Investigating Zircon Shock Microstructures with NanoSIMS

46th Lunar and Planetary Science Conference (2015)
INVESTIGATING ZIRCON SHOCK MICROSTRUCTURES WITH NANOSIMS. C. A. Crow1, B. Jacobsen2, K. D. McKeegan1, and D. E. Moser3 1University of California, Los Angeles (Department of Earth, Planetary,
and Space Sciences, 595 Charles Young Drive E., Box 951567, Los Angeles, CA 90095; [email protected]),
Lawrence Livermore National Laboratory, Livermore, CA, 3University of Western Ontario, London, Canada.
Introduction: The presence of impact shock microstructures has been used to interpret the significance
of lunar and terrestrial zircon Pb-Pb and U-Pb ages [eg.
1, 2]. In end member scenarios of no shock deformation
and complete impact induced recrystallization, the ages
are generally interpreted as the crystallization age and
the impact age, respectively [1, 2]. For cases of intermediate shock deformation, the effects of microstructure formation on the measured isotopic ages are not
clear, especially in the lunar zircon population.
In some terrestrial and lunar zircons, shock microtwins (usually ~1-2 m in width) have been associated
with Pb-loss, but the fact that not all microtwins have
this association leaves their effect on the ages open to
debate [e.g. 1, 2]. Moreover, the small size of these
features has made it difficult to directly investigate
their effects on U-Pb ages by conventional SIMS methods.
Here we report preliminary U-Pb and Pb-Pb analyses of a set of ~5 m lunar zircon microtwins made with
the Lawrence Livermore National Lab (LLNL) NanoSIMS 50. These analyses were undertaken to both (1)
test the spatial limits and feasibility of U-Pb age analyses of zircons using the Hyperion primary beam
source at LLNL, and (2) to investigate the distribution
of U, Pb, and Fe across microtwin structures.
Sample: NanoSIMS analyses were conducted on a
zircon separated from lunar sample 14305 - a clast-rich,
crystalline matrix breccia. The zircon, Z34 “Disco”,
was selected for analysis based on scanning electron
microscopy (SEM) electron backscatter diffraction
(EBSD) images collected at the University of Western
Ontario (Figure 1).
The EBSD analyses revealed that Z34 is a highly
shocked grain containing two crosscutting sets of
shock microtwins, one of which is curved and larger
than typical structures (in some places up to ~ 6 m in
width). The curved microtwins are also associated with
impact melt glass inclusions, typically ~ 1m in diameter, which are easily resolved by the NanoSIMS analyses (Fig. 2). Prior to NanoSIMS analyses, an unshocked
region of Z34 was dated with the UCLA Cameca
IMS1270 giving a 207Pb-206Pb age of 4324 ± 5 Ma (1)
and a 206Pb-238U age of 4231 ± 94 Ma (1).
Figure 1: (Left) EBSD Map of 14305 Z34 “Disco”.
Blue features are curved microtwins; green is a second
crosscutting microtwin; pixilated bands are zones of
less crystalline zircon, most likely high U concentration
primary zoning. (Right) NanoSIMS total ion image
showing location of analyses.
Instrumental Setup. We used a 600 pA 16O- primary beam to obtain a 2x3 m spot size. Previously reported U-Pb NanoSIMS analyses used a 15 m spot
size with a 7 to 9 nA beam [3]. Analyses were collected
in mapping mode where the beam was rastered over a
13 m box, and data was collected from the inner 10
m to exclude edge effects. Each analysis took ~ 2
hours in order to achieve sufficient counting statistics.
Tuning and peak identification was conducted on
NIST610 glass and AS3 zircon standards. Ion intensities were measured on five electron multipliers (EM) by
stepping the magnet over four B-field settings. The
configuration of the trolley system is shown in Table 1.
A high mass resolution scan of 30Si gave a mass resolving power of ~4000 (6000 Cameca units).
