The seismic anomaly beneath Iceland extends down to the mantle transition zone and no deeper   总被引：8，自引：0，他引：8  

G. R. Foulger  M. J. Pritchard  B. R. Julian  J. R. Evans  R. M. Allen  G. Nolet  W. J. Morgan  B. H. Bergsson  P. Erlendsson  S. Jakobsdóttir  S. Ragnarsson  R. Stefansson  K. Vogfjörd 《Geophysical Journal International》2000,142(3):F1-F5

A 3-D teleseismic tomography image of the upper mantle beneath Iceland of unprecedented resolution reveals a subvertical low wave speed anomaly that is cylindrical in the upper 250 km but tabular below this. Such a morphological transition is expected towards the bottom of a buoyant upwelling. Our observations thus suggest that magmatism at the Iceland hotspot is fed by flow rising from the mantle transition zone. This result contributes to the ongoing debate about whether the upper and lower mantles convect separately or as one. The image also suggests that material flows outwards from Iceland along the Reykjanes Ridge in the upper 200 km, but is blocked in the upper 150 km beneath the Tjornes Fracture Zone. This provides direct observational support for the theory that fracture zones dam lateral flow along ridges.  相似文献   

Geographic signatures of North American West Coast estuaries     

Robert Emmett  Roberto Llansó  Jan Newton  Ron Thom  Michelle Hornberger  Cheryl Morgan  Colin Levings  Andrea Copping  Paul Fishman 《Estuaries and Coasts》2000,23(6):765-792

West Coast estuaries are geologically young and composed of a variety of geomorphological types. These estuaries range from large fjords to shallow lagoons; from large to low freshwater flows. Natural hazards include E1 Niños, strong Pacific storms, and active tectonic activity. West Coast estuaries support a wide range of living resources: five salmon species, harvestable shellfish, waterfowl and marine birds, marine mammals, and a variety of algae and plants. Although populations of many of these living resources have declined (salmonids), others have increased (marine mammals). West Coast estuaries are also centers of commerce and increasingly large shipping traffic. The West Coast human population is rising faster than most other areas of the U.S. and Canada, and is distributed heavily in southern California, the San Francisco Bay area, around Puget Sound, and the Fraser River estuary. While water pollution is a problem in many of the urbanized estuaries, most estuaries do not suffer from poor water quality. Primary estuarine problems include habitat alterations, degradation, and loss; diverted freshwater flows; marine sediment contamination; and exotic species introductions. The growing West Coast economy and population are in part related to the quality of life, which is dependent on the use and enjoyment of abundant coastal natural resources.  相似文献   

Petrological geodynamic modeling of mid-ocean ridges     

M. Tirone  G. Sen  J.P. Morgan 《Physics of the Earth and Planetary Interiors》2012

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Photometric calibration of the Swift ultraviolet/optical telescope     

T. S. Poole  A. A. Breeveld  M. J. Page  W. Landsman  S. T. Holland  P. Roming  N. P. M. Kuin  P. J. Brown  C. Gronwall  S. Hunsberger  S. Koch  K. O. Mason  P. Schady  D. Vanden Berk  A. J. Blustin  P. Boyd  P. Broos  M. Carter  M. M. Chester  A. Cucchiara  B. Hancock  H. Huckle  S. Immler  M. Ivanushkina  T. Kennedy  F. Marshall  A. Morgan  S. B. Pandey  M. de Pasquale  P. J. Smith  M. Still 《Monthly notices of the Royal Astronomical Society》2008,383(2):627-645

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The infrared bands of small silicate spheroids     

D. H. Morgan  K. Nandy 《Astrophysics and Space Science》1974,31(2):411-416

We have investigated the effects of the size, shape, and alignment of small silicate spheroids on the wavelength positions of their absorption bands near 10 m and 20 m. These particles are believed to be a major constituent of interstellar grains and are reported to have been observed in circumstellar dust shells. It is found that the bands for spheroids can be displaced by 1 m towards longer wavelengths from their positions for corresponding spheres; and that a further, though smaller, displacement can be superimposed upon this by their subsequent alignment.Communications from the Royal Observatory, Edinburgh, No. 178.  相似文献   

