1

INSIGHT INTO THE EVOLVING COMPOSITION OF

AUGUSTINE VOLCANO’S SOURCE MAGMA FROM A

LOW-K DACITE

A Thesis Presented to the Honors Tutorial College, Ohio

University

In Partial Fulfillment of the Requirements for Graduation from the

Honors Tutorial College with the degree of Bachelor of Science in

Geological Sciences

By Christian Thomas

2018

Dedicated to

Robert J. Thomas

1965-2017

Table of Contents:

Introduction 1

Background 3

Eruptive History 4

Methods 9

Sample acquisition and preparation 9

Microprobe analysis and modeling processes 11

Data and Results 14

Mineralogical relationships 18

Model Results 21

Fe-Ti oxide thermometry 21

Amphibole barometry 22

Plagioclase-amphibole thermobarometry 25

Plagioclase-melt hygrometry 25

Suitability of models vs. potential error 27

Discussion 29

Interpretation of Model Results 30

Patterns and strength of data-trend fit 30

Implications 32

Model data and petrologic comparison 32

Possibilities for unit relations 36

Implications for evolution 37

Conclusions 48

References 50

Appendices 57 1

Introduction:

Augustine Volcano is a volcanic edifice in southern Alaska that has erupted intermittently from the Pleistocene epoch to as recently as 2006 (Waitt and Begét, 2009;

Coombs and Vasquez, 2014). The 1.25 km-tall volcanic cone is part of the Aleutian arc, and is exposed as a 90 km2 island in Cook Inlet, shown in Figure 1. This location has posed problems in the recent past, as the 2006 eruption forced air and sea traffic to be rerouted or closed entirely (Neal et al., 2010). Given that Cook Inlet is host to significant oil industry activity and all sea shipping routes to Anchorage and ash from Augustine is capable of disrupting significant volumes of air traffic, knowledge of how Augustine Volcano could potentially erupt in the future will be beneficial for the physical safety and economic security of the local population and air traffic in the area (Blong, 1984; Neal et al., 2010).

The Augustine volcanic center has been active for at least 25 ka, and 238U-230Th ages for zircons found in dioritic inclusions from the 2006 eruption indicate cooling within a pre-Augustine magmatic region at ~200 ka (Coombs and Vasquez, 2014). The ideal data set for petrologically assessing a long-lived system like this would contain an uninterrupted sequence of rock units, allowing us to observe the volcano’s changing compositional and eruptive dynamics through time. Unfortunately, since large volcanic eruptions only occur intermittently at Augustine, volcanic rocks like tephras and pumices are often poorly indurated, and the island has been subject to many environmental factors detrimental to preservation (e.g., glacial and volcanic erosion; mass wasting). Accordingly, significant gaps exist in the geologic record of Augustine’s eruptions. These time gaps typically appear as unconformities, usually as pyroclastic flow deposits or landslide surfaces in outcrop

(Wallace et al., 2013; Waitt and Begét, 2009). Moreover, these gaps may correspond to 2

Figure 1: Location of Augustine Volcano within Alaska. Note the position of Anchorage NE of Augustine. Figure after Nadeau et al. (2015).

actual gaps in eruptive activity, to units lost to the aforementioned processes, or both. The resulting limited temporal resolution in the rock record is not ideal, but the units discovered and described on Augustine Island still serve as a base for the elucidation of the volcano’s evolution. Research to describe and characterize the presently-exposed stratigraphy is valuable for creating a model of Augustine’s past and future activity.

Small-to-moderately-sized eruptions from the past ~2 ka have been extensively studied (Waitt and Begét, 2009; Tappen et al., 2009; Wallace et al., 2010; Webster et al.,

2010; Coombs and Vasquez, 2014), as have the oldest (by radiometric age dating) known volcanic units, some of which were generated by significantly larger and more explosive eruptions than their modern counterparts (Coombs and Vasquez, 2014; Nadeau et al., 2016,

Nadeau et al. 2015, Zimmer et al., 2010). However, units from the period ~26 to 2.1 ka 3

have not been the subject of in-depth research, with the exception of the high-P2O5 dacite

(Nadeau et al., 2016). The corresponding research gap has made interpretation of

Augustine’s compositional evolution difficult, as much of the volcano’s eruptive history is not known in detail. Although the only eruption that produced particularly voluminous deposits was the ~26 ka rhyolite, material related genetically to this rhyolite was brought to the surface in the 2006 eruption (Coombs and Vasquez, 2014). This lends credence to the possibility that modern eruptions can still draw from ancient magma sources, and more explosive eruptions producing large volumes of ejecta may be possible in the future.

Therefore, this project investigated the petrology of a 6.81 (+/- 0.03) ka dacite from

Augustine Volcano, denoted as “low K2O dacite” (LKD) in the stratigraphy produced by

Wallace et al. (2013). Since this unit’s position in the Augustine stratigraphy is directly above other pyroclastic deposits that have been recently and thoroughly examined, and which correlate to more dangerous eruptions, studying the low-K dacite with the context of earlier eruptive units will advance the scientific community’s understanding of

Augustine’s eruptive history and magmatic evolution.

Background:

The first eruption of Augustine to be observed and described in a scientific manner as it took place occurred in 1883 (Waitt and Begét, 2009); since then, events have been increasingly thoroughly described as they occurred, due at least partially to advances in observational technology. Eruptive events in Augustine’s history prior to 1883 were long known only by their effects on the rock record, including eruptive deposits and unconformities. However, in recent years, work on prehistoric eruptions has expanded our 4 knowledge of Augustine’s history to deposits approximately 2.1 ka in age (Tephra “G”, described in Tappen et al., 2009). Radiocarbon dates for the low-K dacite indicate an age of approximately 6.8 ka, with the next youngest (unnamed) unit at 6.4 ka, and the remaining four known units between the low-K dacite and Tephra “G” are undated

(Wallace et al., 2013). As a result, a span of ~4.5 ka remains effectively uncharacterized, and prior to this study, only one Augustine unit between the ages of 26 and 2.1 ka had been researched in-depth. Studying the low-K dacite in detail is a step toward closing this gap.

Eruptive History:

The oldest materials known to have been erupted from Augustine are coeval rhyolite and basalt deposits dated at ~26 ka (Waitt and Begét, 2009; Wallace et al., 2013).

The rhyolite occurs above the basalt, except for a 20 cm interval where the two are interbedded (Waitt and Begét, 2009). Waitt and Begét (2009) hypothesized that the rhyolite and basalt could have been erupted simultaneously from a peak vent and a submarine flank vent, respectively.

The rhyolite has variable exposure on Augustine Island. The irregular distribution of pyroclastic material around Augustine means that outcrops containing material originating from the same event and of similar compositions can have very different physical characteristics. As such, it is likely that the many (occasionally conflicting) reports of the basal rhyolite on Augustine Island describe the same unit(s) (unpublished pers. comm.). For example, Waitt and Begét (2009) described the rhyolite as a 10 m thick white 5

Tephra Age (y.a.) Juvenile clast types (D = Thickness Whole-rock

Name dominant type) (m) SiO2 (wt%) Unnamed Unknown WHT pumice (D); rare MG 0.6 66.0, 66.2 microvesicular pumice Unnamed 6430 ± 30 LG microvesicular pumice 0.7 64.5 (D), WHT pumice

Low K2O 6810 ± 30 Bone-WHT friable pumice (D) 0.3-1.5 66.5-66.9 dacite

High P2O5 Unknown LG-MG pumice (D) 0.5-1.6 61.9, 62.7, dacite 62.9, 63.2 Rhyolite ~26,000 WHT-CRM pumice (D), LG 2-6 72.0-73.8 finely vesicular pumice, CRM/LG banded pumice

Table 1: Excerpt of the Augustine tephrostratigraphy column published in Wallace et al. (2013) showing the units immediately above and below the low-K dacite (the subject of this work).

pumice with airfall and pyroclastic flow components. More recent work using other exposures of the rhyolite gives a thickness of 2-6 m for the rhyolite, which is more precisely described as consisting of two light-toned pumices that occur both separately and as alternating bands (Wallace et al., 2013). A 238U-230Th age of 26 ka from zircons included in the rhyolite was interpreted as the crystallization age of the unit by Coombs and Vazquez

(2012).

