TENSILE STRENGTHS OF HYDROUS VESICULAR GLASSES - AN EXPERIMENTAL-STUDY

We have measured the pressures of decrepitation of vesicles in synthet ic glasses of feldspar compositions (NaAlSi3O8-KAlSi3O8). Vesicles fil led with Xe do not decrepitate at internal pressures of 160 MPa, indic ating that the unflawed surface of the vesicle wall has an intrinsic s trength > 80 MPa. Vesicles containing CO2 escaped decrepitation and di splayed ductile deformation when the T-g was reached at the maximum P of 200 MPa (indicating an intrinsic strength higher than 100 MPa), Ves icles containing H2O showed dramatically reduced strength, decrepitati ng at internal pressures on the order of 1-5 MPa. The H2O-filled vesic les leaked slowly over periods of several weeks or months, The relativ e 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 rad ial microfractures in the walls of H2O-filled vesicles. We account for the microfracturing with reference to recent studies of chemical-grad ient stress. Our observations may account for a variety of phenomena, which occur wherever hydrous vesicular glasses are formed, including e xplosive decompression of vesicular glassy rock in near-surface volcan ic 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 in vestigating fluid-melt phase equilibria.