Lava shields and fissure eruptions of the Western Volcanic Zone, Iceland: Evidence for magma chambers and crustal interaction (original) (raw)
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Geochimica et Cosmochimica Acta, 2011
The mechanisms and the timescales of magmatic evolution were investigated for historical lavas from the Askja central volcano in the Dyngjufjö ll volcanic massif, Iceland, using major and trace element and Sr, Nd, and Pb isotopic data, as well as 238 U-230 Th-226 Ra systematics. Lavas from the volcano show marked compositional variation from magnesian basalt through ferrobasalt to rhyolite. In the magnesian basalt-ferrobasalt suite (5-10 wt% MgO), consisting of lavas older than 1875 A.D., 87 Sr/ 86 Sr increases systematically with increasing SiO 2 content; this suite is suggested to have evolved in a magma chamber located at 600MPathroughassimilationandfractionalcrystallization.Ontheotherhand,intheferrobasalt−rhyolitesuite(1−5wt600 MPa through assimilation and fractional crystallization. On the other hand, in the ferrobasalt-rhyolite suite (1-5 wt% MgO), including 1875 A.D. basalt and rhyolite and 20th century lavas, 87 Sr/ 86 Sr tends to decrease slightly with increasing SiO 2 content. It is suggested that a relatively large magma chamber occupied by ferrobasalt magma was present at 600MPathroughassimilationandfractionalcrystallization.Ontheotherhand,intheferrobasalt−rhyolitesuite(1−5wt100 MPa beneath the Ö skjuvatn caldera, and that icelandite and rhyolite magmas were produced by extraction of the less and more evolved interstitial melt, respectively, from the mushy boundary layer along the margin of the ferrobasalt magma chamber, followed by accumulation of the melt to form separate magma bodies. Ferrobasalt and icelandite lavas in the ferrobasalt-rhyolite suite have a significant radioactive disequilibrium in terms of (226 Ra/ 230 Th), and its systematic decrease with magmatic evolution is considered to reflect aging, along with assimilation and fractional crystallization processes. Using a mass-balance model in which simultaneous fractional crystallization, crustal assimilation, and radioactive decay are taken into account, the timescale for the generation of icelandite magma from ferrobasalt was constrained to be <$3 kyr which is largely dependent on Ra crystal-melt partition coefficients we used.
Geological Magazine, 2007
Hekla is a Holocene volcanic ridge in southern Iceland, which is notable for the link between repose periods and the composition of the first-erupted magma. The two largest explosive silicic eruptions, H4 and H3, erupted about 4200 and 3000 years ago. Airfall deposits from these eruptions were sampled in detail and analysed for major and trace elements, along with microprobe analyses of minerals and glasses. Both deposits show compositional variation ranging from 72 % to 56 % SiO2, with mineralogical evidence of equilibrium crystallization in the early erupted rhyolitic component but disequilibrium in the later erupted basaltic andesite component. The eruptions started with production of rhyolitic magma followed by dacitic to basaltic andesite magma. Sparse crystallization of the intermediate magma and predominant reverse zoning of minerals, trending towards a common surface composition, indicate magma mixing between rhyolite and a basaltic andesite end-member. The suggested model i...
Geophysical Research Letters, 1991
Precise measurements of U and Th concentrations and Sr, Th and O isotopes in a suite of samples from the 1783-84 Lakagigar eruption, reveal an extreme homogeneity in the 15 kin3 lava flow. However, geochemical constraints suggest that the quartz-tholeiite magma results from the assimilation of approximately 20% of the lower crust by a mantle derived olivine-tholeiite magma. The constant magma composition thus implies vigorous convection and efficient mixing before eruption, probably in a reservoir at the crustmantle boundary. The same deep reservoir probably also fed a shallow magma chamber below Grimsv6tn, inducing simultaneous activity in both Lakagigar and Grimsv6tn central volcano. Introduction From the 8th of June 1783 and for the following 8 months, about 15 km 3 [Thordarson and Self, 1991] of basaltic lava were erupted from the Lakagigar crater row. This is the most voluminous 'lava eruption that has been historically recorded. The basalt composition is typical of an evolved quartznormative tholeiite, of which about 10% consists of phenocrysts phases: plagioclase (An90-An55), olivine (Fo86-Fo68) and augitic pyroxene [Gr6nvold, 1984; Metrich et al., 199!]. The major element composition of the lava flow is uniform, and is identical to that of tephras from the Grimsv•3tn central volcano [Gr6nvo!d, 1984; Sigurdsson and Sparks, !978; Gr•Snvold and Johannesson, 1984].
