µID-TIMS: A Focused Ion Beam (FIB)–Femtosecond (Fs) Laser Microsampling Protocol for Spatially Resolved High-Precision Zircon Geochronology 

Abstract

High-precision (CA-ID-TIMS) U-Pb geochronology of individual growth zones of zircon has been a long-awaited milestone in the geochronological community. Focused ion beam (FIB) and femtosecond (fs) laser machining monitored in real time by CL-SEM both show promise as techniques for physical extraction of target zircon domains in preparation for spatially resolved ID-TIMS dating. In this contribution, we test a novel laboratory protocol for zircon microsampling using an in-house multi-ion plasma (Ar-Xe) FIB and fs laser, and showcase first µID-TIMS zircon dates. We first examine the chemical impacts of protective metal coatings (Cr, Pt-Pd and C) used for ion milling on the U-Pb systematics of a low-Pb and a low-U zircon. We then present high-resolution transmission electron microscope (TEM) images of a zircon surface irradiated by ion and fs laser beams to show the contrasting extent of structural damage induced by the two techniques at standard microsampling conditions. Potential Pb-loss/U-gain in the nanometer-wide ion damaged layer in zircon is mapped by atom probe tomography (APT). Subsequently, we showcase the FIB workflow for extracting a number of microsamples of the Mud Tank and GZ-7 reference zircon spanning the sizes expected in future applications using natural zircon. We present first results of spatially resolved high resolution (µID-TIMS) dating of the Mud Tank and GZ-7 microsamples, and explore the achieved analytical accuracy and precision. Finally, we discuss the feasibility of conducting a µID-TIMS study on natural zircon given zircon features (i.e., age, U content, and volume of target domain) and research objective, and discuss benefits and limits to our approach.