Hawaiian basalt and Icelandic rhyolite: Indicators of differentiation and partial melting   总被引：1，自引：0，他引：1  

B. D. Marsh  B. Gunnarsson  R. Congdon  R. Carmody 《International Journal of Earth Sciences》1991,80(2):481-510

In spite of the voluminous basaltic volcanism on the island of Hawaii, rhyolite is not produced. Iceland, on the other hand, exhibits common rhyolitic volcanism amounting to some 10–12% of its surface rocks. This contrast is investigated using the fundamental igneous processes exhibited by sheet-like Hawaiian lava lakes and Shonkin Sag laccolith in Montana. Highly differentiated, residual melts normally reside within inwardly advancing solidification fronts and are generally inaccessible to eruptive processes. Only when a large initial phenocryst population is present, from which a thick basal cumulate can rapidly form, is it possible to supply highly differentiated melt into the active (i.e., eruptable) portion of the magma chamber. Although there is protracted control of differentiation at Hawaii by settling of olivine, further differentiation occurs within the solidification fronts. Only by repeated transport and holding is it possible to differentiate beyond the critical composition of the leading edge of the solidification front ( 7% MgO and 51.5% SiO₂). Crystal size distributions (CSDs) for Hawaii and Shonkin Sag are used to demonstrate the inferred physical and chemical processes of solidification, including the kinetics of crystallization.A ubiquitous feature of these basaltic bodies is the formation of coarse veins and segregations of refined melt and granophyres within the upper solidification front. It is this fundamental bimodal feature which is the key to understanding Icelandic silicic volcanism.Rhyolites in Iceland occur mainly as a bimodal population with basalts associated with central volcanoes. Rhyolites, granophyres, and felsites are common, with the intrusions often being layered. Ash flows and true granite-like intrusions are rare. The voluminous silicic lavas at Torfajokull central volcano contain disequilibrium phenocryst assemblages. This, and the disagreement in oxygen isotopic values between rhyolites and basalts, reflects extensive partial melting of the heterogeneous basaltic crust of Iceland to produce these rhyolites. Relatively small, chemically distinct, and spatially intimate silicic bodies are formed by concentrating granophyric segregations from earlier cycles of solidification. This process is also reflected in the layered granophyric instrusion of Slaufrudalur in eastern Iceland. Slaufrudalur is an unvented subterranean caldera, equivalent in igneous processes and style to the subaerial Torfajokull caldera.Hawaii is dominated by fractional crystallization due to crystal settling and does not produce rhyolite. Iceland's tectonics allow continual and extensive reprocessing of thin, hot basaltic crust which produces rhyolite by concentrating original silicic segregations and veins and by partially melting intermediate extrusives, which have subsided deep into the crust.

---

Zusammenfassung Auf Hawaii treten, trotz intensiven Basalt-Vulkanismusses, keine Rhyolithe auf. Auf Island dagegen ist Rhyolith, mit 10–12% des anstehenden Gesteins, verbreitet. Dieser Kontrast wurde anhand grundlegender magmatischer Prozesse untersucht, wie sie in flachen Lava-Seen Hawaiis und im Shonkin Sag Laccolith Montanas auftreten. Hochdifferenzierte Restschmelzen verbleiben innerhalb langsam nach innen vorrückender Erstarrungsfronten und sind meist unerreichbar für eruptive Prozesse. Nur wenn anfänglich bereits große Mengen von Einsprenglingen vorhanden sind, die rasch am Boden der Magmenkammer akkumulieren, kann eine hochdifferenzierte Schmelze in den aktiven (d.h. eruptiven) Teil der Magmenkammer gelangen. Obwohl auf Hawaii die Differentiation durch die Kristallisation von Olivin anhaltend kontrolliert wird, findet an der Erstarrungsfront weitere Differentiation statt. Nur durch wiederholten Transport und zeitweiliges Verharren ist es möglich, über die kritische Zusammensetzung der vordersten Erstarrungsfront hinaus zu differenzieren (ca. 7% MgO und 51,5% SiO₂). An Kristallgrö-ßenverteilungen (CDS) von Hawaii und Shonkin Sag können die angenommenen physikalischen und chemischen Prozesse der Kristallisation und die Kristallisationskinetik gezeigt werden. Ein weit verbreitetes Merkmal dieser Basaltkörper ist die Bildung grobkristalliner Gänge und Absonderung von stark differenzierten Schmelzen und Granophyren innerhalb der oberen Erstarrungsfront. Diese ausgeprägt bimodale Charakteristik ist der Schlüssel zum Verständnis des sauren isländischen Vulkanismus.Isländische Rhyolithe treten meist in bimodaler Verbreitung mit Basalten in Zusammenhang mit zentralen Vulkanen auf. Rhyolithe, Granophyre und Feisite sind häufig, in oft geschichteten Intrusionen. Ignimbrite und echte Granitintrusionen sind selten. Die großen Mengen SiO₂-reicher Laven am Torfajokull-Zentralvulkan enthalten Ein-sprenglinge, die sich nicht im Gleichgewicht mit der Matrix befinden. Dies, und die unterschiedlichen delta-¹⁸O-Werte von Rhyolithen und Basalten, zeigen, daß ausgeprägtes teilweises Aufschmelzen der heterogenen Basaltkruste von Island zur Produktion dieser Rhyolithe führte. Relativ kleine, nahe benachbarte saure Körper, die aber deutliche Unterschiede in ihrem Chemismus aufweisen, werden gebildet durch die Konzentration granophyrischer Teilschmelzen aus früheren Kristallisationszyklen. Dieser Vorgang wird auch widergespiegelt in der »layered intrusions« von Slaufrudalur in Ostisland. Slaufrudalur ist eine geschlossene unterirdische Kaldera, deren magmatische Prozesse und Baustil der subaerischen Torfajokull-Kaldera entsprechen.Die Prozesse in Hawaii sind dominiert von gravitativer Kristallisationsdifferentiation und es werden keine Rhyolithe produziert. Die isländische Tektonik führt zu kontinuierlicher starker Wiederaufarbeitung von dünner, heißer basaltischer Kruste. Dabei wird, durch die Konzentration ursprünglicher saurer Teilschmelzen und Gänge und durch die teilweise Aufschmelzung intermediärer Intrusiva, die tief in die Kruste abgesunken sind, Rhyolith produziert.

