Download 2 Introduction. Planet Earth`s internal structure and the processes

Introduction. Planet Earth’s internal structure and the processes that give rise to it are thought to be
known to a first order, as a result of geophysical and geological data and the interpretation of this data.
Since sampling of the Earth’s interior below a few kilometers has only been possible through the
eruptive products from volcanism and results of seismology, the geochemistry of erupted lavas plays an
important role in constraining the Earth’s structure and history. The geochemistry of the lavas is mostly
well explained by the theory of plate tectonics, and the related large-scale convection of the mantle,
erupting at the boundaries of the Earth’s tectonic plates. However, a different type of volcanism erupts
away from plate boundaries, which requires a different process generating magma and supplying the
volcanoes. Known as tropical vacation destinations like Hawaii to most, these volcanoes are known as
“hotspot” volcanoes to geologists, named after the likely cause for volcanism away from plate
boundaries (likely to be anomalously hot, which generates magma). For the last four decades, the
temperature anomaly has been explained with an upwelling hot plume of mantle material from the deep
mantle, and this has been part of teaching material for introductory geology classes. In stark contrast, a
group of well-established scientists have recently begun to argue against the existence of such mantle
plumes, instead arguing for an overall warmer mantle, that feeds eruptions wherever the plates happen to
crack and break. The controversy that has followed is of great importance to our understanding of the
Earth’s interior, because mantle plumes require a particular set of physical conditions to exist, and their
existence or lack thereof thus
significantly impacts our understanding
(of conditions) of the Earth’s dynamic
interior. Some of the fuel for this
controversy comes from a couple of
island groups that do not fit the
“plume” model perfectly. One of these
groups is the Line Islands, located south
of Hawaii (Figure 1), and the currently
available
(age)
data
requires
geochemical fingerprinting to establish
the origin for the volcanic chain. I
therefore propose to guide a MSc.
student in using the Geology
Department’s facilities to measure the
geochemical composition of an existing
set of samples from the Line Islands,
for which some key (age) data is
already available.
Radiogenic isotopes in geochemistry.
In order to fingerprint the mantle
material that produced the erupted
lavas, usually the relative proportion of
radiogenically
produced
isotopes
(atoms with a different mass of the
same element) compared to stable
isotopes of the same element are
measured. For example, the radioactive
Figure 1. Google Earth image of the Pacific Ocean floor, where
blue colors show deep ocean floor, while yellow and orange
colors are shallow. The Line Islands are outlined by a red
ellipse. The black lines indicate the age of the ocean floor.
Since the Line Islands run parallel to the age contours, they
were thought to have formed at a mid-oceanic plate boundary,
however volcano ages suggest a more complicated origin.
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decay of uranium (U) generates two different isotopes of lead (Pb) with atomic mass 206 and 207 (206Pb,
207
Pb). If U is given a significant amount of time, a lot of radiogenic 206Pb and 207Pb will be generated.
The actual amount generated can be gauged by measuring the radiogenic isotopes against the stable
isotope, 204Pb, which will remain unchanged over time. Therefore, the proportions of 206Pb and 207Pb
versus 204Pb provide an estimate of time (commonly used to provide true ages for rocks). In addition, the
amount of U compared to Pb is important, because if there was hardly any U to begin with, it will not
produce significant amounts of 206Pb and 207Pb, thus leaving proportions of the 206Pb and 207Pb versus
204
Pb unchanged. In short, the measured Pb isotope proportions in volcanic rocks yield an integrated
result of time for decay and initial relative proportions of U to Pb. These two factors are variable in the
mantle source that provides the magma erupted at the surface, and therefore can be and have been used
to track processes in the Earth’s interior. Combining the measurement of Pb isotopes with measurements
of the daughter products of other radioactive systems (e.g. strontium - Sr, neodymium - Nd, hafnium Hf) makes it possible to constrain a number of different processes affecting the mantle through geologic
time. As a result, it is relatively common to obtain multiple isotope measurements on a subset of the
studied lava samples.
The combined isotope data for sample sets from a variety of different “hotspot” volcanic systems
define a significant range in measured isotope ratios. In a multi-dimensional plot of (Pb, Sr, Nd) isotope
ratios, the data are confined within a tetrahedron, whose corners define isotopic compositions that have
been associated with different dynamic processes within the Earth (Zindler and Hart, 1986; Hart et al.,
1992; Figure 2). More importantly, the different volcanic systems take up different areas within the
tetrahedron, making it possible to distinguish different “hotspots” by their geochemical composition.
This is important in areas where volcanoes of different ages and origins are located in the same region,
since the combination of geochemical compositions and ages can help unravel what volcanoes belong
together and what process lies at their origin.
