PETROLOGY OF THE 1877 ERUPTION OF COTOPAXI VOLCANO, ECUADOR: INSIGHT ON MAGMA EVOLUTION AND STORAGE

Megan Saalfeld

A Thesis

Submitted to the Graduate College of Bowling Green State University in partial fulfillment of the requirements for the degree of

MASTER OF SCIENCE

May 2018

Committee:

Daniel F. Kelley, Committee Co-Chair

Kurt S. Panter, Committee Co-Chair

John Farver ii ABSTRACT

Daniel F. Kelley, Committee Co-Chair

Kurt S. Panter, Committee Co-Chair

Cotopaxi is a stratovolcano in the Northern Andes Volcanic Zone, and has a history of bimodal volcanism, alternating between rhyolite and andesite. With Cotopaxi reawakening in

2015 after 138 years of quiescence, the question of what is occurring beneath the surface becomes especially poignant. To contribute to this question, it is productive to look to the volcano's recent past. This work characterizes the mineralogy and geochemistry of the recent eruptive products of Cotopaxi, with emphasis on the two pulses of the 1877 eruption.

Additionally, pressures and temperatures are estimated for magmas prior to all eruptions. This will allow a better understanding of the magma plumbing system and its evolution over time.

Over the past 500 years Cotopaxi has had five major eruptive events (VEI 3-4), which occurred in 1532, 1742, 1744, 1768, and 1877, and included pyroclastic surges, scoria flows, and lahars.

After the initial pulse of the 1877 eruption and the subsequent lahars, a second pulse of magma produced a pyroclastic density current, containing scoria clasts up to 1 meter in diameter. All samples collected from these eruptions are basaltic-andesite to andesite (56-59 wt. % SiO2), with a mineral assemblage of pl + opx + cpx + mag ± ol. Plagioclase from all samples range from

An47 to An78 and show both normal and reverse zoning. Normally zoned crystals exhibit greater compositional variation between cores and rims than reversely zoned crystals (median Ancore-

Anrim 8% vs 4%, respectively). The presence of disequilibrium textures such as zoned plagioclase and mingled magma indicate that magma mixing plays a role in magma genesis in addition to crystal fractionation. Pyroxenes occur as augite and enstatite and do not exhibit significant iii zoning. The very similar petrologic character of these deposits suggests that they were sourced from a relatively long-lived magma system that underwent differentiation and replenishment between eruptions. Fractional crystallization modeling shows that 17% crystallization of plagioclase (9.8%) + clinopyroxene (3.5%) + olivine (1.6%) + apatite (0.1%) is necessary to produce the observed variation. However, some samples show a better fit with mixing trends than crystallization trends, so it is likely that both processes are occurring. Thermobarometric data indicate that magma storage occurred at temperatures of 1000-1150°C and pressures ranging from 0.20 GPa (during the 1877 eruption) to 0.43 GPa (during the 1532 eruption), which is equivalent to depths of 7 and 15 km (± 9 km), respectively. Data from geodetic and seismic studies indicate that magma was injected at a depth of 4 to 5 km which caused ground deformation and seismic unrest from 2001 to 2002. While thermobarometry is poorly constrained, these results suggest that magma storage has become progressively shallower over time. iv ACKNOWLEDGMENTS

I would like to thank the staff of the Instituto Geofisico in Ecuador for collecting and providing samples, and Patty Mothes for inspiring this project. To Craig Grimes from Ohio

University for providing assistance with XRF analysis, Tom Darrah at Ohio State University for allowing me access to his lab, Erica Maletic for assistance with ICP-MS, and Gordon Moore at

University of Michigan for technical support with EMPA. Microprobe analysis was funded by the Richard D. Hoare Research Scholarship (BGSU). Above all, thank you to my advisors and committee member for teaching me so much over the past two years and for helping me to become a better scientist. v

TABLE OF CONTENTS

Page

CHAPTER 1. INTRODUCTION ...... 1

CHAPTER 2. GEOLOGIC SETTING ...... 3

CHAPTER 3. METHODS AND SAMPLE CHARACTERIZATION ...... 6

Field Observations ...... 6

Sample Collection ...... 7

Analytical Methods ...... 8

CHAPTER 4. RESULTS ...... 10

Petrography ...... 10

Textures Indicative of Magma Mixing ...... 12

Mineral Chemistry ...... 12

Plagioclase ...... 12

Pyroxene ...... 13

Major and Trace Elements ...... 13

Glass Compositions ...... 13

Whole Rock Compositions ...... 14

Geochemical Variability Between Eruptions...... 16

Barometry, Thermometry, and Hygrometry ...... 17

Barometry ...... 17

Thermometry...... 18

Hygrometry ...... 19 vi

CHAPTER 5. DISCUSSION ...... 20

Magma Mixing vs. Fractionation ...... 20

A System of Fractionation and Recharge ...... 24

Magma Storage Conditions...... 26

Models of Evolution ...... 28

CHAPTER 6. CONCLUSIONS ...... 29

REFERENCES...... 30

APPENDIX A. TABLES ...... 35

APPENDIX B. MICROPROBE ANALYSIS OF PLAGIOCLASE AND PYROXENE COMPOSITIONS...... 48

APPENDIX C. FIGURES ...... 55 1

CHAPTER 1. INTRODUCTION Subduction zone volcanism is generated with variable inputs from the partial melting of the subducting slab and sediment, the overlying mantle, and the continental crust (Hickey et al.,

1986; Hildreth and Moorbath, 1988; Tatsumi, 1989; Hidalgo et al., 2012). While andesite is one of the most abundant magma compositions occurring at continental volcanic arcs, it has been shown experimentally (Nicholls & Ringwood, 1972; Mysen et al., 1974) and with natural samples (Reubi & Blundy, 2009) that the majority of andesites are not primary melts. Rather, andesitic magmas form as the product of differentiation processes such as crystal fractionation, assimilation and fractional crystallization (AFC) and magma mixing. These processes impart chemical and textural signatures on the resulting lavas (Panter et al., 1997; Schiano et al., 2010;

Garrison et al., 2011; Lee and Bachmann, 2014). By determining the role that each of these processes play in the differentiation of magma, we strive to better understand shallow level magma evolution in the context of recent eruptive activity at Cotopaxi.

By focusing efforts on a short time interval, one can provide a more detailed account of the processes occurring that have created chemical diversity within the most recent eruptive cycles. Characterizing the storage and evolution of recent magmatism at this classic subduction zone stratovolcano can provide insight into processes occurring here and at similar volcanoes.

Cotopaxi is one of the most well-known and well-studied volcanoes in South America. The stratigraphy of its eruptive products has been well documented (Hall and Mothes, 2008; Garrison et al., 2011), and the stratigraphic relationships in the field are very clear, making it easy to distinguish different pulses of eruptions. These factors make Cotopaxi a prime candidate for investigating the magmatic processes that dominate subduction zone volcanism and that cause 2 magmatic variation in the upper crust. This study builds upon previous research by adding geochemical data and petrologic analyses for andesitic eruptions of the past 500 years. 3

CHAPTER 2. GEOLOGIC SETTING

Cotopaxi volcano is a composite volcano located in the Northern Volcanic Zone of the

South American Andes, 60 km south of Quito, Ecuador (Figure 1). Cotopaxi reaches to 5,897 m above sea level and is one of four currently active volcanoes in Ecuador. The Andes are a continental volcanic arc formed by the subduction of the Nazca Plate beneath the South

American Plate. The Nazca Plate off the coast of Ecuador contains the Carnegie Ridge, an aseismic ridge which marks the trace of the Galapagos hotspot. The subduction of this region of the oceanic crust corresponds to a decrease in subduction angle from 35° north of the Carnegie

Ridge to 25° beneath Ecuador (Guillier et al., 2001). The flatter subduction angle causes the volcanic arc in Ecuador to widen and split into two cordilleras, the Western Cordillera and the

Eastern Cordillera (Figure 1), dominated by dacitic and andesitic products, respectively (Hall et al., 2008; Hall and Mothes, 2008). The two cordilleras are separated by the Inter-Andean Valley.

Cotopaxi, along with other large stratovolcanoes such as Tungarahua, Antisana, and Sangay, define the Eastern Cordillera. Cotopaxi is unique in that it has exhibited a history of bimodal volcanism, alternating between andesitic and rhyolitic periods of activity, without intermediate dacitic products (Hall and Mothes, 2008).

The stratigraphy of Cotopaxi has been well documented and absolute ages have been determined with the earliest eruptive phase occurring between 560 and 420 ka (Hall and Mothes,

2008). The eruptive history has been divided into two stages; Cotopaxi I (560-420 ka) and

Cotopaxi II (13.2 ka – Present), which are separated by a long repose interval, during which the regional Cangahua and Chalupas (211 ka) ignimbrite sequences were deposited. The more recent

Cotopaxi II is then divided into sub-stages Cotopaxi IIA (13.2 - 4 ka) and IIB (4 ka – present).

Cotopaxi IIA predominantly consists of a sequence of rhyolitic composition lavas, block-and-ash 4 flows associated with dome collapses, and pyroclastic deposits related to eruption column collapse. The current eruptive period, Cotopaxi IIB, began 4,000 years before present (BP) and marked the most recent transition from rhyolite to andesite (Hall and Mothes, 2008). From 4,200 to 2,100 years BP lava emission is estimated to have been 1.65 km3/ka. Lava emissions have increased from then until the present, at an estimated 1.85 km3/ka (Hall and Mothes, 2008).

Additionally, the frequency of eruptive cycles has increased over time. Early eruptive cycles of

Cotopaxi IIB averaged 300-400 years/cycle (4000-2200 years BP), while intermediate cycles

(2200-900 years BP) averaged 100-150 years/cycle (Hall and Mothes, 2008). Historical eruptions have had both long periods of repose (390 years prior to 1532 CE, and then 208 years prior to 1742 CE) as well as very short intervals (24 years between 1744 CE and 1768 CE).

The most recent large volume, high explosivity eruption of this period began June 26th,

1877. Field relationships and historical accounts show that the event produced two pulses of scoria-bomb pyroclastic flows (Wolf, 1878; Hall and Mothes, 2008). The initial pulse of this eruption produced a strombolian-style eruption with pyroclastic flows that partially melted

Cotopaxi's summit glaciers and triggered massive lahars that reached the Pacific Ocean 500 km away in less than 18 hours (Wolf, 1878). This initial pulse yielded a syn-lahar scoria deposit

(Figure 2).

The second pulse of the 1877 eruption produced a scoria flow which extends north from the crater for approximately 7 km, along the west side of Ingaloma, and overlies the lahar and scoria deposits produced during the first pulse (Figures 2 and 3A). The scoria flow is a post-lahar pyroclastic density current which is indicative of a vulcanian boiling-over event, associated with an open vent. Low-level fire fountaining likely produced the large, unbroken cauliflower texture scoria clasts (Hall et al., 2013; Rader et al., 2015). An eyewitness account by Wolf (1878) 5 describes the eruption as “a dark foam-like cloud boiling over the rim of the crater and descending all sides of the volcano, like the boiling over of a pot of rice.”

Cotopaxi’s most recent activity occurred from April 2015 to 24 January 2016. This eruption consisted of periods of ash emissions, degassing, and seismic unrest. This eruption emitted ~8.6 x 105 m3 of ash and a VEI of 2 (Bernard et al., 2016). The first ash emissions began on August 14th, 2015, and ash plumes from these blasts rose ~7.8 km above the crater (Bernard et al., 2016). Ash from this event was analyzed by Gaunt et al. (2016), and is included in this study for comparison. 6

CHAPTER 3. METHODS AND SAMPLE CHARACTERIZATION

Field Observations

Two main outcrops were chosen for study: Quebrada Saquimala and west of Ingaloma.

Samples from the 1532-1768 eruptions, and samples from the first pulse of the 1877 eruption were collected from Quebrada Saquimala, while samples from the second pulse of the 1877 eruption were taken from near Ingaloma (Table 1). The syn-lahar and post-lahar scoria flows produced during the two pulses of the 1877 eruption (1877-1 and 1877-2) were chosen for study, due to the change in morphology between the first (syn-lahar) and second (post-lahar) pulses.

The 1877 post-lahar scoria flow is approximately 7 km long and varies in thickness, but is on average ~4 meters thick. The top of the flow is composed of mostly spherical scoriaceous bombs ranging from 0.5-2 meters in diameter, with most being around 1 meter. The bombs display a cauliflower bomb texture. There is a 1-2 cm weathered rind around the exterior of the clasts, but the interior is mostly unaltered (Figure 3B). The scoria contains abundant fine vesicles, with concentric banding around the core, and pockets of larger vesicles. Iron oxide alteration occurs as a thin film along the walls of some vesicles. The sides of the flow often have levees that consist of these larger clasts and are up to 1 meter above the body of the flow. The interior of the flow is ~40% ash with larger clasts ranging from 0.5-10 cm. There are several types of lithic clasts in the matrix of the scoria flow: 1) red scoria clasts that were entrained within the flow from oxidized material exposed below the terminus of the summit glacier (see Figure 3A); 2) black, rounded, devitrified scoria, which are most likely entrained material from a previous eruption based on the higher degree of devitrification relative to the vitreous juvenile scoria clasts; 3) brown, vitreous scoria that are interpreted, based on the freshness of the glass, to indicate that this is juvenile material; 4) dense dark grey andesitic blocks, which have a vitreous 7 luster, and prismatic jointing. These dense andesitic blocks are interpreted as being recycled juvenile material, likely a conduit plug formed before or between pulses of the eruption. They often have pieces of foamy material stuck to the sides of them, possibly indicating that the plug was forming contemporaneously with the rising of the foamy magma. At the distal end of the scoria flow, the uppermost boulder-sized scoria clasts are visible, but there is no topographic relief between the scoria flow and the underlying lahar as there is up slope. Here, the scoria flow most likely sunk into the still soft lahar deposit, indicating that the scoria flow was likely deposited soon after the lahar (B. Bernard, personal communication, June 2017).

Pyroclastic deposits from the 1532-1768 eruptions as well as from the first pulse of the

1877 eruption outcrop in the valleys along the southern flank of Cotopaxi (Figure 2). The deposits are mostly scoria, with some deposits containing smaller amounts of pumice and lithic clasts from volcanic plugs (Table 1). The 1532 deposit has a mingled magma characteristic, with both brown and black scoria clasts (Hall and Mothes, 2008). The 1742 deposit is a pyroclastic flow of black scoria with no color variation. The 1744 deposit is a partially welded pyroclastic fall. The top of the deposit is oxidized red, and the bottom of the deposit contains lithic andesitic blocks. Clasts from this eruption have been separated into grey pumice, black pumice, plug andesite, and black scoria. The 1768 deposit is a pyroclastic fall deposit that includes clasts of scoria and white pumice lapilli. The first pulse of the 1877 eruption produced a pyroclastic flow with black scoria.

Sample Collection

Fieldwork to collect samples from the scoria flow of the second pulse of the 1877 eruption was undertaken in June 2016. Samples from the centers of ten scoria bombs were collected over 1 km along the length of the distal portion of the flow (Figure 2). Sampling was 8 conducted on this portion of the flow because the stratigraphic relationship is very clear, with the scoria flow sitting directly on top of the 1877 lahar deposit generated during the first pulse of the eruption (Figure 3). The samples from 1532, 1742, 1744, 1768, and 1877-1 were collected from

Quebrada Saquimala (Figure 4), by staff at the Instituto Geofisico in 2015. This site was chosen because the exposed stratigraphy allowed for the identification and sampling of all five deposits at one site.

