ABSTRACT

ENCLAVE FORMATION, MIXING, AND ERUPTION TRIGGERING: A CASE STUDY OF THE , LASSEN VOLCANIC CENTER,

The exact manner in which 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 in the Cascade Arc. Mineral and bulk 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

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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 WITHIN CHAOS CRAGS ERUPTIVE PRODUCTS (DATA DISC) ...... 134 APPENDIX H: MAJOR OXIDE COMPOSITIONS OF 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

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APPENDIX J: LOG-LINEAR VESICLE SIZE DISTRIBUTION GRAPHS FOR CHAOS CRAGS MAFIC ENCLAVES (DATA DISC) ...... 137 APPENDIX K: PROPORTIONS OF MIXING FOR CHAOS CRAGS ERUPTIVE PRODUCTS AS DETERMINED BY LINEAR MASS- BALANCE CALCULATIONS (DATA DISC) ...... 138 LIST OF TABLES

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Table 1. Percentage of Host-derived plagioclase (An20-59) present within Chaos Crags mafic enclaves, percentage of enclave-derived plagioclase (An>60) present within Chaos Crags host lavas, and percentage of rhyolitic crystalline mush-derived plagioclase (An<20) present within Chaos Crags host lavas and mafic enclaves...... 74 Table 2. Experimental Starting Compositions of Phase Relations Studies Compared to Chaos Crags Mafic Enclaves ...... 79

LIST OF FIGURES

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Figure 1. Simplified geologic map of the Chaos Crags [after Clynne and Muffler (2010)] overlain on 1 MOA Shasta Co. DEM (USGS National Elevation Dataset)...... 6

Figure 2. Phase stability diagram of Chaos Crags Group 1 pyroclastic flows ...... 11 Figure 3. (a) Thermal and rheological enclave formation model of the Chaos Crags...... 12

Figure 4. (a) Enclave formation and magma mixing model of the Chaos Crags. .. 14 Figure 5. Bubble growth behavior as illustrated in log-linear VSD plots [after Mangan and Cashman (1996)]...... 20 Figure 6. TAS Classification of Chaos Crags eruptive products (after LeBas 1986). MAC data obtained from Michael Clynne (written comm., February 2014)...... 24

Figure 7. Variations in wt.% SiO2 and wt.% MgO by dome (A-F)...... 25 Figure 8. Compositional variation diagrams of wt.% major oxides within Chaos Crags eruptive products (after Harker 1909). SEE = 1.0 wt.%. .. 26 Figure 9. Measured An components of plagioclase vs. predicted An components of plagioclase from bulk rock compositions in which plagioclase reside...... 28 Figure 10. BSE images of plagioclase phenocrysts populations within Chaos Crags eruptive products and measured An component along sampling transects...... 29

Figure 11. Compositional variations in clinopyroxene...... 32 Figure 12. Measured EnFs components of clinopyroxene vs. predicted EnFs components of clinopyroxene from bulk rock compositions in which clinopyroxene reside...... 33 Figure 13. Measured DiHd components of clinopyroxene vs. predicted DiHd components of clinopyroxene from bulk rock compositions in which clinopyroxene reside...... 34 Figure 14. (a) Measured CaTs components of clinopyroxene vs. predicted CaTs components of clinopyroxene from bulk rock compositions in which clinopyroxene reside...... 35 x x

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Figure 15. BSE images highlighting disequilibrium textures of clinopyroxene within Chaos Crags mafic enclaves...... 36

Figure 16. Compositional variations in orthopyroxene...... 37 Figure 17. BSE images highlighting disequilibrium textures of orthopyroxene within Chaos Crags eruptive products...... 38 Figure 18. Orthopyroxene Mg number vs. whole rock Mg-number of material in which orthopyroxenes reside...... 39 Figure 19. Fo contents of olivine vs. whole rock Mg-number of material in which reside...... 40 Figure 20. BSE images highlighting disequilibrium textures of olivine within Chaos Crags eruptive products...... 41 Figure 21. Amphibole Mg number vs. whole rock Mg-number of material in which reside...... 42 Figure 22. BSE images highlighting disequilibrium textures of hornblende within Chaos Crags eruptive products...... 43 Figure 23. Contrast in groundmass textures between (A) Group 1 (ppl) and (B) Group 2 (xpl) host lavas...... 45 Figure 24. Representative textural and vesicular variations within mafic enclaves. See text for discussion...... 49 Figure 25. Representative variations in groundmass textures and mineralogy between Group 1 (A-D) and Group 2 (E-H) mafic enclaves...... 50 Figure 26. Variation diagrams of selected major elements vs. silica (after Harker 1909) within individual mafic enclaves...... 53

Figure 27. Textural variations within selected individual mafic enclaves...... 54

Figure 28. Vesiculation of eruptive products by dome. SEE = 4.3%...... 58

Figure 29. Mafic enclave diameter vs. percent vesiculation by volume...... 59 Figure 30. Log-linear VSD plots of mafic enclaves which display similar nucleation and growth trends as their host lavas...... 60 Figure 31. Log-linear VSD plots of mafic enclaves displaying trends of coalescence, nucleation, and growth which differ greatly from their host lavas...... 61 xi xi

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Figure 32. Whole Rock wt.% SiO2 vs. measured An component of plagioclase within natural Chaos Crags samples from this study and experimental plagioclase from referenced studies...... 64 Figure 33. Crystallization temperatures (°C) of Chaos Crags Eruptive Products...... 65 Figure 34. Crystallization temperatures (°C), pressures (kbar), and depths (km) of Chaos Crags eruptive products...... 66

Figure 35. Calculated densities of Chaos Crags eruptive products by dome...... 67 Figure 36. Frequency distributions of plagioclase An contents within Chaos Crags host lavas and mafic enclaves...... 70 Figure 37. Degrees of mixing between -MELTS approximated parental rhyolite and estimated parental basalt...... 75 Figure 38. Compositional variation in olivine within Chaos Crags mafic enclaves...... 80 Figure 39. Approximation of sequence of crystallization for Chaos Crags calculated mafic end-member by Rhyolite-MELTS (Gualda et al. 2012)...... 81 Figure 40. BSE images of pre-eruptive vesicle population candidates present within Chaos Crags mafic enclaves and their accompanying log- linear VSD plots...... 94 Figure 41. Representative effects of vesiculation on calculated pre-eruptive densities of Chaos Crags mafic enclaves...... 96 Figure 42. Variation diagrams of selected major elements vs. silica (after Harker 1909) within individual mafic enclaves...... 99 Figure 43. Host-derived phases and vesiculated rhyolitic melt present in enclave rims...... 100 Figure 44. Log-linear VSD plots of selected mafic enclaves demonstrating variations in nucleation rates, growth rates, and bubble behavior within individual mafic enclaves...... 102 Figure 45. Model for Magma Mixing and Enclave Formation within the Chaos Crags...... 109

INTRODUCTION

The study of eruption triggering mechanisms has great value as an assessment of volcanic hazards. Several mechanisms are known to induce volcanic eruptions, including: seismic triggering (Gregg et al. 2011), surface triggering by (Manga and Brodsky 2006), roof uplift and crack propagation (Gregg et al. 2011), advective overpressure (Manga and Brodsky 2006), mixing of compositionally and thermally contradistinctive magmas (Clynne 1999), chamber overturn (Pallister et al. 1992), and mafic recharge (Kent et al. 2010, Huppert et al. 1982). Processes involving external trigger mechanisms, such as seismic and surface triggering, are more easily observed and directly correlated to an eruption than those which are completely internalized within the magma chamber; such internalized triggers instead rely on the development of eruption triggering models to identify particular mechanisms with increased precision (Slezin 2003). Mafic recharge into the base of crustal reservoirs has been widely recognized as a completely internalized eruption trigger mechanism (Sparks et al. 1977, Clynne 1999, Pallister et al. 1992, Kent et al. 2010, Huppert et al. 1982). Although much research has been conducted on this process, the exact manner in which this trigger mechanism operates remains unknown, because it is not yet possible to directly observe recharge processes or obtain quantitative data during recharge events (Slezin 2003). Petrologic evidence from numerous volcanics associated with island arcs and orogenic belts indicates that the mixing of magmas have the capacity to occur only a short period of time prior to eruption (Huppert et al. 1982). Thus, the inability to directly observe mixing processes within the chamber has generated debate about whether magma mixing within the chamber is the trigger for volcanic eruptions, or if eruption triggers magma mixing 2 2

within the chamber and conduit (Bindeman and Podladchikov 1993, Woods and Cowan 2009, Wood et al. 2012, Huppert et al. 1982, Eichelberger 1975, Jackson et al. 2011). Addressing these issues is imperative for a complete understanding of volcanic hazards. The Lassen Volcanic Center (LVC) is the southernmost group of active volcanoes within the , and displays a Pleistocene to Holocene stratovolcano with a series of dacitic lava domes and associated tephra and pyroclastic deposits (Heiken and Eichelberger 1980, Christiansen et al. 2002, Clynne and Muffler 2010). The Chaos Crags, a group of six lava domes within the LVC, may be representative of lavas produced by other recent eruptions of the LVC; the majority of flows produced at other young vents are characterized by similar mafic enclaves and disequilibrium mineral assemblages (Clynne 1999, Clynne and Muffler 2010, Heiken and Eichelberger 1980, Feeley et al. 2008). The presence of mafic enclaves has characterized the Chaos Crags as a type locality for the mixing of two end-member magmas in the Cascade Arc (Heiken and Eichelberger 1980, Wallace and Bergantz 2005). Numerous investigations of the 1915 eruption have culminated in the development of a complex eruption triggering model highly dependent upon mafic recharge, magma vesiculation, melt density modifications, and chamber instability (Clynne 1999, Salisbury et al. 2008, Jackson et al. 2011). Studies of recharge and eruption triggering have been applied to the Chaos Crags eruptive sequence, but mineral chemistry and vesicle size distributions were not incorporated into these studies (Tepley et al. 1999, Hootman 2011, Jackson et al. 2011); thus, our understanding of this system remains incomplete. This project attempts to further refine the mafic recharge triggering model first proposed by Tepley et al. (1999) and refined by Hootman (2011) by considering the effects of 3 3

vesiculation on the pre-eruptive densities of contrasting magmas within the chamber, and then using petrologic arguments derived from mineral compositions and observed variations within individual mafic enclaves. It is the aim of this study to contribute to the understanding of magma mixing, magma mingling, processes of mafic enclave formation, and eruption triggering mechanisms. This study uses the Chaos Crags as a test case to compare the feasibility of various eruption triggering mechanisms. Pre-eruptive magmatic densities and the pressures and temperatures of magma storage are calculated in order to answer several questions, such as: what kinds of end-member magmas were present in the pre-eruptive system? Were recharge magmas sufficiently vesiculated so as to be less buoyant than overlying dacitic magmas? And, did crystallization of recharge magmas occur before, during, or after magma mixing?

PREVIOUS STUDIES OF THE CHAOS CRAGS, LASSEN VOLCANIC CENTER, CALIFORNIA

Previous investigations of the Chaos Crags have concentrated on identification, extent, and sequence of eruptive products (Williams 1931, Crandell et al. 1974, Heiken and Eichelberger 1980, Christiansen et al. 2002, Clynne et al. 2002, Clynne and Muffler 2010), the internal structures of lava domes (Watts et al. 2013), volumetric distribution of inclusions (Heiken and Eichelberger 1980, Clynne and Muffler 2010, Hootman 2011), mineralogical assemblages (Heiken and Eichelberger 1980, Tepley et al. 1999, Christiansen et al. 2002), geochemical and petrological analyses of eruptive products (Heiken and Eichelberger 1980, Tepley et al. 1999, Christiansen et al. 2002, Wallace and Bergantz 2005, Collins et al. 2012, Underwood et al. 2012, Klemetti et al. 2013), categorization and distribution of enclave types (Clynne et al. 2002, Hootman 2011), determination of pre-eruptive storage conditions of host magmas (Klemetti et al. 2013, Quinn 2014), and petrogenic models of enclave formation and magma mixing (Tepley et al. 1999, Klemetti et al. 2013, Andrews and Manga 2014).

Geologic Setting The Cascade Range of the northwestern United States is a continental situated above the Cascadia subduction zone, extending from Mount Meager in southwestern British Columbia southward to the Lassen Volcanic Center (LVC) (Salisbury et al. 2008). The LVC is the southernmost group of active volcanoes within the Cascade Range, displaying a Pleistocene to Holocene stratovolcano and a series of dacitic lava domes with their associated tephra and pyroclastic deposits (Heiken and Eichelberger 1980, Tepley et al. 1999, Feeley et al. 2008). 5 5

Volcanism at the LVC commenced ~825 Ka, and continues to present day, with the most recent event being the 1915 eruption of Lassen Peak (Clynne et al. 2012). Volcanic activity may be divided into three major sequences: the Rockland caldera complex (~825 to 609 Ka), Brokeoff Volcano (590 to 385 Ka), and the Lassen domefield (~300 to 0 Ka) (Clynne and Muffler 2010). Eruptive activity within the Lassen domefield may be divided into three sequences based upon age and silica content: the Bumpass (~300 to 190 Ka) and Eagle Peak (~70 Ka to present) sequences are predominantly dacitic to rhyodacitic dome-forming lavas, characterized by the presence of mafic enclaves and disequilibrium mineral assemblages; the Twin Lakes sequence (~310 to 240 Ka, and ~90 Ka to present) differ from other products of the Lassen domefield in that they produce hybridized basaltic-andesitic to andesitic lavas (Clynne and Muffler 2010). Gravity studies of the LVC reveal a shallow, low density, ~5 km diameter mass (Pakiser 1964, Clynne et al. 2012) modelled to be the slowly crystallizing residual magma from the Bumpass sequence (Klemetti et al. 2013). Sporadic revitalization of this crystalline mush by mafic recharge events is interpreted to have fed LVC volcanism over the past 90,000 years (Clynne et al. 2012, Klemetti et al. 2013). The youngest eruption in the Eagle Peak sequence, the Chaos Crags are found on the northern flank of the dormant Brokeoff stratovolcano, located on the western perimeter of the Lassen volcanic region (Heiken and Eichelberger 1980, Clynne and Muffler 2010). The Chaos Crags and nearby Lassen Peak are underlain by ~4 km of ≤3.5 Ma , <300 m of Cretaceous sedimentary rocks, and a crystalline Sierran basement (Underwood et al. 2012, pers. comm., Michael Clynne, June 2014). The Chaos Crags comprise six dacitic to rhyodacitic lava domes (A-F) within the LVC, their associated tephra and pyroclastic deposits, and debris from dome collapse (Fig. 1) (Clynne and Muffler 2010); the total 6 6

volume of erupted products is ~2 km3 (Heiken and Eichelberger 1980, Clynne and Muffler 2010). The six domes represent a complex eruption sequence beginning

1,103 ±13 years BP and ending approximately 1060 years BP; Dome C experienced collapse and produced debris known as the Chaos Jumbles in a series of three successive rockfalls that all occurred at 278 ±30 years BP (Tepley et al. 1999, Clynne et al. 2008).

Figure 1. Simplified geologic map of the Chaos Crags [after Clynne and Muffler (2010)] overlain on 1 MOA Shasta Co. DEM (USGS National Elevation Dataset).

Petrology and Geochemistry Lavas of the Chaos Crags are porphyritic hornblende-biotite to characterized by a mostly disequilibrium phenocryst assemblage, a significant decrease in SiO2 wt% over the eruption sequence, and the presence of vesiculated and undercooled hybrid basaltic- enclaves that increase in volumetric abundance as the eruption sequence progresses (Tepley et al. 1999, Christiansen et al. 2002, Clynne and Muffler 2010). In addition to visually distinct 7 7

mafic enclaves, host lavas also contain numerous microscopic enclaves and clumps of enclave material interpreted to be the product of enclave disaggregation, increasing in number over the eruption sequence (Tepley et al. 1999, Underwood et al. 2005). Comprehensive lithologic and petrographic descriptions of the Chaos Crags lavas are given in detail in Christiansen et al. (2002). Host lavas contain ~35-40 percent by volume (vol.%) phenocrysts, comprised of ~25% plagioclase, 4% biotite, 4% amphibole, and 2% (Christiansen et al. 2002, Underwood et al. 2012). Disequilibrium textures and mineral assemblages are common within Chaos Crags host lavas, increasing in prevalence over the eruption sequence (Tepley et al. 1999). Host lavas feature partially resorbed normally zoned sodic plagioclase phenocrysts, unresorbed sodic plagioclase with weakly reversely- zoned rims, heavily reacted sodic plagioclase phenocrysts with calcic overgrowth rims, mottled and partially resorbed amphibole often featuring corona rims, pseudomorphic replacement of magnesiohornblende by pyroxene, opacitized amphibole and biotite, antecrystic zircon, anhedral to subhedral quartz, and reacted olivine displaying iddingsite alteration and/or hornblende overgrowths (Heiken and Eichelberger 1980, Tepley et al. 1999, Christiansen et al. 2002, Underwood et al. 2012, Klemetti et al. 2013) Eruptive products may be separated into two distinct suites based upon

SiO2 wt.% and volume percentage of mafic enclaves: Group 1 lavas (Dome A, Dome B, and their associated pyroclastic flows), which are rhyodacitic (>69 wt.%

SiO2), with approximately 2% mafic enclaves by volume; and Group 2 lavas

(Domes C, D, E, and F), which are rhyodacitic to dacitic (<69 wt.% SiO2), containing up to ~20 vol.% mafic enclaves (Tepley et al. 1999, Clynne et al. 2002, Watts et al. 2013). The abundance of disequilibrium textures within host lavas 8 8

increase over the eruption sequence, as enclave disaggregation is more common within Group 2 lavas (Christiansen et al. 2002).

