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New insights on the origin of flow bands in obsidian

By
Jonathan M. Castro
Jonathan M. Castro
Department of Mineral Sciences, Smithsonian Institution, P.O. Box 37012, NMNH MRC-119, Washington, D.C. 20013-7012, USAcastro@volcano.si.edu.
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Donald B. Dingwell
Donald B. Dingwell
Department für Geo- und Umweltwissenshaften, Universität München, Theresienstraße 41 III, 80333 München, Germany
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Alexander R.L. Nichols
Alexander R.L. Nichols
Department für Geo- und Umweltwissenshaften, Universität München, Theresienstraße 41 III, 80333 München, Germany
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James E. Gardner
James E. Gardner
Department of Geological Sciences, The University of Texas at Austin, 1 University Station C1100, Austin, Texas 78712-0254, USA
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Published:
January 01, 2005

We examined the textures, volatile contents, and cooling histories of microlite-defined flow bands in several rhyolitic obsidians in order to test whether textural variations between bands could be ascribed to different degassing and cooling histories, and to assess the timing and location of band formation. Flow bands are defined by variations in microlite number density (NV ) and size. For each mineral phase examined, smaller average crystal sizes characterize the microlite-rich bands in contrast to microlite-poor bands, which contain relatively larger crystals of lower NV . Magmatic H2O concentrations of microlite-rich and microlite-poor bands show no statistical difference between the textures. Calorimetric measurements yield similar glass transition temperatures and cooling rates for adjacent bands. These observations suggest that microlite heterogeneities could not have developed during late stage cooling and degassing during flow emplacement, as such textural variations imply distinct cooling and/or degassing histories. Rather, textural heterogeneities must have formed during flow in the conduit.

Hydrothermal annealing experiments were conducted on natural fragmented rhyolite in order to simulate the welding process in silicate melt and to provide first-order estimates of the time scales and deformation required to produce flow bands. Flow bands formed in experiments conducted at H2O-vapor pressures of 50 and 100 MPa, and for temperatures ranging from 800 to 850 °C. In each case, bands formed as a result of redistribution of oxide-rich domains and grain boundary coatings in annealed glass powders that underwent viscous deformation. Experiments suggest that bands may form on relatively short time scales (∼7 d) and for small bulk strains (∼1). Band formation may be promoted by high melt-H2O concentrations, shear stress, and viscous and frictional heating.

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GSA Special Papers

Kinematics and dynamics of lava flows

Michael Manga
Michael Manga
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Guido Ventura
Guido Ventura
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Geological Society of America
Volume
396
ISBN print:
9780813723969
Publication date:
January 01, 2005

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