Analytical Methods

Ten tephra samples from the second pulse of the 1877 eruption as well as twenty samples from the five prior eruptions dating back 500 years were analyzed for major element concentrations, and nineteen of those samples were analyzed for trace element concentrations.

Additionally, 6 representative samples were analyzed for mineral chemistry (Table 1). Samples here are referred to by the year they were erupted (e.g. samples from the 1532 CE eruption are referred to as 1532, etc., and samples from the first and second pulses of the 1877 eruption are referred to as 1877-1 and 1877-2, respectively).

Whole rock powders for x-ray fluorescence (XRF) and inductively coupled plasma mass spectroscopy (ICP-MS) were prepared at Bowling Green State University using a jaw crusher and a tungsten carbide puck mill until a flour-like consistency was reached. Crushed samples were picked prior to powdering to ensure that fragments are devoid of saw marks, secondary alteration materials and accidental lithics in order to best evaluate magma geochemistry.

Glass beads for geochemical analysis by x-ray fluorescence (XRF) were prepared at Ohio

University using a 1:3 ratio of sample to flux. A 35.3% Li-tetraborate 64.7% Li-metaborate flux was used, and one drop of LiI non-wetting agent was added prior to fusion. Analyses were conducted on the Rigaku Supermini200 at Ohio University and using the Fundamental 9

Parameters (FP) method and calibrated against 12 USGS rock standards. BHVO-2 (basalt) and

AGV-2 (andesite) were run alongside unknowns during analysis. Duplicate analyses were run for all samples from the 1532, 1742, 1744, 1768, and 1877-1 samples. All major element oxides have an average precision of less than ±2% of the reported value.

Trace element data were obtained via ICP-MS using methods outlined in Eggins et al.

(1996) for two samples from the 1532 eruption, one from the 1742 eruption, four from the 1744 eruption, two from the 1877-1 scoria pulse, and ten from the 1877-2 scoria pulse. Analysis was performed at the WHEEL Noble Gas Laboratory at The Ohio State University. Samples were digested in a mixture of nitric acid and hydrofluoric acid for 6-12 hours, until all particles were dissolved, and then digested a second time in only nitric acid. After all samples were dissolved, they were diluted ~1000x to ensure element concentrations were within range of the detector’s limits (less than few hundred ppm). Standards BHVO-2 (basalt) and AGV-2 (andesite) were prepared to mimic the expected element concentrations based on preliminary XRF data.

Additionally, 6 highly polished, carbon-coated thin sections (50μm) were prepared at

Bowling Green State University for electron microprobe analysis. Major element concentrations from groundmass glass and euhedral plagioclase and pyroxene phenocrysts were obtained at the

EMAL laboratory at the University of Michigan using a CAMECA SX-100 microprobe.

Representative samples from the 1532, 1742, and 1877-1, as well as three samples from the

1877-2 were selected for analysis. Operating conditions were 15 kV accelerating voltage, 5 nA beam current, and 5 μm beam size for plagioclase and matrix glass; 15 kV accelerating voltage,

9 nA beam current, and <1 μm beam size for pyroxene. 10

CHAPTER 4. RESULTS

Petrography

Petrographic analysis was conducted using standard techniques for 10 samples of the

1532 to 1877-1 deposits and 10 samples of the 1877-2 deposit, and results are summarized in

Tables 1 and 2. The samples are porphyritic with phenocrysts varying from 10% to 25% by volume. The phenocrysts assemblage is generally plagioclase > orthopyroxene > clinopyroxene

± magnetite and other opaque oxides. The samples are moderately to highly vesicular (up to 50% vesicles by volume) and have a range of hypocrystalline to holohyaline groundmass. Oscillatory zoning is common in larger plagioclase phenocrysts, and some phenocrysts display sieve textures. Cognate xenoliths occur as subrounded clasts displaying a granular texture composed of pyroxene and plagioclase.

The 1532 deposits contain a mixture of brown and black scoria clasts. The mineral assemblages of these two clast types is different with the black scoria containing plagioclase (5-

10%) + orthopyroxene (2-3%) + clinopyroxene (<1%) + accessory magnetite (<1%), and the brown scoria is composed of plagioclase (~25%) + orthopyroxene (1-2%) + opaque oxides

(<1%). The black scoria has a holohyaline groundmass, with few microphenocrysts, while the groundmass of brown scoria is hypocrystalline, and plagioclase phenocrysts display more complex zoning and sieve textures.

The 1742 eruption ejected molten ballistic bombs, which produced a 2-3 meter thick deposit of light colored breccia clasts set in lava (P. Mothes, personal communication, May,

2015). It has a mineral assemblage of plagioclase (12-15%) + orthopyroxene (~5%) + clinopyroxene (<1%). The pyroclasts are scoriaceous and have a holocrystalline groundmass.

Plagioclase phenocrysts have oscillatory zoning with sieve texture in the centers of crystals. 11

Pyroxenes occur as phenocrysts, microphenocrysts, and cognate xenoliths, which tend to be highly fractured and altered.

The 1744 deposit is divided into 4 components: pyroclastic fall (PF), brown pumice (BP), grey pumice (GP), and dense lithic blocks of conduit plug rocks (PLUG) (Hall and Mothes,

2008; Patty Mothes, personal communication). The 1744-PF deposits contain a phenocryst composition of plagioclase (~20%) + orthopyroxene (1-2%) + clinopyroxene (5-7%), with abundant microphenocrysts of plagioclase and pyroxene. The brown pumice has fewer plagioclase phenocrysts (~8-10 %), and the feldspars are not strongly zoned. Microphenocrysts are abundant and cognate lithics are not present. The grey pumice is abundant in plagioclase phenocrysts and microlites, with very little pyroxene (<1%). Most plagioclase phenocrysts are euhedral and pristine (no resorption textures), with a few displaying oscillatory zoning (Figure

5A).

The black scoria produced during the 1768 eruption contains plagioclase (~10%) + orthopyroxene (1%) + clinopyroxene (1%) + accessory magnetite. The majority of plagioclase phenocrysts are sector zoned or oscillatory zoned, and some phenocrysts have a sieve texture.

The 1877 scoria clasts are moderately to highly vesicular (30-60% vesicles) with a hypohyaline groundmass. The groundmass contains small (1-5μm) microphenocrysts of plagioclase and pyroxene. Phenocrysts total 10-12% by volume. Approximately 60-75% of the phenocrysts are euhedral plagioclase (0.5-1 mm). Several of the larger plagioclase phenocrysts are weakly zoned, and rarely, plagioclase phenocrysts display strong zoning (Figure 5A, C). Most plagioclase are pristine, while a small population have strongly resorbed cores (Figure 5F). Orthopyroxene accounts for 15-25% of the phenocryst population, and the remaining 10-15% is clinopyroxene.

Pyroxene phenocrysts are generally euhedral (Figure 5B), and are approximately 0.2 mm in 12 diameter. Pyroxenes usually occur within cognate xenoliths, which sometimes include smaller plagioclase phenocrysts (Figure 5D, E).

Textures Indicative of Magma Mixing. Hall and Mothes (2008) note that eruptions from 1532 and later have a much more mafic character than the other andesites of the Cotopaxi

IIB sequence, and whole rock analyses from this study supports this as discussed below. They further suggest that this more mafic character could be due to an injection of mafic magma into a more evolved magma prior to the onset of activity in 1532. Within each of the 1532 samples, there is petrographic evidence of magma mingling with inclusions found within scoria range from ~1-2 mm in diameter (Figure 6). The inclusions show both sharp and gradational contacts with the host material. The contacts are generally smooth but irregular and vesicles are concentrated within the host material along the contact margins (Figure 6). Additionally, the

1768 sample exhibits a mingled texture, both in hand sample and in thin section (Figure 6). In hand sample, the 1768 scoria clearly shows patchy zones of brown and black, and preferential oxidation of zones within the sample. Thin section analysis of this deposit shows patchy zones of a dark, less vesicular material. Phenocrysts of olivine are not identified in the inclusions, however the glassy groundmass with small crystals of plagioclase are reminiscent of tachylite

(quenched basaltic glass) characteristics reported by Bae et al. (2012).

Mineral Chemistry

Plagioclase. Plagioclase crystals from the 1532-BR, 1877-1 and 1877-2 samples range in composition from An43 to An78, which mostly fall within the labradorite and bytownite fields

(Table 3, Figure 7). Cores range from An47 to An78, whereas rims range from An43 to An71, and samples contain both normal and reverse zoning. The cores of normally zoned plagioclase range from An53 to An78, where the cores of reversely zoned crystals range from An21 to An70 (Table 13

4). The rims of normal and reverse plagioclase range from An52 to An67 and An47 to An72, respectively. Pyroclastic deposits from 1532 have the most calcic plagioclase, with compositions

An63 to An77 (rims and cores). The 1877-1 and 1877-2 pulses have similar plagioclase compositions of An43 to An67, and An47 to An65, respectively. The 1742 eruption was poor in plagioclase, and no plagioclase compositions were collected.

Pyroxene. Clinopyroxene and orthopyroxene are present in all samples. The clinopyroxene occurs as a high-Ca augite (Wo40-43, En39-45, Fs12-19) (Table 3, Figure 8). There is one outlier of pigeonite (Wo13, En61, Fs26) in the 1532 sample. Clinopyroxenes have magnesium content expressed as Mg# (Mg# = 100*Mg/Mg+Fe in mol %) which range from 67 to 78. Some phenocrysts show weak (<1 wt.% MgO) reverse zoning.

Orthopyroxene is slightly more abundant (~15% of phenocrysts) than clinopyroxene

(~10%), and is classified as enstatite (Wo2-3, En62-74, Fs23-34) (Table 3, Figure 8). The MgO concentration is slightly lower than the clinopyroxene, with Mg # ranging from 65 to 76. There is some heterogeneity in Fe content with ferrosillite (Fs) ranging from 23 to 34%. No significant zoning is found.

Major and Trace Elements

Glass Compositions. Glass analyses for pyroclastic material from 1532, 1742, 1877-1, and 1877-2 eruptions are given in Table 5 and Figures 9 and 10. These analyses are compared to glass compositions from the 2015 eruption (Gaunt et al., 2016). Glass compositions are andesitic to dacitic (Figure 9), spanning a range of 56-68 wt.% SiO2. The eruptions of 1532 and 1877-1 have andesitic compositions (61-64 wt.% SiO2). The 1742 and 1877-2 eruptions span the entire range of values (60-68 wt. % SiO2). Glass from the 2015 eruption have SiO2 contents between 63 and 67 wt.% (Figure 9). The general trend of all glass samples shows decreasing MgO, FeO, and 14

CaO with increasing SiO2 content, while K2O shows an increase in concentration with increasing

SiO2. Trends in Al2O3 and Na2O concentrations with increasing SiO2 are less coherent but generally decrease and increase, respectively, with increasing SiO2 content (Figure 10).

Whole rock compositions analyzed by XRF show a more restricted compositional range relative to groundmass glass analyses by EMPA. Glass compositions have increasing concentrations of TiO2, K2O, and P2O5 with increasing silica, indicating that those elements are concentrated in the melt during crystallization (Figure 10). Conversely, the glass has decreasing concentrations of Al2O3, MgO, CaO, and FeO with increasing silica (Figure 10).

Whole Rock Compositions. Whole rock major and trace element data are presented in

Table 6 and Figures 9-12. The eruptive products of the past 500 years are comprised of medium-

K2O basaltic andesite and andesite (55.6-58.7 wt.% SiO2). These samples show decreasing concentrations of MgO, CaO, FeO, TiO2, MnO and P2O5 with increasing SiO2 content.

Conversely, K2O, Na2O and Al2O3 concentrations have a positive correlation with SiO2. There is no systematic variation between composition and age. The least evolved samples in the suite (56 wt. % SiO2) were produced during the 1768 eruption. The black pumice from the 1532 eruption is the most evolved composition (59 wt. % SiO2) of the sampled suite, while the brown pumice is more mafic in composition (56 wt. % SiO2). The 1744 samples also show some variation in silica content, ranging from 57 to 58 wt. % SiO2. Both of the 1877 pyroclastic deposits have homogenous compositions that are tightly clustered at approximately 58 wt. % SiO2 (Table 6,

Figure 11). Major element compositions of magmas erupted in the past 500 years are comparable to the more mafic end of the Cotopaxi IIB data array (Figure 11).

The major and trace element compositions of the 1532-1877 samples fall within the range of other andesites from Cotopaxi IIB (Garrison et al., 2011, Garrison et al., 2006), and again are 15 towards the mafic end of the data range. Trends in trace element data for samples analyzed in this study follow the same trend as Garrison’s Cotopaxi IIB samples, with the exception of Y, which has a slightly lower concentration than the main trend, and Ba and Nd, which have overall higher concentrations within the same range of SiO2 wt.% relative to the pre-1532 andesites

(Figure 12). However, Yb and Lu (which are likely to have a similar bulk mineral-melt distribution as Y) are within range of previous Cotopaxi results. High Field Strength Elements

(HFSE; e.g., Zr, Nb), Large Ion Lithophile Elements (LILE; e.g., Rb, Ba), and Light Rare Earth

Elements (LREE; e.g., La, Nd) have a positive correlation with increasing SiO2 (Figure 12).

Conversely, Cr and Ni are negatively correlated with SiO2, although there is some scatter.

Middle and Heavy Rare Earth Elements (e.g., Sm and Yb, respectively) and Sr have no discernable correlation with changing SiO2 concentration.

On primitive mantle normalized diagrams Cotopaxi samples show enriched LILE, pronounced positive K, Pb, Sr anomalies, and paired negative anomalies for Nb and Ta (Figure

13A, B), which is typical of arc magmas (Tatsumi et al., 1986; McCulloch and Gamble, 1991).

These characteristics are likely a remnant signature of input of fluid from the subducting slab

(Brenan et al., 1994; Tatsumi and Kosigo, 1997). Elements with low fluid mobility (such as Nb and Ta) do not get transported with the slab fluid and are therefore depleted in the melt (Schmidt et al., 2004). Likewise, elements that are fluid mobile get transported with the fluid and incorporated into the magma. Elements such a Pb and Sr, which are incompatible, then get enriched in the remaining melt as crystallization occurs (Davidson, 1996; Hawkesworth et al.,

1997).

On chondrite and primitive mantle normalized multi-element plots, samples analyzed in this study show relatively uniform concentration patterns that are similar to previous Cotopaxi 16

IIB compositions (Figures 13 and 14). Rare earth element contents show a smooth pattern that are LREE enriched relative to HREE (Figure 14), which is a typical signature of subduction zone magmas (Davidson, 1996). Ratios of La/Lu for the early Cotopaxi IIB data have much greater range in values relative to the 1532-1877 samples (45-120 and 70-90, respectively). Normalizing the 1532-1877 samples normalized to an average trace element concentrations of the Cotopaxi

IIB suite (AVG CTX) shows near unity, however, a few samples show slight enrichments in

REEs (Figure 14B) and slight depletions in Rb, Th, U, Pb and Y (Figures 13B).

Geochemical Variability Between Eruptions

Characterizing the chemical composition of erupted products with respect to age can help evaluate the timing and duration of fractional crystallization and recharge events.

Glass compositions are highly variable compared to whole rock compositions.

