Diversity and Petrogenesis of Rhyolites from an Intra-oceanic Arc

167 Pages·2017·10.75 MB·English

WWeesstteerrnn WWaasshhiinnggttoonn UUnniivveerrssiittyy
WWeesstteerrnn CCEEDDAARR
WWU Graduate School Collection WWU Graduate and Undergraduate Scholarship
Summer 2018
DDiivveerrssiittyy aanndd PPeettrrooggeenneessiiss ooff RRhhyyoolliitteess ffrroomm aann IInnttrraa--oocceeaanniicc AArrcc::
EEvviiddeennccee ffrroomm IIOODDPP SSiittee UU11443377,, IIzzuu BBoonniinn RReeaarr--aarrcc aanndd
SSuurrrroouunnddiinngg AArreeaa
Luan J. (Luan Jean) Heywood
Western Washington University, [email protected]
Follow this and additional works at: https://cedar.wwu.edu/wwuet
Part of the Geology Commons
RReeccoommmmeennddeedd CCiittaattiioonn
Heywood, Luan J. (Luan Jean), "Diversity and Petrogenesis of Rhyolites from an Intra-oceanic Arc:
Evidence from IODP Site U1437, Izu Bonin Rear-arc and Surrounding Area" (2018). WWU Graduate School
Collection. 759.
https://cedar.wwu.edu/wwuet/759
This Masters Thesis is brought to you for free and open access by the WWU Graduate and Undergraduate
Scholarship at Western CEDAR. It has been accepted for inclusion in WWU Graduate School Collection by an
authorized administrator of Western CEDAR. For more information, please contact [email protected].
Diversity and Petrogenesis of Rhyolites from an Intra-oceanic Arc: Evidence from IODP
Site U1437, Izu Bonin Rear-arc and Surrounding Area

By

Luan Jean Heywood

Accepted in Partial Completion
of the Requirements for the Degree
Master of Science

ADVISORY COMMITTEE

Dr. Susan DeBari, Chair

Dr. Susanne Straub

Dr. Jackie Caplan-Auerbach

Dr. Brian Rusk

GRADUATE SCHOOL

Dr. Gautam Pillay, Dean
Master’s Thesis

In presenting this thesis in partial fulfillment of the requirements for a master’s degree at
Western Washington University, I grant to Western Washington University the non-exclusive
royalty-free right to archive, reproduce, distribute, and display the thesis in any and all forms,
including electronic format, via any digital library mechanisms maintained by WWU.

I represent and warrant this is my original work, and does not infringe or violate any rights of
others. I warrant that I have obtained written permissions from the owner of any third party
copyrighted material included in these files.

I acknowledge that I retain ownership rights to the copyright of this work, including but not
limited to the right to use all or part of this work in future works, such as articles or books.

Library users are granted permission for individual, research and non-commercial reproduction
of this work for educational purposes only. Any further digital posting of this document requires
specific permission from the author.

Any copying or publication of this thesis for commercial purposes, or for financial gain, is not
allowed without my written permission.

Luan Heywood

August 13th, 2018
Diversity and Petrogenesis of Rhyolites from an Intra-oceanic Arc: Evidence from IODP
Site U1437, Izu Bonin Rear-arc and Surrounding Area

