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Mid-Holocene Eruptive Activity of the Hekla Volcanic System

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Mid-Holocene Eruptive Activity of the Hekla Volcanic System 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 Copyright © 2018 Daníel Freyr Jónsson All rights reserved 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
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