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Examensarbete vid Institutionen för geovetenskaper Degree Project at the Department of Earth Sciences
ISSN 1650-6553 Nr 442
Submarine Alteration of Seamount Rocks in the Canary Islands:
Insights from Mineralogy, Trace Elements, and Stable Isotopes
Undervattensomvandling av basaltiska bergarter från Kanarieöarna: insikter från mineralogi,
spårämnen och stabila isotoper
Aduragbemi Oluwatobi Sofade
INSTITUTIONEN FÖR GEOVETENSKAPER
D E P A R T M E N T O F E A R T H S C I E N C E S
Examensarbete vid Institutionen för geovetenskaper Degree Project at the Department of Earth Sciences
ISSN 1650-6553 Nr 442
Submarine Alteration of Seamount Rocks in the Canary Islands:
Insights from Mineralogy, Trace Elements, and Stable Isotopes
Undervattensomvandling av basaltiska bergarter från Kanarieöarna: insikter från mineralogi,
spårämnen och stabila isotoper
Aduragbemi Oluwatobi Sofade
ISSN 1650 - 6553
Copyright © Aduragbemi Oluwatobi Sofade
Published at Department of Earth Sciences, Uppsala University (www.geo.uu.se), Uppsala, 2018
Abstract Submarine Alteration of Seamount Rocks in the Canary Islands: Insights from Mineralogy, Trace Elements, and Stable Isotopes Aduragbemi Oluwatobi Sofade
Seamounts play an important role in facilitating the exchange of elements between the oceanic lithosphere and the overlying seawater. This water-rock interaction is caused by circulating seawater and controls the chemical exchange in submarine and sub-seafloor rocks and also plays a major role in determining the final composition of these submarine rocks.
This investigation is designed to evaluate the (i) degree of alteration and element mobility, (ii) to identify relations between alteration types and (iii) to characterise the chemical processes that take place during seafloor and sub-seafloor alteration in the Central Atlantic region.
The investigated submarine rocks are typically altered and comprise calcite and clay minerals in addition to original magmatic feldspar, olivine, pyroxene, quartz, biotite, and amphibole.
Elemental analyses show that submarine rocks with high water-rock ratio have experienced near complete loss of Si and alkali elements to seawater but are enriched in calcium and phosphorous. In addition, there is a strong enrichment of trace elements such as Sr, Ti, Rb and trivalent REEs in altered submarine samples that are likely residual in character. Oxygen and hydrogen isotopic values indicate a low temperature alteration process at less than 50 ℃. Nannofossils were present in one sample and investigation suggests that the seamount south of El Hierro evolved from a young Canary activity rather than the early Cretaceous magmatic events as has been argued previously.
Keywords: Canary Islands, seamounts, submarine alteration, trace elements, nannofossils
Degree Project E1 in Earth Science, 1GV025, 30 credits Supervisors: Valentin. R. Troll and Frances Deegan Department of Earth Sciences, Uppsala University, Villavägen 16, SE-752 36 Uppsala, Sweden.
ISSN 1650-6553, Examensarbete vid Institutionen för geovetenskaper, No. 442, 2018
The whole document is available at www.diva-portal.org
Populärvetenskaplig sammanfattning Undervattensomvandling av basaltiska bergarter från Kanarieöarna: insikter från mineralogi, spårämnen och stabila isotoper Aduragbemi Oluwatobi Sofade
Djuphavsberg spelar en viktig roll för att underlätta utbytet av element mellan den oceaniska litosfären och det överliggande havsvattnet. Interaktionen mellan vattnet och bergarterna orsakas av cirkulerande havsvatten och kontrollerar det kemiska utbytet i undervattensbergarterna och som även spelar en viktig roll för att bestämma de slutliga produkterna i dessa bergarter.
Undersökningen syftar till att (i) utvärdera graden av kemisk omvandling och rörlighet av elementen, (ii) identifiera samband mellan olika omvandlingstyper och (iii) att karakterisera de kemiska processer som äger rum vid kemisk omvandling av bergarter vid och under havsbottnen i Centralatlanten.
De undersökta undervattensbergarterna är generellt kemiskt omvandlade och består av kalcit och lermineral utöver ursprungligt magmatiskt fältspat, olivin, pyroxen, kvarts, biotit och amfibol.
Elementanalyser visar att de undervattensbergarter med en hög vatten-berg kvot har förlorat i stort sett all Si och nästan alla alkaliska element till havsvattnet medan en anrikning har skett av kalcium och fosfor. Dessutom har det i de omvandlade undervattensproverna skett en tydlig anrikning av spårämnena Sr, Ti, Rb och av trivalenta sällsynta jordartsmetaller. Syre- och väteisotopvärden indikerar en omvandlingsprocess vid låga temperaturer mindre än 50 °C. I ett prov fanns nannofossiler och en undersökning av dessa tyder på att djuphavsberget söder om El Hierro bildades under en yngre vulkanisk aktivitet än den magmatiska aktivitet som tidigare föreslagits som ägde rum under perioden Krita.
Nyckelord: Kanarieöarna, djuphavsberg, undervattensomvandling, spårämnen, nanofossiler
Examensarbete E1 i geovetenskap, 1GV025, 30 hp Handledare: Valentin R. Troll och Frances M. Deegan Institutionen för geovetenskaper, Uppsala universitet, Villavägen 16, 752 36 Uppsala (www.geo.uu.se)
ISSN 1650-6553, Examensarbete vid Institutionen för geovetenskaper, Nr 442, 2018
Hela publikationen finns tillgänglig på www.diva-portal.org
Table of Contents Page
1. Introduction……………………………………………………………………..... 1
1.2. Previous research work………………………………………………. 1
2. Geological setting………………………………………………………………… 3
2.1 Geological background………………………………………….......... 3
2.1.1 Geology of Gran Canaria Seamount………………............. 4
2.1.2 Geology of Tenerife Seamount……………………….......... 4
2.1.3 Geology of La Palma Seamount…………………………… 5
2.1.4 Geology of El Hierro Seamount…………………………… 5
2.1.5 Geology of Las Hijas Seamount…………………………… 6
2.1.6 Geology of Hijo de Tenerife Seamount………..................... 6
2.1.7 Geology of Tropic Seamount……………............................ 6
3. Analytical Methods………………………………………………………………. 8
3.1 Sample Preparation……………………………………........................ 8
3.1.1 Sample preparation and processing………………………... 8
3.1.2 Sample description………………………………………… 9
3.2 Mineralogy…………………………………………………………… 10
3.3 Whole-rock major, trace and rare earth elements…………………..... 10
3.4 Mineral mapping……………………………………………………... 10
3.5 Stable isotopes………………………………………………………... 10
3.6 Nannofossils examination……………………………………….......... 11
4. Results…………………………………………………………………………….. 12
4.1 Petrography…………………………………………………... ……… 13
4.2 Mineralogy…………………………………………............................ 21
4.3 Major and trace elements concentrations…………………………….. 21
4.3.1 Loss and gain of major oxides……………………………...32
4.3.2 Trace element concentration…………………………......... 32
4.4 Rare earth element chemistry…………………………........................ 33
4.5 Stable isotopes………………………………………........................... 34
4.6 Distribution of nannofossils at seamount south of El Hierro………… 38
5. Discussions………………………………………………………………………... 42
5.1 Mineralogy…………………………………………………………… 42
5.2 Element fluxes…………………………………………..……............. 42
5.2.1 Loss and gain of major oxides…………………………….. 43
5.2.2 Trace elements mobility…………………………………… 43
5.3 Rare earth element chemistry…………………………..…………….. 44
5.4 Stable isotopes………………………………………………..………. 45
5.5 Volcanic evolution of El Hierro Seamount…………………............... 46
6. Conclusions……………………………………………………………….……… 48
7. Acknowledgements………………………………………………………………. 49
8. References……………………………………………………………………….... 50
Appendix A………………………………………………………………………..56
Appendix B………………………………………………………………………..59
1
1 Introduction
The study of seamounts and isolated volcanic structures on the seafloor provides us with a better
understanding of submarine volcanism and post-magmatic alteration processes prevailing in the
oceanic crust. Dredging into these seamount and submarine flank materials has greatly helped in
furthering our understanding of physical and chemical changes as well as prevailing conditions during
the alteration of these submarine rocks on the seafloor (Staudigel and Schmincke, 1984). The exchange
between seawater and submarine basaltic rocks at both high and low temperatures is fundamental in
studying the alteration pattern and the degree of elemental fluxes within this system (Hofmann, 1998;
Wheat and Mottl, 2000). These alteration processes change the physical and chemical composition of
the entire oceanic system (Staudigel and Hart, 1983), necessitating the understanding of the
geochemistry of alteration processes in the submarine ocean island environment.
The major and trace element mobility will give us an overview of the patterns and degree of
alteration in submarine rocks. By comparing the alteration related chemical changes relative to the
unaltered sample suite provided in this project, we intend to study the mobility of trace elements in the
altered, moderately altered, and unaltered to slightly altered seamount samples from the Canaries. The
main focus will be to understand the mobility of these elements in altered submarine samples relative
to their unaltered equivalent, as well as their alteration products (Utzmann et al., 2002). In this report,
twenty-two (22) dredged samples from the Meteor M43-1 cruise in 1998 were selected for geochemical
analyses. Of these, five samples contain fresh basaltic rocks, ten (10) are mildly altered, and seven (7)
are intensely altered submarine rocks. The samples come from six seamounts in the Canary Island
Canary Province.
We determine the major and trace element mobility, the isotopic compositions of these
submarine volcanic edifices, as well as the volcanic evolution of the Canary Island Seamount Province
from the examination of nannofossils. A recent study shows that the metabolism of microbes also
enhances the alteration of basaltic glass (Staudigel et al., 1995). These processes are said to also deplete
the concentration of silica, calcium, and sodium by enhancing the precipitation of secondary minerals,
e.g. zeolite and clay minerals that are rich in iron and magnesium (Berger et al., 1994).
1.2 Previous research work
Previous studies on sub-seafloor alteration showed that there is evidence for fluxes in major elements
in altered basaltic glass, while only a few studies have been conducted to explain the trace element
mobility during hydrothermal transformation of basaltic material during seafloor alteration (Crovisier
et al., 1983, Berger et al., 1994; Zierenberg et al. 1995; Utzmann et al., 2002). Alteration products such
as zeolite and phillipsite do not always incorporate trace elements with the exception of rubidium.
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However, the degree of accumulation of rare earth elements in altered glass increases as a result of the
atomic number and ionic field strength (Utzmann et al., 2002). This means that the retention and release
of trace elements depend on the physiochemical conditions during the alteration of these basaltic rocks.
Young oceanic crust reacts with circulating seawater during cooling and releases elements into the
ocean or acts as a sink for dissolved ions from seawater (Seyfried and Mottl 1982; Thompson, 1983).