The ages play an important role in this type of study, because “hotspot” volcanoes typically form a
chain of progressively older volcanoes as a result of continued volcanic activity “burning a hole” into
the bottom of a moving tectonic plate, building volcanoes that are slowly displaced from the source that
feeds them. Therefore, hotspot volcanoes can be expected to show a chain of age-progressive volcanoes
with a similar geochemical composition.
Plate tectonics and hotspots. A key factor in the comparison of ages and geochemical compositions for
different volcanoes is thus knowledge of how the Earth’s tectonic plates have moved through time.
Based on a number of different datasets, ranging from geological to physical (GPS) measurements, plate
motion models (e.g. Altamimi et al., 2002; Wessel and Kroenke, 2008) have been constructed over the
last few decades, allowing for reconstruction of the original eruptive location of volcanoes, given the
volcano’s age and current location. As a result, a complex region with volcanoes of various ages can be
interpreted in terms of origin by taking each volcano’s age and reconstructing where plate motion
models suggest the volcano would have been when it first erupted. Volcanoes that were fueled by the
same “hotspot” should all reconstruct to roughly the same geographical location, while non-related
volcanoes will reconstruct in entirely different locations. This approach was successfully used by Konter
et al. (2008) to interpret the origin of submarine volcanoes in the western Pacific Ocean, an area
crowded with submarine volcanoes. Multiple different sets of volcanoes, all older than fifty million
years, reconstruct in three different clusters of eruptive locations, centered on three recently active
volcanoes in the southwest Pacific Ocean (Figure 2). However, to truly prove a genetic relationship
between the old volcanoes and recent volcanism the geochemical compositions should also match within
each cluster.
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Figure 2. Map of the
southwest Pacific Ocean
floor (color shows depth).
Symbols show reconstructed
eruptive
locations
for
different “hotspot” volcanic
chains. Isotopic compositions, shown in color (see
inset), highlight compositional similarity within
clusters with recent volcanism (stars), and differences between clusters. Map
after Konter et al (2008).
The geochemical comparisons are inherently multi-dimensional in nature and prompted Konter et al.
(2008) to create a color-coding for the isotopic compositions of the measured lavas, where each corner
of the isotopic range of all Earth’s hotspot volcanoes (tetrahedron shape; Figure 2) are represented by a
different color. Each sample is assigned a color, the saturation of which is dependent on how close the
sample is to the corners in isotopic space. As is visible in Figure 2, the combination of reconstructed
eruptive locations based on volcano ages and the colors based on the geochemical compositions
produces three clear “hotspot” locations of distinct compositions. Therefore, the results of this study
showed that the volcanoes in this region actually are sourced in three focused areas despite the variable
ages found for neighboring volcanoes, a feature found in few hotspot volcanic systems that has been
used to argue against the existence of mantle plumes (e.g. Foulger and Natland, 2003). As a result, the
work by Konter et al. (2008) stirred up quite a controversy with the researchers that have abandoned the
plume model (and was highlighted on their website). Two other volcanic systems that define ambiguous
age progressions are the Samoan volcanic chain and the volcanoes of the Line Islands. Currently funded
and recent research of the PI and colleagues is unraveling the origin for Samoan volcanism (Koppers et
al., 2008; Konter and Jackson, 2010; Jackson et al., 2010). However, for the Line Islands only age data
are available, while it appears that this chain of volcanoes consists of at least four or five groups of
volcanoes.
Motivation for the proposed study. The Line Islands are some of the oldest hotspot volcanoes in
Pacific Ocean basin, making them important for our understanding of the Earth’s mantle nearly one
hundred million years ago, yet they are poorly understood in terms of their origin (Winterer, 1976;
Schlanger et al., 1984; Epp, 1989). The currently available age data can be interpreted as either
widespread volcanism along the entire chain during multiple time spans in geologic history, or
overlapping volcanic chains resulting from multiple hotspots (Figure 3). If volcanism was active only
during particular intervals in history, and the erupting volcanoes all along the island chain were
genetically related shown by distinct compositions during each particular time period, these data would
suggest a non-plume origin for the Line Islands. Alternatively, if there is no relationship in terms of
origin between synchronously erupting volcanoes, and instead there is a relationship between location in
the chain and age (i.e. an age progression), arguments can be made supporting the eruption of multiple
plume-fed volcanoes constructing one long and complex volcanic chain. Geochemical data is required
to distinguish between these two possibilities, where we might expect consistent geochemical (isotopic)
compositions within the four or five potential hotspot volcanic chains. If plume-fed hotspots are not the
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cause of the Line Islands, we might expect
random compositions along potential hotspot
volcanic chains. This proposal, therefore,
requests
funds
to
obtain
isotope
measurements at UTEP to distinguish
between these options, and the different
mantle structure they imply (Figure 4). Due
to the large variation in hotspot lava isotope
compositions (described above), the type of
fingerprinting proposed here is best done
with a combination of isotope systems (Sr,
Nd, Pb, Hf).