Mafic Enclaves of the Chaos Crags Williams (1931) first documented the presence of conspicuous mafic enclaves in Chaos Crags lavas, terming them “cognate” xenoliths and noting their shared mineralogy with host lavas (Heiken and Eichelberger 1980). Mafic enclaves are easily distinguished from their host lavas in hand specimen by their darker color, smaller grain size, and higher crystal content (Heiken and Eichelberger 1980); they range in diameter from ~5 cm to ~1 m (Christiansen et al. 2002), but smaller fragments (<5 cm) of broken or disaggregated enclaves are present, and range in size downward to individual enclave-derived crystals. Mafic enclaves are geochemically and petrologically diverse. Mafic enclaves within Group 1 lavas are basalt (~52 wt.% SiO2) to basaltic-andesite

(~57.5 wt.% SiO2); mafic enclaves from Group 2 lavas are basaltic-andesite, ranging from ~54 wt.% SiO2 to ~62 wt.% SiO2 (written comm., Michael Clynne, February 2014). Mafic enclaves exhibit a variety of textures, from fine-grained porphyritic, to sparsely phyric, and even medium-grained aphyric (Heiken and Eichelberger 1980, Tepley et al. 1999, Christiansen et al. 2002). Enclave groundmasses are microvesiculated pyroxene ± hornblende-plagioclase with Fe-Ti oxides and interstitial glass; phenocrysts of plagioclase and lesser clinopyroxene and olivine are present in nearly all enclaves (Heiken and Eichelberger 1980, Tepley et al. 1999, Christiansen et al. 2002). Mafic enclaves are characterized by disequilibrium mineral assemblages and microtextures, and include mineral phases originating from both the original enclave melt and the rhyodacitic host. Tepley et al. (1999) characterized three 9 9

populations of plagioclase within enclaves: normally zoned unreacted calcic microlites and microphenocrysts, heavily sieve-textured partially resorbed sodic phenocrysts and microphenocrysts with little to no overgrowth rim, and partially resorbed sodic plagioclase with thin (<3 µm) sieve-textured resorption rims surrounding a clear core and overgrown by thick (10-20 µm) strongly normally- zoned calcic overgrowth rims. In addition to variably resorbed and reacted plagioclase, mafic enclaves also feature resorbed quartz phenocrysts with clinopyroxene reaction rims, glomeroporphyroclasts of anorthite and augite- rimmed olivine, partially resorbed and reacted biotite phenocrysts, mottled and partially resorbed amphibole often featuring corona rims, pseudomorphic replacement of magnesiohornblende by pyroxene, and antecrystic zircon (Eichelberger 1978, Heiken and Eichelberger 1980, Tepley et al. 1999, Underwood et al. 2012, Klemetti et al. 2013). The presence of a small number of multiple-generation enclaves (enclaves which contain enclaves, termed “intra-enclaves” for purposes of this study) was also noted by Christiansen et al. (2002). Heiken and Eichelberger (1980) observed that mafic enclaves often display a concentric zonation of disequilibrium textures, in that some enclaves exhibit a decrease in grain size towards their rim, indicative of increased cooling rate and attributed to the significant undercooling and quenching of those enclaves (Heiken and Eichelberger 1980, Feeley et al. 2008). Quenching of enclaves decreases over the eruption sequence, with mafic enclaves found in Group 1 lavas more often presenting chilled and crenulated margins than mafic enclaves within Group 2 lavas (Tepley et al. 1999). 10 10

Pre-Eruptive Storage Conditions of Host Lavas Recent analyses of 238U-230Th ages, Eu/Eu, and Hf concentrations of antecrystic zircons culled from Twin Lakes and Eagle Peak eruptive products suggest the presence of a large, low-temperature, highly evolved crystalline mush underlying much of the LVC; periodic small-scale injections of mafic magma heated and rejuvenated the crystal mush in a limited area surrounding the injection site (Klemetti et al. 2013). 238U-230Th ages, Eu/Eu, and Hf concentrations of zircons separated from Twin Lakes and Eagle Peak eruptive products were used to establish zircon ages and storage conditions for slow crystallization of the nascent pluton. 238U-230Th ages of zircon interiors within Chaos Crags lavas yield an age range of 44 ±4.5 to ~280 Ka; zircon rim ages range from 17.5 ± 7.9 to ~99 Ka. Two distinct populations of zircon were identified: Group A zircons underwent uninterrupted cooling and crystallization conditions immediately following cessation of Bumpass sequence volcanism (T = 674-764°C); Group B zircons were periodically subjected to higher temperatures (702-817°C) as a result of mafic recharge events, experiencing episodes of dissolution during crystal growth (Klemetti et al. 2013). Phase equilibrium experiments of pumice from Group 1 pyroclastic flows were conducted by Quinn (2014). Experimentally-produced major phases of glass, quartz, plagioclase (An35), biotite, hornblende, Fe-Ti oxides, and minor apatite were analyzed by FTIR, SEM and EPMA; pre-eruptive storage conditions of

Chaos Crags host magmas were estimated at: 4.3-5.3 wt.% H2O at fluid saturation,

NNO + 1.5-2.5 fO2, 770 ±10°C, and 145 ±25 MPa (Fig. 2). Amphibole and biotite geospeedometry were used to calculate ascent rates for Chaos Crags eruptive products: Group 1 lavas ascended quickly, with decompression occurring in <48 hrs and minimum ascent velocities of 0.1 km/hr; Group 2 lavas ascended more 11 11

slowly, with decompression times >48 hrs and ascent velocities of <0.1 km/hr Quinn (2014).

Figure 2. Phase stability diagram of Chaos Crags Group 1 pyroclastic flows. Gray shaded region represents experimentally determined storage conditions [from Quinn (2014)].

Petrogenic Models of Enclave Formation and Magma Mixing Andrews and Manga (2014) conducted thermal and rheological modeling of dike intrusion and subsequent breakup to develop an enclave formation model for the Chaos Crags (Fig. 3). X-ray tomographic scans of 5 cm cores of host and enclave samples were used to determine percent crystallinity of eruptive products; 12 12

MELTS simulations (Ghiorso and Sack 1995, Asimow and Ghiorso 1998) were performed to determine phase assemblages, compositions, heat capacities, and percent crystallinities of Crags enclaves and host lavas at P = 200 MPa, Thost =

885-800°C, and Tenclave = 1190-1105.6°C (Andrews and Manga 2014). Mixing styles for initial intrusion conditions of a mafic dike into a more magma chamber were predicted contingent upon the calculated rheological characteristics of timing of host convection, shear stresses acting upon the mafic/silicic interface, dike strength, and crystal fraction. This enclave formation model suggests that one or more meter scale or smaller dikes intruded into a more silicic magma, producing numerous, frequently angular enclaves within the Crags host lavas (Andrews and Manga 2014).

Figure 3. (a) Thermal and rheological enclave formation model of the Chaos Crags. Intrusion of mafic dike into a more silicic host magma. (b) Cooling and heat transfer from dike, establishment of a thermal gradient and convection within host magma. (c) Solidification of dike prior to convection of host magma and dismemberment of solidified dike, forming mafic enclaves (from Andrews and Manga 2014). 13 13

Tepley et al. (1999) used geochemical and petrographic evidence of disequilibrium mineral assemblages and 87Sr/86Sr ratios of plagioclase phenocrysts to produce an enclave formation, magma mixing, and eruption triggering model for the Chaos Crags eruption sequence (Fig. 4). This enclave formation and magma mixing model posits that enclaves are formed by incomplete mechanical mixing of basalt and rhyodacite within the magma chamber. Phenocrysts originally crystallized from the host are integrated into basaltic-andesitic magma during the hybridization process, becoming trapped and resorbed during enclave formation. Post-enclave formation and dispersal within the overlying more silicic magma, shear stresses act upon larger, less-quenched enclaves, disaggregating them and releasing small bits of enclave components (including resorbed host- derived phenocrysts) into the host rhyodacite (Tepley et al. 1999). 14 14

Figure 4. (a) Enclave formation and magma mixing model of the Chaos Crags. Injection of fluidized basalt into base of partially crystallized rhyodacitic magma chamber. (b) Homogenization of a hybridized basaltic-andesitic layer through viscous shearing along the mafic/ interface and entrainment of host-derived phenocrysts. (c) Instability of the basaltic-andesitic magma is caused by crystallization, cooling, and vesiculation of the hybridized layer; enclaves of hybridized magma are released into the overlying rhyodacitic magma. (d) Enclaves of differing diameters, containing host-derived phenocrysts, are variably quenched upon release into the host magma. (e) Viscosity contrasts between host and enclave magmas shear apart larger enclaves; disaggregation of enclaves releases resorbed host-derived phenocrysts and enclave materials into the host magma (from Tepley et al. 1999).

MATERIALS AND METHODS

Analytical procedures for the proposed project are modeled after that of Jackson et al. (2011). Host and enclave samples from each dome were collected in June 2012 and June 2013. Enclaves from each dome were selected at random; within each dome, an attempt was made to collect enclaves from each of the following size increments: (1) broken enclaves or enclave fragments up to 5 cm, (2) 5 cm to 10 cm, (3) 10 cm to 15 cm, (4) 15 cm to 20 cm, and (5) greater than 20 cm. Enclaves were visually examined to determine enclave core and rim boundaries (and intra-enclave boundaries where present); cores and rims of enclaves were treated as separate samples for purposes of whole rock geochemical and vesicle size distribution analyses in order to determine if vesicular or compositional variations within individual enclaves were present.

Whole Rock Geochemical Analyses Whole rock geochemical compositions were determined using a Phillips Analytical Wavelength Dispersive X-Ray Fluorescence Spectrometer. Analysis of major oxide components (SiO2, TiO2, Al2O3, Fe2O3, MgO, MnO, CaO, Na2O,

K2O, P2O5, and Cr2O3) was conducted using the fused glass bead method (Busby et al. 2008). Care was taken to remove visually distinct disaggregated enclave fragments from host lavas prior to analyses, but since individual crystals in felsic host materials may be enclave-derived, a perfect separation of enclave and host is not feasible. Sample powders were ground for 5 to 10 min in a tungsten carbide shatterbox vessel, and calcined for 10 min at 900 °C. Fused beads were prepared using a 1:6 ratio of calcined sample powder to prefused flux (35% Li-tetraborate, 65% Li-metaborate, manufactured by Claisse) with 6 drops LiI added as a releasing agent, and fused using the Claisse Fluxy fusion machine. USGS rock 16 16

powder standards AGV-2, RGM-1, STM-1, SDC-1, BHVO-2, BCR-2, W-2, QLO- 1, GSP-2, and DTS-2, and synthetic standards composed of different proportions of: Al2O3, SiO2, Fe2O3, MgO, NaH2PO4(H2O), and KH2PO4 were used as calibration standards (Busby et al. 2008); Standard Error Estimate for calibrations was ±1.0 wt.% of standard major oxide values for each component analyzed.

Quantitative Chemical Analyses Quantitative chemical analysis of glass and plagioclase, clinopyroxene, orthopyroxene, olivine, and amphibole crystals was conducted at the University of California, Davis using a Cameca SX-100 electron microprobe and at the U.S. Geological Survey (USGS) Menlo Park using a JEOL JXA-8900 electron microprobe. X-ray intensities were converted to wt.% concentrations using automated CITZAF matrix corrections (Armstrong 1995). Polished thin sections were coated with a 25 nm layer of graphite prior to analysis (Watson 1955) using an Edwards Carbon Evaporator. Major oxide (SiO2, TiO2, Al2O3, FeOtotal, MgO

CaO, Na2O, and K2O) compositions of plagioclase were obtained from individual points and transects across zoned plagioclase in thin section. Analyses were run using a 30 nA beam current at UC Davis and 10 nA beam current at the USGS, with an accelerating voltage of 15 keV, and a 10 μm beam diameter. Count times ranged from 20 to 40 s, with Na and K analyzed first to avoid loss by volatization. Taylor standards of albite, anorthite, orthoclase, fayalite, wollastonite, and bytownite were used as calibration standards (Kent et al. 2010). Major oxide

(SiO2, TiO2, Al2O3, FeOtotal, MnO, MgO, CaO, Na2O, K2O, and Cr2O3) compositions of clinopyroxene, amphibole, and olivine were obtained from individual points in thin section. Analyses were run using a 10 nA beam current at UC Davis and a 15 nA beam current at the USGS, with an accelerating voltage of 17 17

15 keV, and a 1 μm beam diameter. Count time for each point was 10 s, with Na and K analyzed first to avoid loss by volatilization. Major oxide (SiO2, TiO2,

Al2O3, FeOtotal, MnO, MgO, CaO, Na2O, K2O, and Cr2O3) compositions of glass were obtained from individual points across glass in thin section. Analyses were run using a 10 nA beam current at UC Davis and a 5 nA beam current at the USGS, with an accelerating voltage of 15 keV, and a 1 μm beam diameter. Count time for each point was 10 s, with Na and K analyzed first to avoid loss by volatization. Taylor standards of jadeite, chromeaugite, hematite, tremolite, chromite, rutile, and rhodonite were used as calibration standards (Putirka & Condit 2003).

Thermobarometric Calculations of Host Magmas and Mafic Enclaves Plagioclase mineral analyses were used to calculate temperatures of rhyodacitic host magmas prior to eruption using plagioclase-melt equilibrium geothermometry (Putirka 2005, Borg and Clynne 1998, Housh and Luhr 1991); (Ab-An) KD =0.1±0.05 (Putirka 2005) was evaluated for each plagioclase analysis considered. Ambient conditions for pre-eruptive storage of the rhyodacitic host magma are assumed to be 145 ± 25 MPa, or a depth of ~4-5 km Quinn (2014). Host magma temperatures obtained from this study were qualitatively compared to pre-eruptive storage conditions calculated by Quinn (2014) using titanomagnetite- ilmenite pairs to ensure accuracy. Plagioclase, clinopyroxene, orthopyroxene, and olivine mineral analyses were used to calculate temperatures of mafic enclaves prior to eruption using plagioclase-melt equilibrium geothermometry (Housh and Luhr 1991, Borg and Clynne 1998, Putirka 2005), clinopyroxene-liquid thermobarometry (Putirka et al. 1996), orthopyroxene-liquid thermobarometry (Putirka 2008), olivine-liquid 18 18

geothermometry (Putirka 2008), and new amphibole-liquid thermobarometers (Ab-An) (Putirka, written comm.); plagioclase KD =0.1 ±0.05 (Putirka 2005), (Fe-Mg) (Fe-Mg) clinopyroxene KD =0.27 ±0.03 (Putirka 2003), orthopyroxene KD = 0.29 (Fe-Mg) ±0.06 (Putirka 2008), olivine KD = 0.3 ±0.03 (Putirka 2008), and amphibole (Fe-Mg) KD = 0.28 ±0.03 (Putirka, written comm.) were evaluated for each mineral analysis considered. The weight percent of H2O for olivine-liquid geothermometry calculations (4.0 wt.% H2O) was obtained from melt inclusion analyses performed by Collins et al. (2012). Clinopyroxene, orthopyroxene, and amphibole mineral analyses were used to calculate crystallization pressures of mafic components using clinopyroxene-liquid thermobarometry (Putirka et al. 1996), orthopyroxene- liquid thermobarometery (Putirka 2008), and new amphibole-liquid thermobarometry (Putirka, written comm.).

