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of granites from the same field area yielded
Micro-Characterization of
Zircon Crystals:
Insights into Deep Time and the Process
of Mountain Building
Final Report of Student-Faculty
Collaborative Challenge Grant Results
Claire Kauffman
(mentor: Dr. Paul Tomascak,
Department of Earth Sciences)
small but interpretable concentration
variations. Incorporation of Ti in zircon is
known to be temperature sensitive, and we
interpret the variation in concentrations to
reflect a group of granites with distinct
intrusion temperatures, consistent with bulk
elemental and isotopic data from those rocks.
We had originally proposed to measure
oxygen isotope ratios of these crystals, but
Project Summary
The purpose of this project was to characterize
the internal structure and chemical variation of
crystals of the mineral zircon (ZrSiO4) from
granitic rocks of the Appalachian mountain belt
in southwestern Maine. The study is a segment
found that the sample size was too small for
application of the instrumentation at McGill
University. We were fortunate that we could
accomplish similar research objectives by
employing a different analytical technique in
the same laboratory.
of a larger project involving students from
Oswego and Buffalo State College. Our findings
include the following:
(1) The fine-scale internal chemical structure of
zircon crystals from contrasting granite types
were captured using an electron probe
microanalyzer (EPMA). Secondary electron
and cathode luminescence images revealed a
variety of features, most interpreted as igneous
(“primary”). In one sample complex internal
zonation patterns suggest older, inherited
cores. The data allow for the selection of
appropriate grains for continuing U-Pb
geochronology of these samples.
(2) Analyses by laser ablation inductivelycoupled plasma source mass spectrometry of
the titanium (Ti) concentrations of these
zircon samples plus zircon from a related suite
Introduction
In southwestern Maine, a variety of granitic
rocks crop out around Sebago Lake (Figure 1).
Many of these rocks have been studied
previously for elemental and isotopic
geochemistry (Tomascak et al., 1996), but only
recently have the migmatitic rocks of the area
begun to receive detailed study. The principal
goal of these studies is to understand the nature
of crustal melting by defining the relationships
between different generations of granitic rocks,
the primary record of crustal growth in the
continents.
One of the most critical steps in defining
‘genetic’ relations amongst spatially associated
groups of granitic rocks is through precise
geochronological constraints. The mineral zircon
is the primary igneous geochronometer used in
(Speer, 1982; Cherniak and Watson, 2001). As
studies of ancient rocks. In order to achieve high
technology advances, tools become more
precision geochronology, internal complexities
widespread to examine physical and chemical
of zircon crystals need to be mapped out.
features at the micrometer (0.001 mm) scale in
Additionally, the trace element budget of zircon
minerals.
Modern geochronologic analysis of single
can yield information about the chemical and
thermal environment of the magma from which
zircon crystals has pointed out that individual
the zircon crystallized, information that can
grains are frequently internally complex. This
compliment studies of bulk compositions of
complexity can manifest itself in complicated,
rocks.
difficult to interpret, geochronologic data
(Bickford et al., 1981; Mezger and
Krogstad, 1997). To this end, the
characterization of zircon grains by a
micro-chemical technique has become an
important first step in the analytical
process. The approach of choice here is
examination by electron beam methods
(EPMA), which combine submicrometer-scale spatial resolution with
essentially non-destructive measurement
(Hanchar and Miller, 1993). Using EPMA,
individual crystals can be inspected for the
presence of “inherited” cores or
recrystallization features that do not
Figure 1. Geological map of the study area in
southwestern Maine. The locations of the three
samples analyzed by EPMA are noted with x
symbols. The location of the set of pluton margin
samples is noted by the shaded oval.
reflect the igneous history that we endeavor to
date. Hence, potentially aberrant age data can be
viewed in the context of the pre-igneous history
of the rock, yielding further valuable insight into
Zircon is a common accessory mineral in
the mountain-building process.