A Thesis
Presented to
The Faculty of
Western Washington University

In Partial Fulfillment
Of the Requirements for the Degree
Master of Science

by
Luan Jean Heywood
August 2018
Abstract
Magmas from the Izu Bonin rear-arc show a geochemical signature (medium- to high-K and
light rare-earth element [LREE] enriched) that resembles the average composition of the
continental crust. I investigate that continent-like signature by examining a suite of rear-arc-
derived mafic to felsic tephras from International Ocean Discovery Program (IODP) Site U1437,
which was drilled as part of IODP Expedition 350. Volcaniclastic glasses from Site U1437
comprise a compositional series from basalt to rhyolite, aged 1.1-4.4 Ma.
Site U1437 tephras record the timespan when the Izu-Bonin arc began rifting and provide a well-
dated record of changing magmatic compositions through the transition into arc rifting.
Considered with context provided by a comprehensive recontextualization of published analyses
of <7 Ma regional dredged rocks, magmatic compositions are shown to vary in coherent
chronological and spatial trends and can be classified into three series: LREE-depleted volcanic
front series; flat pattern REE rift-related series; and LREE-enriched rear-arc seamount chain-type
(RASC-type) series which includes Site U1437 glasses. Each series has a distinctive
characteristic basalt type whose trace element and radiogenic isotope chemistry matches rhyolite
type from the same region.
Geochemical modeling shows that fractional crystallization is a viable mechanism to generate
<4.4 Ma RASC-type rhyolites, via 70-80% fractional crystallization from a typical rear-arc
basalt. Evidence such as lack of trace element correlation above ~73 wt. % SiO and oxygen
2
isotope systematics show that some degree of open-system processes are involved in rhyolite
petrogenesis; however there is no evidence for large-scale melting of pre-existing crustal
sources. The >12.9 Ma crustal sources inferred to exist within the rear-arc region cannot be the
source for rear-arc rhyolites because of differing La/Yb and ɛHf signatures. Thus, I favor a
model where fractional crystallization is the differentiation mechanism involved in the
generation of intraoceanic arc rhyolites in this region.

iv
Acknowledgements
Thank you to the United States Science Support Program (USSSP) for supporting this project. I
am also grateful for additional funding from the Geological Society of America, Western
Washington University Research and Sponsored Programs, Western Washington University
Department of Geology, and additional conference funding from the Western Washington
University Graduate School and National Science Foundation (GeoPRISMS).
I am profoundly thankful to my advisor and mentor, Dr. Susan DeBari. Throughout my graduate
career, you have shown unlimited encouragement and patience and given me incredible insights
into what it means to be an amazing petrologist, educator, academic and person. Your insights
have inspired me to interrogate and push myself in ways that I never could have envisioned. For
me and others, you have cultivated a space for unbounded intellectual growth.
To my committee members: each of your contributions of time and expertise have uniquely
improved this project and I am deeply grateful for it. Dr. Susanne Straub, thank you for your
meticulous attention to detail, immense knowledge, warm words and commitment. Dr. Jackie
Caplan-Auerbach, thank you for your time, open-door policy and mentorship. Dr. Brian Rusk,
thank you for your technical knowledge, philosophical knowledge and wit.
I would also like to thank the Expedition 350 science team for encouragement and an incredible
pool of ideas, intellect and encouragement. Dr. Jim Gill, thank you for bringing context to my
work. Both you and your body of work have been a delight to dialogue with. I am especially
appreciative to my collaborators, chief among them Dr. Julie Schindlbeck
My project greatly benefitted from guidance and support from numerous administrative and
research staff at Western Washington University. In particular I am grateful for support from
Chris Sutton, Theresa Tripp, Kate Blizzard, Ben Paulson, Charles Wandler, Dr. Nicole
McGowan and Megan Spiegel.
Outside Western, I have greatly benefitted from technical guidance and instrumentation help
from a number of other scientists: Dr. Scott Kuehner, Dr. Brennan Van Alderwerelt, L. Kenny
Horkley, Dr. Andy Ungerer, Dr. Brad Pitcher, Dr. Richard Bradshaw and Dr. Mark Reagan
whose encouragement started my scientific journey.
Finally, throughout all these years my friends and family have shown incredible dedication to
me, for which I am very grateful. I would like to extend a special thanks to my graduate cohort,
especially those who shared my interests in petrology and science education. In particular, Olivia
Anderson, Eliza Andrews, Darian Dixon, Ricardo Escobar-Burciaga, Paige Knapp, Mai Sas and
Stephanie Truitt. And most importantly, Rebecca Morris with whom I have found unparalleled
synergy in matters both personal and petrological.
v
Table of Contents

