Neovolcanic Zone Of Iceland Research Articles

A multicomponent geothermometer for high-temperature basalt settings

For successful geothermal reservoir exploration, accurate temperature estimation is essential. Since reservoir temperature estimation frequently involves high uncertainties when using conventional solute geothermometers, a new statistical approach is proposed. The focus of this study is on the development of a new multicomponent geothermometer tool which requires a significantly reduced data set compared to existing approaches. The method is validated against reservoir temperature measurements in the Krafla and the Reykjanes geothermal systems. A site-specific basaltic mineral set was selected as the basis to compute the equilibrium temperatures. These high-enthalpy geothermal reservoirs are located in the neo-volcanic zone of Iceland where the fluid temperatures are known to reach up to 350 °C at a depth of 2000 m. During ascent, the fluid composition is prone to changes as well as possible phase segregation due to depressurization and boiling. Therefore, to reduce the uncertainty of temperature estimations, reservoir temperature conditions are numerically reconstructed with sensitivity analyses considering pH, aluminium concentration, and steam loss. The evaluation of the geochemical data and the sensitivity analyses were calculated via a numerical in-house tool called MulT_predict. In all cases, the temperature estimations match with the in situ temperatures measured at Krafla and Reykjanes. The development of this method tends to be a promising and precise tool for reservoir temperature estimation. The developed methodology is a fast and easy-to-handle exploration tool that can be applied to standard geochemical data without the need for a sophisticated gas analysis yet obtaining very accurate results.

Recycling of crustal material by the Iceland mantle plume: New evidence from nitrogen elemental and isotope systematics of subglacial basalts

We report new nitrogen (N2) abundance and isotope (δ15N) data for 43 subglacial basaltic glasses from the neovolcanic zones of Iceland, a key locality in studies of mantle plume geochemistry and crust–mantle processes. New helium and argon abundance and isotope data are also reported to supplement previous studies (Füri et al., 2010; Barry et al., 2014), allowing elemental ratios (e.g., N2/40Ar∗ where 40Ar∗=radiogenic 40Ar) to be calculated. Subglacial basaltic glasses with N2>2μcm3STP/g show a wide range in δ15N values, from −2.91 to +11.96‰ (vs. Air), with values >6‰ only observed at one locality in the Eastern Rift Zone. Elemental ratios involving N2, i.e., N2/3He, and N2/40Ar∗, span several orders of magnitude from 2.5×105 to 9.0×107, and 32.8 to 1.46×106, respectively. In contrast, argon isotope ratios (40Ar/36Ar) are limited, ranging from air-like (∼298.6) values up to 1330. Glasses exhibit a wide range in helium isotope ratios (8–26 RA), with clear distinctions between individual rift segments.A number of processes have extensively modified original mantle source N isotope and relative abundance compositions – most significantly air interaction, crustal contamination in some instances, and possibly degassing-induced fractionation. Under the assumption that the starting 4He/40Ar∗ production ratio of Iceland mantle is identical to the depleted MORB mantle (DMM), a filtering protocol for the entire N dataset, based upon 40Ar/36Ar and 4He/40Ar∗ ratios, was adopted to identify samples with unmodified δ15N values. Consequently, we identify 22 samples that define the Icelandic mantle N-isotope distribution (δ15N=−2.29 to +5.71‰). Using the filtered dataset, we investigate simple binary mixing scenarios involving N2/3He–N2/40Ar∗–δ15N variations to identify mantle end-member compositions. Mixing scenarios are consistent with a recycled component in the Iceland mantle source, defined by a high and heterogeneous δ15N end-member. Moreover, this end-member is coupled to the high 3He/4He signature, and is characterized by He depletion and/or the presence of excess N2. These features strongly suggest the presence of recycled crustal N-component(s) integrated into and/or entrained by the Iceland plume source.These new results reveal the highly heterogeneous nature of nitrogen in the hybrid Iceland plume source, consistent with models based on trace elements and radiogenic isotopes that advocate for significant heterogeneity of recycled crustal component(s) sampled by the Iceland plume. A relatively young age of the recycled crustal material (possibly Phanerozoic) is consistent with the association of positive δ15N values and high N2/40Ar∗ ratios with constraints from radiogenic isotopes (e.g., Pb), thus indicating a relatively short time-interval (∼108years) between subduction of crustal material and entrainment by the Iceland mantle plume.

