Observations of multiple core reflections of the PnKP and SnKP type and regional variations at the base of the mantle     

Cedric Wright 《Earth and Planetary Science Letters》1973,19(4):453-460

Seismological results interpreted as evidence for large inhomogeneities near the base of the Earth's mantle below Hawaii have recently been published. It is possible to place constraints on the magnitude of such heterogeneities by identifying seismic phases multiply reflected within the Earth's core. The value of such a simple technique is illustrated by using array recordings of P and S5KP waves that have traversed the bottom of the mantle beneath Hawaii to show that there is no clear evidence for the unusual physical properties attributed to this region of the Earth. Identification of the phase S7KP is also reported.  相似文献   

Magmas,mineralization and sea-floor spreading     

J. B. Wright  Patricia Mc Curry 《International Journal of Earth Sciences》1973,62(1):116-125

Relationships between granite bodies and mineralization in Nigeria and southwest England are cited as examples to support the thesis that ore solutions associated with igneous bodies do not necessarily develop by magmatic crystallization processes, but may form as independent by-products of magma generation. Similar conclusions have been reached for the metal provinces of western North America (Noble, 1970). Some hydrothermal solutions develop by expulsion of connate brines, mobilized by the passage of hot magma through sediments. Others may develop at deeper levels.Long-lived geochemical culminations in the deep crust or upper mantle (Schuiling, 1967) provide a possible source of reactivated metalliferous emanations in both nonorogenic and orogenic environments. An additional source of metals for ore deposits associated with island arcs and continental margin tectonism and igneous activity, may be provided by the mechanism of sea-floor spreading.The bulk of ocean floor sediments is scraped off and acreted onto non-descending crustal plates at subduction zones²). However, if the lowermost layers of these sediments are sufficiently compacted, they may be carried down into the mantle on the descending oceanic plate.They contain considerable amounts of heavy metals, which could be remobilized to reappear as ore deposits in island arcs and mountain chains of Andean type.In this way, the major episode of late Mesozoic underflow of the northeastern Pacific floor beneath western North America may have been responsible for much of the contemporaneous mineralization of that region. If this view is correct, much of the rich mineralization in the Andes may have a similar origin.Differences in amount and composition of sea floor sediment consumed, could account for regional changes in metal provinces along the tectonic grain. Remobilization of metals at different depths along Benioff zones could explain regional changes across the tectonic grain.

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Zusammenfassung Beziehungen zwischen Granitkörpern und Mineralisation in Nigeria und Südwest-England werden als Beispiele angeführt, um die These zu stützen, daß Erzlösungen in Verbindung mit Magmatit-Körpern sich nicht notwendigerweise von magmatischen Kristallisationsprozessen ableiten, sondern sich als unabhängige Nebenprodukte bei der Magmenerzeugung bilden können.Ähnliche Schlußfolgerungen sind für die Erzprovinzen des westlichen Nordamerika gezogen worden (Noble, 1970). Einige hydrothermale Lösungen entwickeln sich durch Ausstoß von connaten Laugen, mobilisiert durch das Durchströmen heißer Magma durch Sedimente. Andere mögen sich in tieferem Niveau entwickeln.Langandauernde geochemische Kulminationen in der tiefen Erdkruste oder im oberen Mantel (Schuiling, 1967) schaffen eine mögliche Quelle von reaktivierten, metallhaltigen Emanationen sowohl unter nichtorogenen als auch unter orogenen Bedingungen. Eine zusätzliche Quelle von Metallen für Erzlagerstätten, die an Inselbögen und Kontinentalrand-Tektonik sowie magmatische Aktivität gebunden sind, mag durch den Mechanismus des Sea Floor Spreading hervorgerufen werden.Die Masse der Ozeanboden-Sedimente wird abgetragen und auf nicht absteigenden Krusten-Plateaus tieferer Zonen angehäuft¹). Wenn jedoch die untersten Schichten ausreichend verdichtet sind, können sie in den Erdmantel auf dem sinkenden Ozean-Plateau hinabgelangen. Sie enthalten beträchtliche Mengen von Schwermetallen, die als Erzlagerstätten remobilisiert in Inselbögen und Gebirgsketten vom Anden-Typus wieder in Erscheinung treten können.Auf diese Weise mag die größere Episode der spät-mesozoischen Unterströmung des nordöstlichen Pazifikbodens unter das westliche Nordamerika für den Hauptanteil der gleichzeitigen Mineralisation dieser Region verantwortlich gewesen sein. Falls dieser Gesichtspunkt richtig ist, mögen viele der reichen Mineralisationen in den Anden einen ähnlichen Ursprung haben.Unterschiede in Menge und Zusammensetzung des aufgezehrten Maaresbodensediments könnten als regionale Veränderungen in Metallprovinzen entlang der tektonischen Struktur gedeutet werden. Remobilisierung von Metallen in verschiedenen Tiefen entlang der Benioff-Zonen könnten regionale Veränderungen quer zur tektonischen Richtung erklären.

