# Combined unspiked K-Ar and 40Ar/39Ar dating applied to the dating of 80-260 ka old Icelandic sub-glacial rhyolites

Herve Guillou, Vincent Scao, Sébastien Nomade, Brigitte van Vliet-Lanoë, Céline Liorzou, Ágúst Guðmundsson

► To cite this version:

Herve Guillou, Vincent Scao, Sébastien Nomade, Brigitte van Vliet-Lanoë, Céline Liorzou, et al. Combined unspiked K-Ar and 40Ar/39Ar dating applied to the dating of 80-260 ka old Icelandic sub-glacial rhyolites. Quaternary Geochronology, 2023, 77, pp.101457. 10.1016/j.quageo.2023.101457. hal-04164905

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-version 22/05/2023 for *Quaternary Geochronology*

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-Total word count (text + abstract only): 7254

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**Combined unspiked K-Ar and <sup>40</sup>Ar/<sup>39</sup>Ar dating applied to the dating of 80 to 260 ka old Icelandic sub-glacial rhyolites.**

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Hervé Guillou<sup>(1,*)</sup>, Vincent Scao<sup>(1)</sup>, Sébastien Nomade<sup>(1)</sup>, Brigitte Van Vliet-Lanoë<sup>(2)</sup>, Céline

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Liorzou<sup>(2)</sup>, Ágúst Guðmundsson<sup>(3)</sup>

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11 (1): Laboratoire des Sciences du Climat et de l'Environnement, LSCE/IPSL, CEA-CNRS-UVSQ,

12 Université Paris-Saclay, Bât 714, CEA-Orme des merisiers, RD 128, 91191 Gif-sur-Yvette Cedex, France

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14 (2) : UMR 6538 CNRS Domaines Océaniques, IUEM, Place N.Copernic, 29480 Plouzané, France

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16 (3) : JFS, Geological Services Raudagerdi 31, 108 Reykjavik, Iceland

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19 (*) Corresponding author:

20 Hervé Guillou

21 LSCE/IPSL, Laboratoire CEA-CNRS-UVSQ

22 Bât 714, CEA-Orme des merisiers, RD 128, 91191 Gif-sur-Yvette Cedex France

23 Phone : 33 1 69 08 03 54

24 Email : herve.guillou@lsce.ipsl.fr

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27 Abstract:

28 We have used two techniques (i.e. K-Ar and ⁴⁰Ar/³⁹Ar) on Icelandic obsidian samples to produce
29 and more specially to estimate the quality and accuracy of the ages that can be obtained. Following a
30 meticulous protocol, we were able to date six rhyolitic eruptions with an accuracy 7 to 40 times
31 better than those obtained previously. Among these six rhyolites are the first published K-Ar and
32 ⁴⁰Ar/³⁹Ar ages of Krafla.

33 The combined K-Ar and ⁴⁰Ar/³⁹Ar approach produces not only highly precise but also accurate
34 ages. Such high precision makes it possible to produce accurate reconstructions of ice thickness at a
35 given location and time, to test whether there was a possible link between deglaciation and rhyolitic
36 volcanism onset in Iceland, and to explore other possible applications of the ⁴⁰Ar/³⁹Ar dating method
37 to paleo-environmental and paleo-climatic reconstruction at Iceland's latitude.

38 Then, we investigate, by combining geochemistry (i.e. determination of major and trace element
39 composition) and geochronology (i.e. dating of rhyolitic eruptions via K-Ar and ⁴⁰Ar/³⁹Ar dating) for a
40 number of Icelandic rhyolitic volcanoes whose activity could be recorded in North Atlantic
41 sedimentary cores as well as in Arctic ice. The aim of this approach is to provide new independent
42 anchors and correlations between climate records. Of the six dated eruptions, we propose that one
43 is record in North Atlantic sediments, the Loðmundur eruption that constitutes one of the
44 Kerlingarfjöll tuyas, which we date at 189.9±1.1 ka and assume to be the source of the tephra
45 recognized in core MD04-2822 at a depth of 3630-3631 cm.

46 **Keywords**: Geochronology, ⁴⁰Ar/³⁹Ar, Unspiked K-Ar, Rhyolite, sub-glacial volcanism, Obsidians

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# 1. Introduction

Products of Icelandic rhyolitic volcanism intercalated as tephra in glacial or oceanic sedimentary records provide a powerful correlation tool to link these climate records on an ocean basin scale, and even beyond. Such correlations between records are essential bricks in the understanding of climatic phenomena and their evolution over geological time. Thus, accurate dating of these tephra using dating methods that are independent of climatic parameters, in other words that are not based on the principles of cyclostratigraphy, is an essential prerequisite in the construction of the climate variations time scale.

One of such level is the North Atlantic Ash Zone II (NAAZ II) layer that temporarily coincides with the Thorsmork ignimbrite eruption $^{40}$Ar/$^{39}$Ar dated at 55.6 ± 2.4 ka (2σ uncertainty; Guillou et al., 2019) then more precisely at 56.14 ± 0.44 ka (astronomically calibrated $^{40}$Ar/$^{39}$Ar age, 2σ uncertainty; Groen et Storey, 2022). It is widely used for the North Atlantic and Arctic inter-archive correlations. Such ash layers, which are potential anchor points, dated by methods independent of climatic parameters remain relatively few. In the INTIMATE framework (Blockley et al., 2012; Davies et al., 2012), the ages of the eight rhyolitic tephra benchmark levels recognized in sedimentary or ice cores for the period 80 - 130 ka (Davies et al., 2014) are broadly based on the GICC05 extended time scale (Wolff et al., 2010). Neither they nor their source have been dated by radio-isotopic methods such as $^{40}$Ar/$^{39}$Ar. The same is true for the majority of the rhyolitic tephra of marine isotopic stages (MIS) 4 and 5 found in some North Atlantic marine cores (Brendryen et al., 2010).

While dating interbedded tephra in climatic archives remains complicated, particularly because of their small grain size and glassy nature which favours weathering and recoil effects during irradiation of samples prior to dating (see below), various works (Clay et al., 2015, Moles et al., 2019, Guillou et al., 2019) have shown that their source areas, often obsidian massifs, can be dated.

However, $^{40}$Ar/$^{39}$Ar dating of young rhyolitic obsidians (i.e., a few tens of thousands of years old) in particular those that emplaced under a thick ice sheet, such as many Icelandic rhyolites, remains a challenge. In such a setting, the sources of isotopic clock disruption are numerous and varied, mostly due to the interaction between the magma and glacier meltwater. These disruptions include excess argon, isotopic fractionation, loss or gain of parent and/or daughter isotopes due to weathering and the recoil effect. Based on these observations, we have chosen to apply to unaltered rhyolites an approach combining two dating methods that we consider complementary: the unspiked K-Ar method and the $^{40}$Ar/$^{39}$Ar method. The unspiked K-Ar method does not require irradiation, avoiding the possible effects of recoil. From the $^{40}$Ar/$^{39}$Ar method, we obtain age spectra and isochrons allowing the assessment of the possible effects of alteration and to highlight the excess or loss of argon as well as the isotopic fractionation. The $^{40}$Ar/$^{39}$Ar approach thus makes it possible to check
that all the basic hypotheses of application of the chronometer are correct. If they are, it validates
both the ⁴⁰Ar/³⁹Ar and K-Ar ages of the samples.

We have applied this double dating approach to rhyolitic eruptions of Prestahnùkur, Krafla and Kerlingarfjöll volcanoes (Fig.1). The dated samples were also analysed for their major and trace element compositions in an attempt to correlate with glacial and/or marine tephra.

## 2. Samples and Methods

### 2.1 *Samples*

Our sampling strategy was focused on the achievement of two objectives: 1- to obtain new accurate and precise "tie-points" (i.e., rhyolitic tephra sources, dated and characterized by their trace and major element contents) allowing inter-archival correlation and 2- to increase the number of dated Icelandic rhyolites to better evaluate the possible link between deglaciation and rhyolitic volcanism.

For this we have focused on three eruptive centers (Fig.1): Prestahnùkur, Kerlingarfjöll and Krafla.
The coordinates of the dated samples are reported in Table 1. Most of these samples have already been dated in previous studies. However, the published ages can be greatly improved by our approach combining unspiked K-Ar dating and ⁴⁰Ar/³⁹Ar dating based on rigorous protocols. We hereby point out that for all of the following, all the errors on the ages, whether they are K-Ar or ⁴⁰Ar/³⁹Ar ages, are reported at 2 sigma to facilitate direct comparison with results from both previous studies and different isotope systems.

**Samples ISLN-180, ISLN-181, ISLN-182, Kerlingarfjöll volcano.** For a detailed presentation of the geology of Kerlingarföll, one can refer to the work of Flude et al. (2010), summarized in Guillou et al., 2019. Three of the twenty-one rhyolitic eruptive units were selected for the combined K-Ar and ⁴⁰Ar/³⁹Ar study. Sample ISLN-180 was collected from the Tindur. This sample is a black, glassy obsidian but with a greasy to waxy lustre, that contains minor amount of sub-millimetric feldspars. The favoured age for Tindur by Flude et al. (2010) is 279 ± 30 ka (⁴⁰Ar/³⁹Ar). Sample ISLN-181 was sampled from Loðmundur. The sample comes from a very fresh large block of black, massive, glassy obsidian interbedded in a set of obsidian with large flow bands. The two glass splits analysed by Flude et al. 2010 (samples K12 a and b) allowed the calculation of a mean age (⁴⁰Ar/³⁹Ar) of 184 ± 20 ka. Sample ISLN-182 comes from East Trollabarmur dated at 249±28 ka (Flude et al., 2010). It was collected from a black massive rhyolitic lava that contains rare millimetric feldspars.

