Download Can you begin by explaining why there Temperature-time-Deformation histories

DR LAURA WEBB
Under pressure
Dr Laura Webb reveals her research, which uses pioneering technology to detect titanium traces in
crystals in order to produce insights into the pressure and temperature histories that form rocks
TitaniQ also provides new avenues for
geoscientists to study how the continental
crust deforms at depth.
Can you begin by explaining why there
is need for further study into PressureTemperature-time-Deformation histories
of rocks? What are the aims of this study?
Studying the pressure-temperaturetime-deformation (P-T-t-D) histories of
rocks yields insights into the physical and
chemical processes operative at depth, the
rates at which they occur, and the tectonic
processes that drive the system. This type
of research can have broader implications,
for example impacting seismic hazards or
natural resource exploration. The field is
constantly evolving and old questions are
often revisited as technology improves and
new techniques are developed. This particular
project investigates the application of a new
thermobarometer, based on the titanium
(Ti) concentration of quartz (‘TitaniQ’),
to unravel the complex deformation and
metamorphic history of rocks in the Strafford
Dome, east-central Vermont.
What are we able to learn from studying
metamorphic rocks? What factors do you
examine in your work?
Rocks record metamorphic histories in their
mineralogy, textural relationships between
minerals, and chemical compositions of
minerals. The latter might involve complex
zoning if a mineral has a protracted history
of growth during changing conditions or
multiple phases of growth. A good example
of such a mineral is garnet, a family of
related nesosilicates, often deep red in colour
and used as gemstones and abrasives. It
can also encapsulate other minerals as it
grows, providing an important record of how
mineralogy, mineral chemistry, and also the
structural fabric of the rock changed over
time. Textural relationships between minerals
provide information on the relative timing
of events and the relationship between
metamorphism and deformation.
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INTERNATIONAL INNOVATION
Who have been the main contributors to
this research? Why is it so important to
encourage collaboration?
How have metamorphic rocks been
traditionally measured? What
advancements have allowed for this
research?
Conventional methods to constrain pressure
and temperature conditions of metamorphism
involve analysis of mineral assemblages and
major element chemistry of minerals inferred
to have been in chemical equilibrium. In
recent years, a series of new trace element
thermobarometers, such as TitaniQ, have
been developed. Titanium is a trace element
in quartz that is able to substitute for silica
in the crystal structure and its concentration
in quartz (measured in parts per million) is
a function of the pressure and temperature
conditions during equilibration and the
effective concentration of Ti in the rock. The
technique is a powerful new tool in P-T-t-D
studies because of the abundance of quartz
in many rock types that comprise continental
crust and the stability of quartz over a
broad range of pressure and temperature
conditions. The microscopic structural fabrics
of quartz-rich domains also record important
information about rock deformation. Thus,
Frank Spear and Jay Thomas at Rensselaer
Polytechnic Institute are co-principal
investigators on this project. Former and
current UVM graduate students, Kyle Ashley
and Patrick Dyess, play key roles as well.
Collaboration is essential to this project
because we are integrating diverse physical
and chemical datasets obtained both in the
field and in the laboratory over a range of
spatial scales with a variety of instrumentation.
This requires an extremely wide range of
knowledge groups, as well as a great deal of
work. Our different areas of expertise dovetail
nicely, and as a team we are far more insightful
than any one of us would be alone.
What has been the greatest challenge for
you as a geologist? Which factors hinder
progress the most?
Political borders are, in some ways, one of the
greatest challenges to geologists. Mountain
belts or basins may extend across borders
and access can be limited or impossible in
these regions. The inability to investigate
key field sites due to political reasons most
certainly hinders the progress of the science,
and is a serious cause for concern across the
international research community. Exporting
and importing samples can also pose a unique
set of challenges and raise questions about
the validity of examining samples without
full contextual knowledge. International field
work can be very challenging logistically and
resource intensive. While I love the travel
aspect of my job, I am happy to have such an
ideal region for a case study for this project
close to home.
