摘要:

We discuss the geological, geophysical, and petrological observations that constrain the nature of mantle convection in plumes, and show how theoretical models of mantle plumes have developed over the past three decades. The large volumes of lava emplaced in geologically short periods as flood basalts are generated mainly by decompression melting of abnormally hot mantle brought to the base of the lithosphere by plumes. We present new results from the application of McKenzieand O'Nions' (1991) rare earth element inversion scheme to the geochemistry of flood basalts to infer the amount of mantle melting and the depth interval over which it occurred. Our survey covers flood basalts from the Archaean to the Tertiary. The abundance of geochemical data from the Siberian Traps (250 Ma) and from the Keweenawan (1095 Ma) enables us to infer the temporal evolution of mantle melting in these two flood basalt provinces. We find that three quarters of the samples from flood basalts can be modeled by single‐stage melting of asthenospheric mantle. The remainder require enrichment by small amounts of metasomatic melts emplaced in the lithospheric mantle by an earlier phase of minor (0.3%) melting at depths where garnet is stable. The mantle melting responsible for flood basalts starts at depths of 110 km or more beneath the surface, and is consistent with enhanced mantle potential temperatures of 1450–1550°C. Melting continues to depths of 70–30 km. At least some lithospheric thinning is required to explain both the geochemistry of the melts and the high rate of generation of flood b

ISSN:0148-0227    

DOI:10.1029/95JB01585

年代:1995

数据来源: WILEY

摘要:

The concept of strength envelopes, developed in the 1970s, allowed quantitative predictions of the strength of the lithosphere based on experimentally determined constitutive equations. Initial strength envelopes used an empirical relation for frictional sliding to describe deformation along brittle faults in the upper portion of the lithosphere and power law creep equations to estimate the plastic flow strength of rocks in the deeper part of the lithosphere. In the intervening decades, substantial progress has been made both in understanding the physical mechanisms involved in lithospheric deformation and in refining constitutive equations that describe these processes. The importance of a regime of semibrittle behavior is now recognized. Based on data from rocks without added pore fluids, the transition from brittle deformation to semibrittle flow can be estimated as the point at which the brittle fracture strength equals the peak stress to cause sliding. The transition from semibrittle deformation to plastic flow can be approximated as the stress at which the pressure exceeds the plastic flow strength. Current estimates of these stresses are on the order of a few hundred megapascals for relatively dry rocks. Knowledge of the stability of sliding along faults and of the onset of localization during brittle fracture has improved considerably. If the depth to the bottom of the seismogenic zone is determined by the transition to the stable frictional sliding regime, then that depth will be considerably more shallow than the depth of the transition to the plastic flow regime. Major questions concerning the strength of rocks remain. In particular, the effect of water on strength is critical to accurate predictions. Constitutive equations which include the effects of water fugacity and pore fluid pressure as well as temperature and strain rate are needed for both the brittle sliding and semibrittle flow regimes. Although the constitutive equations for dislocation creep and diffusional creep in single‐phase aggregates are more robust, few data exist for plastic deformation in two‐phase aggregates. Despite the fact that localization is ubiquitous in rocks deforming both in brittle and plastic regimes, only a limited amount of accurate experimental data are available to constrain predictions of this behavior. Accordingly, flow strengths now predicted from laboratory data probably overestimate the actual rock strength, perhaps by a significant amount. Still, the predictions are robust enough that uncertainties in geometry, mineralogy, loading conditions and thermodynamic state are probably the limiting factors in our understanding. Thus, experimentally determined rheologies can be applied to understand a broad range of topical problems in regional and global tectonics both on the Earth and on other planetary bod

ISSN:0148-0227    

DOI:10.1029/95JB01460

年代:1995

数据来源: WILEY

摘要:

The study of magmatic processes is at a juncture. New continuum models for the complex physics of crystallization and melting with convection and crystal‐melt segregation are now available, and the models are amenable to exploration by numerical methods. These models, which were largely developed in the fields of material science and mechanical engineering, will allow petrologists to consider a wide range of transport phenomena: crystal settling with reaction as fully multiphase flow, combined thermal and compositional buoyancy with realistic material property variations, open system and eruptive behavior and the appropriate system geometry and boundary conditions. The traditional notions of magma chambers as large vats of near liquidus material is being replaced by models where a substantial portion of the chamber may consist of a crystal‐melt mush, subject to reintrusion or mobilization. Percolative flow and reaction in this mush, including contaminants, may be as important as near‐liquidus processes in driving petrological diversity. Thus simple dimensionless numbers are of little utility in describing the vigor or style of magmatic processes. Melting of the crust following intrusion by basalt can be considered in light of two end‐members: the large sill or tabular mafic magma body and intrusion of basalt as sequences of spatially and temporally over lapping dikes. Although more commonly cartooned, the large sill type may not be nearly as efficient as repeated diking in driving crustal melting, magma mixing, and mingling. The greatest challenge for those studying magma dynamics will be to key the models to well‐constrained geological examples and to raise the sophistication of the content of these models in light of well‐defined tests that have geological s

