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Enclave Formation, Magma Mixing, and Eruption Triggering: a Case Study of the Chaos Crags, Lassen Volcanic Center, California

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Enclave Formation, Magma Mixing, and Eruption Triggering: a Case Study of the Chaos Crags, Lassen Volcanic Center, California ABSTRACT ENCLAVE FORMATION, MAGMA MIXING, AND ERUPTION TRIGGERING: A CASE STUDY OF THE CHAOS CRAGS, LASSEN VOLCANIC CENTER, CALIFORNIA The exact manner in which mafic recharge acts as an eruption trigger mechanism at arc volcanoes remains the subject of intense debate. This study addresses the issues of mafic recharge and magma mixing as a trigger for volcanic eruptions by examining mafic enclaves of the Chaos Crags, Lassen Volcanic Center, a type locality for mixing of two end-member magmas in the Cascade Arc. Mineral and bulk rock compositions were examined in order to determine end-member magma compositions and pre-mixing magma densities, with specific consideration given to comparisons between cores and rims of mafic enclaves. Mineral-melt equilibria were used to calculate crystallization temperatures and pressures, which were used in conjecture with measured vesicularities to estimate pre-eruptive magmatic densities for enclaves and host lavas. The wide range of compositions, textures, crystallization P and T, and vesicularities present within Chaos Crags eruptive products strongly support that mafic enclaves of the Chaos Crags were formed by the mixing of a parental basaltic end-member magma with rhyodacitic host magma, and that crystallization of the enclaves occurred to some degree after mixing of these two magmas prior to eruption. This study concludes that although mafic recharge and magma mixing were most likely not the proximal trigger for eruption, they were the ultimate eruption trigger for the 1,103 ±13 years BP eruption of the Chaos Crags sequence. The proximal eruption triggering mechanism for the Chaos Crags system may be attributed to increased overpressure within the chamber caused by volatile ii contributions from the crystallizing mafic magma, leading to fluid saturation, rapid vesiculation, and an increase in pH2O within the host magmas of the Chaos Crags. Melissa Ashley Scruggs August 2014 ENCLAVE FORMATION, MAGMA MIXING, AND ERUPTION TRIGGERING: A CASE STUDY OF THE CHAOS CRAGS, LASSEN VOLCANIC CENTER, CALIFORNIA by Melissa Ashley Scruggs A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Geology in the College of Science and Mathematics California State University, Fresno August 2014 APPROVED For the Department of Earth & Environmental Sciences: We, the undersigned, certify that the thesis of the following student meets the required standards of scholarship, format, and style of the university and the student's graduate degree program for the awarding of the master's degree. Melissa Ashley Scruggs Thesis Author Keith D. Putirka (Chair) Earth & Environmental Sciences Mara Brady Earth & Environmental Sciences Michael A. Clynne United States Geological Survey For the University Graduate Committee: Dean, Division of Graduate Studies AUTHORIZATION FOR REPRODUCTION OF MASTER’S THESIS X I grant permission for the reproduction of this thesis in part or in its entirety without further authorization from me, on the condition that the person or agency requesting reproduction absorbs the cost and provides proper acknowledgment of authorship. Permission to reproduce this thesis in part or in its entirety must be obtained from me. Signature of thesis author: ACKNOWLEDGMENTS I have nothing but the utmost respect, gratitude, and appreciation for my advisor, Dr. Keith Putirka. His infinite patience and invaluable guidance, both inside and outside of the classroom, have helped me to grow and succeed both as a scientist and as a person in more ways than he will ever know. Keith, you have been like a father to me, and I only hope that I can one day repay you for all of the wonderful ways in which you have influenced me. I would also like to thank Dr. Michael Clynne, whose knowledge and expertise regarding Lassen and the Chaos Crags are irreplaceable, and Dr. Mara Brady for her comments and reviews on this manuscript. An innumerable amount of thanks are due to Jeff Rash, Dr. John Wakabayashi, Kerry Workman-Ford, and Dr. Beth Weinman, as well as the many other friends I have made during my time here at Fresno State, for their unwavering moral support both in times of great exasperation and great excitement throughout the duration of this project. Most of all, I would like to thank my daughter Korin, who dutifully withstood many long days in the laboratory and evenings in the library. She has graciously yielded many hours of her childhood in exchange for my research, and I can never repay her for them. This project was generously funded by grants from Fresno State Faculty- Sponsored Student Research and the National Science Foundation (Award No. 1250323). TABLE OF CONTENTS Page LIST OF TABLES ................................................................................................ viii LIST OF FIGURES ................................................................................................. ix INTRODUCTION .................................................................................................... 