Percent Vesicularity and Vesicle Size Distributions Two-dimensional evaluation of eruptive product vesicularities was conducted from cut slabs at California State University, Fresno using the methods of Jackson et al. (2011). Rhyodacitic host and mafic enclave samples collected for this study were imaged on a flat-bed scanner at 1200 dpi resolution; contrast/brightness of digitized slabs was adjusted using Adobe Photoshop CS5 to obtain the best representation of vesicles and crystals present within sample groundmass. Vesicle size distribution (VSD) measurements were performed on these images using IPLab image analysis software; sensitivity restrictions of the IPLab software prevented analysis of digitized vesicles less than 0.003 mm2, such that this population was not included in the data set. BSE images of vesicles within a random sample of ten (10) mafic enclaves were analyzed using IPLab image analysis in order to evaluate the population of vesicles excluded from larger 19 19

digitized slab analyses. BSE vesicularity measurements yielded a large population of extremely small-volume vesicles; however, this smallest vesicle population represented <1% of total vesicularity of each eruptive product examined, and was discarded from the data set. Error due to uncertainty in measurement for VSD analyses was calculated by repeating IPLab image analyses for ten randomly selected mafic enclaves, and calculating the mean percent difference in total vesicularity of all analyses; calculated error for vesiculation measurements due to measurement uncertainty was determined to be 4.3%. Stereological conversion and morphometric analysis methods were used to correct apparent two-dimensional measured vesicle area data to actual three- dimensional diameters. Two-dimensional apparent diameter of vesicles (L2D) was calculated from the measured two-dimensional area of vesicles (A2D) by use of the standard equation: 1/2 L2D=2(A2D/π) (1)

Apparent two-dimensional diameters of vesicles (L2D) were corrected using the stereological methods of Peterson (1996); Peterson’s Eq. 5 was applied to determine the true three-dimensional diameter (L3D) of vesicles:

L3D = L2D(4/π) (2)

The measured value N2D represents the number of vesicles of a given size greater than L2D which intersect the cut slab per unit area analyzed; the number of vesicles of a given size greater than L3D which exist in a given unit volume of the same sample (N3D) can be determined through Eq. 1 of Peterson (1996):

N3D = N2D/L3D (3)

The population density (n), or total number of bubbles of a given L3D per unit volume of magma, may be obtained by approximating the slope of a three- dimensional VSD curve, N3D vs. L3D, as in Eq. 2 of Marsh (1988): 20 20

n(L) = - δN3D/δL3D(L) (4) Mangan et al. (1993) introduced the use of log-linear VSD plots to glean bubble growth behavior in magmas from population density graphs developed in previous studies; this process was refined by Mangan and Cashman (1996) (Fig. 5). Equation 7 of Mangan and Cashman (1996) may be used to interpret log-linear VSD plots: 0 n=n exp(-L3D/GT) (5) where n0 is the volumetric number density of bubble nuclei, G is the mean growth rate of bubbles, and T is the timescale of bubble nucleation and growth. The y- intercept of a log-linear VSD plot is representative of ln(n0); G may be determined from a measurement of the log-linear VSD slope (-1/GT); T may be calculated as: T=d/v (6) where d is the depth of the bubble, and v is magma ascent velocity. Further, bubble nucleation rate (J) may be determined using: J=n0G (7)

Figure 5. Bubble growth behavior as illustrated in log-linear VSD plots [after Mangan and Cashman (1996)]. 21 21

Log-linear VSD plots were produced for each VSD analysis conducted in this study. The non-linear nature of bubble growth, especially in an open, changing system (Blower et al. 2002; Naber et al. 2008; pers. comm., Guilherme Gualda, January 2014) was considered, and VSD plots were fit with an inverse arctangent curve to best approximate trends of bubble growth, and for the purpose of deriving slopes at any given vesicle diameter. Non-linear VSD plots were produced using JMP 11.0.0 Statistical Analysis Software by first plotting a traditional log-linear VSD graph, and the best non-linear fit to the data was determined using the following equation: b y = [A/(arctanL3D) ] – C (8) where A, b, and C are regression coefficients. First derivatives of Eq. 8 allow the determination of the slopes of tangent lines, which can then be used to obtain growth and nucleation rates for bubbles within each mafic enclave.

Calculations of Pre-eruptive Magma Densities Pre-eruptive densities of eruptive products were calculated using the methods of Lange and Carmichael (1990) and Ochs and Lange (1999). Whole- rock geochemical analyses from this study were used for bulk rock compositions for each sample. Pre-eruptive storage conditions used to calculate densities of rhyodacitic host magmas were taken from Quinn (2014). Pre-eruptive temperatures and pressures for mafic enclaves were calculated in this study; where pre-eruptive temperatures were not able to be calculated from quantitative chemical analyses, the plagioclase saturation temperature model of Putirka (2008) was used. The weight percent of H2O wt.% was calculated using Eq. 3 of Jackson (2011): 1/2 H2Owt.%=0.7996+15.347(PGPa) -0.00233T°C+0.06248(Na2Owt.%+K2Owt.%) (9) 22 22

Densities of mafic enclaves were calculated using Eq. 4 of Jackson (2011):

ρvm = fmρm+fvρv (10) where ρvm is the density of a vesiculated magma, fm is the fraction of magma, fv is the fraction of vesicles, ρm is the density of non-vesiculated magma, and ρv is the density of vesicles, calculated using the modified Redlich-Kwong equation of state

(Holland and Powell 1991) for H2O: V = (RT/P) + b – (aR√T)/[(RT+bP)(RT+2bP)] + c√P + dP (11) where a, b, c, and d are parameters provided by Holland and Powell (1991).

RESULTS

In the sections below, geochemical compositions, physical and petrographic features, petrologic characteristics, calculated thermobarometric values, and pre- eruptive enclave densities for eruptive products from the Chaos Crags are discussed. A comprehensive list of Chaos Crags eruptive products analyzed in this study is listed in Appendix A.

Geochemical Compositions of Chaos Crags Eruptive Products

Chaos Crags lavas are dacitic to rhyodacitic (67.5 to 71 wt.% SiO2), characterized by the presence of basaltic-andesitic mafic enclaves. TAS contents (LeBas 1986) were used to determine classification of eruptive products. Mafic enclaves within Group 1 units range from basaltic to andesitic; enclaves within Group 2 units are basaltic-andesite to andesitic, with rare trachy-andesite enclaves present (Fig. 6). At face value, it appears that mafic enclaves become compositionally more evolved throughout the eruption sequence (Figs. 7 and 8), however, it is cautioned that this trend could be affected by sampling locality, as samples collected from Dome D were collected from a single area of talus at the west base of the dome, and samples collected from Dome F were collected from a single locality at the east base of the dome.

Physical and Petrographic Features and Petrologic Characteristics of Chaos Crags Eruptive Products

Disequilibrium Mineral Assemblages Present Within Eruptive Products Chaos Crags eruptive products contain a wide variety of mineral phases, many of which are not at equilibrium with either each other, or the bulk whole 24

Figure 6. TAS Classification of Chaos Crags eruptive products (after LeBas 1986). MAC data obtained from Michael Clynne (written comm., February 2014). 25

Figure 7. Variations in wt.% SiO2 and wt.% MgO by dome (A-F). Grey triangles = host lavas, black circles = mafic enclaves. SEE = 1.0 wt.%. MAC data obtained from Michael Clynne (written comm., February 2014). 26

Figure 8. Compositional variation diagrams of wt.% major oxides within Chaos Crags eruptive products (after Harker 1909). SEE = 1.0 wt.%.

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rock composition of the eruptive product in which they reside. A thorough examination of mineral phases present within Chaos Crags host lavas and enclaves is necessary in order to identify distinct populations of mineral phases and their origins.

Plagioclase phenocryst populations. Plagioclase is the predominant phase within the entire suite of Chaos Crags eruptive products (Appendices C and D), and is most often found in disequilibrium with the eruptive product in which it is contained (Fig. 9). Based on petrographic evaluation of eruptive products in thin section and quantitative chemical analysis of mineral phases, four populations of plagioclase phenocrysts may be found within eruptive products: 1) Unresorbed to slightly reacted and resorbed reversely to oscillatory- zoned sodic plagioclase, with minimal reaction rims (Fig. 10a). Population 1 plagioclase are present within host lavas. 2) Unresorbed to slightly reacted normally to oscillatory-zoned calcic plagioclase (Fig. 10b). Population 2 plagioclase are present within mafic enclaves. 3) Variably resorbed sodic plagioclase characterized by patchy zones and rarely enclosing chadacrysts of hornblende or clinopyroxene (Fig. 10c). Population 3 plagioclase are present within both mafic enclaves and host lavas.

4) Sodic plagioclase with clear sodic (~An25-35) cores and thick (100-500 µm), heavily reacted sieve-textured zones sometimes overgrown by thin, strongly normally zoned calcic rims (Fig. 10d). Population 4 enclaves are present within both mafic enclaves and host lavas.

Figure 9. Measured An components of plagioclase vs. predicted An components of plagioclase from bulk rock compositions in which plagioclase reside. 1:1 line denotes that measured plagioclase are at equilibrium with the bulk rock composition. Dashed lines indicate ±An10 deviation from 1:1 line (after Putirka 2005). Inset provides detail of plagioclase at equilibrium. An# = Ca/(Ca + Na).

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Figure 10. BSE images of plagioclase phenocrysts populations within Chaos Crags eruptive products and measured An component along sampling transects. Red lines across BSE images indicate transect locations, red numbered points denote sampling locations where linear transects were unable to be obtained. See text for discussion. 30

Clinopyroxene. Quantitative chemical analyses determined that the majority of clinopyroxene present within Chaos Crags eruptive products are augite and diopside, with rare pigeonite (Fig. 11, Appendix E). A comparison of measured EnFs, DiHd, CaTs, and CaTi components—within analyzed clinopyroxene with predicted components that would be in equilibrium with their respective bulk whole rock compositions—shows that greater than half of analyzed clinopyroxene grains are not in equilibrium with the bulk rock composition of the eruptive product in which they reside (Figs. 12-14). Clinopyroxene is abundant within mafic enclaves, most often as very fine-grained groundmass (Fig. 15a). Phenocrysts and microphenocrysts (Fig. 15a) are common; infrequently, phenocrysts displaying pseudomorphic replacement by amphibole (Fig. 15d) may be present within mafic enclaves and/or host lavas. Clinopyroxene also forms reaction rims around host-derived quartz phenocrysts (Fig. 15c) within mafic enclaves.

Orthopyroxene. Orthopyroxene is most often present within mafic enclaves as a reaction product surrounding olivine, as a reaction product surrounding amphibole and as pseudomorphic replacement of amphibole, and less often as a fine-grained groundmass within enclaves and host lavas (Figs. 16 and 17, Appendix F). Orthopyroxenes are rarely at equilibrium with the bulk rock composition of the eruptive product in which they are contained (Fig. 18).

Olivine. Olivine is present in the majority of mafic enclaves analyzed, either as euhedral phenocrysts or as groundmass crystals (Appendix G). Few olivine phenocrysts are present within host lavas. Forsterite (Fo) contents of olivine range from Fo69 to Fo83, and more often than not are at disequilibrium with the bulk whole rock composition of the host enclave in which they are entombed 31

(Fig. 19). Unreacted olivine phenocrysts are rare within eruptive products (Figs 20a and 20d); olivine phenocrysts are generally surrounded by reaction rims of pyroxene (Fig. 20b), and occasionally by overgrowths of hornblende (Fig. 20c).

Quartz. Quartz is often present within host lavas as large (up to 7 mm) phenocrysts, and occasionally within mafic enclaves. Quartz phenocrysts within mafic enclaves are highly reacted, sometimes completely removed, and always accompanied by fine-grained rims of augite (Fig. 15c).

Hornblende. Hornblende is abundant in Chaos Crags host lavas as large (up to 2 mm) phenocrysts and microphenocrysts; hornblende is also found in the majority of mafic enclaves as euhedral phenocrysts, glomeroporphyritic clots accompanied by biotite and Fe-Ti oxides, acicular microphenocrysts, and fine- grained groundmass. Hornblende within mafic enclaves may experience pyrometamorphic and/or pseudomorphic replacement by orthopyroxene and/or clinopyroxene, and reaction rims of orthopyroxene (Figs. 21 and 22).

Figure 11. Compositional variations in clinopyroxene.

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Figure 12. Measured EnFs components of clinopyroxene vs. predicted EnFs components of clinopyroxene from bulk rock compositions in which clinopyroxene reside. 1:1 line denotes that measured clinopyroxene are at equilibrium with the bulk rock composition. Dashed lines indicate ±0.03 (1σ) deviation from 1:1 line (after Putirka et al. 2008). Inset provides detail of clinopyroxene at equilibrium.

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Figure 13. Measured DiHd components of clinopyroxene vs. predicted DiHd components of clinopyroxene from bulk rock compositions in which clinopyroxene reside. 1:1 line denotes that measured clinopyroxene are at equilibrium with the bulk rock composition. Dashed lines indicate ±0.03 (1σ) deviation from 1:1 line (after Putirka et al. 2008). Inset

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provides detail of clinopyroxene at equilibrium.

Figure 14. (a) Measured CaTs components of clinopyroxene vs. predicted CaTs components of clinopyroxene from bulk rock compositions in which clinopyroxene reside. (b) (a) Measured CaTi components of clinopyroxene vs. predicted CaTi components of clinopyroxene from bulk rock compositions in which clinopyroxene reside. 1:1 line denotes that

measured clinopyroxene are at equilibrium with the bulk rock composition. Dashed lines indicate ±0.03 (1σ) deviation 35

from 1:1 line (after Putirka et al. 2008). 36

Figure 15. BSE images highlighting disequilibrium textures of clinopyroxene within Chaos Crags mafic enclaves. (a) unreacted clinopyroxene microphenocrysts and groundmass. (b) plagioclase phenocryst enclosing clinopyroxene chadacrysts. (c) reacted and partially plucked quartz phenocryst surrounded by a reaction rim of clinopyroxene. (d) pseudomorphic replacement of clinopyroxene phenocryst by amphibole. pl = plagioclase, cpx = clinopyroxene, gl = glass, qtz = quartz.

Figure 16. Compositional variations in orthopyroxene.

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Figure 17. BSE images highlighting disequilibrium textures of orthopyroxene within Chaos Crags eruptive products. (a) Unreacted fine-grained orthopyroxene groundmass within mafic enclave. (b) Olivine phenocryst surrounded by orthopyroxene corona rim within mafic enclave. (c) Pseudomorphic replacement of hornblende by acicular orthopyroxene within mafic enclave (d) Dehydration reaction rim of orthopyroxene surrounding hornblende within host lava. pl = plagioclase, ol = olivine, cpx = clinopyroxene, gl = glass, hbl = hornblende, opx = orthopyroxene. 39

Figure 18. Orthopyroxene Mg number vs. whole rock Mg-number of material in which orthopyroxenes reside. Equilibrium curve is calculated based on the Fe-Mg opx-liq exchange coefficient of KD(Fe-Mg) = 0.29 ±0.06, and an assumed fO2 buffer at -wüstite (MW). Dashed lines represent 1σ error from Putirka (2008). After Roeder and Emslie (1970). 40

Figure 19. Fo contents of olivine vs. whole rock Mg-number of material in which olivines reside. Equilibrium curve is calculated based on the Fe-Mg exchange ol-liq coefficient of KD(Fe-Mg) = 0.30 ±0.03, and an assumed fO2 buffer at magnetite-wüstite (MW). Dashed lines represent 1σ error from Putirka (2008). (After Roeder and Emslie (1970). 41

Figure 20. BSE images highlighting disequilibrium textures of olivine within Chaos Crags eruptive products. (a) Unreacted olivine micrphenocryst within mafic enclave. (b) Olivine phenocryst surrounded by orthopyroxene corona within mafic enclave. (c) Remnants of partially resorbed olivine phenocrysts surrounded by hornblende overgrowths within mafic enclave. (d) Minimally reacted olivine phenocryst present within host lava. Note beginning stages of corona rim development surrounding phenocryst. pl = plagioclase, ol = olivine, cpx = clinopyroxene, gl = glass, hbl = hornblende. 42

Figure 21. Amphibole Mg number vs. whole rock Mg-number of material in which amphiboles reside. Equilibrium curve is calculated based on the Fe-Mg amph-liq exchange coefficient of KD(Fe-Mg) = 0.28 ±0.03, and an assumed fO2 buffer at magnetite-wüstite (MW). Dashed lines represent 1σ error from Putirka (written comm.). After Roeder and Emslie (1970).

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Figure 22. BSE images highlighting disequilibrium textures of hornblende within Chaos Crags eruptive products. (a) unreacted hornblende groundmass within mafic enclave. (b) glomeroporphyritic clot of hornblende and Fe-Ti oxides surrounding biotite, with rare antecrystic zircon within mafic enclave. (c) pyrometamorphic replacement of hornblende by orthopyroxene within mafic enclave (d) reaction rim of orthopyroxene surrounding hornblende within host lava. hbl = hornblende, pl = plagioclase, gl = glass, bt = biotite, opx = orthopyroxene, zr = zircon.

