CONTEXT OF ASKJA

Studies of postglacial and historic lava flows around Askja have included some scattered bulk rock compositional data (Kuritani et al., 2011; Thorarinsson and Sigvaldason, 1962). However, the largest dataset of XRF geochemical data on lavas in the Askja area was collected by D. McGarvie in the 1980s.

These data include Holocene basaltic lava flows, interglacial shield volcanoes, rhyolite domes, dacite lavas, and historic basaltic lava flows (McGarvie pers. comm.). The bulk rock major and trace element results from these previous studies of Holocene and historic basaltic lavas are presented here in Appendix B.4. Major element and trace element trends were investigated in the same manner as the Austurfjöll lavas to compare the range of melt variation of Austurfjöll massif relative to the longer history of Askja volcano.

The historic effusive activity proximal to Austurfjöll occurred in two main periods during the 1920s and in 1961 (Figure 3.15). The events of the 1920s are more disperse, with multiple small vents and resulting lava flows. Samples in the collection include samples from Bátshraun (1921), Mývatnsveitahraun (1922), Kvíslahraun and Suðurbotnahraun (1922/23), and lavas fed from the western corner of Thorvaldstinudur that extend south and east are here referred to as the Southeastern Flank (1924 and 1929). A scoria cone constructed in 1926 in southern Öskjuvatn produced the island Eyja. This island was not sampled for this study. Samples of the 1961 a’a lava flows are also included in the analyses that are comparable to previously published values (Thorarinsson and Sigvaldason, 1962). Samples of postglacial lavas were collected in the region around Askja from various locations. Samples of historic basaltic lava from the Sveinagjá fissure located 60 km north of Askja that were erupted in conjunction with the 1875 rhyolitic eruption of Askja (Sigurdsson and Sparks, 1978) are included in the comparison.

Figure 3.15 Locations of historic lava flows featured in geochemical analyses as they are distributed around Austurfjöll (hillshade). The locations of samples are approximate due to the absence of GPS data. Labels are

of individual eruptive units from the 1920s. Eyja was not sampled.

The total range of compositions of historic basalts and postglacial lavas is comparable to that of the Austurfjöll suite (Figure 3.16), with a similar linear trend of plagioclase cations and incompatible elements and overall enrichment relative to MAR basalts. Only Mývatnsveitahraun (MYV) and a sub-group of the Southeastern Flank lavas plot outside of the range of the Austurfjöll lavas. These two sets of samples are enriched in alkalis relative to the rest of the samples. The postglacial lava suite alone has as much diversity as the Austurfjöll suite. With the exception of MYV, the 1920s lavas and 1961 lavas have a fairly constricted range of compositions, and plot at the more enriched end of the spectrum of Askja compositions. The Sveinagjá lavas also plot well with the enriched Austurfjöll lava range, but the sample number is limited. The lavas of the southeastern flank have by far the greatest diversity of any Askja sample set, falling into three apparent subgroups. As the sample locations are not well constrained geographical trends for this set of lavas are not yet feasible. One of the southeastern flank sub-groups plots with the other enriched historic lavas. A second southeastern flank lava sub-group is uniquely enriched in K2O, but not other incompatibles. The final subgroup plots with the least enriched Austurfjöll and postglacial lavas and glass-bearing gabbro sample (AG 1). The Holocene spatter samples collected during this study from the margins of Austurfjöll do not match chemically with the historic 1961 lavas, which they are proximal to in the field. They do, how ever, fall into the envelope of postglacial lavas collected by McGarvie, which suggests that they predate the historic activity, confirming their identification as Holocene spatter.

Figure 3.16 Comparison of postglacial and historic lavas (McGarvie, pers. comm) with Austurfjöll suite. Values of Normal MAR basalts are from Jakobsson et al. (2008).

3.7 PETROGENESIS

The geochemical analyses of two gabbro nodules revealed distinct compositions for each sample despite similar crystal populations. In thin section, however, the interaction of the nodule AG 1 with a basaltic melt during transport during eruption is preserved in the form of tachylite glass in void spaces between the outermost crystals of the nodule. The geochemistry of this sample is evolved relative to the uncontaminated gabbro sample AG 289. The presence of the nodules also suggests that Austurfjöll units interacted with a crystal-rich zone before eruption. This may indicate that the gabbro represents crystals that were removed from the evolving tholeiitic melt and accumulated in a magma chamber beneath Askja.

The nodules would be formed where moving melt batches dislodge and capture part of the cumulate.

