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.

2

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.

3

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

4

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

5

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

6

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

8

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)

60

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

61

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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