Formation of secondary phases (e.g. palagonite, clay minerals, carbonates and zeolite) marked
the advanced stage of basaltic alteration and this influences the fluxes of the element in the sub-seafloor
(Staudigel and Hart 1983; Furnes 1984; Thorseth et al. 1991). Thompson (1973) postulated that
elements such as B, Li, Cu, Rb, Cs, Pb, and Light Rare Earth Elements (LREE) are usually being
enriched during submarine alteration of basaltic glass, whereas, noticeable changes in the amount of V,
Co, Ni Cr, Co, Ni, Zn, Sr, Y, Zr, Nb, Ba, Hf and Heavy Rare Earth Elements (HREE) are less consistent,
meaning that they are either enriched or depleted.
From the study of basaltic glass from Santa Maria, Azores. Furnes (1980) concluded that Zr,
Nb, La, Ce, and Nd had been enriched relative to their parent sideromelane. He interpreted this
enrichment as an indication of progressive clay mineral and zeolite formation. Staudigel and Hart
(1983) reported that Zn, Cu, Cr, Ni, Co, and REEs are generally lost during palagonitization, whereas
Rb and Cs are gained. Consequently, the mobility of rare earth elements is also dependent and controlled
by the absorption capacity of the secondary phases that is being precipitated during glass alteration in
the sub-seafloor. Analysis of rare earth elements in Daux et al., (1994) supported these conclusions by
comparing the REE and Th content of slightly crystallized palagonite and authigenic phases with those
of highly crystalline palagonite and authigenic phases, the more crystallized material having the higher
REE and Th content relative to the former.
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2 Geological setting
2.1 Geological background
The submarine samples dredged during the Meteor M43-1 cruise in the Canary Islands Volcanic
Province comprise basanite, alkali basalt, nephelinites; plus various hyaloclastites and sedimentary
rocks. The samples represent both basement shield and other post shield volcanic suites of El Hierro,
La Palma, Tenerife, and Gran Canaria (Schmincke, 1982). In this thesis, we studied samples from
South La Palma ridge, South Hierro ridge, Las Hijas seamount, Tropic seamount, Los Gigantes, Punta
de las Rasca off Barranco de Veneguera, Barranco de Tasartico of Gran Canaria, Hijo de Tenerife
seamount, and from the submarine flanks of Tenerife off Guimar and Anaga. The seamount clusters
southwest of the archipelago (Fig. 1A) have been dated by van den Bogaard (2013) and were found to
range from 91 to 142 million years in age while the Canary Islands are of a younger volcanic episode.
The Canary Volcanic Province rests on Jurassic oceanic crust that was formed during the initial stages
of the opening of the Central Atlantic, representing some of the oldest crust in the oceanic basins of the
globe. It shows magnetic anomalies that are parallel to continental margins. The Canary Islands are
widely interpreted as having originated from a hot spot that pierces this oceanic crust (Carracedo et al.,
1998; Geldmacher et al., 2005; Hansteen and Troll 2003; Troll et al 2015; Zaczek et al., 2015).
More than a hundred seamounts and isolated volcanic structures on the seafloor that range
from a few hundred to several thousands of meters in height make up the Canary Island Seamount
Province (CISP; Staudigel and Clague, 2010). These are said to represent the earliest hotspot
signatures that are found in the north-eastern part of the Africa plate (Morgan, 1983). However, the
evolution and origin of this volcanic province and primitive CISP basalts that were derived from
decompression melting of upwelling mantle is still very controversial (Carracedo et al., 1998;
Carracedo and Troll, 2016). The CISP comprises at least five (5) large seamounts located in the north-
eastern part of the archipelago, with several smaller ones existing between them (Fig. 1A and 1B).
Seamounts distal to this archipelago are relatively older than those closer ranging, up to an age of 68
Ma for the Lars/Essaouira seamount (Geldmacher et al., 2005; van den Bogaard, 2013). The isotopic
signatures for magmatic rocks from the older seamounts are similar to rocks from the Canary Islands
(Geldmacher et al., 2005), which implies that these seamounts have likely originated from the same
mantle source that is feeding today’s Canary Island volcanism.
However, the systematic distribution of ages recorded from seamounts northeast of the Canary
Islands shows much older ages in close proximity to the western Canary Islands. There is, however, no
age trend exhibited from the seamounts in the southwest as opposed to the Canary Islands. The random
age distribution is characteristic of fault-controlled volcanism (Troll et al., 2015). Therefore, it might
be that the earlier Cretaceous magmatic episode represents a distribution of distinct events from the
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later and still active Canary Islands Volcanic Province (Troll et al., 2012; Zaczek et al., 2015; Troll et
al., 2015; Carracedo and Troll, 2016).
2.1.1 Geology of Gran Canaria Seamount
Four of the samples examined in this thesis were dredge from the flanks of Gran Canaria. Gran Canaria
is the central and the third-largest island (about 1560 km2) of the archipelago, after Tenerife and
Fuerteventura (Thirlwall et al., 2000). Gran Canaria is not only characterized by basaltic shield
volcanism and caldera-forming felsic eruptions, but also by abundant intra-caldera and extra-caldera
ignimbrites and spectacular cone-sheets (Donoghue et al., 2008; Troll et al., 2011). The history of Gran
Canaria’s volcanism can be divided into two major cycles of activity: the first is the Miocene or shield
stage at approximately 15 - 10 Ma and second being the post-Miocene rejuvenated stage from 5.5 Ma
to present. The tholeiitic to alkali shield basalts seen in Gran Canaria are from the oldest and largest
subaerial exposed unit (Schmincke, 1982; Hoernle & Schmincke, 1993a, b; Troll and Schmincke; Troll
et al., 2003).
Hyaloclastite tuffs and debris deposits recovered from core samples from five offshore drill
sites indicate that there have been submarine eruptions in the area despite the fact that no seamount has
been observed in Gran Canaria (Schmincke and Sumit, 1998).
2.1.2 Geology of Tenerife Seamount
Six of the samples examined in the thesis were dredge from the flanks of Tenerife. Tenerife is the largest
(3718 km2) and highest (2034 km2) of the Canaries Islands and is comprised of several volcanic shields
that made up the island (Carracedo et al., 1998). It evolved around 11.9 to 3.9 Ma by the coalescence
of at least three shield volcanoes with distinctive magmatic sources (Thirlwall et al., 2000; Deegan et
al., 2012). Outcrops consisting of the remnant volcanoes have been recorded in Roque del Conde
(South), Teno (NW) and Anaga (NE) massifs (Thirlwall et al., 2000; Guillou et al., 2004; Delcamp et
al., 2010; Delcamp et al., 2012). The Roque del Conde massif records radiometric dates between 11.9
Ma and 8.9 Ma, and this represents the earliest stage and the only exposed part of the much larger
subaerial central shield on Tenerife (Guillou et al., 2004). Teno with radiometric dates between 6.3 Ma
and 5.0 Ma and Anaga between 4.9 Ma and 3.9 Ma represent the later stage of the shields that emerged
in the northwest and northeast parts of the present-day Canary Islands (Guillou et al., 2004; Clarke et
al., 2009; Longpré et al., 2009; Walter at al., 2012). Volcanic emissions from the Roque del Conde
(central shield), Teno and Anaga volcanoes are largely basaltic, with abundant alkali basalts and
picrobasalts, basanites and less frequent mugearites, hawaiites and benmoreites (Thirlwall et al., 2000;
Wiesmaier et al., 2011). The break in volcanism and erosion dating back to 2 Ma might have followed
the last eruptions at Anaga after which the rejuvenated volcanism formed the Las Cañadas edifice in
the central part between 1.9 Ma and 0.2 Ma, as well as the later twin stratovolcanoes and Teide Pico
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Viejo after about 0.2 Ma (Ancochea et al., 1990; Troll et al., 2002; Carracedo et al., 2006; Carracedo et
al., 2007; Carracedo et al., 2009). The most recent eruption on Tenerife is recorded in the Northwest
Rift Zone of the central edifices and occurred in 1909 and is of broadly basaltic composition basaltic
composition (Carracedo et al., 1998; Carracedo and Troll 2016).
2.1.3 Geology of La Palma Seamount
Samples analysed in this project were dredged from the southern flank of La Palma Ridge (n = 5). La
Palma is the westernmost island of the Canary Island archipelago (Fig. 1A) and La Palma is the second
youngest island of the archipelago. The seamount in the southern submarine flanks of La Palma has
been dated to 2.11 Ma (van den Bogaard, 2013), but older than the 1.7 Ma subaerial volcanic edifice
of the island (Guillou et al., 2001). Evidence of a Pliocene evolved seamount has been reported in
the submarine structure of southern La Palma (van den Bogaard, 2013). La Palma has an area extent
of 730 km and altitude of 6463 m which makes it one of the highest volcanic edifices on earth. The
island resulted from several volcanic episodes that started in the Miocene with the formation of both
extrusive and intrusive seamount edifices, which were subsequently affected by several dyke intrusive
cycles, giving rise to a ‘basal complex’. This basal complex has been subjected to hydrothermal
alteration (Staudigel and Schmincke, 1984). The basal submarine complex unit of this island is
separated from the subaerial lava of northern Taburiente volcano by an unconformity. The subaerial
episode of volcanic activity progressed to build up the Garafia and Taburiente shields before growing
a long ridge towards the south (Cumbre Nueva and Cumbre Vieja). The size of these volcanic units
decreases from the Taburiente volcano in the north to the Cumbre Nueva in the central zones and the
Cumbre Vieja in the south where most recent volcanic activity is concentrated (Carracedo et al., 2001;
Walter and Troll, 2003; Carracedo and Troll, 2016).
2.1.4 Geology of El Hierro Seamount
Samples from El Hierro used in this thesis were dredged from seamounts south of El Hierro (Fig. 1A
& B). El Hierro is the youngest volcano in the Canary Islands at about 1.12 Ma (Guillou et al., 1996).
It rests on a 3500 m deep oceanic floor (Fig. 1A). Three-armed rift zone systems characterize the build-
up of El Hierro and have been called a “Mercedes star” geometry (Fig.1A & B). Tiñor and El Golfo,
the two main shields of the volcano, have grown to a level of instability leading to massive gravitational
landslides in the area. The last growth stage of El Hierro began 158,000 years ago and is characterized
by volcanism in the rift zone of the volcano and by volcanism within the El Golfo giant collapse
embayment (Guillou et al., 1996; Carracedo et al., 2001; Carracedo et al., 2012). Post landslide
volcanism is comprised of the Tanganasoga volcanic complex, which formed 4000 years ago, and
Montaña Chamuscada cinder cone at 2500 ± 70 years. The Montaña Chamuscada cinder cone is one of
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the last eruptions on El Hierro (Guillou et al., 1996), along with the recent submarine eruption in
2011/2012 (Carracedo et al., 2015; Troll et al., 2015; Berg et al., 2016).