Evolution of CO on Titan     

Ah-San WongChristopher G. Morgan  Yuk L. YungTobias Owen 《Icarus》2002,155(2):382-392

The early evolution of Titan's atmosphere is expected to produce enrichment in the heavy isotopomers of CO, ¹³CO and C¹⁸O, relative to ¹²C¹⁶O. However, the original isotopic signatures may be altered by photochemical reactions. This paper explains why there is no isotopic enrichment in C in Titan's atmosphere, despite significant enrichment of heavy H, N, and O isotopes. We show that there is a rapid exchange of C atoms between the CH₄ and CO reservoirs, mediated by the reaction ¹CH₂+*CO→¹*CH₂+CO, where *C is ¹³C. Based on recent laboratory measurements, we estimate the rate coefficient for this reaction to be 3.2×10⁻¹² cm³ s⁻¹ at the temperature appropriate for the upper atmosphere of Titan. We investigate the isotopic dilution of CO using the Caltech/JPL one-dimensional photochemical model of Titan. Our model suggests that the time constant for isotopic exchange through the above reaction is about 800 Myr, which is significantly shorter than the age of Titan, and therefore any original isotopic enhancement of ¹³C in CO may have been diluted by the exchange process. In addition, a plausible model for the evolution history of CO on Titan after the initial escape is proposed.  相似文献   

The calcium exosphere of Mercury     

Rosemary M. Killen  Thomas A. Bida  Thomas H. Morgan 《Icarus》2005,173(2):300-311

A tenuous calcium atmosphere at Mercury, principally seen in the polar regions, was first observed in July, 1998, using the High Resolution Echelle Spectrograph (HIRES) at the W.M. Keck I telescope (Bida et al., Nature 404, 159, 2000). We report four years of observations of the calcium exosphere of Mercury, confirming the initial findings of a very tenuous atmosphere. These observations show a persistent but spatially variable blue shift, indicating an excess velocity toward the observer of up to 3 km s⁻¹, with an average excess velocity of 2.2 km s⁻¹ above the south pole. In addition, the line profiles reveal a hot corona at the equivalent of 12,000-20,000 K in a thermalized atmosphere, indicating a large range of motion with respect to the observer. The calcium is not confined to the polar-regions: rare and low Ca abundance is seen in the equatorial regions. Strong emission was seen anti-sunward on 3 May 2002. Apparent weak emission on the sunward hemisphere may be due to scattered light from the surface, or may indicate a high latitude source. We show that the likely source of the calcium is either impact vaporization in the form of CaO and clusters, which are subsequently photo-dissociated, or ion-sputtering of atoms, molecules and ions. The column abundance is somewhat, but not strongly, correlated with solar activity. We predict a very hot (probably escaping) oxygen component to the hermean exosphere.  相似文献   

Structural controls on groundwater flow and groundwater salinity in the Spicers Creek catchment,Central West region,New South Wales     

Karina Morgan  Jerzy Jankowski  Geoffrey Taylor 《水文研究》2006,20(13):2857-2871

Saline seepage zone development, and hence the onset of dryland salinity, is a major environmental problem occurring within the Spicers Creek catchment. The primary objective of this paper was to identify previously unmapped faults and show the correlation between these faults and groundwater salinization. As identified from this study, there is a close association between geological structural features and the formation of saline seepage zones. The most saline groundwaters in the catchment were encountered where two geological structures join and form a fault intersection. These saline groundwaters are found at various depths within the fractured aquifers, and changes in groundwater chemistry in the aquifers are associated with the presence of fault zones. ¹⁸O and δ²H stable isotopes, together with ⁸⁷Sr/⁸⁶Sr isotopic ratios, indicate that groundwaters within the fault zones are enriched in ¹⁸O and have a strontium signature similar to seawater. This study identifies several geological structures in the Spicers Creek catchment and demonstrates that groundwaters with the highest salinity arise where fault intersections occur. The results of this study may be used to interpret further the mechanisms leading to seepage zone formation in dryland salinity‐affected catchments located throughout the Central West region of New South Wales, Australia. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

