Figure 5.

Plot of 10 × S/K₂O versus K₂O with 2014 matrix glasses, 1865 melt inclusions and melt glasses, and melt inclusions from nearby Irazú Volcano. Olivine‐hosted melt inclusions (yellow triangles) from the 1865 eruption are sulfur rich and K₂O poor, representing primitive undegassed melt. Matrix glasses from both 1865 and 2014 eruptions have essentially the same composition and have degassed almost all of their sulfur. The inset backscattered electron image shows a fresh‐looking glassy shard (with adhered fine ash) erupted in October 2014 that is suspected to be juvenile. Interestingly, a significant group of pyroxene‐ and plagioclase‐hosted melt inclusions from the 1865 eruption have higher K₂O than the Turrialba matrix glasses and are degassed. Irazú melt inclusions (data from Benjamin et al. [2007] and Wehrmann et al. [2011]) form a parallel trend to the Turrialba glasses. The black bar along the X axis shows the K₂O content of Irazú matrix glasses [Alvarado, 1993], again overlapping with Turrialba compositions but extending to higher K.