Niels Oskarsson

Ash layers from Iceland in the Greenland GRIP ice core correlated with oceanic and land sediments

Four previously known ash layers (Ash Zones I and II, Saksunarvatn and the Settlement layer) all originating in Iceland, have been identified in the Central Greenland ice core GRIP. This correlation of the ash between the different environments is achieved by comparison of the chemical composition of glass shards from the ash. This establishes and confirms detailed correlations between the different types of depositional records and the absolute dating of the younger part of the ice core by counting annual layers dates the eruptions accurately. A precise connection with dates obtained by14C beyond the range of dendrochronology is established which provides an excellent confirmation of230Th-234U dates from corals. Four additional Icelandic ash layers have also been identified in the core but not yet correlated with known ash deposits.

The heterogeneous Iceland plume: Nd‐Sr‐O isotopes and trace element constraints

We present a comprehensive set of Sr, Nd, and O isotope data and trace element concentrations from tholeiitic and alkaline lavas of the neovolcanic zones of Iceland (picrites, olivine and quartz tholeiites, transitional and alkali basalts, differentiated rocks). Variations in the oxygen isotope results allow us to distinguish two groups. The first, which comprises quartz tholeiites and more differentiated rocks usually associated with central volcanoes, has low δ18O values (+5 to +1‰) resulting from interaction with the hydrothermally altered Icelandic crust. The second group, which contains picrites, olivine tholeiites, and alkali basalts, has normal mantle oxygen isotopic compositions (δ18O = +5 to +6‰) which are thought to represent those of the mantle source. Nd isotopic compositions vary greatly, from 143Nd/144Nd = 0.51314 in picrites to 0.51295 in alkali basalts. To produce such a variation for rocks with the chemical compositions of Icelandic volcanics (147Sm/144Nd = 0.12=0.28) requires >200 m.y., a period that greatly exceeds the maximum age of Icelandic crust. Previous models, in which the Sr isotopic variations were explained in terms of evolution of crustal reservoirs, are invalidated, and mantle reservoirs with different Nd and Sr isotopic compositions are indicated. The Iceland data define a linear array in the Sr-Nd isotope diagram which overlaps both mid-ocean ridge basalt and oceanic island basalt fields and indicates mixing between depleted and enriched end-members. Alkali basalts come preferentially from an isotopically and chemically enriched component of the Iceland plume, and picrites come from a more refractory, more depleted portion. Positive Sr, Rb, and Ba anomalies are present in picrites and other lavas with low trace element contents. These anomalies are not correlated with isotopic differences but are nevertheless believed to result from interaction between the parent magmas of these rocks and altered Icelandic crust. This indicates that even the most primitive Icelandic lavas have been contaminated with some crustal material.

Intrusion triggering of the 2010 Eyjafjallajökull explosive eruption

Gradual inflation of magma chambers often precedes eruptions at highly active volcanoes. During such eruptions, rapid deflation occurs as magma flows out and pressure is reduced. Less is known about the deformation style at moderately active volcanoes, such as Eyjafjallajökull, Iceland, where an explosive summit eruption of trachyandesite beginning on 14 April 2010 caused exceptional disruption to air traffic, closing airspace over much of Europe for days. This eruption was preceded by an effusive flank eruption of basalt from 20 March to 12 April 2010. The 2010 eruptions are the culmination of 18 years of intermittent volcanic unrest. Here we show that deformation associated with the eruptions was unusual because it did not relate to pressure changes within a single magma chamber. Deformation was rapid before the first eruption (>5 mm per day after 4 March), but negligible during it. Lack of distinct co-eruptive deflation indicates that the net volume of magma drained from shallow depth during this eruption was small; rather, magma flowed from considerable depth. Before the eruption, a ∼0.05 km3 magmatic intrusion grew over a period of three months, in a temporally and spatially complex manner, as revealed by GPS (Global Positioning System) geodetic measurements and interferometric analysis of satellite radar images. The second eruption occurred within the ice-capped caldera of the volcano, with explosivity amplified by magma–ice interaction. Gradual contraction of a source, distinct from the pre-eruptive inflation sources, is evident from geodetic data. Eyjafjallajökull’s behaviour can be attributed to its off-rift setting with a ‘cold’ subsurface structure and limited magma at shallow depth, as may be typical for moderately active volcanoes. Clear signs of volcanic unrest signals over years to weeks may indicate reawakening of such volcanoes, whereas immediate short-term eruption precursors may be subtle and difficult to detect.

