Magmatic volatiles at rifts and arcs: Sources and fractionation effects

327 Pages·2016·7.12 MB·English

University of New Mexico
UNM Digital Repository
Earth and Planetary Sciences ETDs Electronic Theses and Dissertations
7-10-2013
Magmatic volatiles at rifts and arcs: Sources and
fractionation effects
Joost Maarten de Moor
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de Moor, Joost Maarten. "Magmatic volatiles at rifts and arcs: Sources and fractionation effects." (2013).
https://digitalrepository.unm.edu/eps_etds/16
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J. Maarten de Moor
Candidate
Earth and Planetary Sciences
Department
This dissertation is approved, and it is acceptable in quality and form for publication:
Approved by the Dissertation Committee:
Tobias Fischer , Chairperson

Zachary Sharp , Co-chairperson

Penelope King , Co-chairperson

Charles Mandeville

Charles Shearer
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MAGMATIC VOLATILES AT RIFTS AND ARCS:
SOURCES AND FRACTIONATION EFFECTS

by

J. MAARTEN DE MOOR

B.A., Earth and Environmental Science, Wesleyan University, 2003
M.S., Earth and Planetary Sciences, University of New Mexico, 2005

DISSERTATION

Submitted in Partial Fulfillment of the
Requirements for the Degree of

Doctor of Philosophy
Earth and Planetary Sciences

The University of New Mexico
Albuquerque, New Mexico

May, 2013
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© 2013, J. Maarten de Moor
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DEDICATION

This work is dedicated to my grandparents, Joost and Roelfina de Moor and Herman and
Truus van’t Hof, who have played such important roles in my education
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ACKNOWLEDGEMENTS
I would like to express my deep gratitude to all of those who have supported me
and my research during the course of Ph.D. studies. Firstly, none of this work would have
been possible without my advisors. Dr. Tobias Fischer has been a true mentor during both
my M.S. and Ph.D. and provided me with strong support and encouragement in every
aspect of my development as a volcanologist and geochemist. We have shared many
incredible experiences in pursuit of gas samples, some of which were incomparably awe
inspiring and some of which were simply terrifying. I will always be grateful for Tobias’
generous support, guidance, and trust. Similarly, Dr. Zach Sharp has had a huge
influence on my development as a stable isotopist and geochemist and has been
incredibly supportive during both my M.S. and Ph.D. work. Zach has taught me to
question at a fundamental level and to explore ideas creatively. Working with him has
always been a pleasure and an adventure in itself. Likewise, Dr. Penny King has been a
great mentor and extremely supportive during my Ph.D. work. Her breadth of knowledge
has been inspiring and she has taught me to interrogate data rigorously and critically.
Penny’s support and inspiration has allowed me to accomplish analyses that I would not
have otherwise even considered. I have enormous respect for my advisors and am
extremely grateful for their support.
I would like to thank all of those who have contributed to this work in the field
and laboratory. Drs. Rick Hervig, Roman Botcharnikov, Bernard Marty, Dave Hilton,
Erik Hauri, and Mike Zelenski have been inspiring and supportive in the laboratory and
field and have significantly contributed to my scientific thought processes. Drs. Viorel
Atudorei, Vitchko Tsanev, Mike Spilde, Lynda Williams, Adrian Brearley, and Mehdi
Ali are all sincerely thanked for assistance with analytical methods. I would especially
like to thank Drs. Charlie Mandeville, Liz Cottrell, and John Stix for sharing their
analytical standards, influential discussions, and their work that has been inspirational to
me. Carlos Ramirez, Frederick Mangasini, Jessica Rowland, Dr. Dereje Atalay, Dr. Peter
Barry, and Andrés Ulloa all provided assistance and good company in the field. I would
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like to gratefully acknowledge the support and encouragement from the faculty in the
Department of Earth and Planetary Sciences and the Institute of Meteoritics at UNM. The
people of Engare Sero village, especially Olomolik (“Tall”) and Medimuni Juma, are
thanked for their hospitality and logistical support during fieldwork at Oldoinyo Lengai.
The Afar people of Kusa Wat and Durubu villages are thanked for armed protection,
shelter, and resisting the temptation to shoot us themselves.
This work would not have been possible without financial support. In particular,
the NASA Earth and Space Science Fellowship provided funding in support of my
research from 2009-2012. The MARGINS-NSF program and a GSA student research
grant supported research on Anatahan. Work on Oldoinyo Lengai was funded by NSF
Grant EAR0827352, the National Geographic Society, a GSA student research grant, and
NSF funding in support of the Arizona State University National SIMS Facility. Research
on Erta Ale and Masaya was supported by NSF Grant EAR1049891, the NASA Earth
and Space Science Fellowship, and the Caswell-Silver Foundation.
Finally, I would like to thank my family and friends from the bottom of my heart
for bringing light to my life. Mom, Dad, Zand, Em, Oma Jobs, Oma, Jansie, Diederik,
Andra, Odette, Monique, Liesbelle, Meghan, Zach, Scotty, Matt, Leah, Paul, Big Zach,
Oli, Ant, Tiappa, Carol, Kana, Dylan, Spruce, Ryan, Kym, Amber. Love you all.
vii

