Petrogenesis of subglacial pillow lavas from the flanks of the Bárðarbunga volcano in the

Vonarskarð valley

Maria Monika Repczyńska

Faculty of Earth Sciences

University of Iceland

2020

Petrogenesis of subglacial pillow lavas from the flanks of the Bárðarbunga

volcano in the Vonarskarð valley

Maria Monika Repczyńska

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

Advisors

Dr. Sæmundur Ari Halldórsson Dr. Enikő Bali

Master’s Examiner

Ingvar Atli Sigurðsson

Faculty of Earth Sciences School of Engineering and Natural Sciences

University of Iceland Reykjavik, January 2020

Petrogenesis of subglacial pillow lavas from the flanks of the Bárðarbunga volcano in the

Vonarskarð valley

Petrogenesis of pillow lavas from Vonarskarð valley

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

Geology

Copyright © 2020 Maria Monika Repczyńska

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:

Maria Monika Repczyńska, 2020, Petrogenesis of subglacial pillow lavas from the flanks

of the Bárðarbunga volcano in the Vonarskarð valley, Master’s thesis, Faculty of Earth Sciences, University of Iceland, pp. 147.

Printing: Háskolaprent, Fálkagata 2, 101 Reykjavík

Reykjavik, Iceland, January 2020

Abstract

The Bárðarbunga volcano represents one of the largest and most productive volcanic systems

in Iceland. It is located in central Iceland and it is thought to be situated directly above the

Iceland mantle plume. Several studies have been conducted on early postglacial and

Holocene lavas of Bárðarbunga Volcanic System (BVS), however, due to the hardly

accessible location of the partly ice-covered Bárðarbunga volcanic center, studies of lavas

in the nearest vicinity of the volcano are scarce. This thesis presents a detailed geochemical

study of subglacial pillow lavas from ice-free areas in the western and northwestern flanks

of the volcano.

Major element composition of glassy pillow rims indicates their separation into two different

groups of basaltic magma – more primitive and more evolved group, both of which fall close

to or on a liquid line of descent defined by units from the BVS. The more evolved group also

reveals slight enrichment in the incompatible trace elements when compared to the more

primitive group.

Results of thermobarometric calculations and indications from the spatial distribution of the

pillow lavas, suggest that the more evolved group was stored in a relatively shallow reservoir

(4.7 ± 1.8km), located more or less directly beneath the Bárðarbunga volcano. Eruptions of

the lavas from the more primitive group most likely were fed from a slightly deeper (8.4 ±

2.4 km) and larger reservoir, which is in agreement with studies on other units from the BVS.

Paleo-ice thickness estimates based on dissolved volatile concentrations in pillow rim

glasses, suggest that the studied pillow lavas erupted under a relatively thin and uniform ice

sheet of about 400-600 m. Thus, different levels of enrichment and degree of melt evolution

are most likely not explained by the changes of ice sheet loading, but arise from the

heterogeneities of the mantle underlying the Bárðarbunga volcano, as well as from the

structure of its plumbing system.

Útdráttur

Bárðarbunga er ein af stærstu og afkastamestu eldstöðvum Íslands. Hún er staðsett á miðju

landinu og er talin liggja nánast beint yfir möttulstrók Íslands. Nokkrar rannsóknir hafa verið

gerðar á nútíma hraunum sem tengjast eldstöðvarkerfi Bárðarbungu en mun færri rannsóknir

hafa verið gerðar á hraunlögum í nánd við eldstöðina. Þessi ritgerð fjallar um

jarðefnafræðileg sérkenni bólstrabergs sem finna má undan jökli á íslausum svæðum á vestur

og norðvestur hliðum eldstöðvarinnar.

Efnagreiningar glerbólstra gefa til kynna að skipta megi þeim í tvo ólíka hópa – annan

frumstæðari en hinn þróaðri, sem báðir falla nálægt eða á þróunarlínu basalts, ættuðu úr

eldstöðvarkerfi Bárðarbungu. Þróaðri hópurinn sýnir einnig lítilsháttar aukningu í

snefilefnum þegar þau eru borin saman við frumstæðari hópinn.

Niðurstöður varmaþrýstimæla og vísbendingar frá svæðisdreifingu bólstrabergsins, gefa til

kynna að þróaðri hópurinn staðnæmist í tiltölulega grunnu hólfi (4.7 ± 1.8 km), staðsettu

nokkurn veginn beint undir Bárðarbungu eldstöðinni. Bólstrar sem tilheyra frumstæðari

hópnum komu að öllum líkindum af meira dýpi (8.4 ± 2.4 km), sem er í samræmi við

rannsóknir á nýlegum gosum á sprungusveim Bárðarbungu. Lágmarksþykkt þess

ísaldarjökuls sem ofan á hvíldi við gos, var metin út frá styrk uppleystra gastegunda í

glerbólstrum en niðurstöður gefa til kynna að ísbreiðan var að lágmarki 400-600 m. Því er

ólíklegt að hægt sé að skýra mun í efnasamsetningu glerbólstra af breytingum í fargi

ísaldarjökulsins. Misleitni undirliggjandi möttuls ásamt flókinni uppbyggingu kvikukerfis

Bárðarbungu eru líklegri skýring.

vii

Table of Contents

List of Figures ..................................................................................................................... ix

List of Tables ...................................................................................................................... xv

Abbreviations ................................................................................................................... xvii

Acknowledgments ............................................................................................................. xix

1 Introduction ..................................................................................................................... 1

2 Background ..................................................................................................................... 3 2.1 General overview of Icelandic geology and geochemistry of Icelandic lavas ........... 3

2.1.1 Icelandic Geology .......................................................................................... 3

2.1.2 Geochemistry of Icelandic basalts ................................................................. 4 2.2 The Last Glacial Maximum (LGM) and the deglaciation of Iceland ......................... 5

3 Geological setting and sample details ............................................................................ 7

3.1 Bárðarbunga Volcanic System ................................................................................ 7

3.2 Vonarskarð valley.................................................................................................... 9 3.3 Sample details.......................................................................................................... 9

4 Methods .......................................................................................................................... 13

4.1 Electron Probe Micro Analyzer (EPMA) .............................................................. 13 4.2 Fourier Transform Infrared Spectroscopy (FTIR)................................................. 14

4.3 Inductively Coupled Plasma - Optical Emission Spectroscopy (ICP-OES) ......... 16 4.4 Laser Ablation Inductively Coupled Plasma Mass Spectrometer

(LA-ICP-MS) ........................................................................................................ 16

4.5 Multi Collector Inductively Coupled Plasma - Mass Spectrometer (MC-ICP-MS) ....................................................................................................... 17

4.6 Thermobarometric calculations ............................................................................. 18 4.7 Paleo-ice thickness estimations ............................................................................. 20

5 Results ............................................................................................................................ 23 5.1 Petrography ........................................................................................................... 23 5.2 Major and trace element systematics in whole rock and glass .............................. 25

5.2.1 Major elements............................................................................................. 25 5.2.2 Trace elements ............................................................................................. 31

5.3 Mineral chemistry.................................................................................................. 35 5.3.1 Plagioclase chemistry................................................................................... 35 5.3.2 Clinopyroxene chemistry ............................................................................. 36

5.3.3 Olivine chemistry ......................................................................................... 38 5.4 Volatiles................................................................................................................. 39 5.5 Radiogenic isotopes systematics ........................................................................... 41

6 Discussion ...................................................................................................................... 45

viii

6.1 Affiliation to the Bárðarbunga Volcanic System .................................................. 45 6.1.1 Petrological and petrographical characteristics ........................................... 45 6.1.2 Chemical constraints .................................................................................... 46

6.1.3 Isotopic constraints ...................................................................................... 47 6.2 Origin of the two magma types identified ............................................................ 48

6.2.1 Comparison with well characterized products of the Bárðarbunga Volcanic System: clues to their origin ......................................................... 48

6.2.2 Magmatic evolution in the crust .................................................................. 50

6.2.3 Heterogeneity in the mantle source ............................................................. 57 6.3 Evaluating paleo-ice thickness at the time of eruption ......................................... 59

6.3.1 Volatile degassing ........................................................................................ 60

6.3.2 Paleo-ice thickness estimation ..................................................................... 61 6.4 Sampling diverse magma types beneath the Bárðarbunga volcano during

glacial times .......................................................................................................... 63

7 Implications and conclusions ....................................................................................... 67

References .......................................................................................................................... 69

Appendix A: Overview of the samples ............................................................................ 79

Appendix B: Major, minor and selective trace element analyses in whole rock

samples ........................................................................................................................... 81

Appendix C: Major and minor element analyses of glass and mineral phases ........... 83

Appendix D: Trace element analysis of glass.................................................................. 97

Appendix E: Results of H2O analysis ............................................................................ 113

Appendix F: Results of clinopyroxene-liquid thermobarometry calculations .......... 115

Appendix G: Results of plagioclase-liquid thermometry calculations ....................... 119

Appendix H: Calculated CIPW norms .......................................................................... 123

ix

List of Figures

Figure 1. Neovolcanic zones in Iceland (from Thordarson and Höskuldsson, 2008).

Dashed circle indicates the position of the mantle plume. See text for

details. ................................................................................................................. 3

Figure 2. Modelled Icelandic ice sheet during the Last Glacial Maximum (from

Hubbard et al., 2006).......................................................................................... 5

Figure 3. The stages of the last glaciation of Iceland. a) Last Glacial Maximum, b)

Bølling Ice Sheet; c) Younger Dryas Ice Sheet; d) Preboreal Ice Sheet.