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Variations in Host Lavas Host lavas are rhyodacitic to dacitic (Fig. 6), and exhibit a decrease in wt.%

SiO2 from Dome C to Dome F (Figs. 7 and 8). Group 1 host groundmasses display a pumiceous vitrophyric texture, with phenocrysts of plagioclase, hornblende, biotite, and lesser subhedral quartz and Fe-Ti oxides (Fig. 23a). Three populations of large (1-5 mm) plagioclase phenocrysts are present in Group 1 lavas: Population 2 phenocrysts account for the majority of plagioclase present, with lesser amounts of phenocrysts from Populations 3 and 4. An contents of plagioclase within Group 1 lavas range from An06 to An50, with the majority of plagioclase having values of

Figure 23. Contrast in groundmass textures between (A) Group 1 (ppl) and (B) Group 2 (xpl) host lavas. plag = plagioclase, bt = biotite, hbl = hornblende; Fe-Ti = Fe-Ti oxides. clots composed of tabular to prismatic plagioclase, orthopyroxene, clinopyroxene, and hornblende, reacted hornblende and biotite oikocrysts enclosing unreacted plagioclase chadacrysts, pyroxene-rimmed hornblende, euhedral hornblende oikocrysts enclosing reacted and mottled biotite chadacrysts, and anhedral fragments of enclave-derived olivine and clinopyroxene.

Variations among Mafic Enclaves Mafic enclaves are extremely diverse, ranging from basalt to andesite to trachyandesite (Figs. 6-8). Enclaves are spherical to ellipsoidal in shape, but may present angular boundaries resulting from disruption of mafic foam or post- formation enclave breakage (Michael Clynne, personal comm. May 2014). Enclave diameters range from 4 cm to over 50 cm (Appendix A); enclaves with diameters less than 4 cm are most likely broken fragments of larger enclaves. Enclaves throughout the Chaos Crags eruption sequence display a variety of textures, from medium-grained aphyric to medium-grained slightly phyric to 46

fine-grained porphyritic to fine-grained slightly phyric. Not only are textural and vesicular variations present between domes, but enclave textures vary throughout a singular dome, and even within an individual enclave. Enclaves may exhibit crenulated margins and quenched rims, with the core of the enclave having a larger average grain size than the enclave rim (Fig. 24c), however not all enclaves are quenched or crenulated. Enclaves may also be concentrically zoned with respect to vesicularity, with the core of the enclave exhibiting a greater percentage by volume of vesicles than the enclave rim (Fig. 24a). The presence of intra-enclaves (enclaves within enclaves) was noted in a small population of enclaves (Fig. 24f), with the intra-enclave having a markedly different texture and/or vesicularity than the enclave in which it resides. A small number of enclaves analyzed in this study display three or more concentric zones, each with a different texture (Fig. 24d). A textural and/or vesicular distinction between enclave cores and rims is not, however, always apparent within some enclaves (Figs. 24b and 24e). Despite the variety of mafic enclaves found within the Crags’ eruptive products, no correlations were found between texture, percent vesiculation and temperature, pressure, or whole-rock geochemistry. A comparison of the entire data set, including enclaves from other studies and unpublished data, shows a moderate correlation between texture and enclave diameter in that, on average, the largest enclaves (>40 cm diameter) are coarser- grained (personal comm., Michael Clynne, May 2014). Groundmasses of mafic enclaves are microvesiculated calcic plagioclase ± pyroxene ± hornblende ± olivine, with Fe-Ti oxides and abundant interstitial glass; phenocrysts of calcic plagioclase (sometimes enclosing chadacrysts of clinopyroxene and/or hornblende), clinopyroxene, and olivine are common. Mafic enclaves are characterized by disequilibrium mineral assemblages, often 47

containing highly resorbed and sieved sodic plagioclase phenocrysts with normally zoned calcic overgrowth rims, partially resorbed sodic plagioclase oikocrysts enclosing hornblende chadacrysts, completely to partially-reacted quartz phenocrysts rimmed with augite, mottled and partially resorbed opacite- rimmed biotite, pseudomorphic replacement of hornblende by pyroxene, reacted olivine with pyroxene alteration and hornblende overgrowths, partially resorbed amphibole with pyroxene coronas, glomeroporphyritic aggregates of sodic plagioclase + biotite + hornblende + Fe-Ti oxides + silica-rich glass, accessory apatite, and antecrystic zircon. The occurrence of disequilibrium phases increases over the eruption sequence from Group 1 to Group 2 lavas, and is less prevalent in enclaves with quenched margins. Groundmass textures of mafic enclaves are extremely variable in both Group 1 and Group 2 lavas. Figure 25 provides a representation of the variety of textures found within mafic enclaves. There appears to be no preference for grain size, number of textural zones, presence of intra-enclaves, or vol. % vesicularity in any dome over the eruption sequence. Groundmass textures from Group 1 lavas include: a moderately vesiculated microcrystalline porphyritic basaltic-andesite featuring abundant calcic plagioclase and hornblende microphenocrysts, large (2-8 mm) variably resorbed and sieve-textured host-derived sodic plagioclase and large (3-5 mm) augite-rimmed quartz phenocrysts (Fig. 25a); a minimally vesiculated fine to medium-grained diktytaxitic slightly phyric basaltic-andesite featuring sparsely-distributed medium to large (0.5-3 mm) enclave-derived calcic plagioclase and host-derived resorbed and reacted sodic plagioclase (Fig. 25b); a minimally vesiculated fine to medium-grained diktytaxitic aphyric hornblende clinopyroxene basaltic andesite (Fig. 25c); and a greatly vesiculated fine-grained intergranular porphyritic basaltic andesite featuring medium to large (0.5-6 mm) 48

enclave-derived calcic plagioclase and host-derived heavily reacted and sieve- textured sodic plagioclase with thin calcic overgrowths (Fig. 25d). Groundmass textures from Group 2 lavas include: a minimally-vesiculated medium-grained diktytaxitic slightly phyric andesite featuring sparse medium to large (0.5-3 mm) heavily sieved and reacted host-derived sodic plagioclase with thin calcic overgrowths (Fig. 25e); a greatly vesiculated fine-grained subophitic porphyritic andesite featuring abundant host-derived large (2-8 mm) heavily reacted and sieved sodic plagioclase with thin calcic overgrowth rims, enclave-derived oikocrysts of medium to large (0.5-3) slightly reacted oscillatory zoned calcic plagioclase containing clinopyroxene chadacrysts, and enclave-derived glomeroporphyritic clots of calcic plagioclase + clinopyroxene + olivine (Fig. 25f); a fine-grained diktytaxitic slightly phyric hornblende andesite featuring enclave-derived medium (0.5-2 mm) partially resorbed and reacted normally zoned calcic plagioclase microphenocrysts, host-derived medium to large (0.5-7 mm) heavily resorbed sodic plagioclase with thin calcic overgrowth rims, host- derived medium (0.5-2 mm) partially resorbed sodic plagioclase with thick calcic overgrowth rims, and host-derived medium to large (1-5 mm) heavily reacted and opacitized biotite (Fig. 25g); and a greatly vesiculated fine-grained intersertal porphyritic hornblende andesite featuring host-derived large (2-8 mm) heavily reacted and resorbed sodic plagioclase with thin to no calcic overgrowth rims, blebs of host-derived rhyolitic vesiculated glass containing host-derived phenocrysts, host-derived small (0.25-1 mm) amphibole phenocrysts replaced by orthopyroxene (Fig. 25h). 49

Figure 24. Representative textural and vesicular variations within mafic enclaves. See text for discussion. 50

Figure 25. Representative variations in groundmass textures and mineralogy between Group 1 (A-D) and Group 2 (E-H) mafic enclaves. XPL. plag = plagioclase, bt = biotite, hbl = hornblende, cpx = clinopyroxene, qtz = quartz, opx = orthopyroxene, ol = olivine, gl = glass, Fe-Ti = Fe-Ti oxides. 51

Figure 25, cont’d. Representative variations in groundmass textures and mineralogy between Group 1 (A-D) and Group 2 (E-H) mafic enclaves. XPL. plag = plagioclase, bt = biotite, hbl = hornblende, cpx = clinopyroxene, qtz = quartz, opx = orthopyroxene, ol = olivine, gl = glass, Fe-Ti = Fe-Ti oxides. 52

Geochemical and Petrographic Variations Within Individual Mafic Enclaves Groundmass textures, percent vesicularity, and geochemical composition vary drastically both between and within individual mafic enclaves. The majority of mafic enclaves present minor variations in geochemical compositions from core to rim, albeit within standard estimates of error (SEE). Four of the enclaves analyzed, however, exhibit distinct variations in wt.% major oxides, well outside of the margin of error (Fig. 26). A strong correlation is shown between distance from enclave center and SiO2 content, with the outermost portions of the enclave having the greatest wt.% SiO2 and K2O, and the least wt.%

FeOtotal. The bulk of enclaves analyzed in this study display some wt.% increase in

SiO2 with distance from enclave center; those which do not are within SEE values, with the exception of CC-E-I-10, which notably exhibits a 4.05 wt.% decrease in

SiO2 from enclave core to rim. Geochemical variation within an individual enclave is not necessarily correlated with textural variations within that enclave, nor with groundmass texture; variations are present in both medium-grained aphyric and fine-grained porphyritic enclaves. Enclave CC-C-I-11 has a ~9 wt.% decrease in SiO2 from enclave rim to intra-enclave (Fig. 26); this decrease is accompanied by an increase in grain size and vol. % vesicularity from enclave rim to intra-enclave (Fig. 27f – 27h). However, variations in geochemical compositions for enclave CC-B-I-9 are within SEE, despite the increase and subsequent decrease in grain size and vol. % vesicularity from enclave rim to enclave core (Fig. 27a – 27e). Similarly, enclave CC-D-I-1 displays an increase in vol. % vesicularity from enclave rim to core (Fig. 27i – 27j), without any notable variations in geochemical composition, mineralogy or grain size. 53

Figure 26. Variation diagrams of selected major elements vs. silica (after Harker 1909) within individual mafic enclaves. Note variation in composition between enclave rim and core (and intra-enclave). SEE=1.0 wt.%. See text for discussion. 54

Figure 27. Textural variations within selected individual mafic enclaves. PPL. (A) CC-B-I-9 outer rim. (B) CC-B-I-9 middle rim. (C) CC-B-I-9 inner rim. (D) CC-B- I-9 outer core. (E) CC-B-I-9 inner core. (F) CC-C-I-11 rim. (G) CC-C-I-11 core. (H) CC-C-I-11 intra-enclave. (I) CC-D-I-1 rim. (J) CC-D-I-1 core. plag = plagioclase, hbl = hornblende, cpx = clinopyroxene, ol = olivine, gl = glass, Fe-Ti = Fe-Ti oxides.

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Figure 27, cont’d. Textural variations within selected individual mafic enclaves. PPL. (A) CC-B-I-9 outer rim. (B) CC-B-I-9 middle rim. (C) CC-B-I-9 inner rim. (D) CC-B-I-9 outer core. (E) CC-B-I-9 inner core. (F) CC-C-I-11 rim. (G) CC-C- I-11 core. (H) CC-C-I-11 intra-enclave. (I) CC-D-I-1 rim. (J) CC-D-I-1 core. plag = plagioclase, hbl = hornblende, cpx = clinopyroxene, ol = olivine, gl = glass, Fe- Ti = Fe-Ti oxides. 56

Vesicularity of Chaos Crags Eruptive Products Host lavas display a dramatic decrease in vesiculation from Dome A (6.78%) to Dome C (0.14%), and then increase minutely over the remainder of the eruption sequence (Fig. 28). Mafic enclaves display similar increasing and decreasing trends in vesiculation as host lavas over the eruption sequence. The majority of mafic enclaves are <20 vol. % vesiculated; few enclaves analyzed are >20 vol. % vesiculated (Fig. 28). Mafic enclaves demonstrate no correlation between percent vesicularity and/or geochemical composition. A rough correlation may be made between enclave texture and vol.% vesicularity of an inclusion. Enclaves with diktytaxitic groundmass textures are low to moderately-vesiculated, with small, irregular vesicles filling gaps in between groundmass crystals. Enclaves with groundmasses other than diktytaxitic textures, however, may have low, medium, or high vol.% vesicularities, and may display both small, irregular vesicles filling gaps in between groundmass crystals, or larger spherical or coalesced vesicles which dominate the texture of the enclave. Volume % vesicularity of mafic enclaves roughly decreases with enclave diameter (Fig. 29). Smaller enclaves and fragments of disaggregated enclaves may be as little vesiculated as 1.3 vol.%, or as greatly vesiculated as 40.6 vol.%. Larger diameter enclaves are less vesiculated than smaller enclaves, ranging from 0.7 vol.% to 10.2 vol.% vesiculated.

Vesicle Size Distributions of Eruptive Products Log-linear VSD plots of Group 1 host lavas display trends of coalescence, with decreasing growth rates and vesicle diameters from Dome A to Dome B. Group 2 host lavas are dominated by steady-state nucleation and growth, exhibiting negligible amounts of coalescence. Nucleation and growth rates within 57

Group 2 host lavas fluctuate; nucleation rates within Group 2 host lavas are all lower than nucleation rates of Group 1 host lavas. Mafic enclaves display differing trends of bubble nucleation, growth, and coalescence. Approximately 36% of enclaves display nucleation and growth trends similar to their respective host lavas (Fig. 30). Coalescence dominates in mafic enclaves: approximately 86% of all enclaves analyzed exhibit trends of coalescence, with coalescence in 64% of mafic enclaves disparate than that of their host lavas (Fig. 31). Log-linear VSD plots for host lavas are presented in Appendix I; log-linear VSD plots for mafic enclaves are presented in Appendix J.

Figure 28. Vesiculation of eruptive products by dome. SEE = 4.3%. 58

Figure 29. Mafic enclave diameter vs. percent vesiculation by volume. Dotted line indicates upper limit of vol. % vesicularity. 59

Figure 30. Log-linear VSD plots of mafic enclaves which display similar nucleation and growth trends as their host lavas. n0 = volumetric number density of bubble nuclei, J = nucleation rate of bubbles, G = mean growth rate of bubbles, and T = timescale of bubble nucleation and growth. VSD for enclaves cores are presented in black, VSD for enclave rims are presented in grey, and VSD for respective host samples are presented in red. See text for discussion regarding volatile

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behavior within enclaves.

Figure 31. Log-linear VSD plots of mafic enclaves displaying trends of coalescence, nucleation, and growth which differ greatly from their host lavas. n0 = volumetric number density of bubble nuclei, J = nucleation rate of bubbles, G = mean growth rate of bubbles, and T = timescale of bubble nucleation and growth. VSD for enclaves cores are presented in black, VSD for enclave rims are presented in grey, and VSD for respective host samples are presented in red. See text for discussion regarding volatile behavior within enclaves. 61

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Thermobarometric Calculations of Host Magmas and Mafic Enclaves Samples analyzed in this study are extremely crystalline and display a wide variety of disequilibrium mineral assemblages within all samples, implying that the measured whole rock composition can not necessarily be representative of the true composition of the liquid from which mineral phases crystallized from. Plagioclase An contents, crystallization temperatures, and starting composition wt.% SiO2 from the experimental studies of Baker and Eggler (1983), Rutherford et al. (1985), Hammer et al. (2002), DiCarlo et al. (2006), and Rader and Larsen (2013) were compared against measured plagioclase An contents and whole rock wt.% SiO2 of natural Chaos Crags samples analyzed in this study in order to determine if any of the measured whole rock wt.% SiO2 of measured natural Chaos Crags samples may be representative of a liquid which produced the measured An contents of Chaos Crags plagioclase in this study (Fig. 32). A range of measured An contents exists within the same range as experimental plagioclase, indicating that while the entire natural sample was most likely not a liquid at the time of crystallization, there existed some liquid within the sample that had compositions identical or nearly identical to that of the measured whole rock composition. As such, only phases which approach equilibrium between mineral phases and whole rock compositions were considered to estimate pressures and temperatures of crystallization. Where equilibrium is approached between minerals and whole rock compositions, this study finds that the Chaos Crags eruptive products crystallized over a wide range of temperatures (~975-1175°C, Fig. 33) and pressures (~0-5.6 kbar, Fig. 34) over the entirety of the eruption sequence. Pressure-temperatures estimates from clinopyroxene, orthopyroxene, olivine, and hornblende demonstrate both moderately deep and shallow crystallization. Rhyolite-MELTS 63

(Gualda et al. 2012) was used as a test of P-T estimates, using the least contaminated basaltic whole rock compositions to create a phase diagram in P-T space. Measured mineral compositions and P-T approximations produced by Rhyolite-MELTS yield very similar results, namely that the parental mafic end- member reached clinopyroxene-saturation at depth, followed by olivine saturation at lower pressures and temperatures; orthopyroxene, plagioclase and hornblende saturation occurred at shallow pressures and low temperatures.

