Non-adiabatic motion of charged particles traversing a weak magnetic field: Pitch angle scattering     

Sydney Chapman  Peter C. Kendall 《Pure and Applied Geophysics》1964,59(1):100-122

Summary A charged particle moves with velocityv in a constant non-uniform magnetic fieldH, spiralling with Larmor radiusR. IfR is small compared with the scale lengthL of the field, the magnetic moment associated with the Larmor motion of the particle is nearly constant. Consequently , the (pitch) angle betweenv andH, varies as arcsinH ^1/2. Hence in such adiabatic motion is approximately the same at points on the path whereH has the same value. But the magnetic moment and the pitch angle may differ materially at two such points, each in the region whereR/L is small, if between them the particle traverses a region whereR/L is not small. This region of non-adiabatic motion scatters the pitch angles.Such scattering is investigated for regions of weak field (R large), with and without the presence of a neutral line along whichH=0. Either type of region, it is found, can scatter the pitch angles. This gives support to the theory proposed byAkasofu andChapman to explain why auroral arcs and bands are very thin.The scattering here examined is of interest also in connection with magnetic mirror devices for nuclear energy transformation. It may also have applications to phenomena of solar and stellar atmospheres.  相似文献   

Origin of the 3500-3300 Ma calc-alkaline rocks in the Pilbara Archaean: isotopic and geochemical constraints from the Shaw Batholith   总被引：1，自引：0，他引：1  

M.J. Bickle  L.F. Bettenay  H.J. Chapman  D.I. Groves  N.J. McNaughton  I.H. Campbell  J.R. de Laeter 《Precambrian Research》1993,60(1-4)

Calc-alkaline plutonic rocks, intruded at 3450Ma, comprise a major component of the Shaw Batholith in the Archaean east Pilbara Block, Western Australia. New whole-rock Pb isotopic geochronology confirms the extent of these rocks, but a minor plutonic phase is dated at 3338±52 Ma and represents a second plutonic event of the same age as much of the nearby Mt Edgar Batholith. The Sm----Nd isotopic systematics of the 3450Ma rocks imply their derivation from a heterogeneous source, which probably included a slightly older crustal component as well as a depleted mantle component. The 3338±52 Ma pluton includes components derived from crustal sources older than 3600 Ma. The geochemistry and Sm---Nd isotopic systematics of these rocks are consistent with crustal growth in the early Archaean from upper mantle sources as depleted as the modern upper mantle. The Shaw Batholith calc-alkaline suites exhibit very similar chemical trends on variation diagrams to modern calc-alkaline plutonic rocks which can be modelled by a combination of mixing and fractionation. A suite collected from outcrops displaying prominent igneous layering shows distinct geochemical trends which can be modelled by differentiation into a component enriched in ferromagnesian minerals, principally hornblende, and possibly sphene, magnetite and epidote, and into a leucocratic component containing quartz, plagioclase and K-feld-par. These Archaean calc-alkaline plutonic rocks, in common with rocks from many other Archaean calc-alkaline provinces, exhibit very fractionated REE patterns with depleted HREE contents, a feature considered to result from equilibrium with garnet at depth in lower crustal regions. The geochemistry of the Pilbara Archaean calc-alkaline rocks is identical to the subset of modern continental-margin calc-alkaline plutonic rocks with fractionated REE patterns, such as those from the central and eastern Peninsular Ranges Batholith, western USA. The tectonic setting in which the Archaean calc-alkaline rocks formed is still not known. This reflects both uncertainty associated with the petrogenesis and environments of modern calc-alkaline rocks, as well as the limited knowledge of the precise timing and relationships of plutonic, depositional and tectonic events in the Pilbara Archaean.  相似文献   

Géodynamique Pétrolière. Genèse et répartition des gisements d'hydrocarbures: A. Perrodon. Bulletin des Centres de Recherches Exploration-Production Elf-Aquitaine,Mémoire 2. Co-edition Masson and Elf Aquitaine, 1980, 390 pp, 220 figs, 3 tables and 4 plates,FF 160 (bound)     