Nadeau et al. (2015) did more detailed work on the rhyolite using samples from new exposures generated by landslides associated with the 2006 eruption. That work noted a unit thickness closer to 30 m and identified multiple subtypes of the rhyolite, designating them the white, yellow, and flow-banded pumices. The identifiable phenocryst assemblage comprised plagioclase, orthopyroxene, quartz, Fe-Ti oxides, and amphiboles (Nadeau et 6 al., 2015). Melt inclusions were common in all phenocryst types. Oxide geothermobarometry produced temperatures of 800-830 ºC and 759-887 ºC by the QUILF and Ghiorso and Evans (2008) programs (Andersen et al., 1993). The Holland and Blundy

(1994) thermometer returned values of 845 ºC and 137 MPa for the white pumice and 858

ºC and 192 MPa for the flow-banded pumice (Nadeau et al., 2015).

The unit overlying the rhyolite in the Wallace et al. (2013) tephrostratigraphy is described as a high-P2O5 dacite (hereafter “high-P dacite”). That work described the high-

P dacite as a light- to medium-gray pumice of 0.5-1.6 m thickness containing multiple populations of foreign lithic fragments, many of which carry an orange stain (Table 1). As of this writing, reliable radiometric age dates for the high-P dacite have not been acquired.

This complicates interpretations of the temporal aspect of Augustine’s compositional evolution, as will be discussed further below.

Nadeau et al. (2016) performed electron microprobe, secondary ion mass spectrometry (SIMS), and cathodoluminescence analyses on samples of the high-P dacite.

Results were then compared to the analogous data presented for the rhyolite in Nadeau et al. (2015). Resulting images and data helped to quantify the differences in phenocrysts, melt inclusions, and textures between the rhyolite and the high-P dacite, helping to draw the conclusion that the high-P dacite was most likely derived from the same dacitic crystal mush as the rhyolite. The findings supporting this conclusion include the close resemblance of the high-P dacite’s and rhyolite’s mineral assemblages. Factors indicating differences in the source regions and/or eruptive processes (i.e. source region depths, exposure to other magmas) of the rhyolite and high-P dacite include rims of calcic amphibole on cummingtonites in the high-P dacite (as opposed to euhedral and distinct crystals of both 7 types in the rhyolite), more complex zonation in quartz grains, morphologic and chemical differences in melt inclusions, and the far greater volume of large quartz in the high-P dacite over the rhyolite.

Products of recent eruptions are markedly different from their ancient counterparts.

The 1976 eruption produced ash clouds and pyroclastic flows, which deposited 1-3 meters of loose pumiceous material (Waitt and Begét, 2009; Kamata et al., 1991). A study conducted after the 1986 eruption compared ash from that event to 1976 samples. This work showed a slight increase in silica content compared to 1976 ash (64.3-67.9 wt% SiO2 vs. ~63.4 wt%), but both compositions were similar overall; however, ash fall is not necessarily representative of the full compositional range of eruptive products (Yount et al., 1987). The earliest reported phenocryst assemblage for 1986 ash contained glass, plagioclase, pyroxene, and lithic fragments, but no amphiboles or notable Fe-Ti oxides

(Yount et al., 1987).

More recently, Roman et al. (2006) expanded on these data with newly-collected whole-rock and electron microprobe data for explosive-phase tephras from the 1986 eruption. The whole-rock data showed a greater spread in SiO2 content than was evident in the older ash samples (56.7-63.2 wt%), and electron microprobe surveys revealed the presence of clinopyroxene, orthopyroxene, minor olivine and no amphiboles in equilibrium

(Roman et al., 2006). Pyroxene-hosted melt inclusions had H2O contents of 0.6-7.2 wt%, and exhibited a range of dacitic to rhyolitic compositions (Roman et al., 2006). These authors concluded that the 1986 eruption may have been caused by magma mixing, with the projected components being leftover 1976 magma and a silica-poor component from deeper in the system (Roman et al., 2006). 8

Augustine’s most recent eruption took place in 2006. Multiple lithologies with variable compositions and textures were present within these eruptive products, and were identified as broadly andesitic with some basaltic andesites and dacites (Larsen et al.,

2010). Phenocryst populations were overall similar to those found in 1976 and 1986 deposits, consisting chiefly of plagioclase with lesser pyroxenes, oxides, and minor olivine; amphiboles were mostly present either with reaction rims or as pseudomorphs completely transformed to pyroxenes and oxides (Larsen et al., 2010). Plagioclase was commonly euhedral and zoned, with compositions and zonation patterns based on An content corresponding to five general types (Larsen et al., 2010). Later work by Coombs and

Vasquez (2014) studied dioritic inclusions that were erupted as inclusions in the 2006 andesites; they were found to contain a zircon population with ages very similar to those found in the ~26 ka rhyolite.

Our understanding of Augustine in the present is enriched by research of both historical and ancient units; however, not all known units generated by Augustine have been extensively studied. The current body of research on Augustine indicates that the volcano has erupted a wide compositional variety of volcanic rocks. This work is intended to discover the mechanisms of formation for the low-K dacite and the nature of its relationship to other Augustine units. Interpretations of the complex evolutionary processes at work in Augustine from ~26 ka to the present would benefit from a complete, detailed tephrostratigraphic record; this study of the low-K dacite represents one step toward that goal.

9

Methods:

Sample acquisition and preparation:

Data presented here were derived from a hand sample of the low-K2O dacite pumice. The samples were collected by scientists at the Alaska Volcano Observatory

(AVO) at the point marked as “F4” in Figure 2, designated AT-2905. The pumice fragments were rounded but irregularly-shaped triaxial ellipsoids with longest axes of approximately 3-4 cm on average (Figure 3). We had two thin sections made from the four largest clasts, for a total of eight. We used a trim saw to cut open each specimen and observe the interior surface textures. Our thin sections were later manufactured from these cut surfaces.

Figure 2: Aerial map of Augustine Island from Wallace et al. (2013) showing sample collection sites from four field seasons. The low-K dacite samples used in this work were collected near the point marked “F4”. 10

The sawing process caused several small chunks of dacite to break off from the larger samples, which we later crushed in an agate mortar and pestle to an average grain size of approximately 1 mm. We sorted through this powder to extract crystals suitable for use in grain mounts, to isolate those grains for easier microprobe analysis. We focused on phenocrysts that could host melt inclusions, magnetic oxides, and pumiceous fragments incorporating any of these species. Magnetic material was extracted from the powder for grain mounting by a combination of sorting with a household magnet and removal by hand, while the phenocrysts were extracted by hand with tweezers. The two types of grains were separated because of practical concerns associated with microprobe analysis discussed below.

Our grain mounts consisted of a thin layer of epoxy baked onto a 1” round frosted

(evenly scuffed/scratched) glass slide, with the grains to be analyzed mounted in a second layer of epoxy overlying the first. We made a total of 31 grain mounts, shown in Figure 3, of which 11 contained magnetic oxides and 20 contained possible melt inclusion hosts. We scanned each thin section in visible light to create high-resolution visible light maps to mark grains, grain associations, and textures for later analysis on the electron microprobe.

The larger scale of these printed maps compared to imagery taken with a petrographic microscope camera meant that they were also useful for navigation in the microprobe phase, given that instrument’s narrow field of view.

Final sample preparation and electron microprobe analysis were performed at the

American Museum of Natural History’s (AMNH) Earth and Planetary Sciences department in New York City. Each grain mount was polished; both the thin sections and grain mounts 11

Figure 3: Low-K dacite hand samples showing typical fresh surfaces, phenocryst populations, and vesicle spaces.

were cleaned with rubbing alcohol and coated with ~250 Å of carbon using a Denton DV-

502 vacuum evaporator at the AMNH. The thickness of the carbon coat was visually estimated based on the color of the coated samples (Leica Microsystems, 2013). We prioritized our data collection by selecting the grain mounts and thin sections most likely to produce elemental abundance data useful both for discerning compositional trends in the minerals and melt inclusions, and for use as input data in geologic modeling programs.

