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46th Lunar and Planetary Science Conference (2015)
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SIMULTANEOUS ANALYSIS OF STRONTIUM, ZIRCONIUM, AND BARIUM ISOTOPES IN
PRESOLAR SILICON CARBIDE GRAINS WITH CHILI. T. Stephan1,2,3, R. Trappitsch1,2, A. M. Davis1,2,4,
M. J. Pellin1,2,3,4, D. Rost1,2,3, M. R. Savina2,3, and N. Dauphas1,2,4, 1Department of the Geophysical Sciences, The
University of Chicago, Chicago, IL 60637, USA, 2Chicago Center for Cosmochemistry, 3Materials Science Division, Argonne National Laboratory, Argonne, IL 60439, USA, 4The Enrico Fermi Institute, The University of Chicago, Chicago, IL 60637, USA. ([email protected])
Introduction: After more than five years of designing and building the Chicago Instrument for Laser Ionization (CHILI) [1–6], we present the first analyses of
natural materials. CHILI is a resonance ionization mass
spectrometry (RIMS) instrument that is designed to surpass the capabilities of the previous generation RIMS
instruments CHARISMA [7] and SARISA [8], with
regard to useful yield (~40 %), lateral resolution
(~10 nm), and number of photoionization lasers (six,
allowing measurement of isotopic compositions of several elements at once). As an initial demonstration, we
chose to analyze presolar SiC grains for the isotopic
compositions of Sr, Zr, and Ba, three elements that are
particularly important for understanding the s-process in
asymptotic giant branch stars [9–12].
Samples: Presolar SiC grains from mount KJG#2
were analyzed in this study. The grains were extracted
from the Murchison CM2 meteorite more than 20 years
ago [13]. In contrast to recent work on KJG grains [9–
12], the samples in this study were not additionally
treated with concentrated acids to remove parent-body
or terrestrial contamination. The grains were mounted
on a high purity gold foil by depositing them from a
suspension and pressing them into the gold with a sapphire window. Prior to RIMS analysis, energy dispersive X-ray images of the mount were acquired in a
scanning electron microscope to locate SiC grains on
the gold foil. Twelve grains were selected for this study.
RIMS analysis: CHILI is equipped with six tunable
Ti:sapphire lasers, pumped by three 40 W, 527 nm
Nd:YLF lasers. This allows simultaneous resonance
ionization of three elements with independent twophoton ionization schemes or two elements with threephoton schemes. A three-prism beam combiner brings
all six laser beams with different wavelengths onto a
single line through the analysis chamber. Using a
broadband mirror, the laser beams are reflected so that
they travel twice through a cloud of neutrals, which
were desorbed by a 351 nm Nd:YLF laser beam, focused to ~1 µm using a Schwarzschild optical microscope, and rastered over the sample.
Using previously developed ionization schemes, the
Ti:sapphire lasers were tuned for resonance ionization
of Sr (λ1 = 460.862 nm, λ2 = 405.214 nm [11, 12]), Zr
(λ1 = 296.172 nm, λ2 = 442.533 nm [14]), and Ba (λ1 =
307.247 nm, λ2 = 883.472 nm [15]). Isotopic standards
used in this study are NIST SRM 855a with 180 ppm Sr,
Zr metal, NIST SRM 1264a with 690 ppm Zr, and terrestrial BaTiO3, all of which were assumed to be of
normal terrestrial isotopic composition. Data were corrected for dead time effects [16]; the detector dead time
was found to be 600 ps, which was determined by analyzing a Zr metal standard by ion sputtering with varying primary ion beam current. Instrumental isotopic
fractionation was determined to be smaller than the statistical error of typically a few tens of ‰ except for
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Zr, which showed enhanced laser resonance ionization
by ~210 ‰ due to an odd-even effect as had been previously observed [17] and which is corrected for by analyzing standards.