**Samples ISLN-187, ISLN-188, Prestahnúkur volcano.** McGarvie et al, (2007) and McGarvie (2009) suggested that the Prestahnúkur glaciovolcanic edifice, of homogeneous rhyolitic composition, was built in less than 20 years under an ice sheet of about 700 meters thick, 89 ± 48 ka ago. New ⁴⁰Ar/³⁹Ar dating of this edifice was performed by Clay et al. (2015), on the same two samples dated by McGarvie et al., (2007). The new age thus recommended is 132 ± 19ka. Our two samples were
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collected in two different scree corridors. This gives access to large recently fallen blocks from which
large samples of very fresh massive, glassy, sub-aphyric (less than 3% of infra-millimetric crystals of
feldspars) obsidian were found and collected. Given the monogenic character of this volcano,
wherever the sample is taken it will be a good candidate for dating the eruptive event.

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**Sample ISLN-171, Krafla Volcano:** Krafla is a central volcano ~15 km in diameter and characterized by a 100 km long basaltic fissure swarm (Fig.1). It has a bimodal volcanic character: for long periods of time it produces primarily basaltic fissure eruptions and dyke injections into the fissure swarm, but intermittently erupts semi-silicic to silicic magma as well as ashes. Initial basaltic volcanism is dated at 240 ka (Saemundsson and Pringle, 2003), followed by rhyolitic volcanism divided into three phases identified (Jónasson, 1994) and dated (Saemundsson and Pringle, 2000). The first phase dated at ca. 110 ka may have triggered the collapse of a ~8x6 km caldera. This eruption would have generated the rhyolitic dome of Hágöng and a welded tuff of Dacitic composition derived from the incomplete mixing of a rhyolitic and a basaltic magma. Within this caldera, a second caldera was set up, whose limits are unknown because covered by younger lavas (Árnason et al., 2020). During the second phase (i.e., around 90 ka ago) three subglacial rhyolite domes were emplaced just outside the caldera rim. These are the Jörundur, Hliðarfjall and Gaesafjallarani rhyolites. U-Th geochronology of zircons from these domes produced ages of 88.7±9.9 ka, 83.3±9.2 ka, and 85.5±9.4 ka, respectively (Hampton et al., 2021). The final and youngest phase begins with a small-volume rhyolitic eruption that formed Hrafntinnuhryggur (obsidian ridge) at about 24 ka (Saemundsson, 2008). Krafla has remained an active volcano ever since. Its last major eruption, "The Krafla Fires" created impressive lava flows between 1975 and 1984. Sample ISLN-171 is an aphyric rhyolite with glassy matrix collected on Hliðarfjall, one of the three subglacial ridges at the external rim of the caldera.

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## 2.2 A specific preparation of the samples and a reasoned choice of dating methods.

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The essential information about the two dating methods used here (i.e. Unspiked K-Ar and
$^{40}$Ar/$^{39}$Ar) are given in the supplement material. In this section, it is shown the best way to avoid the
main disturbances of the K-Ar clock and how the $^{40}$Ar/$^{39}$Ar method can highlight them. These
elements are essential to interpret some inter-laboratory age differences on similar samples. The
unspiked K-Ar and $^{40}$Ar/$^{39}$Ar methods are based on the same isotopic chronometer, i.e., the decay of
$^{40}$K (parent isotope) into $^{40}$Ar* (radiogenic daughter isotope). At high temperatures (1200°C for a
typical basaltic magma composition), a postulate of the K-Ar method indicates that $^{40}$Ar* is
continuously evacuated from the magma. During the eruption, lavas and tephra solidify rapidly. $^{40}$K
continues its decay into $^{40}$Ar*, but $^{40}$Ar* will then be trapped in the crystal lattice. Thus, the joint
measurements of the number of remaining parent isotopes ($^{40}$K) and the number of daughter
isotopes formed ($^{40}$Ar*) as well as the knowledge of the decay constants of $^{40}$K, make it possible to
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calculated the time elapsed since the date of the eruption (i.e. the age of the rock). This method
implies critical assumptions to be demonstrated especially when eruptions took place under a thick
ice sheet. First, it is crucial to be sure that despite the high pressure exerted by the ice layer, the
magma has completely degassed as far as ⁴⁰Ar* is concerned. In other words, it must be verified that,
at t = 0 (i.e. date of the eruption and crystallization of the lava), the magma is devoid of ⁴⁰Ar*. In such
case, the potential argon initially trapped in the sample has a ⁴⁰Ar/³⁶Ar ratio equivalent to that of the
actual atmosphere, i.e. 298.56 ± 0.31 (Lee et al., 2006). Such sample does not carry excess argon and
is not affected by mass fractionation that will bias the age determination. This is the "ideal sample"
we are looking for as candidate to date these rhyolitic eruptions. Furthermore, it is also considered
that the rocks or crystals must evolve as a closed system, from their solidification to the day it is
collected. This means that the sample must not have been re-heated, which would cause a non-
quantifiable loss of ⁴⁰Ar*, or altered, which would cause a loss of ⁴⁰Ar* accompanied by a loss or gain
of ⁴⁰K, depending on the type of alteration. In sub-glacial volcanoes where water and magma are in
contact during the eruption, the circulation of post-magmatic hydrothermal fluids is particularly
important and the risks of disturbances of the chronometer are highly probable because syn or post-
magmatic alteration. It is also well known, that during its ascent, magma can carry along with the
juvenile magma, older rocks or crystals from the surroundings. That is the inherited Argon issue. So,
the selection and preparation of the samples as well the choice of the dating methods is guided by an
optimal solution to avoid excess argon, mass fractionation, alteration and inherited argon issues.

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In the field, multi-decimeter to metric blocks are hammered out to the core. The selected part of this
block must be unaltered or as little altered as possible upon macroscopic examination. In the
laboratory, the peripheral traces of alteration as well as undetected fracture zones that may remain
after sampling are removed with a rock saw. The samples are then crushed, sieved to the fraction
0.250 mm - 0.125 mm, then washed in an ultrasonic cleaner in an acetic acid bath (1N concentration)
heated to 60°C for 40 minutes. The samples are then rinsed thoroughly with deionized water to
remove all traces of dust and acid. After drying in the oven at 50°C, this fraction is observed with a
binocular magnifier (x40). If in this fraction there are still some grains showing traces of alteration,
these are systematically eliminated by hand-picking at the highest magnification possible. When
phenocryst and/or xenocrysts are present, they are eliminated by densitometric sorting. Depending
on the desired density of the fraction to be dated, either Bromoform or Diiodomethane is used. After
such careful process and to ensure that the samples are as fresh as possible we measured the degree
of alteration by determining their loss on ignition (L.O.I.), that is a key parameter we use to confirm
that a sample is not altered. Faure and Mensing (1993) showed that for water contents of <1%, Rb,
which is a more mobile element than K, remained stable in the volcanic rocks. Therefore, we
considered that a L.O.I. value lower than 1% is a good indication null or minor alteration. Finally, to
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ensure that our $^{40}$Ar/$^{39}$Ar ages were not biased by alteration effects, we also calculated the Alteration Index (A.I.) of Baksi (2007) for each dated crystal and/or for each step. This index is a measure of the extent of secondary alteration based on $^{36}$Ar and $^{39}$Ar contents to evaluate the freshness of the dated samples. Its calculation is explained in Supplementary Table 1.

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The effects of the inherited argon carried into the sample via older crystals or older rocks from the surroundings can be minimized via an appropriate sample preparation procedure. Indeed, the mineral separation using heavy liquid (Bromoform) and/or magnetic separators we applied to our samples produced aliquots of rhyolitic glass or groundmass free of pheno and/or xenocrysts the most potential carriers of inherited argon.

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The argon known as "excess argon" which has the capacity to bias the dating is the fraction of $^{40}$Ar introduced into the sample during its crystallization. It does not come from the in-situ decay of $^{40}$K within the sample. This excess argon is generally distributed as fluid or glass inclusions in the constituent grains of the rock (Kelley, 2002). Densitometric and magnetic sorting cannot separate this argon from purely radiogenic in-situ argon but the $^{40}$Ar/$^{39}$Ar method offers the ability to distinguish radiogenic argon from excess argon via isochrons and age spectra interpretation by implementing the step-heating technique (Turner et al., 1966). The general shape of the age spectra can be used to determine whether the K-Ar clock has been disturbed or not. An undisturbed sample that has evolved as a closed system since its crystallization will produce concordant spectra build from equivalent $^{40}$Ar/$^{39}$Ar ages (within analytical errors) for each temperature step. In samples containing excess argon, step-heating experiments reveal the melting of fluid inclusions at low temperature, yielding ages too old in the early stages of the age spectrum. The melting of glassy or solid inclusions at high temperature (Esser et al., 1997) gives also too old ages in the last stages of the age spectrum. The age spectrum is then discordant, U-shaped, diagnostic of excess $^{40}$Ar. Alteration, re-heating can also be responsible for disturbed age spectra. The $^{40}$Ar/$^{39}$Ar method also allows the data to be examined using isochrons. In an inverse isochron diagram, the intersection of the isochron with the $^{36}$Ar/$^{40}$Ar axis corresponds to a value of $^{39}$Ar equal to 0. This position is that of a sample devoid of K, thus unable to produce purely radiogenic $^{40}$Ar*. Thus, for a sample without excess $^{40}$Ar*, the isochron intercepts the $^{36}$Ar/$^{40}$Ar axis at an ordinate value of 1/298.56 that is the atmospheric ratio (Lee et al., 2006). In contrast, for a sample containing $^{40}$Ar* in excess, this intercept value will be less than 1/298.56. We can thus, from the inverse isochron diagram, highlight the presence or absence of excess argon. Inversely, in case of mass fractionation, atmospheric $^{36}$Ar incorporation is favoured in the magma (Matsumoto and Kobayashi, 1995). Accordingly, the inverse isochron reveals an initial ratio ($^{40}$Ar/$^{36}$Ar) less than the atmospheric value.

