QUANTITATIVE CONSTRAINTS ON MAGMA MIXING AND ERUPTION DYNAMICS DURING THE 1915 LASSEN PEAK, CA ERUPTION USING THERMODYNAMIC MODELING AND TRANSPORT CALCULATIONS

The May 1915 eruption of Lassen Peak, located at the southern termination of the Cascades (USA), is an archetypical example of magma mixing and potentially eruption triggered by volatile-driven dynamic instability. The eruption serially extruded four products over 3 days: a black glassy dacite dome, banded light (dacitic) and dark (andesitic) pumice followed by light dacitic pumice. Clynne (1999) proposed endmember magmas of a dacite and basalt and suggested crystallization of a hybrid magma, at the contact between the basalt and dacite, resulted in vesiculation that led to andesitic inclusion formation and eruption triggering. We quantitatively reconstruct magma storage and mixing conditions to quantify the phase equilibria of magma mixing, vesiculation of the hybrid magma, and its disruption to initiate the eruption of lava and pyroclastic magma. First, MELTS is used to assess the pressure, temperature, and fO_2, of each endmember, from Clynne (1999), consistent with reported mineralogy. The Magma Chamber Simulator is then used to assess mixing at relevant conditions. Results show that the magmas mix at depths of 3-10 km at ~QFM+1, and the mafic endmember is water rich (3-6 wt% H₂O). Cpx crystallization is a consequence of the mixing, consistent with cpx being found as a microphenocryst of the natural samples. The hybrid magma, representative of the andesitic inclusions, requires a mixing ratio of about 2.5:1 of the basaltic endmember to the dacitic endmember. After ~45% crystallization of the hybrid magma (~200 ^oC of cooling), vesiculation occurs and the bulk composition and mineralogy of the hybrid magma matches that of the natural andesitic inclusions. However, the growth and stability of a foam layer along the endmember magma interface must be quantitatively evaluated to test the efficacy of the overturn hypothesis thought to lead to eruption. The density and viscosity of the natural and simulated magmas will be used to reconstruct the density-viscosity structure of the magma storage zone and its role in the magma withdrawal dynamics. Ultimately, this work begins to elucidate a refined picture of magma dynamics during the 1915 Lassen peak eruption and can be applied to other arc volcanic settings to better understand the nexus of petrogenesis and eruption dynamics.