University of Iowa
Iowa Research Online
Theses and Dissertations
Summer 2015
Modeling the magmatic plumbing system beneath
an off-rift volcanic deposit on Iceland, using textural
analyses and geothermobarometry
David Burney
University of Iowa
Copyright 2015 David Burney
This thesis is available at Iowa Research Online: http://ir.uiowa.edu/etd/1831
Recommended Citation
Burney, David. "Modeling the magmatic plumbing system beneath an off-rift volcanic deposit on Iceland, using textural analyses and
geothermobarometry." MS (Master of Science) thesis, University of Iowa, 2015.
http://ir.uiowa.edu/etd/1831.
Follow this and additional works at: http://ir.uiowa.edu/etd
Part of the Geology Commons
MODELING THE MAGMATIC PLUMBING SYSTEM BENEATH AN OFF-RIFT
VOLCANIC DEPOSIT ON ICELAND, USING TEXTURAL ANALYSES AND
GEOTHERMOBAROMETRY
by
David Burney
A thesis submitted in partial fulfillment of the
requirements for the Master of Science
degree in Geoscience
in the Graduate College of
The University of Iowa
August 2015
Thesis Supervisor: Professor David Peate
Copyright by
DAVID BURNEY
2015
All Rights Reserved
Graduate College
The University of Iowa
Iowa City, Iowa
CERTIFICATE OF APPROVAL
MASTER'S THESIS
This is to certify that the Master's thesis of
David Burney
has been approved by the Examining Committee for the
thesis requirement for the Master of Science degree in
Geoscience at the August 2015 graduation.
Thesis Committee:
David Peate, Thesis Supervisor
Morten Riishuus
Mark Reagan
To Laura and Hazel. Without you this never would have happened.
ii
ABSTRACT
The emplacement of tholeiitic magmas along two NE-SW trending rift zones is the
dominant mechanism of crustal accretion on Iceland. Small volumes of transitional to alkaline
magmas erupt through older crust in several off-rift settings, including the Snæfellsnes Peninsula
in western Iceland where the basement is formed by 6-8 Ma flood basalts. In this study I
investigated how these off-rift magmatic plumbing systems compare to those in the main rift
zones, given the significant differences in crustal structure and degree of crustal extension,
through application of quantitative textural analysis and mineral geothermobarometry. My focus
is Vatnafell, a sub-glacial eruptive unit (414 ± 11 ka) at the western end of the off-rift Ljósufjöll
volcanic system in the Snæfellsnes volcanic zone. Samples are highly phyric (~14%
phenocrysts), with large phenocrysts (1-12 mm) of clinopyroxene, olivine, and plagioclase.
Crystal size distributions for olivine and clinopyroxene both show kinked profiles, indicating two
distinct populations. Glomerocrysts in which large clinopyroxene oikocrysts enclose smaller
rounded olivine chadacrysts are common, and a small horizon strongly enriched in large (> 5
mm) olivine and clinopyroxene crystals was found near the base of the unit. These observations
suggest incorporation of wehrlitic and olivine gabbroic cumulates by the host magma. Analyses
show a bimodal composition for clinopyroxene (cores: mg# 83-88; rims/groundmass mg# 7277), and calculations suggest crystallization of cores at or near the Moho in the deep crust (~25
km). Olivine rim diffusion profiles & CSD slopes have been used to estimate residence times
and ascent rates of the crystalline cargo, and indicate rapid ascent (weeks-months) soon after the
incorporation of the wehrlitic cumulate. These data have shown that the magmatic system
beneath Vatnafell, with its lack of shallow magma reservoirs, and Moho depth crystallization
iii
followed by a rapid ascent and eruption, is unlike magmatic systems present at the main rift
zones.
iv
PUBLIC ABSTRACT
Magmatism on Iceland is dominated by the eruption of large volumes of tholeiitic basalts
at the three major rift zones that represent the locations of plate separation and crustal growth.
Minor amounts of transitional to alkaline lavas are also erupted in off-axis zones, notably along
the Snaefellsnes Peninsula which is >100 km from the rifts. An important question addressed, is
whether the magmatic plumbing systems are significantly different in the off-axis zones
compared to main rifts, given the different tectonic environment (e.g. minimal crustal extension,
thicker lithosphere, lower geothermal gradient, lower magmatic productivity, and a different
crustal structure). A sub-glacial unit (Vatnafell) on Snaefellsnes was chosen for study in detail,
because it contains porphyritic samples (CPX, OL, & PLAG), and fresh hyaloclastite glass.
These allowed for detailed reconstruction of magma ascent paths and magma chamber depths
using mineral based geothermobarometry (CPX) and pressure estimates of glass in equilibrium
with CPX, OL, and PLAG. The resulting calculations revealed a crystallization depth of ~25 km
which corresponds with the base of the crust in this region of Iceland. A lack of shallow magma
reservoirs suggests an incorporation of a crystalline cargo at the base of the crust, followed by a
rapid ascent and subsequent sub-glacial eruption.
v
TABLE OF CONTENTS
LIST OF FIGURES ....................................................................................................................................viii
CHAPTER 1: BACKGROUND AND RESEARCH GOALS................................................................................. 1
Background ......................................................................................................................................... 1
Goals ................................................................................................................................................... 4
CHAPTER 2: FIELD WORK & SAMPLE DESCRIPTIONS ................................................................................ 5
Tectonic Setting of the Field Area ........................................................................................................ 5
Sub-Glacial Eruptions .......................................................................................................................... 6
Field Area ........................................................................................................................................... 7
CHAPTER 3: PETROGRAPHY & TEXTURAL ANALYSES.............................................................................. 12
Petrography ....................................................................................................................................... 12
Vatnafell ........................................................................................................................................ 12
Hyaloclastite Breccia ..................................................................................................................... 14
Basal Basalt Flows ......................................................................................................................... 15
Crystal Size Distributions of Vatnafell ............................................................................................... 15
CHAPTER 4: WHOLE ROCK & MINERAL COMPOSITIONS ........................................................................ 21
Whole Rock Geochemistry ................................................................................................................ 21
Loss on Ignition ............................................................................................................................. 21
Major Element Analysis by ICP-OES ............................................................................................ 22
Trace Element Analysis by ICP-MS ............................................................................................... 23
Whole Rock Geochemistry Results and Discussion ........................................................................ 23
Mineral Compositions ....................................................................................................................... 31
Olivine .......................................................................................................................................... 32
Plagioclase .................................................................................................................................... 33
Clinopyroxene ............................................................................................................................... 35
Glass ............................................................................................................................................. 36
CHAPTER 5: DISCUSSION & CONCLUSIONS............................................................................................ 39
Regional compositions ....................................................................................................................... 39
Crystal Size Distributions .................................................................................................................. 41
Temperatures & Depths of Crystallization ......................................................................................... 43
Thermometers ............................................................................................................................... 44
Barometers.................................................................................................................................... 45
vi
Estimating the Time Scales of the Magmatic Processes ...................................................................... 51
Using CSD’s .................................................................................................................................... 51
Using Stoke’s Law .......................................................................................................................... 52
Using Diffusion Profiles .................................................................................................................. 53
Time Scales Discussion ..................................................................................................................... 55
CONCLUSIONS ....................................................................................................................................... 56
REFERENCES .......................................................................................................................................... 60
APPENDIX .............................................................................................................................................. 63
vii
LIST OF FIGURES
Figure 1.1: Map of Iceland...........................................................................................................................1
Figure 1.2: Schematic modified from Kelley & Barton (2008) showing the possible morphologies of
Icelandic magmatic plumbing systems……………. …………………………………………2
Figure 1.3: The material that makes up the bulk of Vatnafell…...…….…………………………………. 4
Figure 2.1: Simplified geologic map of Snaefellsnes Peninsula showing the locations of the three
main volcanic systems………………………………………….. …………………………....5
Figure 2.2: Vatnafell’s table-top shape, and mossy, scree-covered slopes....….. ………………………...7
Figure 2.3: The internal structure of Vatnafell showing the colonnade and entablature top, underlain
by a thick hyaloclastite unit……………..……………………………………... …………….8
Figure 2.4: A view of the colonnade and entablature structure, surrounded by mossy scree,.. …………...9
Figure 2.5: Hyaloclastite breccia showing the fine grained, and coarse grained textures present…….....10
Figure 2.6: Pillow structures in the hyaloclastite…………….………………………………………... ...10
Figure 2.7: Location of outcrops around Vatnafell……………….……………………………………...11
Figure 2.8: The crystal rich zone found at the base of the colonnade on the NE side of Vatnafell...........11
Figure 3.1: BSE image of an OL chadacryst surrounded by a CPX oikocryst………..……………... ….13
Figure 3.2: BSE image of the groundmass of Vatnafell………….………………………………... ……14
Figure 3.3: Hyaloclastite breccia………………………………….…………………………………….. 14
Figure 3.4: The image of a cut and polished slab, and the resulting traced binary image of CPX... …….16
Figure 3.5: The process of isolating mineral phases from a WDS analysis……………………………...18
Figure 3.6: CSD’s for the three main mineral phases present in Vatnafell, as well as their modal
percentage …………………………………………………………………………………...19
Figure 4.1: Total alkali vs silica diagram showing Vatnafell and surrounding outcrops, and glass
analyses from the hyaloclastite breccia………….….…………………………………... ….24
Figure 4.2: Whole rock MgO vs TiO2 showing multiple parental melt compositions……………... …...25
Figure 4.3: Whole rock La/Sm vs. Nb/Zr of mamjor western Iceland geologic regions……… …….......25
Figure 4.4: Rare Earth element patterns of whole rock analyses taken from Vatnafell and
surrounding outcrops normalized to a chondritic meteorite (McDonough, 1995)……........27
Figure 4.5: Whole rock geochemical data plots for Vatnafell and surrounding outcrops……… ……....27
Figure 4.6: Whole rock data plots from Vatnafell and surrounding outcrops compared to
Snaefellsnes Peninsula……... ……………………………………………………………….29
viii
Figure 4.7: Trace element ratios of Vatnafell and the surrounding outcrops gathered from whole
rock analyses………………...................................................................................................30
Figure 4.8: Fo% (Mg/Mg + Fe) within olivine populations in Vatnafell, and the cumulate
zone………………….……………………………………....................................................33
Figure 4.9: BSE image of a glomerocryst OL showing no zonation when in contact with the CPX
oikocrystsw, and diffuse zonation with the groundmass……………………… ……............33
Figure 4.10: Anorthite % (Ca/Ca + Na) vs FeO wt% showing the different plagioclase populations
present in Vatnafell..……………………………………………..………………………….34
Figure 4.11: An% within the plagioclase populations in Vatnafell..……………………….. ……………34
Figure 4.12: CPX in conatact with groundmass…...………………..………………………… …………35
Figure 4.13: Mg# (Mg/Mg + Fe) vs TiO2 wt% showing the different CPX populations present in
Vatnafell……………………………………………………………………………………..35
Figure 4.14: Mg# (Mg/Mg + Fe) of the different clinopyroxene textures present in Vatnafell…………. 36
Figure 4.15: Glass populations within the hyaloclastite breccia.…………………………………………36
Figure 4.16: MgO vs TiO2 within the glass populations………………………………………………….37
Figure 4.17: Microscopic images of hyaloclastite glass representing populations one and two
(upper) and three (lower) taken in plain polarized (left) and cross polarized light
(right). ……………………………………………………………………………………….38
Figure 4.18: Microscopic images of the glassy pockets within CPX in Vatnafell taken in plain
polarized (left) and cross polarized (right) light…… ……………………………………….38
Figure 5.1: The total alkalis vs silica relationship between the WVZ, Snaefellsnes Peninsula, and
Vatnafell……………………………………………………………………………………..39
Figure 5.2: Trace element ratios show the difference in the depth and degree of melting occurring
at the WVZ, Snaefellsnes Peninsula, and Vatnafell..……………………………… ……….39
Figure 5.3: A hypothetical magmatic system and the type of CSD’s it would produce…… …………....41
Figure 5.4: Temperatures calculated using Loucks equation, and the corresponding Mg# of the
glomerocrysts.……………….…………………………………............................................44
Figure 5.5: The binary pressure calculations on high MgO hyaloclastite glass…… ………....................45
Figure 5.6: The systematic offset of ~3 kbar between the methods of Nimis & Ulmer (1998),
and Putirka (2008)...……………………… ………...............................................................46
Figure 5.7: Geophysical calculations estimating the depth of the Moho…………………………….......48
Figure 5.8: This map shows the depth of kKm to the 1200°C isotherm based on modelling of
geothermal data from bore holes……………………… ……................................................49
ix
Figure 5.9: The depths of crystallization for the CPX present at Vatnafell as calculated using
the methods of Nimis & Ulmer (1998), and Putirka (2008)………………….......................50
Figure 5.10: Anatomy of a CSD plot…………………………………………………………………. …..51
Figure 5.11: The beset diffusion profile measured across the zonation within an olivine
chadacryst. …………………………………………………………………………………..54
Figure A.1: The full field area showing all sample locations…………………………………… ……….63
x
CHAPTER 1: BACKGROUND AND RESEARCH GOALS
Background
Iceland is the on-shore continuation of the Mid-Atlantic Ridge, which is the result of the
North American and European tectonic plates rifting apart. Elevated temperatures in the mantle
cause excess melting, leading to an abnormally thick oceanic crust which allows for the subaerial
exposure of Iceland, as well as the production of high volumes of tholeiitic magmas that are
erupted along the rift zones (Jakobsson et al. 2008). The rifting is not isolated to a single linear
region, but instead branches into three
distinct volcanic zones; the Northern
Volcanic Zone (NVZ), the Western
Volcanic Zone (WVZ), and the Eastern
Volcanic Zone (EVZ) (Figure 1.1). The
NVZ and WVZ are responsible for the
majority of crustal accretion on Iceland,
Figure 1.1: Map of Iceland showing the three main rift
zones (NVZ, WVZ, and EVZ) as well as regions of offrift magmatism.