Resistivity characterization of the Krafla and Hengill geothermal fields through 3D MT inverse modeling

Krafla and Hengill volcanic complexes, located 300km apart, are both known as high-temperature geothermal systems located within neo-volcanic zones of Iceland. This paper demonstrates the utilization of three-dimensional (3D) magnetotelluric (MT) inversions from three different inverse modeling algorithms, which leads to characterizing the electrical resistivity structure of geothermal reservoirs with a much greater level of confidence in accuracy and resolution than if a single algorithm was employed in the data interpretation. These are the first 3D MT inversions of a Krafla MT dataset. The inverted model of electrical resistivity is a classic example of a high-temperature hydrothermal system, with a highly resistive near-surface layer, identified as unaltered porous basalt, overlying a low resistivity cap corresponding to the smectite–zeolite zone. This layer is in turn underlain by a more resistive zone, identified as the epidote–chlorite zone, also called the resistive core, which is often associated with production of geothermal fluids. The electrical structure in the upper 1–2km does not correlate with lithology but with alteration mineralogy. At the location of the IDDP-1 well, which encountered magma at 2.1km depth, the resistivity image shows high resistivity, most likely due to the epidote–chlorite geology and the presence of deeper superheated or supercritical fluids. Two km northwest of the well, however, an intrusive low-resistivity feature is imaged rising from depth, and a plausible interpretation is that of a magma intrusion. One possible explanation for the magma encounter at the IDDP-1 well is the existence of pathways or fissures connected to the magma chamber and intersected by the well. The MT response to these magma pathways is not discernible in the existing data, perhaps because this magma volume is below the threshold of resolvability. The electrical resistivity structure of the Hengill geothermal area also reveals characteristic features of a high temperature geothermal system with two low-resistivity layers. The nature of the uppermost low-resistivity layer and the increasing resistivity below it is attributed to hydrothermal mineral alteration, while the nature of the deep low-resistivity layer, centered over the northeast, is not yet well understood. The geothermal system in the northeast area appears to be shallower than the system manifested in the southwest. 3D MT inversions of Krafla and Hengill data sets show that knowledge of the subsurface electrical resistivity contributes substantially to a better understanding of complex geothermal systems.

Concurrent Mixing and Cooling of Melts under Iceland

The compositions of 75 melt inclusions, their host olivines and 49 whole-rock samples of their carrier lavas have been determined. These compositions were added to a compilation of the trace element composition of 243 melt inclusions from 10 eruptions in the neovolcanic zones of Iceland and used to investigate melt mixing processes. Whereas the compositional variability of whole-rock samples from single eruptions is limited, there are significant compositional differences between eruptions. The compositions of inclusions are more variable than those of whole-rock samples of their carrier lava. On a flow-by-flow basis, the average composition of trace element ratios such as La/Yb in the inclusions is similar to that of their carrier lava. These observations indicate that, for each lava flow, melts with compositions similar to those of the inclusions crystallized and mixed to produce the magma that transported the host olivines to the surface. Although many of the olivines are not in Mg–Fe equilibrium with their host melt, they are not accidental xenocrysts because they crystallized from melts similar to those that mixed to form the carrier magma. The trace element variability of melt inclusions drops with decreasing forsterite content of the host olivine. This relationship is observed both within single flows and in the compilation of data from 10 flows. Concurrent mixing and crystallization dominate the compositional evolution of basaltic melts in lower crustal magma chambers. This coupled mixing and cooling is likely to result from convective motions in magma chambers. The rate of change of the mixing parameter, M, with temperature of the melt is dM/dT = 0·0094 ± 0·0036 per °C. This relative rate may be used to constrain the fluid dynamics of basaltic magma chambers.

Glacial and volcanic history of Icelandic table mountains from cosmogenic 3He exposure ages

Basaltic table mountains in the neovolcanic zones of Iceland have been interpreted as subglacial volcanoes that emerged through an ice sheet. Using their distinctive morphological and lithostratigraphic characteristics, the approximate surface elevation and thickness of the ice sheet at the time of eruption can be determined. We measured cosmogenic 3He concentrations in olivine phenocrysts from subaerially erupted basaltic lava caps of table mountains to determine their exposure ages. We argue that these exposure ages closely approximate eruption ages; the possibility of past snow cover is the main uncertainty. The resulting 3He exposure (eruption) ages, calculated using a locally derived 3He production rate calibration, and comprising 42 individual ages from 13 table mountains, allow reconstruction of ice sheet surface profiles through time. The new 3He chronology indicates that 12 of the 13 dated table mountains experienced their final eruptive phase during the last deglaciation. This eruptive chronology is broadly consistent with the hypothesis that melt production in Iceland is enhanced by pressure release from ice sheet unloading during deglaciation. The clustered distribution of table mountain ages suggests that distinct episodes of ice sheet thinning may have coincided with, or closely followed, the two strongest warming events in the North Atlantic region during the last deglaciation: the Bølling warming (ca 14.5 ka) and the warming at the end of the Younger Dryas.

