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
Department of Geology
iii
___________________
Date
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.
iv
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!
vi
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
vii
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
ix
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 calcalkaline 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
ln (population density)
7
6
5
4
3
2
1
0
-1
0
1
Size
Figure 8. Example of a logarithmic CSD graph of #88-1283.
16
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.
17
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.
19
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.
20
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
23
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%).
24
Table .1 Total Enclave Abundance for each dome at Chaos Crags.
Total Enclave
Abundance
(Vol%)
N = number of
point counts
A
2.3%
4
B
3.9%
125
C
9.0%
26
D
12.0%
17
E
11.8%
28
F
13.9%
28
Dome
Total enclaves (Volume %)
16
13.90
14
11.99
11.79
Dome D
Dome E
12
8.98
10
8
6
4
3.93
2.29
2
0
Dome A
Dome B
Dome C
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
8
Dome B
8
6
6
4
2
0.00
0.76
4
1.52
0.00
0.00
1.21
2
1.59
0.89
0
Type 1
Type 2
Type 3
Type 4
Type 1
10
10
Dome C
8
2.35
Type 2
Type 3
Type 4
Type 5
Dome D
5.62
6
3.71
0.13
8
6
4
0.10
Type 5 0
3.42
4
2.86
2.18
2
2
0.00
0.06
Type 4
Type 5
0
0.00
0.77
0
Type 1
Type 2
10
Type 3
10
Dome E
8
2
Type 2
8.41
6
3.50
Type 3
Type 4
Type 5
0.10
0.18
Dome F
8
6.54
6
4
Type 1
4.15
4
1.49
0.05
0.21
1.05
2
0
0
Type 1 Type 2 Type 3 Type 4 Type 5
Type 1 Type 2
Figure 13: Abundance of different enclave types in each dome.
Type 3 Type 4
Type 5
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
5
0
0
-10
2
4
6
10
10
5
0
-5
-10
2
4
6
8
10
0
2
4
6
8
10
Dome D
10
5
0
-5
0
2
4
6
8
10
Corrected Crystal Size (mm)
15
10
5
0
0
2
4
6
8
Corrected Crystal Size (mm)
10
ln (population size)
ln (population size)
-5
-10
Corrected Crystal Size (mm)
Dome E
-10
0
15
15
-5
5
Corrected Crystal Size (mm)
Dome C
0
Dome B
10
-10
Corrected Crystal Size (mm)
15
ln (population size)
8
ln (population size)
10
-5
15
Dome A
ln (population size)
ln (population size)
15
Dome F
10
5
0
-5
0
-10
Figure 14 . CSD curves for the host dacite of each dome.
5
Corrected Crystal Size (mm)
10
30
Type 1
10
6
2
-6
1
2
3
ln (population size)
4
Corrected Crystal Size (mm)
18
10
6
2
-2 0
1
2
3
4
10
6
2
-2 0
-6
1
2
3
4
Corrected Crystal Size (mm4
Type 4
14
10
6
2
-2 0
-6
Corrected Crystal Size (mm)
18
14
18
Type 3
14
-6
ln (population density)
ln (population size)
14
-2 0
Type 2
18
ln (population density)
ln (population density)
18
1
2
3
4
Corrected Crystal Size (mm)
Type 5
14
10
6
2
-2 0
-6
1
2
3
4
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
Dome A
14
10
6
2
-2 0
-6
ln (population size)
ln (population size)
18
22
18
14
10
6
2
-2
0
-6
2
Dome C
2
3
Corrected Crystal Size (mm)
10
6
2
14
10
6
2
1
2
3
Corrected Crystal Size (mm)
4
1
2
3
4
Corrected Crystal Size (mm)
Dome D
22
18
14
10
6
2
-2
-6
0
1
2
3
4
Corrected Crystal Size (mm)
22
ln (population size)
18
-2 0
-6
14
Dome E
22
ln (popluation size)
4
18
-2 0
-6
4
Corrected Crystal Size (mm)
1
Dome B
22
ln (population size)
ln (population size)
22
Dome F
18
14
10
6
2
-2
-6
0
1
2
3
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).
