Mid-Holocene Eruptive Activity of the Hekla Volcanic System

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Mid-Holocene eruptive activity in the Hekla volcanic system

Daníel Freyr Jónsson

Faculty of Earth Sciences University of Iceland 2018

Mid-Holocene eruptive activity in the Hekla volcanic system

Daníel Freyr Jónsson

60 ECTS thesis submitted in partial fulfillment of a Magister Scientiarum degree in Geology

MS Committee Esther Ruth Guðmundsdóttir Bergrún Arna Óladóttir Olgeir Sigmarsson

Master’s Examiner Magnús Á Sigurgeirsson

Faculty of Earth Sciences School of Engineering and Natural Sciences University of Iceland Reykjavik, May 2018

Mid-Holocene eruptive activity in the Hekla volcanic system 60 ECTS thesis submitted in partial fulfillment of a Magister Scientiarum degree in geology

Faculty of Earth Sciences School of Engineering and Natural Sciences University of Iceland Askja, Sturlugata 7 101, Reykjavik Iceland

Telephone: 525 4000

Bibliographic information: Daníel Freyr Jónsson, 2018, Mid-Holocene eruptive activity in the Hekla volcanic system, Master’s thesis, Faculty of Earth Sciences, University of Iceland, pp. 104.

Printing: Háskólaprent Reykjavik, Iceland, May 2018

Abstract

Hekla volcano is one of the most active volcano in Iceland and is known for its explosive eruptions. Studies on prehistoric tephra have focused on the large plinian eruptions, namely Hekla 5, Hekla 4 and Hekla 3. Here, the aim is to improve the knowledge of other prehistoric eruptions and the volcanic history of Hekla by investigating the mid-Holocene tephra layers Hekla DH (6650 cal yr BP), Hekla Mó (~6060 cal yr BP) and Hekla Ö (6060 cal yr BP).

The Hekla DH, Hekla MÓ and Hekla Ö tephra layers have been described, mapped and their composition analyzed in several soil sections proximal to Hekla proper. The magma composition of these three layers range from rhyolite through andesite and basaltic- andesite to basalt. Hekla DH represents the earliest confirmed eruption of basaltic andesite from the Hekla system, which subsequently has become the dominant product of the system. The basalts (SiO2 ~46-47%) observed in these three Hekla tephra layers consists of two types; one with TiO2 <3 wt% and Al2O3 >15 wt% and the other with TiO2 >3 wt% and Al2O3 <14 wt% for a MgO concentration of >6 wt% and <6 wt% respectively. Chemical composition of basaltic andesite in the Hekla DH tephra indicates that lower oxygen fugacity conditions may have existed in the magmatic system beneath Hekla prior to the Hekla DH eruption, compared to later times. Increased magmatic fugacity during mid- Holocene could have resulted from melting different regions of heterogeneous mantle source due to deglaciation of the area.

New isopach maps are compiled for all three tephra layers. Mapping of the dispersal of the Hekla DH tephra has confirmed a source in the Valagjá area, in the northern part of the Hekla system. With the chemical dataset obtained in this study, a possible relationship between Hekla Ö and a distal Hekla tephra in NW, W and SW-Iceland is suggested. Consequently, the Hekla Ö tephra possibly covers as much as 80% of terrestrial Iceland.

In addition, thirteen tephra layers of mid-Holocene age (4200-7100 cal yr BP) were investigated in a soil section in Tagl and analyzed for source provenance. Eleven of the layers originate from Katla, while the chemical composition of two other layers points to a source in Vatnafjöll. One layer is a silicic Katla tephra layer, with the composition and location in the stratigraphy resembling that of the SILK N-1 tephra, which was deposited around 5800 cal BP.

Ágrip

Hekla er eitt af virkustu eldstöðvarkerfum Íslands og er þekkt fyrir stór sprengigos á nútíma. Rannsóknir á forsögulegri eldvirkni Heklu hafa mikið til beinst að stóru plínísku sprengigosunum Heklu 5, Heklu 4 og Heklu 3. Markmiðið með þessari rannsókn er að auka þekkingu á forsögulegri eldvirkni Heklu með rannókn á gjóskulögunum Heklu DH (~6650 ára), Heklu Mó (~6060 ára) og Heklu Ö (6060 kvörðuð ár).

Gjóskulögin Hekla DH, Hekla Mó og Hekla Ö hafa verið kortlögð og efnasamsetning þeirra í jarðvegsniðum í næsta nágrenni Heklu rannsökuð. Efnasamsetning þessara gjóskulaga spannar ríólít, andesít, basalt-andesít og basalt. Gosið sem myndaði Heklu DH er elsta staðfesta gosið á Heklukerfinu þar sem basalt-andesít kvika kom upp, sem síðar meir hefur verið samsetning meirihluta allra gosefna kerfisins. Basaltið (SiO2 ~46-47%) sem finnst í þessum þrem Heklugjóskulögum er af tvennu tagi; annað með TiO2 <3 wt%, Al2O3 >15 wt% og MgO >6wt% en hitt með TiO2 >3 wt%, Al2O3 <14 wt% og MgO <6 wt%. Efnasamsetning basalt-andesíts í Heklu DH bendir til þess að súrefnisþrýstingur í kvikukerfinu undir Heklu hafi verið lægri fyrir eldgosið sem myndaði gjóskulagið, samanborið við það sem hann hefur verið síðar meir. Breyting á súrefnisþrýstingi kann að hafa átt sér stað í kjölfar hörfun jökuls af svæðinu sem létti af þrýstingi á svæðinu og gæti því hafa orsakað að önnur svæði möttulsins hófu að bráðna.

Ný útbreiðslukort eru gerð fyrir öll þrjú gjóskulögin. Kortlagning á Heklu DH hefur staðfest uppruna gjóskunnar frá Valargjársvæðinu í nyrðri hluta kerfisins. Efnagreiningar sem fengust í þessari rannsókn gætu bent til tengsla Heklu Ö við áður greinda gjósku frá Heklu á Vestfjörðum og víðar á vestanverðu landinu. Því gæti Hekla Ö gjóskan hafa borist yfir allt að 80% af Íslandi.

Þrettán önnur gjóskulög fundust ásamt Heklulögunum þremur í jarðvegssniði í Tagli sem spannar 2900 ára tímabil fyrir 4200 til 7100 árum síðan. Af þessum 13 lögum voru 11 rakin til Kötlu, en efnasamsetning tveggja bendir til uppruna í Vatnafjöllum. Á meðal Kötlulaganna var eitt með dasítsamsetningu, sem líklegast er hið ̴ 5800 ára SILK N-1 gjóskulag.

Table of Contents

List of Figures ...... ix

List of Tables ...... xi

Acknowledgements ...... xiii

1 Introduction ...... 15 1.1 Geological background...... 16 1.1.1 Regional geology ...... 16 1.1.2 Hekla volcanism during the Holocene ...... 17 1.1.3 The Hekla suite ...... 19 1.2 The Hekla DH, Hekla Mó and Hekla Ö tephra layers ...... 20 1.2.1 Hekla DH ...... 20 1.2.2 Hekla Mó ...... 21 1.2.3 Hekla Ö ...... 21

2 Methods ...... 23 2.1 Field work and sampling ...... 23 2.2 Sample preparation ...... 25 2.3 Microprobe and whole rock analysis...... 25

3 Results ...... 29 3.1 Stratigraphy ...... 29 3.1.1 Hekla DH ...... 29 3.1.2 Hekla Mó ...... 31 3.1.3 Hekla Ö ...... 31 3.1.4 The Tagl tephra layers ...... 32 3.2 Dispersal and volumes...... 32 3.3 Geochemistry...... 37 3.3.1 Hekla DH, Hekla Mó and Hekla Ö ...... 37 3.3.2 Whole-rock analysis...... 39 3.3.3 The Tagl tephra layers ...... 40

4 Discussions ...... 43 4.1 Stratigraphy ...... 43 4.1.1 Dispersal and eruptive sites ...... 43 4.1.2 Hekla Ö and relation to distal tephra deposits ...... 44 4.1.3 Palagonized units of Hekla DH...... 48 4.1.4 Stratigraphy of the Tagl section ...... 50 4.2 Geochemistry...... 51 4.2.1 The mid-Holocene magmatic system of Hekla ...... 51 4.2.2 Hekla and Katla basalts ...... 54 4.3 Hekla during the mid-Holocene ...... 55

vii 5 Conclusions ...... 57

6 References...... 59

Appendix 1 ...... 65

viii List of Figures

Figure 1. The volcanic systems of the Eastern Volcanic Zone (EVZ)...... 16

Figure 2. Hekla seen from SW ...... 17

Figure 3. Map of the study area...... 24

Figure 4. Locations of the three craters sampled for whole-rock analysis ...... 25

Figure 5. Simplified illustrations of the Tagl and Áfangagil soil sections ...... 27

Figure 6. Photos from field work ...... 30

Figure 7. Microscopip view of grains from Hekla DH...... 30

Figure 8. Deformed boundary between Hekla Mó and Hekla Ö...... 31

Figure 9. Isopach maps of Hekla DH...... 34

Figure 10. Isopach maps of Hekla Ö ...... 35

Figure 11. Isopach map of the light-colored base of Hekla Ö ...... 36

Figure 12. Isopach map of Hekla Mó ...... 36

Figure 13. Major oxide totals of Hekla DH, Hekla Mó and Hekla Ö...... 38

Figure 14. Results of whole-rock analysis ...... 40

Figure 15. Hekla Ö plotted with distal Hekla tephra layers...... 45

Figure 16. Hekla Ö compared with proposed distal deposit of the tephra in Elliðarárdalur and Lake Hestvant ...... 46

Figure 17. A map showing the decreasing silica content of the Hekla Ö tephra from east to west within terrestrial Iceland...... 48

Figure 18. Microscopic view of grains from Hekla DH...... 49

Figure 19. Differentiation trends s calculated with the Petrolog ...... 52

Figure 20. The intermediate magma compositions of Hekla DH and Hekla Mó ...... 53

Figure 21. The basalts of Hekla DH, Hekla Mó and Hekla Ö ...... 53

Figure 22. Overlap between basalts from Hekla and Katla ...... 54

ix Figure 23. The different rock types erupted from the Hekla system plotted against time...... 56

x List of Tables

Table 1. The prehistoric silicic Hekla tephra layers...... 18

Table 2. Grain size terminology ...... 23

Table 3.The typical composition of each rock type identified in Hekla DH, Hekla Mó and Hekla Ö...... 39

Table 4. Average major oxide combositions of the 13 Tagl tephra layers...... 41

Table 5. The tephrostratigraphy of the Tagl section between Hekla 5 and Hekla 4 ...... 51

Acknowledgements

Volcanism is something that I have always been interested in and I regard it as a privilege to have been able to do my studies within my field of interest. Focusing on Hekla was extra special for me, as I grew up in Selfoss with Hekla always visible in the distance. Many people helped me along the way to make this thesis materialize, for which I am very thankful.

I want to thank my advisor Esther Ruth Guðmundsdóttir for her support and guidance throughout the last two years, especially during the last six months. Esther always had time to help and I could not imagine a better advisor.

Guðrún Larsen is deeply thanked for all her time and hard work. She showed the project immense interest as it developed and provided unconditional support. Her ocean of knowledge and will to share it helped a great deal to improve my work. I have really learned a lot from Guðrún.

Olgeir Sigmarsson and Begrún Arna Óladóttir always made themselves available for me. I want to thank them both for all the useful discussions we had, along with invaluable input during the final phases of the project.

The interest shown by Esther, Guðrún, Olgeir and Bergrún in this project really motivated me and working with them has been a great privilege.

I want to thank Eniko Bali, who assisted me with data processing during the latter stages of the project.

I would also like to mention Hreggviður Norðdahl. After a short, random chat with him three years ago, where he encouraged me to continue my studies in geology, I decided to apply for the MS program. Without his encouragement, this thesis would have never have emerged.

My good feltfélag-friends get special thanks - Magnús Freyr Sigurkarlssnon, Guðbjörn Margeirsson, Einar Sindri Ólafsson, Sigríður Inga Svavarsdóttir, Margrét “Rokk” Gísladóttir and Stefán Ármann Þórðarson. All our lunches, coffee breaks and weekly miðvikudagsfundur really helped keeping the spirit up.

I am also very grateful for having Berglind Hrönn Einarsdóttir by my side during my years of studying.

xiii

1 Introduction

The Hekla volcanic system has been one of the most active systems in Iceland throughout the Holocene. The activity in the system has been highly diverse, with volcanic eruptions ranging from fissure eruptions of basaltic lavas to plinian eruptions of silicic rocks (Thorarinsson 1967a; Larsen and Thorarinsson 1977; Jakobsson 1979; Grönvold et al. 1983; Gudmundsson et al., 1992; Sverrisdóttir 2007; Höskuldsson et al., 2007; Meara, 2011; Janebo et al., 2016). The historical activity of the system is well known, mainly through the extensive work of Thorarinsson (1967a). Most studies on the pre-historic volcanic activity of the Hekla volcanic system have focused on the large, plinian silicic eruptions; Hekla 5, Hekla 4 and Hekla 3, while the activity between these large eruptions has gathered less scientific interest. However, a few studies have focused on the tephra sequence of the Hekla system between the pre-historic plinian eruptions (e.g. Larsen and Vilmundardóttir 1992; Róbertsdóttir 1992; Larsen et al., 2002; Róbertsdóttir et al., 2002; Gudmundsdóttir et al., 2011; Meara, 2011).

The aim of this study is to improve the knowledge on mid-Holocene explosive volcanism in the Hekla volcanic system. Numerous explosive eruptions in the Hekla volcano affected Icelandic society during the middle ages (Thorarinsson, 1967a). The system still has the potential to disrupt modern society by damaging and weakening important infrastructure and cause problems for air traffic, as eruptions in Hekla usually start with a powerful subplinian or Plinian phase. It is thus important to recognize and understand the eruptive history of the system to be better prepared to forecast and cope with future eruptions in the system.

A 3000 year-long time slice of the eruptive history of the Hekla system, between the Hekla 4 and Hekla 5 eruptions 4200 and 7100 years ago, is investigated in this study. The main research questions were; 1) what was the nature of the activity in Hekla in the mid- Holocene and 2) was it different from that of later periods?

Multiple soil sections have been investigated in proximal settings of Hekla volcano. The main focus was on improving the knowledge on the three mid-Holocene eruptions in the Hekla volcanic system; Hekla DH (6650 BP), Hekla Mó ( ̴ 6060 BP) and Hekla Ö (6060 BP) (Gudmundsdóttir et al., 2011).

Additionally, the tephrostratigraphy of a soil section in Tagl (18 km north of Hekla) is investigated to gain information on other explosive eruptions from Hekla and to improve the knowledge of the local stratigraphy between the Hekla 5 and Hekla 4 eruptions.

15 1.1 Geological background

1.1.1 Regional geology

The Hekla volcanic system is located at the western margin of the Eastern Volcanic Zone (Figure 1), right where it intersects with the South Iceland Seismic Zone. The system measures 40 km long and 7 km wide (Jakobson, 1979) and is adjacent to the neighboring Vatnafjöll system to the east, which is sometimes considered as a part of the Hekla system (Jóhannesson and Sæmundsson, 1998). Chemical trends can however be used to distinguish between the two systems (Jakobsson, 1979).

Hekla proper (Figure 2) is the focal point of activity in the system. Hekla is an elongated volcanic ridge that rises 1490 m above sea level. The elongated shape of Hekla is the result of repeated eruptions on the Hekla fissure (Icelandic Heklugjá), a 5.5 km long fissure which runs through the summit of the volcano and all major Hekla eruptions are thought be confined to the Hekla fissure. Out of active Icelandic volcanoes, Hekla is unusual as it has

Figure 1. The volcanic systems of the Eastern Volcanic Zone (EVZ). Modified from Jóhannesson and Sæmundsson (1998). Central volcanoes are H: Hekla, K: Katla, B: Bárðarbunga, G: Grímsvötn, T: Torfajökull, E: Eyjafjallajökull. Here Hekla and Vatnafjöll are shown as one system but Jakobsson (1979) outlines Vatnafjöll in the southestern part as one as a separate system.

16 Figure 2. Hekla seen from SW.

produced large volumes of intermediate as well as silicic magma during the Holocene (Thorarinsson, 1967b; Jakobsson, 1979).

It has been speculated that Hekla is a central volcano in its early stages, as there is no visible caldera formation (Larsen and Thorarinsson, 1977). Furthermore, no geothermal activity is observed at the surface and the system has only a moderately mature fissure swarm (Sigmarsson et al., 1992; Thordarson and Larsen, 2007). Hyaloclastite formations in Hekla are exposed up to 900 m a.s.l., indicating a post-glacial build-up of the edifice (Sigvaldason, 1975; Jóhannesson et al., 1990). The Holocene basalt lava flows from the system are also more evolved in composition compared to basalt formations of Pleistocene age in the area, indicating a more evolved volcanism during the Holocene and that the current plumbing system beneath Hekla was established during the same period (Sigmarsson et al., 1992).

1.1.2 Hekla volcanism during the Holocene

A total of 18 explosive/mixed eruptions have occurred in Hekla during historical time, with the 1104 CE eruption (Hekla 1) being the first historical eruption of the system. Five additional fissure eruptions have also occurred during the same period in the vicinity of Hekla (Thorarinsson 1967a; Höskuldsson et al., 2007). About 40 pre-historic Holocene tephra layers of Hekla origin are also known in Icelandic soil and lake sediments (Larsen and Thorarinsson, 1977; Larsen and Vilmundardóttir 1992; Róbertsdóttir, 1992; (Róbertsdóttir et al., 2002; Jóhannsdóttir, 2007; Larsen and Eiríksson, 2008; Gudmundsdóttir et al., 2011; Gunnarson, 2017; Harning et al., 2018, Óladóttir unpublished data).

The main product of the Hekla system during the Holocene has been basaltic andesite (52- 3 57% SiO2), with an estimated dense rock equivalent (DRE) of 14.3 km . The system has 3 3 also extruded an estimated 7 km of basaltic lava (<52% SiO2), 3.7 km of andesite (57-

17 3 62% SiO2) and 7.2 km of dacite/rhyolite (>62% SiO2) during the Holocene (Jakobsson, 1979; Grönvöld et al., 1983; Gudmundsson et al., 1992, Höskuldsson et al., 2007).

The early Holocene activity in the Hekla system was characterized by basaltic fissure eruptions (Jakobsson, 1979). Multiple basaltic lava flows erupted from fissures south and south-east of Hekla from 8000 to 5000 BP, until the activity migrated to the northern part of the system. The activity has continued in the northern part to the present day, while basaltic activity resumed 1000-1500 BP in the southern part (Jakobsson, 1979). No basaltic lava is known to have been erupted from Hekla proper and the basaltic eruptions are confined to the surrounding plain. Basalts of Pleistocene age are however found in hyaloclastite formations at base of Hekla. The most recent basaltic fissure eruption occurred in 1913 in the Lambafit-Mundafell area east of Hekla (Jakobsson, 1979).

The earliest known eruption of rhyolite magma was the Hekla 5 eruption (7100 cal. BP; Larsen et al., 2013). With a volume of 2.5 km3 of freshly fallen tephra, Hekla 5 was the first in a series of large, infrequent plinian eruptions of silicic tephra which characterized the activity in Hekla for the next 4000 years (Table 1). Including Hekla 5, several major, widespread silicic tephra layers are known from this period; Hekla Ö, Hekla 4, Hekla S and Hekla 3 (e.g. Larsen and Thorarinsson, 1977; Gudmundsdóttir et al., 2011; Larsen et al., 2013 and references therin). The first historical eruption, the Hekla 1 eruption (1104 CE), is comparable to these large plinian pre-historic eruptions (Larsen and Thorarinsson, 1977).

Table 1. The prehistoric silicic Hekla tephra layers. Based on data from Larsen and Thorarinsson (1977), Gudmundsdóttir et al. (2011), Larsen et al. (2013) and Stevenson et al. (2015).

Tephra layer Volume as freshly Area covered on land within Age (cal. yr. fallen tephra (km3) 0.1cm isopach line (km2) BP)

Hekla 5 2.5 62.000 7100

Hekla Ö ̴ 1 59.000 6060

Hekla 4 11.2 78.000 4200

Hekla S 2 19.000 3900

Hekla 3 13.3 80.000 3100

The initial products of the large plinian eruptions have a dacite/rhyolite composition, while the last erupted material is usually basaltic andesite or andesite. This range in silica content is observed in chemically zoned tephra deposits, were the silica content decreases upwards within each deposit. The Hekla 5 tephra is however different from the other major silicic Hekla tephra deposits as it lacks all andesite, which led Sverrisdóttir (2007) to suggest that intermediate magma production in the Hekla system only initiated after Hekla 5. However, Jóhannesson et al. (1994) reported three tephra layers of intermediate composition older than Hekla 5 which they consider of Hekla origin and two early-Holocene andesitic tephra

18 layers ( ̴ 9600 BP and ̴ 9400 BP) of Hekla origin were recently discovered in lake sediments in NW-Iceland (Harning et al., 2018).

Because of the chemically zoned products of Hekla and spatial distribution of eruptive sites of different rock types, it has been speculated that a zoned magma chamber exists beneath Hekla. Sigmarsson et al., (1992) proposed that an elongated, bell-shaped magma chamber lies under Hekla. The magma chamber is likely to be located at significant depth, as lack of S-wave attenuation exclude the possibility of any sizeable magma chamber existing 4-14 km below Hekla (Soosalu and Einarsson, 2004). Petrological modelling from the 2000 eruption also suggested that the erupted magma originated from a depth of 14 km (Höskuldsson et al., 2007). Furthermore, Ófeigsson et al. (2011) detected inflation signals following the last two Hekla eruptions and concluded that it was generated by deep magma storage chamber a depth of 14 to 20 km, which could indicate that the minimum depth of the chamber is at 14 km.

It has also been proposed that a vast magma chamber does not exists beneath Hekla and that magma resides in individual magma pockets in the crustal region, which could explain the observed spatial distribution of different rock types in the area (Chekol et al., 2011). According to that model, basaltic magma is periodically injected from a deep-seated reservoir into the crust where it forms individual pockets. Due to fractional crystallization, the basalt evolves to basaltic andesite while the heat from the magma simultaneously causes partial melting of the surrounding crust, generating dacite melt.

1.1.3 The Hekla suite

The Hekla basalts belong to the transitional alkali series. The basalts are relatively rich in Fe, Ti and K and are sometimes termed Fe-Ti basalts. Hyaloclastite basalt formations of Pleistocene age in the area display a more primitive composition than the Holocene basalts, being higher in MgO content (>6.5%) while having significantly lower contents of Ti and Fe (Jakobsson, 1979; Sigmarsson et al., 1992; Sverrisdóttir unpublished data). Even though the primitive composition observed in the Pleistocene formations is not seen in Holocene lava flows from the system, at least five Holocene tephra layers of Hekla origin display this composition, including the ̴ 9000 BP Hekla VF tephra (Jóhannsdóttir, 2007; Óladóttir, 2009; Harning et al., 2018). This primitive basalt composition has however not been identified in Hekla products younger than 2600 years (Óladóttir, unpublished data).

Isotope studies point to a significantly more crustal contamination in the Holocene basalts, which is likely to occur in a deep-seated reservoir before the basalts evolve by fractional crystallization to basaltic andesite. This temporal change in Hekla basalts from Pleistocene to Holocene has been thought to indicate that the current plumbing system beneath Hekla was likely to only have been established during the Holocene (Sigmarsson et al., 1992).

The basaltic andesite in the Hekla system is produced by fractional crystallization of a basaltic parent magma. Sigmarsson et al. (1992) calculated that the basaltic andesite erupted in the 1970 Hekla eruption was produced by about 60% fractional crystallization of a basalt magma with a composition of Fe-Ti basalts like those erupted around Hekla during the Holocene. In the historical period (after 1104 CE), basaltic andesite with a rather uniform composition of around 54-55% SiO2 has usually been the final composition of each Hekla eruption. The composition of the initial products of each eruption is however

19 dependent on the length of the preceding repose period, i.e. longer repose periods leads to more siliceous initial products (Thorarinsson, 1967a).

The andesites and more silicic rocks of the Hekla system are chemically unrelated to the less evolved basalts and basaltic andesite from the system and belong to the low-alkali tholeiite series (Jakobsson et al., 2008). Isotope studies show that dacite melt in the Hekla system is most likely generated by partial melting of metabasalts in the amphibolite facies beneath Hekla (Sigmarsson et al., 1992). Fractional crystallization of the dacite in the upper part of a magma storage zone can produce rhyolites, while mixing with basaltic andesites forms hybrid magmas of andesitic composition. With a long repose time (>200 years), large volumes of dacite will be created (>1 km3), while a shorter repose period (<100 years) will only allow for small volumes of dacite melt to be formed. In the largest Hekla eruptions, where plenty of dacite is available after a long repose time, thorough mixing between the two end-members (dacite and basaltic andesite) converts the bulk of the basaltic andesite to andesite. In most cases however, the volume of dacite melt is too small to convert all the basaltic andesite to andesite (Sigmarsson et al., 1992).

1.2 The Hekla DH, Hekla Mó and Hekla Ö tephra layers

The three Hekla tephra layers that are the focus of this study are stratigraphically located between two of the major silicic tephra layers from Hekla, Hekla 5 and Hekla 4. Both Hekla DH and Hekla Ö have been chemically analyzed and mapped before (Gudmundsdóttir et al., 2011), while no published data exists for Hekla Mó.

