摘要:

Pallasitic meteorites, as examples of samples derived from deep within their parent bodies, can provide information about conditions that existed within planetary interiors. Olivine crystal shape can be correlated with past deformation. Pallasites containing rounded olivines are free of kink bands and other evidence of plastic flow. Pallasites containing angular crystals, on the other hand, show a clear correlation between features of physical deformation, such as olivine crystal shape, size, degree of fracturing, and percentage of kink banded crystals. Pencil glide on the system {0kl} [100] is the predominant mechanism of plastic flow. Glide on this system in olivine indicates deformation at low strain rates and moderate to high temperatures. This occurred in the parent body while the pallasites were still deeply buried. According to petrofabric analyses, the parent fabrics of all pallasites studied are free of preferred orientations of olivine. Certain grains containing kink bands, however, do show preferred alignments. Kink banding in the pallasites is by a mechanism that is distinct from the shock‐induced kinking of chondrites, achondrites, and mesosiderite

ISSN:0148-0227    

DOI:10.1029/JB078i032p07581

年代:1973

数据来源: WILEY

摘要:

Geoid undulations have been recently computed using a set of spherical harmonic potential coefficients and 1° × 1° mean free‐air anomalies. The latter quantities are used in Stokes's equation out to some spherical radius ψ0from the computation points. The possible error sources considered in this paper were: (1) errors caused by the use of Stokes's equation, (2) earth constant errors, (3) errors in the numerical integration of the Stokes equation, (4) potential coefficient errors with specific estimates made for the Smithsonian Astrophysical Observatory Standard Earth II coefficient set, (5) errors caused by neglecting anomaly information outside ψ0, (6) errors due to the neglect of gravity material in blocks smaller than 1° × 1°, (7) errors due to inaccurate gravity material with specific computations made at four points, and (8) errors due to imperfect knowledge of equatorial gravity. Considering error sources 4 through 8, typical values of a root mean square undulation error (δN) were: ψ0= 0°, δN= 6.5 meters; ψ0= 5°, δN= 3.6 meters; ψ0= 10°, δN= 3.0 meters. For certain ψ0values an equatorial gravity error of ±1 mgal results in the dominant undulation error, indicating that a judicious selection of ψ0is necessary when the undulati

ISSN:0148-0227    

DOI:10.1029/JB078i032p07589

年代:1973

数据来源: WILEY

摘要:

The velocities of rarefaction waves in shock‐compressed MgO have been measured by observing the reduction of the shock front velocity near the sample edges due to the rarefaction waves propagating from the edges. The extent of this ‘edge effect’ is difficult to determine accurately because of its emergent nature. Arrangements sensitive to differences in shock front velocity yielded rarefaction wave velocities close to predicted longitudinal velocities in the high‐pressure shock state. Velocities closer to the hydrodynamic sound speed in the shock state were obtained from less sensitive arrangements. These results can be interpreted in terms of a two‐stage elastoplastic model of the decompression. The longitudinal velocities measured in shock states up to 528 kb imply second pressure derivatives of the elastic modulicij″, given byK0cij″ = −1 ± 15, whereKis

ISSN:0148-0227    

DOI:10.1029/JB078i032p07596

年代:1973

数据来源: WILEY

摘要:

Induction and demagnetization characteristics of thermoremanent magnetization (TRM) have been studied for four synthetic magnetites with mean particle sizes of 0.04–0.22 μm. The experiments are intended to simulate the acquisition and laboratory demagnetization of natural remanent magnetization carried by submicroscopic but non‐single‐domain titanomagnetite particles in igneous rocks. Although hysteresis properties indicate that the magnetites are above single‐domain (SD) size, their weak‐field TRM are SD‐like. TRM intensity, for a 1‐oe inducing field, ranges from 6 to 12 emu/cm3. Median alternating demagnetization fields are between 275 and 400 oe, while blocking temperatures are generally within 50°C of the Curie point. However, neither Néel's SD theory nor Stacey's pseudo‐SD theory can account for the observed dependence of TRM on inducing field, since theoretically TRM should saturate in rather low fields. Néel's two‐domain theory fits the strong‐field data very well but is inadequate in the weak‐field region. These failures indicate that theoretically assumed two‐ and four‐domain structures are not appropriate in particles just above SD size. Blocking temperature data are consistent with well‐developed multidomain (MD) structure (i.e., thermal activation volume associated with a domain wall notably less than particle volume) in 0.22‐μm particles but a wavelike spin structure without recognizable domains in particles ≤0.1 μm in size. The dependence of weak‐field TRM on grain sizedalso supports this interpretation: TRM varies approximately asd−1between 0.1 μm and the pseudo‐SD threshold at 15–20 μm, but below 0.1 μm there is no clear size dependence. Particles with wavelike spin structure have not been recognized previously but may well explain the MD‐lik

ISSN:0148-0227    

DOI:10.1029/JB078i032p07602

年代:1973

数据来源: WILEY

摘要:

Electrical conductivity of serpentinized rocks from the Indian Ocean ridge is 3–4 orders of magnitude higher than that of serpentine‐free peridotite, hornblende gabbro, and basalt from the same area and from the Tonga trench. The enhanced conduction is apparently characteristic of many but not all serpentine‐rich rocks and may be due in part to high porosity and in part to mineral conduction associated with the magnetite formed during serpentinization. Observations of high oceanic crustal conductivity could be explained by the presence of serpentine in these rocks, although this might be ruled out on the basis of temperature, velocity, or local petr

