Source of Ash Zone 1 in the North Atlantic     

C. Lacasse  H. Sigurdsson  H. Jóhannesson  M. Paterne  S. Carey 《Bulletin of Volcanology》1995,57(1):18-32

Geochemical evidence shows that the silicic component of the widespread Ash Zone 1 in the North Atlantic is derived from a major ignimbrite-forming eruption which occurred at the Katla caldera in southern Iceland during the transition from glacial to interglacial conditions in Younger Dryas time. Both trace and major element evidence of the rhyolitic products excludes the Öræfajökull volcano as a source. The high-Ti basaltic component in the marine ash zone can also be attributed to contemporaneous eruption in the Katla volcanic complex. Dispersal of tephra from this event is primarily attributed to the generation of co-ignimbrite ash columns in the atmosphere, with ash fallout on both sea ice and on the ocean floor north and east of Iceland. Owing to the changing ocean circulation characteristics of the glacial regime, including suppression of the Irminger Current and a stronger North Atlantic Current, tephra was rafted on sea ice south into the central North Atlantic and deposited as dispersed Ash Zone 1. Sediments south of Iceland also show evidence of the formation of ash turbidites, generated either by the entrance of pyroclastic flows into the sea, or during discharge of jökulhlaups or glacier bursts from this subglacial eruption.  相似文献   

The Small Unmanned Meteorological Observer SUMO: Recent developments and applications of a micro-UAS for atmospheric boundary layer research   总被引：1，自引：0，他引：1  

Joachim Reuder  Marius O. Jonassen  Haraldur Ólafsson 《Acta Geophysica》2012,60(5):1454-1473

During the last 5 years, the Small Unmanned Meteorological Observer SUMO has been developed as a flexible tool for atmospheric boundary layer (ABL) research to be operated as sounding system for the lowest 4 km of the atmosphere. Recently two main technical improvements have been accomplished. The integration of an inertial measurement unit (IMU) into the Paparazzi autopilot system has expanded the environmental conditions for SUMO operation. The implementation of a 5-hole probe for determining the 3D flow vector with 100 Hz resolution and a faster temperature sensor has enhanced the measurement capabilities.  相似文献   

To the origin of Icelandic rhyolites: insights from partially melted leucocratic xenoliths     

Andrey A. Gurenko  Ilya N. Bindeman  Ingvar A. Sigurdsson 《Contributions to Mineralogy and Petrology》2015,169(5):1-21

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Blue straggler stars in dwarf spheroidal galaxies – II. Sculptor and Fornax     

M. Mapelli  E. Ripamonti  G. Battaglia  E. Tolstoy  M. J. Irwin  B. Moore  S. Sigurdsson 《Monthly notices of the Royal Astronomical Society》2009,396(3):1771-1782

The existence of blue straggler stars (BSSs) in dwarf spheroidal galaxies (dSphs) is still an open question. In fact, many BSS candidates have been observed in the Local Group dSphs, but it is unclear whether they are real BSSs or young stars. Shedding light on the nature of these BSS candidates is crucial in order to understand the star formation history of dSphs. In this paper, we consider BSS candidates in Sculptor and Fornax. In Fornax, there are strong hints that the BSS population is contaminated by young stars, whereas in Sculptor there is no clear evidence of recent star formation. We derive the radial and luminosity distribution of BSS candidates from wide field imaging data extending beyond the nominal tidal radius of these galaxies. The observations are compared with the radial distribution of BSSs expected from dynamical simulations. In Sculptor, the radial distribution of BSS candidates is consistent with that of red horizontal branch (RHB) stars and is in agreement with theoretical expectations for BSSs generated via mass transfer in binaries. On the contrary, in Fornax, the radial distribution of BSS candidates is more concentrated than that of all the considered stellar populations. This result supports the hypothesis that most of BSS candidates in Fornax are young stars, and this is consistent with previous studies.  相似文献   

An Angus/Argo study of the neovolcanic zone along the East Pacific rise from the Clipperton fracture zone to 12°N     