1.2.1 Hekla DH

Earlier studies on the Hekla DH (Hekla Dark Horse, Icelandic Blakkur) tephra had confirmed a Hekla origin for the tephra from inspection of grain characteristics and microprobe analysis. The tephra has a relatively narrow dispersal axis which trends NNE from Hekla and extends offshore, where it has been identified in marine sediments cores off North Iceland. Preliminary volume estimates of the deposit give a volume of ̴ 0.8 km3 of freshly fallen tephra, or ̴ 0.5 km3 of compacted tephra. Dating with soil accumulation rate methods (SAR) in Central and North Iceland indicate that the tephra was deposited 6650 years BP (Gudmundsdóttir et al., 2011).

In terrestrial soil sections in Central and North Iceland, the Hekla DH tephra is stratigraphically located in the lower part of the soil and tephra sequence between Hekla 5 and Hekla 4. The tephra has a dark grey base and a black top and has been described to consist almost entirely of grains in the lapilli range within 50 km from source (Gudmundsdóttir et al., 2011).

20 1.2.2 Hekla Mó

No previously published data exists about the black Hekla Mó tephra. The tephra is likely to have been erupted shortly before the major Hekla Ö eruption (6060 BP), as the former is typically seen as a thin tephra layer located directly beneath Hekla Ö. The contact between the two is generally sharp, while it is however often heavily deformed.

1.2.3 Hekla Ö

The Hekla Ö tephra is known to have a widespread dispersal over terrestrial Iceland. Hekla Ö has been identified in soil sections over 200 km away from Hekla (Óladóttir, 2011; Eddudóttir et al., 2016) and in offshore sediments collected north of Iceland (Gudmundsdóttir et al., 2011). More recently, the tephra has been discovered in sediment cores from Lake Lögurinn in Eastern Iceland (Gudmundsdóttir et al., 2016).

In distal deposits, the silica content of the Hekla Ö tephra generally decreases counter- clockwise around Iceland. Preliminary volume calculations of the tephra indicate a volume of ̴ 1 km3 of freshly fallen tephra, or ̴ 0.6 km3 of compacted tephra. Radiocarbon dating yields an age of 6060 cal. yr BP (Gudmundsdóttir et al., 2011) and soil accumulating rate (SAR) calculations by Óladóttir (2009) also indicate an age of 6100 years BP for Hekla Ö, fitting well with the radiocarbon dates.

Mapping of the tephra had previously confirmed a Hekla origin for Hekla Ö. Inspection of grain characteristics, together with microprobe glass analysis, also show distinct Hekla characteristics (Gudmundsdóttir et al, 2011). The grain size distribution of the Hekla Ö indicates that the eruption began and ended with powerful explosive phase. The nature of explosivity was mainly magmatic or hydro-magmatic (Tómasdóttir, 2015).

2 Methods

2.1 Field work and sampling

The research area for this study was constrained to the proximal area within 30 km away from Hekla where the three tephra layers were traced in multiple soil sections (Figure 3). A soil section in Tagl (19 km NNE of Hekla) was chosen as a key section for this study, were both Hekla DH and Hekla Ö are present and demonstrate multiple units which can be readily identified within each tephra. Hekla Mó is also present in Tagl, while its small thickness prevents the identification of individual units. Along with Hekla DH, Hekla Mó and Hekla Ö, the whole tephra sequence of the Tagl section (13 tephra layers) between the major Hekla 5 and Hekla 4 was studied (Figure 5). In addition to measuring the thickness of the tephra layers, each layer was described in terms of grain-size, grading and other lithological features as well as sampled for chemical analyses. For grain size terminology, see Table 2. To construct isopach maps and calculate volumes for Hekla DH, Hekla Mó and Hekla Ö, unpublished and published field measurements were also used (Óladóttir, 2009; Gudmundsdóttir et al., 2011; Larsen unpublished data).

Table 2. Grain size terminology

Grain size (mm) Nomenclature

>64 Blocks & bombs

32-64 Coarse lapilli

16-32 Medium lapilli

2-16 Fine lapilli

1-2 Coarse ash

0.064-1 Medium ash

<0.064 Fine ash

Samples of Hekla DH, and Hekla Ö were collected from the Tagl section, while Hekla Mó was sampled in Áfangagil, 13 km north of Hekla. To obtain a good overview of the major oxide composition and changes in geochemistry with time of the three tephra layers, each deposit was divided into multiple units and each unit sampled. Units were identified on basis of changes in color, grain size and other features. In total, eight units were identified and sampled in Hekla DH, while four units were identified and sampled in both Hekla Ö and Hekla Mó (Figure 5). Unit HDH-1 represents the first erupted tephra for Hekla DH,

23 Figure 3. Map of the study area. Soil sections used for this study are marked with yellow circles, while red circles show locations of soil sections where Hekla DH, Hekla Mó or Hekla Ö have been identified in previous studies (Larsen, unpublished data) while HDH-8 represents the top of preserved material at that locality. The same applies for Hekla Ö (HÖ-1 to HÖ-4) and Hekla Mó (HM-1 to HM-4). The other 13 visible tephra layers identified in the Tagl section, between the major Hekla 5 and Hekla 4 tephra layers, were sampled for geochemistry to determine the volcanic provenance. Tagl 1 represents the oldest tephra layer in the sequence and Tagl 13 the youngest. The stratigraphic location of all units and tephra layers from Tagl and Áfangil are illustrated on Figure 5. All samples were collected from the center of each unit/layer to avoid contamination from surrounding units or layers.

Samples for whole-rock analysis were collected from three craters in the northern part of the Hekla system. The craters were sampled to check for a compositional match between potential source craters and the Hekla DH tephra, as the craters are all located within the expected eruptive area for the tephra. The Rauðaskál crater marks the southernmost crater sampled, while one nameless crater south of Valagjá was also sampled. A crater row, just south of Valagjá, marks the northernmost crater sampled (Figure 4).

24 Figure 4. Locations of the three craters sampled (triangles) for whole-rock analysis; Rauðaskál (green), nameless crater (blue) and a crater south of Valagjá (orange)

2.2 Sample preparation

All samples collected were wet sieved through 1 mm, 125 μm, 90 μm and 63 μm sieves. The 250 μm size fraction was added for the samples belonging to Hekla DH, Hekla Mó and Hekla Ö. Grains collected in the 125 μm size fraction were used for the microprobe analysis.

The highly compact and indurated nature of three Hekla DH tephra samples (units 1, 2 and 6) made the sieving difficult, as not enough grains were collected within the 125 μm fraction with the conventional method of wet sieving. To acquire enough grains in the correct size range, fragments of the samples were submerged in water into which 5-10 ml of 6% H2O2 was added to remove any possible organic matter that could be holding the grains together. After two days in the H2O2 solution, the three samples were submerged in a 30°C ultrasound bath for 90 minutes to shake the grains free of each other. A few rounds in the ultrasound bath turned out to be adequate, as enough grains became free for wet sieving.

2.3 Microprobe and whole rock analysis

Electron microprobe analysis of the tephra samples was carried out with the JEOL JXA- 820 Superprobe at the Institute of Earth Sciences, University of Iceland. An acceleration voltage of 15 kV, 10 nA beam current and beam diameter of 10 μm were used. Basalt glass (A99, see standard values are given in Table A.1.3 in Appendix 1) was used as a standard. Analyses were made on the standards at a 45-60 point analysis interval to check/monitor for possible drift in the instrument. Point analyses were made on 30 glass grains from each sample belonging to Hekla HD, Hekla Mó and Hekla Ö and 15 point analysis for the other layers identified in Tagl soil section. The point analysis was made on predetermined lines in polished thin sections. When a grain contained microlites or had no smooth surfaces, it

25 was skipped and the next grain on the line analyzed. The dataset was analyzed for microlite contamination and analyses considered to be contaminated were emitted. Analyses with sums of major oxides lower than 95% were also emitted.

The small grain sizes used for microprobe (125µm fraction) adequately cover the spectrum of glass compositions in each sample. However, the number of grains of each composition in the samples does not necessarily reflect the volume fraction of the respective glass compositions in the tephra layers.

Whole-rock analysis on the rock samples collected were done using inductively coupled plasma atomic emission spectroscopy (ICP-AES) on powdered samples, following the methods described in Sigmarsson et al., (2011).

26 Figure 5. Simplified illustrations of the Tagl and Áfangagil soil sections. Sampled units of Hekla DH (HDH), Hekla Mó (HM) and Hekla Ö (HÖ) are indicated as well as numbers representing each of the 13 Tagl tephra layers.

3 Results

3.1 Stratigraphy

3.1.1 Hekla DH

The bulk of the Hekla DH tephra is light- to dark-gray, while the top unit is near-black in color. The main characteristic features of the Hekla DH tephra are three concrete-like units (units 1, 2 and 6 in Tagl) within the deposit (Figure 6-A). These units are completely cemented and consist of fine to medium sized ash grains which are light-gray in color. In the Tagl section, the two compact bottom units made up the bottom 30 cm of the deposit (Figure 5). Prints of trees and leaves are common in these compact units (Figure 6-C). Units one and two both contain internal lenses of slightly coarser grains reaching medium lapilli in size, while the coarser clasts are however all completely sealed within the fine- grained, compact groundmass. Coarser grains from surrounding units can also be seen cemented into the top of the fine-grained compact units.

This compact nature of the Hekla DH tephra is mainly observed in the fine-grained bottom units, while coarser units higher up in the deposit are also slightly cemented, with individual clasts typically being covered by a layer of much finer grains. In thin sections, this feature is observed at microscopic level as glass-grains which are enclosed in a mass of much finer fragments. Individual clasts consisting purely of the cemented smaller fragments can also be seen (Figure 7-B), while alteration at rims of glass grains are sometimes observed (Figure 7-A).

In proximal deposits investigated for this study, the Hekla DH tephra generally becomes coarser upwards. No grading is observed within individual units, but some units have visible lenses of coarser grains. The black top unit (HDH-8) of the Hekla DH tephra is in many aspects different from the rest of the deposit. The top unit is coarser than the rest of the deposit, lacks all fine ash and is free of any consolidation. Measuring 32 cm in thickness in Tagl, the top unit varies in thickness from 10-30 cm within 15 km from the Valagjá crater row.

The maximum measured thickness of Hekla DH was 220-240 cm, 1 km North of Valagjá. At that location, the deposit is exposed at the surface and the color difference between the top unit and the rest of the deposit is clearly visible (Figure 6-B). At the same locality, the deposit contains bombs measuring up to 50 cm in width.

29 Figure 6. Photos from field work. A) The compact bottom units of Hekla DH in Tagl. B) The dark colored top unit of Hekla DH visible at the surface, 1km west of Valagjá. C) Print of a tree branch in the compact bottom unit of Hekla DH in Tagl. D) The light colored Hekla Ö on top of the black Hekla Mó in a soil section in Langalda, 25 km north of Hekla.

Figure 7. Microscopic images of the Hekla DH tephra. A) Alteration rims on otherwise fresh glass. B) Larger grains enclosed in a mass of much smaller fragments.

30 3.1.2 Hekla Mó

The Hekla Mó tephra typically has a thickness less than 10 cm within 30 km away from Hekla. The grain size of the tephra generally decreases upwards, with the biggest clasts ranging up to coarse lapilli in the bottom unit (Figure 5). Only minor color variations can be detected in most sections as the tephra is dark-gray to black. An exception is a section in Ófærugil where the Hekla Mó has a grayish-brown base below the black top.

In proximal soil sections, the Hekla Mó tephra is often observed sandwiched between two tephra layers. A thin, black tephra layer, characterized by elongated or needle-like grains (achnelites), is commonly found beneath Hekla Mó with a very thin soil horizon separating the two, while the much larger Hekla Ö tephra lies directly on top of it (Figure 6-D). The contact between Hekla Ö and Hekla Mó is usually very sharp as the two deposits differ highly from each other in terms of color. However, the boundary is often heavily deformed, as seen on Figure 8.

Figure 8. A heavily deformed boundary between the Hekla Mó tephra and Hekla Ö, as seen in a soil section in Hrauneyjar (30 km north of Hekla).

3.1.3 Hekla Ö

Hekla Ö tephra has a light-colored base unit that can be traced east and north of Hekla, while the deposit becomes significantly darker upwards, having a dark-grey top. However, the topmost part may have been eroded in which case the full range of Hekla Ö tephra is not present in Tagl. In the Tagl section (Figure 5), the light-colored base measures 2-3 cm in thickness and consists of grains ranging in size from coarse ash to fine lapilli. This light- colored base unit characterizes the deposit, as the rest of the deposit has a darker color.

The second unit (5-6 cm) at Tagl has a slightly smaller grain size (medium ash to coarse ash) and gradually becomes darker in color upwards. Unit three consists of mainly coarse ash which is dark-yellow, lenses with slightly coarser grains are visible. The fourth and top unit (HÖ-4) makes up the bulk of the total thickness of Hekla Ö in Tagl and measures 54

31 cm thick. The top unit of the tephra usually has a strong inverse grading in proximal sections, is dark-grey and contains grains ranging in size from coarse ash to medium lapilli.

3.1.4 The Tagl tephra layers

Along with Hekla DH, Hekla Mó and Hekla Ö, 13 tephra layers were identified in the Tagl soil section between Hekla 5 and Hekla 4. For identification, each tephra layer was named, with Tagl 1 representing the oldest tephra in the investigated section and Tagl 13 the youngest. The tephra layers form multiple dark colored bands in the soil with the thickness of individual layers ranging from 0.5 to 4 cm (Figure 5). The tephra layers are all fine- grained, consisting of fine- to medium ash and have only minor visible textural structures or grading. There are, however, two exceptions. Tagl 8 consists of brownish glass, with occasional tiny needle-like grains. Tagl 7, located immediately below Hekla Mó, consists of black tephra containing achnelites, elongated or needle-like shiny grains, similar to those commonly seen in tephra from crater rows in Vatnafjöll.

3.2 Dispersal and volumes

New isopach maps of the Hekla DH, Hekla Mó and Hekla Ö are compiled in this thesis (Figures 9-12). Volume calculations based on the data collected for this study give a volume of ̴ 0.8 km3 of freshly fallen tephra for the Hekla DH tephra and ̴ 1.1 km3 for Hekla Ö, which are roughly the same volumes as calculated by Gudmundsdóttir et al., (2011). A minimum volume of ̴ 0.05 km3 is calculated for the Hekla Mó tephra. Mapping of the deposits indicates that the three tephra layers originate from three different areas in the Hekla system.

The dispersal of the Hekla DH tephra demonstrates that the tephra was erupted from the northern part of the Hekla system, with the dispersal strongly indicating that it was erupted from the Valagjá area. The tephrostratigraphy within the Valagjá explosin crater indicates an age younger than 4000 years but remnants of an older crater row are found farther to the southwest (Figure 4). The axis of maximum thickness of Hekla DH lies to the north from the Valagjá area (Figure 9) and the maximum thickness of the deposit (220-240 cm) was measured just north of the crater row. Due to lack of available soil sections closer to Hekla, the southward limits of the Hekla DH tephra have however not been established.

An isopach map of the Hekla Mó tephra appears for the first time in this thesis (Figure 12). The dispersal limits beyond the 1 cm isopach line are however yet to be established. The deposit reaches a maximum thickness just south of Áfangagil ( ̴ 95 cm) but lacks any obvious axis of maximum thickness. The thickness of the deposit decreases rapidly in all directions from Áfangagil, measuring only 6 cm in Tagl, 3 km to the north. The dispersal of the tephra indicates an eruptive site a few kilometers northeast of Hekla, while an andesitic part observed in the tephra at Áfangagil (chapter 3.3.1) and a brownish-gray base of Hekla Mó in Ófærugil, suggests a second eruptive site on the slopes of Hekla proper. A second source could also explain the lack of distinct thickness axis.

Hekla Ö has two dispersal axis which both point to an origin in the Hekla proper (Figure 10). A minor axis trends east-northeast, while the main axis of the tephra trends in a more northward direction. The light-colored base unit of the tephra follows the minor axis and

32 reaches a maximum thickness of 17 cm, 20 km ENE of Hekla, while the maximum thickness was measured directly north of Hekla. The dispersal of the light-colored base layer of Hekla Ö has been mapped (Figure 11).

Correlations between Hekla Ö and a previously identified Hekla tephra layers in distal drill cores and soil sections are suggested in this thesis. If so, it is likely that the Hekla Ö tephra covers as much as 80% of terrestrial Iceland (Figure 10). This relation between Hekla Ö and the distal Hekla tephra deposits is discussed further in chapter 4.1.2.

33 Figure 9. Isopach maps of the proximal (A) and distal (B) dispersal of the Hekla DH tephra (values in cm). Circles mark soil sections where the tephra has been identified, while the black star marks where the tephra has been identified in a drill core (Gudmundsdóttir et al., 2011; Larsen unpublished data).

34 Figure 10. Isopach maps of the proximal (A) and distal (B) dispersal of the Hekla Ö tephra (values in cm). Circles mark soil sections where the tephra has been identified, while black stars mark where the tephra has been identified in drill cores (Hardardottir et al., 2001; Jóhannsdóttir, 2007; Óladóttir, 2009; Gudmundsdóttir et al., 2011; Sigurgeirsson and Hjartarson, 2011; Gudmundsdóttir et al., 2016; Schomacker et al., 2016; Gunnarson, 2017; Harning et al., 2018, Larsen unpublished data).

35 Figure 11. Isopach map of the light-colored base layer of the Hekla Ö tephra (values in cm). Boxes mark soil sections where the base can be identified.

Figure 12. Isopach map of the Hekla Mói tephra. Arrows indicate dispersal from two potential source areas (values in cm)

36 3.3 Geochemistry

3.3.1 Hekla DH, Hekla Mó and Hekla Ö

A detailed record of the major oxide composition of the Hekla DH, Hekla Mó and Hekla Ö tephra layers has been obtained. The electron microprobe analyses are listed in Appendix 1 and the major oxide composition of the three tephras is plotted on Figure 13.

All three tephra layers contain individual compositional groups which can readily be identified from each other by major oxides contents (Table 3). The groups range in composition from primitive basalt to highly evolved dacite/rhyolite. All three tephra layers contain intermediate or silicic groups as well as basalt. Hekla DH contains basaltic andesite, while the intermediate part of Hekla Mó tephra ranges in composition from basaltic andesite to andesite. Hekla Ö contains highly evolved dacite/rhyolite part along with the basalts but lacks all glass of intermediate composition in the Tagl section.

The basalt in the three tephra layers can be identified into two groups. A group of relatively primitive (> 6.5 MgO wt%) composition is observed in Hekla DH and Hekla Ö, while a more evolved basalt composition, corresponding to the Fe-Ti basalt composition (Jakobsson 1979; Sigmarsson et al., 1992), occurs in all three tephra layers. The more primitive group of basalt displays the same chemical characteristics as the Pleistocene formations around Hekla (Jakobsson, 1979; Sigmarsson et al., 1992). It has also been detected in Holocene Hekla tephra layers, e.g. the ̴ 9000 BP Hekla VF tephra (Harning et al., 2018) and the ̴ 4500 BP Hekla F tephra (Jóhannsdóttir, 2007). Within Hekla DH, a trend is observed between the two basaltic members.

As seen on Figure 13, most of the analysis plots within the field of previously published compositions of Holocene Hekla products. A group of basaltic andesite from Hekla DH does however consistently plot outside of the field, having significantly higher contents of TiO2 and FeO at a given MgO than the typical basaltic andesite from the system. To distinguish between the two groups of basaltic andesite, the high-TiO2 group from Hekla DH will be termed Fe-Ti basaltic andesite in this thesis, as it displays some chemical similarities with the Fe-Ti basalts of the Hekla volcanic system.

All three tephra layers display some chemical zonation in the intermediate or more silicic compositions. No zonation is observed within the basalt of the three tephras. The number of grains displaying basaltic compositions does however vary between units.

Grains of intermediate composition in the Hekla DH tephra show limited variations from unit 1 to 7, consisting of basaltic andesite with occasional grains of Fe-Ti basaltic andesite. In contrast, unit HDH-8 largely consists of the Fe-Ti basaltic andesite which only forms a minor part in the lower lying units.

In Áfangagil, the bulk of all analyzed grains of Hekla Mó display Fe-Ti basalt composition. The silica content of the intermediate part of the tephra ranges from 56 to 61%. Interestingly, the highest silica content analyzed in the tephra in Áfangagil is seen in unit 3. Hekla Mó in Ófærugil (7 km north of Hekla) was not analyzed but the grayish- brown base there could be andesitic. On some plots, the composition of the andesite in

37 Hekla Mó indicates a genetic relation with both groups of basaltic andesite analyzed in the t Hekla DH tephra. This relation is mainly observed on FeO vs MgO and Al2O3 vs MgO (Figure 13).

Figure 13. Major oxide totals, displayed as percentage of total oxide weight, of Hekla DH and Hekla Ö from Tagl and Hekla Mói from Áfangagil. Shaded areas mark previously published compositions of Holocene Hekla products (Jakobsson, 1979; Sverrisdóttir, 2007; Jóhannsdóttir, 2007; Chekol et al., 2011).

38 The most evolved composition (76% SiO2) in the three tephra layers is seen in the light- colored base unit (HÖ-1) of Hekla Ö. The SiO2 content of the silicic part of Hekla Ö is highly zoned and gradually decreases upwards within the deposit. The top unit in Tagl (HÖ-4) has a rather uniform composition at the andesite-dacite boundary (62-63%) SiO2. In the Tagl section, Hekla Ö contains no basaltic andesite, while only one analyzed grain, located in HÖ-1, displays an andesitic composition. Due to the lack of analyzed intermediate compositions, a compositional gap exists in Hekla Ö in the Tagl section between 50-60% SiO2 and 2-4% MgO.

Table 3.The typical composition of each rock type identified in Hekla DH, Hekla Mó and Hekla Ö.

t Group SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 Primitive basalt 46.49 2.64 15.81 13.57 0.21 7.03 10.67 2.58 0.41 0.26 Fe-Ti basalt 47.66 4.05 13.55 14.59 0.22 5.22 10.13 2.72 0.72 0.47 Fe-Ti basaltic 54.96 2.93 12.46 14.19 0.34 3.13 6.97 2.47 1.45 1.35 andesite "typical" Hekla 53.55 2.38 14.37 12.59 0.29 3.65 7.26 3.16 1.06 1.09 basaltic andesite Andesite 60.08 1.77 12.87 11.45 0.28 1.66 5.13 3.18 1.93 0.81 Dacite 68.46 0.42 14.91 5.94 0.19 0.36 3.15 2.87 2.14 0.10

3.3.2 Whole-rock analysis

The four whole-rock samples collected from three craters in the northern part of the Hekla system (Figure 14) all display basaltic andesite compositions which corresponds to most of the basaltic andesite in Hekla DH. One analysis collected from the rims of the Rauðaskál crater usually plots as an outsider, while samples collected closer to Valagjá plot among the Hekla DH tephra.

39 Figure 14. Results of whole-rock analysis from the Rauðaskál crater (green triangles) and two other craters closer to Valagjá (blue and orange triangles) plotted with the basaltic andesite of the Hekla DH tephra. See Figure 3 for location of whole-rock samples.

3.3.3 The Tagl tephra layers

Out of the 13 sampled tephra layers in the Tagl soil section, one has a dacitic composition while the remaining layers display basaltic compositions. The average major oxide composition of each layer, along with source system, is listed in Table 4, while all analyses are given in Appendix 1.

Of the 13 analyzed tephra layers, 11 display chemical characteristics similar to that of the Katla volcanic system (60 km south of Tagl), being relatively rich in Ti and Fe. Individually, the Katla tephra layers are uniform in composition, while collectively ranging somewhat in major oxide composition. The dacitic tephra (Tagl 8) has a uniform composition of 67% SiO2, with only minor ranges in all major oxide contents and is a silicic Katla tephra (SILK) layer (Larsen et al., 2001). The composition and stratigraphic position most closely resembles that of the ̴ 5800 BP SILK-N1 layer (Thorsteinsdóttir et al., 2015).

Tagl 7 and Tagl 13 are the only non-Katla tephra layers, with the chemical composition of both layers indicating a source in the Vatnafjöll system (Jakobsson, 1979).

40 Table 4. Average major oxide combositions of the 13 Tagl tephra layers.

Tephra Volcanic t layer system n SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 Tagl 13 Vatnafjöll 15 46.42 3.74 14.02 15.41 0.25 5.62 9.85 2.76 0.61 0.47 Std dev. 0.52 0.22 0.34 0.45 0.02 0.31 0.41 0.4 0.06 0.05 Tagl 12 Katla 15 47.18 4.27 13.15 15.18 0.22 5.09 9.85 2.93 0.75 0.45 Std dev. 0.37 0.1 0.12 0.2 0.02 0.09 0.13 0.2 0.02 0.02 Tagl 11 Katla 15 48.62 4.09 13.18 14.53 0.23 4.64 9.33 3.05 0.93 0.45 Std dev. 0.76 0.17 0.25 0.44 0.02 0.15 0.27 0.23 0.11 0.03 Tagl 10 Katla 15 47.46 4.26 13.36 15.01 0.22 5.09 9.80 2.72 0.76 0.46 Std dev. 0.27 0.1 0.15 0.11 0.02 0.09 0.06 0.33 0.02 0.03 Tagl 9 Katla 14 47.65 4.38 13.3 15.08 0.22 5.0 9.8 2.69 0.77 0.45 Std dev. 0.37 0.08 0.16 0.27 0.02 0.13 0.12 0.25 0.03 0.03 Tagl 8 Katla 13 66.99 1.32 14.21 6.15 0.2 1.05 3.08 1.71 2.6 0.3 Std dev. (SILK) 0.24 0.03 0.1 0.11 0.01 0.03 0.08 0.26 0.04 0.02 Tagl 7 Vatnafjöll 12 47.25 3.78 14.44 15.77 0.25 5.61 9.33 2.82 0.58 0.43 Std dev. 0.25 0.1 0.2 0.14 0.02 0.11 0.09 0.08 0.02 0.02 Tagl 6 Katla 15 49.25 4.04 13.40 14.36 0.23 4.57 9.2 3.06 0.9 0.54 Std dev. 0.59 0.12 0.14 0.36 0.02 0.17 0.3 0.15 0.05 0.07 Tagl 5 Katla 15 48.83 4.19 13.12 14.82 0.23 4.73 9.40 3.02 0.87 0.53 Std dev. 0.22 0.16 0.36 0.34 0.02 0.10 0.07 0.11 0.04 0.06 Tagl 4 Katla 14 48.24 4.26 13.03 15.23 0.23 4.75 9.53 2.99 0.83 0.53 Std dev. 0.52 0.17 0.38 0.57 0.01 0.15 0.19 0.07 0.05 0.06 Tagl 3 Katla 14 47.87 4.24 13.12 15.04 0.23 4.85 9.74 2.95 0.81 0.47 Std dev. 0.41 0.08 0.24 0.41 0.02 0.15 0.21 0.14 0.03 0.02 Tagl 2 Katla 14 48.39 4.19 13.23 14.52 0.22 4.85 9.37 3.09 0.86 0.5 Std dev. 0.27 0.09 0.16 0.19 0.01 0.09 0.1 0.12 0.02 0.02 Tagl 1 Katla 15 48.99 3.98 13.39 14.11 0.22 4.68 9.07 3.09 0.93 0.52 Std dev. 0.59 0.13 0.12 0.43 0.02 0.19 0.3 0.12 0.07 0.05

4 Discussions

4.1 Stratigraphy

4.1.1 Dispersal and eruptive sites

From the proximal dispersal of Hekla DH, Hekla Mó and Hekla Ö, it is likely that the three tephra layers all originate from different sites within the Hekla volcanic system.