---

Résumé En dépit du volcanisme basaltique volumineux des îles Hawaï, il n'y existe pas de rhyolite. En Islande, par contre, le volcanisme rhyolitique est commun et représente 10 à 12% des roches de la surface. Ce contraste est examiné sur la base des processus ignés fondamentaux présentés par les lacs de lave d'Hawaï et le laccolite de Shonkin Sag au Montana. Normalement, les liquides résiduels hautement différenciés résident à l'intérieur des fronts de solidification qui progressent vers l'arrière et sont généralement à l'abri des processus éruptifs. Ce n'est que dans le cas d'une population initiale abondante de phénocristaux, qui se rassemblent dans un cumulat basai épais, que des liquides hautement différenciés peuvent être fournis à la portion active (c'est-à-dire »éruptible«) de la chambre magmatique. A Hawaï, bien que la différenciation soit continuellement régie par la cristallisation d'olivine, la poursuite du processus a lieu à l'intérieur des fronts de solidification. Ce n'est que par la répétition d'actions de transport et de stagnation qu'il est possible de différencier audelà de la composition critique du front de solidification (±7% MgO et 51,5% SiO₂). A partir de la distribution de la taille des cristaux à Hawaï et à Shonkin Sag, on peut déduire les processus physique et chimique de la solidification, y compris la cinétique de la cristallisation.Une particularité courante de ces corps basaltiques est la formation de veines grenues et de ségrégations de liquides très différenciés et de granophyres à l'intérieur du front supérieur de solidification. Cette manifestation bimodale est la clé qui permet de comprendre le volcanisme siliceux islandais.En Islande, les rhyolites constituent d'ordinaire une population bimodale avec les basaltes centraux. Les rhyolites, les granophyres et les felsites sont fréquents, et souvent sous forme d'intrusions litées. Les coulées ardentes et les vraies intrusions de type granitique sont rares. Les volumineuses laves siliceuses du volcan central de Torfajokull contiennent des assemblages de phénocristaux en déséquilibre. Ce fait, ainsi que la non concordance des isotopes de l'oxygène entre rhyolites et basaltes, traduisent, à l'origine de ces rhyolites, une fusion partielle extensive de la croûte basaltique hétérogène d'Islande. Des corps siliceux relativement petits et chimiquement distincts bien que d'emplacements très voisins se sont formés par concentration de fusions partielles granophyriques lors des premiers cycles de solidification. Ce processus s'exprime également dans l'intrusion granophyrique litée de Slaufrudalur, en Islande orientale. Slaufrudalur est une caldeira souterraine fermée, équivalente par son style et son processus igné à la caldeira subaérienne de Torfajokull.A Hawaï, le phénomène dominant est la cristallisation fractionnée gravitative, sans production de rhyolite. La tectonique de l'Islande permet la régénération continue et extensive d'une mince croûte basaltique chaude. Les rhyolites y sont engendrées par la concentration des veines et ségrégations siliceuses originelles et par la fusion partielle de masses extrusives intermédiaires descendues profondément dans la croûte.

---

, . , 10–12% . , Shonkin Sag Laccolith Montanas. . , , . , . . ( 7% MgO 51,5% SiO₂). (CDS) Shonkin Sag , , . . . . , , . . , Torfajokull , ., ¹⁸O , . , , , , « » («layered intrusions») Slaufrudalur, . , , Torfajokull. . . , , , .

