The hydration and alteration of perlite and rhyolite

The volatile concentrations and thermal characteristics of hydrothermally-altered rhyolitic deposits erupted under Icelandic glaciers have been studied by combined differential scanning calorimetry-thermogravimetric analysis-mass spectrometry (DSC-TGA-MS) and x-ray diffraction (XRD). Samples range from pristine obsidians through to strongly-perlitised and altered fragmental deposits. Four types of samples are determined to have notable differences in total volatile concentrations. These are: obsidians (0.44-3.04 wt. %); perlites (2.15-8.15 wt. %); obsidian-breccias (8.49-9.41 wt. %), and hyaloclastites (3.23-7.78 wt. %). DSC-TGA-MS and textural data indicate that the volatile concentration of the perlitic samples increases as the amount of perlitisation increases. XRD data show that the volatile-rich samples are rich in the low-temperature zeolite minerals heulandite and mordenite. The temperature at which volatile exsolution occurs is shown to decrease as the volatile concentration increases reflecting the speciation of water as well as zeolite mineral growth