Mount St Helens lavas and tephras have carried to the surface abundant xenoliths that include plutonic and non-plutonic lithologies (e.g., Heliker 1995). Among plutonic lithologies, a large fraction of the clasts are from the local Tertiary basement, and likely unrelated to the recent Mount St Helens activity. However, a small proportion of them appear to be co-magmatic with the recent volcanic activity and hence provide direct samples of magma reservoirs within the crust.

The first objective of this part of the project will be to date these inclusions by in-situ U/Th/Pb dating on zircons by SHRIMP at the Stanford-USGS facility (e.g., Lowenstern et al. 2000; Bacon and Lowenstern 2005; Bachmann et al. 2007a). The ones that yield young ages and that contain pyroxenes and amphiboles will be used for the second part of this project: obtaining crystallization pressures and temperatures using existing mineral thermo-barometers (e.g., Putirka 2008; Ridolfi et al. 2010). Despite the complex crystallization history that these clasts have undergone, we expect that some will have equilibrium compositions necessary for accurate thermo-barometric calculations.

We plan to use the existing collections of Mike Clynne and Christina Heliker (10+ samples of such co-magmatic samples and expand the collection by additional sampling over the first couple of years of this project. This dataset on plutonic inclusions can be supplemented and/or compared to data from phenocrysts (particularly amphiboles) in erupted lavas and tephras, which show a large range of pressure (e.g., Fig. 6a of Ridolfi et al. 2010). Such a petrological study would provide an important complement to the geophysical imaging of the region by providing independent depth estimates for where the dominant crystallization sites are in the crust beneath Mount St Helens.