Field investigation of volcanic deposits on Vulcano, Italy using a handheld laser-induced breakdown spectroscopy instrument

Abstract

Laser-induced breakdown spectroscopy (LIBS) is an important analytical technique in a variety of fields ranging from in-situ terrestrial geological field investigations to robotic exploration missions of extraterrestrial bodies such as Mars. In this study, the performance of a commercial handheld LIBS instrument was evaluated during an international summer school in 2019 that focused on the exploration of extreme environments on Earth and in space. Several sites on the Eolian island Vulcano (Italy) were investigated with different spectroscopic methods including LIBS. We focus here on the exploration of one particular outcrop with LIBS, where layered and colored ash deposits were observed. Furthermore, a laboratory study was performed to investigate and validate the effect of varying distance of the instrument to the sample. Unsupervised principal component analysis (PCA) for data exploration showed that elemental variations between the layers of the outcrop can be observed with the LIBS data from the handheld instrument. This was further confirmed by a layer-by-layer analysis of elemental correlations and depositional trends. Geologically relevant major elements such as Si, Al, Ca, Fe, K, Mg, and Na could be identified but also minor and trace elements such as F, Li, Mn, and Sr. Our results also show that the effects of varying distances of the instrument to the sample are critical for the quality of the data acquired and hence pose significant challenges to the analysis and interpretation. We propose a dedicated data pre-processing approach, which includes the masking of emission lines of Ar from the locally induced Ar atmosphere, as a possible solution to overcome this challenge. Overall, this study provides a better understanding of the performance and limitations of a handheld LIBS instrument, particularly in the context of future terrestrial and planetary field investigations.