摘要:

One of the fundamental problems in geophysics is the chemical differentiation of the Earth, which is accomplished through magmatic processes, which in turn are a direct manifestation of convection within the mantle. The framework of plate tectonics is as yet the best model for explaining the different types and styles of magmatic activity.Experimental petrology calibrates the conditions for partial melting of various rock types and defines the conditions of magma crystallization under varied conditions. Laboratory data provide phase boundaries constraining the sites of partial melting in the Earth's interior.

ISSN:8755-1209    

DOI:10.1029/RG026i003p00369

年代:1988

数据来源: WILEY

摘要:

Magma genesis, migration, and eruption have played prominent roles in the chemical differentiation of the Earth. Plate tectonics has provided the framework of tectonic environments for different suites of igneous rocks and the dynamic mechanisms for moving masses of rock into melting regions. Petrology is rooted in geophysics. Petrological and geophysical processes are calibrated by the phase equilibria of the materials. The geochemistry of basalts and mantle xenoliths demonstrates that the mantle is heterogeneous. The geochemical reservoirs are related to mantle convection, with interpretation of a mantle layered or stratified or peppered with blobs. Seismic tomography is beginning to reveal the density distribution of the mantle in three dimensions, and together with fluid mechanical models and interpretation of the geoid, closer limits are being placed on mantle convection. Petrological cross sections constructed for various tectonic environments by transferring phase boundaries for source rocks onto assumed thermal structures provide physical frameworks for consideration of magmatic and metasomatic events, with examples being given for basalts, andesites, and granites at ocean‐continent convergent plate boundaries, basalts and nephelinites from a thermal plume beneath Hawaii, kimberlites in cratons, nephelinites from continental rifts, and anorogenic granites. The fluid dynamics of rock‐melt‐vapor systems exerts strong control on igneous processes and chemical differentiation. Unravelling the processes during subduction remains one of the major problems for understanding mantle heterogeneities and the evolution of conti

ISSN:8755-1209    

DOI:10.1029/RG026i003p00370

年代:1988

数据来源: WILEY

摘要:

Among several things, Professor Wyllie has emphasized the influence of volatile components in many magmatic processes. This underscores the importance of understanding interrelationships among the solid and fluid layers of the Earth for at least two reasons. First, our understanding of the magmatic processes is a prerequisite to understanding the evolution of the Earth's atmosphere. Second, the flux of CO2and other volatiles from volcanoes may have been significant in the geological past and may have led to geological discontinuities. The K‐T boundary has been assigned to this cause by som

ISSN:8755-1209    

DOI:10.1029/RG026i003p00405

年代:1988

数据来源: WILEY

摘要:

The crystal chemistry of 16 groups of multiple‐chain, sheet, and framework silicates is reviewed. Crystal structure drawings are presented to illustrate crystal chemical features necessary to interpret chemical data for each mineral group. The 16 silicate groups considered in this review are the amphibole; nonclassical, ordered pyriboles; mica; pyrophyllite‐talc; chlorite; greenalite; minnesotaite; stilpnomelane; prehnite; silica polymorphs; feldspar; nepheline‐kalsilite; leucite‐analcite; sodalite group; cancrinite group; and scapolite. Electron microprobe analyses should be augmented by independent determinations of Fe2+/Fe3+and H2O for many of the silicate groups discussed and by determinations of CO32−, SO42−, S2−, and Li in some of the others. However, microprobe data augmented as suggested will still be ambiguous for some of the silicate groups considered here because the structures are not completely determined or are variable, with disparate domains and/or structural modulations, e.g., pyriboles, greenalite, minnesotaite, and stilpnomelane. Nevertheless, the most rigorous way to interpret silicate mineral chemical data is based on the crystal struct

ISSN:8755-1209    

DOI:10.1029/RG026i003p00407

年代:1988

数据来源: WILEY

摘要:

Understanding seismic wave propagation in realistic seafloor environments is essential for many problems in marine seismology. Finite difference methods are becoming increasingly popular in solving propagation problems as the limitations of other techniques which apply only at high frequency or for flat‐lying media become fully appreciated. The seafloor problem, a high contrast in Poisson's ratio at a rough sharp interface, is particularly challenging, and many published formulations fail to solve it accurately. The purpose of this paper is to summarize the published finite difference formulations for the elastic wave equation and to outline their applicability to seafloor problem

ISSN:8755-1209    

DOI:10.1029/RG026i003p00445

年代:1988

数据来源: WILEY

摘要:

