摘要:

The self‐diffusion of18O‐enriched oxygen in forsterite was determined by measuring the bulk concentration using the nuclear reaction18O(p,α)15N. Important technical improvements led to a more precise determination of the18O diffusion profiles. These results indicate that the self‐diffusion coefficient D* is independent of the annealing duration and the oxygen partial pressure. In addition, the D* values are found to be lower by nearly 1 order of magnitude than those previously published (preexponential factor D0∼ 2.3×10−6cm2/s, activation energy, ∼

ISSN:0148-0227    

DOI:10.1029/JB088iB01p00613

年代:1983

数据来源: WILEY

摘要:

The rifting of the plate boundary in north Iceland forms new ground fissures and reactivates old ones. Continuous growth of fissures is observed in the crust above a magma chamber beneath the Krafla caldera as it responds to changes in subterranean magma pressure. Rapid episodes of fissure formation and reactivation are observed in conjunction with magma intrusions at depth along the rift zone that constitutes the plate boundary between the North American and Eurasian plates. The Krafla caldera contains several fissures that open during periods of uplift lasting 100 to 300 days and that close incompletely in subsidence episodes lasting several hours to 3 weeks. The average rate of opening differs from one period to another, ranging from 0.1 to 0.5 mm/d. Fissure closing and average closing rates also vary from one subsidence episode to another, ranging from minimal values of 2.1 mm and 2.3 mm/d to maximal values of 49 mm and 18 mm/d, respectively. During the two intrusion episodes of September 1977 and March 1980 the magma migrated toward the south with an average velocity 0.6 and 0.5 m/s, respectively. In the September 1977 episode some fissures in the rift zone widened with a rate greater than 190 mm/d, whereas during the March episode that probably resulted in a thinner intrusion the maximum widening rate was only 50 mm/d. The data suggest that the magnitude of a subsidence episode is determined at least partly by the available excess magma pressure in the chamber. To explain the extensive growth of open fissures and normal faulting in the rift zone, it is necessary to assume that in addition to the intrusion of the main central dike at depth, many small dikes are intruded at shallow depths (less than 1 km).

ISSN:0148-0227    

DOI:10.1029/JB088iB01p00625

年代:1983

数据来源: WILEY

摘要:

An understanding of the factors governing domain wall stability is crucial to any attempt to model remanence in multidomain materials. This paper introduces a technique to the study of multidomain remanence which has not previously seen application in rock magnetism: domain wall resonance (DWR). We show that the variation of the DWR spectra with different RF power levels gives useful, and hitherto unavailable, information about the nature of the potentials which govern domain wall motion. In particular, we demonstrate that these potentials must rise less rapidly than a harmonic potential, contrary to previous models. The possible mechanisms for domain wall pinning are discussed and it is concluded that the likeliest candidates for pinning sites in our samples, at least, are large nonuniform dislocation arrays. Finally, we suggest a possible method for selectively demagnetizing multidomain grains based on DWR techniques.

ISSN:0148-0227    

DOI:10.1029/JB088iB01p00637

年代:1983

数据来源: WILEY

摘要:

The Great Logan Paleomagnetic Loop has been interpreted to be a paleomagnetic signature of the North American plate motion during the Keweenawan igneous activity (1200–1000 Ma). However, the presence of three successive asymmetric polarities, along with new geochemical data from the Mamainse Point area, makes this interpretation untenable. A new interpretation is presented according to which the asymmetric polarities represent long‐term zonal nondipole geomagnetic field components. Wilson's single‐dipole offset model can in general explain the inclinational asymmetries, but it yields intensity ratios (reversed/normal) significantly lower than those observed. A coaxial two‐dipole model is proposed that better takes into account the observed intensity data. This model consists of a geocentric dipole and an offset dipole located at the core‐mantle boundary. The offset dipole simulates the long‐term zonal average of the spherical harmonic nondipole field. If the geocentric dipole reverses while the offset dipole retains its constant polarity, nonantiparallel reversal results. A method is presented by which the depth of the offset dipole and the ratio of the dipole strengths can be determined using only the observed inclinations. The two‐dipole model suggests that inclinational asymmetries are due to a different nondipole/dipole ratio during the normal and the reversed polarity. The most probable variant of the two‐dipole models predicts that this ratio is 35% and 49% for the normal and the reversed polarity, respectively. A new interpretation for the evolution of the Logan Loop is described according to which at least 50% of the hairpin‐shaped loop is produced by oscillation of the dipole strength ratio. Apparent polar wander (APW) is still evident, but the shape of the loop and the APW speed along the loop is reduced. The two‐dipole model is tested by worldwide paleomagnetic dat