Goals and anticipated outcomes. I will use
the funds requested in this proposal to
achieve two major goals. First, I will analyze
a subset (~10 samples) of the available
samples, using the Geology clean laboratory Figure 3. Volcano age versus location (latitude). The wide
and NSF-funded multi-collector mass scatter of the data might be the result of multiple plume-fed
spectrometer. I have set up the laboratory hotspots generating volcanoes in different places along the
chain. Each hotspot would generate a progressively older
techniques required to measure the proposed chain away from the active location, which would generate
different isotope ratios over the last few parallel slopes (orange lines). Alternatively, there may have
years, and the funds requested will mainly been arbitrary volcanic activity along the entire chain at
cover the supplies required for careful sample several different times. Geochemical data can resolve
processing and purification for mass whether the sloped trends and thus plumes are realistic
here.
spectrometry.
The other goal of this proposal will be to
involve a student in this project and train her in the field of isotope geochemistry. A MSc. student that
recently started working with me, Lauren Storm, has expressed interest in this project, and if funded this
project will make up a significant part of her MSc. thesis. Since there are many facets to precise and
effective laboratory procedures, I will work closely with Lauren guiding her through the complete
process from sample dissolution, through purification, and mass spectrometry. My recent MSc student,
Lynnette Crocker, first shadowed me through the entire procedure, and then processed the next set under
my direct supervision. Since samples are run in small sets, a small number of samples are run this way,
and then the remainder can be processed by the student independently, which has proven to be an
effective approach (Crocker and Konter, 2010). Lauren has already started following me on a different
project, which uses the same procedures, preparing her for her own sample sets. The intention is to have
a Lauren present the results of this project at an international scientific conference, such as the Fall
meeting of the American Geophysical Union. Part of the budget reflects this, with some travel funds
requested for her to attend such a meeting.
This research will subsequently be written up in a paper to be submitted to a top-tier refereed
scientific journal (student will be first author). The exposure from presenting and publishing will allow
me to seek external funding for further research of this type from the National Science Foundation
(NSF). The NSF typically requires a pilot study to have been carried out as part of the argument for
more detailed analysis. I anticipate writing such a proposal while we are working on the journal
publication.
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Figure 4. (Top) If multiple plume-fed hotspots created the Line Islands, the mantle would likely contain a
number of focused upwelling plumes. (Bottom) If volcanoes were constructed along the island chain, randomly
in location and composition, a compositionally variable (upper) mantle is required. Dynamically this implies a
very different history and evolution for the Earth’s deep interior. Modified from Jackson et al. (2010).
Work plan, and student involvement. The required laboratory techniques necessary to perform the
proposed research are in place, and I have direct access to the samples from the Line Islands that were
age-dated. Therefore, if this proposal is funded, my student Lauren and I will start immediately with
processing the samples for purification and measurement. Since the techniques will require a number of
standard geochemistry laboratory expendable items, I will immediately start purchasing supplies to carry
out the laboratory work. These items include protective gear for clean room use (e.g. gloves, clothes),
clean acids/reagents, multiple Teflon sample containers for each sample, ion-exchange resin, and
smaller essential equipment such as pipette tips, centrifuge tubes, laboratory wipes and storage
containers. I currently have some supplies available from other projects that will allow us to get started
while the ordered supplies come in. Therefore, we will be able to get started with our sample dissolution
right away. During this time sample splits will also be prepared for concentration analysis, since
concentration data are required to interpret the isotope data. For example, understanding mixing of
magmas requires knowledge of concentrations and isotope ratios to calculate the composition of
potential mixtures. In the laboratory, the related steps to dissolution will take on the order of about a
week, after which the chemical separation and purification will be started in at least two separate sets,
running each isotope system sequentially. This will take on the order of two to three months. However,
after each isotope system is prepared for one of the sample sets, mass spectrometry will be carried out,
to ensure time is not wasted should something have gone wrong during the chemical procedures. During
this entire process, the student will be exposed to highly technical sample preparation and purification,
as well as learning to operate a state of the art multi collector mass spectrometer. Such experience will
provide her with the qualifications to work in a commercial laboratory setting, or to continue on in a
PhD program.
Timeline. In the first 6 months we will focus on the chemical purification of the samples and start with
mass spectrometry. The supplies will be purchased during this period, and sample concentration data
will be obtained during this time. In the following 6 months, mass spectrometry will be finished, and
data interpretation of both concentration and isotopic data will begin in preparation for both a
presentation and a journal article.
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