Observational studies have revealed a rich low‐frequency structure in the atmosphere. A review of the theories, observations, and model studies of this low‐frequency atmospheric variability is presented. On time scales of weeks or longer the atmosphere appears to possess distinct oscillatory behavior in well‐defined and persistent “centers of action.” This behavior is also an endemic feature of surrogate atmospheric data sets emerging from experiments with complicated climate models. Many theories have attempted to determine the dominant physical processes responsible for the low‐frequency variance but have usually failed when compared carefully with observations. For example, simple linear steady state and Rossby wave dispersion theories have been used in an attempt to explain the observed global response to low‐latitude perturbation. However, the observed structures of mature anomalies are often quite distinct from the vertical structures of disturbances predicted in these theories. Also, in general circulation and model studies, the sign of the nonlinear response is not simply related to the sign of the forcing as predicted by linear steady state theories. It is argued that the theories fail because either the full three‐dimensional complexity of the basic state is not considered or its inherent instability structure is not recognized or is, in fact, ignored. It is shown that three‐dimensional instability theory provides a natural generalization and marriage of the zonally averaged instability theory of Charney and Eady and the Rossby wave dispersion theory of Rossby and Yeh. As such, it provides a formalism which may be used to understand a wide variety of atmospheric fluctuations including the locations of eddy flux covariance maxima and storm tracks in both the tropics and extratropics and the generation of blocking, teleconnection patterns, and other quasi‐stationary anomaly features. Attention is focused on two particular mechanisms within this formalism for the formation of quasi‐stationary low‐frequency fluctuations. One of these is the baroclinic‐barotropic dipole instability mechanism in which the formation of quasi‐stationary mature anomalies is initiated by the upstream development of mid‐latitude eastward propagating dipole wave trains which arise through the combined baroclinic‐barotropic instability of the three‐dimensional atmospheric flow. The other is the westerly duct mechanism in which the initiation of low‐frequency variability is caused by tropical disturbances. According to this hypothesis, the longitudinal variation of the basic state flow near the equator causes a ducting of wave energy generated in the tropics to specific zones in the upper tropospheric westerlies; these zones then act as source regions for the emanation of waves into the extratropics. Furthermore, this duct also acts as a waveguide for extratropical modes prop

ISSN:8755-1209    

DOI:10.1029/RG026i003p00459

年代:1988

数据来源: WILEY

摘要:

The articles in the special section on Atmospheric Sciences in Antarctica of this issue complete the series on current research work by American atmospheric scientists in the Antarctic. The other 10 papers in the series were published earlier inReviews of Geophysics(vol. 26, no. 1, pages 41–207

ISSN:8755-1209    

DOI:10.1029/RG026i003p00495

年代:1988

数据来源: WILEY

摘要:

Atmospheric trace gas studies in Antarctica have led to an increased understanding of the global effects from anthropogenic emissions of various compounds. These studies have also contributed to the identification of various atmospheric circulation patterns and mixing mechanisms. Time trend analyses have revealed that although the atmospheric concentrations of F‐11, F‐12, CH3CCl3, and other halocarbons are still increasing, their rate of increase has slowed from peak increasing rates observed in the 1970s. For CO2, CO, and CH4, time trend analyses have also been used to analyze seasonal concentration cycles and to establish relationships between sources and sinks at various times throughout the year. Latitudinal gradient studies have generally revealed that for most compounds with significant anthropogenic sources, atmospheric concentrations are higher in the northern hemisphere than in the southern hemisphere. This is mainly a result of the overwhelming majority of anthropogenic sources existing in the northern hemisphere. Vertical concentration profiles in Antarctica are dependent upon the seasonal variations in circulation patterns associated with final warming and breakup of the polar vortex. Since final warming and breakup of the polar vortex may not always occur at the same time each year, profiles for a given time period from separate years may be different. Stratospheric‐tropospheric air exchange over Antarctica occurs primarily as a result of (1) general circulation patterns with subsidence over the polar regions, (2) stratospheric air injection in the vicinity of jet streams and injection from tropopause folding upstream of troughs, and (3) mountain waves of sufficient magnitude to displace air parcels across the tropo

ISSN:8755-1209    

DOI:10.1029/RG026i003p00497

年代:1988

数据来源: WILEY

摘要:

