Rapid water exsolution, degassing, and bubble collapse observed experimentally in K-phonolite melts

We performed three sets of decompression experiments on a hydrated K-phonolite melt at 880 degrees C in order to study the exsolution and degassing behavior of this low viscosity melt. The range of decompression rates studied was 0.01 MPa/s to 0.25 MPa/s. The pressure range was 200 MPa to 10 MPa. We determined that the melt exsolved water in equilibrium at all pressures at the decompression rates studied using micro-reflectance FTIR, a new method which is particularly well suited to determining the dissolved water concentration in high porosity experimental samples. Below 40-50 MPa at all decompression rates, the samples exsolved water in equilibrium, but the sample porosities deviated from values derived from equilibrium calculations/experiments and instead porosities remained constant or even decreased at lower pressures. The bubbles in all samples were predominantly deformed with features characteristic of partially collapsed bubble textures apparent at low pressures. Analysis of the bubble size distributions and deformation parameters indicate that melt shearing due to bubble growth and attendant sample expansion may cause the bubbles to deform, leading to the formation of bubble chains. The deformation and alignment of bubbles increases connectivity and lowers the porosity at which extensive permeability develops. In this situation, many of the connected bubbles are in contact with external vapor leading to significant degassing at 40-50 MPa. This degassing reduces the sample porosity and creates collapsed bubble textures at low pressures. This behavior differs significantly from that of hydrated rhyolite melts. (C) 2008 Elsevier B.V. All rights reserved.