E. Uchupi  W. C. Schwab  R. D. Ballard  J. L. Cheminee  J. Francheteau  R. Hekinian  D. K. Blackman  H. Sigurdsson 《Geo-Marine Letters》1988,8(3):131-138

Still photographs and video images collected along the Neovolcanic Zone of the East Pacific Rise from 10°15′N to 11°53′N show that recent volcanic sheet flows, possibly less than 100 years old, are superimposed on an older sediment-laden pillow terrane. This recent activity is restricted to a narrow zone that crosses two topographic highs at 10°55′N and 11°26′N and diminishes along-axis away from these highs. The association of recent sheet flows with older flows and collapse structures on the overlapping spreading centers at 11°45′N supports the evolutionary model for the occurrence and evolution of overlapping spreading centers by MacDonald and others (1986, 1988).  相似文献   

Volcanogenic sedimentation in the Iceland Basin: influence of subaerial and subglacial eruptions     

Christian Lacasse  Steven Carey  Haraldur Sigurdsson 《Journal of Volcanology and Geothermal Research》1998,83(1-2)

Cores recovered from the Iceland Basin show evidence of transport and deposition of volcaniclastic sediment from the Eastern Volcanic Zone of Iceland during the Holocene and last glacial period. Three types of deposits have been identified: tephra fall, sediment gravity flows, and bottom-current-controlled deposits. Tephra fall layers contain basaltic glass of composition that suggests Katla volcano as the major source. A chronology of the volcano activity is reconstructed, back to isotopic stage 5d (120,000 yr). Glass chemistry of tephra in sediment gravity flows deposited south of Myrdalsjökull Canyon indicates a source in the Grímsvötn–Lakagígar volcanic system. These volcaniclastic gravity flows were most likely derived from jökulhlaups or large glacial floods, at a time of a more extensive ice cover over the volcanic zone. Deposition of the sediment gravity flows has created a deep-sea fan south of the canyon. Basalt glass composition, age, and depositional environment suggest that one early Holocene turbidite sequence was derived from a large jökulhlaup of the Grímsvötn area. The volcanogenic sediment gravity flows were influenced by a strong contour current, moving across the Katla sediment ridges. The contour current has winnowed the silt fraction and transported it downstream as suspended load. The recovery of numerous silty volcaniclastic layers, enriched in detrital crystals, indicates that they contributed to the sedimentation of contourite drifts.  相似文献   

Two serendipitous low-mass LMC clusters discovered with HST1     

Basílio X. Santiago  Rebecca A. W. Elson  Steinn Sigurdsson  & Gerard F. Gilmore 《Monthly notices of the Royal Astronomical Society》1998,295(4):860-868

We present V and I photometry of two open clusters in the LMC down to V ∼26. The clusters were imaged with the Wide Field and Planetary Camera 2 (WFPC2) on board the Hubble Space Telescope ( HST ), as part of the Medium Deep Survey Key Project. Both are low-luminosity ( M_V ∼−3.5), low-mass ( M ∼10³ M⊙) systems. The chance discovery of these two clusters in two parallel WFPC2 fields suggests a significant incompleteness in the LMC cluster census near the bar. One of the clusters is roughly elliptical and compact, with a steep light profile, a central surface brightness μ _V (0)∼20.2 mag arcsec⁻², a half-light radius r _hl∼0.9 pc (total visual major diameter D ∼3 pc) and an estimated mass M ∼1500 M⊙. From the colour–magnitude diagram and isochrone fits we estimate its age as τ∼(2–5)×10⁸ yr. Its mass function has a fitted slope of Γ=Δlogφ( M )/Δlog M =−1.8±0.7 in the range probed (0.9≲ M /M⊙≲4.5). The other cluster is more irregular and sparse, having shallower density and surface brightness profiles. We obtain Γ=−1.2±0.4, and estimate its mass as M ∼400 M⊙. A derived upper limit for its age is τ≲5×10⁸ yr. Both clusters have mass functions with slopes similar to that of R136, a massive LMC cluster, for which HST results indicate Γ∼−1.2. They also seem to be relaxed in their cores and well contained in their tidal radii.  相似文献   

Rifting episode in north iceland in 1874–1875 and the eruptions of askja and sveinagja     

