HALOGENS IN VOLCANIC SYSTEMS
A thesis submitted to the University of Manchester for the degree of
Doctor of Philosophy in the Faculty of Science and Engineering
2017
Lisa Jepson
School of Earth and Environmental Sciences
Contents
1 Introduction ...... 23
1.1 Aims and Objectives ...... 24
2 Literature Review ...... 26
2.1 Halogens ...... 27
2.1.1 The Halogens as Geochemical Tracers...... 32
2.2 Noble Gases ...... 33
2.2.1 Helium and Argon as Geochemical Tracers ...... 34
2.3 Mantle Reservoirs ...... 35
2.3.1 Mantle Components ...... 36
2.4 Halogen and Noble Gas Fractionation ...... 42
2.5 Oceanic Basalts ...... 46
2.6 Mantle Convection: Whole vs Layered ...... 47
2.7 Mantle Plumes ...... 48
2.8 Fluid and Melt Inclusions ...... 50
2.8.1 Formation of Fluid and Melt Inclusions ...... 51
2.8.2 Analysis of Halogens and Noble Gases in Silicate Melt Inclusions ...... 53
2.9 Summary ...... 54
3 Experimental Methods ...... 57
3.1 Sample Selection and Limitations ...... 57
3.2 Halogen and Noble Gas Analysis ...... 58
3.2.1 Preparation for Irradiation ...... 59
3.2.2 Neutron Irradiation ...... 59
3.2.3 Noble Gas Extraction ...... 60
3.2.4 Noble Gas Isotopic Analysis ...... 66
3.2.5 Air Calibration ...... 68
3.2.6 Blanks ...... 69
2
3.2.7 Data Reduction ...... 69
3.2.8 Conversion of the Noble Gases to the Halogens, K and Ca ...... 71
4 Subducted marine pore fluid signature present within the basalts of Tristan da Cunha ...... 73
4.1 Introduction ...... 73
4.1.1 Tristan da Cunha Geological Setting ...... 73
4.1.2 Origin of the Island Group ...... 75
4.1.3 Ages of Volcanism ...... 77
4.1.4 Geochemistry of Tristan da Cunha ...... 78
4.1.5 Tristan (0.21 Ma) ...... 80
4.1.6 Inaccessible Island (~1 Ma) ...... 81
4.2 Aims ...... 82
4.3 Samples ...... 83
4.4 Experimental Methods ...... 85
4.5 Rock Descriptions ...... 86
4.6 Results ...... 91
4.6.1 Halogens ...... 91
4.6.2 Helium and Argon ...... 96
4.7 Discussion ...... 97
4.7.1 Halogen Fractionation...... 97
4.7.2 Halogen variation between Tristan and Inaccessible Island ...... 102
4.7.3 Possible Components of the Tristan da Cunha Basalts ...... 105
4.7.4 Source of the Inaccessible Island Enrichment ...... 107
4.7.5 Halogen Subduction and Recycling ...... 108
4.7.6 Halogen enrichment by metasomatised lithosphere? ...... 109
4.7.7 A model for heavy halogen enrichment in the Inaccessible Island basalts 110
4.7.8 Implications for Halogens in the Earth’s Mantle ...... 116 3
4.8 Conclusions...... 116
5 Plume-Ridge Interaction in the Azores Archipelago ...... 118
5.1 Introduction ...... 118
5.1.1 Azores General Geological Setting ...... 118
5.1.2 Origin of the Azores Archipelago ...... 120
5.1.3 Ages of the Volcanism ...... 121
5.1.4 Azores Geochemistry ...... 122
5.1.5 Santa Maria (8.12 Ma) ...... 124
5.1.6 São Miguel (4.01 Ma) ...... 124
5.1.7 Terceira (3.52 Ma) ...... 125
5.1.8 Flores (2.76 Ma) ...... 126
5.1.9 Graciosa (2.5 Ma) ...... 127
5.1.10 Faial (0.73 Ma) ...... 128
5.1.11 Corvo (0.71 Ma) ...... 129
5.1.12 São Jorge (0.55 Ma) ...... 129
5.1.13 Pico (0.25 Ma) ...... 130
5.2 Aims ...... 131
5.3 Sample Selection ...... 131
5.4 Experimental Methods ...... 141
5.5 Results ...... 141
5.5.1 Integrated Halogen Releases ...... 142
5.5.2 Olivine vs Clinopyroxene Releases ...... 143
5.5.3 Rock Descriptions ...... 147
5.6 Discussion ...... 149
5.6.1 Siting of the Halogens...... 149
5.6.2 Intra-Island Halogen Variation ...... 151
5.6.3 Halogen Variation across the Azores Group ...... 157
4
5.6.4 The source of the depleted halogen signature on Terceira ...... 162
5.6.5 The source of the eastern São Miguel halogen enrichment...... 162
5.6.6 The source of the halogen isotopic variation on Graciosa ...... 164
5.6.7 The source of the enriched iodine component on Corvo and Flores .. 165
5.6.8 Halogen variation with age ...... 167
5.6.9 Halogen variation with location ...... 170
5.6.10 A tectonic control on volcanism and halogen variation ...... 171
5.6.11 The composition of the Azores plume ...... 175
5.6.12 Can the Azores halogens be correlated to other isotope systems? . 175
5.6.13 Consequences for Azores basalts – a mix of HIMU and EM1/EM2? 178
5.6.14 A heterogeneous source for the Azores basalts ...... 179
5.7 Conclusions...... 181
6 Sedimentary and Altered Oceanic Crust Iodine Signatures in the Canary Islands Basalts ...... 184
6.1 Introduction ...... 184
6.1.1 Geological Setting of the Canary Islands ...... 184
6.1.2 Origin of the Canaries ...... 185
6.1.3 Ages of Volcanism ...... 185
6.1.4 Geochemistry of the Canaries ...... 187
6.1.5 Tenerife (12.0 Ma) ...... 188
6.1.6 La Palma (2.0 Ma) ...... 189
6.1.7 El Hierro (1.2 Ma) ...... 190
6.2 Aims ...... 191
6.3 Sample Selection ...... 191
6.4 Experimental Methods ...... 196
6.5 Results ...... 196
6.5.1 Halogen releases from crushing ...... 197 5
6.5.2 Halogen releases from step heating ...... 197
6.6 Discussion ...... 204
6.6.1 Fractionation of the halogens? ...... 204
6.6.2 The presence of different source components ...... 211
6.6.3 3He/4He and Halogen Ratios ...... 213
6.6.4 Marine Pore Fluid / Serpentine Halogen Signature Released from Fluid Inclusions ...... 214
6.6.5 MORB-like Release from Melt Inclusions ...... 217
6.6.6 A Multi-Component HIMU Source ...... 220
6.7 Conclusions...... 222
7 Conclusions ...... 223
7.1 Future Work ...... 227
8 Appendices ...... 230
8.1 Tristan da Cunha: Halogen, K, and 40Ar/36Ar results from irradiated samples (individual steps)...... 230
8.2 Azores: Halogen, K, and 40Ar/36Ar results from irradiated samples (individual steps)...... 236
8.3 Canaries: Halogen, K, and 40Ar/36Ar results from irradiated samples (individual steps)...... 245
8.4 Azores – collected samples locations (samples analysed for halogens in bold). 251
8.5 Additional Canary Islands graphs ...... 258
9 References ...... 262
Word count: 67,506
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List of Figures
Figure 2.1 The Earth’s major volatile reservoirs and their halogen concentrations and ratios...... 31
Figure 2.2 Pb isotope data from ocean island basalts, from island groups analysed for their halogen ratios in this study ...... 38
Figure 2.3 Sr-Pb isotope data from ocean island basalts, from island groups analysed for their halogen ratios in this study ...... 40
Figure 2.4 Spider diagram, showing the concentrations of trace elements, normalised to BSE, in MORB and OIB ...... 47
Figure 2.5 Inclusion entrapment mechanisms ...... 53
Figure 2.6 Map showing the locations of the Azores, Canaries, and Tristan da
Cunha, together the 3He/4He ratio ranges ...... 56
Figure 3.1 Schematic of MS1 and extraction system ...... 65
Figure 4.1 Location of the Tristan da Cunha group, east of the Mid-Atlantic Ridge . 74
Figure 4.2 TAS plot for Tristan and Inaccessible Island magmas ...... 79
Figure 4.3 a) 208Pb/204Pb vs 206Pb/204Pb, b) 207Pb/204Pb vs 206Pb/204Pb in Tristan plume source magmas ...... 81
Figure 4.4 Approximate locations of the Tristan (Main island) samples ...... 83
Figure 4.5 Selected thin sections from Tristan and Inaccessible Island basalts, showing mineralogy and melt inclusions ...... 90
Figure 4.6 Halogen releases during crushing and heating steps ...... 91
Figure 4.7 Br/Cl (a) and I/Cl (b) ratios from crushing with steps of increasing intensity
...... 92
Figure 4.8 Br/Cl (a) and I/Cl (b) variation from select samples with increasing temperature steps during step heating ...... 93
7
Figure 4.9 Plots of K/Cl against a) Br/Cl and b) I/Cl...... 99
Figure 4.10 Plots of a) Br/Cl, b) I/Cl, and c) K/Cl against 1/Cl...... 100
Figure 4.11 Halogen ratio plot of the Tristan and Inaccessible Island basalts ...... 104
Figure 4.12 Halogen ratio plot with pore fluid, serpentinites, wedge fluid, MORB, bulk
Earth, and seawater data ...... 106
Figure 4.13 Models showing the evolution of Gondwana and the Tristan plume ... 114
Figure 4.14 Halogen ratio plot, showing the addition of the WARS xenoliths ...... 115
Figure 5.1 Geological setting of the Azores archipelago ...... 119
Figure 5.2 Typical field localities for the Azores ...... 133
Figure 5.3 Locations of samples collected and analysed as part of this study from the islands located on the Terceira Rift ...... 138
Figure 5.4 Locations of samples collected and analysed as part of this study, for the islands located on the Azores micro-plate ...... 139
Figure 5.5 Locations of samples for the Atlantic Plate islands ...... 140
Figure 5.6 Halogen release during crushing and heating ...... 141
Figure 5.7 Selected thin sections from Graciosa ...... 149
Figure 5.8 Melt inclusions within olivine and clinopyroxene phenocrysts in the Azores basalts ...... 150
Figure 5.9 Halogen ratios for São Miguel samples ...... 152
Figure 5.10 Halogen ratios for Terceira samples ...... 153
Figure 5.11 Halogen ratios for Graciosa samples ...... 154
Figure 5.12 Halogen ratios for Faial samples...... 155
Figure 5.13 Halogen ratios for São Jorge samples...... 155
Figure 5.14 Halogen ratios for Pico samples ...... 156
Figure 5.15 Halogen ratios for Corvo and Flores samples ...... 157
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Figure 5.16 Halogen ratio plot of the Azores basalts...... 160
Figure 5.17 Halogen ratio plot with altered oceanic crust, sub-continental lithospheric mantle, pore fluid, serpentinites, wedge fluid, MORB, bulk Earth, and seawater data
...... 161
Figure 5.18 The halogen composition of the eastern São Miguel basalts ...... 164
Figure 5.19 Summarising the age, eruption history, 3He/4He, halogen ratios, and halogen signatures for the Azores island group...... 169
Figure 5.20 A possible model of the halogen enrichment in the Azores ...... 174
Figure 6.1 The location of the Canary Island group ...... 184
Figure 6.2 Map of the Canary Islands, with the islands analysed as part of this research highlighted in black...... 186
Figure 6.3 Historical eruptions on Tenerife, La Palma, and El Hierro ...... 189
Figure 6.4 Sample locations from Tenerife, La Palma, and El Hierro ...... 193
Figure 6.5 Halogen release during crushing and step heating...... 198
Figure 6.6 Halogen release from the clinopyroxene separates during crushing and step heating ...... 199
Figure 6.7 Plots of K/Cl against a) Br/Cl and b) I/Cl for the crushing releases ...... 207
Figure 6.8 Plots of a) Br/Cl, b) I/Cl, and c) K/Cl against 1/Cl for the crushing releases
...... 208
Figure 6.9 Plots of K/Cl against a) Br/Cl and b) I/Cl for the heating releases ...... 209
Figure 6.10 Plots of a) Br/Cl, b) I/Cl, and c) K/Cl against 1/Cl for the heating releases
...... 210
Figure 6.11 Thin section images from the Western Canary Islands ...... 212
Figure 6.12 Halogen ratio of the crushing releases plot with pore fluid, serpentinites, wedge fluid, MORB, WARs Xenoliths, bulk Earth, and seawater data ...... 215
9
Figure 6.13 Halogen ratio plot of the stepped heating releases, showing the addition of the WARS xenoliths ...... 219
Figure 7.1 Halogen ratio plots for a) Tristan da Cunha, b) The Azores, c) Canaries
(crushing), and d) Canaries (step heating) ...... 229
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List of Tables
Table 1.1 The mantle source reservoirs and the 3He/4He of the island groups in this study...... 25
Table 2.1 Halogen concentrations (ppm) and ratios (mol/mol) for the Earth’s reservoirs ...... 29
Table 2.2 40Ar/36Ar and 3He/4He for the Earth’s reservoirs...... 35
Table 2.3 Long-lived radioactive decay series used as tracers...... 36
Table 2.4 Isotopic ratios of mantle reservoirs...... 41
Table 3.1 The reactions of the irradiated parent isotopes, producing the nucleogenic noble gas isotopes...... 63
Table 3.2 Irradiation details for the halogen analyses...... 64
Table 3.3 Interfering reactions produced during irradiation, with reactions of interest for analysis of the parent isotope ...... 72
Table 4.1 Pb-Sr-Nd isotope data from Tristan and Inaccessible Islands, Walvis
Ridge, and Paraná Magmatic Province (PMP)...... 76
Table 4.2 Mean and standard deviation for isotope ratios for Paraná Magmatic
Province tholeiites, Walvis Ridge, least evolved Tristan samples, and normal MORB
...... 80
Table 4.3 Samples analysed from Tristan and Inaccessible Island...... 84
Table 4.4 Rock descriptions for the Tristan and Inaccessible Island basalts...... 86
Table 4.5 Halogen, K, and 40Ar/36Ar results from irradiated samples ...... 95
Table 4.6 Helium and Argon results from unirradiated samples ...... 97
Table 5.1 Eruptive histories and locations of the Azores islands ...... 122
Table 5.2 Isotope ratios for the Azores island group ...... 123
Table 5.3 Sample location and descriptions...... 134
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Table 5.4 Halogen, K, and 40Ar/36Ar results from irradiated samples ...... 144
Table 5.5 Rock descriptions from thin section analysis ...... 147
Table 5.6 Halogen data and isotope data from Corvo and Flores samples...... 165
Table 5.7 Halogen ranges measured for each island...... 177
Table 6.1 Radiogenic isotope data from Tenerife, La Palma, and El Hierro...... 188
Table 6.2 Sub-set of samples analysed in this study ...... 192
Table 6.3 Sample locations and descriptions ...... 194
Table 6.4 Halogen, K, and 40Ar/36Ar results from irradiated samples ...... 200
Table 6.5 Halogen ranges yielded from each island, during crushing and step heating...... 211
Table 6.6 3He/4He and halogen ratios measured with the La Palma and El Hierro samples...... 213
Table 6.7 Showing the percentages of the halogens and K released from the crushing step...... 216
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Abstract
The halogens (Cl, Br, I) are moderately volatile elements that exhibit incompatible behaviour during melting, and are hydrophyllic - in addition, iodine is strongly fractionated by biological processes. Although the halogens share similar geochemical properties to the noble gases, the heavy halogens in particular have been underutilized as tracers, because of the analytical difficulties related to determining their low abundances in geological materials. This research presents the first known data on HIMU OIBs (Azores, Canary Islands) and extends the known range for EM1 (Tristan da Cunha) and EM2 (Tristan da Cunha and São Miguel) type basalts. Samples were selected from the Tristan da Cunha group (EM1/EM2), the Azores archipelago (HIMU and EM2), and the Canary Islands (HIMU). The samples were selected on the basis of mantle reservoir and 3 4 He/ He (low, MORB-like (Mid-Ocean Ridge Basalt), moderately high RA), to enable a diverse suite of OIB samples to be analysed. Olivine and pyroxene mineral separates from basalts have been analysed from each of the ocean island groups. The halogens are assumed to be mainly sited in melt (Tristan, Azores, Canaries) and fluid (Canaries) inclusions observed within the mineral phases; noble gases were liberated by a combination of crushing and stepped heating. The Tristan da Cunha basalts show Br/Cl and I/Cl molar ratios that extend from the range previously determined for MORB samples up to the high values characteristic of marine pore fluids, with maximum values: Br/Cl = 3.56x10-3 and I/Cl = 3530x10-6. This range is interpreted to represent a mixing trend between a MORB-like and a subducted marine pore fluid or I-rich sediment signature, in the source of the Tristan da Cunha basalts. The Azores basalts yield I/Cl (0.54-1480)x10-6, with the Br/Cl ratios showing (0.39- 3.37)x10-3. There appears to be some variation between islands, observed in the I/Cl values – with the samples from Graciosa showing the greatest range in I/Cl and Br/Cl ratios, and samples from the western islands (Corvo and Flores) showing the most I-rich ratios, the islands on the Terceira Rift showing the greatest range in halogen ratios, with the islands to the south of the Terceira Rift yielding MORB-like halogen ratios. The variation within the group is proposed to be controlled by the interacting nature of the plume and tectonic environment. Crushing analyses show that the Canaries basalts have a similar range in Br/Cl (0.32-2.00)x10-3 to MORB, but extend to much higher I/Cl values (154-18700)x10-6. The data overlap with data from marine sediments. Analyses indicate the presence of a fluid component, which is not seen in the other OIBs. Heating analyses show a more MORB-like component together with the presence of a marine pore fluid component, with Br/Cl (0.14-173)x10-3 and I/Cl values (12.7-4440)x10-6. The EM1 and EM2 basalts show evidence for recycling of metasomatised SCLM - with a marine pore fluid signature – mixing with a MORB-like component, whereas the HIMU basalts exhibit a three component system with a MORB, OIB, AOC, and marine pore fluid signature.
13
Declaration
Six of the samples from the Tristan Island group were originally analysed as part of my thesis submitted for my MEarthSci Earth Sciences degree at the University of
Manchester in 2010. These data from these six samples have been re-analysed and corrected (due to advances in monitor analysis, and my greater experience in data reduction in removing missed peaks during analysis) as part of this PhD, together with the remaining Tristan samples, which were picked, irradiated, and analysed, or re-analysed (due to technical difficulties in 2009/2010) during my PhD.
Copyright statement
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This page must form part of any such copies made.
14 iii. The ownership of certain Copyright, patents, designs, trade marks and other intellectual property (the “Intellectual Property”) and any reproductions of copyright works in the thesis, for example graphs and tables (“Reproductions”), which may be described in this thesis, may not be owned by the author and may be owned by third parties. Such Intellectual Property and Reproductions cannot and must not be made available for use without the prior written permission of the owner(s) of the relevant
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15
Pico, the Azores. Taken September 2011.
16
“You don’t get the winning-losing thing do you? I’m probably just not explaining it.
Winning is not some game, some result. It’s not about getting more points. It’s not even about the smug satisfaction of beating someone, anyone, anything. It’s actually
about never actually achieving anything. It’s all about the trying, the striving, the
grinding on towards getting a little bit better. In truth I suppose it’s not really about
winning at all, it’s about not giving up. Because that’s when you lose”.
Fingers in the Sparkle Jar, Chris Packham, 2016.
17
To my Mum, without whom this journey may have never begun.
And to my husband, James, without whom this step in the journey may have never finished…
18
About the author
After an almost decade long career culminating as a computer programmer in database and statistical analysis, I was made redundant twice in two years, and decided to use the opportunity to go back to university and study the subject that I had always wanted to learn more about, geology. Many a fossil collecting trip with my Mum as a child had sparked my interest in this fascinating area, and in 2006, I began my four year undergraduate degree in Earth Sciences at the University of
Manchester, graduating with first class honours in 2010. During my MEarthSci, I was given the opportunity to study the halogen geochemistry of the Tristan Island group – a subject area that captured my interest like no other and became my passion – culminating in me researching the subject at greater depth for my PhD.
19
Acknowledgements
I wish to thank firstly my supervisor Professor Ray Burgess for his supervision, encouragement and enthusiasm throughout my PhD, and for his unwavering support and understanding through difficult periods.
My thanks go to Bev Clementson and John Cowpe for their technical help and assistance. I am also grateful to Dave Blagburn, for his patience and help in explaining how everything (and more!) in the lab works, his technical support, and for his assistance and support in sample loading. I would also like to thank Cath Davies for her technical help, and many a chat that picked me up and motivated me when I needed it most.
Sincere thanks go to Dr Vera Fernandes for providing additional samples from the
Azores, but particularly for her excellent help in organising the fieldwork, and for her assistance in the field. Heartfelt thanks also go to Professor Zilda França and
Professor Victor-Hugo Forjaz for their hospitality, information, and discussions during my time on the Azores. Thanks Dr Paul Harrop for providing the Canary Island samples, Dr David Murphy for the Tristan samples and thin sections, and to Dr Felix
Genske for additional Azores samples.
I must also thank my lab buddies, Dr Déborah Chavrit and Dr Olly Warr, for their help, ideas, motivation, support, and much need laughs during our time together in
G.69, and for their ongoing support since moving on to pastures new. Thanks go to everyone in the research group, who provided advice, suggestions, critique, help, and hugs when needed, but particularly Alex Clarke and Francesca McDonald, who were never too busy to listen or check up on me, and also Dr Lorraine Ruzié-
20
Hamilton and Dr Trish Clay. A big thanks to Dr Nicole Spring for reaching out to me and thinking about me when I most needed it.
I would also like to thank Dr Alison Pawley and Dr Tony Adams for their help, support, and encouragement over the years, and for giving me the opportunity to demonstrate on their courses and become a better teacher. And of course, Dr John
Nudds, for many a “blibble” in his office when needed! A special thanks to Dr Maria
McNamara at UCC for offering me a place to work and welcoming me in the department – and for empathising and sending my husband home when I needed his support. Thanks also go to Dr Pat Meere and Dr Richard Unitt at UCC, for granting me access to the microscopes in the School of Biological, Earth, and Environmental
Sciences in order to complete my corrections, and for helping with mineral identification.
Sincere thanks go to Norman Darwen, for his endless support, advice, and down-to- earth attitude over the ten years in which I have been at the University of
Manchester. I would also like to thank all the staff at the Disability Support Office for all their help and advice.
Thanks to my friend Phil Jones, for letting my husband and I live with him rent-free, allowing me to stay within travelling distance of the university, when we suddenly found ourselves homeless and without funding or job in 2014. Thanks also to Dave and Holly Jones for letting me stay with them during my visits back to the UK to work on my PhD in 2015/6.
Finally, I like to thank my Mum and my Dad for their support during my PhD, and for all the cat-sitting, cleaning, financial help, and coffees and meals out when I needed
21 a break. And of course, my husband, Dr James Jepson who was and is always there for me, and perhaps understood best how difficult, and yet rewarding, a PhD can be.
I wish to acknowledge the National Environmental Research Council (NERC) for providing the funding for my PhD, and the Hitchon Fund, the Mineralogical Society of
Great Britain and Northern Ireland, and European Association of Geochemistry for providing me with grants that gave me the opportunity to present my research at various conferences worldwide.
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1 Introduction
The halogens are an important suite of volatiles, which can be used to trace igneous processes and the transport of materials through the Earth’s major reservoirs
(asthenospheric mantle, oceanic crust, crust, sub-continental lithospheric mantle, sediments), and into the oceans (Aiuppa et al., 2009). The Earth’s reservoirs each have distinct halogen compositions, due to the transport and degassing of the halogens over geological time. By studying the halogens in volcanic rocks, we can gain insight into mantle dynamics, such as mantle convection and subduction processes (Kendrick et al., 2014a; Kendrick et al., 2013b; Kendrick et al., 2015;
Kendrick et al., 2014b; Kendrick et al., 2012a; Kendrick et al., 2011). How the halogens are recycled into the mantle is poorly understood, and analysing the concentrations of halogens in igneous rocks can help elucidate these processes.
Although considerable research has been published on the halogen content of mid- ocean ridge basalts (MORB), there is currently a paucity of published data on halogens in ocean island basalts (OIB), particularly with respect to iodine. This is likely to be, at least in part, due to the difficulties in measuring iodine due to its low abundances in geological materials, but these analytical difficulties can be overcome using noble gas mass spectrometry and an extension of the Ar-Ar dating technique, allowing measurement of the halogens at ppb levels (Kendrick, 2012; Ruzié-
Hamilton et al., 2016). Over the course of this research, using this analytical method, there has been increasing data reported for MORB, serpentinites, marine pore fluids, and wedge fluids (John et al., 2011; Kendrick et al., 2013b; Kendrick et al., 2014b;
Kendrick et al., 2012a; Kobayashi et al., 2017; Sumino et al., 2010).
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1.1 Aims and Objectives
The aim of this project was to determine the halogen concentrations and ratios within a suite of ocean island basalts, using samples previously collected, and samples obtained during fieldwork undertaken as part of this project, and to use these data to constrain the halogen composition of the source of ocean island basalts. There is a paucity of data from OIBs relative to MORBs, therefore this research focuses on OIB sources, including deep mantle end-members and recycled components, and how these environments relate to variations observed within the halogens, and placing the halogen variation observed within the context of other geochemical systems.
Samples were selected from the Tristan da Cunha group, the Azores archipelago, and the Canary Islands. The samples were selected on the basis of mantle reservoir
3 4 (EM1, EM2, HIMU), and He/ He (low, MORB-like, moderately high and high RA), to enable a diverse suite of OIB samples to be analysed (Table 1.1).
The Azores fieldwork allowed collection of low 3He/4He EM2 (São Miguel) and moderately high 3He/4He HIMU (Graciosa, Terceira and Pico) samples. These islands were chosen due to their location at a triple ridge junction, and due to their varying geochemistry (HIMU, EM2, and varying 3He/4He). Additional samples were obtained (provided by Dr Vera Fernandes) from São Jorge and Faial, allowing for analyses of all islands to the east of the Mid-Atlantic Ridge (excluding Santa Maria).
Further samples were later obtained from Dr Felix Genske, from the islands of Corvo and Flores, allowing for comparison between the islands located to the west of the
Mid-Atlantic Ridge (on the American Plate), with those located to the east (on the
Azores micro-plate) (Genske et al., 2016; Genske et al., 2014; Genske et al., 2012).
24
The Canary Islands have MORB-like 3He/4He (and are proposed to come from a
HIMU reservoir). Samples were readily available within the research group collection
(collected during fieldwork by Dr Paul Harrop in 1997 and 1998), some of which were already analysed for 3He/4He and 40Ar/36Ar, allowing for analyses from El Hierro, La
Palma, and Tenerife.
The samples from the Tristan da Cunha group of islands have low 3He/4He (and are from an EM1/EM2 reservoir). Samples were available from the Natural History
Museum, London collection (BM 1962 / 128) that were collected during the Royal
Society Expedition to the islands in 1962, which took place shortly after the 1961 eruption on the main island of Tristan (Baker et al., 1964). Samples were chosen from the islands of Tristan and Inaccessible, to establish if there is any difference in the halogen composition of the basalts between the two islands. A spread of localities was chosen across the island of Tristan.
Table 1.1 The mantle source reservoirs and the 3He/4He of the island groups in this study.
Island / Island Group Mantle Reservoir 3He/4He
Tristan da Cunha EM1, EM2 4-6 RA
Canaries HIMU 5.5-9.5 RA
Azores (exc. São Miguel) HIMU 7.2-13.5 RA
Azores (São Miguel only) EM2 <5.1 RA
(Data from Day and Hilton, 2011; Gibson et al., 2005; Graham, 2002; Jean-Baptiste et al., 2009; Moreira et al., 1999; Widom et al., 1997)
25
2 Literature Review
The Earth’s mantle is chemically and isotopically heterogeneous (Hanan and
Graham, 1996; Jackson et al., 2007). The origin of this heterogeneity is unknown, but most authors (e.g. Hofmann, 1997; Jackson et al., 2007) believe that the subduction of continental lithosphere, and oceanic crust and associated oceanic sediments, and pore fluids has led to this variable composition, together with depletion during partial melting of the mantle and crust formation (Carlson, 1995;
Davies, 1990; Sumino et al., 2010).
Mid-ocean ridge basalts (MORB) and ocean island basalts (OIB) sample the Earth’s mantle, and can therefore help us to elucidate mantle processes such as convection and subduction, and give us insight into the composition of their mantle sources.
Through analysing the composition of oceanic basalts, we can identify the nature of the source reservoir and determine the chemical composition and isotopic ratios of mantle reservoirs.
Due to their distinct ratios within the Earth’s major reservoirs (asthenospheric mantle, oceanic crust, crust, sub-continental lithospheric mantle, sediments), the halogens can be used as tracers of mantle processes. Previous work has concentrated on chlorine and bromine, perhaps due to the analytical difficulties in measuring iodine due to its much lower abundance in most geological materials. Although there is increasing information on MORB halogen content, data on OIB are scarce, particularly with respect to iodine, as only a few samples have been reported so far in the literature (Déruelle et al., 1992; Kendrick et al., 2014a; Kendrick et al., 2015).
26
2.1 Halogens The halogens, fluorine, chlorine, bromine, iodine and astatine (F, Cl, Br, I, At), form
Group VII (Group 17 IUAPC) of the periodic table, each having one electron missing from their outer shell making them highly reactive. The halogens are non-metal volatile elements, with their reactivity decreasing down the group (Aiuppa et al.,
2009).
The heavy halogens, Cl, Br, and I, all have large ionic radii and behave as incompatible elements, therefore concentrating in the liquid phase during partial melting and crystallisation (Pyle and Mather, 2009). This liquid phase may be preserved as melt inclusions within silicate minerals, or within basaltic glass (Kent,
2008). Fluorine has the smallest ionic radius of any anion and thus exhibits different geochemical behaviour to the heavy halogens (Aiuppa et al., 2009). Fluorine is a compatible element with a similar ionic radius to oxygen, and may therefore substitute for hydroxyl (OH-) and oxygen within silicate minerals (Aiuppa et al.,
2009).
Astatine forms as part of the 235U and 238U decay series. All of its isotopes have short half-lives, making it the least abundant naturally occurring element (Pyle and Mather,
2009); as such, is not considered further in this review.
Due to their volatility, incompatibility in many silicate minerals, and high solubility in hydrous fluids, halogens degas from the mantle during magmatism and this has led to their concentration in the outer reservoirs of the Earth: the oceans, sediments, crust and MORB-source mantle (Figure 2.1) (Pyle and Mather, 2009). Approximately
73% of Cl resides in the oceans, concentrated in seawater (18,800 ppm) and pelagic clays (21,000 ppm), together with around 51% of Br, with Br split between the crust
(78 ppm for oceanic crust) and seawater (67 ppm) (Déruelle et al., 1992; John et al.,
27
2011; Pyle and Mather, 2009). I is concentrated in organic sediments (60%), as it has an affinity with organic matter, and is concentrated within sedimentary material
(28 ppm in pelagic clays) and shales (19 ppm) within the oceans (Déruelle et al.,
1992; John et al., 2011; Pyle and Mather, 2009).
Schilling et al. (1978) estimated the halogen concentrations in OIB to be 70-257 ppm
Cl (62-81 ppm in exposed islands, 145-461 ppm in submerged) and 0.27-0.69 ppm
Br (0.21-0.30 ppm subaerial, 0.44-1.45 ppm submarine), giving Br/Cl ~1.2x10-3
(mol/mol) (Table 2.1). This ratio is in good agreement with Jambon et al. (1995)
(Br/Cl = 1.1x10-3), who estimated lower concentrations of Cl (35 ppm) and Br (88 ppb) for the primitive mantle. Further work on Azores basalts by Schilling et al.
(1980) extended the range in Cl (30-618 ppm) and Br (0.69-1.76 ppm) (Table 2.1).
Estimates for MORB halogens are better constrained, with analyses by Jambon et al. (1995) on Mid-Atlantic Ridge (MAR) basalts ranging from 49-320 ppm Cl and 60-
1,300 ppb Br; these concentrations are in agreement with earlier work on MAR basalts (glasses and pillow lavas) by Schilling et al. (1980) who measured 21-67 ppm Cl, and 0.1-1.76 ppm Br. MORB halogen ratios range from (0.5-2.8)x10-3 Br/Cl and (4-195)x10-6 I/Cl (Table 2.1) (Burgess et al., 2009; Jambon et al., 1995; Johnson et al., 2000; Kendrick et al., 2014b; Kendrick et al., 2012a).
28
Table 2.1 Halogen concentrations (ppm) and ratios (mol/mol) for the Earth’s reservoirs (DMM = depleted MORB mantle, SCLM = subcontinental lithospheric mantle, BSE = bulk silicate Earth).
Br/Cl I/Cl Reservoir Cl Br I (x10-3) (x10-6)
Seawater 18,800 67 0.05-0.06 1.53 0.95
Marine Pore Fluid 156-20,006 2-258 0.38-218 1-101 3.58-227,000
Wedge Fluids 8-53 0.05-0.24 0.03-0.38 1.7-3.7 700-1,800
Marine Sediments 21,000 70 15-55 1.5 200-700
Marine Shale 180 20 19 49 29,000
Serpentinites 22-2870 0.06-24 0.016-45 1.07-4.64 8-12,100
Altered Ocean Crust 14-875 0.04-2.5 0.0004-0.0379 0.33-1.49 0.83-1.02
Crust 1,900 6.95 1.54 1.6 230
DMM 0.4-10 0.008 0.0008 0.35-8.9 20-560
MORB 49-320 0.06-1.3 0.008 0.5-2.8 4-195
Primitive Mantle 8-38 0.075-0.456 0.0042-0.0133 1.1 -
OIB 30-618 0.09-1.76 0.073-0.61 1.1-1.71 11.6-26.9
EM1 270-860 0.84-3 0.022-0.058 1.13-1.71 11.6-24.9
EM2 590-1450 1.7-3.7 0.028-0.093 1.29-1.55 14.6-26.9
SCLM 3 0.011 0.0004 1.4-4.3 9.4-70.0
CI Chondrite 698 3.56 430 2.26 172
BSE 10-35 0.05-1.08 0.007-0.01 1.77 72 (Data complied from Aiuppa et al., 2009; Burgess et al., 2009; Burgess et al., 2002; Chavrit et al., 2016; Déruelle et al., 1992; Fehn et al., 2006; Fehn et al., 2007a; Fehn et al., 2000; Fehn et al., 2003; Jambon et al., 1995; John et al., 2011; Johnson et al., 2000; Kastner et al., 1990; Kendrick et al., 2013b; Kendrick et al., 2015; Kendrick et al., 2014b; Kendrick et al., 2012a; Lu et al., 2008; Mahn and Gieskes, 2001; Martin et al., 1993; Muramatsu et al., 2007; Muramatsu et al., 2001; Schilling et al., 1980; Schilling et al., 1978; Sumino et al., 2010; Tomaru et al., 2007). Data on I are scarce, but work by Déruelle et al. (1992) on basaltic glasses and basalts (and one phonolite) from a range of settings including MORB, OIB, IAB
(island arc basalts) and BABB (back-arc basin basalts) found a range of I concentrations (3-363 ppb). Most samples had less than 13 ppb I (averaging ~8 ppb), with only two samples > 70 ppb (Déruelle et al., 1992). Samples from Iceland,
Hawaii, and the Society Islands reported I concentrations in the range 7.3-61 ppb
(Déruelle et al., 1992). Most recently, the data field for MORB has been extended, and data have been reported on mantle xenoliths, altered oceanic crust (AOC),
29 wedge fluids, and serpentinites (Table 2.1) (Broadley et al., 2016; Chavrit et al.,
2016; John et al., 2011; Kendrick et al., 2013b; Kendrick et al., 2014b; Kendrick et al., 2012a; Kobayashi et al., 2017; Sumino et al., 2010).
Data from diamonds (Table 2.1), thought to sample the sub-continental lithospheric mantle (SCLM), show Br/Cl values in the range (1.4-4.3)x10-3, with I/Cl mostly ranging from (9.4-70)x10-6; these ratios are comparable to those calculated in MORB
(Burgess et al., 2009; Burgess et al., 2002; Johnson et al., 2000).
30
Figure 2.1 The Earth’s major volatile reservoirs and their halogen concentrations and ratios (references as per Table 2.1).
31
2.1.1 The Halogens as Geochemical Tracers Due to the distinct ratios, which are orders of magnitude different, in the Earth’s reservoirs (Figure 2.1), the halogens are considered to be important tracers of mantle processes and fluid sources (Johnson et al., 2000; Pyle and Mather, 2009).
The halogen ratios (Br/Cl and I/Cl) within melt inclusions, fluid inclusions, and glasses can be used to determine the source reservoir and components of magma, and to trace the recycling of materials through subduction zones back to the Earth’s surface (Pyle and Mather, 2009).
The halogens are proposed to be recycled into the mantle during subduction of serpentinites, oceanic lithosphere, altered oceanic crust, and SCLM (Broadley et al., 2016; Chavrit et al., 2016; John et al., 2011; Kendrick et al., 2013b; Kendrick et al., 2012b; Sumino et al., 2010). During subduction of crust and lithosphere, sedimentary and marine pore fluid hosted halogens may also be dragged down with the subducting slab, enriching the mantle halogen budget (John et al., 2010;
John et al., 2011); a high marine pore fluid halogen signature may also be preserved within serpentinites, where the serpentinising fluid is enriched in halogens (Kendrick et al., 2013b; Kendrick et al., 2011; Sumino et al., 2010), or in
SCLM, as the progressive release of halogens from the downgoing slab releases fluids with a marine pore fluid signature into the overlying mantle (Broadley et al.,
2016). The primitive halogen composition of the mantle is unknown, but recent analyses on EM1 and EM2 basaltic glasses (Kendrick et al., 2015; Kendrick et al.,
2014b) suggest that the OIB source mantle is relatively degassed with respect to the halogens, revealing Br/Cl and I/Cl ratios similar to that observed in DMM and
MORB basalts (Jambon et al., 1995; Kendrick et al., 2012a). It is not well- constrained how the halogens are subducted into the mantle, but there is increasing evidence that the halogens survive subduction and enrich the OIB source, possibly through halogens with a marine pore fluid signature substituting 32 for OH- in the mineral lattice during serpentinisation (Broadley et al., 2016;
Kendrick et al., 2013b; Sumino et al., 2010). Serpentinites have been shown to preserve a marine pore fluid signature to depths of 100 km, in the Higashi-akaishi peridotites (Sumino et al., 2010).
Analyses of the Br/Cl and I/Cl within OIBs and mantle xenoliths, allow for determination of the mantle components present within the halogens in the mantle.
2.2 Noble Gases The noble gases, helium, neon, argon, krypton, xenon and radon (He, Ne, Ar, Kr,
Xe and Rn) form Group VIII (Group 18 IUAPC) of the periodic table. The noble gases all have a full outer shell of electrons, and are the least reactive group of elements, sometimes referred to as the inert gases. Although also known as the rare gases due to their paucity in the solid Earth, helium is the second most abundant element in the universe, after hydrogen (H) (Wieler, 2002). However, due to their high volatility and inert character, the noble gases are depleted in geological materials, and are instead concentrated in the Earth’s atmosphere
(Porcelli et al., 2002a).
The sources of the noble gases in the Earth can be divided into those trapped during the formation of the Earth, primordial, those formed by radioactive decay processes, and those formed by cosmogenic processes (Ballentine et al., 2002).
Argon is the most abundant noble gas on Earth (Porcelli and Turekian, 2003). 40Ar is a product of the β+ decay of 40K (the other product being 40Ca by β- decay) and is still being generated today; whereas 36Ar and 38Ar are essentially primordial
(Porcelli and Pepin, 2003; Porcelli and Turekian, 2003).
Helium is depleted with respect to the other noble gases, as it is not gravitationally bound and therefore escapes the Earth’s atmosphere, with a short residence time
33
(~1 Ma) (Ozima and Podosek, 2002). 3He is mostly primordial, trapped during the
Earth’s accretionary phase (Porcelli and Pepin, 2003). 4He is a radiogenic isotope, formed mainly during the decay of 235U, 238U, and 232Th (Porcelli and Pepin, 2003).
U and Th are incompatible elements, and therefore are concentrated within the outer reservoirs of the Earth. During the decay of 235U, 238U and 232Th, alpha particles are produced. Subsequently, 4He can be produced by the acquisition of electrons by alpha particles (Porcelli et al., 2002b).
2.2.1 Helium and Argon as Geochemical Tracers 3He/4He in geological materials is measured relative to the atmospheric value of
-6 3 4 1.4x10 , = 1 RA (Table 2.2) (Ozima and Podosek, 2002). He/ He in MORB
3 4 averages 8.75 ± 2.14 RA (Graham, 2002), however, He/ He is much more variable in OIB, ranging from below MORB values (~5.5 RA), up to 43 RA (Graham, 2002;
Porcelli and Turekian, 2003). The source of the high 3He/4He in some OIB is assumed to be a primitive mantle reservoir within the deep mantle, that represents the BSE (bulk silicate Earth) (Hilton and Porcelli, 2003; Porcelli and Turekian,
2003). The MORB-like 3He/4He values yielded in OIB samples is possibly due to the recycling of U- and Th-rich components, with ingrowth of 4He, produced during the alpha decay of 235U, 238U, and 232Th, reducing the 3He/4He ratio over time
(Hilton and Porcelli, 2003; Porcelli and Turekian, 2003). 3He/4He in the crust is extremely low at ~0.01 RA (Ozima and Podosek, 2002).
The atmospheric ratio of 40Ar/36Ar is 298.56 ± 0.31 (Lee et al., 2006). In the crust this ratio extends from 299 to 100,000 (Kendrick and Burnard, 2013). MORB values extend even higher, in the range of 28,000-44,000 (Hilton and Porcelli,
2003). OIB values are less than MORB, but fall in the range of 2,600-12,000
(Farley and Neroda, 1998; Hiyagon et al., 1992; Poreda and Farley, 1992;
Valbracht et al., 1997).
34
Table 2.2 40Ar/36Ar and 3He/4He for the Earth’s reservoirs.
40Ar/36Ar 3He/4He
i -6 f Atmosphere 298.56 ± 0.31 1.4x10 (=RA)
a f Crust 299 – 100,000 ~0.01 RA
d c MORB 28,000 - 44,000 8.75 ± 2.14 RA
begh c OIB 2,600 – 12,000 ~5.5 - 43 RA
(Data from Farley and Neroda, 1998b; Graham, 2002c; Hilton and Porcelli, 2003d; Hiyagon et al., 1992e; Kendrick and Burnard, 2013a; Lee et al., 2006i; Ozima and Podosek, 2002f; Poreda and Farley, 1992g; Valbracht et al., 1997h).
2.3 Mantle Reservoirs The formation of the crust from the upper mantle has led to the residual mantle becoming depleted in incompatible trace elements (Carlson, 1995). Recycling of crust, lithosphere, seawater, marine pore fluids, and sediments has contributed to the mantle being less chemically and isotopically differentiated than would be expected in the absence of plate tectonics (Carlson, 1995; Hofmann, 1997).
There is often a poor correlation between 3He/4He in OIBs with the other radiogenic isotopes, which suggests that the He is sampling different mantle components to the other isotopes, suggesting mixing between different source components within the mantle. However, picrites from Baffin Island (up to 3He/4He
3 4 87 86 = 49.5RA) show a strong correlation between He/ He and Sr/ Sr and
143Nd/144Nd (Stuart et al., 2003). Some of the highest 3He/4He ratios observed (up
3 4 87 86 to He/ He = 40 RA in Iceland) are associated with intermediate Sr/ Sr ratios
(White, 2015). The chemical variations within OIBs and MORB have been proposed to be sampling different mantle components, based on end-members observed Sr-Nd-Pb isotope space (87Sr/86Sr, 143Nd/144Nd, 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb) (White, 2015; Zindler and Hart, 1986).
35
2.3.1 Mantle Components Table 2.3 Long-lived radioactive decay series used as tracers (after Hofmann, 1997).
Parent Daughter Main Decay Half-life (yr) Tracer ratio Nuclide Nuclide Mode(s) (radiogenic/ nonradiogenic) 147Sm 143Nd α 106 x 109 143Nd/144Nd 87Rb 87Sr β- 48.8 x 109 87Sr/86Sr 176Lu 176Hf β- 35.7 x 109 176Hf/177Hf 187Re 187Os β- 45.6 x 109 187Os/188Os 40K 40Ar β+ 1.25 x 109 40Ar/36Ar 232Th 208Pb α, γ 14.01 x 109 208Pb/204Pb 238U 206Pb α, γ, F 4.468 x 109 206Pb/204Pb 235U 207Pb α, γ, F 0.738 x 109 207Pb/204Pb
The mantle “end-members” were initially proposed by Zindler and Hart (1986), who suggested four isotopically distinct mantle components: (DMM) depleted
MORB mantle, (HIMU) high-µ, (EM1) enriched mantle 1, and (EM2) enriched mantle 2; these are based on Sr-Nd-Pb isotope characteristics (Hart et al., 1992).
These components contribute to the sources of oceanic basalts (OIB and MORB), in varying proportions (Hofmann, 1997). In addition to the four components,
Zindler and Hart (1986) described a fifth component, PREMA, or prevalent mantle, originally thought to be a mix of DMM, HIMU, EM1 and EM2 (Figure 2.2) (and possibly BSE), but later revised to exclude EM2 (Faure, 2001). Current thinking suggests that there are six mantle end-members: 1) EM1, 2) EM2, 3) HIMU, 4)
DMM, 5) PREMA, and 6) LOND (White, 2015). LOND is a relatively new term, not
yet commonly adopted in the literature, but represents a source that yields low εNd for a given 87Sr/86Sr (White, 2015). PREMA is thought to have a more primitive source, representing the higher 3He/4He ratios observed in OIBs (White, 2015).
36
These mantle components are defined by where they lie within Sr-Nd-Pb isotope space, each having distinct isotopic ratios of 143Nd/144Nd, 87Sr/86Sr, 206Pb/204Pb,
207Pb/204Pb, and 208Pb/204Pb (Hofmann, 2007). These isotope ratios can be used as tracers of recycling, magmatic, and mantle processes (Table 2.3) (Hofmann,
2007). Although these mantle components were initially proposed as mantle end- members (Zindler and Hart, 1986), most ocean island basalts show a mixing between two or more of these components, including the island groups of Tristan da Cunha, the Azores, and the Canary Islands (Figure 2.3). Although there is general consensus that recycled material is introduced into the lithosphere and deep mantle through subduction and mantle convection (White, 2015), there is little agreement on the source of the variation within the proposed mantle end- members, however, possible contaminants are discussed in the following sections.
2.3.1.1 DMM DMM is defined as having the highest 143Nd/144Nd (>0.5132), and the lowest
87Sr/86Sr (<0.7025), 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios (Carlson, 1995;
Hofmann, 1997; Zindler and Hart, 1986). DMM is understood to have formed by depletion of the primitive mantle during crust formation, as the incompatible elements were extracted into the oceanic and continental crust from the fertile mantle (Carlson, 1995). DMM is the source of MORB (Workman et al., 2004).
2.3.1.2 HIMU 238 204 HIMU (µ = ( U/ Pb)t=0) is enriched in U and Th relative to Pb, with very high
206Pb/204Pb ratios (>20), low (unradiogenic) 87Sr/86Sr, and intermediate 143Nd/144Nd
(and 187Os/188Os >0.134) (Carlson, 1995; Stracke et al., 2005; Zindler and Hart,
1986). This enrichment of the source in incompatible U and Th is proposed to have originated from recycling and long term (1.4-2 Ga) storage of ancient oceanic crust (Jackson et al., 2007; White, 2010; Willbold and Stracke, 2006; Workman et al., 2004; Zindler and Hart, 1986). 37
Localities where HIMU basalts are found include the Azores, St Helena, Ascension
Island, Guadalupe Island, and Canary Islands in the Atlantic Ocean, Balleny
Islands in the Southern Ocean, and the Austral Islands, Mangaia Island and
Rurutu Island in the Pacific Ocean (Day and Hilton, 2011; Faure, 2001; Hofmann,
1997).
Figure 2.2 Pb isotope data from ocean island basalts, from island groups analysed for their halogen ratios in this study (data from the Earthchem databases). Tristan da Cunha lies between EM1, EM2, and PREMA, with the Azores and Canary Islands showing a mixture of EM1, EM2, HIMU, and PREMA. 2.3.1.3 EM1 and EM2 Both EM1 and EM2 are enriched in incompatible elements relative to the BSE, and are thought to contain recycled continental and oceanic material, including upper and lower continental crust, and subducted metasomatised oceanic lithosphere
(Carlson, 1995; Farley et al., 1992; Hofmann, 2007; Jackson et al., 2007; White,
2010; Willbold and Stracke, 2006). The origin of EM1 has been suggested to be from the recycling of delaminated subcontinental lithosphere, pelagic sediments,
38 or lower continental crust (Hofmann, 2007). The origin of EM2 is also controversial, but it has been proposed to contain recycled metasomatised oceanic lithosphere (Workman et al., 2004), ancient recycled continental crust
(Jackson et al., 2007), or recycled terrigenous sediments (Hofmann, 2007).
EM1 has very low 143Nd/144Nd and 206Pb/204Pb (<18), relatively low 87Sr/86Sr
(<0.706), and relatively high 208Pb/204Pb; EM2 has a higher 87Sr/86Sr ratio (>0.706) and higher 206Pb/204Pb (≈18.8) (Table 2.4) (Carlson, 1995; Hofmann, 2007).
Localities which have EM1 type lavas present include Tristan da Cunha (and the related Walvis Ridge and Gough Island) in the Atlantic Ocean and Pitcairn Island in the Pacific Ocean (Faure, 2001; Hofmann, 2007; Kendrick et al., 2014b). EM2 type basalts are also found at Tristan da Cunha (but not the Walvis Ridge) and at
Samoa, Marquesas Islands, and Society Islands in the Pacific Ocean, and São
Miguel (Azores) in the Atlantic Ocean (Faure, 2001; Hofmann, 2007; Kendrick et al., 2015; Kendrick et al., 2014b; Moreira et al., 1999; Moreira et al., 2012). Both
EM1 and EM2 are concentrated in OIB just south of the equator, a feature known as the DUPAL anomaly (Hofmann, 1997).
39
Figure 2.3 Sr-Pb isotope data from ocean island basalts, from island groups analysed for their halogen ratios in this study (data from the Earthchem databases). Tristan da Cunha lies between EM1and PREMA, with the Azores showing a mixture between HIMU and EM2, and the Canary Islands a mixture of HIMU, and PREMA.
40
Table 2.4 Isotopic ratios of mantle reservoirs.
DMM HIMU EM1 EM2 PREMA BSE
3He/4He 8e 5-9.5e <15f <7f
40Ar/36Ar 41,500g <2,300h <2,200h
18.5- 206Pb/204Pb 18b 20.5-21.5 <18a ≈18.8a 19.65b
15.5- 207Pb/204Pb ~15.45i ~15.9 i ~15.6 i ~15.7 i 15.57b
208Pb/204Pb ~37.25 i ~41 i ~39 i ~40 i 38.4-39.2b
0.7025cd- 0.7045- 87Sr/86Sr <0.7025a 0.7028c <0.706a >0.706a 0.7052b 0.7050b
0.5127b- 143Nd/144Nd >0.5132a 0.5128c 0.5124c 0.5126c 0.5132cd
187Os/188Os >0.134a ≈0.1275a
(Data from Carlson, 1995a; Farley et al., 1992b; Graham, 2002e; Hanan and Graham, 1996f; Hanyu et al., 2011h; Tucker et al., 2012g; White, 2015i; Wilson, 1993c; Zindler and Hart, 1986d; data from (i) are approximate average values; PREMA values include those for FOZO, C, and PHEM). 2.3.1.4 PREMA and FOZO, C and PHEM – one component? Zindler and Hart (1986) suggested a fifth mantle component, prevalent mantle
(PREMA) which is intermediate between HIMU, DMM and EM1 (Faure, 2001).
PREMA is suggested to originate from the mixing of these three components (and possibly BSE), formed by partial melting of the plume head and adjacent lithospheric mantle (Faure, 2001; Zindler and Hart, 1986).
The terms FOZO (focus zone), C (common component) and PHEM (primitive helium mantle) have been used in addition to PREMA, to represent a mixed common component (Hofmann, 2007). FOZO is proposed to be a mixture of DMM and HIMU (Wilson, 1993). PHEM is thought to represent a primitive, or
3 4 undegassed mantle source, and is characterised by high He/ He (>24 RA), near bulk Earth 87Sr/86Sr (0.7042-0.7052) and 143Nd/144Nd (0.51265-0.51280), and radiogenic Pb – 206Pb/204Pb ~18.5-19.0, 207Pb/204Pb ~15.5-15.57, and 208Pb/204Pb
~38.4-39.2 (Table 2.4) (Farley et al., 1992). C is proposed to be a component 41 common to all MORB, defined as 87Sr/86Sr ~0.703-0.704, 206Pb/204Pb ~19.2-19.8,
207Pb/204Pb ~15.55-15.65, and 208Pb/204Pb ~38.8-39.6 (Hanan and Graham, 1996).
Similar to PHEM, it is suggested to be a mixture of DMM, EM1, EM2 and HIMU
(Hanan and Graham, 1996).
The similar isotopic ratios of FOZO, PHEM and C, together with PREMA has led to the suggestion that there is little difference between these components (Hofmann,
2007; White, 2015).
2.4 Halogen and Noble Gas Fractionation When comparing halogen ratios and relating the measured ratios to possible recycled and mantle components, it is important to establish if the halogens fractionate from one another, prior to their entrapment in the host minerals as melt and fluid inclusions, as any fractionation from one another would alter the halogen ratios. Balcone-Boissard et al. (2010) analysed a series of magmas, including samples from the Azores, for their halogen rations in melt inclusions and glassy matrix, which showed that while halogen fractionation is observed in calc-alkaline magmas, the halogen ratios (Br/Cl and I/Cl) remained constant during differentiation and degassing of phonolitic and trachytic melts from the magma reservoir to eruption, and they propose this conclusion would also hold for basaltic magmas. It is also observed that that H2O degassing has little effect on the halogen ratios during plinian eruptions (Balcone-Boissard et al., 2010).
