Eliminating Laser Induced Elemental U-Pb Fractionation using low sample volume multi-shot ablation protocols

The interaction of incident laser radiation and sample substrate is complex and difficult to predict. Natural zircons areoften both structurally and chemically heterogeneous in 3-dimensional space. Encountering growth-related, structural micro-heterogeneities, inclusions and chemical complexities is almost inevitable when employing ‘conventional’ static, high-frequency laser sampling protocols often lasting several tens of seconds at a time.

 

A multi-shot approach to laser ablation by contrast implements a minimal sample exposure time to incident laser radiation by applying multiple 1 Hz shots in delayed succession to a single sampling site. This process can be conceptualised as a “slowing down” of a high-frequency (10-20 Hz) static Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) analysis. Each laser pulse applied in this manner, produces signal peak which is distinct albeit transient. The ability to integrate and collate signal pulses for a small number of consecutive laser shots (10-30 shots), as opposed to continuously pulsing the laser, produces highly precise age determinations (<1% reproducibility, 2slevel) on small sample volumes (~695µm³ on 91500 zircon standard). The multi-shot LA-ICP-MS protocol employed here effectively eliminates ‘downhole’ fractionation as the resultant craters are extremely shallow (as shallow as ~553nm 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.