Lagging mantle convection,the geoid and mantle structure     

Geoffrey F. Davies 《Earth and Planetary Science Letters》1984,69(1):187-194

In numerical models of convection incorporating migration of a simulated subduction zone, the main descending flow lags far behind the migrating trench, and a geoid low is associated with the main descending flow. This provides physical plausibility for the suggestion by Chase and Sprowl that present very long-wavelength (degree 2–4) geoid lows are associated with Mesozoic trench locations, and suggests further that the present long-wavelength geoid, deep mantle structure and hotspot distribution may be straightforward consequences of plate evolution since the Upper Paleozoic.  相似文献   

A new solid state diffusion model applied to inverse zoning and diffusion rims in minerals     

Friedemann Freund  Bruce V. King 《Physics and Chemistry of Minerals》1984,11(3):113-124

Any oxide and silicate mineral which is nominally anhydrous but crystallized in the presence of H₂O incorporates traces of H₂O in solid solution. In the case of MgO it can be shown that OH^? pairs convert into H₂+O ₂ ^2? . If the H₂ molecules are lost, the O ₂ ^2? remain in the lattice as excess oxygen stabilized by excess cation vacancies. When the O ₂ ^2? anions decay either thermally or by decompression unbound O^? states (positive holes) are generated which lead to surface charges and subsurface space charge layers. Calculated space charge profiles are presented. O^? concentrations as small as 10–20 ppm suffice to create electric surface fields of the order of 4·10⁷ V·m^?1. The diffusion mechanism which derives from these premises incorporates novel features: the cation diffusion is coupled to the counterdiffusion of unbound and vacancy-bound O^? states. The cation diffusion is predicted to be very fast because first, it is field-enhanced (electrochemically driven) and second, it is not rate-limited by the intrinsic cation vacancy concentration nor by the counter-diffusion of other cations. The model may apply to cases of inverse zoning and diffusion rim formation in minerals under certain P-T conditions.  相似文献   

Computer simulations of the structural and physical properties of the olivine and spinel polymorphs of Mg2SiO4     

Geoffrey D. Price  Stephen C. Parker 《Physics and Chemistry of Minerals》1984,10(5):209-216

The aim of the work presented is to develop a computer simulation technique which will predict the structure and physical properties of forsterite and ringwoodite, the major mantle-forming polymorphs of Mg₂SiO₄. The technique is based upon energy minimization, in which all structural parameters are varied until the configuration with the lowest energy is achieved. The lattice energy and physical properties (e.g. elasticity and dielectric constants) are calculated from interatomic potentials, which generally include electrostatic and short-range terms. We investigate several types of traditional potential models, and present a new type of model which includes partial ionic charges and a Morse potential to describe the effect of covalency on the Si-O bond. This new form of potential model is highly successful, and not only reproduces the zero-pressure structural, elastic and dielectric properties of forsterite and ringwoodite, but also accurately describes their pressure dependence.  相似文献   

Biological marker compounds as indicators of the depositions! history of the Maoming oil shale     

Simon C. Brassell  Geoffrey Eglinton  Fu Jia Mo 《Organic Geochemistry》1986,10(4-6)