erupting tholeiitic magmas in response to
decompression melting of the
asthenospheric mantle beneath the actively rifting lithosphere (Jakobsson et al. 2008). The EVZ
is the result of rifting actively propagating to the south into older crust, and erupts transitional to
alkali magmas (Jakobsson et al. 2008). Published geophysical and geochemical data indicate
that magmatic plumbing systems beneath the rift zones generally show magma storage at the
Moho, as well as in one or more smaller magma chambers in the mid to shallow crust (e.g.
Figure 1.2. Kelley & Barton, 2008).
1
Small volumes of volcanism on Iceland occur in regions away from the main rift zones
and are referred to as off-rift volcanic zones (e.g. Jakobsson et al. 2008). These off-rift zones are
related to distinct tectonic conditions (e.g. Peate et al. 2010). Snaefell & Oraefajokull are central
volcanoes close to the rift flank of the NVZ near the inferred plume axis, and above a potential
continental microplate (Torsvik et. al. 2015), and Vestmannaeyjar are the offshore islands at the
tip of the actively propagating EVZ to the SW (Jakobsson et al. 2008). Snaefellsnes Peninsula in
the west is quite distinct being oriented perpendicularly to, and located over 100 km away from,
the WVZ (Figure 1.1). Snaefellsnes Peninsula is not related to rift propagation or influence of
the mantle plume, and is experiencing no significant extensional forces (Figure 1.1) (Sigurdsson,
1970). Sigurdsson (1970) was the first to describe the orientation of faults on the Snaefellsnes
Peninsula as being the result of unequal rifting rates between the NVZ and the WVZ. This
creates a right-lateral transform motion
that runs the length of the peninsula, and is
believed to be the primary reason for
volcanism along Snaefellsnes Peninsula
(Sigurdsson, 1970). Similar transcurrent
Figure 1.2: Schematic modified from Kelley & Barton
(2007) showing the possible morphologies of Icelandic
magmatic plumbing systems. Hyaloclastites in the rift
regions offer the opportunity for shallow crustal magma
chambers (right), while the crustal structure of
Snaefellsnes Peninsula may only have sub-MOHO
chambers with little to no shallow crustal storage (left).
volcanic zones can be found along midocean spreading ridges, often manifesting
as a linear chain of seamounts running
perpendicular to the active spreading ridge.
Unlike mid-ocean ridge transcurrent volcanic zones, the Snaefellsnes Peninsula is over 100 km
away from the nearest spreading zone and has large silicic central volcanoes. The relationship
2
between this tectonic environment and magmatic evolution along Snaefellsnes Peninsula has
never been fully understood.
The Snaefellsnes region is tectonically distinct from the main rift, in that melting is not
being driven by plate separation, and the thicker lithosphere limits the extent of decompression
melting. Compared to the rift zones, there is a lower geothermal gradient as indicated by the
absence of high-T geothermal fields, and lower magmatic productivity (eg. Martin &
Sigmarsson, 2007; Thordarson & Höskuldsson, 2008). The crust beneath Snaefellsnes is
dominated by massive flood basalts of the Tertiary Basalt Formation (>7 Ma). This contrasts
with the rift zones that contain extensive Plio-Pleistocene (<3 Ma) hyaloclastite deposits in the
upper 5 km or so of the crust (Bindeman et al. 2009). These differences in proportion of
hyaloclastite, crustal thickness, and crustal age influence how magma ascends and stalls within
the lithosphere. For example, the thicker lithosphere and lower geothermal gradient at
Snaefellsnes may lead to significant sub-Moho crystallization, while the absence of thick
sequences of structurally weak hyaloclastite deposits in the upper crust may lead to no or lesser
development of mid to shallow crust magma chambers compared to the main rifts. Although
there are many published studies on the depths of crystallization and magma chambers beneath
the main rift zones, little work has been done on the off-axis volcanic regions.
3
For this project, I have selected a
small monogenetic sub-glacial volcanic unit
in the central part of Snaefellsnes Peninsula
for detailed study named Vatnafell. The
porphyritic and glassy nature of the Vatnafell
samples makes them great candidates for
2 cm
mineral and glass geothermobarometry
(Figure 1.3), using methods that have
Figure 1.3: The material that makes up the bulk of
Vatnafell is very crystal rich with the phases of CPX,
Ol, and PLAG.
already proven to be successful on the main
rift lavas (e.g. Kelley & Barton, 2008; Maclennan et al. 2001).
Goals
The primary goals of this research project were:
1.) To map the detailed field relationships of units in the Vatnafell area to establish the full
extent of Vatnafell, and use this information to constrain emplacement mechanisms, temporal
relationships, and collect samples of all main lithological units to determine if they are all
cogenetic.
2.) To conduct textural analyses to identify individual mineral populations that reveal the
magmatic crystallization and migration habits in the lithosphere prior to eruption.
3.) To quantify the compositions of these unique mineral populations and use those data to
determine temperatures and depths of crystallization in order to map out the plumbing system
beneath Vatnafell.
4
CHAPTER 2: FIELD WORK & SAMPLE DESCRIPTIONS
Tectonic Setting of the Field Area
The Snaefellsnes off-rift volcanic zone is composed of Plio-Pleistocene and younger
volcanic units lying unconformably on older Neogene (>7 Ma) lava flows. The Neogene flows
are the result of a paleo-rift that jumped eastward around 7 Ma to form the NVZ & WVZ
(Hardarson, 1993). The younger Plio-Pleistocene & Quaternary volcanics are subdivided into
three E-W trending volcanic systems (e.g. Jakobsson et al. 2008), Ljosufjoll in the east, the
smaller Lysuskard in the middle, and the peninsula culminates with the Snaefellsjokull volcanic
system in the west which is dominated by the Snaefellsjokull central volcano (Figure 2.1). The
Ljosufjoll volcanic system is larger and more dominant than the Lysuskard system. Beginning
~50 km west of the WVZ, it is ~80 km long, and tapers from ~15 km wide to ~1-3 km wide at
Figure 2.1: Simplified geologic map of Snaefellsnes Peninsula showing the locations of the three main
volcanic systems. The star is the location of Vatnafell, the focus of this study.
5
the western end (Figure 2.1). A central volcano, Ljosufjoll, has erupted more evolved rhyolitic
& trachytic magmas (Flude et al. 2008), but a majority of the eruptions are monogenetic low
volume, alkaline basalts (Hardarson, 1993). The eruptive patterns of Lysuskard, the smallest
volcanic system, mimic those of Ljosufjoll with monogenetic, alkaline basalts being the
dominant product, with some rhyolites erupted centrally. All post-glacial eruptions (< 12,000
years ago) in the Ljosufjoll volcanic system consist of monogenetic transitional to alkaline basalt
cinder cones and lava flows. Although both zone in the west. These systems erupt transitional
to alkaline basalts, with an overall increase of alkalinity from east to west along Snaefellsnes
Peninsula (Hardarson 1993, Sigmarsson 1992, Jakobsson et al. 2008). Lysuskard is ~30 km
long, and although it has an E-W trend in the east, it dog-legs north to have a NW-SE trend in the
west. Lysuskard shows compositional similiarities to the Snaefellsjokull system and is not as
clearly defined as the Ljosufjoll system. Although Lysuskard and Ljosufjoll are linear in shape,
they do not show the same large-scale fissure style eruptions that are present in the rift zones
(Hardarson, 1993). Some eruptions may begin as smaller fissure eruptions, but quickly reduce to
localized scoria cones (Hardarson, 1993).
Sub-Glacial Eruptions
The steep sided, flat topped shape of Vatnafell suggests it has a sub-glacial origin.
Widespread glaciation in Iceland began 3 million years ago with over 20 glacial periods recorded
in the geologic record. At least 10 of these glacial periods have been found in the geologic
record on Snaefellsnes Peninsula (Hardarson, 1993). These glacial and interglacial periods can
be seen in the morphology of the volcanic deposits. Subaerial eruptions are unrestricted by ice,
and therefore are able to form shield or cone-like morphologies. Subglacial eruptions must form
6
accommodation space by melting the confining glacial ice, which results in distinctive
hyaloclastite sequences and steep sided table mountains (Jakobsson & Gudmundsson, 2008).