4
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
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
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
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
Dome A
LC89-1512
Dome B
36
Sample #
Type
SiO2
Al2O3
FeOt
MgO
CaO
Na2O
K2O
TiO2
P2O5
MnO
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
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
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
btw
1&2
53.26
19.35
7.48
4.96
10.27
2.77
0.81
0.71
0.09
0.14
59.56
17.62
5.16
3.77
7.14
4.31
1.38
0.63
0.13
0.17
2
btw
1&2
56.25
18.21
6.50
4.52
8.80
3.15
1.33
0.76
0.19
0.13
59.28
17.20
5.56
3.73
6.86
4.11
1.83
0.82
0.37
0.12
H
69.73
15.34
2.70
1.36
3.12
4.21
2.84
0.41
0.16
0.07
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
2&3
btw
2&3
57.10
18.43
6.93
3.78
8.15
3.37
1.08
0.74
0.13
0.13
54.46
19.20
7.24
4.60
9.40
2.95
0.99
0.72
0.15
0.14
55.77
18.83
6.87
4.34
8.93
3.11
1.03
0.68
0.16
0.13
LT97-23I
2
btw
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
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
57.64
17.77
6.61
4.09
7.76
3.44
1.39
0.74
0.27
0.14
LT96-25I
2
btw
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
Dome C
LC81-661A
Dome D
LC86-961
LT96-8I
LT96-9I
LC01-2363
Dome E
LC84-637
LT96-11I
LT96-22I
Dome F
37
CaO vs SiO2
12
Type 4
Type 3
10
Type 2
CaO
8
6
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
15
FeOt
Al2O3
20
10
5
0
50
60
70
9
8
7
6
5
4
3
2
1
0
80
50
60
80
70
80
SiO2
SiO2
3.0
6
2.5
5
2.0
4
MgO
K2O
70
1.5
3
1.0
2
0.5
1
0
0.0
50
60
SiO2
70
80
50
60
SiO2
6
5
Na2O
4
3
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
CaO (wt%)
Described as
transitional between
6
4
2
Host
Type 4
Type 3 this study
Type 1
Type 5
Type 4 this study
Type 2
Type 1 this study
Type 3
Type 2 this study
0
50
55
60
SiO2 (wt%) 65
70
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.
75
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 endmember (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 14. 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
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
Dome
F
F
F
F
F
F
C/D
C/D
C
C/D
C/D
D
D
D
D
D
B/D
B
B
B
B
B
B
B
B
B
B
B
B
B
B
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
Northing Easting
626361 4487829
626362
626325
626413
626506
624323
624325
624453
624392
624347
624336
62432
624327
624318
624317
624247
624225
624196
624172
624135
624109
624162
624191
624189
624226
4487668
4487508
4487413
4487463
4487326
4487324
4487410
4487316
4487289
4487252
4487206
4487184
4487166
4487116
4487062
4487011
4486970
4486915
4486840
4486758
4486711
4486665
4486699
4486437
624232
4486398
624273
4486347
59
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
624286
4486288
624323
4486233
624332
4486181
624346
4486134
624347
4486122
624353
4486088
624363
4486044
624381
4485968
624400
4485940
624431
4485844
624481
4485771
624499
4485725
624582
4485687
624650
4485637
624692
4485576
626158
626142
626118
626100
626100
626105
626071
626051
625957
4486735
4486683
4486612
4486508
4486506
4486446
4486286
4486180
4486013
60
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
B
A
A
B
B
B
B
B
B
C
E
E
E
E
E
E
E/F
E
E
E
E
E
E/F
E/F
B/E/F
E/F
F
F
F
F
F
F
F
C
C
C
C
C
C
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
10T
625887
625864
625861
625568
625748
626161
626135
626088
626085
624914
625024
625164
625200
625249
625466
625664
625788
625941
626025
625959
625972
625963
626017
626116
626135
626214
626231
626261
626270
626331
626391
626448
626371
623301
623339
623475
624590
624522
624469
4485938
4485678
4485653
4485677
4485678
4486805
4486853
4486880
4486934
4488281
4488233
4488274
4488320
4488472
4488442
4488347
4488320
4488241
4487053
4487067
4487111
4487135
4487151
4487116
4487054
4487079
4487176
4487262
4487325
4487375
4487482
4487604
4487886
4488035
4488877
4488993
4488135
4488053
4487892
61
110
111
112
113
114
115
C
C
C
C
10T
10T
10T
10T
624575
624669
624775
624903
4487914
4487837
4487851
4488105
62
APPENDIX B
Hand Sample List
Hand samples collected by Dr. Michael Clynne of the USGS and by Crystal Hootman
Textural
Enclave
type
Sample
#
Location
Host/Enclave
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
4
85-717
B
E
87-1235
871235A
871235B
871235C
871235D
C
H
C
E
1
C
E
2
C
E
5&3
C
E
3
88-1283
B
E
1
89-1511
931963A
A
E
1
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
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
09-17
A
E
09-18
B
H
09-19
B or E?
