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The Magmatic Evolution of Oceanic Plateaus: a Case Study from the Azores

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The Magmatic Evolution of Oceanic Plateaus: a Case Study from the Azores The magmatic evolution of Oceanic Plateaus: a case study from the Azores Dissertation zur Erlangung des Doktorgrades der Mathematisch-Naturwissenschaftlichen Fakultät der Christian-Albrechts-Universität zu Kiel vorgelegt von Christoph Beier Kiel 2006 Ich versichere an Eides statt, daß die vorliegende Abhandlung ausschließlich unter Verwendung der angegeben Hilfsmittel entstanden ist und, abgesehen von der Beratung durch meine akademischen Lehrer, nach Inhalt und Form meine eigene Arbeit darstellt. Des weiteren habe ich weder diese noch eine ähnliche Arbeit an dieser oder einer anderen Hochschule im Rahmen eines Prüfungsverfahrens vorgelegt oder veröffentlicht. Kiel, den ...................... ………………………………………… Christoph Beier Referent/in: ...................................................................................................................... Koreferent/in: .................................................................................................................... Tag der mündlichen Prüfung: .................................................................................. Zum Druck genehmigt: Kiel, .................................................................................. Der Dekan TABLE OF CONTENTS Vorwort .......................................................................................................................................XI Einführung und Zusammenfassung ............................................................................................ XIII Introduction and Summary ......................................................................................................... XV CHAPTER I GEOCHEMICAL AND GEOCHRONOLOGICAL CONSTRAINTS ON THE VOLCANIC EVOLUTION OF THE AZORES PLATEAU 1.1 ABSTRACT .......................................................................................................................................... 1 1.2 INTRODUCTION .................................................................................................................................. 2 1.3 GEOLOGICAL SETTING ..................................................................................................................... 2 1.4 METHODS .......................................................................................................................................... 4 1.4.1 Sampling and sample treatment 4 1.5 RESULTS ............................................................................................................................................ 9 1.5.1 Major elements, trace elements and isotopes 9 1.5.2 Age dating 11 1.6 DISCUSSION ..................................................................................................................................... 11 1.6.1 Partial melting processes of the Azores Plateau magmas 11 1.6.2 Source heterogeneities 15 1.7 DYNAMICS OF THE AZORES PLATEAU ............................................................................................ 18 1.8 ACKNOWLEDGMENTS ..................................................................................................................... 20 REFERENCES .......................................................................................................................................... 21 V TABLE OF CONTENTS CHAPTER II MELTING AND MANTLE SOURCE VARIATIONS ALONG AN ULTRASLOW SPREADING RIFT: THE TERCEIRA AXIS, AZORES 2.1 ABSTRACT ........................................................................................................................................ 27 2.2 INTRODUCTION ................................................................................................................................ 28 2.3 GEOLOGICAL SETTING ................................................................................................................... 28 2.4 METHODS ........................................................................................................................................ 30 2.4.1 Sampling and sample treatment 30 2.5 RESULTS .......................................................................................................................................... 38 2.5.1 Major and trace elements 38 2.5.2 Radiogenic isotope compositions 40 2.6 DISCUSSION ..................................................................................................................................... 41 2.6.1 Magma generation along the Terceira axis 41 2.6.2 Mantle sources and mixing along the Terceira axis 45 2.6.3 Comparison to other ultraslow ridges 48 2.7 CONCLUSIONS .................................................................................................................................. 49 2.8 ACKNOWLEDGMENTS ...................................................................................................................... 50 REFERENCES .......................................................................................................................................... 50 CHAPTER III RELICS OF A SUBDUCTED SEAMOUNT: THE SÃO MIGUEL MANTLE SOURCE 3.1 ABSTRACT ........................................................................................................................................ 55 3.2 INTRODUCTION ................................................................................................................................ 56 3.3 GEOLOGICAL SETTING .................................................................................................................... 56 3.4 METHODS ........................................................................................................................................ 57 3.5 RESULTS .......................................................................................................................................... 64 3.6 DISCUSSION ..................................................................................................................................... 68 VI TABLE OF CONTENTS 3.6.1 The Sete Cidades mantle source 69 3.6.2 The Nordeste mantle source 72 3.6.2.1 Sediments 73 3.6.2.2 Subcontinental lithosphere 74 3.6.2.3 Melt-rock interactions 75 3.6.2.4 Subduction of enriched basalts and trachytes 76 3.7 CONCLUSIONS .................................................................................................................................. 78 3.8 ACKNOWLEDGMENTS ...................................................................................................................... 78 REFERENCES .......................................................................................................................................... 79 CHAPTER IV MAGMA EVOLUTION OF THE SETE CIDADES VOLCANO, SÃO MIGUEL, AZORES 4.1 ABSTRACT ........................................................................................................................................ 85 4.2 INTRODUCTION ................................................................................................................................ 86 4.3 GEOLOGICAL BACKGROUND ........................................................................................................... 86 4.3.1 Geological setting of São Miguel 86 4.3.2 The stratigraphy of Sete Cidades volcano 88 4.4 SAMPLING STRATEGY AND ANALYTICAL METHODS ........................................................................ 89 4.4.1 Sampling and sample treatment 89 4.4.2 Major and trace element analysis 98 4.4.3 Mineral chemistry 98 4.4.4 Nd-Sr isotopes 99 4.5 PETROLOGY AND MINERAL CHEMISTRY ......................................................................................... 99 4.5.1 Petrology 99 4.5.1.1 Primitive rocks 99 4.5.1.2 Intermediate rocks 99 4.5.1.3 Evolved rocks (trachytes) 100 4.5.2 Mineral chemistry 102 4.5.2.1 Olivine 102 VII TABLE OF CONTENTS 4.5.2.2 Clinopyroxene 102 4.5.2.3 Feldspar 103 4.5.2.4 Spinel and iron-titanium oxides 104 4.5.2.5 Amphibole 104 4.5.2.6 Biotite 104 4.5.2.7 Apatite 104 4.6 GEOCHEMISTRY .................................................................................................................. 105 4.6.1 Whole-rock compositions 105 4.6.2 Glasses 107 4.6.3 The caldera outflow deposit 107 4.7 DISCUSSION ......................................................................................................................... 107 4.7.1 Primitive magmas of Sete Cidades and their formation 107 4.7.2 Magma ascent and fractional crystallization processes of the Sete Cidades magmas 109 4.7.2.1 Geothermobarometry 109 4.7.2.2 Oxygen fugacity 111 4.7.2.3 Fractional crystallization and modelling using MELTS 111 4.7.2.4 Effects of fractional crystallization processes on incompatible element ratios 112 4.7.3 Time-scales of magma evolution and ascent beneath Sete Cidades 114 4.7.4 Temporal variations in the petrogenesis of the Sete Cidades lavas 116 4.7.5 Evidence for magma mixing? 117 4.8 CONCLUSIONS ..................................................................................................................... 119 4.9 ACKNOWLEDGMENTS ........................................................................................................... 120 REFERENCES ............................................................................................................................. 120 APPENDIX TABLES ..................................................................................................................................... A-I CURRICULUM VITAE ........................................................................................................
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