Cosmic-ray exposure history at the Apollo 16 and other lunar sites: lunar surface dynamics     

R.J Drozd  C.M Hohenberg  C.J Morgan  C.E Ralston 《Geochimica et cosmochimica acta》1974,38(10):1625-1642

Concordant ⁸¹K-Kr exposure ages for four station 11 breccias indicate an age of 50.3 ± 0.8m.y. for North Ray Crater. Ray structures visible from orbital photography suggest that stations 8 and 9 should contain a substantial amount of South Ray ejecta. Concordant ⁸¹Kr-Kr exposure ages at these sites indicate an age for South Ray Crater of 2.04 ± 0.08m.y. Surface effects (tracks, surface angularities, and micro-crater populations) show good general agreement with this young an age, but discrepancies on a sample-by-sample basis seem to indicate that extensive pre-surface irradiations must have occurred. A detailed pre-surface exposure history is derived for the parent boulder of samples 69935 and 69955. It is suggested that secondary impacts play a major role in near-surface regolithic stirring. Widespread pre-surface irradiation would in fact be expected if most of the newly excavated material had been transported to the surface by secondary impacts rather than by the South Ray event itself.  相似文献   

Phosphorus distribution between potassic alkali feldspar and metaluminous haplogranitic liquid at 200 MPa (H<Subscript>2</Subscript>O): the effect of undercooling on crystal–liquid systematics     

George?B.?Morgan VIEmail author  David?London 《Contributions to Mineralogy and Petrology》2005,150(4):456-471

To evaluate the applicability of P₂O₅ concentration in potassic alkali feldspar as a monitor of P₂O₅ in melt for undercooled systems, crystal–melt partitioning for P was evaluated via feldspar growth experiments in P-bearing ((3 wt% P₂O₅), water-saturated haplogranitic liquids at 200 MPa, with liquidus undercoolings (ΔT) of 25, 50, 100, 200, and 300°C. Increasing undercooling in the range ΔT=25–200°C shows an evolution of crystal morphologies, from euhedral and well-filled individuals at ΔT=25–50°C to radial clusters with increasingly skeletal habit at greater undercooling. Experiments at ΔT=100–200°C also document the development of P- (up to (9 wt% P₂O₅) and Si-enriched, more alkaline boundary layers adjacent to crystals. Experiments at ΔT=300°C show an additional change in crystallization fabric in which spherulites of skeletal crystals form in open (vapor) space created by the dissolution of bulk silicate, and compositional boundary layers are not observed. We interpret the changes in reaction products at ΔT=300°C to indicate conditions below a glass transition; hence, partition coefficients were not determined for this undercooling. Values of K _d(P)^Kfs/melt from experiments at ΔT=25–200°C, calculated from pairs of crystal and immediately adjacent liquid compositions (including boundary layers at higher undercooling), are mostly in the range of 0.25–0.55 and show no effective change with increased undercooling. Essentially no change in K _d(P)^Kfs/melt with undercooling apparently stems from an interplay between boundary layer composition and a change in the substitution mechanism for P in feldspar from AlPSi₋₂, common in peraluminous to metaluminous liquids near equilibrium, to increasing proportions of ([ ],P)(M⁺,Si)₋₁ with increased undercooling. Bulk glass and liquid beyond boundary layers in experiments with significant percentages of crystallization are homogeneous, and show pronounced fractionation primarily due to the removal of an orthoclase component. Because crystallization was still in progress in experiments with ΔT≤200°C, compositional homogeneity in the bulk liquid requires extremely rapid diffusion of most haplogranite components (Na, K, and Al), apparently resulting from chemical potential gradients stemming from the removal of components from the liquid by crystal growth. Similar homogeneity and bulk fractionation in experiments with ΔT=300°C requires rapid diffusive equilibration for the alkalis even at temperatures below an apparent glass transition. Unlike the haplogranite components, P is only concentrated in liquid boundary layers (ΔT≤200°C) or low-density aqueous vapor (ΔT=300°C) adjacent to crystals. Hence, the P₂O₅ contents of melt inclusions likely are not representative of bulk melt concentrations in significantly undercooled systems (ΔT≤50–100°C).  相似文献