DETERMINING CRYSTALLIZATION DYNAMICS OF RECYCLED TEPHRA THROUGH REHEATING EXPERIMENTS UNDER NACL SOLUTION

Explosive basaltic volcanoes with low mass eruption rates often have insufficient energy to expel clasts beyond the vent rim, resulting in clasts falling back into the vent. Pyroclasts may be reheated, or even encapsulated in more juvenile melt and ejected again - i.e., recyled, producing microcrystalline-rich areas surrounded by microlite-poor matrix. Such microcrystalline textures have been observed within tephra in low mass eruption rate volcanoes around the world, including the submarine volcano, NW Rota-1, Mariana Arc. Interestingly, the matrix glass of the microlite-rich areas of NW Rota-1 samples are highly enriched in chlorine (≤2wt% Cl). Reheating experiments can simulate the recycling process and constrain crystallization dynamics to understand the influence of NaCl upon cyrstallization. Preliminary findings from reheating experiments show the extent of crystallization increases with greater temperatures and, to a lesser degree, longer timescales.

In this study, we have reproduced recycling textures through reheating basaltic tephra in a 3.5% NaCl aqueous solution as an analog to submarine recycling, and compare our results to untreated 'dry' experiments. All experiments were run in a 1atm tube furnace over timescales of 5-60 minutes and temperatures 700°C-1100°C. SEM imaging and EDS element mapping were used to analyze the extent of reheating-induced crystallization and determine mineral phases produced.

Reheating-induced crystallization starts at ≥1000°C with incipient crystallization occurring at ≤ 6 minutes, transitioning to localized areas of crystallization at ≥ 30 minutes and extensive crystallization (no matrix glass remains) at ≥1100°C and ≤ 5 minutes. Mineral phases present include plagioclase, pyroxenes, and oxides, all present at T ≥ 1000°C and ≥ 30 minutes. Reheated samples saturated in NaCl solution show similar timescales for crystallization as untreated samples, but a greater extent of crystallization, including localized microcrystalline-rich areas (similar to inclusions found in NW Rota-1 tephra), a texture seemingly unique to NaCl influenced samples, suggesting interaction with NaCl influences crystallization. Chlorine concentrations do not appear to be elevated within the microcrystalline-rich areas, as found in NW Rota-1 microcrystalline inclusions.