R.E. Chapman 《Sedimentary Geology》1981,30(3):237-238

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Strontium isotopic contamination and oxidation during ocean floor hydrothermal metamorphism of the ophiolitic rocks of the Troodos Massif,Cyprus     

E.T.C. Spooner  H.J. Chapman  J.D. Smewing 《Geochimica et cosmochimica acta》1977,41(7):873-890

Twenty-six new high precision ${}^{87}\text{Sr}{}^{86}\text{Sr}$ratio determinations and existing analyses are used to discuss the strontium isotopic composition of the Upper Cretaceous ophiolitic rocks of the Troodos Massif, Cyprus. Relative to initial magmatic ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios (0.70338 ± 0.00010 to 0.70365 ± 0.00005), the hydrothermally metamorphosed pillow lavas and dyke complex have been contaminated by isotopically heavier strontium.This observation confirms the hypothesis that hydrothermal metamorphism was a consequence of sea water-rock interaction, since sea water was the only readily accessible reservoir of isotopically heavier strontium. The fact that metagabbros and altered trondhjemites were also Sr isotopically contaminated shows that sea water penetrated approximately 2 km into the oceanic crust represented by the ophiolitic sequence.The amount of Sr isotopic contamination requires that the bulk sea water: rock ratio was at least ~15:1 and shows that water-rock interaction occurred in a flow system. The degree of oxidation decreases with increasing depth. This shows that the vertical component of fluid flow was downward. The absolute bulk water/rock ratio (for water at S.T.P.), as estimated from the oxidation profile, may have been as large as ~3 × 10³:1 —a large figure which independently confirms that rocks showing strong δ¹⁸O shifts have interacted with large volumes of water.The sites of discharge of the hot fluid, which must have come out of the system, are identified as the cupriferous pyrite ore deposits. This process of mass transfer corresponds to hydrothermal convection in a permeable medium with an open upper boundary surface.  相似文献   

The utilization of North sea hydrocarbons as raw materials for petro-chemical manufacture     

Keith Chapman 《Geoforum》1977,8(4):169-182

The oil and gas reserves of the North Sea have principally been regarded as sources of energy and little attention has been paid to their potential as raw materials for chemical manufacture. This paper reviews the technical and economic possibilities presented by the North Sea hydrocarbons as petro-chemical feedstocks. In particular, an attempt is made to predict changes which may occur in the geography of petro-chemical production in North West Europe if full advantage is taken of these possibilities.  相似文献   

Long wavelength thermal convection between non-conducting boundaries     

C.J. Chapman  S. Childress  M.R.E. Proctor 《Earth and Planetary Science Letters》1980,51(2):362-369

Non-linear Rayleigh-Bénard convection in a fluid layer is considered as a model of convection in the Earth's upper mantle. Previous studies have shown that when the temperature is held fixed at one of the boundaries of the layer, convection takes place in cells of width of the order of the layer depth or less. We investigate the effects of a different thermal boundary condition, in which the flux of heat is held fixed on both layer boundaries; then if this flux is just greater than that required for the onset of convection, motion takes place on horizontal scales much greater than the layer depth. An analytical treatment of the equations, based on an expansion in the depth-to-width ratio of the cells, shows that cells of a definite horizontal scale are the fastest growing according to linearised theory, but that these cells are unstable to ones of larger wavelength than themselves. Thus the dominant wavelength lengthens with time. The results hold whether the heat flux is generated internally of comes from beneath the layer. These results produce flow patterns similar to those found when the heat flux is much greater than the critical value. The results have important consequences for the understanding of mantle convection.  相似文献   

Pyrope-spinel (alkremite) xenoliths from kimberlite     

P. H. Nixon  N. A. Chapman  J. J. Gurney 《Contributions to Mineralogy and Petrology》1978,65(3):341-346

Xenoliths consisting of two thirds pyrope (Py₇₃Alm₁₄Gr₁₃-Py₁₅Alm₁₈Gr₃₁) and one third hercynite-bearing spinel with minor chromium, from Bellsbank and Jagersfontein kimberlites, South Africa, are compared with similar rocks, alkremites, from the Udachnaya pipe, U.S.S.R. From published experimental data and textural relationships these formed as early dense cumulates in aluminous mantle melts under restricted pressure conditions equivalent to about 75 km depth. At greater pressures very pyrope-rich garnets (Py₈₀) are capable of being formed. The garnet spinel xenoliths are considered to have become separated from the magma prior to crystallisation of clinopyroxene, whereas complete uninterrupted fractionation and reaction would produce the more common griquaites (eclogites).  相似文献   