DR LAURA WEBB
Rock technology
A team at the University of Vermont has been working to uncover
information on plate dynamics, whilst also endeavouring to create
and hone new investigative techniques in order to aid future studies
QUARTZ HAS A plethora of uses, ranging from
gemstones to piezoelectric applications in watch
mechanisms. Yet, it is its presence as a common
mineral that has captured the attention of a
research group at the University of Vermont,
who are eager to investigate its potential in plate
tectonic studies.
The group has made swift progress and has
discovered that quartz is able to provide a
detailed history of sedimentary rock deformation
and metamorphism. For example, their studies in
Vermont have shown that a burial of sedimentary
rocks along an ancient continental margin are
then followed by the tectonic burial of the rocks
to a great depth during continental collision.
Finally the rocks rise back up to the surface.
Through their research, they have discovered
that these violent processes appear to happen
over a very short timescale, in contradiction
to what could be expected for such a complex
process. Quartz is key to uncovering each stage of
formation, and the team is able to demonstrate
their Pressure-Temperature-time-Deformation
(P-T-t-D) history by studying the patterns of
titanium concentrations in quartz found in
different segments of rock, including in garnet
and rock matrix. Profiling the presence of titanium
in each of these formations provides an accurate
assessment of the pressures and temperatures
that the rock has been exposed to, revealing its
history and providing a basis for inferences about
the evolution of continental crust.
The researchers have employed numerous
innovative techniques in order to uncover the
detailed histories of the rocks that they are
studying, providing significant advances in
understanding rock formation processes. Prior
to the in-depth measure of titanium using
thermobarometry via secondary ionisation
mass spectrometry, the team has utilised
electron backscatter and cathodoluminesence
(CL) imaging. The latter has proved to be a
particularly important tool for this project.
Previous studies have determined that the CL
intensity of quartz in the blue wavelengths is
strongly indicative of the titanium concentration
in that quartz. The method has allowed the team
to quickly determine titanium concentrations
within a single quartz grain, as well as between
neighbouring grains. This manifests itself in
differently graduated images when treated
with CL. A grain which appears with a dark core
and a bright rim in CL imaging demonstrates a
higher concentration of titanium near the rim,
allowing inferences about changes in heat and
pressure. Furthermore, the darker core is likely
to have formed earlier in the history of the rock,
so relative timing can be calculated from this
technique. These assumptions can then be tested
in more detail through the other techniques open
to the researchers, meaning that they can initially
survey many rocks before narrowing their study.
INVESTIGATING VERMONT
The team has chosen to pursue their field work
near to the laboratory, which has provided a
number of advantages for the investigation. The
Strafford Dome Complex in Vermont provides
metamorphic rocks with complex deformation
histories of exactly the kind the researchers need
as part of their work. Building on the P-T-t-D
framework established by other studies, the group
are able to exploit these techniques because of
the wide presence of quartz within the rocks. Dr
Laura Webb, who is leading the investigation,
FIGURE 1. EXAMPLES OF DEFORMED AND METAMORPHOSED SEDIMENTARY ROCKS IN THE STRAFFORD DOME,
EAST CENTRAL VERMONT
WWW.RESEARCHMEDIA.EU 121
INTELLIGENCE
CONSTRAINING P-T-T-D PATHS OF
METAMORPHIC TECTONITES WITH THE
TITANIQ THERMOBAROMETER
OBJECTIVES
• To develop powerful new tools for geologists
to constrain pressure temperature-timedeformation (P-T-t-D) in order to understand
the tectonic and petrologic histories of many
crustal rocks
• To investigate the strain field associated with
P-T conditions and provide insight into the
physical processes associated with quartz reequilibration with respect to Ti distributions
• To determine not just the temperature, but
also the depth at which a fabric is developed
KEY COLLABORATORS
Frank Spear and Jay Thomas, Rensselaer
Polytechnic Institute, USA
Graduate Students:
Kyle Ashley, UVM Geology, MS, 2011
Patrick Dyess, UVM Geology, MS candidate
FUNDING
National Science Foundation –
Award No. 0948529
CONTACT
Dr Laura E Webb
Department of Geology
University of Vermont
180 Colchester Ave
Burlington
VT 05405
USA
T +1 802 656 8136
E [email protected]
https://sites.google.com/site/
laurawebbuvmgeology/
www.uvm.edu/~geology/?Page=faculty/
webb.php
LAURA E WEBB is Assistant Professor of
Geology at the University of Vermont. Her
research interests include the evolution of
plate boundaries, and how continents deform
internally in response to plate boundary
processes.
