TEXTURAL ANAYLSIS OF MAFIC ENCLAVES AS AN INSIGHT INTO MAGMA MIXING PROCESSES AT CHAOS CRAGS, LASSEN VOLCANIC CENTER, CALIFORNIA

Crystal Dawn Hootman B.A., California State University, Sacramento, 2007

THESIS

Submitted in partial satisfaction of the requirements for the degree of

MASTER OF SCIENCE

in

GEOLOGY

at

CALIFORNIA STATE UNIVERSITY, SACRAMENTO

SUMMER 2011

TEXTURAL ANAYLSIS OF MAFIC ENCLAVES AS AN INSIGHT INTO MAGMA MIXING PROCESSES AT CHAOS CRAGS, LASSEN VOLCANIC CENTER, CALIFORNIA

A Thesis

by

Crystal Dawn Hootman

Approved by:

______, Committee Chair Dr. Lisa Hammersley

______, Second Reader Dr. Diane Carlson

______, Third Reader Dr. David Evans

______Date

ii

Student: Crystal Dawn Hootman

I certify that this student has met the requirements for format contained in the University format manual, and that this thesis is suitable for shelving in the Library and credit is to be awarded for the thesis.

______, Department Chair ______Dr. David Evans Date

Department of Geology

iii

Abstract

of

TEXTURAL ANAYLSIS OF MAFIC ENCLAVES AS AN INSIGHT INTO MAGMA MIXING PROCESSES AT CHAOS CRAGS, LASSEN VOLCANIC CENTER, CALIFORNIA

by

Crystal Dawn Hootman

The Chaos Crags are a series of volcanic domes located in Lassen Volcanic

Center, southernmost Cascades Range. The six domes erupted approximately 1100 years

ago. The host rock is dacite, which is compositional similar in all domes at (66-69 wt. %

SiO2), and differs from the mafic enclaves that range in composition from (53-61 wt. %

SiO2). The enclaves result from two distinct and thermally different magmas mixing and

can provide an insight into the processes of magma mixing. Five texturally different

enclaves types were identified. To determine abundance of the enclaves in each dome,

113 point count stations were completed in the dome complex talus slopes. Previously

collected and new samples were photomircrographed and plagioclase crystals were hand

traced to be processed in Crystal Size Distribution (CSD), which determines nucleation and growth time of crystals. Previously completed geochemical data was used to determine if the enclaves and host were similar or different in composition. The results of observation at Chaos Crags were 1) The total abundance of enclaves increases with eruption of domes. 2) There are distinctive abrupt increases in the total abundance of enclaves between eruption of domes B and C, domes C and D.

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3) There are more modest increases in the total abundance of enclaves between eruption of domes A and B, domes E and F. 4) Although it seems likely that all enclave types are present in each dome, changes in distribution of enclave type seem to correlate with the increase in total abundance. 5) Host dacites show a narrow range in composition while enclaves show a mixing trend from a more mafic source toward the host dacite. 6) There is a clear link between enclave type and geochemistry. The most mixed enclave types are type 1. The least mixed are types 3 and 4. The magma mixing model proposed is one of repeated injections of small batches of mafic magma, either injected as a fountain or ponded at the base of the magma chamber. Also the mafic magma injections are the suggested cause of eruption. Enclaves present are directly related to the type of recharge event. Disaggregation of the enclaves occurred in the conduit during the eruption. This thesis was just an initial step using CSD and geochemical data leading to some surprising results and further research should be conducted.

______, Committee Chair Dr. Lisa Hammersley

______Date

v

ACKNOWLEDGMENTS

This thesis would have never been completed without the support of my graduate advisor, Dr. Lisa Hammersley. She was patient with me during the times I could not work on my thesis, yet would remind me that I could finish and remind me of my goal that I wanted that to teach. She helped my understanding of want it takes to be a scientist. I feel privileged to be Lisa’s first graduate student. I would also like to thank my committee members, Dr. Diane Carlson and Dr. David Evans, willing to take time out of their summer schedule to help me complete my Master’s Degree. I will always be appreciative, especially to Dr. Diane Carlson, throughout the last nine years, who would witness my change from being a horrible field mapper in Field Geology to signing off on my Master’s Degree. This project would have not even been started, if it were not for Dr. Michael Clynne of the USGS graciously allowing me to use his samples collected from Chaos Crags before I collected my own sample set. Christiana Stout, an undergraduate student at the time I started my project, introduced me to the Chaos Crags field area. Christine O’Neill was an undergraduate that also had a project at the Chaos Crags Jumbles and it was a privilege to spend time in the field and determining the enclave types with her. To my friend Melinda Fredericksen, she helped me complete field work and was excellent company out in the field. I appreciate the National Association of Geoscience Teachers for giving me a scholarship that helped with field work and tuition. Also I need to thank my son Justin, he means the world to me. Lastly, I need to thank my better half, Ed Shakespeare, he was encouraging me every day to finish my thesis, eventually it sunk in and I completed the task in hand. It feels incredible that my thesis is done!

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TABLE OF CONTENTS Page

Acknowledgments...... vi List of Tables ...... ix List of Figures ...... x Chapter 1. INTRODUCTION ……………...……………………………………………………….... 1 1.1 Purpose of the Study…………………...……………………………………….... 1 1.2 Geologic Setting of the Lassen Region ...... 2 1.3 Geology of the Chaos Crags ...... 5 2. METHODS ...... 8 2.1 Identification of Enclave Types ...... 8 2.2 Field Work ...... 8 2.3 Crystal Size Distribution Analysis (CSD) ...... 11 3. RESULTS ...... 16 3.1 Classification of Enclave Types ...... 16 3.2 Enclave abundance in the Chaos Crags ...... 23 3.3 Crystal Size Distribution ...... 27 3.4 Geochemistry ...... 33 4. DISCUSSION ...... 41 4.1 Magma Mixing Models...... 41 4.2 The Lassen Peak Magma Mixing Model ...... 42 4.3 Mixing studies at Chaos Crags………………………………………………… .. 45 4.4 Observations from Chaos Crags………………………………………………… 47 4.5 Chaos Crags Magma Mixing Model…………………………………………… 50 5. CONCLUSION………………………………………………………………………… 56 Appendix A. Point Count Locations ...... 58 Appendix B. Hand Sample list ...... 62 Appendix C. Theory of Crystal Size Distribution ...... 64 Appendix D. Background to CSDCorrections Program ...... 67

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Appendix E. Individual Crystal Size Distribution Graphs ...... 68 References ...... 166

viii

LIST OF TABLES Page

1. Table 1 Total Enclave Abundance for each dome at Chaos Crags…………… 24

2. Table 2 Major Element Composition of Host and Enclave Rocks…………… 35

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LIST OF FIGURES Page

1. Figure 1 Location map of the major volcanoes in Cascade Range.…………….. 3

2. Figure 2 Simplified Geologic Map of Chaos Crags, Lassen Volcanic Center,

California…………………………...………………………………………….. 6

3. Figure 3 Point count grid with 361 points and example of point count sample

location.………………………………...……………………………………… 9

4. Figure 4 Location map of point count locations for each of the domes at Chaos

Crags…………………………..…...... 10

5. Figure 5 (A) Photomicrograph of sample #88-1283, which is enclave type 2 in

Dome B. …………………………………………………...………………….. 12

6. Figure 6 An example of the CSDCorrections Program data entry page from

http://depcom.uqac.ca/~mhiggins/csdcorrections.html……………...... …….. 14

7. Figure 7 Crystal shape of 3D sections calculated from the 2D image by using

short axis and normalized frequency.…………………………………………. 15

8. Figure 8 Example of a logarithmic CSD graph of #88-1283…………………... 15

9. Figure 9 Different textural types of enclaves present at Chaos Crags…………. 17

10. Figure 10 Different types of enclaves margins observed in various dome

locations…………………………...…………………...…………………….. .19

11. Figure 11 Disequilibrium textures present in the host and enclaves………….. 23

12. Figure 12 Abundance of enclaves by dome…………………………………….24

13. Figure 13 Abundance of different enclave types in each dome...…….. ..……. .26

x

14. Figure 14 CSD curves for the host dacite of each dome….……...……………. 29

15. Figure 15 CSD curves for each enclave type………………………….………. 30

16. Figure 16 CSD curves for enclave types for each dome……..………..………. 33

17. Figure 17 CaO vs. SiO2 host dacite and enclave samples analyzes as part of this

study for CSD………………………………………………………………… 37

18. Figure 18 A comparison of major element compositions at Chaos Crags of host

dacite (diamonds) and mafic enclaves (squares) …………………...... ……… 38

19. Figure 19 CaO vs. SiO2 for all samples analyzed by Dr. Michael Clynne …… 39

20. Figure 20 Vesiculation model of the mafic foam layer ………………………. 42

21. Figure 21 Magma mixing and enclave formation model for Lassen Peak…..... 44

22. Figure 22 Enclave formation model proposed for Chaos Crags by Tepley et

al., (1999)……………….………………………………………….………... 46

23. Figure 23 Magma mixing model for Chaos Crags………………...………….. 52

xi 1

Chapter 1

INTRODUCTION

1.1 Purpose of the Study

Magmatic recharge of mafic magma into a shallow reservoir has been shown to be the trigger for volcanic eruptions, yet our understanding of this process is still incomplete (e.g. Crater Lake (Bacon, 1986); Pinatubo (Pallister et al., 1992); Lassen Peak

(Clynne, 1999); El Chichón (Tepley et al., 2000)). Mafic enclaves (also called magmatic inclusions or inclusions) provide the physical evidence that mixing has taken place in a magma chamber between two distinct compositionally and thermally different magmas and can be used as a tool to provide insight into the process of magma mixing (Heiken and Eichelberger, 1980; Bacon, 1986; Clynne, 1999, Browne et al., 2006; Feeley et al.,

2008). Good outcrop exposure and the abundance of several different textural types of mafic enclaves make Chaos Crags an ideal location to study magma mixing processes.

