Glassy tephra of Yasur Volcano, Vanuatu: A magnetic, petrographic, and crystallographic study and implications for devitrification

Rock magnetic properties are commonly used to reveal information on magnetic mineral phases, magnetic grain sizes, and physical grain sizes in natural materials. In tephra deposits, these parameters potentially vary as a result of primary and post-depositional processes such as devitrification. Since many rock magnetic properties and the process of devitrification are both grain size and composition dependent, trends in magnetic properties of tephra deposits with depth may correlate with iron-bearing crystallite evolution over time.;This study has examined trends in magnetic properties of a series of andesitic tephras and the glass fractions of these tephras with depth. Samples collected from increasing depths through stratified ash beds near Yasur volcano exhibit remarkable homogeneity with depth; field observations indicated no observable change in color, bulk density, or overall clast size distribution to 15.54 m. Previous geochemical investigations revealed a fairly narrow range in composition of the bulk deposits and the glass fraction with depth. Rock magnetic analyses have been complemented with petrographic, crystallographic, and mineralogical methods including microscopy, Scanning Electron Microscopy (SEM), X-Ray Diffraction (XRD), Infrared Spectroscopy, and Transmission Electron Microscopy (TEM). Two tephra samples (at 7.54 and 15.54 m) have been radiocarbon dated.;Iron-bearing crystallites were not detected at any depth using the combination of methods in this study, therefore the hypotheses could be neither proven nor discarded. Relationships between magnetic properties of the tephra and petrography have been explored. Magnetic grains of primary origin were found to carry the magnetic properties of both the tephra and the glass fraction. Trends in magnetic parameters vary little with depth, but variations may be attributed to subtle changes in magma composition over time or changes in eruptive history (e.g., eruption velocity, plume height, wind velocity, etc.) that are not detectable by macroscopic observations. Two radiocarbon dates, 22,935 +/- 188 b.p. (15.54 m) and ∼9442 b.p. (7.54 m) were used in combination with the direct observation that the surface tephra corresponds with the present-day deposit to constrain the results of the study.