Microprobe analysis and modeling processes:

We used a Cameca SX-100 microprobe to collect elemental compositional data for minerals and melt inclusions in the low-K dacite. We focused on individual minerals and inclusions since bulk rock chemistry was already determined with X-ray fluorescence

(XRF) by the AVO. Data were acquired using the microprobe’s wavelength-dispersive spectroscopy (WDS) mode; we used the less-accurate, but also less-destructive, energy- dispersive spectroscopy (EDS) mode for preliminary characterization of ambiguous or unknown minerals. Different calibrations were required to obtain useful compositional data for the silicate phenocrysts and melt inclusions than for the Fe-Ti oxides. The microprobe 12

Figure 4: Grain mounts of Fe-Ti oxides (left) and melt-inclusion-bearing phenocrysts (right). Only the grain mounts containing the coarsest oxides were analyzed.

measured Mg, Si, Al, Fe, Ti, Mn, and S in both calibrations; the oxide calibration added V,

Cr, Ni, and Cu to this set of elements, while the phenocryst calibration instead measured

Na, K, P, Cl, and Ca. Elemental abundances were reported as weight percent oxides (all Fe reported as FeO). Two sets of beam conditions were required for the melt inclusion data analysis due to the tendency of ions like Na+ to be driven off from hydrous glasses under the higher current of condition 2 (Table 2, this work; Borom and Hanneman, 1967). We analyzed a total of 280 grains, of which 52 were oxides and 228 were silicates.

Compositional data from the microprobe was used to model the conditions of the original magma chamber using several petrological modeling programs. These programs modeled a range of environmental parameters such as: temperature, pressure, oxygen fugacity (fO2), and H2O content for different phenocrysts. The Ghiorso and Evans (2008) model for Fe-Ti oxide equilibria uses experimental data on the iron-titanium exchange in 13

Calibration Oxide calibration Phenocryst Phenocryst

parameter calibration calibration

(condition 1) (condition 2)

Beam voltage (keV) 15 15 15

Beam current (nA) 20 2 10

Beam diameter (µm) 3 5 5

Table 2: Beam conditions for microprobe analysis.

ferromagnesian oxide solid solution systems to predict the temperature and oxidation state at which oxide pairs equilibrated. We used this model to constrain the temperature and fO2 values at crystallization for magnetite-ilmenite pairs in the low-K dacite. Pressure values from amphiboles were calculated using the models of Hammarstrom and Zen (1986),

Hollister et al. (1987), Johnson and Rutherford (1989), and Schmidt (1992). Additional data on temperature, pressure, fO2, and the water content of the melt were obtained from the Ridolfi et al. (2010) amphibole geothermobarometer. All five amphibole geobarometers are based on the correlation between total Al in amphiboles and their pressures of equilibration. The Ridolfi hygrometer is also based on Al in amphiboles, and its thermometer and oxygen fugacity calculator use Si and Mg as input data, respectively.

The Holland and Blundy (1994) method was used to obtain pressure and temperature data for touching plagioclase-amphibole pairs. The method is predicated on the substitution of various major cation species (e.g. K, Na, Ca, Mg, and Si) into the appropriate sites in calcic amphiboles. Finally, the Lange et al. (2009) plagioclase-melt inclusion hygrometer used experimental data on the interaction between silicate melts and the albite-anorthite solid solution to yield values for the weight percent of H2O in the low-K dacite’s parent melt. 14

Data and Results:

The hand samples of the low-K dacite studied here consist of pebbles of pumice 2-

3 cm in diameter, collected from the southwest flank of Augustine Volcano. The groundmass is off-white when fresh, weathering to a yellow-brown color. Fe oxides

(distinguished by their dark color and magnetic properties), quartz, feldspar, and amphibole were all present as subhedral to euhedral phenocrysts. Typical phenocryst sizes were 0.025-

0.05 mm, and the largest grains in any given sample did not exceed 1-2 mm in diameter.

Some samples exhibited flow textures, expressed by elongated vesicles and glass fragments, while others showed no preferred orientation direction (Figure 5A).

Petrographic study of and compositional data for the thin sections built on and confirmed our basic observations of the samples at macroscale. Figure 5A shows a typical view of the low-K dacite in thin section, albeit with a higher proportion of phenocrysts to matrix than is common. Mineral phases identified under the petrographic microscope included quartz, plagioclase feldspar, at least two different amphibole types, orthopyroxene, and apatite; elemental data from the microprobe confirmed that the Fe- oxides present are magnetite and ilmenite. Zircon and anhydrite are also present as small inclusions, as were Fe-sulfides (the latter within larger magnetite grains).

We plotted whole-rock and microprobe-derived geochemical data for plagioclase- and quartz-hosted melt inclusions from the low-K dacite on a total alkali vs. silica (TAS) plot, shown in Figure 6; the plot also contains melt inclusion and whole-rock data for the

Pleistocene basalt, the three rhyolite lithologies, and the high-P dacite for comparative

15

Figure 5: Photomicrographs of flow banding (Figure 5A) and matrix glass on a plagioclase (Figure 5B) found within the low-K dacite.

purposes. All 52 plotted LKD-hosted melt inclusions are rhyolitic. The quartz-hosted melt inclusions show higher silica contents than their plagioclase-hosted counterparts, but overall similar alkali contents; this may be explained by contamination by melting host grains, as will be discussed later.

The matrix between phenocrysts, described as aphanitic in hand sample, remains difficult to classify mineralogically in thin section. In some areas with relatively coarse groundmass, the appearance of the matrix is consistent with glass containing microlites of feldspar, quartz, and amphiboles. Flow structures are present in multiple thin sections within the matrix, but are not obvious at the scale of a full thin section (Figure 5A). Halos of bubbles commonly occur as rims on feldspars and amphiboles (Figure 5B); similar features are present in the rhyolite (Figure 4D in Nadeau et al., 2015).

Melt inclusions of various sizes were common in feldspars and quartz, and were also very rarely found in amphiboles. We found melt inclusions in varying numbers, from large clusters of small inclusions to well-preserved inclusions up to ~75 µm in diameter. A few inclusions contained daughter crystals; these crystals were too small to be analyzed 16

Figure 6: Total alkali vs. silica plot for whole-rock and melt inclusion data from various ancient Augustine units. WR = whole rock compositional data, MI = melt inclusion compositional data. Low- K dacite data from this study, rhyolite and basalt data from Nadeau et al. (2015), and high-P dacite data from Nadeau et al. (2016).

with the microprobe. We observed multiple sets of melt-inclusion-rich bands within zoned feldspars; one such band is visible in Figures 7A and 7B.

The only feldspar found in the low-K dacite is plagioclase, with a typical composition of An45 to An70 and Or<1. Individual phenocrysts have diameters up to 2.0 mm, commonly exhibit subhedral form, and often contain melt inclusions, as previously noted. Zonation parallel to preserved crystal faces is prominent in some of the larger phenocrysts; at least one anhedral grain contains three compositionally distinct zones, one of which exhibits sieve texture (Figure 7C). 17

Figure 7: Three examples of grain relations and textures with implications for the compositional evolution of Augustine. Figure 7B is a higher-magnification view of the white-outlined area in Figure 7A.

We observed multiple types of amphibole in the low-K dacite, which were distinguished based on their physical, optical, and chemical properties. Cummingtonite

(the only low-Ca amphibole noted) appeared as anhedral to subhedral phenocrysts with two cleavage planes, 1st-order interference colors, and a brown color in plane light

(subhedral examples are shown in Figure 8A). Some pyroxenes appeared similar to cummingtonites, but the two minerals were distinguished by microprobe data and cleavage

(where present). The remainder of the amphibole population comprised various calcic amphiboles. Tschermakites and ferri-tschermakites appeared as amphiboles with 2nd-order interference colors, yellow-green color in plane light, two well-defined cleavage planes, 18

Figure 8: Examples of three types of amphibole found in the low-K dacite. Figure 8A depicts a pair of cummingtonites; Figure 8B, a pyroxene/hornblende pair; and Figure 8C, a zoned calcic hornblende (tschermakite/hastingsite).

and subhedral to euhedral form. Hornblendes often showed two excellent cleavage planes,

1st-order interference colors, and subhedral to euhedral form. A typical magnesiohornblende is shown in Figure 8B, and a zoned hastingsite-tschermakite in Figure

8C.

