摘要:

Magnetostriction of synthetic polycrystalline magnetite and titanium‐rich titanomagnetites has been measured from 25°C to their Curie temperatures. These are the first reported measurements of magnetostriction above room temperature for titanomagnetites. The magnetostriction constants are measured with a recording rotating field magnetostrictometer, which uses a strain gage bridge technique and two rotating SmCo5permanent magnets. Magnetostriction is recorded continuously as the sample is slowly heated and cooled in a fixed magnetic field. Measurements were made on sintered polycrystalline samples of magnetite, titanomagnetite (Fe3‐xTixO4,x=0.0, 0.4, and 0.6), and an aluminum‐magnesium substituted titanium‐rich titanomagnetite (AMTM60). The thermal dependence of the polycrystalline magnetostriction constant λs(T) varies as (1‐T/Tc)1.3for titanium‐rich titanomagnetites and as (1‐T/Tc)0.9for pure magnetite. the polycrystalline magnetostriction constant is well approximated by a power law equation in terms of the saturation magnetization, λs(T)αMs(T)n, wheren=2.31 (TM0), 3.47 (TM40), 2.75 (TM60), and 2.80 (AMTM60). The magnetostriction results for AMTM60 are in excellent agreement with previous predictions based on do

ISSN:0148-0227    

DOI:10.1029/92JB01846

年代:1993

数据来源: WILEY

摘要:

A theoretical basis is given for two empirically derived methods of three‐axis alternating field (AF) demagnetization of static samples used initially by Zijderveld and Dunlop to remove “spurious” components of magnetization produced by the AF. It is shown theoretically that both methods of treating the experimental results to reveal the natural remanent magnetization (NRM) vector are precisely those which algebraically eliminate gyroremanent magnetization (GRM). By implication, the “spurious” remanences found by these and other workers are almost certainly of gyromagnetic origin and not remanences produced as a result of shortcomings in instrumentation. It is further demonstrated that by suitably treating the data obtained by either three‐axis method, GRM can be identified and used to provide information about the anisotropy

ISSN:0148-0227    

DOI:10.1029/92JB01849

年代:1993

数据来源: WILEY

摘要:

We have quantified effects of experimental deformation on the magnetic properties of a set of synthetic “calcite sandstone” samples containing magnetite. The deformation was carried out in a microcomputer‐controlled apparatus that adjusted the applied differential stress as needed to maintain a constant strain rate of 10−5s−1. Most samples were deformed under dry conditions, but a few were deformed with a pore fluid present; the samples deformed under dry conditions required substantially higher differential stresses. Macroscopically ductile shortening strains of up to 25% produced the following irreversible changes in magnetic properties: (1) increased bulk coercivity, remanence coercivity, and mean anhysteretic remanence susceptibility; (2) decreased mean low‐field susceptibility; (3) decreases in the component of remanence parallel to shortening; (4) smaller decreases for most samples in the component normal to shortening, resulting in a net “rotation” of the remanence away from the shortening axis; (5) larger decreases in the normal component in a few samples, resulting in a net “rotation” of the remanence towards the shortening axis; (6) increased magnetic anisotropy; and (7) increased “deformation” of initial magnetic ellipsoids. A comparison of data for samples deformed under dry and wet conditions (higher and lower differential stresses, respectively) indicates that remanence reorientation and susceptibility anisotropy are controlled primarily by bulk strain (i.e., rotation and displacement of particles), whereas coercivity and anhysteretic anisotropy are controlled dominantly by microstrain

ISSN:0148-0227    

DOI:10.1029/92JB01028

年代:1993

数据来源: WILEY

摘要:

The present study investigates changes in the magnetic properties of hematite, more precisely low‐Ti titanohematite, in Icelandic subaerial basalts during low‐temperature metamorphism (<300°C). The results have implications for detection of secondary magnetization directions in basalts and perhaps for seafloor magnetic anomaly modeling. Samples were collected both from outcrops and from the Iceland Research Drilling Project (IRDP) core and represent 3.6 km of crust. The natural remanent magnetization (NRM) unblocked below 590°C during thermal demagnetization was assigned to magnetite and that unblocked above 610°C to hematite. Unblocking temperatures of hematite gradually increased in the laumontite zeolite zone from the original low range of 610°–650°C to 650°–680°C. We attribute this increase to the migration of a small amount of initial Ti out of the hematite lattice. Titanium migration is consistent with microprobe analyses. The intensity of the hematite magnetization correlated with that of magnetite until the top of the laumontite zeolite zone (1.8 km depth). Below this depth, the intensity of the magnetite magnetization decreased and correlation became poor due to alteration of magnetite to nonmagnetic minerals. Both magnetite and hematite appeared to retain their primary directions of magnetization to the bottom of the laumontite zeolite zone even though Ti had migrated out of the lattices. If field‐controlled chemical remanent magnetization is not produced during Ti migration, then seafloor basalts undergoing the same processes may also faithfully retain primary directions of remanence. Below 3.1 km depth in the epidote metamorphic zone, large secondary components in some samples could not be completely removed by thermal demagnetization to 550°C and were attributable to a thermo‐viscous remanent magnetization (thermo‐VRM) in magnetite. The durability of the hematite magnetization suggests that the identification of hematite components in subaerial basalts may help reveal subtle secondary components contained in magnetite directions

