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The Magmatic-Hydrothermal Transition in Peralkaline Rhyolite Magma at Terceira, Azores

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The Magmatic-Hydrothermal Transition in Peralkaline Rhyolite Magma at Terceira, Azores The Magmatic-Hydrothermal Transition in Peralkaline Rhyolite Magma at Terceira, Azores by Caitlin Beland A thesis submitted in conformity with the requirements for the degree of Masters of Applied Science Department of Earth Sciences University of Toronto © Copyright by Caitlin Beland 2014 The Magmatic-Hydrothermal Transition in Peralkaline Rhyolite Magma at Terceira, Azores Caitlin Beland Masters of Applied Science Department of Earth Sciences University of Toronto 2014 Abstract The geochemistry of quartz-hosted melt (MI) and fluid inclusions (FI) in quartz syenite from Terceira, Azores was investigated to provide insight into late-stage evolution of peralkaline melts and the behaviour of high field strength (HFSE) and rare-earth elements (REE) at the magmatic- hydrothermal transition. Crystalline and hydrous MI analyzed by laser ablation-inductively- coupled plasma mass-spectrometry (LA-ICP-MS) show extreme magmatic enrichment of HFSE and REE. Sanidine crystallization resulted in enrichment of the melt in HFSE, REE and volatiles. Halite-saturated FI analyzed by LA-ICP-MS show lower total REE abundances than melts, and a general enrichment in HREE. Comparison of REE distribution patterns of MI and miarolitic zircon and monazite suggest late-stage melt evolution by monazite, then zircon and pyrochlore fractionation. Microthermometry of FI suggests maximum trapping conditions of 675°C, 120 MPa. The residual evolved to very volatile-rich compositions and initially exsolved a hydrosaline melt that was diluted to lower salinities by aqueous-fluid exsolution on cooling. ii Acknowledgments Special thanks to Professor Jim Mungall for patience, assistance and guidance. Thanks to Professor Jake Hanley at Saint Mary’s University for assistance and guidance with data processing and many helpful discussions. Also, many thanks to Dr. Colin Bray for guidance with microthermometry, and Dr. Duane Smythe for general guidance with analytical techniques. Last but not least, infinite thanks to my father especially for teaching me the values of perseverance and determination, my family and Simon Urbain for their continued support. iii Table of Contents Acknowledgements .......................................................................................................................iii Table of Contents ...........................................................................................................................iv List of Tables ................................................................................................................................vii List of Figures ..............................................................................................................................viii List of Appendices .........................................................................................................................ix Chapter One: Introduction .............................................................................................................1 Origin of peralkaline melts ................................................................................................2 Solubility of incompatible lithophile elements, volatiles and their magmatic enrichment in peralkaline melts .....................................................................................5 Solubility of the HFSE in peralkaline melt ............................................................5 Solubility of volatiles in peralkaline melt ..............................................................6 Influence of volatiles on solubility of the HFSE in peralkaline melt .....................7 Magmatic enrichment of the HFSE in peralkaline melt .........................................8 Solubility of the HFSE in aqueous fluid and their hydrothermal transport ........................8 Solubility of the HFSE in aqueous fluid .................................................................8 The role of aqueous fluids in the genesis of HFSE deposits .................................10 Partitioning behaviour of the HFSE between silicate melt and aqueous fluid .................11 Chapter Two: Research paper prepared for the Journal of Petrology ...........................................13 Abstract .............................................................................................................................13 Introduction .......................................................................................................................14 Geology of Terceira...............................................................................................18 Materials and Methods ......................................................................................................21 Sample collection and petrography .......................................................................21 Microthermometry ................................................................................................22 LA-ICPMS ............................................................................................................22 iv Electron Microprobe .............................................................................................26 Petrographic Observations ................................................................................................27 Quartz syenite .......................................................................................................27 Mineral compositions ............................................................................................33 Quartz-hosted inclusions .......................................................................................37 Fluid inclusions .........................................................................................38 Melt inclusions ..........................................................................................40 Hydrous melt inclusions ...........................................................................41 Fluid Inclusion Microthermometry ...................................................................................43 Compositions of Melt and Fluid Inclusions ......................................................................47 REE .......................................................................................................................47 Element ratio variations ........................................................................................50 Discussion .........................................................................................................................52 Phase assemblages ................................................................................................52 Late-stage mineralogy of Terceira quartz syenite and inferred compositional features of the residual liquid ............................................52 Miaskite or agpaite? ..................................................................................53 Intensive parameters .............................................................................................57 Oxygen fugacity........................................................................................57 Pressure and temperature of entrapment of melt and fluid inclusions.......60 Temperature of quartz crystallization .......................................................64 Compositions of minerals, melts and fluids ..........................................................65 Boundary layer effects and post-entrapment crystallization .....................65 Miarolitic monazite and zircon compositions ...........................................66 Melt inclusion compositions .....................................................................67 Fluid inclusion compositions ....................................................................70 v FI-MI pairs and distribution coefficients ..................................................72 Conclusions ......................................................................................................................80 References ........................................................................................................................82 Chapter Three: Conclusions and future work ..............................................................................93 References …...………………...………………………………………………………………..96 Appendix A: Microthermometry Results ....................................................................................111 Appendix B: LA-ICPMS Data of Fluid and Melt Inclusion Analyses .......................................117 Appendix C: Abandoned/Failed Components of Research ........................................................127 vi List of Tables Table 1: Comparison of analytical results of Mungall & Martin (1996) and the present study on sample P16 ....................................................................................................................................25 Table 2: EMP operating conditions ..............................................................................................26 a: monazite analyses .........................................................................................................26 b: zircon analyses ..............................................................................................................27 Table 3: A list of HFSE-bearing minerals found in miarolitic cavities ........................................30 Table 4: Ti abundances in different zones (seen in CL) of
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