The Eocene Maoming oil shale from Guangdong Province occurs as a laterally uniform stratigraphic section, typically 20–25 m thick, from which the aliphatic hydrocarbon constituents of six representative samples were investigated using GC and C-GC-MS. The sediments evaluated included the basal lignite, a vitrinite lens from the overlying claystone, and four intervals from the massive oil shale bed. As expected, the lignite and vitrinite differ markedly from the oil shales. The lignite is dominated by bacterial hopanoids and components of higher plant origin, including C₂₉ steroids and triterpenoids such as oleanenes. Visually, the oil shale samples show corroded and degraded phytoclasts, spores, wispy particles of fluorescent organic material attributable to dinoflagellates and, especially in the uppermost sample, colonial algal bodies. The distributions of biological markers in the oil shales show many features in common, notably a dominance of dinoflagellate-derived 4-methylsteroids, and a significant proportion of higher-plant derived n-alkanes with marked odd-over-even carbon number predominance. Overall, they exhibit several features that resemble characteristics of the Messel shale. The hydrocarbons of the lowest shale horizon suggest that there may have been a gradual transition between deposition of the original peat and the subsequent oil shales. The aliphatic hydrocarbons of the uppermost shale are dominated by a number of C₃₁ and C₃₃ botryococcane homologues and other unusual branched alkanes possibly derived from green algae. All of the samples are immature. Overall, molecular and microscopic examination of the stratigraphic succession of the Maoming oil shale suggests a shallow, lacustrine environment within which peats were deposited. This lake subsequently deepened to support abundant algal populations, especially dinoflagellates, culminating in a dominance of botryococcoid algae.  相似文献   

Mastodon-bearing springs and late Quaternary geochronology of the lower Pomme de Terre Valley, Missouri : By C. Vance Haynes, Jr., 1985. Geological Society of America Special Paper 204. ix + 35 pp     

W. Geoffrey Spaulding   《Quaternary Research》1986,25(3)

  相似文献   

Crustal outgassing and LILE enrichment in major lithosphere structures,Archean Abitibi greenstone belt: evidence on the source reservoir from strontium and carbon isotope tracers     

R. Kerrich  B. J. Fryer  R. W. King  L. M. Willmore  E. van Hees 《Contributions to Mineralogy and Petrology》1987,97(2):156-168

Major structural discontinuities in the Abitibi greenstone belt acted as conduits for outgassing of the Archean crust, as reflected in fixation of a select group of lithophile elements including Si, C, K, Rb, Ba, Li, Cs, B and Pb, in metasomatized faults. For two of the largest structures, the Destor-Porcupine (DP) and Kirkland Lake — Cadillac (KC) fault zones 6×10¹⁵ g Si, 3×10¹⁵ g CO₂ and 10¹⁵ g K were introduced into the faults during expulsion of an estimated 6×10¹⁸ g aqueous fluids. Strontium isotope ratios of tourmaline, piemontite, actinolite and scheelite mineral separates, characterized by Rb/Sr0.02, are concordant with respect to ⁸⁷Sr/⁸⁶Sr initial ratios over local sectors of the faults. The Sr isotope data record geographic variations which, from east to west on the KC fault is 0.7031–0.7041 (Val d'Or), 0.7008–0.7022 (Bourlemaque), 0.7017–0.7019 (Bousquet), 0.7029–0.7031 (Noranda), and 0.7013 to 0.7015 (Kirkland Lake). At Timmins, on the PD fault, ⁸⁷Sr/⁸⁶Sr initial ratios cluster at 0.7010 to 0.7020. Metasomatised fault zones are systematically more radiogenic than contiguous host lithologies, and imply a source reservoir (0.7010 to 0.7041) generally more radiogenic than the upper mantle at 2690 Ma (0.700±0.001), or contemporaneous volcanic rocks of mafic to ultramafic composition (0.700 to 0.7012). Whereas certain minerals are concordant and retentive, Rb-Sr isochrons based on suites of rocks at progressive intensities of metasomatism, have been systematically reset over an elpased time of 200 Ma after termination of outgassing, due to disturbance accompanying incremental displacements on structures.Carbon isotope compositions of ferroan dolomites in faults are tightly clustered along local fault sectors, but also display a marked provinciality: from east to west ¹³C=–6.0 to –8.5 (Malartic), –8.0 to –9.0 (Cadillac), –2.0 to –4.5 (Kirkland Lake), and –0.5 to –3.5 (Timmins). The observed provinciality of both ¹³C values and ⁸⁷Sr/⁸⁶Sr initial ratios is interpreted to reflect compositional heterogeneities in a radiogenic sialic crust and the green-stone belt supracrustal sequence, both of which supplied volatiles, magmas and lithophile elements to the fault structures during late stage transpressive tectonics.  相似文献   

