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..............................................................................................