granitic rocks. Uranium-lead (U-Pb) dating of
When zircon crystallizes from a magma, it
zircon permits ages precise to better than 1 Myr
incorporates numerous minor and trace
for rocks in the 300-400 Myr range. Zircon is
elements. The incorporation of these elements
additionally notable for its enormous resistance
depends somewhat on the activity of the
to both high temperature alteration and diffusion
elements in the magma as well as on the
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temperature, with higher crystallization
mineral surface, which generates an array of
temperature translating to “freer” inclusion of
secondary
exotic elements. Among the elements that may
bombarded atoms. The first mode of analysis is
substitute into zircon by this process is Ti, which
for secondary electron imaging. This is not a
has identical electrical charge to Zr and is
chemical technique, but allows the user to
similar in ionic radius. It has been observed that
evaluate the texture of the surface of grains. The
Ti concentrations in igneous zircon can be used
energies of the secondary X-rays are measured
as a thermometer: concentration is a
in a spatially resolved spectrometer, allowing for
crystallization temperature proxy (Watson and
the fine-scale chemical imaging of crystals. In
Harrison, 2005).
this
mode
X-rays
(called
characteristic
back-scattered
of
the
electron
imaging, or BSE) enrichment in elements with
high atomic number shows up as bright regions,
Methods
Zircon makes up < 0.001% of most granites.
and regions enriched in lighter elements show up
Hence, the physical removal and concentration
dark. A third mode, cathode luminescence (CL),
of these tiny (predominantly < 0.1 mm) crystals
allows another window into chemical variations.
from rocks is non-trivial. Using methods
In this case instead of highlighting variation in
established in the Oswego labs, prior to the
atomic number, CL shows enrichment in of a
beginning of this project Ms. Kauffman made
group of “activator elements” such as several of
zircon separates from a set of key granite
the lanthanides. Images from BSE and CL are
samples. For part of the current work her
used complimentarily. Under the direction of
samples
zircon
research faculty at Syracuse, Ms. Kauffman was
separates from a group of related granites from
able to carry out the image acquisition fully
elsewhere in the field area in Maine.
independently.
were
supplemented
with
For in situ analyses, hand-picked zircon
Acquisition of precise Ti concentrations in
crystals were first encased in an inert epoxy
tiny zircon crystals requires a spatially resolved
mount and polished to achieve a surface capable
method of analysis. Electron probe is generally
of being micro-characterized. A benefit to this
not sensitive enough to adequately quantify the
technique is that individual crystals can later be
low levels of Ti that may be present (e.g., < 50
plucked from the mount and dissolved for U-Pb
ppm, or parts per million). Thus we elected to
geochronology, part of the larger study.
examine the selected grains by plasma source
The first analytical tool used was the newly-
mass spectrometry (ICP-MS), with samples
upgraded EPMA at the Department of Earth
introduced by ablation from an ultraviolet laser.
Sciences at Syracuse University. This instrument
Dr. William Minarik of the Department of Earth
focuses a beam of electrons on the polished
and Planetary Sciences at McGill University
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gave Ms. Kauffman the necessary tutorial in
Crystals examined in this study were the smaller
operation of the instrument, and she lead the
grains, with an expectation that the larger
acquisition of the analyses, with her faculty
crystals have a greater likelihood of inheritance
advisor present to help. Unlike electron probe
and internal complications. Sample SG01-35b is
measurement, the laser beam disintegrates a
a two-mica granite associated with migmatites in
finite volume of sample, with more destruction
the central portion of the study area. Zircon from
for longer ablation time. The 40 micrometer
this sample was mainly in the form of colorless,
laser spot was sufficiently large that only one or
faceted prisms, often showing igneous zonation
some times two analyses per crystal were
in visible light (Figure 2). Sample SG01-78 is a
possible.
muscovite granite from the eastern portion of the
study area. Zircon in this sample was in stubbier
forms that the other two samples.