Journal of Volcanology and Geothermal Research, 2016
The complex processes occurring in the initial phases of an eruption are often recorded in the products of its opening stage, which are usually characterized by small volume and limited dispersal, and thus generally poorly studied. The 2010 eruption of Eyjafjallajökull (Iceland) represents a unique opportunity for these investigations thanks to the good preservation of tephra deposits within the ice/snow pack. A detailed geochemical investigation on the glassy groundmass of single ash clasts disclosed a population of fragments with unusual high 87 Sr/ 86 Sr (up to 0.70668) for Icelandic magmatism, and anomalous elemental composition with respect to most of the juvenile material of the eruption. This suggests that during its rise, before intruding into the ice cover, magma at a dyke tip selectively assimilated hydrothermal minerals with seawater-related, high-Sr isotopic ratios (zeolites, silica phases, anhydrite) hosted in altered volcanic/epiclastic rocks. According to the observed precursory seismicity, only restricted to few hours before the onset of the eruption, this process could have accompanied subcritical aseismic fracture opening during the days before the eruption, possibly related to stress corrosion-cracking processes, which enhanced the partial dissolution/melting and subsequent selective assimilation of the host rocks. Introduction Studies of explosive eruptions are generally based on the products of their paroxysmal phases, recognised and correlated at the scale of the whole dispersal area (
1998
Miocene to Holocene sediments cored at Sites 918 and 919 in the Irminger Basin contain windblown ash layers derived from Icelandic volcanoes. They represent the most explosive volcanism of the rift system. In contrast, only those eruptions that produced lava flowing far from the rift zone are now exposed on Iceland due to subsidence close to the rift axis. Sediments older than 3.2 Ma contain little fresh glass, due to the hydrating effect of diagenetic fluids, although one basaltic layer dated at 10.5 Ma and three rhyolitic layers dated at 12.1−12.7 Ma were found to contain fresh glass. Major, trace, and rare earth element data have been collected from aphyric glass shards from these ash layers, using a combination of electron and ion microprobe technology. The ashes are dominantly basaltic and tholeiitic in character and are principally derived from the rift zone, with minor input from off-axis sources, probably the Snaefellsnes Peninsula. Their trace element characteristics are consistent with derivation from a mantle source similar to that below Iceland today, although the degree of this enrichment, as modeled by Zr/ Nb, can be seen to be variable over short periods of time and to have greater range than that found in exposed Icelandic lavas of the same age. The variations are not attributable to fractional crystallization. The ashes indicate that either different volcanoes were erupting compositionally variable lavas at any one time or that the composition varied rapidly with time, or both. These variations are homogenized by magma mixing prior to eruption in the large volume flows preserved in the Tertiary lava flows exposed in Iceland. Chemical variation seen in the ash layers may be due to variation in the degree of mantle melting, mantle heterogeneity, or derivation of melt from different depths within the melting column. Melting is thought to have commenced within a garnet-bearing mantle and continued in a spinel-bearing zone.
On the recent bimodal magmatic processes and their rates in the Torfajökull-Veidivötn area, Iceland
2008
Historical bimodal composition eruptions spanning Torfajokull central volcano and neighbouring Veidivotn fissure swarm in southeastern Iceland provide clues about the nature and timing of basalt and rhyolite petrogenesis and eruption at an active divergent plate boundary. This study focuses on lavas and tephras of the last two regional eruptions in 871 and 1477 AD, using samples that approximate mafic and felsic endmember compositions relative to regional mixing trends in literature data from the area. Whole rock and mineral U-Th-Ra isotopic compositions demonstrate both rapid petrogenetic timescales and limited compositional variation in the basalt and rhyolite magma sources. Rhyolites display the greatest (230)Th excesses (<= 17%) at only slightly lower ((230)Th/(232)Th) activity ratios than co-eruptive Veidivotn basalts. Both magma types display small but significant (226)Ra excesses (<= 10% and <= 60% in rhyolites and basalts, respectively). U-series isotopic and trace element data are consistent with crustal melting dominated by young (<= 60 kyr), K-metasomatized mafic protoliths similar in composition to the Veidivotn tholeiites as source of the 871 and 1477 rhyolites. Young protolith ages are inconsistent with published models that call on old materials as source rocks for Icelandic rhyolites (e.g., old silicic segregation tenses of isostatically subsided lavas). Zero-age U-Th and few-ka Ra-Th mineral-whole rock isochrons indicate that crystals formed shortly prior to eruption, consistent with petrographic and compositional indicators that they are phenocrysts, and suggest that the rhyolite melts are of Late Holocene age.