MAGMATIC VOLATILES AT RIFTS AND ARCS:
SOURCES AND FRACTIONATION EFFECTS

by

J. MAARTEN DE MOOR

B.A., Earth and Environmental Science, Wesleyan University, 2003
M.S., Earth and Planetary Sciences, University of New Mexico, 2005
ABSTRACT
Mantle degassing is a fundamental process that modifies the chemical
composition of the atmosphere, crust, and deep Earth. Elements are returned to the
mantle from surface reservoirs through subduction at convergent margins. Melts
generated in the mantle carry dissolved volatiles from depth and upon decompression
exsolve a fluid or vapour phase. These volatiles drive explosive eruptions, change
climate, and form economic metal deposits. Thus, the sources and processes involved in
degassing of the mantle and melts are a crucial topic in geochemistry.
This dissertation is composed of four chapters, each contributing original and new
insights into sources and processes involved in mantle degassing. Chapters 1 and 2 focus
on volatiles and the origin of carbon-rich magmatic emissions in the East African Rift,
which is perhaps the least understood tectonic system on Earth in terms of mantle
degassing. East Africa hosts a diverse array of enigmatic magma compositions, best
exemplified by Oldoinyo Lengai volcano in Tanzania, which is the only active
carbonatite volcano on Earth and an important alkaline endmember magmatic system. In
Chapter 1 (published in Earth and Planetary Sciences in January, 2013) we show that
nephelinite melts at Oldoinyo Lengai are the most carbon-rich natural silicate melts
known to science. However, we argue that the mantle source for these melts is not
unusually rich in carbon. Rather, extreme degrees of fractional crystallization and the
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high carbon solubility in these alkali-rich melts are responsible for the high observed
carbon contents. We also show that the Oldoinyo Lengai magmatic system is rich in
water, contrary to prior assumption, and that water degassing plays a fundamental role in
the eruptive behaviour and magmatic evolution at Oldoinyo Lengai. The appendix to
Chapter 1 contains additional data (including carbon, oxygen ,and sulfur isotope
compositions and assessment of sulfur behaviour in the magma system) from Oldoinyo
Lengai that have not been published.
Chapter 2 presents the first bulk gas and nitrogen isotope compositions from the
Rungwe Volcanic Province, which is the southernmost volcanic manifestation of rifting
in East Africa. In this paper (currently in press in Chemical Geology), we show that the
gases emitted at Rungwe are CO -rich, presenting a hazard to the inhabitants of this
2
fertile and thus heavily populated area. We find that the gas compositions record high
temperatures in the deeper hydrothermal system, which could be a valuable geothermal
resource for local economic development. The nitrogen isotope compositions and gas
ratio tracers are consistent with an upper mantle source for the gases, and this signature is
strongest in the central part of the province where the intersection of deep crustal
structures provide direct conduits for mantle degassing. Interestingly, the mantle
signatures are associated with the lowest temperature emissions in the region. The
structures provide conduits for melts as well as gases, leading to the coincidence of
mantle gas emission and volcanic edifice building in the central Rungwe Volcanic
Province. These high elevation mountains are the recharge zones for the shallow aquifer
and the mantle gases thus equilibrate with cold meteoric water close to the surface.
Chapters 3 and 4 are studies of S degassing at rift and arc volcanoes through the
use of S isotopes. Sulfur is extremely important for eruption prediction because SO is
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the only gas routinely measured remotely at active volcanoes. Explosive eruptions can
inject large quantities of S into the stratosphere where it oxidizes to sulfate, which can
cause decadal climate cooling. Chapter 3 examines S degassing during an explosive
eruption at Anatahan ( Mariana Arc) that emitted about 250 ktons of SO in the first ten
2
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days of the eruption. The results of this study show that the source of erupted sulfur was
from ultimately from the mantle, with little addition from subducted seawater sulfate, or
the pre-eruptive hydrothermal system. Sulfur isotopes fractionate during the degassing
process, and the change in sulfur isotope compositions through the eruption are consistent
with closed system degassing of a magma body.
Chapter 4 presents a detailed assessment of the S cycle at persistently degassing
basaltic volcanoes. In this study, we constrain the conditions of degassing (oxygen
fugacity, temperature) as rigorously as possible to utilize S isotope compositions of gases
and melts to address equilibrium versus non-equilibrium degassing and Earth’s sulfur
cycle. We find that S degassing is not an equilibrium process and that S partitioning into
the gas phase is associated with a kinetic fractionation effect reflected in the S isotope
compositions. Erta Ale, a reduced magmatic system, convincingly demonstrates a kinetic
effect because S2- (rather than SO 2-) is the dominant S species in the melt. Equilibrium
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degassing should result in preferential partitioning of the heavy S isotope into the gas,
however we observe that the gas is isotopically lighter than the melt. This is consistent
with faster diffusion of the light isotope. At Masaya, an oxidized magmatic system, the
equilibrium and kinetic isotope fractionation effects both favor isotopically light gas,
which makes the contributions from equilibrium versus kinetic effects more ambiguous.
However, under steady state conditions the gas phase is representative of the sulfur
isotope composition of the source and the isotope composition of gas from Masaya
indicates recycling of oxidized S through the subduction zone. Mass balance calculations
show that only a small fraction of the S subducted at the Central American arc is returned
to surface reservoirs. However, the flux of oxidized S from the subducted slab is high
enough to rapidly oxidize iron in the mantle wedge, providing an explanation why arc
magmas are more oxidized than those at rifts. Finally, the S retained in the subducted
slab is isotopically light, potentially carrying the signature of microbial life into the deep
mantle.