ISSN:0148-0227    

DOI:10.1029/JB100iB01p00305

年代:1995

数据来源: WILEY

摘要:

The core‐mantle boundary (CMB) sets boundary conditions for processes occurring within the core. Thus the history of the geomagnetic field is intimately connected with the history of the CMB: information about one can often provide information about the other. It is a simple matter to separate sources of external origin from those of internal origin, but there is no unique way to specify the location of the internal sources. Indirect arguments suggest that sources within Earth's outer core strongly dominate the field for spherical harmonic terms with degree less than about 14 and that crustal sources play an increasing role in harmonics with higher degrees. Building on the pioneering studies of Roberts and Scott and of Backus in the 1960s, the velocity field of the core fluid at the top of the outer core can be estimated from geomagnetic secular variation data. However, even with appropriate assumptions about location of the magnetic field source, there is a serious nonuniqueness in inversion of the secular variation data to velocity field because magnetic field lines are not changed by movement of conductive fluid parallel to those field lines. Additional assumptions about the flow are therefore required, sometimes leading to quite different estimates in the pattern of fluid flow. The resulting velocity field estimates appear to be sensitive to conditions (e.g., small lateral variations in temperature) at the CMB. Consideration of mantle dynamics suggests that large changes in the CMB conditions probably occur on a 10‐ to 100‐m.y. timescale. Secular variations with periods shorter than a million years, but longer than several years, almost certainly originate from processes operating in the outer core; unfortunately, there is not yet consensus as to what those processes are. Longer‐period variations in the paleomagnetic record, including nonstationarities in the rates of magnetic field reversals, in paleointensities, and in paleosecular variation, may reflect changes in CMB conditions. Because these boundary conditions are controlled primarily by mantle dynamics, there have been several speculations regarding causal links between changes in Earth's lithosphere and changes in Earth's magnetic field. Finally, the evidence linking lateral variations at the CMB to perceived systematics in polarity transition data is found to be intriguing, but insuf

ISSN:0148-0227    

DOI:10.1029/94JB02362

年代:1995

数据来源: WILEY

摘要:

For more than 200 years the origin of Earth's magnetic field was attributed to permanent magnetization. Even today no single argument (e.g., that Earth's deep interior is too hot to sustain permanent magnetization) conclusively rules out the permanent magnetization hypothesis. Nevertheless, when all the evidence is considered, this hypothesis can be safely discarded and replaced with an electric current (dynamo) hypothesis. Surprisingly, this can be done even though there is no adequate dynamo model for Earth. The development of geodynamo models began with the disk dynamo of Larmor in 1919 and expanded to include many classes of models, such as αω, α2, α2ω, Taylor state, and Model Z dynamos. Because of mathematical difficulties associated with solving the many coupled partial differential equations of dynamo theory, numerous simplifying assumptions are made. The majority of numerical dynamo models assume a three‐dimensional velocity field in an inviscid fluid and use mean field theory to solve for axisymmetric magnetic fields. There is also an increasing number of intermediate and strong field models emerging, in which feedback from the magnetic field to the velocity field is permitted. Nevertheless, these models still require several simplifying assumptions and there are many additional problems. For instance, many core parameters are difficult to estimate; there is debate on whether the top of the core is stably stratified and on the effects such stratification might have; what effects the presence of an inner core have; and whether the coupling across the coremantle boundary significantly affects the geodynamo. Perhaps it is not surprising that dynamo theoreticians, faced with large difficulties in mathematics and many uncertainties in physics, essentially choose to ignore input from fields such as paleomagnetism. However, it is precisely because of such difficulties that paleomagnetism can provide valuable constraints to narrow the range of viable dynamo models. For example, paleomagnetism ultimately should provide constraints on the velocity and magnetic field symmetries of dynamos; determine whether the geodynamo is in the weak, intermediate, or strong field regime; determine if there is a fundamental difference in dynamo processes during superchrons when reversals of the magnetic field essentially cease; and provide valuable information on the growth of the inner core and its possible stabilizing effects on geodynamo pro

ISSN:0148-0227    

DOI:10.1029/94JB02361

年代:1995

数据来源: WILEY

摘要:

The discovery of seafloor hydrothermal systems approximately two decades ago has led to a major reassessment of the Earth's thermal and geochemical budgets and has revolutionized our understanding of biological processes. This review traces the development of the study of seafloor hydrothermal systems from the indirect evidence provided by conductive heat flow anomalies to the discovery of ≈ 350°C black smoker vents on the East Pacific Rise at 21°N. Although the review focuses on physical characteristics and processes, it outlines some key characteristics of vent fluid chemistry that provide constraints on physical models. Ridge crest systems have thermal power outputs ranging from 10 to 104MW. They are transient systems, driven by magmatic heat sources, but episodic events such as megaplumes, the interplay between focused and diffuse venting, and other aspects related to their thermal, chemical, and biological evolution remain poorly understood. Advances will be made by continuing exploration and discovery to determine the full range of possible phenomena both on and off axis and in different tectonic settings. In order to understand the complete, integrated ridge system, however, future studies must include long‐term monitoring of an active system, deep drilling into the reaction zone, and mathematical modeling that incorporates both physical and chemical constr