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Even if a sample seems ideal to avoid all the ⁴⁰Ar/³⁹Ar dating biases (i.e., alteration, mass fractionation, excess or inherited argon), there is still a fragility, especially for very poorly crystallized materials (i.e. obsidians) that can be subject to the recoil effect. Neutron activation can have undesirable side effects, including the recoil of ³⁹Ar and ³⁷Ar atoms from their K and Ca targets. (Turner & Cadogan 1974; Foland et al. 1984, Jourdan and Renne, 2014 and references therein). The loss of ³⁷Ar can affect the age of Ca-rich samples through interference corrections on ³⁶Ar producing an underestimated ⁴⁰Ar*/³⁹Ar<sub>K</sub> and, therefore, falsely too young ages. Another issue may be the recoil of the daughter atom (i.e. ³⁹Ar) over a short distance. This redistribution of ³⁹Ar and potential loss of ³⁷Ar from the grain lattice, result in complex age spectra that are difficult to interpret and can make the calculated ages erroneous. That is the case for glassy and cryptocrystalline rocks such as many Icelandic rhyolites. Unlike the ⁴⁰Ar/³⁹Ar method, the K-Ar method does not require irradiation, thus avoiding this recoil effect, but it fails to evidence mass fractionation, alteration, excess and inherited argon. Thus, if the same sample gives a K-Ar age equivalent to the ⁴⁰Ar/³⁹Ar age for which the absence of alteration, excess argon, or fractionation would have been clearly established, the dating result can be considered strongly reliable. This is why we have implemented hereafter an experimental approach combining the K-Ar and the ⁴⁰Ar/³⁹Ar step-heating.

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## 3. Results

### 3.1 Geochemistry results

The composition of the six dated rhyolites is reported on a total-alkali-silica (TAS) plot (Fig. 2; Le Bas et al., 1986). Whole rock chemistry is given in Table 2. Five of the six samples lie in the rhyolite field of the TAS plot. ISLN-182 lies at the boundary of the dacite and rhyolite fields, and is significantly higher in iron and calcium than the other samples. ISLN-182 is therefore a rhyodacite.

The calculated alkali index (molar (Na₂O + K₂O)/Al₂O₃) are very close to 1.0. Thus, the rhyodactite and the five rhyolites plot at the boundary separating the subalkaline and peralkaline domains.

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### 3.2 Unspiked K-Ar results

Supplementary Table 2 reports the ⁴⁰Ar/³⁶Ar ratios measured and ⁴⁰Ar* values calculated from the analyses of the six K-Ar dated samples. Standard error values (Supp. Table 2) are close to or lower than 1%. This illustrates the good stability of the measurement conditions during these experiments. The unspiked K-Ar technique was here capable of precision of 0.2 to 1.3% for amounts of ⁴⁰Ar* between 30.0 and 7.0% of the total ⁴⁰Ar. Unspiked K-Ar ages are reported in Table 3. The K-Ar ages of samples ISLN-180, ISLN-181 and ISLN-182 define an age range of 187.5 ± 4.3 – 258.9 ± 6.1 ka for the three eruptions of the Kerlingarfjöll. The Rhyolite of Krafla Volcano (sample ISLN-171) is dated at
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94.4 ± 2.6 ka. K-Ar ages of samples ISLN-187 and ISLN-188, both from Prestahnúkur volcano, are equivalent at 2σ level (84.3 ± 2.4 and 85.7 ± 2.8 ka).

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3.3. $^{40}$Ar/$^{39}$Ar results

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**Samples ISLN-180, ISLN-181, ISLN-182, Kerlingarfjöll volcano.** The six-separate step-heating experiments (Fig.3) conducted on these three samples (two experiments on each) allow the calculation of plateau age with 98.7 to 100.0% of total gas released (Table 4, Supplementary Table1). This is evidence that the K-Ar clock of these samples is not disturbed and that the calculated ages are reliable. The $^{40}$Ar* contents for the steps constituting the plateaus are relatively high considering the relatively young eruption ages. They range from 3.8% to 24.6%, 9.9% to 82.4% and 6.5% to 34.5% for ISLN-180, ISLN-181 and ISLN-182 respectively. There is also no evidence of $^{40}$Ar* excess or mass fractionation, as the $^{40}$Ar/$^{36}$Ar intercept values calculated for the six inverse isochrons (Fig.3, Table 4) are within uncertainties the current atmospheric value.

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According to the L.O.I. values (0.19 to 0.42%, Supplementary Table 3), all samples are considered unaltered. This observation is confirmed if we consider the A.I. values (Supplementary Table 1). A.I. is a more sensitive indicator of alteration. Except for one step (FG-2286), most of them have an A.I. value below the threshold value and very few of them equal to the threshold. All these controls confirm that the $^{40}$Ar/$^{39}$Ar dates calculated from the step-heating experiments (i.e., 191.6 ± 1.9 ka for Loðmundur, 236.4 ± 4.5 ka for Tindur and 257.6 ± 5.7 ka for Trollabarmur) are reliable and geologically significant, corresponding to the eruption ages.

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**Samples ISLN-187, ISLN-188, Prestahnúkur volcano.** Almost all the extracted gas (96.4 to 100.0%) during the 4 step-heating analysis (2 experiments per sample) was used to calculate the plateau and inverse isochron ages (Table 3, Fig.4). The values of $^{40}$Ar* vary from 1.2 to 97.6%. For the steps defining the plateaus, these values are in general higher than 50% (Supplementary Table 1). Both samples are very fresh with L.O.I. values lower than 0.26% (Table 2). Within the exception of the lowest temperature steps, which in any case gave off only small quantities of gas, all the others are characterized by extremely low A.I. values. As none of the inverse isochrons show evidence of mass fractionation or excess argon, the age we propose for Prestahnúkur volcano is the weighted average calculated from the four step-heating experiments, 85.4 ka ± 1.3 ka.

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**Sample ISLN-171, Krafla Volcano.** Almost all the gas extracted during the two step-heating experiments was used to define the plateau and calculate an age. The values of Ar* vary from 9.8 to 50.7% and are high enough to date with a precision better than 2% this rhyolitic eruption. The absence of excess argon is confirmed by the similarity of the plateau and inverse isochron ages,
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respectively 93.0 ± 1.8 and 92.5 ± 3.9 ka (Fig.5). This sample is not altered as evidenced by its L.O.I.
value of 0.97% and its A.I. values well below the threshold. Thus, we propose an age of 93.0 ± 1.8 ka
for the Hliðarfjall subglacial ridge.

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## 4. Discussion

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### 4.1 Comparison with previously published ages

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First, we observe the remarkable agreement between the K-Ar and ⁴⁰Ar/³⁹Ar ages for the six samples
dated jointly by these two methods (Table 5). One of the fundamental assumptions in K-Ar and
⁴⁰Ar/³⁹Ar dating is that the system has remained closed since the material was produced, so that no
Ar or K has either entered or left the sample since formation. In all dated samples, the successive
heating steps define an age plateau. It indicates that the ratio ⁴⁰Ar/³⁹Ar remained constant
throughout the different heating steps. This enables us to define and calculate a plateau age and
demonstrates that the rock has evolved as a closed system since its formation. This ensures the
validity of the K-Ar ages and explains their consistency with the ⁴⁰Ar/³⁹Ar ages. This agreement also
attests to a complete degassing with respect to Argon as well as to the absence of recoil effect during
the irradiation of the samples although they are mostly obsidians. Based on this observation, we
propose for these six samples, to combine the K-Ar ages with the ⁴⁰Ar/³⁹Ar ages to calculate the best
age of the corresponding eruptions, that will be the weighted average of the K-Ar and ⁴⁰Ar/³⁹Ar ages.
Thus, an age of 191.0 ± 1.7 ka is retained for ISLN-181, 258.2 ± 4.2 ka for ISLN-182, 93.5 ± 1.5 ka for
ISLN-171 and 85.2 ± 1.1 ka for the sub-glacial Prestahnúkur volcano when combining the four ages
from samples ISLN-187 and ISLN-188. The difference between the K-Ar and ⁴⁰Ar/³⁹Ar ages of sample
ISLN-180 is higher than for the other samples. Nevertheless, they are equivalent at 2σ. The weighted
mean age of 239.1 ± 3.6 ka with a mean square weighted deviation (MSWD) of 2.5 and a small
probability of fit (p) value of 0.085.