Calculated Densities of Chaos Crags Eruptive Products Calculated densities of Chaos Crags host magmas from the various domes are similar, with a range of 2.28 g/cm3 – 2.30 g/cm3. Calculated vesiculated bulk densities of mafic enclaves are highly diverse. The majority of mafic enclaves have a vesiculated density less than that of their respective host magmas, however, approximately 15% of mafic enclaves are more dense than the host magma in which they reside (Fig. 35). Enclave density increases with enclave diameter in a similar manner to vol. % vesiculation trends for mafic enclaves. Larger enclaves, having lower-than-average vesicularities (Fig. 29), have greater bulk densities than smaller diameter enclaves, which have a range of vesicularities. No relationship could be established between enclave density and geochemical composition, groundmass texture (sans vesiculation), or position within the eruption sequence.

Figure 32. Whole Rock wt.% SiO2 vs. measured An component of plagioclase within natural Chaos Crags samples from this study and experimental plagioclase from referenced studies. Secondary axis denotes temperature of experimental plagioclase. Red polygon illustrates an equilibrium range of An and T as extrapolated from experimental studies.

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Figure 33. Crystallization temperatures (°C) of Chaos Crags Eruptive Products. H2Ool = 4.0 wt.%. SEE for each phase is as follows: clinopyroxene = ±33°C; plagioclase = ±23°C; olivine = ±19°C; orthopyroxene = ±26°C; hornblende = ±30°C.

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Figure 34. Crystallization temperatures (°C), pressures (kbar), and depths (km) of Chaos Crags eruptive products. SEE for each phase is as follows: clinopyroxene = ±1.7 kbar, ±33°C; orthopyroxene = ±2.6 kbar, ±26°C; hornblende = ±2.5 kbar, ±30°C.

Figure 35. Calculated densities of Chaos Crags eruptive products by dome. Host lava densities calculated at T=770°C, P=145 MPa, H2O wt.% = 4.80, and ɸ=0.1. Densities of mafic enclaves calculated individually at P=145 MPa, T=average 1/2 plagioclase temperature for that enclave, H2O wt.% = 0.7996+15.347(PGPa) -0.00233T°C+0.06248(Na2Owt.%+K2Owt.%), ɸ=0.5, and %vesiculation= measured vol.% vesiculation for that enclave. SEE = 5.3%.

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DISCUSSION

The petrologic and geochemical evidence presented illustrate that although mafic recharge and magma mixing may have been the ultimate trigger for the

1,103 ±13 years BP eruption of the Chaos Crags volcanic sequence, the proximal cause of the eruption was most likely an increase in overpressure within the chamber induced by volatile contributions from the shallow crystallization of mafic magma, leading to fluid saturation, rapid vesiculation, and an increase in pH2O within the host magmas of the Chaos Crags. In this section, the geochemical, petrologic data, and textural data presented within the Results are discussed and interpreted as strongly supporting a model for the origin of Chaos Crags magmas as the complex mixing and mingling of two distinct magmas: a parental basaltic end-member magma not seen within the Chaos Crags eruptive products, and a parental rhyolite end-member consisting of remobilized crystalline mush (Klemetti et al. 2013). A magma mixing and eruption triggering model for the Chaos Crags system is then proposed, accounting for variations within geochemical composition, volume % vesicularity, and groundmass texture both between and within individual enclaves.

Geochemical Constraints on Magma Mixing and Identification of Parental End-member Magmas The formation of mafic enclaves has been thoroughly documented in the literature, with several mechanisms for enclave formation proposed. The presence of mafic enclaves is most often attributed to magma mixing of basalt and rhyolite end-members, as in Eichelberger (1975) and Bacon (1986). Geochemical compositions of Chaos Crags host lavas are rhyodacitic, and decrease in wt.% SiO2 to over the entirety of the eruption sequence (Fig. 6). 69

Klemetti et al. (2013) suggest that a slowly crystallizing mush is present underneath the LVC, remnant from Bumpass sequence volcanism, which is locally heated and remobilized by mafic recharge, leading to Eagle Peak and Twin Lakes Sequence eruptions. Our work, as shown below, indicates that this end-member may be much higher in SiO2 than any of the erupted products. Host lavas are intruded by recharge magmas as represented by basalt to andesite mafic enclaves (with rare trachyandesite). The majority of whole rock compositions fall on a linear trend indicative of binary mixing, but with a “Daly gap” (the frequently observed absence of intermediate compositions in systems composed of both basalts and more evolved magmas, such as ) (Mourtada-Bonnefoi and

Provost 1999) located between ~60wt.% SiO2 and ~67 wt.% SiO2 (Fig. 6). Mixing is also supported by the presence of mixed crystal populations and classic examples of mineral reaction and dissolution textures. A minor enclave population is present within Group 2 lavas which exhibit elevated levels of K2O+Na2O and 87Sr/86Sr relative to the remainder of the enclaves; this population may have been contaminated by Mesozoic basement rock underlying the Chaos Crags magma chamber (personal communication, Michael Clynne, March 2014). Measured geochemical compositions and petrographic analyses of host lavas demonstrate that erupted host lavas are contaminated to differing degrees by microscopic bits of disaggregated enclaves and enclave-derived phenocrysts – as such, host lavas cannot represent the composition of the parental rhyolite end- member. However, mineral compositions provide a record of the pre-mixing felsic end-member magma. The majority of plagioclase analyses from host lavas yield

An components of An25-35 (Fig. 36, Appendix C) – much more sodic than plagioclase predicted to be in equilibrium with erupted host lavas using Putirka 70

Figure 36. Frequency distributions of plagioclase An contents within Chaos Crags host lavas and mafic enclaves. Host lavas are represented in darker colors, mafic enclaves are represented in lighter shades. An component = Ca / (Ca + Na). 71

(2005). It is possible to estimate the composition of the magma parental to An25-35 crystals through experimental studies that produce such low An compositions, as well as pre-eruptive storage conditions established by Quinn (2014), and estimated by Rhyolite-MELTS (Gualda et al. 2012) (Appendix K). In the same manner as host lavas, all mafic enclaves from the Chaos Crags display geochemical and petrologic evidence of varying degrees of contamination from the more felsic host magmas (Figs. 9-22). Histograms of the compositions of plagioclase phenocrysts, microphenocrysts, and groundmass crystals express the degree of mixing between host lavas and the mafic component (Table 1, Fig. 36). Table 1 and these histograms demonstrate that: 1) mafic enclaves show much greater degrees of contamination by host-derived phases (6-37% contamination) as compared to contamination of host lavas by enclave-derived phases (0-27% contamination), and 2) that intermediate liquids were produced, most likely by magma mixing, and that such liquids cooled sufficiently to be multi-saturated. Group 1 eruptive products are significantly less cross-contaminated than Group 2 lavas, due to the lesser degree of enclave disaggregation within Group 1 lavas. Visual petrographic analysis of the most mafic enclave analyzed, CC-UPF-I-1 core (51.88 wt.% SiO2, 4.72 wt.% MgO), yields ~5 vol.% host-derived phenocrysts of heavily reacted sodic plagioclase. Unreacted plagioclase phenocrysts within CC-UPF-I-1 are extremely calcic, with compositions ranging from An75 to An93 (Appendix D). Plagioclase-liquid equilibria calculations

(Putirka 2005) predict compositions of An90 in equilibrium with the whole-rock geochemical composition of CC-UPF-I-1 core, hinting that the parental basaltic end-member for the Chaos Crags system is slightly more mafic than enclave CC- UPF-I-1. The parental basaltic end-member is estimated to contain 51 wt.% SiO2 (Appendix K), by mass balance using enclave CC-UPF-I-1 core, given a mixing 72

proportion of ~5 wt.% parental rhyolite end-member magma with 76 wt.% SiO2. Mass-balance calculations between the estimated parental basaltic end-member magma and a rhyolite end-member estimated using Rhyolite-MELTS reinforce the findings of Tepley et al. (1999), in that Chaos Crags eruptive products are the result of differing degrees of mixing and hybridization between the two end- members (Fig. 37, Appendix K), with the exception of the small population of andesitic to trachy-andesitic mafic enclaves within Group 2 lavas which exhibit 87 86 elevated levels of K2O+Na2O and Sr/ Sr relative to the remainder of the enclaves; these levels have been attributed to contamination by wall rock, and therefore their compositions cannot be solely attributed to mixing. Group 1 host lavas contain 24-29 wt.% of enclave basalt end-member, whereas Group 2 host lavas are more hybridized, with up to 28-32 wt.% of mafic end-member. Group 2 host lavas contain greater proportions of disaggregated mafic enclave materials than Group 1 host lavas, which may partially account for the greater degree of hybridization within Group 2 lavas. Perhaps more interesting, though, is that the host felsic magmas appear to have experienced a greater degree of mixing, but at the same time contain a smaller proportion of enclave-derived crystals when compared to the numbers of felsic end-member crystals found in the enclaves. A scenario which may account for the greater degree of hybridization of host felsic lavas compared to mafic enclaves is that the first recharge event(s) which heated and remobilized the rhyolite crystalline mush underlying the Chaos Crags occurred well prior to eruption. This “first contact” between end-member basalt and the rhyolite end-member must have been of significant volume and momentum to reasonably homogenize the remobilized crystalline mush and shift its bulk composition towards a more dacitic composition (Eichelberger et al. 2006), producing a large volume of hybridized rhyodacite, which became 73 progressively more contaminated over the remainder of the eruptive sequence through disaggregation of mafic enclaves. Enclaves actually found within Chaos Crags eruptive products formed by subsequent recharge and mixing with this already-mixed rhyodacitic magma. This scenario, however, argues against recharge as the proximal trigger for eruption; if the initial recharge event triggered eruption, mafic enclaves present within Group 1 lavas would have experienced similar degrees of contamination as host lavas. Mafic enclaves contain a broad range of mixing between end-member magmas. Enclaves within Group 1 lavas contain a smaller proportion of end- member rhyolite than Group 2 lavas, most likely due to the lesser amount of enclave disaggregation and contamination with host materials. It should be noted, however, that even the most mafic of enclaves presents a small degree of hybridization with the approximated end-member rhyolite (Fig. 37).

Table 1. Percentage of Host-derived plagioclase (An20-59) present within Chaos Crags mafic enclaves, percentage of enclave-derived plagioclase (An>60) present within Chaos Crags host lavas, and percentage of rhyolitic crystalline mush- derived plagioclase (An<20) present within Chaos Crags host lavas and mafic enclaves. Calculated percentages of “0” for are most likely not truly zero, and may be attributed to the rarity of such phases present within eruptive products.

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Figure 37. Degrees of mixing between Rhyolite-MELTS approximated parental rhyolite and estimated parental basalt. Composition of parental basalt end-member represented by 51 wt.% SiO2 and y-intercept. Dashed red line represents simple binary mixing trend; solid red marks indicate wt.% of parental rhyolite contributed to the eruptive product.

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Petrologic Constraints on Magma Mixing, Mingling, and Enclave Formation

Crystallization Pressures and Temperatures of Host Magmas Based on the presence of zircons in Chaos Crags rhyodacites and dacites that range in age from 17.5 ± 7.9 to ~99 Ka for zircon rims and 44 ±4.5 to ~280 Ka for zircon interiors, it is inferred that the magma body underlying the Chaos Crags and surrounding area is the low-temperature, highly-evolved crystalline mush left over from 300 Ka to 190 Ka Bumpass sequence volcanism (Klemetti et al. 2013). Quinn (2014) estimates pre-eruptive storage conditions of host lavas to be 4.3-5.3 wt.% H2O saturation, NNO + 1.5-2.5 fO2, 770 ±10°C, and 145 ±25 MPa (Fig. 2). Experiments conducted on lower pyroclastic flow pumice produce titanomagnetite-ilmenite rims indicating equilibration temperatures ranging from 760°C to 807°C Quinn (2014), much lower than the average plagioclase crystallization temperatures (900± 23°C) for Group 1 host lavas, as recorded by phenocryst rims and groundmass crystals in this study. It is possible that the higher host magma temperatures recorded by plagioclase phenocryst rims and groundmasses in this study resulted from a combination of heat influx from the mafic component and latent heat resulting from decompression-induced groundmass crystallization (Pallister et al. 2008), or that Group 1 lavas were erupted at ascent rates greater than those that would have allowed for re- equilibration of Fe-Ti rims with the host magma.

Crystallization Pressures and Temperatures of Mafic Enclaves Thermobarometric calculations of phases present within mafic enclaves demonstrate that nearly all crystallization of host and mafic magmas occurs at shallow depths. The mafic component is cpx-saturated, crystallizes very few 77 phenocrysts at high temperatures and pressures with most crystallization occurring between T=975-1150°C and P=0.5-2 kbar. Calculated crystallization pressures and temperatures of clinopyroxene suggest that a small fraction of the parental mafic component was crystalline at P> 8 kbar. The lowest clinopyroxene P-T estimates are T = 1026°C and P = 0.2 kbar (Figs. 33 and 34). The mafic component becomes ol-saturated between 1074°C and 1043°C, and continues crystallization until ~1030°C. Crystallization of orthopyroxene and hornblende begin at 1030°C— 975°C (reaching saturation at different temperatures throughout the eruption sequence) and less than 0.8 kbar, followed shortly thereafter or contemporaneously by calcic plagioclase at ~1020°C. Clinopyroxene-melt equilibria and orthopyroxene-melt equilibria P-T estimates of P<3 kbar and T=1020-1150°C were calculated from clinopyroxene and orthopyroxene reaction rims surrounding host-derived phenocrysts of quartz and amphibole (Fig. 15, Fig. 17, Fig. 22, Appendix E, Appendix F) It is inferred that magma mixing occurs mostly at conditions of <2 kbar and 1020-1150°C. The sequence of crystallization obtained from thermobarometric calculations (i.e., clinopyroxene preceding olivine) is supported by several lines of evidence. Compositions of mafic enclaves and the calculated parental basalt end-

77 member were compared to starting compositions of the experimental studies of basaltic andesite performed by Baker and Eggler (1983), Sisson and Grove (1993), Gaetani et al. (1994), Moore and Carmichael (1998), DiCarlo et al. (2006), Pertermann and Lundstrom (2006), Almareev et al. (2013), Blatter et al. (2013), and Rader and Larsen (2013) (Table 2). Mafic enclaves from Chaos Crags are most like the experimental bulk compositions of Almareev et al. (2013), which at

H2O>4.0 wt.% and fO2=QFM + 3.3 produce magnetite at the liquidus, and where clinopyroxene is the first silicate phase to become saturated. These bulk 78 compositions, and mafic enclaves from the Chaos Crags, have higher CaO, FeO, and Al2O3, and lower MgO, compared to the compositions studied by Gaetani et al. (1994) and Moore and Carmichael (1998), and these contrasts appear to stabilize augite relative to olivine and plagioclase. The low MgO contents of the Chaos Crags mafic enclaves (3.2-5.18 wt. % MgO) (Fig. 8) and low NiO contents of olivine within mafic enclaves (Fig. 38) show that even our most mafic samples are highly differentiated. However, such differentiation must have occurred at depth below the shallow Chaos Crags plumbing system. Our calculated Chaos Crags mafic end-member is similar to starting compositions from Sisson and Grove (1993), and it is almost certain that considerable olivine fractionation occurred prior to recharge of the Chaos Crags system, but none of the olivines recovered from natural enclave samples provide a direct record of such crystallization. As a further check on our P-T estimates, Rhyolite-MELTS (Gualda et al. 2012) was used to model the crystallization history of the Chaos Crags calculated parental mafic end-member (Fig. 39). Saturation temperatures and pressures were modeled in 25 bar and 25°C increments, over the range of T=950-1250°C and P=0.1-7 kbar. Modeled phase saturations closely approximately the sequence of

78 crystallization seen in natural Chaos Crags mafic enclaves.

Table 2. Experimental Starting Compositions of Phase Relations Studies Compared to Chaos Crags Mafic Enclaves

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Figure 38. Compositional variation in olivine within Chaos Crags mafic enclaves. Outlined field are olivines calculated to be in equilibrium with denoted source compositions. Lower Ni (ppm) and Fo contents of Chaos Crags olivine are

representative of shallow crystallization of a mafic magma differentiated at depth (after Ruprecht and Plank 2013). OIB = 80 ocean island basalt, MORB = mid-oceanic ridge basalt, HI = Hawaiian basalt.

