73-8 Using the Chemical Composition of Cryptotephra as a Fingerprint for Miocene Volcanic Deposits in the Central Montana Renova Formation

Session: Using Volcanic Deposits to Help Us Understand Volcanic and Magmatic Processes (Posters)

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Volcanic eruptions can eject tephra, including ash and lithics, thousands of miles from the eruption site. “Cryptotephra” refers to fine ash deposited far from the volcanic center, composed of microscopic volcanic glass and mineral fragments. The chemistry of the glass can be used to determine volcanic origin. In our study, nine Miocene samples were collected in southwestern Montana from the upper Renova Formation, a preserved extensional terrestrial basin which preserves multiple ash beds and fluvial-mobilized volcanic material. Volcanics preserved in the Renova Formation are anomalous as the volume of material preserved is greater than known active volcanics (primarily including the Cascades and Snake River Plains) at the time of deposition. This study aims to determine the origin and model deposition of the cryptotephra. Three samples have K-Ar sanidine ages of 9.33 – 10.35 Ma, and the remaining samples will also be dated. Volcanic glass fragments were picked manually and analyzed for trace element concentrations (Cr, Ni, Zn, Rb, Sr, Y, Zr, Nb, Ba, Pb, Th, U, and REEs) by ICP-MS. Trace element data were compared with temporally concordant, regional volcanic rock logged on the EarthChem database. Trace element concentrations of our samples show strongest correlation with volcanic rocks of the Snake River Plains. Most late Miocene regional volcanic data in the EarthChem database have juvenile signatures (ex. low Rb/Sr < 0.5), however, our sample glasses and some silicic rocks of the Snake River Plains have higher Rb/Sr ratios, up to 2.5, along with low Sr/Y ratios, < 10. To better model the volcanic transport, SEM will be used to measure the major element compositional data and predict the melt viscosity of the glasses. This is done in conjunction with another study of the morphology and Raman spectral signatures of the cryptotephra glasses currently being conducted by another member in our laboratory group to ultimately determine transport of the cryptotephra in Miocene sedimentary basins. This study shows how cryptotephra can be used as a tool to predict the behavior of ancient volcanoes and how that volcanic material traveled over time following an eruption. There are currently volcanoes in the Western United States that could potentially erupt and studying ancient eruptions could help us predict and prepare for the outcome of these eruptions.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9111

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