From Norðdahl et al. (2008). ............................................................................. 6

Figure 4. Bárðarbunga Volcanic System. Red circle denotes the studied area. B –

Bárðarbunga volcano; Þ -Þjórsa; Bár – Bárðaldalur; Kis – Kistufell; H –

Holuhraun; Hg – Hágöngur. From Larsen and Guðmundsson (2014). ............ 8

Figure 5. Simple geological map of the studied area with sample locations. Modified

from Hjartarson et al. (2019). .......................................................................... 10

Figure 6. Examples of CO2 and H2O-rich samples in mafic melts from Shishkina et al.

(2014) showing the peak positions of carbonate and water bands. ................. 15

Figure 7. a) Representative mid-infrared spectra of two analyses from this study; b)

Carbonate bands cannot be seen in any of the analyses in this study, thus

the concentration of CO2 is below the detection limit (< 30 ppm); c) peak

position of OH+H2O band. ............................................................................... 15

Figure 8. Influence of CO2 concentration on volatile saturation pressure of H2O in

basaltic magmas. Calculated by VolatileCalc (Newman and Lowenstern,

2002). ................................................................................................................ 21

Figure 9. BSE image of a glass shard with dispersed microcrysts of olivine,

plagioclase and clinopyroxene. Few vesicles are present. ............................... 24

Figure 10. a) BSE image of a glass shard with abundant microcrysts forming an

aggregate of clinopyroxenes, olivines and plagioclases, with subophitic

texture. Note small melt inclusion entrapped in a plagioclase crystal in the

bottom right of the glass shard. Clinopyroxene crystals show various types

of zoning. White arrow indicates a clinopyroxene crystal with normal

zonation; b) Clinopyroxene with hour-glass zonation and ingrowing

plagioclase. ....................................................................................................... 24

Figure 11. TAS classification diagram of analyzed samples based on their whole rock

and glass composition. The purple line indicates the boundary between

alkaline (top) and subalkaline (bottom) rock series based on Rollinson

(1993). ............................................................................................................... 26

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x

Figure 12. AFM classification diagram presenting division of calc-alkaline rock series

form tholeiitic rocks series, based on relative proportions between oxides

of alkali, iron and magnesium in rock composition. Dividing line based on

Irvine and Bargar (1971). ................................................................................ 26

Figure 13. Ol-Pl-Qz and Ol-Cpx-Qz pseudoternary projections in the tholeiitic field

of Ol-Cpx-Qz-Ne basalt tetrahedron after Yoder and Tilley (1962)

projected from clinopyroxene and plagioclase, respectively. .......................... 27

Figure 14. Variation of TiO2 vs. Mg# for glass and whole rock samples from each

location. The error bars denote 2SD for whole rock (left) and glass (right)

composition based on the measured standards. ............................................... 28

Figure 15. Fenner type diagrams of the major elements’ oxides vs. MgO wt.% in whole

rock and glass samples. Error bars show 2SD for whole rock (left side of

plot) and glass (right side of the plot) samples. ............................................... 29

Figure 16. a-d: Variation of selected trace elements as a function of MgO wt.% in

whole rock and glass samples; e) Variation of Y (ppm) vs. Zr (ppm) in

whole rock and glass samples. Error bars show 2SD for whole rock (left

side of plot) and glass (right side of plot) samples.......................................... 32

Figure 17. Primitive mantle normalized (McDonough and Sun, 1995) multi-element

diagram for glass samples. Two groups can be distinguished. ........................ 34

Figure 18. Chondrite normalized (McDonough and Sun, 1995) REE diagram for glass

samples. ............................................................................................................ 34

Figure 19. A ternary classification diagram of plagioclase endmemebers (Deer et al.,

1963). Plagioclases from both groups are classified as bytownites,

however four most primitive plagioclases are on the border of anorthite

field. .................................................................................................................. 35

Figure 20. a) Histogram showing anorthite content in analyzed plagioclase

microcrysts. Note clear bimodal distribution between the two groups; b)

Rhodes diagram of Ca# number in averaged glass composition vs. An [%]

content in plagioclases. The dashed lines show the KD values in a range

between 1 - 1.3 and denote the equlibrium between the melt and minerals.

.......................................................................................................................... 36

Figure 21. A ternary classification diagram of pyroxene endmembers (Morimoto et

al., 1988). Minerals from more primitive group are mostly endiopsides,

however bimodal distribution can be observed with few crystals falling on

diopside area. Clinopyroxenes from the second group, are solely augitic

in composition, with bimodal distribution occurring among them as well. ..... 36

Figure 22. a) Al2O3 / TiO2 ratio plotted with Mg# of clinopyroxene in order to

minimize the chemical differences arising from sector zoning in the

crystals. Clinopyroxenes from the more primitive group have higher Mg#

and Al2O3 / TiO2 ratio when compared to the second group; b) Cr2O3 vs.

Mg# in clinopyroxenes. Minerals from the more primitive group have

xi

significantly higher Cr2O3 concentration, which also varies among them

due to the sector zoning. ................................................................................... 37

Figure 23. Melt – clinopyroxene equilibrium checks. a) Rhodes diagram of Mg#

number in averaged glass composition vs. Mg# in clinopyroxenes. The

dashed lines show the KD values in a range between 0.24 – 0.3 and denote

the equilibrium between the melt and minerals; b) Observed vs. predicted

DiHd components. Dashed lines indicate ±0.06 shift from 1:1 line; c)

Observed vs. predicted EnFs components. Dashed lines indicate ±0.05

shift from 1:1 line; d) Observed vs. predicted CaTs components. Dashed

lines indicate ±0.03 shift from 1:1 line. ............................................................ 38

Figure 24. a) NiO vs. Fo (%) in olivine microcrysts. A slight positive correlation can

be observed; b) Rhodes diagram of Mg# number in averaged glass

composition vs. Mg# in olivine microcrysts. The dashed lines show the KD

values in a range between 0.27 – 0.33 and denote the equlibrium between

the melt and minerals........................................................................................ 39

Figure 25. Measured H2O vs. averaged H2O concentration in glass samples. Error

bars indicate 2SD for MgO and ±10% for H2O. .............................................. 40

Figure 26. a) Measured H2O vs. MgO wt.% in glass samples. Dashed trendline shows

negative correlation of H2O and MgO; b) Measured H2O vs. K2O wt.% in

glass samples. Dashed trendline shows positive correlation of H2O and

K2O; c) Measured H2O vs. Ce concentration in glass...................................... 41

Figure 27. Isotopic variations in glasses and groundmass in the samples from this

study and other samples from Iceland: a) 143Nd/144Nd vs. 87Sr/86Sr; b) 176Hf/177Hf vs.87Sr/86Sr; c) 143Nd/144Nd vs. 176Hf/177Hf. The isotopic data

from Iceland are from Harðardóttir (2019). .................................................... 42

Figure 28. Major and trace elements compositional fields based on the figure from

Óladóttir et al. (2011a, b). a) Plot of FeO vs. K2O for the samples from

this study and other volcanic systems from the region; b) Plot of Sr/Th vs.

Th of the samples and other volcanic system from the region. The glass

samples from this study clearly fall into BVS field. .......................................... 46

Figure 29. Isotopic variations in glasses and groundmass in each of the locations and

other samples from BVS. a) 143Nd/144Nd vs. 87Sr/87Sr; b) 176Hf/177Hf

vs.87Sr/86Sr; c) 176Hf/177Hf vs. 143Nd/144Nd. BVS data are from: Kempton et

al. (2000); Breddam (2002); Kokfelt et al. (2006); Halldórsson et al.

(2008); Peate et al. (2010); Manning and Thirwall et al. (2014);

Sigmarsson and Halldórsson (2015); Svavarsdóttir et al. (2017);

Halldórsson et al. (2018). ................................................................................. 48

Figure 30. a) Al2O3 vs. MgO for all published basaltic glasses from BVS. Error bars

are the size of the symbols; b) K2O vs. MgO for all published basaltic

glasses from BVS. Error bars show 2 SD of the standard for the analyses

from this study. ................................................................................................. 49

xii

Figure 31. Primitive mantle normalized multi-element diagram of the average trace

element compositions of samples from this study, Kistufell (Breddam,

2002) and 2014-15 Holuhraun (Halldórsson et al., 2018). ............................. 50

Figure 32. a) Histogram showing distribution of pressure calculated by cpx - melt

barometer of Neave and Putirka (2017). SEE of the barometer 1.4 kbar;

b) Calculated depth vs. Cpx Mg#. SEE of the depth estimate equals 5 km. ..... 51

Figure 33. Pressure vs. temperature calculated by cpx – liq thermobarometer. The

error bars show SEE of the thermobarometer – 1.4 kbar and 45℃. ............... 52

Figure 34. Histograms showing most frequent: a) temperatures calculated by cpx -

melt thermometer of Putirka (2008); b) temperatures calculated by plag -

melt thermometer of Putirka (2008). ................................................................ 52

Figure 35. Major elements fractional crystalization modelling. Calculated line of

descent from Kistufell averaged starting composition are for 0.001, 2 and

4 kbar pressure. a)Al2O3/TiO2 vs. MgO; b) CaO/Al2O3 vs. MgO.................... 54