Figure 39. Approximation of sequence of crystallization for Chaos Crags calculated mafic end-member by Rhyolite- MELTS (Gualda et al. 2012). Saturation temperatures and pressures modeled in 25 bar and 25°C increments, over the

range of T=950-1250°C and P=0.1-7 kbar. Calculated P and T for natural samples overlain on diagram. 81

82

Textural and Mineralogical Constraints on Magma Mixing The presence of crenulated and quenched enclave margins (Fig. 24) within a portion of the mafic enclaves, accompanied by disequilibrium mineral assemblages within both mafic enclaves and host lavas (Figs. 9-22) are indicative of varying degrees of mixing between host lavas and less-evolved basalt prior to complete crystallization and solidification of the mafic component. The numerous groundmass textures present within mafic enclaves (Fig. 25) support a variety of thermal and compositional contrasts between enclaves and host lavas at the time enclave material is introduced into the overlying silicic magma. In the below section, variations in vesicularity and groundmass textures within the variety of Chaos Crags mafic enclaves are discussed, as well as how these variations may aid in the development of a magma mixing model for the Chaos Crags volcanic system.

Disequilibrium Mineral Assemblages Present in Eruptive Products Host lavas of the Chaos Crags are strikingly porphyritic. Mafic enclaves found within host lavas are dominantly porphyritic, with few, larger-grained mafic enclaves being aphyric to slightly phyric in nature. The majority of phenocrysts present within eruptive products display complex zoning and reaction textures indicative of disequilibrium with the liquid composition of the eruptive products in which they are contained. The presence of a disequilibrium mineral assemblage within Chaos Crags eruptive products has been well-documented by Heiken and Eichelberger (1980), Tepley et al. (1999), Christiansen et al. (2002), Wallace and Bergantz (2005), and Underwood et al. (2012). Here, a concise interpretation of the key disequilibrium textures and phases relative to this study is presented. 83

Plagioclase. Population 1 (Fig. 10a) plagioclase phenocrysts (An 25-55) are present within host lavas, and display reversely to oscillatory-zoning with minimal sieved reaction textures, consistent with an origin of a silicic magma which was slowly heated. Population 2 (Fig. 10b) plagioclase crystals are present within mafic enclaves, commonly with extremely high An contents (An75-90) pointing to a more primitive basalt as their growth environment; this population is sometimes found with sieve-texture rims present. When Population 2 plagioclase presenting sieve-textured zones are present within mafic enclaves, they exhibit calcic overgrowth rims consistent with reequilibration of the crystal with the enclave liquid composition after a heating event (Tepley et al., 1999). Population 3 (Fig. 10c) phenocrysts originate in the host magma and are common within both mafic enclaves and host lavas, but are present to a lesser extent within Group 1 rhyodacitic host lavas and the least contaminated mafic enclaves. Patchy Population 3 plagioclase are characterized by highly fritted and sieved cores of

~An05-30, with occasional growth zones of higher An content and calcic overgrowth rims; this population is extremely small and was most likely part of the crystalline mush leftover from Bumpass sequence volcanism, being reintroduced into the Crags system upon remobilization of the mush by mafic

83 recharge (Klemetti et al. 2013) Population 4 (Fig. 10d) plagioclase (An 25-55) phenocrysts and microphenocrysts are the most common disequilibrium population present within both enclaves and host lavas; occasionally, this population will display oscillatory zoning and multiple zones of sieve textures, suggestive of multiple mixing events throughout the life of the phenocryst (Tepley et al., 1999). Host-derived plagioclase phenocrysts and microphenocrysts from Populations within mafic enclaves are generally characterized by clear, normally 84

zoned overgrowth rims of An75-An50 (Tepley et al., 1999, this study). The presence of strong, normally-zoned calcic overgrowth rims on phenocrysts and microphenocrysts with An<50 cores suggests that host-derived plagioclase were incorporated into the mafic component some time period prior to eruption, allowing for equilibration of the mineral to its new conditions.

Olivine. Olivine is present within both mafic enclaves and host lavas, with

Fo contents ranging from Fo69-Fo83. The majority of olivine are at disequilibrium with the bulk rock compositions of their eruptive products (Fig. 19). Disequilibrium olivine within eruptive products may exhibit overgrowths of hornblende (Fig. 20c) or reaction coronas of orthopyroxene (Fig. 20b), preserving olivine cores. Corona rims of orthopyroxene surrounding olivine develop as a response of the CaO-Na2O-MgO-Al2O3-SiO2 system to changes in Na and H2O during cooling of a melt in low-P environments (Turner and Stüwe 1992). Hornblende overgrowths on olivine, common within Chaos Crags eruptive products, occur after original development of a corona rim from an increase in pH2O; hornblende may partially or completely replace orthopyroxene in corona rims, depending on the amount of H2O present within a system after initial corona formation (Turner and Stüwe 1992). The presence of an individual enclave- 84

derived olivine phenocryst was noted in Dome D host lavas exhibiting minimal reaction with the surrounding host lava (Fig. 20d), indicating that disaggregation of the enclave it originated from occurred either immediately preceding or during eruption, as no well-defined corona rim surrounds the phenocryst.

Pyroxenes. Of particular importance to this study is the presence of clinopyroxene and orthopyroxene reaction rims surrounding host-derived phenocrysts present within mafic enclaves. The presence of augite-rimmed 85 partially resorbed quartz phenocrysts (Fig. 15c) within mafic enclaves has been previously addressed by Heiken and Eichelberger (1980) and Tepley et al. (1999) to be the result of quartz-bearing host lavas being incorporated into enclave material. Orthopyroxene corona rims surround olivine phenocrysts within mafic enclaves (Fig. 20b), and less frequently within host magmas (Fig. 20d). Orthopyroxene is also infrequently present in Chaos Crags eruptive products as a product of amphibole dehydration (Underwood et al., 2012). Pseudomorphic replacement of host-derived hornblende phenocrysts and microphenocrysts by acicular orthopyroxene (and lesser clinopyroxene) may occur when heating of a host-derived hornblende dehydrates the mineral, producing a solid-state reaction product of optically aligned, acicular pyroxene which retain the shape of the original hornblende crystal (Mazzone et al. 1987, Underwood et al. 2012). The extent to which a host-derived hornblende has been replaced may be a function of either time, as pseudomorphic replacement of hornblende by orthopyroxene progresses from the outer portions of the crystal inwards to the hornblende core (Underwood et al. 2012); orthopyroxene-replaced hornblende phenocrysts and microphenocrysts which had spent lesser amounts of time in contact with hotter

85 magmas would be less replaced, surrounded by only a rim of orthopyroxene, but leaving a relatively unreacted core (Figs. 17d and 25h). Complete replacement of the host-derived hornblende (Fig. 17c) would result from longer times entrained in and/or in contact with hotter magmas. In a similar manner, the differing degrees of corona rim development surrounding olivine phenocrysts present within eruptive products indicate variable degrees of mixing that take place over the extent of the thermal history of mafic enclaves. 86

The pressures and temperatures at which clinopyroxene and orthopyroxene reaction products develop may provide insight as to the thermal history and conditions of mixing events. Clinopyroxene reaction products within the Chaos Crags system form at P=0-3 kbar and T=1080-1150°C, whereas orthopyroxene reaction products occur at lower temperatures and pressures, at P=<1 kbar and T=947-1050°C. The different P and T between clinopyroxene and orthopyroxene reaction products support a magma mixing model wherein mixing occurs over a wide range of pressures and temperatures, throughout the enclave formation process. 1) Clinopyroxene reaction rims form surrounding host-derived quartz phenocrysts at higher T and P, most likely from the entrainment of host magmas and host-derived phenocrysts through turbulent convection along the mafic-felsic interface immediately after injection of the mafic component into the magma chamber (Tepley et al. 1999). 2) Pseudomorphic replacement of hornblende by acicular pyroxene and development of corona rims surrounding olivine phenocrysts form during later stages of enclave formation, but prior to complete crystallization of the enclave. The presence of vesiculated rhyolitic glass

86

containing host-derived phenocrysts within enclave rims supports the incorporation of host-derived melt and crystals, either at the time of or immediately after dispersal into the rhyodacitic host, at a point when the enclave is only a partially crystalline mush. Pseudomorphic replacement of hornblende forms from in situ degassing of host-derived hornblende due to heating upon its incorporation into the mafic enclave (Mazzone et al. 1987, Underwood et al. 2012). 87

Groundmass Textures of Eruptive Products The wide variety of groundmass textures present in mafic enclaves of the Chaos Crags (Fig. 25) are a function of compositional and thermal differences between the parental basalt end-member and the overlying host rhyodacite. Four dominant textures are present within Crags enclaves: (1) medium-grained aphyric, (2) medium-grained slightly phyric, (3) fine-grained porphyritic, and (4) fine- grained slightly phyric. Groundmass textures present within mafic enclaves, however, are a continuum of these textures, varying throughout a singular dome, and even within individual enclaves (Figs. 25 and 27). Enclaves experience vastly different degrees of undercooling, most likely attributed to their position within the magma chamber immediately after injection of the liquid mafic component (Bacon 1986, Coombs et al. 2002, Martin et al. 2006a). Enclaves with acicular and skeletal groundmasses experienced greater undercooling-induced crystallization than enclaves with larger-grained, more equant groundmasses, implying that groundmasses with greater width/length ratios for individual crystals experienced lower ΔT between enclave and host; this suggests that larger-grained enclaves may have originally been positioned at slightly deeper levels of the hybridized basaltic-andesite layer (Coombs et al.

87

2002, Martin et al. 2006a). The absence of quenched margins on some of the fine- grained porphyritic and fine-grained slightly phyric mafic enclaves may be attributed to shearing and disaggregation of enclaves post-enclave dispersal into the overlying rhyodacitic host. Fine-grained enclaves which spent a longer amount of time within host magmas prior to eruption may have been initially quenched as a result of high ΔT and ΔC, but would be subjected to thermally-induced convection within the high-viscosity host magma for a longer period of time, thus increasing the probability for disaggregation by the shearing off of their outermost 88 rims (Martin et al. 2006b). Enclaves exhibiting quenched margins most likely did not reside within host magmas for long periods of time prior to eruption, preserving their quenched textures.

Vesiculation of Eruptive Products Mafic enclaves of the Chaos Crags exhibit a wide range of vesicularity across the entire eruptive sequence. Mafic enclaves display similar increasing and decreasing trends in vesiculation as host lavas over the eruption sequence (Fig. 28). The majority of mafic enclaves are <20 vol. % vesiculated; few enclaves analyzed are >20 vol. % vesiculated (Fig. 28). Vesiculation of Group 1 host lavas are incipiently vesicular (Houghton and Wilson 1989), ranging from 2.5-6.7 vol.% vesiculation, and decreasing in vol.% vesicularity over the eruption sequence. 63% of mafic enclaves present within Group 1 lavas are less vesiculated than the rhyodacitic host in which they reside. Group 2 lavas are non-vesicular (<2 vol.% vesiculated) (Houghton and Wilson 1989); 99% of mafic enclaves present within Group 2 lavas display greater vol.% vesicularity than their respective host lavas. The vesiculation seen within an erupted volcanic rock may be interpreted as a signature of volatile behavior prior to and during an eruption event. As magma propagates upwards during eruption, bubble nucleation and growth rates increase 88

in sync with the velocity of magmatic ascent, provided that volatile saturation has been reached and bubbles have nucleated (Klug and Cashman 1994). Large numbers of small vesicles represent continuous nucleation and growth, whereas small numbers of large vesicles represent coalescence (Mangan et al. 1993). Coalescence may occur either pre-eruptively or syneruptively (Mangan et al. 1993, Blower 2002, Castro et al. 2012); the distinction between pre-eruptive and 89 syneruptive coalescence remains the topic of contentious debate (Klug and Cashman 1994, Gualda and Anderson 2007, Genareau et al. 2013). The presence of pre-eruptive vesicles in a magmatic system is significant because such a population can increase the buoyancy of a magma (Eichelberger 1980), increase overpressure (Cashman 2004), cause mass and heat transfer (Gualda and Anderson 2007), and provide nucleation sites for crystal growth (Cashman 2004). Log-linear VSD plots for eruptive products aid in determining volatile behavior (Mangan and Cashman 1994), and may give information on volatile contents of magmas and relative residence times of vesiculated magmas within the chamber prior to eruption (Shea et al. 2010).

Volatile behavior and bubble growth within host lavas. The vesicularity of Chaos Crags host lavas directly correlates to magmatic ascent rates. During ascent, if volatiles are unable to equilibrate during decompression due to high magmatic ascent rates, bubbles nucleate homogenously at high rates and are unable to adequately degas as the change in pressure demands, recording a pumiceous texture with high vol. % vesicularity and low crystallinity in the eruptive product (Sparks 2003, Cashman 2004, Gonnermann and Houghton 2012). Log-linear VSD plots of Group 1 host lavas (Appendix I) demonstrate medium to large corrected 89

vesicle diameters, and show trends of coalescence (Mangan and Cashman 1996, Shea et al. 2010), especially in Dome A host lavas. Coalescence within Group 1 host lavas is most likely syneruptive, or may even have occurred post-eruption (Mangan et al. 1993). This study concludes that Group 1 host lavas preserve vesiculation textures (Fig. 23a) and vol. % vesicularity (Fig. 28) consistent with magmatic ascent rates of >0.1 km/hr as estimated by Quinn (2014). 90

If volatiles are allowed greater time to mobilize and escape from the chamber, they are able to degas efficiently, resulting in heterogenous nucleation of bubbles, a low to moderate vol.% vesicularity and a more crystallized groundmass (Sparks 2003, Cashman 2004). Degassing-induced crystallization of microlites is commonly reported in effusive eruptions, and may act as an inhibitor to further vesiculation and coalescence of a magma (Klug and Cashman 2004). Log-linear VSD plots of Group 2 host lavas (Appendix I) show that Group 2 host lavas are dominated by steady-state nucleation and growth, with negligible amounts of coalescence. Coalescence within Group 2 host lavas may have been inhibited by degassing-induced crystallization (Klug and Cashman 1994), as supported by the lower vesicularities (Fig. 28) and abundance of microlites and microcrystalline groundmasses present (Fig. 23b) in Group 2 host lavas. This study determine that Group 2 host lavas preserve vesiculation and crystallinity textures consistent with the lower magmatic ascent rates of <0.1 km/hr as estimated by Quinn (2014).