ISSN:0148-0227    

DOI:10.1029/92JB01253

年代:1993

数据来源: WILEY

摘要:

Scanning electron microscope and Scanning transmission electron microscope observations of thin sections and separates of the Devonian Onondaga and Helderberg limestones and Ordovician Trenton limestone in New York state allow us to identify three types of magnetite: pseudoframboids, nonspherical magnetite, and fine‐grained magnetite. Magnetite was observed replacing pyrite occurring as crystals in spherical pseudoframboids or with nonspherical shapes. Fine‐grained magnetite, consisting of aggregates of one or more rounded single crystals, approximately 2000 Å (200 run) in diameter, could not be observed in thin sections of New York carbonates due to its small size but was found in magnetic extracts. Hysteresis measurements of magnetic extracts verify that fine‐grained magnetite is capable of carrying remanent magnetizations. However, pseudoframboidal magnetite and nonspherical magnetite are polycrystalline and consist of assemblages of single to pseudosingle domain‐sized crystals that also can be carriers of the remagnetization. These data, in combination with observations from other localities, collectively imply that the Alleghenian remagnetization is carried by the fine‐grained magnetite, although pseudoframboidal and nonspherical magnetite may also contribute. Thus the Alleghenian remagnetization is a chemical remanent magnetization due to authigenic magnetite. Crystallization of magnetite was mediated by fluids, with dissolution and crystallization activated by stress during the Alleghenian Orogeny. The origin of such fluids is unknown, but they may have originated through crustal scale fluid migration. Tectonically induced brine migration due to emplacement of thrust sheets is preferred over a meteoric source of

ISSN:0148-0227    

DOI:10.1029/92JB01323

年代:1993

数据来源: WILEY

摘要:

The amplitudes of marine magnetic anomalies show a clear worldwide pattern of systematic variation when viewed as a function of age. Globally, the amplitudes decrease with age over the first 20 to 30 million years, a phenomenon that has been attributed to the low‐temperature oxidation of the magnetic mineral titanomagnetite in the upper extrusive rocks. In oceanic crust older than 40 million years, however, the amplitudes of the magnetic anomalies increase with increasing age and remain at elevated levels for the entire period between 80 and 160 million years. In order to examine the processes responsible for this elevated crustal magnetization in older oceanic crust, we compiled all of the existing rock magnetic data from relevant Deep Sea Drilling Project and Ocean Drilling Program sites that sampled extrusive rocks from “normal” ocean crust. This compilation shows that the laboratory measurements of the magnetization of the upper basement rocks closely reflect that of the anomaly amplitudes, both in the decrease due to oxidation in the first 20 to 30 million years, and in the increase in magnetization in older (>80 million years) crust. This positive correlation between anomaly amplitudes and upper crustal magnetization supports the argument that a major source of the marine magnetic anomalies is in the upper extrusive volcanic rocks. Further, the Curie temperature data show that oxidation of the magnetic minerals in oceanic basalts occurs largely within the first 30 million years, and does not increase significantly beyond that point. Finally, the strong positive correlation between the intensity of the magnetization of the drill core samples and the saturation magnetization argues that the higher magnetic anomaly amplitudes over older crust are related to a change in an intrinsic property of the upper crust. Specifically, we propose that the observed elevated magnetization of older ocean crust is due primarily to an increased abundance of magnetic FeTi oxides in the older crustal rocks. This large increase in abundance of the FeTi oxides may be related to a systematic increase in bulk FeTi content in older tholeiitic basalts, or, more likely, is due to a difference in the partitioning of the iron and titanium content between the silicate and oxide p

ISSN:0148-0227    

DOI:10.1029/92JB01322

年代:1993

数据来源: WILEY

摘要:

To constrain theories for the dynamic evolution of global ice mass through the late Neogene, it is important to determine whether major changes in the record were gradual or rapid. Of particular interest is the evolution of the near 100‐kyr ice age cycle in the middle Pleistocene. We describe and apply a new technique based on multiple taper spectral analysis which allows us to model the time evolution of quasi‐periodic signals. This technique uses both phase and amplitude information and enables us to address the question of abrupt versus gradual onset of the 100‐kyr periodicity in the middle Pleistocene. We analyzed three long (>2.6 m.y.) time series from Deep Sea Drilling Project (DSDP) site 607 (midlatitude Atlantic) and Ocean Drilling Program (ODP) site 677 (equatorial Pacific). The published time scales for these sites differ significantly, influencing the interpretation of the mid‐Pleistocene climate transition. When the data series are expressed on the same time scale, there is evidence for a coherent δ18O signal at both sites in the eccentricity and obliquity frequency bands, consistent with variations in global ice volume as the causative factor. If the Shaddeton et al. (1990) time scale for ODP 677 is accepted, the amplitude match between the δ18O obliquity cycle and the 65°N insolation derived from the recent astronomical solution of Laskar (1988, 1990) is excellent for timest≲2.3 Ma. If the Ruddiman/Raymo time scale for DSDP 607 is accepted, the 41‐kyr δ18O cycle has enhanced amplitude between 1.0 and 1.5 Ma, relative to the late Pleistocene (t<1.0 Ma), implying a nonlinear Earth system response to obliquity insolation cycles. We do not find compelling evidence for an abrupt change in the 100‐kyr δ18O signal, although we cannot rule it out at this time. If the Shackleton et al. (1990) time scale for ODP 677 is accepted, our three δ18O records are consistent with a low‐amplitude 100‐kyr cycle between 1.2 and 2.6 Ma, whose local period of oscillation alternates between the 95‐ and 124‐kyr eccentricity periods, and a gradual increase in the 100‐kyr δ18O signal between 0.5 amd 1.0 Ma. The DSDP 607 time scale is more favorable to an abrupt jump in amplitude for the 95‐kyr δ18O envelope, but not in the 124‐kyr envelope. Rather, long‐period δ18O fluctuations appear phase‐locked with the 124‐kyr eccentricity cycle some 300–400 kyr prior to its growth in amplitude and phase‐lock with the 95

ISSN:0148-0227    

DOI:10.1029/92JB01815

年代:1993

数据来源: WILEY

摘要:

High‐resolution, deep‐tow multichannel seismic data are used to investigate the detailed structure of sediments containing methane hydrate. These data support thick, laterally extensive layers of methane hydrate‐bearing sediment underlain by a bottom simulating reflector (BSR) and spatially discontinuous zones of hydrate within the sediments above the BSR depth where no BSR is present. These data resolve normal faults which extend from the surface through the BSR with apparent offsets of up to 20 m. A phase inversion identified at the top of the BSR shows that the material immediately beneath the BSR has anomalously low velocity, consistent with a layer of sediment containing free methane gas. The fault offsets along the BSR are consistent with a pressure change of ∼200 kPa (∼2 bars) across the f

ISSN:0148-0227    

DOI:10.1029/92JB01706

年代:1993

数据来源: WILEY

摘要:

Cylindrical samples of granite were deformed at 26ºC, constant confining pressure (60 MPa), and constant pore pressure (20 MPa). Axial and volumetric strain were determined from changes in the output of resistance foil strain gauges bonded to the rock surface. In addition, dc electrical resistivity was measured parallel to the sample axis. During each experiment (typically lasting from 1–2 weeks), the deviatoric stress σdapplied to the sample was cycled between 70% and 90% of the short–term failure strength. The bulk of the experiments were conducted in the secondary or “steady state” creep regime. Inelastic volumetric strain rate was found to obey the law , where and (compressive stresses are negative). TheCcoefficient represents a strain‐hardening‐like term. The stress dependence is of the same form as the stress dependence measured for mode I crack growth in double cantilever beam experiments. The observed creep behavior is analyzed in terms of stress corrosion and crack growth models including a formulation based on energy release rate for characteristi

ISSN:0148-0227    

DOI:10.1029/92JB01828

年代:1993

数据来源: WILEY

摘要:

The electrical conductivity along [100] of single crystal San Carlos olivine was measured as a function of temperature between 1100° and 1200°C and oxygen fugacity between 10−6and 10+0.5Pa (at 1200°C), and either with (“pyroxene‐buffered”) or without (“self‐buffered”) an added natural pyroxene buffer from a San Carlos lherzolite. Under these temperature and ƒO2conditions, electrical conduction in the self‐buffered sample is attributed to polarons (Fe*) and electrons (e′) and in the pyroxene‐buffered sample is attributed to polarons(Fe*) and magnesium vacancies(V″Mg). Over the range of temperature and ƒO2investigated, the electrical conductivity of the self‐buffered sample is given by σsb[100]= 2.27(S/m)e−0.55(eV)/kTƒO20.18+306.3(S/m)e−2.25(eV)/kTƒO2−0.18and for the pyroxene‐buffered sample by σpb[100]= 0.18(S/m)e−0.34(eV)/kTƒO20.17+15.2(S/m)e−1.3(eV)/kTwherekis Boltzmann's constant,Tis in Kelvin, and ƒO2is in atmospheres. The conductivity of the pyroxene‐buffered sample is lower than that of the self‐buffered sample, primarily as a result of a decrease in the polaron concentration. the electrical conductivity of both samples was found to decrease irreversibly once the samples experienced an oxygen fugacity more reducing than approximately the wüstite‐magnetite buffer. Electron microprobe analyses indicate that this effect results from loss of iron from the olivine samples to the iridium electrodes. A series conduction model based on the observed compositional gradient adequately accounts for the magnitude of the irreversible conductivity decrease and limits the thickness of any surficial pyroxene phase to<0.1 μm. Mantle temperature profiles based on laboratory measurements of self‐buffered samples predict temperatures of the order of between 25° and 150°C colder, depending on the ambient oxygen fugacity, tha

ISSN:0148-0227    

DOI:10.1029/92JB01584

年代:1993

数据来源: WILEY