Experimental constraints on bubble interactions in rhyolite melts: implications for vesicle size distributions

We have studied interactions between bubbles of two distinct size classes i n rhyolite melts experimentally decompressed between 200 and 80 MPa. The fi rst set of 'decompression' bubbles has a size range (R-dec) of 1-11 mu m an d is formed from nucleation and growth upon isothermal decompression of the melt. The larger populations of 'hydration' bubbles are on average 30-40 m u m in radius (R-hyd) and are formed from pore spaces present that were fil led with water vapor during the saturation runs prior to the decompression experiments. The first type of interaction results in the elongation of dec ompression bubbles oriented radially around the larger hydration bubbles. T he degree of elongation increases both as a function of distance and with i ncreasing ratio of hydration to decompression bubble size (R-hrd/R-dec). Th e second type of interaction studied results in a reduction of the size of decompression bubbles located within a range of distances from 10 to 65 mu m from a hydration bubble surface, relative to the modal size of the unaffe cted bubbles in the same sample. In addition, within an average distance of 10 mu m, melt next to the hydration bubble surface is depleted in decompre ssion bubbles. Our results indicate that concentration gradients in the mel t are probably responsible for bubble size reduction and the depleted zones , because the predicted time scales for Ostwald ripening are much longer th an those of the experiments. These effects persist even to the lowest endin g pressures studied (80 MPa), which indicates that size distributions of sm all bubbles may be affected by concentration gradients in the depleted melt shell surrounding large bubbles. Large bubbles present in an ascending mag ma, prior to a subsequent nucleation event, could therefore affect the grow th of the smaller bubble population occurring within the depleted melt shel l of the larger bubbles, and produce a bimodal vesicle size distribution. E longated decompression bubbles may be strained as a result of melt flowing away from the much larger hydration bubbles as they grow. Estimates of capi llary number (Ca) plotted against deformation (D-f) indicate that bubbles i n water-rich rhyolite melts are deformable, even at small sizes (1 mu m) an d small values of Ca. Our results show a different trend of D-f with Cn tha n previous studies in non-geological systems predict, indicating that visco sity effects may be important. The preservation of deformation textures dep ends strongly on relaxation time, explaining the lack of deformation textur es in less viscous natural lavas. (C) 2000 Elsevier Science B.V. All rights reserved.