SUB-MICRON U-PB GEOCHRONOLOGY OF ZIRCON AND THE EFFECT OF RADIATION DAMAGE ON DEPTH-DEPENDENT FRACTIONATION: RESULTS FROM DISCRETE MICRO-VOLUME LASER ABLATION DEPTH PROFILING WITH A SINGLE-COLLECTOR ICP-MS

The use of UV laser ablation ICP-MS in the geochronology and sedimentary provenance community has risen exponentially in the last two decades yet standard ablation methods do not adequately allow exploration of the three-dimensional nature of geochemical reservoir materials. 2D rastering protocols are limited by spot diameter during static ablation and signal convolution during dynamic rastering. We present a revised method for zircon U-Pb analysis utilizing a discretized depth profile technique employing a Thermo Element XR single-collector ICPMS coupled to a Photon Machines Analyte 193H ArF excimer laser ablation system. By utilizing bursts of 5 laser pulses, a sample-standard bracketing approach, and constructing 3-D fractionation correction surfaces we achieve an average depth resolution of ~0.55µm for a 35µm spot size (~2.4ng zircon/analysis) and accurate ages for individual analyses with 2σ internal errors of ~6%. A typical sample contains 15 analysis cycles separated by an appropriate washout time with weighted mean 2σ internal errors of ~2.5%, reduced χ² (MSWD) values near unity at the sample level, and total depths of 7-10µm. We demonstrate that calculating isotope ratios by integrating the total counts for each analysis is preferred over cycle-by-cycle ratios and is easily implemented for a single-collector system. A 183 sample round-robin analysis session of zircons with known ages demonstrates our ability to resolve ages from ~18-1200Ma over a >100x range of U and Th concentrations.

An observed positive correlation between Raman spectroscopic metrics of crystal lattice damage and ablation pit depth measured by vertical scanning interferometry suggests radiation damage significantly modifies the laser ablation process. These changes scale with alpha dosage and effective Uranium (eU) suggesting that the amount of accumulated radiation damage exerts a fundamental control on ablation efficiency. Consequently, significant mismatch between the damage profiles of zircon standards and ‘unknowns’, or variations within the standard itself, can lead to overdispersed ages and thus may exert a fundamental control on the ultimate accuracy and precision of any laser ablation analysis.