E
09-20
C
H
09-21
C
E
4
3
2
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),
𝑛(𝐿) =
𝑑𝑁(𝐿)
𝑑𝐿
(1)
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),
𝜕(𝑉𝑛) 𝜕(𝐺𝑣𝑛)
+
= 𝑄𝑖 𝑛𝑖 − 𝑄𝑜 𝑛𝑜
𝜕𝑡
𝜕𝐿
(2)
where Vn is crystal population per volume, Gvn is growth rate for crystal population per
volume, Q is flux rate (cm3/s), 𝑄𝑖 𝑛𝑖 is inflow and 𝑄𝑜 𝑛𝑜 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),
𝑁𝑜 =
3⁄
4
𝒯𝑜
𝐶𝑛 ( )
𝐺0
(3)
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),
𝐺
1⁄
4
𝐿𝑜 = 𝐶𝐿 ( 𝒯𝑜 )
0
(4)
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),
𝑡𝑐 = 𝐶𝑡 (𝐺𝑜3 𝒯𝑜 )−1⁄4
(5)
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
11
10
9
ln (population density)
8
7
6
5
4
3
2
1
0
-1
0
1
Size
69
Dome A #84-428 host 4x included
Ellipse Major axis
12
11
10
9
8
7
6
5
4
3
2
1
0
-1
1
Size
70
Dome A #84-428 enclave in host 4x excluded
Ellipse Major axis
11
10
9
8
7
6
5
4
3
2
1
0
-1
0
1
Size
71
Dome A #84-428 enclave in host 4x included
Ellipse Major axis
11
10
9
8
7
6
5
4
3
2
1
0
-1
0
1
Size
72
Dome A #84-428 host second spot 4x excluded
Ellipse Major axis
11
10
9
8
7
6
5
4
3
2
1
0
-1
-2
0
1
Size
2
73
Dome A #84-428 host second spot 4x included
Ellipse Major axis
11
10
9
8
7
6
5
4
3
2
1
0
-1
-2
0
1
Size
2
74
Dome A #85-715 enclave 4x excluded
Ellipse Major axis
8
7
6
5
4
3
2
1
0
-1
0
1
Size
75
Dome A #85-715 enclave 4x included
Ellipse Major axis
8
7
6
5
4
3
2
1
0
-1
0
1
Size
76
Dome A #85-715 enclave second spot 4x excluded
Ellipse Major axis
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
-1
0
1
Size
77
Dome A #85-715 enclave second spot 4x included
Ellipse Major axis
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
-1
-2
-3
0
1
2
Size
3
78
Dome A #85-715 enclave 10x excluded
Ellipse Major axis
12
11
10
9
8
7
6
5
4
3
2
0
0.05
0.1
0.15
0.2
0.25
Size
0.3
0.35
0.4
0.45
0.5
79
Dome A #85-715 enclave 10x included
Ellipse Major axis
12
11
10
9
8
7
6
5
4
3
2
0
0.05
0.1
0.15
0.2
0.25
Size
0.3
0.35
0.4
0.45
0.5
80
Dome A #89-1511 enclave 4x excluded
Ellipse Major axis
12
11
10
9
8
7
6
5
4
3
2
1
0
-1
0
1
Size
81
Dome A #89-1511 enclave 4x included
Ellipse Major axis
12
11
10
9
8
7
6
5
4
3
2
1
0
-1
0
1
Size
82
Dome A #89-1511 enclave second spot 4x excluded
Ellipse Major axis
11
10
9
8
7
6
5
4
3
2
1
0
-1
-2
0
1
Size
2
83
Dome A #89-1511 enclave second spot 4x included
Ellipse Major axis
11
10
9
8
7
6
5
4
3
2
1
0
-1
-2
1
Size
2
84
Dome A #89-1511 enclave 10x excluded
Ellipse Major axis
20
19
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