2.1.5 Geology of Las Hijas Seamount
Samples were also dredged from a small group of seamounts named the Las Hijas (‘the daughters’).
These are located 70 km southwest of El Hierro. The name implies a seamount that is growing to
represent the next island in the Canary archipelago (Rihm et al., 1998). However, van den Bogaard
(2013) dated the dredged trachyte sample from the flanks of this seamount and recorded a radiometric
age of approximately 142 Ma, which meant that they ranked among the oldest seamount in the Canary
Volcanic Province. This evidence led him to suggests a new name, Las Bisabuelas (‘the great-
grandmothers’) for this seamount group (Fig. 1A & B). If active, the Las Hijas seamount may be able
to form a new subaerial island within the next 500,000 years, assuming a standard Canary Island growth
rate during the shield stage of volcanism (Rihm et al., 1998).
2.1.6 Geology of El Hijo de Tenerife Seamount
El Hijo de Tenerife seamount (‘son of Tenerife’) is located between Tenerife and Gran Canaria (Fig.
1A) and it has been dated at 0.2 Ma (van den Bogaard, 2013). It is still unclear whether the seamount
will break the surface to form the eighth Canary Island. By taking insights from previously determined
growth rates of the shield stage of Gran Canaria (Schmincke and Sumita, 1998). This can also be used
to explain the growth of this young seamount that lies in the channel between Tenerife and the steeper
flank of Gran Canaria (Schmincke and Graf, 2000). Although, no major indication of an oceanic fault
has been reported in the profile system of this volcano (Krastel and Schmincke, 2002b), active seismic
signature around the area suggests that Hijo de Tenerife is gradually growing into an active submarine
volcano.
2.1.7 Geology of Tropic Seamount
The Tropic Seamount was also sampled. It is located at the south-western end of the Canary Island
Seamount Province and erupted from 119 Ma to 114 Ma with possible late-stage eruptions until ~ 60
Ma (van den Bogaard, 2013). It is the most isolated of the Saharan group of seamounts (Fig. 1A & B).
It lies about 100 km southwest of the main group and rises from a depth of 4300 m. The Tropic seamount
is a four-armed star that resembles the "NATO" star with directions Southwest, Northwest, Northeast,
and Southeast (Halbach et al., 1983). Trachyte was recovered from the Southwestern flanks of the
seamount, this indicates that trachyte forms a very significant part of Tropic seamount, at least at depths
of about 200m. The overlying conglomerates and flat top indicates that the Tropic seamount was
previously an oceanic island that has been eroded to about 1000 m in height (Halbach et al., 1993).
7
Figure 1. Maps showing the Canary Islands Seamount Province A. The Canary Islands and the Canary Island Seamount Province (modified after Carracedo and Troll, 2016).
The Canary Islands and their associated seamounts extend to the northeast and show different ages with a systematic distribution over the past ~ 65 Ma. In contrast, the
Cretaceous seamounts to the southwest are scattered with respect to their ages (A) and likely follow an ancient oceanic fracture, which would also explain their seemingly
random age distribution (e.g. van den Bogaard 2013; Feraud et al., 1980). B. Overview map showing the ship track of Meteor 43/1 cruise to the Canary Islands in 1998 within
and south of the Canary archipelago and sample points and number of samples taken from each location (Schmincke and Graf, 2000)
Las Hijas
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3 Analytical methods
3.1 Sample preparation
3.1.1 Sample description
A total of 48 rock samples from the Meteor M43-1 Cruise (DECOS, Destruction, and Construction of
Seamounts) to the Canary Islands in 1998 were prepared for petrographical, mineralogical and
geochemical observations. More than 60% of the stations contained volcanic rocks and the samples
vary widely in vesicularity, chemical and mineralogical composition, grain size, structures,
emplacement mechanisms, and degree of alteration. The most common rock types are alkali basalt and
basanite (Schmincke et al., 1998). These dredged samples were recovered from seamounts and from
the flanks of the western Canary Islands, especially the submarine rift systems. These include the South
La Palma ridge (n = 5), South Hierro Ridge (n = 6), the Las Hijas seamounts (n = 10), Tropic Seamount
(n = 6), Los Gigantes (n = 3), Punta de la Rasca (n = 2) near Tenerife off Barranco de Veneguera,
Barranco de Tasartico (n = 4) in Gran Canaria, Hijo de Tenerife (n = 13), and the submarine flank of
Tenerife off Guimar and Anaga (n = 1). Detailed sample descriptions are presented in Table 1.
Simultaneously, these sites were also mapped by swath bathymetry and sub-bottom profiling within, as
well as north and south of the Canary archipelago (Fig. 2 & 3) using parasound and a bathymetric
multibeam system (Schmincke and Graf, 2000). Rocks that were dredged during the cruise comprise
altered basalts, trachyte, vesiculated basalts, abundant carbonate sediments, volcanoclastic tuff with
large clasts of coral shells, felsic lappilistones, basaltic lava and scoria, hemipelagic sediments,
limestones, basaltic lappilistones, as well as felsites and rhyolites. (see Table 1).
Figure 2. Meteor ship used during the M43-1 to the Canary Islands (photo: https://www.ldf.uni-hamburg.de)
9
Figure 3. Photograph of the three main dredge types used on board during the M43-1 cruise exercise: drum dredge
and 2 chain dredges of different design, each of the dredges helps to collect different sample types at a different
point in the subsea floor (images from Schmincke and Graf, 2000).
3.1.2 Sample preparation and processing
Seamount samples (n = 48) were processed for the purpose of this project. Hard and blocky specimens
were cut to sizes suitable for polished thin sections at the Department of Earth Science, Uppsala
University, Sweden. Several samples contained variably consolidated sediments, which could be at a
risk of dissolving in the cooling waters of the saw. These samples were separated and stabilized in
epoxy before being sent for thin section preparation. Resin and hardener were mixed under safe
laboratory conditions to the ratio of 7:1. The mixture was poured into the epoxy holder. Prior to this,
the inner lining of the epoxy holder was lubricated using vaseline in order to prevent the hardened epoxy
from getting stuck to the surfaces of the epoxy holders. The unconsolidated rock specimens were gently
allowed to sink into the epoxy and kept in the oven at 100 °C for about 2 - 3 minutes, aiding the rapid
escape of air bubbles. The samples were allowed to remain in the epoxy for 48 hours before being
removed and sent out for polished thin-section preparation. Rock samples (n = 22) were selected based
on the evident physical properties such as hardness, colour, and texture. These samples represent the
different degrees of alteration, for example, fresh basalt, mildly altered basalts, and altered submarine
rock samples. These rock specimens were crushed using a jaw crusher under clean labouratory
conditions, which prevents contamination of samples. Furthermore, a fraction of the crushed samples
were pulverised using an agate mortar and pestle in the geochemistry labouratory at Uppsala University,
still under clean laboratory procedures. Mortar, pestle, and equipment were cleaned with acetone before
the next sample was processed.
10
3.2 Mineralogy
5mg of pulverised sample (n = 22) were selected and analysed at the Natural History Museum in
Stockholm for Powder X-Ray Diffraction (pXRD) analysis using a PANalytical X’pert diffractometer
that is equipped with an X’celerator silicon-strip detector, which was used to analyse the mineralogical
components. The instrument was operated at 45 kV and 40 mA using Ni-filtered Cu-Kα radiation (λ =
1.5406 Å). Samples were run between 5 – 70° (2θ) for 20 minutes in step sizes of 0.017° in continuous
scanning mode while rotating the sample. Data were collected with "divergent slit mode" and converted
to "fixed slit mode" for Rietveld refinement, using High Score plus 4.7.
3.3 Whole-rock major, trace and rare earth elements
Fresh, hand-picked, and representative fragments of crushed rock samples (n = 22) were sent to Actlabs
(Activation Laboratories Ltd), Ancaster, Ontario, Canada for major and trace element analysis.
LiBO2/Li2B4O7 fusion digestion inductively coupled plasma emission spectrometry (ICP - ES) was
used for analysis of major elements, while 4 acid digestion inductively couple plasma mass
spectrometry (ICP - MS) was used for trace element and rare earth elements analysis. Loss on ignition
(i.e. volatile content) was determined by calculating the mass difference after ignition at 1000 °C. Iron
content is reported as Fe2O3 (T). Method detection limits (MDL) are 0.01 wt. % for all major elements
as oxides except for MnO (0.001 wt. %) and TiO2 (0.002 wt. %). Trace element MDLs are in the range
of 0.001 ppm to 30 ppm.
3.4 Mineral mapping
Backscattered electron (BSE) images from selected thin sections were acquired using the FEG-Electron
Microprobe JXA-8530F Jeol Hyperprobe at the Department of Earth Sciences, Uppsala University.
Prior to analyses, all thin-sections were carbon coated to reduce charging imposed by the electron beam
of the microprobe. Measurements were conducted under a standard operating conditions of 15 kV
accelerating voltage and a 10 nA beam current with counting times of 10s on peaks and 5s on
background.
3.5 Stable isotopes
Five (5) mg of whole rock powder (n = 22) was sent for oxygen and hydrogen isotope and water
extraction analyses at the University of Cape Town in South Africa. D/H and 18O/16O were determined
by using a Finnigan MAT 252 mass spectrometer. D/H determination, the method described by
Vennemann and O’Neil (1993) was employed. The powders were degassed on a conventional silicate
vacuum line at 200 °C before pyrolysis. Water was generated from about 50 mg of an internal biotite
standard (CGBi, δD = -9 ‰ and analysed in duplicate in every sample. The recommended procedure
11
by Coplen (1995) were used for D/H determination by using an internal water standard (CTMP, δD = -
9‰ ) to calibrate the raw data to SMOW standard. Water concentrations were calculated from the
voltage measured on a mass 2 collector of the mass spectrometer using sample inlet volume as
recommended by Vennemann and O’Neil, (1993).
For oxygen analysis, whole-rock samples were dried at 50 °C and degassed in a vacuum on a
convectional silicate line at 200 °C (see Harris & Vogeli 2010). Silicate minerals were reacted with 10
kPa of ClF3 for 3 h at 550 °C. The liberated O2 was converted to CO2 using a platinized carbon rod at
high temperature. The internal quartz standard NBS - 28 (δ18O = 9.64 ‰ ) was used in normalising the
raw data to the standard mean ocean water (SMOW) scale (Coplen et al., 1983). All oxygen data is
reported in the standard δ18O notation relative to the standard mean ocean water (SMOW) where δ =
(Rsample / Rstandard – 1) ×1000 and R = 18O/16O. For H-isotope, all carbonate was first removed by
reaction with diluted HCl. Two samples were analysed for carbonate; 681-1 and 773-5a which had no
carbonate present and analytical error were estimated to be ± 0.1 ‰ ( 1σ), ± 2 ‰ ( 1σ), and 0.10 wt. %
for δ18O, δD, and H2O respectively in all the samples.