Compositions range from basaltic andesite to dacite (56 to 68 wt. % SiO2). Samples from the

1532 eruption have the least evolved glass (1532-BR), with SiO2 concentrations ranging from 56 to 63 wt. %. Additionally, the 1532 eruption has high MgO and CaO, and low K2O concentrations. Samples from the 1742, 1877-1, and 1877-2 eruptions have no discernable trend with age and glass compositions span the entire range of concentrations for all major elements.

The range of whole rock geochemical compositions from the past 500 years is relatively restricted with small variations in major elements, and very little variation in minor elements

(Figure 15). Whole rock major and minor element concentrations show variation of ~2 wt. %

SiO2, and < 0.2 wt. % variation for oxides that are < 2 wt. % of rock. Much of the variation is defined by samples 1744-BP and 1532-BL which have higher SiO2 contents by 2 wt. % (Figure

11), and depletion of 0.5 wt. % MgO, FeO, and CaO, relative to samples from the other eruptions

(Figures 11 and 15). 17

Whole rock trace element concentrations are relatively homogeneous between eruptions as well. Exceptions to this are Ba, Sr, and Rb which show a total variation in the sample suite of

13, 106, 65, and 8 ppm, respectively (Figure 12). These variations mostly occur in the 1744 eruption, which also shows higher concentrations in Co, Cr, Ni (Figure 16). Similarly, concentrations of Ba, Rb, and Ce are lower in the 1744 eruption. Additionally, there is a smaller inverse peak in the 1877-1 samples, where Co, Cr, and Ni have lower concentrations, and Ba, and Ce have higher concentrations than the other samples (Figure 16). Other trace elements that have concentrations of < 40 ppm in the rock vary by < 5 ppm (often < 1 ppm), and trace elements that have concentrations of 100-150 ppm vary by < 30 ppm (Figure 16).

Barometry, Thermometry & Hygrometry

Geothermobarometry calculations using mineral-melt pairs (Putirka, 2008) were performed to estimate the temperature, pressure and water content of the magma to constrain pre-eruptive storage conditions and to determine any changes in storage conditions over time.

Orthopyroxene-melt pairs were checked for equilibrium using the Rhodes diagram, which compares the observed Fe-Mg exchange coefficient with a standard value of 0.29 ± 0.06 (Rhodes et al., 1979). Plagioclase-melt pairs were used to estimate temperature and water content due to the relatively low associated error. Analysis was done on rims and cores of euhedral crystals that did not show signs of resorption.

Barometry. Orthopyroxene and liquid pairs were used to calculate magma storage temperature and pressure using the Al-in-orthopyroxene barometer (Putirka, 2008; Equation 29a)

(Figure 17). This method was chosen because it had the lowest margin of error, and orthopyroxene was present and in equilibrium with glass in all samples. 18

The highest mean pressure is from the more primitive 1532-BR eruption (0.429 GPa; n =

1). However, the 1742 eruption yields similar pressures (0.412 ± 0.03 GPa; n = 4). The first and second pulses of the 1877 eruption yields the lowest pressure estimates at 0.235 ± 0.21 GPa (n =

5) and 0.201 ± 0.17 GPa (n = 8). Pressure estimates for the 1877-1 and 1877-2 eruptions are variable and range from 0.08-0.38 and 0.02-0.32 GPa, respectively. For each eruption, values are reported as an average with an error of two standard deviations (2σ). Calibration data for this method yield a standard error estimate of ±0.26 GPa. Pressure calculations of orthopyroxene- liquid pairs appear to show a trend of decreasing pressure with time (Figure 17), however, when considering the error, the pressure difference between the 1532 and 1877 eruptions is less than the range of error, and therefore are considered highly speculative. Overall, magma storage depths for the Cotopaxi lavas of the past 500 years is approximately 9.5 ± 8.7 km or within the upper 18 km of crust (when assuming a crustal density of 2.8 g/cm3).

Thermometry. The composition of plagioclase rims that are in equilibrium are used to calculate pre-eruptive temperatures of the magma. Equilibrium is checked using the Ab-An exchange, and by comparing the equilibrium constant to a standard value of 0.1 ± 0.05 (Putirka,

2008). Pressure does not have a large impact on temperature calculations. For example, an increase of 1 kbar will only increase the temperature estimate by ~5°C, which is less than 1 standard deviation. Because of this, pressure is held constant at 4 kbar (0.4 GPa) for all samples.

This results in a temperature estimate for all eruptions of 1113 ± 25°C (n = 21) (Figure 18).

Temperature estimates are constant across all eruptions (1532-1877) within the range of error, and thus this estimate includes all data from the 1532 eruption, and both pulses of the 1877 eruption (1742 plagioclase phenocrysts were not analyzed). The orthopyroxene-liquid geothermometer yields similar temperature estimates of 1085 ± 35°C (n = 22). 19

Hygrometry. A plagioclase-melt hygrometer (Putirka, 2008; Equation 25b) is used to calculate water content in the melt. Results of 1.0 ± 1 wt.% H2O is found for all samples.

Andean arc andesites (54-60 wt.% SiO2) have water contents ranging from 0 to 5 wt. % (Grösser,

1989; Droux and Deyalole, 1996; Hickey-Vargas et al., 2016; GeoRoc database: http://georoc.mpch-mainz.gwdg.de/georoc/). The low water content calculated may explain why amphibole is not present in the samples studied. Water contents of ~4-5 wt.% are needed for hornblende to be stable (Moore and Carmichael, 1998).

20

CHAPTER 5. DISCUSSION

Fractional crystallization and magma mixing are known to be common processes which act to produce andesitic magmas from primary melts at continental arc volcanoes (Lee and

Bachmann, 2014 and references therein). This study further illustrates the importance of fractionation and mixing processes on differentiation of magma in the upper crust. These processes can help us to understand how magmatism at continental subduction zones affects the composition of the crust and to provide insight into possible eruption triggers.

Magma Mixing vs. Fractionation

Numerous models for magma differentiation have been proposed to explain chemical variations in subduction zone volcanism (e.g. Vogt, 1921; Wickman, 1943; Wright and Fiske,

1971; Michael 1983). In this study, two fundamental processes were examined, fractional crystallization and magma mixing, to interpret the compositional variability in this sample suite.

Least-squares mass balance modeling (Bryan et al., 1969) is used to estimate the amount of fractionation that would need to occur to produce the most evolved composition in this suite from a hypothetical parent composition. The model predictions require 17-23% crystallization to evolve the magma from 56 to 59 wt.% SiO2, which is within range of experimental results by

Lee and Bachmann (2014), which require a change in crystallinity of 17-20% depending on pressure and water content. A parent composition was chosen from the most mafic sample within the suite (1532-BR) and the most felsic sample (1532-BL) was chosen as the daughter composition (Table 6). These end member compositions are from the same eruption and likely represent the compositions of a primitive magma that recharges the system and the composition of an evolved magma that forms by fractionation of the more primitive magma. Because of this, the 1532-BL sample is a good daughter lava end member and is used for modeling fractional 21 crystallization from the primitive magma composition. Additionally, because these two compositions are present in the same sample, they provide excellent end member compositions for modeling magma mixing within this suite.

Starting with the most evolved daughter lava composition (1532BL-1), and adding back mineral compositions of phenocrysts that are present in the samples (Table 3, Appendix A),

Model A shows that 23% crystallization (12.4% plagioclase, 2.4 % pyroxene, 6.9% hornblende, and 1.1% magnetite) could produce most of the chemical variation in this suite (Table 7, Model

A). Trace element concentrations were modeled using mineral-melt partition coefficients derived from phenocryst-groundmass experiments of lavas with similar composition (Table 8). The best solution for fractional crystallization using this mineral assemblage results in a sum of squares of residuals equal to 0.008. This model requires the crystallization of amphibole which is not present in the phenocryst assemblage of erupted lavas. However, the lack of amphibole phenocrysts may be due to its breakdown to minerals pyroxene, plagioclase and Fe-oxides as water is lost during magma ascent (Jakes and White 1972; Rutherford and Hill, 1993). However, hygrometry estimates of 1.0 ± 1 wt. % H2O, are below the stability range for hornblende (Moore and Carmichael, 1998), which might indicate that amphibole was not a stable phase in the magma prior to ascent.

Calculated concentrations for trace elements show an adequate fit for most elements except Ni and Cr (Table 7, Model A). The calculated concentrations of Ni and Cr are too low in comparison to observed values, meaning that the model is under predicting the amount of olivine and clinopyroxene that are crystallizing. When olivine is added into the fractionating assemblage, the difference in Ni content between observed and calculated is much better (Table

7, Model B). This solution requires 18% crystallization, with a crystallization assemblage of 22

9.8% plagioclase, 0.8% clinopyroxene, 5.2% hornblende, 1.1% magnetite, and 1.4% olivine and the major-minor element solution provides a good sum of the squares of residuals equal to 0.029

(Table 7, Model B).

Accessory minerals such as apatite have been shown to have a significant effect on trace element concentrations in fractionated calc-alkaline magmas (Green, 1981; Zielinski and Frey,

1970; Price and Taylor, 1973). To help resolve trace element concentrations in the model predictions, a third model (Table 7, Model C) without hornblende but with apatite was tested, and results in a crystallizing assemblage of 10.2% plagioclase, 3.5% clinopyroxene, 1.6% magnetite, 2.0% olivine, and 0.1% apatite. This model is the best predictor of trace element concentrations relative to Models A and B. Specifically Ba, Zr, Sr and the LREE-MREEs (La,

Ce, Sm, Eu, Tb) show a good fit with observed concentrations, however, this is at the expense of a poorer fit with Ni relative to Model B. From Model A to Model C there is a 5% decrease in crystallization (from 23% to 17%) required to produce the observed variation from 56 to 59 wt.%

SiO2. Lee and Bachman (2014) predict a change in crystal content of ~18% with this change in silica content for a magma crystallizing from a basaltic source. This estimate of crystallinity is lower than the crystallization predicted by Model A, but matches well with the predictions of

Model C, which further supports that Model C provides a good estimation of fractional crystallization for this system.

Fractional crystallization models were also evaluated graphically, along with magma mixing, on trace element plots. When comparing ratios of highly incompatible vs. moderately incompatible elements (Figure 19) model fractional crystallization curves provide a good fit to the data trend but only when Nd and Ce behave compatibly. This conflicts with the least-squares mass balance models (Table 7, Models A and B) that found the bulk distribution of Nd and Ce to 23

be incompatible (D < 0.3). A solution is to include a small amount of apatite (Kd ap-melt Nd = 32.8 and Ce = 21.1; Fujimaki, 1986) to the fractionation assemblage (Table 7, Model C) which results in an increased compatibility of Nd and Ce. All models that were tested show a good fit for major elements (sum of squares of residuals < 0.03). Model C however, provides the best predictions for trace element concentrations (Table 7; Figure 19), and is therefore the favored model for fractionation. Additionally, Model C provides a reasonable assemblage of fractionating phases based on the minerals observed as phenocrysts and accessory minerals that are common in andesitic rocks (i.e. olivine and apatite).

Mixing can also provide a solution for some of the geochemical variation in the sample suite (Figure 19). To help distinguish between fractional crystallization and mixing trends a plot of Rb/Ni vs. Rb is employed and shown in Figure 20. The model curve for mixing between endmember 1532 (-BR and -BL) compositions appears to better explain the variability in the

1744 samples. To further test if this model is reasonable a hybrid composition is calculated and compared with the composition of samples 1744-BP and 1877-1. The calculated hybrid for 1744-

BP is produced by combining 81.6% of 1532-BL (felsic endmember) with 18.4% 1532-BR

(mafic endmember) and compared to sample 1744-BP in Figure 21. Likewise, the 1877-1 hybrid is created by a mixture of 65.0% 1532-BR and 35.0% 1532-BL. The models provide a very good fit to observed data and support mixing to explain geochemical variation in the sample suite.

It has been proposed by Schiano et al. (2010) that all geochemical variation in rocks from other volcanoes in Ecuador (Tungarahua, Iliniza, and Pichincha) can be attributed to simple mixing without calling upon fractional crystallization. They also suggest that mixing is likely to explain magma evolution at other Ecuadorian volcanoes including Cotopaxi. Using ratios of highly incompatible/moderately incompatible vs. highly incompatible trace elements, trends 24 show a better fit with mixing trends than fractional crystallization trends (Figure 19), indicating that mixing is a dominant process in producing the variation between 1532-1877. Plotting ratios of highly incompatible and moderately incompatible trace elements vs. highly incompatible trace elements (i.e. Th/Nd vs. Th and Ba/Ce vs. Ba, Figure 19) produces positive linear trends for mixing, and sub-horizontal to moderately positive trends for fractional crystallization. Samples from this study fall along a positive linear trend, indicating that Ce and Nd are more compatible than the fractional crystallization models predict (Figure 19). This indicates that the models are still underpredicting accessory minerals such as apatite, or that the samples are being influenced by magma mixing.

For this suite, neither fractional crystallization nor mixing provides a solution for all of the samples, suggesting that both mixing and fractionation are controlling the observed variation.

The occurrence of recharge and mixing is supported by trends in whole rock chemistry, which show peaks in elements associated with more mafic magmas (MgO and Sr) (Figure 22).

Additionally, textural analysis reveals evidence of incomplete homogenization in the 1532 and

1768 samples, suggesting recent recharge. Support or fractional crystallization is derived from least squares mass balance modeling, which provide good estimations of the amount of crystallization needed to reproduce the range of chemical compositions. Finally, graphical representations of the mixing and fractional crystallization models show that both processes are relevant during this time period to explain petrologic variation and magma evolution.

A System of Fractionation and Recharge

A small-volume, closed magma system with an andesitic composition will fully crystallize within 1700-5000 years if recharge does not occur (Blake and Rogers, 2005). The production of chemically similar andesitic magmas at Cotopaxi over long time scales (>4000 25 years) and the presence of disequilibrium textures suggests that the magma system is long-lived

(Hawkesworth et al., 2004; Cooper and Kent, 2014) and experiences recharge events which replenish the thermal and chemical budget of the magma system (Tepley et al., 2000; Grogan and Reavy, 2002).

Chemical compositions are similar across all ages, which suggests the presence of a long- lived open system experiencing ongoing fractionation punctuated by recharge events and eruptions. Long-lived open andesitic systems with recharge have been documented at volcanoes such as Kamkatcha (Izbekov et al., 2004) and Nevado Sabancaya (Gerbe and Thouret, 2004).

The variations that occur within this suite are not correlated with age (i.e. no trend toward more mafic or more felsic compositions over time), which further suggests that recharge by more mafic magmas may have taken place over this period rather than sampling of an evolving magma chamber over a continuous period of fractionation. Major element concentrations show that these samples are the most mafic within the range of the other Cotopaxi andesites (Figures 10 and 11), which suggests that there may have been a recharge event prior to the 1532 eruption which mixed a new, more mafic magma into the system. This mixing event produced the mingled textures and mixed clast compositions (brown and black scoria, BR and BL, respectfully, Figure

19) that is characteristic of the 1532 deposit. Petrographic and geochemical data, such as mingled textures (Figure 6) and an increase of both MgO and Sr (Figure 21), indicate that there may have been another recharge event leading to the 1768 eruption, causing the erupted lavas to be more mafic. This is further supported by the presence of disequilibrium textures in some plagioclase phenocrysts, such as zoning and resorbed cores (Figure 5 and Table 4), and mingled magma preserved in the 1532 and 1768 samples (inclusions of dark, glassy material with few voids, Figure 5). Garrison et al. (2011) suggest there have been at least 16 recharge events over 26 the past 13.2 kyr based on fluctuations in MgO and Sr concentrations. The variations observed in the samples from this study are subtler (<1.0 wt. % MgO and <100 ppm Sr) than the variations observed by Garrison et al. (2011). Slight heterogeneities in crystal content of the sampled aliquot could cause small variations in major and minor elements. For example, if the sampled section had slightly more plagioclase, it would increase the strontium concentration and decrease the MgO concentration. Likewise, more pyroxene would result in higher MgO and lower Sr concentrations. However, the coupled increases in MgO and Sr during the 1532 and 1768 eruptions and the corresponding depletions during the 1744 and 1877 eruptions indicate that the observed variations are not solely being caused by crystal heterogeneity. When compositional variation is plotted for MgO and Sr for eruptions of this suite, there are peaks in the 1532-BR and the 1768 compositions for both MgO and Sr (Figure 21). This suggests that recharge occurred prior to both the 1532 and 1768 eruption events. The subtly of the compositional variation might indicate that the intruding magma is only slightly more mafic than the existing magma, or that the volume of the intruding magma is small compared to the existing magma.