We performed point counts on 4 thin sections, counting plagioclase, amphibole, pyroxenes, oxides, quartz, vesicles, and aphanitic groundmass; the results are shown in

Table 3. Phenocrysts of all types are a relatively small proportion of each thin section; most of the area of a given slide is taken up by a mix of vesicle space and aphanitic groundmass like that shown in Figure 5A.

Mineralogical relationships:

Processes associated with compositional evolution, particularly those that deviate from a direct, closed-system evolutionary pattern, are often evidenced by textural and structural features of individual mineral grains. We noted several grains with unusual features likely caused by changing conditions within the low-K magma chamber. Some of these grains may be interpreted more simply than others, but all show evidence of 19

TS#: Plagioclase Amphibole Pyroxene Oxides Quartz Total points

1a 66.67% 25.40% 0% 6.35% 1.59% 63

2a 55.38% 29.23% 1.54% 13.85% 0% 65

3b 65.42% 20.56% 5.61% 7.48% 0.93% 107

4a 71.28% 20.21% 0% 5.32% 3.19% 94

Table 3: Point-counting data for phenocrysts in four thin sections of the low-K dacite. Raw counts have been converted to percentages.

significant changes in the physical conditions of the parent magma, including potential magma mixing. Magma mixing is often invoked as a mechanism for producing magmatic variability (e.g., Webster et al., 2010; Nadeau et al., 2015), and the nature of its involvement with the low-K dacite, if any, can be inferred through petrographic evidence like that described below.

Plagioclase is the most common phenocryst species in the low-K dacite (Table 3), and relatively large grains (~1 mm) tend to exhibit oscillatory zonation. The zones noted were often very thin (<20 µm), but limitations on our instrument time required the use of larger profile steps than was ideal for fully characterizing the oscillatory zonation. We took compositional profiles of three zoned plagioclase grains, but two in particular showed striking patterns. The grain pictured in Figure 7A is a large, nearly euhedral zoned plagioclase with a melt-inclusion-rich zone near its rim. Most points in our profile of this grain fall between An40 and An50, with a region of An40-45 near the profile’s midpoint and a pronounced spike to An60 near the grain’s rim. This area of high An content overlaps with the previously-mentioned melt-inclusion-bearing zone (Figure 9); the small size of the melt inclusions precluded microprobe analyses of the inclusions themselves. Other grains show similar profile patterns, but lack the defined association of melt inclusions with a high-An zone (Figure 9). 20

Figure 9: Chart of the An content in the zoned plagioclase grains 4b37 (Figs. 9A and 7A) and 1a11a (Figure 9B) as a function of distance from its core. Data points are separated by 12 and 25 microns in Figs. 9A and 9B, respectively.

Not all zoned plagioclase observed in the low-K dacite took the form shown in

Figure 7A. Figure 7C depicts a single plagioclase grain, showing one sieve-textured zone and clean plagioclase surrounding the sieve zone on two sides of the broken grain.

Microprobe analyses of each zone showed that the grain progresses from An76 in the inner section to An57 in the sieve zone, with An42 in the outer zone. Since sieve textures are 21 associated with disequilibrium in a magmatic system, this grain likely records a portion of a magma-mixing process (Tsuchiyama and Takahashi, 1983; Nelson and Montana, 1992).

Amphibole-amphibole pairs also provide useful context for past changes in stable mineral assemblages. Figure 7D shows an anhedral, heavily fractured cummingtonite grain at the core of a subhedral tschermakite grain. The contact between the grains is sharp, and does not appear to contain melt glass or other minerals; this may indicate a rapid transition between stable crystallizing assemblages. Cummingtonite cores surrounded by more calcic amphiboles were also present in the high-P dacite, taking a variety of forms; some exhibited euhedral form and contained pyroxene inclusions, while others appeared similar to the example shown here (Nadeau et al., 2016).

Model Results:

Fe-Ti oxide thermometry:

Analysis of all 8 thin sections and the Fe-Ti-oxide-bearing grain mounts yielded a total of 14 potential magnetite-ilmenite pairs. Before performing any modeling, we verified the equilibrium status of each pair using the Bacon and Hirschmann (1988) test. 13 of these pairs produced useful data through the Ghiorso and Evans (2008) program for calculating temperature and oxygen fugacity; these data are plotted in Figure 10. The average values for these quantities were 784 ºC and ∆NNO=1.47.

Although it utilizes amphiboles, not oxides, the Ridolfi et al. (2010) model does offer additional oxygen fugacity and temperature data. The model cannot calculate ∆NNO for low-Ca amphiboles; as such, we were restricted to 22 amphibole data points corresponding to 14 different grains, one of which contained zones with a variety of 22

Figure 10: Log fO2 vs. temperature plot for the 13 valid magnetite-ilmenite pairs found in the low K2O dacite, as well as comparable data for other ancient and historical Augustine units.

compositions. The inner region of this grain (depicted in Figure 8C) is responsible for the highest-temperature cluster of data points in Figure 11, ranging from 1031 to 1041 ºC.

Figure 11 shows that ∆NNO tends to decrease with increasing temperature, similar to the trend in oxide data shown in Figure 10. The lower-temperature data points correspond almost entirely to hornblendes, whereas the higher-temperature points are classified by the

Ridolfi program as tschermakite-pargasites and hastingsites.

Amphibole barometry:

We acquired a total of 66 data points from the thin sections corresponding to both individual amphibole grains and zones within larger grains; their compositions are plotted 23

Figure 11: Chart of temperature and oxygen fugacity values calculated by the Ridolfi et al. (2010) model for low-K dacite amphiboles (red markers), and by the Ghiorso and Evans (2008) model for oxide pairs (blue markers). The cross at the bottom of the figure depicts the calculated uncertainty for data in that area.

in Figure 12. 39 of these points plot as varieties of magnesio-hornblende, 14 as cummingtonites, seven as tschermakites, four as hastingsites, and two as tschermakitic hornblendes, according to the nomenclature method of Rock and Leake (1984). All four hastingsite points were obtained from a profile across a single zoned amphibole grain; thus, only a portion of one amphibole grain was hastingsite. The amphibole population we measured in the low-K dacite has a much lower frequency of high-Al2O3 amphiboles overall than any other ancient lithology. The low-K dacite’s cummingtonites are more

MgO-rich and Al2O3-poor than most other ancient Si-rich units, except for some cummingtonites from the high-P dacite.

We used multiple models to calculate crystallization pressures for the amphiboles found in the low-K dacite. The first four models returned internally consistent results, with 24

Figure 12: Plot of Al2O3 vs. MgO weight percent content in amphiboles in the low-K dacite. The data cluster around five regions which correspond to the amphibole names listed.

predictable differences between each set of data. The Johnson and Rutherford (1989) model always returned the lowest pressure for each amphibole, while the Schmidt et al. (1992) result was the highest for most grains. The Hammarstrom and Zen (1986) and Hollister et al. (1987) models were generally very similar, but the Hollister et al. (1987) pressures were even higher than those calculated by the Schmidt et al. (1992) method for amphiboles with

43 wt% SiO2 or less. The Ridolfi et al. (2010) model also calculates pressure data for individual amphibole grains; however, as shown in Figure 13 below, the modeled pressures are uniformly significantly lower than those of the other four algorithms.

25

Figure 13: Chart of pressures vs. SiO2 content calculated for calcic amphiboles in the low-K dacite by the Hammarstrom and Zen (1986) and Ridolfi et al. (2010) models, shown in blue and red, respectively. The other three amphibole pressure models discussed in the text are omitted for clarity.

Plagioclase-amphibole thermobarometry:

The Holland and Blundy (1994) thermobarometer requires compositional data from pairs of plagioclase and amphibole grains in equilibrium to function. We collected data on a total of 22 plagioclase-amphibole pairs; after excluding low-quality data, only four pairs remained. The resulting temperatures and pressures are displayed in Figure 14 below.

Three of the amphiboles are described by the Rock and Leake (1984) method as ferri- magnesio-hornblendes, while the other is a ferri-subcalcic-magnesio-hornblende.