Kendrick et al. (2012b) reported that backarc basin basalts (BABB) from the
Manus backarc basin yielded constant Br/Cl and I/Cl ratios over a range of MgO.
As the halogen ratios do not vary as a function of MgO, it is assumed that the halogens did not fractionate from one another by partial melting or fractional crystallisation, and the measured halogen signature is therefore thought to represent the mantle source. A further study on MORB (Kendrick et al., 2012a)
42 again supports that the halogens were not fractionated from one another, in the presence of a CO2-rich fluid, or during partial melting and fractional crystallisation, with the halogen signature again thought to represent the mantle source. As such, it is not thought that the halogens undergo significant fractionation from one another in the mantle (Balcone-Boissard et al., 2010; Bureau et al., 2016; Bureau et al., 2000; Kendrick et al., 2012a), but fractionation of the halogens may occur in the presence of halogen-bearing mineral phases, including apatite, amphibole, mica, and other hydrous minerals (Johnson et al., 2000). Mantle apatites are thought to contain ~3 wt% F, <1 wt% Cl, and 30 ppm Br, thus crystallisation of apatite could remove potentially F>Cl>Br from the melt/fluid phase, enriching the residual melt in the remaining halogens, leading to an increase in I/Cl and Br/Cl
(Aiuppa et al., 2009; Burgess et al., 2009; Johnson et al., 2000). It is not thought that crystallisation of basalts significantly fractionates the halogens from one another, due to the mineral phases involved in basaltic petrogenesis (Balcone-
Boissard et al., 2010). Strong fractionation is however reported from dehydration reactions in subducting slabs, with breakdown fluids enriched in Br and I relative to
Cl, and the downgoing slab becoming progressively more depleted in the heavy halogens (Kendrick et al., 2011).
Fractionation can also occur during evaporation (e.g. halite precipitation) or brine formation, but this process is not thought to occur at mantle depths and pressures
(Martin et al., 1993); however marine brines evolve to high I/Cl ratios (Table 2.1) which are useful in tracking the movement of halogens through the subduction cycle. Addition or removal of halogens may occur at shallow levels, through melt- rock interaction, or magma contamination from meteoric or seawater (Pyle and
Mather, 2009).
43
Although there is increasing information on halogen fractionation within the upper mantle, particularly in relation to MORB and diamond analyses (e.g. Burgess et al., 2009; Kendrick et al., 2012a; Safonov et al., 2007), there is a paucity of data regarding the fractionation of the heavy halogens from one another in the lower mantle. Experimental studies suggest that Cl solubility depends on melt composition, with Cl decreasing as a melt evolves from basaltic to felsic compositions; the presence of H2O-, CO2-, or S-bearing fluids can also cause fractionation of the halogens (Aiuppa et al., 2009). At shallower pressures (0.2
GPa and 900°C), experiments completed by Bureau et al. (2000) suggest that Br is approximately twice as compatible in a hydrous fluid phase than Cl, and I an order of magnitude more compatible than Cl (Df/m 8.1±0.2 Cl, Df/m 17.5±0.6 Br, Df/m
104±7 I), which would suggest that I would be more degassed than Br and Cl in
H2O-rich systems at these depths, resulting in an increased I/Cl ratio in the remaining melt phase. In H2O-poor systems, it has been reported that there is minimal degassing of Cl at shallow (~35m) depths (Edmonds et al., 2009).
Experiments completed at 0.9-1.8 Ga (T=1150-1450°C) by Mungall and Brenan
(2003) show that in the presence of silicate and sulphide melts, I is more
sul/sil sul/sil compatible than Br and Cl in the sulphide melt (DI = 0.15, DBr = 0.026,
sul/sil DCl = 0.038). This suggests that in sulphur saturated systems, an increase of
I/Cl (together with a decrease of Br/Cl) would be seen in the sulphide melt, with a reduction of I/Cl in the silicate melt.
Analyses on alkaline felsic magmas show that fractionation is less efficient than proposed by experimental fluid-melt partition coefficients (Balcone-Boissard et al.,
2010), and that the difference between partitioning observed in samples analysed for their halogen ratios, and those determined in experimental conditions, can vary by a factor of five. Although there is increasing data on Cl (and F), more data are
44 needed with respect to the heavy halogens, to establish if the halogens undergo significant fractionation from one another in the OIB source region.
Solubility of the noble gases in basaltic magmas decreases with increasing atomic mass, and similar to the halogens, is dependent on melt composition (Graham,
2002). During magma degassing, the heavier noble gases will partition preferentially into the exsolved volatile rich fluid/gas phase, resulting in a depletion in the residual melt (Graham, 2002). The volatile content (H2O, CO2) of melts also influences noble gas solubility, with small amounts (3 wt%) of H2O increasing He solubility (by a factor of 3), and significantly increasing the solubility of the heavier noble gases compared to anhydrous melts (Graham, 2002). The effect of CO2 is less well constrained, but its presence is thought to decrease the solubility of the noble gases in a melt (Graham, 2002). As the noble gases have low solubilities, any exsolution of a vapour phase during magma degassing will result in the degassing of noble gases (Hilton and Porcelli, 2003). However, an anhydrous
CO2-rich magma will retain its dissolved He more effectively than a H2O-rich magma, as the solubility of the noble gases in the magma will decrease
(dramatically for the heavier noble gases) as the water exsolves (Graham, 2002).
The solubility of the noble gases in mineral phases is less well known, but experimental partition coefficients (D = concentration in crystal/concentration in melt) and those measured from olivine-rich MAR basalts, suggest D < 1, indicating a preference for the melt phase during crystallisation or partial melting (with
0.0058 < DHe < 0.07, and 0.003 < DAr < 0.15 in olivines) (Graham, 2002; Hilton and
Porcelli, 2003). The partitioning behaviour of the noble gases has resulted in the distinct elemental ratios within the Earth’s reservoirs, thus like the halogens they are sensitive tracers of mantle and magmatic processes (Hilton and Porcelli,
2003).
45
2.5 Oceanic Basalts Mid-ocean ridge basalts (MORB) are produced by the partial melting of mantle peridotite beneath oceanic spreading ridges (Wilson, 1993). Less chemical and isotopic variation is observed in MORB, which are more chemically uniform than ocean island basalts (OIB) (Davies, 1990). MORB originates from the upper mantle and samples the DMM mantle component, an end-member which has become depleted in incompatible elements over time due to the extraction of the
Earth’s crust (Carlson, 1995). The chemical composition of basalts varies depending on their source region, the degree of partial melting of the source rock, fractionation of the magma, and interaction with surrounding rocks (contamination) and other magmas and fluids, with the heterogeneity in OIB suggesting that OIB samples a less homogenous region in the mantle than MORB. Hotspot OIBs are thought to be caused by mantle plumes (see section 2.7), sourced from the core- mantle boundary, 670km seismic discontinuity, or from within the upper mantle
(Montelli et al., 2004), with other OIBs sourced from shallower depths (Niu et al.,
2011). OIB rarely have end-member component compositions; often their compositions reflect mixing between the three OIB end-members (EM1, EM2, and
HIMU) and DMM (Wilson, 1993). OIB lead isotopes overlap the MORB field, but also extend to more extreme values (Hofmann, 2007). MORB tend to be more radiogenic in Nd and Hf isotopes, but less radiogenic in Sr isotopes (Hofmann,
2007).
OIB are more chemically and isotopically heterogeneous than MORB (Carlson,
1995). The major and trace element geochemistry differs between OIB and MORB
(Hofmann, 2007), with most OIBs enriched in incompatible elements, with respect to MORB, with MORB depleted in K, Ti, Pb, Rb, Ba, Th, U, and the LREEs (Figure
2.4) (White, 2010). The enrichment in OIB is proposed to be due to the OIB source being less depleted in incompatible elements, and because OIB is often produced 46 by lower degrees of partial melting than MORB (Hofmann, 2007); during partial melting, the incompatible elements will partition into the melt phase, thus during low degrees of partial melting, the source and the melt will vary greatest in their incompatible element concentrations, with the melt being enriched with respect to the source. Recycling of the Earth’s crust back into the mantle may also have re- fertilised the OIB source over time – suggesting that the trace element composition and isotope ratios observed in OIB are due to a mixture of these three factors.
Figure 2.4 Spider diagram, showing the concentrations of trace elements, normalised to BSE, in MORB and OIB (figure modified from White, 2010).
2.6 Mantle Convection: Whole vs Layered Two main models for mantle convection exist, based on seismic and gravity data, and geochemistry (e.g. nobles gases and lithophile element isotope composition)
(Graham, 2002; Hilton and Porcelli, 2003). The first model considers that convection in the upper and lower mantle are independent from one another,
47 separated by a boundary layer at the 660-670 km seismic discontinuity, where the spinel phase transforms into perovskite (Graham, 2002; Hilton and Porcelli, 2003;
Lebedev et al., 2002). This model considers that initially the entire mantle had the same isotopic and elemental composition, but that the upper mantle then degassed (forming the Earth’s atmosphere), whereas the lower mantle was isolated (apart from a small flux from OIB) and thus remains primitive or undegassed (Hilton and Porcelli, 2003). Therefore, the differences between MORB and OIB geochemistry can be accounted for in that MORB samples a depleted upper mantle source, whereas OIB sample a deeper relatively undepleted or primitive lower mantle, representative of the BSE (bulk silicate Earth), which remains geochemically isolated from the upper mantle (Graham, 2002; Porcelli and Ballentine, 2002). A modification of the layered mantle model suggests interaction between the reservoirs, through an input of noble gases into the upper mantle within fluxes of upwelling material from the lower mantle, and from atmospheric noble gases present in subducted material (Hilton and Porcelli, 2003).
Seismic tomographic evidence provides support for the whole mantle convection model, as it has shown that some subducted slabs have penetrated the 670 km discontinuity, suggesting that a boundary separating upper and lower mantle convection is not present at this depth (Hilton and Porcelli, 2003; Porcelli and
Ballentine, 2002). However, we must find some way in which distinct mantle domains remain isolated from whole mantle convection, in order to explain the heterogeneity observed in OIB geochemistry (Hilton and Porcelli, 2003).
2.7 Mantle Plumes Mantle plumes are areas of hot, upwelling, buoyant material in the mantle, which erupt at hotspots at the Earth’s surface (Koppers, 2011), and are thought to be
100-300°C hotter than the surrounding upper mantle (Hawkesworth and
48
Scherstén, 2007; White, 2010). Some hotspots have associated flood basalts, e.g.
Paranã-Etendeka continental flood basalts (Tristan plume) and the Azores plateau
(Azores plume), whereas other hotspots, e.g. the Canary Islands, do not have related flood basalt provinces.
There is increasing data on the seismic imaging of mantle plumes, with some plumes imaged proposed to be sourced near to the seismic discontinuity at the
670km mantle boundary, others from the core-mantle boundary, and other plumes with no well-constrained depth profile (Montelli et al., 2004). The Azores plume and the Canary Islands plume are imaged down to the core mantle boundary, suggesting a deep source for the plumes; the Azores plume shows a lack of resolution in the mid-mantle (>660km) (Adam et al., 2013), but this may be due to it linking with the Canary Islands plume at depth (Montelli et al., 2004). The Tristan plume has recently been imaged by Schlömer et al. (2017), who propose a 100km wide plume extending down to 250km, which may be the remnant of a dying mantle plume.
Although mantle plumes are not thought to be fixed in position (Montelli et al.,
2006), they remain relatively fixed with respect to the motion of the plates over the surface expression of the plume. The movement of the plates over hotspots can cause a hotspot track, typical of that seen at the Hawaiian Islands, where the oceanic islands are progressively older moving away from the present plume location (White, 2010). This progressive ageing away from the plume is observed at the Canary Islands, and to some extent, Tristan da Cunha, outlined by the
Walvis Ridge. However, there is no correlation between the ages of the Azores
Islands, with reference to the proposed plume location (suggested by Adam et al.
(2013) to be beneath the island of Terceira). This may be due to the complex
49 setting of the Azores at the triple ridge junction, whereby plume material may travel along the strike of the Terceira Rift.
It is suggested that the high 3He/4He ratio (greater than MORB) observed in some ocean island settings, e.g. the Azores (except São Miguel), Iceland, and Hawaii, may be due to a relatively undegassed component, from a deep reservoir
(Hofmann et al., 2011). This suggests that the mantle has not completely homogenised over time, with the variations observed within OIB sampling a more primitive component, as well as recycled material, and MORB; both of which may be entrained as the plume rises to the surface. Analysing samples from ocean islands may allow us to constrain the sources present within OIB.
2.8 Fluid and Melt Inclusions
Fluid inclusions are small inclusions of melt, pure water, brines, or gases that are trapped within crystals as they grow from a fluid or melt (Roedder, 1984). The term fluid inclusion refers to any inclusion that was fluid at the time of its entrapment, however some authors retain the term fluid inclusion for those inclusions which remain in the fluid form, and use the term melt inclusion for now solidified inclusions which contain glass or crystallised phases (Roedder, 1984), the latter terminology used herein. Silicate melt inclusions are small blebs of melt trapped within the host silicate mineral during crystallisation (Frezzotti, 2001). These blebs are preserved as inclusions of glass, and may also contain gas bubbles and daughter mineral phases (Frezzotti, 2001).
Most fluid inclusions are between 1-10 µm in size, but can range from <0.2 µm to over 100 mm (however rarely larger than 1 mm); with smaller inclusions being far more abundant than larger ones (Roedder, 1984).
50
2.8.1 Formation of Fluid and Melt Inclusions
As a crystal grows, fluid or melt may become trapped within irregularities in the crystal structure (Roedder, 1984). Inclusions can form at the crystal melt interface, in defects in the lattice structure of the host crystal, and during skeletal growth or rapid post-dissolution growth of a crystal (Kent, 2008); these inclusions are known as primary inclusions as they were trapped during crystallisation (Figure 2.5)
(Bodnar, 2003; Kent, 2008; Roedder, 1984). Secondary inclusions can occur when melt or fluid enters a fracture in a crystal, and the fracture subsequently heals, trapping the melt or fluid (Roedder, 1984). Pseudosecondary (or primary- secondary) inclusions are those trapped when a fracture occurs during crystal growth (Bodnar, 2003). The term “fluid inclusion assemblage” (FIA) refers to a group of inclusions which were trapped simultaneously, which can be observed optically if inclusions bound zoning within minerals, or are present along healed fractures (Bodnar, 2003; Kent, 2008).
The majority of melt inclusions found within silicate minerals are primary as they are trapped during crystallisation, although secondary inclusions may be trapped during the healing of fractures and often appear as trails within the host crystal
(Frezzotti, 2001). Within basalts, inclusions may be found in any of the major mineral phases; in plagioclase inclusions often line the zonal boundaries within the mineral, but inclusions tend to be more randomly distributed within pyroxene and olivine (Frezzotti, 2001).
As olivine is one of the first minerals to crystallise from a melt, it is often used to analyse volatiles within melt inclusions. Lithophile trace elements, e.g. Cl, Br, and
I, are relatively incompatible in olivine, and therefore not much of any of these elements will be incorporated into its mineral structure; it can therefore be assumed that the measured concentrations of these elements comes solely from 51 within the fluid inclusions and melt inclusions (Kent, 2008), making olivine an ideal phase for bulk analysis of the halogen ratios within melt and fluid inclusions. It has been suggested that volatile abundances within melt inclusions are relatively unaffected by shallow degassing, and that the concentrations of the incompatible elements are better preserved (Kent, 2008; Qin et al., 1992). Melt inclusions within olivine can be considered a closed system with respect to their incompatible elements, due to their slow diffusion rates (Schiano, 2003). It is therefore not expected that there would be any post-entrapment fractionation of the halogens, or contamination from crustal materials, and thus Br/Cl and I/Cl ratios measured within (melt inclusions trapped within) olivine crystals are thought to be representative of the magma source.
Melt inclusions can provide insight into the undifferentiated composition of the magma, particularly with respect to volatiles, and magma evolution (Frezzotti,
2001). Many volatiles degas during magma ascent and eruption, but the phenocryst surrounding the silicate melt inclusion prevents the gases from escaping, and also offers protection from weathering and alteration (Kent, 2008;
Lowenstern, 2003).
As such, the study of silicate melt inclusions within crystals can provide information on the composition magma at the time of entrapment, and help determine the abundances of halogens and other volatile elements (e.g. noble gases, H2O, CO2, S) within the parental magma, and thus the source region
(Aiuppa et al., 2009). Melt inclusions can also help elucidate degassing processes and provide information on the recycling of subducted material (Frezzotti, 2001).
52
Figure 2.5 Primary inclusions (a-e), and secondary inclusions (f); top and bottom figures in each panel show early and late stage crystal growth; a) crystal growth after dissolution, b) inclusions forming at the crystal-melt interface, c) inclusions hosted in defects in crystal lattice, d) textural re-equilibration and overgrowth following dendritic growth, e) overgrowth of skeletal crystals, inclusions form symmetrically, f) inclusion trail along a healed fracture (redrawn from Kent, 2008). 2.8.2 Analysis of Halogens and Noble Gases in Silicate Melt Inclusions
Volatiles can be extracted from melt inclusions by in vacuo crushing or heating of a bulk sample, or laser ablation prior to analysis. Crushing has been observed to only release a small percentage (~1%) of the gas from melt inclusions, although crushing effectively releases gases trapped from within fluid inclusions (Villa,
2001). The small release from melt inclusions can be problematic when analysing the noble gases (and also the halogens) due to their low abundances. Stepwise heating of bulk samples can overcome the problems associated with crushing, as this will release volatiles trapped within melt inclusions (Villa, 2001).
After release, the gas can be analysed on a mass spectrometer, using an extension of the Ar-Ar dating technique (see section 3.2). Concentrations of the halogens can also be determined via the same method, but the samples must be
53 irradiated via neutron irradiation to artificially produce isotopes of the noble gases
(Kendrick, 2012). Concentrations of the noble gases can then be used to determine the original abundances of the halogens, using monitor minerals
(Kendrick, 2012; Ruzié-Hamilton et al., 2016).
This technique has previously been used (e.g. Broadley et al., 2016; Chavrit et al.,
2016; Sumino et al., 2010) to analyse halogens (and noble gases) present in fluid and melt inclusions in mantle xenoliths, OIBs, and altered oceanic crust, and to determine the source of the halogen ratios present within the samples. The advantage of measuring melt and fluid inclusions by this bulk analysis method is that it has lower detection limits with respect to the heavier halogens (Br and I) when compared to other techniques (e.g. electron microprobe), which can only measure the more abundant (F, Cl) halogens (Roedder, 1990). Analyses of fluid and melt inclusions within serpentinites, basalts, and mantle xenoliths have been shown to contain recycled components, with the halogens in the fluid and melt inclusions preserving the halogen signature of the source; therefore this technique allows the halogens to be used as geochemical tracers of alteration, subduction, and recycling into the mantle reservoirs.
2.9 Summary There are still relatively few data in the literature reporting the halogen ratios within
OIBs. Recently, samples have been reported from EM1 and EM2 mantle end- members, from the Pitcairn and Society seamounts, and Samoa (Kendrick et al.,
2015; Kendrick et al., 2014b), which all report a MORB-like OIB signature. No known data have been reported on halogen ratios HIMU-type basalts.
In order to gain a better understanding of the halogen components present in the mantle, to determine if a recycled signature is present within OIBs, and to establish if OIB is geochemically different to MORB with respect to its halogen 54 ratios, it was important to target OIBs from a variety of settings, which show a range across the mantle components (e.g. EM1, EM2, HIMU). It is unknown if there is a correlation between 3He/4He ratios and halogen signature within the mantle, therefore islands with a range of 3He/4He ratios were targeted.
The island groups of Tristan da Cunha, the Canaries, and the Azores are all located in the Atlantic Ocean, with Tristan da Cunha, and the Canaries situated on the African Plate, east of the Mid-Atlantic Ridge (Figure 2.6). The Azores are situated at an active triple junction between the North American, Eurasian and
African Plates, both on the east and the west side of the Mid-Atlantic Ridge. Both the Canaries and the Azores are located in the Northern Hemisphere, with the
Tristan da Cunha group located in the Southern Hemisphere, 400km east of the
Mid-Atlantic Ridge. These islands show a range in He-Sr-Pb isotope space, with the Canary Islands and the Azores (except São Miguel) proposed to have a HIMU mantle component. However, there two island groups show a range in 3He/4He ratios, with the Canary Islands being more MORB-like and the Azores ranging to higher than MORB values, perhaps indicating a more primitive source. São Miguel
3 4 has low He/ He (<5.1 RA), proposed to be due to the presence of an EM2 component, whereas Tristan da Cunha shows a mixture of EM1 and EM2 characteristics. In OIBs, 3He/4He weakly correlates with other isotopes, suggesting a mixed source for these islands – a source which may be able to be resolved by analysing the halogen ratios present.
55
Figure 2.6 Map showing the locations of the Azores, Canaries, and Tristan da Cunha, together the 3He/4He ratio ranges (Day and Hilton, 2011; Graham, 2002; Jean-Baptiste et al., 2009; Moreira et al., 1999) and proposed mantle components, based on Sr-Nd-Pb isotope space (White, 2015; Zindler and 3 4 Hart, 1986). He/ He classifications based on reference to MORB ratio (8.75 ± 2.14 RA).
56
3 Experimental Methods
3.1 Sample Selection and Limitations
Fieldwork to the Azores was undertaken during a two week period in September
2011. The islands of São Miguel, Pico, Terceira, and Graciosa were selected as the target islands for sample collection, as samples were already available from
São Jorge and Faial. The samples were selected by mainly targeting sites with previously published data from other isotope systems (Beier et al., 2006; Beier et al., 2007; Calvert et al., 2006; Elliott et al., 2007; Madureira et al., 2005; Moreira et al., 1999). The samples were collected mainly from roadside cuttings and coastal and lakeside exposures (due to the presence of dense vegetation on the islands), and were chosen based on the presence of visible olivine phenocrysts. Over 80 olivine-bearing basalt samples were collected: 20 from each of the islands of São
Miguel, Terceira, and Pico, and 24 from Graciosa. Before leaving each island, the samples were catalogued, and posted back to the UK for later analysis.
The project has a number of limitations related to sample selection criteria:
The sample selection aimed to achieve geographical spread at the expense
of stratigraphic framework
The relatively short interval of fieldwork was designed to accommodate
visiting several islands, rather than detailed collection from a single volcanic
source
For ease of sampling and efficiency, samples were collected primarily from
roadside cuttings, and coastal and lakeside locations, were exposure was
optimum; and in particular targeting areas where samples were collected
from in previously published studies
57
The samples analysed in this study had not been previously analysed,
however, in order to overcome this limitation, samples, when possible, were
obtained and collected from the same geographical areas as previously
published samples (Beier et al., 2006; Calvert et al., 2006; Elliott et al.,
2007; Madureira et al., 2005; Moreira et al., 1999)
Further samples were obtained from the islands of São Jorge and Faial (provided by Dr Vera Fernandes), and Corvo and Flores (supplied by Dr Felix Genske)
(Genske et al., 2016; Genske et al., 2012).
Samples from Tristan da Cunha and the Canary Islands were selected from a set of previously collected samples, with the aim of sampling a wide geographical area on each island. Approximate locations are used for the Tristan da Cunha samples, as due to the age of the collection and the sampling strategy used, local name localities were used, rather than specific localities or coordinates.
3.2 Halogen and Noble Gas Analysis
Olivine and pyroxene crystals are handpicked using a binocular microscope, and prepared for irradiation (as detailed in section 3.2.1). The halogen (Cl, Br, I), K,
Ca, and noble gas (Ar, Kr, Xe) content of these grains are measured using an extension of the Ar-Ar dating technique and noble gas mass spectrometry.
Olivine was analysed as it is one of the first mineral phases to crystallise from a melt; pyroxene was also selected because it is a major mineral phase and provides a comparison with olivine – to establish if both phases contain the same generation of melt inclusions, and to determine if the pyroxene phase is sampling more differentiated melts.
58
3.2.1 Preparation for Irradiation
Olivine and pyroxene crystals are handpicked under a binocular microscope, and then washed in acetone and deionised water to remove any surface contaminants.
The crystals are then dried at ~100°C and weighed, wrapped in aluminium foil
(and re-weighed) and sealed in evacuated quartz tubes, together with a hornblende flux monitor, Hb3gr, of known age (1.0736±0.0053 Ga) and K
(1.247±0.008 wt%), Ca (7.45 wt%), and Cl (2379 ppm) content, Scapolite standard with known halogen content (C = 3.1±1 wt%, Br = 538±33 ppm, and I = 1054±178 ppb), and an enstatite achondrite meteorite I-Xe standard (Shallowater) of known age (4.56±0.00004 Ga) (Hohenberg, 1967; Jourdan et al., 2006; Kendrick, 2012;
Roddick, 1983).
3.2.2 Neutron Irradiation
Prior to isotopic analysis, the samples were irradiated in position B2W of the
SAFARI-1 reactor, NESCA, Pelindaba, South Africa (irradiation MN07a), or the
RODEO facility, High Flux Reactor (HFR), NRG Petten, The Netherlands
(MN2011a, MN2012b, and MN2013a), and held on site for approximately six weeks, before being returned to Manchester. Irradiation generates the following
39 38 37 80 128 neutron-induced isotopes: ArK, ArCl, ArCa, KBr and XeI, with a conversion efficiency, determined from the monitor minerals (section 3.2.1), of about 1 atom in
104 of the parent element (Table 3.1). For the halogens the current detection limits are 6.6x10-9 g Cl, 6.9x10-13 g Br, and 1.0x10-12 g I.
The abundances of the neutron-derived isotopes are then measured and used as a proxy for the parent isotopes. This method allows the natural noble gas isotopes
(e.g. 40Ar, 36Ar, 84Kr, 129Xe) to be measured at the same time as the neutron- derived isotopes. The fast and thermal neutron fluxes are then determined using
59 the flux monitors. The abundance of the neutron-derived isotopes produced depends on the initial abundance of the parent isotope, the neutron cross-section, and the neutron flux that the samples were exposed to in the nuclear reactor
(Table 3.2).
3.2.3 Noble Gas Extraction
After irradiation, the samples are removed from the quartz irradiation tubes and loaded either into the in vacuo crushing device, or the furnace loading system
(“Christmas Tree”) for extraction of the noble gases. Crushing is used to release gases from any fluid inclusions present, whilst step heating releases gases from melt inclusions, in addition to any remaining fluid inclusions. Initially, a subset of the samples to be analysed are both crushed and step heated, to establish the presence of halogens within fluid and melt inclusion phases. If the crushing step releases small amounts of halogens, further samples will be step heated only, as near to the detection limit errors are increased due to the program being unable to find the mass peaks. The halogen abundance of the host mineral is assumed to be very low.
3.2.3.1 Crushing
Samples are crushed in vacuo using modified Nupro® valves (Stuart et al., 1994).
The samples are placed in steel crucibles and manually crushed using a steel pestle attached to the Nupro® valve. The noble gases are extracted from the samples using one or more steps of increasing crushing intensity. Each sample is manually crushed, turning the valve fully by hand, and then unscrewing the valve.
This process is repeated up to a total of 20 times, with the crushers being gently tapped between each crush, in order to agitate the grains between each step.
Laboratory experiments on unirradiated samples have been observed to show
60 typically 20-30% of the sample is crushed to <100μm. Following extraction, the gases are purified for a minimum of five minutes (up to fifteen minutes) using a
SAES NP10 Al-Zr getter at 450°C to remove active gases (e.g. CO2, H2O, CH4 and other hydrocarbons).
The gases are then released into the manifold where they are gettered for another five minutes at 250°C using a SAES ST220 Al-Zr-V getter, whilst the charcoal finger remains isolated from the gases and is cooled to ~-196°C using liquid N2
(Figure 3.1). After five minutes, the noble gases (Ar, Kr, Xe) are trapped onto the charcoal finger over another five minute period. The manifold is then isolated from the extraction line and crushers, and the charcoal finger heated to 120°C to expand the noble gases into the mass spectrometer inlet manifold for a final purification step. After five minutes, the gases are admitted into the mass spectrometer for analysis. The gases are gettered at room temperature during analysis, using a SAES NP10 Al-Zr getter.
3.2.3.2 Heating
Crushed residues and uncrushed samples are heated in an all metal low blank
UHV Ta-resistance extraction furnace for 30 minutes. Samples are individually dropped into the furnace for gas extraction from the “Christmas Tree” (Figure 3.1).
The samples are heated in several temperature steps, the first usually a low temperature step (~600°C) to remove any adsorbed gases. A final high temperature step (~1600°C) is run in order to release any residual gases.
During heating, the gases are gettered in the furnace at 450°C using a SAES
NP10 Al-Zr getter, to remove any active gases. After heating, the samples are released into the manifold, for further gettering at 250°C. The procedure used for analysis after crushing is then followed.
61
It has been observed that pyroxenes cause a temporary memory effect in the furnace due to their high Ca (37Ar) content. Therefore, olivine samples are analysed prior to the pyroxenes.
62
Table 3.1 The reactions of the irradiated parent isotopes, producing the nucleogenic noble gas isotopes.
Parent Parent Neutron Cross Yield Isotopic Reaction Corrections Isotope Section (σt) (Branching) Abundance
39K 0.9326 2.098 b 39K(n,p)39Ar(β-)39K 1.0 Ca
37Cl 0.2422 432.9 mb 37Cl(n,γ)38Cl(β-)38Ar 1.0 K, (36Ar)
Radioactive 40Ca 0.9694 407.6 mb 40Ca(n,α)37Ar(β-)37Cl 1.0 decay
(84Kr U- 79Br 0.5069 11.00 b 79Br(n,γ)80Br(β-)80Kr 0.917 corrected)
(84Kr U- 81Br 0.4931 2.691 b 81Br(n,γ)82Br(β-)82Kr 1.0 corrected)
127I 1.0000 6.202 b 127I(n,γ)128I(β-)128Xe 0.940 (129Xe)
63
Table 3.2 Irradiation details for the halogen analyses.
Irradiation Irradiation Analyses Samples Reactor Alpha ± Alpha Beta ± Beta J ± J FBr FI Name Date Completed
06/07/09- Tristan 11 MN07a 01/06/2007 Pelindaba 0.521875 1.26E-04 3.899993 2.57E-02 0.016404 8.81E-05 1.1 1.3 20/10/09
08/07/11- Tristan 22 MN2011a 10/05/2011 Petten 0.506732 5.81E-05 9.214438 3.24E-02 0.00726 1.47E-05 1.23 1.57 17/11/11
14/01/13- Canaries3 MN2012b 14/05/2012 Petten 0.494114 4.65E-04 9.671056 6.10E-02 0.006528 2.42E-05 1.67 2.07 28/06/13
09/08/12- Azores 14 MN2012b 14/05/2012 Petten 0.494114 4.82E-04 9.671056 6.10E-02 0.006528 2.42E-05 1.67 2.07 20/12/12
23/10/13- Azores 25 MN2013a 26/07/2013 Petten 0.492497 4.58E-04 9.452988 4.65E-02 0.006278 1.04E-05 1.68 1.58 20/02/14
1. Tristan 1 = BM 1962 128 / 112, 114, 446, 473, and 480. 2. Tristan 2 = BM 1962 128 / 60, 186, 341, 482, 484, and 646; P20(3) 3. Canaries = All Tenerife, La Palma, and El Hierro samples 4. Azores 1 = G11-01, 15, and 18; P11-13, 19,and 20; SMi11-10, and 19; T11-05, 16 (Px only), and 18 5. Azores 2 = G11-04, 08, 17, and 20; P11-06, and 12; SMi11-05, 08, 13, 13X, 17, and 20; T11-02, 10, 16 (Ol only), 19, and 20; all São Jorge, Faial, Corvo, and Flores samples
64
Figure 3.1 Schematic of MS1 and extraction system (F = furnace, LP = laser port, AL = aliquot).
65
3.2.4 Noble Gas Isotopic Analysis
The custom built MS1 single sector magnetic focussing mass spectrometer is used for isotopic analysis (Figure 3.1). The MS1 has a Baϋr-Signer high transmission ion source, a 90-deg radius flight tube, and two ion detectors – a
Faraday detector and an electron multiplier. Higher abundance Ar isotope measurements are made on the Faraday detector, with less abundant Kr and Xe measured on the electron multiplier. The system is operated under ultra-high vacuum (<10-9 mbar) using three ion pumps (on the extraction line, manifold and mass spectrometer). The system is initially pumped down using a rotary pump to
10-2 mbar, and a turbo pump to 10-6 mbar. The turbo pump also maintains the vacuum (~10-6) in the furnace jacket.
The electromagnetic field and data acquisition on MS1 are controlled using a
FORTRAN-90 SPEC program. Measurements are made in instrument divisions (1 div = 1x10-4 volts). Divisions are converted to abundances manually, using the data obtained during air calibration.
By adjusting the magnetic field, the program initially centres on the m/z 40 peak on the Faraday detector. Measurements are then made of the remaining isotopes of
Ar (m/z 39, 38, 37, 36) relative to this peak by “peak hopping”. Mass peaks 35 and
41 are also measured, to detect any interference from Cl and hydrocarbons respectively.
After measuring the Ar isotopes, the peak centre for 80Kr is then found and the other Kr isotopes (m/z 82, 84, 86) are measured on the electron multiplier, by finding peaks relative to this reference peak. Finally, the peak centre for 132Xe is found, and measurements of the Xe isotopes (m/z 128, 129, 130, 131, 134, 136) are made relative to this reference peak, again using the electron multiplier.
66
Baseline readings are also measured off-mass for each of the noble gases and these are subtracted from the peak measurements automatically during data reduction. Once all the isotopes have been measured the scan is complete; seven scans are made of each analysis, taking around 45 minutes to complete. During data reduction, the peaks are regressed to the inlet time of the gas into the mass spectrometer, to obtain a consistent set of readings.
Data reduction is undertaken using the SPEC program, which records the time which the sample was admitted into the mass spectrometer, the time of every isotopic measurement, and the peak intensity measurements. Once the final scan is finished, the data are reduced by extrapolation back to the inlet time (t = 0). This is done for each isotope by plotting of graph of the time of the measurement (on the x-axis) against the value of the measurement (y-axis) and performing a regression. Anomalous points, which can occur when the mass peak has been missed (e.g. due to magnetic field drift) can be removed manually in order to increase the accuracy and reduce the error of the extrapolation.
Three different regressions are calculated for each isotope; abundance (A), ratio
(R), and mean (M). For (A), a regression is performed on the measured abundance of the isotopes, and extrapolated back to the start of the analysis to give the initial abundance. Measurements may rise or fall during the analysis, due to desorption and adsorption; this regression allows for this and provides an initial abundance. For (R), a regression is performed on the ratio of the scarce isotope
(e.g. 36Ar) to an abundant reference isotope (40Ar or 84Kr). Using the derived abundance of the reference isotope at inlet time, the abundance of the scarce isotope is then calculated from the ratio. (M) calculates the mean abundance of the isotope over the period in which the measurements were acquired.
If (A) and (R) are in agreement, the regression with the lowest error is chosen. 67
3.2.4.1 Errors
Errors on the isotopic measurements are derived by the SPEC program. During each data acquisition cycle, 20 readings are taken for each isotope. The isotopic measurements and associated errors (1σ) are the mean and standard deviation of the inter-quartile range of these 20 measurements. The inter-quartile range is used in order to exclude any anomalous readings. After the seven data acquisition cycles are complete, each average measurement is then plotted against the time of the measurement, and the isotopic value obtained by error-weighted least squares fitting of a line back to inlet time.
Errors on manually calculated values, for example ratios and concentrations, are derived by propagating the errors from the original measurement error derived by the SPEC program.
3.2.5 Air Calibration
Air calibrations are run on a daily basis, whereby a known volume of air is analysed in the mass spectrometer to ascertain the sensitivity and mass discrimination. This gives an absolute concentration of Ar in cm3/div, where 1 div =
1x10-4 volts (the absolute sensitivity is 4x10-4 amps/torr). An aliquot of air at
5.5x10-3 torr (volume 1.071 cm3) is extracted using a metering volume from a previously filled air reservoir bottle. The air sample is expanded into the manifold, and then analysed as described previously. The 40Ar, 84Kr and 132Xe isotopes are measured in order to determine sensitivity. Typical sensitivities are 2x10-12 cm3
STP/div 40Ar, 4x10-15 cm3/div 84Kr, and 3x10-15 cm3/div 132Xe. This sensitivity is used to convert the sample voltage measured on the mass spectrometer to cubic centimetres of gas at STP.
68
3.2.6 Blanks
Samples for analysis via step heating are initially loaded in the furnace “Christmas
Tree” glass loading system. During sample loading, the “Christmas Tree” loading system is exposed to the atmosphere, therefore after the samples are loaded, but prior to them being released into the furnace, the furnace is “baked out” to reduce the amount of adsorbed gases present within the furnace. The furnace is heated to high temperature (~1700°C) over a number of increasing temperature steps; this procedure is repeated a number of times to reduce the furnace blank.
After the bake out procedure is completed, a number of blanks are measured, typically at the temperature steps which will be used during gas extraction of the samples. Blank procedure follows the sample analysis procedure, with the exception that no sample is loaded into the furnace. Blank corrections are applied where necessary, but these are typically low enough (<10% of the amount of noble gases released from the samples) to not be applied. Should the blank remain high, another furnace bake out can be run until the measured blank is sufficiently low.
Blanks were also measured on the crushers on samples that were crushed.
Blanks were typically completed on a weekly basis during sample analysis - before a batch of samples are analysed and after the batch is completed.
3.2.7 Data Reduction
After regression using the SPEC program is complete, the raw data are input into
37 an Excel spreadsheet and corrected for mass discrimination, ArCa decay, neutron
38 interference reactions and ArCl interference.
69
3.2.7.1 Mass Discrimination
The MS1 mass spectrometer favours the heavier isotope of Ar, 40Ar (as the heavier ions have slightly more kinetic energy), and therefore reports a high ratio
40 36 of Ar/ Arair during air calibration. The atmospheric argon ratio measured during air calibration is typically in the range of 297-300. The difference in the air calibration ratio and the known air ratio of 298.6 (Lee et al., 2006) is then used to normalise the sample measurements of 36Ar, 38Ar and 39Ar, with the 40Ar measurements remaining unchanged.
37 3.2.7.2 ArCa Correction
A correction (Table 3.3) may need to be applied due to the decay of 37Ar
37 37 ( Ar(ε) Cl; t1/2 35.04 days), dependent on how quickly the samples are analysed after irradiation. The conversion of 40Ca to 37Ar is monitored using the hornblende
Hb3gr standard for which the Ca content is known.
3.2.7.3 Neutron Interference Corrections
Irradiation of Ca and K produces additional amounts of Ar isotopes via interfering reactions (Ca produces 36Ar and 39Ar, K produces 38Ar and 40Ar). Pure zero-age salts of potassium sulphate (K2SO4) and calcium fluoride (CaF2) are irradiated with the samples to measure the production rate of Ar from these interfering isotope reactions. Isotopic ratios are determined from the salts and a correction of the Ar isotopes is applied.
36 3.2.7.4 ArCl Interference
A further correction is made on 36Ar, which is produced from the decay of 36Cl. 36Cl is produced via neutron capture and gamma decay of 35Cl during irradiation
(35Cl(n,γ)36Cl(β-)36Ar). Although 36Cl decays slowly due to its long half-life (301,000
70
± 4,000 years), if there are large amounts of Cl present in a sample, and a long delay (>1 year) between irradiation and analysis, a significant amount of 36Ar can still be produced prior to analysis, as 35Cl is the most abundant isotope of Cl.
3.2.8 Conversion of the Noble Gases to the Halogens, K and Ca
In order to convert the noble gas abundances into their parent isotope concentrations, a conversion factor is determined using a standard of known composition. Using previously outlined procedures (Hohenberg, 1967; Jourdan et al., 2006; Kendrick, 2012; Roddick, 1983; Ruzié-Hamilton et al., 2016), production of Xe from I is determined from the Shallowater monitor (with known I content and
129Xe/127I), production of the nobles gases from the parental abundances of K and
Ca is determined from the hornblende Hb3gr standard, and the production of the nobles gases from Cl and Br is determined using the scapolite BB1 monitor.
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Table 3.3 Interfering reactions produced during irradiation, with reactions of interest for analysis of the parent isotope highlighted in bold (modified from Kelley, 2002). Interfering reactions highlighted in red are corrected for post-analysis.
Ar Isotope Ca K Ar Cl
36Ar 40Ca(n,nα)36Ar 35Cl(n,γ)36Cl(β-)36Ar
37Ar 40Ca(n,α)37Ar 39K(n,nd)37Ar 36Ar(n,γ)37Ar
39K(n,d)38Ar 38Ar 42Ca(n,nα)38Ar 40Ar(n,nd)38Cl(β-)38Ar 37Cl(n,γ)38Cl(β-)38Ar 41K(n,α)38Cl(β-)38Ar
42Ca(n,α)39Ar 39K(n,p)39Ar 38Ar(n,γ)39Ar 39Ar 43Ca(n,nα)39Ar 40K(n,d)39Ar 40Ar(n,d)39Cl(β-)39Ar
43Ca(n,α)40Ar 40K(n,p)40Ar 40Ar 44Ca(n,nα)40Ar 41K(n,d)40Ar
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4 Subducted marine pore fluid signature present within the basalts of Tristan da Cunha
4.1 Introduction
4.1.1 Tristan da Cunha Geological Setting
The island group of Tristan da Cunha, located in the south Atlantic Ocean (37°05’ south, 12°17’ west), consists of six volcanic islands, Tristan (also known as Main
Island, and referred to as “Tristan” hereafter), Inaccessible Island, the Nightingale
Islands (Nightingale Island, and the islets of Middle Island and Stoltenhoff Island), and Gough Island, the latter being approximately 400km south of the main island group (Figure 4.1). The islands are located 430km east of the Mid-Atlantic Ridge in the Atlantic Ocean. The Tristan da Cunha group of islands is thought to have formed due to volcanism associated with the Tristan mantle plume tail (Gibson et al., 2005). The head of the Tristan mantle plume is believed to have impacted
Gondwanaland prior to 134 Ma, forming the Paraná-Etendeka flood basalt province (Gibson et al., 2005; Thiede and Vasconcelos, 2010).
Tristan is still volcanically active, whereas Nightingale and Inaccessible are now in their erosional phases (Gass, 1967). The most recent eruption on Tristan occurred in October 1961 (Baker et al., 1964).
There is a paucity of literature on the geology of the island group, perhaps due to its remote location and access difficulties to the islands of Inaccessible and
Nightingale; the most detailed review to date remains the report from the 1962 expedition (Baker et al., 1964). The 1962 Royal Society expedition (Baker et al.,
1964) took place after the 1961 eruption on the Settlement Plain of Tristan. The expedition completed a geological survey of the island of Tristan, including geological mapping, with a shorter survey of islands of Inaccessible and
Nightingale also undertaken. The first geological mapping was completed of the 73
Tristan volcanic centres, with particular emphasis on the 1961 eruptive centre
(Baker et al., 1964). Nearly 700 samples were collected from Tristan, Inaccessible
Island, and Nightingale; a sub-set (n=29 Tristan; n=6 Inaccessible; n=8
Nightingale) of which was analysed for its petrography and major and trace element geochemistry, classifying the lavas in the alkali basalt-trachyte series
(later revised by le Roux et al. (1990) to the basanite-phonolite series). It is suggested that the olivine basalts present on Tristan da Cunha represent the high potassium parental magma, and that the intermediate and more evolved lavas present within the island group are derived from this parental magma (Baker et al.,
1964).
Figure 4.1 Location of the Tristan da Cunha group, east of the Mid-Atlantic Ridge (modified and redrawn from Ernesto et al., 2002)
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4.1.2 Origin of the Island Group
4.1.2.1 Paraná-Etendeka
The Tristan hotspot (sometimes referred to as the Walvis hotspot) is thought to have impacted the Gondwana continent, prior to the breakup of South America and Africa, and the opening up of the South Atlantic Ocean in the early
Cretaceous (Gibson et al., 2005). The impact of the plume head and subsequent volcanicity is thought to have led to the formation of the Paraná-Etendeka flood basalt province, today found in Brazil, South America (Paraná) and Namibia,
Africa (Etendeka) (Gibson et al., 2005).
Earlier research on the Paraná-Etendeka flood basalt province suggested an age of 138-127 Ma, with continuous magmatism over this period (Gibson et al., 2005;
Stewart et al., 1996; Turner et al., 1994). However, more recent re-analyses on the samples previously analysed by Turner et al. (1994) and Stewart et al. (1996) yield a much shorter period of volcanism beginning around 134.8-134.1 Ma, with basaltic magmatism occurring over <1 Ma (Thiede and Vasconcelos, 2010); these dates are confirmed in recent U-Pb dating of 134.3±0.8 Ma for the onset of volcanism, and can therefore be assumed to be the more precise dates (Janasi et al., 2011).
Analyses by Marques et al. (1999) from the Paraná Magmatic Province (PMP) flood basalt samples show three distinct groups in Pb isotope space (Table 4.1).
The variation in the isotopes across the PMP is proposed to relate to the mixing between EM1, EM2, and DMM mantle end-members (Marques et al., 1999).
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Table 4.1 Pb-Sr-Nd isotope data from Tristan and Inaccessible Islands, Walvis Ridge, and Paraná Magmatic Province (PMP).
Location 206Pb/204Pb 207Pb/204Pb 208Pb/204Pb 87Sr/86Sr 143Nd/144Nd 0.70495– 0.51247– Tristan 18.19–18.74 15.49–15.58 38.34-39.21 0.70584 0.51259 0.70414– 0.51252– Inaccessible 18.60–18.76 15.54–15.59 38.93-39.24 0.70505 0.51267 0.70362- 0.51240- Walvis Ridge 17.55-18.47 15.46-15.54 38.07-38.87 0.70502 0.51293 LTiB northern 17.76-17.81 15.52-15.54 38.08-38.16 - - PMP LTiB southern 0.70466- 18.09-18.48 15.60-15.67 38.19-38.67 - PMP 0.70801 HTiB northern 17.62-17.81 15.51-15.54 38.02-38.11 - - PMP HTiB southern 17.35-17.56 15.49-15.51 37.85-37.92 - - PMP (data complied from Cliff et al., 1991; Graham et al., 1992; Kurz et al., 1982; le Roex et al., 1990; Marques et al., 1999; Salters and Sachi-Kocher, 2010)
4.1.2.2 Walvis Ridge
After the initial and rapid eruption of the Paraná-Etendeka flood basalts, eruption is thought to have continued, with volcanism from the remaining plume tail forming the Walvis Ridge (O'Connor and Duncan, 1990).
The Walvis Ridge is a submarine ridge, 2,800km in length and approximately
500km wide, which extends from the Etendeka province in Africa, nearly all the way to the Tristan group of islands in the South Atlantic (Adam et al., 2007; Salters and Sachi-Kocher, 2010). At its north-eastern end, it rises 2-3km above the seafloor, displaying a ridge morphology; this morphology changes heading southwest as the ridge splits into two branches each composed of individual seamounts (Adam et al., 2007).
The South Atlantic spreading axis is thought to have migrated westwards away from the Tristan hotspot around 80-70 Ma, with a transition from on-axis to intraplate volcanism (Adam et al., 2007; O'Connor and le Roex, 1992). The Walvis
Ridge formed as the African plate drifted northwards over the Tristan plume (Adam et al., 2007). As such, the oldest section of the ridge is located to the northeast, and the youngest at the southwest end of the chain (Adam et al., 2007; O'Connor
76
and Duncan, 1990). The Walvis Ridge basalts show an EM1 type signature, proposed to have formed by ancient (~4.2 Ga) metasomatism of a low degree melt
(Salters and Sachi-Kocher, 2010) or due to metasomatised subcratonic lithospheric mantle, with additions from shallow-recycled delaminated metasomatised lithosphere (Gibson et al., 2005). Samples analysed by Salters and Sachi-Kocher (2010) yield variations in Sr-Nd-Pb isotope space when compared to the Paranã Magmatic Province (Table 4.1).
The present day plume axis is thought to be beneath Gough Island, or further south (Gibson et al., 2005).
4.1.3 Ages of Volcanism
All of the islands in the group are thought to have formed during the last 18 Ma, with Tristan being the youngest (Cliff et al., 1991). The main shield building phase of the Tristan volcano was short-lived, occurring between 0.2-0.1 Ma (McDougall and Ollier, 1982). Recent work by Hicks et al. (2012) dates the ages of the parasitic centres from 118-3 ka, with eruptions from the summit ranging from 81-5 ka, suggesting that summit eruptions continued after the establishment of the parasitic cones. There appears to be no spatio-temporal link between the eruption localities, nor with the magmatic composition on Tristan (Hicks et al., 2012).
Inaccessible Island and Tristan have K/Ar ages of ~1 Ma and 0.21 Ma respectively
(O'Connor and le Roex, 1992). The oldest island in the group is Middle Island
(Nightingale Island group), which has a K/Ar date of 18 Ma; although it is expected that this age is an overestimate, due to more recent revisions of the ages of
Tristan and Inaccessible Island (Baker et al., 1964; Cliff et al., 1991). K/Ar dates for Gough Island give an age for the onset of subaerial volcanism as approximately 2.5 Ma (O'Connor and le Roex, 1992).
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4.1.4 Geochemistry of Tristan da Cunha
The island group is understudied with respect to isotopic ratios - perhaps due to its remote location, and difficulties and timescales in arranging access to the island - although there is some more recent data on the REE (Gibson et al., 2005) and more data from the island of Tristan (Baker et al., 1964; le Roex et al., 1990), when compared to Inaccessible Island (Cliff et al., 1991). It is proposed that there is an EM1 component present in the mantle source for the Tristan da Cunha basalts, a component that is attributed to either input from pelagic sediments, shallow-recycled delaminated subcontinental lithospheric mantle (SCLM), or lower continental crust (Gibson et al., 2005; le Roex, 1986; Weaver et al., 1986; Weaver et al., 1987). It is well established that the source of Tristan da Cunha is heterogeneous and data show that there may be a mixing of EM1 and EM2 compositions in the Tristan mantle source, with a contribution from deep-recycled metasomatised lithosphere or oceanic crust (le Roex et al., 1990; Richardson et al., 1982; Wilson, 1993). It is postulated that there may have been more than one enrichment event in the Tristan mantle source (le Roex, 1985), which may have resulted in the mixture of mantle components proposed to be present (Gibson et al., 2005; le Roex, 1986; Weaver et al., 1986; Weaver et al., 1987) in the Tristan plume magmas.
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Figure 4.2 TAS plot for Tristan and Inaccessible Island magmas (data compiled and plotted from Baker et al., 1964; Cliff et al., 1991; le Roex et al., 1990). The geochemistry of Tristan and Inaccessible Island magmas are very similar in
Pb-Sr-Nd isotope space, and show comparable trace element compositions (Cliff et al., 1991; le Roex et al., 1990). The magmas are alkaline and silica undersaturated, however, the Inaccessible lavas are less alkali rich in comparison to the Tristan samples (Figure 4.2); the Tristan samples also trend towards lower
SiO2 (Tristan 41-62%; Inaccessible: 46-63%) (Baker et al., 1964; Cliff et al., 1991; le Roex et al., 1990). Although the Tristan and Inaccessible Island magmas yield similar Pb-Sr-Nd isotope ratios, these differ significantly from the Paraná Magmatic
Province samples, and samples analysed from the Walvis Ridge (Marques et al.,
1999), suggesting that the Tristan plume has evolved, or is chemically heterogeneous and sampling different mantle end-members over time (Table 4.2)
(le Roex et al., 1990).
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Table 4.2 Mean and standard deviation for isotope ratios for Paraná Magmatic Province tholeiites, Walvis Ridge, least evolved Tristan samples, and normal MORB (Marques et al., 1999).
Location 206Pb/204Pb 207Pb/204Pb 208Pb/204Pb 87Sr/86Sr 143Nd/144Nd
Normal MORB 18.3 ± 0.3 15.49 ± 0.04 37.9 ± 0.3 0.7026 (±2) 0.5131 (±1)
Tristan 18.58 ± 0.09 15.54 ± 0.02 39.06 ± 0.09 0.70501 (±6) 0.51254 (±3)
Walvis Ridge 17.9 ± 0.3 15.49 ± 0.02 38.5 ±0.3 0.7046 (±4) 0.5125 (±1)
LTiB n PMP 17.78 ± 0.03 15.53 ± 0.01 38.12 ± 0.03 0.7058 (±1) 0.51246 (±3)
LTiB s PMP 18.20 ± 0.07 15.61 ± 0.01 38.32 ± 0.10 0.7052 (±5) 0.51269 (±8)
HTiB n PMP 17.65 ± 0.02 15.52 ± 0.01 38.05 ± 0.04 0.7058 (±2) 0.51242 (±3)
HTiB s PMP 17.45 ± 0.09 15.50 ± 0.01 37.89 ± 0.03 0.7053 (±5) 0.51239 (±5)
4.1.5 Tristan (0.21 Ma)
Tristan is the youngest island (0.21±0.1 Ma) in the group and the only island in the group to have been active in historical times (McDougall and Ollier, 1982) – in the past 15 ka eruptions have only occurred from parasitic vents on the flanks of the main volcano (which total over 30 in number), with the most recent eruption occurring near to the settlement in the north of the island in 1961 (Baker et al.,
1964; Gass, 1967; Hicks et al., 2012; O'Connor and le Roex, 1992).
The island of Tristan consists of interbedded layers of basalts and associated pyroclastics, ejected mainly from the central cone; the pyroclastics having formed due to the magma interacting with either a shallow water table, or a crater lake
(Baker et al., 1964). The dominant rock type on Tristan is basanite, but ranges from ankaramitic basanites through to phonolites (le Roex et al., 1990). The main shield building phase was short-lived occurring 0.2-0.1 Ma, with the volcano now rising 5.5km above the sea bed, with 2km of subaerial exposure (Baker et al.,
1964; Hicks et al., 2012). Recent Ar/Ar dating by Hicks et al. (2012) of the post- shield building parasitic centres ranges from 118±4 ka to 3±1 ka (n=15). Little erosion has taken place on Tristan, with the main areas of erosion being around the sea cliffs and on the upper slopes of the island.