3.6 Nannofossils examination
Two rock samples, 638-14, a carbonate sediment with volcanic clasts from the South La Palma Ridge
and 674-2, a ‘basaltic lappilistone’ from South El Hierro ridge were preferentially selected based on
their fossil contents for nannofossils examination. To remove potential contamination with modern
sediments, samples were immersed in a 10% solution of hydrogen peroxide for 24 hours and
subsequently cooked in the same solution for 10 minutes.
Samples were rinsed thoroughly with water and dried in an oven at 50 °C. A small amount of
sediment was scraped off the clean sample. The powder produced from this process was put on a glass
slide and evenly distributed with a drop of water (smear slide). The glass slide was then dried on a
heating plate. The dry sediment powder was permanently mounted using Norland Optical Adhesive and
a cover glass. Prepared slides were analysed for coccoliths under a light microscope at 1000x
magnification at the Department of Earth Science, Uppsala University. From the cleaned sediment
samples, small pieces were carefully broken off to expose fresh surfaces for scanning electron
microscopy (SEM). These small pieces were mounted on aluminium stubs with carbon adhesive (Leit
C), coated with a gold-palladium alloy and then studied using a Zeiss Supra 35VP field emission
scanning electron microscope operating at 5 kV at the Evolutionary Biology Centre, Uppsala
University. The aim was to take high-resolution images of coccolith and foraminifera assemblages in
order to be able to assess and provide precise identification of all the common morphotypes that are still
within the recognisable species level and their geological ages. The identified, age diagnostic specimens
were determined from the established classification scheme for nannofossils.
12
4 Results
4.1 Petrography
Results for petrographical observations for some selected submarine rock samples are summarised in Table 1 and Figure 4, 5 and 6.
Table 1. Petrographic Description and Visually Estimated Degree of Alteration
Sample
ID
Sample type Location Degree of
alteration
Petrographic description
679-1 Felsic lappilistone South Hierro Ridge Moderately
altered
A relatively well sorted, pumiceous lappilistone. The lappilistones are held together by a matrix
of cement filling its open pore spaces. It has smaller vesicle at the margin that becomes lager
towards the interior with strong palagonite alteration and secondary minerals.
679-2 Felsic tuff with intermediate
components
South Hierro Ridge Moderately
altered
It consists of altered pyroxene with few anhedral amphibole, feldspar, and biotite crystals.
681-1 Hemipelagic sediments South El Hierro
Ridge
Moderately
altered
It contains pyroxene phenocryst, with a glass groundmass and lots of nannofossils.
689-1 Felsic lava Las Hijas Seamount Unaltered Felsite, possibly trachyte, with a mildly altered glass and few fresh phenocrysts of feldspar.
687-3 Felsite Las Hijas Seamount Unaltered It contains feldspar, with few mafic inclusions and opaque mineral.
689-2 Amphibole bearing felsite with
mafic inclusions
Las Hijas Seamount Moderately
altered
Highly to moderately altered sample with phenocryst of plagioclase, entirely felsic, fine to
trachytic texture.
689-3a Felsic clast- supported breccia Las Hijas Seamount Moderately
altered
It consists of scattered feldspar microlite that has been altered to clay.
689-3c Felsite breccia with manganese
crust
Las Hijas Seamount Very altered Highly altered sample with few vesicles.
689-6a Felsic lava Las Hijas Seamount Unaltered It is rich in feldspar and a few phenocryst of pyroxene.
703-1b Limestone, basaltic lapillistone,
felsite
Tropic Seamount Very altered It contains many altered clast of different sizes cemented by carbonate, with some fresh
igneous inclusion.
13
703-2 Basaltic lapillistone, carbonate
cemented matrix
Tropic Seamount Very altered Dark, well sorted lapillistone with poorly defined vesicles and a portion of freshly preserved
igneous inclusions. Minor to strong palagonitic alteration, it contains two different clasts,
altered mafic rock and a trachyte or carbonate.
727-1 Felsite Los Gigantes Very altered It consists of altered plagioclase, amphibole, and biotite crystals.
733-1c Fine-grained basalt and
volcaniclastics
Punta de la Rasca Mod. altered It is rich in plagioclase with very few crystals of well-preserved pyroxene and biotite, it also
contains shards of altered mineral along the veins of the specimen, probably a sample
undergoing an early stage of alteration.
733-2 Fine-grained basalt and
volcaniclastics
Punta de la Rasca Very altered It consists of strong palagonite alteration with highly altered plagioclase, pyroxene, and
amphibole crystal.
736-4 Rhyolite Off Barranco de
Veneguera and
Barranco de
Tasartico
Unaltered This consists of subhedral feldspar crystals, pyroxene, and amphibole.
755 Vesicular bombs of intermediate
composition
Hijo de Tenerife Moderately
altered
This sample consists of large vesicles with moderately altered crystals of feldspar and
megacryst of altered pyroxene.
755-8 Vesicular bombs of intermediate
composition
Hijo de Tenerife Moderately
altered
This contains a lot of vesicles with very few crystal of feldspar and megacryst of altered
pyroxene.
756-1 Vesicular bombs of intermediate
composition
Hijo de Tenerife Moderately
altered
This contains a lot of vesicles with very few crystal of feldspar and megacryst of altered
pyroxene.
759-1 Vesicular bombs of intermediate
composition
Hijo de Tenerife Moderately
altered
This contains a lot of vesicles with very few crystal of feldspar and megacryst of altered
pyroxene and deformed amphibole.
773-5a Carbonate sediments with volcanic
clasts
Submarine flank
Tenerife, off
Guimar and Anaga
Moderately
altered
It contains a lot of calcite and chlorites
773-5b Carbonate sediments with volcanic
clasts
Submarine flank
Tenerife, off
Guimar and Anaga
Altered It comprises of carbonates and volcaniclastics
14
A B
C D
E F
G H
15
L M
N O
16
Figure 4 (A - U). Pictures of submarine rock samples in hand specimen A). 773-5 (Submarine flank of Tenerife,
off Guimar and Anaga) - A carbonate sediment with volcanic clasts B). 775-8 (Hijo de Tenerife) - Slightly vesicular
bomb of intermediate composition C. 689-6a (Las Hijas Seamount) - Felsic lava D). 756-1 (Hijo de Tenerife) -
Slightly vesicular bomb of intermediate composition E). 689-2 (Las Hijas Seamount) - Amphibole bearing felsites
with mafic inclusions F). 703-1 (Tropic Seamount) - Limestone, with basaltic lappilistone G). 755 (Hijo de Tenerife)
- Slightly vesicular bomb of intermediate composition H). 727-1 (Los Gigantes) - Felsites I). 687-1 (Las Hijas
Seamount) - Felsic lava. J). 773-2 (Submarine flank of Tenerife, off Guimar and Anaga) - Carbonate sediments with
volcanic clast K). 689-1 (Las Hijas Seamount) - Felsite L). 687-3 (Las Hijas Seamount) - Felsite M. 689-3a (Las
Hijas Seamount) - Felsic clast supported Seamount N. 736-4 (Gran Canaria, off Barranco de Veneguera and
Barranco de Tasartico) O). 759-1(Hijo de Tenerife) - Bombs and volcaniclastics of intermediate composition P).
733-1c (Punta de la Rasca) - Fine grained basalts and volcaniclastics Q). 679-2 (South El Hierro Seamount) - Felsic
tuff with intermediate components. R). 673-1 (South El Hierro Ridge) - Basaltic lappilistone with manganese crust
S). 703-2 (Tropic Seamount) - Basaltic lappilistone T). 689-3c (Las Hijas Seamount) - Felsite breccia with
manganese crust U). 753-1 (Hijo de Tenerife) - Slightly vesicular bomb of intermediate composition.
R S
T U
P Q
17
(b)
(c) (d)
(e) (f)
(g) (h)
(a)
palagonite
palagonite
palagonite
palagonite
palagonite
palagonite
palagonite
18
Figure 5 (a - n). Photomicrographs showing stages of alteration in the representative samples (a - b) 678-2 (South
Hierro ridge) - very altered felsic lappilistone with syenite fragments with deformed plagioclase and palagonite
(c) 674- 4 (South Hierro ridge)- very altered basaltic lappilistone with palagonites. (c - e) 703-2 (Tropic seamount)
- A basaltic lappistone with palagontic mineralization with manganese crust. (f - i) 727-1( Los Gigantes) - Early
stage pelagonitic alteration with deformed amphibole crystals (j - n) 743-3 (Gran Canaria, off Barranco de
Veneguera and Barranco de Tasartico) - very altered basaltic lappilistone with carbonate and zeolite matrix with
early stage palagonitic alteration with maganese crust.
(i) (j)
(k) (l)
(m) (n)
palagonite
manganese crust
19
(b)
(c) (d)
(a)
(e) (f)
20
Figure 6 (a - l). SEM images showing true and false colour alteration textures of altered submarine rocks (a - d)
736-4 (Gran Canaria, off Barranco de Veneguera and Barranco de Tasartico) - BSE image showing alteration
textures of slightly altered specimen with fresh crystals of plagioclase (e - f) 759-1 (Hijo de Tenerife) BSE image
showing alteration textures of very altered submarine samples with clay minerals along the veins of the rocks (i -
l) 703-3 (Tropic seamount) - BSE image showing alteration textures of very altered specimen with clay minerals.
(g) (h)
(k) (l)
(i) (j)
21
4.2 Mineralogy
XRD analyses (Table 2 and figures 3B, 3C, and 7A) show the distribution of primary and secondary
minerals in the submarine samples. Sample ‘773-5b’ is the most altered sample with ~ 95% calcite
(Fig. 9A). Phillipsite also comprised about 24% of the sample (e.g. 733-2), with lots of amorphous
minerals, possibly clay. In fact, the entire altered rock suite contained at least one primary igneous
mineral, mainly feldspar. The moderately altered seamount samples are more enriched in primary
minerals such as quartz, feldspar, pyroxene, and small amounts of calcite (< 1%) (Table 2; Fig. 7B).
Slightly altered seamount samples contained predominately plagioclase, orthoclase, pyroxene, and
amphibole characteristic of fresh basaltic rock assemblages (Sumita and Schmincke, 1998).
Results from petrographical (Fig. 5) and SEM (Fig. 6a - l) analyses further outlined the
alteration products from these submarine rocks. Fig. 4A showed that most of the altered rock
assemblages are composed of secondary minerals such as palagonite, calcite, and chlorites while the
slightly altered mineral assemblages consist of fresh plagioclase, quartz, olivine, pyroxene, amphiboles,
and biotite (see Table 2; Fig. 5a - h). SEM images (Fig. 6g - l) showed that alteration was initiated by
hydrothermal fluids and this opened up veins of these seamount samples leaving tails of clay and other
secondary minerals along the cracks. Distinct relicts of hydrothermal fluid flow direction (Fig. 6e - l)
indicates a fluid-rock interaction (Cabrera Santana et al., 2006).