Plagioclase-melt hygrometry:

The Lange et al. (2009) hygrometer provides an estimate of the H2O content in a plagioclase-hosted melt inclusion, and requires the An and Ab content of the host grain, the major-element-oxide composition of the melt inclusion, and ambient 26

Figure 14: Chart of the pressures and temperatures calculated by the Holland and Blundy (1994) method for four plagioclase-amphibole pairs found in the low-K dacite.

pressure/temperature conditions at the time of crystallization. We ran the model on 39 inclusions, using the average temperature from the Ghiorso and Evans (2008) model and the average pressure from the Holland and Blundy (1994) program described above; the values used were 784 ºC and 361.5 MPa, respectively. Figure 15 displays the result of these calculations, plotted against the weight percent of SiO2 in the plagioclase host; the average

H2O content was 6.76 wt%. Lange et al. (2009) recommend a minimum temperature and maximum pressure of 825 ºC and 300 MPa to ensure accurate results; our values are outside these limits. These data are included here because they remain informative in spite of potential errors, as will be discussed below.

Although not necessarily controlled by the same factors as melt inclusion water content, the relative concentration of H2O in amphiboles is inextricably linked to the water 27

Figure 15: Chart of the modeled H2O content in 39 plagioclase-hosted melt inclusions in the low-K dacite, computed by the Lange et al. (2009) model. The SiO2 content displayed is that of the host plagioclase.

content of the amphibole’s parent melt. The Ridolfi et al. (2010) model calculates the weight percent of H2O both in amphibole grains and in their parent melt; the amphibole

H2O contents are limited structurally and thus only show a small range, but there is more variation in the modeled content of the melt (Figure 16). A correlation between temperature and H2O content is evident; increasing temperatures tend to decrease the solubility of water in silicate melts, but pressure (which, in natural settings, increases with temperature according to depth) increases volatile solubility limits. Both temperature and pressure increase with depth, yielding competing solubility effects, but experimental work indicates that the pressure effect is stronger, which explains the patterns we observe here (Yamashita

1999).

Suitability of models vs. potential error:

The models we used in this work are calibrated for a variety of silicic compositions 28

Figure 16: Chart of calculated crystallization temperatures for amphiboles in the low-K dacite, plotted against the modeled H2O content of each grain’s parent melt. All data calculated by the Ridolfi et al. (2010) model. The dotted and dashed lines represent the maximum and minimum stable temperatures for most of the amphiboles used by Ridolfi et al. (2010).

and physical parameters, including dacites. However, the specific requirements of each model do not always coincide exactly with our data, which may introduce error in calculated values beyond that stated by the models’ authors.

The Lange et al. (2009) hygrometer employs a wide range of parameters to calculate melt inclusion H2O contents, and requires that still more be constrained to ensure the accuracy of the data it calculates. The program works best when pressures are less than

3 kbar, temperatures are between 825 and 1230 ºC, the plagioclase host has anorthite contents between An37 and An93, the melt inclusions have silica contents between 45 and

75 weight percent, and the melt itself has between 0 and 7 weight percent H2O (Lange et al., 2009). Results from other models for these parameters are mixed; we used the average temperature from the Ghiorso and Evans (2008) oxide pair method, which, at 784 ºC, is 29 slightly below the recommended interval. The pressure used for the hygrometer calculations, 361.5 MPa, was the average of the four values obtained from the Holland and

Blundy (1994) plagioclase-amphibole pair method; this is higher than the stated upper limit, but the authors state that application of the hygrometer to moderately higher-pressure conditions should not increase error to unacceptable levels (Lange et al., 2009). The actual compositional parameters of the low-K dacite used in the hygrometer are well within the recommended limits, although the calculated H2O contents extend to 8.5 wt% at the maximum.

The Ridolfi et al. (2010) model uses data from calcic amphiboles to calculate the temperature, pressure, oxygen fugacity, and H2O content of the melt that gave rise to the individual amphibole. The uncertainties associated with each quantity are not insignificant, as Figures 11, 16, and 19 show, and further issues have been raised with the suitability of the model for high-silica targets (Pamukcu and Gualda, 2013). However, since the values for temperature, pressure, ∆NNO, and H2O weight percent computed for the low-K dacite melt by other models are well within the stated limits for the Ridolfi et al. (2010) model, we present its results here to offer a broader perspective on the potential origins of the unit.

Discussion:

The low-K dacite, as part of a series of deposits erupted by the same volcano, is best considered with its predecessors in mind. We present interpretations of our data in two main parts; our interpretation of model results pertaining chiefly to the low-K dacite is followed by a synthesis of data and interpretations from this work and others, culminating in a theoretical model of origin for the low-K dacite. 30

Interpretation of Model Results:

This study is intended to shed light on the evolution of Augustine Volcano as a whole; the relationship between the dacite and other Augustine units is a necessary component in any interpretation of this process. Comparisons between potentially related units like these benefit from the inclusion of all available data; we do so here by considering petrographic impressions, model-calculated physical parameters, and theories from other research. The results of the modeling process are certainly informative on their own, but are most useful when discussed in context of petrographic data, other model results, and with an understanding of their inherent limitations.

Patterns and strength of data-trend fit:

Most of our data plot along well-defined trendlines, with little scatter or deviations from the mean. Temperature and oxygen fugacity data for Fe-Ti oxides calculated by the

Ghiorso and Evans (2008) model follow the trend in fO2-T space established by data from recent eruptions, the high-P dacite, and the 26 ka rhyolite; specifically, the low-K dacite records higher temperatures and more reducing conditions than most rhyolite data, but lower temperatures and more oxidizing conditions than the historical eruptions, consistent with long-term trends at Augustine. Data of the same type calculated for amphiboles by the Ridolfi et al. (2010) model appear to correspond to a similar ∆NNO curve as the oxides, but extend the curve to higher temperatures and show a tendency toward higher ∆NNO than the oxides at any given temperature. This apparent discrepancy could be caused by one or two factors, one being that the two minerals record different sets of conditions and 31 the other being calculation errors in the models. Venezky and Rutherford (1999) showed that Fe-Ti oxides can re-equilibrate over a span of days to weeks when the conditions of their environment change, a significantly shorter timeframe than for amphiboles. If the oxides measured here re-equilibrated prior to or during eruption, then the differences between model results for oxides and amphiboles are reasonable -- the lower temperatures recorded by the oxides could correspond to eruptive conditions, while the amphiboles would still preserve their initial pressures and temperatures of earlier crystallization at depth.

Different measures of the H2O content in the low-K dacite melt plot closely; although the Lange et al. (2009) hygrometer uses melt inclusion compositional data, and the Ridolfi et al. (2010) model is based on amphiboles, both methods return calculated water contents in the 5.5-9 wt% range. The data for plagioclase-hosted melt inclusions lie between 5.6 and 8.0 wt% H2O (Figure 15). The calculated values are for the water content in melt inclusions and the parent melt of amphibole grains, respectively, but are similar.

Plagioclase- and quartz-hosted melt inclusions in the high-P dacite had H2O contents in the

5.5-7.5 wt% range, as measured by SIMS; plagioclase-hosted melt inclusions from the rhyolite ranged from 3.9 to 5.7 wt% H2O (Nadeau et al., 2016; Nadeau et al., 2015). The general similarity of high-P dacite melt inclusions’ water contents to those in the low-K dacite may indicate that both are derived, at least in part, from material richer in H2O than the rhyolite, given its low water contents relative to the two dacites. Given that water solubility in magmas increases with pressure, and therefore depth, the water-rich material was likely sourced from greater depths than the rhyolite (Moore et al., 1995). 32

The prospect of deeply-sourced high-H2O magma naturally raises the question of its composition. The Lange results indicate an increase in melt inclusion water content with decreasing silica in the host plagioclase, and the Ridolfi results show an increase in temperature for the same increase in H2O content. The correlation between lower silica content and higher temperature in high-H2O species is consistent with derivation from the mafic, high-temperature magmatic component of the Augustine system referenced before.