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Tristan samples show 143Nd/144Nd (0.51247–0.51259, n=22), high 87Sr/86Sr
(0.70495–0.70584, n=23), 208Pb/204Pb = 38.34-39.21 (n=16), 207Pb/204Pb = 15.49–
15.58 (n=16), 206Pb/204Pb = 18.19–18.74 (n=16) (Figure 4.3) (le Roex et al., 1990).
3 4 He/ He ranges from 4.0-6.3 RA (n>5) (Graham et al., 1992; Kurz et al., 1982).
Figure 4.3 a) 208Pb/204Pb vs 206Pb/204Pb, b) 207Pb/204Pb vs 206Pb/204Pb in Tristan plume source magmas from Paraná Magmatic Province (open triangles), Walvis ridge (open diamonds), Inaccessible Island (squares), and Tristan (circles) (data from Cliff et al., 1991; le Roex et al., 1990; Salters and Sachi- Kocher, 2010; closed symbols show unpublished data from Dr David Murphy, measured from the samples in this study). 4.1.6 Inaccessible Island (~1 Ma)
Inaccessible Island lies 22 miles to the south-west of Tristan, and is the remnant of the NE sector of a volcanic cone; it is currently in its erosional stage, with the island being the much smaller remnant of a large volcanic cone (Cliff et al., 1991;
Gass, 1967). The island consists of interbedded basaltic lavas and pyroclastics, similar to those reported on Tristan – again with numerous parasitic cinder cones
(Baker et al., 1964). The oldest reliable date for the Inaccessible Island volcanism
81
is proposed to be 1.81±0.6 Ma (n=1), with most flows dated from 0.95-0.72 Ma
(n=5) (Chevallier et al., 1992) .
Samples from Inaccessible Island are isotopically similar to Tristan, with low
143Nd/144Nd (0.51252–0.51267, n=11), high 87Sr/86Sr (0.70414–0.70505, n=11),
208Pb/204Pb = 38.93-39.24 (n=11), 207Pb/204Pb = 15.54–15.59 (n=11), 206Pb/204Pb =
18.60–18.76 (n=11) (Cliff et al., 1991).
4.2 Aims
Data on the halogen content of ocean island basalts (OIBs) are scarce, particularly with respect to iodine. A total of 12 samples were analysed for their halogen (Cl,
Br, and I) content, using an extension of the Ar-Ar dating technique: eight from
Tristan, and four from Inaccessible Island. In addition to the halogens, the samples were also analysed by Dr David Murphy, to determine the 3He/4He and 40Ar/36Ar ratios.
The aims of the study were: 1) to establish the halogen ratios (Br/Cl and I/Cl) within the Tristan da Cunha group basalts, 2) to determine the source of the halogens within the basalts, 3) to see if the halogens within the Tristan and
Inaccessible Island basalts show a recycled signature as indicated in previous studies of other isotope systems, 4) to expand the global dataset on the halogen content within EM1 and EM2 type OIBs. This study provides the first data on the halogens within the Tristan da Cunha group, and increases the known data on halogen ratios within EM1 and EM2 source OIBs.
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4.3 Samples
The samples were collected during the 1962 Royal Society Expedition to the island group (Baker et al., 1964). Nearly 700 samples were collected from the island group, a subset (11) from the Natural History Museum, London collection
(BM 1962 128) were selected from Tristan (Figure 4.4) and Inaccessible islands for analysis in this study (Table 4.3). A further sample from Inaccessible (P20(3)) was provided by Dr David Murphy, which was collected during a field trip to
Inaccessible Island in the 1980s.
Figure 4.4 Approximate locations of the Tristan (Main island) samples (map modified from le Roex et al., 1990; McDougall and Ollier, 1982). Sample numbers are from the BM 1962 128 collection held at the Natural History Museum, London (Baker et al., 1964) Dates added from Hicks et al. (2012) and McDougall & Ollier (1982).
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Table 4.3 Samples analysed from Tristan and Inaccessible Island.
Phases Sample Island Location Rock Type Mineralogy Analysed
BM 1962 Boulder, Boatharbour Tristan Ankaramite* Cpx, Ol, Plag Ol, Cpx 128/60 Bay
BM 1962 Tristan Sandy Point Flow Porphrytic Olivine Basalt* Cpx, Ol, Plag Ol 128/112
BM 1962 Boulder, Sandy Point Tristan Ankaramite* Cpx, Plag, Ol Ol 128/114 Flow
BM 1962 Ridge where the goat Porphrytic olivine basalt / Tristan Cpx, Ol, Plag Ol, Cpx 128/186 jumped off trachybasalt*
BM 1962 Between Round Hill Tristan Trachybasalt* Cpx, Ol, Plag Ol 128/341 and Cave Gulch
BM 1962 Tristan First Gulch Olivine Basalt* Ol, Cpx, Plag Ol 128/482
BM 1962 Vesicular Porphrytic Tristan Burntwood Cpx, Ol, Plag Ol, Cpx 128/484 Trachybasalt*
BM 1962 Tristan Big Beach Olivine-bearing Basalt Ol, Cpx Ol, Cpx 128/646
BM 1962 Inaccessible - Olivine-bearing Basalt Ol, Cpx, Plag Ol 128/446
BM 1962 Inaccessible - Olivine-bearing Basalt Ol, Cpx, Plag Ol 128/473
BM 1962 Inaccessible - Olivine-bearing Basalt Ol, Cpx, Plag Ol 128/480
P20 (3) Inaccessible - Olivine-bearing Basalt Ol, Cpx Ol, Cpx
Ol = olivine, Cpx = clinopyroxene, Plag = plagioclase. * denotes rock types described by Baker et al. (1964), which have now been revised (e.g. le Roex et al., 1990) from the alkali basalt-trachyte series to the basanite-phonolite series, on the basis of their high alkali content. These would now all classify as olivine basanites, however, the original description has been retained to show the evolution trend of the samples. Samples were selected from a variety of locations on Tristan (8) as well as four samples from Inaccessible Island, to establish if there is any variation in the halogen content between the islands, and the assumed temporal ranges between the islands. Samples were selected on the basis of the presence of an olivine (and 84
clinopyroxene) phenocryst phase, and to sample a wide range of locations and volcanic centres, where locality information was known.
4.4 Experimental Methods
Samples were prepared, irradiated, extracted, and analysed as per section 3.2.
The samples and monitors were irradiated in two batches: the first batch (Tristan
1) at the SAFARI-1 reactor, NECSA, Pelindaba, South Africa, and the second batch (Tristan 2) at the Petten reactor, Netherlands.
Post-irradiation, the samples were either crushed (2-4 steps) and/or step heated
(600-1,600°C) in vacuum to release their trapped gases into the MS1 mass spectrometer. Four samples from the earlier irradiation were both crushed and step heated (600°C, 800°C, 1000°C, 1200°C, 1400°C, and 1600°C), however as the majority of the gases (>80%) were released during step heating, later samples were only step heated, reducing to three steps (600°C, 1250°C, 1600°C) for the second irradiation.
Unirradiated olivine and clinopyroxene separates were analysed by Dr David
Murphy in Manchester, using a MAP-215 mass spectrometer, to determine the
3He/4He and 40Ar/36Ar, using previously outlined procedures (e.g. Burgess et al.,
1998).
A selection of the samples was also analysed using optical microscopy (Table
4.4), to establish the presence of melt inclusions within the samples (Figure 4.5).
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4.5 Rock Descriptions Table 4.4 Rock descriptions for the Tristan and Inaccessible Island basalts.
Phases Sample Rock Type Description Analysed for Halogens Tristan BM1962 128/60 olivine-basalt Phenocryst Minerals (80%): cpx ol, cpx 45%, ol 30%, plag 15%, opqs 10% Groundmass (20%): plag (+ opqs + ol + cpx) Description: Ol 1-5mm, euhedral to subhedral. Cpx 2- 4mm, euhedral, showing some twinning. Plag <1mm, euhedral laths, multiple twinning, no visible zoning. MIs visible in ol crystals, both primary and secondary (along fractures). MIs also present in cpx, possible primary or secondary. Some plag and cpx crystals enclosing earlier formed ol crystals. Porphyritic texture. BM1962 128/112 olivine-basalt Phenocryst Minerals (20%): cpx ol (70%), plag (20%), ol (10%) Groundmass (80%): plag laths (+ opqs + ol + cpx) Description: Ol 0.5-2mm, subhedral to anhedral. Cpx 1- 4mm, euhedral to subhedral, some twinning present. Plag euhedral to subhedral laths, 0.5-1mm, some fractured phenocrysts. Possible resorption observed in some ol and plag crystals. Cpx enclosing earlier formed ol, plag, and opqs. MIs visible in ol and plag.
BM1962 128/114 olivine-basalt Phenocryst Minerals (10%): cpx ol (55%), opqs (20%), ol (15%), plag (10%) Groundmass (90%): plag laths (+ opqs + ol ± px) Description: Ol 1.5-mm (average 1.5mm), euhedral to subhedral. Cpx 0.5-4mm (average 1.5mm), enclosing euhedral to subhedral opqs. Opqs euhedral to subhedral, tabular, 0.5-1mm (average 1mm). MIs visible in ol; possible MIs observed in cpx.
BM 1962 128/186 olivine-basalt Phenocryst Minerals (10%): cpx ol and cpx (60%), opqs (25%), ol (15%) Groundmass (90%): plag laths (+ opqs + ol ± cpx) Description: Ol rare, 0.5 -
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Phases Sample Rock Type Description Analysed for Halogens 1.0mm. Cpx 1-2mm, fractured euhedral to subhedral. Opqs 0.5-2mm, some skeletal and resorbed. Plag only present in the groundmass, as laths.
BM 1962 128/341 olivine-basalt Phenocryst Minerals (5%): plag ol (50%), ol (25%), cpx (20%), opqs (5%) Groundmass (95%): plag laths (+ opqs + ol ± cpx) Description: Ol 0.5-1.0mm, euhedral to subhedral. Cpx 1- 2mm (average 1.5mm), euhedral to subhedral, twinning. Plag 0.5-1.0mm (average 0.5mm), multiple twinning in 2 directions. Rare opqs 0.25- 1.0mm (average 0.5mm), euhedral to subhedral, tabular. Rare small vesicles ~0.25mm. MIs present within olivine crystals. Groundmass shows weak preferred orientation of plag laths.
BM 1962 128/482 olivine-basalt Phenocryst Minerals (15%): ol ol (65%), cpx (20%), opqs (10%), plag (5%) Groundmass (85%): plag laths (+ opqs + ol) Description: olivine 1-3mm (average 1.5mm), euhedral to subhedral, some oscillatory zoning. Cpx 1-2mm (average 1mm), euhedral, some resorption and fracturing. Plag 0.5-1.0mm (average 0.5mm), multiple twinning. Opqs 0.25- 1.0mm (average 0.5mm), euhedral to subhedral, tabular. MIs present in both ol and cpx, possibly secondary. Rare vesicles present ~0.5mm.
BM 1962 128/484 vesicular Phenocryst Minerals (25%): cpx ol, cpx olivine-basalt (70%), ol (15%), plag (15%), opqs (<1%) Groundmass (75%): glassy with plag laths (+ ol + cpx) Description: Cpx 0.75-2.0mm (average 1mm), euhedral to subhedral, zoning and resorption present. Ol 1-2mm (average 2mm), euhedral to subhedral, some resorption. Plag 0.25-1.5mm (average 0.75mm), euhedral to subhedral, multiple twinning. Rare opqs 0.25-0.5mm 87
Phases Sample Rock Type Description Analysed for Halogens (average 0.25mm), euhedral to subhedral (mainly subhedral), some tabular. MIs present in both cpx and ol. Mainly secondary in cpx, possible primary in ol (larger secondary MIs, with smaller possible primary MIs). Vesicles present 1-4mm (average 1mm), often coalescing with one another.
BM 1962 128/646 olivine basalt Phenocryst Minerals (10%): cpx ol, cpx (70%), ol (15%), opqs (10%), plag (5%) Groundmass (90%): plag laths (+ opqs + ol ± cpx) Description: Ol 0.5-1.0mm (average 0.75m), euhedral to subhedral. Cpx 1-2mm (average 2mm), euhedral, some fracturing. Opqs 1-2mm (average 1.5mm), euhedral to subhedral, tabular. Plag 0.5- 1.0mm (average 0.5mm), euhedral laths, multiple twinning. MIs present in ol and cpx, possibly secondary.
Inaccessible BM 1962 128/446 porphyritic Phenocryst Minerals (35%): cpx ol olivine-basalt (40%), ol (30%), plag (30%) Groundmass (65%): very fine grained / glassy (+ plag laths + ol) Description: Ol 1.0-2.5mm (average 1.5mm), mainly euhedral (to subhedral). Cpx 0.5-2mm (average 1mm), euhedral, twinning, some zoning present. Plag 0.25-1mm (average 0.75mm), euhedral laths, often grouped in clusters, multiple twinning. MIs observed in ol.
BM 1962 128/473 vesicular Phenocryst Minerals (50%): cpx ol porphyritic (50%), plag (30%), ol (20%) olivine-basalt Groundmass (50%): almost completely glassy (+ plag laths + ol) Description: Cpx 2-4mm (average 2mm), euhedral, some resorption, zoning. Ol 0.5-4.0mm (average 1mm), euhedral to subhedral, some resorption (rounding, embayment), some alteration to serpentine. Plag 0.25-1.0mm (average 0.75mm), euhedral
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Phases Sample Rock Type Description Analysed for Halogens laths, multiple twinning. MIs present in cpx (primary MIs) and ol (primary and secondary MIs). Vesicles 0.25-1.0mm (average 0.5mm). Some cpx crystals enclosing earlier formed ol.
BM 1962 128/480 vesicular Phenocryst Minerals (25%): ol ol porphyritic (75%), cpx (20%), plag (5%) olivine-basalt Groundmass (75%): plag laths (+ opqs + ol ± cpx) Description: Olivine 0.25- 1.0mm (average 0.75mm), euhedral to subhedral, often in clusters, rare alteration to serpentine. Cpx 0.25-2.5mm (average 0.5mm), euhedral, twinning, some zoning. Rare plag ~1mm, euhedral laths, multiple twinning. Primary and secondary MIs observed in ol. Vesicles 0.25-1.0mm (average 0.5mm).
Rock Description key: ol = olivine, cpx = clinopyroxene, plag = plagioclase, opqs = opaque minerals, MIs = melt inclusions. See Table 4.3 for original rock types, as described by Baker et al. (Baker et al., 1964).
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Figure 4.5 Selected thin sections from Tristan and Inaccessible Island basalts, showing mineralogy and melt inclusions ((a-b) BM 1962 128/480 (Inaccessible), (c-d) BM1962 128/112 (Tristan), (e-f) BM 1962 128/473 (Inaccessible), (g-h) BM 1962 128/60 (Tristan)).
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4.6 Results
4.6.1 Halogens
4.6.1.1 Crushing Releases
Due to the small amounts of halogens and potassium released during the crushing steps, only four samples (from the initial radiation, Tristan 1) were crushed, all of which were olivine separates. The majority of the gases were released during the step heating steps (Figure 4.6), with <20% of the total releases during the crushing steps. As such, and as there was no major variation in the halogen ratios between the crushing and heating steps, further samples were step heated only.
Figure 4.6 Halogen releases during crushing and heating steps.
Little variation was observed within each sample in the Br/Cl and I/Cl ratios during the crushing steps (e.g. BM 1962 128 / 446 and BM 1962 128 / 473) (Figure 4.7) with increasing intensity, therefore later crushing analyses were reduced from four steps of increasing intensity, to two steps (e.g. BM 1962 128 / 114).
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Figure 4.7 Br/Cl (a) and I/Cl (b) ratios from crushing with steps of increasing intensity (samples BM162 128/446, 473, from Inaccessible Island, sample 114 from Tristan). 4.6.1.2 Step Heating Releases
Samples were initially step heated in 200°C steps, from 600-1600°C, to establish if there was wide variation in the halogen ratios between the different heating steps.
Due to the limited variation (usually under one order of magnitude) observed in the halogen ratios within each sample during these steps (Figure 4.8), further samples were step heated in three steps (600°C, 1250°C, and 1600°C) for efficiency.
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Figure 4.8 Br/Cl (a) and I/Cl (b) variation from select samples with increasing temperature steps during step heating (samples BM162 128/446, 473, 480, and P20(3) from Inaccessible Island, sample 114 from Tristan). 4.6.1.3 Integrated Halogen Releases
The Inaccessible Island olivine samples range from (95.1-3530) x 10-6 I/Cl (Table
4.5), but exhibit a much smaller range of Br/Cl, from (1.42-3.56) x 10-3. The highest
Br/Cl and I/Cl is observed in sample BM 1962 128/446, with the lowest Br/Cl and
I/Cl seen in P20(3). This sample was also analysed for the halogen content within 93
clinopyroxene separates, which showed similar values at 1.58x10-3 Br/Cl, and
115x10-6 I/Cl.
The Tristan olivine samples range from (6.68-187)x10-6 I/Cl and (0.98-3.36)x10-3
Br/Cl, with sample BM 1962 128/482 having both the lowest Br/Cl and the lowest
I/Cl, sample BM 1962 128/114 the highest I/Cl, and BM 1962 128/186 the highest
Br/Cl (Table 4.5). From the clinopyroxenes, a range of (6.68-42.8)x10-6 I/Cl is observed (Table 4.5), together with Br/Cl ranging from (0.97-1.15)x10-3. The clinopyroxenes exhibit slightly lower Br/Cl ratios than the olivine separates from the same sample, with the I/Cl ratios often similar (within a factor of 3) – the exception being sample BM 1962 128/186 which shows an order of magnitude difference in the I/Cl ratios between the olivine (68.0x10-6) and the clinopyroxene
(6.68x10-6) separates (Table 4.5).
The maximum I/Cl reported for the Tristan samples is an order of magnitude lower than observed in the highest Inaccessible Island samples.
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Table 4.5 Halogen, K, and 40Ar/36Ar results from irradiated samples (for individual steps, see Appendix 8.1).
Cl ± Cl ± Br ± I ± K Br/Cl (x 10-3 I/Cl (x10-6 K/Cl ± 40Ar/ ± 40Ar/ Sample Analysis Island Phase Br ppb I ppb K ppm ppm ppm ppb ppb ppm molar) molar) (molar) K/Cl 36Ar 36Ar
60 Heat T Ol 2.89 0.16 7.72 0.60 0.34 0.03 292.31 9.86 1.19 ± 0.14 33.0 ± 3.32 92 6 308 8
Heat T Cpx 12.12 0.15 29.03 0.37 1.22 0.14 751.99 2.48 1.06 ± 0.02 28.1 ± 3.24 56 1 259 9
112 Heat* T Ol 5.62 0.12 18.36 0.71 1.45 0.15 368.51 1.12 1.45 ± 0.06 71.9 ± 7.57 60 1 271 4
114 Crush + Heat* T Ol 1.77 0.19 10.37 0.29 1.29 0.13 773.81 3.02 2.52 ± 0.28 187 ± 28.8 340 36 300 3
Heat T Ol 3.19 0.24 24.09 1.97 0.78 0.07 310.52 11.64 3.36 ± 0.37 68.0 ± 8.22 88 7 625 12 186 Heat T Cpx 19.54 0.62 42.86 3.40 0.47 0.04 471.74 15.36 0.98 ± 0.08 6.68 ± 0.55 22 1 312 11
341 Heat T Ol 1.79 0.10 4.87 0.39 0.13 0.01 116.99 4.13 1.21 ± 0.12 19.8 ± 2.16 59 4 324 8
482 Heat T Ol 6.69 0.31 15.54 1.50 0.17 0.02 759.10 29.90 1.03 ± 0.11 7.22 ± 0.75 103 6 311 14
Heat T Ol 9.88 0.55 43.12 3.33 1.65 0.42 548.87 20.82 1.94 ± 0.18 46.7 ± 12.0 50 3 446 15 484 Heat T Cpx 109.89 0.53 283.54 3.29 5.93 0.66 4944.99 15.29 1.15 ± 0.01 15.1 ± 1.69 41 0 304 3
Heat T Ol 8.85 0.30 22.12 1.62 0.91 0.07 1364.68 45.19 1.11 ± 0.09 28.7 ± 2.42 140 7 333 9 646 Heat T Cpx 4.14 0.12 9.71 0.17 0.63 0.08 583.18 1.91 1.04 ± 0.04 42.8 ± 5.85 128 4 300 5
446 Crush + Heat* I Ol 2.91 0.17 24.54 0.67 41.32 1.44 73.12 0.62 3.56 ± 0.21 3530 ± 223 20 1 297 2
473 Crush + Heat* I Ol 14.28 0.32 86.38 1.88 11.24 0.49 861.36 3.72 2.78 ± 0.08 189 ± 9.11 42 1 295 2
480 Crush + Heat* I Ol 0.66 0.10 4.86 0.16 1.45 0.33 100.29 0.69 3.25 ± 0.51 609 ± 166 136 21 288 2
Heat I Ol 1.75 0.05 5.60 0.15 0.59 0.04 238.28 1.29 1.42 ± 0.05 95.1 ± 6.50 124 3 330 2 P20 (3) Heat I Cpx 4.71 0.07 16.76 0.17 1.93 0.08 913.75 4.27 1.58 ± 0.03 115 ± 5.28 176 3 295 4
Sample number prefix = BM 1962 128/; T = Tristan, I = Inaccessible; Ol = olivine, Cpx = clinopyroxene. *denotes samples run (gas extracted) previously by author (Abbott, 2010), with the results subsequently re-reduced (for greater accuracy) and re-interpreted during this study.
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4.6.2 Helium and Argon
Helium and argon analyses were completed by Dr David Murphy on unirradiated olivine and clinopyroxene separates (Table 4.6), which can be compared to argon analyses (40Ar/36Ar) obtained during the halogen analyses on the irradiated samples (Table 4.5). Both sets of analyses show the Tristan and Inaccessible
Island basalts exhibiting an air-like to low crustal 40Ar/36Ar ratio. The Tristan unirradiated samples range from 312-743 40Ar/36Ar, with the irradiated samples showing a similar range from 259-625 40Ar/36Ar. The Inaccessible Island basalts range from 313-533 40Ar/36Ar for the unirradiated samples, with the irradiated samples showing a narrower more air-like range from 288-330 40Ar/36Ar. The difference between the irradiated and unirradiated samples may be due to 36Ar being generated from neutron interference reactions in Ca, or the presence of a more air-dominated signature, perhaps due to irradiation damage. The larger errors observed are due to low 36Ar content.
The helium data reported show slightly more radiogenic helium ratios when compared to MORB (~8RA), with the Tristan samples ranging from 2-5.8RA, and
3 4 the Inaccessible Island basalts ranging from 2.1-6.2RA. Minor variations in He/ He between olivine and clinopyroxene mineral fractions in same sample may be due to different trapping times, or due to a variation in the U-Th content between the mineral phases; a similar variation has been observed by Day and Hilton (2011) in
Canary Island samples.
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Table 4.6 Helium and Argon results from unirradiated samples (unpublished data provided by Dr David
Murphy) and irradiated (*) samples (this study).
3 4 40 36 40 36 Island Sample Phase He/ He (R/RA) Ar/ Ar Ar/ Ar*
Tristan BM1962 128/60 Olivine 2 ± 8.8 361 ± 274 308 ± 8
Tristan BM1962 128/60 Clinopyroxene 3.3 ± 4.1 357 ± 19 259 ± 9
Tristan BM1962 128/112 Olivine 5.1 ± 0.9 743 ± 36 271 ± 4
Tristan BM1962 128/112 Clinopyroxene 5.4 ± 1 387 ± 38 -
Tristan BM1962 128/114 Olivine 3.3 ± 1.7 378 ± 16 300 ± 3
Tristan BM1962 128/114 Clinopyroxene 4.9 ± 1.3 381 ± 24 -
Tristan BM1962 128/186 Clinopyroxene 5.8 ± 1.1 364 ± 9 312 ± 11
Tristan BM1962 128/341 Olivine 4.9 ± 0.9 312 ± 13 324 ± 8
Tristan BM1962 128/341 Clinopyroxene 3.9 ± 3.1 348 ± 17 -
Tristan BM1962 128/482 Olivine 5.7 ± 1.3 328 ± 17 311 ± 14
Tristan BM1962 128/482 Clinopyroxene 2.5 ± 3.6 334 ± 5 -
Tristan BM1962 128/484 Olivine 5.1 ± 3.3 413 ± 43 446 ± 15
Tristan BM1962 128/484 Clinopyroxene 5.7 ± 2 378 ± 54 304 ± 3
Tristan BM1962 128/646 Clinopyroxene 3.9 ± 4.2 366 ± 28 300 ± 5
Inaccessible BM1962 128/446 Olivine 2.1 ± 2.4 315 ± 11 297 ± 2
Inaccessible BM1962 128/446 Clinopyroxene 6.2 ± 1 377 ± 29 -
Inaccessible BM1962 128/473 Olivine 5.8 ± 0.6 533 ± 21 295 ± 2
Inaccessible BM1962 128/473 Clinopyroxene 5.1 ± 0.8 401 ± 15 -
Inaccessible BM1962 128/480 Olivine 5.9 ± 2.5 313 ± 3 288 ± 2
Inaccessible BM1962 128/480 Clinopyroxene 5.4 ± 2.3 337 ± 4 -
Inaccessible P20(3) Olivine 4.5 ± 7.8 313 ± 9 330 ± 2
Inaccessible P20(3) Clinopyroxene 4.9 ± 2.3 403 ± 30 295 ± 4
4.7 Discussion
4.7.1 Halogen Fractionation
Experimental data show that the halogens behave as incompatible elements at mantle temperatures and pressures, exhibiting very low halogen partition coefficients between the melt and the crystallising phases (e.g. olivine and clinopyroxene) (Fabbrizio et al., 2013; Joachim et al., 2015). Fluorine is more compatible than chlorine within mantle minerals, and although experimental data
(Dcrystal/melt) is not available for Br and I due to the extremely low abundances within
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the mineral phases, it is expected that this trend continues with increasing ionic radius, thus DF > DCl > DBr > DI. Due to the low halogen partition coefficients, it is not expected that the heavier halogens (Cl, Br, and I) are significantly fractionated from one another during partial melting or crystallisation, with the halogens being concentrated in the melt phase. Recent data from Kendrick et. al (2012a) on
Macquarie Island basalts confirm this, showing that the halogens and potassium are largely unfractionated from one another in basalts during partial melting and crystallisation, however it is suggested by Ruzié-Hamilton et al. (2016) that the halogens can fractionate from one another during partial melting, particular with respect to I, the most incompatible of the halogens. Without major element data, it is not possible to determine the presence of fractional crystallisation or partial melting within the samples, however, as Cl (Ionov et al., 1997) is more easily taken into minerals (e.g. apatite) at mantle pressures and depths than Br and I, we can use Cl concentrations as a proxy for these effects.
In the Tristan and Inaccessible Island basalts, there is a lack of correlation between chlorine concentration and K/Cl, Br/Cl, and I/Cl (Figure 4.9 and Figure
4.10), suggesting that the halogens have not been significantly fractionated from one other during partial melting and crystallisation. Experimental results suggest
sul/sil that in the presence of a sulphide melt, I is less incompatible (DI = 0.15) than
sul/sil sul/sil Cl (DCl = 0.038) and B (DBr = 0.026) (Mungall and Brenan, 2003), with D tentatively proposed to increase with increasing pressure. No sulphide bearing minerals are observed in the samples from Tristan and Inaccessible Island, and it is therefore not expected that the variation seen in the halogen ratios is due to fractionation in the presence of a sulphide melt.
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Figure 4.9 Plots of K/Cl against a) Br/Cl and b) I/Cl. The lack of a correlation suggests that the halogens have not being fractionated from one another during partial melting or crystallisation processes.
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Figure 4.10 Plots of a) Br/Cl, b) I/Cl, and c) K/Cl against 1/Cl. The lack of a correlation suggests the absence of any significant fractionation resulting from degassing processes.
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Degassing of the basaltic melt during magma ascent and at low pressures is another mechanism that could cause fractionation of the halogens, and must be considered as all the samples were erupted subaerially. Bureau et al. (2016; 2000) showed that during decompression the halogens partition strongly into the fluid phases, with Dfluid/melt increasing by an order of magnitude with increasing ionic radius (Cl 8.1±0.2, Br, 17.5±0.6, I 104±7). However, in the Tristan and
Inaccessible Island basalts, it is notable that there is enrichment in I/Cl (~2 orders of magnitude compared to MORB) with respect to Br/Cl (discussed in section
4.6.2), which is the opposite to expected if the observed variation in halogen ratios is due to degassing during magma ascent and storage in crustal magma chambers.
The presence of H2O and CO2 vapour phases also influence halogen degassing within basaltic melts, with the addition of H2O increasing the halogen diffusivity into the vapour phase, and CO2 increasing Cl solubility into the melt phase (Alletti et al., 2009). At low pressures, halogens will partition into a vapour phase – in a H2O- poor system, degassing will occur at shallower depths, and halogen partitioning is not thought to be significant until pressures of <1 Mpa (~35m) (Edmonds et al.,
2009). Again, this mechanism is not thought to have significantly fractionated the halogens within the Tristan and Inaccessible Islands, as a reduction in the Br/Cl and I/Cl ratios would be expected, but the opposite is observed with I/Cl trending to higher values. Multi-step degassing will fractionate the halogens, but this will always lead to lower concentrations of the heavy halogens with respect to Cl, and this is not observed in the Tristan da Cunha basalts.
Finally, with the exception of sample P20(3), those samples in which both olivine and clinopyroxene phases were analysed show similar halogen ratios (Table 4.5)
– suggesting that a similar component is trapped within melt inclusions in both 101
olivines and clinopyroxenes, and that crystallisation is not causing the halogens to fractionate from one another. The difference in concentrations between the olivine and clinopyroxene separates is thought to be as a result of a greater population of melt inclusions within the clinopyroxene separates, perhaps due to the cleavage planes within the clinopyroxenes acting as nucleation sites for melt inclusions.
Comparing the evolution of the samples with their halogen ratios, there does not appear to be a correlation between the halogens, and how compositionally evolved the sample is, with the more evolved samples e.g. BM 1962 128/186, BM
1962 128/341, and BM 1962 128/484 overlapping in their Br/Cl ((1.15-3.36)x10-3) and I/Cl range ((6.68-68.0)x10-6), with the less evolved (e.g. BM 1962 128/60 and
BM1962 128/114) samples (Br/Cl = (1.06-2.52)x10-3; I/Cl = (28.1-187)x10-6).
The lack of evidence for halogen fractionation within the Tristan and Inaccessible
Island basalts suggests that the evolution of the Tristan basalts has not affected the halogen ratios, and that the halogen ratios measured in the samples are representative of the Br/Cl and I/Cl within the magma source.
4.7.2 Halogen variation between Tristan and Inaccessible Island
The Tristan samples with the lower Br/Cl and I/Cl ratios overlap with the known field for MORB (Figure 4.11), and lie close to previously analysed samples from
EM1 and EM2 basalts from Pitcairn and Society seamounts (Kendrick et al.,
2014b) and EM2 basalts from Samoa (Kendrick et al., 2015). It is possible that there may be a MORB component present within the Tristan basalts, perhaps due to entrainment of MORB as the Tristan plume rises through the upper mantle.
However, it must be considered that the halogen composition of the Tristan plume may be MORB-like, as observed at Pitcairn, Society, and Samoa (Kendrick et al.,
2015; Kendrick et al., 2014b) suggesting that the halogens have become
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homogenised in the plume source mantle due to mixing through long term subduction and recycling. This suggests the common halogen component reported across the EM1 and EM2 source basalts at Pitcairn, Society, and Samoa
(Kendrick et al., 2015; Kendrick et al., 2014b) may also be present at Tristan da
Cunha, and these samples therefore extend the field for EM1 and EM2 OIBs.
Four of the Tristan samples lie outside the known field to MORB, together with all of the Inaccessible Island basalts (Figure 4.11). Whilst the Tristan samples trend towards higher Br/Cl ratios than observed in MORB samples, the Inaccessible
Island basalts yield both higher Br/Cl and higher I/Cl ratios. It is possible that this elevation may be due to either a spatial or temporal variation, with Inaccessible
Island being the older of the two islands. The most recent volcanism known from
Inaccessible Island is dated at 0.72±0.04 Ma (n=1), from the Blendon Hall intrusion, thought to be the youngest unit on the island (Chevallier et al., 1992), which occurred prior to any known sub-aerial eruptions on Tristan (0.21 Ma). It is therefore suggested that the Inaccessible Island basalts show a change in the composition of the Tristan plume over time. This change has been observed in other isotope systems, when comparing Tristan da Cunha, together with the other
Tristan plume sites (Paraná-Etendeka and Walvis Ridge).
Focusing on the cause of this variation, it is observed that the samples from
Inaccessible Island range well outside the known fields for MORB, and both EM1 and EM2 basalts, and therefore cannot be explained by the presence of a single
MORB-like or OIB component - and thus we must find another component, or components, to explain the elevation in the heavy halogens, with respect to chlorine, observed in the Inaccessible Island basalts.
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Figure 4.11 Halogen ratio plot of the Tristan and Inaccessible Island basalts (additional data from Bruland and Lohan, 2003; Fehn et al., 2006; Jambon et al., 1995; Kendrick et al., 2014a). Error bars smaller than symbol when not seen.
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4.7.3 Possible Components of the Tristan da Cunha Basalts
A possible source proposed to be present in the Tristan plume is oceanic crust
(Richardson et al., 1982). The known field for halogens within altered oceanic crust (AOC) (Chavrit et al., 2016) is shown in Figure 4.12. Cl-rich amphibole is formed during low temperature hydrothermal alteration, fractionating the halogens and causing lower halogen ratios to be present in AOC than observed in unaltered
MORB (Chavrit et al., 2016). On Figure 4.12 it can be observed that the field for
AOC lies at lower Br/Cl and I/Cl values than the majority of the Tristan samples, although there is some overlap within the MORB range and the lower Br/Cl samples with AOC altered at shallower depths, therefore this sample could be considered to be MORB- or OIB-like. It is possible that we are seeing a component of subducted AOC present within lower Br/Cl and I/Cl samples, however it is more likely that these lower samples are simply just MORB-like or OIB, without needing to invoke the presence of AOC. However, another component must account for those samples trending to higher values.
It has been proposed that the heterogeneity observed within the Tristan group is due to the presence of delaminated SCLM (Gibson et al., 2005). From Figure 4.12, it is observed that although some of the samples with higher Br/Cl ratios overlap with the known field for SCLM from diamond analyses (Burgess et al., 2009;
Burgess et al., 2002; Johnson et al., 2000), the majority of the samples lie either at lower Br/Cl values, or higher I/Cl.
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Figure 4.12 Halogen ratio plot with pore fluid, serpentinites, wedge fluid, MORB, bulk Earth, and seawater data from literature (additional data from Anders and Ebihara, 1982; Bruland and Lohan, 2003; Chavrit et al., 2016; Fehn et al., 2006; Fehn et al., 2000; Fehn et al., 2003; Fehn et al., 2007b; Jambon et al., 1995; John et al., 2011; Kastner et al., 1990; Kendrick, 2012; Kendrick et al., 2013a; Kendrick et al., 2014a; Lu et al., 2008; Mahn and Gieskes, 2001; Martin et al., 1993; Muramatsu et al., 2007; Muramatsu et al., 2001; Sumino et al., 2010; Tomaru et al., 2007). Error bars smaller than symbol when not seen.
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4.7.4 Source of the Inaccessible Island Enrichment
Although the Tristan and Inaccessible Island data show parallel trends on the TAS diagram (Figure 4.2) there is only a small difference in SiO2 (wt%) difference between the basalts from the two islands. Since SiO2 shows the most variation in a suite of magmatic rocks during fractional crystallisation, the small difference in
SiO2 content means it is unlikely that halogens are unlikely to have been significantly fractionated by this process. In addition, there is an overlap between the two of the Tristan samples which yield higher halogen ratios, and the
Inaccessible Island basalts – suggesting that the source of the halogens sampled in these Tristan basalts is the same as observed in the Inaccessible Island samples.
In Figure 4.12 it can be observed that the Inaccessible Island samples extend well into the known field for marine pore fluids. It is therefore possible that there is a contribution from marine pore fluids present within the Inaccessible Island basalts.
As seawater interacts with biogenic iodine-rich sediments within pore spaces, the
I/Cl (and to some extent, the Br/Cl) ratio of the trapped pore fluid rises, giving the range shown in Figure 4.12. The high halogen ratios observed in the Inaccessible
Island olivines are consistent with a recycled halogen signature, and may therefore be explained by a marine pore fluid component within the source of the Tristan da
Cunha magmas. This trend from MORB values to marine pore fluid values suggests a possible mixing trend between a MORB or OIB source and marine pore fluids.
This suggests that the Inaccessible Island basalts are sampling a source with a marine pore fluid signature. It must be established how these marine pore fluids are being subducted and subsequently mixed into the source, and whether 107
Inaccessible Island is sampling a local length-scale heterogeneity or a heterogeneous mantle plume.
It should also be noted that the location of the Tristan group is not proximal to any present day subduction sites, being located 400km from the MAR, and other isotopic systems (e.g. 206Pb/204Pb and 208Pb/204Pb) suggest that pelagic sediment is not an input within the Tristan da Cunha basalts (Gibson et al., 2005); however, if I-rich sediment or marine pore fluids are introduced into the mantle source, this would not be seen in Pb isotope space.
4.7.5 Halogen Subduction and Recycling
Recent work by Sumino et al. (2010) and Kobayashi et al. (2017) on exhumed mantle wedge peridotite shows that marine pore fluid halogens, can be subducted and recycled to depths of at least 100km in a closed system, without significant fractionation of the halogens from one another. It is suggested that the pore fluid signature present within subducted peridotites is due to the serpentinisation of the peridotite by pore fluids, with the halogens substituting for OH within the serpentine lattice, or due the entrapment of marine pore fluids along grain boundaries within the peridotite (Kendrick et al., 2013b; Sumino et al., 2010). It is not known if the halogens can be recycled to depths greater than 100km without significant fractionation from one another, as it would be expected that the heavy halogens would be lost preferentially to Cl (I>Br>Cl) due to their increased ionic radii and decreasing compatibility (Kendrick et al., 2013b). However, it is proposed that halogens can be subducted effectively up to depths of 150-200km, at which stage antigorite breaks down (Kendrick et al., 2013b; Ulmer and Trommsdorff,
1995). As such, it is not known if halogens, particularly the heavy halogens, could be subducted to the depths required for them to be incorporated within the plume source. The subduction of volatiles into the mantle is controlled by dehydration 108
reactions; any fluid component in the downgoing slab will be preserved to greater depths during fast subduction in cold subduction zones (van Keken et al., 2011). It is therefore possible that deep recycled halogens are being observed within the plume source. Alternatively, the Tristan plume may be homogenous with respect to the halogens, and the enrichment present within the Inaccessible Island basalts is due to the plume sampling a local length-scale heterogeneity at lower depths beneath the island, of which the potential sources are discussed in the following section.
4.7.6 Halogen enrichment by metasomatised lithosphere?
Gibson et al.’s (2005) model on the evolution of the Tristan plume proposes that initial melts from the Tristan plume head were uncontaminated, however later melts, present on the Walvis Ridge, contain contributions from entrained shallow- recycled delaminated metasomatised SCLM, with recent subaerial basalts containing entrained MORB and deep recycled SCLM. Although some Tristan samples may show a contribution within the halogens from recycled SCLM within the Tristan basalts, this is not, if present, considered to be a major component, and therefore there must be another, more I-rich, end-member present to account for the higher I/Cl ratios that are present in the Inaccessible Island basalts.
As dehydration reactions occur in the downgoing slab, halogen-rich fluids would migrate from the slab into the overlying mantle wedge. It is possible that the marine pore fluid signature present within the Inaccessible Island basalts results from the release of halogen-rich fluids during dehydration that have preserved the marine pore fluid signature, and subsequently migrated into the SCLM. However, this proposal does not overcome the problem of the absence of a local subduction zone, which is still needed in order to recycle the marine pore fluids back into the mantle. 109
4.7.7 A model for heavy halogen enrichment in the Inaccessible Island basalts
In order to explain the presence of a marine pore fluid signature in the
Inaccessible Island basalts, together with the absence of any proximal subduction zone, the long term tectonic setting of the area must be considered (Figure 4.13), to establish if the mantle below Tristan da Cunha potentially has undergone one or more enrichment events with respect to the halogens.
Based on a model by Gibson et al. (2005), where it is proposed that entrained
SCLM and recycled SCLM are present to explain the variation in Pb isotope space between Paraná-Etendeka, Walvis Ridge, and Tristan da Cunha, a model is proposed. During the closure of the Mozambique ocean (573 Ma) (Figure 4.13a) and the subsequent suturing of East and West Gondwana (Figure 4.13b) (Kröner and Stern, 2005), oceanic lithosphere would have been subducted into the mantle, together with marine sediments and marine pore fluids. As the downgoing slab was subducted to greater depths and temperatures, dehydration reactions would have occurred, expelling fluids from the downgoing slab. It is proposed that these fluids would eventually be enriched in the heavy halogens as serpentine underwent chrysotile to lizardite to antigorite transition (Kendrick et al., 2013b;
Kendrick et al., 2011); initially with the slab expelling an I-rich fluid, becoming more
Br-rich with increasing depth. If these fluids migrated through the overlying mantle, they could metasomatise the overlying SCLM, leading to an enrichment of the heavy halogens, with respect to Cl, in the SCLM. It is proposed that this mechanism enriched the overlying SCLM and that this signature was preserved in the SCLM long term, until the break-up of Gondwana (Figure 4.13c) due to the impact of the Tristan plume head (~134 Ma) (Gibson et al., 2005; Thiede and
Vasconcelos, 2010). This event led to the creation of the Paraná-Etendeka
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continental flood basalt province, in present day South America and Africa, as well as the opening of the South Atlantic Ocean (Figure 4.13d) (Gibson et al., 2005).
During this period, the metasomatised SCLM was delaminated, causing local heterogeneities within the upper mantle, and potentially the deeper subduction of the SCLM into the mantle. It is proposed that delaminated SCLM, with a preserved marine pore fluid signature, was recycled into the source of the Tristan plume, and that this heterogeneity is sampled initially by the Walvis Ridge magmatism (Figure
4.13e), and subsequently the Inaccessible Island basalts (Figure 4.13f i); with later
Tristan magmas less contaminated by SCLM (Figure 4.13g). This would cause a marine pore fluid-like halogen signature to overprint the OIB halogen signature, leading to the elevated I/Cl and Br/Cl signatures observed within the Inaccessible
Island basalts, and may also account for the increased Br/Cl signature observed in three of the Tristan basalts. Although the evolution of the Tristan plume is supported by Pb isotope ratios, similar Pb ratios are reported between the Tristan and Inaccessible Island basalts, suggesting that the halogens are sampling a heterogeneity not seen in other systems. There is no correlation between the halogen ratios in the Tristan basalts and Pb isotope data, suggesting a different source component for the halogens, supporting a marine pore fluid signature.
Comparing the Inaccessible Island basalts to previously published data by
Broadley et al. (2016) on SCLM xenoliths from the Western Antarctic Rift System
(WARS), it is observed (Figure 4.14) that the SCLM can preserve long term (100-
500 Ma) a marine pore fluid signature that has been incorporated during subduction-related metasomatism of the SCLM. The Inaccessible Island samples are in good agreement with the range reported in the WARS xenoliths, suggesting that recycled metasomatised SCLM is a potential source for the enriched halogen signature present in the Inaccessible Island basalts. 111
An alternative model is that the Tristan and Inaccessible Island magmas are sampling serpentinised oceanic lithospheric mantle, which has preserved a marine pore fluid signature (Figure 4.13f ii). As the plume rises through the mantle, mixing with the serpentinised mantle could cause the Tristan da Cunha magmas to inherit this halogen pore fluid signature. Both SCLM (6.01 RA) and oceanic lithosphere (4-
3 4 7.08 RA) overlap in the their range of He/ He ratios (Day and Hilton, 2011; Hilton and Porcelli, 2014; Moreira and Kurz, 2001), and are in agreement with the higher values reported from the Tristan da Cunha basalts, therefore either contamination of the earlier magmas with local serpentinised lithospheric mantle, or by the entrainment of older delaminated metasomatised SCLM in the magma source, could be the cause of the enriched halogen signature present in the Inaccessible
Island basalts.
Using REE data measured in the Inaccessible Island, modelling by Gibson et al.
(2005), where the MORB source is mixed with subducted oceanic crust and associated sediments, produces a good overlap with the HREEs, but cannot alone produce the variation observed in the LREEs in Inaccessible Island. The
Inaccessible Island basalts yield a higher concentration of LREEs than in the convecting mantle source, whilst the HREEs show a lower concentration when compared to the MORB source. If the modern day Tristan plume was sampling subducted oceanic crust, it would be expected that the LREEs measured in the
Inaccessible Island basalts would have a lower concentration than in the convecting mantle; however, in this model, the reverse is observed (Gibson et al.,
2005). Halogens measured in altered oceanic crust (Chavrit et al., 2016) show lower Br/Cl ratios ((0.33-1.49)x10-3) and I/Cl ratios ((0.83-1.02)x10-6) than observed in MORB (Br/Cl (0.52-2.8)x10-3, I/Cl (4-195)x10-6) – this together with the
REE data from Gibson et al. (2005) suggests than another source is needed to
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support the higher than MORB halogen ratios and LREE concentrations observed in the Inaccessible Island basalts.
An alternative model by Gibson et al. (2005) shows that the basalt REE compositions can be accounted for by combining a convecting mantle source plus
20% delaminated SCLM – showing a strong correlation with both the LREEs and the HREEs measured in Inaccessible Island basalts. When these data and models are considered alongside the halogen data, particularly the variation observed within the Br/Cl and I/Cl ratios in the Inaccessible Island basalts, it is suggested the recycling of metasomatised delaminated SCLM provides a stronger model for the source of the material being sampled by the Tristan plume – with the metasomatising fluid being a halogen-rich marine pore fluid, which was preserved during metasomatism and subsequent subduction and recycling, causing the higher than MORB halogen ratios observed in the Inaccessible Island basalts.
It is therefore proposed, based on the halogen ratios, 3He/4He data, and Pb isotopes that the Tristan plume comprises of three end members: 1) an EM1 and
2) an EM2 end-member, indistinguishable within the halogens, but shown to be present in Sr, Nd, and Pb isotopes, and 3) recycled delaminated metasomatised
SCLM.
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Figure 4.13 Models showing the evolution of Gondwana and the Tristan plume, proposing a possible explanation for the elevated heavy halogen signature present in the Inaccessible Island basalts (expanded and based on a model by Gibson et al., 2005; with additional age and tectonic data from Granot and Dyment, 2015; O'Connor and Jokat, 2015; Sankaram, 2007; Thiede and Vasconcelos, 2010). Two possible models are proposed for the Inaccessible Island halogen enrichment: f i) continued entrainment of delaminated metasomatised SCLM, or f ii) contamination of magmas with serpentinised oceanic lithosphere. 114
Figure 4.14 Halogen ratio plot, showing the addition of the WARS xenoliths from Broadley et al. (2016), representing metasomatised SCLM (other references as per Figure 4.12). Error bars smaller than symbol when not seen. 115
4.7.8 Implications for Halogens in the Earth’s Mantle
As there is a strong overlap between the halogen ratios observed on Tristan and the previously reported EM1 and EM2 source basalts from Pitcairn, Society, and
Samoa, it is proposed that these basalts are typical of EM1 and EM2 basalts, and extend the known values of EM1 and EM2 previously reported (Kendrick et al.,
2015; Kendrick et al., 2014b). The halogens show a dominantly MORB-like or OIB- like in composition, and this component suggests that either entrainment of
MORB, as the plume material rises, is overprinting the plume halogen signature, or that the mantle source of EM1 and EM2 type basalts has become homogenised over time, due to long term subduction and recycling. It is expected that the latter is more plausible, as a large amount of MORB would need to be entrained by the source of the basalts in order to overprint an enriched mantle halogen signature, and it is expected that the entrained material would be volumetrically minimal in comparison to the plume material.
The Tristan da Cunha basalts extend the range of EM1 basalts into the field previously reported for EM2 basalts, suggesting that, based on halogens alone, it is impossible to distinguish between the two mantle end members.
4.8 Conclusions
The samples with lower halogen ratios lie close to the values for MORB,
suggesting that the MORB and plume sources are similar with respect to
their halogen ratios, with enrichment of the halogens causing a
heterogeneous source.
The Tristan da Cunha samples are dominantly MORB-like in halogens, in
good agreement with previously reported EM1 and EM2 type basalts,
extending the known field for EM1 and EM2 basalts, and suggesting that
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the OIB source and MORB mantle have become homogenised with respect
to the halogens, as a result of long term subduction and recycling.
The Inaccessible Island basalts show a strong overlap in their halogen
ratios with marine pore fluids, suggesting a marine pore fluid signature is
present within the source of the Tristan da Cunha basalts, proposed to be
from the recycling of metasomatised SCLM into the source of the basalts.
The Inaccessible Island basalts show the highest I/Cl ratios, suggesting that
the source of the Tristan plume has continued to change chemically over
time since the onset of subaerial volcanism, in good agreement with
previously reported Pb data from Paraná-Etendeka, Walvis Ridge, and
Tristan da Cunha.
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5 Plume-Ridge Interaction in the Azores Archipelago
5.1 Introduction
5.1.1 Azores General Geological Setting
The Azores archipelago is located in the North Atlantic Ocean, between 38°22’ and 38°36’N latitude and 28°00’ and 28°W longitude (França et al., 2006b). The
Azores comprises of nine volcanic islands, in descending age order: Santa Maria
(8.12±0.85 Ma), São Miguel (4.01±0.05 Ma), Terceira (3.52 Ma), Flores (2.76 Ma),
Graciosa (2.5 Ma), Faial (0.73 Ma), Corvo (0.71 Ma), São Jorge (0.55±0.06 Ma), and Pico (0.25 Ma) - spanning a distance of 500km from west to east. The islands are located on a triple ridge (R-R-R) junction, with Corvo and Flores situated to the west of the Mid-Atlantic Ridge (MAR) on the North American plate. The remaining islands are located to the east of the MAR, on the proposed Azorean micro-plate
(Madeira and Ribeiro, 1990), sited between the Eurasian and African plates. The islands of Graciosa, Terceira, and São Miguel are located on the Terceira Rift, with the islands of Faial, Pico, São Jorge, and Santa Maria just to the south of the Rift.
The islands are divided geographically into three groups, the Occidental Group
(Flores and Corvo), the Central Group (Faial, Pico, São Jorge, Graciosa and
Terceira), and the Oriental Group (São Miguel and Santa Maria) (Millet et al.,
2009).
The islands are situated on an active plate Ridge-Ridge-Ridge (RRR) triple junction, between the North American, Eurasian and African Plates, in the Atlantic
Ocean (Figure 5.1) (Ancochea et al., 1990; Beier et al., 2007). This triple junction is thought to have been active for 45 Ma (Georgen, 2008). Flores and Corvo are situated on the North American Plate, to the west of the Mid-Atlantic Ridge (MAR), which is spreading at 25mm y-1 in this region (Freire Luis et al., 1994). The
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remaining islands are located east of the MAR on the proposed Azorean micro- plate between the Eurasian and African plates, with Graciosa, Terceira and São
Miguel on the boundary between the plates (Ancochea et al., 1990; Madeira and
Ribeiro, 1990; Moreira et al., 1999). This boundary, known as the Terceira Rift, formed ~1 Ma ago, and is thought to be a divergent boundary spreading at 2-4mm y-1 (Beier et al., 2008; Georgen, 2008). The East Azores fault zone (EAFZ), to the south of the islands, trends EW linking the MAR to the Gloria Fault (Madeira and
Ribeiro, 1990). The islands to the west of the MAR, Corvo and Flores, lie on a
NNE-SSW striking ridge that runs subparallel to the MAR (Genske et al., 2012).
The remaining islands are dominated by NW-SE striking rifts, parallel to the
Terceira Rift (Genske et al., 2012).
Figure 5.1 Geological setting of the Azores archipelago (figure redrawn from Millet et al., 2009 with additions from Madeira and Ribeiro, 1990 and Moreira et al., 1999). The Azores rocks are predominantly basaltic, but range from alkali basalt, to basaltic trachyandesite and trachyandesite, to alkali-rich rhyolite in composition,
119
with pyroclastic pumice and ash deposits also present (Faure, 2001; França et al.,
2006b; Ridley et al., 1974).
Although extensive research has been undertaken into the geochemistry of São
Miguel and Terceira (e.g. Beier et al., 2006; Calvert et al., 2006; Elliott et al., 2007;
Madureira et al., 2005), little work has been completed on the remaining islands.
Data are rare for Faial, and Flores, and there is a particular paucity of information regarding Corvo and Graciosa, with only a few papers published on these islands
(e.g. França et al., 2006a; Genske et al., 2016; Genske et al., 2012; Larrea et al.,
2012; Larrea et al., 2014). There is an absence of halogen data in the literature for the archipelago, providing an opportunity for extensive study on the islands.
5.1.2 Origin of the Azores Archipelago
Volcanism on the Azores Plateau is proposed to be related to the Azores mantle plume, which is thought to originate from the core-mantle boundary, as shown by a reduced seismic velocity indicating increased temperatures extending to the base of the mantle; however, some authors suggest a shallower origin for the plume
(Courtillot et al., 2003; Millet et al., 2009; White, 2010). Initial volcanism is thought to have occurred on the Azores Plateau ~36 Ma ago, with subaerial volcanism taking place from ~8 Ma ago to the present day (Cannat et al., 1999; Millet et al.,
2009).