Helium, oxygen, strontium and neodymium isotopic relationships in Icelandic volcanics

Abstract 3He/4He ratios have been obtained for basaltic, intermediate and acid volcanic glasses from Iceland. Basaltic glasses exhibit a wide range of 3He/4He ratios (4 3He/4He does not correlate with either 87Sr/86Sr or 143Nd/144Nd ratio and radiogenic components of He, Sr and Nd have apparently been decoupled. Interaction of Icelandic magmas with hydrothermally altered and older Icelandic crust is the preferred explanation for variable and often low δ18O values. It is suggested that primary 3He/4He ratios may have been modified by incorporation of radiogenic helium developed within the Icelandic crust to impose a larger range of 3He/4He ratios on the erupted products than was actually inherited from the mantle beneath Iceland. Intermediate and acid samples have all been severely contaminated by atmospheric helium, most probably at very shallow levels within the crust.

The interaction between volcanic gases and tephra: Fluorine adhering to tephra of the 1970 hekla eruption

Abstract The mass distribution and sorting of tephra produced in the plinian phase of the 1970 Hekla eruption was controlled by the particle size distribution, the height of the eruption column, and velocity of transport. Near the volcano the mass distribution of soluble fluorine was controlled by particle size of the deposits, but approaches the mass distribution of the tephra at longer distances. Adsorbed soluble fluorine reaches a maximum at a distance from the volcano determined by the velocity of the transporting medium. SEM studies show the soluble fluorine to be chemically adsorbed on the surface of tephra particles. The adsorption is shown by experiment to occur at temperatures below 600°C in the cooling eruption column. Evaluation of reactions in the eruption column leads to the conclusion that formation of water soluble compounds adhering to tephra is principally controlled by environmental factors and to a lesser degree by the composition of the volcanic gas phase.

Fertilizing potential of volcanic ash in ocean surface water

The fertilization potential of newly erupted and well-preserved ash from the 2000 Hekla eruption in Iceland was measured for the first time by flow-through experiments. As previously shown, (1) the North Atlantic Ocean, including the subarctic seas surrounding Iceland, is the largest net sink of the world’s oceans for atmospheric CO 2, owing to biological drawdown during summer; (2) almost complete consumption of phosphate in chlorophyll-rich areas of the North Atlantic Ocean might limit primary production; and (3) in the southern Pacific Ocean and parts of the equatorial Pacific Ocean iron might limit primary production. We found through laboratory experiments that volcanic ash exposed to seawater initially releases large amounts of adsorbed phosphate, 1.7 mmol·g 21 ·h 21 ; iron, 37.0 mmol·g 21 ·h 21 ; silica, 49.5 mmol·g 21 h 21 ; and manganese, 1.7 mmol·g 21 ·h 21 . Dissolution of acid aerosols adsorbed to the surface of the ash caused the high initial release of major and trace elements. Because of the instantaneous dissolution of adsorbed components when newly erupted volcanic ash comes in contact with the ocean surface water, macronutrients and ‘‘bioactive’’ trace metals are released fast enough to become available to support primary production.

Characterization of Eyjafjallajökull volcanic ash particles and a protocol for rapid risk assessment

On April 14, 2010, when meltwaters from the Eyjafjallajökull glacier mixed with hot magma, an explosive eruption sent unusually fine-grained ash into the jet stream. It quickly dispersed over Europe. Previous airplane encounters with ash resulted in sandblasted windows and particles melted inside jet engines, causing them to fail. Therefore, air traffic was grounded for several days. Concerns also arose about health risks from fallout, because ash can transport acids as well as toxic compounds, such as fluoride, aluminum, and arsenic. Studies on ash are usually made on material collected far from the source, where it could have mixed with other atmospheric particles, or after exposure to water as rain or fog, which would alter surface composition. For this study, a unique set of dry ash samples was collected immediately after the explosive event and compared with fresh ash from a later, more typical eruption. Using nanotechniques, custom-designed for studying natural materials, we explored the physical and chemical nature of the ash to determine if fears about health and safety were justified and we developed a protocol that will serve for assessing risks during a future event. On single particles, we identified the composition of nanometer scale salt coatings and measured the mass of adsorbed salts with picogram resolution. The particles of explosive ash that reached Europe in the jet stream were especially sharp and abrasive over their entire size range, from submillimeter to tens of nanometers. Edges remained sharp even after a couple of weeks of abrasion in stirred water suspensions.