The 1742 eruption has low concentrations of Sr, but relatively high concentrations of MgO. The decreasing concentration of MgO and Sr measured from the1768 to 1877 samples (Figure 21) may then indicate crystal fractionation of mafic phases following these recharge events. This supports the idea that fractional crystallization was ongoing in the interim periods between the recharge events.

Magma Storage Conditions

Experimental studies on volcanic systems of similar compositions can provide first-order approximations of magma storage conditions at Cotopaxi. Tungarahua, which has a very similar bulk composition and phenocryst assemblage (Samaniego et al., 2011; Hall et al., 1999), has 27

been estimated to have magma temperatures of 950-1000°C at 3 wt.% H2O (Samaniego et al.,

2011). Ridolfi et al. (2008) have constrained magma chamber depths at Reventador to 8.2-11.3 km (~ 2.3 to 3.0 kbar). For samples at 2 kbar and with oxygen fugacity between NNO+2 and

NNO+3, amphibole (which is absent in the Cotopaxi samples that are investigated in this study) has been shown to be stable at temperatures below 950°C and above water contents of 5.5 wt.%

(Martel et al., 1999). The calculated pressure and temperature conditions presented above agree with these ranges from other studies.

The orthopyroxene and glass pair thermobarometer yielded the best estimates (lowest margin of error) of pressures for the samples studied (Figure 17). Although pressure estimates are poorly constrained, they show a general trend towards lower pressure over time from 0.43

GPa (1532-BR) to less than 0.20 GPa (1877). Both pulses of the 1877 eruption show a wide range of pressures (0.38-0.08 and 0.32-0.02 GPa, respectively). Although highly speculative because of the large error (±0.26 GPa), this wide range could suggest that magma storage is a system of interconnected stacked sills and dikes, which would be conducive to recharge.

Hickey et al. (2015) used deformation and seismicity data from the 2001-2002 period of unrest to estimate magma storage depths. Their model suggests that an oblate magma body was injected at depths of 4-5 km (1-2 kbar). This agrees with the trend of decreasing pressure from

1532-1877 and could further support the idea that the magma storage system has been migrating towards shallower depths. Additionally, Riviera et al. (2017) constrained magma depths from the

2015 eruption phase using InSAR and GPS data. They determined that magma from an inclined sheet intrusion, extending from 12.1 to 5.5 km below the summit, fed the eruptions that lasted from August-November 2015. These geophysical models for recent activity reinforce the depth 28 ranges that are derived here from thermobarometric calculations of erupted lavas in this study.

Models of Evolution

From the results of modeling presented above, we can construct a chronology of events

(Figures 23 and 24). The proposed chronology shows a recharge event prior to the 1532 eruption, which produces the mafic whole rock composition and the mingled magma texture. This is followed by fractional crystallization, producing the more evolved 1742 and 1744 deposits.

Another recharge event prior to 1768 pushes the system back toward a mafic composition, resulting in the more primitive compositions and mingled textures produced during the 1768 eruption. Finally, fractional crystallization continues again to evolve the magma toward more felsic compositions and produce the 1877 lavas. Glass compositions from the 2015 ash deposits are dacitic, suggesting that the magma chamber has continued to evolve since the 1877 eruption.

The conceptual models show these processes occurring in an idealized balloon-shaped magma chamber, however in reality the system is most likely a complex system of interconnected sills and dikes, where recharge and mixing is possible between sills.

29

CHAPTER 6. CONCLUSIONS

The lavas erupted from Cotopaxi over the past 500 years are basaltic andesite, with compositions ranging from 56-59 wt. % SiO2 and 5-6 wt. % K2O + Na2O, and the samples show no continuous trend with age. The deposits contain ~15-20% phenocrysts with an assemblage of plagioclase + orthopyroxene + clinopyroxene + magnetite. The 1877 eruption exhibits a change in morphology between the first and second pulses from more fragmented scoria clasts to large

1-meter scoria bombs, respectively. This change is not accompanied by a change in chemical composition and is likely due to changes in eruptive style between early and late stages of the eruption.

Fractional crystallization and magma mixing both play a role in creating the compositional variability of lavas erupted at Cotopaxi Volcano since 1532. The 1877 and 1742 deposits have compositions that are reproduced by modeling 10-20% crystallization of the most mafic sample in the suite (1532-BR), which is supported by the presence of unzoned and pristine phenocrysts of plagioclase and pyroxene. The strong presence of disequilibrium textures such as reverse zoning, sieve textures, and mingled magmas in the 1532, and 1768 samples indicate that recharge events are occurring within the magma system. This is supported by the fit of magma mixing curves with the observed geochemical data.

Estimates of magma storage depths from Al-in-orthopyroxene thermobarometry indicate that magmas are being stored at a range of depths, from 4.3 kbar (~15 km) in 1532 to 0.17 kbar

(<1 km) in 1877, and at temperatures of ~1078°C. The range in pressure estimates for the 1877-1 and 1877-2 could indicate that magma is being stored at a range of depths as a system of dikes or sills. 30

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APPENDIX A. TABLES

Table 1. Sample characterization and summary of analyses conducted.

Sample Description Location Analysis 1532-BL Black scoria Quebrada Saquimala XRF, ICP-MS 1532-BR Brown scoria Quebrada Saquimala XRF, ICP-MS, EMPA 1742 Black scoria Quebrada Saquimala XRF, ICP-MS,EMPA 1744-BP Black pumice Quebrada Saquimala XRF, ICP-MS 1744-GP Grey pumice Quebrada Saquimala XRF, ICP-MS 1744-PF Black scoria Quebrada Saquimala XRF, ICP-MS 1744-PLUG Dense andesitic plug rock Quebrada Saquimala XRF, ICP-MS 1768 Scoria/pumice pyroclastic fall Quebrada Saquimala XRF 1877-1 (BL) Black scoria Quebrada Saquimala XRF, ICP-MS, EMPA 1877-1 (BR) Brown scoria Quebrada Saquimala XRF, ICP-MS 1877-2 (1) Scoria megaclast from top of flow W. of Ingaloma XRF, ICP-MS 1877-2 (2) Scoria megaclast from top of flow W. of Ingaloma XRF, ICP-MS 1877-2 (3) Scoria megaclast from top of flow W. of Ingaloma XRF, ICP-MS, EMPA 1877-2 (4) Scoria megaclast from top of flow W. of Ingaloma XRF, ICP-MS 1877-2 (5) Scoria megaclast from top of flow W. of Ingaloma XRF, ICP-MS 1877-2 (6) Scoria megaclast from top of flow W. of Ingaloma XRF, ICP-MS, EMPA 1877-2 (7) Scoria megaclast from top of flow W. of Ingaloma XRF, ICP-MS 1877-2 (8) Scoria megaclast from top of flow W. of Ingaloma XRF, ICP-MS 1877-2 (9) Scoria megaclast from top of flow W. of Ingaloma XRF, ICP-MS 1877-2 (10) Scoria megaclast from top of flow W. of Ingaloma XRF, ICP-MS, EMPA XRF = X-ray fluorescence, ICP-MS = Inductively coupled plasma mass spectroscopy, EMPA = Electron microprobe analysis. Samples here are referred to by the year they were erupted (e.g. samples from the 1532 CE eruption are referred to as 1532, etc., and samples from the first and second pulses of the 1877 eruption are referred to as 1877-1 and 1877-2, respectively). 36

Table 2. Summary of petrography

Sample GM % Vesicles Phenocrysts % Pheno Plag Textures Mingling 1532-BL Hypohy. 25 Plag > Opx > Cpx + Mt 10-15 Strong resorption Yes 1532-BR* Holohy. 40 Plag > Opx + Mt 10-12 Strong resorption Yes 1742* Holohy. 30 Plag > Opx > Cpx 15-20 Resorption, Oscillatory Zoning No 1744-BP Hypohy. 20 Plag > Opx > Cpx + Mt 10-12 Strong resorption, Oscillatory zoning No 1744-GP Hypocry. 40 Plag + Opx + Mt 15-20 Oscillatory Zoning No 1744-PF Hypocry. 5 Plag >> Cpx > Opx 50 Resorption, Oscillatory Zoning No 1744-Plug Hypocry. 0 Plag > Mt > Cpx > Opx 40 Moderate resorption No 1768 hypohy. 45 Plag > Opx = Cpx + Mt 5-10 Resorption Yes 1877-1* Hypohy. 25 Plag > Opx > Cpx + Mt 10-15 Mild resoprtion No 1877-2* Hypohy. 30 Plag > Opx > Cpx + Mt 10-15 Resorption, Oscillatory Zoning No * Indicates sample that was analyzed by EMPA GM = Groundmass; Pheno = phenocryst; Plag = plagioclase; Opx = orthopyroxene; Cpx= clinopyroxene Mt = magnetite; Hypohy = hypohyaline; Holohy = holohyaline; Hypocry = hypocrystalline 37

Table 3. Representative mineral chemistry for plagioclase and pyroxene phenocrysts from electron microprobe analysis. Sample 1532 1532 1877-1 1877-2 1877-2 1877-2 1877-2 analysis rim core core interm. rim core core plagioclase

SiO2 51.59 47.86 53.12 52.37 55.60 49.89 52.15 TiO 0.04 0.00 0.04 0.05 0.03 0.04 0.04

Al2O3 28.63 31.78 27.52 28.81 26.74 30.28 29.73

Fe2O3 0.71 0.12 0.30 FeO 0.00 0.42 0.35 0.74 0.54 0.56 0.68 MnO 0.07 0.02 0.01 0.00 0.00 0.00 0.06 MgO 0.10 0.04 0.06 0.07 0.06 0.03 0.10 CaO 12.98 15.54 11.36 12.68 9.95 14.20 12.70

Na2O 4.02 2.38 4.77 4.51 5.79 3.01 4.21

K2O 0.14 0.08 0.28 0.18 0.29 0.11 0.14

P2O5 0.03 0.04 0.00 0.00 Total 98.29 98.25 97.82 99.43 99.05 98.11 99.81 Structural formulae on the basis of 8 oxygens Si 2.388 2.232 2.459 2.389 2.529 2.325 2.372 Ti 0.002 0.000 0.001 0.002 0.001 0.001 0.001 Al 1.562 1.747 1.502 1.548 1.433 1.663 1.594 Fe+3 0.026 0.004 0.011 0.028 0.021 0.000 0.026 Fe+2 0.000 0.016 0.014 0.000 0.000 0.022 0.000 Mn 0.003 0.001 0.000 0.000 0.000 0.000 0.002 Mg 0.007 0.003 0.004 0.005 0.004 0.002 0.007 Ca 0.644 0.777 0.563 0.619 0.485 0.709 0.619 Na 0.361 0.216 0.428 0.398 0.510 0.272 0.372 K 0.008 0.005 0.016 0.011 0.017 0.007 0.008 An 63.556 77.915 55.878 60.230 47.904 71.775 61.945 Ab 35.607 21.622 42.489 38.744 50.430 27.536 37.213 Or 0.837 0.464 1.633 1.026 1.666 0.689 0.842 An = (Ca/(Ca + Na + K))*100 Ab = (Na/(Ca + Na + K))*100 Or = (K/(Ca + Na + K))*100 Fe 2+ is measured by microprobe, and Fe3+ is calculated using charge balance. All values reported as wt. %. 38

Table 3. (continued) Sample 1532 1742 1877-2 1877-2 1532 1742 1877-1 analysis rim rim core rim rim core rim clinopyroxene orthopyroxene

SiO2 51.36 50.64 51.99 51.76 52.57 54.37 54.05

TiO2 0.68 0.74 0.55 0.46 0.26 0.22 0.30

Al2O3 2.92 3.42 2.20 3.43 2.98 1.43 1.84

Cr2O3 0.00 -0.01 -0.01 0.05 -0.01 0.03 0.00

Fe2O3 0.86 1.48 0.45 0.70 0.38 0.00 0.00 FeO 8.13 8.47 8.97 7.14 15.45 17.40 16.38 MnO 0.20 0.16 0.22 0.15 0.40 0.41 0.43 MgO 14.55 14.30 15.08 15.31 25.14 25.10 25.47 CaO 20.52 19.54 19.57 20.30 1.51 1.54 1.67

Na2O 0.32 0.44 0.33 0.37 0.11 0.05 0.01 Total 99.53 99.19 99.36 99.67 98.80 100.56 100.15

Structual formulae on the basis of 6 oxygens Si 1.916 1.899 1.942 1.915 1.927 1.971 1.960 Ti 0.019 0.021 0.016 0.013 0.007 0.006 0.008 Al 0.128 0.151 0.097 0.149 0.129 0.061 0.079 Cr 0.000 0.000 0.000 0.001 0.000 0.001 0.000 Fe+3 0.024 0.042 0.013 0.020 0.010 0.000 0.000 Fe+2 0.254 0.266 0.280 0.221 0.473 0.528 0.497 Mn 0.006 0.005 0.007 0.005 0.012 0.013 0.013 Mg 0.809 0.799 0.840 0.845 1.374 1.357 1.377 Ca 0.820 0.785 0.783 0.805 0.059 0.060 0.065 Na 0.023 0.032 0.024 0.027 0.008 0.004 0.001 Wo 43.01 41.50 40.89 42.59 3.10 3.08 3.34 En 42.42 42.24 43.83 44.69 71.66 69.79 71.03 Fs 14.57 16.25 15.29 12.72 25.24 27.13 25.62 Wo = (Ca/Ca + Mg + Fe(total))*100 En = (Mg/Ca + Mg + Fe(total))*100 Fs = (Fe(total)/Ca + Mg + Fe(total))*100 Fe 2+ is measured by microprobe, and Fe3+ is calculated using charge balance. All values reported as wt. %. 39

Table 4. Core and rim compositions of zoned plagioclase phenocrysts.

Reverse Normal

Sample AnRim AnCore Sample AnRim AnCore 1532 71.84 70.43 1532 67.12 74.93 1877-1 46.83 21.99 1532 63.56 77.91 1877-1 66.70 55.88 1877-2 56.25 71.78 1877-2 58.12 47.25 1877-2 51.71 53.05 1877-1 57.49 55.00 1877-2 57.53 65.06

Complex

Sample AnRim Intermediate AnCore 1877-1 42.75 61.96 60.56 1877-2 60.92 60.23 55.25 59.67 57.40 40

Table 5. Glass compositions from electron microprobe analysis.