ISSN:0148-0227    

DOI:10.1029/JB088iB01p00645

年代:1983

数据来源: WILEY

摘要:

At each node of a network‐type survey, two surveyed values are obtained. In the absence of errors, these two values should be equal, providing that the quantity being surveyed is, for all practical purposes, time independent. When a low‐frequency, time‐dependent error enters the system, it can be indirectly observed in the form of surveyed‐value differences occurring at the network nodes. This paper describes a technique for reducing such an error. The method consists of using a conjugate gradient projection algorithm to find the unbiased, discrete function of minimum weighted variation which produces the observed nodal differences. After inspecting the discrete solution, a suitably chosen continuous function can be fitted to the discrete data and used to reduce the time‐dependent error. This technique has been successfully used to reduce the radial ephemeris error present in SEASAT geoid height data (Cloutier, 1981). Future applications include the removal of the diurnal variation present in magnetic surveys and the removal of the nonlinear gravimeter drift present in gravit

ISSN:0148-0227    

DOI:10.1029/JB088iB01p00659

年代:1983

数据来源: WILEY

摘要:

We give here a direct solution of this problem by converting, in an indirect way, the relevant integral equations over each block to their equivalent line integrals taken along and around the boundary curves of the block. Results of sample computations given clearly demonstrate the convergence and stability of these line integrals. We also computed geoidal parameters due to a 30°×30° inner zone alone at 71 grid locations in Cameroon, using 1°×1° anomalies and integration steps of 30″ for the four innermost blocks and 180″ for the rest of the blocks and compared these with equivalent values from the midpoint method. This comparison yielded significant differences—rms ΔN= 2.2 m, ΔNmax= 4 m; rms Δξ = 2.9″, Δξmax= 10.5″; rms Δη = 2.0″, Δηmax= 6.7″—that reflect the poor accuracy of the center point method, especially for 1°×1°

ISSN:0148-0227    

DOI:10.1029/JB088iB01p00665

年代:1983

数据来源: WILEY

摘要:

Tracking data from synchronous earth's satellites can be analyzed to provide the resonant geopotential coefficients, mainlyC22andS22which represent the ellipticity of the earth's equator. Very accurate tracking techniques (LASER, Doppler) could in principle be used to improve the present knowledge of these coefficients but nongravitational perturbations on the satellite orbit, in particular solar radiation pressure, cannot be modeled to the same high accuracy unless the satellite is an ad hoc geophysical satellite. On the other hand, geosynchronous satellites currently used for telecommunication purposes can be easily tracked with moderate size large field telescopes measuring the satellite angular coordinates with respect to some catalogue field stars. The accuracy is at least 2 arc sec (≃ 400 m), i.e., much worse than the accuracy of the LASER data but surely comparable to longitude errors due to the radiation pressure model. Moreover, optical tracking does not need any onboard device and is easily accomplished. We conclude that the organization of a worldwide optical tracking campaign would allow an improvement in the knowledge of the long‐wavelength geoid by at least one order of magnitude, and this is very interesting for many geophysical purpo

ISSN:0148-0227    

DOI:10.1029/JB088iB01p00669

年代:1983

数据来源: WILEY

摘要:

I have inverted the recently completed set of seismic arrival times from the Apollo lunar seismic network to estimate the average seismic velocities in three sections of the lunar mantle: two for the upper mantle and one for the middle mantle. The method used is a variation of the linearized least squares inversion where the inversion is accomplished in steps. The estimated average velocities in the upper mantle decrease fromVp= 7.74 km/s andVs= 4.49 km/s in the section above 270‐km depth toVp= 7.46 km/s andVs= 4.25 km/s in the section between 270‐ and 500‐km depth, confirming the earlier finding of negative gradients based on seismic amplitude variations. The average velocities in the middle mantle between the depths of 500 km and 1000 km ofVp= 8.26 km/s andVs= 4.65 km/s are significantly higher than those in the upper mantle, contradicting earlier estimates based on more limited data. The higher velocities may suggest initial melting of the moon down to at least 1000‐k

ISSN:0148-0227    

DOI:10.1029/JB088iB01p00677

年代:1983

数据来源: WILEY