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The consistency between our K-Ar and ⁴⁰Ar/³⁹Ar ages being established, we can now compare our
ages to those published (Table 5). Our ages for the Tindur, Loðmundur and Trollabarmur eruptions
(samples ISLN-180, ISLN-181 and ISLN-182 from Kerlingarfjöll) are quite consistent with the ⁴⁰Ar/³⁹Ar
ones of Flude et al., (2010). In their study, experiments on three splits from sample Ker00/34, an
obsidian of Pitchstone type from Tindur, produced ages of 285±50 ka, 246±48 ka and 312±54 ka.
Even if their two first ages are equivalent to the age we calculate for the Tindur eruption (ISLN-180:
239.1 ± 3.5ka), we note that their results are relatively scattered and that our ages are 7 to 12 times
more accurate. In Guillou et al., (2019), we highlighted important differences in analytical protocols
that could explain why our ages are more accurate. In the study of Flude et al. (2010), of the three
stages (400°C, 1200°C, and 1600°C), only the 1200°C stage was used to calculate the ⁴⁰Ar/³⁹Ar age by
325 arbitrarily setting the initial trapped argon composition at the atmospheric one (i.e., 295.5 in their 326 study). This approach, which is similar to a "total Ar/Ar fusion" method as acknowledged by Flude et 327 al. (2010) has certainly produced significant ages but these have high errors. So, in the following, we 328 will use our more precise and accurate ages that rely on a consensus of experimental and analytical 329 protocols (Schaen at al., 2021) as the "best ages" for the rhyolites of the Kerlingarfjöll.

330 The comparison exercise is more complex for the dating of the Prestahnúkur (ISLN-187, ISLN-188). 331 While being 40 times more precise, our ages are compatible with those of McGarvie et al. (2007) 332 calculated on the basis of step-heating experiments on two obsidian samples. For the same 333 monogenetic volcano, Clay et al., (2015) propose an age of 132 ± 19 ka that differs significantly from 334 ours. What then could be the sources of an overestimated age such as that determined by Clay et al., 335 (2015)? The apparent too old age can be due to an excess of ⁴⁰Ar* carried in crystals such as older 336 feldspars, either formed well before the eruption, or torn from the surrounding and incorporated 337 into the magma during its eruptive ascent to the surface. It is possible, although highly speculative, 338 that such crystals could be present in the aliquots analysed by Clay et al. (2015), if these could not be 339 removed either by magnetic or densitometric sorting. The presence of such feldspars that could not 340 have been separated from the glass at the size of the fraction analysed was noted by Clay et al. 341 (2015). Non-optimal analytical conditions can also be mentioned. In the case of these very young 342 glasses, it is of course essential to measure the ³⁶Ar of the sample and the blank accurately because 343 limited accuracy in measuring small amounts of ³⁶Ar could have an impact on value of the 344 determined age (P. Clay, Pers. Comm.). In the case of the Prestahnúkur samples analysed by Clay et 345 al. (2015), the ³⁶Ar signals of the samples were small and the blank corrections proportionally large. 346 The sensitivity of the determined age to this correction is significant, so even small changes could 347 have impacted the age results. An indication of this probable difficulty for Clay et al., (2015) to 348 measure the ³⁶Ar with precision lies in the values calculated for the ⁴⁰Ar/³⁶Ar intercepts of their 349 isochrons. In contrast to our results, they deviate significantly from the modern atmospheric value, 350 varying between 58 ± 20 and 431 ± 100. Clay et al, (2015) consider that this could either indicate that 351 the data is poorly constrained or that some samples contain a trapped Ar component with ⁴⁰Ar/³⁶Ar 352 below that in modern air. Let us note moreover three of the six committed experiments did not allow 353 Clay et al. (2015) to trace a proper isochron. For these reasons, we consider that the age of 132 ± 19 354 ka proposed by Clay and collaborators in 2015 was overestimated and the age of the Prestahnúkur 355 eruption should be 85.2 ± 1.1 ka for.

356 For samples ISLN-180, ISLN-187, and ISLN-188, it can be argued that these scree samples are hardly 357 representative of the landforms we wish to date and that this sampling is a fundamental error. 358 Admittedly, scree sampling can introduce error into our work because we cannot be completely sure
359

that the samples belong to the volcano we wish to date, especially in Iceland where moving ice has brought rocks back and forth for thousands of years. To counter this objection, we rely on field observations and geochemical and geochronological data. On the field, we made sure that our samples belonged to the majority facies and that it was equivalent to the facies described in previous work. The ages obtained here, although much more precise, are identical to those published for the Tindur (Flude et al., 2010) and Prestahnúkur (McGarvie et al., 2007). The chemical composition of major and trace elements as well as correlation plots between elements for samples ISLN-180, ISLN-187 and ISLN-188 and equivalent samples analysed by Flude et al. (2010) and McGarvie et al. (2007) are reported in Suppl. Table 3. The compositions of major elements in ISLN-180 and KER00/34 are perfectly similar and the trace element compositions show also good agreement. We therefore conclude that our sample ISLN-180 is representative of Tindur. The SiO₂ contents of ISLN-187 and ISLN-188 are slightly lower, but still close to those of PK1-7. For other major elements, they are very similar. Concerning trace elements, compositions of ISLN-188 and PK1-7 are very close. Although slightly different, those of ISLN-187 can be considered as close to identical. This supports the argument that ISLN-188 and ISLN-187 belong to Prestahnúkur.

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As far as we know, our K-Ar and ⁴⁰Ar/³⁹Ar ages of Krafla are the first ones published along with their detailed experimental data. Indeed, the previous ⁴⁰Ar/³⁹Ar ages (Saemundsson et al., 2003) are not presented with sufficient details (i.e., are they plateau or isochron ages, what about possible excess of argon, calibration standards... ?) to be compared with other ⁴⁰Ar/³⁹Ar ages or ages from other isotope systems. Our combined K-Ar and ⁴⁰Ar/³⁹Ar approach allows us to date the Hliðarfjall rhyolite at 93.5±1.5 ka. This age is about 10 ka older than the U-Th age obtained from isotopic analysis of zircons from the same rhyolite (i.e. 83.3±9.2 ka, Hampton et al., 2021). Nevertheless, the two ages agree at 2σ. This result is an encouragement to promote the joint use of these two isotopic systems to improve the precision and accuracy of both techniques, especially when applied to Icelandic rhyolites.

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## 4.2 Refined paleoclimatic and paleoenvironmental applications of the K-Ar clock thanks to more precise ages

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Increased precision on ages allows for the potential contributions of ⁴⁰Ar/³⁹Ar dating of Icelandic rhyolites to paleoclimatic and paleoenvironmental studies. Tuffen (2001) developed a model based on the height of the tuyas (i.e., the flat-topped, steep-sided subglacial volcanoes where our samples were collected) to provide constraints on the minimum thickness of the syn-eruptive ice sheet and hence ice sheet elevation. The precise dating of tuyas should allow to accurate reconstructions of ice thickness at a given place and time. The use of rhyolitic tuyas for these reconstructions has been

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limited until now, notably because of the difficulties in producing sufficiently precise dates (McGarvie et al., 2007; Clay et al. 2015). Our new $^{40}$Ar/$^{39}$Ar ages, whose associated errors are between 1 and 5 ka, can significantly contribute in modelling the growth and decay rates of Iceland's ice sheets, because they provide accurate anchor points indicating the minimum ice-thickness at a precise location and time. Estimates of ice thickness at Kerlingarfjöll, proposed by Stevenson et al. (2009) based on the volatile content of the glasses have values of at least 550 m and possibly much. Combining our dates with this estimate, we can propose that during the MIS 7 interglacial stage, the Kerlingarfjöll was covered by an ice sheet about 550 meters thick. At Prestahnúkur, the ice was at least 700m thick during the eruption (McGarvie et al., 2007). Thanks to our precise $^{40}$Ar/$^{39}$Ar age we can anchor an ice thickness at this location at the end of MIS5.

Only about 10-12% of outcropping rocks in Iceland are of silicic composition, most of them being basaltic (Walker, 1966). Indeed, basaltic lavas can be dated by the same techniques, but in such young and low K lavas, the amount of 40Ar* are too low to produce ages with high precision. Therefore, the study of a possible link between climate and volcanism at the latitude of Iceland based only on rhyolitic volcanism is necessarily biased. Despite this, investigating whether rhyolitic volcanism develops preferentially during glacial periods or during interglacial periods allows us to understand whether it is cadenced by successive glacial loads and discharges between 80 and 260 ka. In previous works (Flude et al., 2008; 2010; Clay et al., 2015), the errors on the $^{40}$Ar/$^{39}$Ar eruption ages were too high for eruption dates of rhyolitic units to be related solely to a glacial or interglacial period. Thanks to the improved accuracy and precision obtained here, we can now try to determine whether some climatic periods were more favourable than others for the onset of rhyolitic volcanism.