0.05
0.1
0.15
0.2
0.25
Size
0.3
0.35
0.4
0.45
0.5
85
Dome A #89-1511 enclave 10x included
Ellipse Major axis
20
19
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
0.05
0.1
0.15
0.2
0.25
Size
0.3
0.35
0.4
0.45
0.5
86
Dome B #84-443 host 4x excluded
Ellipse Major axis
6
5
4
3
2
1
0
-1
-2
-3
-4
-5
0
1
2
Size
3
87
Dome B #84-443 host 4x included
Ellipse Major axis
6
5
4
3
2
1
0
-1
-2
-3
-4
-5
0
1
2
Size
3
88
Dome B #84-443 host second spot 4x excluded
Ellipse Major axis
8
7
6
5
4
3
2
1
0
-1
-2
-3
-4
0
1
2
Size
3
89
Dome B #84-443 host second spot 4x included
Ellipse Major axis
13
12
11
10
9
8
7
6
5
4
3
2
1
0
-1
-2
-3
-4
0
1
2
Size
3
90
Dome B #85-717 enclave 4x excluded
Ellipse Major axis
8
7
6
5
4
3
2
1
0
-1
-2
0
1
Size
91
Dome B #85-717 enclave 4x included
Ellipse Major axis
8
7
6
5
4
3
2
1
0
-1
-2
0
1
Size
92
Dome B #85-717 enclave second spot 4x excluded
Ellipse Major axis
6
5
4
3
2
1
0
-1
-2
-3
0
1
Size
93
Dome B #85-717 enclave second spot 4x included
Ellipse Major axis
9
8
7
6
5
4
3
2
1
0
0.05
0.1
0.15
0.2
0.25
Size
0.3
0.35
0.4
0.45
0.5
94
Dome B #88-1283 enclave 10x excluded
Ellipse Major axis
10
9
8
7
6
5
4
3
2
1
0
-1
0
1
Size
95
Dome B #88-1283 enclave 10x included
Ellipse Major axis
10
9
8
7
6
5
4
3
2
1
0
-1
0
1
Size
96
Dome B #88-1283 enclave second spot 10x excluded
Ellipse Major axis
8
7
6
5
4
3
2
1
0
-1
-2
-3
-4
0
1
2
Size
3
97
Dome B #88-1283 enclave second spot 10x included
Ellipse Major axis
6
5
4
3
2
1
0
-1
-2
-3
-4
-5
-6
0
1
2
3
Size
4
5
98
Dome C #84-434 host glomerporphyric 4x excluded
Ellipse Major axis
7
6
5
4
3
2
1
0
-1
-2
-3
-4
-5
-6
-7
-8
-9
0
1
2
3
4
Size
5
6
7
8
99
Dome C #84-434 host glomerporphyric 4x included
Ellipse Major axis
7
6
5
4
3
2
1
0
-1
-2
-3
-4
-5
-6
-7
-8
-9
0
1
2
3
4
Size
5
6
7
8
100
Dome C #84-434 host second spot 4x excluded
Ellipse Major axis
11
10
9
8
7
6
5
4
3
2
1
0
-1
-2
-3
-4
-5
-6
0
1
2
3
Size
4
5
101
Dome C #84-434 host second spot 4x included
Ellipse Major axis
11
10
9
8
7
6
5
4
3
2
1
0
-1
-2
-3
-4
-5
-6
0
1
2
3
Size
4
5
102
Dome C #87-1235 host 4x excluded
Ellipse Major axis
10
9
8
7
6
5
4
3
2
1
0
-1
-2
-3
-4
-5
-6
0
1
2
3
Size
4
5
103
Dome C #87-1235 host 4x included
Ellipse Major axis
10
9
8
7
6
5
4
3
2
1
0
-1
-2
-3
-4
-5
-6
0
1
2
3
Size
4
5
104
Dome C #87-1235 host second spot 4x excluded
Ellipse Major axis
10
9
8
7
6
5
4
3
2
1
0
-1
-2
-3
-4
-5
-6
0
1
2
3
Size
4
5
105
Dome C #87-1235 host second spot 4x included
Ellipse Major axis
10
9
8
7
6
5
4
3
2
1
0
-1