Field Area
Vatnafell is centrally located along the Snaefellsnes Peninsula (64.91429°N,
22.90840°W), between two small lakes (Hraunsfjardarvatn to the west, & Baularvallavatn to the
250 m
Figure 2.2: Vatnafell’s table-top shape, and mossy, scree-covered slopes.
east) on the western edge of the Ljosufjoll volcanic zone (Figure 2.7). The location of Vatnafell,
~20 km away from the silicic centers for the two neighboring volcanic zones of Ljosufjoll and
Lysuskard, limits influences from nearby well developed polygenetic central volcanoes which
may have more robust, long lived plumbing systems (e.g. Kokfelt et al. 2009; Flude et al. 2008).
The only published work on Vatnafell is by Guillou et al. (2010) who dated the main colonnade
unit (see below) using the unspiked K-Ar method, and revealed an eruption date of 414 ± 11 ka
which corresponds to the end of the MIS12 glaciation, transitioning into the MIS11
interglaciation.
7
The main edifice of Vatnafell is a round table mountain (~1 km in diameter), sometimes
called a tuya, and this morphology is consistent with a subglacial eruption (Jakobsson &
Gudmundsson, 2008). It is a basalt formation consisting of a basal hyaloclastite breccia, overlain
250 m
Figure 2.3: The internal structure of Vatnafell showing the colonnade and entablature top, underlain by a
thick hyaloclastite unit and basal basalt flows.
by a single subaerial lava flow that forms the upper half of this 200m tall steep sided deposit and
consists of a colonnade and entablature morphology (Figures 2.2 & 2.3). The columnar jointing
is relatively even in width, although with a slight taper from the bottom (~3/4 m) to the top (~1/2
m) of the colonnade and entablature sequence (Figure 2.3). The colonnade is not a flat, uniform
sequence within Vatnafell, but it is present in some areas and obscured by scree in others (Figure
2.4), while it undulates across the paleo-topography present during eruption (Figures 2.2 & 2.3).
Observations across the relatively flat top of Vatnafell show that the entablature columns are
oriented radially outwards in all directions, and there is an absence of any overlying subaerial
units. Guillou et al. (2010) initially interpreted the colonnade as an intrusion into the
surrounding hyaloclastite, however the presence of a previously undescribed entablature unit, as
well as the absence of any overlying units does not necessarily exclude the possibility of
Vatnafell being an extrusive feature. Similar colonnade and entablature lava units have been
described in Antarctica from sub-glacial environments, and have been interpreted to be sub-
8
glacial eruptions beneath relatively thin (~150 – 200 m thick) ice (e.g. Smellie 2008). While
these Antarctic eruptions are often capped with a sub-aerial lava allowing for ice thickness
estimates to be made, the lack of a sub-aerial contact at Vatnafell means that a definitive
intrusive/extrusive interpretation cannot be made. A monogenetic, mostly confined, subglacial
eruption is the likely eruptive scenario in order to create the geologic sequence present at
Vatnafell, but any geologic evidence of complete intrusive confinement is no longer present and
so that distinction cannot be made with any certainty.
The columnar units at Vatnafell are
composed of phyric, three phase basalt
consisting of clinopyroxene, olivine, and
plagioclase. A thin (~ ½ m) yet distinct
crystal rich zone can be found in one
location near the base of the colonnade
unit (Figure 2.8). This zone is highly
crystalline (~40-50% phenocrysts), and
Figure 2.4: A view of the colonnade and entablature
structure, surrounded by mossy scree. Dr. Morten
Riishuus (in the blue coat) for scale.
comprised almost entirely of CPX & OL
with PLAG only occurring as microlites within the groundmass. The largest difference is the
crystal density (~60% in the crystal rich zone compared to ~15% in the colonnade), with the
petrographic descriptions and CPX & OL compositions being the same as the columnar unit
above (see Chapter 3).
9
The base of this columnar section
and cumulate zone has a sharp contact
with a thick (~15m) hyaloclastite breccia
unit. This hyaloclastite breccia unit is
mostly covered by talus from the columnar
unit above, but outcrops in several
locations around the entirety of the main
edifice (Figure 2.5). The largest exposure
Figure 2.5: Hyaloclastite breccia showing the fine
grained, and coarse grained textures present. Dr. Ingrid
Peate for scale.
of the hyaloclastite breccia is southwest of
Vatnafell, and can be seen along ~ ½ km
of the lake shore just above the dam. Other exposures are limited to intermittent outcrops just
east and northwest of Vatnafell. This breccia contains coarse sand to cobble sized basaltic clasts
supported by a light to dark brown hyaloclastite glass matrix. The base of this unit shows a
distinct glassy contact with many pillow structures present (Figure 2.6).
The hyaloclastite unit is cross-cut by a nearly vertical dike (182°S, 87° Dip), and underlain by
basaltic lava flows. Although most of these basaltic
flows outcrop in different areas from one another,
the best exposures are located within the stream
south of Vatnafell (Figure 2.7). Field observations
show that these sub-horizontal flows are ~ 4m thick,
20 cm
lay stratigraphically on top of one another, and are
Figure 2.6: Pillow structures in the
hyaloclastite.
separated by a highly vesiculated horizon and a
10
Figure 2.7: Location of outcrops around Vatnafell. Small Figure 2.8: The crystal rich zone found at
the base of the colonnade on the NE side of
stars indicate whole rock sample locations.
Vatnafell.
sharp contact. The other basalt exposures around Vatnafell are small isolated topographic
undulations to the north, and to the south of Vatnafell (Figure 2.7). All surrounding basalt
outcrops (including the dike) are extremely aphyric, with ~2% phenocrysts of plagioclase and
olivine. Exact sample locations, and location descriptions can be found in the appendix (Table
A.1).
11
CHAPTER 3: PETROGRAPHY & TEXTURAL ANALYSES
Petrography
Vatnafell
The colonnade and entablature units are petrographically identical and show no abrupt
field contacts, just a change in column width and orientation, so they will be collectively called
“Vatnafell”. When discussing the crystalline cargo in Vatnafell such terms as phenocryst,
xenocryst, and antecryst will not be used as their geologic definitions rely on genetic
relationships between the crystals and the host melt. Instead, the term “macrocryst” will be
applied to any crystal visible with the naked eye (>1 mm), the term “groundmass” to crystals < 1
mm, and the term “glomerocryst” to any multiphase macrocrysts (following terminology in
Neave et al. 2014). Abreviations will be used for the three mineral phases present;
clinopyroxene = CPX, olivine = OL, and plagioclase = PLAG. The highly phyric Vatnafell
basalt (~15% macrocrysts) contains three phases with a majority (~65%) of the macrocrysts from
~1mm – 15mm.
All of the mineral phases >8 mm (and some >1 mm) in size are comprised entirely of
multi-phase glomerocrysts. These glomerocrysts are predominantly CPX oikocrysts enclosing
smaller rounded OL chadacrysts. The CPX oikocrysts all show zonation in compositionally
sensitive back scatter electron (BSE) images (Figure 3.1), as well as disequilibrium textures such
as embayments and reaction rims when in contact with the groundmass. The rounded OL
chadacrysts show no zonation when in contact with the surrounding CPX oikocryst, yet show
zonation in BSE images when adjacent to groundmass (Figure 3.1). A small proportion (<10%)
12
of the glomerocryst population is
comprised of PLAG, OL, and ± CPX.
CPX
When CPX is present in these PLAGOL
bearing glomerocrysts, it is still the
dominant encapsulating oikocryst, with
the PLAG chadacrysts showing the same
CPX
characteristics as the OL chadacrysts
(rounded, and in textural equilibrium with
Figure 3.1: BSE image of an OL Chadacryst (center)
surrounded by a CPX oikocryst. Note the zonation
present in both mineral phases when in contact with the
groundmass (diffuse in the OL, & sharp in the CPX).
the CPX but not the groundmass). If
CPX is not present in the PLAG-bearing
glomerocrysts, then the PLAG takes on a more subhedral, tabular shape and is evenly mixed
(~50/50%) with anhedral to subhedral OL.
Of the macrocryst populations (1 mm - 15 mm), about 35% are single phase phenocrysts.
These phenocrysts are CPX (50%), OL (40%), and PLAG (10%). Most of the CPX phenocrysts
(~90%) show the same textural patterns as the CPX present in the glomerocrysts. They are
anhedral, and show disequilibrium textures with thin growth rims. A small proportion (~10%) of
the CPX phenocrysts show a more subhedral crystalline shape. The OL phenocrysts show a
similar pattern as the CPX phenocrysts with most being rounded and in disequilibrium. The
largest difference is that the rounded OL phenocrysts do not show late stage growth rims. The
pattern continues with the OL, where ~10% have a more subhedral crystalline shape.
13
Details of the groundmass are best seen in
BSE images. The groundmass crystals are fine
grained (< 100 µm) and consist of tabular PLAG
(50%), blocky CPX (30%), blocky OL (15%), and
interstitial Fe-Ti oxides (5%). There is no glass
present, and no preferred orientation of any
Figure 3.2: BSE image of the groundmass of
Vatnafell.
mineral phase (Figure 3.2).
Hyaloclastite Breccia
The hyaloclastite breccia that underlies Vatnafell consists of subangular to angular basalt
clasts supported by a glassy matrix. The clasts range in size from ~4 mm to ~10 cm, and range
in vesicularity from massive to scoracious. The clasts are generally aphanitic with a slight
variation in the modal percent of the sparse OL and PLAG
macrocrysts present, and a lack of CPX macrocrysts
(Figure 3.3). The matrix is almost entirely glass, oriented
in rounded blobs ranging in color from clear to dark
brown, and shows evidence of post emplacement
amygdulization. The lightest colored glass shows the
most vesiculation, the least crystallization of microlitic
PLAG, and no crystallization within the vesicles. The
darkest glass shows the least vesiculation, the most
microlitic PLAG growth, and the most crystallization
Figure 3.3: Hyaloclastite breccia.
within the vesicles (Figures 4.7-4.8).
14
Basal Basalt Flows
The basalt flows that are located stratigraphically beneath Vatnafell, the hyaloclastite
breccia, and a dike that cross cuts the hyaloclastite, are more aphanitic than Vatnafell, containing
only about ~2-4 % phenocrysts of OL, PLAG, and CPX. Field relationships are not readily
apparent as these basalt flows are only found in isolated outcrops and contain no significant
petrographic differences aside from a small outcrop of a vesiculated horizon within the stream
south of Vatnafell. Samples were taken from various outcrops and whole rock major and minor
elemental analyses were used to determine the exact field relationships. Whole-Rock
geochemistry has been able to split the four outcrops into five chemically distinct basalt flows,
and establish the relationship between the feeder dike and the surrounding basalt flows (see Ch 4,
Whole Rock Geochemistry).