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INTERNATIONAL INNOVATION
The team use numerous innovative
techniques in order to uncover the
detailed histories of the rocks that
they are studying, pushing forwards
understanding of rock formation
realises that practical considerations are an
important part of this research: “Conducting
these studies locally allows us to focus our
resources on the intensive laboratory component
of the research”. By carefully allocating the
available assets, the team have been able to test
several hypotheses concurrently. These include
whether quartz included in garnet records the
pressure and temperature conditions experienced
early in the rock’s history, and if the quartz
found outside of the garnet records subsequent
metamorphism. Due to the many assumptions
which have to be met for an accurate estimation
of pressure and temperature, this case study is set
to reveal the potential, and also the limitations,
of this technique. The team hope that their work
will be able to both reveal the history of the
Strafford Dome complex, and delineate the use
of the techniques they have employed.
PROBING PLATES
For Webb, this work is simply the continuation
of a career which has been dedicated to the
investigation of rocks which return from deeply
subducted continental material. This work has
meant a broad investigative remit, covering
ancient collisional mountain belts in China, as
well as modern counterpart examples in south
eastern Papua New Guinea. The insights from
these prior studies have also played into Webb’s
understanding of the ways in which plates
interact: “Results from these studies suggest
that transient microplates within the larger plate
tectonic system may play a key role in plate
dynamics”.
Webb has also been investigating the internal
deformation of plates due to boundary forces,
including faults in Mongolia driven by the collision
between India and Asia. These considerable topics
are addressed through process-focused studies
which utilise the radiometric dating of rocks. This
allows investigators to accurately quantify the
timing of events and speed of tectonic processes.
To this end, Webb is fronting the construction
of a 40Ar/39Ar geochronology laboratory at the
University of Vermont. Given that the need
for age control geoscience currently outstrips
the facilities available, this development seeks
to redress the balance, providing equipment
which will produce the data required for such
investigations.
ACHIEVING CALIBRATION
A significant portion of the research effort
have been dedicated to accurately calibrating
their thermobarometer. Data developed over a
matter of days in a laboratory must replicate
samples which take millions of years to form,
across a multitude of geological timescales.
In order to obtain accurate results, the team
have had to compare their results using
different published calibrations for samples
with known P-T-t-D histories. This has assisted
them in producing replicable results which
should be of use for subsequent investigations.
The production of datasets for known mineral
standards, and for quartz crystals with a known
titanium concentrations, also offer significant
benefits for researchers. Moreover, the group
has endeavoured to ensure the replicability of
results from measured titanium concentrations
within the same quartz grain, as well as across
grains in similar textural contexts from the same
sample, and from different samples with shared
histories. By using this thorough approach, the
team is greatly improving available technologies
for geological studies.
GLOBAL INVOLVEMENT
In a commitment to advancing geological
science as a whole, Webb and her colleagues will
utilise the Database System for Metamorphic
Geochemistry ‘MetPetDB’. This comprises
a well-organised and open-access digital
collection, creating archives which a wide
range of researchers are able to access as
part of their work. Such openness is essential
for encouraging further investigations and
preventing research provincialism. MetPetDB
focuses on metamorphic petrology, and has
been developed by Rensselaer Polytechnic
Institute, who have received support from the
National Science Foundation. By contributing
their own work to this database, the group
at Vermont are part of a wider movement in
academia to provide open data for research,
encouraging ongoing innovation and support
across the world. Building on the many
achievements of their studies thus far, they
hope to continue driving innovations in research
on metamorphic tectonites through their use of
pioneering techniques.