Mapping of the Hekla DH tephra indicates that it was erupted in a fissure eruption in the northern part of the Hekla system, most likely from the Valagjá area. Results of whole- rock analysis of samples collected from the southernmost crater in the Valagjá area give a compositional match with the tephra, further supporting a source in Valagjá area. The Valagjá crater row is, however, thought to have been formed in a more recent eruption, as the tephrostratigraphy inside the crater row indicates an age younger than the Hekla DH eruption (6650 BP). The sampled craters (Rauðaskál and a nameless crater) further south from Valagjá (Figure 4) do display chemical similarities with the Hekla DH tephra, demonstrating the continuous intermediate magma production from Hekla proper to Valagjá. However, the dispersal of Hekla DH makes it unlikely that the tephra originates from those craters.

The dispersal of Hekla Mó, along with the microprobe analysis, confirm an origin in the Hekla system. Although it is difficult to establish the exact eruptive site within the system for the tephra, it is suggested that it was erupted from at least two vents, located a few kilometers apart.

Hekla Mó lacks any obvious dispersal axis and only covers a small area. Furthermore, the deposit maintains a rather uniform thickness of 4-5 cm over a relatively large area within the 1 cm isopach line. However, it must be considered likely that the andesitic part of the tephra was erupted from Hekla proper, as eruptive sites of andesite and more silicic rocks in the Hekla system are thought to be confined to the Hekla fissure (Jakobsson, 1979; Sigmarsson et al., 1992).

In Hekla Mó at Áfangagil, units HM-1 and HM-2 consist of basalt and basaltic andesite, with only one grain displaying an andesitic composition of 57.5% SiO2. In unit HM-3, an andesitic part appears and reaches the highest silica content (61%), dropping slightly in unit HM-4. The lack of andesite in the first erupted products could indicate that the first phase of the eruption occurred in vents northeast of Hekla. In later phases of the eruption, new fissures could have opened at the flanks of Hekla proper, or within the edifice, allowing andesite to be erupted. This is supported by the shape of the isopach lines on Figure 12, which can be explained by two thickness axes spaced 4-6 km apart.

The Hekla Mó tephra could therefore have been erupted from multiple vents within the Hekla system, which were active simultaneously. This behavior would then resemble that

43 of the 1970 Hekla eruption, where multiple fissures, located up to 7 kilometers apart, opened up within a few hours from the onset of the eruption, SW and NNE of Hekla (Thorarinsson and Sigvaldason, 1972). More data is however needed to establish the exact eruptive sites for Hekla Mó.

The Hekla Mó tephra is a small tephra layer with a minimum volume of only 0.05 km3 of freshly fallen tephra. This volume is of the same order of magnitude as some of the tephra layers erupted in the smallest historical eruptions of Hekla. In the historical eruptions, the bulk of the erupted volume is usually in the form of lava flows. Whether the Hekla Mó eruption transitioned into an effusive eruption is not known but should be considered as a possibility. The highly deformed boundary between the Hekla Mó and Hekla Ö is the result of a very short interval between the two eruptions (months/years). The deformed boundary between Hekla Mó and Hekla Ö suggests that Hekla Mó was still fresh and uncompacted when the much larger Hekla Ö erupted and deposited on top of it, causing the deformation of the Hekla Mó tephra.

Both the dispersal and chemistry of the tephra confirm that Hekla Ö was erupted from Hekla proper. A change in wind direction, occurring as the eruption proceeded, resulted in the two dispersal axes and extensive dispersal over terrestrial Iceland (Figure 10). The high proportion of basalt grains within the 125-250 µm size range of the tephra is somewhat unusual for the silicic Hekla eruptions and might possibly result from an explosive basaltic eruption occurring simultaneously somewhere within the volcanic system.

To avoid extrapolating, volume calculations were made from the measured thicknesses of each of the tephra layers in terrestrial soil sections, thus representing only minimum volumes for each layer. The calculations were done in this manner as the western dispersal limits of Hekla Ö are still relatively uncertain and the dispersal of Hekla Mó is still unknown beyond the 1 cm isopach line.

The volumes calculated in this study give roughly similar volumes for Hekla DH as calculated by Gudmundsdottir et al. (2011). These same volumes result from the source area being moved further north (from Hekla proper to the Valagjá area). Due to the northward dispersal of the Hekla DH, that would normally result in decreased volume for the deposit. However, the maximum measured thickness of the deposit in this study was 220 cm (1 km west of Valagjá), which is significantly thicker than the 90 cm measured by Gudmundsdóttir et al. (2011).

4.1.2 The relation of Hekla Ö to distal tephra deposits

The Hekla Ö tephra was erupted in a plinian eruption in the Hekla volcano 6060 BP and has a minimum volume of ̴ 1.1 km3 of freshly fallen tephra. The existence of Hekla Ö has been confirmed in soil sections up to 200 km away from Hekla (e.g. Óladóttir et al., 2011; Eddudóttir et al., 2016) and in drill cores from Lake Lögurinn, 280 km to the NE (Gudmundsdóttir et al., 2016). The chemical record of Hekla Ö obtained with this study is however the most detailed yet and can be used to correlate the proximal record with potential distal Hekla Ö deposits.

A tephra of Hekla origin, named the Brattihjalli tephra, was recently identified in drill cores collected from lakes in Vestfirðir, Northwest Iceland. Schomacker et al. (2016) speculated on a relationship with the Hekla Ö tephra, but further data was needed to

44 Figure 15. Hekla Ö plotted with Brattihjalli tephra (Schomaker et al., 2016), Hekla 5 (Sverrisdóttir, 2007), T-Tephra/Hekla T from Bæjarvatn (Harning et al., 2018) and the QUB-602 sample (Pilcher et al., 2005). confirm that. Harning et al. (2018) proposed that the Brattihjalli tephra is a ̴ 9000 year old Hekla tephra layer, termed Hekla VF (Hekla Vestfirðir, formerly Hekla AlB-1 from Jóhannsdóttir, 2007). The Hekla VF tephra is purely basaltic in composition, containing the two groups of basalt compositions (primitive and Fe-Ti basalt) observed in Hekla Ö and other Hekla tephra layers. The correlation between Hekla VF and Brattihjalli was mainly established from the basaltic part of the tephra layers and stratigraphic position, but Schomaker et al. (2016) identified the Brattihjalli tephra close to the interface between glaciolacustrine and non-glacial sediments in two lakes, which might indicate an early- Holocene age for the tephra. It is likely that the proposed Brattihjalli tephra in Lake Neðra- Eyvindarvatn is in fact the Hekla VF tephra, as it lacks all silicic grains at that locality and is located just above the glaciolacustrine/non-glacial sediment interface.

However, the stratigraphic position and the approximate age of the Brattihjalli tephra in lake sediments in Brattihjalli, Skorarvatn, Reykjafjördur, Dagverdardalur and Nedra- Hvalárvan in Vestfirðir, fits with Hekla Ö. There the Brattihjalli tephra also displays chemical similarities with Hekla Ö, having a silicic group of andesite and dacite while containing basalt grains of rather primitive composition. When the major oxide composition of the Brattihjalli tephra is compared to the Hekla Ö samples collected in Tagl, the two tephra layers share near identical chemical characteristics (Figure 15). The Brattihjalli tephra is therefore suggested here to be a distal deposit of the Hekla Ö tephra.

The correlation between the Brattihjalli tephra and Hekla Ö can be further supported by the identification of Hekla Ö in Lake Barðarlækjartjörn and a soil section in Kagaðarhóll, both in Northwest-Iceland (Eddudóttir et al., 2016). The silicic part of Hekla Ö in

45 Figure 16. Analyses of Hekla Ö from Tagl compared with proposed distal deposit of the tephra in Ellidarárdalur (Sigurgeirsson and Hjartarson, 2011) and Lake Hestvatn (T- tephra/Hekla-T, Hardardóttir et al., 2001). The analyses from Lake Hestvatn are averages.

Barðarlækjartjörn displays an average silica content of 60.9%, while the basaltic part contains both primitive basalt and Fe-Ti basalt (Eddudóttir et al., 2016; unpublished data) which is consistent with Brattihjalli tephra and Hekla Ö in the Tagl section.

Sigurgeirsson and Hjartarson (2011) discovered a potential Hekla Ö deposit in a few soil sections within the capital region of Iceland, 100 km west of Hekla. In the Elliðarárdalur Valley within Reykjavik, the tephra can be found beneath the 5200 BP Leitahraun lava. It was also traced in soil sections in Mosfellsbær and Kjalarnes and has and estimated age of ̴ 6000 BP (Sigurgeirsson and Hjartarson, 2011). In the Elliðarárdalur Valley, the tephra contains rather uniform andesite of 60% SiO2, while few grains display the Fe-Ti basalt composition (Figure 16) (Sigurgeirsson and Hjartarson, 2011; unpublished data). Both the stratigraphic position and chemical composition of this tephra layer in the capital region support a correlation with Hekla Ö. The andesitic composition also fits the expected decrease in SiO2 content of the silicic part of the tephra from east to west around Iceland, thus the existence of Hekla Ö in Southwest-Iceland is confirmed.

Closer to Hekla, Hardardóttir et al. (2001) identified a Hekla tephra layer (T-tephra) of mid-Holocene age, occupying a stratigraphic position between Hekla 5 and Hekla 4 in a drill core collected from lake Hestvatn in South-Iceland, 50 km west of Hekla. The tephra mainly has a primitive basalt composition, while a few grains display an andesitic composition. Hardardottir et al. (2001) speculated on a possible origin of the andesitic grains from Hekla and suggested the Vestmannaeyjar system for the basaltic grains. In more recent studies, the T-tephra (also termed Hekla T) has been traced and used as a regional tephra marker in West- and Northwest-Iceland (Jóhannsdóttir, 2007; Gunnarson, 2017; Harning et al., 2018). The T-tephra has been dated between 5580 to 6100 BP (Jóhannsdóttir, 2007; Gunnarson, 2017).

The andesite composition analysed by Hardardóttir et al. (2001) in the T-tephra in Lake Hestvatn (58-60% SiO2, Figure 16) is similar to the analysis of Sigurgeirsson and Hjartarsson (2011) from the capital region. When compared to Hekla Ö analysis from Tagl, both the basaltic part and the andesite of the tephra compositionally match Hekla Ö. The similar ages, as well as identical composition of basalt and andesite, thus both strongly support a relation between the T-tephra and Hekla Ö

46 Except for the andesite of the T-tephra in Lake Hestvatn (Hardardottir et al., 2001), the published data shows no andesite or dacite in the tephra, except for one andesite grain analyzed by Harning et al. (2018) from lake Bæjarvatn in Vestfirðir (Figure 15). However, the lack of intermediate and silicic analysis in the T-tephra might result from the high proportion of basalt grains observed in the tephra. In Tagl, 70-80% of analyzed grains in the 125-250 µm fraction of each unit of Hekla Ö have a basaltic composition. Consequently, 15-20 point analysis of the T-tephra could possibly miss all grains of andesite or more silicic composition. It is also possible that the approach to grain selection for analysis is different. The method used in this study, where grains on the selected lines are analyzed, (see 2.3) is likely to include all grain types in each sample.

Even though the T-tephra is widespread and has been identified in multiple drill cores from lakes west and northwest from Hekla, it has never been identified in soil sections close to Hekla. Thus, from the identical composition of the basaltic part, dispersal and similar ages, it is proposed in this thesis that the T-tephra (also Hekla-T) is in fact a part of Hekla Ö.

These findings in Northwest- and South-Iceland, as well as within the capital region, indicate that the last phase of the Hekla Ö eruption ejected andesitic tephra as well as basaltic tephra, which was subsequently carried west and northwest from Hekla. As result of that, Hekla Ö lacks nearly all andesite in the Tagl section due to its location NNE of Hekla proper.

Because of the eastbound dispersal of Hekla Ö, Gudmundsdóttir et al. (2011; 2016) speculated that Hekla Ö might have reached as far as Scandinavia. More recently, Plunkett and Pilcher (2018) proposed that a highly silicic tephra (73-76% SiO2), found in a peat bog in Norway and previously identified as Hekla 5 (sample QUB-602 from Pilcher et al., 2005), is the Hekla Ö tephra. When compared to the chemical record of Hekla Ö obtained with this study and published data for Hekla 5, QUB-602 does however more resemble Hekla 5 (Figure 15). If Hekla Ö did in fact reach Scandinavia, it is thus yet to be identified there.

It had previously been noted that the silica content of the silicic part of Hekla Ö in distal deposits decreases counter-clockwise around Iceland, caused by a change in wind direction as the eruption went on (Gudmundsdóttir et al., 2011). Highly silicic products from the opening phase were mainly carried to the east and northeast from Hekla (Figure 10). This is observed in Lake Lögurinn in East-Iceland, where the Hekla Ö tephra has a silica content of 72% (Gudmundsdóttir et al., 2016), corresponding to the base of Hekla Ö (unit HÖ-1 in Tagl), which has an eastern dispersal from Hekla. The intermediate and silicic grains of the Brattihjalli deposit in NW-Iceland typically contain 58-63% SiO2, thus corresponding to the last preserved products in Tagl (unit HÖ-4). In a soil section in North- Iceland, the silicic part of Hekla Ö ranges mainly from 62-68% in silica content (Gudmundsdóttir et al., 2011) which falls into the gap between the intermediates in Northwest-Iceland and the highly-silicic part in East-Iceland. Only in the Tagl section has the near-full compositional range of Hekla Ö been analyzed in detail within a single section, allowing for most of the distal records to be correlated to individual units of the tephra layer within the proximal region. The decreasing silica content of Hekla Ö counter- clockwise around Iceland is illustrated on Figure 17.

47 Figure 17. A map showing the decreasing silica content of the Hekla Ö tephra from east to west within terrestrial Iceland. Lines are the isopach lines of Hekla Ö. Numbers display analysed silica content of the tephra from seven localities. Dataset includes data from the present study, Gudmundsdóttir et al. (2011), Sigurgeirsson and Hjartarson (2011), Gudmundsdóttir et al. (2016), Eddudóttir et al. (2016) and Schomacker et al. (2016).

If the correlations between Hekla Ö and the distal deposits proposed in this thesis are confirmed, Hekla Ö covers as much as 80% of terrestrial Iceland, which is a much greater dispersal than previously thought (Gudmundsdóttir et al., 2011).

4.1.3 Palagonized units of Hekla DH

The compact nature of the Hekla DH tephra is considered to result from palagonitization of basalt grains in the small to medium ash size range (<1 mm) within the tephra at ambient temperature. Palagonitization of mafic glass is thought to mainly occur due to water alteration of sideromelane at ambient temperature, resulting in light-colored palagonite (Stroncik and Schminke, 2002). The high proportion of ash-size basalt grains within the tephra has accelerated the palagonitization of units 1, 2 and 6, as larger surface areas are exposed, resulting in completely cemented units. In the surrounding units, the fine-grained fragments only cover the larger grains, partially cementing the deposit. The total lack of consolidation of HDH-8 (Figure 18) can, however, be explained by the low number of basalt grains within the unit, as palagonitization is confined to basalt glass. For known Hekla tephra layers, this high level of compactness is thought to be unique to Hekla DH (Larsen, personal communication).

48 Figure 18. Microscopic view of grains from units HDH-2 (A) and HDH-8 (B) in the 250 µm to 1 mm size range. Most grains in unit HDH-2 are light colored as result of palagonization, which is not seen in the black HDH-8.

The high proportions of basalt grains within the 125-250 µm grain size range of the Hekla DH tephra hint at a highly efficient fragmentation during the explosive eruption. It is possible that the explosivity was mainly caused, or enhanced, by external water, resulting in the highly but varying compact nature of the deposit. However, the first erupted tephra was still hot enough to become indurated at distance of >20 km from source in the Valagjá area. This would be better explained by a vapor-rich magmatic plume. During a powerful opening phase, the bulk of the erupted magma was fragmented into fine- or medium sized ash grains, resulting in the completely cemented bottom units (HDH-1 and HDH-2). In later phases of the eruption, highly fragmented basaltic and intermediate tephra was erupted along with larger clasts. In the resulting vapor-rich plume, the larger grains mixed with the ash particles, resulting in armored lapillis. After deposition, the larger grains became highly- to completely cemented within the palagonized matrix. The lack of grains in the small to medium ash size range in unit HDH-8 points to a much less efficient fragmentation during the final phases of the explosive eruption, possibly indicating a different eruptive site.

The Hekla DH eruption occurred during a warm period 6650 BP, during the Holocene climatic optimum (e.g. Andresen and Björck, 2005). In proximal deposits, prints of leaves are commonly seen within the completely cemented bottom units, while casts of trees branches are often seen in the deposit. This major eruption is therefore likely to have been devastating for the local vegetation. The environmental impact by the Hekla DH eruption could also have reached over a relatively large area, as the tephra layer maintains a thickness of minimum 40-60 cm along the axis of maximum thickness to Búðarháls, 15 km north of Valagjá.

49 4.1.4 Stratigraphy of the Tagl section

The stratigraphy of the Tagl soil section between the major Hekla 5 and Hekla 4 tephra layers consists of a total of 16 tephra layers (including Hekla DH, Hekla Mó and Hekla Ö) erupted from three volcanic systems within the Eastern Volcanic Zone; Hekla, Katla and Vatnafjöll.

Hekla 5, Hekla DH, Hekla Ö and Hekla 4 are dated tephra markers within the Tagl section (Table 5). Additionally, the composition and stratigraphic location of the silicic Katla layer in the section (Tagl 8) resembles that of the SILK N-1 tephra, which was deposited around 5800 BP (Larsen et al., 2001; Thorsteinsdóttir et al., 2015). The 16 tephra layers were deposited in the Tagl area with an average interval of ̴ 180 years over a period spanning 2900 years. The average depositional interval of tephra layers between Hekla 5 and Hekla Ö was much shorter (ca. 100 years) than the average interval between Hekla Ö and Hekla 4 (>300 years). This indicates that the preservation potential of tephra layers in the area was greatly affected by the deposition of Hekla Ö, or a reduction in eruption frequency.

In sections further to the east, that also cover this time slice and where the Hekla DH and Hekla Ö are not as dominant, e.g. at Herbjarnarfell, the number of tephra layers is twice that of the Tagl section (Larsen, unpublished data). Correlation of the Tagl section to other published records, such as Hestvatn and Hvítarvatn (Jóhannsdóttir, 2007), was therefore not carried out in detail due to less preservation at the Tagl section. However, because of its proximity to the Hekla and Vatnafjöll volcanic systems, along with the high level of activity in the Katla volcanic system throughout the Holocene (Óladóttir et al., 2008), the stratigraphy of the Tagl section does provide a good archive of mid-Holocene explosive eruptions within the EVZ. Katla is responsible for the highest number of the tephra layers within the deposit, while thicker and coarser layers originate from the proximal Hekla system.

Table 5. The tephrostratigraphy of the Tagl section between Hekla 5 and Hekla 4.

Tephra Source Composition Thickness Age (cal. References layer system (cm) yr. BP) Hekla 4 Hekla 4200 Larsen et al. (2013) Tagl 13 Vatnafjöll Basalt 2.5 Tagl 12 Katla Basalt 2.5-3.5 Tagl 11 Katla Basalt 3.5-4 Tagl 10 Katla Basalt 1.3-2 Tagl 9 Katla Basalt 1 Tagl 8 Katla Dacite + basalt 0.5 ̴ 5800 Larsen et al. (2001); (SILK-N1) Thorsteinsdóttir et al. (2015) Hekla Ö Hekla Basalt+ dacite/rhyolite 50 6060 Gudmundsdóttir et al. (2011) Hekla Mó Hekla Basalt + basaltic 6 ̴ 6060 andesite/andesite Tagl 7 Vatnafjöll Basalt + basaltic andesite 1.5-2 Tagl 6 Katla Basalt 1.50 Tagl 5 Katla Basalt 2.0 Tagl 4 Katla Basalt 0-1.3 Basalt + basaltic Gudmundsdóttir et Hekla DH Hekla andesite 153 6650 al. (2011)

Tagl 3 Katla Basalt 0.8 Tagl 2 Katla Basalt 2.3-2.8 Tagl 1 Katla Basalt 1.2-1.8 Hekla 5 Hekla 7100 Larsen et al. (2013)

4.2 Geochemistry

4.2.1 The mid-Holocene magmatic system of Hekla

The Fe-Ti basaltic andesite of the Hekla DH tephra consistently plots outside of the previously published data of Hekla compositions (Figure 13). The composition is slightly more evolved (lower average MgO) than that of the other group of basaltic andesite within the same tephra layer, which is more akin to the typical basaltic andesite from later Hekla eruptions. Compared to this latter basaltic andesite group, the Fe-Ti basaltic andesite is rich in TiO2 and FeO, while having lower Al2O3 and K2O contents. The formation of this particular magma can only be speculated about from major element systematics only.

Modelling of fractional crystallization from a primitive basalt composition of the Hekla DH tephra, using the Petrolog 3.1.1. program (Danyushesvky and Pletchov, 2011), indicate

51 Figure 19. Differentiation trends from primitive basalt as calculated with the Petrolog program at varying oxygen fugacity (triangles), plotted with the Hekla DH tephra, including the Fe-Ti basaltic andesite. Two sigma standard error is displayed. that the two groups of basaltic andesite could have been formed at different magmatic oxygen fugacity (fO2) beneath Hekla. The modelling was performed by calculating differentiation trends from the most primitive basalt composition observed in Hekla DH for fractional crystallization process with varying fO2 (log QFM of -2, -1, 0 and +1; where log QFM are log 10 units below, at and above the QFM (quartz-fayalite-magnetite) oxygen fugacity buffer) at a fixed pressure of 6 kbar. The depth chosen roughly corresponds to the estimated minimum depth expected of a magma storage region beneath Hekla (Soosalu and Einarsson, 2004; Höskuldsson et al., 2007). The calculations show that the Fe-Ti basaltic andesite can be reproduced at fO2 buffered by QFM, whereas the more typical basaltic andesite of the Hekla system is most likely formed under significantly +3 higher fO2 (Figure 19). With lower fO2, the activity of ferric iron (Fe ) is lower and consequently the formation and fractional crystallization of ulvospinel (magnetite-ilmenite t solid solution) from the magma is delayed, causing increased TiO2 and FeO concentration at given MgO in the magma.

An increase in fO2 in early- to mid-Holocene in the magmatic system beneath Hekla could have resulted from melting of different regions of a heterogeneous mantle source. The deglaciation of the area is likely to have lowered the lithostatic pressure and thus decreasing the depth of the liquidus of the mantle. Since the mantle is probably of heterogeneous composition, melting at decreased pressure will not only be larger but also cause melting of more refractory and reduced lithologies. How far into the Holocene larger mantle melting due to the deglaciation lasted is not known. It is thus suggested that an increase in fO2 in the Hekla system reflects preferential melting of fertile mantle, compared to the early Holocene when the de-compressional effects of the deglaciation were at maximum. Changes in melting conditions following the deglaciation of Iceland have previously been proposed in the Northern Volcanic Zone (e.g. Slater et al., 1998).

The lack of a mixing relationship between the individual groups of basaltic andesite within the Hekla DH tephra (Figure 13 and Figure 20) might indicate that the two magmas were stored separately at depth prior to eruption. The Hekla DH eruption was a major fissure eruption, depositing approximately 0.8 km3 of freshly fallen tephra. The length of the

52 Figure 20. The intermediate magma compositions of Hekla DH and Hekla Mó plotted together. The Hekla Mó tephra displays a genetic relationship with both groups of basaltic andesite from the Hekla DH tephra.

fissure which erupted is not known but it is likely that different segments were active within certain periods throughout the eruption. Consequently, magma from a different storage region could erupt without mixing together, resulting in the abrupt changes observed in both composition and nature of the tephra layer from units HDH 1-7 to HDH-8 in the Tagl section.