After 11.2 cal. kyr BP, the Icelandic ice sheet retreated very rapidly, and by 8.7 cal. kyr BP, the ice sheet had disintegrated (Ingólfsson et al., 2010 and references therein). The 10 ka old rhyolitic eruption of Askya occurred during this rapid change to present-day climate that caused the ice to melt and isostatic rebound (Sigvaldson, 2002). Rhyolitic eruptions from Oraefajökull, Katla, and Hekla eruptions that are recorded in the Gröenland, GRIP, and NGRIP ice cores occurred during the MIS 2 - MIS 1 transition (Abbott and Davies, 2012; Cook et al., 2022 (Fig.6)). Similarly, the violent eruption of the Thorsmörk rhyolitic ignimbrite is synchronous with the MIS 4 - MIS 3 transition.

To study the tempo of rhyolitic volcanism relative to the glaciation-deglaciation alternation during older periods, it is crucial to accurately compare the chronology of rhyolitic eruptions with that of deglaciations. To this end, we have plotted in Figure 6 our new ages as well as those published (Guillou et al., 2019; Groen and Storey, 2022) obtained from interior rhyolites, those from rhyolitic tephra layers described in marine cores, indirectly dated by isotope stratigraphy, and those sampled in ice cores, on the stacked $\delta^{18}$O benthic record of Lisieki and Raymo (2005). From Figure 6, two
patterns clearly emerge. As shown previously, rhyolitic activity occurs during deglaciation between MIS 2 and MIS 1. In contrast, during MIS 5, most activity occurs during transitions between warmer and cooler substages, namely between the MIS 5.5/MIS 5.4, MIS 5.3/MIS5.2 transitions and finally during the MIS 5.1/MIS 4 transition. A similar pattern prevails during MIS 7. Rhyolitic volcanism is also persistent during cold periods (i.e., MIS 6 and MIS 8). Furthermore, the fact that the rhyolites dated here are subglacial in facies and that at the same time the age of some of them coincides with warm periods (i.e., MIS 5 and MIS 7) indicates the persistence of an ice cap on top of Kerlingarfjöll, Prestahnúkur, and Krafla volcanoes even during the warmest periods. This persistence remains because currently nearly 10% of Iceland is covered by glaciers, including the Jangjökull glacier near Prestahnúkur and the Hofsjökull glacier near Kerlingarfjöll. From these observations, we can conclude that the rhyolitic volcanism that took place between 80 and 260 ka in Iceland occurred during both cold and warm periods and seems to be independent of the alternation of glaciation / deglaciation.

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4.3 Rhyolitic tephra and temporal framework

To be used as absolute time markers allowing the correlation of sedimentary and or glacial archives at great distances, two characteristics of the tephra are crucial and must be known with accuracy and precision: their chemical compositions (i.e., chemical fingerprinting) and their ages or more correctly, the age of their sources. While dating tephra source areas remains challenging, as pointed out by Brauer et al. (2014), it is important to persevere by taking advantage of new technical developments in dedicated $^{40}$Ar/$^{39}$Ar mass spectrometry dating, a method that is independent of climate parameters. Until now, the use of Icelandic rhyolitic tephra has been most often based on only one of the "two legs" of tephrostratigraphy, namely the geochemical component (Abbott et al., 2011, 2012, 2014, 2018; Bourne et al., 2015; Brendryen et al., 2010; Haflidson et al., 2000; Hibbert et al., 2014). Indeed, so far, the precision on the ages of the source areas has remained generally too low for the tephra they emitted to be imposed as other emblematic rhyolitic tephra (i.e., Vedde ash, NAAZII...) as correlation tool. The precision on the ages reached here, as in our previous study (Guillou et al., 2019) is consistent with that of isotope stratigraphy setting the chronology of the marine record and with multiple methods setting the time frame of the glacial record. As a result, we can discernibly test whether one or more of the rhyolites dated here have the same composition and age as some tephra layers found in either the North Atlantic marine or glacial records. Thus, we can try to find out, from chemistry and dating, from which Icelandic volcanic complex the magma forming these tephra layers comes from.

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For the 85-93 ka dated rhyolites (ISLN-187, ISLN-188, and ISLN-171), we used the available North Atlantic tephrostratigraphic framework (Davies et al., 2014) as well as the ages and chemical
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compositions available in Brendryen et al. 2010; Abbott et al. 2011; and Abbott et al. 2013 (Supplementary Tables 4 and 5). We choose to standardized the major element analyses to 100% to optimize comparison. Based on oxide-oxide correlation diagrams, we cannot relate our rhyolites to any contemporary tephra whether intercalated in the marine or glacial record.

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Silicic volcanic rocks in Iceland occur as lavas and domes, or as tephra layers, welded tuffs and ignimbrites. They were formed during effusive and explosive eruptions. Many of the central volcanoes produced silicic volcanic rocks in both types of eruptions. Prestahnùkur is an effusion-dominated rhyolitic tuya that seems to have erupted entirely within an ice cap (McGarvie, 2009). As a result, this type of tuya cannot be the origin of tephra layers. It makes sense, therefore, that the contemporary tephras described in NGRIP (Abbott and Davies, 2012) and MD04-2822 (Abbott et al., 2013) cannot have Prestahnùkur as their source. This is supported by the clearly different chemical compositions between Prestahnùkur and these tephras.

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Stevenson (2005) observed in several rhyolitic tuyas at Kerlingarfjöll that, from the earliest to the latest stage, eruptive materials were produced during initially intense and gradually decreasing interactions between meltwater and rhyolitic magma. These interactions favour explosive eruptions of airborne pyroclasts that can be transported over great distances and deposited as tephra in the sedimentary and glacial climatic archives. The Tindur sequence shows no evidence of an explosive eruption phase (Stevenson et al., 2009). Therefore, none of the tephras archived in the ice and sediment cores can be correlated with this tuya. Similarly, we did not identify any tephra of age and composition corresponding to the Trollabarmur eruption. On the other hand, with respect to Lodmundur tuya, Hibbert et al. (2014) propose that it be considered as one of the potential sources of the MD04-2822 MIS 6 ash layer identified at a depth of 3630-3631 cm. This depth corresponds to an age of 181 ± 6 ka. Sample ISLN-181 that is ⁴⁰Ar/³⁹Ar dated at 189.9 ± 1.1 ka is then of the same age and have similar contents in SiO₂, TiO₂, Al₂O₃, CaO and Na₂O (Supp. Table 6). The difference between their Na₂O/K₂O ratios is related to the K₂O content (i.e., 3.41 in ISLN-181, 4.35% in the marine tephra). The assumption that the Loðmundur is a potential source of this MIS 6 Tephra is then confirmed by our new dating and by similar major element contents between ISLN-181 rhyolite and MD04-2822 3630-3631 tephra. Thus, our ⁴⁰Ar/³⁹Ar age, that is about six times more accurate than the one proposed by Hibbert et al., (2014) can be used to refine the time frame of marine cores that contain this tephra.

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At first glance, it may seem frustrating that only one of the six rhyolites we dated here can be used as tie points for inter-archive correlations. Yet, this observation must be tempered by the fact that some of the rhyolites dated here may be related to yet-to-be-discovered tephra layers. It is also important not to overlook the fact that the comparisons of chemical compositions that we make between terrestrial samples and marine tephra layers may be biased because the methods for determining

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major element compositions are not the same (i.e., ICP-AES for terrestrial samples, EPMA for marine tephra) and that they ideally should be achieved using the same method and/or machine but also coupled with trace elements measurements.

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## 5. Conclusion

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In this paper, we hope to have demonstrated that it is possible to date Late Quaternary obsidian from Iceland with a precision and accuracy far superior to what has been done so far. To accomplish this, we have emphasized sample quality and in particular the importance of dating only samples that show little or no evidence of alteration. The combination of this quality control, rigorous sample preparation and mass spectrometry protocols contribute to improving the geological significance of the dates.

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With this procedure, we were able to date six new Icelandic rhyolites with a precision 7 to 40 times better than that achieved in previous studies. Such high precision opens potential contributions of $^{40}$Ar/$^{39}$Ar dating of Icelandic rhyolites to paleoclimatic and paleoenvironmental studies, in particular to produce accurate reconstructions of ice thickness at a given location and time. An additional result that builds on our new data is that there is no systematic synchronization between deglaciation and rhyolitic activity at least during the period 80 - 260 ka.

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At the same time, our new ages help to increase the number of geochemical and geochronological characterized eruptions that can be used as tools for large-scale correlation of the climate record. Accordingly, we propose an additional eruption as a temporal marker, the Loðmundur eruption that is a potential source of the MIS 6 tephra layer recovered from marine core MD04-2822 at depth 3630-3631 cm. Therefore, we consider the provision of these additional attachment points to be a significant advance in the construction of a "multi-archive" chronology and an improvement in the chronology of the North Atlantic record both inland (ice cores) and in marine basins.

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We are convinced that additional dating of rhyolitic eruptions following our approach will make these eruptions an essential tool in the reconstruction of ancient climates and environments.

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Acknowledgements: H.G. is thankful to P. Clay for providing comments and information on her chronological study and Icelandic rhyolites and to D. McGarvie for providing information on the geology of Torfajökull and their sample locations. H. Guillou thanks the three anonymous reviewers for their evaluation of an earlier version of this article and the anonymous one who revised this one. This work was supported by the French Atomic Commission (CEA) and the CNRS.

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618

619

29 - Lisieki, L.E., Raymo, M.E., 2005. A Pliocene-Pleistocene stack of 57 globally distributed benthic $^{18}$O records. Paleooceanography 20, doi: 1029/2004PA001071.