Geometry and mode of emplacement of the Thverartindur cone sheet swarm, SE Iceland

The exposed interior of the Thverartindur igneous centre is dominated by a dense circumferential swarm of centrally inclined sheets. The swarm is hosted by a stratified lava–hyaloclastite sequence that is tectonically rotated into a rift-parallel monocline, shown by an increased degree of tilting (∼5–25°) towards the neovolcanic zone of Iceland. In this paper, measured orientations and thicknesses of a total of 745 mafic cone sheets from nine sampling locations are statistically constrained and spatially analysed in both their present configuration and corrected for a uniform 13°NW tilt of the swarm. Eight sheet dip trajectories, back-rotated to different levels of erosion, indicate a bowl- and slightly fan-shaped swarm geometry that converges onto a common magma source located ∼3 km below the pre-erosional surface; a geometry that bears striking resemblance to Gudmundsson's [J. Volc. Geotherm. Res. 35 (1998) 179–194] boundary element model of the maximum compressive stress field around a pressurised sphere. An estimated ∼7.2-km-wide and subcircular model source zone at such shallow crustal depth predicts a slightly flattened and 4.0±0.5-km-thick spheroidal magma chamber with a maximum volume of 140±50 km 3. Marked declines in sheet densities along the inner and outer margins of the swarm suggest that most sheets originated from a relatively narrow source zone, compared to the swarm's estimated ∼2–3 km width at the surface. Improved statistical analysis, furthermore, reveals systematic spatial variations in sheet thickness distributions which behave in accordance with the bowl-shaped geometry for the Thverartindur swarm. Thus, a significant ∼0.1 m/km decrease in the average sheet thickness up through the swarm is thought to reflect an overall upward narrowing of sheets, while a relatively low number of <0.5-m-thick sheets in the uppermost part of the swarm arguably reflects the subsurface thermal arrest of more than a third of all sheets injected from the model source.

Volcanism on the Rockall continental margin

Continental break-up in the northern North Atlantic was accompanied by massive volcanism. This produced sequences of basalt flows erupted close to sea-level, which are imaged as seaward-dipping reflectors on seismic reflection profiles. They comprise the upper 4-5 km of transitional crust in both the Edoras Bank and Hatton Bank areas of the Rockall continental margin. Detailed surveys show that the reflectors are convex-upward on dip lines but are sub-horizontal on strike lines. They are probably produced by aerially extensive basalt flows extruded from linear fissure vents aligned parallel to the rift axis. Along the Rockall margin two sequences of seaward-dipping reflectors occur, separated by an 'outer high' near the foot of the continental margin. The landward set of basalt flows are emplaced above stretched and intruded continental crust, while the seaward set span the region from highly distended continental crust to fully oceanic crust. The outer high is capped by basalt flows but exhibits little coherent internal reflectivity. We discuss possible origins of the outer high as a residual block of thinned and intruded continental crust or as a central volcano cut by fissure vents similar to those found in the neovolcanic zones of Iceland.

Helium isotopic systematics within the neovolcanic zones of Iceland

Helium isotopic compositions have been measured in a suite of sub-glacial basaltic glasses from the neovolcanic zones of Iceland. The 3He/4He ratios vary between 8 and 26.2× atmospheric; the higher ratios are generally consistent with a relatively primitive undegassed mantle source for the helium. Several sampling tests indicate that sub-glacial basaltic glasses retain a representative sample of magmatic helium, much like mid-ocean ridge basalts. The overall variations, high observed helium concentrations (up to 1.6×10−6 cm3 STP/g), and young age of Icelandic crust argue strongly against crustal contamination as a primary control of helium isotopes in the basaltic lavas. Each volcanic zone has a distinctive basaltic 3He/4He isotopic signature: 18–26.2× atmospheric in the eastern volcanic zone and central Iceland, 12–16× atmospheric in the western volcanic zone, and 8–11 in the northeastern volcanic zone. The limited range of 3He/4He ratios within each zone, variability between volcanic zones in 3He/4He, and temporal aspects of Icelandic volcanism are interpreted within the framework of a plume model for the mantle beneath Iceland. The preferred model involves variable contributions from a plume source (i.e. undegassed) to each of the Icelandic volcanic zones. The fact that the highest 3He/4He ratios are observed in eastern and central Iceland suggests that volcanism in these areas has the greatest contribution from the plume.