Early studies of the abundance of enclaves at Chaos Crags suggest that the volume of magma mixing into the system increased during the eruption of domes (Stout, 2007).

Questions remain as to whether the enclaves represent a single mixing event or whether the formation of each dome was preceded by a unique mixing event.

This study aims to more completely understand the magma mixing processes that occurred during the eruption of Chaos Crags. The study is made up of three components:

(1) identification and sampling of different textural types of enclaves from each dome, (2) field mapping the distribution for each different textural type of enclave for each dome 2

(measured in talus slopes of the domes), and (3) a detailed petrographic study of the sampled material using crystal size distribution (CSD) and geochemical composition.

While this study is a focused study of the volcanic domes at Chaos Crags, the results could provide insight in magma mixing that is widely applicable to other locations; both ancient and currently active volcanic systems. In particular, a better understanding of the timescales of magma mixing may be important in assessing volcanic hazard.

1.2 Geologic Setting of the Lassen Region

The Chaos Crags are located in the Lassen Volcanic Center (LVC), the

southernmost active volcanic system in the Cascade Range (Figure 1). Subduction of the

Explorer, Juan de Fuca, and Gorda plates beneath the North American plate is associated

with the development of the Cascades (Guffanti and Weaver, 1988). Specifically in the

Lassen region, volcanism has been influenced by subduction of the Gorda plate,

migration of the Mendocino triple junction and Walker Lane, and extension in the Basin

and Range Province (Guffanti et al., 1990; Blakely et al., 1997, Unruh et al., 2003). Late

Quaternary northwest trending normal faults extend from Medicine Lake Highlands to

south of Lassen and connect the southern Cascades Range to the Walker Lane and Basin

and Range Province (Guffanti et al., 1990; Blakely et al., 1997; Unruh et al., 2003). The

Lassen volcanic region is located between the Lake Almanor and Hat Creek grabens

(Guffanti and Weaver, 1988).

3

Figure 1. Location map of the major volcanoes in Cascade Range. Lassen Peak and Chaos Crags are found in the Lassen Volcanic Center and are designated by the white triangle. Modified from Clynne (1990).

4

Volcanic rocks types found in the Lassen region include basalt, basaltic andesite,

andesite, dacite and rhyolite. Two types of basalt are represented in the region: 1) a low

potassium olivine tholeiite basalt (LKOT); and 2) a calc-alkaline basalt. The calc- alkaline basalts represent typical subduction-related melts. A possible origin for the discontinuous LKOT volcanism in the Lassen region is the propagation of northwest trending faults of the Walker Lane seismic area that extends slip back to the Cascadia subduction zone (Guffanti et al., 1990; Unruh et al., 2003). The youthful faults may also be the source for the growth of long-lived volcanic centers (Guffanti et al., 1990).

Volcanism in the region can be classified into two classes: short-lived coalescing volcanoes compositionally ranging from basaltic to andesitic; and long-lived centers compositionally ranging from basaltic to dacitic (Clynne, 1990; Guffanti et al., 1990).

Over the last 3 Ma, five different volcanic centers have been identified within the Lassen region, but older ones probably existed (Clynne, 1990; Guffanti et al., 1990). The volcanic centers identified in the region, in order of age are Snow Mountain, Dittmar,

Yana, Maidu, and Lassen. LVC has an active hydrothermal system progressing to terminal stage, while the four older centers have extinct hydrothermal systems.

Evolution of the LVC is divided into three phases, stage one and two (600-400 ka), involved the growth and destruction of the Brokeoff volcano, an 80 km3 andesitic

stratovolcano. Stage 1 lavas tend to be heterogeneous while stage 2 lavas are

homogenous (Clynne, 1990). Stage three (< 400ka) consists predominantly of silicic

volcanism. Stage 3 began with the eruption of the Rockland sequence, a widespread

rhyolitic pumice fall and ash flow (with an estimated volume of 50 km3), then continued

5 with the Bumpass (250-200 ka) and Loomis sequences (100-0 ka) (Clynne, 1990).

Recent silicic volcanic domes that consist of similar magma mixing textures include the

28.3 ka Lassen Peak (Turrin et al., 1998), which was last active from 1914-1917, and the

1.1 ka Chaos Crags (Clynne and Muffler, 1989).

1.3 Geology of the Chaos Crags

The Chaos Crags are a series of young silicic volcanic domes, named A-F, located north of Lassen Peak (Figure 2). Eruption of the domes began 1230 years ago with three pyroclastic flows that traveled down the Manzanita and Lost Creek drainages (Heiken and Eichelberger, 1980). This was followed 1129 years ago by more pyroclastic flows, named A and B and eruption of dome A. Approximately 1062 years ago eruption of a larger pyroclastic flow C destroyed dome A, which is believed to have blocked the conduit. Domes B-F were then emplaced sequentially. After a hiatus and unrelated to volcanic activity, three rockfall avalanches collapsed from dome C, 275 years ago

(Clynne and Muffler, 1989). The total volume of erupted materials at Chaos Crags is approximately 2km3 (Tepley et al., 1999).

6

Figure 2. Simplified Geologic Map of Chaos Crags, Lassen Volcanic Center, California.

The petrology of lavas at Chaos Crags has been described by Heiken and

Eichelberger, (1980); Clynne and Muffler, (1989); and Tepley et al., (1999). The dominant rock type at Chaos Crags is a porphyritic hornblende-biotite dacite containing a variety of mafic enclaves. Based on slight differences in host rock composition and the number and size of enclaves, Tepley et al. (1999) grouped the lava into groups 1 and 2.

Group 1 lavas contain Dome A and B and their associated pyroclastic flows. Group 2 lavas contain Dome C, D, E, F and associated dome collapse deposits. The phenocryst

7

assemblage in both groups, comprise 30% of the rock and includes plagioclase, quartz,

biotite, hornblende, hypersthene, and low-Ti titanomagnetite in a glassy to devitrified groundmass; which is similar to Lassen Peak.

Enclaves are individual blobs of mafic lava that have been undercooled with

respect to the silicic lava after mixing between the two different compositional magmas.

Previous studies at Chaos Crags by Tepley et al. (1999) indentified three different types

of enclaves, a fine grained porphyritic, a coarse grained non-porphyritic, and a medium

grained porphyritic. Common phenocryst phases in the enclaves are plagioclase,

hornblende, orthopyroxenes, clinopyroxenes, opaques and some rare olivine. Related to

mixing processes are xenocrysts, which are reacted host phenocrysts found within

enclaves (Bacon, 1986). Xenocrysts in Chaos Crags enclaves include hornblende and

quartz that exhibit resorption rims, plagioclase and hornblende can have sieve textures

and plagioclase may have new crystal growth rims. Tepley et al. (1999) observe that

enclaves in the earlier domes have a fine-grained texture, crenulated margins and little

vesiculation while enclaves in the later domes have more vesiculation and many do not

have crenulated margins due to disaggregation.

8

Chapter 2

METHODS

2.1 Identification of Enclave Types

Prior to conducting field work, samples of enclaves from prior studies (personal

communication by Dr. Michael Clynne of the United States Geological Survey and Dr.

Lisa Hammersley) were examined in hand sample and thin section. The enclaves were

classified into five types based on texture and mineralogy. Type 1 enclaves are characterized by an aphantic appearance with abundant small vesicles and large host plagioclase. Type 2 enclaves have a distinctive fine grained “salt and pepper” appearance in hand sample. Type 3 enclaves are easily distinguished by the presence of acicular

hornblende crystals up to 3mm in length and a plagioclase groundmass. Type 4 enclaves

are identified by the presence of visible olivine crystals. Type 5 enclaves display a coarse

grained “salt and pepper” appearance. Complete descriptions of the texture and

mineralogy of the five enclave types are provided in Chapter 3 (results).

2.2 Field Work

Field work commenced in the summer of 2007 and lasted through the summer of

2011. The primary method used in the field was point counting. Outcrop scale point

counting has been shown to be an effective method for determining the distribution of

enclaves in the field (Wolfe et al., 2007; Feeley et al, 2008). The point count process involved taping a 1m2 grid on to an outcrop (Figure 3). Grid lines were spaced 5 cm

9 apart, with a total of 361 points where the lines crossed. When an enclave was present on an intersection of the grid it was counted and classified into its textural type. This method provided a volume estimate for each enclave type. Each time a point count was completed a photograph was also taken.

Figure 3. Point count grid with 361 points and example of point count sample location.

Due to the steep and craggy nature of the Crags, point counting was not conducted on the domes themselves but on large boulders within the talus slope of each dome. Feeley et al. (2008) show that enclave distribution can vary within a dome due to flow regimes. The talus slopes represent an averaging of material from different points on the dome and it is assumed that they give a fair representation of the total abundance of enclaves.

10

A total of 113 point count stations were recorded over the six domes (Figure 4).

The distance between most point count stations was approximately 100m. A station consists of two points counts completed on two different boulders in the vicinity of the

GPS coordinate to get an average. The exact location of each point station is given in

Appendix A.

1000m

Figure 4. Location map of point count locations for each of the domes at Chaos Crags.