1532-1 1532-2 1532 -3 1532-4 1532-5 1742-1 1742-2 1742-3 1742-4

SiO2 54.88 60.60 60.32 61.34 60.33 60.90 56.28 66.10 61.61

TiO2 0.29 0.78 0.73 0.78 0.72 1.02 0.97 1.36 1.24

Al2O3 24.70 16.45 16.35 16.00 16.36 16.11 12.51 13.45 13.65 FeO 2.04 5.01 6.09 5.51 4.61 5.57 10.85 4.52 7.21 MgO 0.85 1.82 3.29 3.22 2.00 0.68 6.21 0.43 2.61 MnO 0.05 0.05 0.11 0.16 0.19 0.24 0.33 0.00 0.11 CaO 10.40 5.06 4.92 4.75 5.08 4.66 6.52 2.22 4.39

Na2O 4.21 4.84 4.69 4.42 4.73 4.92 3.62 4.33 4.41

K2O 0.70 2.07 1.99 2.15 1.90 2.53 1.69 4.40 2.64

P2O5 0.05 0.14 0.25 0.22 0.25 0.37 0.29 0.55 0.42 Total 98.19 96.94 98.83 98.62 96.27 97.10 99.35 97.53 98.41

1742-5 1742-6 1877-1-1 1877-1-2 1877-1-3 1877-1-4 1877(3)-2 1877(3)-3 1877(3)-4

SiO2 58.62 64.20 55.92 61.27 61.80 61.78 62.29 62.68 66.87

TiO2 0.80 1.22 0.07 1.11 0.78 0.93 0.51 0.79 0.86

Al2O3 16.15 12.81 25.81 14.07 16.41 15.26 18.83 17.90 15.68 FeO 5.84 6.71 0.68 8.19 4.42 6.20 3.43 3.07 3.40 MgO 2.97 1.62 0.08 2.72 1.44 2.27 0.78 0.34 0.61 MnO 0.12 0.26 -0.11 0.20 -0.02 0.15 0.04 0.03 -0.01 CaO 5.68 3.46 9.41 3.99 5.18 4.52 5.15 5.21 3.09

Na2O 4.62 3.95 5.99 4.87 4.90 4.40 5.97 5.43 4.47

K2O 1.82 3.81 0.44 2.10 2.03 2.73 1.74 1.90 3.30

P2O5 0.30 0.43 -0.01 0.46 0.40 0.31 0.24 0.38 0.47 Total 96.99 98.55 98.28 99.09 97.42 98.62 98.97 97.74 98.73

1877(3)-5 1877(3)-6 1877(3)-7 1877(6)-2 1877(6)-5 1877(6)-6 1877(6)-7 1877(10)-1 1877(10)-2

SiO2 62.88 65.45 64.31 65.11 60.08 63.42 64.03 64.59 65.16

TiO2 0.80 1.67 0.78 1.20 1.06 1.20 0.81 0.88 1.20

Al2O3 15.92 11.00 17.84 15.32 14.93 16.02 13.93 17.01 14.99 FeO 5.55 9.06 2.46 4.74 7.65 5.52 6.96 3.44 4.13 MgO 1.90 0.91 0.49 0.55 3.04 0.61 3.54 0.36 0.43 MnO 0.18 0.24 0.03 0.06 0.08 0.08 0.15 0.02 0.01 CaO 4.89 3.75 4.86 4.01 3.90 4.71 3.74 4.33 3.80

Na2O 5.07 2.96 5.28 4.48 4.21 4.70 3.87 5.26 4.82

K2O 1.88 2.95 2.15 2.63 2.05 2.31 2.57 2.52 2.92

P2O5 0.33 0.67 0.29 0.55 0.31 0.47 0.44 0.42 0.53 Total 99.39 98.67 98.49 98.65 97.31 99.04 100.03 98.82 98.00

All iron is reported as Fe2+, and all values are reported as wt.%. (#) indicates sample number and -# indicates analysis point. 41

Table 5 (continued). Glass compositions from electron microprobe analysis.

1877(10)-3 1877(10)-4 1877(10)-5 1877(10)-6 1877(10)-7 1877(10)-8 1877(10)-10

SiO2 65.47 64.51 62.85 64.33 65.80 63.16 62.08

TiO2 1.14 1.06 0.98 0.77 1.19 0.72 0.70

Al2O3 16.04 16.28 17.22 16.65 14.72 16.46 16.77 FeO 3.55 3.92 4.03 3.23 4.17 5.23 5.15 MgO 0.38 0.37 0.41 0.55 0.39 1.83 1.80 MnO 0.14 -0.04 0.14 0.06 0.07 0.12 0.08 CaO 3.92 4.27 4.94 4.25 3.75 5.20 5.52

Na2O 4.81 4.97 5.55 5.13 4.52 4.72 4.90

K2O 2.77 2.65 2.02 2.69 3.34 2.15 2.03

P2O5 0.54 0.49 0.35 0.41 0.53 0.32 0.25 Total 98.75 98.47 98.50 98.07 98.48 99.91 99.28

All iron is reported as Fe2+, and all values are reported as wt.%. (#) indicates sample number and -# indicates analysis point. 42

Table 6. Major, minor and trace element analyses of whole rock.

Sample 1877-2(1) 1877-2(2) 1877-2(3) 1877-2(4) 1877-2(5) 1877-2(6) 1877-2(7) 1877-2(8) 1877-2(9) 1877-2(10)

SiO2 57.10 57.09 57.23 57.04 57.14 56.96 57.00 57.13 57.06 57.23

TiO2 0.81 0.80 0.82 0.80 0.80 0.81 0.79 0.81 0.82 0.81

Al2O3 17.60 17.70 17.64 17.59 17.68 17.59 17.53 17.49 17.37 17.67

Fe2O3(Total) 7.60 7.53 7.59 7.54 7.49 7.60 7.66 7.62 7.66 7.52 MgO 3.59 3.66 3.65 3.70 3.60 3.71 3.81 3.72 3.74 3.58 MnO 0.11 0.11 0.11 0.11 0.11 0.11 0.11 0.11 0.11 0.11 CaO 6.89 6.95 6.92 6.95 6.92 6.92 7.01 6.95 6.86 6.90

Na2O 3.92 3.85 3.92 3.90 3.92 3.91 3.86 3.84 3.87 3.90

K2O 1.41 1.39 1.41 1.39 1.40 1.38 1.38 1.39 1.41 1.42

P2O5 0.24 0.23 0.24 0.23 0.24 0.24 0.23 0.24 0.24 0.24 Total 99.27 99.32 99.52 99.24 99.29 99.23 99.38 99.29 99.12 99.38

P 1137 1149 1135 1128 1120 1141 1124 1129 1141 1141 Cr 20.65 23.26 21.50 23.73 21.54 22.77 23.55 24.12 23.82 19.94 Mn 699 721 693 720 682 717 707 702 725 692 Co 145.0 136.9 96.1 135.3 99.1 92.5 92.4 90.8 93.0 93.7 Ni 18.07 17.49 16.14 17.69 16.56 16.67 16.78 16.55 17.17 16.12 Cu 2139 2196 2308 2266 2191 2236 2243 2323 2343 2296 Zn 115.4 120.3 116.2 109.9 108.1 108.2 118.0 111.1 127.4 114.4 Rb 29.57 29.84 29.50 30.17 29.62 30.27 28.95 30.10 30.31 30.25 Sr 592 624 604 624 608 630 609 616 617 611 Y 11.92 11.94 11.88 11.99 11.56 12.10 11.75 11.68 12.06 11.87 Zr 116.0 117.5 118.2 118.6 118.3 121.0 117.8 116.9 121.5 123.9 Nb 4.49 4.53 4.55 4.59 4.60 4.69 4.64 4.56 4.79 4.65 Cs 1.16 1.16 1.15 1.13 1.12 1.13 1.12 1.14 1.16 1.17 Ba 637 638 630 642 637 647 636 653 655 671 La 16.35 16.53 16.23 16.16 16.12 16.16 16.01 16.49 16.52 16.80 Ce 35.22 35.66 35.31 34.48 35.05 35.28 35.13 35.79 36.27 36.47 Pr 4.33 4.33 4.30 4.23 4.24 4.26 4.25 4.28 4.38 4.36 Nd 17.99 18.20 17.70 17.77 17.54 17.79 17.70 17.65 18.29 18.21 Sm 3.80 3.83 3.72 3.69 3.69 3.74 3.72 3.73 3.82 3.78 Eu 1.17 1.17 1.15 1.15 1.15 1.16 1.15 1.16 1.17 1.17 Gd 3.56 3.51 3.49 3.43 3.44 3.53 3.44 3.51 3.54 3.50 Tb 0.52 0.52 0.52 0.51 0.50 0.50 0.51 0.51 0.52 0.52 Dy 2.74 2.75 2.70 2.62 2.63 2.68 2.66 2.68 2.73 2.69 Ho 0.56 0.55 0.55 0.54 0.54 0.55 0.54 0.55 0.55 0.54 Er 1.49 1.49 1.46 1.44 1.42 1.45 1.45 1.48 1.48 1.45 Tm 0.22 0.22 0.22 0.22 0.22 0.22 0.22 0.22 0.22 0.22 Yb 1.29 1.29 1.28 1.26 1.24 1.26 1.26 1.27 1.29 1.27 Lu 0.20 0.19 0.20 0.19 0.19 0.19 0.19 0.19 0.20 0.19 Hf 3.16 3.19 3.10 3.11 3.08 3.12 3.09 3.10 3.17 3.18 Ta 0.30 0.30 0.29 0.29 0.29 0.28 0.29 0.29 0.29 0.29 Pb 9.13 6.65 6.39 6.72 7.08 6.10 6.37 6.32 6.65 6.76 Bi 0.03 0.00 0.00 0.03 -0.02 0.02 0.01 -0.01 0.06 0.01 Th 3.35 3.34 3.31 3.27 3.28 3.34 3.26 3.29 3.38 3.37 U 1.26 1.24 1.25 1.22 1.22 1.22 1.19 1.23 1.25 1.25 All iron reported as Fe3+, major elements reported as wt.%, trace elements reported as ppm. Major and trace elements determined by XRF and ICP-MS, respectively. Paranthetical numbers (i.e. 1877-2(1)) indicate sample number. 43

Table 6 (continued). Major, minor and trace element analyses of whole rock.

Sample 1532-BL 1532-BR 1742 1744-GP 1744-PF 1744-BP 1744-PLG 1768 1877-BL 1877-BR

SiO2 58.72 55.97 56.56 56.66 56.78 58.11 57.17 55.62 56.93 56.89

TiO2 0.80 0.84 0.82 0.78 0.83 0.69 0.82 0.83 0.81 0.80

Al2O3 17.96 17.55 17.64 17.41 17.99 17.53 17.81 17.43 17.64 17.40

Fe2O3(Total) 7.10 7.98 7.80 7.68 7.74 7.28 7.66 7.87 7.57 7.71 MgO 3.15 3.95 3.93 3.65 3.44 3.25 3.60 4.03 3.68 3.84 MnO 0.10 0.12 0.11 0.12 0.11 0.12 0.11 0.12 0.11 0.12 CaO 6.31 6.99 6.98 6.67 6.85 6.42 6.89 6.89 6.86 6.95

Na2O 4.05 3.93 3.87 3.87 3.92 3.97 3.88 3.86 3.90 3.88

K2O 1.60 1.28 1.41 1.28 1.44 1.36 1.43 1.26 1.40 1.38

P2O5 0.22 0.25 0.21 0.23 0.21 0.23 0.21 0.26 0.23 0.22 Total 100.01 98.86 99.33 98.36 99.29 98.95 99.58 98.16 99.14 99.19

P 1100 1202 1039 1153 991 1251 987 1110 1089 Cr 9.87 36.89 28.53 33.28 12.33 35.57 16.98 20.65 27.12 Mn 686 728 715 754 683 827 715 694 719 Co 75.4 110.3 101.6 207.2 60.2 92.2 70.9 74.9 74.3 Ni 8.46 18.09 15.22 17.66 8.93 14.29 9.66 14.94 15.79 Cu 2215 2308 2329 2183 2215 2214 2350 2212 2239 Zn 112.0 120.2 135.1 162.3 105.3 138.4 131.4 119.6 151.2 Rb 35.89 23.76 31.74 25.80 32.46 31.60 32.54 29.34 28.77 Sr 611 648 578 607 591 644 592 602 586 Y 12.19 11.28 11.57 11.22 11.43 13.26 11.59 11.40 11.40 Zr 130.7 109.7 113.5 107.8 109.6 134.1 110.0 116.7 113.8 Nb 5.11 4.57 4.24 4.22 4.09 4.90 4.33 4.53 4.41 Cs 1.43 0.90 1.22 1.01 1.26 1.27 1.07 1.17 1.16 Ba 748 617 679 619 688 724 697 673 653 La 18.14 15.50 15.97 14.69 15.76 16.35 15.10 16.67 15.83 Ce 39.65 35.31 35.05 33.12 34.24 36.85 33.76 36.98 35.33 Pr 4.62 4.29 4.15 4.00 4.10 4.49 4.03 4.36 4.21 Nd 18.79 18.18 17.25 16.93 17.13 18.83 16.70 18.22 17.51 Sm 3.84 3.85 3.72 3.61 3.62 4.04 3.64 3.88 3.73 Eu 1.17 1.20 1.16 1.12 1.15 1.26 1.15 1.16 1.14 Gd 3.53 3.54 3.49 3.35 3.43 3.75 3.38 3.44 3.43 Tb 0.51 0.52 0.51 0.49 0.50 0.56 0.50 0.51 0.51 Dy 2.70 2.66 2.66 2.57 2.63 2.98 2.62 2.62 2.67 Ho 0.55 0.54 0.55 0.52 0.54 0.61 0.53 0.53 0.54 Er 1.47 1.42 1.45 1.39 1.45 1.64 1.43 1.43 1.44 Tm 0.22 0.21 0.22 0.21 0.22 0.25 0.22 0.22 0.22 Yb 1.29 1.20 1.26 1.22 1.27 1.46 1.28 1.25 1.28 Lu 0.20 0.19 0.19 0.19 0.19 0.23 0.19 0.19 0.19 Hf 3.41 2.90 3.03 2.89 2.91 3.47 2.94 3.09 3.05 Ta 0.33 0.28 0.27 0.28 0.26 0.32 0.28 0.29 0.28 Pb 7.41 5.64 6.87 6.14 4.71 7.00 6.61 6.64 6.26 Bi 0.13 0.14 0.15 0.28 0.01 0.28 0.16 0.10 0.13 Th 4.01 2.56 3.59 2.63 3.67 3.17 3.51 3.37 3.30 U 1.48 0.92 1.22 1.01 1.26 1.25 1.24 1.23 1.20 All iron reported as Fe3+, major elements reported as wt.%, trace elements reported as ppm. Major and trace elements determined by XRF and ICP-MS, respectively. Paranthetical numbers (i.e. 1877-2(1)) indicate sample number. 44

Table 7. Least-squares mass balance fractionation solution.