620

621

622

30 - McGarvie, D.W., Stevenson, J.A., Burgess, R., Tuffen, H., Tindle, A.G., 2007. Volcano-ice interactions at Prestahnúkur, Iceland: Rhyolite eruption during the last interglacial-glacial transition. Annals of Glaciology, 45, 38-47.

623

624

31 - McGarvie, D., 2009. Rhyolitic volcano-ice interactions in Iceland. J. Volc. Geoth. Res., 185, 367-389.

625

626

627

32 - Moles, J.D., McGarvie, D., Stevenson, J.A., Sherlock, S.C., Abbott, P.M., Jenner, F.E., Halton, A.M., 2019. Widespread tephra dispersal and ignimbrite emplacement from a subglacial volcano (Torfajökull, Iceland). Geology, 47, 577-580.

628

33 - Sæmundsson, K., 2008. Krafla, Geological Map, 1:25,000; Landsvirkjun: Reykjavík, Iceland.

629

630

34 - Sæmundsson, K., Malcolm, P., 2003. Evolution of the Krafla Central Volcano, North Volcanic Zone, Iceland. AGU Fall Meeting Abstracts. 84. 1052-.

631

632

633

634

635

636

637

35 - Schaen, A.J., Jicha, B.R., Hodges, K.V., Vermeesch, P., Stelten, M.E., Mercer, C.M., Phillips, D., Rivera, T.A., Jourdan, F., Matchan, E.L., Hemming, S.R., Morgan, L.E., Kelley, S.P., Cassata, W.S., Heizler, M.T., Vasconcelos, P.M., Benowitz, J.A., Koppers, A.A.P., Mark, D.F., Niespolo, E.M., Sprain, C.J., Hames, W.E., Kuiper, K.F., Turrin, B.D., Renne, P.R., Ross, J., Nomade, S., Guillou, H., Webb, L.E., Cohen, B.A., Calvert, A.T., Joyce, N., Ganerød, M., Wijbrans, J., Ishizuka, O., He, H., Ramirez, A., Pfänder, J.A., Lopez-Martínez, M., Qiu, H., Singer, B.S., 2021. Interpreting and reporting 40Ar/39Ar geochronologic data. Bull. Geol. Soc. of Amer., 133,461-487.

638

639

640

36 - Sigvaldason, G.E., 2002. Volcanic and tectonic processes coinciding with glaciation and crustal rebound: An early Holocene rhyolitic eruption in the Dyngjufjöll volcanic centre and the formation of the Askja caldera, north Iceland. Bull. Volc., 64, 192-205.

641

642

643

37 - Stevenson JA, Smellie JL, McGarvie DW, Gilbert JS, Cameron BI., 2009. Subglacial intermediate volcanism at Kerlingarfjöll, Iceland: magma-water interactions beneath thick ice. J. Volc. Geoth. Res. 185:337-351

644

645

38 - Tuffen, H. 2001. Subglacial rhyolite volcanism at Torfajökull, Iceland. (PhD Thesis, The Open University).

646

647

39 - Turner, G., Miller, J. A., Grasty, R. L., 1966. The thermal history of the bruderheim meteorite. Earth Planet. Sci. Let., 1, 155-157.
648 41 - Turner, G., Cadogan, P. 1974. Possible effects of <sup>39</sup>Ar recoil in <sup>40</sup>Ar/<sup>39</sup>Ar dating. Geoch. Cosm.

649 Acta, 5, 1601-1615.

650 42 - Walker, G.P.L., 1966. Acid volcanic rocks in Iceland. Bull. Volc., 29, 376-402.

651 43 - Wolff, E.W., Chappellaz, J., Blunier, T., Rasmussen, S.O., Svensson, A., 2010. Millennial-scale

652 variability during the last glacial: the ice core record. Quat. Sci. Rev., 29, 2828-2838.

653
Figure 1: Location map of the dated samples. White stars refer to the dated samples.

Figure 2: Total-alkali vs. silica content of the dated samples.

Figure 3: $^{40}$Ar/$^{39}$Ar Age spectra and Inverse isochron diagrams depicting $^{40}$Ar/$^{39}$Ar experimental results from obsidian or groundmass splits from samples ISLN-180, ISLN-181, ISLN-182. Uncertainties are ± 2σ. Grey filled boxes are not included in the plateau age calculations. Uncertainties are ± 2σ. Grey ellipses correspond to data not included in the inverse isochron calculations.

Figure 4: $^{40}$Ar/$^{39}$Ar Age spectra and Inverse isochron diagrams depicting $^{40}$Ar/$^{39}$Ar experimental results from obsidian splits from samples ISLN-187 and ISLN-188. Uncertainties are ± 2σ. Grey filled boxes are not included in the plateau age calculations. Uncertainties are ± 2σ. Grey ellipses correspond to data not included in the inverse isochron calculations.

Figure 5: $^{40}$Ar/$^{39}$Ar Age spectra and Inverse isochron diagrams depicting $^{40}$Ar/$^{39}$Ar experimental results from groundmass splits from sample ISLN-171. Uncertainties are ± 2σ. Grey filled boxes are not included in the plateau age calculations. Uncertainties are ± 2σ. Grey ellipses correspond to data not included in the inverse isochron calculations.

Figure 6: Lisiecki and Raymo (2005) δ$^{18}$O benthic stack, with the timing of the Icelandic rhyolitic eruptions. The error bars of individual rhyolitic eruptions are here reported at 2 σ. MIS refers to Marine Isotopic Stages. Squares correspond to tephra sampled in marine cores, crosses to tephra from ice cores and circles inland eruptions.

Table 1: GPS coordinates of the samples.

Table 2: Inductively coupled plasma-atomic emission spectrometry (ICP-AES) separated groundmass major (oxide %) and trace (ppm) element compositions of the dated samples (analytical for major and trace elements methods are detailed in Cotten et al. (1995)).

Table 3: New Unspiked K-Ar ages of the Icelandic samples. Ages relative to AC's at 1.1848 ± 0.0006 Ma, Renne et al. (2017) and total $^{40}$K decay constant of Renne et al. (2011) decay constants. Errors are full external errors.

Table 4: Summary of $^{40}$Ar/$^{39}$Ar data from incremental heating experiments. N: number of increments used. Uncertainties ±1σ. Complete set of $^{40}$Ar/$^{39}$Ar measurements for all samples is in Data Repository.

Table 5: Comparison of our geochronological data with published data.

Supplementary Table 1: $^{40}$Ar/$^{39}$Ar data of step heating experiments on samples ISLN-187, ISLN-188, ISLN-171, ISLN-181, ISLN-180 and ISLN-182, relative to 1.1848 ± 0.0006 Ma ACs-2 flux standard.
692

Supplementary Table 2: Raw data from unspiked mass spectrometric measurements of samples. IR =
$^{40}$Ar/$^{36}$Ar isotopic ratio. Subscript atm.ref refers to the atmospheric reference used as an isotopic
reference for the determination of the radiogenic content under identical mass-discrimination
conditions. Subscript cal.Dose refers to a manometrically-calibrated dose of atmospheric argon used
to convert beam intensities into atomic abundances.

693

694

695

696

697

Supplementary Table 3: Comparison of our geochemical data with published data. Composition of PK1-7 is calculated from the mean of the 6 compositions determined in the McGarvie et al. (2007), study. Composition of KER00/34 is from Flude et al., 2010.

698

699

700

Supplementary Table 4: Major (oxide wt%) element compositions of ISLN-187 and ISLN-188 samples, along with potential correlative tephra. TiO₂ vs. SiO₂, Al₂O₃ vs. SiO₂, Na₂O vs. SiO₂, Alkali ratio vs. SiO₂, TiO₂ vs. Al₂O₃, CaO vs. Al₂O₃ and TiO₂ vs. CaO compositional variation diagrams comparing analyses from rhyolites ISLN-187 and ISLN-188 to contemporaneous rhyolitic tephra identified in sequences from North Atlantic marine core MD04-2822 and NGRIP ice core.

701

702

703

704

705

Supplementary Table 5: Major (oxide wt%) element compositions of ISLN-171 sample, along with potential correlative tephra. TiO₂ vs. SiO₂, Al₂O₃ vs. SiO₂, Na₂O vs. SiO₂, Alkali ratio vs. SiO₂, TiO₂ vs. Al₂O₃, CaO vs. Al₂O₃ and TiO₂ vs. CaO compositional variation diagrams comparing analyses from rhyolite ISLN-171 to contemporaneous rhyolitic tephra identified in sequences from North Atlantic marine cores MD99-2289, MD04-2822 and NGRIP ice core.

706

707

708

709

710

Supplementary Table 6: Major (oxide wt%) element compositions of ISLN-181 samples along with potential correlative tephra. TiO₂ vs. SiO₂, Al₂O₃ vs. SiO₂, Na₂O vs. SiO₂, Alkali ratio vs. SiO₂, TiO₂ vs. Al₂O₃, CaO vs. Al₂O₃ and TiO₂ vs. CaO compositional variation diagrams comparing analyses from rhyolite ISLN-181 to contemporaneous rhyolitic tephra identified in sequences from North Atlantic marine core MD04-2822, 985, P57-7.

