐of‐growth theorems are deduced from the quadratic integral. Approximations for small and large values of the wave number are deduced from the integral equation. It is shown that disturbances of sufficiently short wavelength in typical flows are always unstable but that they have a rate of growth that vanishes directly with the wavelength. Finally, it is shown that disturbances of all wavelengths in typical flows are always unstable for sufficientlysmallwind speeds. These conclusions lead to the conjecture that small disturbances in typical flows may be unstable for almost all wavelengths and all wind s

ISSN:8755-1209    

DOI:10.1029/RG002i001p00155

年代:1964

数据来源: WILEY

摘要:

Exploratory data from more than 1000 analyses of the distribution of deuterium in waters of the North American continent and the surface oceans contiguous to the continent are presented. The elementary theory of the processes that appear to explain the changes in the deuterium content of natural waters is developed. Quantitative expressions of the deuterium fractionation that can be expected to occur are presented for all phases of the hydrologic cycle from the evaporation of water from the oceans, its precipitation as rain and snow, and its travel back to the sea. Processes such as the freezing of water under equilibrium and nonequilibrium conditions, the evaporation of water from closed lakes and from lakes with an outlet, the formation of fog, frost, and dew are also discussed. The regional characteristics of the surface waters of North America are described and interpreted as reflecting the history of the water in the course of the hydrologic cycle.

ISSN:8755-1209    

DOI:10.1029/RG002i001p00177

年代:1964

数据来源: WILEY