Figure 36. Fractional crystallization model of La/Yb vs. MgO made with Petrolog 3

software. Averaged composition of Kistufell and the composition of the

most primitive sample were used as starting compositions. ............................ 55

Figure 37. a) Plot of Sr/Sr* vs. MgO for the samples from this study, Kistufell and

Holuhraun. Error bars for the smaples from this study are the size of the

symbols; b) 87Sr/86Sr vs. MgO for the samples from this study, Kistufell and

Holuhraun. Error bars for the smaples from this study show 2SE for 87Sr/86Sr and 2SD for MgO. .............................................................................. 56

Figure 38. Incompatible trace element ratios in the samples from this study, Kistufell

and Holuhraun: a) La/Yb vs. Zr/Y; b) Sm/Yb vs. Nd/Y. The similarity of the

samples from the more evolved group and Holuhraun is evident. Error

bars are the size of the symbos. ........................................................................ 57

Figure 39. a) 87Sr/86Sr vs. Nb/Zr in the samples from this study, Kistufell and

Holuhraun; b) 143Nd/144Nd vs. Nb/Zr of the samples from this study,

Kistufell and Holuhraun Error bars indicate 2SE for the samples from this

study. ................................................................................................................. 58

Figure 40. La/Yb vs. MgO of the samples from this study, Kistufell 2014-15 Holuhraun

and 2014-15 Holuhraun melt inclusions. ......................................................... 59

Figure 41. H2O/Ce ratio vs. MgO. Grey dashed lines show the area of undegassed

MORB samples (150-280), blue dotted lines show the area of undegassed

BVS samples (180 – 253). The average H2O/Ce of undegassed samples

from this study is 219±14. ................................................................................ 61

Figure 42. A histogram showing distribution of MgO wt.% concentration in glasses

from BVS as a function of time. The data come from this study, 2014-15

Holuhraun (Halldórsson et al., 2018), Holocene tephras (Óladóttir et al.,

2011a, b) , Early-Holocene Saxi, Fontur, Brandur cones (Carracciolo et

xiii

al., 2019), Ljósufjöll pillow lavas (Carracciolo et al., 2019), Kistufell

mountain (Breddam, 2002). The samples from both identified here groups

belong to subglacial formations, however the composition of the more

evolved group is clearly distinct from the other units. ..................................... 64

Figure 43. Schematic illustration summarizing the results and observations. ................... 68

xiv

xv

List of Tables

Table 1. Representative average major elements composition in glass (EPMA) ................ 30

Table 2. Representative average trace elements composition in glass (LA-ICP-MS) ....... 33

Table 3. Measured averaged H2O concentration for 14 glass samples along with 1SD .... 39

Table 4. Results od radiogenic isotopes analysis of the basaltic glasses and the

measured standards .......................................................................................... 43

Table 5. Parameters used for paleo-ice thickness estimates and the results of the

calculations. ...................................................................................................... 62

Table S. 1 Sample location, sample type and analytical overview. ..................................... 79

Table S. 2 Major, minor and trace element analyses in whole rock samples from

Vonarskarð valley. ........................................................................................... 81

Table S. 3 Chemical analysis of glass. ................................................................................ 83

Table S. 4 Chemical analysis of plagioclase microcrysts. .................................................. 88

Table S. 5 Chemical analysis of clinopyroxene microcrysts. .............................................. 90

Table S. 6 Chemical analysis of olivine microcrysts. .......................................................... 95

Table S. 7 Trace element analysis of basaltic glasses from this study and the USGS

standards (BCR-2, BHVO-2). ........................................................................... 97

Table S. 8 All measurements of H2O in basaltic glasses from this study. ......................... 113

Table S. 9 Results of clinopyroxene – liquid thermobarometry calculations for the

samples from this study. .................................................................................. 115

Table S. 10 Results of plagioclase – liquid thermobarometry calculations for the

samples from this study. .................................................................................. 119

Table S. 11 Calculated CIPW norms for whole rock and glass samples from this study.

........................................................................................................................ 123

xvi

xvii

Abbreviations

An% - Anorthite, Ca/(Ca+Na+K)

BSE – Back-scatter Electron

BVS – Bárðarbunga Volcanic System

Ca# – Calcium number, Ca/(Ca+Na+K)

cal. kyr BP – calibration kilo/thousand years Before Present

EPMA – Electron Probe Micro Analysis

En% – Enstatite, (Mg/(Mg+Fe+Ca)) x100

EVZ – Eastern Volcanic Zone

Fo% – Fosterite, (Mg/(Mg+Fe)) x 100

FTIR – Fourier Transform Infrared Spectroscopy

HFSE – High Field Strength Elements

HREE – Heavy Rare Earth Elements

ICP-OES – Inductively Coupled Plasma Optical Emission Spectroscopy

KD – Partition Coefficient

LA-ICP-MS – Laser Ablation Inductively Coupled Plasma Mass Spectrometer

LGM – Last Glacial Maximum

LREE – Light Rare Earth Elements

MC-ICP-MS – Multi-Collector Inductively Coupled Plasma Mass-Spectrometer

Mg# - Magnesium number, (Mg/(Mg+Fe)) x 100

MORB – Mid Ocean Ridge Basalt

NVZ – Northern Volcanic Zone

ppm – parts per million

REE – Rare Earth Elements

SD – Standard Deviation

SE – Standard Error

SEE – Standard Error of Estimate

SISZ – South Iceland Seismic Zone

TAS – Total Alkalis vs. Silica

xviii

WVZ – Western Volcanic Zone

wt.% - weight percentage

QFM – Quartz-Fayalite-Magnetite buffer

xix

Acknowledgments

This thesis would not exist without help and support from a number of people, to whom I

am very thankful.

Firstly, I would like to thank my supervisors Sæmundur Ari Halldórsson and Enikő Bali, for

giving me the opportunity to conduct this project and all the insights, help, time and patience

at every stage of it.

I wish to thank the whole team of ING PAN in Kraków. I am especially grateful to Robert

Anczkiewicz for introducing me to isotope geochemistry and allowing me to learn it while

using my rock samples. The experience that I gained during the work at the Geochronology

and Isotope Geochemistry Laboratory enriched my learning outcomes from the MS program

at the University of Iceland and the obtained trace element and radiogenic isotope data added

a lot of value to this thesis. Many thanks to Dariusz Sala and Milena Matyszczak for the time

spent helping me with the sample preparation and MC-ICP-MS and LA-ICP-MS analyses. I

also owe my thanks to Anna Zagórska, Izabela Kocjan and Tomek Siwiecki for help in

preparation of the sample mounts. Not only I have learnt a lot while working with all of you,

but I also had really great time. Dziękuję.

Thanks to Páll Einarsson, Ásta Rut Hjartardóttir and Markus Koleszar for collecting the rock

samples and allowing me to study them in my MS project.

I want to thank Guðmundur Heiðar Guðfinnsson for the assistance with EPMA analysis. I would also like to thank Jóhann Gunnarsson Robin for help in preparing the samples for ICP-OES and

running the analysis.

I am grateful to Albert Þorbergsson, Árni Hjartarson and Ingibjörg Kaldal from ÍSOR for

sharing the shapefiles of the geological map of Central Iceland.

I also want to thank Geoffrey Kiptoo Mibei and Dario Ingi Di Rienzo for assistance with

crushing the samples. Thanks to Eemu Ranta and Alberto Caracciolo for help with sample

preparation for FTIR analysis.

Last but not least, I must thank my family and friends for their endless support and

encouragement during the time of this project.

xx

1

1 Introduction

Basalts are the most common igneous rock type on the Earth’s surface. Because of their

mantle-derived nature, they carry unique information about the chemical composition of the

mantle. Basaltic volcanism occurs most often at mid-ocean ridges, where due to the

divergent plate boundaries and mantle convection, partial melting happens in the response

to decreased pressure in the upwelling mantle. Basaltic volcanism is also common in oceanic

islands, arising from the activity of mantle plumes. In this regard, Iceland due to its unique

position on both, a junction between two tectonic plates and a mantle plume, is one of the

few places where it is possible to study heterogenous components of the mantle and the

interactions between them.

Located in central Iceland and thought to be situated above the mantle plume, the

Bárðarbunga Volcanic System is one of Iceland´s largest and most productive volcanic

systems. Recently, this area has been a subject of a number of studies, mostly because of the

recent 2014-15 Holuhraun event, which was the largest basaltic fissure eruption in Iceland

since Lakagígar from 1783-84. It was one of the best monitored eruptions worldwide, which

resulted in abundant volcanological, geophysical and geochemical investigations. It also

brought attention to the complex geology of the area and the possible connections of BVS

with other volcanic systems, which led to further geochemical characterization and

fingerprinting of products from that area. However, majority of the petrological and

geochemical studies conducted so far, have been focused on the products found on its fissure

swarms, especially the southern Veiðivötn swarm. As a result, the area close to the volcanic

center (the Bárðarbunga volcano) is not very well known.