4.3 Major and trace element
The major and trace-element concentrations of very altered, altered, and slightly altered seamount
samples from the Canaries are presented in Table 3, 4, and 5 respectively. The very altered seamount
samples have very low silica contents of 12.94 - 38.54 wt. % (Table 1), the altered sample has a SiO2
concentration that ranges from 40.51 - 60.56 wt. % (Table 2). The slightly altered sample suite have the
highest silica range of 60.08 - 68.10 wt. % (Table 3). The TiO2 concentration range from 0.75 - 3.75 wt.
% in very altered samples, which is very high relative to 0.75 - 2.48 wt. % and 0.41 - 0.94 wt. % recorded
for altered and slightly altered seamount samples, respectively. This shows a high enrichment of
titanium in the alteration products relative to fresh rock samples. Al2O3 concentration in very altered
sample range from 4.28 - 1.00 wt. %, the altered samples have an alumina content ranging from 9.20 -
17.29 wt. %. The slightly altered samples, however, have the highest concentration of aluminium
ranging from 14.87 - 18.40 wt. %. Fe2O3t is highest in the very altered submarine samples, ranging
from 2.52 - 9.80 wt. %, the altered samples have an iron concentration ranging from 3.34 - 8.31wt%
while slightly altered submarine samples contain 3.20 - 5.10 wt. % of Fe2O3t. MnO concentration is
highest in very altered samples, ranging from 0.06 - 1.07 wt. %, altered samples contain 0.092 - 0.74
wt. % while the slightly altered samples contain the lowest manganese oxide ranging from 0.05 - 0.25
wt. %. MgO is more enriched in the very altered samples with concentration of 1.61 - 5.85 wt. % relative
to 0.82 - 7.00 wt. % and 0.35 - 0.50 wt. % for altered and slightly altered submarine specimen,
22
respectively. The CaO concentration lies between 10.63 and 40 wt.% in the very altered samples, and
are scattered over a low silica range, indicating that there is an enormous enrichment of calcite in the
alteration products relative to the 1.61 - 13.79 wt.% and 0.72 - 2.98 wt.% in altered and slightly altered
samples, respectively. A similar trend has been observed in altered basaltic glass shards (Utzmann et
al., 2002). Both sodium and potassium show low concentration in the very altered samples from 0.85 -
4.05 wt. % for Na2O and 0.80 - 4.22 wt. % for K2O relative to 1.45 - 6.75 wt. % for Na2O and 0.16 -
7.95 wt. % for K2O and 5.88 - 6.43 wt. % for Na2O and 3.93 - 4.81 wt. % for K2O for altered and
slightly altered samples, respectively. P2O5 concentration ranges from 0.21 - 2.19 wt. % in the very
altered samples and from 0.16 - 7.95 wt.% in altered samples while the concentrations of P2O5 in slightly
altered submarine rock lie between 0.14 - 1.10 wt. %. Loss on ignition (LOI) is highest in the very
altered samples and ranges from 15.27 - 18.33 wt. % while the altered samples have 4.34 - 18.33 wt.
%. LOI values are expectedly low in the slightly altered submarine rocks, ranging between 0.22 - 2.04
wt. %.
Plots for fluid immobile (TiO2) and fluid mobile elements (Na2O, K2O, CaO, Sr, and Rb) versus
silica are shown in (Fig. 10a - g). Plots for fluid immobile (TiO2 and Zr) and fluid mobile (Na2O, k2O,
Pb, Sr, and Rb) versus Nb are shown in (Fig. 9a - g). Mass balance plot for some selected trace elements
are shown in (Fig. 11). Plotted in all these fields are data from hydrothermally altered tuff (Donoghue
et al., 2008) and zeolite composition (Utzmann et al., 2002) for comparison. Most of the unaltered to
slightly altered seamount samples show a very linear correlated magmatic trend relative to the very
altered samples analysed in this study (Hansteen and Troll 2003; Donoghue et al., 2008). Mass balance
plots for trace elements distribution in the seamount sample suite show the enrichment trends for most
elements during submarine alteration.
23
Table 2. Distribution of minerals in altered, moderately altered and unaltered seamount samples from XRD results
Qtzh- Hydrothermal quartz, Plg-Plagioclase, Orth-Orthoclase, Ol-Olivine, Cpx-Clinopyroxene, Amph-Amphibole, Calc-Calcium, Fapt-Flouroapatite, Pgskt-Palygorskite, Mag-Magnetite, Antgt-Antigorite, Ilm-Ilmenite,
Kae-Kaersutitite, Phps-Phillipsite (zeolite), Fsp-Ferrosilite, Gyp-Gypsum, Chl-Chlorite, Mags-Magnesite, Rht- Richterite, Amp min-Amorphous minerals.
Degree of
alteration
Primary minerals Secondary minerals
Qtzh Plag Orth Ol Cpx Bt Amp Rht Cal Dol Kae Fapt Pgskt Mag Antgt Ilm Fsl Phps Gyp Chl Mag Amph min
Very altered X X X X
689-3c X
703-1b X X X X X
703-2 X X X
733-2 X X X
773-5b X X X X
687-1 X X X
679-1 X X X X
Altered
679-2 X X X X
755 X X X X
755-8 X X X X
689-2 X X
756-1 X X X X
759-1 X X X X X
681-1 X X X X X
733-5a X X X X
733-1c
X X X X X
Slightly altered
736-4 X X X
687-3 X X X
689-1 X X X
689-3a X X X
689-6a X X
24
25
26
Figure 7 (A - C). Pie chart showing distribution of primary and secondary alteration minerals in representative
rock samples from submarine rocks. (A) Distribution of alteration minerals in the very altered samples, mainly
consisting of calcite alteration. (B) Distribution of primary and secondary minerals in the altered samples, the
degree of alteration ranges from albite alteration to primary igneous compositions. (C) Slightly altered phase with
a fresh igneous rock composition consisting of clinopyroxene, plagioclase and biotite. Full data are provided in
Table 2 (note: wedges are not labelled for quantities <1%).
27
Table 3. Major element composition of very altered seamount samples
Elements 679-1 703-2 703-1b 733-2 773-5b 727-1 689-3c
wt. %
SiO2 38.55 21.69 22.02 34.32 12.76 37.77 12.94
TiO2 3.22 2.741 2.57 3.76 0.685 1.72 0.75
Al2O3 11.00 6.34 7.47 14.34 4.36 13.52 4.28
Fe2O3 9.81 9.79 9.68 12.41 2.53 6.09 2.53
MnO 0.06 0.12 0.05 1.07 0.047 0.16 0.09
MgO 2.63 5.86 1.62 2.79 2.45 3.93 2.57
CaO 10.64 26.23 26.38 7.72 40.52 14.30 40.02
Na2O 1.14 0.86 1.32 3.53 1.15 4.06 1.28
K2O 4.23 0.82 1.57 1.83 0.74 2.74 0.80
P2O5 0.77 0.77 2.19 0.56 0.22 0.33 0.27
LOI 18.10 24.58 25.31 17.76 34.58 15.27 33.98
Sum 100.29 99.60 100.29 100.10 100.10 99.80 99.11
ppm
Co 30 48 24 157 5 12 5
Ni 130 230 80 300 20 40 20
V 188 173 158 159 52 119 54
Zn 90 90 120 290 40 100 40
Ce 56.4 74.5 198 150 39.8 111 34.8
La 43.7 36.8 149 45.6 23.9 65.5 20.3
Nb 43 49 37 100 30 103 28
Ga 16 11 14 13 9 16 8
Pb 5 5 5 12 5 5 5
Rb 77 18 33 22 15 55 13
Ba 40 106 69 229 158 529 156
Sr 84 250 243 733 847 797 946
Th 3 3.4 8.1 6.5 2.7 8.9 2.5
Y 46 16 56 37 10 20 9
Zr 281 217 421 496 127 413 117
28
Table 4. Major element composition of altered seamount samples
Elements 759-1 681-1 733-1c 689-3a 689-2 679-2 687-1 755 755-8 756-1 773-5a
wt.% SiO2 51.91 41.61 48.59 60.56 55.28 54.68 40.51 52.88 50.42 52.01 53.77
TiO2 1.23 0.75 2.49 0.67 1.19 1.54 0.50 1.23 1.16 1.18 0.92
Al2O3 14.43 11.19 17.09 17.29 16.72 16.11 9.20 14.43 13.75 14.27 16.37
Fe2O3 8.41 4.745 8.94 4.56 5.17 6.99 3.34 8.31 8.06 8.12 7.09
MnO 0.40 0.75 0.31 0.05 0.06 0.11 0.16 0.42 0.40 0.57 0.09
MgO 2.84 7.01 3.16 0.82 1.23 2.53 4.60 2.49 2.664 2.47 2.39
CaO 3.56 9.21 6.88 1.80 5.54 3.35 13.79 3.45 3.36 3.33 1.61
Na2O 6.62 1.47 5.08 5.52 5.00 4.11 1.62 6.35 6.22 6.76 4.15
K2O 3.35 2.13 2.33 3.61 3.37 3.15 1.71 3.55 3.56 3.65 2.12
P2O5 0.93 2.89 0.94 0.18 2.18 0.57 7.95 0.87 0.81 0.92 0.16
LOI 6.13 18.34 4.37 4.82 4.34 6.62 15.93 5.49 9.35 6.14 10.68
Sum 99.62 100.10 100.29 99.78 100.19 99.54 98.71 99.06 99.45 98.82 98.74
ppm
Co 8 65 22 1 4 9 12 7 7 12 3
Ni 40 580 20 20 20 20 90 20 20 20 20
V 39 61 133 8 74 104 44 48 39 39 14
Zn 250 150 120 110 170 110 90 250 250 270 210
Ce 414 81.7 192 182 161 164 39.8 384 401 416 125
La 215 175 99.6 109 84.8 104 189 203 211 221 69.9
Nb 264 37 127 120 113 87 15 232 265 261 128
Ga 31 17 21 30 27 22 16 33 31 33 26
Pb 9 14 7 5 5 5 5 10 9 10 5
Rb 71 53 35 32 33 34 63 72 69 83 28
Ba 1663 119 876 1246 809 384 92 1552 1558 1594 741
Sr 2816 206 1287 231 382 363 311 2640 2573 2720 285
Th 27.20 7.9 10.1 14.1 12.5 9.1 5.6 25.7 26.1 27.1 12.7
Y 54 224 45 75 34 34 216 54 55 56 24
Zr 1707 109 437 608 542 608 126 1697 1743 1793 1364
29
Table 5. Major element composition of slightly altered seamount samples
Elements 687-3 736-4 689-6a 689-1
wt. %
SiO2 60.65 68.10 62.61 60.08
TiO2 0.94 0.76 0.42 0.56
Al2O3 17.41 14.87 18.40 17.42
Fe2O3 5.10 3.34 3.20 3.83
MnO 0.07 0.26 0.05 0.11
MgO 0.35 0.50 0.46 0.37
CaO 2.31 0.72 1.42 2.98
Na2O 5.88 6.43 6.35 6.17
K2O 4.81 3.93 4.63 4.80
P2O5 0.46 0.14 0.19 1.10
LOI 1.69 0.22 2.04 1.88
Sum 99.48 98.65 99.57 98.70
ppm
Co 1 1 1 1
Ni 20 20 20 20
V 32 8 20 14
Zn 120 160 70 120
Ce 187 250 142 207
La 105 119 75.9 122
Nb 119 100 69 93
Ga 28 31 29 31
Pb 5 6 5 5
Rb 50 90 46 67
Ba 994 591 1053 1084
Sr 253 42 196 212
Th 12.2 14.8 17.6 13.6
Y 81 65 14 54
Zr 1671 608 430 732
30
Figure 8 (a - g). Plots of fluid-immobile (TiO2) and fluid-mobile (K2O, CaO, Na2O, Pb, Sr, Rb) elements versus
SiO2 for the very altered, altered, and slightly altered seamount samples. Hydrothermally altered tuff from Gran
Canaria (Donoghue et al., 2008) and zeolite composition (Utzmann et al., 2002) were also plotted. The slightly
altered samples in this study show a close to linear trend on all plots relative to the altered and moderately altered
samples that are more dispersed. Note that all trace elements are reported in ppm and all oxides in wt. %. Slightly
altered samples are clouded in black, altered are clouded in blue and very altered samples in pink. Green and
purple clouds mark Donoghue et al., (2008) and Utzmann (2002) samples, respectively.
