Shallow sampling by multi-shot laser ablation and its application within U-Pb zircon geochronology

Abstract

The interaction of incident laser radiation and sample substrate is complex and difficult to predict. Natural zircons are often both structurally and chemically heterogeneous in 3-dimensional space. Encountering growth-related, structural micro-heterogeneities, inclusions and chemical complexities is almost inevitable during a static ablation of several tens of seconds. A multi-shot approach to laser ablation described here implements a minimal sample exposure time to incident laser radiation by applying multiple 1 Hz shots to a single sample location in delayed succession. This process can be conceptualised as a "slowing down" of a high-frequency (5-20 Hz) static laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis until each laser pulse is distinct albeit transient. The ability to integrate and collate signal pulses for a small number of consecutive laser shots, as opposed to continuously pulsing the laser, produces precise age determinations (~1% reproducibility, 2σ level) on small sample volumes (704 ± 23 μm3 on 91500 zircon standard). The multi-shot LA-ICP-MS protocol employed here significantly reduces the effect of 'downhole' fractionation as the resultant craters are extremely shallow (as shallow as 0.56 ± 0.02 μm on 91500 zircon standard) and maintain an aspect ratio of 1. Further benefits include a reduced probability of thermally induced effects (e.g., substrate melting), plasma loading, and the potential for signal mixing (with depth) in a heterogeneous sample.

An Improved In Situ Zircon U‐Pb Dating Method at High Spatial Resolution (≤ 10 μm Spot) by LA‐MC‐ICP‐MS and its Application

“…Comparison with previous studies for an analysis spot size of ≤ 10 µm Table S21 summarises and compares several analytical protocols with high spatial resolution of ≤ 20 µm in diameter or ≤ 10 µm in depth for the accuracy and precision of LA-(MC)-ICP-MS U-Pb zircon ages (Bauer and Horstwood 2018, Chew et al 2019, Corbett et al 2020, Cottle et al 2009, 2012, Gehrels et al 2008, Hattori et al 2017, Johnston et al 2009, Kooijman et al 2012, Lana et al 2017, Mukherjee et al 2019, Sack et al, 2011, Wu et al 2020, Xie et al 2017. A few studies have been carried out with a spot size of ≤ 10 µm (Johnston et al 2009, Sack et al 2011, Xie et al 2017, Mukherjee et al 2019, Wu et al 2020), which will be discussed below.…”

Atomic spectrometry update – a review of advances in environmental analysis

“…To reduce fractionation effects in U–Pb age determinations of zircons by LA-ICP-MS , Corbett et al 270 applied multiple 1 Hz shots to a single sample location in a standard ablation cell (volume ca. 8 cm 3 ).…”

Two reliable calibration methods for accuratein situU–Pb dating of scheelite

“…[7][8][9][10][11] Moreover, low laser sampling rate ($3 Hz) and energy density (<3 J cm À2 ), short ablations ($30 s, avoiding the formation of deep craters), and large crater diameters (>32 mm) were considered for in situ U-Pb dating to minimize matrix effects. 7,12,13 Due to the lack of matrix-matched standards, in situ U-Pb dating of scheelite has been applied in two previous studies using NIST glasses as reference materials. 14,15 From these two studies, the characteristics of scheelite samples, i.e., relatively high and variable radiogenic U concentrations (<1 to >10 ppm) and low common Pb concentrations (<0.01 ppm), offer us a new insight that scheelite is amenable to U-Pb geochronology.…”