A new approach to single shot laser ablation analysis and its application to in situ Pb/U geochronology

Abstract

A novel approach to laser ablation Pb/U geochronology is presented that allows accurate determination of isotope ratios from a single pulse of a 193 nm laser. Data are acquired using a low volume ablation cell that facilitates: (1) production of a high density particle stream; and (2) a short (∼0.5 s) sample washout time. Isotope ratios from an individual laser pulse are calculated by integrating the baseline-subtracted total number of counts for the entire pulse and assigning an internal uncertainty based on counting statistics. This ‘total signal integration’ method eliminates the effects of differing detector response times, particularly in multi-collector-inductively coupled plasma-mass spectrometry (MC-ICP-MS), providing an alternative means to quantify transient signals. Data from reference zircons indicate that it is possible to consistently measure 206Pb/238U and 207Pb/206Pb ratios with external reproducibilities of 2% and 2.8% (2SD) respectively, using a similar amount of material to standard static ablation protocols. Decreasing sample consumption to ∼14 ng zircon (∼75% less than the ‘normal’ ablated mass) results in only a modest increase in the uncertainty to ∼5% on the 206Pb/238U ratio. By analysing consecutive laser pulses from the same ablation site, isotopic depth profiles can be generated with a depth resolution of ∼0.1 µm pulse−1. This technique offers a new opportunity to identify complexities within accessory minerals that were previously beyond the spatial resolution of laser based geochronology methods.