Very altered samples
Donoghue et al., 2008
Utzmann et. al., 2002
Slightly altered
Altered samples
(a) (b)
(c) (d)
(e) (f)
(g)
31
Figure 9 (a - g). Plots of fluid-immobile (TiO2and Zr) and fluid-mobile (K2O, Na2O, Pb, Sr, Rb) elements versus Nb for
submarine rock samples. Data from hydrothermally altered tuff from Gran Canaria (Donoghue et al., 2008) and zeolite
composition (Utzmann et al., 2002) were also plotted. The slightly altered samples in this study showed a close to linear trend
on all plots relative to the very altered and altered samples. This indicates strong mobilisation of these elements during
seawater-rock interaction. The altered and moderately altered samples are generally more enriched in the fluid mobile elements
relative to the unaltered seamount samples in this study. Note that all trace elements are reported in ppm and all oxides in wt.
%. Slightly altered samples are clouded in black, altered are clouded in blue, and altered samples in pink. Green
and purple clouds mark Donoghue et al., (2008) and Utzmann, (2002) samples, respectively.
Very altered samples
Slightly altered samples
Altered samples
Utzmann et. al., 2002
Donoghue et al., 2008
8
(a) (b)
(c) (d)
(e) (f)
(g)
32
4.3.1 Loss and gain of major oxides
Alteration processes are generally associated with the loss of certain elements. In order to be able to
effectively calculate the loss and gain of elements during alteration, elements such as titanium, for
instance, are usually regarded as immobile elements during rock alteration and are therefore commonly
used as reference elements (Staudigel and Hart, 1983; Faure, 1998). When using titanium as a reference
element, the passive enrichment factor is given by the ratio Ti altered samples / Ti fresh rocks and the gain-loss
factor of a given element (E) by (E altered sample / E fresh samples) / (T altered sample / Ti fresh sample). Elements that
have been gained or lost relative to Ti will have gain/loss factors above and below 1, respectively (Fig.
10). Even though titanium may be an ideal element in many cases, it may itself have been gained in this
system. In this calculation, the average composition of the submarine rock was used.
Figure 10. Mass balance plot for major oxides. Chemical changes (gain of >1 and loss of <1 of oxides and
elements) during the alteration of fresh basalts to the altered phase with a constant Ti content ((E altered / E fresh) /
(Ti altered / Ti fresh); E = wt. % of element). The gain and loss calculations show that most of the major elements (Si,
Na, K, Mn, Al, and Fe) have been lost during alteration. The mass balance calculation was based on the average
composition of all the submarine rock samples.
4.3.2 Trace element mobility
Mass balance plots for trace elements (Fig. 11) show that there is a strong enrichment in Sr, and to an
extent in Zr, Nb, Gd, Dy, and Y in altered submarine rocks. This strongly supports the evidence
presented by Utzmann et al., (2002) that transition metals and REE are enriched in the alteration
products of slightly altered submarine rocks during low-temperature seawater-rock interaction relative
to the unaltered submarine suite in this study. There is also a slight deviation from the original parent
compositions of basalt from similar geological setting (e.g. Fig. 19).
33
Figure 11. Mass balance plot for trace elements. Chemical changes (gain of >1 and loss of <1 of oxides and
elements) during the alteration of fresh basalts to the altered phase with a constant Ti content (E altered / E fresh / Ti
altered / Ti fresh); E = wt. % of element). The gain and loss calculations show that most of the major elements (Rb,
Nb, Ce, Pr, Sr, Nd, Sm, Zr, Gd, Dy, and Y) have been gained during alteration. The mass balance calculations
was based on the average composition of all the submarine rock samples.
4.4 Rare earth element chemistry
The average REE distribution normalised to chondrite composition shows that moderately altered
samples are more enriched in LREE relative to the altered and unaltered equivalent (Fig. 12). A similar
trend is seen in the HREE trend. Rare earth element composition from altered glass shards from Gran
Canaria (Utzmann et al., 2002) also plot in the same field, following a trend similar to altered seamount
samples in this study. This further supports that REEs are generally less enriched in altered basaltic
suite (Guy et al., 1999; Utzmann et al., 2002). Trace elements therefore get mobilised in highly altered
rocks, but not in moderately altered samples.
Figure 12. Chondrite normalised rare earth elements average distribution pattern of altered, moderately altered
and unaltered basaltic samples. REE data of altered glass (Utzmann et al., 2002) and average basanite
concentration from Deegan et al., 2012 are also plotted. The data of the chondrite normalised samples are from
Sun and McDonough, 1989.
34
4.5 Stable isotopes
The δ18O and δD compositions and water concentrations of the very altered, altered, and slightly altered
seamount samples are given in Table 6. The very altered seamount samples have anomalously high
δ18O values between 12.8 ‰ and 27 ‰ (n = 6), the δ18O values for the altered lie between 10.8 ‰ -
30.6 ‰ (n = 11), while the slightly altered seamount sample have considerably low δ18O ratios between
6.2 ‰ - 8.6 ‰ (n = 4; Fig. 12A & B). The δD values for the very altered samples range between -67 ‰
and -193 ‰ (n = 6), that for altered sample lie between -31 ‰ and -110 ‰ (n = 13) while that of the
slightly altered seamount samples have δD values between -117 ‰ and -124 ‰ (n = 3; Fig. 13A & B).
Water concentration as high as 3.8 wt. % and 4.6 wt. % characterize very altered and altered seamount
samples. Anomalously low H2O content values were also recorded for some of the very altered to altered
samples with ranges from 0.47 - 0.74 wt. %. This could be due to reheating of initially emplaced
submarine rocks, leading to degassing of water in these rocks. The water concentration is less than 0.4
wt. % in the slightly altered samples (Fig. 13B).
35
Table 5. Stable isotopes composition and H2O wt. % content of submarine rocks
Sample ID.
δ18O‰
δD‰
H2Owt. %
Very altered
679-1
25.9
-193
3.81
689-3c 27 -168 0.47
703-2 19.2 -67 3.13
733-2 18.2 -105 2.64
727-1 12.8 -84 0.70
773-5b 25.7 -178 0.46
Altered
679-2 13.2 -80 0.85
681-1 - -95 4.59
687-1 20.6 -42 4.59
689-2 12.2 -102 0.67
689-3a 10.8 -110 1.01
689-2 12.2 -102 0.67
755-8 9.9 -31 0.83
756-1 10 -101
0.79
703-1b 23.6 -86 4.05
755 10.9 -103 0.98
759-1 9.1 -96 0.74
773-5a - -89 2.84
Slightly
altered
687-3 8.6 -117 0.36
689-1 8.3 -123 0.22
689-6a 8.7 -124 0.26
733-1c 7.0 -98 0.32
736-4 6.2 - -
36
Figure 13 (A and B). (A) Bar chart showing the range of whole rock δ18O values obtained for very
altered, altered and slightly altered Canary Island seamounts samples. Unaltered ignimbrite from Gran
Canaria (Donoghue et al. 2008) with values close to the unaltered samples in this study, and the δ18O
(‰) ranges of mantle rocks (Taylor, 1986) are also shown. (B) Plot of whole-rock LOI wt. % versus
δ18O for altered, moderately altered, and unaltered samples. The unaltered samples have much lower LOI
and δ18O values, relative to the moderately to altered seamount samples.
A
B
37
Figure 14 (A and B). (A) Bar chart showing the range of whole rock δD values obtained for very altered, altered, and
slightly altered submarine samples. Estimated δD range for ambient meteoric water at the alteration site in Gran Canaria
(Donoghue et al. 2008) are shown. (B) Plot of whole-rock δD versus H2O wt. % for very altered, altered, and slightly altered
samples. The altered to very altered samples have an elevated water concentration relative to the unaltered samples, which is
indicative of large interaction with the meteoric water. The general trend also show that the average concentration of δD and
H2O wt. % are depleted in the slightly altered samples relative to the very altered and altered samples due loss of water
during degassing.
B
A
38
Figure 15. Plot of whole-rock δ18O versus δD for very altered, altered, and slightly altered seamount samples.
Also shown are the Global Meteoric Water Line (GMW), the meteoric water lines for North (GCN) and South
Gran Canaria (GCS; Gonfiantini, 1973; Donoghue et al., 2010), the kaolinite line (KL; Savin and Epstein, 1970),
and the hydrated volcanic glass line (HVG; Taylor, 1986). The fields for present day Gran Canaria water (Javoy
et al., 1986), Standard Mean Ocean Water (SMOW), and magmatic water (Taylor, 1986) are also shown for
reference.