The location, depth, and origin of the Azores mantle plume are controversial. Most authors agree that the present day plume location is below the Central Island
Group (Graciosa, Terceira, Pico, Faial, and São Jorge) but the exact location is widely debated, with suggestions including below Faial and Pico (Georgen, 2011),
Terceira (Bourdon et al., 2005), and São Jorge (Millet et al., 2009). A more recent publication by Adam et al. (2013) which models the heat anomalies in the Azores
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mantle using seismic wave data, suggests that there may be two upwellings of hot mantle below the Azores – one sited below Graciosa-Faial-Pico-São Jorge, and the other below Terceira-São Miguel. However, there is general consensus that a plume-ridge system is the cause of the volcanic activity and rifting present on the
Azores plateau.
With regards to the depth and origin of the Azores plume, seismic data and modelling suggests that the plume extends to a minimum depth of the 660km boundary (e.g Adam et al., 2013), with geochemical data, particularly the 3He/4He data (e.g. Jean-Baptiste et al., 2009) and primitive Ne isotope signature
(Madureira et al., 2005), suggesting that the Azores basalts are sampling a deep mantle component with a primitive noble gas signature.
5.1.3 Ages of the Volcanism
The islands are located on the Azores Plateau, an oceanic plateau which began to form ~36 Ma ago (Cannat et al., 1999). The Azores islands have been active volcanically since the Miocene, with the earliest subaerial volcanism exposed on
Santa Maria, and eruptions continuing through the Holocene and to the present day (Madeira and Ribeiro, 1990). São Miguel, Terceira and Flores emerged during the Pliocene, with the remaining islands emerging during the Pleistocene (Madeira and Ribeiro, 1990; Millet et al., 2009).
The ages of the islands, which do not exhibit a linear age trend, from oldest to youngest are: Santa Maria 8.12±0.85 Ma, São Miguel 4.01±0.50 Ma, Terceira 3.52
Ma, Flores 2.76 Ma, Graciosa 2.5 Ma, Faial 0.73 Ma, Corvo 0.71 Ma, São Jorge
0.55±0.06 Ma and Pico 0.25 Ma (Table 5.1) (Abdel-Monem et al., 1975; Feraud et al., 1980; Madeira and Ribeiro, 1990; Millet et al., 2009). Over 30 eruptions have been recorded in historic times, both submarine and on the islands of Faial, Pico,
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São Jorge, Terceira and São Miguel, with the most recent subaerial eruption having occurred at Capelinhos, Faial in 1957-8 (Table 5.1) (Madeira and Ribeiro,
1990; Millet et al., 2009; Oversby, 1971).
Table 5.1 Eruptive histories and locations of the Azores islands (*denotes estimated age).
Last recorded Island Location Group Age subaerial eruption Santa Maria Azores micro-plate Oriental 8.12±0.85 Ma 2.84±0.04 Ma São Miguel Terceira Rift Oriental 4.01±0.05 Ma 1713 Terceira Terceira Rift Central 3.52 Ma 1761 Flores Atlantic Plate Occidental 2.76 Ma 3.0±0.1 ka Graciosa Terceira Rift Central 2.5 Ma 3.9±1.4 ka Faial Azores micro-plate Central 0.73 Ma 1957/8 Corvo Atlantic Plate Occidental 0.71 Ma 80-100 ka* São Jorge Azores micro-plate Central 0.55±0.06 Ma 1808 Pico Azores micro-plate Central 0.25 Ma 1720 (Abdel-Monem et al., 1975; Feraud et al., 1980; França et al., 2006a; Larrea et al., 2014; Madeira and Ribeiro, 1990; Millet et al., 2009; Sibrant et al., 2015) 5.1.4 Azores Geochemistry
The basalts of the Azores exhibit moderately high 3He/4He, with the exception of
São Miguel, which has low 3He/4He similar to Tristan da Cunha (White, 2010). At least two mantle components appear to be present in the Azores; an EM2 component observed in the São Miguel basalts, and a HIMU signature in the remaining islands (França et al., 2006b; Moreira et al., 1999; Widom and Shirey,
1996).
The isotopic ratio of 87Sr/86Sr ranges widely from 0.7032 to 0.7054 across the archipelago, with the highest ratios observed in eastern São Miguel (Faure, 2001).
In addition to the global mantle end-member components, isotopic variations observed in the Azores basalts have been further defined by four local end- members: (SM), defined by the eastern (most radiogenic) São Miguel samples, which has enriched Sr-Nd isotope ratios and corresponds to the most radiogenic
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Pb measured in the São Miguel samples; (T), defined by the Terceira samples, which has intermediate Sr-Nd ratios (87Sr/86Sr ~0.70375 and 143Nd/144Nd
3 4 208 204 ~0.51295), He/ He ~11.5RA, and also has high radiogenic Pb ( Pb/ Pb
~39.56, 207Pb/204Pb ~15.67, and 206Pb/204Pb ~20.51) and excess in 20Ne and 21Ne
(compared to MORB); (D) with depleted Sr-Nd-Pb isotopes which plot in the field of local MORB; this end-member is common to all islands and is defined by the join between the two trends observed on São Miguel and Terceira respectively;
(F), represented by the Faial basalts, with unradiogenic Os (187Os/188Os ~0.11)
(Millet et al., 2009; Schaefer et al., 2002).
Table 5.2 Isotope ratios for the Azores island group
Island 3He/4He 206Pb/204Pb 207Pb/204Pb 208Pb/204Pb 87Sr/86Sr 143Nd/144Nd 187Os/188Os (RA) Santa Maria 7.7 19.01–19.28 15.56–15.58 39.01–39.30 - - -
0.703202- 0.512781- 0.13413- São Miguel 5.2-6.1 19.47-19.91 15.59-15.73 39.25-40.16 0.704470 0.512956 0.14196* Eastern São 0.704220- 0.512620- <5.1 20.00 15.75 40.33 - Miguel 0.704990 0.512774 0.703483- 0.512900- 0.12558- Terceira 9–13.5 20.02 15.64 39.35 0.703565 0.512960 0.13875 0.703268- 0.512920- 0.13298- Flores 8.04 19.55-19.73 15.61-15.63 39.23-39.53 0.703770 0.512948 0.14180
Graciosa 8.1-11.2 19.80 15.60 39.18 - - -
0.703770- 0.512768- 0.11019- Faial 7.2-8.5 19.13-19.71 15.54-15.64 38.75-39.31 0.704012 0.512927 0.13058 0.703372- 0.512920- Corvo - 19.57-19.78 15.61-15.63 39.26-39.48 - 0.730432 0.512930 0.703399- 0.512831- São Jorge - 19.34-20.51 15.63-15.97 39.05-39.56 - 0.703638 0.512940 0.703540– 0.512813– 0.12012- Pico 7.5–10.3 18.12–20.44 15.29–15.72 37.44-39.79 0.703860 0.512928 0.13885 *sample locations unknown (data compiled from Elliott et al., 2007; Genske et al., 2014; Genske et al., 2012; Jean-Baptiste et al., 2009; Madureira et al., 2005; Moreira et al., 1999; Schaefer et al., 2002; Widom et al., 1997).
The SM end-member may represent delaminated SCLM or recycled sediments, T appears to contain a HIMU component, and D may be upper mantle from the MAR
(Millet et al., 2009). The source of F is unknown, however the low Os ratios combined with the heaviest B isotopes might suggest the presence of an old fluid and melt depleted component (Millet et al., 2009). 123
5.1.5 Santa Maria (8.12 Ma)
Santa Maria is the most southerly and oldest island in the group, with initial subaerial volcanism occurring from 8.12 Ma (Abdel-Monem et al., 1975; Millet et al., 2009). Two periods of volcanic activity have taken place on the island, from
8.12–6.08 Ma, and at 4.3 Ma, with a period of quiescence in between, during which time a shelly limestone (Coquina limestone) was deposited (Ridley et al.,
1974). The last eruption took place 2.84±0.06 Ma (Sibrant et al., 2015).
One sample analysed from the island by Moreira et al. (1999) showed 3He/4He =
206 204 7.7±0.3 RA (Table 5.2). Santa Maria basalts show Pb/ Pb = 19.01–19.28
(n=3), 207Pb/204Pb = 15.56–15.58 (n=3), and 208Pb/204Pb = 39.01–39.30 (n=3)
(Moreira et al., 1999).
5.1.6 São Miguel (4.01 Ma)
São Miguel consists of four large volcanoes, Sete Cidades, Agua de Pau (also known as Fogo), Furnas, and Nordeste, from west to east (Beier et al., 2007;
Guest et al., 1999). The shield building stage of the Sete Cidades volcano lasted from >210 ka to 36 ka, when caldera forming eruptions began to occur (ending
16Ka ago) (Beier et al., 2006).
The basalts on São Miguel exhibit highly variable trace element and isotopic ratios
(Beier et al., 2007; Moreira et al., 1999). Variations in isotopic ratios are observed from west to east on São Miguel, with the west having more depleted isotopic signatures, similar to, for example, Pico, and the east having more enriched compositions; thus the isotopic ratios of Sr, Nd and Pb increase from west to east
(with the rocks increasing in age eastwards) (Elliott et al., 2007; Faure, 2001). The
3 4 eastern part of the island has low He/ He (<5.1 RA) (n=15) and relatively high lead ratios (206Pb/204Pb = 20.00, 207Pb/204Pb = 15.75, and 208Pb/204Pb = 40.33; n=3) 124
(Moreira et al., 1999). Eastern São Miguel (Nordeste) has some of the highest
87Sr/86Sr ratios observed in the Azores, ranging from 0.704220-0.704990 (n=6), but some of the lowest 143Nd/144Nd (0.512620-0.512774; n=6) (Genske et al.,
3 4 2014). He/ He ranges from 5.23 to 6.09 RA (n=6) in central and western São
Miguel (Jean-Baptiste et al., 2009). Lower lead ratios are seen in western São
Miguel (206Pb/204Pb = 19.47-19.90, 207Pb/204Pb = 15.59-15.73, and 208Pb/204Pb =
39.254-40.162; n=14) (Moreira et al., 1999; Widom et al., 1997).
The source of the geochemical variation in the basalts has been proposed to be due to the recycling of ancient oceanic crust (Beier et al., 2007) or magmatic underplating (Elliott et al., 2007).
The most recent eruption occurred on São Miguel in 1713 (Madeira and Ribeiro,
1990).
5.1.7 Terceira (3.52 Ma)
The island of Terceira consists of three overlapping stratovolcanoes, Santa
Barbara, Guilherme Moniz, and Cinco Picos, from west to east, with the Pico Alto
Volcanic Center (part of Guilherme Moniz) to the central north (Calvert et al., 2006;
Madureira et al., 2005). Santa Barbara is the youngest volcano, and Cinco Picos the oldest (Calvert et al., 2006). Numerous basaltic vents are also present, clustered in the region between Santa Barbara and Guilherme Moniz (Calvert et al., 2006).
The Cinco Picos volcano completed its main cone building activity around 370-380 ka (Calvert et al., 2006). Activity began at the Guilherme Moniz volcano prior to
270 ka and continued until at least 111 ka, with eruptions from Pico Alto occurring from 9000 to 1000 years ago (Calvert et al., 2006). The Santa Barbara volcano began erupting within the last 100,000 years, with its most recent eruption in 1761 125
at Misterio dos Negros (Calvert et al., 2006; Madeira and Ribeiro, 1990). A submarine eruption occurred in 1998-2000, ~10km from the west coast (Calvert et al., 2006). Both Pico Alto and Santa Barbara are considered to be active
(Madureira et al., 2005). Little is known about the early evolution of the island, perhaps due to the burial of older deposits by more geologically recent lava flows and pyroclastics (Calvert et al., 2006; Madureira et al., 2005).
The collapse of the upper part of the Cinco Picos edifice occurred after 370 ka, forming the largest caldera on the Azores, measuring 7x9km (Calvert et al., 2006).
Cinco Picos is now in its erosional phase, and is covered by basalts and pyroclastic deposits, from post caldera cinder cones, and Guilherme Moniz
(Calvert et al., 2006). The caldera of Guilherme Moniz may have formed due to multiple edifice collapses (Calvert et al., 2006).
3 4 The Terceira basalts show He/ He ratios ranging from ~9–13.5 RA (n=16) (the largest range in the Azores), 20Ne/21Ne >11.2 (n=26), and relatively high lead ratios (206Pb/204Pb = 20.02, 207Pb/204Pb = 15.64, and 208Pb/204Pb = 39.35; n=12)
(Jean-Baptiste et al., 2009; Madureira et al., 2011; Madureira et al., 2005; Moreira et al., 1999). 87Sr/86Sr has ratios similar to EM1 (0.703483-0.703565; n=3), and some of the highest 143Nd/144Nd observed in the Azores (0.512900-0.512960; n=3); however to date, only three samples have been analysed for these isotopes
(Genske et al., 2014). The source has been proposed to be mixing between
MORB and plume material, or lower mantle and recycled oceanic crust (Madureira et al., 2005; Moreira et al., 1999).
5.1.8 Flores (2.76 Ma)
The volcanic deposits on the island of Flores are divided into two major complexes: (BC) The Base Volcanic Complex , which includes the submarine and
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emergent volcanism; and (VC) The Upper Volcanic Complex which comprises subaerial activity (Azevedo and Portugal Ferreira, 2006). Initial submarine volcanism occurred from 2.2–1.5 Ma, followed by emergent activity from 1.0–0.75
Ma, and subaerial eruptions from 0.4–0.003 Ma (Azevedo and Portugal Ferreira,
2006). The most recent eruption occurred 3.0±0.1 ka (Azevedo and Portugal
Ferreira, 2006). Flores, together with Corvo, lies on a NNE-SSW ridge, subparallel to the MAR (Genske et al., 2012).
There is a paucity of published geochemical data on Flores, however data has recently been published on 12 samples, a sub-set of which (5: FL-09-23, FL-09-
26, FL-09-32, FL-09-41, FL-09-42) have been analysed for their halogen content as part of this PhD project (Genske et al., 2016; Genske et al., 2014; Genske et al., 2012). EM1 like 87Sr/86Sr (0.703268-0.703770; n=12) ratios are observed, together with HIMU-like 143Nd/144Nd (0.512920-0.512948; n=12), and FOZO- and
HIMU-like 206Pb/204Pb (19.5483-19.7268; n=12) (Genske et al., 2016; Genske et al., 2014). 207Pb/204Pb ranges from 15.6090 to 15.6264 (n=12) and 208Pb/204Pb from 39.2281 to 39.5278 (n=12) (Genske et al., 2016; Genske et al., 2014).
3 4 He/ He has been measured in one sample (8.04±0.12 RA) (Jean-Baptiste et al.,
2009).
5.1.9 Graciosa (2.5 Ma)
Graciosa is the northernmost island of the central group, situated on the Terceira rift, to the west of Terceira (Larrea et al., 2014). Its elongated shape is thought to be tectonically induced and related to Terceira rift volcanism (Hildenbrand et al.,
2008; Sibrant et al., 2014). Recently published data suggests that the minimum age for the onset of subaerial volcanism is 1.05 Ma, with the formation of the Serra das Fontes shield volcano (Larrea et al., 2014). The island comprises of three
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volcanic complexes: 1) the older volcanic complex, consisting of hawaiitic lava flows, 2) the intermediate volcanic complex, consisting of trachytic lava flows and pyroclastic material, and 3) the younger volcanic complex, composed of basaltic to trachytic lava flows and pyroclastic deposits (Sibrant et al., 2014). The most recent eruption was 3.9±1.4 ka (Larrea et al., 2014).
Isotopic data from only two samples have been reported in the literature (3He/4He
206 204 207 204 208 204 = 8.1-11.2 RA, Pb/ Pb = 19.80, Pb/ Pb = 15.60, and Pb/ Pb = 39.18) relating to Graciosa (Jean-Baptiste et al., 2009; Moreira et al., 1999).
5.1.10 Faial (0.73 Ma)
Faial is a symmetrical volcano with a 2km caldera (Ridley et al., 1974). The island has been divided into four geomorphological units; the central Caldera Volcano,
Capeolo Peninsula in the east, Pedro Miguel Graben to the northeast, and the
Horta Platform in the southeast (Machado et al., 2008). It has been suggested that the magmas show mixing between PREMA, EM2 and HIMU end-members
(Machado et al., 2008). Isotopic ratios measured in olivine and clinopyroxene
3 4 206 204 grains include He/ He ranging from 7.2 to 8.53 RA (n=3), Pb/ Pb = 19.130–
19.707 (n=10), 207Pb/204Pb = 15.541–15.642 (n=10), and 208Pb/204Pb = 38.746–
39.314 (n=10) (Jean-Baptiste et al., 2009; Moreira et al., 1999). 87Sr/86Sr has ratios similar to EM1 (0.703770-0.704012; n=3), and HIMU-like 143Nd/144Nd
(0.512768-0.512927; n=3) (Genske et al., 2014).
The most recent eruption occurred at Capelinhos, Faial in 1957-8, an eruption which increased the island’s length by 1km (Madeira and Ribeiro, 1990; Millet et al., 2009; Ridley et al., 1974).
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5.1.11 Corvo (0.71 Ma)
Corvo is an asymmetrical island, with similar stratigraphy to Flores, comprising of the Basal Volcanic Complex and the Upper Volcanic Complex (UVC) (França et al., 2006a; Genske et al., 2012). The ages of the different units on the island are poorly constrained, with little isotopic data available, although it is suggested that the initial subaerial volcanism may have occurred 1.5-1.0 Ma (França et al.,
2006a; Genske et al., 2012). The last eruption is thought to have taken place on the flank of Vila Nova do Corvo, approximately 80-100 ka (França et al., 2006a).
Only recently has extensive geochemical data been published on the island of
Corvo (Genske et al., 2016; Genske et al., 2014; Genske et al., 2012), expanding the previously published preliminary data by França et al. (2006a). Eight samples were analysed for Pb, Sr, and Nd, a sub-set of which (5 samples: C-09-01, C-09-
05, C-09-06, C-09-07, C-09-18) have been analysed for halogens as part of this
PhD project (Genske et al., 2012). Data reported are similar to those observed on
Flores, with 87Sr/86Sr again falling within the range of EM1 (0.703372-0.730432; n=8), HIMU-like 143Nd/144Nd (0.512920-0.512930; n=8), and FOZO- and HIMU-like
206Pb/204Pb (19.5710-19.7826; n=8) – with 207Pb/204Pb ranging from 15.6143 to
15.6268 (n=8) and 208Pb/204Pb from 39.2576 to 39.4768 (n=8) (Genske et al.,
2016; Genske et al., 2014). There are no known 3He/4He data.
5.1.12 São Jorge (0.55 Ma)
The island of São Jorge is approximately 55x7km, with three main volcanic systems present: Serra do Topo, Rosais, Manadas and Serra do Topo, from west to east (Hildenbrand et al., 2008; Millet et al., 2009). Serra do Topo is thought to be the oldest system on São Jorge (Hildenbrand et al., 2008). Large coastal erosion and stream incision has taken place on the island (Hildenbrand et al.,
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2008). The most recent eruption occurred in 1808 (Madeira and Ribeiro, 1990;
Millet et al., 2009).
São Jorge basalts show a range of 206Pb/204Pb from 19.347 to 20.511 (n=21), with limited 207Pb/204Pb (15.626–15.673; n=21) and 208Pb/204Pb (39.050–39.564; n=21) variation (Millet et al., 2009). Topo basalts have higher Sr isotope ratios (87Sr/86Sr
= 0.703705–0.703765; n=9) and variable Nd (143Nd/144Nd = 0.512900–0.512983; n=9), whilst the basalts from Manadas and Rosais have lower Sr ratios (87Sr/86Sr =
0.703399–0.703638; Manadas (n=7) lower than Rosais (n=5)) and less variable
Nd (143Nd/144Nd = 0.512914–0.512940; n=12) (Millet et al., 2009). The basalts have been interpreted as having a HIMU plume signature mixed with a depleted
MORB end-member, suggesting recycling of delaminated subcontinental mantle into the plume source (Millet et al., 2009). Recent research shows a similar range of 87Sr/86Sr (0.703502-0.703481; n=3) with similar 143Nd/144Nd values (0.512831-
0.512903; n=3) (Genske et al., 2014). There are no known 3He/4He data.
5.1.13 Pico (0.25 Ma)
Pico is the youngest island in the Azores, comprising of a stratovolcano (Pico) which rises to 2351m above sea level (França et al., 2006b; Mitchell et al., 2008).
The most recent eruption occurred in 1720 (França et al., 2006b; Madeira and
Ribeiro, 1990). The island comprises of three main volcanic complexes: (CVM) the
Montanha Volcanic Complex in the west, (CVSR-P) the São Roque-Piedade
Volcanic Complex in the east, and (CCVT-L) the Topo-Lajes Volcanic Complex to the south centre of the CVSR-P (França et al., 2006b). After emergence of the island, volcanic activity was located in the Topo shield volcano, and in the
Montanha composite volcano (França et al., 2006b).
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Isotope ratios range from 143Nd/144Nd = 0.512813–0.512928 (n=4), 87Sr/86Sr =
0.703540–0.703860 (n=4), 208Pb/204Pb = 37.44-39.79 (n=8), 207Pb/204Pb = 15.29–
206 204 3 4 15.72 (n=8), Pb/ Pb = 18.12–20.44 (n=8), and He/ He = 7.5–10.3 RA (n=8), suggesting a mixing between DMM and HIMU, with a small input from EM1 or
EM2 (França et al., 2006b; Genske et al., 2014; Moreira et al., 1999).
5.2 Aims
Data on the halogen content of ocean island basalts (OIBs) are scarce, particularly with respect to iodine, and there are no known data reported on HIMU OIBs. The aims of the study were: 1) to establish the halogen ratios (Br/Cl and I/Cl) within the
Azores basalts, 2) to determine if there is any systematic variation in the halogens between the islands, and in particular to compare the halogen content between the islands to the west of the Mid-Atlantic Ridge and those to the east, 3) to establish if the isotopic variation observed in other isotopic systems is also present within the halogens on the island of São Miguel, 4) to determine the source of the halogens within the basalts, 5) to expand the global dataset on the halogen content within
EM2 type OIBs, and to present the first halogen data for HIMU OIBs.
5.3 Sample Selection
Samples were obtained from eight of the nine islands of the Azores group (Table
5.3). The majority of samples were collected during a two week field trip to the
Azores (Figure 5.2). Over 80 samples were collected from the islands of São
Miguel, Terceira, Pico, and Graciosa – a subset (27) of which were analysed as part of this project (Figure 5.3 and Figure 5.4).
A further five samples were obtained from the islands of Faial (3) and São Jorge
(2) (Fernandes), together with a sample suite of 11 previously characterised (Pb
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and Os isotopes, rock types) samples from Corvo (6) and Flores (5) (Figure 5.4 and Figure 5.5) (Genske et al., 2016; Genske et al., 2014; Genske et al., 2012).
Samples were selected from different volcanic centres, to provide an understanding of any changes over timescales of millions of years, and from islands that are located close to and further from the MAR – providing a spatial section moving away from the ridge and the plume, in order to capture any variations that may be associated with the end-members (HIMU, EM2) determined in other isotope systems.
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Figure 5.2 Typical field localities for the Azores, showing a) a beach locality on Pico, and b) a roadcutting on Graciosa.
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Table 5.3 Sample location and descriptions. Samples from Graciosa (G11-XX), Pico (P11-XX), São Miguel (SMi11-XX), and Terceira (T11-XX) collected during field work (for full list of collected samples, see Appendix 8.4). Samples from Corvo (C-09-XX) and Flores (Fl-09-XX) provided by Dr Felix Genske and described by Genske et al. (2012)
Sample # Location Lat Long Description Rock Description
G11-01 Fenais 39° 1'53.70"N 27°57'23.23"W Roadcutting Olivine-bearing basalt
G11-04 Termas do Carapacho 39° 0'43.96"N 27°57'37.97"W Beach outcrop Olivine-bearing basalt
G11-08 Caldeira 39° 5'17.63"N 28° 0'2.12"W Roadcutting Olivine-bearing basalt
G11-15 Near Porta da Barra 39° 5'17.63"N 28° 0'2.12"W Beach outcrop Olivine-bearing basalt
G11-17 Ponta da Barca 39° 5'35.69"N 28° 2'55.25"W Roadcutting Olivine-bearing basalt
G11-18 Baía da Vitória 39° 4'47.94"N 28° 3'20.49"W Roadcutting Olivine-bearing basalt
G11-20 Tanque 39° 2'41.97"N 28° 1'44.95"W Roadcutting Olivine-bearing basalt
P11-06 Manhenha 38°24'41.38"N 28° 2'7.34"W Beach outcrop Olivine-bearing basalt
P11-12 Near Silveira 38°25'0.70"N 28°17'50.82"W Roadcutting Olivine-bearing basalt
P11-13 Pic de Filipe 38°27'38.00"N 28°18'30.75"W Roadcutting Olivine-bearing basalt
P11-19 Lajido, near airport 38°33'26.17"N 28°25'57.37"W Beach outcrop Olivine-bearing basalt
P11-20 Lajido, near airport (flow below P11-19) 38°33'25.91"N 28°25'56.69"W Beach outcrop Olivine-bearing basalt
SMi11-05 1km N Agua da Retorta, on road to Nordeste 37°46'20.91"N 25° 9'0.15"W Outcrop at roadside Olivine-bearing basalt
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Sample # Location Lat Long Description Rock Description
SMi11-08 After Santo Antonio de Nordestinho 37°51'8.11"N 25°13'10.13"W Roadcutting Olivine-bearing basalt
SMi11-10 Ferraria (carpark at Termas da Ferraria) 37°51'41.09"N 25°51'3.87"W Boulders Olivine-bearing basalt
SMi11-13 Mosteiros 37°53'58.92"N 25°49'4.25"W Wave cut platform Olivine-bearing basalt
SMi11-17 Sete Cidades 37°52'44.49"N 25°46'58.79"W Outcrop Olivine-bearing basalt
SMi11-19 Near Pico Dr. Ferreira 37°47'26.54"N 25°35'40.22"W Roadcutting Olivine-bearing basalt
SMi11-20 Pinhal da Paz 37°47'22.69"N 25°38'46.89"W Outcrop in path Olivine-bearing basalt
T11-02 Veredas 38°41'33.04"N 27°14'28.19"W Outcrop in wall at roadside Olivine-bearing basalt
T11-05 Pico das Caldeirinhas 38°44'16.84"N 27°15'7.30"W Outcrop in field, below cinder cone Olivine-bearing basalt
T11-10 Biscoitos 38°48'1.22"N 27°15'5.40"W Roadcutting Olivine-bearing basalt
T11-16 Ponta Negra 38°40'44.97"N 27° 3'19.81"W Beach outcrop Olivine-bearing basalt
T11-18 Porto Judeu 38°38'44.22"N 27° 8'25.51"W Roadcutting Olivine-bearing basalt
T11-19 South of Pico do Areeiro 38°42'51.09"N 27°11'26.92"W Roadcutting Olivine-bearing basalt
T11-20 Caldeira 38°46'56.59"N 27° 7'19.99"W Beach outcrop Olivine-bearing basalt
SJ-101/07 - - - Olivine-bearing basalt
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Sample # Location Lat Long Description Rock Description
SJ-103/07 - - - Olivine-bearing basalt
FA-02/07 - 38°33'10.75"N 28°37'28.84"W - Olivine-bearing basalt
FA-29/07 - 38°31'22.35"N 28°39'5.35"W - Olivine-bearing basalt
FA-38/07 - 38°36'41.60"N 28°45'40.36"W - Olivine-bearing basalt
C-09-01 - 39°40'30.72"N 31° 6'13.32"W - Olivine (alkali) basalt
C-09-05 - 39°40'49.08"N 31° 5'47.40"W - Olivine basalt
C-09-06 - 39°40'49.08"N 31° 5'47.40"W - Olivine (alkali) basalt
C-09-07 - 39°40'49.08"N 31° 5'47.40"W - Tephrite
C-09-13 - 39°42'18.36"N 31° 5'53.88"W - Olivine (tholeiite) basalt
C-09-18 - 39°40'24.96"N 31° 6'32.40"W - Tephrite
FL-09-23 - 39°30'53.64"N 31°13'41.52"W - Olivine (alkali) basalt
FL-09-26 - 39°30'53.64"N 31°13'41.52"W - Olivine (alkali) basalt
FL-09-32 - 39°27'29.16"N 31°15'52.56"W - Olivine trachybasalt
FL-09-41 - 39°25'21.36"N 31°14'30.48"W - Olivine (alkali) basalt
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Sample # Location Lat Long Description Rock Description
FL-09-42 - 39°25'27.84"N 31°14'24.36"W - Olivine (alkali) basalt
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Figure 5.3 Locations of samples collected (circles) and analysed (filled circles) as part of this study from the islands located on the Terceira Rift (modified from Forjaz et al., 1990; Moreira et al., 1999). Major volcanic centres shown with dates in ka (with number of samples (n=?) shown when dates not well resolved) (dates from Abdel-Monem et al., 1975; Beier et al., 2006; Calvert et al., 2006; Feraud et al., 1980; Larrea et al., 2014; Moreira et al., 2012). Nordeste (São Miguel) represents the Azores EM2 end-member. 138
Figure 5.4 Locations of samples collected (circles) and analysed (filled circles) as part of this study, together with locations (where known) of samples supplied Dr Vera Fernandes (stars) (Faial and São Jorge), for the islands located on the Azores micro-plate (modified from Forjaz et al., 1990; França et al., 2006b; Genske et al., 2012; Machado et al., 2008; Millet et al., 2009; Moreira et al., 1999). Major volcanic centres shown with dates in ka (with number of samples (n=?) shown when dates not well resolved) (Feraud et al., 1980; Hildenbrand et al., 2012).
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Samples for analysis were initially selected to give a spread of locations across each island, and on the basis of the presence of enough volume of unaltered olivine phenocrysts present with the sample. Samples from São Miguel were selected on the basis of their expected EM2 (Nordeste samples) and HIMU mantle end-member (western and central volcanic centres) components, as observed in other isotope systems.
Initially, three samples from São Miguel, Terceira, Pico, and Graciosa were irradiated and analysed to see if there was any differences immediately apparent between these islands, and thus to establish which islands to focus on in detail.
However, initial analyses did not highlight any differences across the island group, so samples from all islands (totalling 7 Graciosa, 5 Pico, 8 São Miguel, and 7
Terceira) were selected for further analysis – together with the samples obtained from the remaining four islands (Table 5.3).
Figure 5.5 Locations of samples supplied by Dr Felix Genske (stars), for the Atlantic Plate islands (modified from França et al., 2006a; Genske et al., 2012). Major volcanic centres shown with dates in ka (Azevedo and Portugal Ferreira, 2006; Feraud et al., 1980).
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5.4 Experimental Methods
The samples were prepared, irradiated, extracted, and analysed as per section
3.2. The samples were irradiated at the Petten research reactor in the
Netherlands. Thirty samples were crushed prior to step heating. Samples were then step heated, in 1-3 three steps from 600-1600°C. Due to low releases of the halogens from crushing, the remaining samples were step heated only, using the same temperature steps. After data reduction, blank-corrected data was compared to the uncorrected data – the results were within error of one another, and therefore blank corrections were not necessary and not applied. However, an initial correction for air was applied during data reduction.
A subset of the samples were also analysed using optical microscopy, to establish the mineralogy and the presence of melt inclusions.
5.5 Results
Figure 5.6 Halogen release during crushing and heating (a = Cl ppm, b = Br ppb, c = I ppb, d = K ppm). As a result of the low release during crushing, further samples (not shown) were step heated only.
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Samples were initially crushed in two steps of increasing intensity, reducing to one step for later samples, as only a small amount (<20%) of halogens was released during the crushing steps (Figure 5.6). The limited release during the crushing steps suggests that the majority of the halogens are sited in melt inclusions, rather than fluid inclusions, confirming observations made on the samples during thin section analysis, where melt inclusions were seen to be present within the olivine and clinopyroxene phases, but no fluid inclusions were observed. Due to this, and the limited variation between the halogen ratios obtained from crushing and step heating, later samples were step heated only. As such, only the integrated ratios
(crushing + step heating total for samples which were both crushed and step heated, and total step heating release for samples which were only heated) are discussed here. After crushing, samples were step heated in one to three steps of increasing intensity (e.g. 600°C, 1400°C, and 1600°C).
5.5.1 Integrated Halogen Releases
The Azores basalts range from (0.54-1480)x10-6 I/Cl, with a smaller range observed in Br/Cl from (0.39-3.37)x10-3 (Table 5.4). The widest range within the individual islands is observed in samples collected from Graciosa, which range from (9.28-930)x10-6 I/Cl, and (0.90-3.37)x10-3 Br/Cl, with sample G11-20 having both the highest Br/Cl (both within Graciosa and overall) and the highest I/Cl observed from Graciosa. The highest I/Cl is observed on Corvo in sample C-09-13
(1480±0.12)x10-6, which shows a range of (75.2-1480)x10-6 I/Cl and (0.82-
1.68)x10-3 Br/Cl.
Little variation (maximum one order of magnitude) is observed within the islands of
Pico, São Jorge (with only two samples analysed), Faial, and Flores, with Pico ranging from (1.36-49.0)x10-6 I/Cl and (0.78-1.91)x10-3 Br/Cl, São Jorge from
(20.4-57.4)x10-6 I/Cl and (1.03-1.05)x10-3 Br/Cl, Faial from (7.26-74.3)x10-6 I/Cl 142
and (0.93-1.10)x10-3 Br/Cl, and Flores from (30.6-494)x10-6 I/Cl and (0.72-
1.99)x10-3 Br/Cl. The lowest Br/Cl (0.39±0.05)x10-3 is observed on Terceira, which ranges from (0.39-1.51)x10-3 Br/Cl and (8.69-298)x10-6 I/Cl, with the lowest I/Cl
(0.54±3.77)x10-6 on Flores.
Samples from western and central São Miguel range from (1.02-2.20)x10-3 Br/Cl and (2.25-42.4)x10-6 I/Cl. The two samples from eastern São Miguel (SMi11-05 and SMi11-08) show the highest I/Cl ratios from the island at (159-344)x10-6 I/Cl, with sample SMi11-05 also giving the highest Br/Cl (2.80±0.07)x10-3 observed on the island.
5.5.2 Olivine vs Clinopyroxene Releases
Six samples were analysed for their halogen content in melt inclusions trapped in both olivine and clinopyroxene phases: two from Graciosa (G11-01, G11-15), one from Terceira (T11-16), one from São Miguel (SMi11-19), and two from Pico (P11-
13, P11-20). All samples yield similar Br/Cl ratios (same order of magnitude) in both the olivine and clinopyroxene analyses. The samples from Pico have similar
I/Cl in both the olivine and clinopyroxene separates, again within the same order of magnitude. The Graciosa samples show an order of magnitude higher I/Cl in the clinopyroxene analyses than seen in the olivine separates (G11-01OL
(9.28±0.34)x10-6 and G11-01PX (172±16)x10-6; G11-15OL (29.3±3.0)x10-6 and
G11-15PX (110±15)x10-6). The samples from Terceira and São Miguel yield an order of magnitude lower I/Cl in the clinopyroxene separates than observed in the olivine separates.
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Table 5.4 Halogen, K, and 40Ar/36Ar results from irradiated samples (for individual steps, see Appendix 8.2). Ol = olivine, Cpx = clinopyroxene.
Cl ± Cl Br ± Br ± I ± K Br/Cl I/Cl Sample Analysis Phase Island Irradiation I ppb K ppm ppm ppm ppb ppb ppb ppm (x10-3 molar) (x10-6 molar)
Heat Ol G Azores 1 5.06 0.12 11.70 0.16 0.17 0.00 165.28 1.07 1.03 ± 0.03 9.28 ± 0.34
G11-01 Heat Cpx G Azores 1 0.76 0.07 5.02 0.07 0.47 0.01 6.08 4.57 2.93 ± 0.27 172 ± 16.4
G11-04 Heat Ol G Azores 2 3.47 0.03 7.05 0.04 0.90 0.01 223.61 1.24 0.90 ± 0.01 72.6 ± 1.13
G11-08 Heat Ol G Azores 2 3.08 0.03 10.48 0.07 0.54 0.03 176.31 0.76 1.51 ± 0.02 49.3 ± 2.34
Crush + Heat Ol G Azores 1 1.90 0.16 12.41 0.15 0.20 0.01 15.32 1.07 2.89 ± 0.25 29.3 ± 3.04
G11-15 Crush + Heat Cpx G Azores 1 4.35 0.56 22.14 0.20 1.71 0.05 95.65 5.57 2.26 ± 0.29 110 ± 14.5
G11-17 Heat Ol G Azores 2 1.96 0.05 5.40 0.01 0.66 0.01 68.22 0.40 1.23 ± 0.03 94.4 ± 2.85
G11-18 Crush + Heat Ol G Azores 1 7.53 0.24 28.45 0.15 1.37 0.04 152.85 4.03 1.68 ± 0.05 50.7 ± 2.28
G11-20 Heat Ol G Azores 2 1.49 0.24 11.28 0.06 4.95 0.06 344.38 2.28 3.37 ± 0.54 930 ± 149
P11-06 Heat Ol P Azores 2 12.50 0.08 30.43 0.14 0.45 0.01 311.17 2.07 1.08 ± 0.01 10.1± 0.19
P11-12 Heat Ol P Azores 2 6.70 0.11 28.78 0.10 0.57 0.01 55.09 0.79 1.91 ± 0.03 23.7± 0.71
Crush + Heat Ol P Azores 1 4.33 0.19 9.34 0.18 0.33 0.02 112.81 6.90 0.96 ± 0.05 21.3 ± 1.61
P11-13 Crush + Heat Cpx P Azores 1 7.53 0.32 22.09 0.28 0.28 0.02 252.19 2.53 1.30 ± 0.06 10.4± 0.77
P11-19 Crush + Heat Ol P Azores 1 4.60 0.34 8.07 0.05 0.02 0.03 113.35 2.85 0.78 ± 0.06 1.36 ± 1.99
Crush + Heat Ol P Azores 1 3.81 0.43 8.96 0.11 0.27 0.02 68.16 4.92 1.05 ± 0.12 20.0 ± 2.88
P11-20 Crush + Heat Cpx P Azores 1 40.38 0.37 100.37 1.25 7.08 0.16 1224.97 8.85 1.10 ± 0.02 49.0 ± 1.18
SMi11-05 Heat Ol SM Azores 2 0.68 0.02 4.26 0.03 0.83 0.01 15.92 0.24 2.80 ± 0.07 344 ± 9.67
SMi11-08 Heat Ol SM Azores 2 0.99 0.04 2.75 0.07 0.57 0.01 52.18 0.49 1.23 ± 0.06 159 ± 7.80
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Cl ± Cl Br ± Br ± I ± K Br/Cl I/Cl Sample Analysis Phase Island Irradiation I ppb K ppm ppm ppm ppb ppb ppb ppm (x10-3 molar) (x10-6 molar)
SMi11-10 Crush + Heat Ol SM Azores 1 9.10 0.32 45.05 0.32 0.32 0.02 102.73 3.96 2.20 ± 0.08 9.70± 0.60
SMi11-13 Crush + Heat Ol SM Azores 2 3.24 0.04 9.48 0.05 0.49 0.02 160.97 0.73 1.30 ± 0.02 42.4 ± 1.74
SMi11-13X Crush + Heat Ol SM Azores 2 0.81 0.02 2.80 0.03 0.09 0.02 10.36 0.77 1.53 ± 0.04 30.4 ± 6.51
SMi11-17 Heat Ol SM Azores 2 1.59 0.03 4.90 0.02 0.53 0.01 27.39 0.57 1.37 ± 0.03 92.6 ± 2.29
Crush + Heat Ol SM Azores 1 2.48 0.32 5.66 0.04 0.13 0.02 158.52 4.25 1.02 ± 0.13 14.1 ± 3.01
SMi11-19 Crush + Heat Cpx SM Azores 1 15.19 0.31 34.70 0.46 0.12 0.06 648.07 4.76 1.02 ± 0.02 2.25 ± 1.18
SMi11-20 Heat Ol SM Azores 2 3.55 0.04 8.54 0.03 0.50 0.01 197.14 0.92 1.07 ± 0.01 39.2± 0.77
T11-02 Heat Ol T Azores 2 2.27 0.05 5.12 0.04 0.24 0.01 57.69 0.64 1.00 ± 0.02 29.7 ± 1.82
T11-05 Crush + Heat Ol T Azores 1 6.40 0.87 16.54 0.18 0.43 0.03 125.27 8.44 1.15 ± 0.16 18.6 ± 2.87
T11-10 Heat Ol T Azores 2 3.47 0.04 6.76 0.04 0.12 0.00 86.56 0.21 0.87 ± 0.01 9.52± 0.19
Heat Ol T Azores 2 1.56 0.02 5.32 0.04 1.67 0.03 50.25 0.38 1.51 ± 0.02 298 ± 7.17
T11-16 Crush + Heat Cpx T Azores 1 84.80 0.69 188.49 2.35 3.85 0.12 4979.22 26.42 0.99 ± 0.01 12.7± 0.42
T11-18 Crush + Heat Ol T Azores 1 5.17 0.56 4.57 0.18 0.16 0.01 108.31 6.56 0.39 ± 0.05 8.69± 0.98
T11-19 Heat Ol T Azores 2 1.90 0.06 2.89 0.02 0.09 0.02 91.45 1.00 0.68 ± 0.02 13.4 ± 3.66
T11-20 Heat Ol T Azores 2 1.09 0.03 2.33 0.03 0.22 0.01 45.52 0.45 0.95 ± 0.03 55.8 ± 3.51
SJ-101/07 Crush + Heat Ol SJ Azores 2 5.36 0.23 12.42 0.14 1.10 0.10 128.21 2.25 1.03 ± 0.05 57.4 ± 5.99
SJ-103/07 Crush + Heat Ol SJ Azores 2 3.43 0.09 8.14 0.04 0.25 0.02 175.28 1.48 1.05 ± 0.03 20.4 ± 1.40
FA-02/07 Crush + Heat Ol FA Azores 2 1.32 0.10 2.75 0.07 0.19 0.03 36.57 1.44 0.93 ± 0.07 40.6 ± 7.08
FA-29/07 Heat Ol FA Azores 2 3.56 0.03 8.15 0.22 0.95 0.04 147.31 0.42 1.02 ± 0.03 74.3 ± 3.18
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Cl ± Cl Br ± Br ± I ± K Br/Cl I/Cl Sample Analysis Phase Island Irradiation I ppb K ppm ppm ppm ppb ppb ppb ppm (x10-3 molar) (x10-6 molar)
FA-38/07 Crush + Heat Ol FA Azores 2 25.14 0.15 62.07 0.17 0.65 0.04 709.62 3.44 1.10 ± 0.01 7.26± 0.45
C-09-01 Heat Ol C Azores 2 11.49 0.10 21.14 0.09 9.98 0.09 174.73 1.62 0.82 ± 0.01 243 ± 3.01
C-09-05 Heat Ol C Azores 2 3.42 0.04 12.93 0.10 1.23 0.02 111.20 1.77 1.68 ± 0.02 100 ± 1.80
C-09-06 Crush + Heat Ol C Azores 2 0.92 0.04 2.01 0.04 1.21 0.06 14.38 0.57 0.97 ± 0.05 369 ± 25.6
C-09-07 Crush + Heat Ol C Azores 2 0.39 0.04 0.97 0.02 0.28 0.01 3.52 0.20 1.10 ± 0.11 203 ± 65.5
C-09-13 Heat Ol C Azores 2 0.67 0.06 2.28 0.03 3.55 0.04 44.22 0.40 1.52 ± 0.13 1480 ± 124
C-09-18 Heat Ol C Azores 2 6.00 0.08 17.26 0.05 1.61 0.02 89.61 0.44 1.28 ± 0.02 75.2 ± 1.34
FL-09-23 Crush + Heat Ol FL Azores 2 0.78 0.07 3.51 0.02 1.38 0.07 46.65 0.59 1.99 ± 0.17 494 ± 49.1
FL-09-26 Heat Ol FL Azores 2 4.64 0.05 7.53 0.02 1.27 0.01 181.76 0.96 0.72 ± 0.01 76.4 ± 1.13
FL-09-32 Heat Ol FL Azores 2 16.26 0.12 51.20 0.06 17.31 0.17 453.10 1.35 1.40 ± 0.01 297 ± 3.64
FL-09-41 Heat Ol FL Azores 2 0.34 0.03 0.69 0.02 0.00 0.00 6.50 0.57 0.89 ± 0.08 0.54 ± 3.77
FL-09-42 Crush + Heat Ol FL Azores 2 4.40 0.10 8.34 0.11 0.48 0.06 161.52 2.29 0.84 ± 0.02 30.6 ± 3.80
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5.5.3 Rock Descriptions Table 5.5 Rock descriptions from thin section analysis (ol = olivine, cpx = clinopyroxene, plag = plagioclase, opqs = opaque minerals, MI = melt inclusions). * from Genske et al. (2012).
Phases analysed Sample Rock type Description for halogens Phenocryst Minerals (70%): plag (55%), cpx (20%), ol (15%), opqs (5%) Groundmass (30%): plag (+ opqs + ol + cpx) Ol 0.5-2mm, subhedral, resorbed – embayment. Cpx 0.5-4mm (average 1.5mm), euhedral to subhedral, some twinning, resorption (less than observed G11-01 Olivine-basalt in ol) rounding of euhedral crystals. Plag 1-4mm (average 1.5mm), euhedral to ol, cpx subhedral laths, oscillatory zoning, multiple twinning, some fractured crystals. MIs in ol (primary and secondary). MIs (primary) concentrated around rims in cpx. Outcrop: large ol phenocrysts (up to 8mm), small vesicles present (Figure 5.7) Phenocryst Minerals (30%): plag (50%), ol (45%), cpx (5%) Groundmass (70%): plag laths (+ opqs + ol) Ol 0.5-3mm (average 1.5mm), euhedral to anhedral (mostly subhedral), resorption. Cpx 1.0-1.5mm (average 1.5mm), euhedral to subhedral, fractured, G11-15 Olivine-basalt ol, cpx some resorption. Plag 0.25-3mm (average 0.75mm), euhedral to subhedral laths, possible alteration around rims to clay minerals. MIs present in ol and cpx (both primary and secondary).
Phenocryst Minerals (15%): ol (75%), cpx (15%), plag (10%) Groundmass (85%): plag laths (+ opqs + ol ± cpx) Ol 0.5-2mm (average 1mm), euhedral to subhedral, some resorption. Cpx 0.5- G11-18 Olivine-basalt 1mm (average 0.5mm), euhedral to subhedral. Plag 0.25-0.75mm (average ol 0.5mm), euhedral laths. MIs observed in both ol and cpx. Vesicles 0.5-1.5mm (average 0.5mm). Outcrop: ol phenocrysts up to 10mm.
Phenocryst Minerals (10%): cpx (45%), ol (35%), plag (20%) Groundmass (90%): plag laths (+ opqs + ol ± cpx) Cpx 2-4mm, subhedral, resorbed. Ol 0.5-2.0mm, subhedral, resorbed. Plag P11-13 Olivine-basalt 0.5-1mm, subhedral and resorbed (euhedral in groundmass), multiple twinning, ol, cpx some zoning. Primary and secondary MIs observed in ol. MIs (possible secondary) present in cpx. Possible compacted vesicles present (Figure 5.7).
Phenocryst Minerals (20%): plag (80%), ol (15%), cpx (5%) Groundmass (80%): plag laths (+ opqs + ol ± cpx) Plag 0.5-2.0mm (average 1.5mm), euhedral laths, often in clumps. Ol 0.25- P11-19 Olivine-basalt ol 0.5mm (average 0.5mm), euhedral to subhedral, some resorption. Rare MIs in ol. Vesicles present 0.5-2.0mm (average 0.5m), rounded.
Phenocryst Minerals (35%): cpx (60%), ol (40%) Groundmass (65%): plag laths (+ opqs ± ol ± cpx) Ol 1-4mm (average 2.5mm), euhedral to subhedral, some resorption P11-20 Olivine-basalt (anhedral, embayed). Cpx 1-5mm, euhedral (some resorption and rounding), ol, cpx twinning. High density of MIs observed in ol – possible secondary. Vesicles (~35%) 0.5-1.0mm (average 0.5mm), often coalescing.
Phenocryst Minerals (25%): ol (70%), cpx (30%) Groundmass (75%): plag laths (+ opqs + ol) Ol 1.5-3mm (average 2mm), subhedral to anhedral, resorption (rounding). Cpx SMi11-08 Olivine-basalt 2-6mm (average 3mm), euhedral to subhedral. MIs in both ol (primary and ol secondary) and cpx (secondary). Large, abundant phenocrysts present in outcrop of ol and cpx (up to 15mm).
Phenocryst Minerals (20%): ol (45%), cpx (40%), plag (15%) Groundmass (80%): plag laths (+ opqs + ol + cpx) Ol 0.25-2.0mm (average 1mm), euhedral to subhedral, resorption (some rounding). Cpx 0.25-1.5mm (average 1mm), euhedral, twinning. Plag 0.25- SMi11-19 Olivine-basalt ol, cpx 2.0mm (average 0.75mm), euhedral laths. MIs observed in ol (primary and secondary) and cpx (possibly secondary). Preferred orientation of plag laths in groundmass (Figure 5.7).
Phenocryst Minerals (<10%): plag (50%), cpx (40%), ol (10%) Groundmass (>90%): plag laths (+ opqs + ol ± cpx) Plag 0.25-0.75mm (average 0.5mm), euhedral laths, twinning. Cpx 0.25- T11-05 Olivine-basalt 0.5mm (average 0.25mm), euhedral to subhedral, zoned rims. Ol 0.1-0.5mm ol (average 0.2mm), euhedral to subhedral. Vesicles (10%) 0.25-0.75mm (average 0.25mm), compacted.
Phenocryst Minerals (25%): ol (50%), cpx (45%), plag (5%) T11-16 Olivine-basalt Groundmass (75%): plag laths (+ opqs + ol ± cpx) ol, cpx Ol 0.5-2.0mm (average 1mm), subhedral to anhedral, glomerocrysts, 147
Phases analysed Sample Rock type Description for halogens resorption (rounding, embayment), some skeletal crystals. Cpx 0.5-2.0mm (average 1mm), euhedral to subhedral, resorption (embayment). Plag 0.5- 2.0mm (average 0.5mm), euhedral to subhedral laths twinning. Possible MIs in ol (not observed in cpx).
Phenocryst Minerals (<5%): ol (50%), cpx (35%), plag (15%) Groundmass (>95%): plag laths (+ opqs + ol ± cpx) Ol 0.5-2.0mm, euhedral to subhedral, rare phenocrysts, resorption, possible T11-18 Olivine-basalt ol myrmetic texture. No MIs observed. Vesicles (15%) 0.5-2.0mm (average 0.75mm). Preferred orientation of plag laths in groundmass.
Phenocryst Minerals (<5%): cpx (50%), ol (30%), plag (20%) Groundmass (>95%): plag laths + ol (+ opqs ± cpx) Mostly microphenocrysts, with rare phenocrysts. Ol 0.25-2.0mm (average 1mm), subhedral, resorbed (rounding and embayment). Cpx 1.0-1.5mm FA-02/07 Olivine-basalt ol (average 1mm), rare, euhedral. Rare vesicles 0.25-0.5mm (average 0.25mm). MIs observed in ol (primary and secondary). Preferred orientation of plag laths in groundmass.
Phenocryst Minerals (50%): plag (50%), ol (25%), cpx (25%) Groundmass (50%): plag laths + ol (+ opqs ± cpx) Plag 0.5-2.0mm (average 1.5mm), euhedral laths, rare resorption, multiple FA-29/07 Olivine-basalt twinning. Ol 1.5-3.0mm (average 2mm), subhedral to anhedral, resorption ol (rounding). Cpx 1.5-3.0mm (average 2mm), euhedral to subhedral, fractured. Plag laths showing preferred orientation.
Phenocryst Minerals (20%): plag (50%), cpx (40%), ol (10%) Groundmass (80%): plag laths + glass (+ ol ± cpx) Plag 0.25-2.0mm (average 0.75mm), euhedral laths, multiple twinning. Cpx 1- FA-38/07 Olivine-basalt 3mm (average 2mm), euhedral to subhedral, some resorption, possible zoned ol rims. Ol 0.25-0.5mm (average 0.5mm), subhedral, some rounding. MIs present in ol.
Phenocryst Minerals (60%): plag (60%), cpx (30%), ol (10%) Groundmass (40%): glassy with plag laths (+ ol + cpx) Plag 0.5-3.0mm (average 0.75mm), euhedral laths, multiple twinning. Cpx 1- SJ-01/07 Olivine-basalt 2mm (average 2mm), euhedral to subhedral, some embayment. Ol 0.25- ol 1.0mm (average 1mm), euhedral to subhedral. MIs observed in ol (secondary and possible primary).
Phenocryst Minerals (50%): ol (40%), cpx (40%), plag (20%) Groundmass (50%): plag laths with interstitial ol + cpx Ol 0.75-1.0mm (average 1.0mm), euhedral, some resorption. Cpx 0.5-3mm SJ-03/07 Olivine-basalt ol (average 2mm), zoned rims, simple twinning. Plag 0.75-2.0mm (average 1mm), euhedral laths, multiple twinning.
Fresh lava flow with 15% cpx (pheno- and xenocrystals up to 7 mm), 5% C-09-01* Alkali basalt altered Ol (52 mm), 15% ol Pl phenocrysts (53 mm),51% Fe–Ti oxides C-09-06* Alkali basalt - ol C-09-07* Tephrite - ol C-09-09* Alkali basalt Fresh dense lava flow, evenly distributed Ol and Cpx (10%) up to 7mm in size ol Lower flow of an eruption sequence, up to 1 cm sized vesicles (15%), Cpx up C-09-18* Tephrite to 7mm (10%), 15% Pl phenocrysts (51 mm) and xenocrystals (53 mm), 3% ol altered Ol, dense matrix Fresh, vesiculated but dense basalt flow, occasional mafic cumulates (51%), FL-09-23* Alkali basalt ol 3% Cpx, 3% Pl,51% Ol, Fe–Ti oxides disseminated (1 mm) Fresh basaltic flow at the base of a sequence, fine-grained, 3% Cpx (52 mm), FL-09-26* Alkali basalt ol 5% Pl 52 mm), 1% Ol (51 mm) Fresh mafic composition, 4% Ol xenocrystals up to 3 mm, 2% Cpx (53 mm), FL-09-32* Trachybasalt ol occasional (1%) Pl laths up to 2 mm Fresh porous basalt, cm-sized Cpx 2%, vesicular matrix, 1% large Pl (58 mm) FL-09-41* Alkali basalt ol xenocrystals enclosing small Cpx, 1% Ol phenocrysts (51 mm) Vesicular fresh basalt, 5% Cpx up to 5 mm, 3% Pl crystals (53 mm), 1% FL-09-42* Alkali basalt ol altered Ol, occasional Fe–Ti oxides
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Figure 5.7 Selected thin sections from Graciosa (a-b G11-01), Pico (c-d P11-13), and São Miguel (e-f SMi11-19). Ol = olivine, Cpx = clinopyroxene, Plag = plagioclase.