1 PREVIOUS STUDIES OF THE CHAOS CRAGS, LASSEN VOLCANIC CENTER, CALIFORNIA ............................................................................. 4 Geologic Setting ................................................................................................ 4 Petrology and Geochemistry ............................................................................. 6 Pre-Eruptive Storage Conditions of Host Lavas ............................................. 10 Petrogenic Models of Enclave Formation and Magma Mixing ..................... 11 MATERIALS AND METHODS ........................................................................... 15 Whole Rock Geochemical Analyses ............................................................... 15 Quantitative Chemical Analyses ..................................................................... 16 Thermobarometric Calculations of Host Magmas and Mafic Enclaves ......... 17 Percent Vesicularity and Vesicle Size Distributions ...................................... 18 Calculations of Pre-eruptive Magma Densities .............................................. 21 RESULTS ............................................................................................................... 23 Geochemical Compositions of Chaos Crags Eruptive Products ..................... 23 Physical and Petrographic Features and Petrologic Characteristics of Chaos Crags Eruptive Products ........................................................... 23 Geochemical and Petrographic Variations Within Individual Mafic Enclaves ............................................................................................... 52 Vesicularity of Chaos Crags Eruptive Products .............................................. 56 Thermobarometric Calculations of Host Magmas and Mafic Enclaves ......... 62 Calculated Densities of Chaos Crags Eruptive Products ................................ 63 DISCUSSION ......................................................................................................... 68 vi vi Page Geochemical Constraints on Magma Mixing and Identification of Parental End-member Magmas ............................................................ 68 Petrologic Constraints on Magma Mixing, Mingling, and Enclave Formation ............................................................................................. 76 Textural and Mineralogical Constraints on Magma Mixing ........................... 82 Mixing Constraints Required by Vesicular and Compositional Zonation Within Individual Mafic Enclaves ....................................................... 97 Magma Mixing, Enclave Formation, and Eruption Triggering Model ......... 105 CONCLUSION .................................................................................................... 111 Recommendations for Further Studies .......................................................... 113 REFERENCES ..................................................................................................... 115 APPENDICES ...................................................................................................... 126 APPENDIX A: COMPREHENSIVE LIST OF CHAOS CRAGS ERUPTIVE PRODUCTS ANALYZED IN THIS STUDY .......................................... 127 APPENDIX B: MAJOR OXIDE WHOLE ROCK GEOCHEMISTRY OF CHAOS CRAGS ERUPTIVE PRODUCTS (DATA DISC) .................... 129 APPENDIX C: MAJOR OXIDE COMPOSITIONS OF INDIVIDUAL PLAGIOCLASE WITHIN CHAOS CRAGS HOST LAVAS (DATA DISC) ......................................................................................................... 130 APPENDIX D: MAJOR OXIDE COMPOSITIONS OF INDIVIDUAL PLAGIOCLASE WITHIN CHAOS CRAGS MAFIC ENCLAVES (DATA DISC) ........................................................................................... 131 APPENDIX E: MAJOR OXIDE COMPOSITIONS OF CLINOPYROXENE WITHIN CHAOS CRAGS ERUPTIVE PRODUCTS (DATA DISC) .... 132 APPENDIX F: MAJOR OXIDE COMPOSITIONS OF ORTHOPYROXENE WITHIN CHAOS CRAGS ERUPTIVE PRODUCTS (DATA DISC) .... 133 APPENDIX G: MAJOR OXIDE COMPOSITIONS OF OLIVINE WITHIN CHAOS CRAGS ERUPTIVE PRODUCTS (DATA DISC) .................... 134 APPENDIX H: MAJOR OXIDE COMPOSITIONS OF AMPHIBOLE WITHIN CHAOS CRAGS ERUPTIVE PRODUCTS (DATA DISC) .... 135 APPENDIX I: LOG-LINEAR VESICLE SIZE DISTRIBUTION GRAPHS FOR CHAOS CRAGS HOST LAVAS (DATA DISC) ........................... 136 vii vii Page APPENDIX J: LOG-LINEAR VESICLE SIZE DISTRIBUTION GRAPHS FOR CHAOS CRAGS MAFIC ENCLAVES (DATA DISC) .................
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