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Summary The convection in a rapidly rotating electrically conducting, fluid horizontal layer of non-constant stratification, permeated by an inhomogeneous magnetic field, is studied. In this connection, a temperature model of the layer is constructed, which creates a structure such that part of the layer is unstably and a part stably stratified. The results obtained are applied to the conditions in the fluid Earth's core.
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The Cenozoic Mormon Mountain Volcanic Field (MMVF) of northern Arizona is situated in the transition zone between the Basin and Range and the Colorado Plateau. It is composed of alkalic to sub-alkalic basalts and calcalkalic andesites, dacites, and rhyodacites. Despite their spatial and temporal association, the basalts and the calcalkalic suite do not seem to be co-genetic. The petrogenesis of primitive MMVF basalts can be explained as the result of different degrees of partial melting of a relatively homogenous, incompatible element-enriched peridotitic source. The variety of evolved basalt types was the result of subsequent fractional crystallization of olivine, spinel, and clinopyroxene from the range of primitive basalts. Crustal contamination seems to have occurred, but affected only the highly incompatible element abundances. The formation of MMVF calcalkalic rocks is most readily explained by small to moderate amounts of partial melting of an amphibolitic lower crust. This source is LREE-enriched but depleted in Rb and relatively unradiogenic Sr (87Sr/86Sr 0.7040). Calcalkalic rhyodacites may also be derived from andesitic parents by fractional crystallization. The overall petrogenesis of the MMVF complex is the result of intra-plate volcanism where mantle-derived magmas intrude and pass through thick continental crust. 相似文献
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Major-element, trace-element and isotopic compositions of approximately 1200 basalts (< 53 wt. % SiO2) from intra-oceanic island arcs have been compiled to assess the nature and possible sources of primitive island-arc basalts (IAB). The chemical characteristics of IAB are examined with reference to those of mid-ocean ridge basalts (MORB) and intraplate oceanic basalts (IPB). Major-element compositions of primitive [] IAB and MORB are similar, but differ significantly from IPB. In general, IAB do not have higher Al2O3, lower TiO2 or a lack of Fe enrichment compared to primitive MORB but many do have greater K2O contents. Differences in major- and minor-element contents between more evolved IAB and MORB result from the dominance of plagioclase + olivine crystal fractionation in MORB magmas vs. clinopyroxene + olivine controlled fractionation in IAB suites. This difference in crystallization history may be related to the higher PH2O or greater depth of crystallization of IAB magmas compared to those inferred for MORB.IAB are characteristically enriched in large-ion-lithophile (LIL) elements and depleted in high-field-strength ions (e.g., Zr, Nb and Hf) relative to normal MORB (N-type) and IPB. The enrichment of some LIL elements (e.g., Sr, Rb, Ba and Pb) relative to the rare-earth elements in IAB is difficult to explain by simple partial melting alone and suggests a multistage petrogenesis involving an LIL-enriched component. Low abundances of high-field-strength ions in evolved IAB are explicable in terms of fractional crystallization, but the cause for consistently low abundances in primitive IAB remains problematic.Island-arc lavas contain greater concentrations of volatiles and have higher and Cl/F ratios than either MORB or IPB, suggesting involvement of a slab-derived volatile component. However, this is not consistent with data which indicate that only near-trench volcanics have been significantly affected by dehydration of the oceanic crust.Sr-, Nd-, Pb- and O-isotopic data, in conjunction with the trace-element data, clearly indicate that IAB are derived from heterogeneous, LIL-depleted mantle sources most similar to those which give rise to enriched MORB (E-type). The marked shift towards higher in IAB compared to oceanic lavas with similar values cannot be explained simply by the addition of radiogenic Sr from the slab. Variable degrees of contamination from a crustally-derived sedimentary component is consistent with the isotopic and trace-element data from a number of arcs. However, the lack of correlation between LIL/REE ratios and more radiogenic isotopic ratios suggests that this enrichment/contamination process is complex. A multi-stage petrogenetic model involving subducted oceanic crust (± sediments), dehydration/volatile transfer, and partial melting of metasomatized mantle beneath island arcs is considered the most reasonable, although least constrained, method to generate a variety of primitive IAB. 相似文献
