Phase equilibrium experiments at 0.5 GPa and 1100-1300 °C on a basaltic andesite from Arenal volcano, Costa Rica

We present results from piston-cylinder experiments on a synthetic composition of basaltic andesite that corresponds to lavas erupted from the ongoing eruption at Arenal volcano, Costa Rica, in order to shed light on magmatic processes at upper crustal depths beneath Arenal. The starting composition represents the least evolved basaltic andesite from the initiation of stage 2 of the current eruption. Anhydrous and hydrous experiments were conducted at 0.5 GPa and 1100-1300 °C: the principal phases encountered were melt, plagioclase, orthopyroxene and clinopyroxene of variable CaO content. Glass and plagioclase compositions change in a consistent manner with decreasing temperature for both hydrous and anhydrous experiments. The phase equilibria dictate that Arenal magmas must have contained > 2 wt.% H₂O in order for the erupted rocks to have once represented liquid compositions at a relatively high temperature (1200 °C) and > 4 wt.% H₂O if the melt was at the lower temperatures (≤ 1150 °C) that are more likely for the Arenal system. However, anorthite-rich plagioclase phenocrysts (> An₈₅) commonly found in Arenal lavas cannot be accounted for by any reasonable permutation of higher temperature and water content. The close correspondence of the phase compositions (rims of plagioclase, orthopyroxene) and crystallinity observed in stage 2 lavas from Arenal and a hydrous experiment with 2 wt.% water in the melt provides evidence for Arenal magmas ponding and equilibrating at 1150 °C and ∼ 12-14 km depth. The conclusion that Arenal lavas reflect equilibration between observed minerals and a melt with ∼ 2 wt.% H₂O at 0.5 GPa, ∼ 1150 °C, argues that these bulk compositions are unlikely to have ever reflected fully molten liquids.