Results
Zircon grains from three samples were
Zircon grains from sample SG02-15 are
analyzed by EPMA, and images were acquired
primarily chemically homogeneous, and many
for multiple grains from each sample. Sample
are unzoned with minimal CL activation (Figure
SG02-15 is a two-mica granite from the western
3). Where cores are apparent, they show simple
part of he study area. Its zircon population was
(igneous) zonation in CL but not in BSE.
composed of both larger slightly dark crystals
and smaller more pristine grains (Figure 2).
Figure 2. Bulk zircon separates from SG02-15
(left) and SG01-35b (right), viewed in plane
polarized light. Scale bars are 100 micrometers
long. Note the two distinct size populations in
SG02-15.
Figure 3. EPMA images from two zircon grains in
sample SG02-15 (BSE=top, CL=bottom).
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Grains from sample SG01-35b show an
The three samples documented for internal
overall similar lack of zonation to SG02-15,
chemical variations were also analyzed for Ti
although some grains show rhythmic (igneous)
concentration by LA-ICP-MS. In addition to
zonation in BSE, although cores are not
these, a set zircon grains from five granite
apparent. Zircon crystals from sample SG01-78
samples from the northeast margin of the Sebago
show a variety of internal features not in
pluton were examined, as they were made
common with the other two samples. Most
available by a separate ongoing student project.
grains show a distinct, zoned core. This zoning
The overall precision of the method, as defined
is in some samples simple and rhythmic, but in
by repeat analyses of a benitoite standard, was
others it is complex and boundaries between
±4.5% (1 sd). The sample data are reported in
zones are cuspate, suggesting resorption during
Table 1.
metamorphism or detrital transport and later new
overgrowth (Figure 4).
Table 1. Titanium concentration data for
samples from this study
mean
sample
Ti (ppm)
± 1 sd
n
2.8
6
33
7
14
9
27
5
12
4
3.6
3
7.0
5
112
6
[range]
SG02-15
13.4
[5.5-15.4]
SG01-35b
51
[16.5-96.5]
SG01-78
11
[1.6-2.7]
LL1077
25
[4.7-65.3]
LL178
15
[7.2-32.1]
LL164-55
5.2
[3.1-9.4]
LL153-87
Figure 4. EPMA images of zircon crystal from
sample SG01-78 (top=BSE, bottom=CL). Note the
complex CL zoning in the core of the crystal,
common in grains from this sample.
23.8
[15.6-32.0]
LL300-54
122
[16.9-295]
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bulk rock Ti concentration can be effectively
used to constrain the activity of Ti in the
Interpretations
The imaging studies of zircon grains suggests
magmas from which these crystals derive, the
that only a small proportion of grains from
variability in concentrations of crystals from the
sample SG01-78 will be viable for U-Pb
same sample poses a problem for application of
geochronology.
the geothermometer, as each sample will yield a
The Ti analyses produced a number of
large range in crystallization temperatures as a
conclusions. The samples exhibited a wide range
result. This phenomenon, as it applies to other
of concentration, from extremely low (3.1 ppm)
igneous thermometers, has lead to discussion as
to rather high for rocks of these bulk
to whether the maximum temperature is most
compositions (295 ppm). Indeed, among crystals
likely to be “true,” with lower temperatures
from a single sample the variability was
reflecting variable degrees of re-equilibration,
typically large (e.g., standard deviations outside
diffusion or other secondary effects (Watson and
variation
analytical
Harrison, 1983). Ultimately we are likely to use
uncertainty). The one sample with large enough
a model approach that will define temperatures
zircon crystals to assess core versus rim relations
relative to a variety of geologically reasonable
(SG01-78) showed correlated variability: the
melt compositions (Ferry and Watson, in press).
more recent external zircon growth (“rim”) had
That work is ongoing.
expected
purely
from
The five samples from the Sebago pluton
consistently higher concentrations than the
crystal cores; in one instance the offset was by a
margin
factor of seven.