ISSN:0148-0227    

DOI:10.1029/JB078i032p07614

年代:1973

数据来源: WILEY

摘要:

Ultrasonic shear wave birefringence is used as a positive indicator of elastic anisotropy. Polarized shear waves are generated by conversion of a compressionalPwave to a shearSwave at a free surface (Jamieson and Haskins, 1963). TheP‐Sconversion technique is preferred over the direct use of AC‐cut quartz shear wave transducers because the wave amplitudes are considerably greater in the conversion technique. Even though theP/Swave amplitude ratio for theP‐Sconversion transducers is 1/80, thePwave has sufficient amplitude for recognition. The rotation of the sample allows identification and measurement of the travel times of two orthogonally polarized shear waves. Relative difference in the two shear wave travel times positively establishes homogeneous anisotropy, even though the sample may be nearly isotropic in thePwave mode. ThusPwave velocity and travel path distance need not be known in order to detect elastic anisotropy. Shear wave birefringence is not restricted to any particular scale, and a minimum of one propagation direction may be used to establish anisotropy. A suite of 20 rock samples of various lithologies was measured according to this technique. All samples were found to be anisotropic. These results point out that elastic anisotropy is probably the rule rather than the exception in describing the elastic behavior of rock materials of hand sample size. It is therefore of interest to determine at which scale, if any, elastic anisotropy is no longer the rule but the exception. The use of shear wave birefringence may aid in answering this que

ISSN:0148-0227    

DOI:10.1029/JB078i032p07623

年代:1973

数据来源: WILEY

摘要:

Upper and lower bounds on the adiabatic bulk modulus are calculated using energy principles for two cases. In the high‐frequency case, deformation is adiabatic on the scale of a single grain, and temperature may vary from grain to grain. Bounds are found to be given by the Voigt and Reuss averages of the components of the adiabatic stiffness and compliance tensors. At lower frequencies, deformation is adiabatic overall, but temperature of adjacent grains has sufficient time to reach an equilibrium value. Bounds at high frequencies for olivine at room temperature are found to be nearly equal numerically to the low‐frequency bou

ISSN:0148-0227    

DOI:10.1029/JB078i032p07631

年代:1973

数据来源: WILEY

摘要:

Theoretical bounds on the strain energy due to internal stress in polycrystalline materials are found using energy principles. These bounds are used to derive bounds on bulk thermal expansion; the expressions depend only upon the single‐crystal stiffness and compliance tensors, the thermal expansion tensors, and the partial volumes of the mineral components. A description of the geometric configuration of the phases is not required in the analysis. Calculations based on published data for corundum, rutile, calcite, and quartz show that bounds on thermal expansion for polycrystalline calcite, which is the most anisotropic of the four minerals, differ by only 6%. The specific heat of the polycrystalline forms of these minerals is found to be very nearly equal to the single‐crystal values. Theory is used to find hot‐pressing pressure and temperature required for minimizing internal stresses at room conditions and to estimate internal stresses in surface rock that is free of internal stress at

ISSN:0148-0227    

DOI:10.1029/JB078i032p07637

年代:1973

数据来源: WILEY

摘要:

Magnetic properties of oceanic basalts from Deep‐Sea Drilling Project (DSDP) sites 36, 54, 57, 77, 83, and 84 are described. Secondary components of magnetization were found to be significant at many of these sites. The basalts were placed in deuteric oxidation class 1 on the basis of opaque mineralogy observations. These, together with the analysis of irreversible high‐field thermomagnetic curves measured in air, indicated that the dominant magnetic minerals were nonstoichiometric titanomagnetites. The remanent magnetizations were stable against af demagnetization, having median destructive fields (mdf) of 250–550 oe and Königsberger ratios of 10–50. These magnetic properties are compatible with some of the basic assumptions of sea floor spreading. However, the remanent inclinations, although stable, did not give interpretable paleolatitudes for the sites, as has also been observed in other studies of partially oriented submarine basalts. Class 1 basalt from hole 57 had a coarse grain size and lower Curie temperatures than the other basalts. Its remanence was very unstable (mdf of 40 oe) and could be accounted for in direction, intensity, and stability by a viscous remanent magnetization (VRM) acquired in situ during the Brunhes normal polarity epoch. Similar unstable magnetizations have been observed in several other DSDP basalts. The VRM of the oceanic crust is proposed as an explanation of some magnetic ‘qu

ISSN:0148-0227    

DOI:10.1029/JB078i032p07647

年代:1973

数据来源: WILEY

摘要:

A thermodynamic treatment is given of the local propagation of a coherent interface between two nonhydrostatically stressed crystals. The form of the strain increment undergone by an element of crystal as the interface propagates through it is restricted by the condition of displacement continuity. This restriction generally implies stress discontinuity at the interface. Thus coherent transitions in nonhydrostatically stressed crystals cannot generally take place at homogeneous stress. A criterion for the equilibrium of two crystals at a coherent interface is derived. This reduces to the criterion given in Kamb's (1959) treatment of dry recrystallization if the shear components of the strain increment parallel to the interface vanish.

ISSN:0148-0227    

DOI:10.1029/JB078i032p07661

年代:1973

数据来源: WILEY