H. Sigurdsson  R. S. J. Sparks 《Bulletin of Volcanology》1978,41(3):149-167

In 1874 and 1875 the fissure swarm of Askja central volcano was activated during a major rifting episode. This rifting resulted in a fissure eruption of 0.3 km³ basaltic magma in Sveinagja graben, 50 to 70 km north of Askja and subsequent caldera collapse forming the Oskjuvatn caldera within the main Askja caldera. Five weeks after initial collapse, an explosive mixed magma eruption took place in Askja. On the basis of matching chemistry, synchronous activity and parallels with other rifted central volcanoes, the events in Askja and its lissure swarm are attributed to rise of basaltic magma into a high-level reservoir in the central volcano, subsequent rifting of the reservoir and lateral flow magma within the fissure swarm to emerge in the Sveinagja eruption. This lateral draining of the Askja reservoir is the most plausible cause for caldera collpse. The Sveinagja basalt belong to the group of evolved tholejites characteristie of several Icelandic central volcanoes and associated fissure swarms. Such tholeiites, with Mgvalues in the 40 to 50 tange, represent magmas which have suffered extensive fractional crystallization within the crust. The 12% porphyritic Sveinagja basalt contains phenocrysts of olivine (Fo_62–67), plagioclase (An_57–62), clinopyroxene (Wo₃₈En₄₆Wo₁₆) and titanomagnetite. Extrusion temperature of the lava, calculated on the basis of olivine and plagioclase geothermometry, is found to be close to 1150°C.  相似文献   

Geochemistry of the Lesser Antilles volcanic island arc   总被引：1，自引：0，他引：1  

G.M Brown  J.G Holland  H Sigurdsson  J.F Tomblin  R.J Arculus 《Geochimica et cosmochimica acta》1977,41(6):785-801

New analyses of 1518 rocks for major and certain trace elements are used to examine chemical variations between the 15 larger volcanic islands of the Lesser Antilles island arc. The depth to the top of the subduction zone dipping westward at about 40° lies about 100km below all the volcanoes of the arc. Most of the sampled eruptions are post-Miocene (5-1 m.y.) although south of Martinique, the Oligocene-Miocene and the younger arc are superimposed.There is a chemical variation along the arc axis, from alkalic (southern) through calc-alkalic (central) to tholeiitic (northern) volcanic suites. Three islands are examined in detail as type examples of this variation, i.e. Grenada (south), Dominica (centre), and St. Kitts (north). The Grenada suite includes basanites, alkalic basalts, and subalkalic basalts, andesites and dacites. The subalkalic basalts, andesites and dacites each fall into three chemical groupings along the axis of the arc, distinguished especially by K, Zr, Ni and Cr abundances. The whole Lesser Antilles assemblage is characterised by low K abundances and low K/Rb ratios, compared with other island arcs.The magmas are believed to have evolved through processes of partial melting and crystal fractionation. Partial melting of garnet Iherzolite at about 100km depth in a relatively ‘fertile’ zone of upper mantle in the southern sector, above the subducted slab of basaltic ocean crust, could have produced the undersaturated alkalic magmas. In the central and northern sectors, where the crustal structures are more complex, partial melting may have occurred within more ‘barren’ upper mantle, to produce tholeiitic and calc-alkalic magmas depleted in certain trace elements. In either case, water was probably added to the melted zone from the subducted and hydrated oceanic crust, since the whole arc assemblage was erupted explosively and the rocks are rich in A1₂O₃, plagioclase is very calcic, and amphibole is an important phase. The second process was crystal fractionation at low pressure, as evidenced by the abundance of cumulate xenoliths. Separating phases for the southern volcanoes were olivine, calcic augite and Cr-spinel, followed by hornblende, anorthite and Ti-magnetite at lower temperatures. There is little evidence of the higher-temperature fractionation controls for the central and northern volcanoes.  相似文献   

Helium isotopic systematics within the neovolcanic zones of Iceland     

Mark D. Kurz  Peter S. Meyer  Haraldur Sigurdsson 《Earth and Planetary Science Letters》1985,74(4):291-305

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