ISSN:0148-0227    

DOI:10.1029/95JB01240

年代:1995

数据来源: WILEY

摘要:

In large magma chambers, gradients of temperature and composition develop due to cooling and to fractional crystallization. Unstable density differences lead to differential motions between melt and crystals, and a major goal is to explain how this might result in chemical differentiation of magma. Arriving at a full description of the physics of crystallizing magma chambers is a challenge because of the large number of processes potentially involved, the many coupled variables, and the different geometrical shapes. Furthermore, perturbations are caused by the reinjection of melt from a deep source, eruption to the Earth's surface, and the assimilation of country rock. Physical models of increasing complexity have been developed with emphasis on three fundamental approaches. One is, given that large gradients in temperature and composition may occur, to specify how to apply thermodynamic constraints so that coexisting liquid and solid compositions may be calculated. The second is to leave the differentiation trend as the solution to be found, i.e., to specify how cooling occurs and to predict the evolution of the composition of the residual liquid and of the solid forming. The third is to simplify the physics so that the effects of coupled heat and mass transfer may be studied with a reduced set of variables. The complex shapes of magma chambers imply that boundary layers develop with density gradients at various angles to gravity, leading to various convective flows and profiles qf liquid stratification. Early studies were mainly concerned with describing fluid flow in the liquid interior of large reservoirs, due to gradients developed at the margins. More recent work has focused on the internal structure and flow field of boundary layers and in particular on the gradients of solid fraction and interstitial melt composition which develop within them. Crystal settling may occur in a surprisingly diverse range of regimes and may lead to intermittent deposition events even with small crystal concentrations. Incorporating thermodynamic constraints in the study of the dynamics of settling has only just begun. Many dynamical phenomena have been found using theoretical arguments, laboratory experiments on analog systems, and numerical calculations on simplified chemical systems. However, they have seldom been applied to natural silicate melts whose phase diagrams and important physical properties such as thermal conductivity and chemical diffusion coefficients remain poorly known. There is a gap between model predictions and observations, as many models are designed to explain large‐scale features and many observations deal with the local texture and mineral assemblages of the rocks. This review stresses the relevance to the geological problem of the work carried out in parallel in other disciplines, such as physics, fluid dynamics, and metallurg

ISSN:0148-0227    

DOI:10.1029/95JB01239

年代:1995

数据来源: WILEY

摘要:

In this paper we compare lava temperatures measured using Cr‐Al thermocouples or infrared spectrometry with estimated quenching temperatures based on the glass geothermometry calibration of Helz and Thornber (1987). Comparative data are available for the April 1982 and September 1982 summit eruptions, the Pu'u O'o east rift eruption (1983–1986), all three eruptions at Kilauea, and the 1984 Mauna Loa eruption. The results show that quenching temperatures, based on the MgO contents of Kilauean glasses (TMgO), lie within ±10°C of field measurements using the infrared spectrometer for 85% of the samples. Where a Cr‐Al thermocouple was used, 90% of the field measurements lie within +1° to −11°C ofTMgOfor samples withTfield>1130°C. Samples whereTfield<1130°C show larger divergence. The uncertainty inTMgOby itself is ±10°C, so the level of agreement between field measurements andTMgOis very good for Kilauean lavas. Systematic comparison of field measurements of temperature with glass geothermometry for the 1984 Mauna Loa eruption suggests that, although the field and glass temperatures lie within ±10°C of each other, the KilaueanTMgOcalibration is nevertheless not appropriate for Mauna Loa glasses and that actual quenching temperatures for Mauna Loa samples will lie 10°–20°C higher than would be predicted from the Kilauea calibration curve. Consideration of possible effects of variable volatile content suggest that in most cases these are small. Samples erupted early in an eruption may reflect preeruptive water contents different enough to affectTMgOsignificantly, but later spatter samples and all flow samples appear to have equilibrated at low enough water contents for the calibration to be applicable. We conclude that the MgO‐based geothermometer can be applied to glassy Kilauean samples to give temperatures that generally will lie within ±10°C of a field measurement. Plots of glass MgO content versus time, if a suitable sample base is available, should give a thorough, quantitative record of the thermal history

