The evolution of the Peach Spring Tuff magmatic system as revealed by accessory mineral textures ...

311 Pages·2010·4.74 MB·English

by  Ayla Pamukcu

THE EVOLUTION OF THE PEACH SPRING TUFF MAGMATIC SYSTEM AS REVEALED BY
ACCESSORY MINERAL TEXTURES AND COMPOSITIONS

By

Ayla Susan Pamukcu

Thesis
Submitted to the Faculty of the
Graduate School of Vanderbilt University
in partial fulfillment of the requirements
for the degree of

MASTER OF SCIENCE
in
Earth and Environmental Science

August, 2010

Nashville, Tennessee

Approved:
Professor Guilherme A. R. Gualda
Professor Calvin F. Miller
To Supereruptions
Without you this project would never have been done. No joke.
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ACKNOWLEDGEMENTS

I owe much thanks to the many people who have helped this work come to be. I owe
particularly considerable gratitude to Dr. Guilherme Gualda and Dr. Calvin Miller, without whose
unfailing enthusiasm and patience I would not have completed this work. I can only hope to some
day have the level of curiosity and energy that they constantly exude. I have wholeheartedly
appreciated their guidance and have learned so very much over the course of this process.
I could also not have completed this work without the extraordinary help of my fellow
MESSY students, namely Lily Claiborne, Lindy Colombini, Danny Flanagan, and Tamara Carley. I
am also indebted to Mark Rivers, Joe Wooden, and Rosanne Delapp, who were extremely helpful
guides in performing the various analyses conducted throughout this project. Funding support
from this project came from National Science Foundation grants EAR-0911726 and EAR-
0711109.
Thanks are also due to the faculty and staff of the Earth & Environmental Sciences
Department at Vanderbilt. My questions were always accepted and answered with interest and
excitement. I bow down to Aaron Covey, who made sure I was supplied with the technological
means to do this project.
The other graduate students of the EES department were also vital to keeping me going.
I would particularly like to thank Andrew Roberts for always being up for an adventure, a walk, or
eating outside, and for getting that dead decaying mouse out from under my kitchen sink.
Finally, I could not have done this at all without the love and support of my family, friends,
and the PB. I particularly thank my mom, dad, and brother for weathering the trials and
tribulations of having a daughter/sister interested in geology, namely for dealing with the fact that
she thinks rocks are the coolest things ever and goes gallivanting around the world to look at
them. I also give many thanks to the PB, who (foolishly) still thinks I’m smart and who is endlessly
curious about the world, which makes me more so. My dearest friends from Blue Bell are also
due enormous thanks for always providing me with laughs, amongst many other things. Thank
you all for always listening to my ramblings and being ever encouraging of my insanity.
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TABLE OF CONTENTS

Page
DEDICATION....................................................................................................................................ii
ACKNOWLEDGEMENTS................................................................................................................iii
LIST OF TABLES............................................................................................................................vi
LIST OF FIGURES.........................................................................................................................vii
Chapter
I. THE PEACH SPRING TUFF..................................................................................................1
Introduction.......................................................................................................................1
Geologic Background.......................................................................................................2

II. METHODS..............................................................................................................................5

Samples............................................................................................................................6
Analytical Methods...........................................................................................................7
Bulk density determinations, thin section documentation and crystal separation.....7
Differential Absorption X-ray Tomography (DAT) and Crystal Size Distributions
(CSD).........................................................................................................................8
Scanning Electron Microscope (SEM).....................................................................10
Laser Ablation Inductively Coupled Plasma Mass Spectrometer
(LA-ICPMS)..............................................................................................................11
Reverse Geometry Sensitive High Resolution Ion Microprobe
(SHRIMP-RG)..........................................................................................................13
Whole rock geochemistry........................................................................................15
III. RESULTS.............................................................................................................................14
Bulk density determinations...........................................................................................16
Whole rock geochemistry...............................................................................................17
Phenocryst assemblages...............................................................................................17
Trace element compositions..........................................................................................18
Glass and whole rock compositions and trends......................................................18
Zircon and sphene compositions...................................................................................21
Rare earth elements................................................................................................21
Trace element variations.........................................................................................26
Ti-in-zircon and Zr-in-sphene thermometry.............................................................32
Textures..........................................................................................................................36
Crystal size distributions..........................................................................................36
Qualitative textural features of minerals in the Peach Spring Tuff..........................39

IV. DISCUSSION........................................................................................................................44