A total of 38 new samples were collected during field work. Sampling focused on creating a complete set of enclave types for each dome. Collected samples ranged from 3

11

to 6 inches and were selected based on determined textural difference. Some of the new

samples were thin sectioned for petrography and use in Crystal Size Distribution

Analysis. A complete list of samples is provided in Appendix B.

2.3 Crystal Size Distribution Analysis (CSD)

Crystal size is a function of nucleation and growth time (Marsh, 1988, 1998).

The theory of Crystal Size Distribution (CSD) was developed by two chemical engineers,

Randolph and Larson (1971), as a quantitative approach to measure the crystallization process independent of an exact kinetic theory. CSD was first applied to geologic systems in two different studies: the development of CSD theory (Marsh, 1988) and application to igneous rocks (Cashman and Marsh, 1988). The equations developed by Randolph and

Larson and adapted by Marsh form the foundation of CSD theory. A summary of the background and formulas for CSD analysis can be found in Appendix C.

Digital photomicrographs of thin sections were analyzed for representative host rock and enclaves from each dome at Chaos Crags. Maximum information for rock textures is achieved by selecting many different spots in thin sections for each dome

(Higgins, 2000). Individual plagioclase crystals were traced by hand using tracing paper and afterwards verified with the use of a polarizing microscope (Figure 5). Tracing by hand removes any bias and allows for a more accurate total crystal number (Martin et al.,

2006). For example, digital analysis programs are more likely to count touching crystals

as a single crystal.

12

A B

10x 10x

Figure 5. (A) Photomicrograph of sample #88-1283, which is enclave type 2 in Dome B. (B) Example of hand traced plagioclase overlay.

The tracings were scanned at 300 dpi, converted to TIFF’s and opened in the program Image J, a public domain Java image processing program that can calculate dimension and area of objects in an image. (ImageJ is downloadable for free at

http://rsbweb.nih.gov/ij/.) Image J was used to calculate volume, average size, and

location sites of crystals in each thin section tracing. Data from Image J was inserted as a

plug-in into the CSDCorrections 1.39 program (Figure 6; Higgins, 2000; downloadable

for free at http://wwwdsa.uqac.uquebec.ca/~mhiggins/CSD.html), which converts 2D

data into 3D based on assumptions regarding the crystal shape. These assumptions are

entered by the user in the form of (S: I: L), S=shortest crystal dimension, I= intermediate

13 crystal dimension, and L= longest crystal dimension. A 1:1:1 ratio reflects cubes, 1:1:3, acicular crystals, and 1:10:10, tablets (Figure 7). The CSD analysis is typically presented as a logarithmic scale graph that compares size against population density of crystals and evenly distributes maximum intersection size across the total crystal size range (Figure 8;

Marsh 1988, Marsh, 1998). To obtain an accurate 3D shape using the CSDCorrections program, at least 200 phenocrysts must be represented and have a R2 value over 0.8 to be reliable (Morgan and Jerram, 2006). R2 is determined as a fractional variation that best fits an example in the database of 703 shapes with crystal habits. For more information on the background of the CSDCorrections program, see Appendix D.

14

Figure 6. An example of the CSDCorrections Program data entry page from http://depcom.uqac.ca/~mhiggins/csdcorrections.html.

15

Figure 7. Crystal shape of 3D sections calculated from the 2D image by using short axis and normalized frequency. From Morgan and Jerram (2006).

Ellipse Major axis 10

9

8

7

6

5

4

3

ln (population density) 2

1

0

-1

0 1 Size

Figure 8. Example of a logarithmic CSD graph of #88-1283.

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Chapter 3

RESULTS

3.1 Classification of Enclave Types

The enclaves were classified into five types based on texture and mineralogy

(Figure 9). Type 1 enclaves are characterized by an aphantic appearance. Very fine

grained crystals of plagioclase and hornblende comprise a gray groundmass. Large

reacted plagioclase crystals (from the host dacite) up to 6mm in length are present as well as rarer hornblende crystals up to 6mm are also present. Vesicles up to 3mm in size are common in type 1 enclaves. Type 2 enclaves have a fine grained “salt and pepper”

appearance. Plagioclase and hornblende are blocky in shape. Reacted host plagioclase

crystals, up to 5mm in length are present. Type 3 enclaves are easily distinguished by the

presence of acicular hornblende crystals up to 3mm in length and a plagioclase

groundmass. Occasional reacted host plagioclase crystals are present. Type 4 enclaves

have visible olivine crystals up to 3mm in size. The groundmass consists of both blocky

and acicular hornblende crystals up to 2mm. Plagioclase crystals also are both blocky

and acicular. Reacted plagioclase crystals are up to 4mm in length. Type 5 enclaves

display a coarse grained “salt and pepper” appearance. Hornblende crystals are blocky

and up to 2mm.

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A B

C D

E

Figure 9. Different textural types of enclaves present at Chaos Crags. A) Type 1; B) Type 2; C) Type 3; D) Type 4; E) Type 5

Enclave size rarely exceeds 1m in length, but can also be as small as 10mm, (the

smallest enclaves that could be recognized in the field). Weathering in some of the

domes also made distinguishing the different types of enclaves difficult. Dome C is

highly altered especially in the jumbles debris avalanche field with the dacite and mafic

enclaves altered to a salmon color. Dome A, B, and E also have experienced more

18 weathering in certain locations, such as talus slopes debris covered in lichens due to other geomorphological controls, such as precipitation.

The margins of the enclaves can vary from crenulated to disaggregated (Figure

10). Crenulated margins have a convoluted, wavy appearance. Disaggregation of enclaves occurs during transport of mostly cooled enclaves within the magma chamber.

Disaggregated margins are those where smaller portions of the enclave are found in the host rock close to the margin. Some enclaves have sharp margins. These may represent the cores of enclaves that have disaggregated and then been transported. It appears that enclaves are more crenulated and less disaggregated in domes A and B. In domes C-F, the enclave margins are less crenulated and more disaggregated.

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A B

C D

E

Figure 10. Different types of enclave margins observed in various dome locations. (A) Chilled margin from Dome B, note pen for scale (B) A disaggregated margin from Dome D, note 12 inch ruler for scale (C) A crenulated margin from Dome C, also note the inherited plagioclase host dacite in the enclave. Size of enclave is 15cm (D) Crenulated margins between the two different textural types of enclaves. Size of enclave is 30cm. (E) Disaggregated enclaves in Dome B, note field notebook for scale.

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Disequilibrium textures are present in both host and enclaves. Common disequilibrium textures include sieved crystals, embayments, compositional zoning, and resorption rims (Figure 11).

The phenocryst phases present in the host and enclaves are plagioclase, amphibole, orthopyroxene, clinopyroxene, biotite, quartz, olivine and iron oxides.

Plagioclase is the most common phenocrysts, present as large crystals up to 7mm. The crystals are typically euhedral to subhedral and can vary in habit from prismatic to acicular. The larger crystals plagioclase crystals in the enclaves are incorporated host dacite crystals that exhibit unreacted or reacted textures. Sieve textures and zoning in both host and enclaves are quite common. Sieve textures can vary from just a few microns from the rim to the entire crystal.

Amphiboles usually are 1mm in size but can be larger and vary from anhedral to euhedral. The crystals exhibit good cleavage planes. The majority of the amphibole phenocrysts exhibit opaque reaction rims of, most likely, magnetite. Orthopyroxene is rare, but can be found in glomerocrysts. When present, it ranges from 0.1mm to 0.5mm in size and is anhedral to subhedral. Clinopyroxenes range from 0.1mm to 1mm in size and exhibit a crystal shape from anhedral to euhedral. Occasionally, clinopyroxene can be found intergrown with amphibole. Biotite crystals range in size from 0.1mm to 1mm, but rarer crystals up to 2mm are present. Good cleavage is present and grains can range from anhedral to euhedral. Quartz is not that common, but when present can range in size from 1mm to 5mm. The crystals are usually embayed and anhedral to subhedral.

Olivine is also rare, but ranges from 0.1mm to 1.0mm, with an occasional larger crystal

21 up to 4mm in the type 4 enclave. The crystals are anhedral in enclaves but can be euhedral in the host. Oxides range in size from 0.1mm to 0.5mm.

22

A B

C D

E F

G H

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Figure 11. Disequilibrium textures present in the host and enclaves. All pictures were taken under crossed polars at 4x magnitude. (A) Sieve textured plagioclase with new growth rim from Dome A #85-715. (B) Eroded plagioclase phenocrysts from Dome B #85-717. (C) An outer rim sieve texture plagioclase from Dome C #84-434. (D) Biotite with an opaque reaction rim in Dome D #84-435. (E) A composition zoned plagioclase at Dome F #84-455. (F) A small glomerporphyric clast in Dome D #84-435 (G) Compositionally normal zoned host plagioclase crystals and hornblende crystals from Dome F #84-455 (H) A compositionally zoned, sieve textured new growth rim plagioclase in Dome F #93-1963A.

3.2 Enclave abundance in the Chaos Crags

From the initial eruption of Dome A through to final eruption of Dome F, enclave abundance increases from 2.3-13.9 vol.% (Table 1 and Figure 12). This observation is consistent with the conclusions of Stout (2007) that the abundance of enclaves increased throughout the eruption of Chaos Crags. The increase in total abundance is not monotonic. There is a clear jump from Dome B to Dome C (3.9% - 9.0%). A second significant jump occurs between the eruption of Dome C and Dome D (9.0% - 12%).

More modest jumps appear to occur between eruptions of Dome A and B (2.3% - 3.9%) and between the eruption of Dome E and Dome F (11.8% - 13.9%).