In a few analyses, the andesite of the Hekla Mó tephra displays a possible genetic relation with the Fe-Ti basaltic andesite of the Hekla DH tephra, which is best seen on FeO vs MgO and Al2O3 vs MgO (Figure 20). No other known Hekla products have similar composition as the peculiar Fe-Ti basaltic andesite. The Hekla Mó eruption occurred approximately 500 years after the Hekla DH eruption and no eruption is known to have occurred in the Hekla system between the two. The relation of the andesites of the Hekla Mó tephra with the Fe-Ti basaltic andesite of the Hekla DH tephra possibly indicates that a batch of residual Fe-Ti basaltic andesite magma mixed with more typical basaltic andesites of the Hekla system at some depth following the Hekla DH eruption. That hybrid magma later evolved to andesite prior to being erupted in the Hekla Mó eruption. This might have removed all residual and available Fe-Ti basaltic andesite magma from the system, as no younger Hekla products display that composition.

Unlike the basaltic andesites of Hekla DH, where no relation is observed between the two groups of intermediate magma compositions, a continuous magma evolution trend is

Figure 21. The basalts of Hekla DH, Hekla Mó and Hekla Ö. An evolution trend is observed from the most primitive (high MgO) to the more evolved basalts within Hekla DH.

53 observed in the basalt part of the tephra. The basalts of Hekla DH ranges highly in composition, with the MgO content varying 5-7.5% (Figure 21). A similar range is seen in the Hekla Ö tephra, while only in the Hekla DH tephra can a full trend be observed. This trend reflects a failure by the young magma system of Hekla to fully evolve and mix available primitive mantle melt, and derived basalt thereof. In later times, evolved and relatively well mixed Fe-Ti basalts have dominated the basaltic activity of the Hekla system, while the primitive basalt composition has only been observed in a handful of tephra layers in the last 9000 years (Jóhannsdóttir, 2007; Óladóttir, 2009 and unpublished data; Gunnarson, 2017; Harning et al., 2018)

4.2.2 Hekla and Katla basalts

In the samples collected from the Tagl section, the basaltic Katla tephra layers display similar chemical characteristics as some the most evolved Fe-Ti basalt composition observed in Hekla DH, Hekla Mó and Hekla Ö (Figure 22). The Katla tephra layers do however display consistently higher TiO2 and K2O contents than the Hekla basalts, but not enough to readily separate the two. This near-identical composition of two different volcanic systems could therefore result in distal tephra to be erroneously assigned to a specific volcanic system.

In the three Hekla tephra layers, the Fe-Ti basalt group always occurs along with other compositional groups which display distinct Hekla characteristics. This also applies to purely basaltic tephra layers from Hekla, such as the Hekla VF tephra, where both primitive basalts and evolved Fe-Ti basalts occur together (Harning et al., 2018).

The Fe-Ti basalt ranges significantly in composition within all three Hekla tephra layers. For example, the TiO2 content of the Fe-Ti basalt in the Hekla Ö tephra ranges from 3.5- 4.5%. When compared, the Katla tephra layers are individually much more uniform in nature, usually containing only one compositional group and displaying only minor ranges in most major oxides. Collectively, the Katla tephra layers do however range somewhat in major oxides (Óladóttir et al., 2008). The uniformity of each Katla layer results in much lower standard deviations of each major oxide contents on average than that of the Fe-Ti basalts of the individual Hekla tephra layers. This is best seen in the standard deviations for TiO2 and CaO, where Fe-Ti basalt of the Hekla tephra layers display 2-3 times greater standard deviations compared to individual basaltic Katla tephra layers.

Figure 22. Overlap between basalts from Hekla and Katla. Based on data collected in this study.

54 4.3 Hekla during the mid-Holocene

The Hekla Ö eruption was the second in line of large, silicic eruptions that occurred infrequently in Hekla over a 4000 years long period, following the Hekla 5 eruption. Hekla DH and Hekla Mó however represent an intra-cyclic activity between Hekla 5 and Hekla Ö, both being characterized by relatively smaller volumes and less evolved compositions, possibly analogous to most of the historical eruptions of Hekla. Hekla DH and Hekla Mó also display largely similar chemical compositions with products from the historical eruptions in the system, while displaying larger ranges.

The Hekla DH eruption is the earliest confirmed eruption of basaltic andesite in the Hekla system (Figure 23), occurring in the Valagjá area in the northern part of the system. The record of prehistoric basaltic andesite lava flows from Hekla proper is poor and the age of the oldest lava flows is not known. Hekla 4 produced relatively small amount of basaltic andesite as did Hekla S (Sverrisdóttir, unpublihed data) some 300 years later. Following the Hekla 3 eruption, basaltic andesite has become more prominent (e.g. Róbertsdóttir et al. 2002). In historical times, basaltic andesite of a rather uniform composition has been the main product of Hekla proper. The Hekla DH tephra does contain basaltic andesite which is very similar to the products of the most recent Hekla eruptions. The tephra is however slightly different from the known intermediate Hekla tephra layers, as it partly consists of the peculiar Fe-Ti basaltic andesite, which was most likely formed within magmatic conditions which have not been present in more recent times, as discussed in chapter 4.2.1.

Following the onset of basaltic andesite production from Hekla during the mid-Holocene, eruptions of primitive basalt compositions have practically ceased. The primitive basalt composition dominates the hyaloclastite formations of Pleistocene age in the Hekla area (Jakobsson, 1979; Meyer et al., 1985; Sigmarsson et al., 1992; Sverrisdóttir unpublished data), while that composition is also observed in several tephra layers of early-and mid- Holocene age (Jóhannsdóttir 2007; Óladóttir, 2009; Gunnarson, 2017; Harning et al., 2018). In Hekla DH and Hekla Ö, the primitive basalt was erupted along with more evolved Fe-Ti basalt. The Fe-Ti basalts have dominated the basaltic activity in the system ever since, with all known basaltic lava flows from the system displaying that composition (Jakobsson, 1979; Meyer et al., 1986; Sigmarsson et al., 1992; Chekol et al., 2011). Thus, a change is observed in the basaltic activity of the Hekla system around 7000-6000 years BP (Figure 23). This change occurred concurrently with the initiation of intermediate magma production in the system, which subsequently has become the dominant product of the Hekla system.

Sigmarsson et al. (1992) proposed that the current plumbing system beneath Hekla was only established during the Holocene. The commencement of basaltic andesite magma production in the system during the mid-Holocene, along with changes in the basaltic activity during the same period, might indicate the current plumbing system emerging following the Hekla 5 eruption. The three tephra layers focused on in this study might therefore have been erupted from a young plumbing system, which later has matured and established more regular activity and purely erupted evolved lavas of basaltic and intermediate compositions between the large, silicic eruptions.

55 Figure 23. The different rock types erupted from the Hekla system plotted against time. Circles indicate known and dated occurrences of the respective rock type, while shaded areas indicate the period when the respective rock type has been erupted during the Holocene. The dataset does not cover all known eruptions in the Hekla system but gives an overview. Dataset includes this study and available data for the Hekla system (Jakobsson, 1979; Róbertsdóttir et al., 1992, 2002; Larsen and Vilmundardóttir, 1992; Larsen et al., 2002; Jóhannesdóttir, 2007; Sverrisdóttir, 2007 and unpublished data; Gudmundsdóttir et al., 2011; Chekol et al., 2011; Meara, 2011; Gunnarsson, 2017; Harning et al., 2018; Óladóttir, 2009 and unpublished data).

56 5 Conclusions

The proximal dispersal of the Hekla DH, Hekla Mó and Hekla Ö tephra layers has been mapped. Mapping of the Hekla DH tephra has confirmed that the tephra was erupted from the Valagjá area. Whole-rock analysis of samples collected from crater rims within the southwestern most crater row, further support a source in the Valagjá area, giving a compositional match with basaltic andesite in the tephra. The Hekla Mó tephra, which was erupted shortly before the 6060 BP Hekla Ö, was erupted from multiple vents within the Hekla system. The two dispersal axes of Hekla Ö confirm that the tephra was erupted from Hekla proper. Hekla DH and Hekla Ö were both major eruptions on the volcanic system, having calculated volumes of 0.8 km3 and 1.1 km3 of freshly fallen tephra, respectively. The Hekla Mó was however a minor event, producing only a minimum of 0.05 km3 of tephra.

The detailed chemical record of Hekla Ö obtained with this study can be used to establish a relation between individual units of Hekla Ö in the proximal Tagl section to distal deposits within terrestrial Iceland. Consequently, a relation between Hekla Ö and distal Hekla deposits in Northwest-, West-, Southwest- and South-Iceland is suggested, pulling the western dispersal margins of the tephra much further west than previously thought. When compared to the much larger, silicic Hekla tephra layers, Hekla Ö has a modest volume of 1.1 km3 but great dispersal, possibly covering as much 80% of Iceland. That coverage is similar with that of Hekla 3 and Hekla 4, while the volume of the two tephra layers is in both cases is more than ten times greater.

The three tephra layers were erupted at a time when Hekla was evolving from a volcano characterized by primitive basalts to a volcano dominated by evolved volcanism. The Hekla DH eruption 6650 BP was the first confirmed eruption of basaltic andesite from the Hekla system, which subsequently has become the most important product of the system. t The occurrence of basaltic andesite, rich in FeO and TiO2 (termed Fe-Ti basaltic andesite in this thesis), within the tephra is coherent with magmatic oxygen fugacity beneath Hekla being significantly lower prior to the Hekla DH eruption compared to that of later times. An increase in the magmatic oxygen fugacity during early- to mid-Holocene could have resulted from earlier deglaciation of the area, which is likely to have caused melting of different regions of a heterogenous mantle. Further modeling is however needed to confirm this.

Furthermore, both Hekla DH and Hekla Ö contain a basalt part which ranges highly in composition (5-7,5% MgO). Only in the Hekla DH tephra can a continuous evolution trend be observed however. These high ranges in basalt compositions demonstrate a failure by the Hekla system to fully evolve and mix all available primitive mantle melt prior to erupting during the mid-Holocene. In later times, relatively evolved and well mixed basalt, displaying the Fe-Ti basalt composition, has dominated the basaltic activity in the system.

57 Along with Hekla DH, Hekla Mó and Hekla Ö, 13 tephra layers were identified in a soil section in Tagl. All 13 layers were erupted from volcanic systems within the EVZ and provide a good overview of the local mid-Holocene tephrostratigraphy. Ten of the thirteen layers are basaltic Katla layers, while one is a silicic Katla layer (SILK). The composition of the two remaining layers indicate a source in Vatnafjöll.

58 6 References

Andreasen, C., & Björck, S., 2005. Holocene climate variability in the Denmark Strait region. Geografiska Annaler 87 A (1), 159-174.

Chekol, A.T., Kobayashi, K., Yokoyama, T., Skaguchi, C., Nakamura, E., 2011. Timescales of magma differentiation from basalt to andesite beneath Hekla Volcano, Iceland: Constraints from U-series, disequilibria in lavas from the last quarter-millenium flows. Geochimica et cosmochimica acta 75, 256-283.

Danyushevsky, L.V., Pletchov, P. 2011. Petrolog 3: Integrated software for modelling crystallization processes. Geohchemistry, Geophysics, Geosystems, G3, 12/7. 37 p. Q07021, doi:10.1029/2011GC003561.

Eddudóttir, S.D., Erlendsson, E., Tinganelli, L., Gísladóttir, G., 2016. Climate change and human impact in a sensitive ecosystem: the Holocene environment of the Northwest Iceland highland margin. Boreas 45, 715-728.

Guðmundsdóttir, E.R., Larsen, G., Björck, S., Ingólfsson, Ó., 2016. A new high-resolution Holocene tephra stratigraphy in eastern Iceland: Improving the Icelandic and North Atlantic tephrochronology. Quaternary Science Review 150, 234-249.

Gudmundsdóttir, E.R., Larsen, G., Eiríksson, J., 2011: Two new Icelandic tephra markers: The Hekla-Ö tephra layer,~6060 cal. yr BP and Hekla-DH tephra layer, ~6650 cal. yr BP – Land-Sea correlation of Mid-Holocene markers. The Holocene 21; 629-639. DOI: 10.1177/0959683610391313.

Guðmundsson, A., Óskarsson, N., Grönvold, K., Sæmundsson, K., Sigurðsson, O., Stefánsson, R., Gíslason, S.R., Einarsson, P., Brandsdóttir, B., Larsen, G., Jóhannesson, H., Thordarson, T., 1992. The 1991 eruption of Hekla, Iceland. Bulletin of Volcanology 54, 238-246.

Gunnarson, S., 2016. Holocene Climate and Landscape Evolution in the west Central Highlands, Iceland. Master´s thesis, University of Iceland, Reykjavík.

Grönvold, K., Larsen, G., Einarsson, P., Thorarinsson, S., Sæmundsson, K., 1983. The eruption of Hekla 1980-1981. Bulletin of Volcanology 46, 349-363.

Hardardóttir J, Geirsdóttir Á, Thordarson T, 2001. Tephra layers in a sediment core from Lake Hestvatn, southern Iceland: implications for evaluating sedimentation processes and environmental impacts on a lacustrine system caused by tephra fall deposits in the surrounding watershed. In: J.D.L. White and N.R. Riggs (Ed.), Volcaniclastic Sedimentation in Lacustrine Settings. Internat Assoc Sedimentolog (IAS). Spec. Publ. Blackwell Science, Oxford: 225-246.

59 Harning, D.J., Thordarson, T., Geirsdóttir, Á., Zalzal, K., Miller, G.H. 2018. Provenance stratigraphy and chronology of Holocene tephra from Vestfirðir, Iceland. Quaternary Geochronology 46, 59-76.

Höskuldsson, Á., Óskarsson, N. Pedersen, R., Grönvold, K., Vogfjörð, K., Ólafsdóttir, R. 2007. The millenium eruption of Hekla in February 2000. Bulletin of Volcanology 70, 169- 182.

Jakobsson, S., 1979. Petrology of recent basalts of the Eastern Volcanic Zone, Iceland. Acta Natura Islandica 26.

Jakobsson, S., Jónasson, K., Sigurðsson, I.A., 2008. The three igneous rock series of Iceland. Jökull 58, 117-138.

Janebo, M.J., Thorvaldsson, T., Houghton, B.F., Bonadonna, C., Larsen, G., Carey, J.R., 2016. Dispersal of key subplinian-Plinian tephras from Hekla volcano, Iceland: implications for eruption source parameters. Bulletin of Volcanology 78, 66.

Jóhannesson, H., Grönvold, K., Sveinbjörnsdóttir, Á.E., 1994. Ófærugil ravine. Tephrocronology below Hekla 5. Abstracts, Geoscience Society of Iceland, Spring Meeting, 13.

Jóhannesson, H., Sæmundsson, K., 1998. Geological map of Iceland, 1:500.000. Bedrock Geology. Reykjavík.

Jóhannesson, H., Sæmundsson, K., Jakobsson, J., 1990. Geological map of Iceland, sheet 6, South Iceland. 1:250.000. Icelandic Museum of Natural History and Iceland Geodetic Survey, Reykjavík. Jóhannsdóttir, G.E., 2007. Mid Holocene to late glacial tephrochronology in West Iceland as revealed in three lacustrine environments. Master´s thesis, University of Iceland, Reykjavík.

Larsen, G., Newton, A.J., Dugmore A.J., Vilmundardóttir, E.G., 2001: Geochemistry, dispersal, volumes and chronology of Holocene silicic tephra layers from the Katla volcanic system, Iceland. Journal of Quaternary Science 16, 119-132.

Larsen G., Sverrisdóttir G., Hjartarson H., Einarsson P., Jóhannesson H., 2013. Hekla, in: Sólnes, J., Sigmundsson, F., Bessason, B., (ed), Náttúruvá á Íslandi - Eldgos og jarðskjálftar, 189-209. Viðlagatrygging Íslands/Háskólaútgáfan, Reykjavík.

Larsen, G., Thorarinsson, S., 1977. H4 and other acid Hekla tephra layers. Jökull 27, 28- 46. Larsen, G., Vilmundardóttir, E.G., 1992. Hekla tephra layers from th period 2900-1800 BP: H-x, H-y and H-z. Abstracts, Geoscience Soc. Iceland Spring Meeting, 28-29.

60 Larsen, G., Vilmundardóttir, E.G., Róbertsdóttir, B.G., 2002. SE-trending Hekla tephra sectors erupted between 2660 and 2880 14C yrs BP. Characteristics of potential marker horizons outside Iceland. Abstracts, The 25th Nordic Geological Winter Meeting, 121.

Meara, R.H., 2011. Geochemical fingerprinting of Icelandic silicic Holocene Tephra layers. PhD dissertation, University of Edinburgh, Edinburgh. 338 pp. Meyer, P.S., Sigurdsson, H., Schilling, J.G., 1985. Petrological and geochemical variations along Iceland´s Neovolcanic zones. Journal of Geophysical Research 90, 10043-10072.

Ófeigsson, B.G., Hooer, A., Sigmundsson, F., Sturkell, E., Graenthin, R., 2011. Deep magma storage at Hekla volcano, Iceland, revealed by InSAR time series analysis. Journal of Geophysical Research 116:1-15 https://doi.org/10.1029/2010JB007576.

Óladóttir, B.A. 2009. Holocene eruption history and magmatic evolution of the subglacial Vatnajökull volcanoes, Grímsvötn, Bárðarbunga and Kverkfjöll, Iceland. PhD dissertation, Université Blaise Pascal, Clermont-Ferrand and University of Iceland, Reykjavík. 139p.

Óladóttir, B.A., 2011. Holocene volcanic activity at Grímsvötn, Bárdarbunga and Kverkfjöll subglacial centres beneath Vatnajökull, Iceland. Bulletin of Volcanology 73, 1187-1208. doi:10.1007/s00445-011-0461-4.

Óladóttir, B.A., Sigmarsson, O., Larsen, G., Thordarson, T., 2008. Katla volcano, Iceland: Magma composition, dynamics and eruption frequency as recorded by Holocene tephra layers. Bulletin of Volcanology 70, 475-493.

Pilcher, J., Bradley, S.R., Francus, P., Anderson, L., 2005. A Holocene tephra record from Lofoten, Arctic Norway. Boreas 34, 136-156.

Plunkett, G., Pilcher, J. R., 2018. Defining the potential source region of volcanic ash in northwest Europe during Mid- to Late Holocene. Earth Science Reviews 179, 20-37.

Róbertsdóttir, B. G., 1992. Þjú forsöguleg gjóskulög frá Heklu, HA, HB, og HC (Three prehistoric tephra layers from Hekla volcano, HA, HB and HC). Abstracts, Geoscience Soc. Iceland Spring Meeting, 6–7.

Róbertsdóttir, B.G., Larsen, G., Eiríksson, J., 2002. A new detailed stratigraphical and geochemical record of 30 tephra layers from the Hekla volcanic system, Iceland 2980-850 cal. BP. The 25th Nordic Geolocical Winter Meeting. Reykjavík.

Schomacker, A., Brynjólfsson, S., Andreassen, J.M., Guðmundsdóttir, E.R., Olsen, J., Odgaard, B.V., Hakansson, L., Ingólfsson, Ó., Larsen, N.K., 2016. Quaternary Science Reviews 148, 68-84.

Sigmarsson, O., Condomines, M., Fourcade, S., 1992. A detailed Th, Sr and O isotope study of Hekla: differentiation processes in an Icelandic volcano. Contributions to Mineralogy and Petrology 112, 20-34.

Sigmarsson, O., Vlastelic, I., Andreasen, R., Bindeman, I., Devidal, J.L., Moune, S., Keiding, J.K., Larsen, G., Höskuldsson, Á., Thordarson, T., 2011. Remobilization of silicic

61 intrusion by mafic magmas during the 2010 Eyjafjallajökull eruption. Solid Earth 2, 271- 281.

Sigurgeirsson, M.Á., Hjartarson, Á., 2011. Gjóskulög og fjörumór á berghlaupi við Sjávarhóla á Kjalarnesi. Náttúrufræðingurinn 81, 123–130

Sigvaldason, G.E., 1975. Um bergfræði Heklu. Náttúrufræðingurinn 44, 153-170.

Slater, L., Jull, M., McKenzie, D., Gronvöld, K., 1998. Deglaciation effects on mantle melting under Iceland: results from the northern volcanic zone. Earth and Planetary Science Letters 164, 151-164.

Soosalu, H., Einarsson, P., 2004. Seismic constraints on magma chambers at Hekla and Torfajökull volcanoes, Iceland. Bulletin of Volcanology 66, 276-286.

Stevenson, J., Larsen, G., Thordarson, T., 2015. Physical volcanology of the prehistoric Hekla 3 and Hekla 4 eruptions, Iceland. EGU General Assembly id.4207.

Stroncik, N.A., Schminke, H.U., 2002. Palagonite – a review. Internaional Journal of Earth Sciences 91, 680-697.

Sveinbjörnsdóttir, Á.E., Heinemeier, J., Kristensen, P., Rud, N., Geirsdóttir, Á., Harðardóttir, J., 1998. 14C AMS datings of Icelandic lake sediments. Radiocarbon 40, 865-872.

Sverrisdóttir, G., 2007. Hybrid magma generation preceding Plinian silicic eruptions at Hekla, Iceland: evidence from mineralogy and chemistry of two zoned deposits. Geological Magazine 144 (4), 643-659.

Thorarinsson, S., 1967a. The eruptions of Hekla in historical times. In: Einarsson, T., Kjartansson, G. and Thorarinsson, S., (ed) The Eruption of Hekla 1947–48. Soc Sci. Islandica 1, 1–177.

Thorarinsson, S., 1967b. Hekla and Katla. The share of acid and intermediate lava and tephra in the volcanic products through the geological history of Iceland. In: Björnsson, S., (ed) Iceland and mid-ocean ridges. Soc Sci Islandica 1, 190-197.

Thorarinsson, S., Sigvaldason, G.E., 1972. The Hekla eruption of 1970. Bull. Volcanol. 36, 269–288.

Thordarson, T., Larsen, G., 2007. Volcanism in Iceland in historical time: Volcano types, eruption style and eruptive history. Journal of Geodynamics 43, 118-152.

Thorsteinsdóttir, E.S., Larsen, G. and Gudmundsdóttir, E.R., 2016. Grain characteristics of silicic Katla tephra layers indicate a fairly stable eruption environment between 2800 and 8100 years ago. Jökull 66, 69-82. Tómasdóttir, S., 2015. Gjóskulagið Hekla Ö í Tagli. BS thesis, Faculty of Earth Sciences, University of Iceland, 78p.