5.6 Discussion
5.6.1 Siting of the Halogens Thin section observations show the presence of melt inclusions within the olivine and clinopyroxene phenocrysts within the Azores basalts (Figure 5.8). Fluid inclusions were not observed in the thin sections, and the majority of the halogens were released during the step heating, rather than in the crushing phases; suggesting that the halogens are sited mainly within melt inclusions hosted within the olivine and clinopyroxene crystals. 149
Figure 5.8 Melt inclusions (MI) within olivine (ol) and clinopyroxene (cpx) phenocrysts in the Azores basalts (a-d P11-20, e-f G11-15). Yellow lines highlight possible melt inclusion trails. Six of the samples were analysed for their halogen content in both olivine and clinopyroxene phases, with two of the samples having similar halogen ratios in each phase, two samples yielding higher I/Cl in the clinopyroxene analyses, and two samples showing higher I/Cl in the olivine analyses. This suggests that the melt inclusion assemblages in the two phases may be sampling halogens from different sources. The presence of a Cl-bearing phase, such as amphibole or apatite, would cause the I/Cl ratio in the residual melt, and therefore also in melt 150
inclusions trapped in later formed crystals, to rise (Chavrit et al., 2016); however, no Cl-bearing phases were observed during the thin section analyses. Although amphibole and apatite have been observed in some Corvo and Flores samples
(França et al., 2006a; Genske et al., 2012), these phases were not observed in samples analysed for halogen content (Genske et al., 2012), and are therefore it is not thought that fractional crystallisation is the cause of the halogen variation observed within these samples.
Many of the olivine crystals show features of resorption (Figure 5.8c, e), suggesting that they crystallised earlier from the melt than the clinopyroxene crystals, and stagnated in the magma chamber for a period of time prior to eruption. The differences in the halogen ratios in the melt inclusion assemblages in olivine and clinopyroxene crystals may be due to the olivines sampling the magma before any interaction, e.g. with the magma chamber walls, or prior to the refreshing of the magma chamber with a fresh batch of magma. All the clinopyroxene analyses show a higher concentration of halogens present within the separates; this is thought to be due to a higher density of melt inclusions within the clinopyroxene crystals, due to nucleation and trapping of melt inclusions on cleavage planes.
5.6.2 Intra-Island Halogen Variation On the island of São Miguel, it is observed the highest I/Cl ratios are present in the eastern end of the island (samples SMi11-05 and SMi11-08), in the Nordeste volcanic centre (Figure 5.9). The I/Cl ratios here are an order of magnitude higher, when compared with the samples analysed from the central and western areas of the island. This is the oldest volcanic centre on the island, and the area in which previously reported samples in other geochemical systems have yielded an enriched component (e.g. Moreira et al., 1999).
151
Figure 5.9 Halogen ratios for São Miguel samples (bracketed figures show clinopyroxene measurements on the same samples; “x” denotes xenolith analysed from same sample). References as per Figure 5.3. Although the dates of the other volcanic centres on São Miguel (and indeed the other islands in the Azores group) are not well constrained, the variation shown could possibly be an age-related variation, as the Nordeste volcanic centre is extinct and last thought to have been active ~700 Ma. It is also possible that this variation is due to a local length-scale anomaly beneath São Miguel, as proposed by other authors (e.g. Beier et al., 2007; Elliott et al., 2007).
In the central and western volcanic centres, there seems to be little correlation between age, location, and halogen ratios, with both zone 2 and Sete Cidades showing overlapping I/Cl ratios over two orders of magnitude.
Overlapping halogen ratios are again observed across the volcanic centres on the island of Terceira (Figure 5.10), which shows no correlation between volcanic centre, location, age, and halogen ratios. The lowest and highest I/Cl values are yielded from the Cinco Picos volcanic centre; however, lower Br/Cl values are also observed in from the Pico Alto volcanic centre.
152
Figure 5.10 Halogen ratios for Terceira samples (bracketed figures show clinopyroxene measurements on the same samples). References as per Figure 5.3. On the island of Graciosa, the highest I/Cl ratio is observed on the NW volcanic platform (Figure 5.11), which is possibly the oldest region on the island (n=1 samples dated). This area again shows a range of halogen ratios that overlap with other volcanic centres on the island, providing little correlation of the halogen ratios with volcanic centres, location, and age.
153
Figure 5.11 Halogen ratios for Graciosa samples (bracketed figures show clinopyroxene measurements on the same samples). References as per Figure 5.3. A range of halogen ratios is observed across Faial (Figure 5.12), but due to the small number of samples analysed for their halogen content (n=3), and the small number of samples in the literature dating these volcanic centres (n=6), it is impossible to ascertain if there is any relationship between the halogens and sample location, age, or volcanic centre. No sample providence is known for the
São Jorge samples (Figure 5.13), and again the dates of the volcanic centres are poorly constrained, with ages of only four samples reported in the literature
(Feraud et al., 1980).
154
Figure 5.12 Halogen ratios for Faial samples. References as per Figure 5.4.
Figure 5.13 Halogen ratios for São Jorge samples. References as per Figure 5.4. Little variation is observed in the halogen ratios across Pico (Figure 5.14), with four samples yielding I/Cl in the same order of magnitude across the island, and only P11-19 showing lower I/Cl values.
155
Figure 5.14 Halogen ratios for Pico samples (bracketed figures show clinopyroxene measurements on the same samples). References as per Figure 5.4. The highest I/Cl in the Azores island group is observed on the island of Corvo
(sample C-09-13), in the northern part of the island (Figure 5.15). Lower I/Cl ratios are observed in the southern samples, however with the limited number of samples analysed from the northern (n=1) and southern (n=5) areas, it is impossible to ascertain if this variation is related to location or volcanic centre. The ages of the volcanic centres on Corvo are unknown, therefore the variation of the halogens on Corvo cannot be correlated to the age of the eruptive centres.
156
Figure 5.15 Halogen ratios for Corvo and Flores samples. References as per Figure 5.5. The highest I/Cl values preserved on Flores are from the northern (FL-09-23 and
FL-09-26) and western (FL-09-32) areas, with the more centrally-located samples showing a lower range of halogen ratios (Figure 5.15). Again, it is difficult to relate this variation to age, location, or volcanic centres, due to the limited age resolution of Flores. The distribution does not appear to be related to magma evolution, as the most compositionally evolved sample (FL-09-32, trachybasalt) shows a range which overlaps with the less evolved alkali basalts (FL-09-23 and FL-09-26). The lowest I/Cl ratios are shown in remaining alkali basalts (FL-09-41 and FL-09-42), indicating that the halogen ratios observed are not related to fractionation processes.
5.6.3 Halogen Variation across the Azores Group It is observed that samples from each island lie within, or close to, the known field for halogens observed in MORB (Figure 5.16). This suggests that there is a
MORB-like component common to all the islands. The central islands of Pico,
Faial, and São Jorge all exhibit similar halogen signatures, which show a MORB-
157
like range in composition, and are also consistent with those previously reported in the literature from EM1 and EM2 type mantle sources in Pitcairn and Society
Islands (Figure 5.16) (Kendrick et al., 2014b). Only one sample, P11-12 (Pico), deviates from the MORB field, trending to higher Br/Cl values. This lack of variation could suggest that the halogen isotopic signature of the plume is observed in the basalts from the central islands, suggesting that there is little difference in the halogen compositions of MORB and OIB. Alternatively, the plume material is mixing with MORB mantle, and overprinting the plume halogen signature with a MORB-like halogen signature.
The two remaining central islands, Terceira and Graciosa, both share this common
MORB-like component. However, Graciosa trends to much higher I/Cl and Br/Cl ratios, exhibiting the greatest range in halogen signatures across the entire archipelago, ranging from a MORB-like signature, overlapping with the serpentinite field, and extending to a marine pore fluid-like signature (Figure 5.17). One sample from Terceira (T11-16) also extends into the serpentinite field. Unique to Terceira is that lower Br/Cl ratios are observed in three of the samples. These samples are the only Azores samples which lie within the known field for AOC, altered oceanic crust (Chavrit et al., 2016) (Figure 5.17). This could indicate the presence of a recycled AOC component within the Terceira basalts, as proposed by Moreira et al. (1999), where high lead isotopic ratios were observed coupled with an unradiogenic or primitive helium ratio (R/RA=11.3), similar to that observed
(R/RA=9.5-11.5) by Madureira et al. (2005). Moreira et al. (1999) also observed similar helium and lead values for Pico and Terceira, however, the presence of an
AOC-like halogen signature is only observed in three of the Terceira samples, and is not seen at all within the Pico samples, where all the samples lying within the range observed for MORB.
158
The eastern island of São Miguel also shows a range of halogen signatures
(Figure 5.16). Similar to the central island group, some samples show a MORB- like signature, and also extend into the serpentinite field (Figure 5.17). Samples from the Nordeste volcanic centre (eastern São Miguel) overlap with both the serpentinite and marine pore fluid fields (Figure 5.18). This is in agreement with variations observed in other isotopic systems, where the eastern basalts show a more radiogenic lead signature, together with radiogenic helium (R/RA <5.1)
(Moreira et al., 1999). If eastern São Miguel represents an EM2 mantle end- member as previously proposed (Widom et al., 1997), these two samples would extend the range previously observed for EM2 halogens by Kendrick (2014b), for
Br/Cl, but particularly for I/Cl, which would increase by an order magnitude.
On Flores, MORB-like halogen ratios are observed, trending to slightly higher I/Cl ratios, overlapping with the serpentinite and marine pore fluid fields (Figure 5.16).
The highest I/Cl ratios in the Azores group are shown on the other western island,
Corvo (Figure 5.16). Although the Br/Cl ratios remain MORB-like, the I/Cl ratios trend to almost two orders of magnitude higher values than that previously seen in the literature for MORB, extending into the known field for serpentinites. This suggests the presence of an I-rich sediment or marine pore fluid-like component within the Corvo (and the higher Flores) samples (Figure 5.17).
The variations across the island group suggest that there three components present within the halogens in the Azores basalts: a) a MORB-like signature, b) a serpentinite or marine pore fluid signature, and c) an AOC signature.
159
Figure 5.16 Halogen ratio plot of the Azores basalts (additional data from Bruland and Lohan, 2003; Fehn et al., 2006; Jambon et al., 1995; Kendrick et al., 2014a). Error bars, where not seen, are smaller than symbol. 160
Figure 5.17 Halogen ratio plot with altered oceanic crust (AOC), sub-continental lithospheric mantle (SCLM), pore fluid, serpentinites, wedge fluid, MORB, bulk Earth (CI), and seawater data from literature (additional data from Anders and Ebihara, 1982; Bruland and Lohan, 2003; Chavrit et al., 2016; Fehn et al., 2006; Fehn et al., 2000; Fehn et al., 2003; Fehn et al., 2007b; Jambon et al., 1995; John et al., 2011; Kastner et al., 1990; Kendrick, 2012; Kendrick et al., 2013a; Kendrick et al., 2014a; Lu et al., 2008; Mahn and Gieskes, 2001; Martin et al., 1993; Muramatsu et al., 2007; Muramatsu et al., 2001; Sumino et al., 2010; Tomaru et al., 2007). Error bars, where not seen, are smaller than symbol. 161
5.6.4 The source of the depleted halogen signature on Terceira
Whilst the majority of the samples analysed from Terceira seem to represent the common MORB-like plume source observed across the island group, three of the samples trend towards lower halogen ratios, overlapping with the known field for
AOC. Lower halogen ratios can result due to seawater contamination, either pre- or post-eruptive – however, we can exclude alteration of the erupted basalts, as the samples collected and analysed were free from any visible alteration, and the basalts trend towards lower Br/Cl than observed in seawater (Figure 5.17). Having excluded alteration of the Terceira basalts, we must consider the possibility of the presence of
AOC within the source of the Terceira basalts. During subduction, altered oceanic crust, enriched in Cl with respect to Br and I, was subducted into the mantle. As the downgoing slab dehydrated, due to the greater compatibility of Cl, I and Br would have been preferentially expelled from the slab, decreasing the halogen ratios further. If this AOC was later incorporated into the plume material, either due to deep recycling or due to entrainment in the shallower mantle, this would cause the depleted halogen signatures observed in the Terceira samples. This would indicate that if an AOC signature is observed within the halogens, it is local to Terceira.
Alternatively, this variation could be a temporal, rather than spatial, variation.
5.6.5 The source of the eastern São Miguel halogen enrichment
The enrichment observed within the two samples from the eastern end of the island, particularly in sample SMi11-05, suggest the presence of either I-rich sediments or a marine pore fluid component locally, within the source of the São Miguel basalts
(Figure 5.18). It has previously been proposed that the length-scale variation observed in eastern São Miguel is either due to the recycling and long term storage of ancient oceanic crust and/or sediments (Beier et al., 2007; Elliott et al., 2007;
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França et al., 2006b). Pre-Grenville subduction is proposed to have recycled oceanic crust and sediments into the SCLM beneath the Grenville Continent, which was then delaminated during Jurassic rifting and sea floor spreading, and incorporated into the mantle beneath the Azores archipelago (Moreira et al., 1999; Pegram, 1990). If the ancient oceanic crust halogen signature was dominant in the entrained delaminated
SCLM, we would expect that the São Miguel basalts would overlap with the known field for AOC (Chavrit et al., 2016), with reduced halogen ratios with respect to the
MORB field. However, we are seeing an increase within both the Br and I content in the São Miguel basalts, showing an increased Br/Cl but particularly I/Cl ratio. This can be explained by the incorporation of a sediment or marine pore fluid signature into the SCLM prior to delamination, and the subsequent introduction of this signature into the mantle within the delaminated SCLM beneath eastern São Miguel, resulting in a local length-scale (km) heterogeneity, in agreement with the model proposed by Moreira et al. (1999). If this heterogeneity is sampled by the rising plume material, this would result in the eastern São Miguel basalts showing higher
Br/Cl and I/Cl ratios in comparison with the central and western basalts, which is observed within the samples. Alternatively, it may be that the signature observed in the Nordeste basalts is due to heterogeneity within the plume material, suggesting a deeper recycling of a marine pore fluid signature into the mantle, perhaps incorporated within serpentinites. As the marine pore fluid halogen signature is observed on other islands, particularly Corvo and Graciosa, it must be considered that the heterogeneity observed in eastern São Miguel is not, as indicated by other isotopic systems, a local length-scale heterogeneity.
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Figure 5.18 The halogen composition of the eastern São Miguel basalts, and the central and western São Miguel basalts. Error bars, where not seen, are smaller than symbol. 5.6.6 The source of the halogen isotopic variation on Graciosa
Graciosa is one of the least studied of the Azores islands, with only one known sample previously analysed, yielding MORB-like helium (R/RA=8.1) (Moreira et al.,
1999). It shows the greatest range of halogen ratios within the island group, with three samples lying within the known field for MORB, a further two samples yielding values similar to those previously observed for EM1 and EM2 basalts, and the remaining four samples trending towards higher Br/Cl and I/Cl ratios. However, the samples that yield the highest halogen ratios exhibit ratios similar to those observed within the eastern São Miguel basalts. It would therefore appear that a similar halogen rich component is present within the Graciosa basalts, as observed in eastern São Miguel. Due to the lack of a correlation between the halogen ratios and the spatio-temporal variation within the island group, it may be a similar situation to the eastern São Miguel basalts, in that the Graciosa samples with the higher I/Cl and
Br/Cl ratios are sampling a local heterogeneity within delaminated SCLM, that preserved a pore fluid halogen signature during pre-Grenville subduction,
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underneath Graciosa. Alternatively, the pore fluid signature may be present within heterogeneous plume material, perhaps due to the subduction of halogen-rich marine sediments or marine pore fluids. Unlike observed at São Miguel, where the anomalously high halogen samples are restricted to the Nordeste volcanic centre in the eastern part of the island, the Graciosa samples with higher halogen ratios are located in different regions of Graciosa, suggesting that only some samples are sampling the heterogeneity. Further work would need to be completed to see if the variation can be resolved temporally.
5.6.7 The source of the enriched iodine component on Corvo and Flores
Corvo and Flores are the most unusual of the Azores islands, in that they are located on a different plate to the proposed mantle plume location, situated to the west of the
MAR on the Atlantic plate, with the remainder of the island group located to the east, on the Eurasian plate and the Azores micro-plate. The samples analysed here represent the first known samples from the Azores to be characterised for both their halogen content, and Pb and Os isotopes (Table 5.6).
Table 5.6 Halogen data (this project) and isotope data (Genske et al., 2016) from Corvo (C-09-XX) and Flores (FL-09-XX) samples.
Br/Cl I/Cl Sample 206Pb/204Pb 207Pb/204Pb 208Pb/204Pb 187Os/188Os (10-3) (10-6) C-09-01 0.82 ± 7.93 243 ± 3.01 19.7826 ± 0.0012 15.6268 ± 0.0013 39.4768 ± 0.0041 0.1284 ± 0.0005 C-09-06 1.68 ± 0.02 100 ± 1.80 19.6487 ± 0.0009 15.6168 ± 0.0008 39.3519 ± 0.0021 - C-09-13 1.52 ± 0.13 1480 ± 124 19.7436 ± 0.0010 15.6208 ± 0.0010 39.4174 ± 0.0030 - C-09-18 1.28 ± 0.02 75.2 ± 1.34 19.7662 ± 0.0009 15.6282 ± 0.0008 39.4472 ± 0.0022 0.1370 ± 0.0005 FL-09-23 1.99 ± 0.17 494 ± 49.1 19.5715 ± 0.0008 15.6124 ± 0.0007 39.2679 ± 0.0022 - FL-09-26 0.72 ± 7.72 76.4 ± 1.13 19.5486 ± 0.0016 15.6077 ± 0.0016 39.2281 ± 0.0044 - FL-09-32 1.40 ± 0.10 297 ± 3.64 19.6437 ± 0.0009 15.6240 ± 0.0007 39.3462 ± 0.0019 - FL-09-41 0.89 ± 0.83 0.54 ± 3.77 19.5265 ± 0.0007 15.6143 ± 0.0007 39.2490 ± 0.0021 - FL-09-42 0.84 ± 0.02 30.6 ± 3.80 19.5166 ± 0.0014 15.6132 ± 0.0017 39.2408 ± 0.0057 0.1292 ± 0.0002
When comparing the halogen variation measured in the Corvo and Flores samples, together with the Pb isotopes measured in the same samples, a correlation is not
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apparent (Table 5.6). Although the Corvo samples with the highest I/Cl ratios also show the highest 206Pb/204Pb (19.7436-19.7826), the lowest Corvo I/Cl ratio
((75.2±1.34)x10-6), still has a much higher 206Pb/204Pb ratio (19.7662±0.0009) than measured in the Flores sample with the highest I/Cl ratio (206Pb/204Pb =
19.5715±0.0008; I/Cl = (76.4±1.13)x10-6). It appears that on Corvo and Flores, there is no correlation between the halogen values observed and the measured ratios in
Pb and Os, suggesting that the halogens are decoupled from other isotope systems.
However, the range of 206Pb/204Pb falls indicates a mixing between HIMU and EM2, confirming that a mixture of mantle end-members is observed across the Azores archipelago.
Due to their location to the west of the MAR, and the differing age and thickness of the lithosphere here, it may be expected that these two islands would vary most from the central group. While they do indeed show increased I/Cl (and to a lesser extent, increased Br/Cl) ratios with respect to MORB, particularly on Corvo, the halogen ranges observed on the two western islands are not as great as that observed on the anomalous central island of Graciosa. Although trending to slightly higher I/Cl values, the highest samples from Corvo and Flores show approximately the same range as those observed on Graciosa. However, there is less of an overlap on the western islands with the MORB field, particularly on Corvo, when compared to Graciosa. A slightly larger sample set was analysed on Graciosa, and perhaps, more importantly, a larger spatial range of samples was obtained from Graciosa than from Corvo and
Flores – and this may account for less MORB-like samples being observed on Corvo and Flores.
Genske et al. (2016) suggest that the local variation observed on the western islands is due to a local low degree partial melt from an enriched source component auto-
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metasomatising the plume mantle underneath Corvo and Flores. This could explain the relative enrichment of the I with respect to Cl, as, due to their incompatibility, any low degree of melting would enrich the melt I>Br>Cl. However, perhaps a simpler explanation is that the basalts on Corvo and Flores are sampling the same heterogeneity observed on São Miguel and Graciosa – either a heterogeneity with the plume material itself, preserving a marine pore fluid halogen signature, or entrainment of delaminated SCLM, which has preserved a marine pore fluid-like signature and is incorporated into the Azores basalts as the plume material rises. It is therefore proposed that the enrichment observed on the western islands is similar to that observed on São Miguel and Graciosa; however as these four islands are situated on either side of the MAR and are not contemporaneous, the cause of the similar halogen signature across these islands must be investigated.
5.6.8 Halogen variation with age
Comparing the halogen ratios of the older islands of Graciosa, Terceira, São Miguel, and Flores (Figure 5.19), we can see that these islands yield Br/Cl and I/Cl values which overlap with both the MORB field, and the serpentinite and marine pore fluids field. The younger islands of Faial, Pico, and São Jorge, all lie within the MORB field, suggesting that there may be an age related component driving the differences in the halogen ratios between the islands. It may be expected that initial melts from the plume source would be more diverse, as incompatible elements, such as the halogens, would be released during lower degrees of partial melting, with later melts progressing to a more depleted, and MORB-like signature.
Both of the islands on the Atlantic Plate, Corvo and Flores, yield similar halogen ratios to one another, and indeed overlap with the range shown by the older islands on the Eurasian Plate. Geochemical ages for Corvo are rare, and the age of this
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island is the most disputed, ranging from 1.5-1.0 Ma (França et al., 2006a) for the onset of subaerial volcanism, to 0.71 Ma (Millet et al., 2009). If we assume the age proposed by França et al. (2006a) to be accurate, this would shift the age of Corvo to be intermediate between the older islands, and the younger islands – perhaps supporting that the composition of the plume derived melts has changed over time.
However, if the geochemical age proposed by Millet et al. (2009) is the more accurate age, this would place Corvo with the younger islands – meaning that the variation in halogen geochemistry seen between the islands cannot be explained solely due to temporal variation.
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Figure 5.19 Summarising the age, eruption history, 3He/4He, halogen ratios, and halogen signatures for the Azores island group (SERP = serpentinite, MPF = marine pore fluids) (Halogen data this study; helium data and ages from Azevedo and Portugal Ferreira, 2006; França et al., 2006a; Jean-Baptiste et al., 2009; Larrea et al., 2014; Machado et al., 2008; Madeira and Ribeiro, 1990; Millet et al., 2009; Moreira et al., 1999; Sibrant et al., 2015). 169
5.6.9 Halogen variation with location
The present day plume is thought to be located below either Terceira, Pico, or Faial
(Genske et al., 2016; Millet et al., 2009; Moreira et al., 1999). If any variation is observed laterally, we would expect to observe the greatest isotopic deviation from the plume signature to occur in those islands situated furthest from the plume source, i.e. the western islands of Corvo and Flores, and the eastern island of São
Miguel. São Miguel and the western islands of Corvo and Flores are located approximately equidistant from the proposed location of the present day plume, and therefore may be expected to show the most variation from the central group.
Initially, the results from Corvo, Flores, and eastern São Miguel may suggest that there is greater variation with distance from the plume location (Figure 5.19), however, the MORB-like signature within central and western São Miguel, coupled with the marine pore-fluid signature observed on Terceira and Graciosa, suggest that a simple lateral variation does not explain the halogen ranges observed within the
Azores basalts.
Looking at the halogen variation with respect to the ages of the individual islands does not clarify the observed halogen variation. The oldest island analysed, São
Miguel (4.01 Ma), shows significant west to east variation within the halogen ratios, echoing that observed in other isotopic systems, and highlighting the existence of the two proposed mantle end-members within the Azores, and the island of São Miguel itself (e.g. Moreira et al., 1999; Widom et al., 1997). Terceira (3.52 Ma) shows less variation, being MORB-like, with the exception of the three samples that show a
AOC-like component. Both Flores (2.76 Ma) and particularly Graciosa (2.5 Ma) show
I/Cl values extending beyond the MORB field and into the halogen marine pore fluid field. Whereas samples from Faial (0.73 Ma) lie within the values previously
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observed for MORB, much higher I/Cl values are observed from Corvo (0.71 Ma), with the Br/Cl values remaining MORB-like. Looking at the youngest islands of São
Jorge (0.55 Ma) and Pico (0.25 Ma), a return to MORB-like values is observed.
As such, there does not appear to be a correlation between the age of the islands, and the halogen ratios, ruling out a temporal variation in the Azores basalts; nor is there a strong lateral variation observed. This suggests that neither spatial nor temporal variation is the cause of the variation in halogen ratios observed within the
Azores archipelago.
If the islands are grouped into three groups, with Corvo and Flores, both situated to the west of the MAR in one group, the islands located on the Terceira Rift (Graciosa,
Terceira, and São Miguel) into another group, and the remaining islands, located to the south of the Terceira Rift (Faial, Pico, and São Jorge) into a third group, a pattern is observed in the halogen ratios in each of the groups. Both Corvo and Flores yield similar Br/Cl and I/Cl ratios, extending from the MORB field into the field observed for serpentinites. The islands located on the Terceira rift all exhibit a similar range of halogen ratios to one another, extending from the MORB field into the marine pore fluid field; the exception being a three samples from Terceira that extend to lower
Br/Cl and I/Cl values, overlapping with the field observed for altered oceanic crust.
Seismic evidence suggests that the source of the Azores plume at depth is located beneath Terceira, with the plume material shearing southwest as it rises to shallower depths in the upper mantle.
5.6.10 A tectonic control on volcanism and halogen variation The three islands that lie on the Terceira Rift, namely Graciosa, Terceira, and São
Miguel, all show halogen ratios which extend from the MORB field, into the
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serpentinite and marine pore fluid fluids, with some of the lower values from Terceira also extending down into the known field for altered oceanic crust. These three islands differ in their halogen range from the three central islands that are located off the ridge axis to the south. Faial, Pico, and São Jorge all yield values within the known range from MORB, with only one sample from Pico extending out of this field into the field for serpentinites and SCLM. These results could suggest a tectonic relationship between the halogen composition of the melts on the islands. If the present day plume head is located beneath the island of Terceira, the plume material could exploit the Terceira Rift system, using the rift as a conduit allowing flow of plume material to the other islands (Graciosa and São Miguel) which are also located along the Terceira Rift.
Lateral flow of plume material along the Terceira Rift may also explain why similar
I/Cl values are observed on Corvo and Flores, even though they lie to the west of the
MAR. The predominant rift systems on the islands to the east of the MAR are parallel to the Terceira Rift, whilst the predominant direction on the islands of Corvo and
Flores strike roughly N-S; that is, sub-parallel to the MAR. However, transform faults intersecting the MAR strike E-W, and extend west to the islands of Corvo and Flores.
These transform faults may allow the flow of plume material from the Terceira Rift system to the western islands, or the presence of contemporary lavas on either side on the MAR may be explained by the mantle plume material crossing the ridge (ridge jumps) (Sleep, 2002). Alternatively, there may be more than one domain of delaminated SCLM with an enriched halogen signature, as proposed in the model by
Moreira et al. (1999), with the SCLM being present within the mantle on either side of the MAR, as a result of seafloor spreading. This may be entrained as the melts from the plume migrate through the upper mantle underneath Corvo and Flores (Figure
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5.20). If the plume itself is enriched with regards to halogens, or has heterogeneous enriched domains, the high halogen ratios present on Corvo can be explained without the need to invoke a separate enriched domain in the upper mantle.
It must be considered that the source of the MORB component within the Azores basalts is MORB itself, rather than a depleted plume component. The Azores is situated on an active triple ridge spreading system, where there is decompression upwelling of MORB magma, producing new oceanic crust at the spreading ridges.
However, it may be expected that if MORB was the dominant component present in the Azores, that the islands located on the Terceira rift would show more MORB-like values, being situated on an active, albeit slow-moving spreading ridge. However, the opposite is in fact observed, with the islands on the Terceira Rift extending to higher Br/Cl and I/Cl values, whilst those islands of the central group situated to the south of the rift show the more MORB-like values. It is suggested that this variation is explained by the change in the composition of the plume source over time, with initial melts, represented by the samples from Graciosa, Terceira, São Miguel, Flores, and perhaps Corvo, representing the more enriched plume material, with later melts, observed on the younger intraplate islands of Faial, Pico, and São Jorge, representing the depleted plume signature. Conversely, if the present day plume location is indeed situated beneath the island of Terceira, MORB may be entrained during the lateral migration of plume material to the central islands located to the south of the Terceira Rift, with the MORB signature overprinting any enriched or primitive plume signature. Seismic imaging shows that the plume material is sheared south-westerly from Terceira to the southern central islands, in agreement with this hypothesis (Yang et al., 2006).
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Figure 5.20 A possible model of the halogen enrichment in the Azores modified after Moreira et al. (1999) and Genske et al. (2016). AOC and sediments/marine pore fluid signature are incorporated into the SCLM during pre-Grenville subduction. Subsequent delamination of SCLM preserves signature, and is entrained by rising plume material, with possible incorporation of marine pore fluid signature into the deeper mantle and plume source.
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5.6.11 The composition of the Azores plume
Seismic evidence suggests that the modern day location of the Azores plume is below the island of Terceira (Yang et al., 2006). Geochemical evidence supports this data, as Terceira has some of the highest 3He/4He observed on the Azores, together with a primitive Ne isotope signature (Madureira et al., 2005). As such, if we assume that the MORB signature of the central island group is representative of the source of the Azores basalts, we can therefore assume that the MORB-like signature observed in Faial, Pico, and São Jorge represents: a) that the Azores mantle plume is depleted in Br and I halogens with respect to Cl, or b) that a MORB-like signature is overprinting the plume signature. If the Azores mantle plume is indeed depleted with respect to the halogens, this suggests that the Br and I are depleted in the mantle source, perhaps due to long term mixing of the lower mantle and DMM, and the subduction of seawater into the mantle source. Alternatively, the MORB-like signature observed may be due to the entrainment and mixing of the primitive plume material with depleted MORB mantle, due to both the lateral migration of MORB from the MAR and Terceira Rift, and due to the migration of the plume material across the
MAR to the western islands. This MORB-like signature, found across the island group, is in agreement with the Azores end-member (D) observed by Millet et al.
(2009).
5.6.12 Can the Azores halogens be correlated to other isotope systems? Comparing the halogen data with data from other geochemical systems, it appears that the halogens are decoupled from other isotope systems, as there is no correlation in the halogens versus Sr-Nd-Pb isotope space; something which is also observed within the noble gases. The islands with the highest 206Pb/204Pb (>20),
Terceira, São Miguel (Nordeste), Pico, and São Jorge, exhibit different halogen
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signatures, with Pico and São Jorge having a MORB-like halogen signature, Terceira extending from AOC to marine pore fluids, and São Miguel from MORB to marine pore fluids. It does not seem possible to reconcile the halogens with Pb isotope systematics; there does not seem to be a correlation between Pb isotopes, 3He/4He, and the halogens. However, this may be expected, as the higher halogen ratios are sampling a marine pore fluid signature, which would not be expected to be rich in U,
Th, or Pb.
There may, however, be a correlation between the halogen ratios as Os isotopes.
Schaefer et al. (2002) observed that 187Os/188Os ratios vary symmetrically throughout the Azores, in a pattern proposed to be reflecting the mantle plume (Table 5.7). The highest 187Os/188Os ratios are seen on Flores (0.13298-0.14180) and São Miguel
(0.13414-0.14196), with Terceira and Pico extending upwards from subchondritic values (0.12558-0.13875 Terceira; 0.12012-0.13885 Pico), and Faial yielding the lowest values (0.11019-0.13058). This symmetry is similar to what is observed in the halogens – with São Miguel and Flores showing the highest values (in these five islands), Terceira intermediate, and Pico and Faial having MORB-like halogen signatures. These higher 187Os/188Os ratios indicate the presence of an enriched component, in agreement with the halogen data for these islands.
He isotopes show mixing between MORB and primitive values, providing evidence of a mixed source for the Azores basalts. Both the halogens and 3He/4He show variation on small (intra-island) length scales; a feature which is not unusual in a ridge setting, and is thought to represent mixing of MORB, a primitive mantle source, and recycled lithosphere and crustal material (Graham et al., 2014).
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Table 5.7 Halogen ranges measured for each island.
Island Location Group Br/Cl (10-3) I/Cl (10-6) 3He/4He 206Pb/204Pb 207Pb/204Pb 208Pb/204Pb 87Sr/86Sr 143Nd/144Nd 187Os/188Os (RA) 0.703202- 0.512781- 0.13413- São Miguel Terceira Rift Oriental 1.02-2.20 2.25-42.4 5.2-6.1 19.47-19.91 15.59-15.73 39.25-40.16 0.704470 0.512956 0.14196* Eastern São Oriental 0.704220- 0.512620- Terceira Rift 1.23-2.80 159-344 <5.1 20.00 15.75 40.33 - Miguel 0.704990 0.512774 0.703483- 0.512900- 0.12558- Terceira Terceira Rift Central 0.39-1.51 8.69-298 9–13.5 20.02 15.64 39.35 0.703565 0.512960 0.13875
Graciosa Terceira Rift Central 0.90-3.37 9.28-930 8.1-11.2 19.80 15.60 39.18 - - -
Azores 0.703770- 0.512768- 0.11019- Faial Central 0.93-1.10 7.26-74.3 7.2-8.5 19.13-19.71 15.54-15.64 38.75-39.31 micro-plate 0.704012 0.512927 0.13058 Azores 0.703399- 0.512831- São Jorge Central 1.03-1.05 20.4-57.4 - 19.34-20.51 15.63-15.97 39.05-39.56 - micro-plate 0.703638 0.512940 Azores 0.703540– 0.512813– 0.12012- Pico Central 0.78-1.91 1.36-49.0 7.5–10.3 18.12–20.44 15.29–15.72 37.44-39.79 micro-plate 0.703860 0.512928 0.13885 Atlantic 0.703268- 0.512920- 0.13298- Flores Occidental 0.72-1.99 0.54-494 8.04 19.55-19.73 15.61-15.63 39.23-39.53 Plate 0.703770 0.512948 0.14180 Atlantic 0.703372- 0.512920- Corvo Occidental 0.82-1.68 75.2-1480 - 19.57-19.78 15.61-15.63 39.26-39.48 - Plate 0.730432 0.512930 (additional isotope data from Elliott et al., 2007; Genske et al., 2014; Genske et al., 2012; Jean-Baptiste et al., 2009; Madureira et al., 2005; Moreira et al., 1999; Schaefer et al., 2002; Widom et al., 1997)
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5.6.13 Consequences for Azores basalts – a mix of HIMU and EM1/EM2?
The halogens identify three components present with the Azores basalts: 1) a common MORB-like component, observed across the archipelago, 2) an AOC component on Terceira, and 3) a marine pore fluid component present in the basalts of Corvo, Flores, Graciosa, and São Miguel. This common MORB-like component is also observed in EM1 and EM2 mantle end-members. As the halogens present in the Azores basalts indicate a range of values, trending from the AOC field, through those known from EM1 and EM2 basalts and MORB, into a marine pore fluid-like signature, it is proposed that the Azores basalts do not reflect a simple HIMU source.
If the MORB-like basalt halogen ratios present in OIBs from the Azores, Tristan da
Cunha, Pitcairn, and Society Islands (Kendrick et al., 2014b) are as a result of long term degassing and halogen depletion of the plume source mantle resulting in a
MORB-like halogen signature for OIB, and/or mixing with the DMM mantle, the mantle-end members would show similar halogen ratios. However, the proximal location of the Azores to the MAR may represent an anomaly within worldwide
HIMU OIBs – in that plume source material may be mixing with the MORB source, potentially overprinting any halogen signature present with the Azores plume itself.
Alternatively, it may be that the Azores plume is a HIMU-bearing plume, but due to the complex tectonic setting, with interaction between the ridges and MORB mantle, the HIMU halogen signature is being overprinted with a depleted mantle signature. Due to the complexity of the tectonic setting with a triple ridge system, delaminated enriched SCLM, and plume being present, it is difficult to resolve the halogen signature of the primitive plume material, in order to establish the halogen characteristics of the HIMU source. HIMU basalts are categorised by the presence of recycled oceanic crust and sediments, therefore the halogen AOC signature 178
present on Terceira may be best representative of the HIMU source present within the Azores plume.
It is not known if the halogens can be subducted deep into the mantle beyond the subduction zone environment, although studies have confirmed that they subducted to depths of at least 100km (Sumino et al., 2010). If it is assumed the mantle is depleted with respect to halogens, this would require the reintroduction of halogens to the source via subduction, in order to observe the distinct range of halogen signatures within the Azores basalts – and the range of sources, from
AOC to marine pore fluid, suggests multiple components have reintroduced halogens into the mantle source.
5.6.14 A heterogeneous source for the Azores basalts
Variation observed both intra-island and across the island group can be explained by invoking a homogenous plume source with local length-scale heterogeneities from entrainment of previously subducted materials or it may be that what is actually being sampled is a heterogeneous mantle plume with mixed components being sampled at plume source depth. It is possible that earlier melts from the plume were relatively more enriched in halogens. With time, the plume material may have become more depleted with respect to the halogens, explaining why the younger islands (excluding Corvo, where the ages are not well resolved) show a
MORB-like signature.
The halogen data, combined with previously published data from other isotope systems (e.g. Béguelin et al., 2017; Beier et al., 2007; Madureira et al., 2005;
Moreira et al., 1999; Moreira et al., 2012) support a heterogeneous plume model.
Béguelin et al. (2017) propose a three component model for the Azores group: 1) an ancient recycled oceanic crust component, represented by São Miguel and the
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João de Castro seamount, 2) an enriched end-member (TSJ), observed on
Terceira and São Jorge, with a component of recycled younger (<632 Ma) oceanic crust, and 3) a common Azores component, found across the archipelago. It is suggested that the multiple eruption centres sample different domains within a heterogeneous Azores plume (Béguelin et al., 2017).
The common Azores component proposed by Béguelin et al. (2017) is consistent with the MORB-like halogen component, which is present on the Western, Central, and Eastern Islands. In addition, the proposed TSJ AOC component (Béguelin et al., 2017), based on Pb and Hf isotopes, is also indicated by the halogens, best represented by the lower Br/Cl halogen ratios observed on Terceira. However, the two proposed models (Béguelin et al., 2017; this study) differ when the third component, ancient AOC, is considered – as this component is not required to explain the halogen ratios in the São Miguel basalts.
Although the presence of ancient AOC on São Miguel cannot be ruled out on the basis of the halogen data, the addition of AOC to the Azores common component, would have the effect of reducing the Br/Cl and I/Cl values, giving rise to an array of halogen ratios between AOC and MORB-like values. However, the third component present within the halogens, on São Miguel and also on Corvo, Flores, and Graciosa, increases the Br and I to higher values, thus supporting a marine pore fluid halogen component – an additional component which is not observed in the Hf-Nd-Pb isotopes. The marine pore fluid halogen component would not be tracked by Hf and Pb isotopes, therefore the halogens provide us with unique insight into the subduction and recycling of a marine pore fluid signature.
It is therefore proposed that the halogens, together with other isotope systems, support a heterogeneous plume model, with distinct domains being sampled in the
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central islands (AOC), and across the island group (a common, MORB-like component). The enriched marine pore fluid component, although not present on every island, is present across the island group, particularly on the Western
Islands (Corvo and Flores), but also on the central island of Graciosa, and in eastern São Miguel. This distribution cannot be explained if each of the islands is sampling a distinct domain within a heterogeneous mantle plume, and therefore it is considered more likely that in addition to the plume heterogeneity, the islands are also sampling halogen enriched domains in the mantle, proposed to be delaminated SCLM, which has preserved a marine pore fluid halogen signature.
5.7 Conclusions
It is proposed that there are three components present within the
halogens of Azores group, represented by a MORB-like halogen
signature, an AOC halogen signature, and a marine pore fluid signature.
The MORB-like composition observed across the island group, but
particularly in the central islands of Faial, Pico, and São Jorge is
proposed to be due to either the Azores mantle plume being depleted
with respect to the halogens, or due to the entrainment and mixing of
MORB with the plume material, as it ascends through the mantle and
migrates laterally.
The marine pore fluid signature component observed in the eastern São
Miguel basalts is suggested to be due to the recycling of sediments into
the mantle, either through delaminated SCLM below the island resulting
in a local length-scale heterogeneity.
This sediment or marine pore fluid component is also proposed to be the
cause of the halogen variation present on Graciosa, Terceira, Corvo,
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and Flores, suggesting the presence of several enriched SCLM halogen
domains in the mantle underlying the Azores group.
The depleted signature seen in three Terceira samples may be due to
the proposed subduction of AOC, representing a heterogeneous plume
source, which has remained distinct and unmixed over time.
The elevated halogen ratios observed on the western islands of Corvo
and Flores is proposed to be related to the migration of plume material
either along the Terceira Rift and MAR rift systems, with the plume
material sampling previously subducted I-rich sediments or marine pore
fluids, or due to entrainment locally of delaminated SCLM with a pore
fluid signature.
It is proposed that the variation seen across the Azores archipelago is
due to the different eruption centres sampling different domains in the
heterogeneous plume, together with the incorporation of halogen
enriched delaminated SCLM, present in the mantle beneath the Azores.
Although other geochemical systems support that the Azores
represents, at least in part, a HIMU mantle end-member, it is observed
that the halogen signature within the Azores plume is indistinguishable
from EM1 and EM2, perhaps due to interaction of plume material with
the complex tectonic setting of the Azores plateau at a triple-ridge
setting, and heterogeneity within the plume material itself. The Azores
basalts sample more than one end-member, with HIMU mixing with EM1
and EM2, as supported by other isotope systems.
The variation in halogen ratios observed in the Azores is similar to that
observed in the Inaccessible Island basalts, indicating that the Azores
HIMU end-member is not observed in the halogens, or that the Azores
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plume is a mixture of HIMU and EM1/2 end-members, together with
MORB.
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6 Sedimentary and Altered Oceanic Crust Iodine Signatures in the Canary Islands Basalts
6.1 Introduction
6.1.1 Geological Setting of the Canary Islands
The Canary Islands archipelago is comprised of seven large volcanic islands (with several smaller islands), Fuerteventura, Lanzarote, Gran Canaria, Tenerife, La
Gomera, La Palma, and El Hierro. The islands are located in the Atlantic Ocean,
100km west of the African continental shelf (Figure 6.1) (Carracedo et al., 1998).
The islands span a distance of approximately 500km from the western to the eastern islands (Anguita and Hernán, 2000; Geyer and Martí, 2010).
Figure 6.1 The location of the Canary Island group, located off the west coast of Africa in the Atlantic ocean (redrawn and modified from Carracedo and Day, 2002). The Canaries are situated on Jurassic oceanic crust on the African Plate, a slow moving plate thought to be moving at rates of up to 20mm a-1 (Carracedo et al.,
184
2001b). It is thought that the rate of plate movement has decreased significantly over the last 31 Ma (O'Connor and le Roex, 1992).
6.1.2 Origin of the Canaries
The origin of the Canary Islands has been highly debated in the past, partially due to earlier (1990s) dating of the islands exhibiting a lack of linear age progression within the island group (Anguita and Hernán, 2000; Carracedo et al., 1998; Patriat and Labails, 2006). Most authors believe that the volcanism of the island group is due to a mantle plume or hotspot. Early dating of the Canary Islands did not support a linear hotspot track, however, more recent dating by Paris et al. (2005) provides a younger age for La Gomera, which together with ages from Gran
Canaria, Tenerife, El Hierro, and La Palma by Guillou et al. (2001; 2004a; 1996;
2004b), provide a much better fit to the classic linear plume model.
It is thought that activity first occurred around 68 Ma at Lars/Essaouira seamount
(Troll and Carracedo, 2016a). Sr-Nd-Pb isotopes suggest the source of the
Canaries volcanism is a mix of HIMU, EM and DM, with some authors also reporting a PHEM source (discussed below) (Anguita and Hernán, 2000).
6.1.3 Ages of Volcanism
Extensive K/Ar dating has been completed on the rocks present on the Canary
Islands, giving accurate ages for the onset of subaerial volcanism on each of the islands (Carracedo et al., 1998). As of 1998, over 450 K-Ar dates had been published, constraining the ages of initial subaerial volcanism: Fuerteventura 22.0
Ma, Lanzarote 15.5 Ma, Gran Canaria 14.5 Ma, Tenerife 12.0 Ma, La Gomera 9.4
Ma, La Palma 2.0 Ma and El Hierro 1.2 Ma (Figure 6.2) (Carracedo et al., 1998;
Carracedo and Day, 2002; Paris et al., 2005).
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Figure 6.2 Map of the Canary Islands, with the islands analysed as part of this research highlighted in black. Work by Carracedo et al. (2001b) has shown that the islands show a linear age (Ma) progression, typical of ocean island groups (redrawn and modified from Carracedo et al., 2001b; Carracedo and Day, 2002; Guillou et al., 2004a; additional age data from Paris et al., 2005). Eruptive gaps occur in the geological histories on Fuerteventura, Lanzarote, and
Gran Canaria during the Middle-Lower Miocene (Carracedo et al., 1998).
However, volcanic activity has continued uninterrupted since subaerial emergence on the younger islands of Tenerife, La Gomera, La Palma and El Hierro
(Carracedo et al., 1998; Paris et al., 2005). However, there is a notable gap between the emergence of La Gomera, and the two youngest islands of La Palma and El Hierro, which are still in their shield stages (Carracedo et al., 2001b).
The evolution of the emerged islands can be divided into three main stages: the shield stage; the gap stage; and the post-erosional rejuvenated volcanism stage
(Carracedo et al., 1998). The stage prior to emergence is often referred to as the pre-shield or seamount stage (Carracedo et al., 1998). It should be noted, however, that this terminology is not universal, and other authors use the term basal complexes for the pre-shield stage, shield constructs for the shield stage and post shield cones for the rejuvenated stage (Anguita and Hernán, 2000).
186
The islands of Tenerife, La Palma, and El Hierro are currently in their pre-gap shield stage, with Tenerife probably approaching its gap stage; whilst La Palma and El Hierro are thought to be in the most active phase of shield stage volcanism
(Carracedo et al., 1998). La Gomera is currently in its gap stage, and the remaining islands of Fuerteventura, Lanzarote and Gran Canaria in their post- erosional rejuvenated stage of volcanic activity (Carracedo et al., 1998). It is thought that each stage represents a new batch of magma rising from the mantle
(Anguita and Hernán, 2000).
Magma composition is varied on the Canaries (from basanite to tholeiitic basalts, with some felsic volcanism on Gran Canaria and Tenerife), with distinct differences in magma types during each stage of volcanism (Carracedo et al., 1998). Overall basaltic magmatism is dominant (Carracedo et al., 1998); during the shield and rejuvenated stages, volcanism is predominantly basaltic (picrites, tholeiites and basanites) with associated differentiated lavas (phonolite and trachytes)
(Carracedo et al., 1998).
6.1.4 Geochemistry of the Canaries
As only the western Canaries (El Hierro and La Palma) and Tenerife are studied as part of this project, only these islands are discussed in detail.
The Canaries are thought to have a HIMU mantle end-member component present, but there is also evidence for mixing between HIMU and enriched mantle together with depleted mantle (Day and Hilton, 2011; Day et al., 2010; Hilton et al.,
2000; Simonsen et al., 2000). The HIMU end-member observed in the Canary
Islands is proposed to be due to the subduction and recycling of oceanic crust and gabbroic lithosphere (Day and Hilton, 2011; Day et al., 2010).
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Table 6.1 Radiogenic isotope data from Tenerife, La Palma, and El Hierro.
3He/4He 206 204 207 204 208 204 87 86 143 144 Island Pb/ Pb Pb/ Pb Pb/ Pb Sr/ Sr Nd/ Nd (RA) 15.517- 39.331- 0.7028– 0.51277– Tenerife 0.88-6.91 19.3-20.1 15.640 39.767 0.7032 0.51296 19.527- 15.592- 39.277- 0.703057- 0.512894- La Palma 4.15-8.80 20.152 15.659 40.014 0.703964 0.512966 19.109- 15.531– 38.217- 0.702871– 0.512890– El Hierro 0.80-9.72 19.848 15.634 39.598 0.703144 0.513003 (data compiled from Day and Hilton, 2011; Hilton et al., 2011; Hilton et al., 2000; Mouatt, 2006; Simonsen et al., 2000). 6.1.5 Tenerife (12.0 Ma)
The island of Tenerife is the largest and highest island in the Canaries, comprising: 1) the Anaga shield volcano to the northeast, 2) the Teno shield volcano to the west, 3) the Caldera de Las Cañadas to the centre of the island, 4) with Teide nested within it, together with 5) outcrops of the Central shield volcano to the north and south of the island (Troll and Carracedo, 2016d). The summit of
Pico del Teide, a stratovolcano in its shield or post-erosional stage, rises to over
3700m above sea level (Carracedo et al., 2001b; Carracedo et al., 1998). The most recent eruption occurred at Chinyero in 1909 (Figure 6.3) (Carracedo, 1994).
Tenerife exhibits a moderate-high 143Nd/144Nd range of 0.51277–0.51296 (n=59),
HIMU-like 87Sr/86Sr (0.7028–0.7032, n=60), high 208Pb/204Pb = 39.331-39.767
(n=44), 207Pb/204Pb = 15.517-15.640 (n=44), radiogenic 206Pb/204Pb (19.3-20.1, n=44) (Simonsen et al., 2000). 3He/4He shows a radiogenic to MORB-like range
(0.88-6.93, n=16) (Mouatt, 2006). The HIMU end-member, which is present alongside EM1-type material, is proposed to be due to the recycling and long-term storage of ancient oceanic crust (Simonsen et al., 2000).
188
Figure 6.3 Historical eruptions on Tenerife, La Palma, and El Hierro (modified and redrawn from Carracedo, 1994; Carracedo and Day, 2002; Carracedo et al. 2012; Troll and Carracedo, 2016d; with additional data from Carracedo et al., 2007; Carracedo et al., 2012). 6.1.6 La Palma (2.0 Ma)
The island of La Palma is in its shield stage, and consists of three main volcanic units: 1) the basal complex, 2) the older volcanic series, and 3) the Cumbre Vieja series (Carracedo et al., 2001b; Klügel et al., 2000). Seven historical eruptions have occurred on La Palma, with the most recent eruption in 1971 (Figure 6.3) from the Teneguía vent, at the southern tip of the island (Carracedo, 1994). The island shape is elongated N-S and thought to be controlled by rifting, with the
Taburiente volcano to the north, and the Cumbre Vieja volcano to the south
(Carracedo et al., 2001b; Troll and Carracedo, 2016c). The island exhibits a north-
189
south linear age progression, with the youngest volcanism to the south of the island (Troll and Carracedo, 2016c).
It is proposed by Day and Hilton (2011) that there is a recycled gabbroic lithosphere component present within samples from La Palma. Samples from La
Palma show moderately high 143Nd/144Nd (0.512894-0.512966), HIMU-like
87Sr/86Sr (0.703057-0.703964), high 208Pb/204Pb = 39.277-40.014 (n=6),
207Pb/204Pb = 15.592-15.659 (n=6), and high 206Pb/204Pb = 19.527-20.152 (n=6)
(Day et al., 2010; Mouatt, 2006). 3He/4He shows a radiogenic to MORB-like range from 4.15-8.19 RA (n=43) (Day and Hilton, 2011; Mouatt, 2006).
6.1.7 El Hierro (1.2 Ma)
El Hierro is the youngest of the Canary Islands, with the onset of subaerial volcanism from ~1.2 Ma (Carracedo et al., 2012; Guillou et al., 1996). Similar to La
Palma, El Hierro is in its shield stage (Carracedo et al., 2001b). There are three main volcanic units on El Hierro: 1) The Tiñor volcano (with a remnant remaining on the NE of the island), representing the first subaerial volcanism on El Hierro, 2)
The El Golfo volcano, to the NW of the island, and 3) the youngest unit, The Rift volcanism, related to a three arm rift system (Carracedo et al., 2001a; Troll and
Carracedo, 2016b).
The latest eruption occurred off the coast of El Hierro in October 2011 (Figure
6.3), prior to which there had been no activity on the island since 1793, when an eruption occurred at the Volcan de Lomo Negro vent to the west of the island
(Carracedo et al., 2001b; Carracedo et al., 2012). The embayed shape of the island is due to the collapse of the Tiñor and El Golfo volcanoes, and giant landslides related to rifting (Troll and Carracedo, 2016b).
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Samples analysed from El Hierro by Day et al. (2010) and Mouatt (2006) show moderately high 143Nd/144Nd (0.512890–0.513003, n=24), HIMU-like 87Sr/86Sr
(0.702871–0.703144, n=21), moderately high 208Pb/204Pb = 38.727-39.348 (n=24),
207Pb/204Pb = 15.531–15.617 (n=24), and moderately high 206Pb/204Pb = 19.109-
3 4 19.666 (n=24). He/ He shows a radiogenic range from 0.80-9.72 RA (n=39), trending to higher than MORB values (Day and Hilton, 2011; Hilton et al., 2000;
Mouatt, 2006). There is evidence for the recycling of basaltic oceanic crust present within the source for the El Hierro basalts (Day and Hilton, 2011).
