Tensile strengths of hydrous vesicular glasses: An experimental study

We have measured the pressures of decrepitation of vesicles in synthetic glasses of feldspar compositions (NaAlSi₃O₈-KAlSi₃O₃). Vesicles filled with Xe do not decrepitate at internal pressures of 160 MPa, indicating that the unflawed surface of the vesicle wall has an intrinsic strength >80 MPa. Vesicles containing CO₂ escaped decrepitation and displayed ductile deformation when the T_g was reached at the maximum P of 200 MPa (indicating an intrinsic strength higher than 100 MPa). Vesicles containing H₂O showed dramatically reduced strength, decrepitating at internal pressures on the order of 1-5 MPa. The H₂O-filled vesicles leaked slowly over periods of several weeks or months. The relative stability of the inclusions is strongly dependent on the quench rate, with rapidly quenched inclusions showing greater stability over long periods of time. Microscopic examination revealed the presence of radial microfractures in the walls of H₂O-filled vesicles. We account for the micro fracturing with reference to recent studies of chemical-gradient stress. Our observations may account for a variety of phenomena, which occur wherever hydrous vesicular glasses are formed, including explosive decompression of vesicular glassy rock in near-surface volcanic environments, spontaneous decrepitation of vesicular basaltic glass dredged from the seafloor ("popping rocks"), and rapid loss of H₂O from synthetic vesicular glasses produced in laboratory experiments investigating fluid-melt phase equilibria.