Volatile behavior and bubble growth within mafic enclaves. Mafic enclaves of the Chaos Crags are variably vesiculated, ranging from 0.69 vol. % vesiculated to 40.62 vol.% vesiculated. Mafic enclaves display the same overall trends in vesiculation as host lavas, decreasing in overall vesicularity from Domes A-C, and 90

then increasing over the remainder of the eruption sequence (Fig. 28). Unlike host magmas, which may be thought of as open systems, unbroken mafic enclaves behave as a system closed to the exchange of volatiles and preserve their volatile contents, providing an excellent record of volatile behavior within magmatic systems (Eichelberger 1980, Bacon 1986). Log-linear VSD plots of mafic enclaves show a variety of volatile behaviors (Appendix J). All enclaves exhibit trends of syneruptive steady-state nucleation and growth, however, only 91

86% of mafic enclaves analyzed in this study display trends of bubble coalescence (Mangan and Cashman 1996, Shea et al. 2010). Approximately 36% of enclaves display similar nucleation and growth trends as their respective host lavas (Fig. 30), an expected consequence of their syn-eruptive histories. However, 64% of mafic enclaves analyzed in this study display nucleation and growth trends contrary to that of their host lavas (Fig. 31). Highly vesiculated enclaves displaying trends of coalescence are present in low-vesicularity host lavas dominated by steady-state nucleation and growth. Disparate vesiculation trends between mafic enclaves and their resident host lavas may be explained by the “closed system” nature of mafic enclaves, which allows for thermal equilibration between enclaves and host magmas, but prevents the exchange of volatiles (Eichelberger 1980, Bacon 1986). It should be noted, however, that mafic enclaves within the same dome (thus experiencing eruption at the same time) display differing trends of nucleation, growth, and coalescence (Appendix J). The question then becomes, what may account for some enclaves within the same eruption to experience coalescence of volatiles forming large vesicles, while others only show syn-eruptive nucleation and growth of small vesicles? The presence of dixtytaxitic groundmass textures within enclaves of the Chaos Crags

91 indicates that at least some portion of the enclave population was formed before volatile saturation and vesiculation of the mafic magma (Martin et al. 2006a). Yet other enclaves within the Chaos Crags exhibit large, spherical to compound vesicles which clearly precede enclave crystallization and show evidence of bubble coalescence, most likely induced by slow rates of crystallization-induced exsolution of volatiles within the mafic component, and coalescence of those volatiles causing the destruction of the partially-formed crystalline framework within finer-grained enclaves (Martin et al. 2006b). However, this fails to explain 92 why fine-grained enclaves are variably vesiculated, and do not demonstrate similar trends of nucleation rate, growth, or coalescence. This author proposes that one or both of two scenarios may account for the differences in volatile behavior and bubble growth between fine-grained mafic enclaves of the same eruptive dome:

1) H2O contents of the mafic magma were spatially heterogeneous, as a result of the dynamic nature of the mixing layer at the mafic/felsic interface, resulting in uneven supersaturation and exsolution of volatiles in different enclaves, depending on the spatial location of their origin within the magma chamber; or 2) Exsolution of volatiles and bubble formation in different enclaves occurred at different time periods prior to and/or during eruption, resulting in different populations of bubbles. The timing of bubble formation and coalescence is not easily constrained; evidence has been presented for pre-eruptive (Gualda and Anderson 2007, Castro et al. 2012), syneruptive (Klug and Cashman 1994, Castro et al. 2012, Genareau et al. 2013), and post-eruptive (Klug and Cashman 1994) coalescence of bubbles. At what point volatile saturation occurs within a system has a significant effect on

92 magmatic dynamics and eruptive behavior (Klug and Cashman 1994, Blower 2002, Cashman 2004, Gualda and Anderson 2007). A pre-eruptive vesicle population indicates H2O saturation and volatile exsolution within a magma (Sparks 2003), may drive homogenous nucleation of bubbles (Gualda and Anderson 2007), can aid in the transfer of melt, crystals, heat and volatiles across compositional boundaries (Thomas et al. 1993, Gualda and Anderson 2007), and may cause degassing-induced crystallization and rheological stiffening (Sparks 2003). 93

Gualda and Anderson (2007) presented evidence for a pre-eruptive vesicle within the Bishop rhyolite, identified by an abundance of magnetite vesicles in direct contact with the walls of the vesicle. The argument is presented that as the attachment of a magnetite crystal to a bubble is energetically favorable as compared to the attachment of silicate phases to the bubble wall, a pre-eruptive vesicle formed from the melt and captured crystals of magnetite as it moved through the melt (Gualda and Anderson 2007). This study presents two vesicles within the Chaos Crags mafic enclaves which are in direct contact with Fe-Ti oxides, and identify them as candidates for a pre-eruptive vesicle population (Fig. 40). The presence of Fe-Ti oxides in direct contact with vesicles suggests that a pre-eruptive vesicle population is present within the Chaos Crags mafic enclaves, which demonstrates trends of bubble coalescence. It should be cautioned, however, that the 2-D BSE images obtained may not be representative of the true nature of the relationship between Fe-Ti oxides and vesicles, and advise that an evaluation of this relationship be conducted using the methods of Gualda and Anderson (2007) in order to conclusively determine the presence of a pre-eruptive vesicle population. The spectrum of enclave vesicularities (Fig. 28), differences in bubble

93 behavior between mafic enclaves of the Chaos Crags (Appendix J), and the identification of candidates for a pre-eruptive vesicle population suggest that H2O contents of the Chaos Crags mafic component were heterogeneous, causing some mafic enclaves to reach volatile saturation at earlier times than others and resulting in a number of enclaves with large, coalesced vesicles, which reside within thesame chamber as mafic enclaves which reached volatile saturation at later times and do not contain large, coalesced vesicles.

94

Figure 40. BSE images of pre-eruptive vesicle population candidates present within Chaos Crags mafic enclaves and their accompanying log-linear VSD plots.

(a) Vesicles present within CC-E-I-14 core. Note Fe-Ti oxides directly in contact 94 with vesicle population. Log-linear VSD plot for CC-E-I-14 displays trends of coalescence in both enclave core and enclave rim. (b) Vesicle present within CC- E-I-18 core. Note Fe-Ti oxide directly in contact with vesicle. Log-linear VSD plot for CC-E-I-18 displays a trend of coalescence in enclave core, with lesser coalescence in enclave rim. V = vesicle, Fe-Ti = Fe-Ti oxides, n0 = volumetric number density of bubble nuclei, J = nucleation rate of bubbles, G = mean growth rate of bubbles, and T = timescale of bubble nucleation and growth. VSD for enclaves cores are presented in black, VSD for enclave rims are presented in grey, and VSD for respective host samples are presented in red.

95

Weight percent H2O of the mafic component was derived from the olivine melt inclusion measurements of Collins (2012). At 4.0 wt.% H2O (Collins 2012), basalts and basaltic reminiscent of natural Crags enclave compositions would become supersaturated and begin to exsolve H2O and vesiculate at a minimum of P≈125 MPa (Baker and Alletti 2012), or ~4.6 km depth. This is consistent with the findings of Bindeman and Bailey (1994), which demonstrate that a hydrous basaltic magma containing 3.0 wt.% H2O may vesiculate at depths of ~4 km, becoming less dense than overlying rhyodacitic magma.

This author posits that H2O saturation, exsolution of volatiles, and subsequent vesiculation of mafic enclaves provide the mechanism to lower the majority of Chaos Crags calculated enclave densities to less than that of the overlying host magmas (Eichelberger 1980, Huppert et al. 1982, Bindeman and Podladchikov 1993) (Fig. 41a) at minimum P≈125 MPa (Baker and Alletti 2012), resulting in the flotation and dispersion of mafic enclaves into the overlying host magma. It is well documented that vesiculation of mafic enclaves is sufficient to decrease the density of mafic enclaves such that they become buoyant enough to rise into an overlying, less dense, silicic host within a stratified magma chamber (Eichelberger 1980, Huppert et al. 1982, Bacon 1986, Bindeman and

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Podladchikov 1993, Thomas et al. 1993, Bindeman and Bailey 1994, Coombs et al. 2002). In rare cases, however, calculated vesiculated densities of mafic enclaves fail to become less dense than the overlying silicic magma (Fig. 41b). A review of the literature demonstrates rare cases where non-vesiculated (and thusly, greater density) mafic enclaves are present within eruptive products (Gençalioğlu- Kuşcu and Floyd 2001). This suggests that although the predominant mechanism for dispersal of mafic enclaves within host magmas is a decrease in density driven by enclave vesiculation and dispersal within the overlying silicic host magma, 96

96

Figure 41. Representative effects of vesiculation on calculated pre-eruptive densities of Chaos Crags mafic enclaves. (a) Measured 7.61 vol.% vesicularity is sufficient to lower the vesiculated density of sample CC-UPF-I-1 to less than that of the overlying rhyodacitic host magma. (b) Measured 0.69 vol.% vesicularity is insufficient to lower the vesiculated density of sample CC-C-I-1 to less than that of the overlying rhyodacitic host magma.

97 other mechanisms may operate to incorporate mafic enclaves into overlying silicic hosts (Kouchi and Sunagawa 1985). This author suggests a scenario in which this small population of more dense, minimally-vesiculated enclaves originates at spatially lower positions within the mafic layer of the Chaos Crags stratified magma chamber, and may be either entrained in the overlying rhyodacitic host upon convection within the host magma, or are incorporated into the overlying silicic magma upon eruption.

Mixing Constraints Required by Vesicular and Compositional Zonation Within Individual Mafic Enclaves

Variations in Geochemical Composition Minor variations in enclave compositions from core to rim are present within all mafic enclaves, albeit within standard estimates of error (SEE). Within enclaves that exhibit variations in geochemical composition outside of the margin of error, a strong correlation is shown between distance from enclave center and

SiO2 content, with the outermost portions of the enclave having the greatest wt.%

SiO2 and K2O, and the least wt.% FeOtotal (Fig. 26). One sample analyzed, enclave

CC-E-I-10, exhibits a 4.05 wt.% decrease in SiO2 from enclave core to rim. 97

Compositional variations of individual enclave cores, rims, and intra- enclaves (where present) plot on linear mixing trends with host magmas (Fig. 42), suggesting mixing with host magmas as the mechanism for compositional zoning within individual enclaves. Intra-enclaves are present within a small number of mafic enclaves, having distinct sharp contacts with the enclave in which they reside; intra-enclaves may have a more mafic composition and are more vesiculated than the rest of the enclave in which they reside. 98

The presence of several host-derived phases and blebs of rhyolitic melt accompanied by host-derived phenocrysts found within enclave rims (Fig. 43) demonstrates that during the enclave formation process, host-derived components are introduced into mafic enclaves. Calcic overgrowth rims (Fig. 43a-b), vesiculation of rhyolitic melt (Figs. 43a-c and 43e), and incomplete pseudomorphic replacement of hornblende by orthopyroxene (Fig. 43e) suggest that the entrainment of host-derived phases and glass are not syneruptive. However, few enclaves which display crenulated margins captured this process syneruptively. Enclave CC-F-I-10 (Fig. 43f) contains an unreacted plagioclase phenocryst of composition An28 and a phenocryst of unreacted hornblende currently in the process of entrainment into the enclave, demonstrating that enclave formation is not a singular event, but rather continuous throughout the thermal history of the magma chamber.

Variations in Vesicularity from Enclave Core to Rim Mafic enclaves are concentrically zoned with respect to vesicularity. In all but two of the enclaves analyzed in this study, vol. % vesicularity increases towards the interior of the enclave, i.e., enclave cores are more vesiculated than

98 enclave rims (Fig. 27). Where present, intra-enclaves have a greater vol.% vesicularity than any part of the enclave in which it is enclosed. Often, the contrasts in vesicularity are great enough to be seen in hand specimen (Figs. 24a, 24c-d, and 24f). A vesicular distinction between enclave cores and rims is not, however, always apparent in hand specimen within some enclaves (Figs. 24b and 24e), or may be within the range of uncertainty (4.3 %). 99

99

Figure 42. Variation diagrams of selected major elements vs. silica (after Harker 1909) within individual mafic enclaves. Note variation in composition between enclave rim and core (and intra-enclave). Error bars denote 1.0 wt.% error. ΔT based on estimated Tenclave – Thost using the plagioclase-melt equilibria geothermometer of Putirka (2005). SEE Tplag = ±23°C. Red dashed lines illustrate linear mixing between host lava and the most mafic enclave component. 100

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Figure 43. Host-derived phases and vesiculated rhyolitic melt present in enclave rims. Note the presence of host-derived phases commonly included within vesiculated blebs of rhyolitic melt. (a) Population 4 plagioclase phenocrysts adjacent to a vesiculated bleb of rhyolitic melt within rim of Enclave CC-A-I-5. (b) Population 4 plagioclase phenocrysts enclosed within and adjacent to vesiculated rhyolitic melt within quenched rim of Enclave CC-B-I-9. (c) bleb of rhyolitic melt within rim of Enclave CC-C-I-11. (d) Glomerophenocrystic clot of biotite, hornblende, Fe-Ti oxides, and antecrystic zircon within quenched rim of Enclave CC-E-I-12. (e) Incomplete pseudomorphic replacement of hornblende by orthopyroxene enclosed within bleb of rhyolitic host melt within rim of Enclave CC-E-I-18. (f) Syneruptive entrainment of host-derived plagioclase and hornblende into rim of Enclave CC-F-I-10; yellow dashed line indicates rim of enclave prior to phenocryst entrainment; yellow dotted line indicates edge of enclave including host-derived phases. Reddish-pink ink is present within photomicrographs. Pl=plagioclase, hbl = hornblende, zr = zircon, bt = biotite, opx = orthopyroxene, gl = glass. 101

Log-linear VSD plots of enclave rims as compared to enclave cores (and intra-enclaves, when applicable) demonstrate that variations in bubble nucleation, growth, and coalescence are present within individual enclaves (Fig. 44, Appendix J). Within an individual mafic enclave, volatiles may coalesce, grow in response to syneruptive decompression (steady-state nucleation and growth), or exhibit a combination of both behaviors. If log-linear VSD plots display both steady-state nucleation and growth and coalescence within the same enclave, volatiles within the enclave core (and intra-enclave, if present) are dominated by coalescence; while volatiles within the enclave rim only grow in response to syn-eruptive decompression.

Processes of Enclave Formation Which May Produce Variations Within Individual Mafic Enclaves Highly vesiculated, more mafic cores within mafic enclaves have been previously attributed to the process of gas-filter pressing (Anderson et al. 1984, Bacon 1986, Sisson and Bacon 1999). Gas-filter pressing concentrates mafic components and volatiles within the center of enclaves, and expels residual rhyolitic melt from crystallization, which migrates towards enclave rims and may be extruded from the enclave into the host magma (Bacon 1986). However, there 101 are several key lines of evidence missing in natural Chaos Crags samples, which would support differentiation by gas filter-pressing as the lone mechanism by which enclaves develop compositional, textural, and vesicular variations: 1) Gas filter-pressing produces large vesicles in enclave cores, filled by segregated rhyolitic melt (Anderson et al. 1984, Bacon 1986). The small number of blebs of rhyolitic glass preserved within Chaos Crags mafic

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Figure 44. Log-linear VSD plots of selected mafic enclaves demonstrating variations in nucleation rates, growth rates, and bubble behavior within individual mafic enclaves. Note (a) Enclave CC-A-I-12. Vesicles in enclave rim show steady-state nucleation and growth; vesicles in core display a coalescence trend. (b) Enclave CC-C-I-11. Vesicles in enclave rim show steady-state nucleation and growth; vesicles in core display a coalescence trend; vesicles in intra-enclave display coalescence and are of greater diameter than those in the enclave core. (c) Enclave CC-F-I-14. Vesicles in enclave rim show steady-state nucleation and growth; vesicles in core display a coalescence trend, and have a significantly larger diameter than vesicles within enclave rims. n0 = volumetric number density of bubble nuclei, J = nucleation rate of bubbles, G = mean growth rate of bubbles, and T = timescale of bubble nucleation and growth. 103

enclaves are accompanied by host-derived phenocrysts, and are often preserved in enclave rims. 2) Groundmass textures of enclaves which have experienced gas filter- pressing exhibit chilled margins due to the greater ΔT between enclave rim and host magma (Bacon 1986). Enclaves exhibiting chilled margins and a decrease in grain size from enclave core are present within the Crags enclaves, but are not ubiquitous. 3) Gas filter-pressing results in enclaves with mafic cores and more silicic rims (Bacon 1986). Enclave CC-E-I-10 records a more mafic rim, and a more silicic core. 4) Where gas filter-pressing produces compositional zonation of enclaves, a “halo” of rhyolitic melt is often present around enclaves, as this residual melt is expelled from enclaves into the surrounding host (Bacon 1986). Rhyolitic melt halos surrounding mafic enclaves are not evident in Chaos Crags host lavas. 5) Compositionally-zoned enclaves produced by gas filter-pressing are the product of in situ differentiation of homogenous, whole enclaves; compositions of cores and rims of individual enclaves should plot on a

103 mixing line with segregated melt glasses, and not display trends of

linear mixing with host magmas (Bacon 1986). Compositional variations of individual Chaos Crags mafic enclave cores, rims, and intra-enclaves (where present) plot linearly on bivariate mixing trends with host magmas (Fig. 42). 6) Gas filter-pressing does not account for the presence of intra-enclaves which are rare, but present within the Chaos Crags mafic enclaves. Intra-enclaves are often enclosed within the rim of the enclave in which 104

they reside (Fig. 24f), and are of a more mafic composition than other enclave zones. Gas filter-pressing produces compositionally-zoned enclaves with the most mafic part of the enclave at the core. The linear mixing trends between the most mafic and most felsic portions of compositionally-zoned mafic enclaves, presence of intra-enclaves within enclave rims, presence of calcic overgrowth rims on host-derived plagioclase and partial pseudomorphic replacement of hornblende accompanied by blebs of rhyolitic melt within enclave rims, and syneruptive entrainment of host-derived phases into mafic enclaves lead this author to posit that mechanical mixing of mafic enclaves and host magmas, not in situ differentiation due to gas filter- pressing, is the mechanism responsible for the compositional variations present in mafic enclaves. This study attributes the concentric zonation of vesicles within individual mafic enclaves to the effects of crystallization on volatile concentration, as suggested in other in-depth studies of mafic enclaves (Martin et al. 2006a). Mafic enclaves with large, coalesced vesicles concentrated in enclave cores are generally fine-grained slightly phyric to porphyritic; medium-grained aphyric to slightly phric enclaves, often with diktytaxitic groundmass textures, may exhibit a

104 concentric zonation with respect to vol.% vesicularity, but often to a lesser extent

than fine-grained enclaves, and often within the margin of error for vesicularity measurements. The rapid crystallization of fine-grained enclaves, especially those displaying quenched rims, would drive volatiles towards the center of the inclusion where they would have time to nucleate, grow, and coalesce (Martin et al. 2006b). 105