Appendix 1

Table A.1.1 Electron microprobe analysis results Sample Tephra Volcanic Analysis name layer system SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 Total date Tagl 13 Vatnafjöll 45.99 3.78 14.23 16.28 0.27 5.50 9.54 2.62 0.67 0.50 99.38 11.2.2017 46.50 3.94 13.35 15.95 0.25 5.44 9.85 2.99 0.64 0.52 99.43 46.48 3.77 13.96 15.77 0.27 5.36 9.67 2.87 0.63 0.55 99.33 46.06 3.79 14.13 15.57 0.23 5.62 9.71 2.97 0.60 0.43 99.11 47.51 3.93 14.34 15.53 0.23 5.58 9.99 1.37 0.64 0.44 99.56 45.98 3.64 14.22 15.50 0.23 5.69 9.61 2.92 0.61 0.49 98.89 46.27 3.86 14.23 15.48 0.22 5.64 9.66 2.86 0.60 0.47 99.29 46.38 3.81 14.13 15.48 0.24 5.91 9.61 2.93 0.57 0.50 99.57 46.34 3.81 14.28 15.39 0.26 5.51 9.61 2.85 0.58 0.46 99.09 46.01 3.72 13.92 15.36 0.26 5.72 9.64 2.75 0.60 0.44 98.42 45.87 3.54 14.38 15.32 0.26 5.88 9.79 2.84 0.57 0.47 98.92 46.81 3.69 13.72 15.15 0.27 5.73 10.11 2.90 0.59 0.47 99.44 46.02 3.64 14.28 15.07 0.25 5.75 9.94 2.82 0.60 0.44 98.81 47.52 4.11 13.27 14.93 0.21 4.76 9.83 2.91 0.75 0.47 98.76 46.50 3.13 13.92 14.30 0.25 6.14 11.22 2.74 0.46 0.34 99.00 Mean 46.42 3.74 14.02 15.41 0.25 5.62 9.85 2.76 0.61 0.47 99.13 stdv 0.52 0.22 0.34 0.45 0.02 0.31 0.41 0.40 0.06 0.05 0.33

65 Sample Tephra Volcanic Analysis name layer system SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 Total date Tagl 12 Katla 47.79 4.34 13.15 15.17 0.25 5.16 9.92 3.06 0.72 0.43 100.00 11.2.2017 47.77 4.47 13.38 15.57 0.23 5.20 9.82 2.63 0.79 0.47 100.33 47.50 4.14 13.16 14.94 0.23 5.02 9.74 2.81 0.76 0.46 98.77 47.48 4.26 13.15 15.14 0.22 5.04 9.90 2.99 0.74 0.49 99.41 47.27 4.21 13.31 15.11 0.22 5.01 9.87 3.02 0.74 0.46 99.22 47.25 4.34 13.34 15.08 0.21 5.13 9.71 2.88 0.75 0.41 99.10 47.21 4.14 13.07 15.06 0.19 5.12 9.84 3.00 0.76 0.47 98.87 47.20 4.32 13.07 15.12 0.25 4.85 9.76 2.96 0.76 0.44 98.73 47.20 4.29 13.09 15.33 0.21 5.24 10.10 2.34 0.71 0.42 98.93 47.16 4.36 13.06 15.10 0.23 5.08 9.99 3.09 0.77 0.47 99.31 47.14 4.38 13.24 15.50 0.23 5.10 10.03 3.12 0.77 0.48 100.00 46.82 4.27 13.00 15.06 0.22 5.08 9.90 3.01 0.75 0.43 98.55 46.73 4.25 12.97 14.91 0.23 5.18 9.65 3.04 0.77 0.48 98.22 46.68 4.13 13.11 15.12 0.21 5.07 9.74 2.95 0.76 0.43 98.19 46.57 4.21 13.17 15.46 0.23 5.09 9.79 3.02 0.73 0.45 98.71 Mean 47.18 4.27 13.15 15.18 0.22 5.09 9.85 2.93 0.75 0.45 99.09 stdv 0.37 0.10 0.12 0.20 0.02 0.09 0.13 0.20 0.02 0.02 0.64

Sample Tephra Volcanic Analysis name layer system SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 Total date Tagl 11 Katla 47.50 4.40 12.75 15.00 0.23 4.79 9.78 3.18 0.86 0.42 98.91 11.2.2017 48.35 4.33 12.90 14.87 0.22 4.46 9.36 3.01 0.90 0.47 98.88 47.50 4.33 13.15 15.16 0.22 4.61 9.52 3.12 0.85 0.46 98.92 48.33 4.17 13.28 14.58 0.24 4.74 9.52 3.14 0.92 0.45 99.37 48.05 4.17 13.14 14.82 0.28 4.84 9.62 3.11 0.87 0.50 99.40 48.09 4.16 13.26 14.50 0.22 4.68 9.55 2.90 0.82 0.42 98.59 48.83 4.14 13.48 14.60 0.23 4.56 9.21 2.93 0.92 0.40 99.30

66 48.59 4.08 13.26 14.76 0.25 4.74 9.46 3.14 0.85 0.43 99.56 48.46 4.01 13.10 14.11 0.21 4.38 9.16 3.23 0.92 0.48 98.06 49.54 4.00 13.46 15.06 0.24 4.83 9.21 2.31 1.26 0.42 100.33 49.25 3.95 13.17 14.17 0.20 4.63 9.20 3.15 0.97 0.43 99.12 48.14 3.93 12.67 14.68 0.22 4.82 9.56 3.03 0.87 0.44 98.36 49.56 3.92 13.43 13.90 0.20 4.52 8.96 3.18 1.01 0.44 99.12 49.14 3.89 13.29 13.99 0.22 4.60 9.06 3.13 0.92 0.48 98.72 50.04 3.86 13.41 13.80 0.21 4.43 8.82 3.18 1.02 0.46 99.24 Mean 48.62 4.09 13.18 14.53 0.23 4.64 9.33 3.05 0.93 0.45 99.06 stdv 0.76 0.17 0.25 0.44 0.02 0.15 0.27 0.23 0.11 0.03 0.54

Sample Tephra Volcanic Analysis name layer system SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 Total date Tagl 10 Katla 48.09 4.37 13.32 15.09 0.24 5.00 9.82 1.62 0.80 0.49 98.84 11.2.2017 47.15 4.35 13.02 14.98 0.25 4.92 9.81 2.92 0.75 0.48 98.63 47.39 4.34 13.25 15.08 0.20 5.00 9.85 2.93 0.73 0.46 99.23 47.53 4.32 13.24 15.19 0.23 5.08 9.76 2.92 0.74 0.46 99.48 47.29 4.32 13.30 14.97 0.21 5.08 9.75 2.84 0.76 0.44 98.96 47.55 4.30 13.46 14.82 0.22 5.05 9.81 2.65 0.79 0.44 99.10 47.16 4.30 13.46 14.95 0.22 5.19 9.82 2.71 0.77 0.42 99.01 47.60 4.28 13.47 15.05 0.23 5.18 9.78 2.61 0.76 0.46 99.41 47.12 4.28 13.41 14.99 0.21 5.22 9.78 2.79 0.76 0.43 98.99 47.71 4.26 13.28 14.92 0.22 5.06 9.77 2.75 0.73 0.50 99.21 47.45 4.21 13.41 14.95 0.22 5.03 9.73 2.65 0.75 0.45 98.85 47.30 4.21 13.19 15.00 0.25 5.18 9.78 2.93 0.74 0.51 99.09 47.57 4.20 13.52 14.97 0.20 5.13 9.85 2.77 0.73 0.46 99.40 47.23 4.16 13.56 15.00 0.20 5.18 9.96 2.97 0.76 0.46 99.49 47.74 3.97 13.47 15.26 0.23 5.02 9.74 2.77 0.76 0.44 99.40 Mean 47.46 4.26 13.36 15.01 0.22 5.09 9.80 2.72 0.76 0.46 99.14

67 stdv 0.27 0.10 0.15 0.11 0.02 0.09 0.06 0.33 0.02 0.03 0.26

Sample Tephra Volcanic Analysis name layer system SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 Total date Tagl 9 Katla 47.91 4.54 13.17 15.49 0.21 4.73 9.77 2.85 0.82 0.46 99.95 11.2.2017 47.40 4.44 13.31 14.84 0.24 5.12 9.68 2.71 0.74 0.47 98.94 47.40 4.44 13.49 14.87 0.21 5.11 9.83 2.63 0.79 0.41 99.17 47.34 4.43 13.33 14.90 0.22 5.17 9.78 2.86 0.77 0.45 99.26 47.42 4.42 13.36 14.87 0.21 5.09 9.67 2.39 0.76 0.49 98.68 47.82 4.41 13.23 15.04 0.21 5.00 9.93 2.87 0.77 0.48 99.76 47.56 4.39 13.46 15.08 0.24 4.95 9.95 2.94 0.78 0.41 99.76 47.39 4.38 13.23 14.99 0.21 4.91 9.84 2.58 0.73 0.47 98.74 47.50 4.37 13.23 15.13 0.25 5.02 9.72 2.67 0.77 0.46 99.12 48.62 4.35 13.59 15.62 0.23 5.10 9.97 2.03 0.70 0.44 100.65 47.60 4.33 13.00 15.11 0.23 4.74 9.74 2.95 0.80 0.48 98.98 47.28 4.30 13.14 14.93 0.19 5.06 9.83 2.67 0.76 0.40 98.56 48.15 4.25 13.45 15.46 0.23 4.99 9.95 2.86 0.78 0.48 100.60 47.70 4.24 13.16 14.72 0.20 4.99 9.57 2.59 0.75 0.44 98.36 Mean 47.65 4.38 13.30 15.08 0.22 5.00 9.80 2.69 0.77 0.45 99.32 stdv 0.37 0.08 0.16 0.27 0.02 0.13 0.12 0.25 0.03 0.03 0.72

Samp le Tephra Volcanic Analysis name layer/Unit system SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 Total date Tagl 21 SILK-N1 Katla 67.00 1.35 14.36 5.92 0.18 1.05 3.10 1.87 2.56 0.33 97.72 11.2.2017 67.47 1.30 14.35 6.11 0.20 1.02 3.05 1.98 2.53 0.30 98.32 67.05 1.32 14.33 6.12 0.19 1.09 3.16 2.02 2.59 0.28 98.14 67.05 1.32 14.29 6.11 0.18 1.09 3.13 1.26 2.65 0.30 97.38 66.91 1.28 14.21 6.41 0.22 1.04 3.12 1.71 2.64 0.33 97.88

68 67.39 1.28 14.21 6.04 0.19 1.00 3.03 1.90 2.61 0.29 97.94 66.72 1.30 14.18 6.21 0.18 1.04 3.15 1.98 2.62 0.33 97.71 66.66 1.27 14.18 6.20 0.21 1.04 3.10 1.82 2.58 0.33 97.39 67.11 1.36 14.17 6.17 0.22 1.08 3.10 1.61 2.61 0.32 97.74 66.77 1.30 14.17 6.22 0.19 1.09 3.17 1.64 2.57 0.27 97.40 66.85 1.36 14.16 6.13 0.20 1.07 2.89 1.44 2.57 0.28 96.95 67.10 1.37 14.15 6.10 0.18 1.02 3.00 1.28 2.66 0.29 97.14 66.85 1.32 14.01 6.17 0.20 1.05 3.03 1.72 2.58 0.28 97.21 Mean 66.99 1.32 14.21 6.15 0.20 1.05 3.08 1.71 2.60 0.30 97.61 stdv 0.24 0.03 0.10 0.11 0.01 0.03 0.08 0.26 0.04 0.02 0.41

47.48 4.23 13.55 14.89 0.23 4.99 9.80 2.66 0.79 0.50 99.13 46.34 3.69 14.69 14.91 0.27 5.82 9.66 2.81 0.58 0.41 99.18 46.91 3.96 14.12 14.90 0.25 5.41 9.73 2.74 0.69 0.46 99.15 Mean 46.91 3.96 14.12 14.90 0.25 5.41 9.73 2.74 0.69 0.46 99.15 stdv 0.57 0.27 0.57 0.01 0.02 0.42 0.07 0.08 0.10 0.05 0.03

Sample Tephra Volcanic Analysis name layer system SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 Total date HÖ-4 Hekla Ö Hekla 63.32 0.86 15.34 9.06 0.26 1.04 4.46 1.80 1.68 0.35 98.17 11.2.2017 62.99 0.81 15.34 8.93 0.30 0.94 4.40 1.56 1.73 0.29 97.30 62.71 0.88 15.47 9.50 0.30 1.09 4.70 2.04 1.63 0.42 98.74 62.56 0.87 15.37 9.18 0.25 1.07 4.56 1.69 1.61 0.38 97.54 62.52 0.82 15.06 9.30 0.27 0.98 4.66 1.74 1.69 0.39 97.43 62.46 0.87 15.29 9.61 0.28 1.07 4.53 1.96 1.67 0.32 98.05 62.26 0.89 15.29 9.69 0.27 0.99 4.53 1.63 1.72 0.40 97.67 Mean 62.69 0.86 15.31 9.32 0.28 1.03 4.55 1.77 1.68 0.37 97.84 stdv 0.36 0.03 0.13 0.29 0.02 0.05 0.10 0.17 0.04 0.05 0.51

69 47.49 5.18 11.93 16.76 0.24 4.22 9.17 2.74 1.00 0.57 99.30 47.54 4.49 12.75 15.14 0.30 4.70 9.57 2.38 0.94 1.01 98.82 47.76 4.37 13.36 15.00 0.23 4.89 9.56 2.60 0.73 0.44 98.95 47.18 4.34 13.23 15.26 0.22 5.15 9.87 2.74 0.76 0.49 99.24 47.55 4.29 13.51 14.98 0.22 5.11 10.00 2.68 0.73 0.48 99.55 47.89 4.22 13.55 14.55 0.20 5.03 9.60 2.70 0.80 0.44 98.98 48.61 4.21 13.54 14.64 0.24 4.69 9.45 2.76 0.87 0.48 99.49 49.29 4.08 13.63 14.18 0.22 4.59 9.35 2.83 0.91 0.42 99.50 47.50 4.03 13.60 14.77 0.20 5.56 11.00 2.84 0.61 0.36 100.48 46.98 3.98 13.59 14.72 0.22 5.44 10.72 2.68 0.58 0.40 99.31 47.06 3.85 13.83 14.22 0.19 5.76 10.91 2.60 0.58 0.40 99.40 47.27 3.79 13.49 14.55 0.21 5.61 10.88 2.98 0.59 0.36 99.73 47.30 3.70 13.76 13.97 0.19 5.82 10.92 2.75 0.56 0.38 99.35 47.47 3.67 14.01 13.67 0.21 5.61 10.24 2.78 0.62 0.35 98.63 47.80 3.62 13.80 14.66 0.24 5.11 9.71 2.74 0.73 0.56 98.96 Mean 47.65 4.12 13.44 14.74 0.22 5.15 10.06 2.72 0.73 0.48 99.31 stdv 0.60 0.40 0.51 0.71 0.03 0.48 0.66 0.13 0.15 0.16 0.44

46.64 2.37 16.21 12.62 0.21 7.40 10.90 2.62 0.40 0.22 99.60

Sample Tephra Volcanic Analysis name layer system SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 Total date HÖ-3 Hekla Ö Hekla 70.97 0.26 14.43 4.37 0.16 0.19 2.56 2.90 2.33 0.06 98.24 11.2.2017 68.20 0.43 15.10 6.21 0.22 0.36 3.30 2.87 2.08 0.14 98.90 67.07 0.46 15.03 6.69 0.22 0.47 3.23 3.30 2.02 0.14 98.62 66.76 0.47 15.23 6.91 0.20 0.48 3.48 2.58 2.03 0.13 98.27 66.33 0.47 14.85 6.94 0.23 0.47 3.52 3.12 1.94 0.14 98.02 Mean 67.87 0.42 14.93 6.22 0.21 0.39 3.22 2.95 2.08 0.12 98.41 stdv 1.87 0.09 0.31 1.08 0.03 0.13 0.39 0.27 0.15 0.03 0.35

70 47.11 3.97 13.54 14.95 0.23 5.53 11.13 2.88 0.56 0.43 100.33 47.08 3.77 13.48 14.64 0.22 5.56 10.90 2.92 0.58 0.36 99.51 46.81 3.76 13.45 14.43 0.20 5.83 10.98 2.85 0.51 0.33 99.15 47.51 3.67 13.99 13.98 0.18 5.44 10.70 2.91 0.61 0.37 99.36 47.52 3.65 13.70 13.81 0.22 5.53 10.47 2.99 0.64 0.42 98.95 47.38 3.63 13.81 13.80 0.21 5.60 10.64 3.09 0.62 0.43 99.22 47.62 3.57 13.73 13.79 0.20 5.63 10.69 3.00 0.64 0.44 99.31 46.89 4.53 12.34 15.78 0.25 5.09 10.43 2.95 0.63 0.39 99.28 46.53 4.47 12.41 16.05 0.23 5.12 10.19 2.79 0.63 0.43 98.84 Mean 47.16 3.89 13.38 14.58 0.22 5.48 10.68 2.93 0.60 0.40 99.33 stdv 0.37 0.36 0.60 0.86 0.02 0.24 0.29 0.09 0.04 0.04 0.43

46.32 2.75 15.89 13.70 0.23 6.87 10.66 2.60 0.42 0.28 99.73 46.67 2.74 15.76 13.63 0.19 7.16 10.75 2.53 0.41 0.30 100.14 46.67 2.73 15.80 13.62 0.19 6.96 10.62 2.62 0.41 0.24 99.87 46.57 2.71 15.70 13.45 0.23 6.75 10.78 2.46 0.40 0.23 99.28 46.24 2.71 15.77 13.65 0.19 7.11 10.69 2.63 0.39 0.27 99.65 46.57 2.66 15.78 13.68 0.24 6.80 10.59 2.43 0.42 0.28 99.45 46.57 2.65 15.85 13.59 0.19 6.95 10.29 2.74 0.41 0.26 99.50 46.48 2.64 15.81 13.63 0.21 7.12 10.71 2.67 0.43 0.25 99.95 46.45 2.58 15.85 13.03 0.19 7.32 10.64 2.48 0.40 0.26 99.19 46.46 2.55 15.71 13.55 0.22 7.17 10.71 2.66 0.42 0.23 99.68 46.40 2.49 15.90 13.72 0.20 7.13 10.79 2.52 0.41 0.23 99.79 46.44 2.44 15.92 13.53 0.19 7.03 10.86 2.59 0.40 0.23 99.63 Mean 46.49 2.64 15.81 13.57 0.21 7.03 10.67 2.58 0.41 0.26 99.66 stdv 0.13 0.10 0.07 0.18 0.02 0.17 0.14 0.09 0.01 0.02 0.27

71 Sample Tephra Volcanic Analysis name layer system SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 Total date HÖ-2 Hekla Ö Hekla 70.23 0.39 14.58 5.41 0.18 0.29 2.77 2.52 2.35 0.08 98.80 11.2.2017 68.36 0.38 15.05 5.83 0.19 0.36 3.13 2.95 2.07 0.10 98.43 68.19 0.40 14.82 5.87 0.18 0.33 3.21 2.89 2.16 0.11 98.16 68.04 0.44 15.01 6.32 0.18 0.38 3.29 3.02 2.08 0.10 98.87 67.49 0.47 15.10 6.28 0.21 0.43 3.34 2.97 2.06 0.11 98.45 Mean 68.46 0.42 14.91 5.94 0.19 0.36 3.15 2.87 2.14 0.10 98.54 stdv 1.04 0.04 0.21 0.37 0.01 0.05 0.23 0.20 0.12 0.01 0.29

46.71 3.86 13.13 14.78 0.21 5.52 10.72 2.77 0.58 0.40 98.67 46.88 3.84 13.29 14.57 0.19 5.46 10.74 2.92 0.59 0.36 98.84 47.39 3.65 13.34 13.76 0.24 5.62 10.58 2.81 0.61 0.41 98.41 47.13 3.79 13.35 14.77 0.22 5.54 10.66 2.84 0.61 0.42 99.34 46.93 3.86 13.38 14.48 0.23 5.62 10.85 2.83 0.59 0.40 99.17 46.52 3.78 13.59 14.53 0.23 5.36 10.89 2.93 0.57 0.40 98.80 47.74 3.53 13.65 13.84 0.22 5.50 10.37 3.00 0.68 0.44 98.97 47.55 3.72 13.69 14.02 0.21 5.51 10.46 3.06 0.66 0.40 99.28 47.28 3.36 13.73 13.87 0.23 5.64 10.84 2.85 0.61 0.35 98.75 47.58 3.56 13.74 13.97 0.23 5.41 10.67 2.96 0.63 0.42 99.17 47.63 3.74 13.82 14.05 0.21 5.77 10.78 2.94 0.62 0.45 100.01 47.82 3.63 13.85 13.90 0.21 5.62 10.50 2.88 0.65 0.42 99.48 47.21 3.72 13.87 14.01 0.25 5.80 10.90 2.88 0.61 0.34 99.59 47.58 3.70 13.88 14.09 0.21 5.58 10.51 2.82 0.64 0.35 99.37 47.43 3.63 13.95 13.94 0.21 5.56 10.54 2.82 0.66 0.42 99.16 Mean 47.29 3.69 13.62 14.17 0.22 5.57 10.67 2.89 0.62 0.40 99.13 stdv 0.39 0.14 0.26 0.35 0.01 0.12 0.17 0.08 0.03 0.03 0.41

72 47.14 2.83 14.91 13.63 0.20 6.07 10.35 2.82 0.53 0.30 98.78 47.18 2.87 15.23 13.75 0.25 6.04 10.33 2.70 0.50 0.27 99.12 46.53 2.65 15.59 13.42 0.23 6.76 10.82 2.71 0.46 0.27 99.44 46.40 2.68 15.67 13.64 0.18 7.14 10.72 2.62 0.40 0.29 99.73 46.24 2.71 15.77 13.58 0.21 7.04 9.94 2.64 0.41 0.28 98.82 46.01 2.62 15.79 13.20 0.21 7.30 10.86 2.66 0.40 0.28 99.33 46.48 2.70 15.82 13.48 0.23 6.79 10.51 2.66 0.42 0.27 99.36 46.49 2.55 15.86 13.00 0.19 7.00 10.92 2.57 0.40 0.19 99.17 46.41 2.70 15.95 13.46 0.18 6.93 10.77 2.56 0.41 0.26 99.63 Mean 46.54 2.70 15.62 13.46 0.21 6.79 10.58 2.66 0.44 0.27 99.27 stdv 0.38 0.10 0.34 0.23 0.02 0.45 0.32 0.08 0.05 0.03 0.33

Sample Tephra Volcanic Analysis name layer system SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 Total date HÖ-1 Hekla Ö Hekla 76.05 0.11 11.14 1.28 0.04 0.01 0.49 2.58 3.84 0.01 95.55 11.2.2017 75.21 0.19 13.15 2.68 0.13 0.06 1.33 2.86 3.04 0.06 98.70 70.32 0.62 14.13 4.38 0.16 0.26 2.45 2.91 2.59 0.27 98.10 66.65 0.46 15.08 6.50 0.25 0.43 3.40 3.31 2.00 0.09 98.17 62.57 0.74 15.20 8.51 0.26 0.90 4.38 3.01 1.68 0.26 97.51 60.08 1.77 12.87 11.45 0.28 1.66 5.13 3.18 1.93 0.81 99.16 Mean 68.48 0.65 13.60 5.80 0.19 0.55 2.86 2.98 2.51 0.25 97.86 stdv 6.55 0.60 1.54 3.79 0.09 0.63 1.78 0.26 0.82 0.30 1.27

47.13 3.58 13.82 13.64 0.24 5.88 10.93 2.89 0.56 0.37 99.04 46.87 3.71 13.70 13.75 0.21 5.69 10.83 3.04 0.60 0.38 98.78 46.13 3.64 13.39 13.75 0.21 5.68 10.63 3.03 0.64 0.45 97.55 47.36 3.66 13.70 13.95 0.23 5.67 10.72 2.91 0.59 0.39 99.18 47.57 3.59 13.67 13.74 0.21 5.63 10.57 2.94 0.62 0.39 98.93 46.80 3.94 13.17 14.62 0.25 5.60 10.52 2.97 0.58 0.41 98.87

73 46.78 3.84 13.39 14.30 0.22 5.60 10.75 3.04 0.59 0.43 98.93 47.10 3.67 13.67 13.84 0.27 5.57 10.51 2.97 0.64 0.42 98.65 46.05 3.81 13.34 13.56 0.22 5.54 10.95 2.93 0.59 0.44 97.42 46.79 4.03 13.00 15.36 0.24 5.50 10.50 2.76 0.60 0.45 99.24 47.06 4.07 13.35 15.16 0.23 5.26 10.14 3.07 0.66 0.51 99.51 Mean 46.88 3.78 13.47 14.15 0.23 5.60 10.64 2.96 0.61 0.42 98.74 stdv 0.46 0.17 0.26 0.63 0.02 0.15 0.23 0.09 0.03 0.04 0.66

46.20 2.69 15.50 13.58 0.21 7.10 10.66 2.61 0.40 0.29 99.24 46.46 2.64 15.74 12.96 0.18 7.33 10.92 2.66 0.39 0.23 99.51 46.34 2.64 15.52 13.53 0.20 7.18 10.67 2.75 0.42 0.29 99.55 46.26 2.61 15.99 13.33 0.22 7.33 10.67 2.59 0.40 0.23 99.62 46.28 2.56 15.59 13.60 0.20 7.24 10.72 2.68 0.41 0.29 99.58 46.29 2.52 15.78 13.08 0.21 7.28 10.72 2.53 0.39 0.29 99.09 Mean 46.31 2.61 15.69 13.35 0.20 7.24 10.73 2.64 0.40 0.27 99.43 St .dev 0.09 0.06 0.19 0.27 0.01 0.09 0.10 0.08 0.01 0.03 0.22

Tephra Volcanic Analysis Sample layer system SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 Total date H-Mó Hekla Mó Hekla 60.14 1.40 15.06 10.09 0.27 2.02 5.63 3.09 1.60 0.51 99.82 10.2.2017 59.86 1.46 15.05 10.33 0.27 2.21 5.75 3.18 1.44 0.54 100.09 58.55 1.54 14.64 11.10 0.29 2.22 5.66 2.92 1.45 0.76 99.14 57.32 1.94 14.71 12.11 0.29 2.99 6.44 2.69 1.31 0.66 100.46 56.84 2.19 13.47 12.19 0.31 2.44 5.96 3.04 1.57 0.76 98.77 55.62 2.18 15.27 11.91 0.30 3.19 7.35 2.88 1.44 0.63 100.77 55.22 2.30 14.12 12.58 0.25 3.20 6.61 3.11 1.21 0.69 99.29 Mean 57.65 1.86 14.62 11.47 0.28 2.61 6.20 2.99 1.43 0.65 99.76 stdv 1.94 0.38 0.63 0.97 0.02 0.50 0.64 0.17 0.14 0.10 0.73

74 47.22 4.11 13.05 15.41 0.25 4.90 9.80 3.04 0.69 0.45 98.92 48.10 4.17 13.52 15.38 0.23 5.09 10.22 3.04 0.72 0.47 100.94 48.01 4.08 13.26 15.33 0.25 4.79 10.00 2.99 0.72 0.48 99.91 47.00 4.07 13.33 15.12 0.26 5.07 10.14 3.03 0.68 0.43 99.13 47.95 4.15 13.51 15.05 0.24 5.08 9.73 2.96 0.69 0.48 99.84 47.75 4.07 13.13 15.03 0.25 4.99 9.77 3.07 0.71 0.42 99.19 48.32 4.02 13.53 15.02 0.21 5.49 10.93 2.68 0.77 0.37 101.34 46.99 3.93 13.19 14.79 0.20 5.49 10.74 2.96 0.60 0.43 99.32 47.19 4.13 12.82 14.44 0.27 5.04 10.03 3.00 0.69 0.51 98.12 47.96 3.79 14.03 14.36 0.21 5.33 10.85 2.99 0.59 0.39 100.50 47.50 3.78 13.48 14.31 0.21 5.81 10.87 2.83 0.60 0.38 99.77 47.95 3.77 13.69 14.28 0.23 5.63 10.81 2.83 0.59 0.40 100.17 47.78 3.62 13.83 14.25 0.21 5.57 11.02 2.80 0.58 0.35 100.00 47.90 3.80 13.58 14.24 0.20 5.58 11.04 2.88 0.56 0.44 100.22 47.60 3.68 13.64 14.23 0.22 5.68 11.01 3.03 0.55 0.36 100.00 47.05 3.81 13.51 14.22 0.25 5.67 10.68 2.91 0.57 0.38 99.04 47.28 3.71 13.84 14.14 0.20 5.78 10.81 2.86 0.57 0.41 99.59 47.66 3.63 13.71 14.12 0.22 5.76 10.86 2.89 0.53 0.35 99.73 Mean 47.62 3.91 13.48 14.65 0.23 5.38 10.52 2.93 0.63 0.42 99.76 stdv 0.41 0.19 0.31 0.48 0.02 0.34 0.48 0.10 0.07 0.05 0.76