Anomalous focal mechanisms: tensile crack formation on an accreting plate boundary

Conventional double-couple solutions do not satisfactorily account for many small magnitude earthquakes in the Hengill geothermal area in the Neovolcanic Zone of Iceland. The far-field radiation pattern is predominantly compressional but a few dilatational arrivals have been recorded. We prefer the interpretation that these events are due to tensile crack formation at a shallow (1–7 km) depth within a cooling intrusive body. Volume considerations indicate that initial fracturing of the rock is seismic but that subsequent widening proceeds aseismically. Other reports of anomalous radiation patterns from earthquakes on the Mid-Atlantic Ridge describe events of a similar type, that is, exhibiting predominantly compressional radiation patterns and a reduced dilatational field1–4.

Spatial and temporal variation in the trace element geochemistry of the Eastern Iceland Flood Basalt Succession

Concentrations of Zr, Y, Nb and rare earth elements (REE) are reported for basaltic lavas from the Tertiary to Recent flood basalt succession of eastern Iceland. The samples are drawn from a series of over-lapping vertical profiles through the westward dipping succession. The trace element results show that more primitive (low Zr, Y, Nb and REE) lavas are more abundant in the topographically higher parts of the lava pile. This part of the pile consists of the attenuated edges of overlapping lava units, which thicken as they are traced westward down-dip. The results are thus interpreted in terms of the spatial variation in the chemistry of individual lava units. Lavas erupted from near the spread-center for each eruptive unit are commonly more evolved Variation of this type is observed at the present day in the neovolcanic zone of Iceland and also controlled the pattern of volcanism in at least two other extensional igneous provinces, Skye and Troodos. In assessing temporal variation in chemistry of the erupted lavas, allowances must be made for the primary spatial variation noted above and the secondary chemical effects of the varying degrees of zeolitization. When these allowances are made, there appears to have been no systematic change in the chemistry of the flows with time.

Rare-earths in Icelandic neovolcanic rocks

Seven volcanic rocks from neovolcanic zone of Iceland were analysed for rare-earths (RE). The bulk partition coefficients are estimated from combination of the studied samples. In contrast with the mid-ocean ridge basalts which show the solid-type RE patterns, four of five arbitrarily chosen Icelandic basalts appear to have liquid-type RE patterns. The occurrence of relatively pristine lava with relative unfractionated RE abundances to chondrite is also considered to be associated with the peculiar tectonics of Iceland.

On heat flow in Iceland

Data on subsurface temperature conditions from borehole measurements in various parts of Iceland were presented. The subsurface temperature field is in many areas found to be controlled by the movement of water in the upper part of the crust, and so is also the surface heat flow by conduction. Data were presented to show that conditions for free convection of water may well exist in the neovolcanic zone of Iceland. It is suggested that this phenomenon may account for the anomalous heat flow pattern observed on the Mid-Atlantic Ridge. For a more detailed account of this paper the reader is referred to the following: Gudmar Palmason, 1967,On heat flow in Iceland in relation to the Mid-Atlantic Ridge. In:Iceland and mid-ocean ridges (Ed.S. Bjornsson), Vis. Isl. (Societas Scientiarum Islandica), Reykjavik.

Some problems of volcanism in Iceland

Until two decades or so ago the petrography of Iceland seemed to present a rather simple picture. About 99 per cent of the rocks were regarded as basaltic, 1 per cent as rhyolitic and rocks of intermediate composition as nearly non-existent. Recent research has changed this picture. Central volcanoes producing acid and intermediate lava and tephra have played a considerable role both in the Tertiary and Quaternary. Contrary to the lava massproducing basalt volcanoes, fed by a deep-seated and rather uniform basalt magma source, the differentiated central volcanoes are normally fed by separated magma chambers at smaller depth, where differentiation takes place between the eruptions, resulting in intermediate and acid products which bridge the “immiscibility gap” between the basalts and the rhyolites. Eight to ten per cent of the postglacial lavas and tephras and about 25 per cent of the historical ones are acid or intermediate. Opinions still differ as to whether the existence of these rocks can be explained solely as the result of gravitational differentiation of parent basalt magma or has to be explained partly by a remelting of a continental layer beneath the plateau-basalts. There are, however, some rather strong geological evidences in favour of the existence of such a layer and they seem at the moment not to be contradicted by geophysical research. The Surtsey eruption has demonstrated the close relation between the craterrows, shieldvolcanoes and tablemountains and convincingly shown how greatly external circumstances may influence volcanic activity and the shape of a volcano. The eruption focused interest on the neovolcanic zone of Iceland as a supra marine part of the Mid-Atlantic rift-ridge zone, where volcanic activity has taken place under similar conditions as along its submarine parts, viz. under water (glacial meltwater) when the Pleistocene ice-sheets blanketed the volcanic activity. Similar morphological types of volcanoes can therefore be expected in both cases, viz. steepsided mounds and long steepsided ridges.