Open AccessCooling process recorded in subglacially erupted rhyolite glasses: Rapid quenching, thermal buffering, and the formation of meltwater
Author(s) -
Wilding Martin C.,
Smellie John L.,
Morgan Sally,
Lesher C. E.,
Wilson Lionel
Publication year - 2004
Publication title -
journal of geophysical research: solid earth
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.67
H-Index - 298
eISSN - 2156-2202
pISSN - 0148-0227
DOI - 10.1029/2003jb002721
Subject(s) - glass transition , amorphous ice , meltwater , mineralogy , activation energy , relaxation (psychology) , geology , thermodynamics , materials science , analytical chemistry (journal) , amorphous solid , chemistry , glacial period , composite material , geomorphology , physics , social psychology , organic chemistry , chromatography , psychology , polymer
The thermal histories of two subglacial rhyolite glasses from Torfajökull complex of south central Iceland have been determined from changes in the relaxation of enthalpy in the calorimetric glass transition interval. Heat capacity measurements of bulk glass samples from Rauðufossafjöll (sample number JS.1.1) and Bláhnúkur (sample number JS.2.1) by differential scanning calorimetry show marked differences in the temperature of the onset of glass transition ( Tg ) on reheating. The average Tg is 750 K for JS.1.1, whereas it is 850 K for JS.2.1. These differences in the onset of Tg are related to quench rate through considerations of the activation energy and timescales for structural relaxation modeled, using the Kohlrausch‐Williams‐Watt function. Multiple glass samples from the two Torfajökull locations recover different average values and distributions of a fictive temperature, T f , which we interpret in terms of differences in the thermal histories of the glasses. In both glass samples the range of fictive temperatures and, by implication, apparent cooling rates, is lower than those estimated for a glass rapidly quenched by contact with cold (glacial melt) water and requires that the glass was annealed at temperatures in the glass transition interval associated with relaxation times of the order of 1000–10,000 s. The complex cooling histories of the Torfajökull samples show that the products of subglacial eruptions may be held at elevated temperatures for several hours and, as a result, may drive vigorous convection in the ice‐bound vaults above erupting rhyolite deposits.