The lattice dynamics and thermodynamics of the Mg2SiO4 polymorphs     

Geoffrey D. Price  Stephen C. Parker  Maurice Leslie 《Physics and Chemistry of Minerals》1987,15(2):181-190

We use an approach based upon the Born model of solids, in which potential functions represent the interactions between atoms in a structure, to calculate the phonon dispersion of forsterite and the lattice dynamical behaviour of the beta-phase and spinel polymorphs of Mg₂SiO₄. The potential used (THB1) was derived largely empirically using data from simple binary oxides, and has previously been successfully used to model the infrared and Raman behaviour of forsterite. It includes ‘bond bending’ terms, that model the directionality of the Si-O bond, in addition to the pair-wise additive Coulombic and short range terms. The phonon dispersion relationships of the Mg₂SiO₄ polymorphs predicted by THB1 were used to calculate the heat capacities, entropies, thermal expansion coefficients and Gruneisen parameters of these phases. The predicted heat capacities and entropies are in outstandingly good agreement with those determined experimentally. The predicted thermodynamic data of these phases were used to construct a phase diagram for this system, which has Clausius-Clapeyron slopes in very close agreement with those found by experiment, but which has predicted transformation pressures that show less close agreement with those inferred from experiment. The overall success, however, that we have in predicting the lattice dynamical and thermodynamic properties of the Mg₂SiO₄ polymorphs shows that our potential THB1 represents a significant step towards finding the elusive quantitative link between the microscopic or atomistic behaviour of minerals and their macroscopic properties.  相似文献   

Late Cretaceous genesis of the Kula plate     

Mark T. Woods  Geoffrey F. Davies 《Earth and Planetary Science Letters》1982,58(2):161-166

We propose that the Pacific-Kula ridge began spreading approximately 85 m.y. B.P., during Late Cretaceous time. Extrapolation of the Great Magnetic Bight backwards in time results in an implausible ridge configuration. This implies that plate velocity vectors for the Pacific, Kula, and Farallon plates were not constant during this interval. Evidence for splitting of the Kula plate from the Pacific plate along the Chinook trough is the relationship between the north-striking Amlia and Adak fracture zones, the Chinook and Emperor troughs, and the magnetic lineations south of the Aleutian trench. If this hypothesis is correct, it will require that Mesozoic reconstructions of the Pacific basin and their relation to Cenozoic reconstructions be re-examined. A previously unrecognized Mesozoic plate may be required. We propose calling this the Izanagi plate.  相似文献   

The American oyster as a coastal zone pollution monitor: A pilot study     

Geoffrey I. Scott  David R. Lawrence 《Estuaries and Coasts》1982,5(1):40-46

The Condition Index [(dry meat weight)(100)/(internal cavity volume)] has been analyzed and compared in the American oyster,Crassostrea virginica, from two South Carolina estuaries, one of which was considered polluted by coliform bacteria standards. During the warmer months, oysters from the unpolluted habitat showed a consistently and significantly higher Condition Index. Best fit and power function regressions of Condition Index on total coliform bacteria levels conform with previously cited effects of pollutants on oysters. Condition Index droped markedly as the incidence of total coliforms increased toward levels of 100 per 100 ml of water. Our results and the known sensitivity of oysters to a spectrum of pollutants suggest that their condition could be used to monitor waterborne pollution in coastal zone areas.  相似文献   

Chronology and development of the 1980 earthquake at El Asnam (Algeria): a postscript     

Y. Thommeret  G.C.P. King  C. Vita-Finzi 《Earth and Planetary Science Letters》1983,63(1):137-138

Radiocarbon dating has now confirmed the alluvial chronology that was used in assessing the extent of deformation during the 1980 El Asnam earthquake. It also supports the suggestion that earlier movements were related to a hypocentre further to the east.  相似文献