Abstract
Obsidian is volcanic glass that results from the rapid cooling of silica-rich melt. Nanoscale crystallites precipitate out of the melt prior to solidification and remain embedded in the amorphous matrix. These crystallites provide information on the flow kinetics and composition of the melt. Due to the sparsity and size of nanolites, studies often focus on supramicron crystallites. This research takes advantage of the conchoidal fracture of obsidian by knapping samples with nanometer-thin edges for transmission electron microscopy characterization. Nanolites in the amorphous matrix are studied using energy-dispersive spectroscopy (EDS) and electron diffraction. Certain alkali and alkaline-earth cations exhibit patterns of depletion near Fe-oxide nanolites. EDS is used to identify nanolites and variations in the composition of the matrix. Parallel beam diffraction and radial distribution function analysis of nearest-neighbor distances determine average bond lengths in the matrix near nanolites, showing that nanolites influence the nearby short-range ordering and atomic character of the matrix. Analysis reveals decreased mean nearest-neighbor distances in the matrix adjacent to nanolites compared to the bulk. Our methods exhibit the required sensitivity to detect variations in the composition and structure near nanolites, and our findings indicate that obsidian nanolites contribute to quantifiable localized changes in the amorphous structure.
Highlights
Obsidian is an amorphous volcanic mineraloid of variable composition that forms when viscous magma of high silicon content cools rapidly before the development of a crystal structure (Jerram & Petford, 2011)
Taking advantage of obsidian’s conchoidal fracture nature, flint knapping techniques can produce thin obsidian samples for transmission electron microscopy (TEM) analysis without introducing pitting, contamination, or dislodging crystallites. Using this method of sample production, Fe-oxide nanolites were identified in the amorphous matrix of obsidian sourced from the Warner Mountains
Obsidian nanolites are of geologic significance because of the information they contain about the kinetics and thermodynamics of the parent melt from conduit ascension to emplacement and with respect to emerging research on the role they play in heterogeneous bubble nucleation and volcanic explosivity
Summary
Obsidian is an amorphous volcanic mineraloid of variable composition that forms when viscous magma of high silicon content cools rapidly before the development of a crystal structure (Jerram & Petford, 2011). During the process of cooling, undercooling produces a variety of crystallites, including nanolites, which are nanoscopic euhedral crystals embedded within the amorphous matrix. The geologic utility of microlite (micron-scale crystallite) characterization has been proven through studies relating microlite crystallographic orientations to the flow direction of the matrix (Manga et al, 2018), correlating bulk obsidian color to microlite compositions (Camargo et al, 2016) and approximations of the cooling rate based on the size and spacing of hostgrown microlites (Bowen, 1956; Sano & Toramaru, 2017).
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