6.2 Aims
The aims of the study were: 1) to establish the halogen ratios (Br/Cl and I/Cl) within the Canary Island basalts, 2) to determine if there is any systematic variation in the halogens between the islands, 3) to determine the source components of the halogens within the basalts, 4) to establish the first halogen data for HIMU OIBs, alongside the Azores samples (Chapter 5). A total of 19 samples were analysed for their halogen content (Cl, Br, and I): Four from
Tenerife, five from La Palma, and nine from El Hierro.
6.3 Sample Selection
Samples were selected from a previously collected sample suite. Samples were selected based on the presence of olivine or clinopyroxene phenocryst phases, sampling three different islands, giving a broad coverage of the islands to analyse inter-island variation, and where possible, to select samples from a range of locations and volcanic centres across each island. These samples were collected for helium analyses (Table 6.2), and were briefly described by Dr Paul Harrop
(Table 6.3). The phenocryst phases were previously separated prior to this project, with no hand specimen samples remaining, or thin sections available.
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Table 6.2 Sub-set of samples analysed in this study, previously analysed and described by Dr Paul Harrop (unpublished data).
Sample 3He/4He 40Ar/36Ar
97LP8 7.11 ± 0.28 -
97LP9 7.18 ± 0.18 -
97LP10 8.83 ± 1.03 304 ± 0.9
98EH1 7.01 ± 0.30 -
98EH2 7.51 ± 0.27 -
98EH10 7.81 ± 0.49 -
Four samples were selected from two different locations on Tenerife, including
Teide, five samples were selected from southern La Palma (Cumbre Vieja volcano), and ten samples from a spread of locations on El Hierro (Figure 6.4).
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Figure 6.4 Sample locations from Tenerife, La Palma, and El Hierro (redrawn from Carracedo and Day, 2002; Carracedo et al., 2012; Troll and Carracedo, 2016d).
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Table 6.3 Sample locations and descriptions (Fsp = feldspar, Ol = olivine, Plag = plagioclase, Cpx = clinopyroxene). Rock types described by Dr Paul Harrop (unpublished data).
Phases Sample Island Location Grid Reference Rock Type Phenocrysts Analysed
97TF39 Tenerife Mña del Estrecho, near summit 326800 3131000 Plag basanite? - pahoehoe Fsp, Ol Ol
97TF40 Tenerife Mña del Centeno, 10m from summit 326700 3130400 Olivine-basalt (aa) Ol, Cpx, Fsp Ol
97TF41 Tenerife Mña del Centeno, 10m from summit 326700 3130400 Olivine-basalt (aa) Ol, Cpx, Fsp Ol
97TF54 Tenerife N of Mña Mosnaza 347000 3130700 Tephro phonolite? - aa block Plag, Cpx Cpx 700m along Barquita Track - La Birigoyo channel levee (furthest 97LP7 Palma south channel) 221750 3168450 Olivine-basalt (aa) Ol, Cpx Ol, Cpx La Punta Salemera - 200m to south of 97LP8 Palma quarry 229900 3163475 Olivine-basalt (aa) Ol, Cpx Cpx La 5m south of edge of quarry, top of 97LP9 Palma quarry 229850 3163375 Olivine-basalt (aa) Cpx, Ol Ol, Cpx La 3rd bend up Pirs Road, ridge above 97LP10 Palma (track off to right) 219625 3162225 Phonolite Fsp, Cpx, Ol Ol La Mña Cabrera, 40m above road, just 97LP12 Palma north of Goat Farm 219875 3159775 Olivine-basalt (aa) Fsp, Cpx, Ol Ol, Cpx
98EH1 El Hierro 1793? Flow 189800 3074200 Olivine-basalt (pahoehoe) Ol, Cpx Ol
98EH2 El Hierro 1793 flow 189800 3074200 Olivine-basalt (pahoehoe) Ol, Cpx Ol 194
Phases Sample Island Location Grid Reference Rock Type Phenocrysts Analysed
98EH3 El Hierro Mña de las Calcocas, near summit 190100 3070400 Olivine-basalt Ol, Cpx Ol, Cpx
98EH4 El Hierro Mña de las Calcocas, near summit 190100 3070400 Olivine-basalt Ol, Cpx Ol, Cpx
98EH5 El Hierro Flow above Mña de Orchilla 190500 3069200 Olivine-basalt (pahoehoe) Ol, Cpx Ol
98EH7 El Hierro - 191800 3075100 Olivine-basalt (aa) Cpx, Ol Ol, Cpx La Restinga Ravas from Mña de el 98EH8 El Hierro Julan 205100 3063900 Olivine-basalt (pahoehoe) Ol Ol Mña de La Empalizada, on track 98EH1 heading up hill - channel to E of 0 El Hierro track 201700 3065200 Olivine-basalt (aa) Ol, Cpx Cpx 98EH1 Aa flow 10m NW of road above 2 El Hierro Guanche Caves/lava tubes 204500 3076700 Olivine-basalt (aa) Ol, Cpx Ol, Cpx 98EH1 6 El Hierro Tamadusa, La Cancela flow 214900 3081300 Olivine-basalt (aa) Cpx, Ol Ol
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6.4 Experimental Methods
The samples were prepared, irradiated, extracted, and analysed as per section
3.2. The samples and monitors were irradiated at the Petten reactor, Netherlands.
Post-irradiation, the samples were crushed (one step of 20 crushes) and step heated in 2-3 steps (600-1,600°C) in vacuum to release their trapped gases into the MS1 mass spectrometer. After data reduction, blank-corrected data were compared to the uncorrected data – the results were within error of one another, and therefore blank corrections were not necessary and not applied. However, an initial correction for air was applied during data reduction.
6.5 Results
The halogen releases observed from crushing in the Canary Islands (Table 6.4) basalts (Cl 0.0-0.9ppm, Br 0.1-3.4ppb, I 0.0-20.1ppb), are much larger when compared to the Tristan da Cunha (Table 4.5) and the Azores (Table 5.4) basalts.
The Tristan da Cunha basalts show a much lower halogen release (0.1-2.3ppm Cl,
1.1-15.7ppb Br, and 0.1-3.9ppb I), together with the Azores basalts (0.0-2.7ppm
Cl, 0.1-6.6pb Br, and 0.0-0.5ppb I). With respect to I, the Canaries samples released over five times more halogens than the maximum observed in the Tristan samples, and over 400 times than observed in the Azores. Although not as extreme in step heating, the Canaries samples also released more halogens (Cl
0.5-83.2ppm, Br 1.2-152.1ppb, I 0.4-56.7ppb) than the Tristan da Cunha samples
(Cl 0.5-1ppm, Br 3.7-70.6ppb, I 1.2-37.4ppb), and more Cl and I than the Azores samples (Cl 0.2-81.7ppm, Br 0.3-180.9ppb, I 0.0-6.6ppb).
Unlike the Tristan and the Azores samples, the Canary Islands samples from La
Palma and El Hierro consistently yielded 1-2 orders of magnitude higher I/Cl in the crushing releases (Table 6.4), when compared to the step heating releases. Due to the analyses revealing significant differences between the crushing and step 196
heating release, together with up to 16% of the halogens released during crushing, the data are therefore reported separately for the crushing and heating steps
(Figure 6.5 and Figure 6.6). Integrated results are shown for reference in Table
6.4, together with the crushing and total heating releases.
6.5.1 Halogen releases from crushing
Crushing release of halogens from Canaries basalts give I/Cl between (154-
18700)x10-6 I/Cl, with a more restricted range in Br/Cl from (0.32-2.00)x10-3 (Table
6.4). The smallest ranges are observed on Tenerife, with (154-2750)x10-6 I/Cl and
Br/Cl (0.32-1.90)x10-3. The lowest I/Cl is observed on El Hierro, which ranges from
(64.3-2870)x10-6 I/Cl, and (0.41-1.69)x10-3 Br/Cl. The highest ratios for both Br/Cl and I/Cl are observed in the samples from La Palma, which show a range of (219-
18700)x10-6 I/Cl and (0.74-2.00)x10-3 Br/Cl.
6.5.2 Halogen releases from step heating
The step heating release range of (12.7-4440)x10-6 I/Cl, is an order of magnitude lower than observed in the crushing releases, but shows a similar range in Br/Cl from (0.14-1.73)x10-3 (Table 6.4). The Tenerife samples show similar ratios to the crushing releases, with (22.2-4440)x10-6 I/Cl and Br/Cl (0.14-1.73)x10-3 observed.
The lowest I/Cl is again recorded from El Hierro, ranging from (12.7-202)x10-6 I/Cl, and (0.84-1.08)x10-3 Br/Cl. The highest ratios for both Br/Cl and I/Cl are observed in the samples from La Palma, which show a range of (49.7-917)x10-3 I/Cl and
(0.55-1.00)x10-3 Br/Cl.
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Figure 6.5 Halogen release during crushing and step heating for the olivine separates (a = Cl ppm, b = Br ppb, c = I ppb, d = K ppm). Error bars, where not seen, are smaller than bar lines. 198
Figure 6.6 Halogen release from the clinopyroxene separates during crushing and step heating (a = Cl ppm, b = Br ppb, c = I ppb, d = K ppm). Error bars, where not seen, are smaller than bar lines. 199
Table 6.4 Halogen, K, and 40Ar/36Ar results from irradiated samples (for individual steps, see Appendix 8.3).
Temp°C / Br/Cl I/Cl Sample Phase Analysis Cl ppm ± Cl ppm Br ppb ± Br ppb I ppb ± I ppb K ppm ± K ppm K/Cl (molar) ± K/Cl 40Ar/36Ar ± 40Ar/36Ar Crushes (x10-3molar) (x10-6molar)
Crush 20 0.04 0.01 0.15 0.01 0.04 0.01 1.10 0.21 1.47 ± 0.37 242 ± 66.0 22.6 6.8 284 6
97TF39 Ol Heat Total 0.45 0.18 1.20 0.04 0.45 0.02 47.04 1.11 1.19 ± 0.48 280 ± 113 95.4 38.3 295 3
Crush + Heat Total 0.55 0.18 1.53 0.05 0.54 0.02 49.54 1.21 1.24 ± 0.41 273 ± 90.8 82.0 27.1 293 2
Crush 20 0.08 0.02 0.17 0.02 0.08 0.01 1.45 0.12 0.91 ± 0.26 264 ± 73.8 15.7 4.5 297 12
97TF40 Ol Heat Total 1.06 0.05 2.42 0.04 0.82 0.03 61.69 1.27 0.91 ± 0.05 203 ± 12.2 52.4 2.7 304 5
Crush + Heat Total 1.27 0.13 2.92 0.06 1.06 0.03 65.95 1.32 1.02 ± 0.10 232 ± 24.1 47.2 4.8 301 6
Crush 20 0.04 0.01 0.18 0.01 0.41 0.01 0.81 0.19 1.90 ± 0.42 2750 ± 599 17.8 5.6 256 10
97TF41 Ol Heat Total 1.03 0.06 4.04 0.10 16.68 0.76 76.23 1.35 1.73 ± 0.11 4440 ± 341 67.4 4.3 296 6
Crush + Heat Total 1.04 0.10 4.76 0.10 18.34 0.76 78.46 2.26 2.03 ± 0.19 4930 ± 501 68.5 6.7 290 5
Crush 20 0.34 0.01 0.24 0.03 0.19 0.02 5.36 0.21 0.32 ± 0.04 154 ± 15.4 14.5 0.7 253 18
97TF54 Cpx Heat Total 35.78 0.28 11.21 0.14 2.84 0.12 316.71 2.85 0.14 ± 0.00 22.2 ± 0.97 8.0 0.1 294 5
Crush + Heat Total 36.63 0.28 11.80 0.16 3.31 0.13 330.29 2.89 0.14 ± 0.00 25.2 ± 1.01 8.2 0.1 285 6
Crush 20 0.11 0.03 0.19 0.02 0.09 0.01 nd nd 0.75 ± 0.23 219 ± 66.0 nd nd 318 3
Ol Heat Total 4.10 0.11 9.06 0.04 0.99 0.04 80.78 1.74 0.98 ± 0.03 67.8 ± 3.38 17.9 0.6 296 6
Crush + Heat Total 4.42 0.15 9.58 0.06 1.24 0.04 78.92 2.21 0.96 ± 0.03 78.5 ± 3.85 17.9 0.6 313 3 97LP7 Crush 20 0.31 0.02 1.40 0.01 0.37 0.02 2.48 0.08 2.00 ± 0.13 333 ± 25.1 7.2 0.5 442 20 Cpx Heat Total 15.17 0.12 31.35 0.26 2.70 0.09 329.41 1.63 0.92 ± 0.01 49.7 ± 1.64 19.7 0.2 296 1
Crush + Heat Total 16.07 0.14 35.42 0.26 3.78 0.10 336.63 1.65 0.98 ± 0.01 65.7 ± 1.77 19.0 0.2 315 2
Crush 20 0.30 0.03 0.74 0.03 20.09 1.39 1.36 0.09 1.09 ± 0.12 18700 ± 2336 4.1 0.5 464 3 Cpx 97LP8> Heat Total 18.60 0.16 41.47 0.40 56.69 1.17 385.23 2.45 1.00 ± 0.01 852 ± 19.0 18.8 0.2 324 7
Crush + Heat Total 19.27 0.17 43.12 0.41 101.65 3.32 388.27 2.45 0.99 ± 0.01 1470 ± 49.9 18.3 0.2 404 4
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Temp°C / Br/Cl I/Cl Sample Phase Analysis Cl ppm ± Cl ppm Br ppb ± Br ppb I ppb ± I ppb K ppm ± K ppm K/Cl (molar) ± K/Cl 40Ar/36Ar ± 40Ar/36Ar Crushes (x10-3molar) (x10-6molar)
Crush 20 0.73 0.01 1.85 0.02 6.29 0.14 5.86 0.27 1.12 ± 0.02 2400 ± 66.6 7.3 0.3 321 3 Cpx 97LP8< Heat Total 83.24 0.54 152.13 0.74 44.05 1.22 1193.52 5.45 0.81 ± 0.01 148 ± 4.20 13.0 0.1 310 7
Crush + Heat Total 85.37 0.54 157.50 0.74 62.29 1.29 1210.53 5.51 0.82 ± 0.01 204 ± 4.41 12.9 0.1 316 3
Crush 20 0.10 0.01 0.17 0.02 0.60 0.02 1.43 0.10 0.74 ± 0.13 1630 ± 190 12.6 1.6 325 6
Ol Heat Total 4.88 0.05 10.17 0.06 3.91 0.11 105.24 0.69 0.93 ± 0.01 224 ± 6.84 19.6 0.2 303 2
Crush + Heat Total 5.18 0.06 10.68 0.09 5.67 0.13 109.44 0.75 0.92 ± 0.01 306 ± 8.04 19.2 0.3 308 2 97LP9 Crush 20 0.21 0.02 0.66 0.01 3.57 0.08 1.23 0.22 1.40 ± 0.11 4800 ± 366 5.4 1.0 367 3 Cpx Heat Total 20.96 0.16 46.43 0.38 28.12 2.01 445.91 2.09 0.99 ± 0.01 375 ± 27.0 19.4 0.2 316 9
Crush + Heat Total 21.57 0.17 48.28 0.38 38.55 2.02 449.11 2.88 1.00 ± 0.01 500 ± 26.5 18.9 0.2 352 4
Crush 20 0.16 0.03 0.42 0.10 0.35 0.02 1.15 0.34 1.15 ± 0.33 596 ± 107 6.5 2.2 280 12
97LP10 Ol Heat Total 3.65 0.12 4.49 0.04 4.37 0.14 83.07 4.44 0.55 ± 0.02 335 ± 15.8 20.7 1.3 298 10
Crush + Heat Total 3.88 0.13 5.11 0.15 4.87 0.15 84.76 4.46 0.58 ± 0.03 351 ± 15.8 19.8 1.2 295 8
Crush 20 0.09 0.02 0.16 0.00 2.35 0.60 0.37 0.15 0.81 ± 0.23 7400 ± 2782 3.8 1.9 720 29
Ol Heat Total 2.52 0.11 5.64 0.09 8.47 0.38 51.16 1.30 0.98 ± 0.05 917 ± 58.0 18.5 0.9 323 9
Crush + Heat Total 2.71 0.13 6.04 0.09 14.15 1.50 50.14 1.41 0.99 ± 0.05 1460 ± 169 16.8 0.9 442 10 97LP12 Crush 20 0.29 0.02 0.68 0.02 3.22 0.82 1.36 0.22 1.04 ± 0.09 3100 ± 829 4.3 0.8 338 4 Cpx Heat Total 26.12 0.26 48.92 0.19 21.99 0.56 377.59 2.69 0.83 ± 0.01 236 ± 6.38 13.1 0.2 300 13
Crush + Heat Total 26.87 0.27 50.67 0.20 30.30 2.21 381.11 2.75 0.84 ± 0.01 315 ± 23.2 12.9 0.2 322 6
Crush 20 0.08 0.02 0.12 0.02 0.09 0.02 0.74 0.13 0.66 ± 0.21 316 ± 121 8.0 2.7 296 2
98EH1 Ol Heat Total 3.51 0.06 8.63 0.11 0.38 0.03 77.33 1.08 1.08 ± 0.02 26.2 ± 2.61 19.2 0.4 292 9
Crush + Heat Total 3.67 0.10 8.94 0.12 0.62 0.07 79.21 1.13 1.08 ± 0.03 47.5 ± 5.46 19.6 0.6 295 3
98EH2 Ol Crush 20 0.06 0.01 0.12 0.01 0.64 0.02 1.01 0.09 0.82 ± 0.10 2870 ± 337 14.5 2.1 480 22
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Temp°C / Br/Cl I/Cl Sample Phase Analysis Cl ppm ± Cl ppm Br ppb ± Br ppb I ppb ± I ppb K ppm ± K ppm K/Cl (molar) ± K/Cl 40Ar/36Ar ± 40Ar/36Ar Crushes (x10-3molar) (x10-6molar)
Heat Total 3.09 0.07 7.56 0.17 0.78 0.03 60.61 1.61 1.05 ± 0.03 65.9 ± 2.87 16.9 0.5 306 7
Crush + Heat Total 3.11 0.12 7.89 0.17 2.59 0.05 63.44 1.63 1.13 ± 0.05 233 ± 10.3 18.5 0.9 338 7
Crush 20 0.06 0.01 0.22 0.02 0.13 0.02 0.70 0.19 1.53 ± 0.29 561 ± 118 10.2 3.2 914 33
Ol Heat Total 4.95 0.11 11.72 0.02 0.82 0.03 103.54 2.57 1.05 ± 0.02 46.1 ± 2.02 19.0 0.6 340 16
Crush + Heat Total 5.07 0.11 12.13 0.04 1.06 0.04 104.88 2.60 1.06 ± 0.02 58.3 ± 2.72 18.8 0.6 573 18 98EH3 Crush 20 0.78 0.02 2.38 0.02 1.90 0.24 2.21 0.23 1.36 ± 0.03 681 ± 88.4 2.6 0.3 365 3
Px Heat Total 22.58 0.18 54.92 0.46 4.21 0.30 401.57 4.36 1.08 ± 0.01 50.1 ± 3.70 16.1 0.1 297 8
Crush + Heat Total 24.75 0.21 62.01 0.46 9.87 0.78 408.16 4.41 1.11 ± 0.01 111 ± 8.90 15.0 0.2 345 3
Crush 20 0.07 0.02 0.07 0.01 0.17 0.02 0.80 0.11 0.41 ± 0.15 653 ± 212 10.2 3.3 403 9
Ol Heat Total 6.15 0.11 14.55 0.21 0.83 0.03 120.61 1.97 1.05 ± 0.02 37.6 ± 1.38 17.8 0.4 321 6
Crush + Heat Total 6.35 0.12 14.74 0.22 1.30 0.07 122.88 2.00 1.03 ± 0.03 57.2 ± 3.26 17.6 0.4 363 5 98EH4 Crush 20 0.46 0.02 0.98 0.02 2.91 0.36 1.77 0.12 0.95 ± 0.04 1770 ± 228 3.5 0.3 338 2 Cpx Heat Total 33.99 0.27 72.42 0.46 7.76 0.21 657.13 2.95 0.95 ± 0.01 63.9 ± 1.78 17.6 0.2 306 4
Crush + Heat Total 34.84 0.28 74.23 0.46 13.17 0.70 660.41 2.96 0.95 ± 0.01 106 ± 5.69 17.2 0.2 323 2
Crush 20 0.25 0.01 0.29 0.01 0.14 0.01 1.01 0.32 0.51 ± 0.04 158 ± 10.7 3.6 1.2 318 1
98EH5 Ol Heat Total 9.65 0.17 18.20 0.18 1.37 0.05 137.76 2.33 0.84 ± 0.02 39.7 ± 1.55 13.0 0.3 289 5
Crush + Heat Total 10.43 0.17 19.09 0.19 1.81 0.05 140.85 2.53 0.81 ± 0.02 48.5 ± 1.59 12.3 0.3 303 3
Crush 20 0.31 0.01 0.59 0.02 0.17 0.01 0.55 0.26 0.86 ± 0.04 159 ± 13.6 1.6 0.8 344 31
Ol Heat Total 5.58 0.14 12.89 0.16 3.51 0.08 123.55 2.21 1.03 ± 0.03 176 ± 6.22 20.1 0.6 300 5
98EH7 Crush + Heat Total 5.94 0.15 13.60 0.16 3.72 0.09 124.21 2.23 1.02 ± 0.03 175 ± 5.88 19.0 0.6 307 6
Cpx Crush 20 0.90 0.04 3.41 0.03 1.89 0.24 2.41 0.57 1.69 ± 0.08 588 ± 78.2 2.4 0.6 304 2 Heat Total 13.25 0.17 26.97 0.17 9.58 0.27 232.28 2.36 0.90 ± 0.01 202 ± 6.29 15.9 0.3 287 8
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Temp°C / Br/Cl I/Cl Sample Phase Analysis Cl ppm ± Cl ppm Br ppb ± Br ppb I ppb ± I ppb K ppm ± K ppm K/Cl (molar) ± K/Cl 40Ar/36Ar ± 40Ar/36Ar Crushes (x10-3molar) (x10-6molar)
Crush + Heat Total 14.83 0.19 32.97 0.18 12.90 0.49 236.52 2.56 0.99 ± 0.01 243 ± 9.82 14.5 0.2 301 2
Crush 20 0.12 0.02 0.15 0.01 0.06 0.01 0.85 0.18 0.57 ± 0.12 142 ± 41.8 6.5 1.8 625 60
98EH8 Ol Heat Total 4.70 0.10 9.14 0.10 0.40 0.02 88.95 2.02 0.86 ± 0.02 23.5 ± 1.33 17.2 0.5 301 7
Crush + Heat Total 5.05 0.12 9.58 0.11 0.57 0.04 91.44 2.09 0.84 ± 0.02 31.7 ± 2.56 16.4 0.5 416 16
Crush 20 0.33 0.01 0.59 0.03 0.96 0.12 1.12 0.17 0.80 ± 0.05 816 ± 108 3.1 0.5 319 5 Cpx 98EH10 Heat Total 17.34 0.14 39.16 0.13 7.97 0.15 295.33 3.59 1.00 ± 0.01 128 ± 2.62 15.5 0.2 363 16
Crush + Heat Total 18.30 0.15 40.88 0.17 10.75 0.39 298.58 3.62 0.99 ± 0.01 164 ± 6.06 14.8 0.2 334 6
Crush 20 0.33 0.01 0.94 0.02 0.08 0.01 2.00 0.12 1.28 ± 0.04 64.3 ± 4.82 5.6 0.4 450 11
Ol Heat Total 7.76 0.11 18.26 0.14 0.43 0.03 159.67 2.37 1.02 ± 0.02 12.7 ± 1.01 18.5 0.3 302 10
Crush + Heat Total 8.42 0.17 20.52 0.15 0.61 0.03 164.47 2.39 1.08 ± 0.02 20.3 ± 1.11 17.7 0.4 376 8 98EH12 Crush 20 0.27 0.01 0.73 0.04 0.20 0.01 1.38 0.07 1.20 ± 0.09 203 ± 13.4 4.6 0.3 319 5 Cpx Heat Total 15.74 0.18 35.38 0.17 2.16 0.05 297.67 2.45 1.00 ± 0.01 38.4 ± 1.03 17.2 0.2 261 8
Crush + Heat Total 16.31 0.18 36.93 0.19 2.58 0.06 300.60 2.46 1.01 ± 0.01 44.1 ± 1.07 16.7 0.2 287 5
Crush 20 0.04 0.01 0.08 0.01 0.06 0.01 0.49 0.11 1.02 ± 0.34 485 ± 157 12.0 4.5 311 27
98EH16 Ol Heat Total 6.59 0.07 16.02 0.11 1.47 0.04 139.12 1.64 1.05 ± 0.01 59.9 ± 1.93 18.9 0.3 331 8
Crush + Heat Total 6.63 0.10 16.15 0.11 1.56 0.04 139.82 1.65 1.08 ± 0.02 65.8 ± 211 19.1 0.4 325 10 97TFxx = Tenerife, 97LPxx = La Palma, 98EHxx = El Hierro
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6.6 Discussion
6.6.1 Fractionation of the halogens?
Due to the different compatibilities between the halogens, during partial melting or fractional crystallisation the halogens can become fractionated, particularly with respect to the heavy halogens, with I being the most incompatible due to its large ionic radius. As such, during partial melting, I will preferentially partition into the fluid phase (I>Br>Cl), whilst Cl is more readily accommodated in crystal structures.
Therefore it must be considered if the halogen signature that is observed is simply due to fractionation during partial melting or fractional crystallisation.
Another method that may also lead to the fractionation of the halogens from one another is the degassing of magmas during their ascent through the mantle and crust, particularly during stagnation and storage in magma chambers (Bureau et al., 2016; Bureau et al., 2000). Fluid and melt inclusion analyses indicate that during magma ascent, the Canary Islands magma stagnates and pools both initially in the upper mantle, where it is stored long term, and finally, for a shorter period of time in the lower crust to Moho (Galipp et al., 2006; Gurenko et al., 1996;
Hansteen et al., 1998). However, in a H2O-poor system (H2O-poor basaltic magmas, e.g. Kilauea), it is not thought that the halogens undergo significant fractionation from one another at >1 MPa (Edmonds et al., 2009). Data from
Hansteen et al. (1998) on fluid inclusions from e.g. La Palma and El Hierro show that the inclusions within olivine and clinopyroxene crystals were formed at pressures greater than 0.2 GPa (0.2-0.68 GPa), and therefore it is not thought that the halogens present in the Canary Island basalts have undergone significant fractionation from one another. In addition, the samples that exhibit the highest I/Cl ratios are from La Palma, where the final stagnation level for the Cumbre Vieja magmas, the region analysed here, is at 15-26km depth (Hansteen et al., 1998).
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There is no evidence on La Palma for long-lived shallow magma chambers in which fractionation or degassing occurred (Galipp et al., 2006). This suggests that the halogen signature exhibited by the Canary Islands basalts is not as a result of fractionation of the halogens from one another due to degassing.
In Figure 6.7 to Figure 6.10, no correlation is observed between chlorine concentration and K/Cl, Br/Cl, and I/Cl, suggesting that the halogens have not been significantly fractionated from one other during partial melting and crystallisation. As such, it is proposed that the halogen signature observed in the
Canary Island basalts is representative of the source of the basalts.
It is noted that there is a difference between the ratios revealed in the clinopyroxenes and the olivines, with the clinopyroxenes showing approximately an order of magnitude higher Cl concentrations. This may be due to the clinopyroxenes sampling a different component within the Canary Island basalts, or maybe due to a higher population of the melt and fluid inclusions present with the basalts. This second scenario may be expected, as experimental studies show that the cleavage planes in clinopyroxenes allow for easier post-entrapment leakage and re-equilibration of fluid and melt inclusions (Hansteen et al., 1998).
Samples analysed by Hansteen el al. (1998) from El Hierro and La Palma support this hypothesis, with the secondary fluid inclusions present within olivine crystals often yielding higher densities than fluid inclusions present in clinopyroxenes from the same samples, indicating that inclusion leakage and density resetting may have occurred. Despite the differences between the two phases, there is considerable overlap between the halogen ratios reported in the olivines and
-3 clinopyroxenes, with the olivines ranging from (0.41-1.90)x10 Br/Cl and 64.3-
7400)x10-6 I/Cl in the crushing step, compared to the clinopyroxene range of (0.32-
2.00)x10-3 Br/Cl and (154-18700)x10-6 I/Cl; with the heating steps yielding (0.55- 205
1.73)x10-3 Br/Cl and (12.7-4440)x10-6 I/Cl in the olivines, and (0.14-1.08)x10-3
Br/Cl and (22.2-852)x10-6 in the clinopyroxenes (Table 6.5). Within the same samples, the olivine and clinopyroxenes vary by a maximum of an order of magnitude I/Cl.
From the halogen data, it is not possible to ascertain if the melt and fluid inclusions in the Canary Island basalts are primary or secondary, due to the bulk extraction nature of this technique. Hansteen et al. (1998) observed both primary and secondary inclusions present in the basalts from La Palma and El Hierro; however
He data (Day and Hilton, 2011) show that the inclusions present are primary and reflect the mantle source of the Canary Island basalts. Both primary and secondary CO2-rich inclusions have been observed by Hansteen et al. (1998) on the basis of inclusion densities. High pressure inclusions are primary and lower pressure ones are secondary, the latter either representing re-equilibration, or growth of inclusions are shallower levels (Hansteen et al., 1998). The inclusions are interpreted to indicate accumulation of magmas at Moho depths beneath the
Canary Islands (Hansteen et al., 1998). Helium isotope data obtained by in vacuo crushing (Day and Hilton, 2011) shows a uniform 3He/4He suggesting both primary and secondary inclusions contain the same composition; this is also consistent with the halogen crushing data. The variation in halogen ratios between the fluid and melt inclusions may be due to heterogeneity within the plume source, or due to the plume entraining halogen-enriched domains present in the mantle beneath the Canary Islands.
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Figure 6.7 Plots of K/Cl against a) Br/Cl and b) I/Cl for the crushing releases. The lack of a correlation suggests that the halogens have not been fractionated from one another during partial melting or crystallisation processes.
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Figure 6.8 Plots of a) Br/Cl, b) I/Cl, and c) K/Cl against 1/Cl for the crushing releases. The lack of a correlation suggests the absence of any significant fractionation.
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Figure 6.9 Plots of K/Cl against a) Br/Cl and b) I/Cl for the heating releases. The lack of a correlation suggests that the halogens have not being fractionated from one another during partial melting or crystallisation processes.
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Figure 6.10 Plots of a) Br/Cl, b) I/Cl, and c) K/Cl against 1/Cl for the heating releases. The lack of a correlation suggests the absence of any significant fractionation.
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6.6.2 The presence of different source components
A much smaller range of values is observed in both Br/Cl and I/Cl for the heating releases in comparison with the releases recorded from crushing (Table 6.5). This suggests that the halogens are sited in two different phases within the Canary
Island basalts. Fracture release from the silicate phases (melt or mineral) is negligible (Dickinson et al., 1992), therefore crushing is releasing halogens trapped within a fluid inclusion assemblage, and step heating is liberating halogens trapped within melt inclusions (together with any remaining fluid inclusions). La Palma shows the biggest variation between crushing and step heating releases, with an order of magnitude higher I/Cl ratios shown in the crushing data than in the heating releases. The Tenerife samples show the least variation between the crushing and heating steps.
With the exception of Tenerife, a much narrower range of Br/Cl ratios is observed in the heating releases than seen in the crushing releases. It is expected that the release during crushing represents halogens released from fluid inclusions (with no release from melt inclusions). The majority of the stepped heating releases are from halogens sited within melt inclusions, together with any halogens not released from fluid inclusions during the crushing step. Previous studies have shown that olivine and clinopyroxenes from the younger islands (Gran Canaria, La
Palma, and El Hierro) are rich in fluid inclusions (Figure 6.11), together with the presence of melt inclusions (Galipp et al., 2006; Gurenko et al., 1996; Hansteen et al., 1998).
Table 6.5 Halogen ranges yielded from each island, during crushing and step heating.
Island Phase Br/Cl (x10-3) (crushing) Br/Cl (x10-3) (heating) I/Cl (x10-6) (crushing) I/Cl (x10-6) (heating) Ol 0.91 - 1.90 0.91 - 1.73 242 - 2750 203 - 4440 Tenerife Cpx 0.32 0.14 154 22.2 Ol 0.74 - 1.15 0.55 - 0.99 219 - 7400 67.8 - 917 La Palma Cpx 1.09 - 2.00 0.81 - 1.00 333 - 18700 49.7 - 852 Ol 0.41 - 1.53 0.84 - 1.08 64.3 - 653 12.7 - 176 El Hierro Cpx 0.80 - 1.69 0.90 - 1.08 203 - 2870 38.4 - 202
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Figure 6.11 Thin section images from the Western Canary Islands (a-e Gran Canaria, f-h La Palma) showing the presence of both fluid (a-h) and melt (b) inclusions within the samples. Image taken from Hansteen et al. (1998).
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6.6.3 3He/4He and Halogen Ratios Six of the samples from La Palma (n=3) and El Hierro (n=3) have previously been analysed for their 3He/4He isotope data by Dr Paul Harrop, and are reported in
Table 6.6. There does not appear to be a direct correlation between the halogen ratios and 3He/4He; samples 97LP8 and 98EH1, which have the highest and lowest I/Cl respectively (of the samples in which 3He/4He was also measured), yield the lowest 3He/4He ratios (7.11±0.28 and 7.01±0.30 respectively). This may be due to there being several components present within the Canary Island basalts, with different components present in the fluid and melt inclusion assemblages.
Table 6.6 3He/4He (*measured in cpx) and halogen ratios measured with the La Palma and El Hierro samples (He isotope data from Dr Paul Harrop).
3 4 Sample Phase Step Br/Cl I/Cl He/ He (RA) (x10-3 molar) (x10-6 molar)
Crush 1.09 ± 0.12 18700 ± 2336 Cpx 97LP8> Heat 1.00 ± 0.01 852 ± 19.0 Crush + Heat 0.99 ± 0.01 1470 ± 49.9 7.11 ± 0.28 Crush 1.12 ± 0.02 2400 ± 66.6 Cpx 97LP8< Heat 0.81 ± 0.01 148 ± 4.20 Crush + Heat 0.82 ± 0.01 204 ± 4.41 Crush 0.74 ± 0.13 1630 ± 190 Ol Heat 0.93 ± 0.01 224 ± 6.84 Crush + Heat 0.92 ± 0.01 306 ± 8.04 97LP9 7.18 ± 0.18* Crush 1.40 ± 0.11 4800 ± 366 Cpx Heat 0.99 ± 0.01 375 ± 27.0 Crush + Heat 1.00 ± 0.01 500 ± 26.5 Crush 1.15 ± 0.33 596 ± 107 97LP10 Ol Heat 0.55 ± 0.02 335 ± 15.8 8.83 ± 1.03* Crush + Heat 0.58 ± 0.03 351 ± 15.8 Crush 0.66 ± 0.21 316 ± 121 98EH1 Ol Heat 1.08 ± 0.02 26.2 ± 2.61 7.01 ± 0.30* Crush + Heat 1.08 ± 0.03 47.5 ± 5.46 Crush 0.82 ± 0.10 2870 ± 337 98EH2 Ol Heat 1.05 ± 0.03 65.9 ± 2.87 7.51 ± 0.27* Crush + Heat 1.13 ± 0.05 233 ± 10.3 Crush 0.80 ± 0.05 816 ± 108 Cpx 98EH10 Heat 1.00 ± 0.01 128 ± 2.62 7.81 ± 0.49 Crush + Heat 0.99 ± 0.01 164 ± 6.06
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6.6.4 Marine Pore Fluid / Serpentine Halogen Signature Released from Fluid Inclusions
In Figure 6.12 it is observed that the samples from all three islands are distinct from MORB, trending from MORB-like values up to two orders of magnitude higher than MORB with respect to their I/Cl ratios. The samples from La Palma range to the highest values, almost an order of magnitude higher than observed for El
Hierro and Tenerife. However, there is an overlap in halogen ratios between the islands, suggesting that there is a common component(s) in the basalt from the three islands.
The samples also exhibit a wider range of Br/Cl ratios than observed in MORB, displaying values both lower than MORB, with similar Br/Cl ratios as observed in
AOC (Chavrit et al., 2016), with a few samples trending to values higher than
MORB.
In Table 6.7 it is shown that there is a greater proportion of the overall I released during crushing, than there is from Cl and Br, most notable in the La Palma and El
Hierro samples. This perhaps suggests a different source for some of the I within the Canary Islands samples, than the source of the Cl and Br. One possible source for this enrichment is I-rich organic sediments, or marine pore fluids, which are enriched in I due to sediments, or serpentinised oceanic lithosphere which has preserved a marine pore fluid signature. Comparing the samples to the marine pore fluid field (Figure 6.12), it is observed that the samples have a similar range in I/Cl ratios. The Canary Islands samples also exhibit a good overlap with previously reported halogen data from serpentinites measured from sea-floor mid- ocean ridge, passive margin, and fore-arc settings (Kendrick et al., 2013b), where marine pore fluids are thought to have been the serpentinising fluid, causing the serpentinites to take on a marine pore fluid-like signature.
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Figure 6.12 Halogen ratio of the crushing releases plot with pore fluid, serpentinites, wedge fluid, MORB, WARs Xenoliths, bulk Earth, and seawater data from literature (additional data from Anders and Ebihara, 1982; Broadley et al., 2016; Bruland and Lohan, 2003; Chavrit et al., 2016; Fehn et al., 2006; Fehn et al., 2000; Fehn et al., 2003; Fehn et al., 2007b; Jambon et al., 1995; John et al., 2011; Kastner et al., 1990; Kendrick, 2012; Kendrick et al., 2013a; Kendrick et al., 2014a; Lu et al., 2008; Mahn and Gieskes, 2001; Martin et al., 1993; Muramatsu et al., 2007; Muramatsu et al., 2001; Sumino et al., 2010; Tomaru et al., 2007) (see appendix 8.5 for additional graphs). Error bars, where not seen, are smaller than symbols. 215
Table 6.7 Showing the percentages of the halogens and K released from the crushing step.
% released from crushing Phase Sample Cl Br I K Ol 9.0% 10.9% 7.9% 2.3% 97TF39 Ol 7.3% 6.6% 8.8% 2.3% 97TF40 Ol 3.9% 4.2% 2.4% 1.0% 97TF41 Ol 2.7% 2.1% 8.3% 0.0% 97LP7 Ol 2.1% 1.7% 13.3% 1.3% 97LP9 Ol 4.3% 8.6% 7.3% 1.4% 97LP10 Ol 3.4% 2.8% 21.8% 0.7% 97LP12 Ol 2.3% 1.4% 19.9% 0.9% 98EH1 Ol 2.0% 1.5% 45.3% 1.6% 98EH2 Ol 1.3% 1.8% 13.4% 0.7% 98EH3 Ol 1.1% 0.5% 16.7% 0.7% 98EH4 Ol 2.6% 1.6% 9.5% 0.7% 98EH5 Ol 5.2% 4.4% 4.7% 0.4% 98EH7 Ol 2.5% 1.6% 13.4% 1.0% 98EH8 Ol 4.0% 4.9% 14.8% 1.2% 98EH12 Ol 0.6% 0.5% 4.2% 0.3% 98EH16 3.4% 3.4% 13.2% 1.0% Average Cpx 0.9% 2.1% 6.1% 1.7% 97TF54 Cpx 2.0% 4.3% 12.1% 0.7% 97LP7 Cpx 1.6% 1.7% 26.2% 0.4% 97LP8> Cpx 0.9% 1.2% 12.5% 0.5% 97LP8< Cpx 1.0% 1.4% 11.3% 0.3% 97LP9 Cpx 1.1% 1.4% 12.8% 0.4% 97LP12 Cpx 3.3% 4.2% 31.1% 0.5% 98EH3 Cpx 1.3% 1.3% 27.2% 0.3% 98EH4 Cpx 6.3% 11.2% 16.5% 1.0% 98EH7 Cpx 1.9% 1.5% 10.7% 0.4% 98EH10 Cpx 1.7% 2.0% 8.3% 0.5% 98EH12 2.0% 2.9% 15.9% 0.6% Average
Although there is little overlap with SCLM (Figure 6.12), there is a good overlap with the I/Cl ratios when the Canary Island basalts are compared to previously reported data on the West Antarctic Rift System (WARs) (Figure 6.12) (Broadley et al., 2016). These samples contain xenoliths of enriched SCLM, which show elevated I/Cl ratios, proposed to be from the introduction and retention of the 216
halogens into the overlying SCLM during subduction, as halogen-bearing fluids are released from the downgoing slab during subduction (Broadley et al., 2016). It may therefore be possible that there is a component of metasomatised SCLM present with the Canary Island basalts.
6.6.5 MORB-like Release from Melt Inclusions
Compared to the crushing releases, the halogen ratios show a much narrower range during stepped heating (Figure 6.13). Samples from El Hierro overlap strongly with the known field for MORB, with the samples from La Palma trending from MORB-like, to an order of magnitude higher in their I/Cl ratios. The samples also lie close to OIB (EM1 and EM2) samples reported in the literature by Kendrick et al. (2014b) and the MORB-like HIMU basalts yielded from the Azores (Chapter
5).
One sample from Tenerife (97TF54 cpx) yields lower Br/Cl ratios in both the crushing and heating steps than observed in the other samples. This sample is more evolved (phonolite) than the majority of the samples. However, it is not thought that the difference in the Br/Cl ratio is due to the sample trapping a more evolved magma, as this difference is not observed in the other phonolitic sample,
97LP10. In addition, during fractional crystallisation, it would be expected that Cl would preferentially be removed from the melt in respect to the heavier halogens, as Cl-bearing phases (e.g. amphibole) began to crystallise; this would result in an increased Br/Cl ratio, however the opposite is observed. It is possible that the variation in this sample is due to a higher Cl content than observed in the other samples.
217
Comparing the samples to the known fields for serpentinites marine pore fluids
(Figure 6.13), it is observed that the Canary Island samples trend to similar I/Cl values as seen in serpentinites and marine pore fluids, although they yield order of magnitude lower I/Cl than shown in the crushing releases. As crushing is not
100% efficient at liberating all fluid inclusions, it is likely that a proportion of the fluid inclusions only released their halogens during step heating. Therefore, the heating results show both the halogens released from melt inclusions together with the halogens released from the remaining fluid inclusions. This would increase the halogen ratios from the step heating releases to higher I/Cl values, trending towards those observed in the crushing releases. As such, it is proposed that the halogen ratios from the melt inclusions are more MORB-like, with the I-rich contribution coming from the remnant fluid inclusions.
Unlike the crushing results, the halogens released from step heating do not show a strong overlap with the marine pore fluid and serpentinite fields, suggesting that the high I/Cl signature is found primarily in the fluid inclusions, with a more MORB- like signature, observed in the melt inclusions. It is therefore proposed that the melt inclusions are showing a MORB-like or OIB halogen component, not observed in the crushing releases from fluid inclusions.
218
Figure 6.13 Halogen ratio plot of the stepped heating releases, showing the addition of the WARS xenoliths from Broadley et al. (2016), representing metasomatised SCLM (other references as per Figure 6.12) (see appendix 8.5 for additional graphs). Error bars, where not seen, are smaller than symbols. 219
6.6.6 A Multi-Component HIMU Source
It is proposed that there are two components present within the halogens from the
Canary Island basalts: 1) A MORB-like component, seen in the melt inclusions, and 2) a marine pore fluid component, observed from the fluid inclusions.
The presence of these two components is consistent with published data from other isotope systems (Day and Hilton, 2011; Day et al., 2010; Simonsen et al.,
2000). Day and Hilton (2011) proposed a four component system, with components from recycled AOC, recycled oceanic lithosphere, a HIMU component, and a MORB-like component. Day and Hilton (2011) measured
3He/4He in samples from El Hierro and La Palma, in both olivine and clinopyroxene separates. In the majority of samples, the olivine and clinopyroxene separates are in equilibrium, showing similar 3He/4He ratios, thought to be trapping unmodified mantle-derived He signatures (Day and Hilton, 2011). The samples yield a range
3 4 of He/ He, from 4.15-9.72 RA, averaging 7.6±0.8 RA for La Palma, and 7.7±0.3 RA for El Hierro, when the clinopyroxenes are excluded. The average He ratios are in agreement with the higher I/Cl ratios reported in this study, as 3He/4He ratios of
6.75-7.08 RA indicate the presence of a recycled oceanic lithospheric component
(Hilton and Porcelli, 2014), which would have a marine pore fluid or serpentinite halogen signature. A MORB-like signature is also observed both in the halogens in this study and in the 3He/4He in samples from Day and Hilton (2011), suggesting that a MORB-like component is present in the Canary Island samples. In addition, a subducted oceanic crust signature is present within the 3He/4He isotopes measured in the Canary Islands, with samples measured by Mouatt (2006) extending to <1.0 RA.
The differences in the halogen ratios (particularly I/Cl) between the islands of La
Palma and El Hierro, are also in observed He-O isotope systematics (Day and 220
Hilton, 2011; Day et al., 2010), where it is proposed that a recycled oceanic crust component and a depleted mantle component is present in the El Hierro basalts, with the La Palma basalts additionally showing a contribution from recycled lithosphere. This is broadly in agreement with the halogen data, where higher I/Cl ratios are observed on La Palma, overlapping with the serpentinite field, and the El
Hierro basalts showing a greater MORB-like component.
This mixing between the four isotopically distinct components may explain why there is no correlation in the El Hierro and La Palma basalts between 3He/4He and halogen ratios in Sr-Nd-Os-Pb isotope space.
The different components observed across the islands, together with the fluid and melt inclusions trapping different components, suggests that the Canary Islands
HIMU source has remained heterogeneous with respect to the halogens, and has not become homogenised over time. The source of the Canary Island basalts has remained unmixed and is proposed to be preserving a depleted MORB-like signature, together with the addition of a recycled marine pore fluid or sediment signature, present in subducted serpentinised lithosphere, and ancient oceanic crust – typical of HIMU basalts.
It is not known if the halogens can be subducted into the mantle source for the
HIMU basalts, although research has suggested that the halogens can be preserved, particularly in cold fast subduction zones, down to depths of at least
200km when antigorite breaks down (Kendrick et al., 2013b; Kendrick et al., 2011;
Sumino et al., 2010; Ulmer and Trommsdorff, 1995; van Keken et al., 2011). It is proposed that either a) the halogens are being subducted into the HIMU plume source, where they remain relatively unmixed prior to their return to the surface, or b) that as the plume material rises, it samples and entrains heterogeneous pockets
221
of mantle that are enriched with a marine pore fluid halogen signature. In either model, the halogens must remain relatively unmixed in order to explain the differences and various components observed from the fluid and melt inclusion phases.
6.7 Conclusions
The halogens in the Canary Islands represent a two component mantle
source, containing contributions from a marine pore fluid signature, and a
MORB-like component.
The crushing and heating releases are sampling different components, with
the crushing sampling marine pore fluids released from fluid inclusions, and
the heating sampling a MORB-like or OIB component, released from melt
inclusions.
It is proposed that these differences rise from a heterogeneous source,
which has been enriched in the halogens during one or more subduction
events, and which has not completely mixed and homogenised with time.
It is suggested that AOC was subducted into the HIMU mantle source,
together with serpentinite with a marine pore fluid signature, and I-rich
sediments.
These data expand on the Azores samples (Chapter 5), together providing
the first known data on the halogen ratios present within HIMU source
basalts.
222
7 Conclusions
Ocean island basalts provide a unique window into mantle composition, and give insight into the subduction, recycling, and mixing of the halogens. The data in this study significantly expand the known data for halogens for ocean island basalts, providing the first halogen Br/Cl and I/Cl data for Tristan da Cunha, the Azores, and the Canary Islands.
The halogen data from Tristan da Cunha group, the Azores, and the Canary
Islands support previously published data (Kendrick et al., 2015; Kendrick et al.,
2014b), which suggest that on the basis of halogens alone, it is not possible at present to determine the mantle source of the basalts in the current halogen OIB dataset, or to distinguish between the EM1, EM2, and HIMU mantle end-members and MORB using halogen ratios. However, the mantle end members can be determined on the basis of 3He/4He ratios, which can be measured in the same samples, or alternatively, by analysing the concentrations of the REEs, in addition to the halogen ratios. It is possible that there are subtle differences in the halogen composition between the mantle sources, which are then overprinted by local mantle compositions within each island group, either from the mantle plume itself, or due to variability from the local mantle underlying the oceanic island systems.
To date, halogen data is known for relatively few EM1, EM2, and HIMU OIBs (e.g.
Kendrick et al., 2014b; Kendrick et al., 2012a) and as the worldwide dataset for halogens continues to expand and be refined, it may become possible to resolve the mantle end-members on the basis of halogen ratios.
Tristan da Cunha represents EM1 and EM2 type basalts; end-members thought to be enriched due to the recycling of delaminated metasomatised subcontinental or oceanic lithosphere, pelagic sediments, or lower continental crust (Hofmann,
223
2007). The samples from the island of Tristan overlap with the MORB field, and are consistent with previously published data on halogens in EM1 and EM2 basalts (Kendrick et al., 2015; Kendrick et al., 2014b), showing a common MORB- like component in EM1 and EM2 basalts from three hotspot locations (Figure 7.1).
However, a simple two component model cannot explain the range observed in the Inaccessible Island basalts, which trend to higher I/Cl values. It is proposed that these basalts sample a local length-scale heterogeneity beneath Inaccessible
Island. As these samples overlap with the known field for marine pore fluids, it is suggested that the signature present is that of a subducted marine pore fluid being entrained locally in the source of the Inaccessible Island basalts. A model is proposed whereby the marine pore fluid signature is preserved in metasomatised
SCLM, which was subducted into the mantle during the closure of the
Mozambique Ocean, and subsequently stored and sampled by the Inaccessible
Island basalts, perhaps due to entrainment of the enriched material as the plume material rises. This is in agreement with other isotope systems (Gibson et al.,
2005), which propose a metasomatised SCLM source present in the Tristan da
Cunha basalts; thus a three-component system is being observed in the
Inaccessible Island basalts, which show a mixture of EM1, EM2, and marine pore fluid halogen signatures. A similar marine pore fluid signature is also observed in the Azores basalts, where SCLM is proposed to be a source component. It is also consistent with the evolution of the Tristan mantle plume over time, as observed in
Pb isotopes (Gibson et al., 2005; Marques et al., 1999; Salters and Sachi-Kocher,
2010).
The MORB-like signature observed in the Tristan samples is thought to be representative of the plume source, suggesting that long term subduction and recycling of the halogens has led to their homogenisation within the mantle,
224
resulting in similar halogen ratios in MORB and OIB. However, the local heterogeneities observed with the Tristan da Cunha group suggests that pockets of halogen-enriched material are replenishing the mantle locally through subduction.
The Azores (except eastern São Miguel) has been proposed to be a HIMU mantle- end member, a source thought to represent the recycling and long term storage of ancient oceanic crust (Jackson et al., 2007; Workman et al., 2004; Zindler and
Hart, 1986). However, the halogen data (Figure 7.1) show an overlap with EM1 and EM2 end-members, MORB, AOC, and marine pore fluids – suggesting a complex and mixed source for the Azores basalts, which cannot be explained by a
HIMU component in isolation. Together with the 3He/4He and Pb data (e.g. Moreira et al., 1999), the halogens support a heterogeneous source, perhaps controlled in part by the complex tectonic setting of the Azores basalts in a triple ridge system.
The Terceira Rift may have a dominant control on the relationship of the magmas between the islands, with the islands on the rift (Graciosa, Terceira, and São
Miguel) showing a similar range of components, from AOC, to a MORB-like or
OIB-like signature, and a marine pore fluid signature – a signature which is also seen in the western island group (Corvo and Flores). The intra-plate islands located to the south of the Terceira Rift show more MORB-like compositions, perhaps due to the entrainment of MORB as the plume material shears southwest from the Terceira Rift to the islands. A more “primitive” He component is also present on the islands, observed most strongly on Terceira, which is thought to be the location of the plume, based on the seismic imaging of the mantle below the
3 4 island group, together with the higher (~13.5 RA) He/ He ratios.
The Canary Islands are also proposed to be HIMU type ocean island basalts, but again a range of halogen ratios is observed. These samples are particularly 225
interesting, in that they show a significant difference between the halogen ratios released during the crushing and step heating samples – a difference which is seen across the islands. The Canary Islands samples trend to higher I/Cl ratios during the crushing releases; this halogen signature is proposed to originate from a serpentinite component, which in turn has preserved a marine pore fluid signature. The heating releases are similar to the common MORB -like component preserved in the Tristan da Cunha and Azores samples. This pattern was observed across all three islands, and across the majority of samples, suggesting that instead of local length-scale heterogeneities being sampled by a homogenous plume, a heterogeneous plume source is being sampled by the Canary Island basalts, comprising of a MORB-like signature, and subducted marine pore fluid signature – with the components enriching the mantle source in halogens through recycling. This similar signature between the islands suggests that the Canary
Islands samples may provide evidence for deeper halogen subduction into the mantle.
A simple end-member (e.g. EM1, EM2, HIMU) mantle composition cannot explain the variation present within ocean island basalt and plume settings. Halogen ratios, together with 3He/4He and Sr-Nd-Pb isotopes, support a mixed composition, sampling these idealised mantle end-members. The results from this study suggest that the Earth’s mantle has a common halogen component, observed across all the island groups, represented by a MORB-like signature present in
EM1 and EM2 type basalts, suggesting that the mantle has become homogenised over time, but is locally heterogeneous with respect to its halogen composition, due to the enrichment by subducted halogen signatures, either sampled at shallower depths by rising homogeneous plume material, or recycled into the deeper mantle, where they remain isolated within a heterogeneous plume. This
226
suggests that the mantle composition and halogen budget is strongly controlled by the subduction and recycling of e.g. subducted AOC, SCLM, oceanic lithosphere, sediments, and marine pore fluids.