Results and Discussion: Six of the twelve grains
selected had sufficient Ba content for isotopic analysis
with CHILI. Five of these six grains also showed sufficient Sr concentration. However, none of the grains had
enough Zr to detect any Zr mass peaks. For KJH grains,
which are larger in diameter by a factor of ~2 than our
KJG grains, a previous study showed that about one
third had enough Zr for isotope analysis with
CHARISMA [17]. According to trace element analyses
by secondary ion mass spectrometry and synchrotron Xray fluorescence, ~50 % of SiC grains are expected to
be enriched in Zr/Si by more than a factor of 2 compared to CI meteorites [18, 19]. Although absolute element concentrations cannot be easily determined with
CHILI, from the analysis of the NIST SRM 1264a with
690 ppm Zr, we infer that Zr isotope analysis should be
possible for such enriched grains. Why no Zr was found
in the present study needs further investigation.
Sr and Ba isotopic ratios, normalized to 86Sr and
136
Ba, respectively, are displayed in Figs. 1 and 2 as δ
values (deviation from standards in ‰). For comparison, Figs. 1 and 2 also show data obtained with
CHARISMA on SiC grains that had undergone acid
cleaning [9–12]. The SiC grains analyzed in the present
study show a similar range of isotope ratios as in the
previous studies. However, there seems to be a tendency
to plot closer to normal than the previously analyzed
grains; residual parent-body or terrestrial contamination
could be responsible for such a trend.
Conclusions and Outlook: CHILI has analyzed its
first natural samples. With regard to sensitivity, lateral
resolution, and mass resolution, we have not yet reached
the performance aimed for. However, we are still learning how to tune the instrument. Some improvements to
be implemented shortly will: (1) increase the sensitivity
by letting the laser light pass eight times through the
analysis chamber; (2) optimize motionless blanking of
the ion gun to achieve high lateral resolution; (3) re-
46th Lunar and Planetary Science Conference (2015)
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place and mechanically align the reflectron in the timeof-flight mass spectrometer for higher mass resolution
and transmission. Even without these improvements,
CHILI is more capable than earlier RIMS instruments.
A wide variety of cosmochemical problems will be explored in the near future, including isotopic studies of
presolar grains, CAIs, and samples from Genesis and
Stardust.
References: [1] Davis A. M. et al. (2009) LPS 40,
#2472. [2] Stephan T. et al. (2010) LPS 41, #2321.
[3] Stephan T. et al. (2011) LPS 42, #1995. [4] Stephan
T. et al. (2012) LPS 43, #2660. [5] Stephan T. et al.
(2013) LPS 44, #2536. [6] Stephan T. et al. (2014) LPS
45, #2242. [7] Ma Z. et al. (1995) Rev. Sci. Instrum., 66,
3168–3176. [8] Veryovkin I. V. et al. (2004) Nucl. Instr.
and Meth. B, 219–220, 473–479. [9] Liu N. et al. (2014)
ApJ, 786, 66-1–66-20. [10] Liu N. et al. (2014) ApJ,
788, 163-1–163-7. [11] Liu N. et al. (2015) ApJ, submitted. [12] Liu N. (2014) Dissertation, The University of
Chicago, 181 pp. [13] Amari S. et al. (1994) GCA, 58,
459–470. [14] Barzyk J. G. et al. (2007) MAPS, 42,
1103–1119. [15] Savina M. R. et al. (2003) GCA, 67,
3215–3225. [16] Stephan T. et al. (1994) J. Vac. Sci.
Technol. A, 12, 405–410. [17] Nicolussi G. K. et al.
(1997) Science, 277, 1281–1283. [18] Amari S. et al.
(1995) Meteoritics, 30, 679–693. [19] Kashiv Y. et al.
(2010) ApJ, 713, 212–219.
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δ( Sr/ Sr) [‰]
Fig. 1: Strontium isotope data of presolar SiC grains
measured with CHILI (red symbols) in comparison with
data measured with CHARISMA (gray symbols) [11,
12]. Error bars are 2σ.
Fig. 2: Barium isotope data of presolar SiC grains
measured with CHILI (red symbols) in comparison with
data measured with CHARISMA (gray symbols) [9, 12].
Error bars are 2σ.