46.95 2.70 16.03 13.12 0.20 7.00 10.83 2.68 0.38 0.24 100.14 46.70 2.68 15.94 13.50 0.21 7.06 10.71 2.70 0.43 0.23 100.16 46.50 2.59 15.86 13.38 0.20 7.13 10.81 2.59 0.41 0.25 99.72 Mean 46.72 2.66 15.94 13.33 0.21 7.06 10.78 2.66 0.41 0.24 ##### stdv 0.23 0.06 0.09 0.19 0.01 0.07 0.06 0.06 0.02 0.01 0.25

75 Sample Tephra Volcanic Analysis name layer system SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 Total date Tagl 7 Vatnafjöll 52.55 3.41 13.54 12.84 0.22 4.01 7.94 3.22 1.09 0.54 99.35 10.2.2017 51.62 3.53 13.61 13.11 0.22 4.14 8.49 3.22 1.14 0.47 99.55 Mean 52.55 3.41 13.54 12.84 0.22 4.01 7.94 3.22 1.09 0.54 99.35 stdv 0.66 0.08 0.05 0.19 0.00 0.09 0.39 0.00 0.03 0.04 0.14

46.93 3.98 13.98 16.07 0.27 5.47 9.42 2.83 0.61 0.45 100.01 47.83 3.94 14.08 15.91 0.25 5.41 9.23 2.76 0.61 0.47 100.49 47.32 3.85 14.50 15.62 0.24 5.74 9.23 2.66 0.57 0.44 100.16 47.40 3.78 14.38 15.57 0.23 5.44 9.33 2.96 0.57 0.42 100.08 47.42 3.76 14.53 15.77 0.25 5.68 9.35 2.91 0.58 0.47 100.72 47.17 3.75 14.56 15.76 0.24 5.59 9.27 2.76 0.59 0.39 100.08 47.16 3.75 14.54 15.73 0.25 5.73 9.40 2.92 0.57 0.42 100.47 47.40 3.74 14.48 15.94 0.23 5.68 9.33 2.85 0.57 0.44 100.66 47.26 3.73 14.50 15.81 0.23 5.64 9.45 2.77 0.60 0.44 100.42 47.06 3.73 14.59 15.70 0.25 5.62 9.45 2.76 0.57 0.44 100.18 46.92 3.70 14.52 15.70 0.30 5.68 9.31 2.85 0.56 0.39 99.94 47.08 3.66 14.57 15.68 0.25 5.68 9.16 2.84 0.55 0.41 99.88 Mean 47.25 3.78 14.44 15.77 0.25 5.61 9.33 2.82 0.58 0.43 ##### stdv 0.25 0.10 0.20 0.14 0.02 0.11 0.09 0.08 0.02 0.02 0.28

46.66 2.53 15.86 13.19 0.22 7.31 10.73 2.63 0.40 0.24 99.77

Sample Tephra Volcanic An alysis name layer system SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 Total date Tagl 6 Katla 48.59 4.25 13.17 14.97 0.24 4.59 9.48 3.02 0.88 0.49 99.68 10.2.2017 49.17 4.17 13.43 14.30 0.23 4.34 9.14 3.03 0.91 0.48 99.20 49.46 4.15 13.40 14.20 0.23 4.66 9.34 3.22 0.89 0.56 100.11

76 48.00 4.15 13.24 14.72 0.24 4.89 9.91 3.00 0.77 0.44 99.36 49.77 4.08 13.41 14.65 0.23 4.49 9.08 3.12 0.93 0.53 100.28 49.44 4.08 13.39 14.14 0.23 4.46 8.74 3.22 0.97 0.55 99.21 49.31 4.08 13.38 14.58 0.25 4.73 9.33 3.05 0.88 0.53 100.12 49.24 4.07 13.10 14.57 0.19 4.68 9.39 3.11 0.90 0.56 99.82 49.63 4.02 13.41 14.39 0.27 4.57 9.21 2.63 0.91 0.60 99.64 49.21 4.02 13.45 14.62 0.23 4.62 9.17 3.10 0.90 0.54 99.86 48.78 3.95 13.60 14.16 0.24 4.69 9.01 3.17 0.91 0.48 99.00 49.33 3.91 13.51 14.14 0.21 4.36 9.10 3.10 0.88 0.59 99.13 49.08 3.91 13.36 14.43 0.23 4.62 9.16 3.15 0.87 0.50 99.31 49.05 3.89 13.59 14.10 0.21 4.59 9.29 2.90 0.88 0.50 99.00 50.70 3.83 13.53 13.47 0.23 4.22 8.62 3.12 1.02 0.74 99.48 Mean 49.25 4.04 13.40 14.36 0.23 4.57 9.20 3.06 0.90 0.54 99.55 stdv 0.59 0.12 0.14 0.36 0.02 0.17 0.30 0.15 0.05 0.07 0.42

Sample Tephra Volcanic Analysis name layer system SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 Total date Tagl 5 Katla 49.18 4.55 12.25 15.66 0.27 4.56 9.40 2.84 0.98 0.68 100.37 10.2.2017 48.66 4.46 12.59 15.10 0.24 4.59 9.32 2.89 0.89 0.59 99.33 48.84 4.28 13.18 14.54 0.19 4.86 9.30 2.86 0.88 0.44 99.37 48.53 4.28 12.78 15.30 0.24 4.52 9.41 3.03 0.90 0.54 99.54 48.46 4.27 12.76 14.91 0.23 4.83 9.53 3.06 0.84 0.50 99.39 48.81 4.22 13.25 14.58 0.21 4.70 9.42 2.92 0.85 0.50 99.46 49.05 4.20 13.31 14.71 0.21 4.72 9.32 3.21 0.88 0.51 100.11 48.89 4.17 13.34 14.59 0.25 4.77 9.50 3.08 0.83 0.51 99.92 48.73 4.14 13.37 14.64 0.24 4.85 9.41 3.15 0.83 0.48 99.84 48.83 4.10 13.39 14.58 0.21 4.77 9.33 3.00 0.83 0.49 99.53 49.06 4.09 13.15 15.12 0.24 4.73 9.44 3.13 0.86 0.55 100.36 48.98 4.08 13.36 14.50 0.22 4.76 9.37 3.00 0.84 0.53 99.64

77 49.08 4.05 13.49 14.67 0.23 4.83 9.41 2.97 0.84 0.52 100.09 48.77 4.02 13.29 14.72 0.22 4.67 9.36 2.99 0.88 0.55 99.47 48.57 3.99 13.27 14.63 0.23 4.76 9.48 3.12 0.89 0.58 99.52 Mean 48.83 4.19 13.12 14.82 0.23 4.73 9.40 3.02 0.87 0.53 99.73 stdv 0.22 0.16 0.36 0.34 0.02 0.10 0.07 0.11 0.04 0.06 0.36

Sample Tephra Volcanic Analysis name layer system SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 Total date Tagl 4 Katla 71.76 0.32 13.52 4.14 0.16 0.09 1.13 3.96 4.07 0.04 99.18 10.2.2017

47.39 4.74 12.21 16.29 0.24 4.56 9.30 2.88 0.89 0.67 99.16 47.27 4.34 12.98 16.20 0.24 4.70 9.33 2.88 0.80 0.53 99.27 47.70 4.44 12.49 15.92 0.23 4.47 9.18 3.02 0.98 0.63 99.06 48.56 4.23 12.94 15.43 0.23 4.71 9.69 2.95 0.80 0.53 100.06 47.98 4.24 12.95 15.25 0.23 4.83 9.41 2.96 0.79 0.49 99.13 48.20 4.35 12.75 15.19 0.19 4.65 9.54 3.03 0.84 0.51 99.25 48.14 4.19 13.40 15.09 0.22 4.93 9.69 3.08 0.79 0.44 99.97 48.62 4.17 13.13 15.05 0.23 4.76 9.68 3.00 0.84 0.53 100.01 48.49 4.15 12.91 15.03 0.24 4.75 9.65 2.92 0.84 0.47 99.45 48.41 4.18 13.22 14.98 0.24 4.68 9.70 3.08 0.84 0.46 99.79 48.51 4.22 13.06 14.86 0.23 4.91 9.83 2.98 0.77 0.51 99.88 49.09 4.15 13.45 14.86 0.24 4.73 9.58 3.03 0.80 0.56 100.49 48.85 4.01 13.31 14.82 0.21 4.81 9.57 2.94 0.84 0.48 99.84 48.14 4.29 13.60 14.22 0.25 5.02 9.30 3.11 0.82 0.58 99.33 Mean 48.24 4.26 13.03 15.23 0.23 4.75 9.53 2.99 0.83 0.53 99.62 stdv 0.52 0.17 0.38 0.57 0.01 0.15 0.19 0.07 0.05 0.06 0.44

78 Sample Tephra Volcanic Analysis name layer system SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 Total date HDH-8 Hekla DH Hekla 55.99 2.74 12.60 12.63 0.41 3.39 8.02 2.09 1.24 1.32 100.43 10.2.2017 55.83 2.83 12.63 13.97 0.37 3.05 6.77 2.95 1.47 1.41 101.28 55.76 2.87 12.68 14.19 0.32 2.98 6.88 2.59 1.41 1.33 101.01 55.63 2.72 12.49 14.21 0.32 2.89 6.65 2.44 1.55 1.27 100.17 55.44 2.78 12.52 14.21 0.33 3.09 6.76 2.57 1.45 1.25 100.40 55.43 2.91 12.60 13.58 0.43 3.46 7.78 1.99 1.31 1.33 100.82 55.41 3.06 11.78 13.90 0.30 3.52 7.28 2.55 1.52 1.45 100.77 55.40 3.03 12.51 14.22 0.35 3.05 6.76 2.53 1.43 1.30 100.59 55.25 2.71 12.72 13.89 0.32 2.91 6.63 2.59 1.60 1.32 99.94 55.15 2.73 12.49 14.19 0.32 2.95 6.41 2.09 1.58 1.37 99.28 55.14 2.93 12.46 14.27 0.34 3.10 6.76 2.37 1.63 1.40 100.40 55.07 2.78 12.36 13.80 0.37 3.20 6.91 2.34 1.52 1.34 99.70 55.06 2.76 12.24 13.48 0.34 3.00 6.67 2.75 1.50 1.33 99.13 55.05 2.88 12.66 14.11 0.35 3.00 6.74 2.56 1.46 1.30 100.12 54.95 2.99 12.55 14.54 0.36 3.22 6.98 2.87 1.42 1.35 101.24 54.94 2.87 12.55 14.10 0.31 3.09 6.75 2.73 1.39 1.30 100.03 54.61 2.99 12.23 14.50 0.34 3.20 6.92 2.42 1.41 1.34 99.96 54.48 3.08 12.47 14.67 0.32 3.21 7.09 2.42 1.46 1.40 100.60 54.34 3.04 12.56 14.79 0.33 3.28 7.06 2.64 1.38 1.36 100.79 54.10 3.29 12.39 15.07 0.30 3.17 7.17 2.25 1.45 1.45 100.65 54.00 3.19 12.49 14.59 0.32 3.32 7.12 2.17 1.43 1.25 99.88 53.52 3.26 11.97 14.97 0.35 2.76 6.98 2.47 1.41 1.51 99.20 53.45 2.97 12.55 14.42 0.34 3.26 7.14 2.39 1.41 1.25 99.18 Mean 54.96 2.93 12.46 14.19 0.34 3.13 6.97 2.47 1.45 1.35 100.24 stdv 0.70 0.17 0.22 0.53 0.03 0.19 0.36 0.24 0.09 0.07 0.64

79 55.26 2.38 13.76 13.28 0.32 3.12 6.82 2.60 1.23 1.13 99.90 54.71 2.09 14.58 12.64 0.28 3.28 6.98 2.96 1.19 1.15 99.86 55.25 1.91 14.86 11.49 0.26 3.13 6.90 2.94 1.19 1.11 99.04 Mean 55.07 2.13 14.40 12.47 0.29 3.18 6.90 2.83 1.20 1.13 99.60 stdv 0.31 0.24 0.57 0.91 0.03 0.09 0.08 0.20 0.02 0.02 0.48

47.68 3.52 13.45 15.91 0.26 5.65 11.32 1.78 0.64 0.36 100.57 47.59 4.08 13.55 15.05 0.25 5.59 10.67 2.12 0.70 0.42 100.01 47.57 3.01 15.30 14.46 0.21 6.61 11.12 1.94 0.51 0.24 100.97 47.12 3.82 13.32 14.64 0.20 5.57 10.89 2.89 0.62 0.43 99.50 Mean 47.49 3.61 13.91 15.02 0.23 5.86 11.00 2.18 0.62 0.36 100.26 stdv 0.25 0.46 0.93 0.65 0.03 0.50 0.28 0.49 0.08 0.09 0.64

Sample Tephra Volcanic Analysis name layer system SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 Total date HDH-7 Hekla DH Hekla 54.69 2.95 12.35 13.68 0.30 3.10 7.14 2.66 1.26 1.33 99.46 10.2.2017

53.72 2.47 14.84 12.42 0.26 3.68 7.27 3.37 1.02 0.87 99.92 54.67 2.30 14.22 12.67 0.29 3.33 6.94 2.96 1.15 1.21 99.73 54.28 2.28 14.13 12.85 0.32 3.40 6.94 3.06 1.20 1.27 99.73 54.68 2.26 14.54 12.11 0.30 3.26 7.14 2.56 1.13 1.15 99.13 54.80 2.23 14.39 12.06 0.28 3.28 6.97 3.18 1.11 1.03 99.33 54.15 2.18 14.48 12.05 0.29 3.34 7.07 3.32 1.09 1.09 99.06 54.72 2.06 14.89 12.03 0.29 3.34 7.10 3.15 1.11 1.02 99.70 Mean 54.43 2.25 14.50 12.31 0.29 3.38 7.06 3.09 1.11 1.09 99.51 stdv 0.40 0.12 0.29 0.34 0.02 0.14 0.12 0.27 0.05 0.13 0.34

47.57 4.00 13.46 14.66 0.26 5.09 9.94 3.01 0.70 0.46 99.15 47.23 3.85 13.71 14.18 0.22 5.78 10.88 2.84 0.59 0.33 99.61

80 47.54 3.82 13.41 14.21 0.24 5.80 11.03 2.95 0.57 0.39 99.96 47.68 3.79 13.66 14.18 0.23 5.62 11.05 2.92 0.57 0.41 100.10 48.54 3.78 13.83 14.53 0.24 5.88 11.18 2.51 0.58 0.37 101.44 47.32 3.75 13.45 14.82 0.25 5.34 10.43 3.00 0.61 0.39 99.36 47.38 3.75 13.36 14.05 0.23 5.59 10.93 2.84 0.59 0.43 99.15 47.65 3.74 13.80 14.25 0.23 5.71 10.80 2.86 0.56 0.39 100.00 47.11 3.73 13.77 13.99 0.22 5.68 10.88 2.75 0.54 0.36 99.02 47.62 3.70 13.75 14.19 0.21 5.71 10.67 2.92 0.56 0.41 99.73 47.48 3.69 13.66 14.24 0.23 5.85 11.07 2.84 0.57 0.47 100.10 47.37 3.68 13.83 14.02 0.24 5.75 10.94 2.91 0.56 0.39 99.69 48.34 3.66 13.99 14.34 0.22 5.78 11.00 2.41 0.57 0.35 100.66 47.53 3.64 13.78 14.06 0.21 5.60 11.01 2.70 0.55 0.40 99.49 47.34 3.64 13.71 13.93 0.24 5.86 11.05 2.84 0.55 0.33 99.49 47.67 3.62 13.96 13.85 0.22 5.48 10.97 2.63 0.58 0.37 99.34 Mean 47.59 3.74 13.70 14.22 0.23 5.66 10.86 2.81 0.58 0.39 99.77 stdv 0.37 0.10 0.19 0.26 0.01 0.21 0.30 0.17 0.04 0.04 0.62

46.97 3.00 14.76 14.20 0.21 6.25 10.79 2.74 0.50 0.29 99.72 46.82 2.96 15.14 13.93 0.23 6.53 10.89 2.67 0.49 0.33 99.99 47.02 2.89 15.02 14.10 0.20 6.38 10.89 2.82 0.50 0.29 100.12 47.35 2.85 15.32 13.79 0.22 6.41 10.83 2.70 0.48 0.29 100.25 46.71 2.77 14.95 13.76 0.19 6.39 10.99 2.80 0.48 0.29 99.33 Mean 46.97 2.89 15.04 13.96 0.21 6.39 10.88 2.75 0.49 0.30 99.88 stdv 0.24 0.09 0.21 0.19 0.02 0.10 0.08 0.06 0.01 0.02 0.37

81 Tephra Volcanic Analysis Sample layer system SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 Total date HDH-6 Hekla DH Hekla 53.61 2.98 12.78 13.61 0.29 3.54 7.30 3.26 1.10 1.41 99.89 10.2.2017 54.24 2.97 12.39 13.57 0.30 3.19 6.89 3.04 1.26 1.46 99.31 Mean 53.93 2.98 12.59 13.59 0.29 3.37 7.10 3.15 1.18 1.44 99.60 stdv 0.45 0.01 0.28 0.03 0.00 0.25 0.29 0.16 0.11 0.04 0.41

52.33 2.61 14.71 12.78 0.26 3.96 7.84 3.33 0.94 0.99 99.76 52.96 2.52 14.71 12.70 0.27 3.81 7.61 3.21 0.98 0.99 99.75 52.96 2.45 14.47 12.41 0.33 3.90 7.65 3.30 0.98 1.00 99.46 53.59 2.43 14.42 12.54 0.32 3.69 7.24 3.25 1.07 1.14 99.69 54.73 2.21 14.76 12.08 0.26 3.20 6.88 3.27 1.13 1.17 99.69 54.45 2.19 14.65 11.31 0.29 3.36 7.16 3.06 1.10 1.20 98.78 53.65 2.07 14.33 12.04 0.29 3.45 7.25 3.21 1.07 1.06 98.42 54.84 2.01 14.74 11.83 0.31 3.02 6.57 3.04 1.15 1.08 98.60 Mean 53.69 2.31 14.60 12.21 0.29 3.55 7.28 3.21 1.05 1.08 99.27 stdv 0.92 0.22 0.17 0.49 0.03 0.34 0.42 0.11 0.08 0.08 0.57

48.31 4.16 13.47 15.28 0.24 5.45 11.05 2.22 0.61 0.45 101.24 47.56 3.94 13.24 14.55 0.21 5.64 11.20 2.77 0.54 0.37 100.02 47.52 3.75 13.50 14.29 0.24 5.77 10.95 2.82 0.60 0.40 99.85 47.48 3.74 13.63 13.93 0.21 5.77 10.66 2.91 0.59 0.39 99.31 47.58 3.73 14.01 13.85 0.23 5.84 10.89 2.94 0.54 0.39 100.00 47.69 3.70 13.59 14.17 0.22 5.67 10.94 2.90 0.57 0.39 99.84 47.02 3.69 13.64 14.33 0.21 5.74 10.85 2.87 0.58 0.36 99.28 47.51 3.65 13.53 13.93 0.22 5.66 10.97 3.06 0.55 0.42 99.49 47.85 3.64 13.65 14.00 0.22 5.58 10.70 2.85 0.58 0.39 99.46 46.79 3.64 13.43 13.86 0.24 5.76 10.81 2.92 0.58 0.38 98.41 47.42 3.62 13.86 14.14 0.22 5.75 11.00 2.59 0.54 0.36 99.49

82 47.38 3.61 13.74 13.82 0.19 5.80 10.91 2.84 0.55 0.35 99.19 47.36 3.61 13.86 13.65 0.19 5.95 10.79 2.87 0.55 0.36 99.19 47.96 3.59 13.95 14.25 0.21 5.86 11.09 2.91 0.55 0.37 100.74 47.38 3.58 13.76 13.90 0.21 5.88 10.79 2.92 0.57 0.41 99.39 46.75 3.35 14.23 15.24 0.23 5.95 10.17 2.70 0.50 0.39 99.51 Mean 47.47 3.69 13.69 14.20 0.22 5.75 10.86 2.82 0.56 0.39 99.65 stdv 0.40 0.17 0.25 0.47 0.02 0.13 0.23 0.19 0.03 0.03 0.65

46.93 3. 08 14.73 13.62 0.25 6.69 10.92 2.50 0.43 0.32 99.47 47.19 2.87 14.97 13.77 0.23 6.51 10.93 2.68 0.49 0.29 99.93 47.07 2.87 15.11 14.20 0.18 6.34 10.96 2.68 0.49 0.29 100.20 46.88 2.73 14.90 13.65 0.23 6.39 10.94 2.67 0.49 0.31 99.19 Mean 47.02 2.89 14.93 13.81 0.22 6.48 10.94 2.63 0.48 0.30 99.70 stdv 0.14 0.14 0.16 0.27 0.03 0.16 0.02 0.09 0.03 0.02 0.45

Tephra Volcanic Analysis Sample layer system SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 Total date HDH-5 Hekla DH Hekla 55.92 1.84 14.91 11.72 0.30 2.74 6.57 3.10 1.26 1.00 99.36 10.2.2017 52.35 2.63 14.64 12.90 0.27 4.14 7.70 3.40 0.95 0.88 99.87 51.68 2.69 14.71 12.93 0.26 4.06 7.85 3.22 0.90 0.94 99.25 53.15 2.28 14.47 12.08 0.28 3.78 7.33 3.24 1.01 1.10 98.71 54.39 2.35 14.07 12.88 0.32 3.74 7.07 2.89 1.14 1.22 100.06 53.01 2.52 14.25 12.64 0.29 3.70 7.38 3.31 1.02 1.12 99.23 52.67 2.47 14.67 12.52 0.28 3.69 7.59 3.29 1.01 0.96 99.16 54.36 2.36 13.43 12.93 0.30 3.58 6.90 2.95 1.20 1.27 99.27 54.42 2.31 14.15 12.75 0.30 3.43 6.91 3.02 1.09 1.30 99.68 Mean 53.55 2.38 14.37 12.59 0.29 3.65 7.26 3.16 1.06 1.09 99.40 stdv 1.32 0.25 0.45 0.43 0.02 0.41 0.42 0.18 0.12 0.15 0.41

83 50.25 2.83 13.39 14.11 0.26 5.63 9.98 2.47 0.41 0.28 99.61

46.59 3.65 12.90 15.92 0.24 5.30 11.05 2.81 0.61 0.33 99.40 47.46 3.69 13.89 15.74 0.24 5.28 9.04 3.04 0.67 0.51 99.57 47.13 3.92 13.34 14.50 0.22 5.43 10.55 3.03 0.63 0.42 99.18 47.54 3.86 13.56 14.21 0.22 5.61 10.82 3.04 0.60 0.39 99.85 47.55 3.62 13.65 14.05 0.24 5.85 11.02 2.76 0.53 0.39 99.66 47.61 3.71 13.80 14.00 0.21 5.77 11.05 2.69 0.55 0.38 99.77 47.21 3.66 13.71 13.87 0.21 5.72 10.74 2.90 0.57 0.37 98.95 47.58 3.59 13.55 13.79 0.20 5.74 10.58 2.93 0.60 0.46 99.03 47.65 3.53 13.87 13.77 0.23 5.95 10.87 2.84 0.57 0.37 99.65 47.14 3.68 13.93 13.75 0.22 5.82 11.07 2.87 0.51 0.41 99.40 47.17 3.65 13.78 13.75 0.21 5.65 10.88 2.65 0.58 0.35 98.67 46.72 3.17 14.78 14.73 0.22 6.15 10.10 2.76 0.50 0.36 99.49 Mean 47.28 3.64 13.73 14.34 0.22 5.69 10.65 2.86 0.58 0.39 99.38 stdv 0.35 0.18 0.44 0.76 0.01 0.26 0.58 0.13 0.05 0.05 0.36

47.08 2.92 14.77 14.00 0.21 6.29 10.94 2.76 0.48 0.30 99.75 46.52 2.87 14.91 14.17 0.19 6.42 10.86 2.71 0.50 0.30 99.46 46.82 2.86 14.73 13.31 0.24 6.36 10.79 2.74 0.49 0.30 98.64 46.54 2.85 15.50 13.21 0.19 6.80 10.98 2.59 0.42 0.27 99.35 46.92 2.83 15.02 13.98 0.21 6.41 10.83 2.76 0.49 0.28 99.73 46.74 2.81 14.84 14.15 0.26 6.41 10.64 2.90 0.50 0.30 99.55 46.48 2.70 15.71 13.72 0.22 6.92 10.65 2.73 0.43 0.36 99.92 47.00 2.68 15.84 13.42 0.20 7.18 10.82 2.71 0.40 0.28 100.53 Mean 46.76 2.82 15.17 13.75 0.22 6.60 10.81 2.74 0.47 0.30 99.62 stdv 0.23 0.08 0.45 0.39 0.02 0.32 0.12 0.09 0.04 0.03 0.54

84 Tephra Volcanic Analysis Sample layer system SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 Total date HDH-4 Hekla DH Hekla 55.79 1.69 14.90 11.17 0.29 2.53 6.15 3.14 1.28 0.89 97.83 9.2.2017 52.19 2.65 13.97 13.21 0.27 3.96 7.42 3.13 1.05 0.95 98.80 52.05 2.61 14.30 13.11 0.27 3.88 7.49 3.13 1.01 0.93 98.78 51.85 2.52 14.22 12.95 0.27 3.70 7.54 3.31 0.97 0.90 98.23 52.32 2.50 14.30 13.02 0.27 3.88 7.47 3.26 1.01 0.92 98.96 51.72 2.50 14.18 13.32 0.29 4.02 7.53 3.30 0.96 1.00 98.82 52.03 2.42 14.25 12.71 0.26 3.80 7.51 3.28 0.97 0.88 98.11 52.81 2.37 14.41 12.43 0.31 3.64 7.39 3.36 1.05 1.02 98.79 53.96 2.32 13.98 12.95 0.29 3.40 6.82 3.04 1.13 1.22 99.11 53.63 2.15 14.35 12.22 0.30 3.30 6.94 3.25 1.08 1.15 98.37 55.08 2.12 14.84 12.27 0.31 3.16 6.84 2.94 1.10 1.07 99.73 55.70 1.84 14.95 11.84 0.32 2.78 6.31 3.22 1.14 1.01 99.10 55.01 1.81 15.04 11.87 0.31 2.87 6.47 3.04 1.31 0.98 98.71 Mean 53.40 2.27 14.44 12.54 0.29 3.46 7.07 3.18 1.08 0.99 98.72 stdv 1.55 0.32 0.37 0.64 0.02 0.49 0.51 0.13 0.11 0.10 0.49