Model A

1532-BR = 0.772 (1532-BL) + 0.124 Plag (An47) + 0.024 Opx (En72) + 0.069 Hbl + 0.011 Mt

SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2OK2OP2O5 NiO Cr2O3 Daughter 59.13 0.81 18.09 6.44 0.10 3.17 3.36 4.08 1.61 0.22 0.00 0.00 Parent 57.08 0.86 17.89 7.32 0.12 4.03 7.13 4.01 1.30 0.25 0.00 0.00 Calculated (P) 57.12 0.88 17.90 7.32 0.09 4.04 7.13 4.02 1.31 0.18 0.00 0.00 Difference -0.01 -0.02 0.00 0.00 0.03 -0.01 0.00 -0.01 -0.01 0.08 0.00 0.00 Sum of Squares of Residuals = 0.008 Trace Element Solutions: Rb Sr Ba Zr Ni Cr Nb Hf Ta Ce Nd Sm D 0.11 0.91 0.05 0.05 0 4.57 0 0.06 0.05 0.18 0.21 0.23 Observed (P) 24.0 653.6 622.2 110.6 18.2 37.2 4.61 2.92 0.28 35.6 18.3 3.88 Calculated (P) 28.7 601.7 588.5 103.0 6.58 25 3.98 2.69 0.26 32.3 15.4 3.17 Difference -4.8 51.9 33.6 7.68 11.7 12.2 0.63 0.23 0.03 3.29 2.88 0.72 1532BL-1(D) 36.2 615.7 753.1 131.6 8.52 9.94 5.15 3.43 0.33 39.9 18.9 3.87

Yb Pb Th U La Eu Tb Lu Dy D 0.36 0.36 0.07 0.01 0.12 0.25 0.34 0.32 0 Observed (P) 1.21 5.68 2.58 0.93 15.6 1.1 0.52 0.19 2.75 Calculated (P) 1.10 6.33 3.18 1.15 14.6 0.86 0.43 0.17 2.18 Difference 0.11 -0.65 -0.60 -0.22 1.07 0.23 0.09 0.02 0.57 1532BL-1(D) 1.30 7.47 4.04 1.49 18.3 1.04 0.51 0.2 2.82

Bulk distribution coefficients (D) calculated using published Kd values (see Table 7). All values are calculated as 100% volatile free, all Fe as FeO. Abbreviations: Plag = plagioclase;Opx = orthopyroxene; Hbl = hornblende; Mt = magnetite; P = Parent; D = Daughter. Hbl and Mt compositions from Garrison et al., 2011. 45

Table 7. (continued) Least-squares mass balance fractionation solution.

Model B

1532-BR = 0.817 (1532-BL) + 0.098 Plag (An57) + 0.008 Cpx (En44) + 0.052 Hbl + 0.011 Mt + 0.014 Ol

SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2OK2OP2O5 NiO Cr2O3 Daughter 59.13 0.81 18.09 6.44 0.10 3.17 3.36 4.08 1.61 0.22 0.00 0.00 Parent 57.08 0.86 17.89 7.32 0.12 4.03 7.13 4.01 1.30 0.25 0.00 0.00 Calculated (P) 57.10 0.91 17.98 7.31 0.09 4.04 7.13 3.89 1.36 0.19 0.00 0.00 Difference -0.02 -0.05 -0.09 0.01 0.03 -0.01 0.00 0.12 -0.06 0.06 0.00 0.00 Sum of Squares of Residuals = 0.029 Trace Element Solutions: Rb Sr Ba Zr Ni Cr Nb Hf Ta Ce Nd Sm D 0.2 1.08 0 0.05 3.42 5.11 1.28 0.21 0.63 0.15 0.04 0.48 Observed (P) 24 653.6 622.2 110.6 18.2 37.2 4.61 2.92 0.28 35.6 18.3 3.88 Calculated (P) 30.8 625.3 616.2 108.7 13.9 22.8 5.45 2.92 0.31 33.6 15.6 3.48 Difference -6.84 28.3 5.99 1.98 4.35 14.4 -0.84 -0.01 -0.02 1.97 2.75 0.4 1532BL-1(D) 36.2 615.7 753.1 131.6 8.52 9.94 5.15 3.43 0.33 39.9 18.9 3.87

Yb Pb Th U La Eu Tb Lu Dy D 0.44 0.38 0.16 0.04 0.08 0.48 0.51 0.4 0 Observed (P) 1.21 5.68 2.58 0.93 15.6 1.1 0.52 0.19 2.75 Calculated (P) 1.16 6.58 3.41 1.23 15.2 0.94 0.47 0.18 2.31 Difference 0.05 -0.9 -0.83 -0.3 0.44 0.15 0.05 0.01 0.45 1532BL-1(D) 1.3 7.47 4.04 1.49 18.3 1.04 0.51 0.2 2.82

Bulk distribution coefficients (D) calculated using published Kd values (see Table 7). All values are calculated as 100% volatile free, all Fe as FeO. Abbreviations: Plag = plagioclase; Cpx = clinopyroxene; Hbl = hornblende; Mt = magnetite; Ol = olivine P = Parent; D = Daughter. Hbl and Mt compositions from Garrison et al., 2011. Olivine composition is average of subduction zone andesites (GeoRoc) 46

Table 7. (continued) Least-squares mass balance fractionation solution.

Model C

1532-BR = 0.826 (1532-BL) + 0.102 Plag (An47) + 0.035 Cpx (En44) + 0.016 Mt + 0.020 Ol + 0.001 Ap

SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2OK2OP2O5 NiO Cr2O3 Daughter 59.13 0.81 18.09 6.44 0.10 3.17 3.36 4.08 1.61 0.22 0.00 0.00 Parent 57.08 0.86 17.89 7.32 0.12 4.03 7.13 4.01 1.30 0.25 0.00 0.00 Calculated (P) 57.02 0.96 17.84 7.29 0.10 4.04 7.13 4.04 1.36 0.24 0.00 0.00 Difference 0.03 -0.10 0.03 0.03 0.02 0.00 0.00 -0.03 -0.06 0.02 0.00 0.00 Sum of Squares of Residuals = 0.018 Trace Element Solutions: Rb Sr Ba Zr Ni Cr Nb Hf Ta Ce Nd Sm D 0.13 1.12 0.02 0.06 2.2 5.99 0.02 0.11 0.11 0.29 0.41 0.67 Observed (P) 24.0 653.6 622.2 110.6 18.2 37.2 4.61 2.92 0.28 35.6 18.3 3.88 Calculated (P) 30.6 629.5 624.5 110.1 10.7 25.8 4.27 2.89 0.28 34.8 16.9 3.63 Difference -6.60 24.10 -2.30 0.50 7.50 11.40 0.34 0.03 0.00 0.80 1.40 0.25 1532BL-1(D) 36.2 615.7 753.1 131.6 8.52 9.94 5.15 3.43 0.33 39.9 18.9 3.87

Yb Pb Th U La Eu Tb Lu Dy D 0.6 0.16 0.14 0.12 0.16 0.51 0.75 0.5 0.25 Observed (P) 1.21 5.68 2.58 0.93 15.6 1.1 0.52 0.19 2.75 Calculated (P) 1.20 6.36 3.43 1.26 15.6 0.95 0.49 0.18 2.45 Difference 0.01 -0.68 -0.85 -0.33 0.00 0.15 0.03 0.01 0.30 1532BL-1(D) 1.30 7.47 4.04 1.49 18.3 1.04 0.51 0.2 2.82

Bulk distribution coefficients (D) calculated using published Kd values (see Table 7). All values are calculated as 100% volatile free, all Fe as FeO. Abbreviations: Plag = plagioclase; Opx = orthopyroxene; Hbl = hornblende; Mt = magnetite; Ol = olivine; Ap = apatite; P = Parent; D = Daughter. Hbl, Mt, and Ap compositions from Garrison et al., 2011. Olivine composition is average of subduction zone andesites (GeoRoc) 47

Table 8. Partition coefficient values used for least squares modeling.

el/min PL CPX OPX OL HB MT AP Rb 0.2 0.03 0.01 0.022 0.33 -- -- Sr 1.67 0.5 0.01 0.31 0.48 -- -- Ba -- 0.1 0.1 -- -- 0.12 -- Zr -- 0.29 0.11 0.016 -- -- 0.636 Ni ------19 6.8 9.6 -- Cr -- 30 10 -- 12.5 93 -- V ------3.4 8.7 -- Nb ------0.11 ------Hf 0.02 0.46 0.11 -- 0.5 0.3 0.73 Ta--0.50.11------Ce 0.07 0.47 0.31 -- 0.32 0.12 21.1 Nd -- 0.86 0.47 -- -- 0.25 32.8 Sm 0.03 1.6 0.46 -- 1.4 0.29 46 Yb 0.02 2 0.77 0.64 1.2 0.24 15.4 Pb ------0.43 ------Th 0.01 0.1 0.14 -- 0.5 0.05 -- U 0.01 ------0.1 -- -- Co -- 5.5 12 -- 2 -- -- Cs -- 0.01 0.01 -- -- 0.39 -- Sc -- 17 4.3 -- -- 1.7 -- Zn -- 2 3.7 10 0.4 5.4 -- La -- 0.28 0.26 -- 0.25 0.22 14.5 Eu 0.18 1.1 0.34 -- 1.2 0.22 25.5 Tb 0.05 2.7 0.69 -- 1.3 0.37 -- Lu -- 2 0.71 -- 1.1 0.32 13.8 Dy ------0.44 34.8 Li 0.3 ------0.3 -- -- Cu ------0.05 -- -- Fe ------26.9 ------Gd ------43.9 Er ------22.7 PLAG = plagioclase (Dostal et al., 1983); CPX = clinopyroxene (Bacon & Druitt, 1988); OPX = orthopyroxene (Bacon & Druitt (1988); HB = hornblende (Dostal et al., 1983); OL = olivine (Ewart & Griffin, 1994); MT = magnetite (Luhr & Carmichael, 1980); AP = apatite (Fujimaki, 1986). 48

APPENDIX B. MICROPROBE ANALYSIS OF PLAGIOCLASE AND PYROXENE COMPOSITIONS.

Plagioclase Analysis Sample 1532 1532 1532 1532 1532 1532 1877-1 1877-1 1877-1 1877-1 1877-1 analysis rim core rim core rim core rim core rim core

SiO2 49.32 49.22 49.91 48.61 51.59 47.86 51.28 53.12 56.67 51.54 51.78 TiO 0.01 0.07 0.05 0.01 0.04 0.00 0.04 0.04 0.05 0.03 0.03

Al2O3 30.18 30.02 29.59 30.66 28.63 31.78 28.92 27.52 25.11 28.30 28.09

Fe2O3 0.55 0.39 0.68 0.79 0.71 0.12 0.00 0.30 0.84 0.96 0.32 FeO 0.00 0.00 0.00 0.00 0.00 0.42 0.42 0.35 0.00 0.00 0.36 MnO 0.10 0.03 0.00 0.05 0.07 0.02 0.10 0.01 -0.06 -0.03 -0.01 MgO 0.08 0.08 0.10 0.08 0.10 0.04 0.06 0.06 0.05 0.10 0.08 CaO 14.57 14.19 13.56 15.16 12.98 15.54 13.47 11.36 8.88 12.56 12.24

Na2O 3.09 3.22 3.60 2.76 4.02 2.38 3.61 4.77 6.26 4.13 4.26

K2O 0.10 0.11 0.11 0.06 0.14 0.08 0.17 0.28 0.46 0.19 0.22

P2O5 Total 97.99 97.33 97.59 98.18 98.29 98.25 98.08 97.82 98.27 97.79 97.36 Structural formulae on the basis of 8 oxygens Si 2.299 2.307 2.329 2.267 2.388 2.232 2.383 2.459 2.594 2.397 2.415 Ti 0.000 0.003 0.002 0.000 0.002 0.000 0.002 0.001 0.002 0.001 0.001 Al 1.658 1.658 1.627 1.685 1.562 1.747 1.584 1.502 1.354 1.551 1.544 Fe+3 0.020 0.014 0.025 0.029 0.026 0.004 0.000 0.011 0.030 0.035 0.012 Fe+2 0.000 0.000 0.000 0.000 0.000 0.016 0.016 0.014 0.000 0.000 0.014 Mn 0.004 0.001 0.000 0.002 0.003 0.001 0.004 0.000 -0.002 -0.001 -0.001 Mg 0.006 0.006 0.007 0.005 0.007 0.003 0.004 0.004 0.006 0.008 0.006 Ca 0.728 0.712 0.678 0.757 0.644 0.777 0.671 0.563 0.435 0.626 0.611 Na 0.279 0.293 0.326 0.250 0.361 0.216 0.325 0.428 0.556 0.373 0.385 K 0.006 0.007 0.007 0.004 0.008 0.005 0.010 0.016 0.027 0.011 0.013 An 71.841 70.429 67.122 74.925 63.5558 77.9146 66.695 55.8775 42.754 61.963 60.560 Ab 27.572 28.921 32.230 24.721 35.607 21.622 32.328 42.489 54.587 36.911 38.132 Or 0.587 0.650 0.648 0.354 0.837 0.464 0.977 1.633 2.659 1.126 1.308 An = (Ca/(Ca + Na + K))*100 Ab = (Na/(Ca + Na + K))*100 Or = (K/(Ca + Na + K))*100 Fe 2+ is measured by microprobe, and Fe3+ is calculated using charge balance. All values reported as wt. %. 49

Plagioclase Analysis Sample 1877-1 1877-1 1877-2 1877-2 1877-2 1877-2 1877-2 1877-2 1877-2 1877-2 1877-2 analysis rim rim rim core core rim rim core

SiO2 53.02 52.59 51.38 52.37 53.48 51.91 52.47 50.67 53.02 55.60 52.21 TiO 0.03 0.07 0.04 0.05 0.05 0.03 0.00 0.05 0.04 0.03 0.04

Al2O3 27.29 27.42 28.79 28.81 27.93 28.13 28.28 29.25 28.42 26.74 28.59

Fe2O3 0.63 0.00 FeO 0.00 0.76 0.80 0.74 0.65 0.60 0.59 0.65 0.77 0.54 0.69 MnO -0.04 -0.03 0.08 0.00 0.00 0.03 0.00 0.00 0.02 0.00 0.03 MgO 0.08 0.10 0.11 0.07 0.04 0.09 0.07 0.09 0.11 0.06 0.12 CaO 11.40 11.68 12.53 12.68 11.47 12.09 11.84 13.22 11.77 9.95 11.92

Na2O 4.99 4.46 4.33 4.51 5.00 4.40 4.74 3.83 4.68 5.79 5.01

K2O 0.26 0.22 0.17 0.18 0.20 0.18 0.18 0.15 0.19 0.29 0.17

P2O5 0.00 0.03 0.00 0.02 0.02 0.02 0.05 0.04 0.03 Total 97.66 97.26 98.22 99.43 98.83 97.45 98.18 97.93 99.06 99.05 98.82 Structural formulae on the basis of 8 oxygens Si 2.454 2.455 2.373 2.389 2.447 2.416 2.418 2.354 2.426 2.529 2.386 Ti 0.002 0.001 0.002 0.002 0.002 0.001 0.000 0.002 0.001 0.001 0.001 Al 1.508 1.489 1.567 1.548 1.506 1.543 1.536 1.601 1.532 1.433 1.540 Fe+3 0.000 0.023 0.031 0.028 0.025 0.023 0.023 0.025 0.030 0.021 0.026 Fe+2 0.030 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 Mn -0.001 -0.002 0.003 0.000 0.000 0.001 0.000 0.000 0.001 0.000 0.001 Mg 0.008 0.007 0.007 0.005 0.003 0.006 0.005 0.006 0.007 0.004 0.008 Ca 0.584 0.565 0.620 0.619 0.562 0.603 0.585 0.658 0.577 0.485 0.584 Na 0.403 0.448 0.388 0.398 0.443 0.397 0.423 0.345 0.415 0.510 0.444 K 0.013 0.015 0.010 0.011 0.012 0.010 0.011 0.009 0.011 0.017 0.010 An 58.385 54.977 60.922 60.23 55.251 59.671 57.403 65.061 57.529 47.9043 56.253 Ab 40.319 43.555 38.113 38.744 43.575 39.291 41.557 34.063 41.381 50.430 42.775 Or 1.297 1.468 0.964 1.026 1.174 1.038 1.040 0.876 1.090 1.666 0.972 An = (Ca/(Ca + Na + K))*100 Ab = (Na/(Ca + Na + K))*100 Or = (K/(Ca + Na + K))*100 Fe 2+ is measured by microprobe, and Fe3+ is calculated using charge balance. All values reported as wt. %. 50