7.1 Future Work
As the halogens were analysed by bulk extraction from whole mineral (olivine and clinopyroxene) separates, rather than from melt and fluid inclusions directly, it was not possible during the course of this study to calculate source concentrations of the halogens within the island groups. In order to calculate the halogen concentrations, it may be possible to analyse the chlorine concentrations within the melt inclusions present within the olivine crystals, using an electron microprobe or SIMS. Once the chlorine concentrations in typical melt inclusions were obtained for the samples, these concentrations could be used, together with the previously obtained halogen ratios, as a proxy for the concentrations of the heavier halogens
(Br, I).
It would also be interesting to date some of the samples, particularly those from the Tristan da Cunha group, in order to confirm if there is a temporal variation present within the halogen ratios; alternatively, to expand the dataset, halogen analyses could be completed on samples previously dated in the literature, e.g.
Hicks et al. (2012). Analyses of the halogen ratios present within the Paraná-
Etendeka flood basalts and the Walvis Ridge, would help to gain further insight into how the Tristan plume has evolved over time.
Although there is increasing data on the halogens, the noble gases, and radiogenic isotopes in OIB and plume settings, there is still a paucity of data in the literature for which analyses have been completed on all these isotopic systems in the same samples. Analyses on the same samples would enable a better
227
understanding of how the halogens, noble gases, and radiogenic isotopes correlate within OIBs. This, together with further analyses of the fluid and melt inclusion phases, including microprobe analyses to estimate the halogen concentrations, alongside 40Ar/36Ar dating of the samples would enable a more comprehensive overview of the behaviour of halogens within OIBs settings. In order to gain further insight of halogen systems in a plume environment, it would be interesting to analyse samples in a plume setting, away from an active ridge, completing a detailed analyses on melt and fluid inclusions, halogens, noble gas
(Ar and He) content, and their radiogenic isotope ratios. From the areas analysed as part of this project, the Canary Islands would be the ideal target area, as it is away from the influence of a triple ridge and MAR setting, unlike the Azores and
Tristan da Cunha, and would therefore potentially allow characterisation of the halogens within the plume source. Further samples would need to be collected, perhaps targeting some of the older islands in the Canaries, alongside the younger ones analysed as part of this study, in order to track the evolution of the plume over time.
228
Figure 7.1 Halogen ratio plots for a) Tristan da Cunha, b) The Azores, c) Canaries (crushing), and d) Canaries (step heating) (additional literature data as per Figure 4.14). 229
8 Appendices
8.1 Tristan da Cunha: Halogen, K, and 40Ar/36Ar results from irradiated samples (individual steps).
Temp ± Cl ± Br ± I ± K Br/Cl I/Cl K/Cl ± 40Ar/ ± 40Ar/ Sample Analysis °C / Island Phase Cl ppm Br ppb I ppb K ppm ± Br/Cl ± I/Cl ppm ppb ppb ppm (molar) (molar) (molar) K/Cl 36Ar 36Ar Crushes
60 Heat 600 T Ol 0.62 0.06 0.50 0.06 0.13 0.02 136.46 7.37 3.61E-04 5.41E-05 6.00E-05 1.14E-05 201 21 415 89
60 Heat 1250 T Ol 1.37 0.14 4.33 0.50 0.12 0.01 116.76 6.21 1.41E-03 2.18E-04 2.38E-05 3.81E-06 77 9 263 18
60 Heat 1600 T Ol 0.90 0.06 2.88 0.33 0.09 0.01 39.09 2.10 1.42E-03 1.86E-04 2.85E-05 3.89E-06 39 3 308 6
60 Heat Total T Ol 2.89 0.16 7.72 0.60 0.34 0.03 292.31 9.86 1.19E-03 1.14E-04 3.30E-05 3.32E-06 92 6 308 8
112 Heat 600 T Ol 1.00 0.03 1.66 0.10 0.43 0.07 131.53 0.74 7.37E-04 4.94E-05 1.20E-04 1.98E-05 119 4 283 12
112 Heat 800 T Ol 2.22 0.08 3.62 0.12 0.27 0.05 104.33 0.62 7.26E-04 3.64E-05 3.43E-05 6.15E-06 43 2 253 6
112 Heat 1000 T Ol 0.39 0.04 3.41 0.13 0.15 0.04 39.22 0.31 3.88E-03 4.69E-04 1.05E-04 3.28E-05 91 11 207 12
112 Heat 1200 T Ol 0.85 0.05 4.49 0.24 0.13 0.04 49.33 0.37 2.35E-03 1.88E-04 4.27E-05 1.33E-05 53 3 227 19
112 Heat 1400 T Ol 0.77 0.04 3.81 0.42 0.28 0.10 34.61 0.26 2.19E-03 2.62E-04 1.00E-04 3.54E-05 41 2 283 16
112 Heat 1600 T Ol 0.39 0.02 1.36 0.48 0.19 0.05 9.50 0.12 1.56E-03 5.64E-04 1.38E-04 3.51E-05 22 1 282 4
112 Heat Total T Ol 5.62 0.12 18.36 0.71 1.45 0.15 368.51 1.12 1.45E-03 6.40E-05 7.19E-05 7.57E-06 60 1 271 4
114 Crush 1 T Ol 0.08 0.00 0.09 0.01 0.03 0.04 0.57 0.14 5.19E-04 7.65E-05 1.20E-04 1.32E-04 7 2 436 30
114 Crush 20 T Ol 0.17 0.06 1.06 0.10 0.05 0.01 10.30 0.19 2.78E-03 9.61E-04 9.04E-05 3.42E-05 55 18 497 18
114 Crush Total T Ol 0.25 0.06 1.15 0.10 0.09 0.04 10.87 0.24 2.07E-03 5.06E-04 9.99E-05 4.87E-05 40 9 481 16
114 Heat 600 T Ol 0.88 0.12 3.21 0.12 0.24 0.04 198.79 1.51 1.62E-03 2.22E-04 7.52E-05 1.70E-05 205 27 293 5
114 Heat 800 T Ol 0.32 0.03 2.10 0.13 0.28 0.05 435.72 2.38 2.91E-03 3.56E-04 2.42E-04 4.76E-05 1232 131 275 12
114 Heat 1000 T Ol 0.11 0.05 1.39 0.10 nd nd 78.34 0.82 5.87E-03 2.64E-03 nd nd 674 299 318 28 230
Temp ± Cl ± Br ± I ± K Br/Cl I/Cl K/Cl ± 40Ar/ ± 40Ar/ Sample Analysis °C / Island Phase Cl ppm Br ppb I ppb K ppm ± Br/Cl ± I/Cl ppm ppb ppb ppm (molar) (molar) (molar) K/Cl 36Ar 36Ar Crushes
114 Heat 1200 T Ol 0.12 0.06 1.09 0.08 0.36 0.10 38.78 0.49 3.86E-03 1.87E-03 8.03E-04 4.48E-04 282 135 277 28
114 Heat 1400 T Ol nd nd 0.89 0.13 0.22 0.02 5.43 0.29 nd nd nd nd nd nd 283 7
114 Heat 1600 T Ol 0.11 0.03 0.54 0.09 0.13 0.04 5.88 0.38 2.15E-03 7.50E-04 3.28E-04 1.36E-04 48 15 287 4
114 Heat Total T Ol 1.52 0.18 9.22 0.27 1.20 0.13 762.94 3.01 2.70E-03 3.28E-04 2.21E-04 3.53E-05 456 54 287 3
114 Crush + Heat Total T Ol 1.77 0.19 10.37 0.29 1.29 0.13 773.81 3.02 2.52E-03 2.78E-04 1.87E-04 2.88E-05 340 36 300 3
186 Heat 600 T Ol 0.18 0.02 0.32 0.05 0.03 0.00 22.79 1.34 7.80E-04 1.61E-04 4.90E-05 8.95E-06 114 17 830 711
186 Heat 1250 T Ol 2.50 0.23 9.63 1.11 0.14 0.02 122.65 7.17 1.71E-03 2.53E-04 1.57E-05 2.43E-06 45 5 416 16
186 Heat 1600 T Ol 0.51 0.06 14.15 1.63 0.60 0.07 165.08 9.07 1.23E-02 1.94E-03 3.30E-04 5.29E-05 292 36 698 4
186 Heat Total T Ol 3.19 0.24 24.09 1.97 0.78 0.07 310.52 11.64 3.36E-03 3.73E-04 6.80E-05 8.22E-06 88 7 625 12
341 Heat 600 T Ol 0.14 0.04 0.25 0.03 0.03 0.01 29.45 2.00 7.86E-04 2.38E-04 6.58E-05 2.33E-05 188 53 268 42
341 Heat 1250 T Ol 1.24 0.09 2.86 0.33 0.04 0.01 59.64 3.24 1.02E-03 1.40E-04 8.30E-06 1.32E-06 44 4 396 46
341 Heat 1600 T Ol 0.40 0.03 1.76 0.20 0.06 0.01 27.90 1.59 1.94E-03 2.77E-04 3.89E-05 5.96E-06 63 6 315 2
341 Heat Total T Ol 1.79 0.10 4.87 0.39 0.13 0.01 116.99 4.13 1.21E-03 1.20E-04 1.98E-05 2.16E-06 59 4 324 8
482 Heat 600 T Ol 2.55 0.19 0.99 0.12 0.02 0.01 492.95 26.19 1.73E-04 2.46E-05 1.97E-06 8.70E-07 175 16 254 43
482 Heat 1250 T Ol 4.33 0.24 12.88 1.48 0.08 0.01 269.77 14.34 1.32E-03 1.68E-04 5.44E-06 7.90E-07 57 4 277 13
482 Heat 1600 T Ol nd nd 1.67 0.20 0.07 0.01 nd nd nd nd nd nd nd nd 378 18
482 Heat Total T Ol 6.69 0.31 15.54 1.50 0.17 0.02 759.10 29.90 1.03E-03 1.10E-04 7.22E-06 7.54E-07 103 6 311 14
484 Heat 600 T Ol 1.52 0.30 8.40 0.98 1.08 0.41 119.34 7.37 2.45E-03 5.55E-04 1.99E-04 8.48E-05 71 14 417 31
484 Heat 1250 T Ol 7.79 0.43 26.48 3.04 0.37 0.04 356.59 19.07 1.51E-03 1.93E-04 1.32E-05 1.74E-06 42 3 610 58
484 Heat 1600 T Ol 0.58 0.16 8.24 0.95 0.20 0.03 72.94 3.92 6.36E-03 1.86E-03 9.67E-05 2.93E-05 115 32 419 7
484 Heat Total T Ol 9.88 0.55 43.12 3.33 1.65 0.42 548.87 20.82 1.94E-03 1.84E-04 4.67E-05 1.20E-05 50 3 446 15
231
Temp ± Cl ± Br ± I ± K Br/Cl I/Cl K/Cl ± 40Ar/ ± 40Ar/ Sample Analysis °C / Island Phase Cl ppm Br ppb I ppb K ppm ± Br/Cl ± I/Cl ppm ppb ppb ppm (molar) (molar) (molar) K/Cl 36Ar 36Ar Crushes
646 Heat 600 T Ol 2.87 0.17 3.84 0.44 0.36 0.04 707.79 37.63 5.94E-04 7.70E-05 3.51E-05 4.74E-06 224 18 402 19
646 Heat 1250 T Ol 2.31 0.14 6.54 0.75 0.16 0.02 276.04 14.71 1.26E-03 1.64E-04 1.95E-05 2.64E-06 108 9 366 27
646 Heat 1600 T Ol 3.66 0.20 11.74 1.37 0.38 0.05 380.84 20.24 1.42E-03 1.83E-04 2.93E-05 4.25E-06 94 7 298 11
646 Heat Total T Ol 8.85 0.30 22.12 1.62 0.91 0.07 1364.68 45.19 1.11E-03 8.97E-05 2.87E-05 2.42E-06 140 7 333 9
60 Heat 600 T Px 2.02 0.09 1.50 0.14 0.40 0.05 177.90 0.98 3.30E-04 3.42E-05 5.54E-05 7.75E-06 80 4 255 13
60 Heat 1250 T Px 8.65 0.11 23.70 0.34 0.57 0.10 524.55 2.13 1.22E-03 2.35E-05 1.85E-05 3.33E-06 55 1 276 36
60 Heat 1600 T Px 1.45 0.02 3.83 0.07 0.24 0.08 49.54 0.80 1.17E-03 2.74E-05 4.71E-05 1.51E-05 31 1 253 4
60 Heat Total T Px 12.12 0.15 29.03 0.37 1.22 0.14 751.99 2.48 1.06E-03 1.88E-05 2.81E-05 3.24E-06 56 1 259 9
186 Heat 600 T Px 7.57 0.41 2.75 0.32 0.14 0.02 223.61 11.96 1.61E-04 2.05E-05 5.16E-06 7.49E-07 27 2 317 67
186 Heat 1250 T Px 5.01 0.28 14.38 1.65 0.11 0.02 104.24 5.68 1.28E-03 1.62E-04 6.37E-06 9.15E-07 19 1 283 33
186 Heat 1600 T Px 6.96 0.37 25.74 2.96 0.21 0.03 143.89 7.79 1.64E-03 2.09E-04 8.56E-06 1.15E-06 19 1 320 3
186 Heat Total T Px 19.54 0.62 42.86 3.40 0.47 0.04 471.74 15.36 9.75E-04 8.34E-05 6.68E-06 5.54E-07 22 1 312 11
484 Heat 600 T Px 30.74 0.35 11.78 0.18 0.71 0.10 1037.82 5.10 1.70E-04 3.19E-06 6.43E-06 8.69E-07 31 0 287 8
484 Heat 1250 T Px 77.65 0.39 268.22 3.28 5.10 0.66 3845.65 14.41 1.53E-03 2.03E-05 1.83E-05 2.36E-06 45 0 404 21
484 Heat 1600 T Px 1.49 0.11 3.54 0.21 0.13 0.02 61.52 0.52 1.05E-03 9.94E-05 2.44E-05 4.75E-06 37 3 280 1
484 Heat Total T Px 109.89 0.53 283.54 3.29 5.93 0.66 4944.99 15.29 1.15E-03 1.44E-05 1.51E-05 1.69E-06 41 0 304 3
646 Heat 600 T Px 0.51 0.09 1.82 0.08 0.39 0.06 289.45 1.46 1.58E-03 2.76E-04 2.11E-04 4.85E-05 513 87 326 5
646 Heat 1250 T Px 2.59 0.07 5.73 0.12 0.22 0.04 260.25 0.90 9.83E-04 3.30E-05 2.33E-05 4.09E-06 91 2 293 7
646 Heat 1600 T Px 1.03 0.05 2.16 0.10 0.03 0.05 33.49 0.84 9.28E-04 6.19E-05 8.12E-06 1.25E-05 29 2 293 10
646 Heat Total T Px 4.14 0.12 9.71 0.17 0.63 0.08 583.18 1.91 1.04E-03 3.58E-05 4.28E-05 5.85E-06 128 4 300 5
446 Crush 1 I Ol 0.26 0.02 1.21 0.07 0.44 0.07 1.83 0.09 2.05E-03 1.97E-04 4.73E-04 8.43E-05 6 1 302 8
232
Temp ± Cl ± Br ± I ± K Br/Cl I/Cl K/Cl ± 40Ar/ ± 40Ar/ Sample Analysis °C / Island Phase Cl ppm Br ppb I ppb K ppm ± Br/Cl ± I/Cl ppm ppb ppb ppm (molar) (molar) (molar) K/Cl 36Ar 36Ar Crushes
446 Crush 3 I Ol 0.08 0.04 0.34 0.04 0.39 0.07 0.50 0.08 1.88E-03 1.02E-03 1.35E-03 7.56E-04 6 3 287 20
446 Crush 10 I Ol 0.15 0.02 1.23 0.13 1.16 0.10 0.92 0.14 3.55E-03 5.79E-04 2.11E-03 3.22E-04 5 1 304 13
446 Crush 20 I Ol 0.33 0.03 2.03 0.16 1.87 0.17 1.32 0.09 2.75E-03 3.61E-04 1.60E-03 2.20E-04 4 0 328 9
446 Crush Total I Ol 0.82 0.06 4.82 0.22 3.87 0.22 4.57 0.20 2.60E-03 2.28E-04 1.31E-03 1.23E-04 5 0 306 6
446 Heat 600 I Ol 0.95 0.04 8.41 0.23 9.03 0.68 26.76 0.27 3.92E-03 2.11E-04 2.65E-03 2.33E-04 25 1 302 2
446 Heat 800 I Ol 0.22 0.05 4.59 0.30 6.03 0.48 17.75 0.25 9.47E-03 2.11E-03 7.81E-03 1.78E-03 75 16 293 13
446 Heat 1000 I Ol 0.22 0.05 2.45 0.39 11.53 0.89 7.30 0.31 5.01E-03 1.34E-03 1.48E-02 3.37E-03 30 7 264 24
446 Heat 1200 I Ol 0.12 0.03 0.78 0.09 8.65 0.71 6.37 0.20 2.83E-03 7.66E-04 1.97E-02 5.03E-03 47 11 342 34
446 Heat 1400 I Ol 0.20 0.09 1.98 0.10 1.77 0.22 4.75 0.25 4.37E-03 1.99E-03 2.46E-03 1.16E-03 21 10 285 6
446 Heat 1600 I Ol 0.37 0.09 1.50 0.29 0.44 0.04 5.63 0.12 1.78E-03 5.60E-04 3.29E-04 8.73E-05 14 3 291 3
446 Heat Total I Ol 2.09 0.15 19.73 0.63 37.45 1.43 68.55 0.59 4.20E-03 3.40E-04 5.02E-03 4.18E-04 30 2 293 3
446 Crush + Heat Total I Ol 2.91 0.17 24.54 0.67 41.32 1.44 73.12 0.62 3.56E-03 2.12E-04 3.53E-03 2.23E-04 20 1 297 2
473 Crush 1 I Ol 0.45 0.01 2.88 0.13 0.11 0.03 0.81 0.07 2.83E-03 1.57E-04 7.02E-05 1.95E-05 2 0 288 21
473 Crush 3 I Ol 0.44 0.06 3.76 0.24 0.20 0.08 1.50 0.17 3.81E-03 6.00E-04 1.30E-04 5.48E-05 3 1 302 7
473 Crush 10 I Ol 0.61 0.01 3.63 0.21 0.19 0.03 3.04 0.12 2.65E-03 1.60E-04 8.93E-05 1.43E-05 5 0 319 10
473 Crush 20 I Ol 0.78 0.02 5.48 0.33 0.26 0.03 4.98 0.11 3.10E-03 1.99E-04 9.30E-05 1.08E-05 6 0 315 10
473 Crush Total I Ol 2.28 0.07 15.75 0.48 0.77 0.10 10.33 0.24 3.06E-03 1.30E-04 9.46E-05 1.22E-05 4 0 307 7
473 Heat 600 I Ol 2.73 0.12 11.19 0.79 0.91 0.13 133.36 1.32 1.82E-03 1.52E-04 9.32E-05 1.36E-05 44 2 291 8
473 Heat 800 I Ol 3.11 0.09 13.59 1.07 2.51 0.20 299.47 2.08 1.94E-03 1.63E-04 2.26E-04 1.89E-05 86 3 293 14
473 Heat 1000 I Ol 1.47 0.12 9.45 0.21 3.99 0.35 134.70 1.68 2.85E-03 2.34E-04 7.57E-04 8.90E-05 83 7 308 8
473 Heat 1200 I Ol 2.61 0.08 17.37 1.08 1.86 0.18 243.01 1.78 2.95E-03 2.06E-04 1.99E-04 2.03E-05 84 3 289 18
233
Temp ± Cl ± Br ± I ± K Br/Cl I/Cl K/Cl ± 40Ar/ ± 40Ar/ Sample Analysis °C / Island Phase Cl ppm Br ppb I ppb K ppm ± Br/Cl ± I/Cl ppm ppb ppb ppm (molar) (molar) (molar) K/Cl 36Ar 36Ar Crushes
473 Heat 1400 I Ol 0.76 0.17 8.52 0.54 0.34 0.05 14.67 1.09 4.99E-03 1.16E-03 1.26E-04 3.42E-05 18 4 277 4
473 Heat 1600 I Ol 1.32 0.16 10.51 0.25 0.85 0.14 25.83 0.72 3.54E-03 4.30E-04 1.80E-04 3.67E-05 18 2 295 2
473 Heat Total I Ol 12.00 0.31 70.63 1.82 10.47 0.48 851.04 3.71 2.61E-03 9.57E-05 2.44E-04 1.29E-05 64 2 293 2
473 Crush + Heat Total I Ol 14.28 0.32 86.38 1.88 11.24 0.49 861.36 3.72 2.78E-03 7.74E-05 1.89E-04 9.11E-06 42 1 295 2
480 Crush 1 I Ol nd nd 0.18 0.02 0.08 0.01 0.26 0.08 nd nd nd nd Nd nd 278 6
480 Crush 20 I Ol 0.15 0.03 1.02 0.08 0.14 0.03 2.28 0.08 2.95E-03 5.60E-04 2.54E-04 6.78E-05 13 2 309 18
480 Crush Total I Ol 0.14 0.04 1.20 0.08 0.22 0.03 2.54 0.12 3.88E-03 1.11E-03 4.39E-04 1.35E-04 17 5 296 11
480 Heat 600 I Ol 0.23 0.04 1.53 0.03 0.61 0.32 19.99 0.41 2.96E-03 4.77E-04 7.48E-04 4.08E-04 79 13 291 1
480 Heat 800 I Ol 0.12 0.02 0.89 0.06 0.28 0.05 49.40 0.35 3.22E-03 5.29E-04 6.29E-04 1.41E-04 365 55 290 13
480 Heat 1000 I Ol 0.05 0.03 0.57 0.04 0.15 0.03 20.48 0.21 4.89E-03 2.74E-03 8.25E-04 4.95E-04 359 200 314 13
480 Heat 1200 I Ol 0.04 0.03 0.36 0.03 0.19 0.03 6.49 0.21 4.32E-03 2.99E-03 1.40E-03 9.97E-04 159 109 272 26
480 Heat 1400 I Ol nd nd 0.05 0.05 0.01 0.01 0.46 0.15 nd nd nd nd nd nd 294 3
480 Heat 1600 I Ol 0.10 0.06 0.25 0.10 0.00 0.02 0.93 0.26 1.11E-03 8.18E-04 5.21E-06 4.52E-05 8 6 273 3
480 Heat Total I Ol 0.53 0.09 3.65 0.14 1.23 0.33 97.75 0.68 3.08E-03 5.66E-04 6.56E-04 2.10E-04 169 30 287 2
480 Crush + Heat Total I Ol 0.66 0.10 4.86 0.16 1.45 0.33 100.29 0.69 3.25E-03 5.10E-04 6.09E-04 1.66E-04 136 21 288 2
P20 (3) Heat 600 I Ol 0.75 0.02 3.78 0.05 0.47 0.03 19.58 0.23 2.24E-03 5.81E-05 1.74E-04 1.14E-05 24 1 303 2
P20 (3) Heat 1250 I Ol 0.45 0.01 1.03 0.12 0.06 0.02 204.19 1.14 1.02E-03 1.17E-04 4.03E-05 1.21E-05 415 11 290 11
P20 (3) Heat 1600 I Ol 0.55 0.04 0.80 0.07 0.06 0.01 14.51 0.56 6.42E-04 7.77E-05 3.26E-05 7.50E-06 24 2 383 4
P20 (3) Heat Total I Ol 1.75 0.05 5.60 0.15 0.59 0.04 238.28 1.29 1.42E-03 5.38E-05 9.51E-05 6.50E-06 124 3 330 2
P20 (3) Heat 600 I Px 0.46 0.03 4.56 0.12 1.35 0.08 37.38 0.54 4.41E-03 3.21E-04 8.25E-04 7.35E-05 74 5 264 12
P20 (3) Heat 1250 I Px 2.72 0.05 7.50 0.08 0.43 0.02 842.15 4.16 1.23E-03 2.58E-05 4.44E-05 2.59E-06 281 5 293 3
234
Temp ± Cl ± Br ± I ± K Br/Cl I/Cl K/Cl ± 40Ar/ ± 40Ar/ Sample Analysis °C / Island Phase Cl ppm Br ppb I ppb K ppm ± Br/Cl ± I/Cl ppm ppb ppb ppm (molar) (molar) (molar) K/Cl 36Ar 36Ar Crushes
P20 (3) Heat 1600 I Px 1.53 0.04 4.69 0.08 0.15 0.02 34.22 0.78 1.36E-03 4.21E-05 2.69E-05 3.21E-06 20 1 305 8
P20 (3) Heat Total I Px 4.71 0.07 16.76 0.17 1.93 0.08 913.75 4.27 1.58E-03 2.84E-05 1.15E-04 5.28E-06 176 3 295 4
235
8.2 Azores: Halogen, K, and 40Ar/36Ar results from irradiated samples (individual steps).
Temp°C Cl ± Cl I/Cl 40Ar/ ± 40Ar/ Sample Analysis / Phase Island Irradiation Br ppb ± Br ppb I ppb ± I ppb K ppm ± K ppm Br/Cl (molar) ± Br/Cl ± I/Cl ppm ppm (molar) 36Ar 36Ar Crushes
G11-01 Heat 1600 Ol G Azores 1 5.06 0.12 11.70 0.16 0.17 0.00 165.28 1.07 1.03E-03 2.74E-05 9.28E-06 3.39E-07 309 24
G11-04 Heat 1400 Ol G Azores 2 2.71 0.02 4.97 0.02 0.82 0.01 206.86 1.22 8.15E-04 7.10E-06 8.44E-05 1.20E-06 268 7
G11-04 Heat 1600 Ol G Azores 2 0.77 0.02 2.08 0.03 0.08 0.01 16.75 0.25 1.21E-03 3.40E-05 3.10E-05 2.63E-06 239 12
G11-04 Heat Total Ol G Azores 2 3.47 0.03 7.05 0.04 0.90 0.01 223.61 1.24 9.02E-04 8.57E-06 7.26E-05 1.13E-06 255 7
G11-08 Heat 1400 Ol G Azores 2 2.17 0.02 7.10 0.06 0.45 0.02 154.56 0.70 1.45E-03 1.69E-05 5.83E-05 3.12E-06 278 1
G11-08 Heat 1600 Ol G Azores 2 0.91 0.03 3.39 0.02 0.09 0.01 21.75 0.30 1.65E-03 5.63E-05 2.79E-05 2.42E-06 300 15
G11-08 Heat Total Ol G Azores 2 3.08 0.03 10.48 0.07 0.54 0.03 176.31 0.76 1.51E-03 1.96E-05 4.93E-05 2.34E-06 284 4
G11-15 Crush 20 Ol G Azores 1 0.01 0.07 0.97 0.03 0.01 0.01 0.64 0.42 3.94E-02 2.44E-01 1.82E-04 1.14E-03 216 61
G11-15 Heat 1600 Ol G Azores 1 1.89 0.11 10.70 0.14 0.19 0.00 14.19 0.77 2.52E-03 1.45E-04 2.78E-05 1.66E-06 303 23
G11-15 Crush + Heat Total Ol G Azores 1 1.90 0.16 12.41 0.15 0.20 0.01 15.32 1.07 2.89E-03 2.45E-04 2.93E-05 3.04E-06 280 26
G11-17 Heat 1400 Ol G Azores 2 1.50 0.05 4.11 0.01 0.60 0.01 59.21 0.30 1.22E-03 3.87E-05 1.11E-04 3.78E-06 269 10
G11-17 Heat 1600 Ol G Azores 2 0.45 0.02 1.29 0.01 0.06 0.01 9.01 0.26 1.26E-03 6.23E-05 3.96E-05 3.84E-06 299 19
G11-17 Heat Total Ol G Azores 2 1.96 0.05 5.40 0.01 0.66 0.01 68.22 0.40 1.23E-03 3.31E-05 9.44E-05 2.85E-06 281 10
G11-18 Crush 5 Ol G Azores 1 0.52 0.07 1.89 0.03 0.07 0.01 2.04 0.60 1.62E-03 2.07E-04 3.88E-05 5.65E-06 265 34
G11-18 Crush 20 Ol G Azores 1 0.40 0.09 1.70 0.03 0.09 0.00 1.24 0.65 1.88E-03 4.26E-04 5.92E-05 1.36E-05 1235 3446
G11-18 Crush Total Ol G Azores 1 0.92 0.11 3.60 0.05 0.16 0.01 3.28 0.88 1.74E-03 2.12E-04 4.77E-05 6.11E-06 371 120
G11-18 Heat 600 Ol G Azores 1 4.00 0.10 11.64 0.10 0.41 0.02 70.18 1.11 1.29E-03 3.55E-05 2.85E-05 1.47E-06 303 34
G11-18 Heat 1250 Ol G Azores 1 1.27 0.04 6.81 0.06 0.63 0.04 74.94 1.63 2.38E-03 8.21E-05 1.39E-04 9.64E-06 259 15
G11-18 Heat 1600 Ol G Azores 1 0.84 0.12 4.45 0.06 0.09 0.00 2.67 3.24 2.34E-03 3.31E-04 2.88E-05 4.34E-06 237 72
G11-18 Heat Total Ol G Azores 1 6.11 0.16 22.90 0.13 1.12 0.04 147.79 3.80 1.66E-03 4.56E-05 5.14E-05 2.39E-06 267 27
236
Temp°C Cl ± Cl I/Cl 40Ar/ ± 40Ar/ Sample Analysis / Phase Island Irradiation Br ppb ± Br ppb I ppb ± I ppb K ppm ± K ppm Br/Cl (molar) ± Br/Cl ± I/Cl ppm ppm (molar) 36Ar 36Ar Crushes
G11-18 Crush + Heat Total Ol G Azores 1 7.53 0.24 28.45 0.15 1.37 0.04 152.85 4.03 1.68E-03 5.37E-05 5.07E-05 2.28E-06 294 33
G11-20 Heat 1400 Ol G Azores 2 1.18 0.23 10.01 0.06 4.91 0.06 336.65 2.28 3.76E-03 7.41E-04 1.16E-03 2.29E-04 292 5
G11-20 Heat 1600 Ol G Azores 2 0.31 0.04 1.28 0.02 0.05 0.02 7.73 0.04 1.86E-03 2.50E-04 4.31E-05 1.65E-05 268 8
G11-20 Heat Total Ol G Azores 2 1.49 0.24 11.28 0.06 4.95 0.06 344.38 2.28 3.37E-03 5.36E-04 9.30E-04 1.49E-04 287 4
G11-01 Heat 1600 Px G Azores 1 0.76 0.07 5.02 0.07 0.47 0.01 6.08 4.57 2.93E-03 2.74E-04 1.72E-04 1.64E-05 104 37
G11-15 Crush 20 Px G Azores 1 0.72 0.27 6.61 0.09 0.20 0.01 0.40 2.92 4.07E-03 1.54E-03 7.68E-05 2.93E-05 305 3
G11-15 Heat 600 Px G Azores 1 3.09 0.30 10.61 0.14 1.37 0.04 94.94 2.25 1.52E-03 1.49E-04 1.23E-04 1.26E-05 380 32
G11-15 Crush + Heat Total Px G Azores 1 4.35 0.56 22.14 0.20 1.71 0.05 95.65 5.57 2.26E-03 2.92E-04 1.10E-04 1.45E-05 314 4
P11-06 Heat 1400 Ol P Azores 2 11.90 0.08 28.36 0.14 0.40 0.00 294.88 2.05 1.06E-03 8.50E-06 9.38E-06 1.01E-07 278 6
P11-06 Heat 1600 Ol P Azores 2 0.60 0.03 2.07 0.02 0.05 0.01 16.29 0.28 1.55E-03 7.40E-05 2.40E-05 3.61E-06 232 5
P11-06 Heat Total Ol P Azores 2 12.50 0.08 30.43 0.14 0.45 0.01 311.17 2.07 1.08E-03 8.58E-06 1.01E-05 1.92E-07 256 4
P11-12 Heat 1400 Ol P Azores 2 2.89 0.06 10.83 0.06 0.27 0.01 43.69 0.62 1.66E-03 3.50E-05 2.59E-05 1.43E-06 293 7
P11-12 Heat 1600 Ol P Azores 2 3.81 0.09 17.95 0.08 0.30 0.00 11.41 0.49 2.09E-03 5.10E-05 2.20E-05 5.94E-07 315 5
P11-12 Heat Total Ol P Azores 2 6.70 0.11 28.78 0.10 0.57 0.01 55.09 0.79 1.91E-03 3.16E-05 2.37E-05 7.08E-07 301 5
P11-13 Crush 20 Ol P Azores 1 0.20 0.03 0.41 0.02 0.01 0.00 3.02 2.07 9.22E-04 1.28E-04 2.03E-05 2.97E-06 131 19
P11-13 Heat 1600 Ol P Azores 1 3.71 0.17 8.07 0.17 0.28 0.02 103.46 2.57 9.65E-04 4.91E-05 2.14E-05 1.81E-06 236 11
P11-13 Crush + Heat Total Ol P Azores 1 4.33 0.19 9.34 0.18 0.33 0.02 112.81 6.90 9.59E-04 4.59E-05 2.13E-05 1.61E-06 218 10
P11-19 Crush 20 Ol P Azores 1 0.51 0.15 0.59 0.01 0.02 0.00 1.84 0.82 5.16E-04 1.50E-04 9.56E-06 2.90E-06 nd nd
P11-19 Heat 600 Ol P Azores 1 2.36 0.12 1.24 0.02 0.01 0.02 26.14 1.68 2.34E-04 1.26E-05 9.15E-07 2.18E-06 298 91
P11-19 Heat 1250 Ol P Azores 1 1.25 0.17 5.45 0.04 0.07 0.01 77.95 1.77 1.94E-03 2.72E-04 1.56E-05 3.66E-06 296 23
P11-19 Heat 1600 Ol P Azores 1 0.10 0.03 0.34 0.00 nd nd 6.01 0.28 1.48E-03 4.48E-04 nd nd 236 34
P11-19 Heat Total Ol P Azores 1 3.71 0.22 7.03 0.04 nd nd 110.10 2.46 8.43E-04 4.94E-05 nd nd 274 24
237
Temp°C Cl ± Cl I/Cl 40Ar/ ± 40Ar/ Sample Analysis / Phase Island Irradiation Br ppb ± Br ppb I ppb ± I ppb K ppm ± K ppm Br/Cl (molar) ± Br/Cl ± I/Cl ppm ppm (molar) 36Ar 36Ar Crushes
P11-19 Crush + Heat Total Ol P Azores 1 4.60 0.34 8.07 0.05 0.02 0.03 113.35 2.85 7.79E-04 5.74E-05 1.36E-06 1.99E-06 314 32
P11-20 Crush 20 Ol P Azores 1 0.33 0.17 0.10 0.02 0.01 0.01 3.15 1.50 1.40E-04 7.39E-05 5.26E-06 9.61E-06 77 28
P11-20 Heat 600 Ol P Azores 1 1.13 0.06 0.64 0.02 0.09 0.00 19.91 0.66 2.52E-04 1.47E-05 2.26E-05 1.52E-06 231 105
P11-20 Heat 1250 Ol P Azores 1 1.51 0.22 7.06 0.09 0.15 0.01 36.63 2.19 2.08E-03 3.01E-04 2.80E-05 4.54E-06 304 118
P11-20 Heat 1600 Ol P Azores 1 0.53 0.17 1.06 0.05 0.02 0.00 5.51 3.23 8.87E-04 2.90E-04 9.40E-06 3.07E-06 129 1
P11-20 Heat Total Ol P Azores 1 3.17 0.28 8.76 0.11 0.26 0.01 62.05 3.96 1.23E-03 1.10E-04 2.29E-05 2.30E-06 202 29
P11-20 Crush + Heat Total Ol P Azores 1 3.81 0.43 8.96 0.11 0.27 0.02 68.16 4.92 1.05E-03 1.19E-04 2.00E-05 2.88E-06 173 24
P11-13 Crush 20 Px P Azores 1 0.23 0.07 0.22 0.01 0.01 0.01 2.92 0.83 4.18E-04 1.27E-04 1.27E-05 9.56E-06 298 12
P11-13 Heat 600 Px P Azores 1 7.14 0.29 21.72 0.28 0.26 0.01 247.18 2.09 1.35E-03 5.83E-05 1.02E-05 6.23E-07 270 11
P11-13 Crush + Heat Total Px P Azores 1 7.53 0.32 22.09 0.28 0.28 0.02 252.19 2.53 1.30E-03 5.73E-05 1.04E-05 7.66E-07 280 8
P11-20 Crush 20 Px P Azores 1 1.19 0.12 3.28 0.12 0.27 0.04 37.20 2.95 1.23E-03 1.30E-04 6.44E-05 1.07E-05 286 0
P11-20 Heat 600 Px P Azores 1 38.15 0.29 94.22 1.23 6.56 0.14 1155.15 6.91 1.10E-03 1.66E-05 4.81E-05 1.11E-06 253 9
P11-20 Crush + Heat Total Px P Azores 1 40.38 0.37 100.37 1.25 7.08 0.16 1224.97 8.85 1.10E-03 1.70E-05 4.90E-05 1.18E-06 285 1
SMi11-05 Heat 1400 Ol SM Azores 2 0.51 0.01 3.75 0.02 0.79 0.01 13.08 0.15 3.25E-03 7.42E-05 4.33E-04 1.13E-05 269 5
SMi11-05 Heat 1600 Ol SM Azores 2 0.16 0.01 0.52 0.03 0.04 0.00 2.84 0.19 1.41E-03 1.29E-04 6.50E-05 7.10E-06 299 18
SMi11-05 Heat Total Ol SM Azores 2 0.68 0.02 4.26 0.03 0.83 0.01 15.92 0.24 2.80E-03 7.26E-05 3.44E-04 9.67E-06 277 6
SMi11-08 Heat 1400 Ol SM Azores 2 0.38 0.01 1.23 0.03 0.38 0.01 37.13 0.44 1.45E-03 5.79E-05 2.87E-04 1.27E-05 306 4
SMi11-08 Heat 1600 Ol SM Azores 2 0.62 0.04 1.52 0.06 0.18 0.01 15.05 0.20 1.09E-03 8.25E-05 8.21E-05 5.95E-06 317 13
SMi11-08 Heat Total Ol SM Azores 2 0.99 0.04 2.75 0.07 0.57 0.01 52.18 0.49 1.23E-03 5.97E-05 1.59E-04 7.80E-06 311 6
SMi11-10 Crush 20 Ol SM Azores 1 0.18 0.03 2.03 0.05 0.02 0.00 0.94 1.16 5.02E-03 7.31E-04 2.72E-05 4.18E-06 340 32
SMi11-10 Heat 600 Ol SM Azores 1 0.93 0.11 0.95 0.05 0.05 0.01 17.31 2.83 4.55E-04 6.02E-05 1.49E-05 3.26E-06 303 170
SMi11-10 Heat 1250 Ol SM Azores 1 2.73 0.29 11.77 0.16 0.10 0.01 79.13 2.01 1.92E-03 2.07E-04 1.06E-05 1.39E-06 298 135
238
Temp°C Cl ± Cl I/Cl 40Ar/ ± 40Ar/ Sample Analysis / Phase Island Irradiation Br ppb ± Br ppb I ppb ± I ppb K ppm ± K ppm Br/Cl (molar) ± Br/Cl ± I/Cl ppm ppm (molar) 36Ar 36Ar Crushes
SMi11-10 Heat 1600 Ol SM Azores 1 5.16 0.05 29.16 0.26 0.14 0.01 4.81 0.60 2.51E-03 3.27E-05 7.37E-06 5.78E-07 707 70
SMi11-10 Heat Total Ol SM Azores 1 8.82 0.32 41.89 0.31 0.29 0.02 101.26 3.52 2.11E-03 7.73E-05 9.15E-06 6.04E-07 425 106
SMi11-10 Crush + Heat Total Ol SM Azores 1 9.10 0.32 45.05 0.32 0.32 0.02 102.73 3.96 2.20E-03 7.87E-05 9.70E-06 6.00E-07 398 69
SMi11-13 Crush 20 Ol SM Azores 2 0.12 0.02 0.43 0.01 0.07 0.01 1.16 0.06 1.57E-03 2.23E-04 1.67E-04 2.96E-05 329 10
SMi11-13 Heat 1400 Ol SM Azores 2 2.94 0.02 8.26 0.05 0.14 0.01 154.65 0.72 1.25E-03 1.17E-05 1.32E-05 8.93E-07 305 2
SMi11-13 Heat 1600 Ol SM Azores 2 0.15 0.02 0.70 0.01 0.26 0.01 4.92 0.03 2.03E-03 3.24E-04 4.80E-04 8.05E-05 292 11
SMi11-13 Heat Total Ol SM Azores 2 3.10 0.03 8.96 0.05 0.40 0.02 159.57 0.72 1.29E-03 1.53E-05 3.64E-05 1.57E-06 301 4
SMi11-13 Crush + Heat Total Ol SM Azores 2 3.24 0.04 9.48 0.05 0.49 0.02 160.97 0.73 1.30E-03 1.69E-05 4.24E-05 1.74E-06 310 4
SMi11-13X Crush 20 Ol SM Azores 2 0.04 0.01 0.24 0.01 nd nd 0.55 0.07 2.46E-03 5.40E-04 nd nd 51 8
SMi11-13X Heat 1400 Ol SM Azores 2 0.61 0.01 1.71 0.02 0.08 0.01 6.15 0.74 1.25E-03 2.26E-05 3.63E-05 3.73E-06 308 7
SMi11-13X Heat 1600 Ol SM Azores 2 0.16 0.02 0.81 0.02 0.02 0.01 3.58 0.19 2.29E-03 2.29E-04 2.84E-05 1.63E-05 317 7
SMi11-13X Heat Total Ol SM Azores 2 0.76 0.02 2.52 0.03 0.09 0.01 9.73 0.77 1.47E-03 3.71E-05 3.47E-05 4.51E-06 311 5
SMi11-13X Crush + Heat Total Ol SM Azores 2 0.81 0.02 2.80 0.03 0.09 0.02 10.36 0.77 1.53E-03 4.19E-05 3.04E-05 6.51E-06 296 5
SMi11-17 Heat 1400 Ol SM Azores 2 1.40 0.03 4.46 0.02 0.44 0.01 24.58 0.55 1.41E-03 2.96E-05 8.83E-05 2.33E-06 324 9
SMi11-17 Heat 1600 Ol SM Azores 2 0.18 0.01 0.43 0.01 0.08 0.00 2.80 0.12 1.04E-03 5.35E-05 1.25E-04 8.36E-06 290 7
SMi11-17 Heat Total Ol SM Azores 2 1.59 0.03 4.90 0.02 0.53 0.01 27.39 0.57 1.37E-03 2.66E-05 9.26E-05 2.29E-06 309 6
SMi11-19 Crush 5 Ol SM Azores 1 0.00 0.07 0.15 0.01 0.00 0.01 0.60 0.98 1.38E-01 2.13E+01 1.18E-03 1.82E-01 311 266
SMi11-19 Crush 20 Ol SM Azores 1 0.13 0.11 0.26 0.01 0.02 0.00 0.08 1.75 8.73E-04 7.36E-04 4.52E-05 3.89E-05 357 1087
SMi11-19 Crush Total Ol SM Azores 1 0.13 0.13 0.40 0.01 0.02 0.01 0.68 2.00 1.37E-03 1.38E-03 4.93E-05 5.65E-05 327 387
SMi11-19 Heat 600 Ol SM Azores 1 1.11 0.15 0.59 0.01 0.02 0.00 26.29 1.54 2.39E-04 3.27E-05 6.19E-06 1.36E-06 283 145
SMi11-19 Heat 1250 Ol SM Azores 1 1.05 0.18 4.02 0.03 0.06 0.00 125.84 1.86 1.71E-03 3.02E-04 1.49E-05 2.83E-06 1455 2389
SMi11-19 Heat 1600 Ol SM Azores 1 0.12 0.03 0.40 0.01 0.01 0.00 5.30 1.41 1.52E-03 3.65E-04 1.93E-05 9.97E-06 180 7
239
Temp°C Cl ± Cl I/Cl 40Ar/ ± 40Ar/ Sample Analysis / Phase Island Irradiation Br ppb ± Br ppb I ppb ± I ppb K ppm ± K ppm Br/Cl (molar) ± Br/Cl ± I/Cl ppm ppm (molar) 36Ar 36Ar Crushes
SMi11-19 Heat Total Ol SM Azores 1 2.27 0.24 5.02 0.04 0.09 0.01 157.43 2.80 9.83E-04 1.04E-04 1.09E-05 1.43E-06 393 108
SMi11-19 Crush + Heat Total Ol SM Azores 1 2.48 0.32 5.66 0.04 0.13 0.02 158.52 4.25 1.02E-03 1.31E-04 1.41E-05 3.01E-06 385 109
SMi11-20 Heat 1400 Ol SM Azores 2 3.04 0.04 6.85 0.03 0.46 0.01 167.01 0.68 1.00E-03 1.42E-05 4.20E-05 8.29E-07 301 15
SMi11-20 Heat 1600 Ol SM Azores 2 0.52 0.02 1.68 0.02 0.04 0.00 30.12 0.61 1.45E-03 4.94E-05 2.25E-05 2.19E-06 239 9
SMi11-20 Heat Total Ol SM Azores 2 3.55 0.04 8.54 0.03 0.50 0.01 197.14 0.92 1.07E-03 1.40E-05 3.92E-05 7.71E-07 269 8
SMi11-19 Crush 20 Px SM Azores 1 1.69 0.02 2.31 0.06 nd nd 53.13 2.58 6.07E-04 1.85E-05 nd nd 287 1
SMi11-19 Heat 600 Px SM Azores 1 12.89 0.30 31.56 0.45 0.31 0.01 575.90 3.22 1.09E-03 3.01E-05 6.75E-06 2.39E-07 344 22
SMi11-19 Crush + Heat Total Px SM Azores 1 15.19 0.31 34.70 0.46 0.12 0.06 648.07 4.76 1.02E-03 2.45E-05 2.25E-06 1.18E-06 288 1
T11-02 Heat 1400 Ol T Azores 2 1.95 0.04 4.21 0.03 0.21 0.01 51.76 0.62 9.60E-04 2.22E-05 3.05E-05 1.60E-06 230 7
T11-02 Heat 1600 Ol T Azores 2 0.32 0.02 0.90 0.02 0.03 0.01 5.93 0.15 1.26E-03 9.56E-05 2.49E-05 8.45E-06 246 3
T11-02 Heat Total Ol T Azores 2 2.27 0.05 5.12 0.04 0.24 0.01 57.69 0.64 1.00E-03 2.26E-05 2.97E-05 1.82E-06 238 4
T11-05 Crush 5 Ol T Azores 1 0.13 0.53 0.06 0.03 0.02 0.00 2.08 4.59 1.99E-04 8.41E-04 4.37E-05 1.83E-04 188 851
T11-05 Heat 600 Ol T Azores 1 2.38 0.39 1.45 0.07 0.09 0.02 41.95 4.60 2.71E-04 4.57E-05 1.09E-05 2.61E-06 790 378
T11-05 Heat 1250 Ol T Azores 1 2.60 0.19 8.68 0.09 0.24 0.03 72.89 2.81 1.48E-03 1.11E-04 2.63E-05 3.40E-06 277 12
T11-05 Heat 1600 Ol T Azores 1 1.25 0.11 6.33 0.13 0.06 0.01 7.53 1.15 2.26E-03 1.97E-04 1.37E-05 2.25E-06 277 19
T11-05 Heat Total Ol T Azores 1 6.23 0.44 16.46 0.17 0.40 0.03 122.37 5.52 1.17E-03 8.47E-05 1.79E-05 1.92E-06 314 15
T11-05 Crush + Heat Total Ol T Azores 1 6.40 0.87 16.54 0.18 0.43 0.03 125.27 8.44 1.15E-03 1.56E-04 1.86E-05 2.87E-06 310 42
T11-10 Heat 1600 Ol T Azores 2 3.47 0.04 6.76 0.04 0.12 0.00 86.56 0.21 8.66E-04 1.06E-05 9.52E-06 1.90E-07 289 5
T11-16 Heat 1400 Ol T Azores 2 1.29 0.02 4.44 0.03 1.56 0.03 44.12 0.33 1.52E-03 2.39E-05 3.37E-04 8.41E-06 286 12
T11-16 Heat 1600 Ol T Azores 2 0.27 0.01 0.88 0.02 0.10 0.00 6.12 0.19 1.45E-03 7.83E-05 1.08E-04 6.87E-06 265 10
T11-16 Heat Total Ol T Azores 2 1.56 0.02 5.32 0.04 1.67 0.03 50.25 0.38 1.51E-03 2.40E-05 2.98E-04 7.17E-06 276 8
T11-18 Crush 20 Ol T Azores 1 0.41 0.27 0.17 0.01 0.01 0.00 3.45 3.16 1.86E-04 1.23E-04 6.30E-06 4.26E-06 151 94
240
Temp°C Cl ± Cl I/Cl 40Ar/ ± 40Ar/ Sample Analysis / Phase Island Irradiation Br ppb ± Br ppb I ppb ± I ppb K ppm ± K ppm Br/Cl (molar) ± Br/Cl ± I/Cl ppm ppm (molar) 36Ar 36Ar Crushes
T11-18 Heat 600 Ol T Azores 1 4.34 0.09 4.22 0.18 0.14 0.00 101.22 1.04 4.32E-04 2.07E-05 9.15E-06 3.63E-07 230 12
T11-18 Crush + Heat Total Ol T Azores 1 5.17 0.56 4.57 0.18 0.16 0.01 108.31 6.56 3.93E-04 4.51E-05 8.69E-06 9.83E-07 208 37
T11-19 Heat 1400 Ol T Azores 2 1.42 0.04 2.15 0.02 0.08 0.00 81.22 0.64 6.75E-04 2.18E-05 1.66E-05 1.10E-06 503 40
T11-19 Heat 1600 Ol T Azores 2 0.48 0.03 0.74 0.01 0.01 0.02 10.23 0.76 6.78E-04 4.79E-05 4.04E-06 1.40E-05 234 9
T11-19 Heat Total Ol T Azores 2 1.90 0.06 2.89 0.02 0.09 0.02 91.45 1.00 6.76E-04 2.03E-05 1.34E-05 3.66E-06 340 14
T11-20 Heat 1400 Ol T Azores 2 0.77 0.03 1.89 0.02 0.21 0.01 41.67 0.43 1.09E-03 4.29E-05 7.74E-05 4.41E-06 270 6
T11-20 Heat 1600 Ol T Azores 2 0.31 0.02 0.44 0.02 0.00 0.01 3.85 0.12 6.23E-04 3.94E-05 2.65E-06 6.74E-06 348 12
T11-20 Heat Total Ol T Azores 2 1.09 0.03 2.33 0.03 0.22 0.01 45.52 0.45 9.53E-04 3.14E-05 5.58E-05 3.51E-06 301 6
T11-16 Crush 20 Px T Azores 1 2.67 0.18 6.40 0.23 0.00 0.07 48.64 1.94 1.07E-03 8.11E-05 4.04E-07 7.83E-06 286 1
T11-16 Heat 600 Px T Azores 1 81.65 0.66 180.93 2.34 3.84 0.08 4921.74 26.32 9.85E-04 1.50E-05 1.32E-05 3.08E-07 333 28
T11-16 Crush + Heat Total Px T Azores 1 84.80 0.69 188.49 2.35 3.85 0.12 4979.22 26.42 9.88E-04 1.47E-05 1.27E-05 4.16E-07 287 1
SJ-101/07 Crush 20 Ol SJ Azores 2 0.58 0.05 1.51 0.08 0.49 0.07 1.90 0.21 1.15E-03 1.16E-04 2.32E-04 4.06E-05 237 30
SJ-101/07 Heat 1400 Ol SJ Azores 2 4.34 0.19 10.19 0.08 0.47 0.04 122.87 2.15 1.04E-03 4.62E-05 3.01E-05 2.94E-06 282 6
SJ-101/07 Heat 1600 Ol SJ Azores 2 0.26 0.12 0.27 0.05 0.00 0.01 2.88 0.58 4.65E-04 2.23E-04 5.16E-06 1.13E-05 211 18
SJ-101/07 Heat Total Ol SJ Azores 2 4.60 0.22 10.47 0.09 0.47 0.04 125.75 2.23 1.01E-03 4.97E-05 2.87E-05 2.91E-06 272 6
SJ-101/07 Crush + Heat Total Ol SJ Azores 2 5.36 0.23 12.42 0.14 1.10 0.10 128.21 2.25 1.03E-03 4.61E-05 5.74E-05 5.99E-06 267 7
SJ-103/07 Crush 20 Ol SJ Azores 2 0.04 0.03 0.19 0.01 0.02 0.01 1.15 0.07 1.89E-03 1.46E-03 1.33E-04 1.16E-04 380 74
SJ-103/07 Heat 1400 Ol SJ Azores 2 3.12 0.08 7.08 0.04 0.23 0.01 166.12 1.45 1.01E-03 2.54E-05 2.04E-05 8.76E-07 308 11
SJ-103/07 Heat 1600 Ol SJ Azores 2 0.26 0.04 0.85 0.01 nd nd 7.86 0.24 1.43E-03 2.12E-04 nd nd 390 18
SJ-103/07 Heat Total Ol SJ Azores 2 3.38 0.09 7.92 0.04 0.23 0.01 173.98 1.47 1.04E-03 2.70E-05 1.87E-05 1.12E-06 343 10
SJ-103/07 Crush + Heat Total Ol SJ Azores 2 3.43 0.09 8.14 0.04 0.25 0.02 175.28 1.48 1.05E-03 2.94E-05 2.04E-05 1.40E-06 348 12
FA-02/07 Crush 20 Ol FA Azores 2 0.09 0.01 0.26 0.02 0.01 0.00 0.99 0.15 1.25E-03 1.86E-04 3.52E-05 7.20E-06 279 4
241
Temp°C Cl ± Cl I/Cl 40Ar/ ± 40Ar/ Sample Analysis / Phase Island Irradiation Br ppb ± Br ppb I ppb ± I ppb K ppm ± K ppm Br/Cl (molar) ± Br/Cl ± I/Cl ppm ppm (molar) 36Ar 36Ar Crushes
FA-02/07 Heat 1400 Ol FA Azores 2 0.41 0.02 0.68 0.01 0.06 0.01 14.79 0.75 7.48E-04 4.23E-05 4.31E-05 7.51E-06 283 11
FA-02/07 Heat 1600 Ol FA Azores 2 0.68 0.09 1.42 0.06 0.10 0.03 19.35 1.17 9.28E-04 1.28E-04 4.10E-05 1.27E-05 276 25
FA-02/07 Heat Total Ol FA Azores 2 1.09 0.09 2.11 0.06 0.16 0.03 34.15 1.39 8.61E-04 7.73E-05 4.17E-05 8.44E-06 279 17
FA-02/07 Crush + Heat Total Ol FA Azores 2 1.32 0.10 2.75 0.07 0.19 0.03 36.57 1.44 9.28E-04 7.29E-05 4.06E-05 7.08E-06 279 8
FA-29/07 Heat 1400 Ol FA Azores 2 3.28 0.03 7.16 0.22 0.68 0.04 136.63 0.36 9.70E-04 3.13E-05 5.77E-05 3.32E-06 293 8
FA-29/07 Heat 1600 Ol FA Azores 2 0.28 0.01 0.99 0.01 0.27 0.01 10.68 0.22 1.59E-03 5.61E-05 2.70E-04 1.34E-05 250 8
FA-29/07 Heat Total Ol FA Azores 2 3.56 0.03 8.15 0.22 0.95 0.04 147.31 0.42 1.02E-03 2.91E-05 7.43E-05 3.18E-06 269 6
FA-38/07 Crush 20 Ol FA Azores 2 0.09 0.01 0.29 0.04 0.01 0.01 0.67 0.16 1.38E-03 2.92E-04 2.34E-05 2.63E-05 283 47
FA-38/07 Heat 1400 Ol FA Azores 2 8.35 0.11 14.18 0.08 0.11 0.01 232.80 1.51 7.54E-04 1.09E-05 3.82E-06 4.70E-07 297 7
FA-38/07 Heat 1600 Ol FA Azores 2 16.63 0.10 47.41 0.14 0.53 0.03 475.69 3.08 1.27E-03 8.10E-06 8.83E-06 5.81E-07 296 4
FA-38/07 Heat Total Ol FA Azores 2 24.98 0.15 61.59 0.16 0.64 0.04 708.49 3.43 1.10E-03 6.99E-06 7.16E-06 4.18E-07 296 4
FA-38/07 Crush + Heat Total Ol FA Azores 2 25.14 0.15 62.07 0.17 0.65 0.04 709.62 3.44 1.10E-03 7.15E-06 7.26E-06 4.46E-07 295 6
C-09-01 Heat 1400 Ol C Azores 2 9.00 0.09 16.50 0.09 9.39 0.09 149.43 1.57 8.15E-04 9.40E-06 2.92E-04 3.95E-06 294 4
C-09-01 Heat 1600 Ol C Azores 2 2.49 0.04 4.64 0.01 0.59 0.02 25.29 0.43 8.28E-04 1.34E-05 6.60E-05 2.99E-06 242 2
C-09-01 Heat Total Ol C Azores 2 11.49 0.10 21.14 0.09 9.98 0.09 174.73 1.62 8.18E-04 7.93E-06 2.43E-04 3.01E-06 262 2
C-09-05 Heat 1400 Ol C Azores 2 2.77 0.04 10.33 0.09 1.12 0.02 88.59 0.95 1.66E-03 2.67E-05 1.13E-04 2.17E-06 285 4
C-09-05 Heat 1600 Ol C Azores 2 0.65 0.01 2.60 0.04 0.11 0.01 22.60 1.49 1.78E-03 4.32E-05 4.72E-05 3.05E-06 278 7
C-09-05 Heat Total Ol C Azores 2 3.42 0.04 12.93 0.10 1.23 0.02 111.20 1.77 1.68E-03 2.32E-05 1.00E-04 1.80E-06 283 4
C-09-06 Crush 20 Ol C Azores 2 0.15 0.02 0.38 0.01 0.14 0.01 0.82 0.09 1.09E-03 1.45E-04 2.56E-04 3.85E-05 330 11
C-09-06 Heat 1600 Ol C Azores 2 0.48 0.02 0.91 0.02 0.64 0.03 9.16 0.40 8.44E-04 3.41E-05 3.77E-04 2.29E-05 279 18
C-09-06 Heat 1700 Ol C Azores 2 0.22 0.02 0.55 0.02 0.36 0.05 4.01 0.37 1.13E-03 1.36E-04 4.67E-04 8.64E-05 292 23
C-09-06 Heat Total Ol C Azores 2 0.69 0.03 1.46 0.03 1.01 0.06 13.17 0.55 9.33E-04 4.41E-05 4.05E-04 3.03E-05 285 14
242
Temp°C Cl ± Cl I/Cl 40Ar/ ± 40Ar/ Sample Analysis / Phase Island Irradiation Br ppb ± Br ppb I ppb ± I ppb K ppm ± K ppm Br/Cl (molar) ± Br/Cl ± I/Cl ppm ppm (molar) 36Ar 36Ar Crushes
C-09-06 Crush + Heat Total Ol C Azores 2 0.92 0.04 2.01 0.04 1.21 0.06 14.38 0.57 9.72E-04 4.69E-05 3.69E-04 2.56E-05 307 9
C-09-07 Crush 20 Ol C Azores 2 0.15 0.02 0.49 0.01 0.20 0.01 0.75 0.12 1.50E-03 2.38E-04 3.88E-04 6.47E-05 345 27
C-09-07 Heat 1600 Ol C Azores 2 0.11 0.02 0.14 0.02 0.01 0.00 1.10 0.09 5.86E-04 1.20E-04 2.35E-05 9.84E-06 209 29
C-09-07 Heat 1700 Ol C Azores 2 0.09 0.01 0.16 0.01 nd nd 1.42 0.07 8.49E-04 1.10E-04 nd nd 239 24
C-09-07 Heat Total Ol C Azores 2 0.19 0.02 0.30 0.02 0.01 0.00 2.52 0.12 7.03E-04 8.70E-05 2.35E-05 9.84E-06 226 19
C-09-07 Crush + Heat Total Ol C Azores 2 0.39 0.04 0.97 0.02 0.28 0.01 3.52 0.20 1.10E-03 1.09E-04 2.03E-04 6.55E-05 279 16
C-09-13 Heat 1400 Ol C Azores 2 0.49 0.04 1.94 0.03 3.46 0.03 41.62 0.39 1.75E-03 1.57E-04 1.97E-03 1.75E-04 300 2
C-09-13 Heat 1600 Ol C Azores 2 0.18 0.03 0.34 0.01 0.09 0.01 2.61 0.09 8.66E-04 1.67E-04 1.40E-04 2.91E-05 279 5
C-09-13 Heat Total Ol C Azores 2 0.67 0.06 2.28 0.03 3.55 0.04 44.22 0.40 1.52E-03 1.27E-04 1.48E-03 1.24E-04 299 2
C-09-18 Heat 1400 Ol C Azores 2 5.51 0.08 15.62 0.05 1.54 0.02 81.97 0.41 1.26E-03 1.87E-05 7.83E-05 1.48E-06 303 3
C-09-18 Heat 1600 Ol C Azores 2 0.49 0.01 1.64 0.01 0.07 0.00 7.63 0.16 1.50E-03 2.86E-05 4.09E-05 1.60E-06 278 7
C-09-18 Heat Total Ol C Azores 2 6.00 0.08 17.26 0.05 1.61 0.02 89.61 0.44 1.28E-03 1.75E-05 7.52E-05 1.34E-06 297 3
FL-09-23 Crush 20 Ol FL Azores 2 0.21 0.01 0.55 0.02 0.08 0.02 0.73 0.14 1.17E-03 9.12E-05 1.06E-04 3.39E-05 353 27
FL-09-23 Heat 1600 Ol FL Azores 2 0.50 0.02 2.75 0.01 1.22 0.06 44.43 0.34 2.46E-03 1.23E-04 6.85E-04 4.69E-05 287 5
FL-09-23 Heat 1700 Ol FL Azores 2 0.03 0.06 0.09 0.00 0.07 0.02 1.33 0.46 1.27E-03 2.35E-03 5.98E-04 1.12E-03 225 15
FL-09-23 Heat Total Ol FL Azores 2 0.53 0.07 2.84 0.01 1.29 0.06 45.76 0.57 2.39E-03 2.97E-04 6.80E-04 9.00E-05 284 5
FL-09-23 Crush + Heat Total Ol FL Azores 2 0.78 0.07 3.51 0.02 1.38 0.07 46.65 0.59 1.99E-03 1.74E-04 4.94E-04 4.91E-05 290 5
FL-09-26 Heat 1400 Ol FL Azores 2 4.16 0.05 6.95 0.01 1.29 0.01 147.55 0.81 7.43E-04 8.54E-06 8.69E-05 1.31E-06 504 30
FL-09-26 Heat 1600 Ol FL Azores 2 0.48 0.01 0.58 0.02 nd nd 34.21 0.53 5.40E-04 1.78E-05 nd nd 280 8
FL-09-26 Heat Total Ol FL Azores 2 4.64 0.05 7.53 0.02 1.27 0.01 181.76 0.96 7.22E-04 7.72E-06 7.64E-05 1.13E-06 419 16
FL-09-32 Heat 1400 Ol FL Azores 2 14.68 0.10 44.97 0.05 15.66 0.17 409.88 0.89 1.36E-03 9.53E-06 2.98E-04 3.83E-06 310 13
FL-09-32 Heat 1600 Ol FL Azores 2 1.58 0.06 6.23 0.03 1.64 0.02 43.22 1.01 1.75E-03 6.48E-05 2.91E-04 1.14E-05 288 11
243
Temp°C Cl ± Cl I/Cl 40Ar/ ± 40Ar/ Sample Analysis / Phase Island Irradiation Br ppb ± Br ppb I ppb ± I ppb K ppm ± K ppm Br/Cl (molar) ± Br/Cl ± I/Cl ppm ppm (molar) 36Ar 36Ar Crushes
FL-09-32 Heat Total Ol FL Azores 2 16.26 0.12 51.20 0.06 17.31 0.17 453.10 1.35 1.40E-03 1.02E-05 2.97E-04 3.64E-06 298 8
FL-09-41 Heat 1600 Ol FL Azores 2 0.33 0.03 0.60 0.02 nd nd 5.99 0.57 8.20E-04 7.48E-05 nd nd 318 58
FL-09-41 Heat 1700 Ol FL Azores 2 0.02 0.01 0.08 0.01 0.01 0.00 0.50 0.06 2.13E-03 1.44E-03 9.25E-05 7.20E-05 147 10
FL-09-41 Heat Total Ol FL Azores 2 0.34 0.03 0.69 0.02 0.00 0.00 6.50 0.57 8.86E-04 8.27E-05 5.42E-07 3.77E-06 242 25
FL-09-42 Crush 20 Ol FL Azores 2 1.23 0.04 3.72 0.08 0.16 0.04 4.26 0.06 1.34E-03 5.66E-05 3.64E-05 9.98E-06 256 19
FL-09-42 Heat 1400 Ol FL Azores 2 0.50 0.04 0.54 0.03 nd nd 23.21 2.14 4.82E-04 5.15E-05 nd nd 332 74
FL-09-42 Heat 1600 Ol FL Azores 2 2.46 0.08 3.44 0.05 0.31 0.03 133.32 0.83 6.22E-04 2.17E-05 3.53E-05 3.42E-06 269 9
FL-09-42 Heat Total Ol FL Azores 2 2.96 0.09 3.98 0.06 0.29 0.03 156.53 2.29 5.98E-04 2.02E-05 2.78E-05 2.87E-06 277 11
FL-09-42 Crush + Heat Total Ol FL Azores 2 4.40 0.10 8.34 0.11 0.48 0.06 161.52 2.29 8.42E-04 2.29E-05 3.06E-05 3.80E-06 272 10