ISSN:0148-0227    

DOI:10.1029/94JB02222

年代:1995

数据来源: WILEY

摘要:

To establish links among domain images, volume changes in magnetization, and mechanisms for pseudosingle‐domain (PSD) behavior, we have studied single platelets of natural hematite (Elba, Italy) in order to compare Bitter patterns resulting from high‐field treatments to the hysteresis behavior of the same individual particles. There is excellent agreement between the fields that cause large changes of a particle's Bitter pattern and the fields that cause large changes of a particle's magnetic moment. When in the saturation remanent state, most particles exhibit patterns that suggest a state of near saturation. Such patterns change very little until the application of a critical back field triggers the appearance of a major Bitter line that nearly bisects the grain. Likewise, in the majority of particles the measured ratio of saturation remanence to saturation moment is large (≅ 0.7), and there is little change in saturation remanence until a critical back field equal to the coercive force (Hc) reverses the moment through a large Barkhausen jump. In most particles whose patterns have been compared to their measured behavior, this large jump on the hysteresis loop occurs in the same critical back field that causes the sudden appearance of a major Bitter line. Consequently, we interpret hysteresis to be governed by the nucleation of walls in the majority of hematite particles studied here. In such particles, Hcincreases as effective grain size decreases, according to Hc∝ deff−0.64, where deff∝ Ms1/3and Msequals the particle's saturation moment. This study leads to four conclusions: (1) Bitter patterns do represent volume domains and the magnetization processes that they imply, (2) nucleation is the dominant hysteresis mechanism in PSD particles of this natural hematite, (3) the grain size dependence of nucleation field Hnin hematite is given by Hn= Hc∝ deff−0.64, and (4) nucleation is a viable mechanism for explaining PSD behavior in other natural magnetic materials, such as magnetite, titanomagnetite,

ISSN:0148-0227    

DOI:10.1029/93JB01228

年代:1995

数据来源: WILEY

摘要:

We present a comprehensive description of the analysis of marine magnetic vector anomalies. The marine magnetic vector field can be measured with a shipboard three‐component magnetometer (STCM). The STCM has the advantage of measuring three components, but it suffers from several types of noise sources because the principles of its measurement are more complex than that of a proton‐precession magnetometer. We have estimated the quality of STCM data, constructed an optimal filter, and evaluated the reliability of the filtered data. Then, we have organized an analytical procedure composed of four steps: (1) the discrimination between two‐ and three‐ dimensional magnetic sources; (2) the determination of magnetic boundaries; (3) the calculation of the boundary strikes; and (4) the estimation of the boundary magnetization contrasts. We show examples of these procedures using the data near the East Pacific Rise 28°S–31°S collected during leg 5 of the GLORIA expedition. All the results of the above processing are merged in order to illustrate the distinct pattern of the seafloor magnetization in the su

ISSN:0148-0227    

DOI:10.1029/94JB02596

年代:1995

数据来源: WILEY

摘要:

A global study of trench flexure was performed by simultaneously modeling 117 bathymetrie profiles (original depth soundings) and satellite‐derived gravity profiles. A thin, elastic plate flexure model was fit to each bathymetry/gravity profile by minimization of theL1norm. The six model parameters were regional depth, regional gravity, trench axis location, flexural wavelength, flexural amplitude, and lithospheric density. A regional tilt parameter was not required after correcting for age‐related trend using a new high‐resolution age map. Estimates of the density parameter confirm that most outer rises are uncompensated. We find that flexural wavelength is not an accurate estimate of plate thickness because of the high curvatures observed at a majority of trenches. As in previous studies, we find that the gravity data favor a longer‐wavelength flexure than the bathymetry data. A joint topography‐gravity modeling scheme and fit criteria are used to limit acceptable parameter values to models for which topography and gravity yield consistent results. Even after the elastic thicknesses are converted to mechanical thicknesses using the yield strength envelope model, residual scatter obscures the systematic increase of mechanical thickness with age; perhaps this reflects the combination of uncertainties inherent in estimating flexural wavelength, such as extreme inelastic bending and accumulated thermoelastic stress. The bending moment needed to support the trench and outer rise topography increases by a factor of 10 as lithospheric age increases from 20 to 150 Ma; this reflects the increase in saturation bending moment that the lithosphere can maintain. Using a stiff, dry‐olivine rheology, we find that the lithosphere of the GDH1 thermal model (Stein and Stein, 1992) is too hot and thin to maintain the observed bending moments. Moreover, the regional depth seaward of the oldest trenches (∼150 Ma) exceeds the GDH1 model depths b