Crystal Size Distributions of Vatnafell
Textural investigations of the mineral phases within Vatnafell included petrographic
observations using optical microscopy, SEM, and EMP, but the main technique was
quantification of crystal size distribution (CSDs). The size and distribution of crystals present in
a lava can reveal many insights into the crystallization processes of a magma body prior to
eruption. CSDs are based on the principle that for a single mineral phase there is a linear,
negative correlation between crystal size and the natural log of the number of crystals present
within individual size intervals (e.g. Higgins, 2006; Marsh, 1998). The slope of this line can be
used to calculate residence times, growth rates, as well as relationships between nucleation and
growth rate (Cashman & Marsh, 1988). Deviations from a single linear trend are an indicator of
open system relationships occurring within the magma body prior to eruption. Arguably the
15
most powerful aspect of CSDs is that they are achievable through relatively inexpensive
methods, and can provide valuable textural insight and recognition of distinct crystal
populations, thereby guiding future compositional analyses.
The highly phyric nature, as well as the large size of the phenocrysts of Vatnafell in this
study, means that thin section analysis alone would not provide a statistically significant number
of crystals for construction of CSDs (typically > 200) and so a series of polished rock slabs were
prepared (Morgan & Jerram, 2006). One columnar piece (Sample name: Plug, collected on the
NW edge of Vatnafell. See “colonnade collection site” on fig 1.10) was cut into four slabs
A
roughly 5 cm thick, each with a roughly 26 x 12
cm surface area. These slabs were then polished
using a vibrating flat-lap, and loose silicon
carbide and aluminum carbide grits, until a high
polish was attained. The slabs were then
scanned using a flatbed scanner at a high
B
resolution (4800 dpi) to produce a JPEG image.
This image was cropped to the largest rectangle
possible to ensure accurate modal percentage
calculations (Figure 3.4 A), and color adjusted
so that each mineral phase was clearly defined.
Individual mineral phases were then traced by
Figure 3.4: (A) The image of a cut and polished
slab, and (B) the resulting traced binary image of
CPX.
hand in Adobe Illustrator, and isolated to form a
binary image (Figure 3.4 B). Each single phase
binary image was then analyzed using the “analyze particles” function in ImageJ
16
(http://imagej.nih.gov/ij/). The resulting data set was a pixel count showing the overall area of
each crystal, as well as the major and minor axis of an ellipse fit to each crystal. The pixels were
then converted to millimeters using a scale etched into the slabs prior to scanning. These data
represent the 2D size and shape of each mineral grain present in the image. To convert the 2D
data into a 3D representation of the crystals present, the crystal shape of each phase must be
taken into consideration (e.g. rounded, oval, tabular, etc) and can be quantified using the
spreadsheet CSD-Slice which applies a numerical value between 1 and 10 to three different axes
named X,Y, and Z (Morgan & Jerram, 2006). CSD Slice found the following values for
Vatnafells three main mineral phases (X, Y, and Z axes respectively); CPX: 1.0, 1.4, & 2.0.
PLAG: 1.0, 1.0, & 1.0. OL: 1.0, 1.2, & 1.6. The CSD Corrections 1.4 program
(http://www.uqac.ca/mhiggins/csdcorrections.html) was used to convert the data into a 3D
representation of each crystal size using statistical methods outlined by Higgins (2000).
17
Figure 3.5: The process of isolating mineral phases from a WDS analysis. This scan was done on
one 2 x 2 mm section of groundmass.
Visually tracing crystals on the large slabs provided data that represented the CSDs of
crystals larger than 1mm (macrocrysts). Below that size (< 1mm), image resolution was
insufficient to readily distinguish crystals which meant that it was difficult to capture the
complete population present. To quantify the sub-mm crystal populations, element maps were
obtained on a thin section using wavelength dispersive spectrography (WDS) done on the
University of Iowa’s electron microprobe (EMP) as this revealed the distribution of each mineral
phase present down to the micron scale (groundmass). Using the EMP’s software, individual
mineral phases were isolated by utilizing their unique chemical compositions: CPX =
Ca(Mg,Fe)(Si,Al)2O6, Olivine = (Mg,Fe)2SiO4, and PLAG = (Ca,Na)(Si,Al)AlSi2O8. Using all
18
five of the EMP’s spectrometers, multiple elements can be measured during one scan and
correlated to one another across the region scanned. By adding or subtracting specific elemental
images, individual mineral phases can be isolated from others. For example, both CPX and
PLAG have Ca, but PLAG has far more Al than CPX so by subtracting Al from the Ca map, the
PLAG will disappear from the image with the Al (Figure 3.5). OL can be isolated by subtracting
Ca from Mg, and since PLAG is the aluminous phase the Al map is clear enough to isolate that
mineral phase through color recognition and thresholding. The resulting images contained many
situations where multiple crystals were touching which, if analyzed in ImageJ, would
inaccurately measure these crystals as one large crystal instead of multiple small crystals, and
skew the final CSDs. This was solved by visually inspecting the binary images and by using a 1
pixel wide eraser to manually separate any touching crystals. The resulting binary image was
Size (mm)
Size (mm)
Size (mm)
Size (mm)
Figure 3.6: CSD’s for the three main mineral phases present in Vatnafell, as well as their modal percentage.
19
then processed using the same methods as with the macrocrysts (ImageJ & CSDcorrections).
1707 CPX crystals were measured for CSDs; 490 crystals were analyzed for the macrocryst size
fraction, and 1217 were analyzed for the groundmass fraction. 1062 OL crystals were measured;
860 for the macrocrysts and 202 for the groundmass, and 1656 total PLAG crystals were
measured; 98 and 1558 for the macrocryst and groundmass fraction respectively. The resulting
CSDs on can be seen in Fig 3.6, and all three mineral phases show a distinct change in slope near
the 1 mm size range. The three distinct slopes between the PLAG populations may be an artifact
of a small sample size, as there are very few PLAG macrocrysts in these rocks (n = 98). The gap
in the CPX trends may also be an artifact of the analytical technique used, as ~1 mm is near the
lower size limit of the resolution needed for macrocryst tracing and the upper size limit of thin
section scanning capabilities.
These data can also be used to determine the abundance of macrocrysts (~14%) and the
relative modal proportions of each mineral phase in the macrocryst populations and in the
groundmass populations. The raw calculation of the modal volume between the three mineral
phases can be seen in figure 3.6. It should be noted that these calculations do not take into
account Fe-Ti oxides, or vesicles. The ratio of OL:CPX:PLAG in the groundmass is 14:24:62,
which is similar to the ratios expected from eutectic, low-pressure crystallization of OL-CPXPLAG saturated basalts (Yang et al. 1996; Passmore et al. 2012). The macrocryst ratio is
33:55:12 (OL:CPX:PLAG), which does not reflect shallow eutectic crystallization, but a shift to
a CPX & OL dominated composition
20
CHAPTER 4: WHOLE ROCK & MINERAL COMPOSITIONS
Whole Rock Geochemistry
Whole rock analyses on 12 samples were done using a combination of ICP-MS at the
University of Iowa’s Department of Earth & Environmental Sciences, and ICP-OES at the
University of Iowa’s Department of Chemistry following the methods described in Reagan et al.
(2013). The samples were cleaned of all organic and weathered material, crushed into pea-sized
pieces using a steel jaw crusher, and powdered using a ceramic ball mill.
Loss on Ignition
Loss on ignition (LOI) was done in order to determine the moisture content, as well as
the H2O content of the samples present from both magmatic and post magmatic processes.
Moisture content is measured by weighing the powdered samples, drying them in a 100°C oven,
then weighing them again. Any drop in weight is considered moisture that has been absorbed
from the atmosphere.
Once dried, the samples are ready for LOI which measures weight change at 1050°C.
This is primarily due to release of water hosted in mineral phases and weight gain through
oxidation of Fe2+ to Fe3+ in the furnace. High values of LOI (i.e. several wt%) can indicate the
presence of clay minerals as a result of post-magmatic alteration of the samples. Studies have
shown that the erupted basalts along Snaefellsnes Peninsula have a very low magmatic water
content (0.1 wt%, Nichols et al. 2002), therefore any significant LOI values are an indicator of
post eruption alteration within these samples. For all intents and purposes, all LOI
measurements were zero (a range of -0.56 to 1.19 % was measured); exact LOI numbers can be
21
found in the supplementary material. Petrographic observations agree with these data, in that
minimal alteration products, such as clays or iddingsite, were observed.
Major Element Analysis by ICP-OES
Preparation for inductively coupled plasma optical emission spectrometry (ICP-OES)
began with a flux fusion digestion of the powdered samples. This process begins with
combining ~0.1 g of sample with ~0.4 g of a LiBO 2 flux. These powders are layered in the order
of flux-sample-flux within a graphite crucible, then inserted into a furnace at 1050°C for 16
minutes (stirring the sample half way through) to melt the sample and flux. This melt is then
removed from the furnace and rapidly quenched in 50 ml of 2% HNO3. Solutions were
sonicated in an ultrasonic bath for several hours to ensure complete dissolution. A 10 mL aliquot
was passed through a 0.2 µm filter to remove any graphite flakes from the crucible, and then
diluted to 100 mL using 2% HNO3. These solutions were run on the University of Iowa’s ICPOES (a Varian ICP-OES 720-EES). All major element calculations below were taken from the
ICP-OES due to the ability to measure Si and Cr with the flux fusion process, since the acid
digestion process used in ICP-MS losses Si as a volatile (SiF4) and the Cr bearing mineral phase
chromite does not digest. Data were corrected for instrument drift and calibrated with
international standards BRP-1, BIR-1, AGV-2, and AC-E, using accepted values from the
GEOREM database (http://georem.mpch-mainz.gwdg.de). Two replicate analyses of the JB-2
standard were used to assess data quality, and the average value of these analyses was found to
be in good agreement with the preferred GEOREM values (See supplementary material).
22
Trace Element Analysis by ICP-MS
Trace elements were analyzed by inductively coupled plasma mass spectrometry (ICPMS), using a Thermo X-series II instrument at the University of Iowa. 0.1g of rock powder was
digested with HF-HNO3 acids, and run in 2% HNO3 at a dilution factor of 5000, after spiking
with internal standards (In, Re). Data reduction included corrections for machine drift, reagent
blanks, oxide interferences and isotopic overlaps. The data were calibrated with four rock
standard reference materials (BHVO-2, BIR-1, BRP-1), using the preferred values from the
GEOREM database (http://georem.mpch-mainz.gwdg.de/). Replicate analyses (n = 8) of the JB2 standard were used to assess data quality. These analyses showed variation of < 4% in both
precision and accuracy except for several elements (Mo, Be, & Th) that have low abundances in
the JB-2 standard (see supplementary data).