47.18 4.45 12.42 17.09 0.26 4.14 9.08 2.88 0.76 0.47 98.73 47.02 3.91 13.40 14.68 0.23 5.64 10.77 2.91 0.63 0.39 99.58 47.08 3.85 13.47 14.18 0.21 5.58 10.79 2.91 0.59 0.41 99.07 46.89 3.82 13.37 14.22 0.22 5.59 10.77 2.84 0.59 0.38 98.69 47.14 3.80 13.47 13.96 0.20 5.46 10.67 2.88 0.58 0.35 98.51 48.13 3.74 14.11 15.21 0.25 5.12 8.94 3.09 0.68 0.46 99.73 47.18 3.60 13.84 13.84 0.21 5.95 10.73 3.02 0.57 0.38 99.33 46.83 3.44 13.28 15.44 0.26 5.65 11.19 2.88 0.56 0.28 99.81 46.55 3.35 14.12 15.68 0.23 5.74 10.24 2.81 0.52 0.32 99.57 Mean 47.11 3.77 13.50 14.92 0.23 5.43 10.35 2.91 0.61 0.38 99.22 stdv 0.43 0.32 0.51 1.05 0.02 0.53 0.80 0.09 0.07 0.06 0.49

85 46.68 3.20 14.91 14.74 0.25 5.99 10.32 2.78 0.51 0.33 99.71 46.46 2.99 14.98 14.03 0.22 6.44 11.06 2.80 0.50 0.27 99.75 46.42 2.77 14.99 13.81 0.22 6.70 10.75 2.77 0.43 0.29 99.15 46.42 2.74 14.84 13.04 0.21 6.71 11.19 2.36 0.42 0.27 98.20 46.37 2.67 15.64 13.50 0.20 7.18 10.50 2.59 0.39 0.25 99.29 45.95 2.61 15.97 12.77 0.18 7.33 10.73 2.42 0.39 0.26 98.61 Mean 46.38 2.83 15.22 13.65 0.21 6.73 10.76 2.62 0.44 0.28 99.12 stdv 0.24 0.22 0.47 0.71 0.02 0.49 0.33 0.19 0.05 0.03 0.61

Tephra Volcanic Analysis Sample layer system SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 Total date HDH-3 Hekla DH Hekla 53.49 3.06 12.85 13.46 0.30 3.67 7.64 2.64 1.01 1.13 99.25 9.2.2017 52.85 3.05 12.63 13.21 0.29 3.38 7.22 3.09 1.08 1.17 97.97 Mean 53.17 3.06 12.74 13.34 0.29 3.53 7.43 2.87 1.05 1.15 98.61 stdv 0.45 0.01 0.16 0.18 0.01 0.21 0.30 0.32 0.05 0.03 0.90

52.47 2.43 14.20 12.74 0.25 3.74 7.35 3.24 1.00 1.01 98.44 53.61 2.42 14.35 12.61 0.31 3.48 7.06 3.13 1.10 1.18 99.25 53.66 2.13 14.75 12.14 0.29 3.24 7.04 3.33 1.03 1.11 98.72 54.45 2.03 14.94 11.98 0.29 2.99 6.58 3.25 1.11 1.00 98.62 55.84 2.01 14.10 12.37 0.32 2.90 6.43 3.21 1.32 1.05 99.55 56.44 1.96 14.36 12.33 0.31 2.78 6.42 2.86 1.29 1.00 99.75 55.61 1.95 14.64 12.19 0.32 2.97 6.38 3.02 1.21 0.98 99.27 55.17 1.87 15.00 11.69 0.27 2.84 6.52 3.39 1.23 1.01 98.99 Mean 54.66 2.10 14.54 12.26 0.30 3.12 6.72 3.18 1.16 1.04 99.07 stdv 1.35 0.21 0.34 0.34 0.02 0.34 0.37 0.17 0.12 0.07 0.46

86 46.62 3.44 14.35 15.42 0.27 5.83 10.04 2.83 0.52 0.30 99.61 47.38 4.06 13.38 15.15 0.26 5.18 9.89 3.23 0.69 0.37 99.59 47.18 4.01 13.05 14.79 0.20 5.46 10.70 2.85 0.57 0.36 99.17 47.23 3.78 13.85 15.81 0.24 5.31 9.15 3.14 0.70 0.47 99.68 47.45 3.72 13.77 14.00 0.21 5.83 10.70 2.89 0.57 0.31 99.46 46.74 3.72 13.70 14.38 0.23 5.54 10.91 2.98 0.59 0.36 99.15 45.69 3.65 12.85 15.78 0.25 5.53 10.89 2.92 0.63 0.35 98.54 47.48 3.61 13.66 13.80 0.20 5.68 10.81 2.93 0.58 0.38 99.13 47.76 3.60 14.05 14.43 0.23 6.10 11.32 2.31 0.62 0.33 100.75 45.13 3.53 13.31 13.64 0.25 6.20 10.37 3.31 0.57 0.34 96.64 47.38 3.52 13.87 13.82 0.23 5.87 10.98 2.89 0.56 0.34 99.47 Mean 46.91 3.69 13.62 14.64 0.23 5.68 10.52 2.93 0.60 0.36 99.20 stdv 0.82 0.20 0.44 0.80 0.02 0.32 0.62 0.26 0.06 0.05 1.00

46.67 3.02 14.68 14.49 0.19 6.26 11.01 2.78 0.51 0.26 99.88 46.44 2.93 14.95 14.00 0.21 6.46 10.84 2.78 0.49 0.25 99.35 46.78 2.83 15.15 14.06 0.22 6.48 10.93 2.66 0.49 0.29 99.89 46.76 2.74 15.51 13.76 0.22 6.89 10.72 2.77 0.41 0.29 100.07 46.81 2.73 15.59 13.65 0.14 6.78 10.77 2.63 0.41 0.29 99.80 46.49 2.73 14.89 14.05 0.22 6.61 10.87 2.67 0.50 0.29 99.32 46.07 2.67 15.57 13.78 0.20 6.78 10.67 2.46 0.43 0.26 98.89 46.11 2.66 15.59 13.51 0.21 7.26 10.70 2.72 0.40 0.30 99.46 46.94 2.44 16.18 12.59 0.18 6.95 10.85 2.59 0.41 0.22 99.35 Mean 46.56 2.75 15.35 13.77 0.20 6.72 10.82 2.67 0.45 0.27 99.56 stdv 0.31 0.17 0.47 0.53 0.03 0.30 0.11 0.11 0.05 0.03 0.38

87 Tephra Volcanic Analysis Sample layer system SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 Total date HDH-2 Hekla DH Hekla 52.92 2.45 13.64 13.38 0.31 3.71 6.93 3.03 1.15 1.19 98.71 9.2.2017 53.41 2.36 13.97 13.02 0.32 3.43 6.86 3.20 1.19 1.17 98.93 53.90 2.32 13.77 13.13 0.33 3.65 6.80 2.84 1.15 1.18 99.07 55.42 2.27 14.15 11.51 0.25 3.21 6.75 3.18 1.14 0.54 98.42 54.58 2.23 14.31 12.39 0.28 3.16 6.72 2.87 1.23 1.24 99.01 53.46 2.21 14.49 12.43 0.32 3.34 6.97 3.32 1.10 1.18 98.81 55.42 2.00 14.52 12.29 0.32 2.99 6.48 3.05 1.32 1.00 99.39 54.77 1.88 14.93 12.10 0.30 2.87 6.56 3.11 1.25 0.96 98.73 Mean 54.24 2.22 14.22 12.53 0.30 3.30 6.76 3.08 1.19 1.06 98.88 stdv 0.95 0.19 0.43 0.61 0.03 0.30 0.17 0.16 0.07 0.23 0.29

47.68 4.12 13.48 15.30 0.25 4.99 10.15 3.02 0.68 0.42 100.08 47.05 3.99 13.23 14.69 0.22 5.38 10.45 3.00 0.60 0.46 99.07 46.78 3.94 13.24 14.44 0.20 5.41 10.69 3.05 0.60 0.41 98.76 47.11 3.92 13.15 14.73 0.22 5.54 10.79 2.91 0.56 0.36 99.29 46.96 3.79 13.57 14.48 0.24 5.64 10.64 2.95 0.61 0.38 99.26 47.35 3.75 13.70 14.31 0.22 5.69 10.92 3.06 0.60 0.41 100.01 46.99 3.69 13.60 14.33 0.21 5.62 10.91 2.80 0.59 0.39 99.13 47.42 3.64 13.72 13.93 0.21 5.68 10.70 2.82 0.60 0.40 99.12 47.68 3.58 14.00 14.20 0.21 5.72 10.69 2.76 0.56 0.38 99.78 47.34 3.58 13.78 13.64 0.22 5.85 11.06 2.96 0.57 0.37 99.37 46.64 3.45 13.59 15.17 0.25 5.67 11.16 2.85 0.57 0.31 99.66 47.19 3.40 14.03 13.87 0.23 5.90 10.96 2.91 0.54 0.33 99.36 Mean 47.18 3.74 13.59 14.42 0.22 5.59 10.76 2.92 0.59 0.38 99.41 stdv 0.33 0.22 0.28 0.50 0.02 0.24 0.28 0.10 0.04 0.04 0.40

88 47.04 2.90 15.18 14.05 0.21 6.43 10.98 2.71 0.50 0.29 100.29 46.86 2.89 14.77 14.06 0.23 6.46 10.94 2.76 0.50 0.30 99.77 46.25 2.89 14.61 14.27 0.24 6.41 11.02 2.72 0.51 0.28 99.20 46.43 2.82 14.99 13.78 0.21 6.52 10.80 2.80 0.47 0.32 99.13 47.07 2.80 15.14 13.87 0.20 6.45 10.98 2.70 0.49 0.30 100.01 46.74 2.71 15.53 13.67 0.20 6.96 10.62 2.68 0.42 0.24 99.78 46.30 2.71 15.76 13.71 0.20 7.01 10.66 2.51 0.41 0.25 99.52 46.39 2.70 15.37 13.57 0.24 6.81 10.63 2.65 0.42 0.27 99.04 Mean 46.64 2.80 15.17 13.87 0.22 6.63 10.83 2.69 0.46 0.28 99.59 stdv 0.33 0.09 0.38 0.24 0.02 0.25 0.17 0.09 0.04 0.03 0.45

Tephra Volcanic Analysis Sample layer system SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 Total date HDH-1 Hekla DH Hekla 55.40 1.91 14.58 11.97 0.31 2.88 6.56 3.35 1.20 1.00 99.15 9.2.2017 55.13 1.94 14.61 12.33 0.31 3.12 6.30 2.91 1.37 1.09 99.11 55.02 1.96 14.92 11.78 0.28 3.05 6.70 3.29 1.16 1.03 99.19 54.16 2.22 14.20 12.34 0.31 3.25 6.84 3.21 1.16 1.25 98.95 54.05 2.18 14.77 11.94 0.29 3.25 6.99 3.24 1.07 1.12 98.91 53.99 1.97 15.04 11.79 0.27 2.93 6.83 3.50 1.15 1.11 98.58 Mean 54.63 2.03 14.69 12.03 0.30 3.08 6.70 3.25 1.19 1.10 98.98 stdv 0.63 0.13 0.30 0.25 0.02 0.16 0.25 0.20 0.10 0.09 0.23

47.01 4.05 13.13 14.93 0.22 5.50 10.77 3.14 0.62 0.41 99.78 48.03 3.98 13.44 14.88 0.23 5.60 11.18 2.50 0.54 0.38 100.76 47.08 3.93 13.28 14.40 0.23 5.51 10.77 2.97 0.60 0.40 99.17 47.02 3.90 13.08 14.41 0.25 5.58 10.83 2.91 0.60 0.43 99.01 47.30 3.86 13.35 14.35 0.23 5.65 10.55 2.92 0.60 0.38 99.19 46.57 3.64 13.28 13.91 0.21 5.55 10.72 2.90 0.59 0.39 97.75

89 47.26 3.63 13.60 14.17 0.24 5.76 10.94 2.88 0.57 0.41 99.45 47.08 3.59 13.67 13.83 0.24 5.84 10.88 2.94 0.56 0.42 99.05 Mean 47.17 3.82 13.35 14.36 0.23 5.62 10.83 2.90 0.58 0.40 99.27 stdv 0.41 0.18 0.21 0.40 0.01 0.12 0.18 0.18 0.02 0.02 0.84

46.50 3.04 14.84 14.12 0.21 6.32 10.78 2.84 0.49 0.30 99.43 46.99 3.01 14.78 14.25 0.21 6.30 10.81 2.80 0.52 0.32 99.99 46.67 2.94 14.96 13.79 0.25 6.47 10.90 2.75 0.52 0.30 99.55 46.88 2.92 14.91 14.07 0.23 6.37 10.91 2.70 0.50 0.29 99.79 46.80 2.89 15.03 14.14 0.20 6.52 10.88 2.80 0.50 0.30 100.06 46.94 2.84 14.94 14.14 0.22 6.38 10.83 2.71 0.50 0.27 99.78 46.62 2.83 14.94 14.14 0.23 6.52 10.78 2.72 0.50 0.26 99.54 46.76 2.80 15.47 13.86 0.22 6.70 10.71 2.75 0.45 0.29 100.01 46.61 2.73 15.57 13.42 0.23 7.32 10.69 2.70 0.41 0.23 99.91 46.46 2.72 15.71 13.56 0.21 7.04 10.67 2.57 0.42 0.26 99.62 46.95 2.69 15.12 14.03 0.22 6.62 10.97 2.81 0.48 0.26 100.15 46.54 2.68 15.61 13.42 0.21 7.16 10.64 2.69 0.42 0.26 99.64 46.76 2.60 15.45 13.68 0.24 7.27 10.68 2.64 0.43 0.30 100.05 Mean 46.73 2.82 15.18 13.89 0.22 6.69 10.79 2.73 0.47 0.28 99.81 stdv 0.18 0.13 0.33 0.29 0.02 0.37 0.11 0.07 0.04 0.03 0.24

Sample Tephra Volcanic Analysis name layer system SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 Total date Tagl 3 Katla 47.93 4.92 11.76 16.96 0.28 4.46 9.47 2.21 0.92 0.72 99.64 9.2.2017

46.94 4.38 12.76 16.27 0.25 4.97 10.30 2.61 0.76 0.45 99.69 48.08 4.35 12.94 15.19 0.23 4.66 9.77 3.13 0.84 0.51 99.69 47.57 4.31 13.10 14.87 0.19 4.89 9.73 3.04 0.83 0.45 98.97 48.11 4.29 13.15 15.01 0.23 4.68 9.85 3.03 0.82 0.50 99.67

90 48.44 4.28 12.65 15.47 0.24 4.72 9.49 3.06 0.84 0.45 99.64 48.18 4.27 13.08 14.76 0.20 4.83 9.63 3.10 0.84 0.47 99.37 47.70 4.25 13.38 14.76 0.23 4.86 9.87 3.01 0.78 0.47 99.32 47.84 4.24 12.87 15.02 0.23 4.84 9.63 2.88 0.82 0.47 98.84 47.76 4.22 13.46 15.03 0.26 4.89 9.43 2.89 0.77 0.46 99.18 48.12 4.20 13.41 14.82 0.25 5.01 9.88 3.06 0.78 0.44 99.97 47.47 4.17 13.21 14.80 0.23 4.95 9.70 2.83 0.82 0.47 98.65 47.68 4.15 13.24 15.02 0.22 4.88 9.80 2.77 0.81 0.49 99.06 47.72 4.12 13.25 14.69 0.23 5.12 9.77 2.94 0.77 0.47 99.09 48.52 4.10 13.17 14.85 0.25 4.56 9.55 2.98 0.79 0.42 99.18 Mean 47.87 4.24 13.12 15.04 0.23 4.85 9.74 2.95 0.81 0.47 99.31 stdv 0.41 0.08 0.24 0.41 0.02 0.15 0.21 0.14 0.03 0.02 0.38

Sample Tephra Volcanic Analysis name layer system SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 Total date Tagl 2 Katla 67.55 0.96 14.62 5.44 0.17 0.76 2.68 3.33 2.99 0.24 98.73 9.2.2017

47.84 4.37 13.03 14.76 0.21 4.79 9.48 3.12 0.86 0.46 98.92 48.38 4.28 13.19 14.39 0.21 4.86 9.30 3.05 0.83 0.50 98.99 48.62 4.26 13.42 14.71 0.22 4.85 9.30 3.21 0.88 0.51 99.98 48.84 4.23 13.29 14.32 0.23 4.84 9.49 2.98 0.82 0.53 99.57 48.41 4.22 13.18 14.44 0.21 4.84 9.40 3.02 0.87 0.52 99.11 48.27 4.21 13.38 14.64 0.20 5.00 9.34 3.10 0.86 0.52 99.52 48.59 4.19 13.20 14.71 0.23 4.80 9.41 3.12 0.86 0.49 99.60 48.17 4.19 12.86 14.62 0.23 4.68 9.28 3.03 0.89 0.50 98.45 48.58 4.16 13.37 14.52 0.22 4.84 9.50 2.99 0.84 0.52 99.54 48.41 4.15 13.35 14.48 0.18 4.93 9.15 3.22 0.84 0.52 99.24 48.46 4.14 13.25 14.06 0.23 4.97 9.44 2.98 0.86 0.51 98.89 48.02 4.14 13.36 14.66 0.21 4.94 9.40 3.19 0.82 0.49 99.23

91 48.24 4.06 13.01 14.41 0.23 4.83 9.39 2.87 0.85 0.48 98.36 48.62 4.03 13.31 14.50 0.21 4.72 9.33 3.31 0.90 0.49 99.42 Mean 48.39 4.19 13.23 14.52 0.22 4.85 9.37 3.09 0.86 0.50 99.20 stdv 0.27 0.09 0.16 0.19 0.01 0.09 0.10 0.12 0.02 0.02 0.45

Sample Tephra Volcanic Analysis name layer system SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 Total date Tagl 1 Katla 48.51 4.18 13.36 14.69 0.24 4.85 9.50 3.02 0.83 0.49 99.67 9.2.2017 48.70 4.13 13.38 14.15 0.21 4.74 9.35 3.06 0.90 0.50 99.12 48.43 4.12 13.18 14.75 0.20 4.87 9.38 3.00 0.82 0.48 99.23 49.47 4.11 13.29 14.10 0.21 4.64 8.99 3.15 0.95 0.53 99.43 48.56 4.09 13.37 14.63 0.25 4.92 9.38 3.18 0.86 0.55 99.79 48.81 4.02 13.54 14.51 0.22 4.80 9.30 3.13 0.85 0.46 99.64 48.65 4.00 13.22 14.44 0.25 4.69 9.15 2.97 0.96 0.54 98.86 48.38 3.99 13.36 14.04 0.22 4.82 9.10 2.96 0.91 0.49 98.27 50.07 3.95 13.44 13.79 0.22 4.60 8.93 3.20 0.95 0.47 99.63 49.63 3.91 13.35 13.75 0.22 4.49 8.97 2.90 1.00 0.52 98.74 48.94 3.91 13.28 14.11 0.23 4.68 9.11 3.31 0.95 0.67 99.18 48.22 3.87 13.59 13.88 0.23 4.60 8.73 3.09 0.89 0.53 97.63 49.48 3.82 13.47 13.89 0.23 4.67 8.99 3.28 1.01 0.52 99.36 49.91 3.81 13.36 13.58 0.24 4.64 8.79 3.05 1.04 0.54 98.96 49.03 3.73 13.59 13.29 0.21 4.15 8.33 2.99 1.04 0.57 96.94 Mean 48.99 3.98 13.39 14.11 0.22 4.68 9.07 3.09 0.93 0.52 98.96 stdv 0.59 0.13 0.12 0.43 0.02 0.19 0.30 0.12 0.07 0.05 0.80

Sample Tephra Volcanic Analysis name layer system SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 Total date HM-4 Hekla Mó Hekla 58.82 1.47 14.78 10.08 0.25 2.18 5.54 3.24 1.44 0.62 98.42 17.3.2017 58.66 1.43 14.79 10.13 0.28 2.06 5.39 3.26 1.47 0.70 98.17 56.03 1.88 14.83 11.50 0.26 2.84 6.58 2.70 1.25 0.79 98.66 Mean 57.84 1.59 14.80 10.57 0.26 2.36 5.84 3.07 1.39 0.70 98.41 stdv 1.57 0.25 0.03 0.81 0.02 0.42 0.65 0.32 0.12 0.09 0.24

47.22 4.22 13.14 15.41 0.21 4.96 9.87 3.18 0.77 0.55 99.52 46.87 4.02 13.10 15.00 0.23 5.08 9.75 3.00 0.65 0.53 98.24 46.81 4.00 13.36 15.00 0.24 5.59 10.26 2.82 0.61 0.48 99.17 46.31 3.79 13.42 13.79 0.22 5.68 10.71 2.85 0.57 0.40 97.73 46.72 3.77 13.45 14.15 0.24 5.65 10.64 2.82 0.56 0.41 98.42 46.52 3.74 13.36 14.33 0.22 5.73 10.81 2.86 0.56 0.44 98.58 47.01 3.72 13.45 14.48 0.20 5.68 10.71 2.96 0.54 0.42 99.17 46.70 3.72 13.66 14.24 0.21 5.68 10.87 2.93 0.60 0.41 99.02 46.76 3.70 13.36 14.36 0.20 5.70 10.99 2.89 0.56 0.37 98.89 47.00 3.69 13.46 14.40 0.26 5.71 10.84 2.92 0.56 0.43 99.26 47.03 3.69 13.45 14.21 0.23 5.61 10.93 2.95 0.58 0.36 99.05 46.65 3.66 13.62 14.08 0.21 5.95 10.85 2.88 0.56 0.38 98.83 46.79 3.63 13.50 14.13 0.23 5.77 10.90 2.91 0.56 0.39 98.81 46.96 3.54 13.58 14.29 0.21 5.75 10.86 2.88 0.56 0.40 99.03 46.87 3.35 13.73 13.84 0.26 5.69 10.72 2.93 0.56 0.40 98.35 Mean 46.81 3.75 13.44 14.38 0.22 5.62 10.65 2.92 0.59 0.42 98.80 stdv 0.22 0.21 0.17 0.44 0.02 0.26 0.38 0.09 0.06 0.05 0.47

93 Sample Tephra Volcanic Analysis name layer system SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 Total date HM-3 Hekla Mó Hekla 61.14 1.44 13.89 9.96 0.26 1.64 4.84 2.87 1.81 0.61 98.46 17.3.2017 61.04 1.40 13.31 10.55 0.27 1.51 4.64 2.95 1.85 0.60 98.12 60.88 1.48 13.78 10.26 0.22 1.67 4.93 3.05 1.82 0.62 98.71 60.39 1.51 13.69 10.48 0.25 1.76 5.03 2.86 1.70 0.71 98.39 54.14 2.25 14.05 12.41 0.27 3.37 7.04 2.82 1.19 0.77 98.30 Mean 59.52 1.62 13.74 10.73 0.25 1.99 5.30 2.91 1.67 0.66 98.40 stdv 3.02 0.36 0.28 0.97 0.02 0.78 0.99 0.09 0.28 0.08 0.22

46.76 3.75 13.44 14.23 0.22 5.81 10.87 2.95 0.58 0.44 99.05 46.89 3.67 13.46 14.19 0.23 5.75 10.95 2.85 0.56 0.38 98.93 46.36 3.75 13.48 13.93 0.20 5.73 10.96 2.74 0.57 0.43 98.15 46.77 3.66 13.32 14.10 0.23 5.72 10.78 2.88 0.57 0.40 98.43 46.59 3.74 13.45 14.23 0.19 5.72 10.68 2.81 0.57 0.39 98.37 46.77 3.75 13.45 14.37 0.21 5.71 10.92 2.79 0.57 0.36 98.91 46.69 3.72 13.61 14.20 0.22 5.68 10.98 2.91 0.58 0.38 98.97 47.02 3.72 13.36 13.96 0.22 5.65 10.89 2.77 0.57 0.39 98.55 46.96 3.66 13.43 14.42 0.21 5.64 11.00 2.91 0.58 0.41 99.22 47.12 3.90 13.70 14.61 0.21 5.61 11.07 2.85 0.57 0.37 100.01 46.35 3.86 13.04 14.46 0.24 5.58 10.67 2.89 0.60 0.44 98.13 46.97 3.59 13.52 14.17 0.18 5.56 10.74 2.96 0.58 0.41 98.67 46.77 3.78 13.32 14.28 0.23 5.56 10.90 2.79 0.60 0.46 98.68 46.69 3.67 13.64 13.91 0.23 5.56 10.96 2.70 0.56 0.37 98.29 46.70 3.59 13.59 14.27 0.22 5.53 10.86 2.95 0.58 0.46 98.75 46.38 3.90 13.08 14.58 0.24 5.51 10.66 3.01 0.62 0.44 98.41 46.67 3.69 13.63 13.90 0.24 5.48 10.91 2.92 0.58 0.38 98.40 46.64 3.81 13.24 14.45 0.21 5.48 10.34 2.84 0.61 0.43 98.05 46.55 3.94 12.97 14.80 0.27 5.45 10.55 2.94 0.62 0.41 98.51

