Fracture strength of dry silicate rocks at high confining pressures and activity of acoustic emission

Three dry silicate rocks, gabbro, dunite and eclogite, were triaxially compressed up to a confining pressure of 3 GPa at room temperature. These rocks exhibited brittle fracture behavior up to the highest confining pressure. The change of the mechanism of fracture in the brittle region is suggested from the measurement of the compressive fracture strength and the activity of acoustic emission. The existence of the “high-pressure brittle-fracture” phase is proposed. The fracture strength increased with increase of confining pressure. The increasing rate of strength was lowered at a value of confining pressure: at about 0.8 GPa on gabbro; at about 1.0 GPa on dunite; and at about 1.5 GPa on eclogite. At lower confining pressures than the above value, the acoustic emission rate began to increase at the onset of dilatancy and increased rapidly followed by fracture as the axial stress was increased. At the higher confining pressures, however, the acoustic emission rate did not increase rapidly before final fracture, and stayed constant to the fracture. The similar behavior was shown on the granite studied previously. It is interesting that the frictional strength forms the boundary between “low- and high-pressure brittle-fracture” phases.     

Review of the geomagnetic secular variations on the historical time scale

In view of the classification of the geomagnetic field into its axisymmetric and non-axisymmetric parts, studies of geomagnetic secular variations on the historical time-scale are reviewed. The westward drift of the geomagnetic field, which is one of the most conspicuous features of its secular variation, is examined first. The non-axisymmetric field during the past several hundred years can be well approximated by the superposition of two constant-magnitude fields, a standing and a drifting field, whose lifetimes are supposed to be longer than 1000 years. It is pointed out that the sectorial term of the non-dipole standing field is small compared with the drifting one. The lack of the n = M = 2 term of the standing field is particularly remarkable.

On the other hand, the equatorial dipole field is likely to consist of two components which are both drifting. One drifts westwards with a normal velocity and the other eastwards with a small velocity.

Besides the pronounced westward drift in an east-west direction, the poleward movements of particular foci of the secular variation are noted. This may, however, be related to the rapid growth of the axisymmetric quadrupole field.

The time variation of the dipole field is briefly examined. As far as the data on the historical time scale are concerned, an antiparallel relationship seems to exist between the variations in the dipole and the quadrupole field. As the dipole moment decreases, the magnitude of the quadrupole moment increases. Finally, characteristic oscillation periods of the dipole field are examined. Although the data are few, a 60–70-year period, a 400–600-year and a 8000-year period emerge as the dominant periods.     

根据地球热历史研究得到的地幔温度模型(英文)

通过求解全球能量平衡方程 ,得到地幔温度剖面。中地幔过渡带有一个温度突然增加 ,因此不能根据近地表数据向下做简单的外推。这个问题可以通过研究地球的冷却过程解决。目前地幔底部的温度值是 2 94 0K ,中地幔过渡带温度为 2 0 4 0K ,穿过这个带有一个 310K的突变。上部边界层底部的温度为     

Paleomagnetic field intensity derived from meteorite magnetization

The paleomagnetic field intensity is estimated with the aid of the Koenigsberger-Thellier method for four ordinary chondrites and one carbonaceous chondrite by assuming that the stable NRM component of these meteorites is attributable to the TRM acquired in a low-temperature range (lower than about 400°C) during their extremely-slow cooling process. The results are summarized in Table IV, where the paleomagnetic field intensity ranges from 0.10 to 0.97 Oe.Possible effects of the extremely-slow cooling rate of meteorites and the secondary TRM acquisition of the surface fusion crust upon the original NRM of the meteorite interior are discussed.     

Meteorite magnetism and the early solar system magnetic field

The ferromagnetism of irons, stony-irons, E-, H-, L- and LL-chondrites and achondrites is due to a metallic phase comprising mostly Fe and Ni and small amounts of Co and P. The ferromagnetic constituent in non-metamorphosed C-chondrites is magnetite, but some metamorphosed C-chondrites contain FeNi metallic grains too.

Among the stony meteorites, the content of metals as determined by their saturation magnetization (I_S) sharply decreases in the order E → H → L → LL → achondrites, whereas the I_S value for magnetite and additional metals in C-chondrites ranges from the I_S value of achondrites to that of L-chondrites.

With an increase of Ni-content in the metallic phase in chondrites of the order E → H → L → LL → C, the relative amount of Ni-poor kamacite magnetization, I_S(), in the total I_S decreases in the same order, from I_S()/I_S 1 for E-chondrites to I_S()/I_S 0 for C-chondrites. Thus, E-, H-, L-, LL- and C-chondrites and achondrites are well separated in a diagram of I_S()/I_S versus I, which could be called a magnetic classification diagram for stony meteorites.

As the surface skin layer of all meteorites is anomalously magnetized, it must be removed and the natural remanent magnetization (NRM) of the unaltered interior only must be examined for the paleomagnetic study. The NMR of C-chondrites is highly stable and that of achondrites is reasonably stable against AF-demagnetization, whereas the NMR of E-chondrites and ordinary chondrites as well as stony-iron meteorites is not very stable in most cases. Although the NRM of iron meteorites is reasonably stable, it is not attributable to the extraterrestrial magnetic field.

