EXPERIMENTAL CRYSTALLIZATION OF BASALTIC TEPHRA UNDER THE INFLUENCE OF NACL: AN ANALOG FOR RECYCLING IN THE SUBMARINE ENVIRONMENT

Basaltic tephra is commonly observed falling back into the vent during low energy eruptions and may be reheated, or even encapsulated in more juvenile melt and ejected again, i.e. recycled. Reheating/recycling of a pyroclast in the vent may induce additional crystallization complicating the crystallinity textures used to determine magma ascent and cooling histories. However, additional crystallization may also provide signatures of recycling that may be identified in the rock record to help determine eruption dynamics.

Recycling of pyroclasts is likely more common in a subaqueous environment due to overlying hydrostatic pressures. At NW Rota-1, Mariana arc in 2006 pyroclasts were observed falling back into the vent and tephra collected had numerous microcrystalline inclusions, suggesting recycling. The matrix glass of those inclusions was highly enriched in chlorine (≤ 2 wt% Cl). The purpose of this study is to determine the timescales and temperatures at which microcrystalline inclusions form during a recycling event, and to understand the assimilation of chlorine into the matrix glass.

Two sets of reheating experiments were conducted using basaltic tephra. One set of samples was soaked in a 3.5% NaCl aqueous solution, then heated to evaporate all water leaving NaCl encrusted on the external surfaces. The second set had no initial processing. Reheating experiments on both sample sets were conducted using a Deltec tube furnace at temperatures (T) of 650-1100°C for timescales of 5-60 minutes. We then used a SEM to image the reheated and control (unheated halves) samples. Preliminary results for the unsalted samples indicate localized crystallization at 1000°C after 10 minutes, and more extensive crystallization at T ≥ 1000°C within ≤30 minutes. The NaCl-encrusted samples appear to initiate crystallization at similar temperatures as the unsalted samples, however, the extent of crystallization appears greater. Specifically, the NaCl-encrusted samples appear to show a greater extent of plagioclase growth, while the unsalted samples are dominated by pyroxene and oxide growth with less pronounced plagioclase growth. Further analyses are needed to determine the extent of NaCl assimilation. However, our preliminary results suggest NaCl affects the extent of crystallization and phases within reheated samples.