Whole Rock Geochemistry Results and Discussion
The ambiguity of the geologic relationships between outcrops in the field necessitated the
use of whole rock geochemistry to group and define each rock unit, as well as to determine if the
lower basalt flows are younger Plio-Pleistocene flows or part of the older Neogene flood basalt
basement. Since the focus of this study is on Vatnafell, the primary use of the whole rock
geochemistry is to determine which outcrops, if any, are the result of the same magmatic event
that formed Vatnafell.
23
When plotted on a
total alkalis vs silica
diagram, all analyses fall
within a transitional alakalic
basalt field (Figure 4.1).
The whole rock data show a
broad range in MgO of 5.7
to 9.5 wt%, and a range in
Figure 4.1: Total Alkalis vs. Silica Diagram showing Vatnafell and
surrounding outcrops (black diamonds), and glass analyses from the
hyaloclastite breccia (grey pluses). These analyses show a transitional
alkalic composition, which is distinct from the sub-alkaline tholeiitic
trend seen at the rift zones.
SiO2 of 46.8 to 48.7 wt%
(Figure 4.5). While
fractional crystallization
may create variations such as this, outcrops with a similar MgO of ~8% show a range in TiO 2 of
1.6 to 2.4% (Figure 4.5). Such a large range in TiO2 shows that there is no single liquid line of
decent between all units which indicates different parental magmas. All samples show similar
chondrite-normalized rare Earth element (REE) patterns, with an overall enrichment in light rare
Earth elements (LREE’s) (Figure 4.4). A positive Eu anomaly can be seen in several REE
patterns. Although this positive Eu anomaly can be caused by accumulation of PLAG crystals in
a sample as PLAG readily accepts Eu2+ into its structure, the lack of PLAG phenocrysts in these
samples indicates that a different parental magma, not an excess of PLAG, is the cause for this
positive Eu anomaly. This specific pattern is characteristic of the compositions seen in the
Snaefellsjokull volcanic system to the west of Vatnafell (Figure 4.4). Variations in incompatible
trace element ratios such as Nb/Zr (0.22 to 0.26), and La/Sm (4.0 to 5.2) are also not affected by
fractional crystallization, and provide more evidence for different parental melts (Figure 4.7).
24
When comparing Vatnafell and
surrounding outcrops to other Icelandic
geologic regions, such as the Snaefellsnes
Peninsula as a whole and the nearest main
rift zone (WVZ), a broad compositional
spectrum can be seen. Variations in La/Sm
Figure 4.2: Whole rock MgO vs TiO2 showing
multiple parental melt compositions.
and Nb/Zr show a clear difference between
Vatnafell, and the WVZ, with Snaefellsnes
Peninsula showing a transitional composition
spanning the two (Figure 4.3). TiO2 also
varies regionally, with Vatnafell having
generally lower TiO2 than the surrounding
Snaefellsnes Peninsula (Figure 4.2).
Figure 4.3: Whole rock La/Sm vs. Nb/Zr of major
western Iceland geologic regions.
The compositional variability seen
regionally across Snaefellsnes and the WVZ, as well as locally between the outcrops present at
Vatnafell, cannot be created by fractional crystallization alone and are the result of differeing
levels of melt generated, over-printed by compositionally distinct parental magmas. Figure 4.3
shows evidence of both different degrees of melt, as well as distinct parental magmas. Large
degrees of melt are created at the rift zones, and produce low La/Sm & Nb/Zr, while lower
degrees of melt create higher La/Sm & Nb/Zr, and the variation of data is evidence of different
parental magmas (e.g. Vatnafell shows a La/Sm variation of 4.0 to 5.2, and a Nb/Zr variation of
0.20 to 0.26) (Figure 4.3). The WVZ shows large degrees of melting, while Snaefellsnes shows
moderate to lower degrees of melting, and Vatnafell and surrounding outcrops were formed by
25
lower degrees of melting. The TiO2 variation is also a factor of both degree of melt produced, as
well as parental magma composition. Lower degrees of melt, distinct parental magma
compositions, as well as fractional crystallization, will produce higher TiO 2. Since the increase
in TiO2 relative to the decrease in MgO does not fall on a single liquid line of decent, fractional
crystallization is not the primary cause for this variation and is attributed to the degrees of melt
generated from compositionally distinct parental magmas at each volcanic region.
The variation within Vatnafell and the surrounding outcrops is apparent when looking at
trace element ratios. A distinct low Nb/Zr isolates the basalt flows in the stream from the other
outcrops, while the high Eu/Eu* & Ba/Th, and lower La/Sm is distinct within the dike. The
overall variation of La/Sm for all samples ranges from 4.0 to 5.2 and lies within analytical error
(Figure 4.7). The same is true for Nb/Zr (0.22 to 0.26), Dy/Yb (2.05 to 2.19), and Eu/Eu* (1.00
to 1.15) (Figure 4.7). These variations indicate that the surrounding basalt outcrops are not a part
of Vatnafell, and originated from a different parental than Vatnafell.
26
Figure 4.4: Rare earth element patterns of whole rock analyses taken from Vatnafell and
surrounding outcrops normalized to a chondritic meteorite (McDonough, 1995).
Figure 4.5: Whole rock geochemical data plots for Vatnafell and surrounding outcrops. Continued on the
next page.
27
Figure 4.5: Continued.
28
Figure 4.6: Whole rock data plots from Vatnafell surrounding outcrops compared to Snaefellsnes Peninsula.
29
Figure 4.7: Trace element ratios of Vatnafell and the surrounding outcrops
gathered from whole rock analyses.
30
Mineral Compositions
All mineral phase compositions were measured using the electron microprobe (EMP) at
the University of Iowa which is a JEOL 8230 electron microprobe with 5 wavelength-dispersive
spectrometers equipped with 8 large format diffracting crystals for measuring trace element
concentrations. All three main mineral phases (CPX, OL, & PLAG) were analyzed using a beam
current of 30 nA with an accelerating voltage of 15 KeV and a 5 micron beam. On-peak count
times ranged from 10-60s depending on the concentration of the element of interest. Glass
compositions were measured at similar conditions, but with a 10 micron beam. CPX and Ol
analyses were calibrated using NBDI, a Smithsonian diopside standard, and astimex Cr-diopside,
while Plag was calibrated using LABR, a Smithsonian labradorite. Reproducibility and data
quality assessments by repeat analyses of the standards, were performed before each analytical
session and these standard data are given in the supplementary material.
Analyses were conducted on both thin-sections and grain mounts, with an emphasis on
thin sections to provide in-situ petrographic context between mineral phases. A columnar block
from the colonnade section of Vatnafell was sliced into 5 slabs (slab 1, slab 2, slab 3, slab 4, and
slab-top). The front and back of each slab were then polished (labeled F or B respectively), and
scanned for CSD analyses. Thin sections were cut from the slabs after scanning, and labeled a,b,
& c when more than one thin section was taken from one slab. The cumulate zone is represented
by sample 1405-BA, while the hyaloclastite breccia is represented by sample 9A. All thin
sections and grain mounts were prepared at the University of Iowa, and finished with a 0.05 µm
alumina polish. All “slab-X” analyses and 1405-BA analyses are from Vatnafell that contains
the three dominant mineral phases to be used for geothermo-barometric calculations (CPX, Ol,
31
and PLAG). Glass analyses were made on thin sections from the glassy matrix within the
hyaloclastite breccia underlying Vatnafell.
The raw EMP data were screened to remove spurious or bad analyses based on several
criteria. The first was the overall totals acquired by the EMP, and any analyses with totals less
than 98%, or over 102% were deleted. Any analyses that did not show the proper major
elements present for the desired mineral phase were most likely due to multiple phases being
excited by the beam, or from the presence of a flaw in the crystal (such as a fracture that is filled
with matrix glass or altered products) and were deleted. The final check was to make sure that
each analysis had the correct stoichiometric balance between the measured cations and oxygens
that are present in each chemical formula, 4 for CPX, 3 for OL, and 5 for PLAG (calculated
against 6, 4, and 8 oxygens respectively). Only analyses that were within ± 0.1 of the proper
cation sums were accepted. All data are given in table A.1.
Olivine
72 analyses were made on the OL populations within Vatnafell and the cumulate zone.
The Ol have been split into three textural populations; groundmass (< 1 mm), macrocryst cores
(interior of crystals > 1 mm), and macrocryst rims (exterior of crystals > 1 mm). Fig 4.4
summarizes the Fo% relationships present between the OL populations. The groundmass
population show no zonation, and are Fo 72 ± 1 (1 st. dev, n = 7). The macrocryst population show
a continuous range from Fo 72 to Fo84 and BSE images reveal diffuse normal zonation from cores
to rims across a distance of ~150 µm, with the cores being Fo 83 ± 1 and the rims being Fo 72 ± 1,
and similar to groundmass compositions. The glomerocryst population is predominantly defined
by CPX oikocrysts enclosing rounded Ol chadacrysts with Fo83±1, however there is a minor
population of glomerocrysts defined by PLAG oikocrysts and the OL chadocrysts are Fo 79±1.
32
Figure 4.8: Fo% (Mg/Mg+Fe) within olivine populations in Vatnafell, and the cumulate zone.
The OL’s within the glomerocrysts show two
CPX
chemical patterns in BSE images. Where the OL
is in contact with the host CPX oikocryst, there is
OL
no zonation present (Fig 4.9). In contrast where
the Ol is in contact with the groundmass, a
CPX
Figure 4.9: BSE image of a glomerocryst Ol
showing no zonation when in contact with the
CPX oikocrysts, and diffuse zonation with the
groundmass.
similar diffuse normal zonation as seen in the
macrocrysts is present, with Fo 82 ± 1 cores to Fo72
± 1 rims
which are the same composition as the
surrounding groundmass (Fig 4.9). A series of
high precision olivine analyses were taken across 5 different glomerocryst OL’s in order to
calculate diffusion profiles.
Plagioclase
A total of 102 analyses were made on plagioclase within Vatnafell (Fig 4.10). No
analyses were gathered from the cumulate zone sample as it does not contain PLAG macrocrysts.
The PLAG was organized into three textural populations; macrocrysts, groundmass, and
glomerocrysts. The plagioclase in Vatnafell show a bimodal relationship with a high anorthite
33
group at An86±3, and a low anorthite group at An73±3. The high anorthite group is made up of the
glomerocrysts and the macrocryst cores, while the low anorthite group is made of the
groundmass and the macrocryst rims (Figure 4.11). BSE imaging did not reveal any zonation
within any of the crystal populations, however chemical analyses show normal zonation from
cores to rims within the macrocrysts and glomerocrysts with the rims being the same An% as the
surrounding groundmass.
Figure 4.10: Anorthite % (Ca/Ca + Na) vs. FeO wt% showing the different
plagioclase populations present in Vatnafell.
Figure 4.11: An% within the plagioclase populations in Vatnafell.