This study focuses on geochemical characterization of subglacial volcanic formations in the

Vonarskarð valley, located within the eastern and north-eastern flanks of the volcano. This

area, although it has not experienced any recent volcanism, can provide important

information about the past magmatic activity around the Bárðarbunga center. The presence

of well-preserved fast-quenched volcanic glass on the rims of subglacially erupted pillow

lavas, allows us to carry out an extensive study on the actual melt compositions, unaffected

by accumulation of crystals. The major, minor and trace element compositions of the melt

along with the radiogenic isotopes, are crucial to characterize the mantle source and

reconstruct magma evolution. Magma storage conditions, such as depth and temperature are

estimated using the chemical composition of minerals and their relations with the enclosing

melt by geothermobarometers. Rapid cooling of the melt during subglacial eruptions,

enables the analysis of volatiles that are trapped in the volcanic glass. It provides information

about the volatile budget of the magma, the extent of degassing, but it can also be used as a

proxy in reconstructing ice-thickness at the time of eruptions. Comparing these data with

selected and well characterized units from the region, facilitates a better understanding of

the spatial and temporal changes of melt delivery into and within the system, as well

assessment of the controlling processes.

2

3

2 Background

2.1 General overview of Icelandic geology and

geochemistry of Icelandic lavas

2.1.1 Icelandic Geology

Iceland is situated at the junction of two tectonic plates – the North American and the

Eurasian plates, with the axial rift zones transecting it from Reykjanes Ridge (RR) in the

south-west to Kolbensey Ridge (KR) in the north-east. Along the plate boundary there are

three main neovolcanic zones- the Western Volcanic Zone (WVZ), the Eastern Volcanic

Zone (EVZ) and the Northern Volcanic Zone (NVZ). The South Iceland Seismic Zone

(SISZ) is situated between the EVZ and the WVZ (Fig. 1). The zones are connected by the

Mid-Iceland Belt, which stretches across the center of the island. Iceland is the only surface

manifestation of the Mid-Atlantic Ridge, because of the extra magma supplies generated by

a hot upwelling mantle plume. The mantle plume is thought to be currently situated below

the central part of the island (Fig. 1). Due to Iceland´s unique location the Icelandic mantle

and its eruptive products have been the subjects of a significant number of studies in the

fields of petrology, volcanology, geochemistry and geophysics (Thordarson and

Höskuldsson, 2008).

Figure 1. Neovolcanic zones in Iceland (from Thordarson and Höskuldsson, 2008). Dashed

circle indicates the position of the mantle plume. See text for details.

4

Volcanism in Iceland is dominated by basaltic effusive eruptions with a minor component

of intermediate and felsic more explosive events. Intermediate and felsic rocks are usually

related to the central volcanoes, which are generally associated with volcanic systems, and

with them they make fundamental units building the volcanic zones. Currently, there are

about thirty active volcanic systems in Iceland, which consist of either a fissure swarm, a

central volcano, or a combination of both (Thordarson and Höskuldsson, 2008). The

traditional discrimination between the volcanic systems is based on tectonic characteristics,

such as location of faults and grabens, petrographic observations as well as major and trace

elements compositions (e.g., Jakobsson, 1978; Sæmundsson, 1979; Guðmundsson and

Högnadóttir, 2007; Óladóttir, 2011; Hartley and Thordarsson, 2013). Due to the structural

complexities and ice cover, delimiting them and estimating their production rates is not

always straightforward. However, the geochemical variability between the individual

volcanic systems may provide critical information in defining their boundaries (e.g.,

Sigmarsson and Halldórsson, 2015; Svavarsdóttir et al., 2017).

2.1.2 Geochemistry of Icelandic basalts

Icelandic basalts are divided into three distinctive groups based on their geochemistry: (i)

depleted tholeiitic basalts, typical for the rift zone, (ii) enriched transitional alkalic basalts

and (iii) alkalic basalts from the off-rift flanks i.e. Snæfellsjökull Volcanic Zone and

Southern part of Eastern Volcanic Zone (Sigmarsson and Steinthórsson, 2006; Halldórsson

et al., 2016). The mantle under Iceland is heterogenous, with at least two different melting

lithologies prevailing – depleted lherzolite component from the upper mantle and a more

enriched garnet pyroxenite component (Sigmarsson et al., 2008). These both sources

generate diverse primitive melts from which magmas crystallize. The differences in

contribution of these two components can be seen clearly in incompatible trace element and

radiogenic isotopes ratios, which are correlated with each other and become more depleted

towards the plume center in the E-W trend and more enriched in the N-S trend (e.g.,

Sigmarsson et al., 2008; Shorttle and Maclennan, 2011). Less radiogenic values of long-

lived isotopic tracers (e.g., 87Sr/86Sr) and more depleted incompatible trace element

signatures in the central Iceland, where the crust is the thickest, are commonly explained by

higher degree of melting of depleted lherzolite component, which dominates over the more

enriched garnet pyroxenite component (Sigmarsson et al., 2008; Koornneef et al., 2011). The

plume material is thought to carry not only old, recycled oceanic crust but also primordial,

undegassed mantle components, as reflected in high 3He/4He in the volcanic products from

central Iceland (Halldórsson et al., 2016; Harðardóttir et al., 2018). The heterogeneities do

not occur only on a regional scale in Iceland and the adjacent Reykjanes and Kolbensey

ridges, but also on a much smaller scale, sometimes even within a single lava flow, as can

be seen in variable incompatible element and isotopic ratios of primitive magmas, crystals

and melt inclusions trapped in them (Maclennan, 2008).

Changes in the geochemistry of Icelandic basalts are also observed as a function of time.

Rapid deglaciation and subsequent isostatic rebound of Iceland at the end of the Last Glacial

Maximum (LGM) around 10 cal. kyr BP (calibration kilo-years before present) (Ingólfsson

et al., 2010), led to increased eruption rates and formation of large shield lavas with highly

variable chemical compositions. Eruption rates in the first 2-5 thousand years after ice sheet

recession are thought to be 8-50 times higher than nowadays. Geochemical studies of

formations from NVZ (Slater et al., 1998; Maclennan et al., 2002) and WVZ (Sinton et al., 2005; Eason et al., 2015), formed in glacial, early-postglacial and late postglacial times,

5

show that volcanic products erupted shortly after disappearance of the ice load have a greater

range of MgO contents and more depleted trace element compositions, when compared to

magmas generated during glaciation and after early-deglaciation phases. Explanations to

account for this include increased decompression melting of the mantle and/or changed

stress in the crust, which results in the eruption of magma stored in shallow reservoirs

(Guðmundsson, 1986; Maclennan et al., 2002).

2.2 The Last Glacial Maximum (LGM) and the

deglaciation of Iceland

Weichselian, the last major glaciation, started around 100 cal. kyr BP and lasted until 10 cal.

kyr BP. Its most extensive expansion, LGM, took place around 25.3 – 14.9 cal. kyr BP

(Ingólfsson et al., 2010). Although evidence of last glaciation, are preserved both on- and

offshore, estimating the precise extent of the ice sheet is not simple. Terrestrial indications,

such as postglacial geomorphology, height of the table mountains, arrangement of glacial

deposits, and directions of glacial striations provide information about the height and extent

and local thickness of the ice sheet. These, combined with data obtained from offshore

morphology and sediments, in addition to surface temperatures, were used by Hubbard et al.

(2006) to create a model of LGM ice sheet. According to that model, the ice sheet extended

towards the break in Icelandic shelf and had an average thickness of about 940 m, with

1500±500 m in the central part of Iceland (Fig. 2).

Figure 2. Modelled Icelandic ice sheet during the Last Glacial Maximum (from Hubbard et

al., 2006).

Research on marine shorelines and radiocarbon dated marine sediments from various places

on Iceland and its shelf, revealed that since late Weichselian, Iceland underwent many

environmental, climatic and landscape changes (Fig. 3). Deglaciation of Iceland was a

relatively rapid process, which started during Bølling age around 15.4 – 13.9 cal. kyr BP.

Global sea level rise and higher temperatures in the ocean around Iceland had led to collapse

6

of the ice sheet´s marine parts, mostly because of intensive calving (Ingólfsson and

Norðdahl, 2001; Norðdahl et al, 2008; Ingólfsson et al. 2010). Around 13.9 cal. ka BP, about

75% of LGM ice sheet was melted (Fig. 3b). Because of low viscosity of asthenosphere

beneath Iceland, shrinking of the ice cap was followed by a quick glacio-isostatic rebound,

and sea regression in the Northeastern and Southwestern coastal areas (Ingólfsson and

Norðdahl, 2001; Norðdahl and Pétursson, 2005; Norðdahl et al., 2008).

However, at the Pleistocene - Holocene transition, Northern Hemisphere cooled again which

caused return of glacial conditions. From 13.9 – 11 cal. kyr BP ice sheet transgression

occurred which reached its maximum in Younger Dryas around 11 cal. kyr BP (Fig. 3c). The

retreat time of Younger Dryas ice sheet is not exactly known, however the ice sheet extent

in early Preboreal times (11.5 -10.1 cal. kyr BP) is thought to have been smaller, by about

20% compared to its size 800 years before (Fig. 3d). Since then, the ice sheet continued to

melt, and the ultimate evidence of its disintegration is subaerial Þjórsá lava, which erupted

around 8.6 cal. kyr BP (Norðdahl and Pétursson, 2005; Norðdahl et al., 2008).

a) b)

c) d)

Figure 3. The stages of the last glaciation of Iceland. a) Last Glacial Maximum, b) Bølling

Ice Sheet; c) Younger Dryas Ice Sheet; d) Preboreal Ice Sheet. From Norðdahl et al., 2008.

