WHERE VOLCANISM AND THE ATMOSPHERE MEET: GAS COMPOSITIONS OF VOLCANIC PUMICE CLASTS

Silicic volcanic eruptions are one of the main vectors through which the crust interacts with the atmosphere and biosphere, and these violent eruptions can have a profound impact on Earth’s climate. During ascent and eruption, volatile species (e.g., H₂O, CO₂) are exsolved from magmas. These gasses may also interact with the atmosphere before being trapped in vesicles below the glass transition. Therefore, volcanic pumice clasts are potential recorders of both atmospheric composition at the time of eruption and of the processes and potential triggers of large volume silicic volcanic eruptions. Whereas melt inclusions in minerals yield critical information on the formation and evolution of volcanic systems, they likely do not record the volatile state of a magma during eruption. In this study we measure gasses trapped in pumice clasts from the Bishop Tuff to test the idea that pumice clasts contain records of atmospheric and volcanic processes and use this data to interpret the volcanic activity that formed the Long Valley Caldera.

The Bishop Tuff was produced by a ~765,000 year old large-volume silicic volcanic eruption. Relatively mineral-free pumice clasts from the Bishop Tuff were placed under 1E-2 mbar vacuum prior to analysis to remove any atmospheric contamination or adsorbed water from the pumice clasts to ensure the gasses measured were those trapped within vesicles. These samples were then placed in stainless steel crushers and pumped to high vacuum (7E-7 mbar) for 2 days prior to being crushed on-line into a calibrated quadrupole mass spectrometer. Quantified major gas species include: H₂O, CO₂, O₂, N₂ and Ar. Binning these species in to atmospheric (N₂, O₂, Ar with minor CO₂) and volcanic (H₂O and CO₂ with minor, un-quantified SO₂) enables us to observe the composition of the atmosphere, the composition of volcanic volatiles during eruption, and the degree to which these components mix during eruption. This novel technique for analysis of gasses trapped in vesicles of pumice clasts has broad potential for elucidating climatic processes in the past as well as the events leading to the eruption of large-volume silicic volcanic systems.