34
Clinopyroxene
The clinopyroxenes within Vatnafell show a bimodal composition similar to the Ol and
PLAG. The clinopyroxenes show a high Mg# population at Mg 83 ± 2, and a low Mg# population
at Mg75 ± 2, with some macrocrysts and glomerocrysts showing intermediate compositions (Fig
4.14). The glomerocrysts are the dominant sub-population within the high Mg# crystals,
although some zonation is present and rim analyses show a similar composition to the
surrounding groundmass population. The macrocrysts show a broad compositional range
encompassing both the high and low Mg#
populations. Most of the clinopyroxenes in the
cumulate zone fall within the high Mg#
compositional range, with a few showing an
intermediate composition, and only one showing a
low Mg# composition. Glomerocrysts and some
Figure 4.12: CPX in contact with groundmass.
macrocrysts show sharp normal zonation into rims
that are in chemical equilibrium with
Cores
2.50
the groundmass population (Fig 4.12).
rims
2.00
TiO2 wt%
Groundmass
A distinct population of low
1.50
1.00
TiO2 analyses all fell within a
0.50
glomerocryst that is a more gabbroic,
0.00
0.70
0.75
0.80
0.85
0.90
PLAG dominated composition (Figure
Mg#
Figure 4.13: Mg# (Mg/Mg + Fe) vs. TiO2 wt% showing the 4.13). The composition of the CPX
different CPX populations present in Vatnafell.
when associated with PLAG has a
35
pattern similar to the OL, where a lower Mg# is seen (Mg83±2 when associated with OL, and
Mg76±1 when associated with PLAG).
Figure 4.14: Mg# (Mg/Mg + Fe) of the different clinopyroxene textures present in Vatnafell.
Glass
The glass from the matrix of the hyaloclastite breccia has been grouped into three distinct
populations. Population one has the most primitive signature with MgO values of 7.3 – 8.4 wt%,
while population two has lower MgO values of 5.7 – 6.9 wt%. Population three is distinct in that
it shows evidence of degassing with low SO3 of essentially zero wt%, compared to populations
one and two that have SO3 wt% values of
0.10 ± 0.03 (Figure 4.15). The three
populations are petrographically distinct as
well (Figure 4.17). Population one are the
lightest colored, open vesicular glassy
nodules, while population two is light
Figure 4.15: Glass populations within the hyaloclastite colored, less vesicular, and shows PLAG
breccia; population one in blue, population two in
orange, and population three in yellow.
microlite crystallization. Population three
36
is a light to medium brown, showing
4.0
Glass
TiO2 (wt%)
3.5
amygdules of a darker brown. When
3.0
comparing MgO to TiO2, it is clear that
2.5
there is a linear trened between the glass
2.0
populations indicating that fractional
1.5
4
5
6
7
8
9
MgO (wt%)
Figure 4.16: MgO vs TiO2 within the glass populations.
The reasonably linear trend indicates fractional
crystallization of the parental melt.
crystallization of a similar parental melt
has taken place (Figure 4.15).
It should be noted that regions
within the glomerocrysts and larger macrocrysts from Vatnafell that were microscopically
interpreted to be glass showed many PLAG microlites, and oxides crystallizing in the BSE
images. It was determined that although these pockets may have been trapped parental melt at
one time, in-situ crystallization has created interstitial glass that is far too evolved to yield
original glass compositions (Figure 4.18).
37
Figure 4.17: Microscopic images of hyaloclastite glass representing populations one and two (upper)
and three (lower) taken in plain polarized (left) and cross polarized light (right).
Figure 4.18: Microscopic images of glassy pockets within CPX in Vatnafell taken in plain
polarized (left) and cross polarized (right) light.
38
CHAPTER 5: DISCUSSION & CONCLUSIONS
Regional compositions
Although whole rock compositions of Vatnafell and surrounding outcrops show small
scale diversity, they fall within the chemical distribution seen in the Plio-Pleistocene off-axis
magmatism along the Snaefellsnes Peninsula. When compared to the nearest main rift zones (the
NVZ & WVZ), it can be seen that Snaefellsnes Peninsula shows an overall trend of higher
alkalinity (higher Na2O + K2O relative to SiO2) (Figure 5.1). According to nomenclature
described by Jakobsson et. al. (2008), this
means that Snaefellsnes Peninsula and
Vatnafell & surrounding outcrops are
defined as transitional basalts, as opposed
to the tholeiitic compositions present along
the main rift zones. This alkali enrichment
is due to the thicker lithosphere of
Figure 5.1: The total alkalis vs. silica relationship
between the WVZ, Snaefellsnes Peninsula, and
Vatnafell.
Snaefellsnes Peninsula forcing any
melting to take place deeper and at lower
degrees preferentially sampling the
enriched mantle component that has a
lower melting temperature, coupled with a
heterogeneous parental melt. Highly
incompatible trace element ratios from the
Figure 5.2: Trace element ratios show the difference in
the depth and degree of melting occurring at the WVZ,
Snaefellsnes Peninsula, and Vatnafell.
39
WVZ, Snaefellsnes Peninsula, and
Vatnafell show this difference in depth
and degree of melting (Figure 5.2). In comparison to the main rift zones, Snaefellsnes Peninsula
shows a broader compositional range within a smaller geographical region. Some of this
compositional range can be explained by the thickening of the lithosphere with increasing
distance from the main rift zone, and reducing the levels of melt which preferentially melts a
more enriched mantle component. However, there are some geochemical trends, such as varying
isotopic ratios (Sr, Nd, Pb, He…) along the length of the peninsula that cannot be explained
through varying the levels of melt generated or by fractional crystallization, and are an indicator
of regional mantle heterogeneity (Hardarson, 1993). Studies have shown that mantle
heterogeneity beneath Iceland is more complicated than a two end-member mixing scenario of
an enriched and depleted mantle source, and isotopic studies on individual volcanic systems must
be done to define regional mantle components (Peate et. al, 2010). Although isotopic analyses
were deemed outside of the spectrum of this study, reasonable evidence from ratios of highly
incompatible elements has been provided showing a heterogeneous mantle source beneath
Vatnafell.
When looking at Vatnafell itself, and the surrounding outcrops, it can be seen that there is
enough compositional diversity to show that there are several different eruptive events within
this small 2 km2 area. Whole rock geochemistry has allowed for the segregation of outcrops into
compositionally distinct lava flows. Vatnafell is compositionally distinct from the underlying
hyaloclasatite breccia, and the surrounding basalt outcrops are all compositionally distinct from
Vatnafell (see chapter 4). Although temporal relationships are hard to define due to the lack of
geochronological analyses (the only being K-Ar dating on the colonnade of Vatnafell dating it at
414 ± 11 ka), the compositional diversity of the outcrops gives a broad perspective on when
these units erupted. Whole rock analyses taken from Vatnafell and the surrounding outcrops
40
show a composition reflecting that of younger Plio-Pleistocene volcanics (e.g. alkali enrichment,
low degrees of melt) vs. the compositions erupted during the Neogene through rift-dominated
tectonics (e.g. tholeiitic, large degrees of melt). Relative field relationships can be inferred based
on stratigraphic relationships, such as Vatnafell is on top of the hyaloclastite breccia, however
the hyaloclastite breccia is cross-cut by a dike. This shows that the dike and Vatnafell are both
younger than the hyaloclastite breccia, however without geochronological data, the relationship
between the dike and Vatnafell cannot be determined. The Ba/Th & Eu/Eu* from the dike are
similar to magmas erupted from the rift showing a different parental magma than Vatnafell.
Crystal Size Distributions
The resulting CSDs on CPX, OL, and PLAG reveal two distinct linear trends with the
change in slope occurring at about the 0.75mm size range, which is accompanied with a gap in
the corresponding crystal size (Figure
3.6). Linear trends present in CSD
analyses are the result of a relationship
between the number of nucleation sites
available, residence time, and the growth
rate of the given mineral species.
Deviations from a single linear trend
indicate a change in crystallization
patterns which are indicative of open
system behavior. New magma
introduced to a crystallizing magmatic
Figure 5.3: A hypothetical magmatic system and the type
of CSD’s it would produce. Marsh, 1998.
system will overprint its own
41
crystallization history onto the CSD analyses. A change in residence time is the simplest
mechanism to facilitate a large change in slope, therefore indicating that the erupted magma
contains two distinct crystal populations; one that had few nucleation sites and a long residence
time, the other that had many nucleation sites and a short residence time. This type of CSD
pattern is indicative of the incorporation of a coarsely crystalline cumulate into the host magma
(Figure 5.3).
CSD’s were measured for the three main mineral phases; CPX, OL, and PLAG.
Although each showed the same broad trend of larger macrocrysts being incorporated into a finer
groundmass host, the three mineral phases had slight differences in their own CSD trends. The
CPX & OL phases predominantly occur together as glomerocrysts, which are a part of the most
primitive CPX & OL crystals (Mg# 82-86, & Fo# 81-83), and represent the incorporation of a
wehrlitic cumulate originating from near Moho depths (see geobarometry results in next section).
Although these multiphase glomerocrysts are the primary reason for the change in slope on the
CPX & OL CSD’s, mono-mineralic macrocrysts are also present and account for many of the
prevalent macrocrysts, and since most of the compositions (Mg# & Fo#) are similar to the
glomerocrysts, as well as calculated depths of crystallization (see geobarometry results in next
section), they are interpreted to be a disaggregated wehrlitic cumulate. Individual CPX & OL
macrocrysts overall show a range of core compositions from Fo# 80-84 for OL, and Mg# 83-87
for CPX, which are important for the geo-barometric calculations as rim analyses represent
equilibrium with the new magma host.
PLAG macrocrysts rarely manifest as multi-mineralic glomerocrysts, but a few instances
of PLAG hosting OL or OL & CPX have been found. The PLAG macrocrysts tend to be
anhedral, rounded fragments, and have a much smaller modal % than CPX & OL (PLAG: 1.69%
42
vs. CPX: 7.72% & OL: 4.63%). These individual PLAG macrocrysts have a range of An% from
An 80-88, while the euhedral PLAG dominated glomerocrysts have a similar compositional
range from An 80-89, although the glomerocrysts are dominated by more primitive compositions
(average of An 87±2), while the individual macrocrysts show a more uniform compositional
spread from An 80-88). While the CPX associated with PLAG yield similar depths as the other
CPX (near Moho), they have a slightly more evolved composition of Mg# 80-82 (compared to
Mg# 82-86 within glomerocrysts), and while a depth cannot be calculated from OL compositions
alone, the OL associated with PLAG shows a similarly slightly more evolved composition (Fo#
79-81, compared to Fo# 81-83 in glomerocrysts). These PLAG macrocrysts most likely
represent a later stage of crystallization from the same depths as the wehrlitic cumulate that was
able to segregate itself from the wehrlitic material due to density differences.