7

3 Geological setting and sample

details

3.1 Bárðarbunga Volcanic System

Bárðarbunga Volcanic System (BVS) is one of the most extensive volcanic systems in

Iceland, commonly considered as a part of the Eastern Volcanic Zone. The central part of it,

the Bárðarbunga volcano, is located in the northwestern edge of Vatnajökull glacier, just

above the presumed mantle plume (Fig. 1, 4). The caldera is about 80 km2 large in size and

it is fully ice covered, rising up to 1200 m above the surrounding bedrock with the highest

point of 2009 m. It also consists of two partially ice covered, mature fissure swarms – a 115

km long Veiðivötn swarm stretching from the volcanic center to the boundary of Torfajökull

Volcanic System in the south and a 55 km long Dyngjuháls swarm in the north (Larsen and

Guðmundsson, 2014) (Fig. 4). It is possible, that the system developed the second central

volcano – Hamarinn, located 20 km SW from Bárðarbunga. The lavas produced in the

eruptions are exclusively basaltic in composition, which is unique as it contrasts with many

other volcanic systems in which more evolved magmas tend to be found near their respective

central volcanoes. According to geophysical data, Bárðarbunga volcano is located above a

dense, gabbro intrusion, with lower density material filling up its caldera. Existence of a

magma chamber is not certain, however if it exists, it could be located at the boundary of

low- and high-density regions. Seismic activity occurs most often at about 5 km depth,

indicating magma movements at this level. Fissure eruptions are possibly fed by lateral flow

from the hypothetical magma chamber of deep-seated reservoir (Óladóttir et al., 2011;

Larsen and Guðmundsson, 2014).

Eruption history of the ice-free parts of the fissure swarms is relatively well constrained

when compared to the history of explosive eruptions which took place in ice covered parts

of Bárðarbunga center and the fissure swarms next to it (Larsen and Guðmundsson, 2014).

The most useful tools to access the rate and extent of basaltic phreatomagmatic eruptions

occurring under ice are tephrochronological studies. A detailed geochemical research on

affinity of the tephra layers surrounding Vatnajökull glacier and the eruption frequency of

the local volcanoes was conducted by Bergrún Óladóttir (2011), which estimated around 350

eruptions in the ice-covered part of BVS during the last 7500 years. There are also few

investigations on pre-Holocene subglacial volcanic formations, i.e. tuyas and hyaloclastites

ridges from BVS. One of them is a detailed study on Kistufell (Fig. 4), a monogenic table

mountain, located NE of Bárðarbunga with the most primitive lavas in the region (Breddam,

2002). Based on morphological observations it is thought to be formed at the end of the

Weichselian glaciation. Another comprehensive study of temporal changes of magma

storage conditions within Veiðivötn fissure swarm was done by Caraciollo et al. (2019).

The lavas and tephra layers in the ice-free SW parts of the BVS fissure swarm are relatively

well examined and mapped. The most voluminous of these, is the 8600 years old, early

postglacial Þjórsá lava, which stretches over an area of 140 km2 (Fig. 4) (Halldórsson et al.,

8

2008). On the other hand, the northern part of the fissure is known to a lesser extent, due to

the hardly accessible terrain and associated challenges related to any fieldwork. The most

recent geochemical studies in the BVS, were conducted by Sigmarsson and Halldórsson

(2015) and Svavarsdóttir et al. (2017). They provided a set of geochemical tracers, including

radiogenic isotopes enabling the likely source provenance of Holocene lavas: Dyngjuháls

and Bárðardalur to be established (Fig. 4).

Holuhraun is the last eruption that occurred within BVS and, during the writing of this thesis,

on Iceland in general. The eruption started in August 2014 and lasted 6 months, ending in

March 2015. It started with a series of earthquakes under the volcanic center, which

subsequently propagated by about 7 km in the SE direction from the caldera, after which

turned by approximately 90o and two weeks later erupted 40 km away in the area which was

thought to be govern by Askja Volcanic System. It was a basaltic fissure eruption, which

produced about 1.44 km3 of lava (Gislason et al., 2015; Geiger et al., 2016; Pedersen et al.,

2017). The dike growth was most likely accompanied by subglacial eruptions as can be

deducted from the presence of ice cauldrons (Sigmundsson et al., 2015; Pedersen et al.,

2017). The lateral magma outflow from the volcanic center into a rift zone caused gradual

caldera collapse, providing a very rare opportunity to observe this phenomenon

(Guðmundsson et al., 2016; Sigmundsson, 2019). Detailed petrological and geochemical

Þ

Bár

Kis

H

Hg

Figure 4. Bárðarbunga Volcanic System. Red circle denotes the studied area. B –

Bárðarbunga volcano; Þ -Þjórsa lava; Bár – Bárðaldalur; Kis – Kistufell; H –

Holuhraun; Hg – Hágöngur. From Larsen and Guðmundsson (2014).

9

studies of the eruption products can be found in Bali et al. (2018), Halldórsson et al. (2018)

and Hartley et al. (2018) to name a few.

3.2 Vonarskarð valley

The investigated lavas come from Vonarskarð area, a valley in central Iceland located by the

northwestern edge of Vatnajökull glacier (Fig. 4, 5). It is located between Tungnafellsjökull

and BVS and it is surrounded by four central volcanoes, out of which three are considered

active (Fridleifsson and Johannesson, 2005). The main part of Tungnafellsjökull volcanic

system is Tungnafellsjökull/Vonarskarð central volcano, which consist of two calderas and

is accompanied by a smaller Hágöngur volcano in the south of the system (Fig. 4). Rhyolites

are common within the Vonarskarð and Hágöngur calderas, which are connected by a swarm

of hyaloclastite ridges (Guðmundsson and Högnadóttir, 2007). The area has not been

volcanically active since the beginning of Holocene, however prominent geothermal activity

occurs around both of the volcanoes. What is more, increased seismic activity was registered

around Tungnafellsjökull glacier in 1996 and 2008-2010, which was suggested to be

associated with magmatic movements at depth and suggested possible connection with the

neighboring Bárðarbunga and Grímsvötn volcanic systems (Björnsdóttir and Einarsson,

2013). Two other surrounding volcanoes are Bárðarbunga in the east and Hamarinn in the

south side of the valley (Fig. 4) (Larsen and Guðmundsson, 2014). Despite the high volcanic

activity in the region, there are no signs of any relatively recent volcanic eruption in

Vonarskarð valley area. The most commonly occurring volcanic products are subglacial

pillow lavas and hyaloclastites, which in several places are covered or surrounded by

younger postglacial lavas of unknown age (Hjartarson et al. 2019).

So far, there have not been many geochemical studies on lavas from Vonarskarð area. Few

samples of postglacial lavas are reported in the papers of Sigmarsson and Halldórsson (2015)

and Svavarsdóttir et al. (2017). Additionally, one sample from Mið-Bálkafell mountain has

been a part of dataset of subglacial glasses included in various projects regarding distribution

of He isotopes (Füri et al., 2010), CO2 (Barry et al., 2014), and paleomagnetic studies

(Cromwell et al., 2015).

3.3 Sample details

A total of 25 samples from several edifices were collected by a team of researchers, involving

Páll Einarsson, Ásta Rut Hjártardóttir and Markus Koleszar during a fieldwork conducted in

August 2017. The main aim of this fieldwork was to describe the morphology and investigate

the origin of the mountains from the area, commonly thought to be tuyas. However, their

small size and lack of lava on their summits questions the relevance of this assumption. A

more detailed study and description of them can be found in the MS thesis of Markus

Koleszar (2019).

The samples were taken mostly from pillow lavas from eight mountains located within the

Vonarskarð valley and several postglacial lavas of unknown age. All the sample locations

are presented on Figure 5. Most of the pillow lava samples have relatively well-preserved

glass on the rims, however cracks and microcrysts are usually present.

10

Figure 5. Simple geological map of the studied area with sample locations. Modified from

Hjartarson et al. (2019).

VON-23

11

Almost all (23 out of 25) samples were crushed to rock powders and analyzed for major and

minor elements composition of whole rock. The targeted samples included 19 subglacial

basalts and 4 postglacial lavas. Overview of the samples can be found in Table S.1 and their

general petrographic features are described in the section 5.1.

Subglacial samples: The samples VON-1 and VON-2 (Fig. 5) come from an unnamed

mountain in the west side of the valley and were analyzed with each of the analytical

techniques (Table S.1). The samples VON-3, VON-4 and VON-12 are from an edifice on

the east to the previous one, however no good quality glass was found in them. VON-7 and

VON-8 were sampled at Mið-Bálkafell mountain, at its slope and summit respectively. Both

of them were analyzed with all of the applied techniques, except for the sample VON-7,

which has not been included in radiogenic isotope analysis. The next examined mountain

was Innsta-Bálkafell, which is located in the north of Mið-Bálkafell (Fig. 5), with the

samples VON-9, VON-10 and VON-11 being analyzed using all the applied methods. The

samples VON-14 – VON-18 were collected from the edifices in the south of the valley:

Fremsta-Bálkafell and Nefsteinn. VON-18 was analyzed with all the methods and the

samples VON-14, VON-16 and VON-17 were not analyzed for radiogenic isotopes

composition. The samples VON-20 – VON 25 were collected from the mountains in the

north of Vonarskarð: Dyngjufell (VON-20, VON-21), Tindafell (VON-22, VON-23) and

altered volcanic formation called Egg (VON-24 and VON-25). The glass in the pillow rims

of the samples from the first two edifices was very well preserved and was analyzed with

every technique. However, the rock specimens from the Egg were almost completely altered,

sample VON-25 had few spots of relatively fresh black glass, which was analyzed for whole

rock composition.

