U-Pb Geo-/Thermochronology Method Development and Techniques
U-Pb geochronology offers one of the most robust and versatile dating techniques available to geologists, and, as a practicing geochronologist, I'm always looking for ways to improve the method. These improvements have ranged from designing techniques to reduce isobaric interferences during thermal ionization mass spectrometry to calibrating reference materials for in situ U-Pb measurements. I also have a keen interest in further developing techniques for dating baddeleyite (ZrO2) with both high-precision and accuracy. This mineral is common in mafic volcanic and plutonic rocks, and the ability to produce reliable high-precision age data from it would open the door to answering a wide range or problems in Earth science.
Cathodoluminescence (CL) image taken by Joel DesOrmeau (UNR) of zircon from the rapakivi intrusive phase of the Golden horn batholith. Approximate length of the grain is 200 micrometers. Zircons separated from this intrusion have homogeneous ages and Hf isotopic compositions, making them ideal reference materials.
Developing Reference Materials for U-Pb Zircon Geochronology
In situ techniques for making U-Pb isotopic measurements offer a faster and cheaper, albeit less precise, alternative to traditional ID-TIMS methods. However, these methods require matrix-matched reference materials to correct for elemental fractionation and to assess external reproducibility. Ideally, these reference materials would closely match unknowns in both age and composition. Such reference materials are readily available for zircon between 1500-400 Ma, but are scarce for the Mesozoic and Cenozoic. Over the past two years, I've been working with Mauricio Ibañez-Mejia (University of Rochester) and collaborating TIMS geochronology laboratories at Princeton University and the University of Kansas and in situ geochronology laboratories at Washington State University, University of Arizona, Chinese Academy of Sciences, Stanford University, and the University of São-Paulo to calibrate a new Eocene reference material (GHR1) for zircon U-Pb and Hf isotopic analyses. There is an inexhaustible supply of these zircons and they should be useful for any researcher conducting in situ analyses of Cenozoic zircon.
Rank order plot of U-Pb dates from collaborating laboratories that use in situ U-Pb geochronological techniques. All of the results show excellent agreement with the CA-ID-TIMS date from MIT and demonstrate that GHR1 zircon is homogenous with respect to U-Pb isotopes.
Baddeleyite (ZrO2) U-Pb Geochronology
Baddeleyite is a common mineral in mafic igneous rocks and can be used for U-Pb geochronology. However, this mineral remains under-utilized in geochronologic studies due to several issues. I have been working to improve techniques for dating baddeleyite in collaboration with Mauricio Ibañez-Mejia (University of Rochester) and Joel DesOrmeau (University of Nevada Reno). These efforts have included helping generate methods for dating baddeleyite by LA-ICP-MS by calibrating a baddeleyite reference material (Ibañez-Mejia et al., 2015). Currently our research team is working to characterize the effects of radiation damage in the mineral. The goal is to ultimately develop methods that will reduce or eliminate Pb-loss in baddeleyite and permit the production of both accurate and precise dates using this mineral. This development would open the door for new research on the timescales of mafic magmatic processes as well as permit detailed geochronology of basalt in stratigraphic sections.
Traditional concordia plots showing the results for ID-TIMS and LA-ICP-MS analyses of baddeleyite from the Ogden gabbro in South Carolina (Ibañez-Mejia et al., 2015). The LA-ICP-MS data is presented showing no correction for matrix-matched fractionation, a correction using zircon, and a correction using a baddeleyite reference material. The large differences in the resulting age using these various corrections demonstrate the importance of well-calibrated reference materials for in situ analyses.
Hacks for U-Pb TIMS Analyses
My main analytical tool remains U-Pb zircon geochronology by thermal ionization mass spectrometry (TIMS) and I'm constantly attempting to shorten analysis time and improve analysis quality for this technique. One persistent problem in the U-Pb TIMS community has been the presence of low-level isobaric interferences under Pb isotopes. These interferences burn off over the first hour of each analysis and result changing isotopic ratios before stable measurements can be made. My working hypothesis is that these interferences represent easily volatilized organic molecules that adhere to the apparatus that holds our sample. Over the course of the run, this apparatus is heated thereby liberating the molecules and causing isobaric interferences. To address this issue, I have started using additional filaments to heat this apparatus prior to the run without heating the sample. This technique removes these molecules without losing any sample material and has virtually eliminated the problem of early isobaric interferences at Princeton University, thereby improving analysis time and quality.
Turret for a TIMS instrument showing the sample holding apparatus with side filaments installed (left) and with only the center filament installed (right). Heating blank side filaments early in the run eliminates the need to "burn off" isobaric interferences.
Comparison between isotopic evolution seen using the standard warmup routine prior to a TIMS measurement of Pb and the evolution seen with the side filament warm up routine. The new routine allows us to use more data during the analysis.