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Table .1 Total Enclave Abundance for each dome at Chaos Crags.

Total Enclave N = number of Abundance point counts Dome (Vol%) A 2.3% 4

B 3.9% 125

C 9.0% 26

D 12.0% 17

E 11.8% 28

F 13.9% 28

Total enclaves (Volume %) 16 13.90 14 11.99 11.79 12 10 8.98 8 6 3.93 4 2.29 2 0 Dome A Dome B Dome C Dome D Dome E Dome F

Figure 12. Abundance of enclaves by dome. Jumps in abundance are indicated by solid lines (significant jumps) and dashed lines (modest jumps).

25

Figure 13 shows the volume abundance of each enclave type for each dome. Only

enclave types 2 and 3 were observed in Dome A. However, it should be noted that one

sample provided by Dr. Michael Clynne was identified as a type 1 enclave and another

identified as a type 4, so these enclaves are likely present but in very small amounts. The

modest jump in total enclave abundance in Dome B is accompanied by the appearance of

enclave types 1, 4, and 5 and an increase in the abundance of type 2 enclaves relative to type 3. The large jump in total enclave abundance between eruption of Domes B and C is marked by an increase in the volume of enclave types 1 and 3 relative to type 2. Type 4 disappears and type 5 remains present in small amounts. The volume of enclave types 2 and 3 increases with the eruption of Dome D with type 2 becoming the most abundant enclave type. Type 1 enclaves decrease in abundance. Type 4 remains absent and the abundance of type 5 increases significantly although this enclave type is still relatively rare. The relative abundance of enclave types is somewhat similar in Dome E, with type 2 remaining the dominant enclave type. The eruption of Dome F is marked by a clear change in the character of the enclaves with type 1 becoming the most abundant type.

26

10 10 Dome A Dome B 8 8

6 6

4 4 1.52 1.59 2 0.76 2 1.21 0.89 0.00 0.00 0.00 0.10 0.13 0 0 Type 1 Type 2 Type 3 Type 4 Type 5 Type 1 Type 2 Type 3 Type 4 Type 5

10 10 Dome C Dome D 8 8 5.62 6 6 3.71 4 2.86 4 3.42 2.35 2.18 2 2 0.77 0.00 0.06 0.00 0 0 Type 1 Type 2 Type 3 Type 4 Type 5 Type 1 Type 2 Type 3 Type 4 Type 5

10 10 Dome E 8.41 Dome F 8 6.54 8

6 6 4.15 3.50 4 4

2 1.49 2 1.05 0.05 0.21 0.10 0.18 0 0 Type 1 Type 2 Type 3 Type 4 Type 5 Type 1 Type 2 Type 3 Type 4 Type 5

Figure 13: Abundance of different enclave types in each dome.

27

3.3 Crystal Size Distribution

Overall, 52 hand tracings of thin sections were completed and processed using

ImageJ and CSDCorrections 1.39. The CSD output for each thin section is provided in

Appendix E. It should be noted that not every enclave type represented in each dome was analyzed for this study. Type 4 and 5 enclaves were hard to collect in the field since they were rarer and usually contained within large boulders. The goal of this study was to assess the relative abundance of different enclave types in each dome and to use CSD on a small set of samples representing each enclave type to determine whether significant differences exist between them that could be connected to magmatic processes. A complete set of enclaves for each dome was collected and will be analyzed as part of a future study by Dr. Hammersley and Dr. Clynne.

Figure 14 shows CSD curves for the host dacite of each dome. These are relatively consistent showing little variation between the domes. All curves are concave upwards indicating at least two plagioclase populations (Martin et al., 2006).

Figure 15 shows CSD curves for the different enclave types. The domes from which each sample was collected are indicated by the color of the line. For most enclave types, the curves are consistent regardless of which dome the enclave was collected from.

Types 1 and 2 show very similar distributions with steep curves indicating large populations of small crystals and fewer large crystals. The curves for type 3 show some variability with two types of curve, one steep like that for types 1 and 2, and one more gentle curve, with a much lower abundance of small crystals. The CSD analysis for the steeper curves was done on higher magnification images, which may have affected the

28 count as fewer crystals were visible. Type 4 enclaves show CSD curves very similar to those for types 1 and 2 but with fewer small crystals overall. The type 4 images analyzed also lack the larger crystals common in types 1 and 2. CSD curves for type 5 enclaves have a lower abundance of smaller crystals and a more noticeably concave upward shape.

29

15 15 Dome A Dome B 10 10

5 5

0 0 0 2 4 6 8 10 0 2 4 6 8 10

ln (population size) (population ln -5 -5 ln (population size) (population ln

-10 -10 Corrected Crystal Size (mm) Corrected Crystal Size (mm)

15 15 Dome C Dome D 10 10

5 5

0 0 0 2 4 6 8 10 0 2 4 6 8 10 -5 -5 ln (population size) (population ln ln (population size) (population ln

-10 -10 Corrected Crystal Size (mm) Corrected Crystal Size (mm)

15 15 Dome E Dome F 10 10

5 5

0 0 0 2 4 6 8 10 0 5 10 -5 -5 ln (population size) (population ln ln (population size) (population ln

-10 -10 Corrected Crystal Size (mm) Corrected Crystal Size (mm)

Figure 14 . CSD curves for the host dacite of each dome.

30

18 Type 1 18 Type 2 14 14 10 10 6 6 2 2

-2 size) (population ln -2 ln (population density) (population ln 0 1 2 3 4 0 1 2 3 4 -6 -6 Corrected Crystal Size (mm) Corrected Crystal Size (mm4

18 18 Type 3 Type 4 14 14

10 10

6 6

2 2

ln (population size) (population ln -2 -2 0 1 2 3 4 density) (population ln 0 1 2 3 4 -6 -6 Corrected Crystal Size (mm) Corrected Crystal Size (mm)

18 Type 5 14 10 6 2 -2

ln (population density) (population ln 0 1 2 3 4 -6 Corrected Crystal Size (mm)

Figure 15. CSD curves for each enclave type. The color of the curves indicates the dome from which each enclave was collected: Blue(small stipples) = Dome A; green(thicker solid lines) = Dome B; red(solid lines) = Dome C; purple(dotted lines) = Dome D; orange(large stipples)= Dome E; teal(thinner solid lines)= Dome F.

31

Figure 16 shows CSD curves each dome. The enclave type is indicated by the color of the line. While not every enclave present in each dome is represented by this data set, it is still worthwhile noting some of the differences apparent between the domes.

The type 1 and 4 enclaves analyzed for domes A and B show very similar curves, relatively steep and lacking a pronounced curvature. The curves for dome C show a greater population of small crystals and the curve is clearly concave upward, indicative of mixing. It is interesting to note that the type 3 enclaves analyzed for dome C are the ones with large populations of small crystals and fit well with other enclave types measured in that dome. The curves for dome D only represent type 3 enclaves and show the gentler slope with fewer small crystals. Only type 2 enclaves are analyzed for domes E and F and they show similar slopes but it appears that the enclaves in dome F have more small crystals.

The CSD data shows that some enclave types have very similar crystal populations even if the larger scale texture of the enclaves may be quite different. Types

1 and 2 are almost identical and strongly similar to type 4. Type 3 enclaves, while showing some variability, appear to be distinct from types 1, 2 and 4. Type 5 also appears to be distinct from the other enclave types. The comparison between domes suggests that there may be some relationship between the texture of the enclave and which dome it in found in. With the somewhat limited data set presented here, it is very difficult to ascertain whether the differences between domes are dominant or between enclave types.

Further analysis of the complete set of samples will help clarify this issue.

32

22 Dome A 22 Dome B 18 18 14 14 10 10 6 6 2 2 ln (population size) (population ln ln (population size) (population ln -2 -2 0 2 4 0 1 2 3 4 -6 -6 Corrected Crystal Size (mm) Corrected Crystal Size (mm)

Dome C Dome D 22 22 18 18 14 14 10 10 6 6 2 2 ln (population size) (population ln ln (population size) (population ln -2 -2 0 1 2 3 4 0 1 2 3 4 -6 -6 Corrected Crystal Size (mm) Corrected Crystal Size (mm)

22 Dome E 22 Dome F 18 18 14 14 10 10 6 6 2 2 ln (population size) (population ln

ln (popluation size) (popluation ln -2 -2 0 1 2 3 4 0 1 2 3 4 -6 -6 Corrected Crystal Size (mm) Corrected Crystal Size (mm)

Figure 16. CSD curves for enclave types for each dome. Type 1 is represented by blue(thinner lines). Type 2 = green(dotted lines). Type 3 = red(solid lines). Type 4 = purple(stippled lines). Type 5 = orange(thicker lines).

33

3.4 Geochemistry

Bulk-rock major element data for samples of host dacite and enclaves were

provided by Dr. Michael Clynne. Analyses were completed at the USGS Analytical

Laboratory in Lakewood, Colorado. Major element concentrations were determined by

wavelength-dispersive X-ray fluorescence analysis following the methods of Taggart et

al. (1987).