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Résumé Les auteurs prennent comme exemple les rapports existant entre les corps granitiques et la minéralisation en Nigérie et dans le sud-ouest de l'Angleterre pour soutenir la thèse que les solutions minéralisantes associées avec les corps magmatiques, ne se développent pas nécessairement à partir de processus de cristallisation magmatiques, mais peuvent se former comme sous-produits indépendants de la génération du magma. Semblables conclusions ont été tirées pour les provinces métalliques de l'ouest de l'Amérique du Nord (Noble, 1970). Quelques solutions hydrothermales se développent par l'expulsion de saumures connées, mobilisées par le passage du magma chaud à travers des sédiments. D'autres peuvent se développer à des niveaux plus profonds.Des culminations géochimiques de longue durée dans la croûte terrestre ou dans le manteau supérieur (Schuiling, 1967) fournissent une source possible d'émanations métallifères réactivées, dans des conditions aussi bien non-orogéniques qu'orogéniques. Une source additionelle de métaux pour les gisements qui, comme l'activité magmatique, sont associés avec les guirlandes d'îles et avec la tectonique propre à la bordure continentale, peut être fournie par le mécanisme de l'expansion du fond des mers.La masse des sédiments du fond océanique est érodée et accumulée sur des plateaux crustaux, non en voie d'affaissements de zones plus profondes³).Quand cependant les couches plus inférieures de ces sédements sont suffisamment compactées, elles peuvent être entraînées dans le manteau sur le plateau océanique en voie d'affaissement.Elles contiennent des quantités considéreables de métaux lourds qui, remobilisés sous la forme de gisements de minerais, peuvent apparaître dans les guirlandes d'îles et les chaînes de montagnes de type andin.De cette façon, l'épisode majeur de la subduction, survenue au Mésozoïque supérieur, du fond du Pacifique NE sous l'ouest de l'Amérique du Nord peut avoir été responsable de la majeure partie de la minéralisation de cette région. Si cette opinion est juste, beaucoup de riches minéralisations rencontrées dans les Andes pourraient avoir une origine similaire.Des différences dans la quantité et la composition du sédiment océanique absorbé pourraient expliquer les changements régionaux dans les provinces métalliques tout le long de la structure tectonique. La remobilisation des métaux à des profondeurs différentes le long des zones Benioff pourrait expliquer les changements régionaux survenant transversalement à la direction tectonique.

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Introduction     

H.E. Wright  M.L. Heinselman 《Quaternary Research》1973,3(3):319-328

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Origin of the younger granites of northern Nigeria     

J. B. Wright 《Contributions to Mineralogy and Petrology》1970,29(1):89-90

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Trace-element patterns at the Cretaceous-Tertiary boundary—Consequences of a large impact     

J. Smit  W.G.H.Z. ten Kate 《Cretaceous Research》1982,3(3):307-332

The earlier discovery of high concentrations of iridium and osmium at the very base of the Tertiary in marine pelagic sequences, has led to the hypothesis that the impact of a large projectile on Earth terminated the Mesozoic era.This paper describes the distribution of other trace elements in samples closely spaced around the Cretaceous-Tertiary boundary. We found an enrichment of Co, Cr, Ni, As, Sb and Se, and a depletion of the rare earth elements (REE), coupled with the anomalously high concentration of Ir and Os. Also, in the enrichment layer, problematic sanidine spherules occur which may be spheres, ablated from the impacting projectile or microtektites.The REE depletion could not only be demonstrated in the basal millimetres of the relatively thick (～ 10 cm) boundary marl bed of the Barranco del Gredero section, Caravaca, Spain, but also in the boundary clay of the Stevns Klint section—the “Fiskeler”—Denmark. In these boundary clay beds are also the highest concentrations of Ir recorded [44 and 86.7 ppb (1 ppb = 10^?9 g/g), respectively].This REE depletion is tentatively connected with the low REE concentration in common meteorites. In that case the basal millimetres are dominated by a meteoritic component and represent a “fall-out” layer of the impact event. This in turn would imply low terrestrial dilution at impact on Earth, suggesting that the projectile might have fallen into the ocean.Identification of the projectile remains speculative; especially the high As, Sb and Se anomalies are problematic. These latter may point to a projectile of uncommon composition, possibly a comet.An integrated scenario of a possible sequence of events is presented, chiefly based on the interpretation of micropaleontological, lithological, geochemical and stable isotope data of the studied sections.The direct and indirect effects of an impact leading to mass extinction constitute a complicated problem and may comprise a sudden temperature rise, a poisoning by cyanide or arsenic, and a few years suppression of sunlight, or preferably, a combination of these effects.  相似文献   