Plagioclase Analysis Sample 1877-2 1877-2 1877-2 1877-2 1877-2 1877-2 1877-2 1877-2 1877-2 analysis core core core rim rim core core core rim

SiO2 49.89 53.84 54.08 53.85 51.81 52.36 52.15 51.44 53.77 TiO 0.04 0.03 0.00 0.05 0.05 0.02 0.04 0.04 0.02

Al2O3 30.28 27.11 27.67 27.22 28.15 28.24 29.73 29.78 28.24

Fe2O3 FeO 0.56 0.59 0.50 0.72 0.84 0.54 0.68 0.72 0.63 MnO 0.00 0.00 0.04 0.05 0.03 0.00 0.06 0.02 0.07 MgO 0.03 0.09 0.05 0.07 0.09 0.09 0.10 0.05 0.03 CaO 14.20 10.59 10.82 10.79 12.06 11.65 12.70 12.68 11.06

Na2O 3.01 6.37 5.34 5.16 4.43 4.62 4.21 4.29 4.81

K2O 0.11 0.25 0.24 0.25 0.18 0.21 0.14 0.15 0.21

P2O5 0.00 0.03 0.01 0.03 0.00 0.01 0.00 0.02 0.00 Total 98.11 98.89 98.74 98.18 97.63 97.75 99.81 99.19 98.83 Structural formulae on the basis of 8 oxygens Si 2.325 2.437 2.470 2.480 2.407 2.425 2.372 2.352 2.463 Ti 0.001 0.001 0.000 0.002 0.002 0.001 0.001 0.001 0.001 Al 1.663 1.446 1.489 1.477 1.541 1.541 1.594 1.605 1.525 Fe+3 0.000 0.022 0.019 0.028 0.032 0.021 0.026 0.028 0.000 Fe+2 0.022 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.024 Mn 0.000 0.000 0.001 0.002 0.001 0.000 0.002 0.001 0.003 Mg 0.002 0.006 0.004 0.005 0.006 0.006 0.007 0.004 0.002 Ca 0.709 0.514 0.529 0.532 0.601 0.578 0.619 0.621 0.543 Na 0.272 0.559 0.473 0.461 0.399 0.415 0.372 0.380 0.427 K 0.007 0.014 0.014 0.015 0.011 0.013 0.008 0.009 0.012 An 71.7753 47.251 52.099 52.846 59.451 57.493 61.9449 61.518 55.282 Ab 27.536 51.432 46.501 45.713 39.467 41.264 37.213 37.628 43.490 Or 0.689 1.317 1.400 1.441 1.082 1.243 0.842 0.854 1.228 An = (Ca/(Ca + Na + K))*100 Ab = (Na/(Ca + Na + K))*100 Or = (K/(Ca + Na + K))*100 Fe 2+ is measured by microprobe, and Fe3+ is calculated using charge balance. All values reported as wt. %. 51

Pyroxene Analysis Sample 1532 1532 1532 1532 1532 1742 1742 1742 1742 1742 1742 analysis rim core rim rim core rim core rim core rim core

SiO2 51.36 52.25 52.57 51.63 51.49 53.38 54.37 51.32 51.04 53.74 54.17

TiO2 0.68 0.55 0.26 0.62 0.29 0.24 0.22 0.58 0.65 0.28 0.26

Al2O3 2.92 2.28 2.98 2.70 1.95 1.21 1.43 2.89 3.25 1.40 1.48

Cr2O3 0.00 0.07 -0.01 0.02 0.00 -0.01 0.03 0.01 0.01 0.01 0.02

Fe2O3 0.86 0.46 0.38 0.83 4.23 0.00 0.00 0.89 0.67 0.08 0.00 FeO 8.13 9.23 15.45 7.95 12.96 16.29 17.40 8.50 8.09 16.40 16.60 MnO 0.20 0.28 0.40 0.25 0.35 0.35 0.41 0.23 0.26 0.44 0.40 MgO 14.55 14.44 25.14 14.55 22.37 25.10 25.10 14.92 14.67 25.33 25.49 CaO 20.52 20.04 1.51 20.70 6.45 1.63 1.54 19.35 19.72 1.67 1.64

Na2O 0.32 0.44 0.11 0.35 0.08 0.06 0.05 0.38 0.40 0.08 0.07 Total 99.53 100.02 98.80 99.60 100.18 98.25 100.56 99.06 98.76 99.42 100.13 Structual formulae on the basis of 6 oxygens Si 1.916 1.943 1.927 1.924 1.894 1.974 1.971 1.921 1.915 1.964 1.966 Ti 0.019 0.015 0.007 0.017 0.008 0.007 0.006 0.016 0.018 0.008 0.007 Al 0.128 0.100 0.129 0.119 0.085 0.053 0.061 0.127 0.144 0.060 0.063 Cr 0.000 0.002 0.000 0.001 0.000 0.000 0.001 0.000 0.000 0.000 0.001 Fe+3 0.024 0.013 0.010 0.023 0.117 0.000 0.000 0.025 0.019 0.002 0.000 Fe+2 0.254 0.287 0.473 0.248 0.399 0.504 0.528 0.266 0.254 0.501 0.504 Mn 0.006 0.009 0.012 0.008 0.011 0.011 0.013 0.007 0.008 0.014 0.012 Mg 0.809 0.801 1.374 0.809 1.227 1.384 1.357 0.833 0.821 1.380 1.379 Ca 0.820 0.798 0.059 0.826 0.254 0.065 0.060 0.776 0.793 0.065 0.064 Na 0.023 0.032 0.008 0.026 0.006 0.005 0.004 0.028 0.029 0.006 0.005 Wo 43.01 42.05 3.10 43.36 12.73 3.31 3.08 40.85 42.03 3.36 3.27 En 42.42 42.16 71.66 42.42 61.44 70.88 69.79 43.83 43.51 70.82 70.84 Fs 14.57 15.80 25.24 14.22 25.83 25.80 27.13 15.32 14.46 25.83 25.89 Wo = (Ca/Ca + Mg + Fe(total))*100 En = (Mg/Ca + Mg + Fe(total))*100 Fs = (Fe(total)/Ca + Mg + Fe(total))*100 Fe 2+ is measured by microprobe, and Fe3+ is calculated using charge balance. All values reported as wt. %. 52

Pyroxene Analysis Sample 1742 1742 1742 1742 1877-1 1877-1 1877-1 1877-1 1877-1 1877-1 1877-2 analysis rim core rim core rim core rim rim rim rim rim

SiO2 50.64 51.53 51.77 51.24 52.90 53.81 53.33 53.51 53.71 54.05 53.88

TiO2 0.74 0.53 0.57 0.63 0.15 0.14 0.29 0.24 0.17 0.30 0.23

Al2O3 3.42 2.70 2.33 3.25 1.21 1.17 0.66 1.32 1.13 1.84 0.83

Cr2O3 -0.01 0.02 0.03 -0.01 0.02 0.03 0.01 -0.01 0.02 0.00 0.02

Fe2O3 1.48 0.75 0.68 1.50 0.00 0.47 0.00 0.00 0.00 0.00 0.00 FeO 8.47 8.78 8.04 7.72 16.84 14.81 20.21 20.60 21.42 16.38 19.05 MnO 0.16 0.29 0.21 0.24 0.35 0.29 0.57 0.50 0.66 0.43 0.57 MgO 14.30 14.42 15.10 14.59 24.64 26.51 22.25 22.84 21.88 25.47 23.81 CaO 19.54 20.01 19.94 20.34 1.44 1.37 1.86 1.62 1.52 1.67 1.41

Na2O 0.44 0.36 0.37 0.39 0.04 0.08 0.04 0.01 0.05 0.01 0.04 Total 99.19 99.39 99.04 99.89 97.59 98.68 99.22 100.62 100.55 100.15 99.85 Structual formulae on the basis of 6 oxygens Si 1.899 1.928 1.936 1.904 1.974 1.967 1.995 1.969 1.989 1.960 1.985 Ti 0.021 0.015 0.016 0.018 0.004 0.004 0.008 0.007 0.005 0.008 0.006 Al 0.151 0.119 0.103 0.142 0.053 0.050 0.029 0.057 0.049 0.079 0.036 Cr 0.000 0.001 0.001 0.000 0.000 0.001 0.000 0.000 0.001 0.000 0.001 Fe+3 0.042 0.021 0.019 0.042 0.000 0.013 0.000 0.000 0.000 0.000 0.000 Fe+2 0.266 0.275 0.251 0.240 0.525 0.453 0.632 0.634 0.664 0.497 0.587 Mn 0.005 0.009 0.007 0.007 0.011 0.009 0.018 0.016 0.021 0.013 0.018 Mg 0.799 0.804 0.842 0.808 1.371 1.444 1.240 1.253 1.208 1.377 1.308 Ca 0.785 0.802 0.799 0.810 0.058 0.054 0.074 0.064 0.060 0.065 0.056 Na 0.032 0.026 0.027 0.028 0.003 0.006 0.003 0.001 0.003 0.001 0.003 Wo 41.50 42.17 41.80 42.62 2.95 2.73 3.82 3.28 3.12 3.34 2.86 En 42.24 42.27 44.05 42.55 70.16 73.56 63.71 64.22 62.54 71.03 67.05 Fs 16.25 15.56 14.16 14.84 26.89 23.71 32.47 32.50 34.34 25.62 30.09 Wo = (Ca/Ca + Mg + Fe(total))*100 En = (Mg/Ca + Mg + Fe(total))*100 Fs = (Fe(total)/Ca + Mg + Fe(total))*100 Fe 2+ is measured by microprobe, and Fe3+ is calculated using charge balance. All values reported as wt. %. 53

Pyroxene Analysis Sample 1877-2 1877-2 1877-2 1877-2 1877-2 1877-2 1877-2 1877-2 1877-2 1877-2 1877-2 analysis core rim core rim core rim core rim core rim core

SiO2 54.02 52.55 51.99 54.55 52.54 51.13 51.92 51.76 52.85 53.29 53.55

TiO2 0.16 0.44 0.55 0.15 0.14 0.55 0.20 0.46 0.20 0.22 0.21

Al2O3 0.78 1.82 2.20 1.59 2.39 3.55 1.37 3.43 1.31 0.94 0.98

Cr2O3 -0.01 0.00 -0.01 0.03 -0.01 0.05 -0.02 0.05 0.02 0.00 0.01

Fe2O3 0.00 0.00 0.45 0.00 0.00 1.52 1.14 0.70 0.00 0.00 0.29 FeO 19.01 8.97 8.97 15.21 19.94 7.15 8.31 7.14 9.31 18.13 16.09 MnO 0.60 0.20 0.22 0.33 0.73 0.19 0.35 0.15 0.39 0.43 0.39 MgO 24.15 15.47 15.08 26.84 23.18 14.96 13.75 15.31 13.46 23.90 25.57 CaO 1.32 19.38 19.57 1.40 0.56 20.13 21.04 20.30 21.14 1.48 1.55

Na2O 0.03 0.31 0.33 0.02 0.03 0.39 0.46 0.37 0.47 0.00 0.04 Total 100.06 99.13 99.36 100.11 99.51 99.62 98.52 99.67 99.13 98.40 98.66 Structual formulae on the basis of 6 oxygens Si 1.984 1.962 1.942 1.964 1.946 1.899 1.965 1.915 1.989 1.986 1.970 Ti 0.004 0.012 0.016 0.004 0.004 0.015 0.006 0.013 0.006 0.006 0.006 Al 0.034 0.080 0.097 0.067 0.104 0.156 0.061 0.149 0.058 0.041 0.043 Cr 0.000 0.000 0.000 0.001 0.000 0.001 0.000 0.001 0.000 0.000 0.000 Fe+3 0.000 0.000 0.013 0.000 0.000 0.042 0.033 0.020 0.000 0.000 0.008 Fe+2 0.584 0.280 0.280 0.458 0.618 0.222 0.263 0.221 0.293 0.565 0.495 Mn 0.019 0.006 0.007 0.010 0.023 0.006 0.011 0.005 0.012 0.014 0.012 Mg 1.322 0.861 0.840 1.441 1.280 0.829 0.775 0.845 0.755 1.328 1.402 Ca 0.052 0.775 0.783 0.054 0.022 0.801 0.853 0.805 0.852 0.059 0.061 Na 0.002 0.023 0.024 0.002 0.002 0.028 0.034 0.027 0.034 0.000 0.003 Wo 2.66 40.46 40.89 2.77 1.15 42.29 44.33 42.59 44.84 3.03 3.10 En 67.53 44.93 43.83 73.78 66.67 43.75 40.30 44.69 39.73 68.02 71.31 Fs 29.81 14.61 15.29 23.45 32.18 13.96 15.37 12.72 15.42 28.95 25.58 Wo = (Ca/Ca + Mg + Fe(total))*100 En = (Mg/Ca + Mg + Fe(total))*100 Fs = (Fe(total)/Ca + Mg + Fe(total))*100 Fe 2+ is measured by microprobe, and Fe3+ is calculated using charge balance. All values reported as wt. %. 54

Pyroxene Analysis Sample 1877-2 1877-2 1877-2 1877-2 1877-2 1877-2 1877-2 1877-2 1877-2 1877-2 analysis core core rim core rim core rim core rim core

SiO2 49.78 53.58 51.65 50.99 53.71 53.70 53.01 53.54 52.16 54.00

TiO2 0.56 0.20 0.44 0.46 0.28 0.18 0.27 0.31 0.54 0.22

Al2O3 2.27 0.81 1.57 1.65 1.39 0.77 1.11 1.59 2.23 0.90

Cr2O3 0.05 -0.02 -0.02 0.00 0.01 0.00 0.01 0.01 0.00 0.01

Fe2O3 2.42 0.00 0.94 1.63 0.00 0.00 0.00 0.00 0.51 0.00 FeO 8.79 15.75 8.04 10.22 16.25 18.20 21.20 16.88 8.35 17.02 MnO 0.33 0.40 0.22 0.45 0.29 0.51 0.57 0.41 0.23 0.37 MgO 13.29 25.61 15.67 13.33 25.34 23.87 21.86 24.97 15.18 24.85 CaO 19.46 1.63 19.33 19.46 1.64 1.59 1.56 1.81 20.08 1.61

Na2O 0.50 0.01 0.26 0.44 0.04 0.05 0.04 0.04 0.32 0.02 Total 97.44 97.97 98.09 98.61 98.96 98.87 99.63 99.55 99.60 99.00 Structual formulae on the basis of 6 oxygens Si 1.915 1.982 1.949 1.943 1.971 1.993 1.980 1.959 1.941 1.989 Ti 0.016 0.006 0.012 0.013 0.008 0.005 0.007 0.008 0.015 0.006 Al 0.103 0.036 0.070 0.074 0.060 0.034 0.049 0.068 0.098 0.039 Cr 0.001 -0.001 -0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.000 Fe+3 0.070 0.000 0.027 0.047 0.000 0.000 0.000 0.000 0.014 0.000 Fe+2 0.283 0.487 0.254 0.325 0.498 0.565 0.662 0.516 0.260 0.524 Mn 0.011 0.013 0.007 0.014 0.009 0.016 0.018 0.013 0.007 0.011 Mg 0.762 1.412 0.881 0.757 1.386 1.321 1.217 1.362 0.842 1.364 Ca 0.802 0.065 0.781 0.794 0.065 0.063 0.062 0.071 0.800 0.064 Na 0.037 0.001 0.019 0.033 0.003 0.004 0.003 0.003 0.023 0.002 Wo 41.84 3.30 40.21 41.29 3.31 3.24 3.21 3.63 41.77 3.26 En 39.76 71.90 45.35 39.36 71.11 67.78 62.68 69.87 43.93 69.88 Fs 18.40 24.81 14.44 19.35 25.58 28.98 34.11 26.50 14.31 26.85 Wo = (Ca/Ca + Mg + Fe(total))*100 En = (Mg/Ca + Mg + Fe(total))*100 Fs = (Fe(total)/Ca + Mg + Fe(total))*100 Fe 2+ is measured by microprobe, and Fe3+ is calculated using charge balance. All values reported as wt. %. 55

APPENDIX C. FIGURES

A

Galapagos Islands B

Carnegie Ridge

Nazca Plate South American Plate

N 160 km

B Figure 1. A) Tectonic setting of Ecuador subduction zone showing Galapagos hotspot and Carnegie Ridge. Dotted lines show the trend of Quito the Western and Eastern Cordilleras, and toothed line indicates the location of the subduction trench. B) Distribution of volcanoes along the two cordilleras. Note location of Cotopaxi along Eastern Cordillera. Digital imagery from Google Earth and NASA Shuttle Radar Topography Cotopaxi Mission (SRTM).