711

712

713

714

715
<figure><img src="image_2.png" /></figure>

N

<figure><img src="image_3.png" /></figure>

<figure><img src="image_4.png" /></figure>

Krafla

Hofsjökull

Jangjökull

Prestahnúkur

Kerlingarfjöll

Vatnajökull

<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /></figure>

100 km
<figure><img src="image_7.png" /></figure>
<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /></figure>
<figure><img src="image_14.png" /></figure>

<figure><img src="image_15.png" /></figure>

<figure><img src="image_16.png" /></figure>

<figure><img src="image_17.png" /></figure>
Age [ka]

K/Ca

<figure><img src="image_18.png" /></figure>

ISLN-171 - KRAFLA - Hliðarfjall

Lab file # FG-2118 to FG-2126, FG-2266 to FG-2275

Cumulative <sup>39</sup>Ar Released [ % ]

<figure><img src="image_19.png" /></figure>

ISLN-171 Exp # 1
Age : 92.1±5.9 ka
Initial <sup>40</sup>Ar/<sup>36</sup>Ar=301.0±13.0
MSWD=0.59

ISLN-171 Exp # 2
Age : 92.7±5.0 ka
Initial <sup>40</sup>Ar/<sup>36</sup>Ar=299.3±9.7
MSWD=0.12
<figure><img src="image_20.png" /></figure>

MIS 1

MIS 2

MIS 3

MIS 4

5.1
ISLN-187
5.3

ISLN-154

ISLN-153

<figure><img src="image_21.png" /></figure>

ISLN-171

<figure><img src="image_22.png" /></figure>

5.2

5.4

5.5

ISLN-95

MIS 5

MIS 6

MIS 7

MIS 8
<table><caption>Table 1: GPS coordinates of the samples.</caption><thead><tr><th rowspan="2">Volcanic Center</th><th rowspan="2">Location name</th><th rowspan="2">Sample</th><th colspan="2">GPS Coordinates</th></tr><tr><th>North</th><th>West</th></tr></thead><tbody><tr><td rowspan="3">KERLINGARFJÖLL</td><td>Tindur</td><td>ISLN-180</td><td>64°38′22.8″</td><td>19°23′39.1″</td></tr><tr><td>Loðmundur</td><td>ISLN-181</td><td>64°39′31.3″</td><td>19°12′45.7″</td></tr><tr><td>Trollabarmur</td><td>ISLN-182</td><td>64°38′42.3″</td><td>19°08′37.7″</td></tr><tr><td rowspan="2">PRESTAHNUKUR</td><td>-</td><td>ISLN-187</td><td>64°34′55.9″</td><td>20°41′42.2″</td></tr><tr><td>-</td><td>ISLN-188</td><td>64°35′0.7″</td><td>20°41′44.1″</td></tr><tr><td>KRAFLA</td><td>Hiðarfjall</td><td>ISLN-171</td><td>65°40′41.3″</td><td>16°52′12.2″</td></tr></tbody></table>
<table><thead><tr><th></th><th>ISLN-171</th><th>ISLN 180</th><th>ISLN 181</th><th>ISLN 182</th><th>ISLN 187</th><th>ISLN 188</th></tr></thead><tbody><tr><th>SiO2</th><td>76.1</td><td>75.4</td><td>73.1</td><td>69.2</td><td>76.4</td><td>76.4</td></tr><tr><th>TiO2</th><td>0.22</td><td>0.18</td><td>0.22</td><td>0.45</td><td>0.11</td><td>0.11</td></tr><tr><th>Al2O3</th><td>11.0</td><td>12.6</td><td>12.4</td><td>13.1</td><td>12.1</td><td>11.6</td></tr><tr><th>Fe2O3</th><td>2.8</td><td>3.0</td><td>3.2</td><td>6.8</td><td>1.8</td><td>1.7</td></tr><tr><th>MnO</th><td>0.050</td><td>0.092</td><td>0.087</td><td>0.208</td><td>0.049</td><td>0.044</td></tr><tr><th>MgO</th><td>0.10</td><td>0.09</td><td>0.05</td><td>0.14</td><td>0.05</td><td>0.04</td></tr><tr><th>CaO</th><td>1.41</td><td>1.08</td><td>0.91</td><td>2.62</td><td>1.02</td><td>0.96</td></tr><tr><th>Na2O</th><td>3.69</td><td>4.47</td><td>4.96</td><td>4.71</td><td>4.22</td><td>3.89</td></tr><tr><th>K2O</th><td>2.72</td><td>3.30</td><td>3.36</td><td>2.62</td><td>3.14</td><td>3.12</td></tr><tr><th>P2O5</th><td>0.019</td><td>0.013</td><td>0.003</td><td>0.068</td><td>0.004</td><td>0.001</td></tr></tbody></table>

<table><tr><th>LOI</th><td>0.97</td><td>0.42</td><td>0.19</td><td>0.28</td><td>0.19</td><td>0.25</td></tr></table>

<table><tr><th>Total</th><td>99.01</td><td>100.60</td><td>98.45</td><td>100.21</td><td>99.05</td><td>98.12</td></tr></table>

<table><tr><th>Sr</th><td>74.7</td><td>66</td><td>69</td><td>160</td><td>80</td><td>74</td></tr><tr><th>Ba</th><td>556</td><td>660</td><td>884</td><td>673</td><td>744</td><td>685</td></tr><tr><th>Sc</th><td>4.7</td><td>4.2</td><td>0.5</td><td>9.4</td><td>1.2</td><td>1.6</td></tr><tr><th>V</th><td>2.0</td><td>2.3</td><td>2.5</td><td>3.3</td><td>3.8</td><td>3.0</td></tr><tr><th>Cr</th><td>2.1</td><td>1.1</td><td>1.6</td><td>1.0</td><td>0.8</td><td>1.9</td></tr><tr><th>Co</th><td>1.3</td><td>0.0</td><td>0.0</td><td>0.6</td><td>0.0</td><td>0.0</td></tr><tr><th>Ni</th><td>1.6</td><td>0.2</td><td>0.6</td><td>0.4</td><td>1.6</td><td>1.5</td></tr><tr><th>Y</th><td>94.1</td><td>105</td><td>134</td><td>108</td><td>111</td><td>109</td></tr><tr><th>Zr</th><td>374</td><td>439</td><td>618</td><td>1026</td><td>208</td><td>212</td></tr><tr><th>Nb</th><td>46.5</td><td>76.8</td><td>137.6</td><td>76.9</td><td>64.5</td><td>64.7</td></tr></table>

<table><tr><th>Agpaitic index</th><td>0.9</td><td>1.0</td><td>1.1</td><td>0.9</td><td>1.0</td><td>1.0</td></tr></table>
<table><caption>Table 3</caption><thead><tr><th>Sample ID<br>Experience n°</th><th>Weight<br>molten<br>(g)</th><th>K*<br>(wt.%)</th><th>40Ar*<br>(%)</th><th>40Ar*<br>(10<sup>-13</sup><br>mol/g)</th><th>40Ar*<br>Weighted<br>mean ±1σ</th><th>(1)Age ± 2σ<br>ka</th></tr></thead><tbody><tr><td><strong>ISLN-182</strong></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>0403</td><td>1.00844</td><td>2.172 ± 0.027</td><td>16.731</td><td>9.930</td><td></td><td></td></tr><tr><td>0414</td><td>1.00469</td><td>"............"</td><td>15.297</td><td>9.613</td><td>9.812 ± 0.056</td><td>258.9 ± 6.1</td></tr><tr><td><strong>ISLN-180</strong></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>0413</td><td>0.99737</td><td>2.737 ± 0.027</td><td>15.662</td><td>11.588</td><td></td><td></td></tr><tr><td>0417</td><td>1.00418</td><td>"............"</td><td>16.609</td><td>11.712</td><td>11.638 ± 0.072</td><td>243.6 ± 5.8</td></tr><tr><td><strong>ISLN-181</strong></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>0402</td><td>1.00314</td><td>2.788 ± 0.029</td><td>30.697</td><td>9.107</td><td></td><td></td></tr><tr><td>0409</td><td>1.00848</td><td></td><td>25.483</td><td>9.149</td><td>9.125 ± 0.0501</td><td>187.5 ± 4.3</td></tr><tr><td><strong>ISLN-171</strong></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>0458</td><td>0.99694</td><td>2.257 ± 0.023</td><td>12.813</td><td>3.719</td><td></td><td></td></tr><tr><td>0465</td><td>2.00548</td><td>"............"</td><td>11.063</td><td>3.669</td><td>3.693 ± 0.035</td><td>94.4 ± 2.6</td></tr><tr><td><strong>ISLN-188</strong></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>0407</td><td>0.98895</td><td>2.593 ± 0.026</td><td>7.224</td><td>3.744</td><td></td><td></td></tr><tr><td>0411</td><td>1.00180</td><td>"............"</td><td>7.694</td><td>4.008</td><td>3.880 ± 0.050</td><td>85.7 ± 2.8</td></tr><tr><td><strong>ISLN-187</strong></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>0404</td><td>1.01346</td><td>2.609 ± 0.026</td><td>16.755</td><td>3.927</td><td></td><td></td></tr><tr><td>0410</td><td>1.01717</td><td>"............"</td><td>16.165</td><td>3.711</td><td>3.842 ± 0.039</td><td>84.3 ± 2.4</td></tr></tbody></table>