Although mafic arc magmas vary significantly in water content, it is possible that, prior to degassing during ascent, the intruding magma considered here could carry more dissolved

H2O than its higher-Si counterpart. Melt inclusion studies of arc magmas have indicated peak water contents of 6-8 wt% in basalts, compared to 1-6% in rhyolites and dacites; given that increases in pressure under crustal conditions tend to increase the saturation limit of magmas of any type, a significant component of the water in both the high-P and low-K dacites may have been derived from a mafic component (Wallace, 2005; Sparks et al.,

1978).

Implications:

Model data and petrologic comparison:

Comparing calculated parameters for the three rhyolite lithologies, the high-P dacite, and the low-K dacite shows that the three units are markedly different in several ways. Fe-Ti oxide data fall on the same trend in temperature-fO2 space for all three units, as shown in Figure 10; the low-K dacite exhibits the highest oxide temperatures of any ancient Augustine unit, some flow-banded rhyolite values excepted. This may indicate that the eruption temperature for the flow-banded rhyolite exceeded that of other ancient units, 33 including the low-K dacite, and thus the rhyolite may have carried more heat than the low-

K dacite from deep parts of the magma chamber system. The high-P dacite and white/yellow rhyolite lithologies show higher ∆NNO than both the low-K dacite and recent

Augustine units.

Melt inclusion data provide a base for additional comparisons between these units.

Compositional data from the low-K dacite’s melt inclusions have a tighter spread than those from the high-P dacite or the rhyolite lithologies, as shown in Figure 6. Quartz-hosted melt inclusions in both dacites show consistently higher SiO2 contents than their plagioclase-hosted counterparts, consistent with melting of the host grains and subsequent

(minor, in the low-K dacite’s case) contamination of melt inclusions (Nadeau et al., 2016).

SIMS was used to directly measure H2O contents for high-P dacite and rhyolite melt inclusions; although we did not do this for the low-K dacite, we do have water contents for

LKD melt inclusions calculated by the Lange et al. (2009) hygrometer. Comparison of these data shows similar, relatively high water contents for both dacite units (5.7-8 wt% and ~5.3-7.8 wt% for the LKD and HPD, respectively), while the rhyolite is comparatively

H2O-poor in all lithologies, most notably the flow-banded pumice with 2-5 wt% H2O

(Nadeau et al., 2015; Nadeau et al., 2016).

Pressures indicated by calcic amphiboles within the white rhyolite are lower than those of the low-K dacite, but the Ridolfi et al. (2010) model returned similar pressures for both the flow-banded rhyolite and for the low-K dacite. Most temperatures and pressures from amphiboles in the rhyolite lithologies are lower than those for the low-K dacite, except for those from the flow-banded lithology, the most mafic of the rhyolites. 34

25

20

15

10 # of counted grains countedof # 5

0

x=An%

Figure 17: Chart of the frequency of occurrence for various plagioclase compositions in the low-K dacite. The mean An content of each profiled grain was calculated and plotted instead of the entire profile to remove bias.

Some textural features in the high-P dacite, like quartz with pyroxene rims, abundant anorthite-rich plagioclase, and the rarity of discrete ferromagnesian phenocrysts, are very different from the low-K dacite; others, like cummingtonites with calcic amphibole rims and zoned plagioclase, are common to both units. Moreover, the range of plagioclase compositions in the low-K dacite in terms of anorthite content (Figure 17) is similar to that in the high-P dacite, but the latter unit has far more high-An feldspar. The population of plagioclase compositions in the low-K dacite is charted in Figure 17. This frequency distribution based on An content is distinct from that of each rhyolite lithology, although the positions of its peaks are similar to those in a similar chart for the white rhyolite (Figure 6 in Nadeau et al., 2015). Some of the oscillatory zonation patterns noted in high-P dacite plagioclase are similar to those shown in Figure 9; the significance of this resemblance is discussed below. 35

Property: Source: Value: Oxygen fugacity (∆NNO): Amphibole (Ridolfi et al., 2010) 0.8-2.0 Oxides (Ghiorso and Evans, 2008) 1.31-1.54 Temperature (ºC): Oxides (Ghiorso and Evans, 2008) 773-795 Amphibole (Ridolfi et al., 2010) 808-1041 Plagioclase-amphibole (Holland and Blundy, 1994) 463-580 Pressure (MPa): Amphibole (Ridolfi et al., 2010) 107-774 Amphibole (Hammarstrom and Zen, 1986) 160-901 Amphibole (Hollister et al., 1987) 143-973 Amphibole (Johnson and Rutherford, 1989) 118-741 Amphibole (Schmidt, 1992) 222-922 Plagioclase-amphibole (Holland and Blundy, 1994) 221-499 Water contents (wt%): Amphibole (Ridolfi et al., 2010) 5.6-8.6 Plagioclase-hosted melt inclusions 5.7-8.5

Table 4: Summary of all calculated values from models based on microprobe data for the low-K dacite.

As previously noted, the low-K dacite contains high-An rims with melt inclusions near the outer edges of at least two zoned plagioclases and calcic amphiboles growing on relatively low-pressure/temperature cummingtonites (Figs. 7A, 7B, 7D, and 9). This combination of features, together with the probable melting of melt inclusion host grains, indicates that the low-K dacite’s parent magma experienced at least one shift in its physical and compositional parameters, likely due to the introduction of a relatively high-Ca, high- temperature magma. Prior research has invoked magma mixing as a critical mechanism in the petrogenesis of the other Augustine units (Nadeau et al., 2015; Nadeau et al., 2016;

Tappen et al., 2009; Larsen et al., 2010; Roman et al., 2006). The high-An rim noted earlier for the large zoned plagioclases is consistent with plagioclase zonation patterns described in the high-P dacite, which were taken as evidence of a mixing event (Nadeau et al., 2016).

Little correlation exists between plagioclase and amphibole zonation patterns for units younger than the low-K dacite and those described in this work; Tappen et al. (2009) found zoned amphiboles with high-Ca cores and low-Ca rims in 2.1-1.0 ka andesitic tephras, which do not match the behavior of the amphibole phenocrysts described here. 36

These more recent tephras also record lower oxygen fugacities and pyroxene abundances than we see in the low-K dacite; given the number of as-yet undescribed units erupted between these tephras and the low-K dacite, attempts to directly link them are difficult to justify. The pattern of most points falling between An45 and An50, except for a lower region of An40-45 near the profile’s midpoint and a pronounced spike to An60 at the edge, does not match any of the zonation patterns described by Larsen et al. (2010) for the 2006 eruption.

Possibilities for unit relations:

The rhyolite, high-P dacite, and low-K dacite are distinct units, considering differences in bulk composition, phenocryst assemblage, and age; however, we can derive more useful interpretations by thinking of these units as parts of a succession, inherently linked to each other. Coombs and Vasquez (2014) linked dioritic inclusions in the 2006 eruption to the 26 ka rhyolite, describing the diorites as the solid residuum of a crystal-rich dacitic mush. The interstitial liquid from this mush would have been the source for the rhyolite, according to this model (Coombs and Vasquez, 2014). Nadeau et al. (2015) mention that the source magma for the white and flow-banded rhyolite pumices may have been blended with a mafic melt bearing high-Al amphiboles, with different proportions of the mafic melt to the original extracted liquid producing different lithologies. This idea was extended to the high-P dacite in Nadeau et al. (2016), where the authors consider the unit as comprising the same dacite mush extract that gave rise to the rhyolite, mixed with a mafic melt over time and residing deeply enough to produce late-stage ferromagnesian minerals. This theoretical model holds true if the magma chamber feeding Augustine 37 consisted of a large chamber connected to a series of feeder dikes, and is consistent with our hypothesis of mafic-silicic magma mixing to give rise to the low-K dacite.

The low-K dacite may represent a continuation of this process of magma mixing, perhaps with a greater proportion of mafic materials introduced into the original dacite mush extract than was the case for the high-P dacite. Such a scenario would account for the outer-zone increases in An content found in zoned plagioclases, the high temperatures and pressures recorded by calcic amphiboles, and the high H2O contents found in amphiboles and plagioclase-hosted melt inclusions, especially if the low-K and high-P dacites are relatively close in eruptive age. Radiometric age dates for the high-P dacite are not currently available; once ages have been calculated for that unit, the plausibility of this idea can be reassessed.

