Atom Probe Tomography of Zircon and Baddeleyite Geochronology Standards

English Abstract:

Atom Probe Tomography (APT) is an analytical technique whereby the positions of elements, and their isotopes, can be reconstructed in 3D, shedding light on nano-scale trace element mobility in geochronology minerals. The sample is extracted from the bulk material and milled using a dual-beam FIB SEM into a needle shape with an apex radius of 50-100nm. Single atoms (or small groups of atoms) are ionized by UV laser pulse from the surface while under the presence of a large electric field and removed from the surface one-at-a-time. Each removal (field evaporation event) is synchronized to a timing pulse and projected to a 2D position-sensitive detector providing ion identification through time-of-flight mass spectroscopy and, ultimately, a composition map of the surface. Done iteratively over many millions of ions, the evolving surface information is reconstructed as a 3D map of individual ions from which information can be extracted for a for a variety of real-space analyses. In general, optimal APT acquisition conditions are material dependent. As APT is a destructive technique, it may be advantageous to perform optimization on non-precious, geochronology standard materials to optimize both data quality and analysis yield prior to sacrificing the true material of interest. In the present work, we explore the effect of various laser-pulse energies on zircon (BR266) and baddeleyite (Phalaborwa) standards. Because limited analysis yield is an issue for APT, possibly exacerbated by deformation microstructure, we also explored some post-specimen-preparation treatments (such as annealing) on both the baddeleyite standard and shocked terrestrial baddeleyites in an effort to increase sample yield and test for induced chemical segregations. Transmission EBSD (TKD) mapping of microtips was also explored to integrate lattice orientation(s) with the 3D APT map. We will show APT analyses from these materials and report on our efforts to improve analysis yield to date.