REASSESSING EMBAYMENT-BASED MAGMA ASCENT RATES FROM SILICIC TO INTERMEDIATE ARC ERUPTIONS

Arc volcanoes are among the most frequently active systems on Earth, where explosive eruptions pose significant hazards in the sky and to communities on the flanks. The explosivity of these eruptions is strongly influenced by the rate of magma ascent in the conduit, a parameter that is challenging to uniquely quantify from storage to surface. One technique that shows promise is modeling volatile (H₂O, CO₂, S) diffusion in crystal-hosted embayments (unenclosed melt inclusions). However, the application of embayments in arc settings remains limited and accompanied by assumptions that ultimately affect estimated ascent rates. We revisit existing embayment datasets from three silicic to intermediate arc eruptions to evaluate the assumptions of (1) linear magma ascent and (2) initial conditions differing from storage values (e.g., no CO₂). We first developed a two-stage decompression model which optimizes three parameters: the initial and final decompression rates and the transition pressure. Preliminary two-stage modeling for the CO₂-free 1991 eruption of Mount Pinatubo (Philippines) has found that volatile gradients in embayments are best fit with an initial slow stage of ascent followed by a final stage over an order of magnitude faster^[1]. We then consider two CO₂-bearing eruptions that have previously been modeled assuming H₂O saturation and linear ascent: the 3600 year BP Santorini (Greece)^[2] and 1980 Mount St. Helens (USA)^[3] eruptions. Using the inferred initial CO₂ concentrations for these eruptions (~200 and ~400 ppm, respectively), we re-model 14 measured volatile gradients in embayments and provide broader ascent rate estimates. Our results indicate that for the CO₂-bearing eruptions, we cannot rule out a prolonged (>10 hours) initial ascent resulting in complete loss of CO₂ from embayments. Further, compared to the one-stage model, our two-stage model better resolves the final, explosive stage of all three eruptions. Our work highlights that embayments can preserve a complete record of magma ascent and should be more widely applied to actively monitored arc settings for an integrated understanding of magma movement from the deep conduit to the surface.

[1] Saalfeld et al. (2021) AGU Fall Meeting.

[2] Myers et al. (2021) Bull. Volc. 83.

[3] Humphreys et al. (2008) Earth Planet. Sci. Lett. 270.