4.6 Distribution of nannofossils at the seamount south of El Hierro ridge
The sample ‘674-2’ comprises a wide variety of calcareous micro and nannofossils, mainly foraminifera
and cocolithophores (Fig. 17 A-C). Two main groups of nannofossils were identified in this project, the
first being the very crystallized group and the later a living group. Most of the coccoliths showed
secondary overgrowth (Fig. 18 E) or heavy recrystallization. A similar case of poorly preserved
coccoliths has been reported in the sedimentary relicts of the El Hierro eruption in 2011 (Zaczek et al.,
2015). A total of seven coccolith taxa and one foraminifera species were identified. The recovered
coccoliths comprise Coccolithus miopelagicus, Calcidiscus leptoporus, Rhabdosphaera clavigera,
Acanthoica quattrospina, Discosphaera tubifera, Gephyrocapsa carribeanica, and Emiliana huxleyi
(Fig. 18 A-F). One age diagnostic foraminifera species, Globoconella terminalis was also identified.
The dominant geological age bracket across this set of calcareous nannofossils can be obtained from
the very recent and well-preserved Emiliania huxleyi (~ 290,000 years) to the oldest, but extinct,
Coccolithus miopelagicus (~ 14 Ma, Miocene age; Fig.16; cf. nannotax.org).
39
Figure 16. Stratigraphic range chart for identified fossil taxa and micro-palaeontological record of some calcareous nannofossils in basaltic lappilistones at South El Hierro
Seamount. The sample contains predominately Jurassic to recent species that define a common Miocene to recent age (14 Ma - 290,000 years). All the nannofossils species
date to the Neogene age. (A) The oldest distribution of nannofossils (B) The recent to modern day distribution of nannofossils.
65,5Ma 2,6 Ma 290,000yrs
Palaeogene Neogene
Quatern
ary
Recen
t
Palaeo
cene
Eocen
e
Olig
ocen
e
Mio
cene
Plio
cene
Than
etian
Selan
dian
Dan
ian
Priab
onian
Bato
nian
Lutetian
Ypresian
Chattian
Rupelian
Messin
ian
Torto
nian
Serrav
alian
Lan
ghian
Burd
igalian
Aquitan
ian
Piacezian
Zan
cian
Ho
locen
e
Pleisto
cene
Species Samples
Cocolithosphores 638-14 674-2
Emiliana huxleyi x
Calcidiscus leptoporus x
Coccolithus miopelagicus x
Rhabdosphaera clavigera x
Acanthoica quattrospina x
Discosphaera tubifera x
Gephyrocapsa carribean x
Foraminifera
x
Globoconella terminalis A
B
40
Figure 17 (a-c). Light micrograph cross-sections of the general distribution of representative smear counts for
nannofossils present in the specimen under varied microscopic settings (a) distribution of nannofossils under plane
polarised light (b) Distribution of foraminifera and cocolithophores assemblage under cross polarised light (c)
Magnified SEM image showing the distribution of coccoliths species in the studied samples. Note the overgrown,
recrystallized, and freshly preserved coccoliths.
(a)
1µm
1µm
(b)
(c)
41
Figure 18 (a-f). SEM images of foraminifera and cocolithophores with variation in the degrees of primary
calcification, secondary dissolution and secondary calcite overgrowth. (a) A well preserved Globoconella
terminalis, foram of possibly Messenia age. (b) Well-preserved Emiliana huxleyi (~ 290,000 years) (c)
Discosphaera tubifera, Pliocene to recent age. (c) Acanthoica quattrospina, Miocene age. (e) Calcidiscus
leptoporus, Miocene age. (f) Rhabdosphaera clavigera, Pliocene to recent. Generally most of the studied species
lie in the Neogene age.
(a) (b)
(c) (d)
(e) (f)
42
5 Discussion
5.1 Petrology and mineralogy
The slightly altered submarine rocks are comprised of main igneous rock forming minerals such as
olivine, amphibole and plagioclase as well as secondary ´´hydrothermal´´ quartz. The very altered
submarine samples are comprised mainly of calcites, phillipsite, kaersutitite; calcium-rich amphibole
and a considerable amount of amorphous minerals that were difficult to identify from the XRD analyses
(Table 2). The very altered submarine rocks also contain an appreciable quantity of original plagioclase,
olivine, and amphibole phenocryst in their groundmass. Petrographical observations also confirmed the
presence of high glittering reddish pink palagonite in the very altered phase (Fig. 5A). The presence of
this alteration product (palagonite) suggests fluid-rock interaction to be the reason for the depleted silica
content (Staudigel et al., 1996; Stroncik and Schmincke, 2001). The presence of these secondary
mineral assemblages such as calcite and clay further suggest very low-temperature water-rock
interaction processes. The general trend of these secondary mineral assemblages also suggests that
phillipsite and calcite represent the last stage of low-temperature alteration of some of the altered sample
suites at temperatures less than 20 ℃ (Alt et al., 1986; Sheppard and Gig, 1996; Deer et al., 1966;
Roberts, 2001).
5.2 Element fluxes
The very altered and moderately altered submarine samples show low SiO2 wt.% concentrations relative
to slightly altered sample suite, hydrothermal tuff (Donoghue et al., 2008), and zeolite (Utzmann et al.,
2002; Fig. 8a - g). This low silica content suggests a low-temperature fluid-rock interaction. Similarly,
an extensive decrease in silica is observed (see Fig. 8a - g) and SiO2 is generally said to be lost during
palagonitization of glass (Kruber et al., 2008).
There is a general decrease in the concentration of the total alkali over a scattered field in the
very altered and moderately altered samples when plotted against silica relative to the slightly altered
samples (Fig. 8c and e). This suggests a loss of Na and K during low temperature alteration. There is a
noticeably high concentration of calcium (Fig. 8d) in the very altered specimen relative to the slightly
altered samples, although this does not apply to the hydrothermal tuff (Donoghue et al., 2008) and
zeolite (Utzmann et al., 2002). This shows that calcium is gained by the altered sample suite during the
low-temperature hydrothermal alteration (see Figure 10).
However, on the plot of immobile elements Zr and TiO2 versus Nb (Fig. 9a & b), the very altered
specimen tends to be generally enriched in the fluid immobile elements i.e. they are enriched due to
being a residue. This correlates with hydrothermally altered tuff (Donoghue et al., 2008) relative to the
more linearly correlated slightly altered samples, indicating a high mobility of trace elements in the
43
very altered and moderately altered submarine rocks. In addition, there is also a considerable mobility
of Pb in the much altered and moderately altered submarine rocks relative to their slightly altered
equivalents (Fig. 9f).
5.2.1 Loss and gain of major oxides
Chemical fluxes are very dependent on the chemical composition of rocks, changes in temperature as
well as the chemistry of fluids. Bednarz et al., (1991) also demonstrates the importance of temperature
on the mobility of elements during alteration. In this study, however, potassium is gained. Potassium is
generally leached from basalt at high temperature (Utzmann et al., 2002) and subsequently gained with
decreasing temperature. Contrary to the report of Schmincke et al., (1982), who suggested the loss of
Mg during low-temperature hydrothermal alteration, Mg is gained here probably due to the
incorporation of Mg from sea water during a restricted fluid accumulation in the altered phase (Berger
at al., 1987).
There is also a noticeable loss of Si, Al, Na and K and a minor enrichment of Ti and Fe during low-
temperature alteration of these basalts (Fig. 10). Similar analyses by Stroncik-True (2000) have also
shown that there is a loss of these elements during the transformation of sideromelane to palagonite in
a low- temperature marine environment.
Calcium leaching from basaltic rocks during submarine alteration is well documented (Seyfried
and Mottl, 1982, Bednarz et al., 1991). Here, there is a general enrichment of calcium. Iron and
manganese are generally affected by redox reaction and might be difficult to discuss in this context.
5.2.1 Trace elements mobility
The mass balance plot for the whole sample suite (Fig. 12) shows a strong enrichment of Sr and, to an
extent, Zr, Nb, Gd, Dy, and Y in altered submarine rocks. This strongly supports the evidence posited
by Utzmann et al., (2002) that transition metals and REE are enriched in the alteration product of fresh
submarine rocks during low-temperature seawater-rock interaction relative to the unaltered rock suite.
The deviation of the three subdivisions of the samples used in this study from the parent composition
field (Fig. 19) further support the mobility of trace elements in the submarine rock suites and that none
of these submarine samples are fresh.
44
Figure 19. The altered submarine rocks from this study and parent basalt and weathered rock samples (Hill et.
al, 2000) plotted on a Ti–Zr–Y discrimination diagram. The parent rock samples are shown as white dots, the
weathered rocks as black dots, and the altered submarine rocks as red cube.
5.3 Rare-earth element chemistry
The distribution of the average REE pattern of very altered, moderately altered, and slightly altered
samples followed a similar trend: they are all more enriched in light rare earth elements (LREE) relative
to heavy rare earth elements (HREE). The moderately altered rocks are anomalously enriched in REE
relative to very altered and unaltered samples. A similar trend has been reported in REE concentration
of saponite and hyaloclastite (cf. Guy et al., 1999).This indicates that the initial concentration of REE
in the fresh phase is less enriched prior to the onset of hydrothermal alteration of these sub-volcanic
rocks (Utzmann et al., 2002). Elevated REE concentrations in the moderately altered samples relative
to the very altered samples (Fig.18) indicate that REE enrichment is significantly dependent on the
degree of rock-fluids fractionation of fresh rocks (Guy et al., 1999) and that release of REE occurs
dominantly after certain a threshold in the degree of alteration has been reached and not after the primary
igneous mineral have been totally altered.
However, a close study of the trend of REE enrichment (Fig. 12) showed that the average
abundance in the very altered samples (as well as altered and moderately altered samples) is close to
those of the unaltered to slightly rocks in this study (Bruque et al., 1980; Berger et al., 1994) and further
emphasizes that the distribution of REE during submarine alteration is dependent on the extent of
seawater-rock interaction.
45
Figure 20. Ce versus La plot. The calculation of the correlation coefficient R2 is based on data from altered,
moderately altered, and unaltered to slightly altered seamount samples. The altered samples showed a very low
correlation coefficient of 0.379, indicating a weak correlation of unaltered to slightly altered samples during
seawater-rock interaction. Also note the high concentration of Ce in moderately altered samples.
The weak correlation of Ce and La (Fig. 20) in the altered samples further supports an
enrichment of REE in the altered submarine rocks. This, however, partially disagrees with some of the
strong correlation existing in the zeolite composition examined by Utzmann et al., (2002). In fact, this
means that there is a favoured enrichment of Ce relative to La in the altered phase (Fig. 20) in a very
low-temperature system since alteration products can readily incorporate trace metals in their crystal
lattice (e.g. phillipsite; Curkovic, et al., 1997).
