Experimental constraints on plagioclase crystallization during H₂O- and H₂O-CO₂-saturated magma decompression

Experiments simulating magma decompression allow the textures of volcanic rocks to be calibrated against known eruptive conditions. Interpretation of natural samples may be complicated, however, by both the decompression path and the composition of exsolving volatiles, which affect the time evolution of crystal textures. Here we present the results of decompression experiments at elevated temperature and pressure designed to assess the effects of degassing path on crystallization of Mount St. Helens rhyodacite. Three families of experiments were employed to simulate varied PH₂O-t trajectories: Single-step, ₂O-saturated decompression (SSD); continuous, ₂O-saturated decompression (CD); continuous, ₂O-CO₂-saturated decompression. Quantitative textural data (crystal abundance, number density, and size) are used to calculate plagioclase nucleation and growth rates and assess deviations from equilibrium in run products. These are the first experiments to quantify feldspar nucleation and growth rates during ₂O-CO₂-saturated decompression. We find that reducing the initial melt water content through addition of CO₂ increases nucleation rates relative to the pure water case, an effect most pronounced at low dP/dt. Moreover, these early formed textural distinctions persist at the lowest pressures examined, suggesting that deep ₂O-CO₂ fluids could leave a lasting textural 'fingerprint' on erupted magmas. Crystals formed prior to decompression during the annealing process also modulate sample textures, and growth on pre-existing crystals contributes significantly to added crystallinity at a wide range of experimental conditions. The phase assemblage itself is a dynamic variable that can be used in conjunction with textural data to infer conditions of magma ascent and eruption. Finally, quantitative textural data from experimental samples are compared with those of natural pyroclasts erupted during the summer 1980 explosive-effusive transition at Mount St. Helens. This comparison supports a model of magma ejected from multiple storage regions present in the upper crust following the May 18 Plinian eruption, such that subsequent eruptions tapped magmas that experienced varied decompression and degassing histories.