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Gustáv Siráň Reviewer V. Bucha Reviewer I. Cupal 《Studia Geophysica et Geodaetica》1972,16(1):100-102
Zusammenfassung Es werden die Wurzeln einer charakteristischen Gleichung für die Randbedingung der Diffusion des toroidalen magnetischen Störungsfeldes berechnet, das in dünner Schicht unterhalb der Grenze Kern — Mantel, als eine Folge von Störung der Winkelgeschwindigkeit dieser Schicht in Anwesenheit des magnetischen Dipolfeldes induziert wird. Die Wurzeln werden für verschiedene Werte des Verhältnisses der Leitfähigkeit des Erdmantels und Erdkerns ausgerechnet. Es wird gezeigt, dass das aus dem Milieu mit der elektrischen Leitfähigkeit in das Milieu mit elektrischer Leitfähigkeit m diffundierende toroidale magnetische Störungsfeld vom Werte m/<10–2 an von dem Verhältnis elektrischer Leitfähigkeiten beider Milieus unabhänging bleibt. 相似文献
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David W. Phelps David A. Gust Joseph L. Wooden 《Contributions to Mineralogy and Petrology》1983,84(2-3):182-190
Major and trace-element whole rock data, Nd and Sr isotopic data, and microprobe data have been collected from a suite of basanites, olivine nephelinites, and olivine melilite nephelinites from the Raton-Clayton volcanic field, New Mexico. Most of the lavas have geochemical characteristics that suggest they are primary upper mantle derived melts. The previously unreported occurrence of Type I and Type II ultramafic xenoliths in some of the lava flows supports this conclusion. All the lavas are strongly enriched in light REE, Sr, Ba, U, Th, and P2O5. 87Sr/86Sr ratios are 0.70394 to 0.70412 and 143Nd/144Nd ratios are equal to an epsilon value of +1.4; the data fall within the Nd-Sr correlation field. Trace-element modeling indicates that the lavas were last in equilibrium with a light-REE enriched mantle with a (La/Yb)N of two to nine. However, the Nd isotopic data indicate a source with a time integrated, chondritic normalized, Sm/Nd ratio of 1.01. To account for this discrepancy a metasomatic enrichment of the source is proposed. The timing of the enrichment event can only be constrained to less than 1 AE ago, and the isotopic composition of the premetasomatized source and the metasomatizing agent cannot be specified. However, geochemical constraints suggest a CO2-rich fluid enriched in incompatible elements as the likely metasomatizing agent 相似文献
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Summary The convection in a rapidly rotating, electrically conducting, horizontal fluid layer, non-constantly stratified and penetrated by an inhomogeneous magnetic field, is studied. The convection is investigated for various ratios of the thickness of the stable and unstable stratified part of the layer. The thermal model of the layer, as well as the analysis of the results have been treated with regard to the physical conditions in the liquid core of the Earth.
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The extrusive products of a Middle to Late Jurassic volcanic event occur throughout a wide area of southern South America. These volcanic rocks are associated in time and space with a series of NNW-trending grabens. The extension that produced the grabens began perhaps in the latest Triassic and continued throughout most of the Jurassic. The Middle to Late Jurassic volcanic rocks represent the culminating event of this period of extension.The Jurassic volcanic rocks described here are dominantly rhyolites and basalts, but flows of intermediate composition are also present. Major element geochemistry on a suite of samples taken from a west-east transect near 44°S latitude shows that these rocks are not related directly to convergent arc volcanism along the margin of South America, but are the products of a separate tectonic/magmatic event that involved significant crustal anatexis.The extension and related volcanism directly preceded the opening of the Rocas Verdes marginal basin along the western margin of Chile and may have led to the initial separation of South America and South Africa. As such, the Middle-to-Late Jurassic extension and volcanism heralded the breakup of part of Gondwanaland. 相似文献