Linking Texture and Degassing in Mildly Explosive Eruptions
Abstract
The explosivity of volcanic eruptions depends on the concentration of volatiles dissolved in magma, the degree magma degasses, and the ascent rate. The interaction of magma and groundwater during phreatomagmatic eruptions arrests magma degassing leading to higher volatile contents, lower vesicularity, fewer microlites in eruptive products, and a more explosive eruptive style. Variations in texture and volatile concentration have been studied in eruptive products, but rarely in concert. Chemically, the degree of degassing can be 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. Textural evidence for degassing can be determined from the size and concentration of vesicles, as well as the crystallinity, crystal size, and area density of microlites found in juvenile clasts. This study presents both textural and chemical data for two mafic monogenetic cones of contrasting eruptive style. The primary objectives are to assess consistency between the methods and elucidate eruption dynamics. Scoria was collected from multiple stratigraphic levels at Jiloa cinder cone and Cerro San Carlos maar near Managua, Nicaragua. Volatile concentrations (H2O, 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 scoria (S>100ppm and H2O>0.09wt%) than scoria collected from the cinder cone (S<60ppm and H2O ~0.09wt%). SEM images show that vesicularity and the abundance of microlites in scoria varies between the two cones and may reflect the degree of degassing and degassing-induced crystallization prior to fragmentation. Digital processing of these images will be performed in order to quantify vesicularity and area density of microlites in scoria. A comparison of chemical and textural analyses will provide a more complete data set from which to evaluate the influence of degassing on eruptive textures and degree of explosivity.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.V33D2674K
- Keywords:
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- 8400 VOLCANOLOGY;
- 8414 VOLCANOLOGY / Eruption mechanisms and flow emplacement;
- 8430 VOLCANOLOGY / Volcanic gases;
- 8499 VOLCANOLOGY / General or miscellaneous