The 1991 eruption of Hekla, Iceland

The eruption that started in the Hekla volcano in South Iceland on 17 January 1991, and came to an end on 11 March, produced mainly andesitic lava. This lava covers 23 km2 and has an estimated volume of 0.15 km3. This is the third eruption in only 20 years, whereas the average repose period since 1104 is 55 years. Earthquakes, as well as a strain pulse recorded by borehole strainmeters, occurred less than half an hour before the start of the eruption. The initial plinian phase was very short-lived, producing a total of only 0.02 km3 of tephra. The eruption cloud attained 11.5 km in height in only 10 min, but it became detached from the volcano a few hours later. Several fissures were active during the first day of the eruption, including a part of the summit fissure. By the second day, however, the activity was already essentially limited to that segment of the principal fissure where the main crater subsequently formed. The average effusion rate during the first two days of the eruption was about 800 m3 s−1. After this peak, the effusion rate declined rapidly to 10–20 m3 s−1, then more slowly to 1 m3 s−1, and remained at 1–12 m3 s−1 until the end of the eruption. Site observations near the main crater suggest that the intensity of the volcanic tremor varied directly with the force of the eruption. A notable rise in the fluorine concentration of riverwater in the vicinity of the eruptive fissures occurred on the 5th day of the eruption, but it levelled off on the 6th day and then remained essentially constant. The volume and initial silica content of the lava and tephra, the explosivity and effusion rate during the earliest stage of the eruption, as well as the magnitude attained by the associated earthquakes, support earlier suggestions that these parameters are positively related to the length of the preceeding repose period. The chemical difference between the eruptive material of Hekla itself and the lavas erupted in its vicinity can be explained in terms of a density-stratified magma reservoir located at the bottom of the crust. We propose that the shape of this reservoir, its location at the west margin of a propagating rift, and its association with a crustal weakness, all contribute to the high eruption frequency of Hekla.

Origin of silicic magma in Iceland revealed by Th isotopes

Th, Sr, Nd, and O isotopes have been determined in a suite of volcanic rocks from Hekla and in a few samples from Askja and Krafla volcanic centers in Iceland. Although {sup 87}Sr/{sup 86}Sr and {sup 143}Nd/{sup 144}Nd ratios are nearly the same for all compositions at Hekla, the ({sup 230}Th/{sup 232}Th) ratios differ and thus clearly show that the silicic rocks cannot be derived from fractional crystallization of a more primitive magma. Similar results are obtained for the Krafla and Askja volcanic centers, where the {delta}{sup 18}O values are much lower in the silicic magma than in the mafic magma. These data suggest that large volumes of silicic rocks in central volcanoes of the neovolcanic zones in Iceland are produced by partial melting of the underlying crust.

Thorium, strontium and oxygen isotopic geochemistry in recent tholeiites from Iceland: crustal influence on mantle-derived magmas

We present new Sr, Th, and O isotopic results on recent olivine tholeiites and quartz tholeiites from the active rift zones in Iceland. Our data show a negative correlation between 87Sr/86Sr ratios and δ18O values, and a positive correlation between (230Th/232Th) ratios and δ18O. These covariations of isotopic ratios strongly support the model proposed by Oskarsson et al. (1982) [1]; which involves a mixing of mantle-derived magmas with crustal silicic magmas produced by melting of old hydrothermally altered rocks under central volcanoes. The hybrid magmas have quartz tholeiite compositions. Thus a large part of the Iceland geochemical anomaly is probably inherited from interaction of the primitive magmas (P-type tholeiites) with the Icelandic crust, and the mantle isotopic heterogeneities in the Icelandic “mantle plume” seem to be rather small.