ISSN:0148-0227    

DOI:10.1029/94JB02468

年代:1995

数据来源: WILEY

摘要:

Near‐liquidus melting relations have been determined for a mafic, plateau‐type, flood phonolite from the Kenya rift at 0.5, 0.7, 0.9, and 1.2 GPa, with H2O added through saturation, and at 0.7 GPa with H2O and CO2added. Mixed‐volatile experiments at 0.7 GPa delineate a near‐liquidus multiple saturation of augite, andesine, phlogopite, oxides, and apatite at 1000°C,XCo2= 0.42, with calcic amphibole melting above 975°C. The multiple saturation and phase assemblage are interpreted to indicate that plateau phonolites were in equilibrium with the residuum of a parental alkali basaltic composition at 0.7 GPa consisting of augite, andesine, titanomagnetite, and olivine (a product of incongruent melting of phlogopite and, possibly, amphibole). This evidence for lower crustal equilibration refutes suggestions that plateau phonolites are low‐pressure differentiates. Their enormous volumes (about 50,000 km3), restricted eruptive period (14–11 Ma), uniform major element compositions, and the paucity of associated mafic‐intermediate rocks also argue against a deep origin by fractional crystallization. A two‐stage process for the origin of the phonolites is consistent with the thermal evolution of the rift. The lower crust was pervasively injected by alkali basaltic magmas during the period of voluminous eruption of early to middle Miocene basalts. Rising isotherms during rift evolution caused subsequent partial melting of this predominantly basaltic lower crust in the late Miocene, generating the p

ISSN:0148-0227    

DOI:10.1029/94JB02160

年代:1995

数据来源: WILEY

摘要:

Geochemical investigations support the petrogenesis of Kenya rift plateau‐type flood phonolites (14–11 Ma) by partial melting of an alkali basaltic material at lower crustal pressures. High‐pressure/high‐temperature experiments on a natural plateau phonolite (Hay and Wendlandt, this issue) document multiple saturation of augite, andesine, titanomagnetite, and phlogopite at 0.7 GPa, 1000°C,XCO2= 0.42, with amphibole appearing at 975°C. A least squares solution to major element modeling, involving subtraction of the compositions of near‐liquidus augite, andesine, titanomagnetite, and olivine (Fo67; hypothesized product of incongruent melting of hydrous phases) from a Kenya Miocene alkali basalt composition, indicates that plateau phonolites can be derived by 15 wt % fusion of this hypothetical parental material (∑R2= 0.07). Alkali basaltic magmas may have injected and/or underplated the lower crust in southern Kenya during prior rift‐related basaltic volcanism (23–14 Ma). Bulk Earth values of (87Sr/86Sr)iand εNdnear zero for four plateau phonolite samples are consistent with a mantle‐derived parental composition. Three of these four samples reflect little, if any, postmelting modification; one sample may have evolved by fractional crystallization (high Rb/Sr, low Ba, Sr and Mg #). A fifth sample may show evidence of assimilation and fractional crystallization processes (elevated radiogenic Sr and Pb, large negative Eu anomaly, and low Ba, Sr, and Mg #). Much of the geochemical variation among plateau phonolite lavas, however, can be ascribed to melting of a predominantly alkali basaltic source with contributions from a lower crustal protolith. A mantle‐derived source is also supported by Sr‐Nd‐Pb isotope data for the phonolites, which indicate that the alkali basaltic source can be described in terms of high U/Pb (HIMU) and enriched mantl

ISSN:0148-0227    

DOI:10.1029/94JB02159

年代:1995

数据来源: WILEY

摘要:

Results of a field study demonstrate that melt flow in the shallow mantle was focused along actively deforming ductile shear zones in the Josephine peridotite in SW Oregon. Intergranular flow of ascending liquids dissolved pyroxene and precipitated olivine, forming zones of dunite replacing harzburgite. Syntectonic formation of dunite was focused along near‐vertical shear zones, with both vertical and dextral displacement, in the shallow mantle at less than 30 km depth and temperatures between 950 and 1100°C. The migrating magmas which formed dunite and related pyroxenite included high‐Mg, calc‐alkaline andesites characteristic of subduction‐related magmatic arcs. Explanations for focusing of melt flow along shear zones include the presence of a pressure gradient between strong and weak material during active deformation (Stevenson, 1989) and rapid infiltration of liquid when grain boundaries are aligned in mantle tectonites (Waff and Faul, 1992). Focused flow of melt may, in turn, lead to localized deformation and the development of shear zones. It is likely that melt‐lubricated shear zones provide an important locus for both melt extraction and strain in the shallow mantle beneath spreading ridges and in subduction‐related m

ISSN:0148-0227    

DOI:10.1029/94JB02063

年代:1995

数据来源: WILEY