Abstract
Characterization of microstructures and perlitic textures may be carried out by combining volcanological and glass technical concepts, with supporting evidence from image analysis. These methods enable investigation of the mechanisms of protracted fracture formation, propagation, and characterization of fracture networks in silica-rich volcanic glass.
Here we present analyses of rhyolitic lava and ignimbrites from different locations and geological ages. Our results allow characterization of the relationship between primary sublinear quench fractures and secondary rounded perlitic fractures, which together form a three-dimensional fracture network.
Completely hydrated samples from the pre-Cenozoic show no variations in volatile fluid content within fractured glass but do show differential depletion in alkali contents, suggesting significant fluid-mediated alteration. In contrast, younger samples are incompletely hydrated and may demonstrate a connection between alkali mobility and rock hydration.
Formation of primary fractures and a first generation of secondary fractures is caused by thermal shock close to the glass transition temperature. Formation and propagation of subsequent fracture generations is related to intrinsic stresses induced by the continued hydration of volcanic glass.
We find that the pore space in rhyolitic rocks is not only influenced by fracture porosity or initial vitric porosity but may increase or decrease during post-emplacement, low-temperature alteration processes.