Postglacial lavas: The samples VON-5 and VON-13 were taken from postglacial lavas from

various places within the valley. The samples VON-6 and VON-19 come from two

postglacial cones in the middle and in the west of the Vonarskarð valley (Fig. 5). All of the

postglacial lava samples were analyzed for the major and minor elements in the whole rock.

12

13

4 Methods

4.1 Electron Probe Micro Analyzer (EPMA)

Major and minor elements in the glass, plagioclase, clinopyroxene and olivine phases were

analyzed at the University of Iceland using the JEOL JXA-8230 Electron Probe Micro

Analyzer which is equipped by 5 wavelength dispersive spectrometers (WDS) and LaB6

electron emitter.

Fresh glass chunks (5-8 per sample) from 14 glass rims of pillow lavas were hand-picked in

order to select the least altered and mineralized shards. Subsequently, they were mounted in

epoxy and hand polished until their surface was flat and smooth. The last step preceding the

EPMA analysis was carbon coating the samples with Cressington 208C high vacuum carbon

coater.

For examination of glass composition several points (8-10) were selected in each sample.

The beam diameter was 10 μm in these analyses. Accelerating voltage was set at 15keV and

the probe current measured on the Faraday cups before the analysis was 10nA.

Measurements were calibrated with natural and synthetic standards provided by Astimex

and the Smithsonian Institute. Besides A99 basaltic glass was analyzed as secondary

standard before and after analyses.

In plagioclase and olivine minerals 5 points were selected in each sample in order to check

if the samples are homogenous. Because of sector zoning in clinopyroxene microcrysts the

number of points was increased to 7-9 in each sample to increase the chance that composition

of at least one of them will be in equilibrium with melt for geothermobarometric calculations.

Focused beam was used during the examination of all the mineral phases and the acceleration

voltage was 15keV. The probe current measured on the Faraday cups before the analysis of

clinopyroxene and olivine was 20nA and 10nA for plagioclase. Secondary standards were

Plag Ast, OLSPM, Cr-Augite for plagioclase, olivine and clinopyroxene, respectively.

The raw data were processed in CITZAF (Armstrong, 1991) and the measurements with

totals below or above the range of 98-101 wt.% were excluded. For more details on the

method see Halldórsson et al. (2018) Supplementary material. The results are laid out in

Appendix C and the averaged glass compositions are in Table 1 in section 5.2.

14

4.2 Fourier Transform Infrared Spectroscopy

(FTIR)

A subset of 14 glasses for pillow lava samples were analyzed for H2O and CO2 content by

Fourier Transform Infrared Spectroscopy (FTIR). The analysis took place at the University

of Iceland using Bruker IFS66 FTIR set to collect mid-infrared spectrum.

Fresh glass chunks from glassy pillow rims were carefully hand-picked, with preference of

choosing only the ones which were not significantly mineralized or altered. Subsequently,

each of the samples was glued by superglue to a glass sample holder and, after drying up,

polished until achieving flat and smooth surface. The next step was to polish the other side

of the samples, and in order to do that the samples needed to be detached from the sample

holder and glued from the already polished side. Again, they were polished until the surface

was smooth and the sample was transparent. Finally, the glue holding the samples was

dissolved in acetone than washed in ethanol and the grains were ready for the measurement.

Prior to analysis, the thickness of each examined glass fragment was measured using

Mitutoyo digital micrometer, with a precision of 0.001 mm.

FTIR spectroscopy collects absorption spectra of the molecules and chemical species from

the sample, by sending IR radiation through it. Absorbance is subsequently converted into

vibrational and rotational energy of the molecules, and results in obtaining a signal on the

detector representing the sample’s spectrum (Devine et al., 1995, Shishkina et al., 2014).

Each absorbance peak in the spectrum is unique for a given chemical species or molecule

which allows its identification (Fig. 6). Determining its concentration in the glass shard can

be calculated by Beer-Lambert’s equation (Eq.1).

Eq.1

𝐶 =100 ∗ 𝐴 ∗ 𝑀

∈∗ 𝑙 ∗ 𝑝

C - concentration

A - absorbance

M - molecular weight of the species in g mol-1

∈- absorption coefficient in L mol-1 L - thickness of the sample in cm

P - density of the sample in kg m-3

Absorption coefficients are taken from Shishkina et al. (2014) and glass densities were

calculated based on Silver et al. (1990) from the compositions analyzed by EPMA.

To obtain the representative average of volatile composition, 2-3 glass chips and 4-8 points

for each of the samples were selected for measurement. Background measurement on KBr

sample holder was taken every hour. IR spectrum was collected from 400 to 4000 cm-1 with

a resolution of 4 cm-1. In order to determine the H2O content, the absorbance of the broad

H2O+OH band at 3500 cm-1 was collected whereas for CO2-concentrations the carbonate

bands in the 1370–1630 cm-1 region was monitored. However, no CO2 bands were observed

which means that the CO2-concentration in these glasses was under detection limit (

15

For more detailed description of the procedure see Haldeman (2018). All the results are

shown in Table S.8 and the averages are shown in Table 3 in section 5.4

Figure 6. Examples of CO2 and H2O-rich samples in mafic melts from Shishkina et al. (2014)

showing the peak positions of carbonate and water bands.

Figure 7. a) Representative mid-infrared spectra of two analyses from this study; b)

Carbonate bands cannot be seen in any of the analyses in this study, thus the concentration

of CO2 is below the detection limit (< 30 ppm); c) peak position of OH+H2O band.

0

0.5

1

1.5

2

2.5

3

3.5

4

1200 1700 2200 2700 3200 3700

Ab

sorb

ance

un

its

Wavenumber (cm-1)

00.5

11.5

22.5

33.5

4

1250 1450 1650

Ab

sorb

ance

un

its

Wavenumber (cm-1)

b)

0

1

2

3

4

2500 3000 3500 4000

Ab

sorb

ance

un

its

Wavenumber (cm-1)

c)

a)

16

4.3 Inductively Coupled Plasma – Optical

Emission Spectroscopy (ICP-OES)

Whole rock composition of 23 samples was examined for major, minor and selected trace

elements. The analyses were done at the Institute of Earth Sciences, University of Iceland

using ICP-OES SPECTRO-CIROS by Johann Gunnarsson Robin. Approximately 50 g of the

fresh parts of the rock samples were crushed in Retch BB100 Mangan jaw-crusher and

subsequently sieved in order to obtain around 1-2 g of 2 mm grain size. Then, the grains

were grinded to around 200 MESH powder in an agate mortar. The next step was mixing of

a 100 mg of the rock powder with 200 mg of LiBO2 in a carbon crucible holder which helps

in enhancing melting of the sample into glass drops. The melting procedure was done in a

high temperature oven at 1000°C for 30 minutes. Standards A-THO, B-ALK and B-THO,

BHVO-1, JA-2, BIR-1 and K1919 were prepared in the same manner as the samples.

A day before the analysis the prepared glass drops were dissolved in the mixture of 5 vol%

HNO3 - 1.33 vol% HCl - 1.33 vol% and saturated H2C2O4 after which put in a rotating

sample holder, until complete dissolution.

International USGS standards K1919, BIR-1 and JA-2 and in-house standards A-THO, B-

THO, B-ALK were used for calibration of the instrument and error calculation. More details

on the analytical procedure can be found in Halldórsson et al. (2018).

The results are presented in Table S.2.

4.4 Laser Ablation Inductively Coupled Plasma

Mass Spectrometer (LA-ICP-MS)

The same sample set of 14 pillow lava glasses analyzed with EPMA for major and minor

elements in glass and the mineral phases was analyzed for the trace elements in glass by LA-

ICP-MS. The analyses were done at the Kraków Research Centre, Institute of Geological

Sciences, Polish Academy of Sciences, with 193 nm excimer (ArF) laser Resolution M50

(Resonetics) coupled with quadrupole ICP-MS 235 XSeriesII (Thermoelectron).

Before the analyses, the mounts were cleaned using 1N nitric acid, acetone and ultra-pure

water. The alignment of the instrument and mass calibration was performed using the NIST

SRM 612 reference glass (Jochum et al., 2011), by inspecting 238U signal and reducing

ThO+/Th+ ratio (kept below 0.5%). The analyses were conducted with 10 Hz pulse

frequency, spot diameter of 100 μm, laser output energy of 100 mJ. Single analyses consisted

of 60 s of background integration followed by 45-s sample integration with the laser firing

and then a 10-s delay to wash out the previous sample. Ablation occurred in pure helium

atmosphere with small addition of nitrogen (up to .005 L/min) in order to enhance sensitivity

of ICP-MS. Analyses were normalized to the international glass standard NIST 612, while

standards BCR-2 and BHVO-2 were used as secondary standards for error determination.

Silica content was used as an internal standard. For more detailed description of the method

see Anczkiewicz et al. (2012). Data reduction was carried out with the GLITTER® software

17

package developed by the Macquarie Research Ltd. (Griffin et al. 2008). To obtain the most

representative average glass composition 8-10 points were analyzed in each sample.

All the results are presented in Table S.7 and averaged sample compositions are presented

in Table 2 in section 5.2.2.

4.5 Multi Collector Inductively Coupled Plasma -

Mass Spectrometer (MC-ICP-MS)

Analyses of strontium, hafnium, lutetium, neodymium, and samarium isotope composition

of 10 glass samples were also performed at the Cracow Research Centre, Institute of

Geological Sciences, Polish Academy of Sciences.