Implications for evolution:

This work has shown that the low-K dacite is likely the product of mixing between a silicic, lower-temperature magma and a more mafic magma from greater depth, where potentially one or both of these phases was an earlier Augustine unit or the parent magma of the same. The observed phenocryst assemblage and grain associations support the contention that the low-K dacite and high-P dacite are the products of similar processes; the low-K dacite would have been exposed to and incorporated more material from the high-temperature, high-pressure mafic component, either by mixing of a greater volume of mafic material or simply by virtue of spending more time at the mixture’s settling depth within the magma chamber (or dike network) prior to eruption. It is also possible that the high-P dacite is a precursor to the low-K dacite; Harker diagrams of whole-rock P2O5, 38

TiO2, and K2O plotted against SiO2 in various Augustine units (Figure 18) place the low-

K dacite away from the modern trendline, intermediate between the high-P dacite and the rhyolite on a separate trend (Wallace et al., 2013). The decreased K2O content in the low-

K dacite relative to the main trend is reflected in the absence of K-feldspar and the low abundances of K-bearing amphiboles like hastingsite; the high-P dacite is even lower in bulk K2O, and contained no hastingsite, and the rhyolite lacked both K-feldspar and hastingsite (Figure 12).

Cummingtonite-cored tschermakite grains discussed earlier (Figure 7D) also contain valuable information regarding the conditions experienced by the low-K dacite.

The four main amphibole barometers we used in this work return pressures of 598-782

MPa for the tschermakite grain, while the Ridolfi model yields estimates of 476 MPa and

950 ºC. Experimental work by Geschwind and Rutherford (1992) indicates that cummingtonite in dacitic systems is stable under conditions of 200-300 MPa and up to 790

ºC. In the most conservative scenario indicated by these pressure estimates, to crystallize the tschermakite at depth with the cummingtonite core, the cummingtonite would have to be transported from ~7.5-11 km to at least 17 km in depth, assuming a crustal density of

2.7 g/cm3, which is likely geologically implausible. Thus, another explanation for this grain association is desirable.

Nadeau et al. (2015) invoked entrainment of calcic amphiboles from depth to explain the presence of both cummingtonite and calcic amphibole in two of the rhyolite lithologies; however, the rhyolite contains discrete phenocrysts of cummingtonite and calcic amphibole, and the cummingtonite-tschermakite grain association considered here 39

Figure 18: Harker diagrams for whole-rock data from the known ancient Augustine units. The rhyolite, high-P dacite, and low-K dacite are indicated by purple triangles, blue diamonds, and orange squares, respectively. Other symbols correspond to various younger Augustine units. After Wallace et al. (2013).

40 is absent. In the high-P dacite, grain associations like that discussed here were explained as the product of a source magma originally crystallizing cummingtonite settling within a single magma storage system and being exposed to a higher-P, more mafic magma with stable tschermakite-pargasite (Nadeau et al., 2016). We offer an explanation that builds on both of these theories, predicated on a two-stage magma mixing process. Grains like that shown in Figure 7D could be produced by the introduction of high-temperature mafic melt into a silicic melt of higher volume, given insertion of the mafic melt sufficiently quickly to preserve the temperature gradient between it and the silicic melt, perhaps as part of a dike network (Figure 20B; Roman et al., 2006). If the resulting initial, hot, hybridized magma equilibrated with the rest of the magmatic system relatively slowly, conditions in localized areas (i.e. temperature and volatile contents) could remain suitable for the resorption of cummingtonite rims and subsequent crystallization of calcic amphiboles onto the cummingtonites, in addition to the growth of pre-existing calcic amphiboles transported with the mafic melt. This hypothesis does depend on the production of calcic amphiboles near or beyond the typical pressure and temperature limits for their crystallization; however, the existence of typically high-pressure amphibole cored by low-pressure amphibole requires such an event to have taken place. The more typical association of hornblende and cummingtonite for an ascending magma – cummingtonite as a rim on hornblende grains, the reverse of the pattern seen here – was noted in dacite pumices from the 1991 eruption of Mount Pinatubo (Pallister et al., 1999).

There is also evidence for such multi-stage magma mixing in plagioclase zonation patterns. As previously noted, the plagioclase shown in Figure 7A is a large, zoned grain which averages ~An45 in composition, with an An61 melt-inclusion-rich zone near its outer 41

Figure 19: Chart of modeled temperatures and pressures for the low-K dacite, as calculated by the Ridolfi et al. (2010) model. The crosses represent the uncertainty in calculated values for their respective regions of the graph. The lower dotted line represents the maximum stable temperature at a given pressure, and the upper dashed line indicates the upper stability limit of amphiboles in equilibrium with most of the volcanic systems referenced by Ridolfi et al. (2010).

edge; the plagioclase in Figure 7C consists of three zones, with anorthite contents of An76,

An57, and An42 in its inner, sieve, and outer zones, respectively. Given the continuous trend of decreasing An content in the sieve-bearing grain, and the anomalous increase in An content of the larger zoned grain, we can reasonably infer that the two grains record different exposure histories. Nelson and Montana (1992) showed that sieve textures in plagioclase feldspar can be generated not only from compositional disequilibrium, but also by rapid decompression in a system of static composition. It is possible that the sieve- bearing grain was originally part of a mafic, high-Ca magma undergoing fractional crystallization, as with the possible mafic dikes, and so was producing lower-An compositions over time (within dikes depicted in Figure 20A). The sieve zone would then 42

Figure 20: Cartoon schematic of the Augustine magmatic system during the generation of the low-K dacite. The hybrid magma in Stage 3 would be erupted as the low-K dacite. 43 have formed at the outer edge of the crystal when the mafic magma was injected into the lower-pressure, more silicic magma referenced before, or potentially during the post- injection equilibration stage, triggering decompressional development of the sieve texture in the corresponding zone (Figure 20B). Lower-An plagioclase would have crystallized onto the sieve zone afterward, in the larger, cooler, more silicic chamber following the second stage of mixing (Figure 20C). If our projection of the inner vs. outer zone is correct, then this pattern is additional evidence of the mixing process described above to explain the amphibole grains in Figure 7D. The second stage of the mixing/pressure-loss event described above was a late-stage occurrence relative to eruption, as is evident from the lack of reaction rims on low-K dacite amphiboles; research on the Mount St. Helens dacites found that the magma in that scenario must have ascended from ~8 km in >5 days in order to produce the observed rimless amphiboles (Rutherford and Hill, 1993). We cannot constrain the timing of the initial injection based solely on reaction rim development, as the local conditions during the first phase of mixing would remain hot and mafic- dominated, and so suitable for continued crystallization of the same amphiboles as before, if any. The introduction and subsequent mixing of the more mafic, high-An component with the main silicic magma may match the An spike and melt entrapment near the edge of the grain shown in Figs. 7A and 7B, as well as the likely melting of various melt inclusion host grains.

The chief insight from a compositional-evolutionary perspective that we draw from these data is that Augustine was still erupting material derived in part from liquid extracts of the dacitic crystal mush nearly 20 k.y. after the first known eruption of that material.

Some communication with ancient lithologies has already been observed in recent 44 eruptions, such as the exhumation of dioritic inclusions coeval to the 26 ka rhyolite in the

2006 eruption; however, that material was found to be solid fragments of wall rock torn from the walls of the Augustine feeder system (Coombs and Vasquez, 2014). That the crystal mush extract was still liquid so long after its initial eruption is further evidence for the introduction of hot, deeply-sourced magmas to the system, which provided sufficient heat input for the silicic melt to retain its mobility and facilitated its eruption. The scenario we consider here is that of mixing between at least two liquid phases, enabling the generation of bulk compositions intermediate to the two components. That the low-K dacite is positioned in between the rhyolite and high-P dacite on the previously-mentioned P2O5,

K2O, and TiO2 Harker diagrams indicates that the low-K dacite may even be the product of mixing between the rhyolite and high-P dacite, or at least their progenitors. Once the gap in research between the low-K dacite and the tephras studied by Tappen et al. (2009) is filled, the extent of the mixing process between these endmember phases can be more fully constrained.

