SPATIAL VARIATIONS IN ANCIENT METEORIC WATER: AN INVESTIGATION OF THE RATTLESNAKE TUFF

Volcanic glass has been used extensively as a paleoclimate proxy. Deuterium (²H) concentrations in glass have been found to be stable over geologic timescales, making δD a reliable proxy for ancient water chemistry. However, continued work revolves around better understanding how different factors affect preserved water in ash. Here, we analyze δD in the Rattlesnake Tuff (RST), a widespread ca. 7 Ma ashflow tuff, and create an isoscape to assess variations in δD across Oregon during that time. Additionally, we examine compositional data from glass shards to explore the relationship between δD and shard composition. The RST exhibits well defined compositional bands owing to its eruption from a zoned magma chamber which may affect δD values and paleoenvironmental interpretations. 16 samples were collected across central and eastern Oregon from various flow units within the RST. Samples were analyzed for δD using a Temperature Conversion Elemental Analysis (TCEA) mass spectrometer and major and minor elements using a Scanning Electron Microscope (SEM). We compare our isotopic results to modern water and published proxy data to better constrain changes in climate and topography across Oregon throughout the Neogene. We also estimate wt% H₂O by calculating excess (non-stoichiometric) oxygen from SEM elemental data. Our results show significant spatial variation in δD values of RST, ranging from –107‰ to –154‰. δD values of ancient glass are similar to modern water near the Cascade Mountains, but become relatively negative to the east near the inferred eruptive center of the RST. We do not observe significant variation in δD among flow units from single locations, nor do we observe a significant relationship between wt% H₂O and major and minor element abundances, supporting the use of volcanic glass as a paleoenvironmental indicator.