  相似文献   

Erratum     

Marteinsdottir  Gudrun; Gunnarsson  Bjorn; Suthers  Iain M. 《ICES Journal of Marine Science》2001,58(1):361

  相似文献   

Reproductive cycles of Mytilus edulis L. on the west and east coasts of Iceland     

Gudrun G. Thorarinsdóttir  Karl Gunnarsson 《Polar research》2003,22(2):217-223

Studies evaluating the reproductive pattern of Mytilus edulis L. were conducted in western and eastern Iceland at two sites at about the same latitude but with different temperature regimes. Mussels were sampled once or sometimes twice a month during two years in Breidifjördur, western Iceland and one year at Mjoifjördur, eastern Iceland. Gonad development was monitored by microscopic observation of thin sections of the gonads. The initiation of gonad development was observed in January in Breidifjördur while in Mjoifjördur some of the animals started developing gonads in October, a month before spawning was over in the population. Spawning started in late June or July, peaked in August and continued until November at both sites. Nutrient reserve stores seemed to be limited and used for initiation of gonad development in winter but were not sufficient for maturation of the gonads. The main growth of the gonads occured in spring in conjunction with phytoplankton blooming and renewal of food resources.  相似文献   

Gravity and S-wave modelling across the Jan Mayen Ridge,North Atlantic; implications for crustal lithology   总被引：1，自引：0，他引：1  

Rolf Mjelde  Inger Eckhoff  Ståle Solbakken  Shuichi Kodaira  Hideki Shimamura  Karl Gunnarsson  Ayako Nakanishi  Hajime Shiobara 《Marine Geophysical Researches》2007,28(1):27-41

The horizontal components from fourteen Ocean Bottom Seismometers deployed along four profiles focused along the western margin of the Jan Mayen microcontinent, North Atlantic, have been modelled with regard to S-waves, based on P-wave models obtained earlier. The seismic models have furthermore been constrained by 2D gravity modelling. High V _p/V _s-ratios (2.3–7.9) within the Cenozoic sedimentary section are attributed to significant porosities, whereas V _p/V _s-ratios in the order of 1.9–2.2 for the Mesozoic and Paleozoic sedimentary rocks indicate shale-dominated lithology throughout the area. The eastern side of the Jan Mayen Ridge is interpreted as a passive, volcanic margin, based on relatively high crustal V _p/V _s-ratios (1.9), whereas lower V _p/V _s-ratios (1.75–1.8) suggest the presence of intermediate composition crust and non-volcanic margin on the western side of the ridge. In the westernmost part of the Jan Mayen Basin, slightly increased upper mantle V _p/V _s-ratios may indicate some degree of serpentization of upper mantle peridotites.  相似文献   

Evolution of oceanic crust on the Kolbeinsey Ridge, north of Iceland, over the past 22 Myr     

Kodaira  Mjelde  Gunnarsson  Shiobara  & Shimamura 《地学学报》1998,10(1):27-31

The evolution of oceanic crust on the Kolbeinsey Ridge, north of Iceland, is discussed on the basis of a crustal transect obtained by seismic experiment from the Kolbeinsey Ridge to the Jan Mayen Basin. The crustal model indicates a relatively uniform structure; no significant lateral velocity variations are observed, especially in the lower crust. The uniform velocity structure suggests that the postulated extinct axis does not exist over the oceanic crust formed at the Kolbeinsey Ridge, but supports a model of continuous spreading along the ridge after oceanic spreading started west of the Jan Mayen Basin. The oceanic crust formed at Kolbeinsey Ridge is 1–2.5 km thicker than normal oceanic crust due to hotter-than-normal mantle from the Iceland Mantle Plume. The observed generally uniform thickness throughout the transect might also indicate that the temperatures of the astheno-spheric mantle ascending along the Kolbeinsey Ridge have not changed significantly since the age of magnetic anomaly 6B.  相似文献   

Precipitation of poorly crystalline antigorite under hydrothermal conditions   总被引：1，自引：0，他引：1  

Ingvi Gunnarsson  Stefán Arnórsson  Sigurethur Jakobsson 《Geochimica et cosmochimica acta》2005,69(11):2813-2828

Magnesium silicate precipitation experiments were carried out in alkaline solutions in the temperature range 39°C-150°C. Titrations were carried out at room temperature where the pH of an aqueous solution containing magnesium and silica was raised to bring about precipitation of a magnesium silicate. The precipitation of the magnesium silicate was rapid. Equilibrium between the solution and the precipitate was attained in a period of less than one hour up to a month at around 90°C, depending on the initial degree of oversaturation. Relative magnesium and silica depletion in the experimental solutions and IR spectra of the precipitate show that the magnesium silicate resembles poorly developed antigorite (p-antigorite). Values for its solubility constant were obtained and an equation describing its solubility in the temperature interval 0°-200°C calculated. The equation is: log K_sp = 9303/T + 3.283, where T is in K, and it is valid for the following reaction: