FIRE AND WATER: A LINK BETWEEN TEXTURE AND DEGASSING IN MILDLY EXPLOSIVE ERUPTIONS

The explosivity of a volcanic eruption depends on the concentration of volatiles dissolved in the magma, the degree the magma degasses, and the ascent rate. Variations in eruptive products related to these factors have been assessed either texturally or chemically. Textural evidence for ascent rate can be obtained from the size and concentration of vesicles, as well as the relative abundance of microlites found in juvenile (magmatic) clasts. The degree of degassing is interpreted by comparing the concentration of volatiles (H₂O, CO₂, S, and Cl) measured in olivine hosted melt inclusions to the concentration of volatiles found in the matrix glass. This study presents both textural and chemical data for two mafic monogenetic cones in Nicaragua of contrasting eruptive style to assess consistency between both methods.

Scoria was collected from multiple stratigraphic levels at Jiloa cinder cone and Cerro San Carlos maar near Managua, Nicaragua. Volatile concentrations (H₂0, CO₂, S and Cl) in matrix glass and olivine hosted melt inclusions were collected by electron microprobe and Fourier Transform Infrared (FTIR) microspectrometry. Volatile concentrations were slightly higher in the maar (S>100ppm and H₂0>0.09wt%) than the cinder cone (S<60ppm and H₂0 around 0.09wt%). SEM images of scoria fragments showed that vesicularity and the relative abundance of microlites also varied between the two cones and may reflect the degree of degassing and degassing-induced crystallization prior to fragmentation. The interaction of magma and groundwater during phreatomagmatic eruptions arrests magma degassing leading to higher volatile contents, lower vesicularity, fewer microlites, and a more explosive eruptive style. The results of this study suggest that textures preserved in explosive eruptions can be linked to chemical measures of magma degassing.