15 km 56

Ingaloma N

Quebrada Saquimala

Figure 2. Map of sampling locations for Cotopaxi deposits from the 1532-1877 eruptions. Grey unit represents synchronous and post- debris flow scoria flows from 1877 (based on mapping by Hall and Mothes, 2008). Red boxes indicate where samples were taken from; Southern samples are from the northwest side of Quebrada Saquimala and include samples from the 1532-1768 eruptions as well as the first pulse of the 1877 eruption. Northern samples (west of Ingaloma) are from the second pulse of the 1877 eruption. Image from NASA SRTM. 57

A

1877 scoria flow

1877 lahar deposit

B

Altered quenched rind

Figure 3. A) View of the deposits on the north side of Cotopaxi (Figure 2) looking south toward Cotopaxi. The red dashed line shows the contact between the scoria flow deposit (formed by the second scoria-bomb pulse), and the underlying lahar (geologist for scale). B) Interior of broken cauliflower-shaped scoria clast, showing crude radial jointing, pockets of high vesicularity, and altered quenched margin. 58

Quebrada Saquimala

1768 1744 1877-1 (Agglutinated PF and bombs)

1742 1532 mingled magma clasts 1744 (plug and white pumice)

Person for scale

Figure 4. Annotated field photo of the 2015 sampling location at Quebrada Saquimala. Ten samples were collected from five eruptions by the staff of the Instituto Geophysico. Two samples were taken from the 1532 deposit (black and brown), one from 1742, four from 1744 (pyroclastic fall, grey pumice, black pumice, and plug rock), one from 1768, and two from 1877 (black and brown). Photo Credit: Patricia Mothes, IGEPN. 59

A B CPX C OPX PL PL PL PL

CPX D E CPX F OPX PL OPX

OPX

PL CPX G H I CPX OPX PL OPX

OPX

PL

Figure 5. Photomicrographs showing different textures of the 1877 scoria flow. A) Pristine oscillatory zoned plagioclase. B) Coexisting euhedral plagioclase and pyroxene phenocrysts C) Pristine zoned plagioclase with pyroxene inclusion. D and E) Pyroxene rich crystal lithcs. F) Plagioclase with resorbed core. G) plane polarized light (PPL) view of D. H) PPL view of E, I) PPL view of F. PL = plagioclase, OPX = orthopyroxene, CPX = clinopyroxene. 60

A B INCL

INCL

1532 1532

C D

INCL INCL

1742 1532

E F

INCL

INCL

1742 1532

Figure 6. Examples of mingled magma textures from the 1532-BR and 1768 samples. A, B and D show examples of sharp, smooth contact between inclusion and host. C, E and F are examples of more diffuse, gradational contacts between inclusion and host. Note the presence of highly resorbed plagioclase phenocryst in F, indicating disequilibrium with the magma. INCL = inclusion 61 An

Anorthite 1532 Bytownite 1742 1877-1 Labradorite 1877-2

Andesine

Oligoclase

Alkali Feldspars Albite

Ab Or Figure 7. Ternary plot of feldspar compositions from electron microprobe analysis (includes core and rim data).

Wo

Augite

Pigeonite

Enstatite Ferrosilite

En Fs

Figure 8. Ternary plot of pyroxene compositions from electron microprobe analysis (includes core and rim data). 62

12 Glass Trachyte 1532 10 1742 Trachy- 1877-1 andesite Trachydacite 8 Basaltic 1877-2 trachy- andesite 2015 Na2O+K2O 6 Whole Rock 1532 1742 4 Basaltic Andesite Dacite 1744 andesite 1768 2 1877-1 1877-2 0 50 55 60 65 70 SiO2

Figure 9. Total Alkali vs. Silica plot of all whole rock and glass analyses (LeBas et al., 1986). All values are reported as wt. % and have been normalized to 100% to account for the presence of volatiles. Data for 2015 ash samples from Gaunt et al., 2016. Grey field indicates range of Cotopaxi IIB andesite compositions from Garrison et al. (2006) and Garrison et al. (2011). 20 7 1532 63 FeO 6 MgO 1742 15 5 1877-1 1877-2 4 2015 10 Whole 3 rock 5 2 1 0 0 11 5 CaO K O 9 4 2 7 3 5 2 3 1 1 0 7 2.0 Na O TiO2 6 2 1.6 5 1.2 4 0.8 3 0.4 2 0.0 30 0.4 Al2O3 MnO 25 0.3

20 0.2

15 0.1

10 0.0 50 55 60 65 70 1.0 SiO2 P2O5 Figure 10. EMPA Glass Analysis: Major 0.8 elements vs. SiO2. Total iron is 0.6 calculated as FeO. All values are shown at wt. %, and have been normalized to 0.4 100% to account for volatiles. Grey fields represent whole rock data for 0.2 samples analyzed in this study. Analysis 0.0 of 2015 ash from Gaunt et al., 2016. 50 55 60 65 70 SiO2 64

9 6 1532 FeO MgO 1742 8 5 1744 7 1768 4 1877-1 6 1877-2 3 Cotopaxi 5 IIB 4 2 3 1 9 CaO TiO2 8 1.0 7 0.8 6 0.5 5 4 0.3

K2O Na2O 2.0 4.5

1.5 4.0

1.0 3.5

0.5 3.0

Al2O3 MnO 20 0.13

18 0.10

16 0.08

14 0.05 55 60 65 SiO2 P2O5 0.30 Figure 11. Whole rock major element 0.25 composition from XRF analysis. Total iron is calculated as FeO. All values are 0.20 reported as wt. % and are normalized to 100% to account for volatiles. Analyses 0.15 for Cotopaxi IIB samples from Garrison 0.10 et al. (2006) and Garrison et al. (2011). 55 60 65 SiO2 65

30 Ba La 1532 900 1742 1744 700 20 1768 1877-1 1877-2 500 10 Cotopaxi IIB 300 0 100 800 Rb Sr 80 700 60 600 40 500 20 400 0 300 25 10 Nd Nb 8 20 6 15 4

10 2 170 19 Y Zr 150 17 130 15 110 13 90 11 70 55 60 65 2.0 SiO Yb 2 Figure 12. Whole rock trace element 1.5 composition from ICP-MS analysis. All trace element concentrations are reported as ppm. 1.0 Analyses for Cotopaxi IIB samples from Garrison et al. (2006) and Garrison et al. (2011). See figure 9 for symbols. 0.5 55 60 65 SiO2 66

Rock/Primitive Mantle Rock/Chondrites 1000 1000 A 1532 1742 A 1744 1768 1877-1 100 100 1877-2 Cotopaxi IIB

10 10

1 1 Rb Th Nb K Ce Pr P Zr Eu Ti Y Lu La Pr Pm Eu Tb Ho Tm Lu Cs Ba U Ta La Pb Sr Nd Sm Gd Dy Yb Ce Nd Sm Gd Dy Er Yb

Rock/AVG CTX Rock/AVG CTX 10 10 B B

1 1

.1 Rb Th Nb K Ce Pr P Zr Eu Ti Y Lu .1 Cs Ba U Ta La Pb Sr Nd Sm Gd Dy Yb La Ce Pr Nd PmSmEu Gd Tb Dy Ho Er TmYb Lu

Figure 13. A) Spider diagram of trace element Figure 14. A) Spider diagram of trace element concentrations normalized to primitive mantle concentrations normalized to chondrite values values after Sun & McDonough (1989). B) after Sun & McDonough (1989). B) Spider Spider diagram of trace element concentrations diagram of trace element concentrations normalize to the average concentrations of normalized to the average concentrations of Cotopaxi IIB andesites. AVG CTX = average Cotopaxi IIB andesites. AVG CTX = average composition of all andesites from Cotopaxi IIB composition of all andesites from Cotopaxi IIB (Garrison et al., 2006; Garrison et al., 2011). (Garrison et al., 2006; Garrison et al., 2011). 67

1420

CaO 12 18 FeO MgO

Al2O3 1016 Na2O MnO

K2O 8 P2O5

TiO2 Weight oxide% Weight 6

4

2

0 1532 1742 1744 1768 1877-1 1877-2

Figure 15. Variation in major element concentrations over time. 68

750

650

550

450

350

250 Co Zn 150 Sr Zr 50 Ba 45 Cr Ni 40 Rb Y 35 Nb

Parts Per Million (ppm) Cs 30 La 25 Ce Pr 20 Nd

15

10

5

0 1532 1742 1744 1768 1877-1 1877-2 Figure 16. Variation in trace element concentrations over time. 69

0.00 0.00

0.05 1.75

0.10 3.50

0.15 5.25 Depth (km) Depth 0.20 7.00

0.25 8.75

Pressure Pressure (GPa) 0.30 10.50

0.35 12.25

0.40 14.00

0.45 15.75 0.50 17.50 1532 17421877-1 1877-2 Figure 17. Pressure calculation results from Al-in-orthopyroxene barometer (Eqn. 29a; Putirka, 2008). Standard error estimate = ± 0.26 GPa. Black Xs indicate the average pressure for each eruption, and error bars show the 2σ error associated with each average. Depth is calculated using a crustal density of 2.8 g/cm3. 70

1200 28a 28b

1150

1100

(°C)

1050

1000

950 1532 1742 1877-1 1877-2

Figure 18. Temperature results using two orthopyroxene thermometers, Equation 28a and Equation 28b (Putirka, 2008). Standard errors for Equations 28a and 28b are ± 28 and ± 48 °C, respectively. Grey field indicates the range of temperature values for all samples. Temperatures calculated using a constant pressure of 0.4 GPa. 71

0.30 2.5 Th/Nd Rb/Nd D = 0.12 DTh = 0.04 Rb D = 1.05 0.25 DNd = 1.05 Nd 2.0 1532-BL 1532-BL 0.7 0.20 0.7 0.8 0.4 0.4 0.9 1.5 0.9 0.6 0.8 0.8 0.15 0.7 0.7 0.9 0.8 0.8 1532-BR 1532-BR 0.9 Th Rb 0.10 1.0 234520 25 30 35 40 23 Ba/Ce 22 21 1532 1742 20 1744 19 1877-1 1877-2 18 0.7 0.8 17 1532-BR 0.9 16 Ba 500 600 700 800 900 Figure 19. Variation diagrams of highly incompatible/moderately incompatible vs. highly incompatible trace elements, with mixing and fractional crystallization (FC) trends. Compatibility of Nd and Ce suggest significant crystallization of apatite. Dots on FC trends indicate the fraction of liquid remaining in increments of 10% (up to 40% crystallization), and dots on mixing curves indicate proportions of mixing endmembers, at 20% increments. 72

1000 Ba

900 0.7 1532 1742 1744 800 1877-1 0.8 1532-BL 1877-2

700 0.9 0.2 0.4 0.6 0.8 600 1532-BR Ba/Ni

30 40 50 60 70 80 90

Figure 20. Models of mixing and fractional crystallization (FC). Dots on fractionalization curves indicate fraction of liquid remaining and have a step of 10%. Dots on mixing curve indicate proportions of mixing endmembers, and are at 20% increments. 73

1000

1744-BP Calculated hybrid 81.6% 1532-BL + 100 18.4% 1532-BR

100 Rock/Primitive Mantle

10 1877-1 Calculated hybrid 35.0% 1532-BL + 65.0% 1532-BR

1 Cs Rb Ba Th U Nb Ta K La Ce Pb Pr Sr P Nd Zr SmEu Ti Dy Y Yb Lu Figure 21. Comparison of the 1744-BP and 1877-1 samples (colored symbols) and calculated hybrids of 1532-BR and 1532-BL samples (black symbols). Trace element compositions have been normalized to primitive mantle compositions. 74

4.20 4.2 1532-BR 1768 4.00 4.0 1742 3.80 3.8 1877 3.60 MgO (wt. %) 3.6 3.40 3.4

3.20 3.2 1744 1532-BL 3.00 3.0

680 680 1532-BR 660 660 1768 640 640 Sr (ppm) 620 620 1532-BL 600 600 1877 580 580 1742 1744 560 560

Figure 22. Compositional variation of the 1532-1877 eruptions. A) Variation in MgO and B) Sr concentrations with each eruption. Peaks in MgO and Sr indicate recharge events. 75

1532 mingled magma Pre-1532

MIXING

1742

1744

1744-BP RECHARGE Time

1768

1877 RECHARGE

2015

Less Evolved More Evolved

Figure 23. Chronology of mixing (recharge) and fractionation (FC) events from 1532-1877 based on whole rock geochemistry and textural analysis (inclusions). A recharge event prior to the 1532 eruption results in the more mafic whole rock composition, and the mingled characteristic of the deposits. Homogenization and fractional crystallization of this more mafic composition result in the increasingly felsic compositions of 1742 and 1744. A second recharge event prior to 1768 again results in a shift toward more mafic compositions and the presence of mingled textures. After 1768, FC has resumed, resulting in the more felsic 1877 eruption and perhaps continuing through the 2015 eruption. 76

2 2 A 2 B C

6 6 6 (km) Depth

10 10 10

14 14 14

Pre-1532 Recharge 18 1532 18 1742 18

2 2 D 2 E F

6 6 6 (km) Depth

10 10 10

14 14 14

1744 Recharge 18 1768 18 1877 18

Figure 24. Schematic model of magma evolution and recharge events with respect to depth estimates from thermobarometry results, at six different time intervals. A) prior to the 1532 eruption, a mafic magma intrudes the evolved magma in the existing chamber, resulting in B) the mingled magma produced during the 1532 eruption. Homogenization and continued fractionation produce the increasingly felsic compositions of C) 1742 and D) 1744. A second recharge event prior to 1768 recharges the chamber and results in E) the more mafic, mingled 1768 magma. F) Shows the composition of the more felsic 1877 magma produced by the resumption of fractional crystallization after the 1768 eruption.