An experimental study of spinel clinopyroxenite xenoliths from the Duncansby Ness vent,Caithness, Scotland     

N. A. Chapman 《Contributions to Mineralogy and Petrology》1975,51(3):223-230

Xenoliths of coarse-grained spinel-clinopyroxenite up to 15 cm in size occur in tuff in an isolated Permian vent on the Caithness coast at Duncansby Ness. Highly altered fragments of chrome-spinel lherzolite and wehrlite are also found in the tuff and in a body of monchiquite within the vent. The spinel-clinopyroxenites consist of aluminous augite and aluminous pleonaste spinel (FeO/MgO = 0.9) and their texture suggests the spinel to have exsolved from the augite. Experiments on representative natural xenolith compositions at 18 kb (dry) indicate that all the spinel in the estimated average bulk composition (Sp_4.9Px_95.1) could have exsolved from an original homogeneous pyroxene. Initial fractionation of such a pyroxene from an alkali basaltic magma at P≥18 kb, 1450-1350 °C, would be followed by spinel exsolution at T< 1290 ° C. A similar origin by fractionation of a highly aluminous augite (± aluminous spinel) at high pressure, with subsequent spinel exsolution is proposed for spinel-clinopyroxenites from alkali basalts elsewhere in the world. The similarity of these xenoliths suggests that such a process may form an important stage in the evolution of some undersaturated basaltic rocks.  相似文献   

Lamb's problem and comments on the paper ‘on leaking modes’ byUsha Gupta     

C. H. Chapman 《Pure and Applied Geophysics》1972,94(1):233-247

Summary In a recent paper,Gupta [5]²⁾ re-examined the significance of leaking modes in Lamb's problem (Lamb [7]). In this paper, we present a brief review of the exact Cagniard-de Hoop solution to this problem, and use these results to examine the question of the leaking mode in more detail. The leaking mode may either cause a separate arrival,P, or influence the shape of other arrivals e.g.SpS. We have attempted to clarify and extend previous results and correct misconceptions which have appeared elsewhere and, therefore, most of this discussion is tutorial in nature.  相似文献   

Processes controlling metal ion attenuation in acid mine drainage streams   总被引：1，自引：0，他引：1  

B.M Chapman  D.R Jones  R.F Jung 《Geochimica et cosmochimica acta》1983,47(11):1957-1973

Two acid mine drainage streams have been investigated by detailed analysis of their sediments and waters, to obtain an understanding of the dominant processes which control the transport and attenuation of heavy metals under conditions of chronic high-level pollutant input. One of the water-courses has a thick hydrous iron oxide crust on its bed, where biotically mediated oxidation of ferrous iron resulted in precipitation of amorphous ferric hydroxide, along with substantial quantities of adsorbed silica, sulphate and Al and lesser quantities of As. Small amounts of K and Pb (and possibly hydronium) jarosites were also present in the sediments. Changes in pH and in the concentrations of Cu, Zn, and Cd appear to be mainly the result of dilution by seeps and tributaries.Although no sediment was recovered during collection of water samples from the second stream, saturation index calculations imply that precipitation should have been occurring. The observed down-stream loss of a number of elements supported this conclusion. The solids predicted to be precipitating were A1(OH)₃, Cu₂(OH)₂CO₃, and Fe(OH)₃. Observed decreases in the concentrations of Cd, Zn and Mn can be accounted for on the basis of dilution alone. However, the additional mechanism of neutralization by higher pH inflows is required to account for the decrease in hydrogen ion concentration downstream.The basis for a potentially useful new technique (congruent element analysis) which enables the identification of conservative components in streams is presented. Comparison of logarithmic concentration versus distance plots delineates the point where chemical removal mechanisms become important for each element.  相似文献