-2
-3
-4
-5
-6
0
1
2
3
Size
4
5
106
Dome C #00-2352 host 4x excluded
Ellipse Major axis
10
9
8
7
6
5
4
3
2
1
0
-1
-2
-3
-4
-5
-6
0
1
2
3
Size
4
5
107
Dome C #00-2352 host 4x included
Ellipse Major axis
10
9
8
7
6
5
4
3
2
1
0
-1
-2
-3
-4
-5
-6
0
1
2
3
Size
4
5
108
Dome C #00-2352 host second spot 4x excluded
Ellipse Major axis
7
6
5
4
3
2
1
0
-1
-2
-3
-4
-5
0
1
2
Size
3
109
Dome C #00-2352 host second spot 4x included
Ellipse Major axis
12
11
10
9
8
7
6
5
4
3
2
1
0
-1
-2
0
1
Size
110
Dome C #84-634 enclave 4x excluded
Ellipse Major axis
9
8
7
6
5
4
3
2
1
0
-1
-2
-3
-4
0
1
2
Size
3
111
Dome C #84-634 enclave 4x included
Ellipse Major axis
9
8
7
6
5
4
3
2
1
0
-1
-2
-3
-4
0
1
2
Size
3
112
Dome C #84-634 enclave second spot 4x excluded
Ellipse Major axis
9
8
7
6
5
4
3
2
1
0
-1
-2
1
Size
2
113
Dome C #84-634 enclave second spot 4x included
Ellipse Major axis
9
8
7
6
5
4
3
2
1
0
-1
-2
-3
-4
0
1
2
Size
3
114
Dome C #87-1235A enclave 4x excluded
Ellipse Major axis
8
7
6
5
4
3
2
1
0
-1
-2
-3
0
1
Size
2
115
Dome C #87-1235A enclave 4x included
Ellipse Major axis
8
7
6
5
4
3
2
1
0
-1
-2
-3
0
1
Size
2
116
Dome C #87-1235A enclave second spot 4x excluded
Ellipse Major axis
9
8
7
6
5
4
3
2
1
0
-1
-2
0
1
Size
2
117
Dome C #87-1235A enclave second spot 4x included
Ellipse Major axis
9
8
7
6
5
4
3
2
1
0
-1
-2
0
1
Size
2
118
Dome C #87-1235B enclave 10x excluded
Ellipse Major axis
20
19
18
17
16
15
14
13
12
11
10
9
8
7
6
0
0.02
0.04
0.06
0.08
0.1
Size
0.12
0.14
0.16
0.18
0.2
119
Dome C #87-1235B enclave 10x included
Ellipse Major axis
20
19
18
17
16
15
14
13
12
11
10
9
8
7
6
0
0.02
0.04
0.06
0.08
0.1
Size
0.12
0.14
0.16
0.18
0.2
120
Dome C #87-1235B enclave second spot 10x excluded
Ellipse Major axis
22
21
20
19
18
17
16
15
14
13
12
11
10
9
8
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
Size
0.08
0.09
0.1
0.11
0.12
0.13
121
Dome C #87-1235B enclave second spot 10x included
Ellipse Major axis
22
21
20
19
18
17
16
15
14
13
12
11
10
9
8
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
Size
0.08
0.09
0.1
0.11
0.12
0.13
122
Dome C #87-1235C enclave 4x excluded
Ellipse Major axis
8
7
6
5
4
3
2
1
0
-1
-2
-3
-4
-5
-6
0
1
2
3
Size
4
5
123
Dome C #87-1235C enclave 4x included
Ellipse Major axis
8
7
6
5
4
3
2
1
0
-1
-2
-3
-4
-5
-6
0
1
2
3
Size
4
5
124
Dome C #87-1235C enclave second spot 4x excluded
Ellipse Major axis
10
9
8
7
6
5
4
3
2
1
0
-1
-2
-3
-4
0
1
2
3
Size
4
5
125
Dome C #87-1235C enclave second spot 4x included
Ellipse Major axis
9
8
7
6
5