The two Antarctic rocket campaigns conducted by American scientists during the past 10 years are described, and the results of these efforts are reviewed. Although the primary objective of both campaigns, the observation of precipitating electrons triggered by the Siple VLF transmitter, was not achieved, significant advancements in understanding upper atmosphere phenomena were made. Standing wave patterns in the ionosphere were observed for the first time by wave experiments flown aboard Nike‐Tomahawk rockets. The same detectors were able to monitor a continuous signal from the transmitter through the neutral atmosphere and into the ionosphere. This in situ observation of a radio wave traversing a neutral and a plasma environment provided unique data for comparison to theoretical studies of wave propagation. SuperArcas measurements provided two distinct types of energetic electron precipitation data: one type was continuous on the time scale of>1 min, and the other was in X ray microbursts created by bursts of energetic electrons stopping in the upper atmosphere with durations<1 s. The burst data were compared to theoretical models of wave‐particle interactions. The observations of continuous electron precipitation with sufficient energy to penetrate the atmosphere to an altitude of 60–80 km were found to be consistent with a model of electron transport from the trapped population to the atmosphere in which gradient and curvature drift in the drift cone plays a principal role. The model requires slow electron pitch angle diffusion of the geomagnetically trapped population and predicts that almost all energetic electron precipitation occurs in the South Atlantic Anomaly. The electron precipitation model is shown to organize all of the SuperArcas data, which include not only measurements at Siple Station, Antarctica, but also those at Kerguelen, anotherL= 4 location in the southern hemisphere, and at northern locations at higher latitudes. A prediction of the model which involves the effect of the dawn‐to‐dusk convection electric field on energetic electron precipitation is described: diurnal modulation is predicted with the maximum electron precipitation intensity when the South Atlantic Anomaly is at local dawn. A test of this prediction made by SuperArcas flights in 1981 provided support for the slow diffusion model. The SuperArcas results on energetic electron precipitation are shown to disagree with a satellite result atL= 4 which requires fast diffusion. Reasons for the discrepancy between the two data sets are discussed, and steps which could be taken to resolve the apparent controversy are suggested. Finally, the suggestion is made that the two rocket campaigns conducted by American scientists in Antarctica during the past decade be regarded as a feasibility study leading to an active rocket research program in Antarctica for the remainder of th

ISSN:8755-1209    

DOI:10.1029/RG026i003p00519

年代:1988

数据来源: WILEY

摘要:

Early in the past decade of U.S. Antarctic research, the whistler method of measuring equatorial electron density was found to agree with in situ satellite electron density measurements by a radio technique. Furthermore, the whistler method of measuring the east‐west component of the convection electric field in the outer plasmasphere was found to agree, under conditions of mapping in a dipole magnetic field, with simultaneous results from incoherent scatter radar. A global model of the east‐west convection electric field in the outer plasmasphere during substorms was developed. The detection of whistlers and their use for magnetospheric diagnostics have been important elements in recent studies of burst precipitation into the ionosphere induced by whistlers and by other transient whistler mode waves propagating in the magnetosphere. Whistlers have also been used to obtain data on theLvalues and equatorial electron densities associated with the propagation paths of signals from the Siple VLF transmitter. The process of untrapping of downcoming wave energy from ducts in the upper ionosphere and the upward repropagation of portions of the energy following reflection in the lower ionosphere lead to the excitation of adjacent ducts as well as to upward propagation in the nonducted mode. Efficient interduct coupling has been found to occur over north‐south ionospheric distances of>1000 km. Studies of the outer limits of observed ducting revealed dayside path radii in the range 6–8REand nightside limits of ∼5.5RE. Ducted propagation beyond the plasmapause was found to occur regularly in the 0000–1800 MLT time range, but with variable rates and at various locations with respect to the plasmapause position. The special features of this propagation are believed to be related to conditions of lightning excitation, ionospheric penetration, and wave‐particle interactions that are special to the region beyond the plasmasphere. New aspects of Siple wave injection experiments were demonstrated by the application of a new phase measurement method to Siple signals that did not exhibit fast temporal growth during passage through the magnetosphere. This method, a refinement of techniques developed previously by New Zealand workers, is capable of detecting fluctuations in phase path with period of ∼10 s and greater and thus can be used to study magnetospheric convection and coupling fluxes along field lines of propagation as well as pulsations associated with ultralow‐frequency perturbations of the geomagnetic field. Additional topics discussed include results from direction‐finding experiments and evidence of the dependence of whistlers upon magnetospheric

ISSN:8755-1209    

DOI:10.1029/RG026i003p00535

年代:1988

数据来源: WILEY