There are multiple sources of evidence to indicate the relationship between melt evolution and macro-phenocrysts of plagioclase and clinopyroxene including: petrology of porphyritic samples, enrichment of Sr, gabbro nodule xenoliths, semi-quantitative mineral compositions of macro-phenocrysts matching gabbro crystals to lava phenocrysts, PER test diagrams of plagioclase and clinopyroxene genetic relationship, and similarities to other macro-phenocryst-rich tholeiites at Icelandic central volcanoes (Furman et al., 1992; Hansen and Gronvöld, 2000). Despite the apparent similarities in magma evolution processes of Austurfjöll lavas, there is still sufficient diversity of the resulting lavas to enable the discrimination of eruptive units. This then suggests that there are likely other controls to this melt evolution, including path effects, recharge, contamination by crust, or interaction with cumulates.

Unit 6 a nd 3 r epresent the most extreme compositions of the spectrum of the Austurfjöll compositions. Unit 3 is the most evolved melt, with low MgO concentrations, enrichment in alkali and incompatible elements, and low phenocryst populations. Unit 6, in contrast, is the least evolved of the eruptive units, with lower incompatible element compositions, and exclusively porphyritic textures. The Unit 6 samples are enriched in CaO and Al2O3 and are lower in TiO2, Na2O, FeO, and K2O relative to Unit 3. This supports the enrichment in calcium-rich plagioclase in the less evolved samples as indicated by the presence of macro-phenocrysts of plagioclase.

The PER analysis and gabbro nodules reflect the likely crystallization and removal of plagioclase and clinopyroxene from the melt in order to produce the Austurfjöll lavas. As a f urther test of this hypothesis, a model was created where the most primitive gabbro (AG 298) was added to Unit 3 until compositions of Unit 6 were reproduced. This test was repeated using Unit 2 as the evolved melt. The test was accomplished by adding a percent of gabbro to the average composition of Unit 3 and renormalized to produce a modeled melt with increasing concentrations of plagioclase and clinopyroxene. The process was repeated for Unit 2 (Figure 3.17). The linear trend of the lava was easily reproduced through the progressive mixing of gabbro with the evolved Unit 3 and Unit 2 melts. There is a slight variation in the slope of the mixing lines produced for Unit 2 and Unit 3, but they define a narrow window, which represents the bulk of Austurfjöll compositions. The Unit 6 lavas represent an interim melt composition that was that was reproduced through the modeled incorporation of a greater percentage of plagioclase and clinopyroxene. However, if the gabbro represents the cumulate of solid removed from a parental melt with a composition similar to Unit 6, t he melt would have to experience over 66% crystallization, according to the Lever Law (Figure 3.17). If instead Unit 5 were to be used as the parent, and Unit 6 were the product of contamination by crystals from the cumulate, the resulting crystallization is on the order of 33%. If the parent were Unit 4 only 10 % fractional crystallization would be required to produce the most evolved melts (Unit 2 a nd Unit 3). Unit 6 lavas are exclusively porphyritic in texture, and Unit 5 has numerous porphyritic units which supports the idea of contamination by phenocrysts. However, porphyritic lavas with a Unit 2 composition were analyzed, and they fit well in the variability expected of the group on t he more evolved end of the compositional range. This exercise remains interesting, but cannot advance beyond speculation without further data including melt inclusions and radiogenic isotopes (e.g. Kuritani et al., 2011) in order to fingerprint the parental melt and the dominance of contamination vs.

crystallization. The accumulation of a cumulate that could produce the observed gabbros would require a significant residence time in the lower crust. Deformation studies (InSar, GPS, Tilt, gravity) over the last three decades have led to models of two discrete sources in the crust below Askja at 3 km and 15 km (de Zeeuw-van Dalfsen et al., 2005; Pagli et al., 2006; Soosalu et al., 2010; Sturkell et al., 2006). These are

potential locations for cumulate accumulation. Evolution of a central volcano including a gabbro accumulation stage and replenishment has been proposed based on dissected massifs on the eastern coast of Iceland (Furman et al., 1992). Crustal reservoirs have been also been suggested for EVZ and NVZ volcanoes from geochemical evidence (Hansen and Gronvöld, 2000). The formation of the cumulate, through the fractional crystallization of plagioclase and clinopyroxene would occasionally be disrupted by recharge and eruptions. Under the right conditions the rising lava would potentially carry macro-phenocrysts derived from a disrupted cumulate dispersed within the melt to produce macro-porphyritic lavas. Alternatively, the cumulates are left undisturbed until later injections of magma disrupt them and carry portions of the cumulate to the surface in the form of gabbro nodule (Furman et al., 1992; Hansen and Gronvöld, 2000). To further elucidate the mechanisms and time scales of this process more must be known about the composition of the melts in the form of radiogenic isotopes, melt inclusions, and more precise crystal compositional data (i.e. microprobe).