Major element composition data for host and enclaves are presented in Table 2

and figures 17 and 18. Figure 17 shows a graph of CaO versus SiO2 wt % for the host and different enclave types that were analyzed by CSD as part of this study. Graphs of

K2O, Na2O, FeOt, Al2O3, and MgO against SiO2 wt. % are shown in Figure 18. The host

dacite does not show much chemical variation, ranging from approximately 66-69 wt. %

SiO2. This is consistent with the CSD analysis of host dacites that showed very little

variation between domes. The enclaves show a wide range in composition from 53 wt.%

SiO2 in the most mafic samples to 61 wt. % SiO2 in the more felsic samples. The samples

form an approximately straight line between the most mafic samples and the host dacites, indicative of mixing. An interesting and somewhat unexpected feature of the geochemical data is that the different enclave types group together chemically. Type 1 enclaves show the greatest degree of mixing, plotting close to the host dacite. Types 3 and 4 are the most mafic samples, representing the least degree of mixing. Type 2 enclaves are chemically intermediate between types 1 and 3. No geochemical data was available for type 5 enclaves analyzed for CSD as part of this study. There are two samples that do not cluster with other samples of the same enclave type: Sample #88-1283 was classified

34 texturally as a type 1 enclave but geochemically it is similar to type 2 enclaves.

Texturally, sample #84-632 has the appearance of a type 3 enclave but is geochemically similar to type 4 enclaves.

35

Table 2. Major Element Composition of Host and Enclave Rocks. These samples were provided by Dr. Michael Clynne. Samples analyzed for CSD as part of this study are highlighted in blue. Sample types are marked H for host and by type for enclaves.

Sample # Type SiO2 Al2O3 FeOt MgO CaO Na2O K2O TiO2 P2O5 MnO Dome A

84-428 H 69.62 15.53 2.65 1.29 3.38 4.33 2.62 0.35 0.13 0.06 85-715 4 53.61 18.91 7.67 5.08 10.16 2.71 0.77 0.69 0.08 0.14 LC89-1512 3 fine 56.05 17.35 6.95 4.62 7.78 3.68 1.45 1.46 0.37 0.12 LT96-3I 2 55.02 18.58 7.02 4.94 9.35 2.83 1.17 0.66 0.15 0.13 LT96-17I 2 55.97 18.93 6.72 4.3 8.63 3.1 1.26 0.67 0.14 0.12 LT96-18I 2 57.55 18.39 6.6 3.77 7.97 3.22 1.44 0.67 0.16 0.13 LT96-26I 2 57.16 18.34 6.49 4.11 8.27 3.16 1.39 0.66 0.14 0.12 LT96-27I 2 57.25 18.34 6.56 4.08 8.3 3.22 1.14 0.67 0.16 0.13 LT96-28I 2 56.45 18.75 6.73 4.01 8.58 3.27 1.11 0.66 0.15 0.13 LT96-29I 2 56.82 18.26 6.6 4.44 8.5 3.1 1.18 0.66 0.15 0.12 LT96-30I 2 56.34 18.58 6.72 4.43 8.56 3.2 1.08 0.66 0.15 0.13 Dome B

84-443 H 69.81 15.60 2.54 1.20 3.31 4.30 2.64 0.35 0.12 0.06 88-1283 1 55.61 18.57 7.76 3.96 8.56 3.35 0.97 0.79 0.12 0.13 85-717 4 53.54 18.91 7.62 5.15 10.31 2.74 0.65 0.70 0.07 0.14 LC88-1281 2 53.65 19.57 7.36 4.67 9.98 2.84 0.87 0.67 0.10 0.13 LC88-1282 2 53.83 19.59 6.76 5.05 10.00 3.03 0.71 0.63 0.12 0.13 LC93-1961 1&2 54.08 18.81 7.64 4.83 9.51 2.88 1.10 0.73 0.11 0.14 LC93-1962 3 fine 55.63 18.61 6.87 4.70 9.04 2.95 1.14 0.66 0.11 0.13 LT96-33I 2 55.60 18.80 6.95 4.47 8.85 3.19 1.02 0.68 0.15 0.13 LT96-34I 2 57.05 18.31 6.53 4.21 8.35 3.18 1.30 0.66 0.15 0.12 LT96-4I 2 57.34 18.37 6.41 4.11 8.18 3.31 1.21 0.65 0.16 0.12 LT96-5I 2 57.25 18.41 6.46 4.14 8.13 3.33 1.22 0.65 0.15 0.12 LT96-6I 2 56.46 18.21 6.79 4.64 8.61 3.16 1.03 0.66 0.15 0.13 LT96-7I 2 56.00 18.60 6.82 4.49 8.80 3.14 1.04 0.68 0.14 0.13

36

Sample # Type SiO2 Al2O3 FeOt MgO CaO Na2O K2O TiO2 P2O5 MnO Dome C

84-434 H 68.88 15.76 2.77 1.46 3.71 4.30 2.50 0.38 0.11 0.06 00-2352 H 68.35 15.81 3.19 1.74 3.91 3.75 2.51 0.43 0.16 0.07 84-634 1 61.39 17.77 5.22 1.91 4.50 4.93 2.66 0.91 0.47 0.12 LC81-661A 3 fine 55.64 18.35 7.13 4.40 9.01 3.06 1.37 0.66 0.09 0.13 LT96-12I 2 57.37 18.35 6.46 4.06 8.31 3.23 1.11 0.68 0.16 0.13 LT96-20I 2 56.28 18.76 6.69 4.18 8.76 3.21 1.04 0.67 0.14 0.13 87-1235A 1 59.81 17.59 5.88 2.56 5.51 4.7 2.17 0.99 0.53 0.12 87-1235B 2 56.74 18.21 6.94 4.19 8.56 3.19 1.1 0.68 0.11 0.12 87-1235C 3&5 55.11 18.81 6.16 4.75 10.11 2.93 1.01 0.63 0.24 0.11 Dome D

84-435 H 67.78 15.81 3.34 1.77 4.08 4.01 2.52 0.42 0.12 0.07 84-632 3 fine 53.26 19.35 7.48 4.96 10.27 2.77 0.81 0.71 0.09 0.14 btw LC86-961 1&2 59.56 17.62 5.16 3.77 7.14 4.31 1.38 0.63 0.13 0.17 LT96-8I 2 56.25 18.21 6.50 4.52 8.80 3.15 1.33 0.76 0.19 0.13 btw LT96-9I 1&2 59.28 17.20 5.56 3.73 6.86 4.11 1.83 0.82 0.37 0.12 LC01-2363 H 69.73 15.34 2.70 1.36 3.12 4.21 2.84 0.41 0.16 0.07 Dome E

84-433 H 66.97 16.26 3.40 1.81 4.48 4.15 2.25 0.41 0.12 0.07 84-635 2 57.91 18.17 6.41 3.91 8.04 3.27 1.26 0.63 0.11 0.13 LC84-637 2 & 3 57.10 18.43 6.93 3.78 8.15 3.37 1.08 0.74 0.13 0.13 btw LT96-11I 2&3 54.46 19.20 7.24 4.60 9.40 2.95 0.99 0.72 0.15 0.14 LT96-22I 2 55.77 18.83 6.87 4.34 8.93 3.11 1.03 0.68 0.16 0.13 btw LT97-23I 1&2 58.78 17.98 6.14 3.69 7.64 3.35 1.35 0.65 0.16 0.12 LT96-24I 2 57.63 18.54 6.26 3.84 8.03 3.35 1.31 0.63 0.15 0.12 Dome F

84-455 H 68.17 16.10 2.98 1.50 4.16 4.23 2.25 0.36 0.11 0.06 93-1963A 2 56.12 18.81 6.86 4.23 8.71 3.11 1.08 0.68 0.12 0.13 LT96-13I 2 57.69 18.59 6.34 3.71 7.88 3.48 1.21 0.67 0.16 0.13 LT96-14I 2 57.64 17.77 6.61 4.09 7.76 3.44 1.39 0.74 0.27 0.14 btw LT96-25I 2&3 54.64 19.12 7.22 4.59 9.26 3.12 0.89 0.71 0.15 0.14 LT96-31I 2 56.92 18.80 6.49 3.91 8.29 3.35 1.16 0.66 0.15 0.13 LT96-32I 2 56.35 18.65 6.75 4.22 8.60 3.25 1.08 0.68 0.15 0.13

37

CaO vs SiO2 12 Type 4 Type 3 10 Type 2 8

6 CaO Host Type 1 4

2

0 50 55 60 65 70 75

SiO2

Figure 17. CaO vs. SiO2 host dacite and enclave samples analyzes as part of this study for CSD. Host samples are represented by black diamonds. Enclave samples are represented by squares. Enclave type is represented by the color of the squares: Type 1 enclaves are blue. Type 2 = green. Type 3 = red. Type 4 = purple. Type 5 samples are not represented here as geochemical data was not available for samples analyzed by CSD.

38

25 9 8 20 7 6

3 15 t 5 O 2 FeO Al 10 4 3 5 2 1 0 0 50 60 70 80 50 60 70 80 SiO2 SiO2

3.0 6

2.5 5 2.0 4

O

2 1.5 3 K MgO 1.0 2 0.5 1

0 0.0 50 60 70 80 50 60 70 80 SiO2 SiO2

6

5 4 O 2 3

Na 2 1 0 50 60 70 80 SiO2

Figure 18. A comparison of major element compositions at Chaos Crags of host dacite (diamonds) and mafic enclaves (squares). Enclave types are color coded: Type 1 = blue; Type 2 = green; Type 3 = red; Type 4 = purple; Type 5 = orange.

39

After observing the relationship between enclave type and geochemistry, a visit

was made to Menlo Park to examine Dr. Clynne’s collection of samples. Hand samples of each of the enclaves for which geochemical data was available were examined and classified into enclave type. Most of the samples were type 2, which might be due to sampling bias (pers. comm. Clynne). Figure 19 shows a plot of CaO vs. SiO2 for all samples examined. Those samples shown in figure 17 are shown with saturated colors.

Samples classified as Menlo Park are shown with paler colors.