Dynamic magma mixing revealed by the 2010 Eyjafjallajökull eruption
Solid Earth Discussions, 2011
Injection of basaltic magmas into silicic crustal holding chambers and subsequent mixing of the two components is a process that has been recognised since the late seventies to have resulted in explosive eruptions. Detailed reconstruction and assessment of the mixing process caused by such intrusion is now possible because of the excep-5 tional time-sequence sample suite available from the tephra fallout of the 2010 summit eruption at Eyjafjallajökull volcano in South Iceland. From 14 to 19 April the tephra contains three glass types of basaltic, intermediate, and silicic compositions recording rapid magma mingling without homogenisation, involving evolved FeTi-basalt and dacite with composition identical to that produced by the 1821-1823 AD Eyjafjallajökull 10 summit eruption. The time-dependent change in the magma composition suggests a binary mixing process with changing end-member compositions and proportions, or dynamic magma mixing. Beginning of May, a new injection of deep-derived basalt was recorded by deep seismicity, appearance of magnesium-rich olivine phenocrysts together with high sulphur output and presence of sulphide crystals. Thus the compo-15 sition of the basaltic injection became more primitive and hotter with time prowoking changes in the silicic mixing end-member from pre-existing melt to the solid carapace of the magma chamber. Decreasing proportions of the mafic end-member with time in the erupted mixed-magma, demonstrate that injections of Mg-rich basalt was the motor of the 2010 Eyjafjallajökull explosive eruption, and that its decreasing inflow terminated 20 the eruption. Significant quantity of silicic magma is thus still present in the interior of the volcano. Our results show that detailed sampling during the entire eruption was essential for deciphering the complex magmatic processes at play, namely the dynamic magma mixing. Finally, the rapid compositional changes in the eruptive products suggest that magma mingling occurs on a timescale of few hours to days whereas the 25 interval between the first detected magma injection and eruption was several months. 3, 2011
The Origin of Rhyolitic Magmas at Krafla Central Volcano (Iceland)
Journal of Petrology, 2021
We present a detailed petrologic study of rhyolites from seven eruptions spanning the full (∼190 ky) history of rhyolitic volcanism at Krafla volcano, northeast Iceland. The eruptions vary widely in size and style, but all rhyolites are crystal-poor (<6 modal%: plagioclase + augite ± pigeonite ± orthopyroxene ± titanomagnetite ± fayalite) and have similar evolved compositions (73.7–75.8 wt% normalized whole-rock SiO2) and trace element patterns. Macrocryst rim compositions from each eruption cluster within a narrow range and are appropriate for equilibrium with their carrier melt. Crystal cores and interiors display complex growth patterns and commonly host resorption surfaces, but compositional variations are slight (e.g. typically <10 mol% An for plagioclase, Mg# <10 for pyroxene), and consistent with an overall trend of cooling and differentiation by crystal fractionation. Although most crystal core and interior compositions are broadly appropriate for equilibrium with m...
Journal of Volcanology and Geothermal Research, 2007
The 1973 eruption of Eldfell volcano, Iceland, appears to have been a short, simple event, but textural and geochemical evidence suggest that it may have had three different magmatic components. The first-erupted fissure magmas were chemically evolved, rich in plagioclase (∼18%) and had shallow, straight crystal size distribution (CSD) curves. The early lavas were less evolved chemically, had lower plagioclase contents (∼ 13%) and steeper, slightly concave up CSDs. The late lavas were chemically similar to the early lavas, but even richer in plagioclase than the initial magmas (∼ 24%) and had the steepest CSDs. There was no chemical evidence for plagioclase fractionation, but compositional diversity could be produced by clinopyroxene fractionation which must have occurred at depth. We propose that the eruption started with old, coarsened (Ostwald ripened) magma left over from a previous eruption, possibly that which produced Surtsey Island ten years earlier. The early flows may be mixtures of small amounts of this old magma with a new, low crystallinity, uncoarsened magma or a completely new magma. The late flows are another new magma from depth, chemically similar to the early flows, but which has grown plagioclase under increasing saturation (undercooling) perhaps during its ascent. All three magmatic components may have originated from the same parent, but had varying degrees of clinopyroxene fractionation, plagioclase nucleation and growth, and coarsening.