ISSN:0148-0227    

DOI:10.1029/95JB01309

年代:1995

数据来源: WILEY

摘要:

We present an improved method for determining statistically significant alignments of pointlike features. One of the principal such methods now in use, the two‐point azimuth method, depends on a homogeneous distribution of points over the region of interest. Modification of this approach by use of the relatively new statistical technique of kernel density estimation permits treatment of heterogeneous point distributions without introducing substantial dependence on choice of the grid employed in the test for significance of apparent preferred orientations. The improved method can selectively reveal alignments on different spatial scales and can suggest the locations of alignments as well as their orientation. We use this method to analyze the spatial distribution of 416 vents, largely of Pleistocene age, in the Pinacate volcanic field, Sonora, Mexico, just east of the northern end of the Gulf of California. Apart from a few sets of aligned cinder cones, the distribution of Pinacate vents appears nearly random on aerial and space photography. However, when treated statistically, old Pinacate vents exhibit structural control trending approximately N10°E throughout the field and in all its subareas. In contrast, vents with ages estimated by comparison with dated cones to be younger than about 0.4 Ma show not only the N10°E control but also N20°W and N55°W alignments significant at the 95% confidence level. The N10° alignment probably reflects the current Basin and Range horizontal stress regime in this particular area, which is atop the mantle magma source of the Pinacate lavas. The N55°W direction is related to a major regional fracture of that orientation passing through the middle of the field and possibly related to normal faults associated with opening of the adjacent Gulf of California. The distribution of vents relative to the fracture trace is consistent with magma having been guided upward along a SW dipping fault plane. The origin of the N20° W alignment is unknown but of pre‐Pleistocene heritage. We found no evidence to support control of the Pinacate vent alignments parallel to rifting or transform directions in the adjacent Gulf. Intrusion along N20°W and N55°W fractures at or since about 0.4 m.y. ago could reflect either a shift in the crustal stress field or an increase in magma pressure in Pinacate conduits that allowed magma to ascend along structures that were not parallel to the maximum horizontal compre

ISSN:0148-0227    

DOI:10.1029/95JB01058

年代:1995

数据来源: WILEY

摘要:

Tephra glasses retrieved from 10 Deep Sea Drilling Project (DSDP) cores around the Mariana arc system make up a remarkable record of explosive volcanism in the Marianas over the past 40 m.y. Major element compositions for approximately 1800 tephra glasses, from basalt through rhyolite, are reported and used to examine the nature and history of this activity. Three maxima of volcanic explosivity, presumably related to especially vigorous volcanism in the Mariana arc, are identified, with the biggest maximum at around 18–11 Ma and two other maxima at 35–24 and 6–0 Ma; the two younger maxima are contemporaneous with peaks in explosive volcanism observed for other western Pacific arcs. Explosive arc volcanism has been predominantly tholeiitic since shortly after arc inception; no boninitic glasses were found. The tephra glasses belong to the low‐ to medium‐K suite, except during an enigmatic phase of medium‐ to high‐K explosive volcanism during the late Miocene (11–7 Ma). Even though Mariana tephra glasses are largely similar in composition to Mariana arc lavas, the tephra show a much larger compositional range than that found for subaerial Mariana arc lavas, which totally lack dacitic to rhyolitic volcanic products. The tephra glasses define a bimodal population in terms of silica content, with a pronounced minimum, or “Daly gap,” around 65–66% SiO2. Mariana tephra glasses are fractionated, with the least evolved glass being Fe‐rich and having a magnesium number (Mg #) (100Mg/Mg+Fe2+) of 55. Basaltic tephra glasses contain ≤16% Al2O3(average 14.3% Al2O2) and are not high‐alumina basalts; this contrasts with the observation that modern Mariana arc lavas contain 15–21% (average 17.4%) Al2O3. The high‐alumina basalt lavas of the Mariana arc probably reflect plagioclase accumulation and not liquid compositions. All tephra glasses plot near low‐pressure cotectics and reaction curves on the subprojection olivine‐clinopyroxene‐quartz; mafic and felsic samples define distinct trends. The mafic trend reflects fractional crystallization of mantle‐derived basaltic magma, whereas the felsic trend may be due either to anatexis of Mariana arc crust or to fractionation of mafic melts. The tephra glass data reinforce the model that the magmatic evolution of the Mariana arc has been dominated by low‐pressure fractionation, perhaps accompanied by anatexis. Episodic changes in melting regime to generate Miocene potassic tephra may be related to changing mantle sources and processes related to episodes of back arc basin spreading. These episodic changes are superimposed on a long‐term increase in potassium that reflects progressive metasomatism of the mantle source. Long‐term increases in K2O contents for Mariana arc magmas inferred from the tephra glass record are 0.004 wt % m.y.−1(mafic), 0.011 wt % m.y.