244
8.3 Canaries: Halogen, K, and 40Ar/36Ar results from irradiated samples (individual steps).
± ± Temp°C Cl ± Cl ± Br ± I ± K Br/Cl K/Cl 40Ar/ Sample Analysis Phase Island Irradiation Br ppb I ppb K ppm ± Br/Cl I/Cl (molar) ± I/Cl K/ 40Ar/ / Crushes ppm ppm ppb ppb ppm (molar) (molar) 36Ar Cl 36Ar
TF39 Crush 20 Ol T Canaries 0.04 0.01 0.15 0.01 0.04 0.01 1.10 0.21 1.47E-03 3.69E-04 2.42E-04 6.60E-05 22.6 6.8 284 6
TF39 Heat 600 Ol T Canaries nd nd 0.43 0.00 0.25 0.02 8.47 0.49 nd nd nd nd nd nd 291 4 34. TF39 Heat 1400 Ol T Canaries 0.41 0.17 0.66 0.04 0.19 0.01 37.48 0.74 7.15E-04 3.01E-04 1.28E-04 5.38E-05 82.8 6 299 3 11. TF39 Heat 1600 Ol T Canaries 0.07 0.04 0.11 0.00 0.01 0.00 1.10 0.67 6.80E-04 3.70E-04 3.58E-05 2.34E-05 14.0 4 297 12 38. TF39 Heat Total Ol T Canaries 0.45 0.18 1.20 0.04 0.45 0.02 47.04 1.11 1.19E-03 4.78E-04 2.80E-04 1.13E-04 95.4 3 295 3 27. TF39 Crush + Heat Total Ol T Canaries 0.55 0.18 1.53 0.05 0.54 0.02 49.54 1.21 1.24E-03 4.11E-04 2.73E-04 9.08E-05 82.0 1 293 2
TF40 Crush 20 Ol T Canaries 0.08 0.02 0.17 0.02 0.08 0.01 1.45 0.12 9.12E-04 2.62E-04 2.64E-04 7.38E-05 15.7 4.5 297 12
TF40 Heat 1400 Ol T Canaries 1.06 0.05 2.18 0.04 0.77 0.03 61.16 0.79 9.14E-04 4.76E-05 2.03E-04 1.22E-05 52.4 2.7 309 4
TF40 Heat 1600 Ol T Canaries nd nd 0.24 0.01 0.05 0.01 0.53 1.00 nd nd nd nd nd nd 293 12
TF40 Heat Total Ol T Canaries 1.06 0.05 2.42 0.04 0.82 0.03 61.69 1.27 9.14E-04 4.76E-05 2.03E-04 1.22E-05 52.4 2.7 304 5
TF40 Crush + Heat Total Ol T Canaries 1.27 0.13 2.92 0.06 1.06 0.03 65.95 1.32 1.02E-03 1.04E-04 2.32E-04 2.41E-05 47.2 4.8 301 6
TF41 Crush 20 Ol T Canaries 0.04 0.01 0.18 0.01 0.41 0.01 0.81 0.19 1.90E-03 4.19E-04 2.75E-03 5.99E-04 17.8 5.6 256 10
TF41 Heat 1400 Ol T Canaries 1.03 0.06 4.00 0.10 16.31 0.76 76.23 1.35 1.73E-03 1.14E-04 4.44E-03 3.41E-04 67.4 4.3 297 4
TF41 Heat 1600 Ol T Canaries nd nd 0.05 0.02 0.37 0.06 nd nd nd nd nd nd nd nd 295 17
TF41 Heat Total Ol T Canaries 1.03 0.06 4.04 0.10 16.68 0.76 76.23 1.35 1.73E-03 1.14E-04 4.44E-03 3.41E-04 67.4 4.3 296 6
TF41 Crush + Heat Total Ol T Canaries 1.04 0.10 4.76 0.10 18.34 0.76 78.46 2.26 2.03E-03 1.94E-04 4.93E-03 5.01E-04 68.5 6.7 290 5
TF54 Crush 20 Px T Canaries 0.34 0.01 0.24 0.03 0.19 0.02 5.36 0.21 3.17E-04 3.80E-05 1.54E-04 1.54E-05 14.5 0.7 253 18
TF54 Heat 1400 Px T Canaries 35.67 0.28 11.21 0.14 2.72 0.12 316.53 1.67 1.40E-04 2.04E-06 2.13E-05 9.63E-07 8.1 0.1 299 4 20. TF54 Heat 1600 Px T Canaries 0.10 0.05 nd nd 0.12 0.01 0.18 2.31 nd nd 3.20E-04 1.62E-04 1.6 4 283 17
TF54 Heat Total Px T Canaries 35.78 0.28 11.21 0.14 2.84 0.12 316.71 2.85 1.40E-04 2.04E-06 2.22E-05 9.67E-07 8.0 0.1 294 5
TF54 Crush + Heat Total Px T Canaries 36.63 0.28 11.80 0.16 3.31 0.13 330.29 2.89 1.43E-04 2.22E-06 2.52E-05 1.01E-06 8.2 0.1 285 6
LP7 Crush 20 Ol LP Canaries 0.11 0.03 0.19 0.02 0.09 0.01 nd nd 7.50E-04 2.28E-04 2.19E-04 6.60E-05 nd nd 318 3
245
± ± Temp°C Cl ± Cl ± Br ± I ± K Br/Cl K/Cl 40Ar/ Sample Analysis Phase Island Irradiation Br ppb I ppb K ppm ± Br/Cl I/Cl (molar) ± I/Cl K/ 40Ar/ / Crushes ppm ppm ppb ppb ppm (molar) (molar) 36Ar Cl 36Ar
LP7 Heat 1400 Ol LP Canaries 3.68 0.06 8.90 0.03 0.96 0.04 76.85 1.44 1.07E-03 1.70E-05 7.31E-05 3.25E-06 19.0 0.5 292 7
LP7 Heat 1600 Ol LP Canaries 0.42 0.10 0.15 0.01 0.03 0.01 3.93 0.97 1.61E-04 3.92E-05 2.11E-05 8.31E-06 8.5 2.9 299 9
LP7 Heat Total Ol LP Canaries 4.10 0.11 9.06 0.04 0.99 0.04 80.78 1.74 9.81E-04 2.73E-05 6.78E-05 3.38E-06 17.9 0.6 296 6
LP7 Crush + Heat Total Ol LP Canaries 4.42 0.15 9.58 0.06 1.24 0.04 78.92 2.21 9.64E-04 3.23E-05 7.85E-05 3.85E-06 17.9 0.6 313 3
LP9 Crush 20 Ol LP Canaries 0.10 0.01 0.17 0.02 0.60 0.02 1.43 0.10 7.40E-04 1.28E-04 1.63E-03 1.90E-04 12.6 1.6 325 6
LP9 Heat 1400 Ol LP Canaries 4.74 0.04 10.02 0.05 3.79 0.11 103.79 0.68 9.39E-04 1.01E-05 2.23E-04 6.90E-06 19.9 0.2 305 3
LP9 Heat 1600 Ol LP Canaries 0.14 0.03 0.15 0.01 0.12 0.01 1.45 0.15 5.01E-04 1.09E-04 2.47E-04 5.24E-05 9.6 2.2 299 3
LP9 Heat Total Ol LP Canaries 4.88 0.05 10.17 0.06 3.91 0.11 105.24 0.69 9.27E-04 1.11E-05 2.24E-04 6.84E-06 19.6 0.2 303 2
LP9 Crush + Heat Total Ol LP Canaries 5.18 0.06 10.68 0.09 5.67 0.13 109.44 0.75 9.16E-04 1.32E-05 3.06E-04 8.04E-06 19.2 0.3 308 2
LP10 Crush 20 Ol LP Canaries 0.16 0.03 0.42 0.10 0.35 0.02 1.15 0.34 1.15E-03 3.29E-04 5.96E-04 1.07E-04 6.5 2.2 280 12
LP10 Heat 1400 Ol LP Canaries 3.15 0.09 4.32 0.04 4.26 0.14 76.94 3.74 6.09E-04 1.90E-05 3.77E-04 1.69E-05 22.1 1.3 292 20
LP10 Heat 1600 Ol LP Canaries 0.49 0.08 0.17 0.01 0.11 0.01 6.13 2.39 1.51E-04 2.59E-05 6.14E-05 1.25E-05 11.3 4.8 301 9
LP10 Heat Total Ol LP Canaries 3.65 0.12 4.49 0.04 4.37 0.14 83.07 4.44 5.47E-04 1.93E-05 3.35E-04 1.58E-05 20.7 1.3 298 10
LP10 Crush + Heat Total Ol LP Canaries 3.88 0.13 5.11 0.15 4.87 0.15 84.76 4.46 5.84E-04 2.58E-05 3.51E-04 1.58E-05 19.8 1.2 295 8
LP12 Crush 20 Ol LP Canaries 0.09 0.02 0.16 0.00 2.35 0.60 0.37 0.15 8.13E-04 2.25E-04 7.40E-03 2.78E-03 3.8 1.9 720 29
LP12 Heat 1400 Ol LP Canaries 2.52 0.11 5.53 0.09 8.25 0.38 51.16 1.30 9.77E-04 4.53E-05 9.17E-04 5.80E-05 18.5 0.9 331 14
LP12 Heat 1600 Ol LP Canaries nd nd 0.12 0.01 0.22 0.02 nd nd nd nd nd nd nd nd 312 9
LP12 Heat Total Ol LP Canaries 2.52 0.11 5.64 0.09 8.47 0.38 51.16 1.30 9.77E-04 4.53E-05 9.17E-04 5.80E-05 18.5 0.9 323 9
LP12 Crush + Heat Total Ol LP Canaries 2.71 0.13 6.04 0.09 14.15 1.50 50.14 1.41 9.89E-04 4.89E-05 1.46E-03 1.69E-04 16.8 0.9 442 10
LP7 Crush 20 Px LP Canaries 0.31 0.02 1.40 0.01 0.37 0.02 2.48 0.08 2.00E-03 1.28E-04 3.33E-04 2.51E-05 7.2 0.5 442 20
LP7 Heat 1400 Px LP Canaries 15.15 0.12 31.32 0.26 2.67 0.09 326.45 1.55 9.19E-04 1.05E-05 4.92E-05 1.63E-06 19.6 0.2 299 1 247 LP7 Heat 1600 Px LP Canaries 0.02 0.04 0.03 0.01 0.03 0.00 2.96 0.51 6.57E-04 1.26E-03 4.04E-04 7.56E-04 131.9 .3 290 2
LP7 Heat Total Px LP Canaries 15.17 0.12 31.35 0.26 2.70 0.09 329.41 1.63 9.18E-04 1.07E-05 4.97E-05 1.64E-06 19.7 0.2 296 1
LP7 Crush + Heat Total Px LP Canaries 16.07 0.14 35.42 0.26 3.78 0.10 336.63 1.65 9.79E-04 1.11E-05 6.57E-05 1.77E-06 19.0 0.2 315 2
246
± ± Temp°C Cl ± Cl ± Br ± I ± K Br/Cl K/Cl 40Ar/ Sample Analysis Phase Island Irradiation Br ppb I ppb K ppm ± Br/Cl I/Cl (molar) ± I/Cl K/ 40Ar/ / Crushes ppm ppm ppb ppb ppm (molar) (molar) 36Ar Cl 36Ar
LP8> Crush 20 Px LP Canaries 0.30 0.03 0.74 0.03 20.09 1.39 1.36 0.09 1.09E-03 1.20E-04 1.87E-02 2.34E-03 4.1 0.5 464 3
LP8> Heat 1400 Px LP Canaries 18.45 0.15 41.47 0.40 56.55 1.17 383.14 1.90 9.99E-04 1.28E-05 8.57E-04 1.91E-05 18.9 0.2 341 8
LP8> Heat 1600 Px LP Canaries 0.15 0.03 nd nd 0.14 0.02 2.09 1.54 nd nd 2.63E-04 6.72E-05 12.7 9.7 294 10
LP8> Heat Total Px LP Canaries 18.60 0.16 41.47 0.40 56.69 1.17 385.23 2.45 9.99E-04 1.28E-05 8.52E-04 1.90E-05 18.8 0.2 324 7
LP8> Crush + Heat Total Px LP Canaries 19.27 0.17 43.12 0.41 101.65 3.32 388.27 2.45 9.94E-04 1.29E-05 1.47E-03 4.99E-05 18.3 0.2 404 4
LP8< Crush 20 Px LP Canaries 0.73 0.01 1.85 0.02 6.29 0.14 5.86 0.27 1.12E-03 2.16E-05 2.40E-03 6.66E-05 7.3 0.3 321 3
LP8< Heat 600 Px LP Canaries 28.10 0.26 19.76 0.23 28.20 1.06 504.85 2.45 3.12E-04 4.64E-06 2.81E-04 1.09E-05 16.3 0.2 298 10
LP8< Heat 1400 Px LP Canaries 54.68 0.47 132.26 0.70 15.78 0.61 689.09 4.13 1.07E-03 1.08E-05 8.07E-05 3.17E-06 11.4 0.1 300 15
LP8< Heat 1600 Px LP Canaries 0.46 0.04 0.10 0.03 0.06 0.01 nd nd 9.80E-05 2.85E-05 3.83E-05 5.28E-06 nd nd 329 12
LP8< Heat Total Px LP Canaries 83.24 0.54 152.13 0.74 44.05 1.22 1193.52 5.45 8.12E-04 6.55E-06 1.48E-04 4.20E-06 13.0 0.1 310 7
LP8< Crush + Heat Total Px LP Canaries 85.37 0.54 157.50 0.74 62.29 1.29 1210.53 5.51 8.20E-04 6.44E-06 2.04E-04 4.41E-06 12.9 0.1 316 3
LP9 Crush 20 Px LP Canaries 0.21 0.02 0.66 0.01 3.57 0.08 1.23 0.22 1.40E-03 1.06E-04 4.80E-03 3.66E-04 5.4 1.0 367 3
LP9 Heat 1400 Px LP Canaries 20.84 0.16 46.43 0.38 27.93 2.01 445.91 2.09 9.90E-04 1.10E-05 3.75E-04 2.71E-05 19.4 0.2 292 7
LP9 Heat 1600 Px LP Canaries 0.12 0.05 nd nd 0.19 0.02 nd nd nd nd 4.46E-04 1.85E-04 nd nd 366 26
LP9 Heat Total Px LP Canaries 20.96 0.16 46.43 0.38 28.12 2.01 445.91 2.09 9.90E-04 1.10E-05 3.75E-04 2.70E-05 19.4 0.2 316 9
LP9 Crush + Heat Total Px LP Canaries 21.57 0.17 48.28 0.38 38.55 2.02 449.11 2.88 9.95E-04 1.11E-05 5.00E-04 2.65E-05 18.9 0.2 352 4
LP12 Crush 20 Px LP Canaries 0.29 0.02 0.68 0.02 3.22 0.82 1.36 0.22 1.04E-03 8.56E-05 3.11E-03 8.29E-04 4.3 0.8 338 4
LP12 Heat 1400 Px LP Canaries 26.05 0.25 48.92 0.19 21.99 0.56 376.35 2.17 8.34E-04 8.57E-06 2.36E-04 6.38E-06 13.1 0.1 298 4 25. LP12 Heat 1600 Px LP Canaries 0.07 0.08 0.00 0.01 nd nd 1.24 1.58 5.70E-06 6.67E-05 nd nd 15.1 3 302 32
LP12 Heat Total Px LP Canaries 26.12 0.26 48.92 0.19 21.99 0.56 377.59 2.69 8.32E-04 8.91E-06 2.36E-04 6.38E-06 13.1 0.2 300 13
LP12 Crush + Heat Total Px LP Canaries 26.87 0.27 50.67 0.20 30.30 2.21 381.11 2.75 8.38E-04 8.94E-06 3.15E-04 2.32E-05 12.9 0.2 322 6
EH1 Crush 20 Ol EH Canaries 0.08 0.02 0.12 0.02 0.09 0.02 0.74 0.13 6.57E-04 2.10E-04 3.16E-04 1.21E-04 8.0 2.7 296 2
EH1 Heat 1400 Ol EH Canaries 3.51 0.06 8.51 0.11 0.33 0.03 74.21 0.74 1.08E-03 2.32E-05 2.62E-05 2.61E-06 19.2 0.4 272 7
EH1 Heat 1600 Ol EH Canaries nd nd 0.12 0.01 0.05 0.00 3.12 0.79 nd nd nd nd nd nd 315 18
247
± ± Temp°C Cl ± Cl ± Br ± I ± K Br/Cl K/Cl 40Ar/ Sample Analysis Phase Island Irradiation Br ppb I ppb K ppm ± Br/Cl I/Cl (molar) ± I/Cl K/ 40Ar/ / Crushes ppm ppm ppb ppb ppm (molar) (molar) 36Ar Cl 36Ar
EH1 Heat Total Ol EH Canaries 3.51 0.06 8.63 0.11 0.38 0.03 77.33 1.08 1.08E-03 2.32E-05 2.62E-05 2.61E-06 19.2 0.4 292 9
EH1 Crush + Heat Total Ol EH Canaries 3.67 0.10 8.94 0.12 0.62 0.07 79.21 1.13 1.08E-03 3.18E-05 4.75E-05 5.46E-06 19.6 0.6 295 3
EH2 Crush 20 Ol EH Canaries 0.06 0.01 0.12 0.01 0.64 0.02 1.01 0.09 8.18E-04 1.01E-04 2.87E-03 3.37E-04 14.5 2.1 480 22
EH2 Heat 1400 Ol EH Canaries 3.09 0.07 7.28 0.17 0.73 0.03 57.46 1.11 1.05E-03 3.28E-05 6.59E-05 2.87E-06 16.9 0.5 338 18
EH2 Heat 1600 Ol EH Canaries nd nd 0.29 0.03 0.05 0.00 3.15 1.17 nd nd nd nd nd nd 285 5
EH2 Heat Total Ol EH Canaries 3.09 0.07 7.56 0.17 0.78 0.03 60.61 1.61 1.05E-03 3.28E-05 6.59E-05 2.87E-06 16.9 0.5 306 7
EH2 Crush + Heat Total Ol EH Canaries 3.11 0.12 7.89 0.17 2.59 0.05 63.44 1.63 1.13E-03 5.09E-05 2.33E-04 1.03E-05 18.5 0.9 338 7
EH3 Crush 20 Ol EH Canaries 0.06 0.01 0.22 0.02 0.13 0.02 0.70 0.19 1.53E-03 2.91E-04 5.61E-04 1.18E-04 10.2 3.2 914 33
EH3 Heat 1400 Ol EH Canaries 4.88 0.07 11.49 0.02 0.76 0.03 102.38 1.29 1.05E-03 1.59E-05 4.34E-05 1.85E-06 19.0 0.4 369 26 33. EH3 Heat 1600 Ol EH Canaries 0.07 0.08 0.22 0.01 0.06 0.01 1.15 2.23 1.43E-03 1.56E-03 2.33E-04 2.55E-04 15.1 3 318 21
EH3 Heat Total Ol EH Canaries 4.95 0.11 11.72 0.02 0.82 0.03 103.54 2.57 1.05E-03 2.26E-05 4.61E-05 2.02E-06 19.0 0.6 340 16
EH3 Crush + Heat Total Ol EH Canaries 5.07 0.11 12.13 0.04 1.06 0.04 104.88 2.60 1.06E-03 2.29E-05 5.83E-05 2.72E-06 18.8 0.6 573 18
EH4 Crush 20 Ol EH Canaries 0.07 0.02 0.07 0.01 0.17 0.02 0.80 0.11 4.11E-04 1.47E-04 6.53E-04 2.12E-04 10.2 3.3 403 9
EH4 Heat 1400 Ol EH Canaries 5.89 0.07 13.99 0.21 0.74 0.03 118.99 1.40 1.05E-03 1.99E-05 3.49E-05 1.27E-06 18.3 0.3 331 8
EH4 Heat 1600 Ol EH Canaries 0.26 0.09 0.56 0.04 0.09 0.01 1.62 1.39 9.70E-04 3.27E-04 9.86E-05 3.36E-05 5.7 5.2 311 10
EH4 Heat Total Ol EH Canaries 6.15 0.11 14.55 0.21 0.83 0.03 120.61 1.97 1.05E-03 2.42E-05 3.76E-05 1.38E-06 17.8 0.4 321 6
EH4 Crush + Heat Total Ol EH Canaries 6.35 0.12 14.74 0.22 1.30 0.07 122.88 2.00 1.03E-03 2.52E-05 5.72E-05 3.26E-06 17.6 0.4 363 5
EH5 Crush 20 Ol EH Canaries 0.25 0.01 0.29 0.01 0.14 0.01 1.01 0.32 5.12E-04 3.51E-05 1.58E-04 1.07E-05 3.6 1.2 318 1
EH5 Heat 1400 Ol EH Canaries 9.36 0.16 17.95 0.18 1.28 0.05 137.58 1.78 8.53E-04 1.69E-05 3.82E-05 1.52E-06 13.4 0.3 287 5
EH5 Heat 1600 Ol EH Canaries 0.29 0.05 0.24 0.04 0.09 0.01 0.17 1.51 3.64E-04 8.47E-05 8.86E-05 1.96E-05 0.5 4.7 294 9
EH5 Heat Total Ol EH Canaries 9.65 0.17 18.20 0.18 1.37 0.05 137.76 2.33 8.38E-04 1.69E-05 3.97E-05 1.55E-06 13.0 0.3 289 5
EH5 Crush + Heat Total Ol EH Canaries 10.43 0.17 19.09 0.19 1.81 0.05 140.85 2.53 8.13E-04 1.57E-05 4.85E-05 1.59E-06 12.3 0.3 303 3
EH7 Crush 20 Ol EH Canaries 0.31 0.01 0.59 0.02 0.17 0.01 0.55 0.26 8.56E-04 4.23E-05 1.59E-04 1.36E-05 1.6 0.8 344 31
EH7 Heat 1400 Ol EH Canaries 5.57 0.13 12.59 0.16 3.46 0.08 123.98 1.07 1.00E-03 2.62E-05 1.74E-04 5.80E-06 20.2 0.5 298 6
248
± ± Temp°C Cl ± Cl ± Br ± I ± K Br/Cl K/Cl 40Ar/ Sample Analysis Phase Island Irradiation Br ppb I ppb K ppm ± Br/Cl I/Cl (molar) ± I/Cl K/ 40Ar/ / Crushes ppm ppm ppb ppb ppm (molar) (molar) 36Ar Cl 36Ar
EH7 Heat 1600 Ol EH Canaries 0.01 0.07 0.29 0.01 0.05 0.01 nd nd 1.84E-02 1.76E-01 2.01E-03 1.92E-02 nd nd 304 10
EH7 Heat Total Ol EH Canaries 5.58 0.14 12.89 0.16 3.51 0.08 123.55 2.21 1.03E-03 2.94E-05 1.76E-04 6.22E-06 20.1 0.6 300 5
EH7 Crush + Heat Total Ol EH Canaries 5.94 0.15 13.60 0.16 3.72 0.09 124.21 2.23 1.02E-03 2.75E-05 1.75E-04 5.88E-06 19.0 0.6 307 6
EH8 Crush 20 Ol EH Canaries 0.12 0.02 0.15 0.01 0.06 0.01 0.85 0.18 5.65E-04 1.16E-04 1.42E-04 4.18E-05 6.5 1.8 625 60
EH8 Heat 1400 Ol EH Canaries 4.53 0.07 8.92 0.10 0.34 0.02 85.14 0.84 8.74E-04 1.72E-05 2.11E-05 1.27E-06 17.0 0.3 291 4 12. EH8 Heat 1600 Ol EH Canaries 0.17 0.06 0.23 0.02 0.05 0.01 3.81 1.84 6.01E-04 2.32E-04 8.82E-05 3.48E-05 20.7 7 318 19
EH8 Heat Total Ol EH Canaries 4.70 0.10 9.14 0.10 0.40 0.02 88.95 2.02 8.64E-04 2.02E-05 2.35E-05 1.33E-06 17.2 0.5 301 7
EH8 Crush + Heat Total Ol EH Canaries 5.05 0.12 9.58 0.11 0.57 0.04 91.44 2.09 8.43E-04 2.18E-05 3.17E-05 2.56E-06 16.4 0.5 416 16
EH12 Crush 20 Ol EH Canaries 0.33 0.01 0.94 0.02 0.08 0.01 2.00 0.12 1.28E-03 4.19E-05 6.43E-05 4.82E-06 5.6 0.4 450 11
EH12 Heat 1400 Ol EH Canaries 7.76 0.11 17.82 0.14 0.35 0.03 157.67 1.65 1.02E-03 1.60E-05 1.27E-05 1.01E-06 18.5 0.3 343 20
EH12 Heat 1600 Ol EH Canaries nd nd 0.44 0.02 0.08 0.01 2.00 1.70 nd nd nd nd nd nd 277 12
EH12 Heat Total Ol EH Canaries 7.76 0.11 18.26 0.14 0.43 0.03 159.67 2.37 1.02E-03 1.60E-05 1.27E-05 1.01E-06 18.5 0.3 302 10
EH12 Crush + Heat Total Ol EH Canaries 8.42 0.17 20.52 0.15 0.61 0.03 164.47 2.39 1.08E-03 2.29E-05 2.03E-05 1.11E-06 17.7 0.4 376 8
EH16 Crush 20 Ol EH Canaries 0.04 0.01 0.08 0.01 0.06 0.01 0.49 0.11 1.02E-03 3.43E-04 4.85E-04 1.57E-04 12.0 4.5 311 27
EH16 Heat 1400 Ol EH Canaries 6.59 0.07 15.52 0.11 1.41 0.04 137.09 1.16 1.05E-03 1.40E-05 5.99E-05 1.93E-06 18.9 0.3 320 8
EH16 Heat 1600 Ol EH Canaries nd nd 0.50 0.02 0.06 0.00 2.03 1.15 nd nd nd nd nd nd 351 18
EH16 Heat Total Ol EH Canaries 6.59 0.07 16.02 0.11 1.47 0.04 139.12 1.64 1.05E-03 1.40E-05 5.99E-05 1.93E-06 18.9 0.3 331 8
EH16 Crush + Heat Total Ol EH Canaries 6.63 0.10 16.15 0.11 1.56 0.04 139.82 1.65 1.08E-03 1.73E-05 6.58E-05 2.11E-06 19.1 0.4 325 10
EH3 Crush 20 Px EH Canaries 0.78 0.02 2.38 0.02 1.90 0.24 2.21 0.23 1.36E-03 2.82E-05 6.81E-04 8.84E-05 2.6 0.3 365 3
EH3 Heat 1400 Px EH Canaries 22.58 0.18 54.68 0.46 4.05 0.30 401.50 1.95 1.08E-03 1.23E-05 5.01E-05 3.70E-06 16.1 0.1 302 8
EH3 Heat 1600 Px EH Canaries nd nd 0.24 0.04 0.16 0.02 0.07 3.90 nd nd nd nd nd nd 286 17
EH3 Heat Total Px EH Canaries 22.58 0.18 54.92 0.46 4.21 0.30 401.57 4.36 1.08E-03 1.23E-05 5.01E-05 3.70E-06 16.1 0.1 297 8
EH3 Crush + Heat Total Px EH Canaries 24.75 0.21 62.01 0.46 9.87 0.78 408.16 4.41 1.11E-03 1.26E-05 1.11E-04 8.90E-06 15.0 0.2 345 3
EH4 Crush 20 Px EH Canaries 0.46 0.02 0.98 0.02 2.91 0.36 1.77 0.12 9.46E-04 3.72E-05 1.77E-03 2.28E-04 3.5 0.3 338 2
249
± ± Temp°C Cl ± Cl ± Br ± I ± K Br/Cl K/Cl 40Ar/ Sample Analysis Phase Island Irradiation Br ppb I ppb K ppm ± Br/Cl I/Cl (molar) ± I/Cl K/ 40Ar/ / Crushes ppm ppm ppb ppb ppm (molar) (molar) 36Ar Cl 36Ar
EH4 Heat 1400 Px EH Canaries 33.94 0.27 72.42 0.46 7.72 0.21 656.41 2.82 9.48E-04 9.53E-06 6.36E-05 1.78E-06 17.6 0.2 317 7 28. EH4 Heat 1600 Px EH Canaries 0.05 0.07 0.00 0.01 0.05 0.00 0.72 0.87 2.28E-06 8.82E-05 2.95E-04 4.57E-04 14.4 2 294 4
EH4 Heat Total Px EH Canaries 33.99 0.27 72.42 0.46 7.76 0.21 657.13 2.95 9.47E-04 9.71E-06 6.39E-05 1.78E-06 17.6 0.2 306 4
EH4 Crush + Heat Total Px EH Canaries 34.84 0.28 74.23 0.46 13.17 0.70 660.41 2.96 9.47E-04 9.52E-06 1.06E-04 5.69E-06 17.2 0.2 323 2
EH7 Crush 20 Px EH Canaries 0.90 0.04 3.41 0.03 1.89 0.24 2.41 0.57 1.69E-03 7.96E-05 5.88E-04 7.82E-05 2.4 0.6 304 2
EH7 Heat 600 Px EH Canaries 4.11 0.11 5.12 0.11 6.07 0.24 66.31 1.45 5.53E-04 1.89E-05 4.13E-04 1.95E-05 14.7 0.5 318 21
EH7 Heat 1400 Px EH Canaries 9.02 0.08 21.64 0.13 3.50 0.13 164.13 1.72 1.07E-03 1.16E-05 1.08E-04 4.26E-06 16.5 0.2 271 14 14. EH7 Heat 1600 Px EH Canaries 0.12 0.10 0.21 0.02 0.01 0.01 1.84 0.73 8.19E-04 7.34E-04 2.91E-05 4.43E-05 14.5 1 289 11
EH7 Heat Total Px EH Canaries 13.25 0.17 26.97 0.17 9.58 0.27 232.28 2.36 9.04E-04 1.30E-05 2.02E-04 6.29E-06 15.9 0.3 287 8
EH7 Crush + Heat Total Px EH Canaries 14.83 0.19 32.97 0.18 12.90 0.49 236.52 2.56 9.88E-04 1.35E-05 2.43E-04 9.82E-06 14.5 0.2 301 2
EH10 Crush 20 Px EH Canaries 0.33 0.01 0.59 0.03 0.96 0.12 1.12 0.17 8.02E-04 5.33E-05 8.16E-04 1.08E-04 3.1 0.5 319 5
EH10 Heat 600 Px EH Canaries 3.55 0.04 2.31 0.05 2.64 0.07 54.79 2.86 2.89E-04 6.94E-06 2.08E-04 5.74E-06 14.0 0.8 492 81
EH10 Heat 1400 Px EH Canaries 13.51 0.13 36.85 0.10 5.35 0.13 241.53 1.57 1.21E-03 1.20E-05 1.11E-04 2.90E-06 16.2 0.2 353 21
EH10 Heat 1600 Px EH Canaries 0.29 0.04 nd nd nd nd nd nd nd nd nd nd nd nd 320 17
EH10 Heat Total Px EH Canaries 17.34 0.14 39.16 0.13 7.97 0.15 295.33 3.59 1.00E-03 8.92E-06 1.28E-04 2.62E-06 15.5 0.2 363 16
EH10 Crush + Heat Total Px EH Canaries 18.30 0.15 40.88 0.17 10.75 0.39 298.58 3.62 9.93E-04 8.88E-06 1.64E-04 6.06E-06 14.8 0.2 334 6
EH12 Crush 20 Px EH Canaries 0.27 0.01 0.73 0.04 0.20 0.01 1.38 0.07 1.20E-03 9.07E-05 2.03E-04 1.34E-05 4.6 0.3 319 5
EH12 Heat 1400 Px EH Canaries 15.64 0.17 35.33 0.17 2.14 0.05 294.62 2.02 1.00E-03 1.17E-05 3.83E-05 1.02E-06 17.1 0.2 260 9 19. EH12 Heat 1600 Px EH Canaries 0.10 0.06 0.05 0.02 0.02 0.01 3.05 1.38 2.23E-04 1.60E-04 4.57E-05 3.53E-05 26.8 7 263 15
EH12 Heat Total Px EH Canaries 15.74 0.18 35.38 0.17 2.16 0.05 297.67 2.45 9.99E-04 1.22E-05 3.84E-05 1.03E-06 17.2 0.2 261 8
EH12 Crush + Heat Total Px EH Canaries 16.31 0.18 36.93 0.19 2.58 0.06 300.60 2.46 1.01E-03 1.22E-05 4.41E-05 1.07E-06 16.7 0.2 287 5
250
8.4 Azores – collected samples locations (samples analysed for halogens in bold).
Sample # Date Location Coordinates Description
G11-01 13/09/2011 Fenais 0417215 4320731 Roadcutting
G11-02 13/09/2011 Fenais 0417287 4319566 Roadcutting
G11-03 13/09/2011 Carapacho 0416755 4318981 Roadcutting
G11-04 13/09/2011 Termas do Carapacho 0416835 4318578 Beach outcrop
G11-05 13/09/2011 Folga 0413478 4319317 Roadcutting
G11-06 13/09/2011 Folga 0413572 4319154 Cliff exposure
G11-07 13/09/2011 Canada Longa 0414530 4321188 Roadcutting
G11-08 13/09/2011 Caldeira 0415203 4320520 Roadcutting
G11-09 13/09/2011 Caldeira / Furna do Enxofre 0415834 4319965 Outcrop at pathside
G11-10 13/09/2011 Caldeira / Furna do Enxofre 0415738 4319924 Roadcutting
G11-11 13/09/2011 Serra das Fontes 0413022 4323992 Roadcutting
G11-12 13/09/2011 NW of Quitadouro 0414297 4324732 Roadcutting
G11-13 13/09/2011 SW of Miradouro 0414654 4323986 Roadcutting
251
Sample # Date Location Coordinates Description
G11-14 14/09/2011 Near Pico Machado 0414452 4325477 Roadcutting
G11-15 14/09/2011 Near Porta da Barra 0413465 4327042 Beach outcrop
G11-16 14/09/2011 Barro Vermelho 0410966 4327945 Beach outcrop
G11-17 14/09/2011 Ponta da Barca 0409311 4327648 Roadcutting
G11-18 14/09/2011 Baía da Vitória 0408684 4326184 Roadcutting
G11-19 14/09/2011 Porto Afonso 0407340 4324744 Beach outcrop
G11-20 14/09/2011 Tanque 0410938 4322271 Roadcutting
G11-21 14/09/2011 Near Esperança Velha 0409913 4322197 Roadcutting
G11-22 14/09/2011 Near Serra Branca 0410751 4320695 Roadcutting
G11-23 14/09/2011 Near Limeira 0412799 4320475 Roadcutting
G11-24 14/09/2011 Near Lagao 0415300 4323504 Roadcutting
P11-01 16/09/2011 São Roque 0385105 4263648 Roadcutting
P11-02 16/09/2011 Near São Roque 0384951 4263630 Outcrop at roadside
P11-03 16/09/2011 Near Cabeço do Redonodo 0386867 4258579 Outcrop
252
Sample # Date Location Coordinates Description
P11-04 16/09/2011 Cabeço do Padre Roque 0400227 4254703 Outcrop at roadside
P11-05 16/09/2011 Cova do Cabo da Canada 0401106 4254824 Roadcutting
P11-06 16/09/2011 Manhenha 0409606 4251980 Beach outcrop
P11-07 16/09/2011 Calheta de Nesquim 0405384 4251155 Roadcutting
P11-08 16/09/2011 Near Foros 0402903 4251695 Roadcutting
P11-09 16/09/2011 Near Pontas Negras 0397826 4252154 Roadcutting
P11-10 16/09/2011 Terra, near Santa Bárbara 0393265 4249662 Roadcutting
P11-11 16/09/2011 Near Lajes 0390864 4249880 Roadcutting
P11-12 16/09/2011 Near Silveira 0386745 4252858 Roadcutting
P11-13 16/09/2011 Pic de Filipe 0385836 4257728 Roadcutting
P11-14 16/09/2011 Nr Cabeço da Passogem 0383844 4254452 Roadcutting
P11-15 16/09/2011 Nr Caetano 0375319 4254368 Roadcutting
P11-16 16/09/2011 Nr São Mateus 0371934 4254997 Roadcutting
P11-17 16/09/2011 Nr Verdelho and Lajidos 0365715 4263871 Beach outcrop
253
Sample # Date Location Coordinates Description
P11-18 16/09/2011 Near Cabeço Chão 0369359 4268182 Roadcutting
P11-19 16/09/2011 Lajido, near airport 0375177 4268624 Beach outcrop
P11-20 16/09/2011 Lajido, near airport (flow below P11-19) 0375193 4268616 Beach outcrop
P11-20a 16/09/2011 Lajido, near airport (flow below P11-19) 0375204 4268623 Beach outcrop
SMi11-01 07/09/2011 Povoação 0655070 4180020 Roadcutting
SMi11-02 07/09/2011 1km west of Faial da Terra 0658655 4178791 Roadcutting
SMi11-03 07/09/2011 1km west of SMi11-02 0658307 4178333 Roadcutting
SMi11-04 07/09/2011 Agua da Retorta 0661730 4181183 Outcrop
SMi11-05 07/09/2011 1km N from Agua da Retorta, on road to Nordeste 0662931 4182185 Outcrop at roadside
SMi11-06 07/09/2011 20m SW from carpark @ Ponta da Madrugada 0663171 4183914 Outcrop at roadside
SMi11-07 07/09/2011 Guilherme ou dos Moinhos 0662575 4189417 Roadcutting
SMi11-08 07/09/2011 After Santo Antonio de Nordestinho 0656644 4190910 Roadcutting
SMi11-09 07/09/2011 1km S of Algarvia 0655022 4190368 Roadcutting
SMi11-10 08/09/2011 Ferraria (carpark at Termas da Ferraria) 0601066 4191057 Boulders
254
Sample # Date Location Coordinates Description
SMi11-11 08/09/2011 200m up road from SMi11-10 601077 4190848 Outcrop
SMi11-12 08/09/2011 Same as SMi11-11 601077 4190848 Outcrop
SMi11-13 08/09/2011 Mosteiros 0603936 4195344 Wave cut platform
SMi11-14 08/09/2011 Sete Cidades 0607339 4193053 Outcrop at lakeside
SMi11-15 08/09/2011 20m from SMi11-14 0607376 4193043 Boulders
SMi11-16 08/09/2011 Sete Cidades 0607079 4193076 Outcrop in path
SMi11-17 08/09/2011 Sete Cidades 0607019 4193073 Outcrop
SMi11-18 09/09/2011 Near Lagao do Fogo 0632666 4181552 Roadcutting
SMi11-19 09/09/2011 Near Pico Dr. Ferreira 0623751 4183525 Roadcutting
SMi11-20 09/09/2011 Pinhal da Paz 0619188 4183337 Outcrop in path
T11-01 10/09/2011 Terra Chã 0477960 4281707 Roadcutting
T11-02 10/09/2011 Veredas 0479025 4282684 Outcrop in wall at roadside
T11-03 10/09/2011 Furnas do Enxofre 0479870 4286524 Roadcutting
T11-04 10/09/2011 Pico Gordo 0477410 4287607 Roadcutting
255
Sample # Date Location Coordinates Description
T11-05 10/09/2011 Pico das Caldeirinhas 0478098 4287735 Outcrop in field, below cinder cone
T11-06 10/09/2011 Gruta do Natal 0476098 4287735 Outcrop
T11-07 10/09/2011 Biscoitos 0476838 4292943 Roadcutting
T11-08 10/09/2011 Biscoitos 0477574 4294817 Beach outcrop
T11-09 10/09/2011 Near T11-08 0477574 4294817 Bomb on beach
T11-10 10/09/2011 Biscoitos 0478161 4294650 Roadcutting
T11-11 10/09/2011 Lagoa da Fajãzinha 0483309 4293847 Boulder field, below cliff exposure
T11-12 10/09/2011 Lagoa da Fajãzinha 0483314 4294006 Boulder
T11-13 10/09/2011 Serreta 0468943 4289855 Outcrop at roadside
T11-14 10/09/2011 Santa Bárbara 0469136 4282664 Outcrop
T11-15 11/09/2011 Porto Martins 0495486 4282867 Roadcutting
T11-16 11/09/2011 Ponta Negra 0495172 4281175 Beach outcrop
T11-17 11/09/2011 Canada do Mato 0490108 4280176 Roadcutting
T11-18 11/09/2011 Porto Judeu 0487780 4277455 Roadcutting
256
Sample # Date Location Coordinates Description
T11-19 11/09/2011 South of Pico do Areeiro 0483410 4285074 Roadcutting
T11-20 11/09/2011 Caldeira 0489388 4292636 Beach outcrop
257
8.5 Additional Canary Islands graphs
Halogen ratio plot of the crushing releases (additional data from Bruland and Lohan, 2003; Fehn et al., 2006; Jambon et al., 1995; Kendrick et al., 2014a). Error bars, where not seen, are smaller than symbols. 258
Halogen ratio of the crushing releases plot with pore fluid, serpentinites, wedge fluid, MORB, bulk Earth, and seawater data from literature (additional data from Anders and Ebihara, 1982; Bruland and Lohan, 2003; Chavrit et al., 2016; Fehn et al., 2006; Fehn et al., 2000; Fehn et al., 2003; Fehn et al., 2007b; Jambon et al., 1995; John et al., 2011; Kastner et al., 1990; Kendrick, 2012; Kendrick et al., 2013a; Kendrick et al., 2014a; Lu et al., 2008; Mahn and Gieskes, 2001; Martin et al., 1993; Muramatsu et al., 2007; Muramatsu et al., 2001; Sumino et al., 2010; Tomaru et al., 2007). Error bars, where not seen, are smaller than symbols. 259
Halogen ratio plot of the integrated stepped heating releases (additional data from Bruland and Lohan, 2003; Fehn et al., 2006; Jambon et al., 1995; Kendrick et al., 2014a). Error bars, where not seen, are smaller than symbols.
260
Halogen ratio plot of the integrated stepped heating releases with pore fluid, serpentinites, wedge fluid, MORB, bulk Earth, and seawater data from literature (additional data from Anders and Ebihara, 1982; Bruland and Lohan, 2003; Chavrit et al., 2016; Fehn et al., 2006; Fehn et al., 2000; Fehn et al., 2003; Fehn et al., 2007b; Jambon et al., 1995; John et al., 2011; Kastner et al., 1990; Kendrick, 2012; Kendrick et al., 2013a; Kendrick et al., 2014a; Lu et al., 2008; Mahn and Gieskes, 2001; Martin et al., 1993; Muramatsu et al., 2007; Muramatsu et al., 2001; Sumino et al., 2010; Tomaru et al., 2007). Error bars, where not seen, are smaller than symbols. 261
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