A new method to deconvolute binary mixture in LA-ICP-MS analyses to quantify the composition of phases smaller than the laser spot size

Abstract

Laser ablation ICP-MS is widely applied to geological materials for the in situ analysis of elemental and isotopic compositions at a tens of micrometers scale. Despite the advances in LA-ICP-MS technology made in the last few decades, analyzing features with a micron-sized diameter still faces technical challenges, in particular, contamination from the host material. Here, we provide a quantification strategy to deconvolute the data signal produced when ablating a mixture of the target material and the host, which enables quantification of features that are smaller than the effective size of the laser spot. Our strategy is to purposely ablate mixtures of the target and host material with varying proportions of the two. The resulting mechanical mixtures define a linear trend in compositional space of which the end points can be determined when at least one element is independently known for both phases (e.g., from microprobe analyses or stoichiometry). The theoretical basis of this approach is described. The methods are evaluated by application to experimental samples displaying liquid immiscibility, both at a >50 μm scale where we show that our methods produce equivalent data to traditional data processing, and to samples where the small liquid droplet size prevents traditional analysis. The methods outlined here allow for quantification of features as small as a few micrometers in size on a standard LA-ICP-MS system, and are ideal to analyse melt or fluid inclusions, crystal zonation as well as other small phases in natural or experimental systems.