12

10 Described as transitional between type 2 and 3

8 Described as transitional between

6 CaO (wt%)CaO

4

2 Host Type 1 Type 2 Type 3 Type 4 Type 5 Type 1 this study Type 2 this study Type 3 this study Type 4 this study 0 50 55 60 SiO (wt%) 65 70 75 2

Figure 19. CaO vs. SiO2 for all samples analyzed by Dr. Michael Clynne. Samples analyzed for CSD for this study are highlighted. The host dacite are represented by diamonds and the mafic enclaves by squares.

40

It can clearly be seen from figure 19 that the relationship between enclave type and geochemistry is more complex than indicated in figures 17 and 18. Type 2 enclaves exhibit a wide range in composition. One group of type 2 samples that plot between type

1 and the main group of type 2 were identified as having textures transitional between type 1 and type 2. Another group of type 2 samples was described as having textures transitional between type 2 and type 3. These plot as a cluster at the more mafic end of the type 2 trend. A small cluster of type 3 enclaves plot within the main type 2 cluster.

These were described in hand samples as “type 3 fine” because although they contained the acicular hornblende diagnostic of type 3 enclaves, the crystals were fine grained.

Type 2 and 3 enclaves both contain hornblende, the main difference being the habit of the hornblende crystals, with type 3 being distinctly acicular. A small cluster of type 2 enclaves plot with the type 3 enclaves at the more mafic end of the spectrum. It is interesting to note that CSD analysis of type 3 enclaves produced two trends. One very similar to type 2, with a steep curve and a large population of small crystals, another with a much gentler curve.

41

Chapter 4

DISCUSSION

4.1 Magma Mixing Models

Magma chambers are not closed systems, but are dynamic open systems that evolve from more mafic to more silicic compositions through some combination of processes such as fractional crystallization, crustal assimilation, and magma mixing

(Sparks et al., 1984; Marsh 1998). Magma mixing is the direct interaction between two compositionally and thermally distinct magmas. Classic studies suggest that mixing will initiate with an injection of hotter less dense mafic magma into a silicic magma chamber

(Eichelberger, 1980; Huppert et al., 1982; Kouchi and Sunagawa, 1983, 1985; Campbell and Turner, 1986).

If direct mixing and hybridization does not occur, the mafic magma cools to the temperature of the silicic magma and a less dense mafic foam interface layer will form from extruded water vapor. If the interface becomes unstable, the mafic foam will vesiculate enclaves into the silicic magma and convection will then cause dispersal of mafic enclaves in the silicic magma (Eichelberger, 1980; Huppert et al., 1982; Figure

20). Campbell and Turner (1985) have shown that, even with a forced injection of the magma, if the magmas are close in density, mixing is little or non-existent. Another factor that can affect mixing is the size of the interface layer (Huppert et al., 1982) and if the enclaves form pre or post dispersal (Coombs et al., 2002). If the layer is only a few

42

centimeters thick, large scale mixing will not occur, but will occur when phenocrysts, vesicles and exsolution of the water vapor decrease the density of the mafic magma.

Figure 20. Vesiculation model of the mafic foam layer. From Eichelberger, 1980.

An intermediate magma may form over a relatively short period of time in the magma chamber. Kouchi and Sunagawa (1983, 1985) performed laboratory experiments, mixing a basaltic and dacitic magma with forced convection and in less than 2 hours, a homogeneous layer of andesite formed in the basalt, along with banded layers in the dacite.

4.2 The Lassen Peak Magma Mixing Model

A classic study of magma mixing worth discussing in detail due to its proximity to Chaos Crags is the 1915 Lassen Peak eruption and initiation of mixing in the volcanic vent. Lassen Peak exhibits some of the same enclaves and disequilibrium texture as

Chaos Crags. It should be noted however, that there are distinct differences, such as the

43 presence of hybridized magma at Lassen Peak. Clynne (1999) proposed that an injection of basaltic andesite magma into the dacite magma chamber as a turbulent fountain formed an andesite foam layer (Figure 21a). By rapidly cooling, the foam layer became unstable and formed enclaves. The less dense enclaves floated into the dacite magma and were distributed by convection currents in the dacite (Figure 21b). Andesite enclaves then disaggregated and mixed in the main part of the dacite chamber to form a black dacite layer. With continued mixing, this layer left the interface to rise through the magma chamber to fracture the wallrock (Figure 21c). Fracturing of the conduit and later eruption suggest that mixing processes can trigger an eruption (Sparks et al., 1977;

Kouchi and Sunagawa, 1985). The eruption ceased with banded pumice and light dacite,

(Figure 21d).

44

Figure 21. Magma mixing and enclave formation model for Lassen Peak. (a) Basaltic andesite magma intruded into the base of a dacite chamber. A mafic foam interface layer forms, vesiculates and produces enclaves (b) Enclaves disaggregate, mix, and form a black dacite layer (c) Black dacite is less dense and rich in volatiles rising up in the magma chamber, fracturing a conduit, triggering an eruption (d) Mixing continues and the eruption ceases with a banded pumice and light dacite. From Clynne (1999).

45

4.3 Mixing studies at Chaos Crags

Tepley et al. (1999) developed a magma mixing model for Chaos Crags based on enclave textures (Figure 22). They conclude that large scale mixing did not occur at

Chaos Crags since rocks of intermediate composition are not present, as they are at

Lassen Peak. They propose that a dacite magma chamber was injected by a basaltic magma that ponded at the base of the dacite. A small portion of dacite and basalt mixed homogeneously at the mafic interface since some of the host phenocrysts are found in the enclaves. Enclaves range in size and texture and some have disaggregated. However, this suggestion does not explain all of the chemical and mineralogical changes seen in Chaos

Crags.

46

Figure 22. Enclave formation model proposed for Chaos Crags by Tepley et al., (1999) (a) Rhyodacite magma chamber is injected with basalt (b) Mixing occurs between the two magmas (c) Formation of a hybrid layer and enclaves (d) All different types of enclaves are now present in host rhyodacite (e) Enclaves disaggregate and disperse resorbed crystals back into host magma.

47

4.4 Observations from Chaos Crags

In order to interpret the magma mixing processes and sequence of events at Chaos

Crags, there are a number of observations from this study that must be explained:

1. The total abundance of enclaves increases as the eruption progressed.

2. There are distinct increases in the total abundance of enclaves between eruption of

domes B and C, domes C and D.

3. There are more modest increases in the total abundance of enclaves between

eruption of domes A and B, domes E and F.

4. Although it seems likely that all enclave types are present in each dome, changes

in distribution of enclave type seem to correlate with the increases in total

abundance.

5. Host dacites show a narrow range in composition while enclaves show a mixing

trend from a more mafic source toward the host dacite.

6. There is a clear link between enclave type and geochemistry. The most mixed

enclave types are type 1. The least mixed are types 3 and 4.

Geochemically, the enclaves form a clear mixing trend between a mafic end- member (53 wt% SiO2) and the dacite host (66-69 wt% SiO2). This suggests that the

mafic input had a relatively consistent composition most closely represented by type 3

and 4 enclaves. Type 3 enclaves are distinguished by the presence of acicular hornblende

phenocrysts. Feeley et al. (2008) note that acicular crystals can form in injected mafic

48

magma that ponds at the base of the magma chamber. Type 4 enclaves contain olivine,

which is also indicative of ponding of the mafic magma and restricted mixing with the

dacitic host. Type 1 enclaves show the most mixed composition. They are fine-grained in

texture with no prominent phenocrysts except for reacted host crystals. Vesicles are

common in type 1 enclaves. It seems likely that type 1 enclaves formed during forceful

injection of mafic magma into the magma chamber. Fountaining formed numerous small enclaves that cooled rapidly. The relatively large surface area of numerous small enclaves allowed for extensive mixing with the dacite host. The presence of large plagioclase crystals from the host indicate incorporation of the dacitic magma into the enclaves as they rose through the magma chamber. Geochemically, type 2 enclaves form a broad trend between the highly mixed type 1 and less mixed type 3 enclaves. Coarser-grained textures suggest a longer mixing time than type 1 enclaves, perhaps in a boundary layer between the mafic magma and the dacite, as suggested by Tepley et al. (1999). Little geochemical data exists for type 5 enclaves. Texturally, they are coarse grained suggesting they cooled more slowly allowing larger blocky phenocrysts of amphibole and plagioclase to form.

The increase in the total abundance of enclaves over the eruptive sequence of

Chaos Crags suggests continued injection of mafic magma into the dacitic magma chamber. The large increases in total enclave abundance between eruption of Domes B and C, C and D, and E and F are accompanied by distinct changes in the population of enclave types. A more modest increase in enclave abundance between eruption of Domes

A and B is also accompanied by a change in enclave type distribution.

49

The relatively modest increase in enclave abundance between eruption of Domes

A and B is accompanied by a marked increase in the abundance of type 1 and 2 enclaves,

which are geochemically the most mixed. The large increase in total abundance of

enclaves between eruption of domes B and C is marked by an increase in the abundance

of type 1 enclaves. Types 2 and 3, which form a continuous trend from relatively

primitive compositions to more mixed compositions also increase, in particular type 3.

CSD curves for Dome C show a large population of small crystals and a markedly

concave upward form that is indicative of extensive mixing. The large increase in total

abundance of enclaves with eruption of Dome D is marked by a large increase in the

amount of type 2 enclaves and a decrease in the amount of type 1 enclaves. Dome E is

very similar to Dome D. CSD curves for Dome D have a more gentle slope with fewer small crystals. Eruption of dome F is marked by a modest increase in the total abundance of enclaves. The distribution of enclave types changes markedly, being dominated by type 1. Type 2 enclaves decrease in abundance. The CSD curves for dome F are steeper than those for dome E, with more small crystals.