Temperatures & Depths of Crystallization
By measuring the precise chemistry of the mineral phases within both textural
populations, the temperatures and pressures of crystallization can be calculated. Many methods
have been developed for calculating temperatures and pressures of crystallization (e.g. Putirka,
2008; Kelley & Barton, 2008) and these methods are heavily dependent on many parameters
related to the volcanic setting, the bulk composition of the parental magma, the phases present,
and the ability to accurately measure those phases. The material present at Vatnafell lends itself
to several thermometers and barometers, and will be discussed below. Many
geothermobarometers require the composition of both a mineral phase and the equilibrium melt
composition. It is difficult to determine a suitable equilibrium melt composition for the cores of
the entrained cumulates at Vatnafell which makes the geothermobarometric calculations not as
straight forward as a magmatic system with closed system behavior.
43
Thermometers
1. Loucks (1996). This method is calibrated at low pressures (< 10 kbar), and is considered
accurate up to 1250°C. It relies on the assumption that OL and CPX (augite) are growing in
equilibrium together, and uses the partitioning of Mg, and Fe between these two phases to
calculate the temperature during mineral growth. The equation derived by Loucks, is:
where the KD term is (Fe/Mg)OL/(Fe2+/Mg)AUG and temperature is in K. The CPX and OL
glomerocrysts, as well as the groundmass, were the best candidates for this method as they both
have OL growing with CPX that are in chemical equilibrium with each other (Figures 3.1 & 4.9).
This method yielded temperatures of
1220 ± 22°C for the glomerocrysts,
and 1191 ± 12°C for the
groundmass. Temperatures for the
glomerocrysts were achieved by
averaging calculations across five
separate glomerocrysts. Each
Figure 5.4: Temperatures calculated using Loucks equation,
and the corresponding Mg# of the glomerocrysts.
Temperatures are based on the partitioning of Mg and Fe
between OL and CPX growing in equilibrium.
individual calculation can be seen in
Fig 5.4. Groundmass temperature
calculations were achieved by averaging calculations between all CPX groundmass analyses, and
all OL groundmass analyses (ten each). The data used to make these calculations can be seen in
44
table A-2. These temperatures show that the CPX & OL glomerocrysts were crystallizing
around 1200-1250°C.
2. Coogan et al. (2014) have developed a thermometer based on high precision OL analyses, and
chemical equilibrium with Cr-spinel inclusions. This method is based on the partitioning of Al
between the Cr-spinel inclusion, and its host OL. While the material from Vatnafell contained
many OL macrocrysts, none of these contained adequate Cr-spinel inclusions.
Barometers
1. Kelley & Barton (200) provided a barometer that can be used on the major element glass or
whole rock compositions that are in chemical equilibrium with OL, CPX, and PLAG. The
composition of the melt that these three mineral phases crystallize together from, changes with
pressure. By using the major element compositions of whole rock or glass analyses, the
pressures of crystallization can be calculated. Since the macrocryst population has shown to be a
foreign incorporation into the host melt, the whole rock analyses are not be representative of a
melt compositoin. Since the modal % of each macrocryst phase has been calculated (see chapter
3), and the precise composition of each macrocryst phase is known (see chapter 4), the
Figure 5.5: The binary pressure calculations on high MgO hyaloclastite glass. CaO/Al2O3 also shows
this binary relationship, which may represent the mixing of two magmas prior to eruption.
45
macrocryst compositions weighted with the modal % can be subtracted from the whole rock
analyses in order to estimate the composition of the host melt which is equal to the groundmass
composition. When this composition is used in Kelley & Barton’s method, a pressure of 7.2
kbars is calculated which converts to a depth of 24.5 km (using a conversion of 3 kbar = 10 km).
The sub-glacial nature of the eruption that created Vatnafell provided a lot of glassy
material that could also be used in the Kelley & Barton (2008) method. The most robust, highest
MgO (7.2 – 8.4 wt%) series of analyses that were taken from the hyaloclastite glass were chosen
for this method as they most likely represent the most primitive glass formed during this
eruption, although one outlier was measured at 9.7 wt% MgO. Individual barometric
calculations done on the glass shows two distinct populations, a deep and a shallow. This
relationship is also reflected in CaO/Al2O3 where the high pressure glass has the lowest
CaO/Al2O3 (Figure 5.5). The average pressure calculated in the deep (low CaO/Al2O3)
population is 5.8 kbar, while the average pressure for the shallow (high CaO/Al2O3) population
is 2.7 kbar. These translate to 19.8 km and 9.2 km respectively.
2. Nimis & Ulmer (1998) developed a barometer that is dependent on the composition of CPX
only. This method is based on the linear relationship between the pressure during crystallization,
and the volume of the unit cell and M1-site
cation. Although this method is useful in
that a liquid composition is not necessary,
the data set in which it was calibrated on had
few analyses in the 0 – 8 kbar range so any
Figure 5.6: The systematic offset of ~3 kbar between
the methods of Nimis & Ulmer (1998), and Putirka
(2008).
measurements within this pressure range are
poorly constrained (Nimis & Ulmer, 1998).
46
Putirka (2008) showed a systematic offset of the Nimis model of about 3 Kbars, and Vatnafell
pyroxene calculations reflect this poor constraint on shallow crystallization, as groundmass was
initially calculated as crystallizing at -3 km. Although a 3 kbar adjustment to the full data set
yields similar numbers to other geobarometers, a different method will be used for final
publication of these data.
3. Putirka (2008) has developed several CPX based geobarometers that are applicable to several
different types of magmatic systems. The geobarometers used on the Vatnafell CPX’s rely on
the composition of the CPX only, since no liquid compositions could be measured that would be
in equilibrium with the cores of the macrocryst population. The equation used does rely on a
temperature being entered into the equation, or an iterative method is used for determining the
pressure of crystallization. The Putirka method was used on the glomerocrysts where the
temperature had been constrained using the methods of Loucks (1996) to calculate the pressures
of crystallization using the equation:
47
The rest of the CPX analyses (mono-mineralic macrocrysts, cores, and rims) were calculated
using Nimis & Ulmer (1998), and then adjusted +3 Kbars to adjust for the systematic offset
measured by Putirka (2008). The results can be seen in Figure 5.9.
Figure 5.7: Geophysical calculations estimating the depth of the Moho. From Kumar et al.
(2007).
These barometric calculations were then compared to geophysical analyses of Iceland,
which determined the Moho to be about 25 km deep along the central region of the Snaefellsnes
Peninsula as reviewed in Savry & Canon-Tapia (2014) (Figure 5.7).
48
Fig 5.8: This map shows the depth in Km to the 1200°c isotherm based on modelling of geothermal data
from bore holes. This corresponds well to the geothermobarometry beneath Vatnafell. From Flovenz &
Saemundsson (1993)
49
Figure 5.9: The depths of crystallization for the CPX present at
Vatnafell as calculated using the methods of Nimis & Ulmer (1998)
& Putirka (2008).
The results of the barometric calculations show that the depths of crystallization have a
bi-modal distribution (Figure 5.6). Most crystallization is taking place from ~20-27 km deep,
which corresponds well with geophysical estimates of the depth of the Moho, with the late stage
growth rims and groundmass crystals grew at near surface conditions. Few PLAG dominated
xenoliths can be seen within Vatnafell, and although none were captured in thin section, isolated
grain mounts allowed for barometric calculations to be done. The undersampled gabbroic
xenolith with CPX of anomalous composition (low TiO2 relative to the CPX in Vatnafell)
yielded a depth of crystallization similar to that of the groundmass and growth rims, and most
likely represents an incorporation of tholeiitic Neogene aged material just prior to eruption since
the low TiO2 indicates crystallization with a low TiO2 melt as seen in Neogene volcanics. This
can be tested with REE analyses using laser ablation ICP-MS on the CPX, and will be done in
future studies. These barometric calculations indicate a complete absence of shallow crustal
magma chambers, and show a single deep magmatic system.
50
Estimating the Time Scales of the Magmatic Processes
There are several methods within these data that can be used to quantify the time scales
involved with the magmatic processes that created Vatnafell from deep crystallization, to
incorporation of the wehrlitic cumulate, followed by the ascent, emplacement, and final
crystallization of Vatnafell. Processes such as the late stage groundmass crystallization can be
quantified using the CSD calculations, the minimum ascent rate can be constrained the size of
crystals in relation to the viscosity of the host melt using Stoke’s Law, and the diffusion of Mg &
Fe across the zonation present within the OL glomerocrysts can be used to calculate the
residence time of the glomerocrysts within the host melt.
Using CSD’s
Within the CSD calculations, there is a fundamental relationship between the slope of the
line calculated, and growth rates & residence time (Figure 5.10). Specifically, the slope = 1/G*T where G is the crystal growth rate, and T is the residence time of that crystal. This
calculation however, is based on the assumption of one batch of
crystallizing magma and no open system behavior. The addition
or loss of crystals will alter the slope of the line calculated by
CSD’s, and this change in slope has no relation to the growth of
crystals but to the proportion of crystals gained or lost. Since
the macrocryst population is comprised of a ripped up cumulate
Figure 5.10: Anatomy of a CSD
plot. Higgins Textures Book. G
= Mineral Growth Rate, Τ =
Residence Time
(crystal enrichment shown in CSD’s, as well as disequilibrium
textures in thin section), the shallower slope cannot be used to
calculate residence times. The groundmass population however, crystallized from the host melt
51
and appears to have not been affected by open system behavior and can be used to calculate
residence times. The slope of the groundmass CSD’s for CPX was calculated to be -26.772, and
using a CPX growth rate of 10-7 – 10-8 cm/s (Dunbar et al. 1995), the residence time of the
groundmass calculates to 4 to 43 days. Assuming all groundmass crystallization began at or near
the time of eruption, this indicates that Vatnafell took roughly one month to cool, and crystallize
the groundmass population (although some groundmass began crystallization during ascent as
well).
Using Stoke’s Law
The minimum ascent rate of the magma from Moho-depth can be calculated using
Stoke’s law which relies on the relationship between the size & density of a given particle, and
the rate at which that particle will settle through a medium with a given viscosity. Using the
entrainment of the wehrlitic glomerocrysts and Stoke’s law:
𝑉=
2 (𝜌𝑝 − 𝜌𝑓 ) 2
𝑔𝑅
9
𝜇
Where V is the terminal velocity of a settling particle (m/s), ρ p is the density of the particle
(kg/m3), ρf is the density of the fluid (kg/m3), μ is the viscosity of the fluid (Kg/m*s), g is the
acceleration due to gravity (m/s2), and R is the radius of the largest glomerocryst measured (m).
The value of 9.8 m/s2 was used for the acceleration due to gravity, and 0.015 m was used for R
which represents the radius of the largest macrocryst found (30 mm in diameter). The value of
3300 kg/m3 was used for ρp based on the average density of augite. The ρf & μ were calculated
using the calculated groundmass composition and the program KWare Magma ver. 2.5 (Wohletz,
K) and found to be 2809 kg/m3 and 8.83 kg/m*s respectively. It should be noted that water
content does effect the viscosity of magma, however Snaefellsnes Peninsula lavas have been
52
shown to be very anhydrous (0.1 wt% H2O: Nichols et al. 2002) and so water content does not
need to be taken into consideration with this calculation.