concentrations, despite the fact that they were
The three primary samples from this study
have
broadly
Ti
spaced no more than several hundred meters
concentrations (TiO2 = 2300 ppm in SG02-15,
based on mineralogy and texture of the rocks, to
2100 ppm in SG01-35b, and 2000 ppm in SG01-
be quite heterogeneous. Bulk rock elemental
78), but they differ somewhat in alumina
data are needed from these samples in order to
saturation (molar Al2O3/(CaO+Na2O+K2O) =
arrive at crystallization temperatures from the
1.52,
zircons.
1.06,
rock
variable
apart. The sample compositions are expected,
and
bulk
similarly
Ti
0.92
uniform
yield
respectively).
This
parameter may be a reasonable means of
inferring melt structure, which has an effect on
Epilogue
Ms. Kauffman presented her results at Quest
the viability of temperature calculations. It is
in the
in 2008 and subsequently completed her B.S. in
thermodynamic model for the Ti-in-zircon
Geology in August, 2008. She has taken a year
thermometer (Watson et al., 2006). Even if the
off from school before beginning graduate
however not currently considered
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calibrations for the Ti-in-zircon and Zr-inrutile thermometers. Contributions to
Mineralogy and Petrology
Tomascak, P.B., Krogstad, E.J., and Walker,
R.J. (1996) The nature of the crust in Maine,
U.S.A.: evidence from the Sebago batholith.
Contributions to Mineralogy and Petrology,
v. 125, p. 45-59.
Watson, E.B., and Harrison, T.M. (1983) Zircon
saturation revisited: temperature and
composition effects in a variety of crustal
magma types. Earth and Planetary Science
Letters, v. 64, p. 295-304.
Watson, E.B., and Harrison, T.M. (2005)
thermometer reveals minimum melting
conditions on earliest Earth. Science, v. 308,
p. 841-844.
Watson, E.B., Wark, D.A., and Thomas, J.B.
(2006) Crystallization thermometers for
zircon and rutile. Contributions to
Mineralogy and Petrology, v. 151, p. 413433.
school, but she continues to work on the zircon
project out of pure interest in the research. She
currently visits the Isotope Geochemistry Lab at
Syracuse University weekly, performing U-Pb
chemical separation in their ultra-clean lab, in
order to acquire high precision U-Pb dates of the
samples she worked on during the SFCCGsupported project. We intend to complete a
manuscript
for
publication
of
both
the
geochronologic data and as the Ti concentration
results in 2009, and Ms. Kauffman is interested
in presenting the results at the Joint Assembly of
the American Geophysical Union (May, 2009, in
Toronto).
References Cited
Bickford, M.E., Chase, R.B., Nelson, B.K.,
Shuster, R.D., and Arruda, E.C. (1981) U-Pb
studies of zircon cores and overgrowths, and
monazite: Implications for age and
petrogenesis of the northeastern Idaho
batholith. Journal of Geology, v. 89, p. 433457.
Cherniak, D.J. and Watson, E.B. (2001) Pb
diffusion in zircon. Chemical Geology, v.
172, p. 5-24.
Hanchar, J.M. and Miller, C.F. (1993) Zircon
zonation
patterns
as
revealed
by
cathodoluminescence and backscattered
electron
images:
Implications
for
interpretation of complex crustal histories.
Chemical Geology, v. 110, p. 1-13.
Mezger, K. and Krogstad, E.J. (1997)
Interpretation of discordant zircon ages: an
evaluation. Journal of Metamorphic
Geology, v. 15, p. 127-140.
Speer, J.A. (1982) Zircon. In: Orthosilicates (ed.
Ribbe, P.H.). Mineralogical Society of
America Reviews in Mineralogy, v. 5, p. 67112.
Ferry, J.M., and Watson, E.B. (in press) New
thermodynamic
models
and
revised
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