(1): Ages relative to AC's at 1.848 Ma, Niespolo et al. (2017) and the total $^{40}$K decay constant of Renne et al., 2011.
<table><caption>Table 4: Summary of ⁴⁰Ar/³⁹Ar data from incremental heating experiments on obsidians and groundmass splits</caption><thead><tr><th rowspan="2">Sample<br>Lab file #.</th><th rowspan="2">wt.<br>(mg)</th><th rowspan="2">K/Ca<br>(total)</th><th rowspan="2">Total Fusion<br>Age (ka)</th><th colspan="4">Age Spectrum</th><th colspan="5">Isochron Analysis</th></tr><tr><th>Increments<br>used (°C)</th><th><sup>39</sup>Ar<br>(%)</th><th>Age ± 2 σ <sup>(1)</sup><br>(ka)</th><th>MSWD</th><th>n/N</th><th>MSWD</th><th><sup>40</sup>Ar/<sup>36</sup>Ar ± 2 σ<br>intercept</th><th>Age ± 2 σ <sup>(1)</sup><br>(ka)</th><th>K-Ar ± 2σ<br>(ka)</th></tr></thead><tbody><tr><td colspan="13"><strong>ISLN-187 - Prestahnúkur volcano</strong></td></tr><tr><td>FG-2127 to FG-2136</td><td>133</td><td>3.42</td><td>89.2 ± 4.3</td><td>597-1195</td><td>100.0</td><td>88.1 ± 3.6</td><td>0.97</td><td>10 of 10</td><td>0.97</td><td>303.2 ± 7.5</td><td>86.3 ± 4.7</td><td></td></tr><tr><td>FG-2245 to FG-2254</td><td>131</td><td>4.39</td><td>85.0 ± 3.0</td><td>665-1119</td><td>99.2</td><td>85.6 ± 1.9</td><td>0.67</td><td>8 of 10</td><td>0.40</td><td>301.4 ± 3.7</td><td>85.1 ± 2.0</td><td></td></tr><tr><td colspan="13">weighted mean plateau and isochron ages from two experiments:</td></tr><tr><td colspan="13"><strong>ISLN-188 - Prestahnúkur volcano</strong></td></tr><tr><td>FG-2137 to FG-2146</td><td>127</td><td>3.68</td><td>89.7 ± 5.0</td><td>737-1197</td><td>100.0</td><td>85.1 ± 3.0</td><td>0.86</td><td>10 of 10</td><td>0.33</td><td>303.1 ± 3.7</td><td>84.0 ± 3.1</td><td></td></tr><tr><td>FG-2255 to FG-2264</td><td>130</td><td>3.60</td><td>83.5 ± 4.8</td><td>665-1119</td><td>96.4</td><td>83.6 ± 2.8</td><td>0.26</td><td>9 of 10</td><td>0.29</td><td>299.1 ± 3.2</td><td>83.5 ± 3.0</td><td></td></tr><tr><td colspan="13">weighted mean plateau and isochron ages from two experiments:</td></tr><tr><td colspan="13"><strong>ISLN-171 -Krafla- Hliðarfjall</strong></td></tr><tr><td>FG-2118 to FG-2126</td><td>127</td><td>2.10</td><td>93.7 ± 3.8</td><td>660-1194</td><td>100.0</td><td>93.0 ± 3.0</td><td>0.52</td><td>9 of 9</td><td>0.59</td><td>301.0 ± 13.0</td><td>92.1 ± 5.9</td><td></td></tr><tr><td>FG-2266 to FG-2275</td><td>123</td><td>1.97</td><td>93.5 ± 3.2</td><td>664-1214</td><td>100.0</td><td>93.0 ± 2.3</td><td>0.11</td><td>9 of 9</td><td>0.12</td><td>299.3 ± 9.7</td><td>92.7 ± 5.0</td><td></td></tr><tr><td colspan="13">weighted mean plateau and isochron ages from two experiments:</td></tr><tr><td colspan="13"><strong>ISLN-181 - Kerlingarfjöll - Loðmundur</strong></td></tr><tr><td>FG-2106 to FG-2116</td><td>127</td><td>3.74</td><td>192.1 ± 1.3</td><td>599-1177</td><td>100.0</td><td>191.6 ± 2.6</td><td>0.26</td><td>10 of 10</td><td>0.30</td><td>299.6 ± 6.9</td><td>191.1 ± 4.3</td><td></td></tr><tr><td>FG-2225 to FG-2234</td><td>125</td><td>3.98</td><td>192.2 ± 1.8</td><td>664-1117</td><td>99.0</td><td>191.7 ± 2.8</td><td>0.51</td><td>8 of 10</td><td>0.21</td><td>302.9 ± 5.5</td><td>190.7 ± 3.1</td><td></td></tr><tr><td colspan="13">weighted mean plateau and isochron ages from two experiments:</td></tr><tr><td colspan="13"><strong>ISLN-180 - Kerlingarfjöll - Tindur</strong></td></tr><tr><td>FG-2157 to FG-2166</td><td>124</td><td>3.26</td><td>235.2 ± 7.6</td><td>659-1212</td><td>99.4</td><td>233.7 ± 6.9</td><td>0.94</td><td>9 of 10</td><td>1.12</td><td>300.5 ± 4.2</td><td>226.7 ± 16.4</td><td></td></tr><tr><td>FG-2286 to FG-2296</td><td>130</td><td>3.36</td><td>238.7 ± 6.5</td><td>605-1214</td><td>100.0</td><td>238.3 ± 5.9</td><td>0.67</td><td>10 of 10</td><td>0.83</td><td>299.1 ± 4.5</td><td>236.8 ± 13.7</td><td></td></tr><tr><td colspan="13">weighted mean plateau and isochron ages from two experiments:</td></tr><tr><td colspan="13"><strong>ISLN-182 - Kerlingarfjöll - Trollabarmur</strong></td></tr><tr><td>FG-2147 to FG-2156</td><td>132</td><td>1.15</td><td>254.9 ± 11.8</td><td>599-1119</td><td>98.7</td><td>259.2 ± 10.7</td><td>0.54</td><td>9 of 10</td><td>0.66</td><td>297.3 ± 6.1</td><td>265.3 ± 31.2</td><td></td></tr><tr><td>FG-2276 to FG-2285</td><td>133</td><td>1.17</td><td>257.1 ± 8.4</td><td>663-1214</td><td>99.9</td><td>257.0 ± 6.8</td><td>0.31</td><td>9 of 10</td><td>0.38</td><td>298.6 ± 3.8</td><td>257.0 ± 11.2</td><td></td></tr><tr><td colspan="13">weighted mean plateau and isochron ages from two experiments:</td></tr></tbody><tfoot><tr><td colspan="13">Ages calculated relative Acs-2 at 1.1848 Ma (Niespolo et al., 2017) and the total ⁴⁰K decay constant of Renne et al., 2011 (1): Full external error.</td></tr></tfoot></table>
<table><thead><tr><th>Sample</th><th>Volcano</th><th>Location</th><th>K-Ar age<br>±2σ (ka)</th><th><sup>40</sup>Ar/<sup>39</sup>Ar age<br>±2σ (ka)</th><th>Combined K-<br>Ar and<br><sup>40</sup>Ar/<sup>39</sup>Ar age<br>±2σ (ka)</th><th><sup>40</sup>Ar/<sup>39</sup>Ar age<br>±2σ (ka)</th><th><sup>40</sup>Ar/<sup>39</sup>Ar age<br>±2σ (ka)</th><th>U-Th age ±2σ<br>(ka)</th><th><sup>40</sup>Ar/<sup>39</sup>Ar age<br>±2σ (ka)</th></tr><tr><td></td><td></td><td></td><td>(This work)</td><td>(This work)</td><td>(This work)</td><td>(McGarvie et<br>al, 2007)</td><td>(Clay et<br>al, 2015)</td><td>Hampton et<br>al., 2021</td><td>(Flude et al.,<br>2010)</td></tr></thead><tbody><tr><td>ISNL-180</td><td>Kerlingarfjöll</td><td>Tindur</td><td>243.6±5.8</td><td>236.4±4.5</td><td>239.1±3.6</td><td></td><td></td><td></td><td>285±50<br>246±48<br>312±54</td></tr><tr><td>ISNL-181</td><td>Kerlingarfjöll</td><td>Loðmundur</td><td>187.5±4.3</td><td>191.6±1.9</td><td>191.0±1.7</td><td></td><td></td><td></td><td>184±20</td></tr><tr><td>ISNL-182</td><td>Kerlingarfjöll</td><td>Trollabarmur</td><td>258.9±6.1</td><td>257.6±5.7</td><td>258.2±4.2</td><td></td><td></td><td></td><td>249±28</td></tr><tr><td>ISNL-187</td><td>Prestahnukur</td><td></td><td>84.3±2.4</td><td>86.1±1.7</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>ISNL-188</td><td>Prestahnukur</td><td></td><td>85.7±2.8</td><td>84.3±2.0</td><td>85.2±1.1</td><td>89±48</td><td>132±19</td><td></td><td></td></tr><tr><td>ISLN-171</td><td>Krafla</td><td>Hliðarfjall</td><td>94.4±2.6</td><td>93.0±1.8</td><td>93.5±1.5</td><td></td><td></td><td>83.3±9.2</td><td></td></tr></tbody></table>