Table 1: NanoSIMS Trolley Configuration
Zr2 O
Calibration. Instrumental U-Pb relative sensitivity
was determined by analyzing AS3 standard zircons
under the same measurement conditions; this is similar
to the protocol used for the IMS-1270 at UCLA and
elsewhere [4]. AS3 zircons have a known, homogene-
46th Lunar and Planetary Science Conference (2015)
ous age of 1099.1 ± 0.2 Ma although the uranium concentration is heterogeneous [5]. A linear fit of the
UO+/U+ versus 206Pb+/238U+ relative sensitivity factor
(measured ratio/true ratio) for the AS3 zircons was
used to infer the RSF of the unknown analyses. The
standards and unknowns had a similar range of UO/U
values, so no extrapolation was necessary.
Data Reduction & Age Calculation. We applied an
EM deadtime correction, a background subtraction, and
a common 204Pb correction. Terrestrial common Pb
values were used for both lunar and terrestrial zircon
corrections because it has previously been shown that
the U-Pb ages of lunar zircons in grain mounts are not
affected by the use of terrestrial vs. lunar common Pb
values [e.g. 6 Supplementary Materials].
The 207Pb-206Pb ages were calculated from these
corrected values using an iterative Matlab code. Uncertainties on these ages are dominated by counting statistics. In contrast, uncertainties on U-Pb ages are a function of both counting statistics and the uncertainty in
the determination of the RSF value.
Results: We collected six analyses of 14305 Z34 three which contained shock microtwins and three in
unmetamorphosed areas. Impact melt glass inclusions
~1 m in diameter (Fig 2), which are associated with
the curved shock microtwins in SEM images, were resolved in the 54Fe/30Si maps (the inclusions were removed for U-Pb and Pb-Pb age calculations). We also
resolved a <5 m high U concentration band (Fig 2),
which most likely results from primary igneous zoning.
The identification of these two features suggest that the
spatial resolution is sufficient to resolve the ~5 m
shock microtwins in the U-Pb age images.
Figure 2: NanoSIMS map of (Left) 54Fe/30Si showing
resolved ~ 1 m impact melt glass inclusions, and
(Right) map of 238U/30Si showing resolved primary
growth band of higher U concentration. Both images
are 10 x 10 m and have a binning size of 3 pixels.
In each of the 206Pb/238U and 207Pb/238U maps we
find no resolvable variation across the twins. This suggests that the process by which these shock microtwins
formed was too rapid to cause appreciable Pb mobility.
Alternatively, the timing of the impact event that produced the microtwins was sufficiently close to the crystallization age of the zircon that we cannot resolve the
age difference within our measurement uncertainties.
The former interpretation would be consistent with UPb results from terrestrial „cold shock‟ micro-twinned
zircons [1].
The average 206Pb-238U age of the NanoSIMS analyses is 4156 ± 77 (1) Ma, which is within error of the
4231 ± 94 (1) Ma age measured by the IMS1270 at
UCLA. Similarly, the 207Pb-206Pb ages range from 4318
± 39 to 4237 ± 58 Ma (1) which is also with in error
of the IMS1270 age of 4324 ± 5 (1). Figure 3 shows
the Concordia diagram for the NanoSIMS analyses of
Z34 “Disco”.
Figure 3: Concordia diagram for NanoSIMS analyses
of lunar zircon 14305 Z34 “Disco” including measurements of regions containing shock microtwins and
those areas devoid of microtwins.
Future analyses will aim to reduce the errors on individual U-Pb and Pb-Pb ages, but our preliminary
conclusion is that formation of these sorts of curviplanar microtwins does not locally result in large degrees
of Pb-loss or U-Pb isotopic age disturbance of impacted lunar zircons. Higher precision measurements
should allow us to test whether other microstructural
domains in this complex grain exhibit clear impactrelated discordance.
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(1993) J. Geophys. Res., 98, 13997-14013. [6] Taylor
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