From these observations it seems likely that there were numerous injections of

mafic magma into the dacite magma chamber beneath Chaos Crags during the eruption

sequence of Domes A-F. An initial injection occurred prior to the eruption of Dome A.

The dominance of type 2 and 3 enclaves suggests the mafic magma ponded at the base of

the magma chamber, forming a mixing boundary layer. The increase of type 1 enclaves and increase of types 2 and 3 in Dome B suggests an overturning of the magma chamber, dispersing enclaves throughout the dacite. This may have been caused by forceful

50 injection and fountaining or simple overturn of the ponded mafic magma caused by vesiculation.

There was a hiatus between the eruption of Domes B and C (Clynne and Muffler,

1989). During this hiatus, another forceful injection of mafic magma intruded the silicic magma chamber, marked by another significant increase in type 1 enclaves. The increased abundance of types 2 and 3, which are more mafic in composition, suggest the mafic magma ponded at the base of the chamber. The CSD curves for Dome C show a distinctive kink suggestive of a new mixing event. Another injection of mafic magma may have occurred prior to eruption of Domes D and E. This is indicated by the large increase in total abundance of enclaves. Type 2 enclaves become the most dominant. It is possible that eruption of Domes D and E were caused by overturn of the mafic layer formed after the eruption of Dome C rather than a separate mixing event (Huppert at el.,

1982, Feeley et al., 2008). Another final injection of mafic magma occurred prior to eruption of Dome F. The dominance of type 1 enclaves suggests that this injection was a forceful injection, forming a fountain of mafic enclaves within the magma chamber.

4.5. Chaos Crags Magma Mixing Model

Figure 23 shows a model of the mixing processes and sequence of events that formed the Chaos Crags.

Type 1 and 2 enclaves are the most abundant enclaves and have direct correlation with a forceful injection of magma. When the magma is injected forcefully into the silicic chamber, it forms a fountain. Type 1 enclaves form first and cool quickly as

51

indicated by their fine-grained appearance. Type 2 enclaves form when the dense mafic

magma falls back to the bottom of the magma chamber and forms a mafic interface

between the magmas. Enclaves vesiculate and form pre-dispersal (Campbell and Turner,

1986; Coombs et al., 2002). Type 3, 4 and 5 enclaves form below the mafic interface at a

slower rate as indicated by their more mafic composition and coarser grain size. The

sieved textures in host phenocrysts were caused by the baking and recooling due to

recharge events. New growth rims formed most likely due to volatile loss in the conduit

during eruption phase (Rutherford and Hill, 1993).

There was an initial dacitic magma chamber (Figure 23 A). The chamber then

received a small forceful injection that caused the pyroclastic flows. More mafic magma

injected and ponded at the base of the chamber and caused eruption of Dome A, the smallest of the domes to erupt with the smallest population of enclaves (Figure 23 C).

Mixing occurred within a mafic foam interface (Eichelberger, 1980) as evidenced by the dominance of enclave types 2 and 3. The significant increase of type 1 enclaves in Dome

B suggests that eruption of Dome B may have been triggered by a second, more forceful injection of mafic magma (Figure 23 D). Although the total population of enclaves in

Dome B is relatively small, this injection of magma may have been one of the largest.

Martin et al. (2006) state that the volume of erupted material is directly proportional to the amount of magma injected. Dome B is the largest of the domes at Chaos Crags.

Another injection of mafic magma caused the increased volume of enclaves seen from Dome B to Dome C. The dominance of type 2 enclaves in Domes C, D and E suggest most mixing occurred across a boundary between ponded mafic magma and the

52 host dacite, perhaps formed after eruption of Dome B. The new influx of magma raises the pressure in the magma chamber causing the wall rock to fracture and eruption of

Dome C (Figure 23 E). More mixing and disaggregation occurred in the conduit during eruption. Mixing was still continuing while the chamber was cooling and when the mixing boundary reached a certain vesiculation level, the layer overturned and Dome D erupted (Figure 23 F). Once again mixing and disaggregation occurred in the conduit during the eruption from the formation of enclave margins. This process was repeated, culminating with the eruption of Dome E (Figure 23 G).

Dome F erupted from a final injection of mafic magma, once again increasing the volume of enclaves (Figure 23 H). Type 1 enclaves recur as the most abundant enclave suggesting forceful injection and fountaining. Once again more mixing and disaggregation occur in the conduit during eruption.

53

54

55

Figure 23. Magma mixing model for Chaos Crags. A) The initial dacitic magma chamber. Plagioclase is represented as white rectangles and hornblende is black rectangles. B) A forceful injection of mafic magma in the dacitic magma chamber causing enclaves to form. Type 1 are represented as blue. Type 2 = green. Type 3 = red. Type 4 = purple. Type 5 = orange. C) The injection of more mafic magma caused the eruption of Dome A and presence of enclaves type 1- 4. D) Another forceful injection of magma caused the eruption of Dome B due to the increase of type 1 enclaves. E) More mafic magma injected and caused the eruption of Dome C and an overall increase of enclaves. F) Another small amount of mafic magma ponded to cause the eruption of Dome D and another increase of overall enclave abundance. G) The overall abundance slightly decreased, so the mafic interface vesiculated and caused the eruption of Dome E. H) The eruption of Dome F was caused by another forceful injection of mafic magma and overall enclave abundance increase again.

56

Chapter 5

CONCLUSION

This study presents field observations, textural and geochemical data that the

Chaos Crags formed through multiple mixing events in a single shallow silicic magma chamber. The magma chamber experienced a series of influxes of mafic magma that either injected as a turbulent fountain or ponded at the base of the silicic chamber. These injections caused the sequential eruption of the domes. Depending on the injection type of fountaining or ponding of mafic magma at the base of the chamber, different types of enclave form. Aphantic enclaves (type 1) form from the forceful injection of mafic magma and are found near the top of the magma chamber. All other enclave types form as a result of ponding of mafic magma. Salt and pepper appearance enclaves (type 2) form within a hybrid interface layer between the two magmas, prior to dispersal.

Enclaves that contain olivine and acicular crystals (types 3, 4 and 5) form below the mafic-felsic interface.

CSD analysis of enclaves shows that to a limited extent, it is possible to distinguish between enclave type by looking at the crystal population. From the limited data available from this preliminary study of the CSD of Chaos Crags enclaves, there appears to be some relationship between the shape of the CSD curves and the domes from which the enclaves were erupted. The clear relationship between enclave texture and geochemistry was a surprising finding of this study and should be explored further.

57

APPENDICES

58

APPENDIX A

Point Count Locations

Point Count Locations of Enclaves at Chaos Crags, Lassen Volcanic National Park

Location Dome Northing Easting 1 F 10T 626361 4487829 2 F 10T 3 F 10T 626362 4487668 4 F 10T 626325 4487508 5 F 10T 626413 4487413 6 F 10T 626506 4487463 7 C/D 10T 624323 4487326 8 C/D 10T 624325 4487324 9 C 10T 624453 4487410 10 C/D 10T 624392 4487316 11 C/D 10T 624347 4487289 12 D 10T 624336 4487252 13 D 10T 62432 4487206 14 D 10T 624327 4487184 15 D 10T 624318 4487166 16 D 10T 624317 4487116 17 B/D 10T 624247 4487062 18 B 10T 624225 4487011 19 B 10T 624196 4486970 20 B 10T 624172 4486915 21 B 10T 624135 4486840 22 B 10T 624109 4486758 23 B 10T 624162 4486711 24 B 10T 624191 4486665 25 B 10T 624189 4486699 26 B 10T 624226 4486437 27 B 10T 28 B 10T 624232 4486398 29 B 10T 30 B 10T 624273 4486347 31 B 10T

59

32 B 10T 624286 4486288 33 B 10T 34 B 10T 624323 4486233 35 B 10T 36 B 10T 624332 4486181 37 B 10T 38 B 10T 624346 4486134 39 B 10T 40 B 10T 624347 4486122 41 B 10T 42 B 10T 624353 4486088 43 B 10T 44 B 10T 624363 4486044 45 B 10T 46 B 10T 624381 4485968 47 B 10T 48 B 10T 624400 4485940 49 B 10T 50 B 10T 624431 4485844 51 B 10T 52 B 10T 624481 4485771 53 B 10T 54 B 10T 624499 4485725 55 B 10T 56 B 10T 624582 4485687 57 B 10T 58 B 10T 624650 4485637 59 B 10T 60 B 10T 624692 4485576 61 B 10T 62 B 10T 626158 4486735 63 B 10T 626142 4486683 64 B 10T 626118 4486612 65 B 10T 626100 4486508 66 B 10T 626100 4486506 67 B 10T 626105 4486446 68 B 10T 626071 4486286 69 B 10T 626051 4486180 70 B 10T 625957 4486013