Figure 3.17 Austurfjöll samples with mixing lines between the most evolved units (2 and 3) to the least evolved gabbro sample. The Lever Law was applied to determine the approximate amount of fractional

crystallization required to create the most evolved melts with different potential ‘parent’ melts. The melts more primitive than the ‘parent’ would be produced by the contamination of parent melt material with

cumulate present in the magma chamber.

The chemistry of the Austurfjöll and Holocene lavas are all enriched in incompatible elements relative to the MAR (Jakobsson et al., 2008; Sigmarsson et al., 2008) as seen in Figure 3.13. Iceland sits along the Mid-Atlantic Ridge, but is 3000 m higher in elevation than the neighboring ridges. The Icelandic crust is 15-35 km thicker than normal oceanic crust and has an enriched chemistry relative to N-MORB lavas (Allen et al., 2002; Gee et al., 1998). The enrichment of both the Austurfjöll and greater Askja lavas matches the expected enrichment of Icelandic tholeiites as a result of the interaction between the MAR and the Icelandic hotspot (Allen et al., 2002; Gee et al., 1998; Jakobsson et al., 2008;

Sigmarsson et al., 2008). The center of the hotspot is typically proposed, through geochemical and geophysical techniques, to be around Bárðarbunga or Kverkfjöll (Schilling et al., 1983; Sigmarsson et al., 2008; Sigvaldason et al., 1974) at the margins of Vatnajökull icecap to the southeast of Askja. The most primitive composition in Iceland, reflecting the greatest influence of the hot spot, occurs at Kistufell (Breddam, 2002), just south of Askja near the location of the estimated thickest Icelandic crust (Allen et al., 2002). Discrimination between mantle sources and residence times of magma batches cannot be resolved without radiogenic isotope data, particularly Sr, Pb and He (Sigmarsson and Steinthórsson, 2007).

The low MgO contents, REE, and abundance of incompatible elements even in the gabbros indicate that the melts that produced these eruptive units were evolved relative to the regional mantle (N-MORB of the MAR). From this knowledge a few qualitative observations can be made. The evolution of the melt is similar to expected compositions of MAR melts that have interacted with the Icelandic Hot Spot. The range in compositions, linear trend, and consistency in mineralogy indicates that the melts are generated using the same repeating process. Further exploration of the peterogensis of the Austurfjöll lavas requires a new sampling campaign focusing on t he identified eruptive units for the purpose of radiogenic isotopic fingerprinting. Further sampling of gabbro nodules and detailed microprobe analysis of phenocryst populations are necessary to reveal the processes which have repeated in generating, storing, and erupting magmas at Askja, and what produces the distinctive characteristics of individual magma batches.

3.8 CONCLUSIONS

The establishment of seven eruptive units, two diamictite units, and two gabbro chemical groups indicates a complexity within the Austurfjöll massif not previously detected (Höskuldsson, 1987; Sigvaldason, 1968; Strand, 1987). A similar trend in compositional diversity is observed in the postglacial lavas that are exposed around the Austurfjöll massif (McGarvie pers. comm.). As some eruptive units were defined using only one geochemical sample, these eruptive units may actually under represent the diversity of the Austurfjöll lavas. Nevertheless, the chemical similarity of eruptive units, such as Unit 2 and Unit 3, which have numerous samples and wide geographic distribution, suggest that the major massif building eruptive phases are well characterized.

The dates presented here are the first for the glaciovolcanic deposits of Askja and indicate a complicated history with ample time for the interaction with multiple glacial advances within the last glacial period (to be discussed further in Chapter 8). The linear trend and PER analysis indicate repeating mechanisms of melt generation dependent on plagioclase and clinopyroxene phenocrysts. However, the range of compositions also suggests there is more than closed-system melt evolution, potentially reflecting path effects, recharge, and / or contamination. To fully reconstruct the peterogensis of Austurfjöll lavas further investigation is required, using such tools as radiogenic isotopes and melt inclusions in order to fingerprint mantle sources and differentiate between mixing and differentiation.

The seven eruptive units of Austurfjöll reveal that Askja is a long lived poly-genetic basaltic central volcano with over 40 ka of glaciovolcanic activity and significant Holocene activity (Annertz et al., 1985; Carey et al., 2009; Hartley and Thordarson, 2012; Sigurdsson and Sparks, 1978; Þorarinsson, 1963; Thorarinsson and Sigvaldason, 1962). The chemical variability of the subaqueous Austurfjöll lavas is similar to the range of lavas produced during the Holocene in subaerial environments. This is clearly a well established system built up of multiple small to moderate volume eruptions.

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