SHRIMP geochronology using an 18O primary beam
Abstract
The key constraint of uranium-lead geochronology is the variation in ionization efficiency of uranium and lead. As the ionization efficiency of Pb is dependent on oxygen availability, a calibration relating the UO/U or UO2/U ratio to the Pb/U ionization efficiency is commonly used. However, these calibrations have historically been limited to errors of about 1%. We have identified the origin of the oxygen in the UO and UO2 species by feeding the primary column duoplasmatron source with 18O2 gas. This creates labeled UO and UO2 isotopologues at nominal masses of 254, 256, 270, 272, and 274, where the 18O isotopologues contain 18O from the isotopically labeled primary beam, and the 16O isotopologues contain oxygen from the natural silicate, phosphate, or oxide geochronology target mineral. The ratio of U18O to U16O depends on the target mineral, and primary ion species (atomic vs. molecular oxygen). In zircon, the variation in U18O vs. U16O can be used to correct for calibration scatter, allowing for more precise and accurate geochronology. This correction only applies to SIMS instruments such as SHRIMP, which can perform uranium-lead geochronology without the use of a third source of oxygen, such as oxygen flooding.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.V33G..06M
- Keywords:
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- 1094 GEOCHEMISTRY / Instruments and techniques;
- 1115 GEOCHRONOLOGY / Radioisotope geochronology