5.4 Stable isotopes
The δ18O values recorded for very altered and moderately altered samples in this study are remarkably
high and ranges from 12.8 - 27 ‰ and 7.0 - 23.6 ‰, respectively, relative to 6.2 - 8.7 ‰ for the unaltered
to slightly samples (Table 5; Fig. 13A & B). The submarine rocks with elevated δ18O values also show
a variety of alteration mineralisation (Figures 5A - D). The slightly altered samples fall within a δ18O
range of 6 - 8‰ (Taylor 1968) for igneous rocks and those of unaltered ignimbrites from Gran Canaria
(Troll and Schmincke, 2002; Donoghue et al., 2008; Berg et al., 2018: in press). Mantle derived basalts
have δ18O values of 5.7 ± 0.2 ‰, which could increase by 1 ‰ through fractional crystallization in a
closed system in ocean islands (Valley et al., 2005). Increase or decrease of this value would indicate
an addition of components in an open system. This helps us to quantitatively determine the low
temperature alteration of minerals and water content of rock (Taylor, 1986; Deegan et al., 2012). In
addition, the very altered and moderately altered samples also show a considerably high LOI vs δ18O
R² = 0.3796
0
50
100
150
200
250
300
350
400
450
0 50 100 150 200 250 300
Ce
(pp
m)
La (ppm)
Altered
Moderately altered
Unaltered
46
trend (Fig. 13B). It thus means that there is a very high loss on ignition in the very altered and
moderately altered rocks relative to slightly altered samples.
Whole-rock values for δD of slightly altered samples lie between -98 and -117 ‰, and have an
H2O content that is less than 0.35 wt.%. This slightly low δD and H2O values can be explained using
Rayleigh-type water exsolution processes that deplete hydrogen and water concentration during
degassing prior to eruption (Fig. 16B; Taylor et al., 1983; Nabelek et al., 1983; Taylor, 1986). The very
altered and moderately altered suite showed δD values that deviate substantially from the mantle range.
The very altered sample suite showed a wide offset from the kaolinite line of Savin and Epstein
(1970) and the hydrated volcanic glass line of Taylor (1968) relative to the slightly altered samples (Fig.
17). Taylor (1968) showed that many rhyolites, rhyolite obsidians, and ignimbrites have very high δ18O
values but none of these very extreme enrichments in δ18O represent a primary magmatic feature,
instead the high δ18O values all resulted from an event that involved low-temperature exchange or
hydration of the volcanic glass by seawater. Hypohyaline rocks containing more than 1 wt. % LOI are
generally not considered a representative fraction of their parent melt but are rather a product of post-
magmatic interaction with sea water (Taylor, 1968; Troll and Schmincke, 2002).
The δD fractionation values for clay minerals and water lie between -30‰ and -20‰ at 20 ℃
and further decrease to +19.7 ‰ at 50 ℃ and +5.6 at 200 ℃ (Sheppard and Gilg, 1996) and show
similar δ18O values during silica- water fractionation of 11.6 ‰ at 200 ℃, +13.1 ‰ at 100 ℃, and 19.7
‰ at 50 ℃. The XRD analyses of the altered submarine samples suite justifies the presence of
secondary alteration minerals such as calcite and amorphous clay (Table 2). This suggests that the very
altered samples must have extensively reacted with seawater blocks (Muehlenbachs et al., 1974;
Hildreth et al., 1994; Taylor 1986), which makes it possible to estimate the alteration temperature of
these altered submarine rocks with very high δ18O values to be ≤ 50 ℃.
5.5 Volcanic evolution of El Hierro seamount
The estimated age range of nannofossils that were examined in this study from south of El Hierro Island
suggests a fairly young submarine age. Our results do not strictly disagree with Ar-Ar ages of 133 Ma
(early Cretaceous) obtained by van Bogaard (2013). The submarine sample (674-2) that was
investigated in this study was dredged at approximately 12 km north from the location of the specimen
(678-2), dated by van den Bogaard (2013). The fossil evidence gives recent to Miocene age (Neogene).
Age correlation of species (Fig. 16) showed two main groups of nannofossils assemblages; the living
and the recycled group. The former were difficult to investigate due to extensive recrystallization of the
fossil group, the specimens from the recycled group seemed to have evolved from much deeper depth
in the seafloor and affected by hydrothermal fluids, at least relative to the living group of nannofossils
assemblage. While, the latter group comprises Emiliania huxleyi (~ 290,000 years) to the much older,
but extinct, Coccolithus miopelagicus (Messinian age, ~14Ma), this marked the age bracket of
47
nannofossils examined in this project. Furthermore, this implies that this seamount south of El Hierro
either evolved from the young Canary activity and not from cretaceous magmatic events. This study
thus further supports the young fossil age (2.5 Ma) recorded in sedimentary strata beneath the basements
of El Hierro Island (Zaczek et al., 2013).
Figure 21. Map of El Hierro showing the sample points for nannofossils samples investigated in this study, van
Bogaard, (2013) in the South of El Hierro ridge, and the 2011-2012 submarine eruption at El Hierro.
48
6 Conclusions
The coordinated analyses of mineralogy, major and trace element concentration, isotopic composition
as well as nannofossils investigation revealed that:
1. All the submarine rocks investigated in this study have undergone different degrees of alteration
and are generally enriched in low temperature alteration minerals such as calcite, chlorite, and
clay. Calcite seems to have replaced the primary minerals in the presence of CO2- rich fluids in
the sub-seafloor at temperatures less than 50 ℃.
2. There is a general enrichment of calcium and phosphorous and loss of Si, Al, and alkaline
elements, Na, K and enrichment of Ca, Mg, Pb, and Sr in these sets of submarine rocks,
suggesting high degree of element mobility during low-temperature seafloor alteration.
3. Transition elements such as Ti, Mn, Fe, Nb, Zr, and REEs are more enriched in the altered
submarine samples due to low-temperature alteration of the oceanic crust, which is probably
due to their preferential affinity for alteration minerals during the seafloor alteration.
4. Isotopic observations show that the investigated submarine rocks have undergone low-
temperature alteration at temperatures less than 50 ℃.
5. The estimated age range of nannofossils specimen that were studied in this research from a
seamount south of El Hierro Island implies that the submarine rocks from the south of El Hierro
evolved from a young Canary event rather than an early Cretaceous volcanic event.
49
7 Acknowledgments
I would like to express my sincere gratitude to my supervisor Prof. Valentin R. Troll for his thorough,
supportive and helpful guidance throughout the duration of this project. I am most grateful for the rare
privilege given me to work on one of your most interesting research samples, and also for all the
inspiration, moral support and encouragements during the period of this thesis.
Special thanks to my co-supervisor Dr. Frances Deegan for her help, encouragements, and giving access
to the geochemistry labouratory, the short courses introduced to us and her review and suggestions on
this project.
To Harri Geiger, I say a big thank you for helping out with the preparation, sending off of samples for
analyses, review of this project, and the wonderful words of encouragement I received from you.
For granting access to the XRD facilities at the Swedish Museum for Natural History in Stockholm, I
would like to acknowledge Dr. Franz Weis and Andreas.
Special thanks to every member of the research team, especially Sophie and Konstantinos, and Erika
for helping with the translation of the abstract to Swedish. This project also benefitted from the sincere
efforts of Dr. Michael Streng and Prof. Jorintje Hendriks during the study of nannofossils and
labouratory investigations. I also like to thank all my colleagues at Geo for being good companions all
through the period of this program.
I wish to express further gratitude to my parents and siblings for their support and love all through the
period of my education.
Most importantly, I would love to give all thanks to God Almighty, the giver of wisdom, for how well
He led me during the period of my studies in Sweden.
Special thanks to the Swedish Institute for the scholarship used in pursing this master degree
programme at Uppsala University.
1
8 References
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APPENDIX A
Figure 1. Photomicrographs showing stages of alteration in the representative samples (a-b) 757-11 (Hijo
de Tenerife) - very altered vessiclated rock and some carbonate minerals (c-d) 755-8 (Hijo de Tenerife)
- Evidence of secondary minerals with relicts of primary feldspar (e) 755 (Hijo de Tenerife) - very altered
intermediate rocks with relicts of fossils (f) 743-3 (Gran Canaria, off Barranco de Veneguera and
Barranco de Tasartico) - very altered basaltic lappillistone with enclaved seconadry mineralization and
early stage alteration along vessicles. (g) 728-2 (Los Gigantes) - carbonate in a cryptocrystaline texture
(h) 759-1 (Hijo de Tenerife) - Slightly altered specimen with amphibole
(a) (b)
(c) (d)
(e) (f)
(g) (h)
58
(a) (b)
(c) (d)
(e) (f)
(g) (h)
59
Figure 2 (a - j). SEM images showing true and false colour alteration textures of altered submarine rocks (a-f)
703-3 (Tropic seamount) - very altered submarine samples with clay minerals along the veins of the rock (g-j)
736-4 (Gran Canaria, off Barranco de Veneguera and Barranco de Tasartico) - BSE image showing alteration
textures of slightly altered seamount specimen with fresh crystals of plagioclase
(i) (j)
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APPENDIX B
Table1. Full sample description from M43-1 cruise report
List Rock Type
638-14 Carbonate sediment with volcanic clast
657-10 Scoriacious alkali basalt
656-1 Carbonate sediments with Alkali basaltic clast
663-5 Tuff to fine lappilistone with felsic and basalt clast
664-4 Basaltic lappilistone with calcareous matrix
669-2 Basaltic lapillistone with carbonate matrix
673-1 Alkali basalt
674-2 Basalt to Carbonate breccias
678-2 Felsite with Inclusion of Intermediate composition
679-2 Felsic tuff with immediate component
681-1 Carbonate sediment with altered basaltic ash clasts
689-1 Felsite
689-3a Felsic clast supported breccias
687-1 Felsite
687-3 Felsite
689-3b Felsite
689-3c Felsite breccia with manganese crust
689-5 Felsite
689-6a Felsite
689-6b Felsite
697-2 Carbonate sediment with highly altered volcanic clast
689-2 Amphibole bearing felsites with more mafic inclusion
703-1a Basaltic lapillistone carbonate cemented matrix
703-1b Basaltic lapillistone carbonate cemented matrix
703-2 Basaltic lappilistone, carbonate cemented matrix
703-3 Alkali basaltic lapillistone cement with zeolites and carbonates
727-2 Basalt
728-2 Carbonate sediment with volcanic clast
733-2 Carbonate sediment with volcanic clast and altered Hyaloclastic
736-4 Rhyolite
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737-1 Carbonate Sediments with volcanic clast
743-3 Basaltic lapillistone with carbonate and zeolite matrix
748-1 Basalt
755 Intermediate rock
755-1 Intermediate rock
755-8 Mafic to Intermediate rock
755-10 Mafic to intermediate rock
755-11 Mafic to Intermediate rock
756-1 Mafic to intermediate rock
757-1 Felsite to intermediate rock
757-2 Felsite to Intermediate Rock
757-3 Lapillistone of basaltic and intermediate composition with a matrix
of manganese crust
757-4 Felsite to Intermediate rock
759-1 Basaltic Lappilistone
759-4 Basaltic Lappilistone
773-1c Alkali basalt
773-2 Basaltic Lapillistone
773-5 Carbonate sediment with volcanic clast
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