The paleointensity for Allende C₃-chondrite is estimated to be about 1.0 Oe assuming that its NRM is of TRM origin. The paleointensity for other reasonably reliable C-chondrites (Orgueil, Mighei, Leoville and Karoonda) is also around 1 Oe.

The paleointensity for two achondrites has been determined to be about 0.1 Oe. The NRM of other achondrites also suggests that their paleointensity is roughly 0.1 Oe.

The NRM of ordinary chondrites is less stable than that of C-chondrites and achondrites so that the estimated paleointensity for ordinary chondrites is less reliable. The paleointensity for comparatively reliable ordinary chondrites ranges from 0.1 to 0.4 Oe.

The paleointensity values of 1 Oe for C-chondrites and 0.1 Oe for achondrites may represent the early solar nebula magnetic field about 4.5 × 10⁹ years ago. A possibility that the paleomagnetic field for achondrites was a magnetic field attributable to a dynamo within a metallic core of their parent planet may also not be rejected.     

A preliminary study on variations in the Gauss coefficients of the geomagnetic potential over several hundred years

Geomagnetic secular variations are examined in terms of time variation in the Gauss coefficients. Major parts of the variations over several hundred years can be represented by a two mode model which consists of a standing and a drifting field. When the Gauss coefficients are plotted on a diagram with g_n^m in the abscissa and h_n^m in the ordinate, the drifting component describes a circle. However, some of the observed coefficients depict an elliptical trajectory rather than a circular one. Improvement of the model is attempted in two different ways. One is to assume time variability of the amplitude of the drifting component. The other is to introduce another drifting mode. Selecting a few spherical harmonic terms, variations in the Gauss coefficients since A.D. 1600 are analysed.When the amplitude of the drifting field is assumed to vary, the observed nature of the elliptical trajectory is well represented. In this case, phase velocity also changes with time. It is large while the amplitude is small, and it is small while the amplitude is large. Three mode models, in which an eastward drifting mode is incorporated, approximate the observed variations as well, not only for the period over several hundred years but also for the last several decades. In this model the westward drifting mode dominates the eastward mode.     

Erratum to: “Partial derivative of love numbers”

A small error in the computations ofOkubo andSaito has been corrected. The correction removes certain previously peculiar results and slightly affects partials within the inner core. The online version of the original article can be found at     

EMRIDGE: The electromagnetic investigation of the Juan de Fuca Ridge

From July to November 1988, a major electromagnetic (EM) experiment, known as EMRIDGE, took place over the southern end of the Juan de Fuca Ridge in the northeast Pacific. It was designed to complement the previous EMSLAB experiment which covered the entire Juan de Fuca Plate, from the spreading ridge to subduction zone. The principal objective of EMRIDGE was to use natural sources of EM induction to investigate the processes of ridge accretion. Magnetotelluric (MT) sounding and Geomagnetic Depth Sounding (GDS) are well suited to the study of the migration and accumulation of melt, hydrothermal circulation, and the thermal evolution of dry lithosphere. Eleven magnetometers and two electrometers were deployed on the seafloor for a period of three months. Simultaneous land-based data were made available from the Victoria Magnetic Observatory, B.C., Canada and from a magnetometer sited in Oregon, U.S.A.Changes in seafloor bathymetry have a major influence on seafloor EM observations as shown by the orientation of the real GDS induction arrows away from the ridge axis and towards the deep ocean. Three-dimensional (3D) modelling, using a thin-sheet algorithm, shows that the observed EM signature of the Juan de Fuca Ridge and Blanco Fracture Zone is primarily due to nonuniform EM induction within the ocean, associated with changes in ocean depth. Furthermore, if the influence of the bathymetry is removed from the observations, then no significant conductivity anomaly is required at the ridge axis. The lack of a major anomaly is significant in the light of evidence for almost continuous hydrothermal venting along the neo-volcanic zone of the southern Juan de Fuca Ridge: such magmatic activity may be expected to have a distinct electrical conductivity signature, from high temperatures, hydrothermal fluids and possible melt accumulation in the crust.Estimates of seafloor electrical conductivity are made by the MT method, using electric field records at a site 35 km east of the ridge axis, on lithosphere of age 1.2 Ma, and magnetic field records at other seafloor sites. On rotating the MT impedance tensor to the principal axis orientation, significant anisotropy between the major (TE) and minor (TM) apparent resistivities is evident. Phase angles also differ between the principal axis polarisations, and TM phase are greater than 90° at short periods. Thin-sheet modelling suggests that bathymetric changes accounts for some of the observed 3D induction, but two-dimensional (2D) electrical conductivity structure in the crust and upper mantle, aligned with the ridge axis, may also be present. A one-dimensional (1D) inversion of the MT data suggests that the top 50 km of Earth is electrically resistive, and that there is a rise in conductivity at approximately 300 km. A high conductivity layer at 100 km depth is also a feature of the 1D inversion, but its presence is less well constrained.