4
3
2
1
0
-1
-2
-3
-4
-5
0
1
2
3
Size
4
5
126
Dome C #87-1235D enclave 10x excluded
Ellipse Major axis
21
20
19
18
17
16
15
14
13
12
11
10
9
8
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
Size
0.08
0.09
0.1
0.11
0.12
0.13
127
Dome C #87-1235D enclave 10x included
Ellipse Major axis
21
20
19
18
17
16
15
14
13
12
11
10
9
8
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
Size
0.08
0.09
0.1
0.11
0.12
0.13
128
Dome C #87-1235D enclave second spot 10x excluded
Ellipse Major axis
19
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
0
0.05
0.1
0.15
0.2
0.25
Size
0.3
0.35
0.4
0.45
0.5
129
Dome C #87-1235D enclave second spot 10x included
Ellipse Major axis
19
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
0
0.05
0.1
0.15
0.2
0.25
Size
0.3
0.35
0.4
0.45
0.5
130
Dome D #84-435 host 4x excluded
Ellipse Major axis
13
12
11
10
9
8
7
6
5
4
3
2
1
0
-1
-2
0
1
Size
2
131
Dome D #84-435 host 4x included
Ellipse Major axis
13
12
11
10
9
8
7
6
5
4
3
2
1
0
-1
-2
0
1
Size
2
132
Dome D #84-435 host second spot 4x excluded
Ellipse Major axis
10
9
8
7
6
5
4
3
2
1
0
-1
-2
-3
-4
-5
-6
0
1
2
3
Size
4
5
133
Dome D #84-435 host second spot 4x included
Ellipse Major axis
10
9
8
7
6
5
4
3
2
1
0
-1
-2
-3
-4
-5
-6
0
1
2
3
Size
4
5
134
Dome D #84-632 enclave 4x excluded
Ellipse Major axis
12
11
10
9
8
7
6
5
4
3
2
1
0
-1
0
1
2
Size
3
135
Dome D #84-632 enclave 4x included
Ellipse Major axis
13
12
11
10
9
8
7
6
5
4
3
2
1
0
-1
0
1
2
Size
3
136
Dome D #84-632 enclave second spot 4x excluded
Ellipse Major axis
9
8
7
6
5
4
3
2
1
0
-1
-2
-3
0
1
Size
2
137
Dome D #84-632 enclave second spot 4x included
Ellipse Major axis
9
8
7
6
5
4
3
2
1
0
-1
-2
-3
0
1
Size
2
138
Dome D #84-632 enclave 10x excluded
Ellipse Major axis
7
6
5
4
3
2
1
0
-1
-2
-3
-4
-5
0
1
2
Size
3
139
Dome D #84-632 enclave 10x included
Ellipse Major axis
7
6
5
4
3
2
1
0
-1
-2
-3
-4
-5
0
1
2
Size
3
140
Dome D #84-632 enclave second spot 10x excluded
Ellipse Major axis
7
6
5
4
3
2
1
0
-1
-2
-3
0
1
Size
2
141
Dome D #84-632 enclave second spot 10x included
Ellipse Major axis
7
6
5
4
3
2
1
0
-1
-2
-3
0
1
Size
2
142
Dome E #84-443 host 4x excluded
Ellipse Major axis
12
11
10
9
8
7
6
5
4
3
2
1
0
-1
-2
-3
0
1
Size
2
143
Dome E #84-443 host 4x included
Ellipse Major axis
12
11
10
9
8
7
6
5
4
3
2
1
0
-1
-2
-3
-4
-5
0
1
2
3
Size
4
5
144
Dome E #84-443 host second spot 4x excluded
Ellipse Major axis
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
-1
-2
0
1
2
3
Size
4
5
145