Measuring isotopic compositions of certain elements requires purification of the samples

from any elements and chemical species, that could interfere with the inspected isotopes

during the analysis. Thus, sample preparation starts with multi-stage digestion procedure in

aggressive acids in order to break the chemical bonds in the samples, which is followed by

chemical clean-up using ion exchange columns.

Approximately 100 mg of clean, fresh glass chips were hand-picked under binocular

microscope and cleaned in ultrasonic bath in acetone, ethanol and MQH2O for 30 minutes

each, plentifully rinsed with water in between. Subsequently, the samples were crushed in

an agate mortar and transferred in one drop of water into clean Teflon beakers produced by

Savillex. The powdered samples prepared for Hf, Lu, Sm, Nd analysis were additionally

mixed with a drop of Lu-Hf and Sm-Nd spikes. The samples were dissolved into a mixture

of 3:1 HF:HNO3 and left for two days on 120ºC hot plate. This step prevents fluorite

formation and destroys silicate bonds. In the next step the samples were treated with 3 mL

of concentrated HNO3 and left on 120°C hot plate until evaporation in order to break down

any residual fluorides. It was followed by adding 6M HCL:0.1% HF to launch dissolution

processes for the next 48 hours, after which they were assumed to be completely dissolved

and in equilibrium with spikes, and subsequently evaporated. As a last step of digestion, they

were treated with 3 mL of 6M HCl and left on a hot plate until complete dryness. USGS rock

standard BCR-2 was treated in the same manner. The detailed description of the procedure

is presented in Anczkiewicz et al. (2004).

Hf, Lu (+Yb) and LREE were first separated on standard cation exchange columns using

Bio-Rad AG50W-X8 resin (200 – 400 mesh size). The used procedure was based on Patchett

and Tatsumoto (1981) with modifications from Anczkiewicz et al. (2004). Hf was separated

from the rest of HFSE (High Field Strength Elements) on Ln-spec R column with procedure

based on Lee et al. (1999). Lu and Light Rare Earth Elements (LREE - Sm, Nd) were cleaned

up on small Ln-spec R column according to modified procedure from Pin and Santos-

Zaldegui (1997). Sr isotopes were separated on a Sr-spec resin (Eichrom) with procedures

mostly based on Thirlwall et al. (2004) and Peryt and Anczkiewicz (2014).

Measurements were carried out using the MC-ICP-MS Neptune by Thermo Scientific in a

static mode with Aridus II as a nebulizing system. The measurement protocol for MC-ICP-

MS follows modified procedures of Thirlwall and Anczkiewicz (2004).

18

Reference standards SRM 987, JMC 475, JNd-1 were used during the analyses of Sr, Hf,

and Nd respectively, in order to monitor the performance of the instrument throughout the

measurement. Because of mass fractionation of the isotopes during the analysis each of the

ratios needs to be corrected for mass bias. This correction can be done by using observed

fractionation between two non-radiogenic isotopes and applying exponential law (Nier 1938;

Russell et al. 1978; Thirlwall and Anczkiewicz, 2004). Hence, strontium instrumental mass

bias was corrected using 86Sr/88Sr ratio of 0.1194, neodymium mass bias was corrected using 146Nd/144Nd ratio of 0.7219 and hafnium mass bias was corrected using 179Hf/177Hf ratio of

0.7325 (Russell et al., 1978). Reproducibility of 87Sr/86Sr ratio of SRM 987 reference

standard was 0.710257±8 (n=5). The obtained results were corrected for Rb interference and

normalized to the recommended SRM 987 87Sr/86Sr ratio of 0.710248 (McArthur 1994;

McArthur et al. 2001). Reproducibility of 144Nd/143Nd ratio of JNd-1 reference standard was

0.512100±3 (n=4). JMC 475 of 177Hf/178Hf yielded 0.7282169±4 (n=4) over the period of

analyses. 176Lu/177Hf errors was 0.5%. All results are presented in Table 4 with the errors

given as two standard error (2SE). Notably, values obtained for USGS standard BCR-2, are

in good agreement with recommended values.

4.6 Thermobarometric calculations

Magma storage conditions, such as pressure and temperature can be estimated using mineral-

melt thermometers and barometers, which use the assumption that the crystal growth occurs

at near-equilibrium conditions (Hammer et al., 2015). These geothermometers and

geobarometers are based on chemical equilibriums with large difference in enthropy (ΔSr)

for temperature estimation or volume (ΔV) for pressure estimation between the reactants and

products (Putirka, 2008). Clinopyroxene-melt (cpx-melt) is one of the most widely applied

geobarometers in mafic melts, which uses pressure sensitive partitioning of jadeite into

clinopyroxene (Eq.2), during which Na and Al take the sites normally occupied by Ca, Fe,

Mg and Si (e.g. Putirka, 2008; Hammer et al., 2015; Neave and Putirka, 2017; Neave et al.,

2019). This exchange equilibrium has the largest ΔV (-23.5 cm3 mol -1) among all the

molecules substituting in clinopyroxene, thus it is a perfect tool in estimating pressure of

mineral-melt equilibration.

Eq.2.

𝑁𝑎𝑂0.5𝑙𝑖𝑞 + 𝐴𝑙𝑂1.5 𝑙𝑖𝑞 + 2𝑆𝑖𝑂2 𝑙𝑖𝑞 = 𝑁𝑎𝐴𝑙𝑆𝑖2𝑂6 𝑐𝑝𝑥

NaAlSi2O6

cpx – jadeite

The newest cpx-melt barometer was established by Neave and Putrirka (2017), based on the

barometer from Putirka (2008). Correct calibration of a barometer is essential in obtaining

the most accurate results. However, earlier barometers calibrated on anhydrous tholeiites

from Icelandic rift zones in the pressures ranging from 1 atm to 5 kbar, and closed-capsule

experiments with pressure in a range from 1 atm to 10 kbar, were significantly

overestimating calculated pressures. The newest barometer of Neave and Putirka (2017)

(Eq.3) was calibrated on experimental data from closed capsule with the pressures in a range

from 1 atm to 20 kbar, which is more relevant to natural magma storage conditions. The

calibration was also performed on both hydrous and anhydrous compositions, and was tested

19

on natural tholeiite samples from Iceland (Neave et al., 2019) including samples from the

Holuhraun eruption from Halldórsson et.al (2018). The standard error of estimate (SEE)

associated with the new barometer is ± 1.4 kbar.

Eq. 3

P(𝑘𝑏𝑎𝑟) = −26.27 + 39.16 𝑇(𝐾)

104 𝑇(𝐾) ln(

𝑋𝐽𝑑𝐶𝑝𝑥

𝑋𝑁𝑎𝑂0.5𝑙𝑖𝑞

𝑋𝐴𝑙𝑂1.5𝑙𝑖𝑞

(𝑋𝑆𝑖𝑂2

𝑙𝑖𝑞)2

) − 4.22 ln (𝑋𝐷𝑖𝐻𝑑𝐶𝑝𝑥 ) +

78.43 𝑋𝐴𝑙𝑂1.5𝑙𝑖𝑞 + 393.81 (𝑋𝑁𝑎𝑂0.5

𝑙𝑖𝑞 𝑋𝐾𝑂0.5𝑙𝑖𝑞 )2

Melt-mineral equilibration temperatures were estimated with Equation 33 from Putrika

(2008) (Eq. 4), which is so far the most precise cpx – melt thermometer (SEE ±45 ºC).

Eq.4.

104

𝑇 (𝐾) (𝐾) = 7.53 − 0.14 ln (

𝑋𝐽𝑑𝐶𝑝𝑥

𝑋𝐶𝑎𝑂𝑙𝑖𝑞

𝑋𝐹𝑚𝑙𝑖𝑞

𝑋𝐷𝑖𝐻𝑑𝐶𝑝𝑥

𝑋𝑁𝑎𝑙𝑖𝑞

𝑋𝐴𝑙𝑙𝑖𝑞) + 0.07(H2Oliq) − 14.9(𝑋𝐶𝑎𝑂

𝑙𝑖𝑞 𝑋𝑆𝑖𝑂2𝑙𝑖𝑞 ) − 0.08

ln (𝑋𝑇𝑖𝑂2𝑙𝑖𝑞

) − 3.62(𝑋𝑁𝑎𝑂0.5𝑙𝑖𝑞

𝑋𝐾𝑂0.5𝑙𝑖𝑞

) − 1.1(𝑀𝑔# 𝑙𝑖𝑞) − 0.18 ln(𝑋𝐸𝑛𝐹𝑠𝐶𝑝𝑥

) − 0.027 𝑃(𝑘𝑏𝑎𝑟)

Additionally, the temperatures of the samples were estimated using plagioclase-melt

thermometer from Putirka (2008) – Equation 24a with SEE ±36ºC (Eq. 5).

Eq.5.