ISSN:0148-0227    

DOI:10.1029/95JB01685

年代:1995

数据来源: WILEY

摘要:

Recent experiments done at low driving frequencies suggest that a large degree of dispersion exists in the measured value of the shear modulus, μ, of α iron at high temperature. Discrepancies between values for μ from ultrasonic measurements and those from low‐frequency torsional measurements have been interpreted in terms of viscoelastic relaxation. However, the ultrasonic data are not in agreement with one another, and the degree of dispersion is not accurately known. We present new high‐temperature data for the elastic moduli of single‐crystal iron (α phase). The elastic moduli were measured using the rectangular parallelepiped resonance method (0.27–0.59 MHz) from room temperature to 925 K. Our data show that the difference in μ at high temperature between ultrasonic‐based measurements and low‐frequency (1 Hz) torsional measurements is only 14 GPa, rather than 29 GPa, as inferred from previous analyses. Thus the possible effects of viscoelastic relaxation are reduced but not eliminated. We find no dispersion in measurements for μ of α iron when considering frequencies ranging from 0.27 to 70 MHz and discuss the possibility that significant viscoelastic effects on measurements of μ at high temperature are limited to fre

ISSN:0148-0227    

DOI:10.1029/95JB01235

年代:1995

数据来源: WILEY

摘要:

Melting temperature measurements of six minerals (stishovite (SiO2), corundum(Al2O3), diopside (CaMgSi2O6), and three perovskites (MgSiO3, CaSiO3, Mg3Al2Si3O12)) at high pressures were carried out in a YAG laser‐heated diamond anvil cell with rhenium metal as an absorber of the laser light. A polished or compressed disc of the sample was in contact with rhenium foil and heated by conduction. Melting was determined by plotting laser power/sample temperature function and looking for the thermal anomaly associated with the fusion of materials. All these solids were found to be highly refractory, requiring quite high temperatures for melting at the lower mantle pressures. The experimental melting results showed that for these minerals, melting temperatures increased with increasing pressure. Our results at low pressures are consistent with the data determined by other techniques (piston‐cylinder, multianvils). The high‐pressure melting of MgSiO3perovskite agreed with the recent measurements by Zerr and Boehler (1993) within experimental uncertainties. Melting temperatures and melting slopes of CaSiO3and Mg3Al2Si3O12perovskites were found to be less than those of MgSiO3perovskite, indicating that the presence of Ca and Al would decrease the melting temperatures of MgSiO3perovskite in the Earth's lower mantle and that this effect will increase with increasing pressures. Melting temperature measurements on stishovite and corundum to pressures of 36 GPa and 25 GPa, respectively, are rep

ISSN:0148-0227    

DOI:10.1029/95JB01864

年代:1995

数据来源: WILEY

摘要:

Transformation textures and kinetics of the olivine to spinel phase transformation were observed in situ in ungasketed samples using a diamond anvil cell (DAC). The low‐temperature kinetic limit for reconstructive transformation from olivine (α) to spinel phase (γ) observed in DAC experiments is approximately 150°C lower than when observed under more hydrostatic conditions. The spinel phase, which in many of the samples is distributed in an annular pattern, exhibits reconstructive textures including grain boundary nucleation, and lack of topotaxy; in some cases it forms lenses similar to those associated with transformation‐induced mechanical failure. Although spinel phase lamellae, formed by a martensiticlike mechanism, are observed in the specimens, the lamellae remain extremely thin (∼10 nm) and do not produce enough spinel to be optically visible. The observation of reconstructive textures within the annular transformed regions leads us to conclude that high shear stress and plastic strain enables reconstructive transformation at temperatures where transformation rates would otherwise be virtually zero. High transient differential stresses and rapid deformation accompany deep earthquakes. Therefore knowledge of the kinetics of this transformation under these conditions is important for understanding the connection between phase transformation and deep earthquakes in subducting lithospher

ISSN:0148-0227    

DOI:10.1029/95JB01578

年代:1995

数据来源: WILEY