Dome E #84-443 host second spot 4x included
Ellipse Major axis
9
8
7
6
5
4
3
2
1
0
-1
-2
-3
-4
-5
-6
0
1
2
3
Size
4
5
146
Dome E #84-635 enclave 4x excluded
Ellipse Major axis
8
7
6
5
4
3
2
1
0
-1
-2
-3
-4
0
1
2
Size
3
147
Dome E #84-635 enclave 4x included
Ellipse Major axis
8
7
6
5
4
3
2
1
0
-1
-2
-3
-4
0
1
2
Size
3
148
Dome E #84-635 enclave second spot 4x excluded
Ellipse Major axis
7
6
5
4
3
2
1
0
-1
-2
-3
-4
-5
0
1
2
3
Size
4
5
149
Dome E #84-635 enclave second spot 4x included
Ellipse Major axis
7
6
5
4
3
2
1
0
-1
-2
-3
-4
-5
0
1
2
3
Size
4
5
150
Dome E #84-635 enclave 10x excluded
Ellipse Major axis
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
-1
-2
-3
0
1
Size
2
151
Dome E #84-635 enclave 10x included
Ellipse Major axis
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
-1
-2
-3
0
1
Size
2
152
Dome E #84-635 enclave second spot 10x excluded
Ellipse Major axis
6
5
4
3
2
1
0
-1
-2
-3
0
1
Size
2
153
Dome E #84-635 enclave second spot 10x included
Ellipse Major axis
6
5
4
3
2
1
0
-1
-2
-3
0
1
Size
2
154
Dome F #84-455 host 4x excluded
Ellipse Major axis
11
10
9
8
7
6
5
4
3
2
1
0
-1
-2
-3
-4
-5
-6
0
1
2
3
Size
4
5
155
Dome F #84-455 host 4x included
Ellipse Major axis
11
10
9
8
7
6
5
4
3
2
1
0
-1
-2
-3
-4
-5
-6
0
1
2
3
Size
4
5
156
Dome F #84-455 host second spot 4x excluded
Ellipse Major axis
11
10
9
8
7
6
5
4
3
2
1
0
-1
-2
-3
-4
-5
-6
0
1
2
3
Size
4
5
157
Dome F #84-455 host second spot 4x included
Ellipse Major axis
11
10
9
8
7
6
5
4
3
2
1
0
-1
-2
-3
-4
-5
0
1
2
3
Size
4
5
158
Dome F #93-1963a enclave 4x excluded
Ellipse Major axis
11
10
9
8
7
6
5
4
3
2
1
0
-1
0
0.05
0.1
0.15
0.2
0.25
Size
0.3
0.35
0.4
0.45
0.5
159
Dome F #93-1963a enclave 4x included
Ellipse Major axis
11
10
9
8
7
6
5
4
3
2
1
0
-1
-2
-3
-4
0
1
2
3
Size
4
5
160
Dome F #93-1963a enclave second spot 4x excluded
Ellipse Major axis
13
12
11
10
9
8
7
6
5
4
3
2
1
0
-1
0
1
Size
161
Dome F #93-1963a enclave second spot 4x included
Ellipse Major axis
13
12
11
10
9
8
7
6
5
4
3
2
1
0
-1
0
1
Size
162
Dome F #93-1963a enclave 10x excluded
Ellipse Major axis
9
8
7
6
5
4
3
2
1
0
-1
-2
-3
-4
0
1
2
Size
3
163
Dome F #93-1963a enclave 10x included
Ellipse Major axis
9
8
7
6
5
4
3
2
1
0
-1
-2
-3
-4
0
1
2
Size
3
164
Dome F #93-1963a enclave second spot 10x excluded
Ellipse Major axis
8
7
6
5
4
3
2
1
0
-1
-2
-3
0
1
Size
2
165
Dome F #93-1963a enclave second spot 10x included
Ellipse Major axis
8
7
6
5
4
3
2
1
0
-1
-2
-3
0
1
Size
2
166
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