Studies of the source of laser-induced isotopic bias in LA-MC-ICP-MS

Coupling laser ablation sampling with MC-ICP-MS analysis allows Cu and Fe isotopic compositions to be measured rapidly in sulphide minerals with 2_ precision as low as 1 part per 10,000 (1 epsilon unit). This precision is much smaller than the variation typical in many sulphide ore deposits, allowing isotopic variations to be detected in a wide variety of mineralised systems on single-grain to deposit-wide scales [1,2,3]. These data offer, for the first time, a direct approach to identifying the source of metals in hydrothermal deposits. Potentially, they also allow isotopic fingerprinting of particular ore depositional environments, which may possess important exploration significance. A major obstacle to performing laser-based metal isotopic analyses is a systematic and ablation-time-dependent bias (up to 40 _) in the isotopic measurements. Correcting this bias can be achieved by referencing to analyses of an isotopically homogeneous, matrix-matched standard. However, the requirement for isotopic mineral standards severely limits the applicability of the technique, and frequent external standardisation reduces sample throughput To determine whether the bias in the laser-based analyses occurs during ablation (i.e., non-isotopically stoichiometric ablation) and/or during post-ablation processes occurring in the ICP, a copper target was ablated under a range of analytical conditions (wavelength, pulse energy, ablation gas). The ablated particulates were filtered from the gas stream, dissolved and analysed on a Neptune MC-ICPMS, employing external normalisation using Ni to correct instrumental mass bias. Biases in the measured isotopic composition of the dissolved particles relative to the target material indicate that isotopic fractionation occurs during the ablation process, but only to a small extent. This leaves open the possibility that isotopic fractionation takes place to a significant degree in the ICP, as demonstrated for elemental fractionation [4], perhaps through the interplay of incomplete ionisation of ablated particulates and a systematic change in ablated particle size distribution during ablation.