Cooling, crystallization, and decompression in the Peach Spring magma
system............................................................................................................................44
Heating and zoning in the Peach Spring Tuff.................................................................47

V. CONCLUSIONS....................................................................................................................50
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Appendix

A. WHOLE ROCK GEOCHEMISTRY OF THE PEACH SPRING TUFF..................................52
B. LA-ICPMS ANALYSES FROM THE PEACH SPRING TUFF..............................................55
C. TRACE ELEMENT COMPOSITIONS FROM SHRIMP-RG OF ZIRCON GRAINS FROM
THE PEACH SPRING TUFF AND CATHODOLUMINESCENCE IMAGES OF ANALYZED
ZIRCON CRYSTALS............................................................................................................90
D. TRACE ELEMENT COMPOSITIONS FROM SHRIMP-RG OF SPHENE GRAINS FROM
THE PEACH SPRING TUFF AND CATHODOLUMINESCENCE IMAGES OF ANALYZED
SPHENE CRYSTALS.........................................................................................................208
E. BLOB3D EXTRACTED DATA USED TO DETERMINE CRYSTAL SIZE
DISTRIBUTIONS................................................................................................................270

REFERENCES.............................................................................................................................301
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LIST OF TABLES

Table Page

1. Sample locations, descriptions, analyses performed, and year collected..............................7

2. Tomographic run letters with corresponding sample size and image resolution....................9

3. SEM Operating Conditions...................................................................................................11

4. LA-ICPMS operating conditions (modified from Colombini 2009)........................................12

5. Measured bulk densities of pumice clasts and intracaldera fiamme....................................16

6. Phenocryst assemblages of PST samples...........................................................................18

7. REE compositions of zircon and sphene crystals used for normalization............................22
vi
LIST OF FIGURES

Figure Page
1. Areal extent of the Peach Spring Tuff.....................................................................................3
2. SiO vs. Sr in CRW, PSTG01C, WSW1, WSW2A, WSW2B and Kingman.........................19
2
3. Whole rock and glass compositions for Kingman (outflow pumice), CRW (intracaldera
fiamma), and WSW1 (mafic enclave)...................................................................................20

4. Chondrite-normalized REE plots of zircon and sphene grains used for zircon- and sphene-
normalized REE plots...........................................................................................................22

5. Zircon-normalized rare earth elements in zircon..................................................................24

6. Sphene-normalized rare earth elements in sphene..............................................................25

7. Zircon-normalized Gd vs. Hf in zircon crystals for (a) KPST01A, (b) WSW2A, (c) WSW2B,
(d) PSTG01C and (e) CRW..................................................................................................27

8. Zircon-normalized Nd v. Hf in zircon for (a) KPST01A, (b) WSW2A, (c) WSW2B, (d)
PSTG01C, and (e) CRW......................................................................................................28

9. Zircon-normalized Yb v. Hf in zircon for (a) KPST01A, (b) WSW2A, (c) WSW2B, (d)
PSTG01C, and (e) CRW......................................................................................................29

10. Sphene-normalized Nd v. Gd in sphene for (a) KPST01A, (b) WSW2A, (c) WSW2B and (d)
PSTG01C..............................................................................................................................30

11. Sphene-normalized Yb and Gd in sphene for (a) KPST01A, (b) WSW2A, (c) WSW2B, and
(d) PSTG01C........................................................................................................................31

12. Ti-in-zircon temperatures......................................................................................................34

13. Zr-in-sphene temperatures...................................................................................................35

14. Crystal size distributions of (a) zircon, (b) sphene, (c) allanite+chevkinite and
(d) magnetite.........................................................................................................................37

15. Phenocryst textures in thin section and crystal separates....................................................40

16. Tomograms and reconstructions of phenocrysts in outflow pumice (a) and intracaldera
fiamma (b).............................................................................................................................41

17. 3D rendition of sphene (yellow), allanite+chevkinite (red) and zircon (white) crystals from
tomography...........................................................................................................................43