Magma Mixing, Enclave Formation, and Eruption Triggering Model The variety of geochemical compositions present, presence of a number of variably reacted and resorbed disequilibrium mineral phases in all mafic enclaves analyzed, continuous range of mineral compositions, continuous range of estimated pressures and temperatures of crystallization, variety of enclave groundmass textures, presence of both quenched and non-quenched rims, high possibility of a pre-eruptive vesicle population, and geochemical and vesicular variations within individual mafic enclaves of the Chaos Crags lead this author to conclude that mafic enclaves of the Chaos Crags were formed by the mixing of a parental basaltic end-member magma with a previously mixed rhyodacitic magma, formed from the rejuvenation of a rhyolitic crystalline mush. As mineral compositions present within Chaos Crags eruptive products are not strictly bimodal, but instead exhibit a continuous range of compositions, it is clear that crystallization occurred to some degree after mixing of these two magmas. Within mafic enclaves, equilibrium clinopyroxene crystals within mafic enclaves have compositions that represent crystallization temperatures ranging from 1017-1152°C, equilibrium olivine crystals within mafic enclaves have compositions that represent crystallization temperatures ranging from 1033-

105 1074°C, equilibrium orthopyroxene crystals within mafic enclaves have

compositions that represent crystallization temperatures ranging from 947-1031°, and equilibrium plagioclase crystals within mafic enclaves have compositions that represent crystallization temperatures ranging from 951-1023°C. Such a wide temperature range must have occurred prior to eruption, not syneruptively. This author posits that mixing of the basaltic end-member and host magmas occurred prior to eruption, and thus was not the proximal trigger for eruption. Instead, this study notes that mafic recharge and magma mixing are the ultimate trigger for 106 volcanic eruption, and attributes the proximal cause for eruption to increased overpressure within the chamber caused by volatile 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. In light of the above-presented constraints on enclave formation and magma mixing, this author presents a magma mixing model which accounts for the range of crystallization temperatures, disequilibrium mineral phases, enclave compositions, textures, and vesicularities present within Chaos Crags mafic enclaves: 1) A high-volume injection of parental basaltic end-member magma was introduced into the highly-evolved crystalline mush underlying the Lassen Volcanic Center, rejuvenating the crystalline mush (Klemetti et al. 2013), and thoroughly hybridizing to form the host rhyodacite. Phenocryst phases within homogenized rhyodacite include Populations 1 and 3 plagioclase, quartz, hornblende, biotite, and antecrystic zircon (Figs. 45a-b). 2) A low-volume injection of parental basaltic end-member magma was injected into the base of the rejuvenated chamber, ponding at the base of

106 the chamber. Rhyodacitic host and host-derived melt were entrained

along the mafic/felsic interface, incorporating them into the basaltic magma, and mixing in various proportions to create a compositionally heterogeneous basaltic-andesite layer (Fig. 45c) (Heiken and Eichelberger 1980, Tepley et al. 1999, Saito et al. 2002, Richer et al. 2004, Eichelberger et al. 2006). 3) Thermal gradients quickly developed within the overlying rhyodacitic host and mafic layers, causing convection within both layers (Snyder 107

2000), and distributing host-derived phenocrysts throughout the basaltic-andesite layer. Undercooling-driven crystallization of basaltic andesite, especially at the top of the layer where there is the greatest thermal and compositional contrast, began, causing second boiling within the melt and formed a dynamic, unstable layer at the top of the basaltic-andesite (Browne et al. 2006). Blobs of highly-vesiculated basaltic-andesite formed at the top of the basaltic-andesite layer; the decrease in density caused by vesiculation allowed highly-vesiculated mushy blobs of mafic magma (mafic enclaves) to rise into the overlying rhyodacitic host (Fig. 45c) (Eichelberger 1980, Thomas et al. 1993, Thomas and Tait 1997, Tepley et al. 1999, Browne et al. 2006, Eichelberger et al. 2006). 4) Convection within the basaltic andesite may have brought more mafic, less hybridized basalt towards the overlying basaltic-andesite, continuously supplying the dynamic basaltic-andesite layer with volatiles, and forming intra-enclaves which were then entrapped within larger blebs of basaltic-andesite magma prior to expulsion into the overlying rhyodacitic host. The composition of enclaves formed at this

107 point was a function of the compositional and percent crystallinity of the

mafic layer at the location they formed at within the basaltic-andesite (Figs. 45c-d) (Saito et al. 2002). 5) Mafic enclaves with fine-grained groundmasses were subject to shearing by the more viscous host magma, causing extremely small fragments (3cm diameter to groups of microlites) to be disaggregated and distributed throughout the host magma (Tepley et al. 1999, Humphreys et al. 2009); disaggregated portions of enclaves are more 108

predominant in Group 2 lavas because of smaller ΔT/ΔC, which allowed for a greater degree of mixing between buoyant enclaves and host magmas (Fig. 45d). 6) Slow cooling in the lower depths of the chamber produced larger- grained basaltic-andesite, which became vesiculated as a result of crystallization-induced volatile saturation (Martin et al. 2006b); these coarser-grained, less vesiculated enclaves rose into the overlying rhyodacitic host if their vesicularity was enough so that they gained buoyancy, or they were entrained within the overlying host magma by convection currents within the overlying host magma (Figs. 45d-e).

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Figure 45. Model for Magma Mixing and Enclave Formation within the Chaos Crags. (a) rhyolitic crystalline mush underlies the Chaos Crags. (b) a large-volume injection of mafic material rejuvenates and hybridizes the crystalline mush, producing the Chaos Crags host rhyodacite lavas. (c) thermal contrasts initiate convection within both the mafic layer and overlying host magmas; a low-volume basaltic recharge into the base of the chamber entrains rhyodacite host magma, producing a heterogeneous basaltic-andesite layer; the mafic component crystallizes and vesiculates, producing mafic enclaves which rise into the overlying host magma. 110

110

Figure 45, cont’d. Model for Magma Mixing and Enclave Formation within the Chaos Crags. (d) basaltic-andesite at slightly deeper levels of the mafic layer slowly crystallize; previously-formed mafic enclaves are distributed throughout the host magma by convection currents. (e) upper portions of highly vesiculated basaltic-andesite are eroded, exposing the more coarse-grained basaltic andesite; less vesiculated, coarser-grained basaltic andesite from the upper portions of the layer are entrained within the overlying host magma by thermal convection currents. 111

CONCLUSION

In this study, the Chaos Crags were used as a test case to compare the feasibility of various eruption triggering mechanisms. Calculated pre-eruptive magmatic densities and the pressures and temperatures of magma storage determined that: 1) The end-member magmas present within the pre-eruptive Chaos Crags

system were a rejuvenated rhyolitic crystalline mush of ~76 wt.% SiO2

and a parental differentiated basalt of ~51 wt.% SiO2. These end- member magmas mixed in various proportions to produce the host magmas of the Chaos Crags. Mafic enclaves were formed through a series of complex mixing and mingling processes between the parental basaltic end-member and already-mixed host magmas. 2) The vesiculation of mafic enclaves present within Chaos Crags eruptive products was not homogenous. Although recharge magmas reached volatile saturation, as evidenced by an abundance of highly vesiculated mafic enclaves, the entire enclave population was not sufficiently vesiculated so as to become less buoyant than the overlying rhyodacitic host magmas. Calculated vesiculated densities of a small population of mafic enclaves remained less dense than overlying host magmas, and were presumably entrained into the overlying silicic host through convection within the overlying host magma. 3) The wide and continuous range of crystallization temperatures of mineral phases present within Chaos Crags mafic enclaves demonstrates that crystallization of recharge magmas began during mixing of the parental basalt end-member and host magmas, and continued after 112

mixing of the two magmas throughout the mingling process, prior to eruption. This author posits that mixing of the basaltic end-member and host magmas occurred some time prior to eruption, and thus was not the proximal trigger for eruption. Instead, this study notes that mafic recharge and magma mixing were the ultimate trigger for volcanic eruption, and attributes the proximal cause for eruption to increased overpressure within the chamber caused by volatile 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. The findings of this study have significant implications for the fields of natural hazard risk assessment and eruption triggering mechanisms. The wide range of estimated crystallization temperatures for phases present within mafic enclaves suggests that some period of time exists between mixing of the parental mafic component and the host rhyodacite magma; further studies of diffusion profiles across phases present within eruptive products may provide a relative timeline for the thermal history of mixing and residence time of magmas within the chamber prior to eruption. Geophysical monitoring of harmonic (volcanic) tremors currently provides the scientific community with a mechanism for the

112 monitoring of magma ascent prior to and during volcanic eruptions (Ferrick et al.

1982). If a timeline could be established for mafic recharge, and the resultant magma mixing and crystallization histories, those timelines could be used in conjecture with monitoring of harmonic tremors, enhancing the possibility for the prediction of volcanic eruptions within lava domes in continental-arc systems. 113

Recommendations for Further Studies Although this study has contributed to the body of work regarding eruption triggering mechanisms, enclave formation, and the Chaos Crags eruptive sequence, much more information can be acquired to improve our understanding of this particular system, and of lava domes within the southern Cascade Range. With respect to analyses of natural samples, additional studies to be completed regarding the Chaos Crags include: 1) Diffusion profiles across plagioclase, clinopyroxene, and olivine present within Chaos Crags eruptive products may help to establish timescales of crystal growth within both mafic enclaves and host lavas, providing a relative timeline for the thermal history of mixing and residence time of magmas within the chamber prior to eruption. 2) Trace element analyses of enclaves cores, rims, and intra-enclaves, which may aid in our understanding of compositional heterogeneities within individual mafic enclaves.

3) Analyses of H2O contents from clinopyroxene, which may aid in

constraining the H2O contents of mafic magmas at various pressures and temperatures.

4) Geothermometry using titanomagnetite-ilmenite pairs within mafic 113

enclaves will determine the precise oxygen fugacity of the mafic component, and may aid in constraining the eruptive temperatures of host magmas. 5) Analyses of P zoning in olivine present within Chaos Crags mafic enclaves may reveal the presence of multiple replenishment events between eruptions within the Chaos Crags eruptive sequence, further 114

our understanding of the enclave formation and eruption triggering processes at work within the Chaos Crags system. To date, there exist no experimental evaluations of the mafic component present within the Chaos Crags system. Experimental determination of phase relationships for the mafic component would greatly constrain the sequence of crystallization and stability of enclave materials prior to introduction within the magma chamber. Additionally, thermodynamic and kinetic modeling of enclave formation should be conducted in order to evaluate the various regimes necessary to produce the complex enclaves seen within the Crags system.

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APPENDICES 127 127

APPENDIX A: COMPREHENSIVE LIST OF CHAOS CRAGS ERUPTIVE PRODUCTS ANALYZED IN THIS STUDY 128 128

Host/ Enclave Host/ Sample No. Enclave Diameter Sample No. Enclave Enclave Diameter CC-A-H-1 H n/a CC-D-I-4 core E 9.6 cm CC-A-I-3 rim CC-D-I-4 rim E 50+ cm CC-A-I-3 core CC-D-I-5 core E 6.9 cm CC-A-I-4 rim CC-D-I-5 rim E 15 cm CC-A-I-4 core CC-D-I-6 core E 3.2 cm CC-A-I-5 rim CC-D-I-6 rim E 5.7 cm CC-A-I-5 core CC-D-I-7 core E 2.9 cm CC-A-I-6 rim CC-D-I-7 rim E 12 cm CC-A-I-6 core CC-D-I-8 core E 3.0 cm CC-A-I-7 rim CC-D-I-8 rim E 3.5 cm CC-A-I-7 core CC-D-I-11 core E 4.9 cm CC-A-I-8 rim CC-D-I-11 rim 11.9 cm CC-A-I-8 core E CC-E-H-1 H n/a CC-A-I-8 intra-enclave 2.0 cm CC-E-H-2 H n/a CC-A-I-9 rim CC-E-I-10 rim E 10.5 cm E ~12.7 cm CC-A-I-9 core CC-E-I-10 core CC-A-I-10 rim CC-E-I-11 rim E 8 cm E 5.8 cm CC-A-I-10 core CC-E-I-11 core CC-A-I-11 rim CC-E-I-12 out rim E 5 cm CC-A-I-11 core CC-E-I-12 in rim E 15.8 cm CC-A-I-12 rim CC-E-I-12 core E 5.5 cm CC-A-I-12 core CC-E-I-13 rim E 10.9 cm CC-A-I-13 core CC-E-I-13 core CC-A-I-13 inner rim E 21 cm CC-E-I-14 rim E 5.4 cm CC-A-I-13 outer rim CC-E-I-14 core CH-CC-09-15 core* CC-E-I-15 rim E ~ 7.6 cm E 6.5 cm CH-CC-09-15 rim* CC-E-I-15 core CC-A-I-16 rim CC-E-I-16 rim E 25 cm CC-A-I-16 outer core E 37 cm CC-E-I-16 core CC-A-I-16 inner core CC-E-I-17 rim E 4.2 cm CC-UPF-H-1 H n/a CC-E-I-17 core CC-UPF-I-1 core CC-E-I-18 rim E 50+ cm E 15.5 cm CC-UPF-I-1 rim CC-E-I-18 core CC-B-H-1 H n/a CC-E-I-19 rim E 6 cm CC-B-I-9 outer rim CC-E-I-19 core CC-B-I-9 mid rim CH-CC-08-15 rim* E 18 cm CC-B-I-9 inner rim E 50+ cm CH-CC-08-15 core* CC-B-I-9 outer core CC-F-3 rim E 12.5 cm CC-B-I-9 inner core CC-F-3 core E CC-B-I-10 rim CC-F-H-4 H n/a E 8.9 cm CC-B-I-10 core CC-F-4 core E 3.0 cm TS DB CC-07-08 rim* CC-F-4 rim E E ~ 11 cm TS DB CC-07-08 core* CC-F-H-5 H n/a CC-B-I-11 rim CC-F-I-10 rim E 7.5 cm E 11.6 cm CC-B-I-11 core CC-F-I-10 core CC-B-I-11 surr. host H n/a CC-F-I-11 rim E 5.6 cm CC-C-H-1 H n/a CC-F-I-11 core CC-C-I-1 rim CC-F-I-12 rim E 40 E 3.4 cm CC-C-I-1 core CC-F-I-12 core CC-C-I-11 rim CC-F-I-13 rim E 7.1 cm CC-C-I-11 core E 50+ CC-F-I-13 core CC-C-I-11 intra-enclave CC-F-I-14 rim E 17 cm CC-D-H-1 H n/a CC-F-I-14 core CC-D-I-1 core CH-CC-09-05 rim E 12 cm CH-CC-09-05 outer CC-D-I-1 rim core E 25 cm CH-CC-09-05 inner CC-D-I-2 core core E 17 cm *samples from Hootman (2011) dataset, courtesy of Lisa CC-D-I-2 rim Hammersley, Ph.D. 129

APPENDIX B: MAJOR OXIDE WHOLE ROCK GEOCHEMISTRY OF CHAOS CRAGS ERUPTIVE PRODUCTS (DATA DISC) 130

APPENDIX C: MAJOR OXIDE COMPOSITIONS OF INDIVIDUAL PLAGIOCLASE WITHIN CHAOS CRAGS HOST LAVAS (DATA DISC) 131

APPENDIX D: MAJOR OXIDE COMPOSITIONS OF INDIVIDUAL PLAGIOCLASE WITHIN CHAOS CRAGS MAFIC ENCLAVES (DATA DISC) 132

APPENDIX E: MAJOR OXIDE COMPOSITIONS OF CLINOPYROXENE WITHIN CHAOS CRAGS ERUPTIVE PRODUCTS (DATA DISC) 133

APPENDIX F: MAJOR OXIDE COMPOSITIONS OF ORTHOPYROXENE WITHIN CHAOS CRAGS ERUPTIVE PRODUCTS (DATA DISC) 134

APPENDIX G: MAJOR OXIDE COMPOSITIONS OF OLIVINE WITHIN CHAOS CRAGS ERUPTIVE PRODUCTS (DATA DISC) 135 135

APPENDIX H: MAJOR OXIDE COMPOSITIONS OF AMPHIBOLE WITHIN CHAOS CRAGS ERUPTIVE PRODUCTS (DATA DISC) 136 136

APPENDIX I: LOG-LINEAR VESICLE SIZE DISTRIBUTION GRAPHS FOR CHAOS CRAGS HOST LAVAS (DATA DISC) 137 137

APPENDIX J: LOG-LINEAR VESICLE SIZE DISTRIBUTION GRAPHS FOR CHAOS CRAGS MAFIC ENCLAVES (DATA DISC) 138 138

APPENDIX K: PROPORTIONS OF MIXING FOR CHAOS CRAGS ERUPTIVE PRODUCTS AS DETERMINED BY LINEAR MASS-BALANCE CALCULATIONS (DATA DISC) Fresno State

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