94 46.50 3.92 13.03 14.62 0.21 5.34 10.52 2.78 0.57 0.44 97.92 Mean 46.71 3.75 13.39 14.28 0.22 5.60 10.81 2.86 0.58 0.41 98.62 stdv 0.22 0.11 0.22 0.26 0.02 0.12 0.19 0.08 0.02 0.03 0.48

46.71 4.01 13.11 14.97 0.23 5.08 9.99 3.15 0.71 0.51 98.47 47.06 4.21 12.83 15.07 0.22 4.93 9.86 3.07 0.71 0.55 98.51 47.38 4.31 12.63 14.54 0.30 4.88 9.18 2.84 0.82 1.12 98.00 46.77 4.16 12.66 15.21 0.25 4.88 9.97 3.10 0.71 0.52 98.23 Mean 46.98 4.17 12.81 14.95 0.25 4.94 9.75 3.04 0.74 0.68 98.30 stdv 0.31 0.13 0.22 0.29 0.04 0.09 0.38 0.14 0.06 0.30 0.24

Sample Tephra Volcanic Analysis name layer system SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 Total date HM-2 Hekla Mó Hekla 57.56 2.13 12.96 12.51 0.29 2.26 5.95 2.75 1.63 1.17 99.20 17.3.2017 56.50 2.26 12.64 12.85 0.29 2.35 5.85 2.92 1.63 1.07 98.36 55.37 2.07 14.62 11.89 0.30 3.16 6.81 3.07 1.16 0.77 99.22 54.42 2.47 13.47 13.42 0.29 3.17 6.64 2.66 1.34 0.90 98.78 Mean 55.96 2.23 13.42 12.67 0.29 2.74 6.31 2.85 1.44 0.97 98.89 stdv 1.36 0.18 0.87 0.64 0.01 0.50 0.48 0.18 0.23 0.18 0.41

46.30 3.75 13.51 13.99 0.22 5.93 10.90 2.89 0.56 0.34 98.38 46.71 3.77 13.42 14.28 0.22 5.78 10.90 2.96 0.56 0.42 99.02 46.24 3.76 13.54 14.20 0.24 5.76 10.81 2.87 0.56 0.45 98.42 46.19 3.77 13.48 14.13 0.24 5.74 10.83 2.94 0.57 0.40 98.29 46.55 3.61 13.41 14.20 0.21 5.74 10.75 2.86 0.56 0.43 98.33 46.67 3.65 13.39 14.40 0.22 5.73 10.80 2.83 0.58 0.42 98.69 46.66 3.63 13.52 14.27 0.22 5.71 10.93 3.03 0.57 0.41 98.95 46.60 3.65 13.16 14.61 0.19 5.70 10.86 2.93 0.60 0.39 98.69 47.54 3.87 13.52 14.80 0.23 5.69 11.21 2.47 0.60 0.44 100.37

95 47.31 4.02 13.52 14.60 0.24 5.62 11.01 2.36 0.60 0.44 99.72 46.27 3.57 13.47 13.83 0.18 5.62 10.97 2.92 0.55 0.39 97.77 46.68 3.87 13.17 13.99 0.21 5.56 10.87 2.87 0.60 0.39 98.21 46.67 3.89 13.02 14.74 0.21 5.55 10.66 3.05 0.59 0.42 98.79 46.47 3.73 14.33 15.67 0.25 5.53 9.36 2.92 0.59 0.42 99.28 46.53 3.68 14.19 15.67 0.24 5.53 9.24 3.00 0.59 0.50 99.17 46.98 3.89 13.33 14.65 0.26 5.51 10.76 2.86 0.58 0.42 99.23 Mea 46.65 3.76 13.50 14.50 0.22 5.67 10.68 2.86 0.58 0.42 98.83 Std 0.37 0.12 0.33 0.54 0.02 0.12 0.55 0.19 0.02 0.03 0.64

46.73 4.20 12.95 15.20 0.24 5.00 9.86 3.22 0.71 0.56 98.67 46.63 4.33 12.48 14.80 0.31 4.99 9.18 2.95 0.76 1.11 97.54 46.72 4.14 12.84 14.99 0.26 4.99 9.96 3.15 0.70 0.49 98.25 46.77 4.08 13.14 15.08 0.25 4.97 9.53 2.98 0.69 0.55 98.04 47.34 4.23 12.68 14.59 0.27 4.95 9.18 3.36 0.79 1.06 98.45 47.01 4.28 12.63 14.32 0.28 4.85 9.10 3.34 0.79 1.09 97.70 Mean 46.87 4.21 12.79 14.83 0.27 4.96 9.47 3.17 0.74 0.81 98.11 stdv 0.26 0.09 0.24 0.33 0.02 0.06 0.37 0.17 0.04 0.30 0.44

Sample Tephra Volcanic Analysis name layer system SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 Total date HM-1 Hekla Mó Hekla 57.05 1.88 13.65 12.45 0.30 2.48 5.83 2.74 1.47 0.89 98.74 17.3.2017

47.52 3.73 14.09 14.07 0.21 5.87 11.34 2.06 0.57 0.36 99.82 46.93 3.86 13.56 14.42 0.22 5.80 10.53 2.87 0.60 0.37 99.16 46.62 3.65 13.59 14.07 0.23 5.75 10.96 2.84 0.56 0.38 98.66 46.67 3.73 13.51 14.29 0.26 5.73 10.95 2.86 0.55 0.40 98.94 46.40 3.71 13.38 14.30 0.20 5.72 10.80 2.97 0.58 0.39 98.44 46.51 3.68 13.59 14.36 0.23 5.71 11.02 2.93 0.58 0.47 99.08

96 46.71 3.89 13.23 14.66 0.25 5.69 10.80 2.69 0.60 0.39 98.91 46.73 3.86 13.39 14.35 0.22 5.67 10.99 2.90 0.59 0.46 99.16 46.25 3.85 13.23 14.61 0.20 5.62 10.76 2.64 0.59 0.41 98.16 46.36 3.88 13.18 14.66 0.24 5.60 10.76 2.79 0.57 0.41 98.44 46.49 3.78 13.12 14.34 0.21 5.58 10.92 3.02 0.61 0.40 98.47 46.42 3.73 13.55 14.21 0.24 5.55 10.74 2.81 0.57 0.37 98.19 46.18 3.79 13.27 14.70 0.21 5.55 10.81 2.87 0.59 0.43 98.40 46.37 3.89 13.25 14.66 0.24 5.54 10.85 2.87 0.59 0.44 98.70 46.58 3.76 13.10 14.31 0.24 5.54 10.54 2.84 0.59 0.43 97.92 46.77 3.80 13.10 14.53 0.21 5.52 10.92 2.88 0.60 0.39 98.72 46.94 3.83 13.21 14.67 0.21 5.51 10.78 2.78 0.58 0.42 98.93 Mean 46.61 3.79 13.37 14.42 0.22 5.64 10.85 2.80 0.58 0.41 98.71 stdv 0.32 0.08 0.25 0.21 0.02 0.11 0.19 0.21 0.02 0.03 0.46

47.10 4.23 13.04 15.37 0.26 5.06 9.95 3.11 0.74 0.49 99.34 47.30 4.41 12.88 14.50 0.28 4.94 9.15 3.15 0.79 1.14 98.54 47.38 3.59 13.32 14.51 0.23 4.93 9.55 3.02 0.83 0.51 97.88 47.30 4.33 12.82 14.69 0.28 4.91 9.17 3.36 0.70 1.10 98.65 47.12 4.36 12.81 14.52 0.28 4.88 9.27 3.51 0.81 1.09 98.66 47.23 4.14 12.81 14.62 0.30 4.85 8.99 3.13 0.83 1.16 98.06 47.78 4.31 12.76 14.71 0.31 4.84 9.04 3.31 0.80 1.03 98.89 47.46 4.02 12.74 14.13 0.27 4.76 8.79 3.20 0.86 1.25 97.48 47.96 4.08 12.91 14.49 0.30 4.59 8.97 2.88 0.86 1.23 98.27 Mean 47.40 4.16 12.90 14.62 0.28 4.86 9.21 3.19 0.80 1.00 98.42 stdv 0.29 0.25 0.18 0.33 0.02 0.13 0.35 0.19 0.05 0.29 0.56

97 Sample Tephra Volcanic Analysis name layer system SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 Total date Áfangagil 1 Vatnafjöll 46.68 3.80 14.32 15.64 0.25 5.64 9.40 2.89 0.59 0.46 99.68 11.2.2017 46.54 3.80 14.16 15.34 0.26 5.59 9.46 2.87 0.61 0.52 99.15 46.30 3.99 13.61 16.02 0.23 5.52 9.32 2.79 0.61 0.45 98.84 45.97 3.82 14.24 15.83 0.22 5.78 9.42 2.82 0.59 0.41 99.11 46.63 3.94 13.90 16.18 0.26 5.42 9.32 2.66 0.67 0.54 99.52 46.70 3.78 14.05 15.96 0.27 5.30 9.36 2.76 0.66 0.45 99.29 45.92 3.70 14.19 15.50 0.24 5.69 9.26 2.88 0.58 0.48 98.44 46.40 3.75 14.28 15.57 0.23 5.77 9.27 2.79 0.59 0.39 99.05 46.30 3.77 14.09 15.90 0.24 5.73 9.09 2.58 0.58 0.43 98.72 46.64 3.67 14.32 15.55 0.28 5.53 9.37 2.86 0.58 0.44 99.23 46.33 3.74 14.04 15.80 0.26 5.64 9.18 2.94 0.55 0.45 98.93 46.73 3.90 14.38 15.49 0.22 5.54 9.45 2.61 0.60 0.36 99.28 46.36 3.74 14.44 15.92 0.23 5.80 9.48 2.87 0.57 0.40 99.82 46.20 3.74 14.36 15.55 0.26 5.78 9.42 2.86 0.58 0.48 99.23 Mean 46.41 3.80 14.17 15.73 0.25 5.62 9.34 2.80 0.60 0.45 99.16 stdv 0.26 0.09 0.22 0.25 0.02 0.15 0.11 0.11 0.03 0.05 0.37

98 Table A.1.2. Measured values of the A99 basalt glass standard. Date Standards SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 Total 9.2.2017 A99v_1 51.22 3.88 12.69 13.13 0.18 4.90 9.05 2.70 0.84 0.40 98.99 A99v_2 51.42 3.91 12.69 13.16 0.20 4.92 9.03 2.65 0.83 0.46 99.28 A99v_3 51.55 3.97 12.70 13.21 0.21 4.94 9.10 2.49 0.84 0.43 99.45 A99h_1 51.04 3.90 12.69 13.31 0.20 5.09 9.28 2.61 0.79 0.46 99.37 A99h_2 51.71 3.80 12.42 13.28 0.19 5.13 9.22 2.60 0.78 0.48 99.62 A99h_3 51.39 3.91 12.70 13.02 0.21 5.20 9.12 2.64 0.82 0.43 99.45 A99n_1 51.01 3.81 12.41 13.20 0.21 5.07 9.17 2.63 0.84 0.46 98.81 A99n_2 51.09 4.04 12.62 13.15 0.20 5.09 9.33 2.70 0.82 0.43 99.47 A99n_3 51.12 3.91 12.75 13.19 0.19 5.10 9.28 2.67 0.84 0.46 99.52

A99v_4 51. 67 3.89 12. 81 13. 27 0.22 4.91 9.21 2.62 0.80 0.42 99. 83 A99v_5 51.57 3.95 12.68 13.21 0.21 4.96 9.09 2.48 0.86 0.45 99.46 A99v_6 51.94 4.02 12.68 13.28 0.20 4.97 9.25 2.44 0.85 0.40 100.03 A99h_4 51.51 3.72 12.74 13.28 0.19 5.09 9.20 2.62 0.82 0.44 99.61 A99h_5 51.39 3.87 12.60 13.20 0.18 4.95 9.11 2.57 0.81 0.44 99.12 A99h_6 51.69 3.83 12.69 13.23 0.21 5.03 9.27 2.54 0.83 0.44 99.75 A99n_4 51.78 3.93 12.90 13.19 0.20 4.93 9.36 2.76 0.84 0.46 100.35 A99n_5 51.13 3.86 12.86 13.22 0.21 5.12 9.37 2.65 0.85 0.44 99.71 A99n_6 51.27 3.85 12.74 13.14 0.19 4.97 9.16 2.79 0.84 0.43 99.38

A99v_7 51. 67 3.97 12. 84 12. 80 0.18 4.77 9.20 2.50 0.84 0.41 99. 18 A99v_8 51.63 3.97 12.97 13.18 0.20 4.91 9.29 2.61 0.86 0.39 100.02 A99v_9 51.54 3.94 12.63 12.93 0.21 4.93 9.16 2.42 0.86 0.43 99.05 A99h_7 51.27 3.73 12.80 13.41 0.23 5.18 9.19 2.73 0.78 0.40 99.72 A99h_8 50.97 3.80 12.53 13.13 0.17 4.98 9.15 2.65 0.78 0.42 98.59

99 A99h_9 51.14 3.89 12.60 13.22 0.19 5.05 9.14 2.65 0.80 0.44 99.12 A99n_7 51.56 3.90 12.70 13.04 0.21 4.86 9.16 2.64 0.81 0.42 99.30 A99n_8 51.22 3.99 12.76 13.28 0.24 5.01 8.88 2.70 0.82 0.41 99.30 A99n_9 50.90 3.92 12.69 13.36 0.21 4.95 9.15 2.64 0.85 0.41 99.08

10.2.2017 A99v_10 52. 45 3.87 12. 71 13. 25 0.18 4.96 9.03 2.48 0.85 0.45 100 .23 A99v_11 51.42 3.86 12.58 12.94 0.17 4.96 9.11 2.71 0.80 0.41 98.97 A99v_12 51.58 3.93 12.87 13.26 0.21 4.81 8.89 2.65 0.87 0.43 99.49 A99h_10 51.97 3.76 12.86 13.20 0.19 5.14 9.16 2.59 0.83 0.39 100.09 A99h_11 51.86 3.96 12.72 13.29 0.25 5.04 9.34 2.49 0.79 0.42 100.16 A99h_12 51.55 3.85 12.83 13.28 0.20 5.11 9.15 2.62 0.82 0.37 99.78 A99n_10 51.87 3.96 12.69 12.62 0.23 4.97 9.22 2.72 0.86 0.43 99.57 A99n_11 51.78 3.87 12.80 13.49 0.21 5.06 9.24 2.68 0.81 0.42 100.36 A99n_12 51.61 3.92 12.75 13.37 0.19 4.90 9.23 2.58 0.82 0.41 99.78

A99v_13 51. 84 3.87 12. 62 13. 12 0.19 4.90 9.19 2.53 0.86 0.39 99. 52 A99v_14 51.48 4.01 12.71 13.21 0.17 4.88 8.76 2.59 0.83 0.41 99.05 A99v_15 51.79 3.99 12.78 13.39 0.22 4.94 9.24 2.52 0.85 0.35 100.07 A99h_13 51.56 3.85 12.55 13.11 0.20 5.13 9.13 2.61 0.82 0.37 99.33 A99h_14 51.49 3.89 12.41 13.26 0.20 5.00 9.01 2.62 0.83 0.42 99.13 A99h_15 51.29 3.92 12.76 13.26 0.20 5.15 9.19 2.55 0.81 0.41 99.55 A99n_13 51.51 3.82 12.83 13.30 0.18 4.94 9.38 2.63 0.83 0.38 99.80 A99n_14 51.41 3.64 12.85 13.26 0.20 4.97 9.25 2.73 0.84 0.38 99.53 A99n_15 51.80 3.89 12.68 13.28 0.20 5.09 9.30 2.49 0.83 0.45 100.00

A99v_16 51. 54 3.98 12. 81 13. 21 0.19 4.99 9.21 2.55 0.86 0.41 99. 74 A99v_17 51.85 3.90 12.79 13.39 0.19 4.88 9.12 2.78 0.87 0.41 100.19

100 A99v_18 51.84 3.98 12.84 13.19 0.21 4.90 9.08 2.47 0.82 0.41 99.74 A99h_16 51.70 3.76 12.70 13.28 0.20 4.86 9.18 2.56 0.79 0.42 99.45 A99h_17 51.84 3.78 12.52 13.11 0.20 5.04 9.10 2.72 0.84 0.38 99.53 A99h_18 51.58 3.87 12.78 13.04 0.21 5.06 9.27 2.52 0.84 0.39 99.57 A99n_16 51.44 3.87 12.71 12.89 0.20 5.04 9.11 2.40 0.81 0.43 98.90 A99n_17 51.30 3.88 12.70 13.24 0.18 4.97 9.14 2.60 0.82 0.40 99.23 A99n_18 51.88 3.73 12.63 13.22 0.22 4.93 9.27 2.65 0.82 0.45 99.80

11.2.2017 A99v_19 51. 18 3.99 12. 78 13. 23 0.23 4.91 9.23 2.68 0.86 0.43 99. 51 A99v_20 51.29 4.01 12.79 13.01 0.19 4.94 8.97 2.49 0.84 0.46 98.99 A99v_21 50.97 3.92 12.79 13.24 0.19 5.07 9.07 2.62 0.83 0.46 99.16 A99h_19 50.94 3.86 12.69 13.19 0.18 5.11 9.22 2.65 0.82 0.44 99.11 A99h_20 50.73 3.88 12.67 13.23 0.22 5.07 9.06 2.55 0.83 0.44 98.68 A99h_21 50.79 3.85 12.70 13.22 0.20 5.02 8.92 2.57 0.84 0.47 98.58 A99n_19 51.09 3.69 12.85 13.15 0.20 5.08 9.32 2.40 0.82 0.44 99.04 A99n_20 51.08 3.87 12.84 13.26 0.23 4.98 9.13 2.58 0.83 0.42 99.22 A99n_21 51.16 3.92 12.76 13.20 0.18 4.97 9.10 2.63 0.85 0.41 99.18

A99h_22 51. 29 3.70 12. 66 13. 03 0.20 5.05 9.38 2.48 0.85 0.40 99. 04 A99h_23 51.16 3.94 12.66 13.20 0.19 5.16 9.22 2.59 0.80 0.38 99.29 A99n_22 51.40 3.96 12.69 13.22 0.22 5.07 9.24 2.46 0.84 0.38 99.49 A99n_23 51.13 3.89 12.88 13.01 0.21 4.95 9.21 2.60 0.81 0.41 99.10 A99n_24 50.72 3.68 12.73 13.34 0.20 5.01 9.10 2.55 0.81 0.45 98.59 A99v_22 51.54 3.87 12.71 13.26 0.21 4.79 9.19 2.46 0.83 0.47 99.33 A99v_23 51.46 4.03 12.46 12.96 0.18 4.70 9.08 2.52 0.82 0.41 98.63 A99v_24 51.40 3.87 12.79 13.38 0.19 4.93 9.08 2.60 0.83 0.40 99.47

101 A99v_25 51.22 3.99 12.92 13.30 0.20 4.95 9.08 2.39 0.84 0.46 99.35 A99v_26 51.15 4.01 12.80 13.35 0.19 4.99 9.19 2.40 0.82 0.39 99.29 A99v_27 51.14 3.95 12.87 13.46 0.19 4.98 9.22 2.49 0.84 0.41 99.55 A99h_25 30.81 3.80 5.96 13.04 0.17 5.40 9.07 2.68 0.80 0.38 72.11 A99h_26 51.21 3.96 12.90 13.12 0.20 5.03 9.21 2.40 0.81 0.42 99.26 A99h_27 51.29 3.95 12.78 13.24 0.21 5.02 9.28 2.58 0.74 0.37 99.47 A99n_25 51.38 3.90 12.76 13.14 0.20 5.01 9.14 2.54 0.82 0.41 99.30 A99n_26 51.17 3.81 12.86 13.26 0.22 5.07 9.10 2.39 0.84 0.39 99.12 A99n_27 51.07 3.97 12.66 13.43 0.18 5.10 9.30 2.62 0.84 0.39 99.56

A99v_28 51. 25 3.87 12. 61 13. 01 0.20 4.80 9.13 2.63 0.85 0.47 98. 82 A99v_29 51.47 3.82 12.86 13.35 0.17 4.91 8.81 2.56 0.84 0.41 99.20 A99v_30 51.59 3.88 12.70 13.27 0.23 4.98 9.03 2.57 0.85 0.40 99.50 A99h_28 33.09 3.81 6.63 13.18 0.17 5.28 9.12 2.70 0.83 0.41 75.22 A99h_29 50.87 3.90 12.68 13.23 0.19 5.12 9.26 2.66 0.80 0.44 99.15 A99h_30 51.16 3.74 12.61 13.25 0.22 5.02 9.33 2.51 0.82 0.41 99.07 A99n_28 51.24 3.90 12.60 13.13 0.20 4.96 9.25 2.49 0.80 0.41 98.99 A99n_29 51.17 4.03 12.79 13.40 0.18 4.87 9.20 2.50 0.83 0.43 99.39 A99n_30 51.42 3.78 12.49 12.93 0.19 4.89 9.10 2.70 0.85 0.43 98.77

A99h _31 29. 10 3.79 5.44 13. 20 0.15 5.25 9.10 2.71 0.80 0.40 69. 94 A99h_32 49.86 3.92 12.21 13.38 0.19 4.97 9.27 2.40 0.80 0.39 97.38 A99h_33 51.03 3.93 12.54 13.07 0.22 5.04 9.03 2.56 0.81 0.45 98.67 A99n_31 50.80 3.86 12.93 13.34 0.18 5.07 9.04 2.59 0.81 0.38 99.00 A99n_32 50.66 3.90 12.64 13.08 0.21 4.88 9.06 2.50 0.81 0.36 98.10 A99n_33 50.83 3.88 12.86 13.19 0.16 4.99 9.20 2.70 0.82 0.44 99.07 A99v_31 51.51 3.93 12.68 13.33 0.20 5.00 9.29 2.25 0.82 0.47 99.48

102 A99v_32 50.88 4.01 12.78 13.25 0.22 4.89 9.17 2.44 0.84 0.47 98.95 A99v_33 51.18 3.99 12.67 13.31 0.21 4.95 9.01 2.38 0.85 0.46 99.01

17.3.2017 A99h_1 50. 45 3.90 12. 30 13. 35 0.19 5.09 9.12 2.69 0.85 0.44 98. 37 A99h_2 50.67 3.94 12.54 13.14 0.19 5.01 9.19 2.52 0.83 0.45 98.47 A99h_3 50.75 3.87 12.61 13.25 0.18 4.87 9.25 2.54 0.84 0.42 98.58 A99n_1 50.69 3.86 12.58 13.12 0.20 4.94 9.01 2.58 0.80 0.41 98.18 A99n_2 51.16 3.93 12.64 13.23 0.19 5.05 9.31 2.64 0.83 0.45 99.43 A99n_3 50.84 3.86 12.53 13.11 0.20 4.99 8.96 2.67 0.83 0.44 98.44 A99V_1 50.95 3.97 12.53 13.20 0.21 4.91 9.05 2.53 0.83 0.44 98.62 A99V_2 51.24 3.98 12.71 13.25 0.17 4.82 9.00 2.47 0.84 0.45 98.93 A99V_3 51.21 4.02 12.71 13.33 0.23 4.94 9.16 2.62 0.83 0.48 99.52

A99h_4 50. 96 3.85 12. 68 13. 32 0.20 5.03 9.16 2.68 0.83 0.43 99. 14 A99h_5 51.07 3.82 12.66 13.12 0.17 5.09 9.16 2.51 0.81 0.45 98.86 A99h_6 51.07 3.92 12.73 13.34 0.20 5.16 9.22 2.58 0.81 0.42 99.46 A99n_4 50.90 3.95 12.66 13.14 0.19 4.97 9.24 2.60 0.85 0.47 98.97 A99n_5 50.85 3.75 12.58 13.22 0.20 4.97 9.20 2.63 0.81 0.45 98.66 A99n_6 51.00 3.84 12.74 13.13 0.20 4.91 9.11 2.62 0.84 0.45 98.83 A99v_4 51.13 4.08 12.75 13.18 0.19 4.89 9.01 2.55 0.85 0.43 99.06 A99v_5 50.89 3.92 12.76 13.20 0.21 4.75 9.23 2.41 0.83 0.45 98.64 A99v_6 50.89 3.87 12.75 13.35 0.21 4.87 9.13 2.53 0.85 0.47 98.92

A99h_7 50. 97 3.88 12. 73 13. 36 0.18 5.01 9.27 2.67 0.79 0.44 99. 30 A99h_8 51.15 3.84 12.60 13.30 0.20 4.99 9.18 2.65 0.82 0.43 99.16 A99h_9 51.16 3.88 12.55 13.30 0.19 4.84 9.19 2.57 0.84 0.48 99.00 A99n_7 51.33 3.79 12.75 13.12 0.21 4.97 9.27 2.63 0.81 0.44 99.32

103 A99n_8 50.99 3.91 12.64 13.17 0.19 5.06 8.97 2.65 0.83 0.43 98.83 A99n_9 51.12 3.95 12.77 13.18 0.21 4.91 8.95 2.55 0.82 0.44 98.90 A99v_7 51.46 3.89 12.64 13.17 0.21 4.89 9.14 2.61 0.83 0.48 99.32 A99v_8 51.15 4.07 12.78 13.27 0.21 4.97 9.26 2.63 0.84 0.42 99.61 A99v_9 51.15 4.07 12.63 13.29 0.20 4.96 9.13 2.65 0.85 0.44 99.37

Table A.1.3. Standard values for the A99 standard at Jeol superprobe at the institute of Earth Science, University of Iceland Standard SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 Total A99 51.06 3.95 12.55 13.35 0.20 5.03 9.17 2.65 0.83 0.43 99.22

104