Extreme isotopologue disequilibrium in molecular SIMS species during SHRIMP geochronology
The current limitation in the accuracy and precision of inter-element analysis in secondary ion mass spectrometry (SIMS) is the ability to find measurable quantities that allow relative differences in ionization and transmission efficiency of secondary ions to be normalized. In uranium–thorium–le...
| Main Authors: | , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2017-12-01
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| Series: | Geoscientific Instrumentation, Methods and Data Systems |
| Online Access: | https://www.geosci-instrum-method-data-syst.net/6/523/2017/gi-6-523-2017.pdf |
| Summary: | The current limitation in the accuracy and precision of inter-element
analysis in secondary ion mass spectrometry (SIMS) is the ability to find measurable quantities that allow
relative differences in ionization and transmission efficiency of secondary
ions to be normalized. In uranium–thorium–lead geochronology, the
ability to make these corrections, or "calibrate" the data, results in an
accuracy limit of approximately 1 %. This study looks at the ionization
of uranium and thorium oxide species, which are traditionally used in U–Pb
calibration, to explore the conditions under which isotopologues, or
molecular species whose composition differs only in the isotopic composition
of one or more atoms in the molecule, remain in or deviate from equilibrium.<br><br>
Isotopologue deficits of up to 0.2 (200 ‰) below ideal mixing are
observed in UO<sub>2</sub><sup>+</sup> species during SIMS gechronological analyses using
the SHRIMP IIe SIMS instrument. These are identified by bombarding natural
U-bearing minerals with an <sup>18</sup>O<sub>2</sub><sup>−</sup> primary beam. The large
anomalies are associated with repeat analyses down a single SIMS sputtering
crater (Compston et al., 1984), analysis of high-uranium, radiation-damaged
zircon, and analysis of baddeleyite. Analysis of zircon under routine
conditions yield UO<sub>2</sub><sup>+</sup> isotopologue anomalies generally within a few
percent of equilibrium. The conditions under which the isotopologue anomalies
are observed are also conditions in which the UO<sub><i>x</i></sub>-based corrections, or
calibration, for relative U vs. Pb ionization efficiencies fail. The
existence of these isotopologue anomalies suggest that failure of the various
UO<sub><i>x</i></sub> species to equilibrate with each other is the reason that none of
them will successfully correct the U  / Pb ratio. No simple
isotopologue-based correction is apparent. However, isotopologue
disequilibrium appears to be a more sensitive tool for detecting high-U
calibration breakdowns than Raman spectroscopy, which showed sharper peaks
for ∼ 37 Ma high-uranium zircons than for reference zircons OG1 and
Temora. U–Th–Sm / He ages were determined for aliquots of reference
zircons OG1 (755±71 Ma) and Temora (323±43 Ma), suggesting that
the broader Raman lines for the Temora reference zircons may be due to
something other than accumulated radiation damage.
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Isotopologue abundances for UO<sup>+</sup> and ThO<sup>+</sup> and their energy spectra
are consistent with most or all molecular species being the product of atomic
recombination when the primary beam impact energy is greater than 5.7 keV.
This, in addition to the large UO<sub>2</sub><sup>+</sup> instrumentally generated
isotopologue disequilibria, suggests that any attempts to use SIMS to detect
naturally occurring isotopologue deviations could be tricky. |
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| ISSN: | 2193-0856 2193-0864 |