Recent eruptions from Augustine have produced less silicic units than those primarily discussed here. Holocene whole-rock data plot mostly as andesites and basaltic andesites, and correspond to generally thinner deposits and lower overall volumes of erupted rock (Figure 6, this work; Waitt and Begét, 2009). If, hypothetically, these units were produced from mixing of the same rhyolitic main-chamber magma invoked for the older units with a mafic component, a much greater proportion of mafic material would be required than was needed to produce the ancient dacites. The more likely explanation is that offered by Roman et al. (2006), who, discussing the 1986 eruption, invoke mixing between leftover dacitic 1976 magma and a mafic magma injected from a deeper reservoir. 45

Larsen et al. (2010) describe a similar process in their account of low-silica andesite generation during the 2006 eruption. Harker diagrams combining Pleistocene, early

Holocene, and modern tephra data show two trendlines, with the Pleistocene rhyolite, early tephras like the HPD and LKD, and the 2006 eruption’s diorite blocks forming their own trend, separate from recent units (Coombs and Vasquez, 2014; Wallace et al., 2013).

Moreover, the low-K dacite is situated near the end of the modern trend, raising the possibility that the LKD is a parent magma for modern units (Figure 18). As the bulk composition and mineral assemblage involved in these recent eruptions are very different from those of the ancient units, we can infer that the Augustine feeder system currently draws from a bulk magma distinct from the older dacite mush liquid extract (Figure 21).

This idea agrees with pressure data from the rhyolite, low-K dacite, and ~2 ka tephras; melt inclusions from the rhyolite indicate a storage pressure of ~260 MPa, while amphibole data from the low-K dacite and ~2 ka tephras show significantly lower pressures of equilibration. Specifically, the lowest-pressure cluster of low-K dacite amphiboles records pressures of 107-186 MPa, according to the Ridolfi et al. (2010) model, and amphiboles in the tephras studied by Tappen et al. (2009) record pressures in the 140-260 MPa range.

Thus, the low-K dacite may represent the physical and chemical transition from deep magma chambers like those that hosted the rhyolite and high-P dacite to the shallower, more mafic dike network associated with recent eruptions by Larsen et al. (2010) and

Roman et al. (2006). Indeed, an InSAR study of surface deformation on Augustine Island after the 2006 eruption found evidence for two different magma bodies beneath the volcano, one at 2-4 km and the other at 7-12 km depth (Lee et al., 2010); the deeper reservoir’s location is consistent with pressure-based estimates of the rhyolite’s source 46

Figure 21: Cartoon of the genetic connections between the ancient Augustine dacite mush, its liquid extract, and solids from the mush erupted in 2006. After Figure 13 in Coombs and Vasquez (2014).

depth (Nadeau et al., 2015), while the shallower depth coincides generally with the depths indicated by studies on the most recent eruptions.

Similar mixing mechanisms have been invoked at other volcanic systems as well.

Geschwind and Rutherford (1992) studied the 3.5 ka Mount Saint Helens eruption using similar techniques to those used here, including consideration of cummingtonite phase equilibria, oxide temperature data, and melt inclusion H2O contents, concluding that the eruption was likely triggered by the intrusion and mixing of a mafic magma with the larger 47 magma chamber. A 1997 study of the ~120 ka history of Vulcano, a volcanic complex north of Sicily, found that the basalts erupted over that time span were derived from at least two different mantle-derived hosts; this conclusion was based partially on the variable mineralogy and Harker diagram trends exhibited by different age groups of the basalts (De

Astis et al., 1997). Venezky and Rutherford (1999) studied dacite erupted by Mount Unzen in 1991, relying heavily on amphibole- and Fe-Ti-oxide-based geothermobarometers similar to those used in this work. These authors were able to derive a depth of storage, pre-eruptive mixing temperatures, and equilibration times for the samples studied, based on petrographic observations, the aforementioned models, and their own experimentally- derived phase equilibria. Other petrologic studies on intermediate volcanic rocks, particularly those in arc settings, corroborate the idea that mixing between silicic and mafic magmas is a critical part of the generation and eruption process of intermediate rocks

(Eichelberger, 1975; Nixon, 1988).

Mixing between magmas of differing compositions has ramifications beyond the simple development of a new magma. A study of the compositional evolution of Medicine

Lake Volcano in California found that, based on inclusions of gabbroic solids within erupted rhyolites, that exsolution of volatiles from crystallizing solids is an important factor in the onset of explosive volcanic eruptions (Grove and Donnelly-Nolan, 1986). Mixing events between relatively hot and cool magmas may promote crystallization of minerals from the hotter magma upon cooling. In systems like Medicine Lake, where the hotter material is basaltic and so carries significant dissolved volatiles like SO2 (Pallister et al.,

1992), mixing events may contribute to the exsolved volatile content of the hybrid magma, and may trigger eruptions. If applicable to a significant extent in the Augustine system, this 48 concept is a critical consideration when evaluating the explosivity of future volcanic eruptions. There has been a decrease in the explosive hazard of recent Augustine eruptions relative to ancient events, given the decrease in silica and corresponding decrease in viscosity of the pre-eruptive magma. However, our data indicate that relatively high H2O contents were present in the low-K dacite, including its mafic minerals; if the mafic material mixing with recent magmas still contains these high concentrations of volatiles, modern-day eruptions may still pose a significant hazard. Moreover, ancient units like the low-K dacite and the rhyolite were thicker than modern deposits, correlating to an overall greater volume of material (Wallace et al., 2013). As previously noted, InSAR data indicate the presence of a magma body in the modern at a similar depth to the Pleistocene rhyolite

(Lee et al., 2010; Nadeau et al., 2015). If this reservoir does exist and contains eruptible silicic magma, high-volume rhyolitic or rhyodacitic eruptions from Augustine may still be possible. As such, additional study of Augustine’s evolution is critical for evaluating its current source region, as well as determining its future threat.

Conclusions:

This work used hand sample and petrographic observations, electron microprobe data, and a variety of geologic models to evaluate the composition, characteristics, and genetic properties of a low-K dacite from Augustine Volcano. Oxygen fugacity data from low-K dacite-hosted oxides plot on the same trend as other ancient Augustine units, which, together with Harker diagram trends, melt inclusion chemistry, and other data sources, connect the low-K dacite to other units in the Augustine tephrostratigraphy.

Elemental ratios, associations of normally-mutually-exclusive minerals, oscillatory 49 zonation patterns, and the spread of amphibole types and corresponding calculated values indicate that multi-stage mixing of high- and low-silica magmas was a critical process in the low-K dacite’s genesis; the mixing would have taken place in a similar manner as that which produced the high-P dacite. The low-K dacite is revealed as one step in Augustine

Volcano’s development, an evolutionary process lasting over 25 ky. The dacite was likely generated by mixing between the same deeply-sourced, high-pressure mafic magma that contributed to the generation of the high-P dacite and a high-silica parent magma similar to that which produced the 26 ka rhyolite. Similar mixing processes are implicated in the petrogenesis of other ancient and modern units in the Augustine succession, and in that of volcanic units from other systems. Further research is needed to fully constrain the extent to which these processes have acted and continue to act in the Augustine system; the status of high-silica components of the Augustine feeder system, including the high-silica component of the low-K dacite, is especially relevant to determining future eruptive hazards.

50

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Appendices:

Appendix A: Microprobe data for low-K dacite samples and calculation results

Appendix B: Total alkali vs. silica plot for low-K dacite melt inclusions and other

Augustine units

Appendix C: Ridolfi et al. (2010) amphibole calculator and results

Appendix D: Holland and Blundy (1994) plagioclase-amphibole thermobarometer and results

Appendix E: Ghiorso and Evans (2008) oxide thermometry results

Appendix F: Amphibole names and Al2O3 vs. MgO graph

All appendices are available as Microsoft Excel 2016 files.

58

This thesis has been approved by

The Honors Tutorial College and the Department of Geological Sciences

Dr. Patricia Nadeau

Professor, Geological Sciences

Thesis Advisor

Dr. Keith Milam

Director of Studies, Geological Sciences

Dr. Cary Frith

Interim Dean, Honors Tutorial College