I'll be in the SW USA for the next week and a half giving talks about SHRIMP and attending AGU. The talks are:
Arizona State University
Thursday 1 Dec time: TBA, Chemistry dept (probably) Title TBA (this one is pretty informal, if you haven't guessed)
University of Texas at Austin
Friday Dec. 2
Understanding the SIMS U/Pb calibration
Jackson School of Geosciences
JGB 3.222 at 11 AM.
American Geophysical Union Fall meeting, Moscone Center, San Francisco
SHRIMP geochronology using an 18O primary beam
Session Title: V33G. Innovations in Isotope Mass Spectrometry and Isotope Metrology II
Session Type: Oral
Start Time: 03:10 PM
End Time: 03:25 PM
Location: Room 3022 (Moscone West)
In preparation for the production of the new SHRIMP IV, a number of experiments were run to characterize the behavior of the U/Pb calibration under various analytical conditions. Repetition of early SHRIMP One work showed that the calibration appears to primarily reflect the dependence of Pb ionization on oxygen activity. In order to constrain the effects of oxygen, further experiments were performed using an 18O primary beam, so that the relative contributions of oxygen from the beam (18O) and the natural samples (16O) could be discerned.
The use of the 18O primary has shown that the ratio of sample oxygen to primary oxygen in the secondary ions varies based on the target mineral and the primary beam impact energy. For baddeleyite, there is also an orientation effect. In some circumstances, the 18O/16O ratio can be used to correct for scatter along the calibration line, allowing sub-percent level accuracy for Paleozoic U/Pb dating of zircon.
The cause of calibration-related uncertainty is still not precisely known, but can probably be related to a number of factors. This work, combined with recent demonstration of SHRIMP geochronology of chemically abraded zircon, suggests several potential ways of improving calibration accuracy. In addition to the standard approaches of high quality sample preparation and wide energy windows, new approaches include the use of 18O to correct for source fluctuations, active change neutralization using a medium energy electron gun, and chemical abrasion. These techniques have not yet been used simultaneously on unknown zircons.
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 18O 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.