60

71 B 10T 625887 4485938 72 A 10T 625864 4485678 73 A 10T 625861 4485653 74 B 10T 625568 4485677 75 B 10T 625748 4485678 76 B 10T 626161 4486805 77 B 10T 626135 4486853 78 B 10T 626088 4486880 79 B 10T 626085 4486934 80 C 10T 624914 4488281 81 E 10T 625024 4488233 82 E 10T 625164 4488274 83 E 10T 625200 4488320 84 E 10T 625249 4488472 85 E 10T 625466 4488442 86 E 10T 625664 4488347 87 E/F 10T 625788 4488320 88 E 10T 625941 4488241 89 E 10T 626025 4487053 90 E 10T 625959 4487067 91 E 10T 625972 4487111 92 E 10T 625963 4487135 93 E/F 10T 626017 4487151 94 E/F 10T 626116 4487116 95 B/E/F 10T 626135 4487054 96 E/F 10T 626214 4487079 97 F 10T 626231 4487176 98 F 10T 626261 4487262 99 F 10T 626270 4487325 100 F 10T 626331 4487375 101 F 10T 626391 4487482 102 F 10T 626448 4487604 103 F 10T 626371 4487886 104 C 10T 623301 4488035 105 C 10T 623339 4488877 106 C 10T 623475 4488993 107 C 10T 624590 4488135 108 C 10T 624522 4488053 109 C 10T 624469 4487892

61

110 111 112 C 10T 624575 4487914 113 C 10T 624669 4487837 114 C 10T 624775 4487851 115 C 10T 624903 4488105

62

APPENDIX B

Hand Sample List

Hand samples collected by Dr. Michael Clynne of the USGS and by Crystal Hootman

Textural Sample Enclave # Location Host/Enclave type 84-428 A H 84-433 E H 84-434 C H 84-435 D H 84-443 B H 84-455 F H 84-632 D E 3 fine 84-635 E E 2 85-715 A E 4 85-717 B E 4 87-1235 C H 87- 1235A C E 1 87- 1235B C E 2 87- 1235C C E 5 & 3 87- 1235D C E 3 88-1283 B E 1 89-1511 A E 1 93- 1963A F E 2 00-2352 C H 07-01 D E 07-02 D E 07-03 D E 07-04 D E 07-05 D E 07-06 D E 07-07 B E 07-08 B E 07-09 B E

63

07-10 B E 07-11 B E 08-01 C E 2 08-02 C E 1 08-03 C E 2 08-04 C E 2 08-05 C H 4 08-06 C E 3 08-07 C H 08-08 C Banded lava 08-09 C H & E 2 08-10 C H & E 3 08-11 C H 08-13 E E 4 08-14 E H 08-15 E E 2 08-16 E E 3 08-17 E E 5 09-01 F E 2 09-02 F H 09-03 F E 3 09-04 F E 2 fine 09-05 F E 2 & 3 09-06 C or D? H 09-07 C or D? E 3 09-08 D E 1 09-09 D E 3 09-10 D E 2 09-11 D E 1 09-12 D E 2 09-13 B E 5 09-14 A E 2 09-15 A E 09-16 A H 3 09-17 A E 09-18 B H 2 09-19 B or E? E 09-20 C H 09-21 C E 3

64

APPENDIX C

Theory of Crystal Size Distribution

The following section is the core of CSD theory. Population density is the overall size and unit volume of crystal numbers. It can be graphed either as a histogram or cumulatively (Figure 1). Population density is given by (Marsh, 1988; p. 278),

where n is the number of crystals, n(L) is the number of crystals per unit length, and (L) is per unit volume of magma.

Figure 1. Examples of population density graphs (A) Histogram. (B) Cumulatively. From Marsh (1988).

The population balance is determined by the rate of new crystal growth during influx and outflux of the population density, and essentially records the birth and death of crystals. Population balance is given by (Marsh, 1988; p. 279),

where Vn is crystal population per volume, Gvn is growth rate for crystal population per volume, Q is flux rate (cm3/s), is inflow and is outflux of crystals.

65

When a batch of magma is injected into a new system, a new residence time is developed and records either a change in volume, recharge rate, or possibly both (Figure

2A). If the new residence time is shorter than the original residence time, old crystals leave the system and the new overall crystal size becomes smaller (Figure 2B). An increase in residence time increases the overall crystal size (Figure 2C).

Figure 2. Residence time. (A) Increase or decrease of time against total population. (B) Residence time decreases and forms smaller crystals. (C) Residence time increases and forms larger crystals. From Marsh (1998).

If nucleation rate and growth rate are known, the rate of nucleation can be

determined. Typical crystal numbers in a sample are given by (Marsh, 1998; p. 555),

66

where CN is constant, To is the nucleating rate and Go is growing rate. This means if

more nucleated crystals are produced, the overall size is going to be limited, whereas, if

crystal nucleation is small, the overall crystal size will be larger. It should be noted that

nucleation does not have to be constant.

Crystal size is determined by (Marsh, 1998; p. 555),

where CL is constant. The longer the crystal resides the in the melt, the larger the crystal

will grow, but is inversely proportional to nucleation rate. Actual crystal size is

dependent on growth rate and crystallization time. Volcanic rocks typically have a

nucleation rate of 10-3 and 10-5 cm-3 s-1 to reach 1 mm diameter (Marsh, 1988, 1998).

Crystallization time is dependent on nucleation and growth rate. It is determined

by (Marsh, 1998, p. 555),

where Ct is constant. The presence of larger crystals will greatly decrease the time needed to crystallize the melt completely. Nucleation and growth adjust to the thermal regime to complete solidification. The degree of undercooling in magma is most likely a minor role in real magmas, since magmas are rarely superheated and the cooling rate is

affected by heterogeneous nucleation (Marsh, 1998). The final CSD graph is a result of

95% crystallization for the system and a decrease is noticeable in smaller crystals as the

melt is diminished (Marsh, 1998).

67

APPENDIX D

Background to CSDCorrections Program

Conversion to 3D is important because crystal habit and roundness are only a

measurement of the crystal intersection in the plane and cannot be used to discuss

petrological processes (Higgins, 2000). Stereological solutions ease the problems that

arise during 2D to 3D conversions, by direct or indirect methods (Higgins, 2000).

Indirect methods include parametric solutions by Peterson (1996), to calculate population

densities as linear variations from the theoretical studies by Marsh (1988). A problem

with this assumption is that natural systems are not always linear and more parameters are needed for any non-isotropic fabric. There are two direct methods: the cut-section effect and Saltikov method. The cut-section effect, in which it is assumed a crystal in 2D will have an intersection that passes through the center of the longest axis. This is problematic since only a sphere has an intersection close to the maximum (Higgins,

2000). Another direct method is the Saltikov method, which uses the intersections of the overall crystal population to indicate true length as a function of intersection lengths.

This method only works well for spheres and near equant shapes (Higgins, 2000).

Higgins (2000) created the CSDCorrections program by modifying the Saltikov Method to use a more complex algorithm to allow for varying crystal habit.

68

APPENDIX E

Individual Crystal Size Distribution Graphs

The CSD graphs were completed for each thin section using the ellipse major axis

measurement, a massive fabric, and 5 bins per decade. Each thin section has an excluded

and included graph. Excluded means if large crystals were not fully contained on the

page they were not counted and included means the crystals were counted. It is noted whether the sample number is excluded or included or if it was the first or second spot on the thin section.

Dome A #84-428 host 4x excluded

Ellipse Major axis 12

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3 ln (population density)

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Dome A #84-428 host 4x included

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Dome A #84-428 enclave in host 4x excluded

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Dome A #84-428 enclave in host 4x included

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Dome A #84-428 host second spot 4x excluded

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Dome A #84-428 host second spot 4x included

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Dome A #85-715 enclave 4x excluded

Ellipse Major axis 8

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Dome A #85-715 enclave 4x included

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Dome A #85-715 enclave second spot 4x excluded

Ellipse Major axis 16

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Dome A #85-715 enclave second spot 4x included

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13 12 11 10 9

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ln (population density) 3 2 1 0 -1

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Dome A #85-715 enclave 10x excluded

Ellipse Major axis 12

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6 ln (population density)

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2 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 Size

79

Dome A #85-715 enclave 10x included

Ellipse Major axis 12

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6 ln (population density)

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2 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 Size

80

Dome A #89-1511 enclave 4x excluded

Ellipse Major axis 12

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Dome A #89-1511 enclave 4x included

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Dome A #89-1511 enclave second spot 4x excluded

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Dome A #89-1511 enclave second spot 4x included

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Dome A #89-1511 enclave 10x excluded

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85

Dome A #89-1511 enclave 10x included

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Dome B #84-443 host 4x excluded

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Dome B #84-443 host 4x included

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Dome B #84-443 host second spot 4x excluded

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Dome B #84-443 host second spot 4x included

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Dome B #85-717 enclave 4x excluded

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Dome B #85-717 enclave 4x included

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Dome B #85-717 enclave second spot 4x excluded

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Dome B #85-717 enclave second spot 4x included

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Dome B #88-1283 enclave 10x excluded

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Dome B #88-1283 enclave 10x included

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Dome B #88-1283 enclave second spot 10x excluded

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Dome B #88-1283 enclave second spot 10x included

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Dome C #84-434 host glomerporphyric 4x excluded

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99

Dome C #84-434 host glomerporphyric 4x included

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Dome C #84-434 host second spot 4x excluded

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Dome C #84-434 host second spot 4x included

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Dome C #87-1235 host 4x excluded

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Dome C #87-1235 host 4x included

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Dome C #87-1235 host second spot 4x excluded

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Dome C #87-1235 host second spot 4x included

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Dome C #00-2352 host 4x excluded

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Dome C #00-2352 host 4x included

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Dome C #00-2352 host second spot 4x excluded

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Dome C #00-2352 host second spot 4x included

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Dome C #84-634 enclave 4x excluded

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Dome C #84-634 enclave 4x included

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Dome C #84-634 enclave second spot 4x excluded

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113

Dome C #84-634 enclave second spot 4x included

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Dome C #87-1235A enclave 4x excluded

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