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CHAPTER I

THE PEACH SPRING TUFF

Introduction
An eruption is deemed a “supereruption” if it is one that deposits a large volume of
material (Sparks et al. 2005, Self 2006) over a relatively short time interval. The tuffs that result
from these eruptions are of great interest to the field of Earth Science, as their existence is
striking evidence that large magma reservoirs exist within the Earth’s crust. Studying these tuffs
can provide important insight into the evolution of such large magma bodies, which is
fundamental to understanding the stability and longevity of such systems, as well as the
mechanism(s) that do (or do not) trigger a supereruption. In addition, they may hold meaningful
information about processes that form and alter the continental crust.
On a broader scale, studying these tuffs is essential for understanding how and why
supereruptions are (or are not) comparable to eruptions of lesser volume, and to what extent
these giant systems are similar to those that form large batholiths. We can relatively easily
monitor and study small ongoing volcanic systems, but we are not able to do this for rare and
short supereruptions. Consequently, by finding and understanding similarities between small and
giant systems we will be better able to prepare for future supereruptions. Volcanic eruptions of all
sizes impact life and society, but supereruptions have the ability to wreak havoc at much larger
scales than smaller eruptions. Thus, despite the fact that such large eruptions are rare, they do
deserve study.
The Peach Spring Tuff (PST) is an example of an enormous pyroclastic deposit formed
by a supereruption (e.g. (Smith & Bailey 1966, Christiansen & Blank 1972, Bailey et al. 1976).
Relative to other known tuffs that are products of super eruptions, such as Oranui or the Bishop
Tuff (Wilson 2001, Wilson & Hildreth 1997), the PST is unusual in its abundance of accessory
minerals like sphene, zircon, and allanite. These minerals are major reservoirs for trace
elements, particularly U, Th, and REE, and can serve as useful geochronometers,
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geothermometers and monitors of magmatic evolution trends. Study of these minerals can place
useful constraints on the conditions in the magma chamber, and their ability to record important
compositional stages of the melt (e.g. via compositional zoning) can be used to understand the
development of the PST system over time.
This primary aim of this study was to assess the history and evolution of the PST system
by using textures and compositions of accessory minerals (sphene, zircon, allanite, chevkinite)
and glasses in pumice clasts and fiamme from various regions of the PST outflow and
intracaldera deposits. This study complements work done by Carley (2010), who used MELTS
modeling and bulk compositions of the same set of pumice clasts and fiamme to investigate the
mechanisms involved in bringing the PST system to an eruptive state.

Geologic Background
The Peach Spring Tuff is a large Miocene ignimbrite located in the southwestern United
States. It was first recognized by Young & Brennan (1974), who described it in the western
Colorado Plateau. Glazner et al. (1986) broadened its known extent to the Mojave Desert, where
field observations and phenocryst assemblages were used to correlate tuff outcrops with the
PST. It has since been found discontinuously in outcrops over a radius of ~360 km (Buesch
1992) around the triple junction of Arizona, Nevada, and California (figure 1). Based upon outflow
exposures known at the time, Buesch (1992) estimated that the PST covered an area of at least
32,000 km2 and had a volume ≥ 640 km3.
The PST represents a geologically instantaneous eruptive event that occurred within a
period of significant regional extension. This, in addition to having an expansive presence
throughout the region, makes the PST an important stratigraphic horizon in a region that
generally lacks similarly useful marker beds.
Originally, the PST was thought to record a single cooling unit (e.g. Young & Brennan,
1976; Glazner et al, 1986); however, recently it has been proposed that two cooling units may
exist (Varga et al. 2004). Sanidine crystals in the PST dated by Ar/Ar have been reported to have
an age of 18.5±0.2 Ma (Nielson et al. 1990, Miller & 1998), but a revised age of 18.66±0.03 Ma
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(using the most recent accepted Fish Canyon sanidine standard age) has recently been obtained
for the PST (Ferguson & McIntosh in prep.).

Figure 1. Areal extent of the Peach Spring Tuff. Red dots indicate sample locations. White circle
indicates the approximate location of the source caldera (Silver Creek Caldera).

The PST is typically strongly welded and varies in thickness (Glazner et al. 1986) from
10-15 m in distal portions (e.g. Barstow, CA) to 60-130 m in more proximal localities (e.g.
Kingman, AZ; Piute Mountains, CA). Young & Brennan (1974) were first to describe the rock
and deemed it a trachyte; however, recent work indicates that the PST outflow is rhyolitic in
composition (Gaudio et al. 2003, Carley 2010). Previous workers have characterized the PST as
containing 4-20% phenocrysts (Young & Brennan 1974) of primarily feldspar (sanidine and
plagioclase), biotite more abundant than hornblende and pyroxene, and rare quartz. Large
sanidine is the predominant feldspar (Glazner et al. 1986)..Accessory minerals include relatively
abundant sphene, zircon and allanite, as well as some possible monazite and chevkinite.
Given the volume of the PST, it is expected that a sizeable caldera (15-20 km diameter)
(Smith 1979) would have been produced from eruption; however, the precise location of the PST
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