V34A-06:
Nanoscale isotope mapping of terrestrial and lunar zircons by atom probe tomography
Wednesday, 17 December 2014: 5:15 PM
Tyler Blum1, David Reinhard2, Michael J Spicuzza1, David Olson2, Matthew A Coble3, Aaron J Cavosie4, Takayuki Ushikubo5, David J Larson2, Thomas F Kelly2 and John W Valley1, (1)University of Wisconsin Madison, Geoscience, Madison, WI, United States, (2)Cameca Instruments Inc., Madison, WI, United States, (3)Stanford University, Stanford, CA, United States, (4)University of Puerto Rico Mayaguez, Dept. of Geology, Mayaguez, PR, United States, (5)Kochi Institute for Core Sample Research, Kochi, Japan
Abstract:
Atom Probe Tomography (APT) allows 3D mapping of elements and isotopes with near atomic resolution, at concentrations down to the 10’s of ppma. Direct characterization of nanoscale chemical heterogeneity down to trace element levels permits novel interrogations of geochemical processes acting at the nm-scale, and further informs us about elemental or isotopic data gathered by larger-scale techniques. We have studied four zircons including a 2.5 Ga xenocryst from a 29 Ma granitoid (ARG2.5), a 4.3 Ga zircon from the quartz monzodiorite clast of lunar sample 15405 (15Q4), and two Jack Hills zircons dated at 4.0 Ga (JH4.0) and 4.4 Ga (JH4.4) (Valley et al. 2014, Nat. Geosci.). JH4.4 and ARG2.5 contain enrichments of Y, REE, and Pb in ~10 nm clusters, in contrast to the uniform distribution of these elements in JH4.0 and 15Q4. Despite varying amounts of nanoscale trace element heterogeneity, all terrestrial zircons have total 207Pb/206Pb ratios consistent with concordant U-Pb ages obtained by SIMS. This proves that zircon cores remained closed U-Pb systems at the micron scale. In JH4.4 and ARG2.5, the 207Pb/206Pb ratio within clusters is a function of the timing of cluster formation, and precludes clusters from being primary features. Instead, the timing of element mobility and cluster formation is linked with known high temperature geological events. The lack of clustering within JH4.0 and lunar zircon 15Q4 suggests no nm-scale element mobility has occurred, and supports the concordant ages obtained by SIMS. The size, shape, distribution and composition of clusters reflect the time-integrated influence of radiation damage, annealing, and temperature on intracrystalline trace element mobility, and can be used to further understand the thermal history of zircons and their structural evolution since crystallization. The combination of spatial resolution and detection limits for APT has exciting potential for many geochemistry and mineralogy studies.