104

𝑇 (𝐾) (𝐾) = 6.4706 + 0.3128 𝑙𝑛 (

𝑋𝐴𝑛𝑝𝑙𝑎𝑔

𝑋𝐶𝑎𝑂𝑙𝑖𝑞

(𝑋𝑆𝑖𝑂2

𝑙𝑖𝑞)2(𝑋

𝑆𝑖𝑂2

𝑙𝑖𝑞)2

) − 8.103 (𝑋𝑆𝑖𝑂2𝑙𝑖𝑞 ) +4.872 (𝑋𝐾𝑂0.5

𝑙𝑖𝑞 ) +

1.5346 (𝑋𝐴𝑏𝑝𝑙𝑎𝑔

) 2 + 8.661 (𝑋𝑆𝑖𝑂2𝑙𝑖𝑞 )2− 3.341 × 10−2 (𝑃(𝑘𝑏𝑎𝑟)) + 0.18047 (𝐻2𝑂 𝑙𝑖𝑞)

In all of the above equations:

T – temperature (K)

P – pressure (kbar)

𝑋𝑖𝑗 – a mole of a cation fraction of i in a phase j

Liq – liquid/melt

Cpx – clinopyroxene

Plag – plagioclase

DiHd – diopside-hedenbergite, Fm – enstatite-ferrosilite (Fm = Fe+Mn+Mg), Jd – jadeite

Ab – albite, An - anorthite

Thermobarometers require input of both melt and mineral compositions in order to estimate

pressure and/or temperature. However, before applying thermobarometric calculations,

several criteria need to be fulfilled. The most important is ensuring whether the mineral –

melt pair is in equilibrium. In cpx-melt thermobarometer this is done by checking if

equilibrium constant KD (Fe-Mg) Cpx-Liq is in a range of 0.27 ± 0.03. If the KD values are

below 0.24 it means that the melt is too Mg-rich and if they are above 0.3 it means that it is

too Fe-rich to be in equilibrium with clinopyroxene (Putirka et al., 2003, 2008). However,

because Fe-Mg exchange equilibrium does not consider e.g. Na-Al or Ca-Na exchange

equilibriums, additional equilibrium check is required. This can be done by comparing

20

measured and predicted mineral components, such as diopside-hedenbergite (DiHd),

enstatite-ferrosillite (EnFs) and Ca-Tschermak (CaTs) in melt – mineral pairs (Putirka, 2008;

Mollo et al., 2013; Neave and Putirka, 2017; Neave et al., 2019). In the new cpx-melt

barometer they were assumed to be in equilibrium if measured DiHd, EnFs and CaTs were

within ±0.06, ±0.05 and ±0.03 of predicted components respectively (e.g. Neave et al.,

2019). In plagioclase-liquid thermometer equilibrium check is more straightforward and

requires insuring that the equilibrium constant KD(Ab-An) pl-liq is in a range 0.28±0.11 for

T>1050ºC and 0.1±0.05 for T

21

nucleation and therefore slow exsolution of the volatiles during rapid magma ascent,

which is rarely a case in basaltic magmas, as it is usually fast enough to keep up with

the magma ascent.

• Sample homogeneity. The presence of bubbles or minerals can change H2O content

in a measured spot, however this is an issue mainly in intermediate and felsic lavas,

where the diffusion rate of volatiles is slower than in basalts, due to lower

temperatures of these magmas.

• Absence of post-quenching hydration. Hydration would usually manifest itself

with a presence of molecular H2O at 1630 cm-1 peak in infrared spectroscopy

analysis.

• Lack of post-quenching movement. Redeposition of the glasses from their

quenching location would provide inaccurate information about the ice thickness in

relation to the sampling elevation.

Once the samples are checked for all the mentioned criteria, their volatile saturation pressure

can be calculated. It can be done by VolatileCalc excel spreadsheet created by Newman and

Lowenstern (2002), which is the most commonly applied method to calculate volatile

saturation pressures and can be used for either basaltic or rhyolitic samples. Except for water

concentration it requires an input of SiO2 and CO2 content of the melt and temperature. The

temperature uncertainties do not affect the calculated pressure. It is in contrast to CO2

concentrations, where even a small addition causes a strong decrease of H2O saturation

value, due to reduction of its fugacity in vapor phase, which leads to an increase of the

estimated quenching pressures (Fig. 8) (Tuffen et al., 2010).

Figure 8. Influence of CO2 concentration on volatile saturation pressure of H2O in basaltic

magmas. Calculated by VolatileCalc (Newman and Lowenstern, 2002).

The calculated volatile saturation pressures provide estimations of the ice/water thickness

under which the samples quenched, which can be calculated by converting the hydrostatic

pressure equation (Eq. 6). However, the interpretation of their meaning is not

straightforward. While calculating the ice-thickness, the kind and weight of the overlying

material needs to be taken into consideration. Subglacially erupted magma may quench

0

10

20

30

40

50

60

70

80

0.00 0.20 0.40 0.60 0.80

Pre

ssu

re (

bar

)

H2O wt.%

20 ppm CO2

0 ppm CO2

22

either under ice, meltwater, previously erupted lava or bedrock, and the combination of all,

which affects the estimated thickness of ice as they all differ in density.

Eq. 6

𝑃 = 𝜌𝑔ℎ

Where:

P – pressure (Pa)

𝜌– ice/water density kg/m3

g – gravity 9.8 m/s2

h – overlying ice/water thickness (m)

In this study 14 glass samples with analyzed H2O concentration were used in paleo-ice

thickness estimations described above. The results of the calculations and their evaluation

can be seen in Table 5 in the section 6.4.2.

23

5 Results

5.1 Petrography

Hand specimens of the majority of collected samples have a 1-3 mm thick glassy pillow rim,

grey, mildly vesicular, cryptocrystalline and/or hypocrystaline groundmass and usually

porphyric texture. One of the most striking features are large abundant, plagioclase

phenocrysts, occurring as either single and usually tabular phenocrysts which reach up to 4-

5 mm in the longest axis or larger rounded mineral aggregates. The second most abundant

phase are smaller phenocrysts of olivine, with maximum size of about 1 mm in diameter,

with subhedral and euhedral forms. Samples VON-14 – VON-18, however, are slightly

different from the rest of pillow lava samples as no large phenocrysts are observed in them

and their texture is more homogenous, when compared to the more porphyric samples. Hand

specimens VON-5, VON-6, VON-13 and VON-19 are aphyric, vesicular lava samples, with

hyaline surfaces. Hand specimens of the samples VON-24 and VON-25 show signs of

prominent alteration with abundant olivine phenocrysts in white, fine groundmass with few

spots of black, fresh-looking glass

The glassy pillow rims, which were inspected in the electron microprobe by back scattered

electron (BSE) images, contain significant amount of microcrysts (defined here as crystals

below 500 μm) of plagioclase, clinopyroxene and olivine, with plagioclase being the most

abundant phase (Fig. 9). The minerals occur individually or in aggregates, forming ophitic-

subophitic texture (Fig. 10a). Microcrysts are less abundant in samples VON: 14, 16. 17, 18,

which also do not contain any microcrysts of olivine. Plagioclase and olivine microcrysts do

not show any zoning, contrary to clinopyroxene, in which sector zoning is apparent (Fig.10

a, b). Small, melt inclusions occur in some minerals (Fig. 10a). Analyzed glass shards have

very few small vesicles, usually between around 50-100 μm (Fig. 9).

24

Figure 9. BSE image of a glass shard with dispersed microcrysts of olivine, plagioclase

and clinopyroxene. Few vesicles are present.

Figure 10. a) BSE image of a glass shard with abundant microcrysts forming an aggregate

of clinopyroxenes, olivines and plagioclases, with subophitic texture. Note small melt

inclusion entrapped in a plagioclase crystal in the bottom right of the glass shard.

Clinopyroxene crystals show various types of zoning. White arrow indicates a clinopyroxene

crystal with normal zonation; b) Clinopyroxene with hour-glass zonation and ingrowing

plagioclase.

Clinopyroxene

Plagioclase

Olivine

Clinopyroxene

Plagioclase

a) b)

25

5.2 Major and trace element systematics in

whole rock and glass

The results of the analyses of major and selected trace elements in whole rock are reported

in Appendix B Table S.2. The results of major element analyses in glass are listed in

Appendix C table S.3 and averaged glass compositions are presented in Table 1. The results

of trace element analyses in glass are shown in Appendix D Table S.7 and the averaged

concentrations are presented in Table 2.

5.2.1 Major elements

Rock Classification

According to Total Alkali versus Silica classification diagram (TAS) examined samples are

classified as basalts belonging to subalkaline magma series based on Rollinsson (1993)

(Fig.11). On AFM ternary diagram, samples plot on the tholeiitic rock series field

determined by Irvin and Baragar (1971) (Fig.12). Based on CIPW (Cross-Iddings- Pirsson-

Washington) weight percentage of normative mineral calculated following the method of

Johanssen (1931) most of the samples are classified as silica saturated olivine tholeiites with

normative hypersthene (Table S.11) according to Yoder and Tilley classification (1962).

Figure 13 presents projections in the tholeiitic Ol-Pl-Di-Q basalt tetrahedron after Yoder and

Tilley (1962). Normative molar mineral proportions used for projections were calculated

following the procedure of Walker et al. (1979). Few of the samples seem to be silica

undersaturated, however it can be an effect of assumed Fe2+/(Fe2++Fe3+) = 0.15 ratio (which

according to Brooks (1976) is the average ratio for reduced basaltic magmas) used in

calculating the normative proportion of silica or analytical uncertainty. The samples follow

clear trends, where the most primitive samples gradually advance towards more evolved

compositions. Projection from diopside and plagioclase show that plagioclase was the main

crystalizing phase, followed by clinopyroxene, which is in accordance with the general

observations in Icelandic tholeiitic basalts.

26

Figure 11. TAS classification diagram of analyzed samples based on their whole rock and

glass composition. The purple line indicates the boundary between alkaline (top) and

subalkaline (bottom) rock series based on Rollinson (1993).

Figure 12. AFM classification diagram present