Submarine Alteration of Seamount Rocks in the Canary

Home , Tropic Seamount

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

DEPARTMENT OF EARTH SCIENCES

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 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 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.

1 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.

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

3 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

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

5 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).

Las Hijas

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)

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)

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. 9

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

10 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 =

– 18 16 (Rsample / Rstandard 1) ×1000 and R = O/ O. 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. %

18 for δ O, 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.

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 Sample type Location Degree of Petrographic description ID alteration 679-1 Felsic lappilistone South Hierro Ridge Moderately A relatively well sorted, pumiceous lappilistone. The lappilistones are held together by a matrix altered 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 South Hierro Ridge Moderately It consists of altered pyroxene with few anhedral amphibole, feldspar, and biotite crystals. components altered

681-1 Hemipelagic sediments South El Hierro Moderately It contains pyroxene phenocryst, with a glass groundmass and lots of nannofossils. Ridge altered 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 Las Hijas Seamount Moderately Highly to moderately altered sample with phenocryst of plagioclase, entirely felsic, fine to mafic inclusions altered trachytic texture. 689-3a Felsic clast- supported breccia Las Hijas Seamount Moderately It consists of scattered feldspar microlite that has been altered to clay. altered 689-3c Felsite breccia with manganese Las Hijas Seamount Very altered Highly altered sample with few vesicles. crust 689-6a Felsic lava Las Hijas Seamount Unaltered It is rich in feldspar and a few phenocryst of pyroxene.

703-1b Limestone, basaltic lapillistone, Tropic Seamount Very altered It contains many altered clast of different sizes cemented by carbonate, with some fresh felsite igneous inclusion.

703-2 Basaltic lapillistone, carbonate Tropic Seamount Very altered Dark, well sorted lapillistone with poorly defined vesicles and a portion of freshly preserved cemented matrix 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 Punta de la Rasca Mod. altered It is rich in plagioclase with very few crystals of well-preserved pyroxene and biotite, it also volcaniclastics 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 Punta de la Rasca Very altered It consists of strong palagonite alteration with highly altered plagioclase, pyroxene, and volcaniclastics amphibole crystal. 736-4 Rhyolite Off Barranco de Unaltered This consists of subhedral feldspar crystals, pyroxene, and amphibole. Veneguera and Barranco de Tasartico 755 Vesicular bombs of intermediate Hijo de Tenerife Moderately This sample consists of large vesicles with moderately altered crystals of feldspar and composition altered megacryst of altered pyroxene.

755-8 Vesicular bombs of intermediate Hijo de Tenerife Moderately This contains a lot of vesicles with very few crystal of feldspar and megacryst of altered composition altered pyroxene. 756-1 Vesicular bombs of intermediate Hijo de Tenerife Moderately This contains a lot of vesicles with very few crystal of feldspar and megacryst of altered composition altered pyroxene. 759-1 Vesicular bombs of intermediate Hijo de Tenerife Moderately This contains a lot of vesicles with very few crystal of feldspar and megacryst of altered composition altered pyroxene and deformed amphibole. 773-5a Carbonate sediments with volcanic Submarine flank Moderately It contains a lot of calcite and chlorites clasts Tenerife, off altered Guimar and Anaga 773-5b Carbonate sediments with volcanic Submarine flank Altered It comprises of carbonates and volcaniclastics clasts Tenerife, off Guimar and Anaga

A B

C D

E F

G H

L M

N O

P Q

R S

T U

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. 16

(a) (b)

palagonite

palagonite palagonite

(e) (f)

palagonite palagonite

(g) (h)

palagonite

(i) (j)

palagonite

(k) (l)

(m) (n)

manganese crust

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.

(a) (b)

(e) (f)

(g) (h)

(i) (j)

(k) (l)

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.

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, 21 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.

Table 2. Distribution of minerals in altered, moderately altered and unaltered seamount samples from XRD results

Degree of Primary minerals Secondary minerals alteration 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 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.

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%).

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

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 TiO 1.23 0.75 2.49 0.67 1.19 1.54 0.50 1.23 1.16 1.18 0.92 2 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

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

(a) Very altered samples (b) Altered samples

Utzmann et. al., 2002

Slightly altered

Donoghue et al., 2008

(e) (f)

(g)

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 (a) Slightly altered samples (b)

Altered samples

Utzmann et. al., 2002

Donoghue et al., 2008

(e) (f)

(g)

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. 31

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).

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. 33

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).

Table 5. Stable isotopes composition and H2O wt. % content of submarine rocks Sample ID. 18 δ O‰ 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 - -

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.

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.

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).

65,5Ma 2,6 Ma 290,000yrs Palaeogene Neogene

Quaternary

Palaeocene Pliocene

Recent

Oligocene

Miocene

Eocene

Danian Selandian Thanetian Ypresian Lutetian Batonian Priabonian Rupelian Chattian Aquitanian Burdigalian Langhian Serravalian Tortonian Messinian Zancian Piacezian Pleistocene Holocene

Species Samples

638-14 674-2

Cocolithosphores Emiliana huxleyi x

Calcidiscus leptoporus x

Coccolithus miopelagicus x Rhabdosphaera clavigera x Acanthoica quattrospina x Discosphaera tubifera x

Gephyrocapsa carribean x B Foraminifera

Globoconella terminalis x A 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. 39

(a) (b)

1µm 1µm

(c)

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) (b)

(e) (f)

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.

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

42 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.

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.

450 Altered 400 Moderately altered 350 Unaltered 300 R² = 0.3796 250

200 Ce Ce (ppm) 150 100 50 0 0 50 100 150 200 250 300 La (ppm) 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 45 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 46 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.

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.

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.

Special thanks to the Swedish Institute for the scholarship used in pursing this master degree programme at Uppsala University.

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.

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APPENDIX A

(a) (b)

(e) (f)

(g) (h)

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)

(e) (f)

(g) (h)

(i) (j)

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

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

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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