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, I 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, I 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. I investigate whether this affects δD values and should be considered in paleoenvironmental interpretations. 16 ash 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 Analyzer (TC/EA) connected to a mass spectrometer and elemental composition using a Scanning Electron Microscope (SEM). I compare my isotopic results to modern water and published ancient water proxy data to better constrain changes in climate and topography across Oregon throughout the Neogene. I also estimate wt. % H2O by calculating excess (non-stoichiometric) oxygen from SEM elemental data. My 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. I do not observe significant variation in δD among flow units from single locations, nor do I observe a significant relationship between wt. % H2O and major and minor element abundances in unprepared samples. Lastly, there is not a significant relationship between prepared glass shard composition and wt. % H2O or δD, supporting the use of volcanic glass as a reliable paleoenvironmental indicator