Abstract .......................................................................................................................................... iv
Acknowledgements ..........................................................................................................................v
List of Figures ............................................................................................................................. viii
List of Tables ................................................................................................................................ ix
1. Introduction ..................................................................................................................................1
2. Regional context for rhyolitic volcanism .....................................................................................3
3. Site U1437 core material .............................................................................................................6
4. Methods........................................................................................................................................7
4.1 Electron microprobe.......................................................................................................8
4.2 LA-ICP-MS....................................................................................................................9
4.3 MC-ICP-MS .................................................................................................................10
4.4 Whole rock XRF/ICP-MS............................................................................................11
5. Results ........................................................................................................................................11
5.1 Glass chemistry ............................................................................................................11
5.2 Isotope data ..................................................................................................................14
5.3 Mineral chemistry and textures ....................................................................................14
5.3.1 Plagioclase ....................................................................................................14
5.3.2 Pyroxenes ......................................................................................................15
5.3.3 Amphibole.....................................................................................................16
5.3.4 Fe-Ti oxides ..................................................................................................16
5.3.5 Other mineral phases .....................................................................................16
5.4 Intensive parameters of crystallization ........................................................................17
6. Discussion ..................................................................................................................................19
6.0 Comparison of Site U1437 tephras to regional magmas .............................................19
6.1 Comparison of Site U1437 tephras to Japanese tephras ..............................................20
6.2 Comparison of Site U1437 tephras to Izu-Bonin dredged rocks .................................21
6.2.1 Volcanic-front volcanism..............................................................................22
6.2.2 Rift-related volcanism (sensu lato) ...............................................................23
6.2.3 Rear-arc seamount chain-type (RASC-type) volcanism (<7 Ma) .................25
6.2.4 Rear-arc origin of Site U1437 tephras ..........................................................27
6.2.5 Potential source for Unit II rhyolites ............................................................28
6.2.6 Across-arc trends in felsic magmas <7 Ma ...................................................29
vi
6.3 Differentiation within the Izu-Bonin rear-arc region ...................................................30
6.3.1 Evidence for fractional crystallization ..........................................................31
6.3.2 Subtraction modeling ....................................................................................32
6.3.3 Trace element modeling ................................................................................34
6.3.4 Role for amphibole .......................................................................................37
6.3.5 Criticisms for fractional crystallization ........................................................38
6.4 Role for crustal melting? ..............................................................................................39
7. Summary ....................................................................................................................................42
Works Cited ...................................................................................................................................45
Appendices ...................................................................................................................................129
Appendix A: EMP, LA-ICP-MS and MC-ICP-MS standards .........................................129
Appendix B: Extended Site U1437 magmatic thermobarometry ....................................143
Appendix C: Major and trace element modeling .............................................................149

vii
List of Figures
Figure 1: Bathymetric map and schematic cross section of the Izu-Bonin subduction zone ........60
Figure 2: Site U1437 core and images of representative study intervals .......................................61
Figure 3: SiO content of Site U1437 glasses ................................................................................62
2
Figure 4: Major element variation diagrams for Site U1437 glasses.............................................63
Figure 5: Trace element variation diagrams for Site U1437 glasses .............................................64
Figure 6: Sr-Nd-Hf-Pb isotope systematics for Site U1437 and Izu-Bonin regional dredges .......65
Figure 7: Microscopic images of typical Unit I and Unit II tephra intervals .................................66
Figure 8: Mineral and glass compositions for Site U1437 tephras ................................................67
Figure 9: Bivariate diagram with Th plotted against Ba/La for Japanese and Izu-Bonin arcs ......68
Figure 10: Bathymetric map of Izu-Bonin arc showing dredged rock classifications ...................69
Figure 11: Major and trace element variation diagrams for Izu-Bonin regional dredges ..............70
Figure 12: Rare-earth element diagram showing Izu-Bonin regional dredges ..............................71
Figure 13: Sr isotope systematics of Izu-Bonin regional dredges .................................................72
Figure 14: Rare-earth element diagram showing Izu-Bonin rift dredges and other dredges .........73
Figure 15: Trace element characteristics of volcanic glasses/rocks fom nearby Site U1437 ........74
Figure 16: Evolution of Izu-Bonin felsic magma compositions with time and space ...................75
Figure 17: Major element subtraction modeling results ................................................................76
Figure 18: Evaluation of model fit for major element subtraction modeling ................................77
Figure 19: Trace element modeling results shown on rare-earth element diagram .......................78
Figure 20: Trace element modeling results shown on bivariate element diagram ........................79
Figure 21: Trace element and isotopic characteristics of Site U1437 rhyolites and potential
crustal sources ....................................................................................................................80

viii
List of Tables
Table 1: Site U1437 glass chemistry .............................................................................................81
Table 2: Site U1437 mineral chemistry, plagioclase ...................................................................119
Table 3: Site U1437 mineral chemistry, pyroxenes .....................................................................121
Table 4: Site U1437 mineral chemistry, amphibole ....................................................................127
Table 5: Site U1437 isotopic analyses .........................................................................................128
ix