Multi-element analysis of minerals using laser ablation inductively coupled plasma time of flight mass spectrometry and geochemical data visualization using t-distributed stochastic neighbor embedding: case study on emeralds

Abstract

In recent years, multi-element chemical analysis has been applied to a broad range of solid samples in mineralogy, geology, environmental science, biology and beyond. In this study, we present a quantification method for the multi-element composition of minerals and a statistical method to investigate chemical similarity among samples. We obtain almost the entire elemental composition simultaneously using laser ablation inductively coupled plasma time-of-flight mass spectrometry (LA-ICP-TOF-MS). A novel concept of “first measure, then determine” which elements are of interest is introduced for multi-element analysis of geological samples. This case study focuses on major, minor and trace element analysis of emerald, a highly relevant mineral in the gemstone trade. In total, 168 samples were analyzed without a priori knowledge of their geographic provenance. They were grouped/clustered solely based on similarities in multi-element concentrations using non-linear unsupervised dimension reduction algorithm t-distributed stochastic neighbor embedding (t-SNE). A comparison with a PCA plot reveals that the application of t-SNE results in better cluster separation. The clusters in the t-SNE plot coincide with the geographic provenance of these emeralds, probably due to unique elemental fingerprints within the geological setting in each provenance. Based on our results, we consider LA-ICP-TOF-MS multi-element data acquisition in combination with t-SNE data visualization a powerful and promising tool in mineralogy and geology. Not only for provenance studies, but when combined with further sample characteristics (e.g. spectroscopic features, host rock composition, geochronology, inclusions) it may assist in understanding the geological formation and setting of minerals within their host-rock and their deposits.