Formation of the Green Tuff,Pantelleria     

J. A. Wolff  J. V. Wright 《Bulletin of Volcanology》1981,44(4):681-690

The strongly peralkaline Green Tuff, Pantelleria, is an example of a thin, densely welded air-fall tuff which mantles an area of at least 85 km². Offshore the tuff is correlated with the Y-6 ash layer in the central Mediterranean Sea, and the total volume of the eruption is estimated at 7 km³ D.R.E. New petrological data suggests that the tuff was erupted from a zoned magma chamber containing a cooler, more fractionated upper zone relative to be bulk of the magma. Analysis of the distribution of accessory lithic fragments in terms of existing models of eruption dynamics indicates emplacement by a plinian-type eruption. It is shown that, due to the low viscosity of pantelleritic ejecta, dense welding can occur at moderate tephra accumulation rates and a rate of the order of 1 cm/minute is suggested for the Green Tuff; this yields an estimate for the eruption duration of rather less than one day. It is predicted that welded tuff should be formed during large plinian eruptions of pantelleritic magma, and therefore that welded airfall tuffs should be common in areas of peralkaline volcanism.  相似文献   

The dali ash,island of rhodes,greece: a problem in interpreting submarine volcanigenic sediments     

J. V. Wright  E. Mutti 《Bulletin of Volcanology》1981,44(2):153-167

In the southern part of Rhodes, Greece, rhyolitic subaqueous pyroclastic deposits are interbedded with Tertiary, deep water, marine sediments. The lowermost and best exposed of these deposits — the Dali Ash — is described here. The deposit has been previously described as a deep water welded subaqueous ignimbrite. This paper shows that there is no evidence of welding, and texture previously reported were misidentified. The Dali Ash consists of a lower massive unit (5 m thick), overlain by a sequence of ash-turbidites (2.5 m thick). The lower unit was deposited by a high concentration turbidity current and the ash-turbidites by dilute turbidity currents. Foraminifera are dispersed throughout the deposit and indicate that all the sedimentary gravity flows were cold water/particulate systems. A palaeomagnetic study also suggests they were deposited cold. The Dali Ash can be interpreted as the lateral equivalent of a subaerial pumiceous pyroclastic flow deposit (ignimbrite). The ash-turbidites then may be redeposited slumps off the submarine slope of the lower massive unit, or, may represent later, smaller pyroclastic flows in the eruption. Other alternatives for the origin of the Dali Ash are fully discussed to show the problems in interpreting submarine volcanigenic sediments. It is possible that the deposits are not even a primary eruptive product and are remobilized pyroclastic debris, slumped, for example, off the sides of a shallow marine rhyolitic tuff ring.  相似文献   

Sovereign Sugar: Industry and Environment in Hawai'i          下载免费PDF全文

Kate A. Berry 《Geographical review》2018,108(2):328-330

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Assessing climate change impacts on California hydropower generation and ancillary services provision     

Kate Forrest  Brian Tarroja  Felicia Chiang  Amir AghaKouchak  Scott Samuelsen 《Climatic change》2018,147(3-4):395-409

Climate change is already affecting species and their distributions. Distributional range changes have occurred and are projected to intensify for many widespread plants and animals, creating associated risks to many ecosystems. Here, we estimate the climate change-related risks to the species in globally significant biodiversity conservation areas over a range of climate scenarios, assessing their value as climate refugia. In particular, we quantify the aggregated benefit of countries’ emission reduction pledges (Intended Nationally Determined Contributions and Nationally Determined Contributions under the Paris Agreement), and also of further constraining global warming to 2 °C above pre-industrial levels, against an unmitigated scenario of 4.5 °C warming. We also quantify the contribution that can be made by using smart spatial conservation planning to facilitate some levels of autonomous (i.e. natural) adaptation to climate change by dispersal. We find that without mitigation, on average 33% of each conservation area can act as climate refugium (or 18% if species are unable to disperse), whereas if warming is constrained to 2 °C, the average area of climate refuges doubles to 67% of each conservation area (or, without dispersal, more than doubles to 56% of each area). If the country pledges are fulfilled, an intermediate estimate of 47–52% (or 31–38%, without dispersal) is obtained. We conclude that the Nationally Determined Contributions alone have important but limited benefits for biodiversity conservation, with larger benefits accruing if warming is constrained to 2 °C. Greater benefits would result if warming was constrained to well below 2 °C as set out in the Paris Agreement.  相似文献   

Climate change impacts on freshwater fish,coral reefs,and related ecosystem services in the United States     

Diana Lane  Russell Jones  David Mills  Cameron Wobus  Richard C. Ready  Robert W. Buddemeier  Eric English  Jeremy Martinich  Kate Shouse  Heather Hosterman 《Climatic change》2015,131(1):143-157

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