The calculation using Stoke’s law yields a minimum ascent rate of 3 km/day. It should
be noted that this is the minimum ascent velocity that will keep the largest macrocrysts from
sinking through the surrounding melt at terminal velocity, and the actual ascent velocity must be
higher in order to carry a crystal cargo to the surface. It should also be noted that density and
viscosity calculations of the surrounding melt do not include the phyric nature of this magma.
Increasing the crystallinity to ~15% of the magma will increase the viscosity by about 1.5, and
reduce the calculated minimum ascent rate.
Using Diffusion Profiles
The entrainment of crystals not in equilibrium with the surrounding melt, will lead to the
diffusion of elements as the system tries to reach a state of equilibrium. Mg & Fe (as well as
other elements) within OL will diffuse at a measurable rate at high temperatures when a
compositional contrast is present. The methods used in this study, based on Mg & Fe diffusion
rates, are taken from Costa & Morgan (2011) and are calculated based on several assumptions.
The diffusion of elements through a solid is a phenomenon that can be easily observed, and very
difficult to quantify. The rates of diffusion measured in OL rely on the crystallographic
orientation, as Mg & Fe will diffuse at different rates depending on fabric of the crystal lattice,
however the orientation of the OL within Vatnafell is not known since all OL’s within the
glomerocrysts are anhedral so these calculations are based on the assumption of diffusion along
the c-axis. Diffusion along the c-axis is the fastest rate of diffusion within the OL crystal lattice,
and any diffusion along a different orientation will yield slower diffusion rates (Costa, 2012).
The rates of diffusion are also heavily dependent on parameters that will change throughout
53
magmatic evolution such as temperature, pressure, and chemical potential. Although several
assumptions are made in these calculations, they still provide valuable insight into the timescales
involved in certain magmatic processes.
For this study, diffusion profiles across OL zonation were measured in order to quantify
the residence time of the large macrocrysts in the transporting host melt prior to eruption. High
precision EMP analyses were done across transects set at a uniform spacing of 19 μm for a
length of 234 μm (the model used required the transect to be condensed from 234 μm to 20 μm).
The total chemical potential across this
234 μm is Fo72 at the rim, to Fo82 at the
core. Assuming O2 fugacity of 1.00E-12
and a temperature of 1230°C, based on
the best profile measured, the residence
time of the OL hosted glomerocrysts is 67
days. Although 6 profiles were measured
Figure 5.11: The best diffusion profile measured across the
in all, five of them failed to produce a full
zonation within an Olivine chadacryst. Times are measured
in seconds starting with initial conditions at 0, all the way
profile from OL core to boundary
up to the profile measured in Vatnafells OL which is
slightly less than 6,000,000 seconds.
conditions within the surrounding
groundmass. The shapes were similar to the only usable profile, and most like would have
yielded similar numbers if suitable for diffusion profile matching. If the assumption is made that
ascent began soon after entrainment at 27 km, this would mean the crystalline cargo would have
had a residence time of 9 days if calculated using Stoke’s law, compared with 67 days from the
diffusion calculation. These estimates are reasonably close considering the assumptions made
54
with both calculations, and indicate timescales of weeks to months from entrainment of the
crystalline cargo to the final cooling of Vatnafell.
Time Scales Discussion
The multiple ascent rates calculated through the different methods helps to illuminate a
more complicated magmatic evolutionary sequence than the geochemistry showed. Although the
groundmass crystallization time of about one month (calculated from the CSD slope) makes
sense, it does not help to explain the magmatic processes going on within the crust of
Snaefellsnes Peninsula presuming it represents crystallization that took place at or near the
surface. Stoke’s Law calculated a number that was different from the diffusion profiles, but that
number is just a minimum estimated ascent rate and does not necessarily disagree with the
diffusion profile calculations considering the amount of assumptions inherent in both
calculations. The assumption of ascent soon after assimilation of the macrocryst population may
not be a valid one. Although the mixing of compositionally distinct magmas has been shown to
be an eruptive mechanism in an arc setting (Kolezar et al. 2012), a tectonically driven eruptive
mechanism would allow for magma mixing to occur prior to a rapid ascent and subsequent
eruption. This model allows for the entrainment of the high Fo macrocryst population within the
less mafic melt to achieve the measured diffusion profiles, followed by an ascent rapid enough to
carry the large crystalline cargo and subsequent crystallization of Vatnafell, all happening on the
order of weeks to months.
55
CONCLUSIONS
Snaefellsnes Peninsula located on western Iceland is the largest off-rift volcanic region.
Vatnafell, a sub-glacial eruptive sequence of a highly phyric (~14% macrocrysts) colonnade and
entablatured basalt, underlain by a thick hyloclastite breccia, has allow for detailed textural and
geochemical quantification, followed by the application of several calculations in order to
determine the depth and timing of events prior to eruption.
Initial textural investigations in the form of CSD’s, show that there is an enrichment of
macrocrysts (crystals above 1 mm in diameter) within Vatnafell. Petrographic and BSE imaging
shows that these macrocrysts all contain disequilibrium textures in the form of anhedral
crystallization shapes, embayments, reaction rims, and diffuse zonation. These observations are
interpreted to be the result of a ripped up wehrlitic (CPX & OL) cumulate incorporated into a
host melt prior to eruption.
Geochemical quantification was done using EMP analyses, and then used to calculate the
depths and temperatures of crystallization, as well as the time scales of events prior to eruption.
The depths of crystallization were calculated based on barometers developed by Putirka (2008),
Nimis & Ulmer (1998), and Kelly & Barton (2008). The methods all yielded slightly varying
results depending on their strengths and weaknesses, yet all showed the same story. A single
pyroxene, composition only equation derived by Putirka (2008) was chosen as the final
barometric calculator. All crystallization of larger macrocrysts was taking place from 20-27 km,
while rims and groundmass crystallized at or near the surface. Several geophysical studies have
estimated the base of the crust to be about 25 km deep in this region of Snaefellsnes Peninsula,
which means that all crystallization of macrocrysts was occurring at the base of the crust with no
56
shallower magma reservoirs present. Temperatures were calculated using the methods of Loucks
(1996) which are based on the partitioning of Mg & Fe between OL and CPX in equilibrium.
The CPX & OL glomerocrysts present in Vatnafell were able to yield robust temperatures, as
several OL’s could be found that had grown in equilibrium with, and completely surrounded by,
a CPX host which had shielded the OL from post entrainment alteration. These temperatures
yielded a range of 1200-1250°C, which corresponds well with geothermal gradient studies
(Figure 5.7).
The time scales of magmatic processes occurring prior to eruption can be calculated with
a certain level of accuracy if several assumptions are made. Residence time calculations that are
inherently involved in CSD’s are only useful under closed conditions, so although CSD analyses
have shown to be somewhat useful for all crystal populations present within Vatnafell, residence
time calculations can only be done on the groundmass populations. These calculations showed
that the main edifice took 4 to 43 days to fully cool and crystalize.
Stoke’s Law is a simple way to calculate the minimum ascent velocity needed to carry a
particle in a fluid. The geochemical data allows for density and viscosity calculations, and
Stoke’s Law will yield the terminal velocity of that particle through a specific fluid. Although
this calculation yield’s a minimum ascent rate of 3 km/day in order to keep the largest
macrocrysts found in Vatnafell in suspension. There are assumptions that are built into Stoke’s
Law that may skew the calculation from reality however, such as the calculation is made for
spherical objects moving through a fluid. In this magmatic system, the particle is not spherical,
and the surrounding melt is so crystalline that it may not fall under the definition of a fluid,
however this calculation yields a reasonable estimate for a minimum ascent rate.
57
The diffusion profiles taken from rim to core across the OL zonation were used to
calculate the residence time of the large macrocrysts after entrainment and prior to eruption. The
methods of Costa (2012) were used, along with several assumptions about temperature, pressure,
crystallographic orientation, and chemical potential. Although these parameters change during
magmatic evolution and migration, they were held as constants for these calculations. To
achieve the wide, diffuse zonation seen within the OL, the macrocrysts would have had to be
within the host melt for about 67 days.
The textural and geochemical analyses, coupled with time scale calculations paint the
picture of the magmatic system that produced Vatnafell. The cool, thick lithosphere present in
this central region of Snaefellsnes peninsula is over 100 km away from main rift zones. The
dominant tectonic forces along this volcanic region is a right-lateral shearing of the lithosphere
coupled with minor rifting. The thick lithosphere at this off-rift setting lacks the weakened
hyaloclastite layers present at the main rift zones that accommodate shallow sill-like magma
reservoirs. The barometric calculations using the geochemical data taken from the CPX
populations show that all crystallization beneath Vatnafell occurred near the base of the crust, or
at the surface right after the eruption. The time scales of these magmatic processes show that
from the incorporation of the macrocryst population, rapid ascent, then eruption of the highly
phyrric material that forms Vatnafell took weeks to months to occurr.
This magmatic system is distinct from the main rift zones in several ways. The rift zones
show high degrees of shallowly crystallizing magma, and erupts relatively aphyrric basalts that
are tholleitic in composition. Vatnafell has been shown to have formed by lower degrees of
melting, crystallizing deep at the base of the crust, and ascending rapidly to form a highly phyrric
transitional basalt. While this study is considered complete, there is more work to be done on
58
Vatnafell in the form of melt inclusions, isotope ratios, and in-situ trace elements within the
macrocrysts which would quantify the composition of the parental melts that the macrocrysts
crystallized from, and help to reconstruct the earlier crystallization habits of this magmatic
system.
59
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62
APPENDIX
Figure A.1: The full field area showing the locations of all samples taken from Vatnafell and the
surrounding flows. Map is oriented with north being up.
63
Latitude (N)
64.91666
64.91588
64.91424
64.91413
Longitude (W)
22.91029
22.90210
22.90397
22.90380
Samples collected
Slabs, DB, 2B, 4C, 4F, Plug, Top-a
1401, 1401B
1402-7
1408
Location Description
Down by the lake shore.
NE of Plug
Cummulate zone
E side hyaloclastite
Base of east hyaloclastite outcrop
Pillows (10=core, 11=glass)
& hyaloclastite
64.91421
22.90462
1409, 1410, 1411
64.91495
22.90901
1412
64.91611
22.90720
1413
64.91137
64.90781
22.90032
22.90933
1414
1415
64.90853
22.91225
1416 A&B
64.90671
64.90826
22.91232
22.91543
1417
1418 A-I
64.90208
22.92186
1419 A & B
64.90996
22.90698
1420
west end of the north side
Hyaloclastite outcrop
east end of the north side
Hyaloclastite outcrop
Basalt SE of plug, next to lake
Basalt from the east end of stream
Pillow from below the dam
(A = glass, B = core)
Basalt S of stream out on its own
Clasts in hyaloclastite breccia
Dike and Chilled margin
(A = margin, B = dike)
Flow S of Vatnafell, between
Vatnafell & stream
Flow N of parking area
A = scoria, B = basalt
1421 A & B
Table A.1: Exact sample locations, as well as relative field locations.
64