Causes and mitigation of U–Pb fractionation during LA-ICP-MS analyses of zircon using nanosecond excimer laser systems

This work investigates the causes of ablation bias in U–Pb dating of zircon by laser ablation inductively coupled mass spectrometry (LA-ICP-MS) and possible methods for correction. Pb/U ablation bias occurs due to the relative volatility of Pb and changes with pulse count (pit depth). Measurements of NIST glass and zircon include significant ablation bias when scanned but since this is constant with time it is possible to determine variations in instrument bias by ablating under fixed conditions. Integrated signal profiles measured using laser pulses at 0.2 Hz combined with modelling of Pb/U fractionation suggest that the earliest Pb/U measurements (first 10 pulses) are affected by decreasing fractionation from a melt pool as it becomes increasingly depleted in Pb. This trend is opposed by deposition of depleted material as fallback, which dominates early signal loss. The fractionation sequence from the first 10 or so pulses is therefore chaotic. Ratios from the following 50 or so pulses show an approximately linear increase in fractionation. Normalized data from these pulses give trends with higher intercepts and lower slopes for standards with higher radiation damage during the same session. Fractionation and signal decay subsequently rise more slowly but remain linear, probably because fractionation is dominated by deposition in the deepening pit. An ablation fractionation model is proposed based on the drop in measured ZrO signal but this cannot be used to estimate accurate bias-free 206 Pb/ 238 U ratios because of the number of unconstrained parameters. The best approach for calibrating against an unknown is a direct comparison of all or part of the standard ratio profile with the sample profile after multiplication by a calibration factor. The factor that results in the best fit should represent a ratio of unbiased 206 Pb/ 238 U between standard and sample. Software is included to process and calibrate data. Data from Precambrian zircon with well-established 207 Pb/ 206 Pb ages suggest that radiation damage below the metamict state results in little bias to discordance. Reverse discordance from metamict zircon appears to be approximately proportional to U concentration. Increased accuracy of 206 Pb/ 238 U ages using nanosecond laser systems can most likely be achieved through instrument design improvements rather than data processing.