Confocal Scanning Laser Microscopic (CSLM) characterization of volcanic rocks

Abstract

Optical and microscopical analyses of surficial and internal textures are key elements of volcanic rock characterization. Yet conventional methods for textural analyses often pose challenges and difficulties in terms of sample preparation and/or time and resources consumption, beside their inherent limitations. Here, we performed Confocal Scanning Laser Microscopy (CSLM) to characterize vesicles in volcanic rocks and their experimental analogs and to compare the results to those of conventional Scanning Electron Microscopy (SEM) investigations. Further, we discuss surface roughness analysis as a possible parameter to analyze different vesicle textures. CSLM obviates the need for sample impregnation (using fluorescent resin) and polishing, it enables fast and automated analyses of samples surfaces at lower costs than conventional techniques. Calculated vesicle size distributions and vesicle number densities of the analyzed samples were compared in order to determine the potential and precision of volume and area information directly obtainable from CSLM measurements in 3D. Monodisperse samples with homogeneous vesicle texture show remarkable similarities in vesicle size and volume distribution, when compared directly with the two methods. For all experimental and natural samples, roughly half of them show VNDs and porosity show the same range, which can be explained by vesicle coalescence. After de-coalescing the images of CSLM for FOAMS analysis, these differences vanish almost entirely for most of the sample types. Combining our textural data and our numerical modeling, we present a universal scaling model to link surface roughness with vesicle number density, porosity, and polydispersivity. These results indicate that CSLM can be a powerful tool to investigate vesicle structures from the millimetric to micrometric scale. The resulting data is both quantitatively and qualitatively equivalent to SEM-derived data for a variety of volcanic rock types. This, combined with the simple sample preparation and handling make CSLM a valid alternative to SEM investigations to characterize vesicles, particles, and any sort of rock property that reflects on topography. Surface roughness can become a new key parameter to interpret porous networks in volcanic rocks and can be applied to other geological fields.